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a Remediation and Containment Branch, Land Remediation and Pollution Control Division, National Risk Management Research Laboratory, USEPA, 26 West Martin Luther King Jr. Drive, Cincinnati, OH 45268
b Animal Manure and Byproducts Lab, Beltsville Agricultural Research Center, USDA-ARS-ANRI, Beltsville, MD 20705
c Department of Soil Science, Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka
* Corresponding author (gangah{at}pdn.ac.lk).
Received for publication October 1, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: -Fe/Mn, easily reducible iron and manganese removed • -FeO, free iron oxide removed • OC, organic carbon • -OC, organic carbon removed
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Indirect methods of characterization (e.g., chemical extractions) and theoretical considerations suggest that metals in biosolids exist as soluble, organically complexed, adsorbed forms coprecipitated with Fe, Al, P, and Ca solid phases (Lake et al., 1984; Jing and Logan, 1992). Lake et al. (1984) stated that metals in biosolids are predominantly associated with solid phases and less than 10% of total metals are soluble and/or exchangeable. Li et al. (2001) demonstrated that Cd sorption for biosolids-amended soils was intermediate to biosolids and the control soil and increased with increasing biosolids application rate. Removal of organic carbon (OC) reduced metal sorption for all samples but did not account for the observed differences between biosolids-amended soils and control, indicating that the increased sorption associated with biosolids application was not limited to the increased OC from the biosolids application. However, their study did not directly evaluate the importance of the Fe and Mn fractions for the increased sorption of metals in biosolids-amended soils.
Surfaces of freshly precipitated metal (hydr)oxides such as Fe, Mn, and Al are known to be highly active sites for sorption of most dissolved metal ion species. Trace metal adsorption to Fe and Al oxides has been extensively studied (Hohl and Stumm, 1976; Kinniburgh et al., 1976; Bolland et al., 1977; Benjamin and Bloom, 1981; Kuo and McNeal, 1984; Weesner and Bleam, 1998). As the high specific surface area of amorphous or colloidal iron oxides is expected to result in higher adsorption capacity, Fe oxides have received the greatest attention. Manganese oxides have received less attention in spite of their lower pHPZC and, therefore, high negative surface charge in the pH range generally found in soils. Fresh biologically oxidized and poorly crystallized or amorphous Mn oxides with larger surface areas for adsorption (Birnie and Paterson, 1991; Nelson et al., 1999) could be important in controlling cationic metal concentrations in soils. Biosolids contain high levels of Fe, Mn, and Al. Therefore, it is possible that inorganic components, especially Fe and Mn phases of biosolids, are responsible for controlling the metal availability in biosolids-amended soil systems. If these hypotheses are correct, removal of Fe and Mn fractions should remove a significant portion of the observed enhanced sorption capacity in biosolids-amended soils compared with the control.
To better predict the environmental fate and mobility of contaminants, it is critical to study the desorption behavior as well as the sorption behavior of contaminants in soil systems. It is almost certain that trace metals sorb onto oxides and other soil constituents specifically (chemisorption) and sorption reactions generally show poor reversibility (Sposito, 1989; McBride, 1989). However, it is unclear whether the surface reaction is genuinely irreversible or simply slow in the reverse direction. One explanation for the irreversible nature of metal sorption is that the activation energies for desorption may be much larger than sorption; thus, rates of desorption at normal soil temperatures are likely to be much slower than rates of sorption (McBride, 1989). Another explanation involves the incorporation of metals into the oxides making them unavailable for desorption (true irreversibility).
The purpose of this study was to investigate the sorption and desorption of Cd in soils with or without biosolids amendments, and to determine the importance of organic matter and easily reducible Fe and Mn fractions on sorption and desorption in these soils. The resulting information is important to understanding the phase or phases responsible for enhanced metal retention observed in biosolids-amended soils and developing effective strategies for in situ stabilization of toxic metals in soils.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Organic Carbon Removal
The sodium hypochlorite (NaOCl) method (Shuman, 1983) was chosen over the hydrogen peroxide (H2O2) method for removal of OC in these materials. The more commonly used H2O2 method causes dissolution of Mn oxides (Jackson, 1956) as well as alkaline earth carbonates and phosphates (Anderson, 1963; Lavkulich and Wiens, 1970), whereas the NaOCl procedure permits the removal of OC effectively with minimal destruction to Mn oxides and other inorganic phases (Anderson, 1963). To a 1-g sample (dry weight basis) placed in a 50-mL plastic centrifugal tube, 20 mL of 0.7 M NaOCl (adjusted to pH 8.5 with HCl) was added and the resulting solution was heated to 90°C in a water bath for 2 h with occasional mixing. After cooling, the sample was centrifuged and the supernatant was collected. For the solid residue this oxidation procedure was repeated and after the second oxidation step, the soil residue (-OC) was washed thrice for approximately 2 min (each) with 30 mL (each) 0.01 M Ca(NO3)2 (pH 7.0) to remove the excess NaOCl left in the sample. After each extraction and washing, samples were centrifuged and the supernatant samples were collected. Separate analyses of these extracting and washing solutions for Fe, Mn, and other metals were performed by ICP–AES. The results were then combined to obtain information on Fe and Mn removed by the extraction procedure. Washed residue (-OC) was used for sorption–desorption experiments. The organic carbon content of the solid residue (-OC) was determined using a TOC analyzer as described before.
