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Development of a simple extraction procedure using ligand competition for biogeochemically available metals of estuarine suspended particulate matter D J. Whitworth, E.P. Achterberg * , V. Herzl, M. Nimmo, M. Gledhill, P.J. Worsfold Department of Environmental Sciences, University of Plymouth, Plymouth, PL4 8AA, UK Received 25 January 1999; received in revised form 8 March 1999; accepted 11 March 1999 Abstract Sorption of trace metals by suspended particulate matter (SPM) in estuarine systems has important implications for the fate of dissolved metals in these waters. This paper describes the development of a single extraction procedure for SPM-associated trace metals, using a ligand competition approach with EDTA as the added complexing ligand. The use of EDTA allows the determination of available particulate trace metals using well de®ned constraints with respect to the competition for trace metals between EDTA and the particles. Incubation experiments showed that equilibrium times between EDTA and particulate material of 72 h were required to reach equilibrium for most of the metals studied (Cu, Zn, Mn, Ni, Co, Al, Fe, Pb and Mg). Optimum conditions included a 0.05 M EDTA concentration and the use of an extractant: particulate matter ratio of 200 : 1 (v : w). Kinetic calculations on data from the incubation experiments were used to calculate the apparent stability constants (K MeS ) for the metal-particulate matter interaction and indicated values ranging from 10 À2.1 for K MgS to 10 À13.5 for K CuS . # 1999 Elsevier Science B.V. All rights reserved. Keywords: Extraction; Metals; Suspended particulate matter; Kinetics; EDTA; Apparent stability constant; Estuary 1. Introduction Trace metal behaviour in estuaries is strongly in¯u- enced by suspended particulate material (SPM). Par- ticle-water interactions of trace metals determine whether they are ¯ushed from an estuary in the dis- solved phase, or because of adsorption onto particles are retained within the internal cycle of the estuary [1]. In many estuaries removal of dissolved metal concen- trations has been observed in the turbidity maximum zone (TMZ), which is an estuarine region with strongly enhanced SPM levels (e.g. >600 mg l À1 in the Tamar estuary (U.K.), salinity 0.5±5 psu) [2,3]. Desorption of SPM bound metals (e.g. Mn and Zn [2] and Cd, Cu and Zn [4]) has been observed in the higher salinity regions of estuaries and has been explained by an increase in major cation concentrations. Suspended particulate matter may consist of biological, organic and mineral phases [5] and each of these phases has a different af®nity for trace metals. Studies involving the determination of trace metals in SPM and sediments often determine total metal concentrations. This approach does not provide infor- Analytica Chimica Acta 392 (1999) 3±17 *Corresponding author. Tel.: +44-1752-233-036; fax: +44-1752- 233-035; e-mail: eachterberg@plymouth.ac.uk 0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0003-2670(99)00285-8 mation about the biogeochemical availability of the particulate matter associated trace metals. For soils and sediments, workers have employed sequential chemical extraction schemes in order to investigate trace metal association with organic and mineral phases in their particles. Commonly employed sequential extraction procedures for sediments include the ®ve step scheme developed by Tessier et al. [6] and the three step BCR scheme [7,8] and variations on these schemes [9,10]. A large number of studies have been published employing the multi-step sequential extraction schemes on sediments [11±13] and soils [14±16]. The sequential extraction procedures have a number of disadvantages limiting their widespread use for studies of SPM associated trace metals. Firstly, the procedures often use 1 g of dry material, an amount which is often dif®cult to obtain for SPM by ®ltration of natural waters. Furthermore, parts of sequential extraction schemes suffer from re-adsorp- tion of the extracted metal onto the residual phases remaining after each extraction step, and a limited speci®city of the reagents for the targeted phases of the soil or sediment [17,18]. In addition, the procedures are labour-intensive and because of the numerous steps involved have enhanced associated errors and an enhanced risk of sample contamination. We, therefore, propose the use of a well de®ned ligand competition procedure for the investigation of non-lattice bound trace metals associated with SPM. Our approach uses EDTA as the added competing ligand. The use of EDTA for soil and sediment extraction procedures has been reported by other workers [16,19]. However, little or no work appears