Journal of Environmental Quality 31:573-580 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Heavy Metals in the Environment
Dissolution Kinetics of Heavy Metals in Dutch Carbonate- and Sulfide-Rich Freshwater Sediments
S. E. J. Buykx*,a,
M. A. G. T. van den Hoopa and
J. P. G. Lochb
a National Institute of Public Health and the Environment, P.O. Box 1, 3720 BA Bilthoven, the Netherlands
b Department of Geochemistry, Institute of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, the Netherlands
* Corresponding author (susan.buykx{at}rivm.nl)
Received for publication March 27, 2001.
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ABSTRACT
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In two sulfide-rich freshwater sediments from the Biesbosch and Kromme Rijn River in the Netherlands differing in carbonate content and acid volatile sulfide (AVS) content, metal and sulfide dissolution kinetics were studied at different acid concentrations by varying both the procedure of acid addition and the extraction time. The establishment of equilibrium was monitored by measuring the pH in time, which reached a near constant value. The equilibrium pH was reached quickly when large amounts of acid were added and slowly when small amounts of acid were added. This observation was confirmed by the yield of extracted metals after either a 45-min or 24-h extraction over a pH range from 0 to 5. The pH factor seemed to be of more influence than time for the dissolution of metals. The amount of extracted metals was highly dependent on the metal itself due to its physico–chemical behavior. Although the sediments studied varied in carbonate content, acid volatile sulfide (AVS), and total metal content, the extracted fraction of metals compared with their total content in the sediment was similar for most metals. Finally, the AVS content as well as the ratio of simultaneously extracted metals (SEM; sum of Cd, Cu, Ni, Pb, and Zn) to AVS decreased with increasing pH. Because the SEM to AVS ratio may be used to set environmental quality criteria for the sediment compartment, this observation is of significance.
Abbreviations: AVS, acid volatile sulfide • AVSs, "standard" acid volatile sulfide • SEM, simultaneously extracted metals
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INTRODUCTION
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SINCE THE MID-1990s, the AVS concept has been introduced in a number of risk assessment studies of anaerobic, heavy metal–polluted freshwater sediments (Hansen et al., 1996; Liber et al., 1996; Nipper et al., 1998). The AVS concept relies on the geochemical process of heavy metals being precipitated by an excess of sulfide as the result of their very low solubility products (Smith and Martell, 1976). This reactive sulfide pool, AVS, consists mainly of amorphous FeS, mackinawite (FeS), and greigite (Fe3S4) (Huerta-Diaz et al., 1993). Under anaerobic conditions the mobility of the metals is thus strongly reduced and toxic effects due to the presence of the metals are negligible (Di Toro et al., 1990, 1992). Although the principles of the AVS concept are based on well-recognized geochemical processes, the applied analytical determination is method-defined. Acid volatile sulfide is determined by the measurement of collected gaseous hydrogen sulfide, 45 min after the addition of hydrochloric acid (to approximately 1 mol L-1) to an oxygen-free sediment suspension. The concentration of extracted metals is determined in the supernatant of the resulting suspension. The ratio of the sum of Cd, Cu, Ni, Pb, and Zn (simultaneously extracted metals, SEM) and the AVS content or the difference between the two is then interpreted as a measure of the potential toxicity of the sediment.
The main idea of this procedure is to obtain information on the speciation of heavy metals under natural conditions relevant to long-term effects for the environment. Direct methods are quite complicated mainly due to the often very low metal concentrations in the pore water even at high total metal contents in the sediment. It is well known that total metal concentrations alone do not provide the necessary information on toxicity. Part of the toxicological effect may be explained from the dissolved fraction, depending on type of organism, environment system, and possible uptake routes (Campbell, 1995).
Changes in pH of natural systems (e.g., due to acid rain) may result in changes of the speciation of heavy metals. In the literature, quite a number of papers deal with the more fundamental aspects of dissolution and its kinetics of pure minerals like goethite (Nowack and Sigg, 1997; Kraemer and Hering, 1997), 
-Al2O3 (Kraemer and Hering, 1997), gibbsite (Lee and Fein, 2000), and hematite (Samson and Eggleston, 1998). In general, dissolution of metals from these minerals depends strongly on the pH of the system, ionic strength, presence of ligands, and temperature. Data for natural systems are very scarce, especially for a system of sulfide-rich freshwater sediment.