Easily Reducible Iron and Manganese Removal
The Tessier et al. (1979) procedure was used for removal of easily reducible Fe and Mn. One gram of sample (dry weight basis) was weighed into 50-mL centrifuge tubes and 20 mL of 0.04 M NH2OH·HCl in 25% HOAc was added. The resulting mixture was placed in a hot water bath maintained at 96°C and digested for 6 h with occasional mixing. After digestion, the sample was centrifuged and the supernatant collected. The solid residue was washed, and extracting and washing solutions were analyzed separately as mentioned previously. Washed residue (-Fe/Mn) was used for sorption–desorption experiments. Additionally, the OC content of the solid residue (-Fe/Mn) was determined using a TOC analyzer as described before to determine the propensity of this extracting procedure to extract OC.
Free Iron Oxides Removal
Some soil samples contained significant amounts of free Fe oxides that could not be extracted by the Tessier et al. (1979) procedure. For these samples, a modified Kunze and Dixon (1986) procedure (citrate–bicarbonate–dithionite, CBD) for the removal of free Fe oxides was used. Twenty milliliters of 0.3 M sodium citrate and 2.5 mL of 0.5 M NaHCO3 were added to a 1-g sample (dry weight basis) placed in a 50-mL plastic centrifugal tube. The suspension was heated to 80°C in a water bath, and then 0.5 g of Na2S2O4 was added. The mixture was stirred constantly for 1 min and occasionally during the next 14 min of digestion at 80°C. After digestion, samples were centrifuged and residue (-FeO) was washed, and the extracting and washing solutions were analyzed as described previously.
Organic Carbon Removal followed by Iron and Manganese Removal (and Vice Versa)
In addition to removal of OC or Fe/Mn, samples were treated to remove OC followed by removal of Fe/Mn (-OC-Fe/Mn), or removal of Fe/Mn followed by removal of OC (-Fe/Mn-OC). The control and lime-treated composted biosolids-amended soil samples were treated to remove free Fe oxides after removal of OC and Fe/Mn (-OC-Fe/Mn-FeO). After each extraction procedure, solid residue was washed, and the extracting and washing solutions were analyzed as mentioned previously. The washed residue was used for the sorption–desorption experiments.
Cadmium Sorption
Intact soil samples (whole), the -OC sample, the -Fe/Mn sample, the -OC-Fe/Mn sample, the -Fe/Mn-OC sample, and the -OC-Fe/Mn-FeO sample were suspended in 20 mL of 0.01 M Ca(NO3)2. Solution pH in the suspension was adjusted to pH 5.5 ± 0.1 using 0.01 M NaOH or 0.01 M HNO3. The suspension volume was adjusted to 25 mL with 0.01 M Ca (NO3)2 and shaken on a platform shaker with a reciprocal movement at 140 rpm. After the pH stabilized (within ±0.1 units) and had been maintained at pH 5.5 ± 0.1 for 24 h, suspensions were centrifuged and the supernatant was filtered through a 0.45-µm filter. Solutions were stored in a refrigerator at 4°C until the analysis. The sample was resuspended in 20 mL of 0.01 M Ca(NO3)2, and Cd standard solution [Cd as Cd(NO3)2·6H2O] was quantitatively added to give four known Cd concentrations (12, 24, 48, and 72 µg of Cd). The pH of the suspension was adjusted to 5.5 with 0.01 M NaOH or 0.01 M HNO3 and the volume was adjusted to 25 mL with 0.01 M Ca(NO3)2. All the sample tubes were sealed and shaken on a platform shaker at 140 rpm for 48 h at 23 ± 1°C. Approximately 2 to 4 h after the shaking started the suspension pH was measured. If pH was not within ±0.1 units of the desired pH level, additional pH adjustment was made. The volume of acid or base used during the adjustment was recorded and added to the total volume for use in final calculation. After a minimum of 48 h of equilibration time or at least 24 h after the pH had stabilized, the pH was checked and verified to be within ±0.1 units of the desired pH. The sample was centrifuged and the clear supernatant filtered through a 0.45-µm filter. If the pH was not within ±0.1 pH units of the desired pH level, the sample was discarded, and the procedure was repeated. Solutions were stored in a refrigerator at 4°C until the analysis. The analysis for Cd was performed by ICP–AES according to USEPA Method 6010B (USEPA, 2001). The amount of Cd sorbed was calculated as the difference between the known initial Cd concentration and final Cd concentration. Solid residue from the sorption experiment was used for the desorption experiments.
