Pergamon 0043-1354(94)00205-3 Wat. Res. Vol. 29, No. 3, pp. 803-809, 1995 Copyright© 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 0043-1354/95 $9.50 + 0.00 SPECIATION OF DISSOLVED CADMIUM: INTERPRETATION OF DIALYSIS, ION EXCHANGE AND COMPUTER (GEOCHEM) METHODS PETER E. HOLM 1., SJUR ANDERSEN2~ and THOMAS H. CHRISTENSEN ~ qnstitute of Environmental Science and Engineering/Groundwater Research Centre, Technical University of Denmark, Building 115, DK 2800 Lyngby, Denmark and :JORDFORSK, Centre for Soil and Environmental Research, 1432 As, Norway (First received February 1994; accepted in revisedform July 1994) Abstract--Equilibrium dialysis and ion exchange methods, as well as computer calculations (GEOCHEM), were applied for speciation of dissolved cadmium (Cd) in test solutions and leachate samples. The leachate samples originated from soil, compost, landfill waste and industrial waste. The ion exchange (IE) method separates dissolved Cd into free divalent Cd (Cd2+) and complexed Cd and furthermore separates the latter into the operationally defined forms: labile, slowly labile and stable complexes. The dialysis (ED) method determines high molecular weight Cd complexes (above 1000mol. wt). For both methods the reproducibility was good. By combining the results of the GEOCHEM calculations in terms of the inorganic complexes, and the IE results, the fractions of free and inorganically complexed Cd were estimated. The IE and ED results furthermore provided information about the organic complexes. Selected environmental leachates showed different Cd speciation patterns as expected. Some leachates were dominated by free divalent Cd (1-70%), some by inorganic complexes (1-87%), and some by organic complexes (7-98%). Key words---cadmium, speciation, leachate, complexes, ion exchange, dialysis, GEOCHEM INTRODUCTION The availability and mobility of cadmium (Cd) in terrestrial environments, e.g. in soils and waste disposal sites, are not only related to concentrations in solution but also to the Cd species present in the solution, i.e. free divalent Cd (Cd z÷), inorganic and organic complexes. The analytical determination of dissolved species is still in its infancy and no method is generally accepted, though many approaches are used. Speciation methods in general, advantages and problems have been reviewed by Florence and Batley (1980). Berggren (1989) studied cadmium species in humic soil solutions by equilibrium dialysis. High molecular weight complexes accounted for 16--43% of dissolved Cd in soil solutions containing dissolved organic carbon (DOC) concentrations of 30 and 160 mgC/1. Lun and Christensen (1989) determined Cd species in solid waste leachates using ion exchange methods. Free divalent Cd accounted for less than 10% of dissolved Cd. In most of the leachates, the operationally defined fractions "labile" and "slowly labile" complexes accounted for 69-100% of Cd in solution. Morrison et aL 0990) found maximum Cd concentrations of 4--13/~gCd/l in urban runoff water, and anodic stripping voltammetry showed that *Author to whom all correspondence should be addressed. wa 29/3-o cadmium was predominantly ionic and inorganically complexed. DOC concentrations in solution ranged up to 41 mgC/l. These results from the literature support the need for speciation methods, but none of the methods alone provides enough information to evaluate the significance of inorganic complexes and the size and apparent stability of the naturally occurring organic complexes. Such information is in particular needed for evaluation of the mobility of Cd in the terrestrial environment, since the majority of the information available only relates to the mobility of free divalent Cd. As shown above, free divalent Cd may only constitute a small fraction of dissolved Cd in the environment. The purpose of this study was to show, by combining the results of existing speciation methods applied to selected solution samples, that it is possible to determine the fraction of free divalent Cd, the fraction of the inorganic and the organic complexes, and to evaluate the size and stability of the organic Cd complexes. The three speciation methods applied were: (i) equilibrium dialysis (referred to as the ED method), which separates complexes according to their size, (ii) ion exchange (referred to as the IE method), which determines free divalent ions and categorizes the complexes according to their apparent stability, and (iii) GEOCHEM computer calculations of the ratio between inorganic complexes and free divalent Cd. 803 804 Peter E. Holm et al. Table 1. Characteristicsof samples used for speciation Solution pH Spcc. cond. (/tS/cm) CaCI2 