to have been carried out using EDTA extraction for SPM-associated metals. The analytical procedure for SPM-associated trace metals reported in this paper complements the ligand competition techni- ques used in our laboratory for the determination of trace metal complexation by dissolved organic ligands in natural waters [20,21]. The proposed extraction scheme allows the application of a well de®ned binding strength, and provides an indication of the biogeochemical availability of particle associated metals. The approach is simple, requires little sample handling, and has a small requirement of SPM (minimum 15 mg). A total digestion using HF complements the ligand competition extraction scheme, and allows the assessment of the contribution of biogeochemically available trace metals to the total particulate metal concentration in SPM. 2. Materials and methods 2.1. Reagents and labware All reagents and wash solutions were prepared in water puri®ed by reverse osmosis (Milli-RO, Milli- pore) followed by ion exchange (Milli-Q, Millipore). Reagents were purchased from Merck and were of AnalaR grade unless otherwise stated. Concentrated acids were puri®ed by sub-boiling quartz distillation and NH 3 was puri®ed through isopiestic distillation. To ensure chemical consistency of the EDTA extrac- tion solutions, a single 2.5 l stock solution of 0.5 M EDTA was freshly prepared and used for all the extraction studies. The pH of the EDTA stock solution was set at pH 7.6 using an appropriate volume of concentrated NH 3 (ca. 6.5 ml). Standard solutions utilised for dissolved metal analysis by inductively coupled plasma-mass spectrometry (ICP-MS) were prepared from Spectrosol standard solutions (1000 mg l À1 Cu, Ni, Co, Fe, Mn, Pb, Al, Mg and Zn), and acidi®ed to pH 2.2 using concentrated HNO 3 (1 ml per 1 ml of solution). When not in use, reagents were stored in high density polyethylene (HDPE, Nalgene) containers at 48 C in the dark. Prior to use, all the sample bottles and reagent containers were soaked in 2% (v/v) Decon 90 for 24 h, then washed in copious quantities of Milli-Q water and transferred to a 50% (v/v) HCl bath and left for one week. They were subsequently rinsed with Milli-Q water and placed in a 20% (v/v) HNO 3 bath. After a further week, the bottles were thoroughly washed with Milli-Q water and stored inside re-seal- able polythene bags. 2.2. Sample treatment Suspended particulate material samples were col- lected during a longitudinal transect in the Scheldt Estuary (Belgium) during a survey with the research vessel Belgica (December 1996). Water samples were collected using 10 l Niskin sampling units deployed at 4 D J. Whitworth et al. / Analytica Chimica Acta 392 (1999) 3±17 3 m depth. After three rinses with Scheldt water, a sample of 2.5 l was collected in an acid washed HDPE container. In the ship's laboratory, the water samples were ®ltered using a polysulfone vacuum ®ltration unit (Nalgene) ®tted with acid washed (1% HCl), pre-weighed membrane ®lters (47 mm diameter, 0.45 mm porosity, cellulose nitrate, What- man). Seawater salts were rinsed from the ®lters containing SPM by washing with 50 ml of Milli-Q water. The ®lters were dried at 458C for 24 h and frozen at À178C for transport to the laboratory in Plymouth. In the laboratory, the ®lters were kept for a further 24 h at 458C and subsequently re-weighed on a precision balance (Sartorius). The weight of SPM on the ®lters was calculated as the difference between the weight of the ®lter containing SPM and the original ®lter. In order to obtain a large quantity of material for optimising a particulate metal extraction protocol using EDTA, approximately 300 g of freshly depos- ited particulate material was collected from the sedi- ment-water interface at Halton Quay on the Tamar Estuary (UK). It was postulated that a surface sedi- ment sample from this locality would closely re¯ect characteristics of estuarine SPM [3]. The sample was collected from the sediment-water interface (<2 cm depth) at a water depth of ca. 30 cm and transferred into a re-sealable polythene bag using a HDPE scrap- per. Air was expelled and the bag was resealed and then stored at 48C for transport to the laboratory. In the laboratory, the particulate material was immediately air dried at 458C for 48 h. In order to achieve a homogenous ®ne grained material, the dried sample was crushed into a ®ne powder (<200 mm) using an acid washed agate mortar and pestle, subsequently placed in a polythene bag and left for 24 h on a motorised end-to-end shaker (Baird and Tatlock, UK) operating at 40 rpm. The organic carbon (OC) content in the Halton Quay particulate material was determined in triplicate by the loss on ignition method [22]. For this purpose