Journal of Environmental Quality 30:1231-1237 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Heavy Metals in the Environment
Assessment of Metal Availability in Smelter Soil Using Earthworms and Chemical Extractions
Jason M. Conder,
Roman P. Lanno* and
Nicholas T. Basta
Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078
* Corresponding author (rlanno{at}okstate.edu)
Received for publication July 10, 2000.
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ABSTRACT
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Chemical immobilization is a relatively inexpensive in situ remediation method that reduces soil contaminant solubility, but the ability of this remediation treatment to reduce heavy metal bioavailability and ecotoxicity to soil invertebrates has not been evaluated. Our objectives were to (i) assess the ability of chemical immobilization amendments (municipal sewage sludge biosolids and rock phosphate) to reduce metal bioavailability and toxicity in a toxic metal-contaminated smelter soil and (ii) evaluate soil extraction methods using Ca(NO3)2 solution or ion-exchange membranes coated with diethylenetriaminepentaacetic acid (DTPA) as surrogate measures of metal bioavailability and ecotoxicity. We treated a soil contaminated by Zn and Pb milling and smelting operations and an uncontaminated control soil with lime-stabilized municipal biosolids (LSB), rock phosphate (RP), or anaerobically digested municipal biosolids (SS) and evaluated lethality of the remediated soils to earthworm (Eisenia fetida Savigny). Lime-stabilized municipal biosolids was the only remediation amendment to successfully immobilize lethal levels of Zn in the smelter soil (14-d cumulative mortality 
15%). Calcium nitrate–extractable Zn in the lethal Zn smelter soil–amendment combinations was 11.5 to 18.2 mmol/kg, compared with the nonlethal LSB amended soil (0.62 mmol/kg). The Ca(NO3)2–extractable Zn-based median lethal concentration (LC50) of 6.33 mmol/kg previously developed in Zn-spiked artificial soils was applicable in the remediated smelter soils despite a 14-fold difference in total Zn concentration. Chelating ion-exchange membrane uptake among the soils was highly variable (mean CV = 39%) compared with the Ca(NO3)2–extraction (mean CV = 1.9%) and not well related to earthworm toxicity.
Abbreviations: DTPA, diethylenetriaminepentaacetic acid • ILL, incipient lethal level (time-independent LC50) • LC50, median lethal concentration • LSB, lime-stabilized municipal sewage sludge biosolids • PRS, Plant Root Simulator ion-exchange membrane • RP, rock phosphate • SS, anaerobically digested municipal sewage sludge biosolids
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INTRODUCTION
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THE mining and smelting of lead (Pb) and zinc (Zn) ore often results in contamination of soil with cadmium (Cd), Pb, and Zn. Metal-contaminated soil often presents an unacceptable risk to human and ecological health and must be remediated (Adriano et al., 1997; Pierzynski, 1997). Commonly used cleanup methods involve excavation and landfilling of smelter-contaminated soil; however, more permanent and less expensive in situ solutions have been favored during the last decade (Iskandar and Adriano, 1997). Chemical immobilization is a relatively inexpensive in situ remediation method in which soil is treated with chemical amendments that reduce metal solubility. Many studies report the use of chemical amendments including organic matter (Pierzynski and Schwab, 1993), alkaline materials (Mench et al., 1994), and phosphates (Boisson et al., 1999; Ma et al., 1994, 1995) for the chemical remediation of metal-contaminated soil. A recent study comparing biosolids, alkaline cement kiln dust, and rock phosphate (carbonated apatite) found that many of these amendments reduced phytoavailability and extractability of heavy metals (Basta and Gradwohl, 2000; Gradwohl, 1998). However, the effect of these amendments on metal bioavailability and toxicity to soil invertebrates was not evaluated.
Toxicity testing using earthworms is a well-developed means of studying the bioavailability and acute toxicity of soil contaminants (Edwards and Bohlen, 1992) and contaminated field soils (Marinussen et al., 1997a). Total heavy metal concentrations may not be directly related to soil organism toxicity due to a number of modifying factors such as pH, organic matter content, and clay content (Beyer et al., 1987; Conder and Lanno, 2000; Hopkin, 1989; Lanno and McCarty, 1997; Ma, 1982; Marinussen et al., 1997b; McLean and Bledsoe, 1992; Morgan and Morgan, 1988; Spurgeon et al., 1997). Cleanup guidelines are often based on total heavy metal content of soil and do not consider what proportion of the total may be biologically available (bioavailable) to organisms, even though environmental risk is related to the bioavailability of heavy metals in soil. The bioavailability of metals cannot be measured directly using chemical analyses; only living organisms can actually determine bioavailability (Lanno and McCarty, 1997). A method of measuring metal availability not involving organisms (reducing expense and variability) that is well related to bioavailability would be an extremely useful screening tool for evaluating metal-contaminated soils.