Cadmium Desorption
Moist solid residue (approximately 1 g, dry weight basis) from the sorption experiments was suspended in 20 mL 0.01 M Ca(NO3)2. The pH of the suspension was adjusted to 5.5 ± 0.1 using 0.01 M HNO3 and 0.01 M NaOH. The volume was adjusted to 25 mL using 0.01 M Ca(NO3)2. All the sample tubes were sealed and shaken on a platform shaker at 140 rpm for 48 h at 23 ± 1°C. About 2 to 4 h after the shaking started, the suspension pH was checked. If pH was not within ±0.1 units of the desired pH level, additional adjustment of pH was made. After a minimum of 48 h of equilibration time or at least 24 h after the pH had stabilized, the pH was checked to verify that it was within ±0.1 units of the desired pH. The sample was centrifuged and the clear supernatant was filtered through a 0.45-µm filter. If the pH was not within ±0.1 pH units of the desired pH level, the sample was discarded, and the procedure was repeated. The solid residue was resuspended in 20 mL 0.01 M Ca(NO3)2 and the desorption experiment as previously described was repeated at least one more time. The analysis for Cd was performed by ICP–AES according to USEPA Method 6010B (USEPA, 2001).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The NH2OH·HCl in the 25% acetic acid (HOAc) extracting procedure removed 84 to 100% of the total soil Mn, suggesting that most Mn in these soils can be extracted by this extracting solution (Table 2). However, Fe removal ranged between 14 to 39% of the total soil Fe, indicating that most of soil Fe existed in forms that this extracting procedure could not remove. Frampton and Reisenauer (1978) reported that the NH2OH·HCl in 25% acetic acid is only capable of extracting amorphous Fe oxides, though it is proven to be an effective extracting solution for Mn from soils (Shuman, 1982). It extracts as much Mn as most of the other extractions used for the extraction of various Fe compounds. Our observations are in agreement with Shuman (1982).
Besides the target materials, Fe and Mn, the NH2OH·HCl in 25% HOAc extraction also removed 5 to 12% of the total soil Al and <1 to 54% of the total soil OC. The NaOCl treatment followed by NH2OH·HCl in 25% HOAc treatment generally increased the amounts of Al removed considerably (7–35% of the total soil Al). This was not the case for the NH2OH·HCl in 25% HOAc treatment followed by the NaOCl treatment. These results indicate that there were some Al compounds in these soils that could only be removed by the NH2OH·HCl in 25% HOAc treatment after treatment to remove OC.
Cadmium Sorption
Effects of biosolids addition on Cd sorption at pH 5.5 are shown in Fig. 2
. These typical sorption isotherms illustrate the linearity of Cd sorption for a soil or fraction of soil, within the range of Cd concentrations used in this experiment. In fact, the isotherm was still linear at an order of magnitude higher solution concentration (data not shown). The coefficients of determination for a sample ranged from 0.94 to 0.99. As illustrated by these isotherms, application of biosolids increased the slope of the sorption isotherm and the increase was different for the different biosolids. These observations are in agreement with previous Cd adsorption studies conducted in our laboratory with different biosolids-amended soils (Li et al., 2001) and support the hypothesis that the addition of biosolids to soil alters the chemical properties of the soil system. We believe that the addition of biosolids adds extra adsorptive phases to soil systems and therefore alters its adsorptive characteristics.
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The slope of the Cd sorption isotherm decreased significantly because of OC removal for both the control soil sample and the biosolids-amended soil sample suggesting that OC is important for the adsorption of Cd by these soils (Fig. 3) . However, removal of the OC fraction did not result in the biosolids-amended soil sample behaving like the control soil. In other words, enhanced Cd sorption due to biosolids application was still apparent after removing the OC (approximately 90%) for lime-treated composted biosolids-amended soil. This indicates that both the OC fraction and the inorganic fraction are responsible for enhanced Cd sorption observed in the biosolids-amended soils. A similar trend was observed for other tested biosolids-amended soils (data not shown) and agrees with the observations of Li et al. (2001) on the importance of the inorganic fraction of biosolids to the increased Cd sorption of biosolids-amended soil. This observation complements those of Mahler et al. (1987) and Brown et al. (1998), which indicated that either inorganic phases in biosolids or recalcitrant OC are responsible for maintaining low plant availability in biosolids-amended soils.