acid was added to the material (10 ml of 1 M HCl to 1 g of particulate matter) to remove inorganic carbon (mainly calcium carbonate), and subsequently the material was dried at 1058C until constant weight was achieved after cooling in a dessicator (weight A). Subsequently, the material was ashed in a muf¯e furnace at 6008C for 8 h, cooled in a dessicator and weighed (weight B). The organic carbon content was calculated as: Percentage OC 100 A À B A (1) 2.3. Use of EDTA as extractant The interaction between an added chelating ligand and metals complexed by naturally occurring ligands in the aquatic environment may be slow. Experiments involving competition for dissolved Cu in seawater between an added metal chelating ligand (salicylal- doxime) and naturally occurring dissolved organic ligands have indicated that the establishment of an equilibrium may take more than 8 h [23]. In order to investigate the rate of interaction between EDTA and particle bound trace metals we performed incubation experiments. The aim of these studies was to establish the minimum time required for the EDTA-particulate metal extraction procedure. The incubation experiments were performed using 2 different ratios of extraction solution to particulate matter (200 : 1 and 2000 : 1 (v : w)), in order to investigate the in¯uence of particulate matter concen- tration on the extraction ef®ciency. Furthermore, two EDTA concentrations (0.005 M and 0.05 M) were investigated. Table 1 includes the experiments that were undertaken. The pH of the EDTA solution was set at 7.6, which is close to the natural pH observed in large parts of estuarine systems. The pH of the EDTA extraction solutions was determined after the experi- ments to ensure that dissolution of material from the particles did not modify the pH of the experiments. No change in pH was observed at the end of the EDTA extraction experiments. Fig. 1 gives a schematic representation of the par- ticle extraction procedures applied during all extrac- tion experiments. Samples were agitated during the incubation period using an end-to-end shaker set at 40 rpm. A centrifuge (Sanyo, Centaur 2, 3000 rpm for 10 min) was utilised to separate the extraction solution from the particulate material. The supernatant was acidi®ed to pH ca. 2 using concentrated HNO 3 (1 ml per 1 ml of solution) to avoid loss of metals onto the wall of the bottles, and then stored at 48C prior to metals analysis by ICP-MS. All experiments were carried out in triplicate using separate fractions of D J. Whitworth et al. / Analytica Chimica Acta 392 (1999) 3±17 5 freshly deposited particulate material collected from Halton Quay or SPM from the Scheldt. 2.4. Other extraction protocols Commonly employed single extraction protocols for marine SPM and sediment particles were utilised to allow comparison with the ef®ciency of the EDTA protocol. The protocols applied included the extrac- tion of metals from particulate material using 1 M HCl for 6 h [24±26], 25% acetic acid at pH 4.5 for 16 h [1], 1 M ammonium acetate at pH 7 for 6 h [19] and 0.05 M EDTA at pH 7.6 for 72 h. The extraction procedures are summarised in Table 1 and Fig. 1 and were applied to certi®ed reference materials (CRMs), BCR 320 (riverine sediment) and BCR Table 1 Procedures used for particle extraction experiments using particulate material from Halton Quay with EDTA, HCl, ammonium acetate and acetic acid as extractants Experiment Reagent Ratio extractant: particle (v : w) Dilution for metals analysis by ICP-MS Incubation time (h) Metals determined 1 0.05 M EDTA 200 : 1 10 1, 4.5,10, 24, 48 Cu, Co, Ni, Zn, Fe, Mg 2 0.005 M EDTA 200 : 1 0 1, 4.5,10, 24, 48 Cu, Co, Ni, Zn, Fe, Mg 3 0.05 M EDTA 200 : 1 10 18, 24, 36, 48, 60, 72 Cu, Co, Ni, Zn, Fe, Al, Pb, Mn, Mg 4 0.05 M EDTA 2000 : 1 10 18, 24, 36, 48, 60, 72 Cu, Co, Ni, Zn, Fe, Al, Pb, Mn, Mg 5 1 M HCl 200 : 1 10 6 Cu, Co, Ni, Zn, Fe, Al, Pb, Mn, Mg 6 25% (v:v) acetic acid 200 : 1 10 16 Cu, Co, Ni, Zn, Fe, Al, Pb, Mn, Mg 7 1 M ammonium acetate 200 : 1 10 6 Cu, Co, Ni, Zn, Fe, Al, Pb, Mn, Mg Fig. 1. Schematic representation of the procedure adopted for particulate metal extraction experiments. 6 D J. Whitworth et al. / Analytica Chimica Acta 392 (1999) 3±17 277 (estuarine sediment) supplied by the Commission of the European Communities, Community Bureau of Reference. Furthermore, a total digestion of the CRMs was performed using HNO 3 and HF, following a method adapted from Rantala and Loring [27], to verify the accuracy of the extraction and analytical procedures. The total digestion was also applied to the SPM from the Scheldt Estuary. For the total digestion, 150 mg of certi®ed reference material, or a pre- weighed ®lter with a known amount of SPM (between 30 and 500 mg) was placed into a 30 ml PTFE decom- position vessel, 10 ml of concentrated HNO 3 was added and the vessel was put on a hotplate (1208C), for a re¯ux period of 24 h. Subsequently, 2 ml of concentrated HF was added and the re¯ux was con- tinued for a further 48 h at 1208C. The vessel was then uncovered and 4 ml of HNO 3 added and the content of the vessel was evaporated to dryness on the hotplate (1208C). 