Investigating phosphate-adsorption behaviour on a real ferrallitic-ferritic soil using a pluralistic approach under non-controlled conditions

Abstract

Context: Ferralsols, which cover approximately 6% of the Earth's continental surface, have unique phosphorus (P) retention properties. Aims:The purpose of the research is to investigate P adsorption properties under noncontrolled conditions using lateritic soil samples, using wet chemical experiments, microscopic, and infrared spectroscopic methods.Methods: Ferralsol samples were analyzed using: 1) kinetic and adsorption isotherms (wet chemical experiment methods), 2) electron microscopy (SEM, TEM) and 3) infrared spectroscopy (ATR-FTIR). Key results:Wet chemical experiments accord with previous studies on lateritic soils where chemisorption mechanisms govern P adsorption. Further, P adsorption itself affect soil Investigating Phosphate Adsorption Mechanisms 2 particles' crystal structure by reductive dissolution of the Fe oxyhydroxide lattice. SEM investigation revealed the location of phosphate-surface complexes in iron-rich areas on soil particles. Infrared spectroscopy (ATR-FTIR) indicated the formation of two inner-sphere complexes: monodentate (FeO)PO2(OH) and bidentate (FeO)2PO(OH) at wavenumber positions 958±5, 1042±5 and 1095±8 cm -1 and 930±5, 983±10, 1005±5 and 1122±9 cm -1 , respectively. Additionally, for the bidentate complex, a band centred at 1030±4 was identified for P concentrations above 500 mgP/L, potentially indicative of a ternary complex. Combined methods suggested the potential involvement of redox mechanisms and other ionic species on the formation and types of P surface complexes.Conclusions: Our approach builds on previous work in this field by showing evidence of complex ionic interactions governing P retention on lateritic soils. Novel insights are evidence of fluctuations in physical and chemical factors with phosphate concentration in solution and adsorption, and suggestion of inner-sphere and ternary surface complexation mechanisms.Implications: Given the wide global distribution of lateritic Ferralsols, our findings have important implications for key emerging challenges relating to P cycling for crop production and environmental impact.