Iron and Aluminium Oxide Mineralogy of Highly Weathered Nigerian Soils and Their Control on Phosphorus and Heavy Metal Dynamics: An Original Mineralogical-Geochemical Framework

This dissertation develops an original mineralogical-geochemical framework that quantitatively links iron and aluminium oxide mineralogy in highly weathered Nigerian soils to phosphorus sorption capacity and heavy metal binding dynamics, providing the mechanistic foundation for predictive geochemical modelling of nutrient and contaminant behaviour in tropical African soils. Nigerian soils in the humid and sub-humid zones are dominated by pedogenic iron and aluminium oxides as the primary mineral reactive surfaces, and their crystallinity, surface area, and surface charge characteristics fundamentally control both nutrient availability and contaminant mobility. This study characterises iron and aluminium oxide mineralogy using X-ray diffraction, selective chemical dissolution (dithionite-citrate-bicarbonate, ammonium oxalate, and sodium pyrophosphate extractions), Mossbauer spectroscopy for iron oxidation state, and BET surface area analysis in 80 representative profiles from the Humid forest (South South), Derived savanna (South West), and Guinea savanna (North Central) zones. Phosphorus sorption and desorption isotherms and heavy metal (Pb, Cd, As, Cr) adsorption envelopes are measured and related to oxide mineralogy parameters. The dissertation proposes the Nigerian Oxide-Controlled Geochemistry Framework (NOCGF) as an original contribution, specifying quantitative relationships between oxide crystallinity indices, surface area, and reactive site density with measured phosphorus sorption maxima and heavy metal distribution coefficients. The NOCGF is implemented as a spreadsheet-based geochemical prediction tool validated against 40 independent soil-test datasets. Keywords: iron oxides, aluminium oxides, soil mineralogy, phosphorus sorption, heavy metal binding.