Chemical Engineering And Processing 47 (2008) 1509–1519

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Available online at www.sciencedirect.com Chemical Engineering and Processing 47 (2008) 1509–1519 Coagulation and flocculation of laterite suspensions with low levels of aluminium chloride and polyacrylamids D´esir´e Dihang a,b , Pierre Aimar b,∗ , Joseph Kayem a , Sylv`ere Ndi Koungou a,b a TEFI Unit-ENSAI/IUT, University of Ngaoundere, P.O Box 455, Ngaoundere, Cameroon b LGC-CNRS-UMR 5503 Universit´ e P Sabatier Toulouse Cedex 9, France Received 28 July 2006; received in revised form July 2007; accepted July 2007 Available online 18 July 2007 Abstract Laterite particles in suspension undergo auto-flocculation as the concentration increases from 160 NTU, and therefore, coagulation and flocculation properties are affected The critical coagulant concentration of laterite by aluminium chloride increases when the initial turbidity is less than 160 NTU, but decreases with the initial turbidity for more turbid ones The maximum concentration is fourth of the standards for potable water In all cases, the critical Zeta potential for coagulation equals ca −20 mV The Zeta potential appears to be a more relevant parameter to study coagulation than the turbidity of the supernatant Flocculation either by non-ionic (PAM-N), cationic (PAM-C), or anionic (PAM-A) high molecular weight polyacrylamids promotes turbidity reduction of pre-coagulated laterite suspensions This turbidity reduction is independent on the amount of polymer added when the suspension is coagulated at the CCC In the other cases, turbidity reduction depends on polymer concentration For suspensions of high turbidity, flocculation does not improve significantly the efficiency as compared to coagulation At low concentration, PAM-N and PAM-A not significantly modify the Zeta potential of the particles, enabling it to remain a relevant parameter to monitor destabilisation by combined coagulation and flocculation Laterite particles are very sensitive to the presence of PAM-C, which induces charge reversal even at very low concentration The critical concentration for flocculation is lower than 0.1 mg/L © 2007 Elsevier B.V All rights reserved Keywords: Laterite; Coagulation; Flocculation; Potable water Introduction Laterite is the major component found in raw water in most tropical regions in Africa, and its removal represents the main objective of the drinking water processes Laterite is a clay material that confers to raw water a red colour and hazy aspect It is also known to be the main vector of arsenic contamination in ground water in many regions of the world Because of the lack of knowledge on this clay and/or inappropriate process, it is common to find suspended particles in tap water, especially during the rainy season Rather than the distribution system (Lehtola et al [1]), investigations reveal the inefficiency of the clarification process, where particle removal is achieved by decantation and ∗ Corresponding author at: LGC-CNRS-UMR 5503 Universit´e P Sabatier Toulouse Cedex 9, France Tel.: +335 15 58 304; fax: +335 15 56 139 E-mail address: aimar@chimie.ups-tlse.fr (P Aimar) 0255-2701/$ – see front matter © 2007 Elsevier B.V All rights reserved doi:10.1016/j.cep.2007.07.002 filtration through sand beds Particle removal is promoted by coagulation with aluminium chloride and by flocculation with polymers In these tropical regions, where water plants are rather old, but where it is necessary to comply with turbidity standards for obvious health issues, it is important to use as little coagulant as possible, in order to fulfil both standards and economical requirements Moreover, the known implication of aluminium in Alzheimer disease makes this issue of using low coagulant level a worldwide problem In drinkable water application, aluminium salt concentration depends on the physicochemical properties influencing salt precipitation So, the salt concentration varies with the pH, ionic force, nature and concentration of ionic species, amount of organic matter, temperature, etc In practical, hydroxide precipitation occurs above 40 ␮M aluminium added but OMS recommended less than 200 ␮g/l of aluminium in treated “potable” water However, using such low coagulant dosage makes it very difficult to locate the concentration giving maximum elimi- 1510 D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 nation of the suspended particles, unless jar test experiments are carried out almost continuously (Gregory and Duan [2], Gregory [3]) The literature abounds with studies on the coagulation–flocculation of clay material, but most of them focus either on concentrated suspensions as generally encountered in mining processes, or on waste water process with high content in organic matter Contrarily, this work investigates destabilisation by coagulation and flocculation of laterite suspensions diluted to over 10-fold the usual cases studied in the literature and where electrokinetics aspects of the suspended particles are important It is aimed in filling the gap in the literature on the destabilization characteristics of laterite suspensions with regards to the classical drinkable water process Materials and experimental methods 2.1 Materials Raw laterite clay was obtained from the banks of the river Bini in Cameroon The sample was washed with ultra pure water and dried The coagulant, aluminium chloride, was prepared daily as g L−1 solution to avoid polymerisation in solution Graciously supplied by FLOERGER, the flocculants, a cationic (FO8990 SEP), an anionic (AH912 SEP) and a non-ionic (FA920 SEP) high molecular weight polyacrylamids, were used for the flocculation of the pre-coagulated laterite suspensions They were prepared as 0.2 g L−1 solutions in distilled water and kept for a week at room temperature Sodium hydroxide and hydrochloric acid, used for pH adjustments, were prepared as 1% (w/w) solutions Potassium chloride was used to set the ionic strength Laterite suspensions were prepared in two steps: first, a dispersion–hydration of the dried powder in distilled water and second a dilution of the obtained suspension to the required turbidity The laterite powder (50 g) is added to water (5 L) under vigorous agitation (750 rpm) and the pH raised to 10 Sodium azide is added for preservation (0.02%, w/w) and mixing is prolonged for 12 h The suspension is then allowed to settle for h and the supernatant is withdrawn It is kept at room temperature and can be used for a week Knowing that raw water turbidity ranges from 22 NTU in the dry season to around 350 NTU during the rainy season, we selected five turbidities (30, 90, 150, 180 and 300 NTU) for our study For this purpose, the supernatant is diluted to the desired turbidity 10−3 M in KCl was added raising the conductivity to around 150 ␮S/cm and the pH was adjusted to All reagents used were of analytical grade Laterite suspensions exhibit negligible organic matter, around mg/L of TOC measured on a TOC Analyzer model VCSN from Shimadzu 2.2 Experimental methods 2.2.1 Characterisation of laterite The particle distribution measurements were carried out using a Malvern Mastersizer 2000 analyser It also gives the measure of specific surface area of the particles in square meter per gram The characterisation of surface structure of the lat- Table Mean hydrodynamic radius and the Zeta potential of the floculants Floculant Mean hydrodynamic radius (nm) Zeta potential (mV) PAM-N PAM-A PAM-C 63.7 74.6 371.0 −5.71 ± −30.0 ± +82.9 ± erite was carried out using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) of the laterite suspension Zeta potential indicating surface charge of the particles was measured with a Malvern Zetasizer The measurement was carried out in various electrolytes, (KCl, NaCl and CaCl2 ) at various concentrations and pH Turbidimetry, light scattering using a Turbiscan-On-Line (Formulaction, France), particle counting (Malvern coulter counter) and dry mass determination by desiccation were used to characterise the properties of laterite suspensions 2.2.2 Characterisation of the floculants We could not obtain the molecular weights from the manufacturer However, we measured the hydrodynamic radius so as to have an idea of the molecular size, as shown in the table (Table 1) High charge density on PAM-C enable it to adopt a more extended configuration in suspension while PAM-A and PAMN, with their low charge density exhibit lower repulsion between charged polymer segment thus a less extended configuration 2.2.3 Coagulation and flocculation study Coagulation and flocculation experiments were carried out in a classical jar test apparatus The flocculants were added after coagulation as it has proven to give better results than other combinations (Hogg [4], Somasundaran [5]) After of agitation at 230 rpm, the coagulant was added drop by drop and left to coagulate particles for The stirring speed was then reduced to 30 rpm for 15 to allow flocs growth and the suspension was allowed to settle for 30 For flocculation of coagulated suspension, the flocculant was added under rapid mixing before the flocs are left to grow The supernatant was further taken out at a fixed cm below the air–liquid interface for turbidity, Zeta potential, and TOC measurements The turbidity