The aim of the present study was to study the variation in physico–chemical behavior of heavy metals and the dissolution of AVS in these systems under different analytical conditions. This was done primarily to obtain more insight in changes in metal speciation due to changing environmental conditions. In addition, by using the commonly used analytical method for the determination of AVS and SEM, the method itself was evaluated as well.
The application of the AVS concept is based on a method-defined analytical procedure, which uses more extreme chemical conditions (pH 0). In this study we want to obtain insight in the results of the experiment under less extreme (more natural) conditions in relation to the commonly used method at pH 0 and a 45-min extraction time.
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MATERIALS AND METHODS
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Sediment Characterization
Freshwater sediments were collected with a scoop from the top 20 cm layer of sediment in the Biesbosch and the Kromme Rijn River at Odijk in the Netherlands in 1995. Experiments were performed in 1995 and 1996. The samples were stored at anaerobic conditions at 4°C in the dark and were intensively mixed for 30 min on a shaking device before use.
The sediment was analyzed for dry weight content by drying the sediment for 24 h at 105°C. Carbonate content was determined by volumetric measurement of CO2 gas after addition of hydrochloric acid (Loeppert and Suarez, 1996) to dry, ground sediment.
Total carbon was analyzed with a CNS analyzer (Fisons Instruments [Milan, Italy] EA 1108) in the dry, ground sediment. The organic carbon content was calculated from the total carbon content and the carbonate (inorganic carbon) content.
The "total" metal contents (Ca, Al, Fe, Mn, Cd, Cu, Ni, Pb, and Zn) were determined in the dry, ground sediment by means of inductively coupled plasma–atomic emission spectrometry (PerkinElmer [Wellesley, MA] Optima 3000 XL) after digestion with aqua regia. Standard solutions were used for the calibration and internal standards (Ga, In) for quality control. Two reference sediments (PACS-1 and CRM-280, obtained from NRC Canada and BCR, respectively) were used to check the digestion method. The measured metal contents in the reference material were in good agreement with those certified. Detection limits were 0.002, 0.07, 0.38, 0.05, 1.48, 0.17, 0.18, 0.62, and 0.60 µmol g-1 dry wt. for Cd, Ni, Pb, Cu, Zn, Fe, Mn, Al, and Ca, respectively.
The AVS concentration in the sediment was determined according to van den Hoop et al. (1997). In short, the extraction procedure was as follows: approximately 4 g of mixed wet sediment was added to 100 mL oxygen-free Milli-Q (Millipore, Bedford, MA). An amount of 10 mL of 12 mol L-1 HCl was added to the resulting sediment suspension. The suspension was stirred constantly and flushed with nitrogen for a period of 45 min. The sulfide in the sediment was transformed into H2S, which was transported by the nitrogen flow to a bottle containing a 0.5 mol L-1 NaOH solution. The collected sulfide formed a blue-colored complex after reaction of a subsample with N,N-diethyl-1,4-phenylene-diammonium-sulfate (DPD) and potassium dichromate. The extinction of this solution was determined spectrophotometrically (in triplicate) at 670 nm (Pharmacia Biochrom [Cambridge, UK] 4060 spectrophotometer). The experimental setup was checked for leakage by determining the recovery of a Na2S solution, of which the concentration was determined by titration with thiosulfate. Recovery was 89% on average; if the recovery was lower than 80% the setup was checked for leakage and the recovery experiment was repeated. Detection limit was 0.1 µmol AVS g-1 dry sediment.
All measurements were performed at least on duplicate samples. The AVS content determined with this standard method at pH 0 and a 45-min extraction time will be referred to as AVSs ("standard" AVS), in order to distinguish between the AVS content determined with different acid concentrations or a 24-h extraction time.
Influence of Acidity on Acid Volatile Sulfide and Extracted Metal Concentrations
The influence of the acidity of the sample solution on extracted metal concentrations was studied in two different ways varying mainly in extraction time (45 min and 24 h, respectively), and in the applied experimental setup.
