Heavy metal contamination of laterites by accumulation of solid wastes

Abstract

A heavy metal is defined as a metal that has the atomic number and specific gravity greater than 20 and 5 Mg m-3, respectively. Heavy metals occur naturally in the soil environment from the weathering of parent materials and also as contaminants at the waste disposing sites. Though, the soil acts as a sink for the majority of heavy metals, the excess uptake of any one of them may limit the plant growth. In this background, the research work entitled “heavy metal contamination of laterites by accumulation of solid wastes” was undertaken during the period 2010-2014. The project aims to investigate the extent of distribution of heavy metals (As, Cd, Cr, Pb, Hg and Co) in waste dumping sites of laterite as influenced by soil and climatic conditions using geostatistical technique, to correlate the activity of major soil enzymes with the contents of heavy metals, to explore the potential of phytoremediation as well as aerobic and anaerobic methods of composting for the removal of heavy metals in solid waste and to evaluate the performance of amaranthus under different decontaminated methods adopted. In order to attain the objectives, four experiments were conducted. The effect of soil and climatic factors on heavy metal accumulation was examined with the collection of soil samples at quarterly intervals for a period of one year from two different waste disposal sites located at Laloor and Kalamassery. For the study on phytoremediation, the crops, vetiver, marigold and sunflower were experimented. Both the aerobic and anaerobic methods of composting were tried for the removal of heavy metals in another set of experiment. A pot culture study was also conducted to assess the growth of amaranthus under different decontaminated conditions. The salient findings are summarized as follows: The percentage distribution of Pb, Ni, Co, Cr and Hg at the different sites of Kalamassery revealed that Cr had the highest distribution followed by others. At Laloor, Pb was found to be dominant for the major dumping sites, whereas Cr at the non dumping site. The geo accumulation index for Hg was recorded as above one for all sites at Kalamassery. The average contents (mg kg-1) of Pb, Co, Ni, Hg and Cr was 15.58, 6.06, 43.18, 0.56, 107.9 at Kalamassery and 82.84, 7.79, 35.54, 0.42 and115.67 at Laloor, respectively. Heavy metal contents were found to be higher during summer than rainy season except for Cr and Cd. In general, soil enzyme activities were higher during quarter II of the sampling period. There was a positive correlation between Fe and urease (0.391*) and Pb and phosphatase (0.350*). The phytoremediation study with the three different crops (sunflower, marigold and vetiver) proved vetiver as a good phytostabilizer compared to others. Vetiver was found to be a translocator of Cr, since Ni, Pb, Co and Hg were mostly stabilized in the root. Marigold translocated all these heavy metals from soil except Co, and sunflower translocated Co, Ni and Cr except Pb. Bioconcentration factor (the heavy metal concentration in the plant/ the heavy metal concentration in the soil) was higher for vetiver compared to sunflower and marigold. Vetiver removed the heavy metals from the waste material in the order Cr>Ni> Co> Pb. Compared to aerobic composting, the anaerobic method proved to be better for the removal of heavy metals from the waste material collected from Laloor. The comparative performance of amaranthus grown in soil under different methods adopted for reducing heavy metal contamination showed that the yield was found to be the highest in the treatment with aerobic compost. The lowest yield was reported for amaranthus grown with phytoremediated material, followed by absolute control. The uptake of major nutrients also followed the same trend as yield. The heavy metal content was higher in the shoots of amaranthus grown with waste material and the accumulation followed the order Cr>Pb>Ni>Co>Hg. In all the treatments under study, the presence of Cr was more dominant in the post harvest soil compared to other heavy metals.