Adsorption and desorption of phosphate on Fe2O3: effect of fulvic acid and pH

Environmental context. Adsorption controls the mobility of chemical species like nutrients in the soil and water environments and forms the basis for some of the methods of treating contaminated waters. Nutrients are introduced into environments where there are large quantities of humic substances adsorbed onto mineral oxides in the soil and sediments but no work has specifically focussed on the effect of adsorbed and free humic substances on the mobility of nutrients, and their adsorption and desorption mechanism on surface sites covered with adsorbed humic substances is still obscure. We believe that our findings and the proposed adsorption mechanism will help advance the technologies and methodologies for the removal of nutrients from wastewater. Abstract. The main objective of the present study was to evaluate the effect of adsorbed fulvic acid (FA) on the mobility of phosphate (Pi) using Fe2O3 as the soil or sediment model. The study showed that adsorbed FA increased the amount of adsorbed Pi as a function of time and pH without it being remobilised. We envisage that Pi adsorbs onto both residual cationic surface sites and onto the adsorbed FA and that additional Pi loads onto the adsorption surface as FA–Pi precipitate. Addition of FA to Fe2O3 with adsorbed Pi resulted in the remobilisation of Pi as FA got adsorbed at pH 5.30 and 6.30. At pH 8.30, FA adsorbed without remobilising Pi. FA is a stronger base than the Pi species at pH 5.30 and 6.30 and should adsorb more strongly. At pH 8.30, FA is thought to bind onto adsorbed Pi. The experimental isotherm data could not be completely fitted into the Freundlich equation. This could be due to the complex adsorption mechanism.