measurements were performed on a Hach 2000N turbidimeter and through light scattering measurements with a Turbiscan-On-Line The Zeta potential was measured on a Malvern Zetasizer All experiments were performed at room temperature The experimental procedure is summarised in Fig The performance of coagulation and flocculation processes is assessed through the turbidity reduction (%) given by (Ozkan and Yekeler [6]): Turbidity reduction (%) = Ti − Tf × 100 Ti (1.1) where Ti (NTU) is the initial turbidity of the suspension and Tf (NTU) is the supernatant turbidity Amounts of coagulant and D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 1511 Fig Protocol for coagulation (a) and coagulation/flocculation (b) of laterite suspension flocculant are expressed both in weight ratio (mass per mass of laterite), associated to molar concentration for the coagulant and mass per volume concentration for the flocculants Results and discussion 3.1 Properties of laterite The investigation of the composition of the laterite by whole-rock analysis and of the main components, using SEM/ microprobe, TEM and XRD, reveals in Fig the presence of platelets of clay material and clusters of iron oxides, corresponding to gibbsite, goethite and hematite titanium oxide The clay composition indicates the presence of high amount of kaolinite and traces of smectite Youngue-Fouateu et al [7]) reported similar results on different samples of laterite from Cameroon In Fig 3, the Zeta potential of the laterite suspensions in absence and presence of different salts is plotted as a function of pH In the presence of CaCl2 , the Zeta potential is constant and independent of the pH Contrarily, in the absence, as well as in the presence of added KCl and NaCl, laterite suspensions show a pH dependence of the Zeta potential The isoelectric point (IEP) is located around pH 3, the Zeta potential is positive below the IEP and negative above Similar Zeta potential is obtained in the absence and in the presence of 10−3 M KCl At pH where the destabilisation tests are performed, the Zeta potential is −35 mV Silica and aluminium oxides have a more prominent influence on the Zeta potential, as indicated by the significantly more negative values measured under alkaline conditions in contrast to acidic one This behaviour is in good agreement with the data on kaolinite and oxides, but the Zeta potential of laterite in the absence of salt is greater in magnitude than the values reported for kaolin (Besra et al [8–10], McFarlane et al [11]) and lower than the values for iron oxides (McGuire et al [12]) The particle size analysis (Fig 4) shows a distribution with an average diameter d50 of approximately 0.25 ␮m, which is smaller than the value obtained by Besra et al [8] The specific surface area calculated from this diameter equals 26.7 m2 /g, and it is very high compared to the result on kaolin (Besra et al [8]) but closer to the one obtained on iron oxides (McGuire et al [12]) It is smaller than the BET area reported on laterite by Blakey and James ([13]) These results suggest that our mode of preparation promotes the maximum of individualization of the suspended particles and a complete elimination of the coarse phase Dispersion properties of laterite suspension examined through the measurement of the turbidity, particle number, light scattered and concentration are given in Fig Graph (a) presents two regions of linear properties that separate at 160 NTU Above this turbidity, the slope is lower The same behaviour is obtained when plotting the turbidity versus the light scattered or the number of particles This suggests that for turbidity smaller than 160 NTU, the suspension behaves as dilute while for higher turbidities, it behaves as concentrated In concentrated suspensions, particles influence each other, and several authors suggest that for clay suspension, the positive charges from the edge face orientate towards the negative charges from the lateral faces in an auto-flocculation reaction (Blakey and James [13]) This observation is important because it can have a critical impact on the destabilisation behaviour of these types of suspension, both on the mechanism involved and the destabilisation results According to our results, this mechanism would be relevant for turbidities higher than 160 NTU 3.2 Coagulation of stable laterite suspension Zeta potential (mV), turbidity reduction (%) are plotted as a function of the coagulant concentration expressed in mass ratio (mg of AlCl3 /g of laterite) and in mass per volume of suspension (␮M of AlCl3 ) for different initial turbidities (Fig 6) As a consequence of particles removal, the turbidity reduction increases with coagulant concentration until it reaches a maximum value, then it decreases with the addition of more coagulant This is often called restabilisation of the system by addition of an excess of coagulant For dilute suspensions, the mass per volume concentration at the critical coagulation concentration CCC increase with the initial turbidity while for concentrated suspension, it decreases On the other hand, for both dilute and concentrated suspensions, the corresponding coagulant mass ratio diminishes as the initial turbidity increases These results confirm the suggestion of an autocoagulation of particles as the suspension turbidity increases As a consequence, the coagulant mass ratio at the CCC at 30 NTU is three folds that at 300 NTU For both cases, the residual turbidity remains high and not acceptable for drinking purposes 1512 D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 Fig Scanning electron micrograph (SEM) (a), transmission electron micrograph (TEM) (b) and corresponding chemical composition (XRD) of global laterite (c); platelet material (d); iron oxides clusters (e) Elements Carbon Silica Oxygen Aluminium Iron Titanium % 13.39 12.54 58.27 13 1.70 0.16 The Zeta potential increases with the amount of coagulant until charge reversal, and stabilization around 30 mV For dilute suspensions, the point of charge reversal (PCR) corresponds to restabilisation (30 and 90 NTU), while it is close to the optimum turbidity reduction for higher turbidity (>150 NTU) suspensions (Fig 6) Turbidity reduction increases with the initial turbidity for dilute suspensions, but it remains constant, however higher, for concentrated suspensions In all cases, the critical Zeta potential is constant and equal to ca −20 mV In Fig 7, the supernatant turbidity is given for various decantation times The results show that, as decantation time increases, the supernatant turbidity decreases and reaches the same maximum value for 30 and 300 NTU suspensions The curves also high- D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 1513 Fig Colloidal titration of a laterite suspension of initial turbidity 150 NTU lights that restabilisation of the suspension, due to excess of coagulant disappears as decantation time increases This suggests that what is called restabilisation would in fact be a kinetics effect 3.3 Discussion on coagulation NMR spectra on the aluminium chloride solution used for coagulation show that only the monomeric octahedral hexahydrate Al3+ (H2 O)6 is added to the laterite suspension In all the jar test experiments, this addition decreases the pH of the suspension to values between 5.5 and 6.5 In this region of pH, there is an intensive formation of in situ Al13 polymers that significantly improves the efficiency of AlCl3 These polymers coexist with lower amounts of monomeric and medium polymerised positively charged aluminium species (Chengzhi Hu et al [14]) Coagulation occurs by particle charge neutralisation and depends closely on the quantity of added coagulant and consequently on the Zeta potential of the suspended particles (Lartiges et al [15], Gregory [16], Wang et al [17], Duan and Gregory [18], Hu et al [14] At the CCC, we observed that the Zeta potential is negative This can be due to soluble silica that lowers the Zeta potential of laterite clay, as it is more negative Fig Particle size distribution of laterite suspension 90 NTU Fig Turbidity (NTU) vs concentration (mg/L) (a) and transmission (%) (b) of laterite suspension than hydrolysis products from aluminium at the same pH (Duan and Gregory [19]) These results suggest a great influence of kinetic aspects in particle size growth due to collision In fact, although the destabilisation process is effective at low initial turbidity, agitation is unable to promote sufficient collision and particle growth, making sedimentation slow and leading to turbid supernatant As the initial turbidity increases (concentrated suspension), kinetic limitations disappear and turbidity reduction becomes high and constant The maximum turbidity removal is obtained at the critical concentration of coagulation; it is around 60% for low turbidities and 90% for high turbidities When the settling time is increased, the turbidity reduction is constant and greater than 95% and the apparent restabilisation of the suspension by excess of coagulant disappears The point of charge reversal (PCR) corresponds to the restabilisation of the system for low turbidity suspensions, but it coincides with the optimum turbidity reduction for concentrated suspensions and long settling times This PCR can therefore not be used as a process