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Department of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO 80523-1170
* Corresponding author (dheil{at}lamar.colostate.edu)
Received for publication October 1, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DOC, dissolved organic carbon • DPASV, differential pulse anodic stripping voltammetry • EC, electrical conductivity
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Several studies, employing measurements of soil pore water collected by either laboratory column leaching experiments or field studies using lysimeters (solution samplers) or drainage tiles, have found that metal concentrations are elevated below the zone of incorporation of biosolids (Sidle and Kardos, 1977; Welch and Lund, 1987; Lamy et al., 1993; McBride et al., 1999; Richards et al., 2000). Soil amendment with municipal solid waste has also been observed to cause elevated metal concentrations in effluents leached through the subsoil (Boyle and Fuller, 1987; Sawhney et al., 1994).
Many researchers have attributed increased metal mobility in amended soils to complexes with DOC released from the biosolids (Gerritse et al., 1982; Christensen, 1985; Lamy et al., 1993; Brown et al., 1997; McBride et al., 1997). The contribution of DOC to metal transport is expected to be magnified as pH increases, although overall mobility is greater at low pH. This is due to the increased solubility (or mobility) of DOC and the very low solubility of metal ions in neutral to alkaline pH soils (Richards et al., 2000). The transport of metals through soil or aquifer materials can be enhanced by DOC derived from sources other than biosolids (Dunnivant et al., 1992; Wang and Benoit, 1996). However, relatively few studies on metal transport in soils amended with biosolids have measured changes in DOC concentrations after the addition of biosolids or provided evidence for a metal–DOC association.
A significant proportion of the water-soluble metals extracted from biosolids and biosolids-amended soils have been found to be associated with organic complexes. Dudley et al. (1987) used gel filtration chromatography to determine the extent of organic complexation of soluble metals in extracts of soils mixed with biosolids. From 80 to 100% of the total water-soluble Cu, 48 to 100% of Zn, and 39 to 100% of Ni was found to be organically complexed. Further evidence of the association between metals and soluble organic matter in amended soils was revealed by the correlation between water-soluble Cd, Cu, and Zn concentrations with absorbance at 650 nm, which is related to DOC concentration (McBride et al., 1997).
Leaching studies indicate that the DOC–metal complexes released from soils amended with biosolids can be transported to a significant depth in the soil. Lamy et al. (1993) observed that both DOC and Cd concentrations collected in drainage tiles located at a depth of 80 cm below the soil surface increased by a factor of approximately three times compared with control samples one month after the addition of biosolids, and then both declined to background levels within one year. Concentrations of Cu, Zn, and Ni were increased by a magnitude of 10- to 20-fold in soil solutions sampled by passive wick lysimeters placed 60 cm below the soil surface (40 cm below the depth of biosolids incorporation) 15 years after amendment with biosolids (McBride et al., 1999). In that same study, differential pulse anodic stripping voltammetry (DPASV) was used to determine that only 30% of water-soluble Zn, 18% of Cd, and 10% of Cu was present as ionic or inorganic complexes, and the remainder of the water-soluble metals was considered to be complexed with DOC.
The definition of DOC is operational and includes all of the dissolved organic materials that pass through a 0.45-µm filter (Buffle et al., 1992). These organic materials include a heterogeneous mixture of both organic colloids and true organic solutes. The humic components of DOC (humic acid and fulvic acid) possess colloidal properties (Stevenson, 1994). Consequently, the mobility of organic colloids present in DOC is expected to depend upon pH, electrolyte concentration, and electrolyte composition (divalent versus monovalent ions) (Tombacz and Meleg, 1990). Temminghoff et al. (1997) demonstrated that the mobility of dissolved humic acid through a soil column was substantially greater after the influent solution was changed from 0.001 M Ca (NO3)2 to demineralized water. This result is consistent with the expected response of colloid stability to a decrease in electrolyte concentration (Sposito, 1984). The colloidal stability of humic acid, because of its greater molecular weight, is more sensitive to changes in solution chemistry than is fulvic acid (Stevenson, 1994). In addition to the complexation of metals by DOC, other possible mechanisms of metal mobilization in biosolids-amended soils include the displacement of exchangeable cations or ion-pair formation resulting from the potentially high soluble salt content of biosolids, and acidification of soil pH (Gerritse et al., 1982; Christensen, 1985; Boyle and Fuller, 1987). The release of exchangeable metals and formation of ion pairs should be amplified in the presence of a high electrolyte concentration, whereas increased electrolyte concentration should minimize metal transport caused by the colloid mobilization mechanism.