solution (10 -2 M) Humic acid solution Soil solution Compost leachate Polluted groundwater Industrial leachate 1 Industrial leachate 2 Industrial leachate 3 7.0 7.0 6.6 6.6 6.8 7.2 7.0 6.5 2300 275 2000 1050 4940 12000 16300 2330 DOC (mgC/l) CdT (/~g/l) 0 25 64 13 264 79 34 3200 Mg (mg/I) Ca (mg/I) 0 I 23 12 36 90 80 10 400 50 400 119 57 300 270 90 9.0 10.1 6.3 8.7 9.2 71 8.9 660 Na (rag/l) CI (mg/l) 0 -24 23 332 2200 2400 40 710 4 45 56 626 2900 3200 60 SO4 (mg/I) 0 l0 90 30 <1 15 20 9 - - N o t analyzed. MATERIALS AND METHODS General All chemicals used were analytical grade (Merck, pro analysis) unless otherwise stated. All plastic and glassware were cleaned and soaked in 2 M HNO3 for at least 12 h, then rinsed with deionised distilled water and dried at 45°C in a convection oven. The dialysis tubing (Spectra/Por® 7 Membranes, molecular weight cutoff of 1000, 45 mm flat width, 6.42 ml/cm) was soaked in a 0.05% sodium azide solution, acid washed in 0.1 M HNO 3 and rinsed with deionised water. When specific or constant pH was needed, adjustments were made by means of small additions of HNO3 or NaOH. ED method principles, calculations and design The ED method separates dissolved Cd in solution (CdT) according to the size of the complexes. The sample constitutes the inner solution in the dialysis bag, and free divalent Cd and complexes with a molecular weight cutoff (MWCO) smaller than 1000, equilibrate over the dialysis tubing membrane into an outer solution identical to the sample with respect to dissolved Cd and major inorganic ions. Complexes larger than 1000 in molecular weight are retained inside the dialysis tubing. At equilibrium, the activities of all ions and complexes smaller than 1000 in molecular weight are identical in the sample (inner solution) and the outer solution. The concentration ofdaigh molecular weight organic metal complexes Cd*Mwin the inner solution at equilibrium equals the difference between the total metal concentration in the inner (Cd0 and in the outer solution (Cdo) at equilibrium: [Cd~Mw] =[Cdl] - [Cdo] [CdT] [Cd~] [CdT] [Cd[] t I I ! Identical: pH A 2+ M 2, Cd, ua , g ,Na* 48 hours I I I cf, SO~ and NO3 activity ! (2) Therefore, the fraction of high molecular weight Cd complexes of the original sample solution is calculated as [CdnMw] [ C d l ] - [Cdo] Outer solution Initial (1) where [ ] is concentration and * refers to the equilibrium situation. Assuming that the concentration of high molecular weight ligands substantially exceeds the concentration of dissolved Cd, the fraction of high molecular weight Cd complexes is the same in the sample solution and in the inner solution at equilibrium [CdnMw] [Cd~Mw] experimental design and allows for the use of relative small sample solutions. The dialysis tubing containing 50 ml sample (inner solution) was suspended in 4.01 solution (outer solution) in a 51 polyethylene bucket with lid. The outer solution was identical to the sample solution with respect to major inorganic ions and also approximately to inorganic Cd (Table 1). Organic matter and, probably, carbonate contributed significantly to the charge balance in some of the leachates. These charge contributions were not quantified, but additional ions were supplied to reach identical specific concentrations in the two solutions. Cd was added to the outer solution in the same concentration as the total dissolved Cd concentration (Cdr) in the sample (inner solution) given in Table 1. The dialysis tubing was fixed with (0.30 mm diameter nylon) lines and a 75 mm tubing closure (Spectra/Por Closures, Spectrum Medical Industries Inc.) at the upper end, and with a 75 mm weighted closure at the lower end. The outer solution was continuously stirred (magnetic stirrer), thus minimizing the dialysis time needed to obtain equilibrium. The dialysis time was 48 h. At equilibrium, the inner solution and 3 subsamples of the outer solution were transferred to polyethylene bottles and prepared for analysis of dissolved Cd and dissolved organic carbon (DOC). The ED method design is shown in Fig. 1. (3) The complementary fraction is defined as the low molecular weight fraction and contains free ions and small complexes. The ED calculations are summarized in Fig. 1. Presumably, the complexing effect of low molecular weight organic ligands is of less importance because the concentrations of these are diluted 80 times in the outer solution. Berggren (1989) presented the experimental design involving the dilution of low molecular weight organic ligands in the outer solution, which is an improvement of the classic ED ~,alculations Highmolecularweight Low molecular weight CdHaw = Cxlr c~ ~ Cd~- Cd 0 Cd= =cdo Cd~ Fig. 1. Equilibrium dialysis method (ED): outline of experimental procedures and calculations for determination of the high molecular weight (> 1000 MW) Cd. Speciation of dissolved cadmium Swaae Chelex (2nYJmln) Rataranea 50.400mgAmbedite 50-400mgAmbedite IdenUcal: Ca2÷,Mg2'" ionicstrength Cd<lOOOl,tg/I ~,,,,,..,~_.