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a The Wyler Department of Dryland Agriculture, The Jacob Blaustein Institute for Desert Research, Ben-Gurion University, Sede Boqer Campus 84990, Israel
b Institute of Soils, Water and Environmental Sciences, The Volcani Center, ARO, P.O.B. 6, Bet Dagan 50250, Israel
c Institute of Soils, Water and Environmental Sciences, The Volcani Center, ARO, P.O.B. 6, Bet Dagan 50250, Israel
* Corresponding author (rayav{at}bgumail.bgu.ac.il)
Received for publication January 12, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CC, complexation capacity • DDW, double-distilled water • DOC, dissolved organic carbon • MW, molecular weight
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Transition metals in the aqueous phase of soils or biosolids can exist as free (hydrated) ions or complexed with organic or inorganic ligands. For example, Hodgson et al. (1965) reported that from 10 to 60% of the Zn in a soil solution was found in complexed form. The higher the organic matter content (e.g., A horizons as compared with B horizons of soils) the higher was the percentage of Zn found in complexed form. Specific metal and environmental conditions such as pH, ionic strength, and potential ligand concentrations interact to determine the distribution of the metal among its various species. As a rule, the apparent stability constants of metal complexes decrease as the ionic strength increases (Martell and Smith, 1976, 1977; MacCarthy and Perdue, 1991). The stability of the complexes is also strongly affected by solution pH (e.g., Sauvé et al., 1997) due to reaction of the ligands with protons, and of the metal cations with hydroxyl groups. Metal solubility usually increases as the pH decreases, with the notable exception of metals present in the form of oxyanions or amphoteric species. Since soil solution properties might change with time (for example following sludge application), the solubility and speciation of metals might also be time-dependent (Dudley et al., 1986; Shuman, 1988; Mo et al., 1999).
Stability constants of transition metal complexes have been determined for natural and artificial solutions (Olomu et al., 1973; Mantoura et al., 1978; Fujii et al., 1982; Sanders, 1983; Stevenson and Fitch, 1986; Stevenson and Chen, 1991). This was often done with insufficient attention to environmental conditions under which the constants were determined. As a rule, Cu, Fe, Ni, and Pb were found to have a strong tendency to form complexes, whereas Cd complexes were weaker. Zinc, Co, and Mn displayed a moderate tendency to form complexes, and their reported stability constants displayed a particular sensitivity to environmental conditions.
Natural aqueous systems often contain unidentified ligands or ligands of undetermined concentrations. It is, therefore, useful to define a complexation capacity, namely the maximum load of transition metals with which potential ligands in the aqueous phase can form complexes. This parameter is determined experimentally as the asymptotical limit of complexed metal concentration, in the presence of excess metal cations. As mentioned above, this value is affected by the solution's properties and, in particular, its pH. MacCarthy and Perdue (1991) suggested that the complexation capacity should be defined at pH = 7 with low ionic strength, using a metal that displays a high tendency to form complexes (e.g., Cu).
The main objectives of the present study were to (i) characterize the speciation of Cu and Zn in the soluble fraction of sewage sludge or of soil–sludge mixtures according to charge, molecular weight (MW), and stoichiometry; (ii) follow the changes in metal speciation with time after sludge application; and (iii) define the complexation capacity of water extracts from sludges or soil–sludge mixtures.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The distribution of metal species according to their charge can be derived from analysis of the solutions after equilibration with the resins using Eq. [1a–c]:
 | [1a] |
 | [1b] |
 | [1c] |
Analysis of Cu, Zn, Ca, Mg, Na, K, and DOC was performed on all solutions that had been equilibrated with the ion exchangers. The amount of ion exchanger used per unit volume of solution was chosen so as to maintain the ion exchange capacity of the added resin at least 10 times higher than the measured concentration of total ions in the solution. Reliability of the procedure was ascertained by checking the extent of removal of Ca, Mg, Na, and K, assuming that virtually all these elements' species were cationic due to their low tendency to form complexes. The extent of complexation of Ca will be discussed later.