control parameter At CCC, the value of the turbidity reduction varies, but the Zeta potential is constant (−20 mV) Therefore, Zeta potential proves to be a more relevant parameter to predict the coagulation of laterite suspensions than the turbidity of the supernatant or the PCR Then, using turbidity to control industrial plants can generate errors as it is dependent on many parameters It can be emphasised 1514 D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 Fig Turbidity reduction after 30 of decantation, TR30min (%) and Zeta potential vs coagulant mass ratio (mg/g) and coagulant concentration (␮M) for laterite suspensions of various turbidities: 30 NTU (a), 90 NTU (b), 150 NTU(c), 180 NTU (d) and 300 NTU (e) that the maximum concentration of aluminium used, 40 ␮M, is five-fold smaller than the standards allowed in processes for drinking water while the minimum value, ␮M is quite equivalent to the standards Definitely, coagulation does not promote sufficient particle removal for water to become potable Table summarises the coagulation characteristics of various laterite suspensions 3.4 Flocculation of coagulated laterite suspension In Fig 8, the turbidity reduction (%) for the flocculation of pre-coagulated laterite suspensions of 30 NTU initial tur- bidity is plotted as a function of the amount of non-ionic PAM-N (a), cationic PAM-C (b), and anionic PAM-A (c) polyacrylamid polymers added At CCC, all polymers promote turbidity reduction, and there is no restabilisation of the suspension with excess of flocculant This particle removal is the maximum achievable, from around 50% for coagulation to 90% for flocculation In addition, PAM-N also induces turbidity reduction for salt concentration lower than CCC, and to a lower extent, slightly above the CCC PAM-C extends the area of efficiency of PAM-N to coagulant concentration slightly greater than CCC Finally, PAM-A provides turbidity reduction at almost all concentrations, except at very low aluminium D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 1515 Fig Effect of settling duration on turbidity reduction for 30 NTU (a) and 300 NTU (b) laterite suspensions (turbidity reduction, TRX , where x is the time of decantation) dosage Contrarily to PAM-N, PAM-C and PAM-A induce a dependence of the turbidity reduction to polymer concentration For high turbidity laterite suspensions (Fig 9), PAM-N promotes turbidity reduction for almost all coagulant concentrations, from around 80% to nearly 100%, but has no effect in the absence of coagulant The critical flocculation concentration (CFC) at CCC diminishes of at least 30-fold with suspension turbidity, from mg/g at 30 NTU to 0.15 mg/g at 300 NTU PAM-C and PAM-A are more effective than PAM-N as they promote turbidity removal on a wider range of coagulant concentration However, as PAM-C and PAM-A are not allowed for drinking water processes, we present only flocculation with PAM-N for laterite suspension of 300 NTU initial turbidity In Figs 10 and 11, the Zeta potentials of the particles in the supernatant of the previously flocculated systems are presented as a function of the coagulant and flocculant concentration For initial high turbidity (300 NTU), all the Zeta potential curves obtained with PAM-N are similar and aligned on the curve obtained for coagulation (Fig 10) For the lowest turbidity suspension (30 NTU), Fig 11 shows that PAM-N and PAM-C tend to increase Zeta potential while PAM-A decreases it At low flocculant concentration (1.5 mg/g of laterite), the Zeta potential curves for coagulation and flocculation are similar for PAM-N and PAM-A Higher amounts of PAM-N (≥5 mg/g) induce a rapid growth of Zeta potential at low aluminium content, and just before the CCC the Zeta potential approaches a plateau value close to mV As a consequence of this plateau value, the flocculation curve intercepts with the coagulation curve at a point below the PCR At this particular point, the coagulated and flocculated particles have equal Zeta potential PAM-C inverted the Zeta potential of the particles, indicating that for the amount of polymer used, we are already overdosing it This behaviour confirms the high charge density of the PAMC, as indicated by the supplier To compare the effect of the three polymers on laterite particles, the turbidity reduction and Zeta potential is plotted versus the flocculant dosage, for a laterite suspension at 30 NTU, coagulated at the CCC (Fig 12) The results show that all polymers exhibit the same efficiency for particle removal at CCC This efficiency is independent of the particle Zeta potential 3.5 Discussion on flocculation PAM-C adsorbs on the laterite particles surface via hydrogen bonding interactions between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups The electrostatic attractions between the positively charged polymer segments and the negatively charged laterite particles promote adsorption and result in raising the particles Zeta potential The amount of polymer adsorbed increases with increasing polymer concentration, generating higher Zeta potential Similar results are reported for flocculation of kaolin with PAM-C (Nasser and James [20]) High molecular weight combined to segment repulsion enable particle bridging by the adsorbed polymer Table properties of coagulated laterite suspension Initial turbidity (NTU) AlCl3 (mg/g laterite) CCC (␮M) Zeta potential at CCC (mV) Residual turbidity at CCC (NTU) Turbidity reduction at CCC (%) Coagulant mass ratio at the PCR (mg AlCl3 /g laterite) Coagulant concentration at the PCR (␮M) Turbidity reduction at the PCR (%) 30 89 −21 11 65 150 17 90 40 17 −21 26 71 59 24 150 38 27 −20 29 86 50 33 82 180 53 40 −21 20 90 60 52 86 300 23 33 −19 15 95 30 40 90 1516 D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 Fig Turbidity reduction (%) vs PAM-N (a), PAM-C (b) and PAM-A (c) concentration of coagulated laterite suspensions of 30 NTU initial turbidity Fig Turbidity reduction (%) vs PAM-N concentration of coagulated laterite suspension of 300 NTU initial turbidity Fig 10 Zeta potential (mV) (%) vs PAM-N concentration of coagulated laterite suspensions of 300 NTU initial turbidity D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 1517 Fig 11 Zeta potential (mV) (%) vs PAM-N (a), PAM-C (b) and PAM-A (c) concentration of coagulated laterite suspensions of 30 NTU initial turbidity Likewise, PAM-A adsorbed on laterite particles via hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, but the amount adsorbed is minimised by the electrostatic repulsion between the particles negative charges and the negative polymer segments The polymer adsorbed results in a shift in position of the plane of shear, hence generating a small decrease in the magnitude of the Zeta potential (Nasser and James [20], Mpofu et al [21]) PAM-A can also adsorb on the positively charged edges of the particles, creating extra negative charges that lower Fig 12 Turbidity reduction (%) and Zeta potential of flocculated laterite suspensions containing AlCl3 at the critical concentration for coagulation, 30 NTU initial turbidity 1518 D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 the overall Zeta potential Moreover, through adsorption on particle negative surface via a polycation (Al3+ , Ca2+ , etc.), PAM-A can similarly lower the particle surface charges The repulsive forces between polymer segments allow the polymer molecules to be extended and to produce loops and tails that promote bridging mechanisms and the formation of large open-structure flocs (Gregory [3], McGuire et al [12]) For PAM-N concentration less than mg/g, adsorption is probably patchy due to a contracted conformation and to the fact that particle surface coverage is less than the optimum As a consequence, there is a little change in the Zeta potential with the amount of PAM-N added This suggests that the adsorbed polymer layer thickness has a minor charge-shielding effect caused by the shift in shear plane at which the Zeta potential is measured Brooks ([22]) noticed that the Zeta potential of particles should increase after adsorption of a neutral polymer, as long as the shear plane is not shifted too far from the negative particle surface, due to the change in the ion distribution in the diffuse double layer The adsorbed layer thickness of PAM-N on iron oxides and kaolin plateaus at approximately 2.3 nm after the addition of 3–5 mg/g of solid (Mpofu et al [21], McGuire et al [12]) Further addition of flocculant can lead to chemisorptions on particle surface (Besra et al [10], Besra et al [23]), that can promote aggregation without having any effect on Zeta potential The results indicate a greater adsorption of PAM-N than PAMA, probably due to adsorption on iron oxides (McGuire et al [12]) Bridging mechanism is enhanced by the high suspension concentration, which enables high molecular weight segments of the PAM-N to bind towards many particles As a consequence of this high molecular weight, the amount of PAM-N used for flocculation diminishes as the suspension turbidity increases Conclusion This work consists in destabilising laterite suspensions by coagulation and flocculation, as usually encountered in the classical potable water process The results pointed out the autocoagulation of laterite clay at turbidity greater than 160 