10 ml of concentrated HNO 3 was added and the content of the vessel was evaporated to dryness at 708C; this step was repeated twice. Then 10 ml of 0.1 M HNO 3 was added to the vessel and the solution was transferred into a 25 ml HDPE volumetric ¯ask containing 0.93 g of H 3 BO 3 . The volumetric ¯asks were made up to volume using 0.1 M HNO 3 and stored in the refrigerator at 48C for subsequent metal analysis by ICP-MS. The total digestion of the CRMs was performed in triplicate. Procedural blanks were pro- cessed and used for correction of particulate metal data. 2.5. Trace metal analysis by ICP-MS The concentration of metals (Cu, Ni, Co, Fe, Mn, Pb, Al, Mg and Zn) in the supernatant after centrifu- gation was determined by ICP-MS using a VG Elemental PQ2 Turbo instrument (Winsford, Cheshire). The spectrometer was ®tted with an Ebdon high solids `V' groove nebuliser (Ar gas ¯ow set at 0.9 l min À1 ) connected to a Scott double pass spray chamber (Ar coolant, 15 l min À1 ) and the plasma gas ¯ow was ®xed at 1 l min À1 . Samples were introduced into the manifold at 1 ml min À1 . Analytes were ionised at 1350 W and the detection dwell time was 10.24 Â 10 À3 s. Fe measurements were determined from the 57 Fe concentration, as 56 Fe could not be directly determined due to poly- atomic interferences (ArO). Sample solutions contain- ing EDTA were diluted with Milli-Q to EDTA con- centrations of 0.005 M, in order to avoid interferences associated with EDTA. For experiments involving EDTA, metal standards were matrix matched to the sample by preparing the standards in 0.005 M EDTA. Calibration was undertaken immediately prior to sam- ple analyses. In addition, the samples and standards were spiked with 115 In (100 mgl À1 ) in order to correct analytical drift during the operation of the spectro- meter. 3. Results and discussion 3.1. EDTA extraction studies Experiments 1 and 2 (see Table 1) were designed to compare the effect of the EDTA concentration and the incubation time on the extraction ef®ciency for 0.005 M and 0.05 M EDTA at a ratio of extraction solution to particulate material of 200 : 1 (v : w; 30 ml EDTA solution with 150 mg of particulate material). Fig. 2 shows the concentrations of Fe, Cu, Zn, Co, Ni, and Mg extracted using EDTA and normalised with respect to the particulate matter concentration, plotted against time for these experiments. The maximum incubation periods used for the EDTA extractions were 48 and 72 h, respectively. The results suggest that after 48 h the extracted concentrations of Fe, Mg and Cu were lower in the 0.005 M EDTA extraction solution compared with the 0.05 M solution. The difference was small and within the experimental error for Zn, Co and Ni. Furthermore, metals were extracted more rapidly and a plateau in metal concentration was reached earlier using the 0.05 M ETDA compared with the 0.005 M EDTA. Therefore, the equilibrium between metals complexed by EDTA, and those asso- ciated with particulate matter, was reached more rapidly using the 0.05 M EDTA solution. The use of a lower EDTA concentration (0.005 M) will, there- fore, extract a lower amount of Fe, Mg and Cu from particulate material, but will also require a longer incubation time to attain equilibrium, compared to a higher EDTA concentration (0.05 M). The in¯uence of the concentration of particulate material on the metal (Me) extraction ef®ciency was investigated by employing 2 different ratios of extrac- tion solution (0.05 M EDTA) to particulate material: D J. Whitworth et al. / Analytica Chimica Acta 392 (1999) 3±17 7 200 : 1 and 2000 : 1 (v : w; 30 ml of EDTA solution with 150 mg and 15 mg of particulate material, respectively). In addition to the metals measured during the previous experiment, Mn, Pb and Al were also determined. The incubation period was 72 h in order to allow more time for the attainment of equili- brium (experiments 3 and 4, Table 1). The results of this experiment are shown in Figs. 2 and 3, and indicate that increasing the extractant to particle ratio from 200 : 1 to 2000 : 1 (v : w) had no effect on the particulate matter-normalised MeEDTA concentra- tions, with the differences between MeEDTA concen- trations within analytical errors (with the exception of Al (Fig. 3(c)). This observation may be explained by the use of an excess concentration of EDTA during the experiments, of which a large fraction was not com- plexed to metal ions. The different behaviour observed for Al may be attributed to the fact that an equilibrium Fig. 2. Concentration of particulate metal (Fe, Cu, Zn, Co, Ni, Mg) extracted using 0.05 M EDTA with an extraction solution to particulate matter ratio of 200 : 1 and 2000 : 1 (v : w), and 0.005 M EDTA with a ratio of 200 : 1, plotted against time. Legend key presented on Fig. 2(a), and number between brackets refers to experiment (see Table 1). Solid curve obtained from model calculations. 