Adsorption of Cadmium, Copper, Lead and Zinc on Organically Amended Soil Fractions Using the Freundlich, Langmuir and Dubinin-Raduskevich Models

Abstract

Background and Objective: High concentration of heavy metals has been discovered in the vegetables in growing area of Kano posing a risk to the soil and its productivity. The feasibility of organic residues such as maize cob and rice husk as adsorbent were evaluated in heavy metal-contaminated soils. The objective of this study was to assess efficacy of using maize cob and rice husk obtained as bio-sorbents in reducing soil solution concentration of heavy metals as well as the influence of contact and residence time on the adsorption of heavy metals: Cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn) by different soil fractions (silt-clay and very fine sand). Materials and Methods: Soil samples were collected from five vegetable gardens in Kano and fractionated into silt-clay and very fine sand fractions. Cu, Pb, Zn in the varying concentration 0, 50, 200 and 400 mg kg–1 and 0, 5, 10 and 50 mg kg–1 Cd were added to the soil along with organic residue. This was incubated for a period of 1 and 7 days. Concentration of the heavy metal after the incubation periods was determined at time intervals of 1, 2, 4 and 8 h in a batch experiment. Data were analyzed using two-way ANOVA. Results: Results obtained showed that sorption increased with time, dosage of the organic residue and incubation. Amongst all the residues, maize cob had better sorption efficiency. The adsorption data obtained were perfectly fitted to the Freundlich adsorption model with an R2 value of 0.99 for Cu, Pb and Zn and the sorption process was found to be a cooperative process between Cu, Pb and Zn. For Cd, the R2 was >0.80 with a favorable and normal adsorption process. Conclusion: Heavy metal sorption by the organic residues at all concentrations was very high. A positive relationship between the rate of adsorption and dosage was obtained indicating that the adsorption process was influenced by increasing the dosage of the residue as well as the shaking period. These amendments which are mostly byproducts of agricultural processing can be used as an excellent source of reducing heavy metal availability in contaminated soils.