In the first experiment with a 45-min extraction time, the setup for the determination of AVSs (as described above) was used with a constant volume of added HCl of 10 mL but with varying hydrochloric acid concentrations (between 12 and 0.084 mol L-1). The sediment suspension was filtered immediately at the end of the 45-min extraction procedure, first through a folded paper filter (Schleicher & Schuell [Dassel, Germany] 595 1/2) and then through a 0.45-µm syringe filter (Millipore Millex HA). The pH was measured in the filtrate. In this filtered extract, the metals Cd, Ni, Pb, and Cu were analyzed on a PerkinElmer 2100 atomic absorption spectrometer. Detection limits were 0.003, 0.013, 0.034, and 0.007 µmol g-1 dry wt. for Cd, Ni, Pb, and Cu, respectively. Zinc, Fe, Mn, and Ca were analyzed on a Spectro Analytical Instruments (Kleve, Germany) Spectroflame M5 inductively coupled plasma–atomic emission spectrometer. Samples were diluted 1 to 50 times with 0.1 mol L-1 HCl. Detection limits were 0.004, 0.008, 0.002, and 0.125 µmol g-1 dry wt. for Zn, Fe, Mn, and Ca, respectively. Acid volatile sulfide was determined as described above.
In the second experiment a much longer extraction time of 24 h was applied in order to attain a pH closer to equilibrium. Hydrochloric acid was added to a sediment suspension at constant temperature and the pH was measured during 24 h. The setup consisted of a double-walled reaction vessel containing 50 mL Milli-Q water. Here, 2 g of wet sediment were added in order to obtain the same sediment to water ratio of 1:25 as in the 45-min experiment. The resulting sediment suspension was flushed continuously with nitrogen to prevent introduction of oxygen. The closed reaction vessel was kept at a constant temperature of 25 ± 0.1°C by a thermostat bath. Temperature and pH were measured with Radiometer (Copenhagen, Denmark) electrodes (glass/calomel, calibrated with buffers pH 4 and 7) connected to a pH–temperature meter (Radiometer PHM 95) connected to a computer for data storage. Two different methods were used for the addition of acid:
Method 1: 5 mL of a HCl solution with a concentration between 0.12 and 1.2 mol L-1 were added once to the sediment suspension and temperature and pH were measured every minute during 24 h.
Method 2: every 10 min during 1000 min a 0.05-mL aliquot of a HCl solution with a concentration between 0.12 and 1.2 mol L-1 was added to the sediment suspension, resulting in a total volume of added HCl of 5 mL. Temperature and pH were measured every minute during 24 h.
After 24 h the sediment suspension was centrifuged at 3000 x g (Du Pont Instruments [Wilmington, DE] Sorvall RC-5B centrifuge) and the supernatant filtered through a 0.45-µm filter (Millipore Millex HA). The filtered extract was analyzed for pH, Ca, Al, Fe, Mn, Cd, Cu, Pb, Ni, and Zn by inductively coupled plasma–atomic emission spectrometry (PerkinElmer Optima 3000 XL). Detection limits were equal to the ones reported above.
Chemicals
All reagents used were of analytical-reagent grade or better. All solutions were prepared using demineralized water, produced by a Milli-Q Reagent Grade system (Millipore). Oxygen was eliminated from the Milli-Q water by purging with nitrogen gas.
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RESULTS AND DISCUSSION
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Sediment Characterization
The sediments studied contain high concentrations of trace metals (see Table 1). According to Dutch environmental quality objectives, both sediments can be classified as heavily contaminated, for Kromme Rijn especially because of high concentrations of Cu, Ni, and Zn and for Biesbosch due to high concentrations of Cd and Zn (VROM, 1999). Acid volatile sulfide concentrations obtained as described in the experimental section are quite high compared with the values usually obtained in risk assessment studies (Di Toro et al., 1990, 1992; Ankley et al., 1991; Allen et al., 1993). In fact, the AVSs content for both sediments is in excess of the total concentration of the heavy metals Cd, Cu, Pb, Ni, and Zn. Without considering other binding phases like organic matter or CaCO3, this means that these metals could be completely bound to the available amount of sulfide. In addition, the freshwater sediments studied are rich in carbonate content, resulting in high pH values. Biesbosch sediment has a higher inorganic carbon (carbonate) content than Kromme Rijn sediment, but lower organic carbon content.
