Effect of chemical amendments on the concentration of cadmium and lead in long-term contaminated soils
Introduction
Heavy metal contamination in the natural soil environment is a major problem for crop quality, human health, and environmental quality, and most of the heavy metals are persistent in soil because of their immobile nature (Chen et al., 1996, Kabata-Pendias and Pendias, 2001). About 1200 sites were listed on the National Priority List (NPL) of USA for the treatment of contaminated soils, and approximately 63% of these sites were regarded as heavy metals contamination (Hazardous Waste Consultant, 1996). Soil pH value, organic matter, clay minerals, oxides of iron or aluminum, redox conditions, and the ageing of contamination controlled the behaviors of the heavy metals in contaminated soil and also affected the uptake of heavy metals by plants (Alloway, 1995, Naidu et al., 1997, Lock and Janssen, 2003, Vig et al., 2003). Thus the sorption of heavy metals in soils under different ionic strength and soil pH value is widely variable for different soils (Naidu et al., 1994). In Taiwan, some rural soils contaminated with Cd, Pb, Cu, and Zn have been identified by Taiwan EPA since 1980s. The contaminations of Cd and Pb in some rural soils of Taiwan were caused by the wastewater discharged from the chemical engineering plants in the industry parks (Chen, 1991).
Some soil remediation techniques were used to immobilize heavy metal in the contaminated soils and to reduce the soluble concentration in soils by the reactions of precipitation, adsorption, or to complex with organic matter in some case studies (Mench et al., 1994, Chen and Lee, 1997, Pierzynski, 1999, Kabata-Pendias and Pendias, 2001). Application of lime materials, compost, phosphate, oxides of iron and manganese, and zeolites significantly reduced the solubility of heavy metals in contaminated soils (McBride and Blasiak, 1979, Sommers and Lindsay, 1979, Kuo and McNeal, 1984, Kuo et al., 1985, Chen et al., 2000, Lee and Chen, 2000, Mench et al., 2000, Basta et al., 2001, Chen et al., 2001, Gupta et al., 2001, Hettiarachchi et al., 2001, Knox et al., 2001, Yang et al., 2001, Mavropoulos et al., 2002). Liming the contaminated soils to reduce the bioavailability of heavy metals is the most widely used remediation treatment. Application of lime materials to the contaminated soil leads to significantly increase the soil pH value because of the release of hydroxyl ion by the hydrolysis reaction of calcium carbonate. Liming can lead to the precipitation of metals as metal-carbonate and significantly decrease the exchangeable fraction of metals in contaminated soil (Knox et al., 2001). This reaction can reduce the bioavailability of heavy metals in soil and also reduce the uptake by plants (Street et al., 1977, Hirsch and Banin, 1990, Chlopecka and Adriano, 1996). Krebs et al. (1998) investigated heavy metal uptake by peas when growing in limed and check soils treated with mineral fertilizer (control), sewage sludge, and pig manure. The peas grown in the limed soils contained lower concentration of Cd, Cu, and Zn in above ground parts than that of plants grown on fertilized, un-limed soils. Friesl et al. (2003) also reported that applying lime significantly decreased Zn uptake by barley (p < 0.5). However, the labile or exchangeable form of metal in contaminated soils may increase and decrease the liming effectiveness after applying lime materials for a long time (Chlopecka and Adriano, 1996, Friesl et al., 2003). Thus frequently and repeatedly applying lime materials are required to increase the effect of lime on metal-contaminated soils.
McBride (1995) reported that the organic matters in sludge buffer soil pH and to avoid heavy metal uptake by plants. Organic matters with suitable reactive groups, such as hydroxyl, phenoxyl, and carboxyl, which effectively controlled the adsorption and complex of heavy metals with soil, and the activity of metals in soils (Alloway, 1995, McBride et al., 1997). Application of organic matter in the metal-contaminated soil can efficiently reduce the concentration of Cd and Zn in the soil solution (Isabelle and Alian, 2001) and also the concentrations of Cu, Mn, and Zn extracted with 0.01 M CaCl2 (de Mora et al., 2003).
Zinc was one of the necessary trace elements for plants and rural soils have optimum Zn content for plant growth. Shen et al. (1996) indicated that the application of Zn can efficiently increase the yield of the Thlaspi caerulescens. Oliver et al. (1996) divided the crops into zinc-efficient and zinc-inefficient species. The zinc-inefficient crops were able to tolerate Zn deficiency and accumulated more Cd than that of zinc-efficient species. They also indicated that the application of Zn in the normal area and Zn deficient areas was effective in reducing the Cd concentration in the wheat grain. When growing in nutrient solution contained low concentration of Cd, a strong antagonistic effect of Zn on Cd accumulation was found in young leaves of lettuce or spinach (McKenna et al., 1993).
The objectives of this study are: (1) to evaluate the effects of chemical amendments on the concentrations of Cd and Pb in soil solution and extract with DTPA or EDTA, and (2) to evaluate the effects of different chemical amendments on the Cd and Pb concentration in the different parts of wheat and total metal uptake in wheat when growing in long-term Cd- and Pb-contaminated soils, more than 20 years, of northern Taiwan.
Section snippets
Basic soil properties of four soils
Four soils contaminated with Cd and Pb in northern Taiwan, highly contaminated Tatan sandy soil site and slightly contaminated Chungfu clayey soil site, were selected for this study. Tatan A and B sandy soils and Chungfu C and D clayey soils were collected from four different sites contaminated by two different chemical engineering plants located at both the Tatan village and Chungfu village for more than 20 years, respectively. The surface soils (0–20 cm) of each contaminated site were sampled,
Soil properties
The cation exchange capacity (CEC) of Chungfu C and D clayey soils were greater than those of Tatan A and B sandy soils because of the greater contents of organic carbon and clay in the Chungfu C and D clayey soils (Table 1). Those mechanisms involved in the adsorption of metals in soil including cation exchange, specific adsorption, co-precipitation, and organic complexation (Alloway, 1995). Adsorption of metals increased with increasing their clay and organic matter contents in the soils (
Conclusions
The application of calcium carbonate and calcium carbonate mixed with zinc oxide (or compost) can significantly reduce the concentration of Cd in soil solution directly sampled with RSMS, and in soil extracted with 0.005 M DTPA or 0.05 M EDTA, or reduce the concentration of Cd in the grain, leaf and stem, husk, or total Cd uptake in all parts of wheat species (p < 0.01). The results of this study also indicated that Cd concentration in soil solution and in soil extracted with DTPA or EDTA were
Acknowledgments
The authors wish to thank the Agricultural Experimental Station of National Taiwan University for providing the greenhouse for pot experiments. This work was partially supported by the Environmental Protection Administration (EPA) of Taiwan.
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