~'~ lOrrL: ~ IOOrng 48.,~u~ I'~ 805 The second part of the method operationally separates the Cd complexes into three fractions, but does not provide a • theoretical measure of the stability of the complexes. The experimental calculations and procedures are summarized in Fig. 2. The method is based on 50 ml samples for the full speciation. Free divalent ions and complexes defined as labile are retained in the column (solution retention time is approximately 2 min in the column). Complexes not redistributed, therefore defined as stable, are in solution after the final batch procedure (48 hours equilibration). However, complexes in solution after the column procedure, but not after the final batch procedure, are defined as slowly labile complexes. Further details on the IE method are given in Christensen and Lun (1989) and Holm et al. (1995). G E O C H E M calculations Cabu~r~ Free divalentions Labilecomplexes Slowlylabilecomplexes Stablecomplexes ~._~+_- cdt-~l. ~ : ~t e.xl, e_~ - Cd; Cd 'C , -cd÷ CdT Cd~c" Cd~- Cd= Car Cdt Cdt CdT Cdt Fig. 2. Ion exchange method (IE): outline of experimental procedures and calculations for determination of the Cd species: free divalent Cd 2+, labile, slowly labile and stable complexes. IF, method principles, calculations and design The IE method separates dissolved Cd into free divalent Cd and 3 complexed fractions. The determination of the free divalent Cd concentration is based upon the equilibrium established between Cd 2+ in solution and Cd 2+ ions that are exchanged onto a weak cation exchange resin (Ca saturated Amberlite). The procedure for the Cd 2+ determination involves a reference experiment. The complexed Cd fraction is separated according to the ability of the metal-ligand equilibria to redistribute, allowing the released Cd 2÷ ions to exchange onto a strong cation exchange resin (Ca saturated Chelex). The procedure for the Cd 2+ determination involves an initial batch experiment together with a reference experiment, both shown in the upper part of Fig. 2. Details can be found in Holm et al. (1995). The reference experiment contains Cd solely added as Cd 2÷. All other conditions (resin weight/sample volume, Ca and Mg concentrations, and ionic strength), except the presence of any ligands, are identical in the sample batch experiment and in the reference experiment. The reference experiment provides the required information on the distribution of free Cd 2+ ions between the resin and the solution under conditions comparable to those of the sample. Assuming that Cd 2+ has the same affinity for the resin in both the sample and the reference experiment, the ratio of Cd 2+ to total Cd in solution can be calculated from the equation: [Cd 2+] [CdT]- [Cds] [Cd~'] (4) [Cd~] [Cds] [Cd~] - [Cd~'] where * refers to the reference experiment and all concentrations refer to dissolved Cd concentrations that can be measured directly (see Fig. 2). Note that this approach is only accurate if the ligand concentration exceeds the total Cd concentration by at least a factor of 5. This assumption is considered fulfilled for most leachates, see also Holm et al. (1995). Inorganic complexes in the samples were calculated with the thermodynamic speciation program GEOCHEM (Mattigod and Sposito, 1979). The program is used in this study only to account for inorganic complexes, as general stability constants for dissolved organic carbon are not available. Input parameters are major anions and cations, total dissolved Cd and the pH (Table 1). The program calculates the ionic strength of the solution and corrects the stability constants to this ionic strength. Output values of the program are the equilibrium concentrations of all species, given as prow of each metal complexed with each anion. The program output was used for calculating the ratio between the inorganically complexed Cd fraction and the fraction of CA2+ in the sample. Samples We selected samples to cover different characteristics with regard to the content of Cd, DOC and inorganic ligands. Both test solutions and leachate samples were considered. The characteristics of all employed solutions and samples are found in Table 1. Apart from the humic acid solution, all solutions and samples containing organic carbon were centrifuged (3500 rpm, 30 min) and decanted to yield solutions