Molecular Weight and Complexation Capacity Determinations
Dissolved Cu and DOC species in the extracts were characterized according to their molecular weight (MW) by gel chromatography. Glass columns having a total volume of 62 mL (1.2-cm i.d.) were filled with gel (Sephadex G-25-80; Sigma), giving a void volume for the packed column of 23 mL (37%). The gel was saturated with Cu before packing, to minimize adsorption of Cu from the eluting sample. Water extracts of the sludge or the soil–sludge mixtures were also fortified with CuCl2 before passing through the gel column, in an attempt to bring the extracts to their full complexation capacity. Copper concentrations, however, were kept below those at which precipitation would be observed. Accordingly, 40 mg L-1 of Cu was added. Samples of 2 mL volume were placed on top of the gel column and eluted with a 5 mg L-1 Cu (as CuCl2) solution in DDW. The leachate was collected in 4.0-mL fractions and analyzed for Cu, DOC, Ca, Mg and, in selected samples, pH.
Complexation capacity (CC) is defined as the maximum load of transition metals with which potential ligands in the aqueous phase can form complexes. As a rule, DOC is assumed to be the dominant source of ligands, and thus the complexation capacity is attributed to the DOC. Apparent complexation capacity of the sludge extract was calculated from the respective concentrations of DOC and complexed Cu. Complexed Cu content was derived using the gel chromatography results, for various dilutions of the sludge extract (the original 2:25 extract and the extract diluted four- and eightfold). The contribution of mineral ligands to the complexation of Cu was estimated from the measured concentrations of inorganic anions (Table 2) coupled with GEOCHEM computations. Thus, it was possible to estimate complexation capacity of the DOC in sludge extracts brought to infinite dilution.
Model-Based Computations
In order to estimate the potential contribution of inorganic ligands to the complexation of Cu, computations using the PC-GEOCHEM model (Parker et al., 1995) were employed. Concentrations of ions included in the calculations are given in Table 2. The Cu(II) concentrations between 5 and 90 mg L-1 were tested and the pH was fixed at 5.5 (a value similar to that of the eluent in the gel chromatography runs). All computations were performed using the measured DOC concentrations, and also in the absence of DOC. The DOC was treated in the computations using the fulvate model of Sposito et al. (1982a).
Incubation Experiments
Sand–sludge solid mixtures were incubated for 15 wk at 25 ± 1°C. Sludge was added to the sand at rates of 1, 6, and 20%, on a dry weight basis. Sand and sludge controls were also incubated under the same conditions. Constant substrate water content (Table 1) was maintained by adding distilled water according to weight loss. Samples were collected and analyzed after 5, 25, 50, and 100 d of incubation. Soluble Cu and Zn were investigated in a paste extract (1:1 soil to water [w/w] ratio) of the 1 and 6% mixtures as well as for the sand control, and in a 2:25 solid to water ratio extract of the pure sludge. The 20% sludge mixture was extracted at both ratios of solid to water. All extracts were passed through a 0.22-µm filter after centrifugation and then underwent ion exchange and gel chromatography determinations as described above for the extracts of the fresh (prior to incubation) samples.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Other investigators have also found that most of the soluble Cu in sludge extracts exists in complexed forms (Hodgson et al., 1965; Fujii et al., 1982; Sanders, 1983; Sauvé et al., 1995). Mingelgrin and Biggar (1986) determined the charge of Cu species in water extracts of sludge using paper electrophoresis. Almost all of the Cu in the saturated extract of sludge was found to be complexed as positive, negative, or neutral complexes. Published data on Zn speciation in sludge extracts is equivocal. Fujii et al. (1982) found that 64% of the Zn in a water extract of a sludge-amended soil was complexed (Cd remaining mostly in cationic form). Lagerwerff et al. (1976) used ionic exchangers for determination of the charge of Cu and Zn species in the leachate of columns packed with sludge. More than 70% of the Cu but only 12% of the Zn in the first fraction of leachate were found to be in complexed form. While the complexed Zn existed almost exclusively as zwitter ion (or amphoteric) species, the complexed Cu was present as amphoteric, negative, and neutral species. In subsequent leachate fractions, nearly all Zn displayed a positive charge, whereas the distribution of Cu species according to charge was similar to that in the first leachate fraction collected. There are overall similarities in distribution according to charge of the species of Cu or Zn in aqueous extracts of sludges investigated by various groups. Furthermore, the different origins of the sludges (and hence their composition), their history before extraction, and dissimilar extraction procedures all contribute to observed differences.