8 D J. Whitworth et al. / Analytica Chimica Acta 392 (1999) 3±17 for Al between EDTA and sorption sites on the particles is attained only very slowly (see below). 3.2. Modelling of EDTA kinetic incubation experiments Experiments 1±4 indicated that the competition between EDTA and particle bound metals was not instantaneous (see Figs. 2 and 3). The data from experiments 1 and 3 (0.05 M EDTA extraction solu- tion to particle ratio of 200 : 1 (v : w)) were, therefore, used to model the interaction between EDTA and metals. The competition between the EDTA and the surface sites of the particles (S) for the trace metals can be described using Eq. (2). The concentration of metal associated with particles is denoted by [MeS] and the EDTA complexed metal concentration by [MeEDTA]. The reaction can be characterised by two rate con- stants: k 1 for the forward and k 2 for the reverse reaction [28]. The time dependent linear differential equation for reaction (2) is expressed by Eq. (3), asuming that the EDTA concentration used is in excess of the metal concentrations, and, therefore, is constant. We also use the assumption that the concentration of S is much greater than the concentration of MeS, and that an increase in concentration of S with time is negligible. MeS EDTA 6 k 1 k 2 S MeEDTA (2) dMeEDTA dt k 1 MeSÀk 2 MeEDTA (3) Using the assumption that at t 0 the amount of the metal complexed with EDTA is zero, the solution to Eq. (3) is MeEDTA k 1 k 1 k 2 MeS t0 À k 1 k 1 k 2 e Àk 1 k 2 t MeS t0 (4) Fig. 3. Concentration of particulate metal (Mn, Pb and Al) extracted using 0.05 M EDTA with an extraction solution to particulate matter ratio of 200 : 1 and 2000 : 1 (v : w). Legend key presented on Fig. 3(a), and number between brackets refers to experiment (see Table 1). Solid curve obtained from model calculations. D J. Whitworth et al. / Analytica Chimica Acta 392 (1999) 3±17 9 With the use of curve ®tting software (CurveFitEx- pert 1.3) and an exponential function of the form y a(1Àe bx ), the concentration of [MeS] t 0 was estimated as being the concentration of [MeEDTA] at equilibrium. Subsequently, a computer programme (written in Turbo-Pascal) was employed to calculate the rate constants (k 1 and k 2 (h À1 )) using Eq. (4) and the data obtained from experiments 1 and 3. The curves obtained from the model calculations are pre- sented in Figs. 2 and 3. Table 2 shows the results of calculations of the minimum time required for the different elements to attain equilibrium with the extraction solution. A commonly used approach to assess the state of equilibrium is the characteristic reaction response time (t resp ), which is de®ned as the time required to achieve 63% of the equilibrium concentration (or the time to reduce the imbalance to e À1 (37%) of its initial imbalance) [29]. We also calculated the minimum times required to achieve 95% (t 95% ) and 100% (t 100% ) of the equilibrium con- centration. Furthermore, the estimated equilibrium concentrations of [MeEDTA] for t 100% determined using this approach are presented in Table 2. Figs. 2 and 3 show that the time required to obtain 95% (t 95% ) of the equilibrium concentration was 30 h or less for all metals except Al (Table 2). For this element we observed a slow desorption process that can be attributed to (a) the release of Al from binding sites due to the competitive action of EDTA, and (b) the slow dissolution of Al from the lattices of clay particles. The slow release of Al is most likely respon- sible for its t 95% value of 149 h. The calculated t 100% for Al was greater than 3000 h, and for Cu and Mn greater than 400 h. Extractions over this period of time are practically impossible, and would most likely result in analytical artefacts including re-adsorption of metals on particles and walls of the sample con- tainer and perhaps bacterial alteration of the metal speciation. An extraction time of 72 h for the experi- ments involving EDTA was, therefore, chosen as the optimum condition, because most of the metal extrac- tions reached a state of at least 95% of their equili- brium within this time period. The kinetic calculations allowed us to determine