Two surrogate methods of measuring metal bioavailability in soils that are quick to perform and relatively inexpensive include (i) weak-electrolyte soil extractions and (ii) ion-exchange membrane metal uptake. Extractions using weak (<1 M) CaCl2 or Ca(NO3)2 solutions have been used successfully as toxicity-related measures of metal availability in soils (Basta and Gradwohl, 2000; Conder and Lanno, 2000; Gradwohl, 1998; Marinussen et al., 1997b; Peijnenburg et al., 1997, 1999; Posthuma et al., 1997; Weljte, 1998). These solutions are hypothesized to extract exchangeable or weakly bound "available" metals in soil (Sloan et al., 1997), which are believed to be available for uptake by soil organisms (Peijnenburg et al., 1999; Posthuma et al., 1997). Conder and Lanno (2000) demonstrated that metal levels in weak Ca(NO3)2 extractions relate well to lethal Cd, Pb, and Zn toxicity in the earthworm E. fetida exposed to metal-spiked artificial soil. In contrast, ion-exchange membranes have a definite advantage over weak electrolyte extractions because they can be deployed in soils with a minimum of soil physicochemical alteration, even under in situ conditions (Liang and Schoenau, 1995, 1996; Qian and Schoenau, 1997). Cation-exchange membranes complex divalent metal ions such as Cd, Pb, and Zn directly from the soil solution, but suffer interference from Ca cations, which compete with heavy metals for membrane uptake (Liang, 1994). Anion-exchange membranes coated with a metal chelator, such as disodium-diethylenetriaminepentaacetic acid (DTPA), can chelate available metals while avoiding Ca saturation due to preferential binding of cationic transition metals (Evangelou, 1998; Liang and Schoenau, 1995). While Liang and Schoenau (1995) found a strong correlation between Cd, Cr, Pb, Ni, and Zn uptake in chelating membranes and uptake for lettuce (Lactuca sativa L.) in soil, Conder and Lanno (2000) found only a weak relationship between ion-exchange membrane uptake and toxicity in the earthworm E. fetida exposed to Cd, Pb, and Zn in spiked artificial soils.
The objectives of this research were to (i) assess the ability of chemical immobilization amendments (municipal sewage sludge biosolids and rock phosphate) to reduce metal bioavailability and ecotoxicity in a toxic, metal-contaminated smelter soil, and (ii) evaluate soil extraction methods using Ca(NO3)2 solution or ion-exchange membranes coated with DTPA as surrogate measures of metal bioavailability and ecotoxicity.
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MATERIALS AND METHODS
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Soil Toxicity Testing
The soil used in this experiment was collected from a site contaminated by Zn and Pb milling and smelting operations near the city of Blackwell in north-central Oklahoma. The soil contained 2.63 mmol Cd/kg, 2.39 mmol Pb/kg, and 195 mmol Zn/kg. Clay content and pH of the study soil were 240 g/kg and 6.4, respectively. Three chemical immobilization amendments were examined: a lime-stabilized municipal biosolid (LSB), North Carolina rock phosphate (RP), and an anaerobically digested municipal biosolid (SS). Physical and chemical characteristics of the amendments are previously described in Gradwohl (1998). Each amendment was thoroughly incorporated into soil (100 g/kg soil) in plastic tubs. The amount of amendment applied was determined from a preliminary study where amendments were added at 10, 30, 100, and 300 g/kg to the Blackwell contaminated soil (Basta et al., 2001). The lowest application that resulted in lettuce growth for some treatments was 100 g/kg. All soil amendments were performed in triplicate. Soil moisture was adjusted to field capacity (0.33 bar, ca. 250 g water/kg) and the soils were incubated at 27°C for 90 d. Soil moisture was maintained and the soils were thoroughly mixed at weekly intervals. An uncontaminated Taloka soil (fine, mixed, thermic Mollic Albaqualt) was used as a reference soil. The Taloka reference soil received the same three chemical immobilization amendments as the metal-contaminated smelter soil.
