Sequential Extraction of Heavy Metals from Soil Samples Collected from Selected Cocoa Farmland in Erijiyan, Ekiti State, Nigeria

Abstract

Heavy metal pollution is of great concern due to their potential harmful effects on man and the environment, as soil and sediment are their primary repositories. Anthropogenic activities such as agricultural practices, industrial activities, and waste disposal have contributed to increases in heavy metal concentrations in soil. It is important to determine the effect of agricultural input on the heavy metal concentrations, mobility and bioavailability on cocoa farms by analyzing soil samples collected from selected locations to determine pH, content of organic matter, presence of water-soluble heavy metals and sequential extraction of heavy metals using standard analytical procedures. Results indicate random variation in the pH and organic matter content of soil with average values of 6.49 ± 0.12 and 2.30 ± 0.11%, respectively. The acid digestion of heavy metals was relatively higher than water-soluble heavy metals with the following mean values: Zn (33.70 > 11.30), Fe (111.71 > 4.58), Mn (1.84 > 1.24), Cd (2.16 > 0.95) and Co (1.66 > 1.01). However, sequential extraction revealed that the heavy metals were distributed in all extractive steps, with the non-residual phase dominating and resulting in possible mobility and bioavailability of those heavy metals. Eight soil samples were randomly collected for analysis of source and mobility of heavy metals at a cocoa farm using a calibrated soil auger, and were immediately placed in sealed polythene bags. The samples were collected at depths from 0 - 15 cm, from eight points at 40 m intervals from the first sample location. The samples were air-dried, ground up and sieved with 4 mm mesh. Each sample was immediately placed in a leached polythene bag and tightly sealed. The pH and organic matter content of soil samples were immediately determined using standard analytical procedures as prescribed by AOAC (12).