The objective of the current study is to determine the contributions of DOC and electrolyte concentration to metal transport in two soils of neutral to alkaline pH, which have a history of amendment with biosolids. We also evaluate the effect of time after biosolids amendment on metal mobility.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Two types of water were used in the leaching experiments. Distilled water was used to represent the relatively low-salinity water from rainfall or snowmelt in the dryland site. A synthetic solution was prepared to simulate the chemical composition of irrigation water sampled at the field location (Table 3) .
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We initially leached the irrigated soil for the first five cycles with the simulated irrigation water. In order to study the effects of leaching with a less saline water on the mobilization of DOC and metals, we then leached the irrigated soil columns with five cycles of distilled water. The order of application of the two types of water to the dryland soil was the reverse, with distilled water applied during the first five cycles and then the simulated irrigation water applied for Cycles 6 through 10. This was done in order to simulate leaching with rainfall and snowmelt water at the beginning of the experiment followed by a change to a saline irrigation water at the end.
After collection, the effluent solutions were stored at 4°C before and after filtration with 0.45-µm Nuclepore polycarbonate filter membranes (Whatman, Clifton, NJ). For the analysis of total metal concentrations only, subsamples of each filtrate were preserved with 0.2% ultrapure HNO3.
Analytical Methods
We measured the pH of the effluent with a glass electrode and an Orion (Beverly, MA) Model 720 pH meter. Electrical conductivity (EC) was measured in the effluent with an Orion Model 108 conductivity meter. We used a Shimadzu (Kyoto, Japan) TOC-500 carbon analyzer to measure DOC in the effluent samples. We used a PerkinElmer (Wellesley, MA) 4100 ZL graphite furnace atomic absorption spectrometer with Zeeman background correction to measure the total dissolved metal concentrations at µg L-1 levels. The samples were analyzed by graphite furnace atomic absorption spectroscopy (GFAAS) with EDL lamps (PerkinElmer, Norwalk, CT) for Pb and Zn and a hollow cathode lamp for Cu with detection limits of 0.5 µg L-1 for Pb and 2.0 µg L-1 for Cu and Zn. These metals were chosen because they were present in greater concentrations than other trace metals in the biosolids.
Differential pulse anodic stripping voltammetry (DPASV) with a thin mercury film–rotating glassy carbon disk (TMF–RGCD) was used to estimate the partitioning of metals between free ions or inorganic complexes versus organically complexed metals in the sub–µg L-1 concentration range (Wang and Benoit, 1996; Bruland, 1989; Rozan et al., 1999). The DPASV system was a Bio-Analytical Systems (West Lafayette, IN) CV-50 W voltametric analyzer connected to an RDE-1 rotating disk electrode. The voltammetry cell consisted of a 30-mL glass cell, a rotating glassy carbon disk (RGCD) electrode where a mercury film was deposited, an Ag–AgCl reference electrode, and a platinum wire counter electrode.
Procedures for analyzing the samples with DPASV were adopted from Bruland (1989). Before the analysis all glassware was soaked in 2% pure ultrapure HNO3 for 24 h, followed by rinsing with ultrapure deionized water (UPDI). The thin mercury film was deposited by adding 5 mg L-1 Hg as Hg(NO3)2 to 30 mL of 0.01 M KCl prepared with high purity KCl (Suprapur; EM Science, Gibbstown, NJ) and UPDI. All solutions were purged and then blanketed with grade 4.8 N2 gas. The Hg film was deposited on the glassy carbon electrode at a deposition potential of -1100 mV and a working electrode rotation rate of 5000 rpm. The potential was then scanned from -1100 mV to -50 mV (sample width, 17 ms; scan rate, 20 mV s-1; pulse amplitude, 50 mV; pulse period, 200 ms; sensitivity; 1 x 10-6 A/V). The deposition and scanning steps were repeated three times, to purify the system of metals, and to ensure that the system was reproducible and ready for calibration.
