Dynamics of heavy metals and their immobilization in zinc smelter-effluent contaminated soils

Long-term use of smelter effluents for irrigating agricultural crops in the nearby vicinity of the smelter plants often results into build up of elevated level of heavy metals and other toxic compounds in soil. This is a matter of concern, because the accumulated metals in soil often transfer to food chain and causes clinical problems in human. Therefore, assessment, monitoring and remediation of metal contaminated soil are of utmost importance, particularly in the smelter-contaminated agricultural lands. The present study assessed the long-term effect of irrigation with zinc smeltereffluent on important soil properties including heavy metal status and metal concentration in the edible parts of the crops grown on smelter effluent-irrigated soils. An attempt was also made to identify the probable solid phases governing the solubility of Zn and Cd in zinc smelter effluent-irrigated soils. The efficacy of lime, single super phosphate (SSP), fly ash and rock phosphate (RP)-Sesbania rostrata leaf in immobilization of zinc (Zn), cadmium (Cd) and lead (Pb) in zinc smelter effluentirrigated soil using Indian spinach (Beta vulgaris L. var. all green), wheat (Triticum aestivum L. var. HD-2967) and lentil (Lens culinaris L. var. Shivalik) as test crops was also evaluated. The effects of amendments on the solubility and distribution of metals in different fractions and their uptake by crops in zinc smelter-contaminated soil were assessed. The effectiveness of amendments was assessed by assessing human health risk for dietary intake of metals through consumption of edible parts of crops grown on smelter-contaminated soil treated with different amendments. For the present study, agricultural lands which have been receiving zinc smelter effluent irrigation for about five decades at Debari, Udaipur were selected. To assess the long-term effect of irrigation with zinc smelter-effluent on soil and plant, fifty nine (59) composite surface soil samples (0-15 cm) were collected from both the farmers’ fields irrigated with zinc smelter effluent and tubewell water. Plant samples, wherever available, were collected from both smelter effluent and tubewell waterirrigated lands. Water samples were also collected from the channel carrying the zinc smelter effluent and tubewells. A greenhouse pot experiment was conducted to assess the effect of amendments on immobilization of metals using Indian spinach, wheat and lentil as test crops. Hazard quotient (HQ) was worked out for assessing risk to human health for intake of metals through consumption of the edible portion of crops. Results indicated that long-term irrigation with zinc smelter effluent resulted into significant build-up of ethylenediaminetetraacetic acid (EDTA) extractable Zn (57.7 fold), Cu (4.51 fold), Fe (3.35 fold), Mn (1.77), Ni (1.20 fold), Pb (45.1 fold) and Cd (79.2 fold) in soils as compared to tubewell water irrigated fields. Total Zn, Cu, Mn, Ni, Pb and Cd content in effluent-irrigated soils was increased by 27.0, 1.60, 1.40, 1.30, 26.2 and 167 fold, respectively over tubewell water-irrigated soils. Among these metals, Cd and Pb concentrations in the edible parts of almost all the crops grown on effluent-irrigated soils were above the safe limit. Assessment of potential ecological risk indicated a very high to moderate ecological risk due to Cd, Pb and Zn in soils close to the immediate vicinity of the smelter plant. On an average, the soil pH dropped by 0.31 units as a result of smelter effluent irrigation. However, effluent irrigation resulted into significant increase in soil organic carbon, electrical conductivity (EC) and CaCO3 content. Increased available potassium (K) and sulphur (S), and reduced available nitrogen (N) and phosphorus (P) were recorded in effluentirrigated soils. Reduced microbial activities in effluent-irrigated soils were also evidenced from the level of microbial biomass carbon and dehydrogenase activity. Identification of solid phases revealed that solubility of Zn and Cd in highlycontaminated soil is limited by the solubility of willemite (Zn2SiO4) in equilibrium with quartz and octavite (CdCO3), respectively. However, in case of tubewell waterirrigated soil, franklinite (ZnFe2O4) in equilibrium with soil-Fe and exchangeable Cd are likely to govern the activity of Zn and Cd in soil solution, respectively. Results of the pot experiment indicated that lime application at 50 g kg was the most effective treatment in reducing the free ion activities of Zn, Cd and Pb in soil. Metals were redistributed from labile pool to relatively immobile pools in soil as a result of lime and fly ash application. Water soluble-exchangeable fraction of Zn, Cd and Pb was reduced to the extent 54.7, 70.0 and 65.7%, respectively due to lime application. The corresponding figures for fly ash were 42.7, 31.2 and 27.1%, respectively. Lime could immobilize EDTA extractable Zn, Cd and Pb to the extent of 38.3, 25.1 and 50.5%, respectively in soil at the harvest of spinach. Whereas, EDTA extractable Zn, Cd and Pb were successfully immobilized by 31.0, 9.81 and 17.9%, respectively in post-harvest soil due to fly ash application. More or less, similar results were recorded in soil after the harvest of wheat and lentil. The order of metal accumulation in edible portion of crops was lentil < wheat < Indian spinach. Among the three crops, only lentil was suitable for cultivation in this Zn-smelter effluentirrigated soil as revealed by risk assessment.