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Table 1. Dry weight, carbonate content, pH, "standard" acid volatile sulfide (AVSs), and total metal content of the sediments studied. Relative standard deviation of the multiple measurements is <5%, representing sample reproducibility and analytical reproducibility.
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Towards Equilibrium pH Values
In the experiment where metal concentrations were determined after a 24-h extraction time, two different methods were used for establishment of an equilibrium pH. In Fig. 1 and 2
results of the pH measurements with these two methods at different added acid concentrations are given.
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Fig. 1. pH vs. extraction time at different added HCl concentrations, after a one-time addition of HCl. Method 1: (a) Kromme Rijn, (b) Biesbosch. Dotted lines are duplicates.
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Fig. 2. pH vs. extraction time at different added HCl concentrations, with intermittent HCl additions during the first 1000 min. Method 2: (a) Kromme Rijn, (b) Biesbosch. Dotted lines are duplicates. For 0.60 mol L-1 the acid was added in 110 and 250 min, respectively, instead of in 1000 min.
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Figures 1a and 1b show the results of Method 1, in which all the HCl was added at the beginning of the experiment and pH was monitored during 24 h for Kromme Rijn and Biesbosch sediments, respectively. This procedure results in an initial large drop in pH after the addition of acid from a pH of about 7.7 (not shown in graph) to a pH of 1.7 to 4.0, depending on the added acid concentration. After this initial drop in pH, the pH increased again and reached approximately constant values after 0.5 to 20 h, assuming that pH is constant within 0.1 pH unit of the final pH. For the usually accepted determination of AVSs the pH will have reached equilibrium after 45 min, as the final pH in the extract is about zero and equilibrium time decreases with decreasing final pH (e.g., for the Kromme Rijn sediment, equilibration time was 0.7 h at final pH 2). The duplicate results show that the reproducibility of the experiment is quite good, as there is only a small difference in final pH values (except for 0.6 mol L-1 in Biesbosch sediment).
In Figures 2a and 2b the results are given for Method 2, in which HCl was added in 0.05-mL aliquots, every 10 min during 1000 min (for most cases), and pH was monitored during 24 h, for Kromme Rijn and Biesbosch sediments, respectively. For Method 2, pH decreased with every acid addition and increased again until the next addition. Overall, a decrease in pH occurred until the addition of acid was stopped. For acid additions with HCl concentrations of 0.48 mol L-1 and 0.60 mol L-1, pH stayed constant after 1000 min, whereas for the other three acid additions pH increased again until the pH measurement was stopped after 24 h and the pH was more or less constant.
Although higher acid concentrations were added to Biesbosch sediment than to Kromme Rijn sediment, higher final pH values were obtained due to the higher buffering capacity of the Biesbosch sediment, which has a larger carbonate content than Kromme Rijn (8.7 and 4.4%, respectively, see Table 1).
Influence of Acidity and Extraction Time on Extracted Metal Concentrations
For the 24-h extraction experiment, the final pH for Method 2 was for most cases only slightly higher than for Method 1 at comparable total added acid equivalents (Fig. 1 and 2; Table 2). Moreover, metal concentrations for the two methods were quite similar at similar pH, indicating that for the present system the method of acid addition does not result in significantly different concentrations of extracted metals. Making use of this observation, concentrations of metals from the two methods were combined and plotted against pH in the filtrate, together with results from the 45-min extraction experiment. An overview of added HCl concentrations and resulting final pH values after 45 min or 24 h (after filtration) are given in Table 2. The plots of metal concentrations vs. pH are shown in Fig. 3
for the Kromme Rijn sediment, from which the following patterns can be observed:
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Fig. 3. Extracted metal concentrations vs. final pH, Kromme Rijn sediment ( = 24-h extraction, x = 45-min extraction, dry wt. = dry weight)
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(i) For all metals studied, the amount of extracted metal depends highly on the final pH of the system. In general, the amount of extracted metal increases with decreasing values of pH. The observed function of extracted metal versus pH differs for different metals, although for some metals (e.g., Fe and Mn), similar patterns are observed. This is most likely due to similarity in their physico–chemical behavior in the sediment.