Molecular Weight of Copper Species
Experimentally, it is convenient to estimate the molecular weight (MW) and separate molecules from one another on the basis of their Stocks effective radius, which is correlated to the molecular weight. This was obtained by the classical gel chromatography (GC) method (Kabzinski, 2000), based on the correlation between pore size of a medium and Kav, which is defined as the inverse logarithmic function of the MW:
 | [2a] |
 | [2b] |
Here, Ve is the volume of leachate drained out of the column at any given time, and V*t is the volume necessary to leach a reference species of a known molecular weight through the column. Vt 
V*t > V0, where V0 is the volume of large, interparticle pores (void volume) and Vt is the total pore volume of the column. The parameters a and b were calculated by assigning Kav = 1 to the MW of hydrated CuCl2 (241 Da), while Kav = 0 corresponds to MW = 5000. Mass balance calculations for DOC demonstrated that the MW values of all organic species present in the solution were within the range separable by the gel used in the present study.
A gel chromatography run of the 2:25 sludge extract enriched with 40 mg L-1 Cu is presented in Fig. 1 . Copper and DOC concentrations as a function of Kav are given in Fig. 1A, and Fig. 1B is a plot of log MW versus Kav. Almost all of the Cu in the soil and sludge solutions was associated with fractions having MW values lower than 1000 Da. Furthermore, most of the Cu species had MW values below 600 Da, but above that of free Cu+2 (Fig. 2A) . The DOC was present in two major peaks: one centered around 3000, and the other around 220 Da. The abundance of low molecular weight DOC is in agreement with the findings of Sachdev et al. (1976), who found that species of MW below 700 Da constituted about 60% of the organic matter in secondary effluents. Partial overlap between the low molecular weight DOC peak and that of the Cu (Fig. 1A) strongly suggests extensive complexation between Cu and low MW DOC species. Alternatively, the fact that Cu was bound exclusively to low molecular weight species is also compatible with a significant contribution of inorganic ligands. GEOCHEM model calculations (see below) suggested that, for the Cu-enriched 2:25 sludge extract, approximately 35% of the soluble Cu was complexed with organic ligands, 25% with inorganic ligands and 40% remained free. The dominant inorganic ligand was sulfate.
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Complexation Capacity of Potential Ligands in Sludge Extracts
The MW distributions of Cu and DOC in leachates of gel columns through which the Cu-fortified sludge extract or its dilutions had been passed are presented in Fig. 2. The chromatogram of a 50 mg L-1 CuCl2 solution in DDW is also given, for comparison.
The apparent complexation capacity (CC) of a solution (not corrected for mineral ligands) was estimated from the integral of the Cu concentration over the entire chromatogram, minus the contribution from free Cu. The peak of the free Cu cation was centered at Kav = 1 and is assumed to be approximately an equilateral triangle. This integral could be divided into the total DOC content of the sample that had been run through the column, to give the CC value. Calculated CC values of the sludge extract and its dilutions are presented in Table 4.
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As dilution increased, a higher fraction of the ligands present in the extract was bound to Cu and "true" complexation capacity was approached. As stated above, the concentration of Cu in samples applied to the columns was kept constant, while the concentrations of the other cations and of potential ligands (in particular DOC) decreased due to dilution. The higher ratio of Cu to the concentration of potential ligands should bring about an increase in the fraction of binding sites that are bound to Cu. The competition of other cations with Cu should also be considerably reduced as dilution increases. Indeed, in the leachates of gel columns through which extract diluted to 1:50 sludge to water ratio was passed, the amount of complexed Ca was only 10% that of the complexed Cu. When extract diluted to a 1:100 sludge to water ratio was passed, the amount of complexed Ca in the leachate was negligible.