the interaction between the metals and the sites on the surface of the particles. For this purpose we separated Eq. (2) into two reactions MeS 6 Me SK MeS SMe H MeS (5) where K MeS is the apparent stability constant, and EDTA Me 6 MeEDTA K H Me H EDTA H MeEDTA EDTA H Me H (6) where K H Me H EDTA H is the conditional stability constant. [EDTA H ] is the concentration of EDTA not complexed by Me, and can be taken as [EDTA], as [EDTA]) [Me]. The conditional stability constant for Eq. (2) can be written as K 2 SMeEDTA MeSEDTA H (7) and is the product of the apparent and conditional stability constants of the separate reactions K 2 K MeS K H Me H EDTA H (8) Table 2 Treshold times and equilibrium concentrations calculated using the kinetic model. Conditional and apparent stability constants (defined in Eqs. (5)±(7)) were obtained from the literature (log K H Me H EDTA H ) [28] and model calculations (log K 2 and log K MeS ) Metal t resp (h) t 95% (h) t 100% (h) eq. conc. (mgg À1 ) Log K H Me H EDTA H Log K 2 Log K MeS Fe 3.5 10 210 768 12.9 7.8 À5.1 Mg 0.8 2.5 50 317 8.0 5.9 À2.1 Mn 7.3 22 442 44.4 13.0 4.5 À8.5 Cu 10 30 608 27.1 17.6 4.1 À13.5 Zn 1.4 4.3 87 23.3 15.7 4.8 À10.9 Pb 8.3 25 503 10.2 16.9 4.8 À12.1 Ni 1.1 3.3 66 1.59 17.8 4.46 À13.3 Co 1.7 5.2 105 0.89 15.5 3.97 À11.5 Al 50 149 3020 245 7.6 3.0 À4.6 10 D J. Whitworth et al. / Analytica Chimica Acta 392 (1999) 3±17 The conditional stability constants for the metal- EDTA reactions (K H Me H EDTA H ) were obtained from the literature [28] and corrected for the side reactions of EDTA and Me at pH 7.6 and ionic strength of 0.1 M (Table 2). The desorption of metals (Me) from surface sites (S) is simply described by our model as a competition between MeS and the sum of the metal EDTA complexes. In practice, only one, or at most two, metal-EDTA complexes will be involved in the equilibrium. The predominant species can be assessed by comparing the -coef®cients that describe the reactions between the metal ion and the EDTA species [30] under the given conditions of pH, temperature and ionic strength. Only the most important MeEDTA complexes have been used for our calculations and are included in Table 2. For most metals considered, the important complex is MeEDTA, although for Al it is Al(OH)EDTA. However, for Fe and Zn both MeEDTA and Me(OH)EDTA are important. In order to simplify the calculation, the predominant complex was used in this study. At pH 7.6 FeEDTA and ZnEDTA formed the predominant species (60 and 90% of the total metal-EDTA complexes, respec- tively), and these were used for further calculations. However, it must be noted that this simpli®cation will result in less certainty for calculations of K FeS and K ZnS . K 2 can be calculated from the forward and reverse reaction rate constants (k 1 and k 2 ) [28,31] K 2 k 1 k 2 (9) and K MeS can then be derived using Eq. (8). The results are presented in Table 2 and show that K MeS generally follows the Irving±Williams order. Accord- ing to this rule, the stability of metal complexes increases in the series [29] Mn 2 ` Co 2 ` Ni 2 ` Cu 2 b Zn 2 . The approach used with EDTA as extractant releases metals into solution from sites with a lower `binding strength' than that of the EDTA ligand. The amount of MeEDTA extracted, therefore, relates to the strength of the MeEDTA complex. In the case of Fe and Al, not just the partition between solid and dissolved phases is important, but also the dis- solution of solid phases, as precipitation and coagula- tion strongly in¯uence their solid speciation. Potentially useful information on metal partitioning between solid and dissolved phases may be gained by comparing apparent stability constants obtained using the method presented here and conditional stability constants measured for naturally occuring dissolved organic material (K H Me H L H ; de®ned using Eq. (10) MeL 6 Me LK H MeL L H Me H Me H L H (10) For example, the current study indicates that the apparent stability constant for Cu (K CuS 10 À13.5 )of the particulate matter from the sediment-water inter- face at Halton Quay, compares well with reported constants for dissolved organic ligands and Cu: 10 À10 ±10 À14 for lacustrine, seawater and estuarine conditions [21,32,33]. This observation may imply that organic ligands on particles are important for the complexation of Cu; sequential extraction schemes on sediments have reported similar ®ndings [18,34]. The Halton Quay particulate material contained an important fraction of OC: 1.01 Æ 0.05%. In natural waters competition may, therefore, occur for Cu between particle surface sites and dissolved Cu com- plexing natural ligands. In the case of Zn, reported conditional stability constants for the dissolved ligands and Zn (between 10 À7.4 and 10 À9.3 ) are some- what higher than for K ZnS (10 À10.9 ). Conditional sta- bility constants for the interaction between dissolved organic ligands and Pb in seawater are of the order 10 À8 ±10 À9 [35]. The lower value for K PbS (10 À12.1 ) would suggest that estuarine particulate matter may actively remove dissolved Pb from solution. A particle reactive behaviour has been observed for dissolved Pb in estuarine systems [36]. Conditional stability con- stants for dissolved ligands and Fe in coastal and oceanic conditions are reported to be ca. 10 À18 ± 10 À23 [31,37]. The apparent stability constant (10 À5.1 ) for FeS is subject to error (see above), how- ever the large difference in stability constant between the dissolved organic ligands and particulate matter for Fe suggests that processes other than complexation (i.e. precipitation, coagulation) determines the beha- viour of Fe in estuarine systems. Further work will need to be undertaken to relate the apparent stability constants to different types of SPM (e.g. riverine, marine, estuarine). This may allow us to link changes in apparent stability constants to different physico-chemical characteristics of SPM. Further- more, the fraction of MeS released by the ligand competition approach will give information about D J. Whitworth et al. / Analytica Chimica Acta 392 (1999) 3±17 11 the geochemical availability of the metals. Inclusion in water quality models of conditional stability constants for the metal-dissolved ligand interactions and the apparent stability constants for metal SPM interac- tions may also result in important improvements in our ability to model contaminant behaviour in natural waters. 3.3. Certified reference materials: total digests and comparison of EDTA extraction with other single extractants Certi®ed reference materials BCR 320 (riverine sediment) and BCR 277 (estuarine sediment) were analysed for total particulate Pb, Zn, Fe, Cu, Co, Ni, Al, Mg and Mn, after digestion using HF and HNO 3 (see Section 2.4) The results of the analysis are pre- sented in Table 3. The data show that the analysed and certi®ed values were in close agreement. Further experiments were performed using the BCR 320 and BCR 277 sediments in order to compare the concentration of exchangeable metals extracted from these sediments using 0.05 M EDTA with other com- monly used single extraction procedures (1 M HCl, 25% (v : v) acetic acid and 1 M ammonium acetate; experiments 5±8, Table 1). Figs. 4 and show the results of these experiments with the extracted metal concentration presented as a fraction of the total metal concentration obtained after total digestion of the sample. Fig. 4 shows that for most metals (with the excep- tion of Al and Zn), 1 M HCl extracted a higher fraction of particulate metal in BCR 320 compared with the other extractants (e.g. Co 44%, Ni 50% and Fe 36%). This observation indicates that metals were not easily removed from the riverine sediment using mild extraction procedures at pH values between 4.5 and 7.6. The low pH of the HCl extraction solution, may have resulted in the dissolution of carbonate phases in the sediment particles, and hence released matrix bound particulate metals. The fraction of par- ticulate metals extracted from the BCR 320 sediment using the 0.05 M EDTA (pH 7.6) and the 25% v : v acetic acid (pH 4.5) extraction solutions were similar (Fig. 4). For most metals, the lowest extraction yield was obtained using 1 M ammonium acetate. The fractions of metal extracted using 1 M HCl were generally similar for BCR 277 compared with BCR 320 (Fig. 5). However, the other extractants showed higher yields for BCR 277. The particulate metals in the BCR 277 estuarine sediment, therefore, appeared to be present in a more available form compared with BCR 320. As was the case for BCR 320, the ammonium acetate extractions with BCR 277 resulted in the lowest yield. Furthermore, the 0.05 M EDTA and 25% acetic acid extractions resulted again in comparable yields. 3.4. Application to SPM from the Scheldt Estuary The 0.05 M EDTA extraction protocol was used to investigate the extractable metal concentrations of SPM in surface water samples from the Scheldt Estu- ary. An incubation time of 72 h was employed and the Table 3 Analysis of total particulate metal concentration in certified reference materials BCR 277 and BCR 320 BCR 277 BCR 320 Certified concentration a Observed concentration Certified concentration Observed concentration a Mn (mg g À1 ) 1.5 a 1.5 Æ 0.008 0.8 a 0.67 Æ 0.02 Mg (mg g À1 )11 a 11 Æ 0.03 21 a 19 Æ 0.5 Al (mg g À1 )51 a 49 Æ 0.8 87 a 80 Æ 0.8 Fe (mg g À1 )46 a 41 Æ 0.4 49 a 37 Æ 0.7 Co (mgg À1 )16Æ 0.8 14 Æ 0.45 19 Æ 0.9 16 Æ 1.9 Ni (mgg À1 )41Æ 4.4 43 Æ 1.6 75 Æ 1.4 N/A Zn (mgg À1 ) 547 Æ 12 536 Æ 31 142 Æ 3 123 Æ 10 Cu (mgg À1 ) 102 Æ 1.6 100 Æ 944Æ 1 36 2.9 Pb (mgg À1 ) 146 Æ 3 152 Æ 6.6 42 Æ 1.6 60 Æ 2.7 a Indicative values (uncertified values provided by BCR); N/A analysis not undertaken; mean Æ SD (n 3). 