free of suspended matter. The humic acid solution was filtered through a 0.2 #m pore size filter (Sartorious Minisart ® NML). Test solutions and those leachates originally containing no Cd were spiked with Cd [stock solutions of Cd(NO3)2] and equilibrated for at least 2 days before centrifugation. Test solutions A 2.10-2M C1- solution was prepared from a 1.0 M CaCI2 stock solution. The solution was spiked with Cd and pH adjusted to 7. Humic acid solution was prepared by dissolving a commercially available humic acid (Roth Chemicals, Humussaure No. 7821) in distilled water. The solution was predialysed (with tubing MWCO 1000) in deionised water under continued stirring for 24 h to remove low molecular weight organic compounds. The predialysed solution was spiked with Ca and Cd. Leachates Leachate representing soil solution was collected from plant pots each containing 1 kg soil. The soil was a sandy loam from the Woburn Market Garden Experiment, U.K., which contained elevated levels of trace metals arising from previous amendments with sewage sludge (McGrath, 1984). Soil solution was collected by leaching with distilled water for 2 days. A compost leachate was generated in the laboratory. About 1 kg of household compost was soaked in 21 of water and equilibrated for 2 days. The leachate was spiked with Cd. Polluted groundwater was collected from the leachate pollution plume a few metres downgradient of the Vejen 806 Peter E. Holm et al. Landfill, Denmark (Lyngkitde and Christensen, 1992). The sample was spiked with CA. Leachates were collected from a waste site containing industrial paint waste. The sea-side site, located in Sandefjord, Norway, reflects the characteristics of the leachates (Andersen, 1991). The three leachate samples from the waste site represent different characteristics: high Cd concentration and high sea salt concentration (Industrial leachate 1), low Cd concentration and high sea salt concentration (Industrial leachate 2), high Cd concentration and low sea salt concentration (Industrial leachate 3). Industrial leachate 2 was spiked with Cd. Analytical methods Soluble Cd was determined by solvent extraction and graphite furnace atomic absorption spectrophotometry (Perkin-Elmer 5000, deutrium background correction, HGA 400 graphite furnace, AS-1 automatic sample injection system). Before analysis all samples were acidified to 10-2 M HNO3. Ca, Mg and Na were determined by flame atomic absorption spectrophotometry (Perkin-Elmer 370). Before analysis, all samples were acidified to 10-2 M HNO3. Ca, Mg and Na in the industrial waste leachate samples were analyzed by inductively-coupled plasma emission spectroscopy (Thermo Jarrel Ash ICAP 1100 with simultaneous detection). CI and SO4 analysis was performed by standard autoanalyzer routine (Technicon Autoanalyzer II). CI and SO4 in the industrial waste leachate samples were analyzed by ion chromatography (Dionex QIC analyzer with AS4ASC column and 1.8mM Na2CO3/1.7mM NaHCO 3 as eluent at a flow rate of 2 ml/min). Dissolved organic carbon (DOC) analysis was performed by standard procedures on a total organic carbon analyzer (Dohrmann DC-80 TOC analyzer). pH was measured by a pH Meter (Hanna Instruments DP 7916R) with a pH combination electrode (Radiometer GK 2401C). Specific conductivity was measured by a conductivity meter (Radiometer CDM 83 Conductivity Meter) with electrode (Radiometer Type CDC 104). RESULTS Reproducibility Reproducibility of the two methods was studied by replicate analysis of four of the solutions: the two test solutions (calcium chloride solution and the humic acid solution) and two leachates (compost leachate and the polluted groundwater). Replicate speciations of all four samples were performed at the same time. Results of these speciations are presented in Table 2. For both methods, the reproducibility was considered good because the determined Cd fractions were the same in the replicates. All results from the replicate speciations are presented as average values of the two determinations in Fig. 3. Test solutions The results of the two speciation methods applied to test solutions and theoretically computed inorganic speciations are shown in Fig. 3. The speciation of the 10 -2 M CaCI 2 solution gave the expected results. The ED method determined 100% low molecular weight Cd. This is in accordance with the fact that the Cd-chloro complexes are smaller than the 1000 MWCO of the dialysis membrane. The IE method yielded 47% Cd 2÷ and 53% labile complexes, which is in close agreement with the computer predictions (49% Cd 2÷, 51% Cd-chloro complexes). The speciation of the predialyzed humic acid solution by the ED