Toward the end of the gel chromatography runs of the sludge extract and of its dilution to a 1:50 sludge to water ratio, the leachates contained far less than the 5 mg L-1 Cu maintained in the incoming eluent (Fig. 2A). This suggests that Cu was removed from the solution as eluent passed through the column. The gel with which the columns were packed had been presaturated with Cu. Therefore, in order to have Cu taken up from the eluent by the gel, some of the initially sorbed Cu had to be removed. Copper was indeed removed from the gel surface and released into solution by exchange with major cations, and in particular Ca. Mass balance calculations indicated that the amount of Cu released into the leachate from the gel surface was equivalent to the amount of the Ca that had originated in the extract sample and then been taken up from the passing solution. The leaching solution that passed through the column, after all components of the extract sample had been leached out, resaturated the gel with Cu at the expense of the Cu content of the eluent. Accordingly, a very broad, low peak of Ca (around 3 mg L-1) appeared in gel chromatograms of the 2:25 extract, after almost four pore volumes of eluent had been passed through the column. Magnitude of this Ca peak decreased as dilution of the extract increased. Exchange between Ca originating in the sludge extract and Cu sorbed to the Cu-saturated gel decreased the concentration of Ca in the column effluent, while increasing the Cu content of the effluent above that of the extract sample applied to the column (Fig. 1A, 2A). Interference by Ca in the measurement of the CC thus was further reduced.
With the most highly diluted sludge extract (1:100 sludge to water ratio), the Cu concentration in the leachate at the end of a run (Fig. 2A) approached the concentration of the incoming eluent (5 mg L-1). This suggests near saturation with respect to Cu of the gel and probably of all complexing species in the Cu-enriched diluted-extract samples. This was expected, due to the decrease in content of all cations except Cu as dilution of the extract increased.
There is at least one more reason (other than reduced competition between Cu and major cations and an increased ratio of Cu to DOC and other potential ligands concentration) why the measured CC approaches the true complexation capacity as dilution of the extract increases. At lower ionic strengths, coiled soluble humic substances are expected to uncoil, and associations of small DOC molecules that may exist at sufficiently high ionic strengths and DOC concentrations (Piccolo and Conte, 2000) are expected to disperse. Sites exposed due to swelling, uncoiling, or dissociation thus become more accessible to Cu (e.g., Mingelgrin and Gerstl, 1993) and increase the measured CC. That the measured CC is inversely related to the ionic strength also has been reported, for example, by MacCarthy and Perdue (1991).
The CC values measured for the sludge extract and its two dilutions were 6.42, 14.62, and 18.36 mM Cu g-1 DOC, respectively (Table 4). A Mitscherlich saturation curve (Peaslee, 1978) was fitted to those data and the maximal complexation capacity (CCmax) was calculated:
 | [3] |
The CC or CCmax values, as defined above, do not take into account the significant contribution of non-DOC species (e.g., mineral anions) to the capacity of the solution to complex cations. Using the GEOCHEM model, it was shown that Cu complexed on DOC constituted 45 to 65% of the total complexed metal for all dilutions of the sludge extract and for all Cu concentrations between 6 and 90 mg L-1. Corrected complexation capacity values, which define the complexation capacity of the DOC, were determined with the aid of the above GEOCHEM calculations (Table 4). The corrected values extrapolated to infinite dilution (Eq. [3]) yielded a maximal complexation capacity for the DOC of 8.75 mM Cu g-1 DOC.
As the dilution of the extract increased, more of the potential binding sites on the DOC became occupied with Cu (Table 4) and, thus, average MW of the DOC should have increased as well. For example, DOC + Cu replaced DOC + H; alternatively, Cu became attached to uncharged sites on the DOC without replacing any moiety. Accordingly, the peak in the gel chromatogram for low MW DOC (the DOC fraction with which Cu was complexed almost exclusively) shifted to a lower Kav value (i.e., a higher MW) as dilution increased (Fig. 2B).
Compared with the cation exchange capacity (CEC) of soil organic matter (1.5–3.5 mM Cu g-1 OC), the complexation capacity determined in the present study for DOC seems high. However, transition metals, particularly Cu, have the capability to bind to sites that are not negatively charged. Hence, uncharged functional groups on the DOC (e.g., weakly acidic groups) may serve as ligands. Also, it is expected that dissolved organic matter will have a higher complexation capacity than the CEC of the soil's stationary organic fraction. This is because of the larger surface area (and hence the larger number of binding sites) exposed per unit weight, along with the higher negative charge density expected in the soluble fraction of organic matter (e.g., Stevenson, 1982).