NTU and the influence on this phenomenon on destabilisation process Another important result is the fact that coagulation can be monitored through Zeta potential measurements, as turbidity removal is always optimum at ca −20 mV, this value being independent of the initial turbidity and other coagulation parameters For low polymer concentration, Zeta potential is also a relevant parameter to study flocculation, as polymer adsorption does not significantly modify the Zeta potential of the particles Coagulation happens by charge neutralisation, the positive hydrolysis product of AlCl3 reducing the global negative surface charge of the particles Turbidity reduction increases with the initial turbidity of the suspension Flocculation mechanism depends on the type and concentration of flocculant All three polymers investigated here are efficient in removing turbidity, and their efficiency is independent on flocculant type at the CCC, PAM-N being the only one agreed for drinking water processes At low flocculant concentration, the amount of flocculant adsorbed on the particles surface is constant and independent on the degree of coagulation This amount is lower than the adsorption capacity of the particle surface making charge neutralisation limited by the amount and sign of charge carried by the polymer Therefore, the magnitude of the particle Zeta potential is governed by coagulant dosage At higher concentration, PAM-C induces charge reversal due to excess charge adsorb at the surface Contrarily, adding more PAM-A or PAM-N has little effect on the Zeta potential, as PAM-A exhibit little adsorption capacity on the negative surface and PAM-N can chemisorbs on the particle surface, resulting in negligible effect on the Zeta potential Acknowledgements The authors wish to thank Formulaction (France) for providing access to the Turbiscan on-line equipment, SNF FLOERGER, (France), for free samples of polyacrylamides and the French Ministry of Cooperation and CNRS for financial support to D.D and S.N.K References [1] M.J Lehtola, T.K Nissinen, I.T Miettinen, P.J Martikainen, T Vartiainen, Removal of soft deposits from the distribution system improves the drinking water quality, Water Res 38 (2004) 601–610 [2] J Gregory, J Duan, Hydrolysing metal salts as coagulants, Pure Appl Chem 73 (2001) 2017–2026 [3] J Gregory, Particles in Water, Properties and Processes, IWA Publishing, London, 2005 [4] R Hogg, Flocculation and dewatering, Int J Mineral Process 58 (2000) 223–236 [5] P Somasundaran, Principles of flocculation, dispersion and selective flocculation, in: Proc Int Symp on Fine Particles Processing, Published by Am Inst Min., Metall., & Pet 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Innovative Research Group (No 50621804) This project was supported by China Postdoctoral Science Foundation also Dr Meiping Tong is also acknowledged for kind assistance in language revision References Amirtharajah, A., Mills, K.J., 1982 Rapid-mix design for mechanisms of alum coagulation J Am Water Work Assoc 74 (4), 2–10 Bertsch, P.M., 1989 Aqueous polynuclear aluminum species In: Sposito, G (Ed.), The Environmental Chemistry of Aluminum CRC Press, Boca Raton, Florida, pp 87–115 Bottero, J.Y., Tchoubar, D., Cases, J.M., Fiessinger, F., 1982 Investigation of the hydrolysis of aqueous solutions of aluminum chloride Nature and structure by small-angle X-ray scattering J Phys Chem 86, 3667–3673 Carlson, K., Via, S., Bellamy, B., Carlson, M., 2000 Secondary effects of enhanced coagulation and softening J Am Water Work Assoc 92 (6), 63–75 Crozes, G.P., White, P., Marshall, M., 1995 Enhanced coagulation: its effect on NOM removal and chemical costs J Am Water Work Assoc 87 (1), 78–89 Dempsey, B.A., Sheu, H., Ahmed, T.T.M., Mentink, J., 1985 Polyaluminum chloride and alum coagulation of clay–fulvic acid suspensions J Am Water Work Assoc 77 (3), 74–80 Edwards, M., 1997 Predicting DOC removal during enhanced coagulation J Am Water Work Assoc 89 (5), 78–89 Edzwald, J.K., 1993 Coagulation in drinking water treatment: particles, organics and coagulants Water Sci Technol 27 (11), 21–35 Hsu, P.H., Cao, D., 1991 Effect of acidity and hydroxylamine on the determination of aluminum with ferron Soil Sci 152, 210–219 Jacangelo, J.G., DeMarco, J., Owen, D.M., Randtke, S.J., 1994 Selected processes for removing NOM: an overview J Am Water Works Assoc 86 (1), 64–77 Lind, C., 1994 Reduce residuals with PAC1 coagulants Public Works, Dec., 43 Matsui, Y., Yuasa, A., Kamei, T., 1998 Dynamic analysis of coagulation with alum and PACl J Am Water Work Assoc 90 (10), 96–106 Odegaard, H., Fettig, J., Ratnaweera, H.C., 1990 Coagulation with prepolymerized metal salts In: Hahn, H.H., Klute, R (Eds.), 1673 Chemical Water and Wastewater Treatment Springer Verlag, New York, pp 189–220 O’Melia, C.R., Yao, K., Gray, K., Tobiason, J.E., 1987 Raw water quality, coagulant selection, and solid–liquid separation Presented at the 1987 AWWA Annual Conference Seminar on Influence of Coagulation on the Selection, Operation, and Performance of Water Treatment Facilities, Kansas City, MO, USA, June 14 Parker, D.R., Bertsch, P.M., 1992 Identification and quantification of the Al13 trideca-meric tolycation using ferron Environ Sci Technol 26, 908–914 Pasrthasarathy, N., Buffle, J., 1985 Study of polymeric aluminum (III) hydroxide solutions for application in wastewater treatment: properties of the polymer and optimal conditions preparation Water Res 19, 25–36 Sinha, S., Yoon, Y., Amy, G., Yoon, J., 2004 Determining the effectiveness of conventional and alternative coagulants through effective characterization schemes Chemosphere 57, 1115–1122 Tang, H.X., 1990 Basic studies of inorganic polymer flocculants Environ Chem (3), 1–12 (in Chinese) Tseng, T., Edwards, M., 1999 Predicting full-scale TOC removal J Am Water Work Assoc 91 (4), 159–170 Tseng, T., Segal, B.D., Edwards, M., 2000 Increasing alkalinity to reduce turbidity J Am Water Work Assoc 92 (6), 44–54 USEPA, 1998 Enhanced Coagulation and Enhanced Precipitative Softening Guidance Manual, EPA, Office of Water and Drinking Ground Water, Washington, DC, pp 20–50 Van Benschoten, J.E., Edzwald, J.K., 1990 Chemical aspect of coagulation using aluminum salts – I: hydrolytic reactions of alum and polyaluminum chloride Water Res 24, 1519–1526 Wang, D.S., Tang, H.X., 2001 Modified inorganic polymer flocculantPFSi: its preparation, characterization and coagulation behavior Water Res 35, 3418–3428 Wang, D.S., Sun, W., Xu, Y., Tang, H.X., Gregory, J., 2004 Speciation stability of inorganic polymer flocculant-PACl Colloids Surf A 243, 1–10 Yan, M.Q., Wang, D.S., Qu, J.H., He, W.J., Chow, C., 2007 Relative importance of hydrolyzed Al(III) species (Ala, Alb and Alc) during coagulation with polyaluminum chloride: a case study with the typical micro-polluted source waters J Colliod Interface Sci 316, 482–489 Yan, M.Q., Wang, D.S., You, S.J., Qu, J.H., Tang, H.X., 2006 Enhanced coagulation in a typical North-china water plant Water Res 40, 3621– 3627 ARTICLE IN PRESS Journal of Environmental Management 85 (2007) 1009–1014 www.elsevier.com/locate/jenvman Nanoparticles in wastewater from a science-based industrial park—Coagulation using polyaluminum chloride M.R Changa, D.J Leea,b,Ã, J.Y Laib a Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan R&D Center for Membrane Technology, Chung Yuan Christian University, Chungli 32023, Taiwan b Received February 2006; received in revised form 28 October 2006; accepted November 2006 Available online January 2007 Abstract The Hsinchu Science-based Industrial Park (HSIP) is the hi-tech manufacturing hub of Taiwan Wastewater from the HSIP contains numerous nano-sized silicate particles whose size distributions peak at and 90 nm A 3–5 mg lÀ1 as Al dose of polyaluminum chloride (PACl) was used in the field to coagulate these particles, but the removal efficiency was low Laboratory scale tests indicated that although PACl coagulation removed 52% of the turbidity and 48% of the chemical oxygen demand (COD) from water, its effect on nano-particle removal was minimal About 58% of the soluble COD was associated with colloidal Si particles A light scattering test and transmission electron microscopy (TEM) demonstrated that the nano-particles agglomerated in approximately linear aggregates of sizes 100–300 nm Prolonged contact between residual PACl and the nano-particles generated large aggregates with sizes of up to 10 mm and a fractal dimension of 2.24–2.63 The results presented herein should be of interest in the processing of ‘‘high-tech’’ wastewater that contains nanosized silica particles r 2006 Elsevier Ltd All rights reserved Keywords: Nano-particles; Coagulation; Fractal dimension; Removal Introduction The Taiwanese government has constructed 96 industrial parks across the island to facilitate industrial development The Hsinchu Science-based Industrial Park (HSIP), established in 1980, is home to 350 companies in, for example, the integrated circuit (IC), computer and peripheral, telecommunications, optoelectronics, biotechnology and precision machinery industries The HSIP has not only substantially affected the development of Taiwan’s economy, but has also gained an international reputation in the semiconductor and related information industries The HSIP currently