method identified 52% low molecular weight Cd which was consistent with the 45% Cd 2÷ determined by the IE method. The difference between the 52% low molecular weight Cd and 45% Cd 2+ is within the uncertainty of the experimental approach but may be explained by the inorganically (3%) and low molecular weight organic complexed Cd fractions. Although the solution was predialyzed, a difference of 2 mgC/l was observed between the inner solution, initially and at equilibrium (Table 3). Thus, the size distribution of humic acids may change upon removal of the smaller size fraction as suggested by Berggren (1989). The IE method identified a significant fraction of labile Cd complexes (47% of the dissolved Cd) but only a minor fraction of slowly labile (9%) and stable (0%) complexes indicating that the major part of the 48% high molecular weight Cd complexes were labile. The speciation of the 10 : M CaCI 2 solution and the predialyzed humic acid solution suggests that the two speciation methods yield consistent results with respect to solutions containing both inorganically and organically complexed Cd. Leachates The computed speciation of the soil solution Table 2. Results of replicate speciations of Cd in four samples by the IE method and the ED method IE method % of dissolved Cd Replicate Cd 2÷ Labile complexes CaCI 2 solution (10 -2 M) A B 45 49 54 51 0 0 1 0 99 100 I 0 Humic acid solution A B 44 45 48 45 8 10 0 0 51 52 49 48 Compost leachate A B 43 47 45 41 11 I1 1 1 77 78 23 22 Polluted groundwater A B 1 0 17 17 56 55 26 28 33 31 67 69 Sample Slowly labile complexes ED method % of dissolved Cd Stable complexes MW < 1000 MW > 1000 Speciation of dissolved cadmium 0 10 I 20 30 40 50 60 70 80 90 100 . IEa I , ¢ , , , , | t 1 0 10 , 20 30 40 50 60 70 80 90 100 i 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 go 100 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 gO 100 80 gO 100 Eo' . EO _iE . . . ' " - ~ 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 807 indicated that the inorganic complexes amounted to only 11% of the concentration of Cd ~+ and inorganically complexed Cd. The ED speciation showed almost no high molecular weight complexes, while the IE method showed 30% labile complexes and identified the remaining part as Cd 2+. These results indicate that the labile complexes consisted of both inorganic and low molecular weight organic complexes. During the dialysis, 50% of the dissolved organic carbon permeated into the outer solution (Table 3), confirming that low molecular weight organic compounds were present in the soil solution. The speciation results obtained for the compost leachate showed that about half of the dissolved Cd was present as free divalent Cd, tess than one tenth as inorganic complexes, about one fourth as low molecular weight organic complexes and one fourth as high molecular weight organic complexes. However, the complexes were all categorized as labile or slowly labile. The stable complex fraction amounted to only 1%. The substantial organic complexation was caused by only 13 mgC/l in the leachate. The computed speciation of Cd in the polluted groundwater, paying attention to only the inorganic ligands, showed that the fraction of inorganic complexes was comparable to the fraction of free divalent Cd. The computed ratio of these two fractions should apply also to the real sample and indicates that the inorganic complexes in the leachate polluted groundwater was as insignificant as the free divalent Cd fraction, which by the IE method was quantified to only 1%. The substantial Cd complexation observed (99%) is, therefore, ascribed exclusively to organic complexes, of which 70% was categorized as high molecular weight complexes (ED) and about 25% as stable complexes (IE). The speciation results for industrial leachates 1 and 2 were very similar. The composition of the solutions were very similar, but Cd was 10 times higher in industrial leachate I than in industrial leachate 2. However, from a theoretical point of view (see for example Christensen et al., 1994), the ratio between complexed and free divalent Cd is independent of the metal concentration, when the ligand is much more abundant than the metal. This relation is convincingly demonstrated by these two industrial leachates, ED 80 90 100 % ot dissolved Cd CAL [] free divalentions Q inorganiccomplexes J ED m low MW<1000 • high MW>1000 J IE [ ] freedivalent Im labile. • slowly • stable [ ions complexes labile complexesJ Table 3. Dissolved organic carbon concentration of the ED inner solutions, initially and at the end of the experiment (equilibrium) DOC inner solution (mgC/I) I complexes I Fig. 3. Cd