Development of an Original Mechanistic Model for Heavy Metal Speciation and Mobility in Niger Delta Contaminated Soils Under Variable Redox Conditions

The environmental fate and ecological risk of heavy metals in the petroleum-contaminated soils of the Niger Delta are critically dependent on the redox dynamics of the soil environment, which oscillate dramatically between oxidising and reducing conditions during seasonal flooding and drying cycles. Existing models for metal speciation and mobility in contaminated soils are largely derived from temperate climate contexts and fail to capture the specific physicochemical and biological dynamics of tropical deltaic soils. This study developed an original mechanistic geochemical model describing heavy metal speciation and mobility in Niger Delta contaminated soils across the full redox spectrum from aerobic to methanogenic conditions. The model integrates thermodynamic equilibrium speciation, kinetically controlled metal-sulphide precipitation and dissolution, metal sorption onto iron and manganese hydroxides with pH-dependent surface complexation constants, and organic matter complexation through competitive binding equations. Field sampling campaigns at contaminated and reference sites in Rivers and Bayelsa States provided geochemical boundary data for model calibration across three seasons. Continuous redox potential, pH, and dissolved metal monitoring using in-situ sensors over 18 months provided kinetic validation data. The model successfully reproduced observed metal mobilisation events during wet season flooding, including lead and cadmium remobilisation driven by reductive dissolution of Fe(III) hydroxide carrier phases. Novel aspects of the model include explicit representation of petroleum hydrocarbon co-contaminant effects on microbially mediated redox reactions. This mechanistic framework represents an original theoretical advance in environmental geochemistry and provides a practical predictive tool for designing remediation strategies in the Niger Delta.

Keywords: heavy metal speciation, redox dynamics, Niger Delta, geochemical modelling, contaminated soils