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.. .DETECTION, FORMATION AND REACTIVITY OF TETRAVALENT LEAD CORROSION PRODUCT (PbO2) AND ITS ROLE IN WATER QUALITY IN DRINKING WATER DISTRIBUTION SYSTEM NAME: ZHANG YUANYUAN... critical role in regulating lead contamination in drinking water, a precise and fast method for its detection is required to determine its abundance in drinking water sample and assess its bioavailability... 95 V SUMMARY Tetravalent lead corrosion product (PbO2) formed from the chlorination of lead- containing plumbing materials (LCPMs) has been linked to lead contamination in drinking water Despite
DETECTION, FORMATION AND REACTIVITY OF TETRAVALENT LEAD CORROSION PRODUCT (PbO2) AND ITS ROLE IN WATER QUALITY IN DRINKING WATER DISTRIBUTION SYSTEM ZHANG YUANYUAN NATIONAL UNIVERSITY OF SINGAPORE 2012 DETECTION, FORMATION AND REACTIVITY OF TETRAVALENT LEAD CORROSION PRODUCT (PbO2) AND ITS ROLE IN WATER QUALITY IN DRINKING WATER DISTRIBUTION SYSTEM NAME: ZHANG YUANYUAN A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL & ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously Zhang Yuanyuan December 2012 I ACKNOWLEDGEMENTS I sincerely express my deep gratitude to my advisor Dr Yi-Pin Lin for his valuable guidance, support and patience throughout my Ph.D study in National University of Singapore I would like to thank Mr Chia Phai Ann, Dr Yuan Ze Liang and Ms Khoh Leng Khim in Department of Chemical and Biomolecular Engineering and Ms Tan Teng Jar in Department of Physics for teaching me how to operate SEM, XPS, BET and XRD Special thanks to members of Dr Lin’s group and the staffs in CEE for their kind help, valuable suggestion and support Appreciation goes to my friends for their support, patience, friendship and encouragement during my Ph.D program Deeply grateful to my family for their love, faith, encouragement and endless supporting!!! Last but not least, the financial support from the National University of Singapore is acknowledged Zhang Yuanyuan II TABLE OF CONTENTS DECLARATION……………………………………………… ACKNOWLEDGEMENTS………………… TABLE OF CONTENTS…………………………… SUMMARY………………………………………………… LIST OF TABLES…………………………………………… LIST OF FIGURES……………… ABBREVIATIONS………… I II III VI VIII X XVI CHAPTER INTRODUCTION AND RESEARCH OBJECTIVES 1.1 Background… 1.2 Objectives… 1.3 Organization of thesis …………… CHAPTER LITERATURE REVIEW… 2.1 PbO2 detection…………… 2.2 PbO2 formation…………… 2.3 PbO2 stability……………………… CHAPTER MATERIALS AND METHODS 17 3.1 Chemicals…… 17 3.2 Iodometric method development… 22 3.3 Lead measurement and recovery tests…… 26 3.4 PbO2 formation experiments 29 3.5 PbO2 reduction experiments in the presence of NH2Cl and Br- 31 3.6 Analytical Methods 34 CHAPTER FAST DETECTION OF LEAD DIOXIDE (PbO2) IN CHLORINATED DRINKING WATER BY A TWO-STAGE IODOMETRICMETHOD 35 4.1 PbO2 measurement in the absence of free chlorine……………… 35 III 4.2 PbO2 measurement in the presence of free chlorine…………… 40 4.3 Interference of PbCO3 particle………………………………… 42 4.4 Interference of Fe2O3 and MnO2 particles……………………… 44 4.5 PbO2 measurement in chlorinated drinking water……………… 48 CHAPTER IODIDE-ASSISTED TOTAL LEAD MEASUREMENT AND DETERMINATION OF DIFFERENT LEAD FRACTIONS IN DRINKING WATER SAMPLES………………… 49 5.1 Recovery tests for chlorinated drinking water with low PbO2 concentrations 5.2 Recovery tests for chlorinated drinking water with high PbO2 concentrations 5.3 49 Total lead measurement using iodide-assisted USEPA method 52 56 CHAPTER DETERMINATION OF PbO2 FORMATION KINETICS FROM THE CHLORINATION OF Pb(II) CARBONATE SOLIDS VIA DIRECT PbO2 MEASUREMENT 58 59 6.1 Influence of Pb(II) solid loading… 6.2 Influence of initial free chlorine concentration… 61 6.3 Influence of DIC…… 6.4 Influence of pH value……… 65 6.5 Determination of rate equations……… 62 66 6.6 Stoichiometry of free chlorine consumption and PbO2 formation 69 6.7 SEM observations…… 71 CHAPTER ELEVATED Pb(II) RELEASE FROM THE REDCUTION OF Pb(IV) CORROSION PRODUCT (PbO2) INDUCED BY BROMIDE-CATALYZED MONOCHLORAMINE DECOMPOSITION 74 7.1 Synergistic influences of Br- and NH2Cl concentration on the reduction of PbO2………………………………… 74 7.2 Influence of pH value… 