Kinetics and energetics of phosphate sorption in a multi-component Al(III)-Fe(III) hydr(oxide) sorbent system.

Multi-component Al-Fe hydr(oxides) are ubiquituous in soil and aquatic environments, where they exhibit biogeochemical controls on nutrients and contaminants. Although, sorption on single-component Al and Fe hydr(oxides) have been extensively studied, limited studies have been done on their multi-component counterparts. In this study, effects of Al/Fe content on the kinetics and energetics of phosphate sorption in a poorly-crystalline co-precipitated mixed Al-Fe hydr(oxide) system were investigated using a combination of traditional batch techniques and flow adsorption calorimetry. Differences in Al/Fe content was found to influence the structural development and anion exchange capacity of the hydr(oxides) and subsequently their phosphate sorption characteristics. Higher structural development decreased phosphate sorption, while higher AEC was associated with increased phosphate sorption, initial sorption rate, and smaller losses in sorption with increasing pH. Results from flow adsorption calorimetry indicated that at pH 4.8 phosphate sorption: (i) occurred irreversibly on anion exchange sites, with a loss of 1.9 moles of AEC per mole of phosphate sorbed, and (ii) was exothermic, with molar heats of adsorption between -25 and -39 kJmol(-1). Molar heats of adsorption were ten times that for anion exchange and independent of hydr(oxide) composition with the amount of energy evolved being directly proportional to the quantity of phosphate sorbed.