speciation results for the samples characterized in Table 1. CAL refers to GEOCHEM calculations, ED to the equilibrium dialysis method and IE to the ion exchange method, aaverage of two determinations. Solution Initially Humic acid solution Soil solution Compost leachate Polluted groundwater Industrial leachate 1 Industrial leachate 2 Industrial leachate 3 25 64 13 264 79 34 3200 At equilibrium 23 32 7.5 80 7.3 6.1 104 808 Peter E. Holm et al. Table 4. InterpretedCd speciesin 6 leachate samples(% of dissolvedCd) Soil Compost Polluted Industrial Industrial Industrial Cd species solution leachate groundwater leachateI leachate2 leachate3 Free divalentCd 70 45 I 19 23a 33 InorganicCd complexes 9 6 1 64 87a 4 Organic Cd complexes 21 49 98 17 7a 63 Organic Cd complexesb --labile 20 37 16 13 6 0 --slowly labile 0 11 55 3 0 63 --stable 1 1 27 I 1 0 Organic Cd complexes¢ --low molecularweight 19 26 30 7 0 40 --high molecularweight 2 23 68 10 7 23 •The sum of these percentagesexceeds 100% (117%), reflectingthe uncertaintyof this approach. bCharacterized with respect to stability/redistributability. CCharacterized with respect to size. where chloride was the dominating ligand and present in similar concentrations in the two leachates. Industrial leachate 3 was characterized by high concentrations of Cd (660 gg/l) of which two thirds was complexed by organic ligands. The dissolved organic carbon content of this leachate was very high (3200 mgC/1), but only 3% of this was characterized as high molecular carbon (see Table 3). However, apparently 23% of the dissolved Cd was associated with this high molecular fraction. The IE method categorized all complexes as slowly labile. DISCUSSION The results of the three speciation methods applied to the selected environmental samples confirm the need for methods to characterize dissolved Cd species. The fraction of free divalent Cd varied from 1 to 70% and the complexes varied between predominantly inorganic and organic complexes. Furthermore the organic complexes varied substantially in terms of size (smaller or larger than 1000 mol. wt) and ability to redistribute upon changes in solution composition (labile, slowly labile, stable). GEOCHEM speciation was performed to evaluate the ratio between the inorganically complexed Cd fraction and the fraction of free divalent Cd. This ratio was applied to the IE results for identifying the significance of the inorganic complexes in the real sample, including the organic ligands. The GEOCHEM calculations and the IE method are useful together because they both identify the fraction of free divalent CD, which is the basis for applying the computer speciation results for the inorganic solution composition to the real sample. Knowing both the free and the inorganically complexed fraction of Cd in the sample, the remaining part must be organically complexed. The organic complexes may in addition be characterized in terms of high or low molecular weight or as stable, slowly labile or labile. This approach has been applied to the speciation results obtained for the environmental samples and the outcome is presented in Table 4. The interpreted speciation results presented in Table 4 reveal that inorganic complexes of Cd are only significant in industrial leachate 1 and 2. In both cases the major ligand is chloride originating from sea water intrusion into the waste disposal site. Very few other inorganic ligands are believed to be of environmental significance at acid to neutral pH values. Table 4 also shows that organic complexes are dominating in the compost leachate, the polluted groundwater and the industrial leachate 3. The characteristics of the organic complexes are, however, different and no simple correlation between the concentration of dissolved organic carbon and the significanoe of organic complexes exists (refer to Table 3 with respect to DOC of the solutions). The organic complexes seem to be fairly labile, or may easily redistribute and lose the complexed Cd upon changes in the solution composition. A substantial fraction of stable Cd complexes was only identified in the polluted groundwater. In this context, a stable complex is defined as a complex that does not lose its Cd within 48 h exposure to an excessive amount of a strong cation exchange resin. In an environmental context, this does not indicate that this fraction of the dissolved Cd is irreversibly bound to the organic compound: the fraction only indicates that the complex is slowly reacting upon changes in the solution composition, and that it may facilitate the transport of Cd in terrestrial environments. 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