A 14-d toxicity test using the earthworm E. fetida was conducted according to a standard protocol (American Society for Testing and Materials, 1997), with noted exceptions, to assess the ability of chemical immobilization amendments to reduce metal bioavailability. For each soil–amendment combination, three 100-g soil replicates were moistened to approximately 80% water holding capacity 24 h prior to the addition of five earthworms per replicate. Mature (clitellate) earthworms weighing approximately 0.2 to 0.4 g were obtained from in-house cultures and allowed to depurate culture bedding from their gastrointestinal tracts for 24 h before exposure to the soil–amendment combinations. Testing was conducted in environmental chambers maintained at 20 ± 1°C under constant light. Earthworm mortality was monitored on a geometric time scale (e.g., 2, 4, 8, 16, 24, 36, 48 h) for the first 48 h (when most mortality occurred), then daily for the remainder of the test. Earthworms were judged to be dead upon observing no movement after gentle stimulation with a blunt probe.
Metal Extraction Using Ion-Exchange Membranes
Ion-exchange membranes were simultaneously exposed to separate soil–amendment combination replicates under the same conditions as in the earthworm toxicity test. Plant Root Simulator (PRS; Western Ag Innovations, Saskatoon, SK, Canada) anion-exchange membranes (polystyrene cross-linked with divinylbenzene) were the ion-exchange membranes used. Plant Root Simulators were converted before use to chelating ion-exchange membranes by complexation of the anionic membrane surface with disodium-DTPA (Liang and Schoenau, 1996). Individual PRSs were then buried in 100 g (dry weight) soil replicates, and moistened as described above for soil–amendment combinations for earthworm exposure. During burial, soils were gently packed around each PRS membrane to ensure that 100% of the membrane's surface was in direct contact with moist soil. After a 1-h exposure period, PRSs were removed from soil, rinsed thoroughly with deionized water to remove soil, and eluted for 1 h with 20 mL of 0.5 M trace metal-grade HNO3 in separate, self-sealing plastic bags. The eluent was analyzed for Cd, Pb, and/or Zn using flame and/or graphite furnace atomic absorption spectroscopy, and the PRSs were regenerated for reuse (Liang, 1994). Two PRS replicates per soil were exposed alongside the earthworm toxicity tests in the same environmental chamber on the first day of the earthworm exposure periods. After regeneration, the PRSs were exposed again on the seventh day. Quality assurance–quality control (QA–QC) procedures included PRS blanks and spikes, conducted with water-rinsed PRSs during the acid elution step.
Soil Analyses
Upon termination of the toxicity tests and PRS exposures, individual soil–amendment combination replicates were stored at -40°C in self-sealing plastic bags until physical and chemical parameters could be measured. Soil pH for all earthworm and PRS soil replicates was measured in the supernatant of a settled 0.01 M CaCl2 soil slurry (10 g dry weight soil/20 mL solution) according to Hendershot et al. (1993). Organic matter content was measured by wet digestion with chromic acid (Yeomans and Bremner, 1988) followed by colorimetric determination (Heanes, 1984). Total metal concentrations of soils were obtained by wet digestion of 1.0 g (dry weight) soil using 5 mL concentrated trace metal-grade HNO3. The digests were then heated to dryness, resolubilized in 15 mL 0.5 M trace metal-grade HNO3, filtered, and brought to a 50-mL volume with 0.5 M trace metal-grade HNO3. To measure weak electrolyte extractable metals, 1.0 g (dry weight) of each soil replicate from the earthworm tests was combined with 20 mL 0.1 M Ca(NO3)2, mixed in a rotary mixer for 4 h at 23°C, and centrifuged at 2500 x g for 15 min. The resulting supernatant was then filtered with a 0.45-µm membrane filter and acidified with 0.5 mL concentrated trace metal-grade HNO3. Total metal digests and Ca(NO3)2 extractions were analyzed by atomic absorption spectroscopy. Quality assurance–quality control measures included duplicate analyses, metal spikes, blanks, and analyses of standard reference soil Sandy Soil B (CRM-SA-B, Environmental Express, Mt. Pleasant, SC). Total metal analyses of the standard reference soil were within performance acceptance limits determined by USEPA 3050 digestion procedures (USEPA, 1995).