Complexation capacity of the DOC in water systems such as rivers, lakes, sewage water, or soil and sludge extracts has been measured or calculated by several researchers, using a variety of procedures. The values reported are in the range 0.03 to 8.4 mM Cu g-1 DOC (e.g., Neubecker and Allen, 1983; Abbt-Braun et al., 1989; Pardo et al., 1994; Gardner et al., 2000). The reported complexation capacity values cover several orders of magnitude, probably because of different procedures used, the metal used to saturate potential binding sites, environmental conditions, and origin of the aqueous phase. It is, however, difficult to find in the published literature a significant correlation between origin of the DOC and reported complexation capacity. The complexation capacity of the DOC at infinite dilution as obtained in the present study is at the upper limit of the reported values.
The Complexation Model
The broad peak of complexed Cu observed in the gel chromatograms (Fig. 1A and 2A) is the product of a composite of complexes, with partly organic and partly mineral ligands. The range of MW values for the Cu complexes suggests that organic ligands complexed with the metal ion could contain between 4 and 30 carbon atoms, assuming one Cu ion per complex. This assumption will be challenged below. If more than one Cu ion is bound in a complex, the ligands would be even smaller. At any rate, the DOC fraction appearing in the peak that centers at Kav 
0.2 (Fig. 2B) and encompasses all DOC of MW > 1000 Da, does not contribute to any significant extent to the complexation of Cu.
In the Cu-fortified sludge extract, Cu concentration was in excess of the concentration of DOC and other potential ligands, and hence a considerable fraction of the Cu was in free-ion form. Estimates based on GEOCHEM calculations indicate that between a third and half of the total Cu in solution remained uncomplexed, depending on dilution of the sludge extract. On the other hand, at lower Cu concentrations (<1 mg L-1), which are more prevalent in the environment, a considerably higher fraction of the Cu in the sludge extract is likely to be complexed. Results summarized in Table 3 demonstrate that practically all of the Cu in the extracts was complexed when the extracts were not fortified with Cu.
The MW at the maximum of the Cu concentration peak in the gel chromatogram of the undiluted sludge extract (Fig. 2A) can be taken to represent the MW of the "most probable" Cu–DOC complex. Subtracting the MW of Cu hydrated with five water molecules from the MW at the peak Cu concentration (154 and 400, respectively) yielded a possible complex of one Cu atom attached to 12 C atoms (assuming the weight ratio of a whole organic molecule to its C content to be 1.7; e.g., Russell, 1956). However, the amount of DOC in the effluent fraction in which the maximum of the Cu peak is located was not sufficient to give a 1:12 Cu to C ratio, even after taking into account that only approximately half the Cu present is complexed with DOC.
The DOC and Cu concentrations in that fraction (after subtracting the amount of Cu complexed with mineral ligands, as computed using the GEOCHEM model), yielded a 3:1 C to Cu molar ratio. In order to achieve this ratio, while maintaining the mean MW of the solutes in the leachate fraction with the highest concentration of Cu (approximately 400 Da), the "most probable" complex would contain 1.9 Cu ions and 5.6 C atoms. In fact, Cu is complexed by a variety of DOC species, having MW values in the approximate range 250 to 750 Da. The important conclusion derived from the calculated "most probable" complex is that the characteristic Cu complex is not composed of a central ion surrounded by ligands but is, more likely, a short DOC string along which cations are attached to functional groups (e.g., carboxyl or amino groups). The number of carbon atoms in the ligands of the "most probable" Cu complex matches their number in small carboxylic (including di- or tri-carboxylic) or amino acids. These acids may be bound to Cu in polydentate fashion (e.g., Tsvetkov and Mingelgrin, 1987) at low Cu concentrations, or attached to two (or more) Cu ions at sufficiently high concentrations of the metal ion. For example, citric acid, a small polycarboxylic acid to which two Cu ions can be attached, would fall within the expected MW range of the Cu–DOC species. Sposito et al. (1982a) mimicked the complexation of metals by DOC by assuming the DOC to be a mixture of nine acids, including amino acids, aliphatic polycarboxylic acids, and aromatic acids with MWs in the range 116 to 192 Da.