(ii) Increasing extraction time results in higher concentrations of extracted metals, especially at higher pH values. At these pH values the system is far from equilibrium after 45 min (see Fig. 1), and consequently metals are not completely extracted. For Ca and Mn, for example, a significant difference in "yield" is observed between the two extraction times at pH values between 4 and 5. However, for pH values below approximately 3, the extracted amount is equal to the total metal content and independent of the presently applied time scale. For the trace metals, the difference in yield occurs over the whole studied pH range, up to the pH where in both experiments concentrations are below the detection limit.
(iii) The variation in the concentration of extracted metals at different pH values is larger than the variation due to a different extraction time. From this observation pH seems to be the dominating factor in the process of dissolution of metals over the time factor.
(iv) The order in which metals are released with decreasing pH values (determined by the pH values at which concentrations become detectable) is Ca 
Mn > Fe > Ni > Zn > Cd > Al > Pb > Cu. For metal sulfide forming metals (Mn, Fe, Ni, Zn, Cd, Pb, and Cu) this order is in good agreement with their decreasing solubility product (Smith and Martell, 1976) (except for Ni and Zn).
(v) The amount of SEM (sum of Cd, Cu, Ni, Pb, and Zn) is predominantly determined by the extracted amount of Zn, which is present in excess of the other heavy metals in the sediment. This is usual for Dutch sediments (Van den Hoop et al., 1997) and is mainly related to their natural background values (Van den Hoop, 1995) despite local elevated levels due to anthropogenic pollution.
(vi) The ratio of extracted metal at pH = 0 and their total content (see Table 1) varies for the different metals: Al 0.08, Ca 1.0, Fe 0.7, Mn 0.9, Zn 0.9, Ni 0.2, Pb 0.9, Cd 0.6, and Cu 0.2. The "low" values for Al and Ni result from their ubiquitous presence in clay minerals, which are not completely digested by the extraction procedure. Calcium and Mn are mainly present as carbonates, which will be completely dissolved at very low pH values resulting in ratios of about 1. The speciation of Fe and Cu is more complex. Iron is known to be present in various amorphous minerals (Morse et al., 1987) that are dissolved at high acidity, whereas part of Fe is built-in in clay minerals and less reactive crystalline sulfides that cannot be liberated with the applied experimental procedure. Although Cu is known not to be present in large quantities in solid phases, it can form very strong complexes with organic material from which it is not released at low pH values. Zinc and Pb are mainly present as sulfides (Buykx et al., 2000), which will dissolve at low pH, resulting in a large ratio of extracted metal to total metal.
Comparison of Sediments
In Fig. 4
the results of extracted metal concentrations in the two sediments are compared for the 45-min experiment, by plotting normalized values against pH. The extracted metal concentrations are expressed as percentages of the total metal concentrations in the two sediments. The results of the 45-min extraction experiment were chosen for the comparison, as the data set of the 24-h experiment is very small for Biesbosch sediment.
The fraction of extracted Al is the same for both sediments and shows a more or less linear decrease with increasing pH. For Fe, however, the extracted fraction is clearly smaller for Biesbosch sediment than for Kromme Rijn sediment, especially at low pH values. Although the carbonate content of Biesbosch sediment is twice as large as for Kromme Rijn, the excess of added acid is so large (especially at pH = 0) that this is unlikely the cause for the difference in extracted metal. The difference might be caused by the type of binding of Fe in the two sediments. Iron in Biesbosch is probably bound as more crystalline sulfides like pyrite, which are less reactive than amorphous sulfides. Calcium, Mn, and the trace metals seem to be extracted in more or less the same quantities in both sediments. Cadmium and Cu in Kromme Rijn sediment seem to be slightly less extracted than in Biesbosch sediment, but not enough data are available for a firm conclusion. As the organic carbon content in Kromme Rijn sediment is higher than in Biesbosch sediment (see Table 1), Cd and Cu might be more strongly bound to organic matter in the former compared with the latter.