The calibration slopes for Zn, Pb, and Cu were obtained with fresh 30-mL 0.01 M KCl solution, also with a 5-min deposition time. Successive standard additions in increments of 0.2 µg L-1 of two standards (Cu–Pb and Zn) were used to overcome the interference between Cu and Zn (Florence, 1986). Addition normally stopped at about 3.5 µg L-1 of Zn, Pb, and Cu. Detection limits were 0.1 µg L-1 for Cu, 0.1 µg L-1 for Pb, and 0.2 µg L-1 for Zn. Each of the three metals was measured separately by setting the deposition potential at a value that was 200 mV more negative than the measured peak potential of that element in order to avoid the detection of organically complexed metals (Florence, 1986). This corresponded to deposition potentials of -1300 to -900 mV for Zn, -650 to -350 mV for Pb, and -400 to -20 mV for Cu. Analysis of the filtered effluent samples was completed by using the same parameters for deposition and stripping as for calibration. We tested this procedure for the speciation of Cu, Pb, and Zn in the presence of the chelate ethylenediaminetetraacetic acid (EDTA) as described by Bruland (1989). Rozan et al. (1999) have verified the accuracy of DPASV for the speciation of Cu in the presence of fulvic acid at concentrations up to 25 mg L-1 as DOC. We obtained similar results for Cu with DPASV and a Cu ion–selective electrode for saturation extracts of our soils recently treated with biosolids.
Data Analysis
All experiments were set up in a split-plot design where water is considered a block in a split-plot design, with biosolids application treatment as the main effect and the cycles as subplots. Experiments were carried out in triplicate. Analytical data included in the statistical analysis were DOC, pH, EC, and total metal concentrations. For the analysis, data were subjected to split-plot analysis with SAS for Windows Version 8.01 (SAS Institute, 1999). The Pearson correlation coefficient was used to study the significance of correlation at the 0.1 probability level. Correlation analysis was first performed over all 10 leaching cycles within each soil and biosolids application treatment (Recent, Previous, and Control). Correlation analysis was then performed for the separate water types used in Cycles 1 through 5 and 6 through 10 both within (by analyzing data from the biosolids application treatments separately) and across (by analyzing data from the biosolids application treatments combined) biosolids application treatments. Correlation analysis across sludge applications for all 10 cycles was not used due to the number of observations (90) that were involved.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The peak DOC concentration for both the recent-addition and previous-addition treatments was found in the second cycle and then decreased through the 10th cycle (Fig. 1). The DOC concentration was significantly greater for the recently amended soil as compared with the previously amended soil and also for the previously amended soil as compared with the control for both Cycles 1 through 5 and Cycles 6 through 10 (Table 4).
The peak EC in the biosolids-amended treatments was observed in the second cycle, indicating that the waterfront from the amended layer (0–10 cm) did not reach the bottom of the columns until the second cycle (Fig. 1). The EC readings decreased as leaching progressed, due to the removal of inherent salts by distilled water, and then increased after switching to irrigation water. We observed significant differences in EC between the two biosolids-amended soils and the control soil (Table 4). Both EC and Zn reached minimum values near the sixth cycle for all three treatments. However, both Cu and Pb reached maximum values during the sixth cycle for the two biosolids-amended soils when EC was at a minimum. Although Cu increased dramatically in the two biosolids-amended soils after switching from distilled to simulated irrigation water beginning with the sixth cycle, the fact that EC did not begin to increase until the seventh or eighth cycles suggests that the Cu release in Cycle 6 may reflect the later stages of leaching with distilled water. The dispersion of Cu-bearing colloids, either organic or mineral, may be responsible for the elevated Cu concentration in Cycle 6. The pH was significantly decreased for the recent-addition treatment compared with the other two treatments for all 10 cycles (Table 4).
The increased concentration of both Zn and Cu and also DOC in the recently amended versus previously amended soils is consistent with previous results, which indicated that leachate concentrations of DOC (Giusquiani et al., 1992), metals (Welch and Lund, 1987; Sawhney et al., 1994), or both DOC and metals (Dudley et al., 1986; Lamy et al., 1993) are the greatest immediately after biosolids addition, and both decrease as leaching progresses.