The above analysis of the "most probable" Cu complex is applicable to conditions of excess Cu. It is, therefore, a description of the behaviour of the Cu–DOC system when it is near the capacity of DOC to bind Cu. At lower Cu concentrations (as is the rule rather than the exception in natural systems, or in soils loaded with sludge or effluent), it is expected that there will be sufficient Cu to interact only with the most strongly complexing DOC species, such as polycarboxylic acids acting as polydentate ligands. Such an interaction should bring about a high percentage of metal complexed and a dominance of anionic complexes, as observed for the sludge extract that was not fortified with Cu (Table 3).
Changes in Transition Metal Species with Time—Incubation Experiments
Addition of sludge to the sand dramatically increased the soluble metal and DOC concentrations in the aqueous extracts (Table 5). These concentrations decreased, however, by 50% or more during the 100 d of incubation for the soil–sludge mixtures. Incubation of pure sludge resulted in a lesser decline in the concentrations of Cu, while soluble Zn concentration did not decline at all.
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2500 Da at peak DOC concentration) decreased dramatically during the first month of incubation, while content of the low MW DOC fraction (MW 600 Da or lower) decreased to a much lesser extent. The MW of the soluble Cu species (Fig. 4)
was in the range 250 to 800 Da, which suggests that Cu in solution tended to bind to low MW ligands. This tendency persisted throughout the entire period of incubation, and behavior of water-soluble Cu and DOC in the extracts of pure sludge and of 6% sludge–soil mixtures displayed a similar dependence on the time of incubation.
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Although some general observations on the effect of incubation on metal species could be made, the results summarized in Fig. 3 through 6 and Table 5 suggest that changes in the properties of the DOC and associated Cu and Zn species with time are not easily predictable. Non-monotonous changes with time were observed for a number of parameters. This observation is not surprising, considering the complexity of the chemical and biological processes that occur in the sludge and in soils loaded with sludge. It does, however, highlight the strong influence of specific conditions and of the exact nature of the sludge and soil on the effects of incubation on the speciation of trace elements and hence on their behavior.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Copper tends to exist, in the extracts of sludge or soils loaded with sludge examined in these studies, as negatively charged species. Such species dominated the extracts througout the entire 100 d of incubation. Zinc, on the other hand, tended to form zwitter ion species. As the incubation period progressed, however, the relative content of positively charged Zn species tended to increase.
The complexation capacity of DOC in the sludge extract, extrapolated to infinite dilution, was 8.75 mM Cu g-1 DOC, while the complexation capacity of the DOC measured in undiluted, Cu-fortified sludge extract (2:25 sludge to water ratio) was 3.66 mM Cu g-1 DOC. It is thus obvious that, in the concentrated extract, DOC was not saturated with respect to its capacity to take up Cu. Such an increase in complexation capacity with dilution could be due to a number of reasons. These include the uncoiling of DOC associations upon dilution (exposing otherwise inaccessible binding sites), decreased competition of Cu with other cations (since fortification with Cu kept its concentration constant while concentrations of all other species decreased with dilution), and a concomitant increase in the Cu to DOC ratio.
A "most probable" Cu–DOC complex was defined for the Cu-fortified undiluted sludge extract. It was found to consist of 1.9 Cu atoms attached to DOC species containing 5.6 C atoms. This outcome suggests that Cu complexes in the aqueous extract of the sludge are composed predominantly of two Cu ions (less frequently one ion), attached to DOC species containing few (most likely five or six) C atoms.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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-amino acids on montmorillonite–Cu–1–Lysine complexes in high-pressure liquid chromatography. Clays Clay Miner. 35:391–395.[Abstract]This article has been cited by other articles:
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  K. A. Merritt and M. S. Erich Influence of Organic Matter Decomposition on Soluble Carbon and Its Copper-Binding Capacity J. Environ. Qual., November 1, 2003; 32(6): 2122 - 2131. [Abstract] [Full Text] [PDF]
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B. P. Jackson, P. M. Bertsch, M. L. Cabrera, J. J. Camberato, J. C. Seaman, and C. W. Wood Trace Element Speciation in Poultry Litter J. Environ. Qual., March 1, 2003; 32(2): 535 - 540. [Abstract] [Full Text] [PDF]
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 P. Fine, A. Hass, R. Prost, and N. Atzmon Organic Carbon Leaching from Effluent Irrigated Lysimeters as Affected by Residence Time Soil Sci. Soc. Am. J., September 1, 2002; 66(5): 1531 - 1539. [Abstract] [Full Text] [PDF]
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