has a secondary biological wastewater treatment works and a physical–chemical treatment plant with a capacity of 86,000 m3 dÀ1 Wastewater from the IC and optoelectronics industries represents 95% of the ÃCorresponding author Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Tel.: +886 2362 5632; fax: +886 2362 3040 E-mail address: djlee@ntu.edu.tw (D.J Lee) 0301-4797/$ - see front matter r 2006 Elsevier Ltd All rights reserved doi:10.1016/j.jenvman.2006.11.013 total flow rate, and 73% of the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD) Fluoride-containing wastewater and chemical mechanical polishing (CMP) wastewater dominate the wastewater stream, in which nano-sized CaF2 and silica constitute the primary particulate phase (Chuan et al., 2002; Yang et al., 2003; Chang et al., 2004) Individual plants in HSIP must treat their wastewater prior to discharge to the works The lack of clean water in Taiwan is such that HSIP companies are legally bound to recover and reuse over 85% of their wastewater A wastewater recycling program has thus been implemented in the park; however, nano-sized particles require high doses of polyaluminum chloride (PACl) to ensure coagulation, which forms a large volume of sludge Moreover, the efficiency of particle removal by conventional coagulation–sedimentation is too low to prevent the fouling of subsequent UF-RO membrane modules (Chang et al., 2006) Although the effluent COD range (30–70 mg lÀ1) has been well below the National Standards (100 mg lÀ1) over the last years, the HSIP has been asked by the local community to reduce its COD ARTICLE IN PRESS M.R Chang et al / Journal of Environmental Management 85 (2007) 1009–1014 a b 120 120 S1 S2 100 Intensity (%) Intensity (%) 100 80 60 40 80 60 40 20 20 1e-1 1e+0 1e+1 1e+2 1e+3 1e+4 1e+5 1e-1 1e+0 1e+1 1e+2 1e+3 1e+4 1e+5 size (nm) size (nm) c d 450 nm 120 120 2.08 mg l-1 100 Intensity (%) 100 Intensity (%) 1011 80 60 40 20 500mg/L PACl 80 450 nm 60 40 20 1e-1 1e+0 1e+1 1e+2 1e+3 1e+4 1e+5 size (nm) 1e-1 1e+0 1e+1 1e+2 1e+3 1e+4 1e+5 size (nm) 800 700 600 500 400 300 200 100 Na Ca Al Si 16 12 12.0 10.0 8.0 0.8 0.6 0.4 0.2 Al Al (0.45µm filtration) Si Si (0.45µm filtration) 14.0 Concentration (mg l-1) Concentration (mg l-1) Fig Size distributions of particles in wastewater samples: (a) Sample S1 filtered with 0.45-mm filter; (b) sample S2 filtered with 0.45-mm filter; (c) effluent from sample S1 coagulated with PACl dose of 2.08 mg lÀ1 followed by 0.45-mm filtration; (d) effluent from sample S1 coagulated with PACl dose of 10.4 mg lÀ1 followed by 0.45-mm filtration 0.0 S1 S2 S3 S1 S2 S3 Fig Concentrations of certain chemical elements in water samples particle size distributions of the 24 and 48 h settling samples were noted (data not shown), indicating that the nano-particles in the water samples were not settleable in the test The metal elements in the filtered water samples detected by ICP-AES included Na, Ca, Si and Al (Fig 3), of which Na and Ca were the products of neutralization and the removal of fluoride in the wastewater pre-treatment units in individual plants The CMP wastewater streams contained mainly Si and Al particles The 0.45 mm filtration removed only 15% of the Si from the water Additionally, the solubility of silica in water is low Therefore, most of the Si in the investigated filtered water samples was in colloidal (o0.45 mm) form According to Fig 3, the concentrations of Na and Ca did not drop after contact aeration or chemical coagulation (Fig 1) The biological unit removed 9% of Si, mostly from the suspended particles fraction (40.45 mm) The chemical coagulation unit effectively removed most of the suspended particles, but the removal rate of the colloidal fraction was limited Although 3–5 mg lÀ1 as Al of PACl was directly added to the chemical coagulation unit (Fig 1), only ARTICLE IN PRESS 1000 100 10 0.1 0.001 0.01 Following aggregation, the I–Q data exhibits a linear correlation over the range 30–300 nm For example, as Fig 6b indicates, linear regression of the I–Q data for a PACl dose of 2.08 mg lÀ1 as Al yields a gradient of À2.2470.06 with r2 ¼ 0.97 The fractal dimensions of the coagulated samples thus estimated are 2.2470.06, 2.3070.07, 2.5670.05, 2.6370.04 and 2.4970.07, for PACl doses as Al of 2.08, 4.16, 6.24, 8.48 and 10.6 mg lÀ1, respectively Restated, the self-agglomeration of fine particles in the 0.45 mm filtered water samples generated aggregates whose fractal dimensionality increased with PACl dose Figs 7a and b show the TEM images of the aggregates in the original and the coagulated water samples (2.08 mg lÀ1 as Al) In the original water sample (Fig 7a), linear aggregates of size 100–300 nm, as determined by the light 0.1 scattering test (Fig 6a), are observed Numerous particles of sizeo10 nm were dispersed in the sample Adding a coagulant caused particle agglomeration to form a large, fractal-like structure The nano-particles in the wastewater of HSIP coagulated to a limited extent (Fig 7a) These particles with a size of 100–300 nm could not be effectively removed from the sedimentation basin The interception efficiency of a normal sand filter is also doubtful However, the noted self-agglomeration of residual fine particles in filtered water samples reveals that retaining the effluent from the coagulation–sedimentation basin for 24 h in a calm hydrodynamic environment, and then rapidly filtering it through sand, may represent a cost-effective way to effectively remove particles in the effluent from HSIP, thereby reducing the associated SCOD (Fig 5) ARTICLE IN PRESS 1014 M.R Chang et al / Journal of Environmental Management 85 (2007) 1009–1014 Conclusions Hsinchu Science-based Industrial Park (HSIP) is a ‘‘high-tech’’ hub for Taiwan’s economy It is home to the integrated circuit (IC), computer and peripheral, telecommunications, optoelectronics, biotechnology and precision machinery industries The wastewater of HSIP contains numerous nanosized silicate particles whose size distributions peak at and 90 nm, which are not effectively removed by the existing works, which use a secondary biological stage and a coagulation–sedimentation unit with PACl as a coagulant Laboratory scale tests revealed that about 58% of the soluble COD was associated with the colloidal Si particles The effective removal of Si nanoparticles is critical to fulfilling the demands of the local community to reduce further COD emission from HSIP Although not significantly affecting the zeta potential, the PACl dose did reduce the residual turbidity of the supernatant from 6.42 NTU to under NTU with a PACl dose of 2.08 mg lÀ1 as Al A further increase in the PACl dose caused an incremental drop in residual turbidity A light scattering test and transmission electron microscopy (TEM) imaging demonstrated that the nano-particles agglomerated to generate linear aggregates of size 100–300 nm With a PACl dose of 2.08 mg lÀ1 as Al, the particles of size 1–5 nm were effectively coagulated Adding more PACl did not greatly change the size distribution of particles Prolonged contact between residual PACl and nano-particles formed large aggregates of size up to 10 mm and a fractal dimension of 2.24–2.63, facilitating COD removal from the wastewater The results presented herein should be of interest in processing ‘‘high-tech’’ wastewater that contains nanosized silica particles Acknowledgment The Center-of-Excellence Program on Membrane Technology, the Ministry of Education, Taiwan, ROC, financially supported this study References Chang, M.R., Chiang, L.I., Lee, D.J., Liu, J.C., Wu, N.M., Chen, W.C., Hsu, B.M., 2004 Conditioning of wastewater sludge from sciencebased industrial park using freezing and thawing Journal of Environmental Engineering, ASCE 130, 1552–1555 Chang, M.R., Lee, D.J., Lai, J.Y., 2006 Nano-particles in wastewater from science-based industrial park-novel thermal treatment Separation Science and Technology 41, 1303–1311 Chuan, T.C., Huang, C.J., Liu, J.C., 2002 Treatment of semiconductor wastewater by dissolved air flotation Journal of Environmental Engineering, ASCE 128, 974–980 Yang, G.C.C., Yang, T.Y., Tsai, S.H., 2003 Crossflow electro-microfiltration of oxide-CMP wastewater Water Research 37, 785–792 Journal of Hazardous Materials 143 (2007) 567–574 Removal of direct dyes by coagulation: The performance of preformed polymeric aluminum species Baoyou Shi a,∗ , Guohong Li b , Dongsheng Wang a , Chenghong Feng a , Hongxiao Tang a a State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O Box 2871, Beijing 100085, China b School of Water Resources and Environment, China University of Geosciences, Beijing 100083, China Received May 2006; received in revised form 22 September 2006; accepted 25 September 2006 Available online 29 September 2006 Abstract Removal of three direct dyes (Direct Black 19, Direct Red 28, and Direct Blue 86) by coagulation with three different Al based coagulants was investigated The main purpose of this paper is to examine the coagulation features of polymeric aluminum coagulants in treatment of dye-polluted waters and the emphasis was placed on the roles of preformed Al species, particularly Al13 The performance of Al13 in coagulation of dyes was observed through jar tests by comparing traditional Al salt, polyaluminum chloride (PACl), and