Waste red mud and natural pumice/volcanic slag particles were surface modified and their selenium adsorption from waters was investigated. Acid activation/heat treatment of original red mud (ORM) particles significantly increased their micropore and external surface area and cumulative volume of pores. Iron oxide coating of pumice/ slags and acid activation of ORM decreased their pH pzc values and increased surface acidity. Selenite/selenate adsorption on iron oxide surfaces and acid activated red mud (AARM) was very fast with approximately first-order adsorption kinetics. Iron oxide coating of pumice/slag and acid activation of ORM particles significantly enhanced their selenite and selenate uptakes. Maximum Se adsorption capacities as high as 6.3 (mg Se/g adsorbent) were obtained by AARM. The extent of selenate uptakes by the surface modified particles was generally lower than those of selenite. Due to competition among Se species and other background water matrix for iron oxide adsorption sites, reduced selenite/selenate uptakes were found in natural water compared to single solute tests. Higher Se uptakes by iron oxide surfaces were found at pH 7.5 compared to pH 8.9, due to increased electrostatic repulsion among iron oxides and Se species at higher pH. The most effective adsorbents among the tested 17 different particles for Se uptake were AARM and iron oxide coated pumice. Se concentrations less than drinking water standards (5-10 mg/L) can be achieved by these particles. These low-cost, natural, or recyclable waste particles appear to be promising adsorbents for Se removal after their surface modification.Abbreviations: AARM, acid activated red mud; BET, Brunauer-Emmett-Teller; DDW, distilled and deionized water; DOC, dissolved organic carbon; IC, iron oxide coated; ICP/MS, inductively coupled plasma/MS; Isp, Isparta pumice; Kula, Kula-Salihli volcanic slag; NOM, natural organic matter; ORM, original red mud; pH pzc , pH of point of zero charge; SEM, scanning electron microscope; SEM-EDX, scanning electron microscope-energy dispersive X-ray spectrometer; SS, steel slag; XRF, X-ray fluorescence Clean -Soil, Air, Water 2012, 40 (7), 735-745 735
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