The reactivity of soil organic fractions towards Cadmium, Calcium, Copper and Zinc

In this project, the physico-chemical characteristics and metal binding for various soil types from a mining area were investigated. Humic substances were extracted from the soil samples using various standard procedures. The physical properties viz: pH, conductivity, particle size distribution, total nitrogen, organic matter, moisture content and exchangeable acidity were determined on the undigested soil samples and potassium, phosphorus, iron, calcium, cadmium, lead and zinc were determined after acid digestion. The mine tailings exhibited some characteristics similar to samples from Bibiani forest (cocoa plantation) e.g.: Organic Matter (New Tailings-0.83 ± 0.05%; Old Tailings-0.53 ± 0.01%; Cocoa Plantation-1.93 ± 0.04%); and Carbon-Nitrogen ratio (New Tailings-12 ± 0.16%; Old Tailings-10.30 ±0.35%; Cocoa Plantation-11.20 ± 0.12%) were similar. The amount of humic acids extracted ranged from 0.09 ± 0.03 mg/L to 0.49 ± 0.01 mg/L, whilst the concentration of orange fulvic acid ranged from 0.11 ± 0.02 mg/L to 0.15 ± 0.09 mg/L. A study of complexation of humic acids by trace metals (Cd, Zn, Ca, and Cu) revealed that metal-humic substances association depends on the metal, the nature of the humic substance and concentration. Calcium was complexed to a higher extent than copper, followed by cadmium and zinc. Complexation of Old Tailings humic substances was stronger than that of Russel Pit humic substances. The relationship between stability constants and pH was discussed in this paper.