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10 to 20 g kg-1 can be detected (Ma et al., 1994). X-ray absorption near edge spectroscopy (XANES) studies (data not shown) revealed that the majority of Fe in the lime-treated composted biosolids had mixed oxidation states of +2 and +3 (magnetite-like) and an oxidation state of +3 (goethite-like).
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Cadmium Desorption
The results from the desorption experiments for the highest Cd sorbed soil samples for the control and two different biosolids-amended soils are shown in Fig. 6
. Desorption experiments were repeated twice and the duration of a single desorption experiment was approximately 2 d. These desorption experiments were conducted using the same background electrolyte as the sorption experiments [0.01 M Ca(NO3)2 at pH 5.5]. Many researchers conduct desorption experiments using either organic chelators (such as ethylenediamine tetraacetic acid) or acidic desorbing solutions. Despite all the other important information offered from these desorption experiments, they do not provide the information on desorption behavior in terms of a irreversible reaction in which the desorption conditions are the same as sorption (Strawn and Sparks, 2000).
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It should be noted that not only Cd sorption but also Cd desorption characteristics of biosolids-amended soils are different from the control soil. Figure 6 shows that a substantial proportion of Cd sorbed by biosolids-amended soils cannot be readily desorbed. For example, in the case of intact samples (whole soil), nearly 37.1% of the sorbed Cd was recovered from the control sample whereas only 15.8 and 10.5% were recovered from lime-treated digested and lime-treated composted biosolids-amended soil, respectively. The removal of OC did not cause the biosolids-amended soils to behave like the control sample, indicating that not only the OC fraction but also the inorganic fraction are responsible for the increased "apparent hysteresis" observed in biosolids-amended soils. Effectiveness of Fe and Mn removal in reducing "apparent hysteresis" was different for lime-treated composted and lime-treated digested biosolids-amended soil. The removal of Fe and Mn appeared to be more important than the removal of OC in removing the differences observed in the desorption characteristics between the lime-treated digested biosolids-amended and control soils (Fig. 6). It has been accepted that the trace metals sorb onto metal oxides by chemisorption showing poor reversibility (Sposito, 1989; McBride, 1994). One explanation noted earlier for the irreversible nature of metal sorption by oxides is that the activation energies for desorption may be much larger than sorption; thus, rates of desorption at normal temperatures are likely to be much slower than rates of sorption. Another explanation involves the incorporation of metals into the oxides, making them unavailable for desorption (true irreversibility). Further, increasing contact time between sorbent and metals may further decrease a metal's ability to desorb from the sorbent (Hogg et al., 1993; Backes et al., 1995). Sorption followed by diffusion of adsorbed ions within the solid oxide material has been proposed as a possible reason for decreased metal desorption with time (Barrow, 1986; Barrow et al., 1989). Therefore, it is reasonable to assume that metals associated with oxides in biosolids-amended soils may be more difficult to desorb compared with the added sorbed Cd in these sorption–desorption experiments. Neither OC removal nor Fe and Mn removal treatments were able to remove the differences between the desorption characteristics of the control and lime-treated composted biosolids-amended soil. As expected, removal of both fractions was more effective in removing those differences than either alone for lime-treated composted biosolids-amended soil. This indicates that increased "apparent hysteresis" in biosolids-amended soils is related to both OC and Fe and Mn fractions. Strawn and Sparks (2000) studied the effects of soil organic matter (SOM) on sorption and desorption behavior of Pb in two soils by treating the soil with sodium hypochlorite to remove the SOM fraction, revealing that removal of SOM plays an important role in decreasing the hysteresis (incomplete or slow desorption reactions) observed in Pb desorption experiments. However, in their study, attempts were not made to observe the importance of Fe and Mn fractions on desorption of Pb.
In this study, our calculations were based on the assumption that all sorbed Cd was from added sorbed Cd in the sorption experiments. The Cd in soils before the sorption experiments should also be considered. This would be more significant for biosolids-amended soils compared with the control soil. If initial Cd had been included, then the differences observed between the control and the biosolids-amended soils would have been greater than reported here; however, that would not change our conclusions. This study provides clear evidences Fe and Mn oxide retention and immobilization of Cd by biosolids.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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-Al2O3. J. Colloid Interface Sci. 55:281–288.Related articles in JEQ:
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