From analyzing effluent from the third cycle and the eighth cycle, the DPASV signals were very small and near or below the detection limits, which were 0.1 µg L-1 for Cu, 0.1 µg L-1 for Pb, and 0.2 µg L-1 for Zn. Therefore, the results showed that more than 99% of Cu and Zn and at least 90% of Pb complexed with either DOC or mineral colloids, and were not present as free metal ions or inorganic complexes, which are DPASV-labile. Temminghoff et al. (1997) found that when the pH was greater than 6.6, the complexation of Cu with DOC comprised more than 99% of the total dissolved element.
The results of the DPASV analysis are consistent with the relationship between Cu and DOC, observed in Fig. 1. The fact that soluble Zn increased with EC suggests that exchangeable Zn may have been released. However, the results of DPASV analysis indicate that the Zn is apparently complexed by DOC or colloids in solution once displaced from exchange sites. The accuracy of the DPASV method that we used has not been validated for speciation of Zn or Pb for extracts of soils recently amended with biosolids, which contain synthetic chemicals in addition to high concentrations of DOC. Therefore, the speciation results for Zn and Pb must be considered tentative and interpreted with caution.
Irrigated Soil
Effluent Cu concentrations were significantly greater for the recent-addition treatments than the previous-addition or the control treatments for all of the cycles after the first, and also for the previous-addition treatment versus the control for both Cycles 1 through 5 and 6 through 10 (Fig. 2
, Table 5) . Zinc was greatest in the fourth cycle for both the previously amended and recently amended soils (Fig. 2). Similar to the dryland soil, effluent Zn concentration was the greatest for the recent-addition treatment, followed by the control and the previous-addition treatment for Cycles 1 through 5 (Table 5). Lead readings were low and highly variable and no differences between treatments were evident (data not shown).
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Electrical conductivity was the highest in the second or third cycle for all three treatments, and after that decreased through Cycle 10 (Fig. 2). As with the dryland soil experiments, peak Zn concentrations were observed near a maximum in EC. Effluent Zn concentration reached peak concentrations during the third or fourth cycle, and then decreased during Cycles 6 through 10 in parallel with EC.
Both the previous and recent amendments with biosolids significantly decreased effluent pH compared with the control (Table 5). This difference in pH could be responsible for the greater Cu concentration in the previous-addition treatment versus the control, even though DOC was very similar for these two treatments.
As for the dryland soil, DPASV measurements indicated that >99% of soluble Cu and Zn were complexed, based on analyses of effluent collected during the third and eighth cycles. This very low concentration of DPASV-detectable Cu and Zn indicates that most of these two metals are present as organic complexes.
Correlation Analysis
Dryland Soil
Correlation analysis within each biosolids application treatment over all 10 cycles for the dryland soil revealed a significant positive relationship between Zn and EC for the control and recent-addition treatments (Table 6)
. This relationship is consistent if a cation exchange mechanism is controlling Zn solubility. Boyle and Fuller (1987) found that the total soluble salt concentration contributed more than DOC concentration in increasing the transport of Zn through soils amended with municipal solid waste. They attributed their results to competition between Zn and soluble salts for nonspecific exchange sites. For Cycles 1 through 5 and 6 through 10 the overall results for each group of cycles showed a negative correlation between Zn and pH; these findings are in agreement with Dutta et al. (1989).
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Effluent concentrations of Pb were negatively correlated with EC within the recent-addition and the previous-addition treatments over all 10 leaching cycles. A cation exchange mechanism should cause a positive relationship between EC and soluble metals, as we found for Zn. This negative correlation between Pb and EC may be the result of the release of Pb-bearing organic colloids under low EC. Similar to Cu, Pb showed a significant positive correlation with DOC when data for all three treatments were combined. The greater dependence of Cu and Pb versus Zn mobilities on DOC is consistent with the greater binding constants of Cu and Pb versus Zn for humic and fulvic acids (Stevenson, 1994).
Irrigated Soil
Similar to the dryland soil, effluent Zn concentrations were positively correlated with EC over all 10 cycles for the control, recent-addition, and previous-addition treatments (Table 7)
. Zinc was also positively correlated with DOC for the recently amended soil over all 10 cycles and across all three biosolids application treatments for Cycles 1 through 5.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Although the measured concentrations of all three metals were below 1 mg L-1 for all effluents collected, the metal mobility was significantly greater in the amended soils, and this could lead to significant redistribution of metals from the mixing layer to the subsoil after several years.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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