Effects of phosphate on mobility of potentially toxic metals in contaminated soils

Akeem A. Abayomi; Olanrewaju J. Omoniyi; Najeem O. Oladosu; Folake Echebiri; Babajide I. Alo

Akeem A. Abayomi Department of Chemistry, University of Lagos, Akoka, Lagos, Nigeria
Olanrewaju J. Omoniyi Department of Chemistry, University of Lagos, Akoka, Lagos, Nigeria
Najeem O. Oladosu Department of Chemistry, University of Lagos, Akoka, Lagos, Nigeria
Folake Echebiri Department of Chemistry, University of Lagos, Akoka, Lagos, Nigeria
Babajide I. Alo Department of Chemistry, University of Lagos, Akoka, Lagos, Nigeria

Keywords: Orthophosphate, Immobilization, remediation, Potentially Toxic Metals

Abstract

Immobilization of potentially toxic metals (PTMs) from contaminated soils using phosphate is one of the remediation measures for controlling leaching of harmful metals in soil into water resources. In this study, sixteen soil samples (4 each) were collected from selected roadsides, farmlands, dumpsites and wetlands for immobilization potential of phosphate on the PTMs at varied pH and phosphate concentrations. PTM concentrations were determined by Atomic Absorption Spectrophotometry and phosphorus concentrations by UV-Visible spectrophotometry. Bioavailable PTMs and bioavailable phosphorus (BAP) were determined before and after immobilization. While the concentrations of total phosphorus in the samples ranged between 8.73 and 204.08 mg kg^-1 in the order of farmland> dumpsite>roadside> wetland, the concentrations of the PTMs were of the order Zn>Pb>Cr>Cu in roadside soil; Zn>Pb>Cu>Cr in farmland soil; Pb>Cu>Zn>Cr in dumpsites soil and Pb>Zn>Cu>Cr in wetland soil. A decrease in the bioavailable PTM concentration (BAPTM) was observed in all the samples after treatment with dipotassium hydrogen phosphate (K₂HPO₄) at varied concentrations and pH, with Pb being the most immobilized metal. The bioavailable concentrations of Pb and Zn were reduced by more than 80% in all the samples, while the concentrations of bioavailable Cr and Cu were reduced by more than 50% and 70%, respectively. The results showed that immobilization of PTMs using phosphate is an effective strategy for controlling the transfer of harmful metals in contaminated soil into proximate water systems.