purified Al13 The results showed that under most cases Al13 had significantly higher efficiency in removal of direct dyes than traditional Al salt and commercial PACl with the exception of Direct Red 28 removal under high pH range The coagulation of direct dyes could be greatly affected by pH Reducing pH was favorable for preformed Al species in a broad pH range For traditional Al coagulant, efficient dye removal only occurred in a relatively narrow pH range of near 6.0 The outstanding coagulation behavior of Al13 could be ascribed to its high charge neutralization ability, relative stability and potential self-assembly tendency © 2006 Elsevier B.V All rights reserved Keywords: Direct dye; Coagulation; Al speciation; Polyaluminum chloride; Al13 Introduction The release of dye compounds from industries of textile dyeing, printing, as well as food and papermaking can cause severe water pollution problems It has been estimated that more than 700,000 tonnes of dyestuff are produced annually, and about 10–15% of these dyes are left in effluents during dyeing processes [1,2] The presence of dyes in water is aesthetically undesirable, even very low concentration of dyes is highly visible On the other hand, dye polluted natural waters can result in serious disturbance to aquatic biosphere due to the reduction of sunlight penetration and depletion of dissolved oxygen Additionally, the majority of synthetic dyes are highly water-soluble azo dyes, which are toxic to some aquatic organisms and may pose serious health threat to human beings It has been found that some azo dyes are able to produce carcinogenic aromatic amines ∗ Corresponding author Tel.: +86 10 62849138; fax: +86 10 62923541 E-mail address: byshi@rcees.ac.cn (B Shi) 0304-3894/$ – see front matter © 2006 Elsevier B.V All rights reserved doi:10.1016/j.jhazmat.2006.09.076 in the process of reductive degradation [3,4] In the recent years, regulations on dye pollutants are becoming more and more stringent world widely Thus, dyes in wastewater have to be removed completely before discharged into receiving waters However, since synthetic dye compounds usually have very complex structure and are intentionally designed to be recalcitrant with poor biodegradability, they are difficult to decolorize by conventional aerobic biological treatments, such as activated sludge process The widely used methods for dyeing wastewater treatment involve many physical–chemical techniques, such as coagulation, adsorption, membrane filtration, and advanced oxidation, etc [5–8] Each treatment method has its advantages and disadvantages Generally, advanced oxidation processes are effective for removal of most dyes, but a common problem with such operations is their relatively high cost in large-scale utilization [1,9] In addition, chemical oxidation usually attacks only the chromophore groups of dyes instead of mineralizing organic dyes completely Moreover, the possible occurrence of some more toxic intermediate products could be of concern Adsorption techniques have much potential in the treatment of 568 B Shi et al / Journal of Hazardous Materials 143 (2007) 567–574 dye-containing waters if high performance and cheap adsorbents are available [10] Membrane filtration has some special features unrivalled by other methods, but the high capital cost and clogging problems associated with this method may limit its application Coagulation/flocculation is one of the most popular unit operations in water and wastewater treatment trains Dye removal by coagulation is not based on the partial decomposition of dye compounds, thus no potentially harmful and toxic intermediates are produced Furthermore, this process can be used in large-scale operation with relatively high operability and cost effectiveness [2,11,12] A limitation of this technique is that some high-soluble, low molecular and cationic dyes might not be effectively removed The disposal of sludge produced by coagulation could be another restriction associated with this technique Although the application of coagulation in water and wastewater treatment has a long history, the mechanisms involved in this process are still not fully understood Coagulation is a very complicated process involving a series of physical–chemical interactions The type of coagulant applied can play important roles in the removal of target pollutants Aluminum and ferric-based salts, such as alum, aluminum chloride, ferric chloride, ferric sulfate, are commonly used traditional coagulants After dosing, aluminum and ferric ions will experience continuous self-hydrolysis and evolve into hydroxide solids finally The in situ formed hydrolysis products and hydroxide solids can neutralize, and/or adsorb particulate and dissolved matters to achieve removal of pollutants The hydrolyzing process of aluminum and ferric salts are subject to water quality con- B Shi et al / Journal of Hazardous Materials 143 (2007) 567–574 569 Fig Chemical structures of dye compounds (a) Direct Black 19, (b) Direct Red 28 and (c) Direct Blue 86 and the signal at 62.5 ppm corresponds to Al13 species (only the central Al atom in Al13 structure could produce resonance signal), the signal at 80 ppm is ascribed to the inner standard of NaAl(OD)4 Other Al species, such as colloidal species could not be observed by 27 Al NMR Obviously, no Al13 was detected from AlCl3 solution, and both Al13 and monomeric Al species existed in PACl As to the purified Al13 , the signal of monomeric species was almost negligible A current study found that the Alb species determined by ferron assay was equivalent to the Al13 species on the condition that the B value (OH/Al molar ratio) was in the range of 1.5–2.5 [25] In this work, the B values of the PACl and Al13 were 2.37 and 2.46, respectively, thus the Alb component determined by ferron assay in both PACl and purified Al13 could be regarded as Al13 species The speciation characteristics of three coagulants are listed in Table As evidenced by 27 Al NMR patterns, the predominant species of AlCl3 was Al monomers (Ala ), accounted for 94.2%; while, the Alb component (Al13 ) of purified Al13 was as high as 95.8%; PACl was consisted of mixed species with more than 50% of Alc 2.3 Jar tests Jar tests were conducted on a program-controlled JTY-4 Jar Tester (Beijing, China) Dye containing test water of 500 ml was transferred into a 800-ml beaker; under rapid stirring of 200 rpm, predetermined amount of coagulant was added, after min, the stirring was changed to 40 rpm with a duration of 15 min; then samples were collected and filtered using 0.45 ␮m membrane filter for residual dye measurement Turbidity was also measured (2100N Turbidimeter, Hach, USA) for some tests after 20 of quiescent settling, samples for turbidity measurement were taken from cm below the surface The pH of test water was adjusted by adding 0.5 mol l−1 HCl and 0.1 mol l−1 NaOH solutions The measurement of Table Al species distribution of three different coagulants Fig Al13 27 Al NMR patterns of coagulant solutions (a) AlCl3 , (b) PACl and (c) AlCl3 PACl Al13 Concentration (mol l−1 Al) pH Ala (%) Alb (%) Alc (%) 0.20 0.20 0.11 2.62 3.65 4.15 94.2 17.6 2.0 5.8 29.9 95.8 0.0 52.5 2.2 570 B Shi et al / Journal of Hazardous Materials 143 (2007) 567–574 pH was carried out using a MP220 pH meter (Mettler-Toledo, Switzerland) 2.4 Measurement of residual dye concentration after coagulation The linear relationships between dye concentration and absorbance at wavelength of λmax were obtained for each dye at pH of 7.80 (Table 1) For residual dye concentration measurement, the pH of filtered samples was adjusted to 7.80 first, and then the absorbance was read at wavelength of λmax The residual dye concentration after coagulation was calculated based on the equations in Table The removal was recorded as the ratio of residual dye concentration and initial test water concentration Results and discussion 3.1 General coagulation behaviors of direct dyes Removal of Direct Black 19 by coagulation was firstly carried out under pH of 7.80, which was the average pH value of tap water The variations of dye removal with dosage are shown in Fig (the dosage was measured as “mol l−1 Al” in this paper) With the increase of coagulant dosage, the removal increased and the curves associated with different coagulants exhibited similar changing trends: slow increase at low dosages, then followed by a rapid increase with dosage; finally the increase became slow again and the curves approached plateau The removal could be reached near 100% at high dosage zone for all three coagulants Before reaching plateau, the coagulation efficiency was: Al13 > PACl > AlCl3 It should be pointed out that no re-stabilization phenomenon (removal reduction with increase of dosage) was observed even the dosage increased to 40 × 10−5 mol l−1 (data are not included) The turbidity evolution with dosage is illustrated in Fig It was found that with the increase of dosage, the turbidity increased first and then decreased Moreover, it was noticed that during the turbidity increasing phase, no observable flocs were formed When the dosage was greater than a certain value (dosage corresponding Fig Coagulation of Direct Black 19 under pH of 7.80 Fig Turbidity changes with dosage for coagulation of Direct Black 19 to the