Use of ArsenX np, a hybrid anion exchanger, for arsenic removal in remote villages in the Indian subcontinent

Abstract

Many of the arsenic removal units operating in remote villages of West Bengal, India now use a hybrid anion exchanger (HAIX) which are essentially spherical anion exchange resin beads containing dispersed nanoparticles of hydrated ferric oxide (HFO). HAIX, now commercially available as ArsenX np, offers a very high selectivity for sorption of oxyanions of arsenic due to the Donnan membrane effect. The sorption columns used in the field for removal of arsenic are either single column or split-column design. The sorption columns allow flow of atmospheric oxygen, thereby promoting oxidation of dissolved Fe(II) species of arsenic-contaminated raw water to insoluble Fe(III) oxides or HFO particulates. Apart from the usual role played by the sorbents like ArsenX np or activated alumina towards arsenic removal, HFO particulates also aid in the treatment process. Each unit is attached to a hand-pump driven well and capable of providing arsenic-safe water to three hundred (300) households or approximately one thousand villagers. No chemical addition, pH adjustment or electricity is required to run these units. On average, every unit runs for more than 20,000 bed volumes before a breakthrough of 50 μg/L of arsenic, the maximum contaminant level in drinking water in India, is reached. In addition to arsenic removal, significant iron removal is also achieved throughout the run. Upon exhaustion, the media is withdrawn and taken to a central regeneration facility where 2% NaCl and 2% NaOH solution are used for regeneration. Subsequently, the regenerated resin is reloaded into the well-head sorption column. Following regeneration, the spent solutions, containing high arsenic concentration, are transformed into solids residuals and contained in a way to avoid any significant arsenic leaching. Laboratory investigations confirmed that the regenerated ArsenX np is amenable to reuse for multiple cycles without any significant loss in capacity.