Data Analysis
All data were checked for homogeneity of variance and normality and transformed as appropriate to meet requirements for ANOVA (Sokal and Rohlf, 1995). Soils were classified as nonlethal if the 14-d cumulative mortality rate was 15%. Toxicity of the Blackwell smelter soil–amendment combinations was assessed by comparing arcsine square root-transformed percent cumulative mortality (Newman, 1995) at each mortality observation. Plant Root Simulator data were analyzed according to a split-plot (repeated measures) test design (Steel et al., 1997). Data not satisfying assumptions for analysis of variance (ANOVA) were analyzed nonparametrically using the Kruskal–Wallis test. Fisher's protected least significant difference (LSD) multiple comparison procedure was also employed to further elucidate differences between means (
= 0.05).
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RESULTS AND DISCUSSION
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Remediation Effectiveness
Blackwell smelter soil was contaminated predominantly with Zn, although Cd and Pb levels were elevated compared with Taloka soil (Table 1). The addition of remediation amendments did not change total Cd, Pb, or Zn levels in Blackwell soil; the remediation goal was not to remove metals from the soil, but immobilize available metal species via changes in soil physicochemical properties. The metals present in the remediation amendments (Gradwohl, 1998) elevated total Cd, Pb, and/or Zn in Taloka soil. The remediation amendments significantly increased organic matter (SS and LSB) and pH (LSB) (Table 1).
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Table 1. Total and Ca(NO3)2-extractable metal concentrations and pH of soil–remediation amendment combinations (mean, n = 3, 95% CI). Within each soil type, means in columns with the same superscript were not significantly different (P > 0.05, Fisher's protected LSD).
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Municipal sewage sludge biosolids amended with lime (LSB) significantly reduced the toxicity of the Blackwell smelter soil to the earthworms, with all Blackwell smelter soil amendment–combinations lethal to the earthworm E. fetida except for LSB (Fig. 1). Applying Fisher's protected LSD tests to transformed mean cumulative mortality at 16, 24, 48, and 72 h, we concluded that the SS amendment was the most toxic Blackwell soil–amendment combination, followed by None and RP. The relative toxicity of the soil–amendment combinations to earthworms was very similar to that of lettuce (Gradwohl, 1998). However, earthworms appeared to be more sensitive as lettuce was able to survive in the Blackwell RP soil–amendment combination. We cannot explain why the Blackwell SS soil–amendment combination was more toxic than the unamended soil, although Edwards and Bohlen (1996) suggest that fresh anaerobic sewage sludge may have additional components aside from heavy metals that are toxic to earthworms. Soil pH appeared to be more important than organic matter in immobilizing bioavailable metals, since LSB (higher pH, lower organic matter) reduced toxicity while SS (lower pH, higher organic matter) seemed to increase toxicity. Sequential extraction results suggest increased pH in soil treated with LSB increased adsorption and/or precipitation of Cd, Pb, and Zn and decreased their availability (Basta et al., 2001). Since Fang and Wong (1999) found that the addition of lime to sewage sludge biosolids reduced DTPA- and water-extractable metals, it follows that LSB-amended soils would also have reduced metal availability compared with SS-amended soils (Basta and Gradwohl, 2000; Sloan and Basta, 1995). The RP amendment did reduce toxicity slightly, but not as effectively as LSB. The main component of RP is carbonated fluorapatite, a mineral that is very effective in immobilizing available Pb (Ma et al., 1993), but not Cd or Zn (Basta et al., 2001). Although RP did not reduce the lethality of the Blackwell soil to earthworms, it was least toxic among the lethal soil–amendment combinations (Fig. 1) and may be useful in reducing long-term bioavailability of metals in some contaminated field soils, especially where Pb is of concern. There was no lethality associated with the three remediation amendments themselves since no mortality rates >15% were observed in the Taloka soil–amendment combinations.
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Fig. 1. Mean percent cumulative mortality (n = 3) of the earthworm Eisenia fetida exposed to Blackwell smelter soil receiving remediation amendments. Data points with the same letter at the same mortality observation time are not significantly different (Fisher's protected LSD, P > 0.05).
