Rapid adsorption of arsenic from aqueous solution by ferrihydrite-coated sand and granular ferric hydroxide

Abstract

The efficiency of As removal by Fe oxyhydroxide-based adsorbents was studied. Water containing micro-molar concentrations of As was cleaned using columns containing ferrihydrite-coated grains of sand or the commercial material granular ferric hydroxide, GFH®. Adsorption of considerable amounts of arsenate and arsenite were achieved on a sub-minute time scale with both adsorbents. Furthermore, efficient adsorption on GFH® can be achieved in seconds of empty bed contact time. Arsenate adsorption was affected by moderate changes in pH, while such an effect was negligible for arsenite. At slightly acidic pH substantially higher amounts of arsenate could be adsorbed. The Fe content of the coated sand was varied and it was found that more As was adsorbed on grains with a higher Fe content, however, the relationship was far from proportional. This is supported by scanning electron microscopy and energy dispersive spectroscopy, which showed that in addition to the occurrence of Fe in compounds coating the flakes, it also occurred adsorbed to the surface. The Fe oxyhydroxide coating was confirmed to be ferrihydrite using EXAFS spectroscopy. Batch experiments with ferrihydrite support the view that almost all can be adsorbed with a sufficient surplus of Fe oxide/hydroxide mineral suspension.