Sorption of Sulfate on Iron Oxide Minerals: Hematite and Goethite

Abstract

The influence of sulfate on terrestrial and aquatic ecosystems depends on the mobility of the sulfate anion in soils. This mobility is determined by several factors, one being the types and amounts of soil constituents. In this study, several iron oxide/hydroxide minerals were evaluated for sulfate sorption characteristics.Hematite and goethite were synthesized and positively identified using x-ray diffraction, mossbauer spectroscopy and scanning electron microscopy. Mineral surfaces were characterized using surface area and zero point of charge measurements, infrared spectroscopy and thermal analyses. Neutron activation and x-ray fluorescence were used to look for impurities. Samples were compared to a natural hematite sample and a synthetic jarosite. Sorption experiments, conducted on mineral suspensions in KNO3 media at room temperature, considered the variables time, ionic strength, solid:solution ratio, pH and sulfate concentration. Sorption was initiated by a fast reaction, followed by a longer, slower one which reached an apparent equilibrium in 24 hours. Sorption was unaffected by sol id:solution ratio and decreased with ionic strength at pH 5 for goethite only. Sorption increased with increasing sulfate concentration and decreasing pH. A sorption maximum was reached by a I 1 minerals except synthetic hematite. Under optimum pH and [SO4]. approximately half of the mineral surface is covered by su1 fate ions. Sulfate was sorbed irreversibly. Only a fraction of sorbed sulfate can be desorbed, an amount which increases with pH. Thermal analyses indicate sulfate to be strongly bonded. The presence of four infrared bands on sulfate treated surfaces indicate direct coordination of the anion to the iron cation. The above evidence, including irreversibility of sorption, supports inner sphere complexing of sulfate. Sulfate sorption on iron oxide/hydroxide minerals is thus a combination of nonspecific electrostatic attraction and mono - multi ligand exchange (including binuclear bridging) which act under different system conditions to form the basis of sulfate sorption behavior. The present observations are important in model ling of environmental systems, such as in the Direct Delayed Response Program Model, due to the significance of irreversibility of sulfate sorption on model assumptions.