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Calcium Nitrate Extractions as Surrogate Measures of Metal Bioavailability
Metal availability as determined by the Ca(NO3)2 extraction was significantly different for Cd, Pb, and Zn among the Blackwell soil–amendment combinations and depicted large differences between the lethal Blackwell soil–amendment combinations and the nonlethal Blackwell LSB and Taloka combinations (Table 1). Zinc had the highest availability of all three metals, on a molar basis, with Ca(NO3)2–extractable concentrations more than 10-fold greater than Cd and Pb in the lethal Blackwell soil–amendment combinations. The 0.1 M Ca(NO3)2 fraction is the most available metal fraction in contaminated soil and has been shown to be well related to earthworm toxicity in metal-spiked artificial soil (Conder and Lanno, 2000) and phytotoxicity to lettuce (Basta and Gradwohl, 2000). Calcium nitrate–extractable Zn levels in the Blackwell soil–amendment combinations are compared with the incipient lethal levels (ILLs: time-independent LC50s) based on Ca(NO3)2–extractable Zn in the Zn artificial soil toxicity test (Fig. 2). Calcium nitrate–extractable Zn levels in the lethal Blackwell soil–amendment combinations were two to three times higher than the ILL (95% CI) of 6.33 (6.17–6.49) mmol Ca(NO3)2–extractable Zn/kg soil measured in artificial soil (Fig. 2). In contrast, Ca(NO3)2–extractable Cd and Pb were present at only 1% of Ca(NO3)2–extractable ILLs: 9.8 (9.4–10.3) and 1.16 (1.11–1.22), respectively (Conder and Lanno, 2000). Thus, we believe that Zn was the most likely source of toxicity. Furthermore, the two lowest lethal exposure levels in the artificial soil Zn test (15.6 and 35.2 mmol Zn/kg soil; Conder and Lanno, 2000) were very similar to the Ca(NO3)2–extractable Zn levels found in the lethal Blackwell soil–amendment combinations (Fig. 2). Time at which 100% cumulative mortality was reached was also very similar (36 to 120 h for lethal field soils, 72 to 144 h for Zn-spiked artificial soils [Conder and Lanno, 2000]). However, Ca(NO3)2–extractable Zn levels in the nonlethal Blackwell LSB combination were far below the ILL for Ca(NO3)2–extractable Zn in artificial soil and Ca(NO3)2–extractable Zn levels in lethal Blackwell soil–amendment combinations. All soil treatments reduced Ca(NO3)2–extractable Cd, Pb, and, with one exception, Zn (Table 1), with the greatest reductions in the Blackwell smelter soil–LSB amendment combination. Compared with Blackwell soil receiving no amendment (None), Ca(NO3)2–extractable Zn in the LSB amendment decreased by a factor of 25, Cd by 10, and Pb by 7. In the other Blackwell soil–amendment combinations, SS and RP, Ca(NO3)2–extractable Cd and Pb decreased by approximately half, while Ca(NO3)2–extractable Zn remained the same in SS, and decreased slightly in RP. As with toxicity, the RP amendment did reduce Ca(NO3)2–extractable metal slightly, but not as effectively as LSB (Table 1, Fig. 1). Treatment of contaminated soil with SS showed slight increases in Ca(NO3)2–extractable Zn but decreases in Ca(NO3)2–extractable Cd and Pb (Table 1). Increased Ca(NO3)2–extractable Zn may be responsible for increased toxicity of the SS treatment compared with the unamended Blackwell soil (Fig. 1). Although nonalkaline organic amendments (i.e., manures) can reduce Zn extracted by neutral salt solutions (Pierzynski and Schwab, 1993), the inability of our SS treatment to reduce Ca(NO3)2–extractable Zn can be attributed to the high Zn content (26 mmol/kg) in the biosolids. The LSB amendment contained Zn levels commonly found in municipal biosolids (USEPA, 1988). Apparently, organic amendments with low Zn are necessary to reduce potentially toxic Zn.
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Fig. 2. Comparison of Ca(NO3)2–extractable Zn in Blackwell contaminated soils, with and without amendments, and lethal Ca(NO3)2–extractable Zn in artificial soil toxicity test (from Conder and Lanno, 2000). The dark line represents the Ca(NO3)2–extractable Zn based, time-independent LC50 (incipient lethal level, ILL) calculated from the artificial soil Zn toxicity test conducted with the earthworm Eisenia fetida; symbols indicate lethal soil–amendment combinations or artificial soils.
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The large differences in the Ca(NO3)2–extractable metal concentrations of the lethal and nonlethal soils, as well as the similarity in lethal Ca(NO3)2–extractable Zn levels of the smelter soil and Zn-spiked artificial soil suggests the possibility of developing universal, soil-independent ILLs for Zn based on Ca(NO3)2–extractable metal levels. Further testing with a wide range of Ca(NO3)2–extractable metal levels in metal-contaminated field soils is necessary to validate this concept.