77 IV 7.3 Influence of Br- concentration…… 79 7.4 Influence of initial NH2Cl concentration……… 81 7.5 Relationship between NH2Cl decomposition and Pb(II) release 83 7.6 Modeling of Pb(II) release 87 CHAPTER CONCLUSIONS AND RECOMMENDATIONS 90 8.1 Conclusions………………………………………………… 90 8.2 Recommendations…………… 93 REFERENCES…… 95 V SUMMARY Tetravalent lead corrosion product (PbO2) formed from the chlorination of lead-containing plumbing materials (LCPMs) has been linked to lead contamination in drinking water Despite the importance of PbO2 in lead contamination is well recognized, several challenges still remain Quantitative determination of PbO2 in water samples has been proven difficult due to the incomplete dissolution of PbO2 in standard sample preservation and acidic digestion procedure Due to the limitation in accurate PbO2 measurement, most studies on its formation were qualitative in nature without providing quantitative kinetic information Hence, it is essential to develop a method that can quickly and accurately detect PbO2 in water samples The stability of PbO2 tends to be impacted by constituents present in drinking water It has been reported that Pb(II) release from PbO2 is associated with NH2Cl decomposition It is known that NH2Cl decomposition can be catalyzed by Br- which is present in many water supplies Whether Br catalyzed NH2Cl decomposition can enhance the release of lead from PbO2 should be explored The objectives of this work were to (1) develop a new method for fast and accurate detection of PbO2 in drinking water, (2) determinate the mechanism and kinetics of PbO2 formation from the chlorination of Pb(II) carbonate solids, and (3) explore the stability of PbO2 in chloraminated water in the presence of Br- to advance our understanding of the role of PbO2 in lead release in drinking water Firstly, a modified iodometric method was developed to detect PbO in water PbO2 can oxidize iodide to form triiodide (I3-), a yellow-colored anion that can be detected by the UV-vis spectrometry Complete reduction of up to 20 VI mg/L PbO2 can be achieved within 10 at pH 2.0 and KI = mg/L A twostage method combining the iodometric step and pH adjustment was proposed to account for the interference of free chlorine presented in water This approach, allows free chlorine to completely react with iodide at ambient pH followed by sample acidification to pH 2.0 to accelerate the iodide oxidation by PbO2 Good recoveries of PbO2 (90 – 111 %) in chlorinated water samples with a concentration ranging from 0.01 to 20 mg/L were achieved The proposed method was then employed to determine the kinetics of PbO2 formation from the chlorination of hydrocerussite (Pb3(CO3)2(OH)2) and cerussite (PbCO3) The obtained rate equations suggest that for both Pb(II) carbonate solids, the formation of PbO2 is first-order with respect to the available Pb(II) solid surface area, free chlorine concentration and OH- concentration, respectively Dissolved inorganic carbon (DIC) was found to inhibit PbO2 formation due to the formation of carbonate-lead surface complexes that protect the surface Pb(II) sites from oxidation The rate of PbO2 formation from the chlorination of hydrocerussite was faster than that of cerussite under the same Pb(II) solid loading However, after normalization of the surface area, the rate constants obtained for both Pb(II) solids are similar The kinetics of PbO2 formation is elucidated for the first time in this study Finally, the stability of PbO2 was investigated in a chloraminated solution containing bromide It was found that Br- was able to enhance Pb(II) release from PbO2 by catalyzing NH2Cl decomposition A single linear correlation between the amount of Pb(II) released and the amount of NH2Cl decomposed either in the absence or presence of Br- was found, suggesting that Br catalyzed VII NH2Cl decomposition and NH2Cl auto-decomposition may generate the same intermediate toward the reduction of PbO2 The modeling of NH2Cl decomposition and Pb(II) release was also attempted to elucidate the mechanism An important implication from the results obtained is that special attention on lead contamination should