12 D J. Whitworth et al. / Analytica Chimica Acta 392 (1999) 3±17 [...]... particulate matter in this part of the estuary may have lost easily exchangeable metal as a result of cation exchange processes, and also may have been diluted with cleaner marine SPM containing low metal concentrations The fraction of Mg extracted using EDTA was small compared with the other metals Particulate Mg is commonly present in clay minerals and, therefore, less accessible to EDTA for extraction... of the particulate material and are, therefore, more loosely bound then residual metals held in the lattices of particles [41] This has important implications for the long term fate of these particle associated trace metals, with possible removal of weakly associated metals by cation exchange upon SPM entering the coastal waters The lowest percentage of extractable metals was observed at high salinities... complexation by dissolved organic ligands in natural waters [20,21] Assessment of the contribution of biogeochemically available trace metals to the total particulate metal concentration in SPM can be performed when the extraction scheme is complemented by a total digestion using HF and HNO3 The proposed extraction scheme has a low requirement of SPM, and is simple and requires a minimal amount of reagents... the application of a ligand competition procedure for the investigation of non- D.-J Whitworth et al / Analytica Chimica Acta 392 (1999) 3±17 15 Fig 6 Concentrations of particulate metals Zn (a) , Ni (b), Cu (c), Mn (d), and Mg (e) obtained using 0.05 M EDTA (exchangeable), and HF and HNO3 (total) from SPM sampled at different salinities in the Scheldt Estuary (Belgium) lattice bound trace metals associated... SPM in the behaviour of metals in these systems Acknowledgements DJW wishes to thank the University of Plymouth for funding his Ph.D studentship EPA gratefully acknowledges Analytica Chimica Acta for the awarded bursary to present a paper at Euroanalysis X (Basel, Switzerland) The authors thank the crew of the R/V Belgica for their assistance during sampling, and Prof R Wollast (University of Brussels)...D.-J Whitworth et al / Analytica Chimica Acta 392 (1999) 3±17 13 Fig 4 Fraction of particulate metals Mg, Mn, Fe, Co, Ni and Cu (a) and Al, Zn and Pb (b) extracted from BCR 320 using 1 M HCl, 25% (v : v) acetic acid, 0.05 M EDTA and 1 M ammonium acetate The digestion using HF and HNO3 represents total particulate metal concentration (100%) Legend presented in Fig 4b EDTA:SPM ratio was 200 : 1 (v : w)... reagents and sample handling and hence has an inherent low risk of sample contamination The occurrence of readsorption of metals during extraction has been reported, where metal initially released by the reagent then re-precipitates or partitions back onto the solid phase [42] Rendell et al (1980) [43] reported that extractions using EDTA are not affected by this problem Our experiments also indicated that... associated with SPM The method uses EDTA (0.05 M) as the added competing ligand and, therefore, has a well de®ned metal binding strength and provides information about the nature of the sorption sites for metals on particles Our approach is along similar lines to the ligand competition tech- 16 D.-J Whitworth et al / Analytica Chimica Acta 392 (1999) 3±17 niques used for determination of trace metal complexation... total particulate metal concentration (100%) Legend presented in Fig 5b relative high proportions of EDTA extractable concentrations for Zn, Ni, Cu and Mn compared to the total SPM metal concentrations, can most likely be explained by the perturbed nature of the Scheldt estuary, with high anthropogenic inputs of metals Metals originating from anthropogenic sources become associated with the surfaces of. .. The total metal concentrations in SPM obtained during our study compared well with previous SPM metal studies carried out in the Scheldt Estuary [36,38] The total concentration of Zn, Ni and Cu in SPM showed a maximum at low salinity (3 psu) The maxima coincided with an increased POC concentration, and may, therefore, have been caused by industrial or urban waste water discharges [39] The increase in . using particulate material from Halton Quay with EDTA, HCl, ammonium acetate and acetic acid as extractants Experiment Reagent Ratio extractant: particle. the application of a well de®ned binding strength, and provides an indication of the biogeochemical availability of particle associated metals. The approach