Plant Root Simulators as Surrogate Measures of Metal Bioavailability
Zinc uptake by PRSs exposed in the Blackwell and Taloka soil–amendment combinations is shown in Fig. 3. Cadmium uptake by PRSs in the Blackwell and Taloka soils was very low (2 and 5% of PRS Zn uptake, respectively), while Pb uptake was even lower (0.8 and 0.6% of PRS Zn uptake, respectively). The mean CV (95% CI) for PRS Zn uptake was 39% (22–56) compared with 1.9% (0.5–3.2) for Ca(NO3)2–extractable Zn, suggesting that Zn availability determined by PRSs was not as precise as Ca(NO3)2–extraction determinations. In addition to reduced precision, PRS measurements were not able to discriminate soil–amendment combinations lethal to earthworms, and thus were poorly related to toxicity, as found by Conder and Lanno (2000) for earthworms exposed to Cd, Pb, and Zn-spiked artificial soils. However, PRSs did indicate high levels of available Zn in all of the Blackwell soil–remediation amendments, despite the very distinct differences in toxicity of these combinations to earthworms. The PRS-chelate, DTPA, may not be suitable for estimating high levels of available metals in soils (Conder and Lanno, 2000). The use of the conventional DTPA soil extraction to measure high levels of available heavy metals may not even be appropriate since it was designed for soil fertility measurements in soils deficient in Zn, iron (Fe), manganese (Mn), or copper (Cu) (O'Connor, 1988). Differences in soil pH may alter the chelating tendencies for individual metals (O'Connor, 1988). When used for metals other than Zn, Fe, Mn, and Cu, the DTPA extraction tends to overestimate plant-available metals (O'Connor, 1988). Its application is also discouraged for estimating plant-available heavy metals in sludge or sludge-amended soils (Hooda and Alloway, 1993; O'Connor, 1988). Regardless, PRSs were able to indicate large differences (two orders of magnitude) in metal bioavailability between the Taloka reference and Blackwell smelter soils.
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Fig. 3. Plant Root Simulator (PRS) chelating ion-exchange membrane Zn uptake in Blackwell smelter and Taloka reference soils receiving remediation amendments. Columns (mean, n = 4, ±95% CI) with the same letter are not significantly different (Fisher's protected LSD, P > 0.05); symbols indicate soil–amendment combinations lethal to the earthworm Eisenia fetida.
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CONCLUSIONS
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The earthworm bioassay was able to assess reductions in metal bioavailability and evaluate chemical immobilization amendment effectiveness. Using this approach, we concluded that LSB is an effective amendment for reducing metal bioavailability to earthworms in contaminated soil, although the effectiveness of LSB may be only temporary (Gradwohl, 1998).
Calcium nitrate–extractable metals appear to be a very promising surrogate measure of metal bioavailability for earthworms in soil. The large differences in the Ca(NO3)2–extractable metal concentrations of the lethal and nonlethal soils, as well as the similarity in lethal Ca(NO3)2–extractable Zn levels of the smelter soil and Zn-spiked artificial soil (Conder and Lanno, 2000) indicates the possibility of developing universal, soil-independent ILLs based on Ca(NO3)2–extractable metal levels. Testing field soils contaminated with metals other than predominantly Zn is needed to further validate the use of weak-electrolyte extractions as surrogate measures of metal bioavailability in soil. Correlation with metal residues in soil organisms (underway for earthworms used in this study) is also necessary.
Although Ca(NO3)2 extractions are easier to perform and better related to earthworm toxicity in soil, PRSs may be useful as an in situ screening tool, and may avoid possible physicochemical alterations of soil involved in weak electrolyte sampling techniques.
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ACKNOWLEDGMENTS
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This research was made possible through a grant from the OSU Environmental Institute to R. Lanno and N. Basta. J. Conder received support in the form of a graduate research fellowship from the U.S. Environmental Protection Agency during part of this study. The authors are appreciative of R. Gradwohl, who provided the remediated soil–amendment combinations for this study. Thanks are due to A. Conder and K. Kejela for assistance with experimental procedures and K. Greer at Western Ag Innovations Inc. for his advice regarding the PRSs. The views expressed in this document do not necessarily represent those of the OSU Environmental Institute, USEPA, or Western Ag Innovations Inc.
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NOTES
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J.M. Conder, current address: Inst. of Applied Science, Univ. of North Texas, Denton, TX 76203.
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ABSTRACT
INTRODUCTION