be paid to water utilities using brackish water and desalinated seawater as their source water which may contain relatively high bromide concentration The developed method for PbO2 measurement may allow in-situ PbO2 detection in the distribution system and facilitate the research requiring accurate PbO2 measurement This study also provides insights into PbO2 redox chemistry which may allow water utilities to develop suitable strategies to control lead contamination and ensure drinking water safety VIII 60 NH2Cl = 54.4 µM (a) NH2Cl = 28.0 µM 50 NH2Cl(µM) NH2Cl = 13.8 µM 40 30 20 10 0 20 40 60 Time (hr) 80 10 100 (b) NH2Cl = 54.4 µM NH2Cl = 28.0 µM Pb(II) µM NH2Cl = 13.8 µM 0 20 40 60 Time (hr) 80 100 Figure 7.7 Effect of initial NH2Cl concentration on the Pb(II) release from PbO2 and NH2Cl decomposition as a function of time in the absence of Br- (a) NH2Cl decomposition, (a) Pb(II) release Experimental condition: PbO2 = 10 mg/L, DIC = mM, pH 7.0, Temp = 25 oC Solid lines represent modeling results 86 7.6 Modeling of Pb(II) release The release of Pb(II) from PbO2 in NH2Cl solutions either in the absence or presence of Br- could be attributed to the reaction between the unidentified intermediates during NH2Cl decomposition and PbO2 We assume that both NH2Cl decomposition pathways produced the same unidentified intermediate (I*) that can reduce PbO2 based on our observation (Figure 7.5) In NH2Cl autodecomposition, I* is formed from the reaction between NHCl2 and H2O and can further react with NHCl2 and NH2Cl to accelerate NH2Cl decomposition (reactions 7, and in Table 2.1) In the presence of Br-, we assume that I* is formed from the reaction between NHBrCl and H2O (analogical to NHCl2 and H2O) and can further react with NHBrCl and NH2Cl with the same rate constants as their auto-decomposition counterparts as below: NHBrCl + H2O → I* + BrI* + NHBrCl→ HOBr + products * k = 4.0 × 105 M-1h-1 (7.1) k = 1.0 × 108 M-1h-1 (7.2) I + NH2Cl → Products -1 -1 k = 3.0 × 10 M h (7.3) Conceptually, the consumption of intermediates (I*) due to the PbO2 reduction reactions could decrease the rate of NH2Cl decomposition However, our results showed that the rate of NH2Cl decomposition in PbO2-containing solution were slightly higher (~10%) than those without PbO2 (Figure 7.1), suggesting that a secondary intermediates (denoted as I*m)) could be formed from PbO2 reduction and it could also react with NHCl2, NHBrCl and NH2Cl (reactions 2, and in Table 7.1) with slightly higher rate constants than those with I* 87 Table 7.1 Proposed reactions for the formation of Pb(II) and reactions of secondary intermediate I* Proposed reactions for the formation of Pb(II) and reactions of secondary intermediate Im* I* + PbO2 → Pb2+ + Im* k1 = 1.3 × 105 m-2·h-1 Im* + NHCl2 → HOCl + Products k2 = 2.0 × 108 M-1·h-1 Im* + NHBrCl → HOBr + Products k3 = 2.0 × 108 M-1·h-1 Im* + NH2Cl → Products k4 = 5.0 × 107 M-1·h-1 Im* denotes the secondary intermediate formed from the reaction of PbO2 with I* Reduction of PbO2 is a surface reaction and its rate should be proportional to the available surface area (23, 25) Thus, the rate of Pb(II) released from PbO2 (reaction in Table 7.1) can be expressed as the following: r = kPbO2 (SA) ([I*]) (7.4) where r denotes the rate of PbO2 released (M∙h-1), kPbO2 denotes the apparent rate constant (m-2∙h-1) (SA) denotes the surface area of PbO2 (m2) [I*] denotes the concentration of the intermediate formed from the NH2Cl auto- and Br catalyzed decomposition (reaction in Table 2.1 and Eq (7.1)) The rate constants of reactions linked to PbO2 reduction (reactions 1-4 in Table 7.1) were determined by the least-square linear regression of experimental data The modeling of NH2Cl decomposition and Pb(II) formation considering both auto-decomposition and Br catalyzed NH2Cl decompositions are shown in Figure 7.2-7.4 It should be noted that the dependences of rate constants and equilibrium constants on temperature were also considered when available [26] k2 (rate constant for reaction in Table 2.1) and kBr (rate constant for reaction in Table 2.1) were found to significantly affect the rate of NH2Cl autodecomposition and Br