Influence of Acidity on Acid Volatile Sulfide Concentrations and Simultaneously Extracted Metals to Acid Volatile Sulfide Ratios
Finally, it is useful to consider the influence of acidity on the extracted amount of AVS (Fig. 5)
and the resulting SEM to AVS ratios (Fig. 6)
, which are usually obtained at pH 0. The results shown in Fig. 5 and 6 are for the 45-min experiment, enabling comparison of the data with the results of the metals shown in the previous paragraph. The dissolution of sulfide precipitates appears to be strongly dependent on the acidity of the sediment. The observed functionality is quite similar to that obtained for the metals (i.e., increasing "yields" at decreasing pH values). For Kromme Rijn the rate of change of AVS yield with pH is higher than for Biesbosch sediment, which is probably due to the larger AVS content of Kromme Rijn sediment. The pH at which no more AVS can be detected (approximately 4.5) is more or less the same for both sediments. As AVS concentrations in Kromme Rijn sediment are larger, but SEM concentrations in both sediments are similar (see Table 1 and Fig. 4), the SEM to AVS ratio is smaller for Kromme Rijn sediment than for Biesbosch sediment. Although both extracted metals and AVS decrease with increasing pH, the resulting SEM to AVS ratio is not constant with changing pH but also decreases with increasing pH.
Implications for the Applicability of the Simultaneously Extracted Metals to Acid Volatile Sulfide Ratio Concept in Risk Assessment Studies
The SEM to AVS ratio method is used in risk assessment studies. The observation that SEM to AVS ratios decrease with increasing pH is of significant practical importance, since the SEM to AVS ratio may be used for the setting of environmental quality criteria for the sediment compartment. The determination at pH 0 with a 45-min extraction time is more or less accepted as the standard method and gives the highest yield in AVS and SEM concentrations and SEM to AVS ratios.
If there are considerations to determine the speciation under less extreme conditions (i.e., more natural conditions), our study shows that this determination is strongly pH- and time-dependent. In order to be able to compare AVS and SEM results from different labs it is important to use a standardized method, as the measured concentrations are method-defined.
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CONCLUSIONS
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With respect to the experimental aspects of this study it can be concluded that the change in pH over time depends on the method of acid addition (added all at once or incrementally) and the amount of acid added. In addition, the "final" pH after 24 h is slightly higher when acid is added drop by drop than when the same amount of acid is added all at once. Both of these observations indicate that the kinetics of pH buffering (i.e., the kinetics of carbonate dissolution) depend on the application method of the acid. In spite of these differences, metal concentrations extracted with the two methods are quite similar at similar pH values.
For both sediments studied the total contents of Al, Fe, Ni, Cd, and Cu are not completely extracted with 1 mol L-1 HCl (pH 0) for either 45 min or 24 h of extraction time, whereas Ca, Mn, Zn, and Pb are. This behavior indicates a difference in speciation of these metals. The extracted metal concentrations decrease with increasing pH. Increasing extraction time results in higher concentrations of extracted metals, especially at higher pH values at which the metals are not completely extracted. The pH seems to be more important than time for the dissolution of metals. The order in which metals are released with decreasing pH values is Ca Mn > Fe > Ni > Zn > Cd > Al > Pb > Cu. For metals that form sulfides (Mn, Fe, Ni, Zn, Cd, Pb, and Cu) this order is in general agreement with their decreasing solubility product. Although the two sediments studied are different in AVSs and carbonate content, the fraction of extracted metal compared with total metal content versus pH is similar for most of the studied metals except Fe, which is probably caused by a difference in Fe speciation between the two sediments. Both measured AVS content and SEM to AVS ratio decrease with increasing pH. As the SEM to AVS ratio may be used for setting environmental quality criteria for the sediment compartment, this observation is of great practical importance.
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ACKNOWLEDGMENTS
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The authors thank Judith Meekes for performing part of the experiments.
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REFERENCES
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ABSTRACT
INTRODUCTION
= Kromme Rijn, 
= Biesbosch)