maximum turbidity in Fig 4), turbidity began to decrease and fine particulate matters could be seen with the increase of dosage When the dosage was high enough, large flocs appeared Fig also shows that during the turbidity increasing phase, coagulants related to higher dye removal also caused higher turbidity (Al13 > PACl > AlCl3 ); in the turbidity decreasing phase, the dye removal efficiencies were all much high, but the settling velocity of flocs formed by AlCl3 was more rapid The coagulation of Direct Red 28 and Direct Blue 86 was examined under the same condition as that of Direct Black 19 Like the coagulation of Direct Black 19, it was also found that with the increase of dosage, turbidity increased first and then decreased with the appearance of appreciable particulates However, the dye removal kept increase within the whole dosage range (Fig 5) The dye removal performance of three coagulants exhibited different characteristics depending on the type of dyes and dosage levels In the coagulation of Direct Red 28, Al13 was Fig Coagulation of Direct Red 28 and Direct Blue 86 under pH of 7.80 B Shi et al / Journal of Hazardous Materials 143 (2007) 567–574 slightly superior to the other two coagulants when the dosage was low With the increase of dosage, AlCl3 became the most efficient one At the high dosage zone, the differences among three coagulants tended to diminish While in the case of Direct Blue 86, Al13 achieved the highest removal within the whole dosage range, and AlCl3 was the poorest coagulant It can also be observed that higher dosages were required for removal of Direct Blue 86 than for removal of other two dyes At the dosage of × 10−5 mol l−1 , the removal of Direct Red 28 had reached more than 90% for all coagulants, but the removal of Direct Blue 86 was only about 60, 70 and 80% for AlCl3 , PACl and Al13 , respectively It indicates that Direct Blue 86 was more difficult to be removed than the other two dyes This phenomenon implies that the treatability of different dyes by coagulation might be associated with the properties of dyes themselves, such as molecular size and chemical structure According to Fig 1, the molecular sizes of both Direct Black 19 and Direct Red 28 are larger than Direct Blue 86 in at least one dimension (the longest dimension of Direct Black 19 molecule had been estimated to be nm [10]) It is well recognized that organic matters with longer molecular chain and larger molecular weight are more favorable for removal by coagulation Fig demonstrates the pH changes after coagulation of Direct Blue 86: pH decreased with the increase of coagulant dosage Once AlCl3 was dosed, the pH was depressed obviously However, when the PACl and Al13 were dosed, only slight pH decrease could be observed It can be explained by the differences of hydrolyzing potentials between monomeric and preformed polymeric Al species AlCl3, mainly consisted of monomeric Al species, has much stronger hydrolyzing tendency than preformed polymeric coagulants once added into test 571 Fig Changes of pH with dosage in coagulation of Direct Blue 86 water, particularly when the pH and alkalinity of test water are at high levels Similar pH changing phenomena were also observed in the coagulation of other two dyes It should be noted that the pH values after coagulation were all higher than 7.0 within the dosage range In order to get more insight into the roles of preformed Al species in removal of dyes, the effect of pH on coagulation performance was further investigated 3.2 Effect of pH on coagulation of direct dyes The removal of each dye under different pH by AlCl3 , PACl and Al13 is presented in Fig The experiments were con- Fig Effect of pH on the removal of direct dyes by AlCl3 , PACl and Al13 (left column: Al dosage of 3.0 × 10−5 mol l−1 ; right column: Al dosage of 5.0 × 10−5 mol l−1 ) 572 B Shi et al / Journal of Hazardous Materials 143 (2007) 567–574 ducted under two dosage levels of low and medium: 3.0 and 5.0 × 10−5 mol l−1 , respectively An obvious feature can be seen from the plots of Fig that the coagulation efficiencies of both PACl and Al13 tended to increase with the decrease of pH and approached almost complete dye removal when the pH was sufficiently low (less than 6.0) At the same time, appreciable and even large flocs were developed rapidly under depressed pH levels However, the removal curves associated with different conditions were more or less different depending on the type of dyes, type of coagulants and level of dosages Generally, Al13 could achieve better removal than PACl within the whole experimental pH range regardless of the type of dyes, and the superiority of Al13 was more obvious under the lower dosage of × 10−5 mol l−1 While in the case of AlCl3 , its coagulation efficiency increased first and then tended to decrease rapidly with the decrease of pH The optimal pH range for AlCl3 varied with the type of dyes and level of dosages Nevertheless, it can be deduced based on the overall results that the maximum removal occurred at pH around 6.0 for AlCl3 , which was in agreement with the results reported by Lee et al [3] It can also be seen that under higher dosage, the coagulation zone of AlCl3 was broader than that under lower dosage In addition, the coagulation zones corresponding to Direct Black 19 and Direct Red 28 were larger than that corresponding to Direct Blue 86 Another feature shown in Fig is that even at the optimal pH for AlCl3 , the coagulation efficiencies of both PACl and Al13 were higher than that of AlCl3 under most cases, particularly under the lower dosage of × 10−5 mol l−1 In order to better understand the coagulation behaviors of three coagulants, the dye removal changes with the increase of dosage were investigated at both pH of 5.8 and 3.5 (Fig 8) It is obvious that Al13 was the most efficient coagulant at both pH levels PACl exhibited significantly higher removals than AlCl3 although the pH of 5.8 was within the coagulation zone favorable for AlCl3 It can be seen that the coagulation features of AlCl3 were markedly different under pH of 5.8 and 3.5 If the dosage was high enough, the dye removal by AlCl3 was able to reach near 100% at pH 5.8 But under pH of 3.5, the coagulation efficiency of AlCl3 was dramatically deteriorated, and the dye removal could not achieve even 60% for all three dyes within the dosage range Furthermore, in the coagulation of Direct Blue 86, the removal curve approached a plateau value of only 50% removal Meanwhile, no observable flocs developed for all the jar tests associated with AlCl3 at pH of 3.5 It has been reported that the Al species was dominated by Ala after dosing at pH of less than 4.0 when traditional Al salt was applied [21,24], thus it could be inferred that monomeric Al species was not effective for direct dye removal With the aid of dye removal measurement, careful observation of the floc developing process indicated that once appreciable flocs were formed, the dye removal could be reached relatively high levels of not less than 80% The 80% removal seemed to be a critical point for removing direct dyes by coagulation In order to more clearly elucidate the effect of pH on the performance of different coagulants, the dosages required to achieve 80% dye removal for different coagulants were obtained by interpolating the corresponding curves of dosage versus removal under different pH conditions (Fig 9) The dosages to achieve 80% removal decreased greatly with the reduction of pH for both PACl and Al13 , and the associated dosages of Al13 were all lower than those of PACl From pH 7.80 to 3.50, the dosages corresponding to 80% removal decreased more than 70% averagely for both PACl and Al13 despite the type of dyes Fig Coagulation of direct dyes by AlCl3 , PACl and Al13 at pH of 5.8 and 3.5 (left column: pH5.8; right column: pH 3.5) B Shi et al / Journal of Hazardous Materials 143 (2007) 567–574 Fig Comparison of dosages required for achieving 80% dye removal at different pH conditions The required dosages of AlCl3 for 80% removal were lower at pH 5.8 than those at pH 7.80, but 80% removal was not able to be reached at pH 3.5 As mentioned above, the type of dyes could affect the coagulation performance of different coagulants To achieve 80% removal, Direct Blue 86 required higher dosage of PACl and Al13 than the other two dyes at pH of 7.80 and 3.50 However, at pH of 5.80, highest dosages of PACl and Al13 were needed for removal of Direct Red 28 When AlCl3 was applied, the highest dosage was with Direct Blue 86 at both pH of 7.80 and 5.80; while Direct Black 19 exhibited the poorest removal at pH of 3.50 (Fig 8) Therefore, in order to achieve high treatment efficiency, the coagulation pH must be optimized based on the function of coagulants and the property of specific dyes at different pH conditions 3.3 Further discussion Many studies have confirmed that traditional coagulants and inorganic polymeric coagulants have much different coagulation 573 behaviors and mechanisms [18–21,26] Charge neutralization is considered to be a prerequisite condition for most coagulation processes to occur In the case of traditional Al salts, charge neutralization is induced by the Al hydrolysis products formed in situ after dosing The in situ formed Al hydrolysis products could aggregate, rearrange