catalyzed NH2Cl decomposition and their values were calibrated by the data obtained from control experiments and determined to be 6.8 × 10-2 h-1 and 1.6 × 108 M-1h-1 which were comparable to those listed in Table 2.1 and 2.2, respectively Based on the least-square regression, apparent rate constant (kPbO2) in Eq (7.4) was determined to be 1.3 × 105 m-2∙h-1 In general, the model provided good simulations of our experimental data, except NH2Cl decomposition at pH 6.0 was overestimated and Pb(II) release at the same pH 88 value was underestimated These discrepancies may be attributed to that the NH2Cl decomposition model was established under neutral to alkaline conditions and may not be directly applicable to pH 6.0 and that the release of Pb(II) resulting from Br induced PbO2 reduction at this pH value was not taken into consideration in the modeling 89 Chapter Conclusions and Recommendations This thesis investigated PbO2 detection, formation and stability relevant to drinking water conditions The findings are summarized in the conclusions and recommendations for the future study are presented below 8.1 Conclusions The conclusions of the research present in this thesis are summarized in the following: (1) PbO2 detection The two-stage iodometric method presented can be used to determine PbO2 in water samples, either in the absence or presence of free chlorine With the optimized KI dose (4 g/L or 24 mM) and pH (pH 2.0), this method can detect 0.01 to 20 mg/L of PbO2 within 10 Based on these preliminary experimental findings, the proposed method may be useful for a variety of field and laboratory applications For example, it is noted that there is no extensive survey of PbO2 occurrence in the distribution system A simple device with similar features to that of the portable chlorine analyzer may be developed based on our findings for in-situ testing of PbO2 In laboratory investigations, the formation of PbO2 from the chlorination of Pb(II) solid phases or Pb2+ has been an important subject However, the rate of PbO2 formation cannot be directly quantified due to the lack of a suitable method allowing quick PbO2 measurement The developed method may overcome previous shortcomings and make the quantification possible It is important to note that the proposed method is not applicable to chloraminated water, due to the incomplete reaction between iodide and monochloramine at neutral to slightly alkaline pH values However, this may be resolved by appropriate filtration of the chloraminated water followed by the quantification of captured PbO2 particles using the procedures developed for disinfect-free solutions Membrane filter with a pore size of 20-30 nm is recommended as 90 PbO2 particles with a size around 100 nm have been discovered in the distribution system [37] Certainly, the same filtration step may also be applied to chlorinated water Special precautions are needed in some conditions For example, the proposed method is limited in the presence of MnO2, which can oxidize iodide at a rate comparable to that of PbO2 This interference may be resolved by coupling our method with AAS or ICP-MS in a way that iodide is used to reduce PbO2 in the acid solution and where Pb2+ is subsequently measured using the AAS or ICP-MS after hot-plate heating for colorless solution in which the presence of MnO2 or Mn2+ does not cause interference (2) Total Pb measurement and Pb fractions differentiation To solve the potential problem occurring in accurate measurement of total lead in high PbO2-containing water samples collected from the distribution system, a method that integrates membrane separation, iodometric PbO2 measurement, strong acid digestion and ICP-MS measurement was proposed Recovery tests using tap water spiked with soluble Pb2+, particulate Pb(II) carbonate and in-situ formed or spiked PbO2 were performed and compared to the results obtained using the USEPA method For samples containing low PbO2 concentrations (0.018-0.076 mg Pb/L), both the proposed procedure and the USEPA method showed promising recoveries of total lead For samples containing high PbO2 concentrations (0.089-1.316 mg Pb/L), the proposed procedure