Mobility of Heavy Metals from the University of Nigeria Sewage-Sludge Disposal Site to the Surrounding Soils and Plants

Abstract:

This study was carried out at the University of Nigeria, Nsukka sewage disposable site. The study was to quantify the mineral contents of sewage sludge and assess their mobility in the soil and plants around UNN disposal site. The experimental layout was a 4x3 factorial in RCBD in which distance from the sewage disposal site and soil depth were the two factors under consideration. The distances from sewage pond were 0 (edge), 100, 300 and 2000 m away from the sewage pond with 2000 m serving as control while soil depths were 0¬¬¬ – 40, 40 – 80, and 80 – 20cm. Soil and plant sampling was carried out from three distances in both sewage site and at the control site. Physico-chemical properties of the soil were determined. Four heavy metals (Cd, Cu, Hg and Pb) were determined from the soil, plant samples as well as the sewage sludge. Results showed that the effects of distance and soil depth were significant (P < 0.05) on pH, exchangeable bases, cation exchange capacity, organic matter, total nitrogen, exchangeable acidity and available phosphorous. At the control site there were no differences among soil properties except organic matter that decreased significantly (P < 0.05) with increase in depth. All the chemical properties determined decreased with increase in depth, except exchangeable acidity which increased with increase in depth. Cadmium, Cu, Hg, and Pb, significantly (P < 0.05) decreased as both distance and depth increased. There was an obvious high heavy metal content in the soil of disposal site when compared with the control soil. In the tissues of the three plants (Manihot spp, Zea mays, Pannicum maximum) studied, Cd, Cu, Hg, Pb concentrations were significantly different (P < 0.05) at 0, 100, 300 and 2000 m distances. Heavy metal content observed in plant tissues at the disposal site was relatively high when compared to that of the control site. Therefore, it follows that the studies at the University of Nigeria Nsukka sewage disposal site results in an increased concentration of soil property, such as organic matter, macro and micro nutrients. The mobility of heavy metals (Cd, Cu, Hg, and Pb) from soil to leaves suggest that all these metals were mobile from soil to plant components. Heavy metal concentrations varied among the tested crop plant, which reflects their differences in their uptake capabilities and their further translocation to the shoot portion of the plant. The bioconcentration factor at both root and leaf tissues of crop plant did not show any abnormal accumulation of heavy metals at sewage site. From the study the heavy metal contents of the sludge, soil and plant species were below toxicity levels when compared to WHO/FAO standards.