and further hydrolyze to amorphous hydroxide precipitate Such freshly formed amorphous solids can further neutralize, adsorb colloidal matters and function as bridges among fine particles At high dosages, the coagulation caused by amorphous hydroxide can be a dominant mechanism, which is the so-called “sweep flocculation” Previous work had demonstrated that Ala species could disappear rapidly and evolve into polymeric species in after dosing, and then further change to hydroxide precipitate gradually However, preformed Al species, such as Al13 and colloidal species could possess relatively high stability after dosing in a broad pH range [24] Therefore, it is difficult for them to form Al hydroxide by way of in situ hydrolysis Due to the high positive charge (+7), Al13 has strong charge neutralization ability for negatively charged colloids or large molecules In addition, Al13 , with nanometer size, has tendency of self-assembly to form large aggregates, which makes it effective to function as bridges among particles Thus, Al13 is considered to be the most efficient species for coagulation/flocculation by some researchers As to the colloidal Al species (Alc component), its charge neutralization ability is much less than that of Al13, although its stability is relatively high too It has been observed that under some circumstances, the coagulation efficiency of colloidal Al species could be higher than that of Al13 species due to the larger size of such species “Electrostatic patch coagulation” has been put forward to interpret the coagulation mechanism of colloidal Al species [19] All the direct dyes used in this study contain sulfonic functional groups, which are negatively charged when dissolved in water The electrostatic repulsion between negative charges on different molecules enhances the solubility of dyes In order to coagulate these dyes, the negative charges of dye molecules need to be sufficiently neutralized It could be one of the reasons that Al13 , with high charge neutralization ability, is superior to the other coagulants in general In the coagulation of Direct Red 28 under pH of tap water (Fig 5), the higher removal by AlCl3 might be due to the “sweep flocculation” mechanism, which is inclined to occur at high dosage and high pH conditions On the other hand, with the decrease of pH, dye protonation processes could lead to reduction of charge density and induce self-aggregation of dye molecules Therefore, less coagulant would be required to destabilize them As noted in Fig 7, the coagulation of the dyes became much more easier with the decrease of pH (with the exception of pH less than 6.0 in the case of AlCl3 ) When AlCl3 was used as coagulant, the changing trend of dye removal with pH implied that the intermediate Al hydrolysis products (such as six-member ring Al species) could play an important role in dye coagulation process When the pH was high (greater than 7.0), the Al hydrolysis could continue very rapidly and amorphous hydroxide would form in a short time Thus, amorphous hydroxide was deemed to be the major product inducing the coagulation/flocculation Due to the 574 B Shi et al / Journal of Hazardous Materials 143 (2007) 567–574 weaker charge neutralization ability, amorphous hydroxide was not effective in dye removal, particularly when the dosage was low When the pH was in the low range (less than 5.0), the Al hydrolysis could be significantly inhibited, and the monomeric and/or oligomeric Al species would exist for a long duration, and these species was not as effective as polymeric Al species in terms of both charge neutralization and particle bridging functions Conclusion Al based coagulants could be used to treat waters polluted by some direct dyes The coagulation performances of traditional Al salt, PACl and purified Al13 were significantly different due to their different speciation characteristics Preformed Al species, particularly Al13 , could play important roles in the coagulation of direct dyes Adjustment of pH was necessary for improving dye removal efficiency and saving coagulant usage For PACl and purified Al13 , the decrease of pH was always beneficial for enhancing dye removal With respect to traditional Al salt, an optimal pH of around 6.0 should be used Generally, purified Al13 had the highest dye removal efficiency The importance of Al13 in coagulation of direct dyes could be attributed to its high charge neutralization ability, relative anti-hydrolysis stability and its nanometer-sized structure with self-assembly tendency Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No 20537020) References [1] T Robinson, G McMullan, R Marchant, P Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour Technol 77 (2001) 247–255 [2] S Papi´c, N Koprivanac, A.L Boˇzi´c, A Meteˇs, Removal of some reactive dyes from synthetic wastewater by combined Al(III) coagulation/carbon adsorption process, Dyes Pigments 62 (2004) 291–298 [3] 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behavior and mechanism between inorganic polymer flocculant and traditional coagulants, in: H.H Hahn, E Hoffmann, H Ødegaard (Eds.), Chemical Water and Wastewater Treatment (IV), Springer-Verlag, Berlin, 1996, pp 83–93 [...]...D Dihang et al / Chemical Engineering and Processing 47 (2008) 1509–1519 [16] J Bratby, Coagulation and Flocculation in Water and Wastewater Treatment, Upland Press, Croydon, 1980 [17] D.S Wang, H.X Tang, J Gregory, Relative importance of charge neutralization and precipitation on coagulation of kaolin with PACI: effect of sulfate ion, Environ... polymerization These products and especially the first one (PACl) are used extensively worldwide during the last two decades, with an ever increasing demand Their properties were intensively examined and have proved to be more efficient in lower dosages and in wider pH, temperature and colloids concentration ranges, than the conventional simpler products, leading to cost and operative more effective treatment... detail (La Mer and Healy, 1963; Mortimer, 1991; Farrow and Swift, 1996b; Hocking et al., 1999; Hogg, 2000) These factors may include:  Slurry properties such as particle size, surface area, surface charge, solution composition, pH and ionic strength  Physical properties of the flocculant, such as molecular weight, charge density and functionality  Dynamic aspects of aggregate rupture and formation... Henderson and Wheatley, 1987) or attributed change to microbial attack (Chmelir et al., 1980), radical attack from residual catalyst (Haas and MacDonald, 1972), disentanglement (Gardner et al., 1978) or conformational changes (Klein and Westerkamp, 1981; Kulicke and Kniewske, 1981; Kulicke, 1986) The latter is generally accepted, with the polymer initially taking an extended conformation and water... indication of the aggregate dimensions, and serve as a guide to the throughput of gravity thickeners They are normally obtained from cylinder tests, where dilute polymer is dosed into a standard cylinder of slurry and mixed, either by inversion or with plungers While such tests are simple and convenient, they suffer from ill-defined mixing The poor mixing can leave polymer-rich and polymer-poor regions, resulting... known in mineral processing applications, was used in this study No evidence of acrylate moieties could be detected by FTIR, 13C NMR or elemental analysis The weight-average molecular mass Mw of this polymer as measured by multi-angle laser light scattering (MALLS) was 20 Â 106 Polymer (1 g) was ‘‘wet’’ with absolute ethanol (2 g) and gently shaken by hand for 30 s and then let stand for 2 min Deionised... Fig 9 Free settling rate and density versus size as measured by FDA for individual aggregates formed in the Shear Vessel at 100 and 200 rpm (72-h-old flocculant stock solution, dosages 40 and 48 g t À 1 at 100 and 200 rpm, respectively) scattering detection, proposed three solution states for polyacrylamide—(i) well-dispersed individual coils, (ii) submicron entangled chains and (iii) supramicron agglomerates... flocculation, rheology and dewatering behaviour of kaolinite dispersions, Int J Mineral Process 71 (2003) 247 268 [22] D.E Brooks, Effect of neutral polymers on the electrokinetic potential of cells and other charged particles, J Colloid Interface Sci 43 (1973) [23] L Besra, D.K Sengupta, S.K Roy, P Ay, Flocculation and dewatering of kaolin suspensions in the presence of polyacrylamide and surfactants,... to introduce silica in aluminium solution, the gelation must be avoided and therefore, the maintenance of poly-silicates solution respective pH should be applied around 2, as the method to handle silicates for coagulants preparation Applying this concept, certain efforts have been made to synthesize and to understand the behaviour and the coagulation efficiency of these coagulants The coagulation performance... result of molecular weight degradation, but rather a change in the polymer’s solution configuration Shyluk and Stow (1969) observed that there was a rapid and a slow stage in the decrease of viscosity of a polyacrylamide solution with time In proposing a mechanism involving both disentanglement and chemical degradation, they also reported for the first time that ageing reduced the ability of the polymer
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