achieved satisfactory recoveries of 91-111 % for total lead while only 44-80 % can be obtained using the USEPA method In addition, the concentrations of Pb in the form of soluble Pb2+, particulate Pb(II) and PbO2 could be determined using the proposed procedure The relatively low recovery of the USEPA method was primarily due to the incomplete dissolution of PbO2 particles during nitric acid digestion and can be improved to acceptable levels by simply adding sufficient KI before strong acid digestion (3) PbO2 formation kinetics 91 When a sufficient oxidation potential is maintained, PbO2 is considered as an excellent sink to control plumbosolvency in the distribution system The kinetics of PbO2 formation from the chlorination of Pb(II) solids presented in this study elucidates the influences of different water parameters (Pb(II) solid loading, free chlorine concentration, DIC and pH) on the rate of PbO2 formation and allows the prediction of PbO2 formation under environmentally relevant conditions, at least in a short period of chlorination An extensive study is warranted to investigate the applicability of the kinetics for the long-term PbO2 formation The inhibition of PbO2 formation by DIC can be successfully explained by the formation of lead-carbonate surface complexes that protect surface Pb(II) from oxidation It is anticipated that the same mechanism may be extended to other ions (4) Elevated Pb(II) release from PbO2 reduction induced by Br catalyzed NH2Cl decomposition Br catalyzed NH2Cl decomposition was demonstrated to be able to cause elevated Pb(II) release from PbO2 and the rate was enhanced by the lower pH value, higher initial Br- and NH2Cl concentrations The intermediate causing the reduction of PbO2 could be identical in NH2Cl auto-decomposition and Br-catalyzed NH2Cl decomposition For distribution systems with historically installed lead-containing plumbing materials, precaution on lead contamination should be taken if considering switching disinfectant from free chlorine to NH2Cl, particularly those in arid and coastal regions with high bromide concentrations in their source waters Systems using RO-desalinated seawater which may contain Br- up to 0.6 mg/L may also pose potential risk of lead release, although the risk could be reduced after mixing with conventionally treated water 92 8.2 Recommendations The results presented in this thesis could aid to accurate measurement of PbO2 and total lead and better understanding of PbO2 formation kinetics in the distribution system Elevated lead release from PbO2 induced by Br-catalyzed NH2Cl decomposition was also demonstrated Future studies needed to improve our understanding in this field are described below (1) The interference of other foreign particles excluded in our study in iodometric method In the developed iodometric method, I3- formed from the oxidation of I- by PbO2 was quantified using UV-vis spectrometer to determine the PbO2 concentration based on 1:1 stoichiometry between the PbO2 consumption and I3formation Foreign particles (e.g., flaking Pb detached from plumping materials during water flushing or metal oxide (e.g., CuO) may exist in the PbO2-contained water They may result in high turbidity and thereby interfere with UV-vis absorbance measurement Further investigation on the detection of PbO2 in particle-contained water using iodometric method is needed (2) The recovery of total lead in water using proposed method Particulate Pb(II) carbonate and Pb(IV) solids are the major corrosion products in the drinking water distribution system where LCPMs are used Other particulate Pb, such as pure Pb, PbO, Pb3O4, Pb3(PO4)2 may exist in the corrosion scale and enter drinking water via physical detachment The influence of these particulate Pb on the PbO2 quantification using the proposed iodometric method and accurate differentiation of these particulate Pb species should be further investigated (3) The effect of orthophosphate on the PbO2 formation kinetics Our experiments showed that PbO2 can be formed from the chlorination of Pb(II) corrosion products It has been reported 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