Selenium is an essential micronutrient for humans and animals. However, it is also one of the
most challenging trace elements because of its narrow gap between toxic (400 µg/day) and beneficial concentration (40 µg/day). Selenium is involved in both environmentally and biologically contaminated systems, particularly, selenium oxyanions (selenite (SeIV) and selenate (SeVI)). In drinking water, the EU established a concentration of 10 μg/L as the standard level of selenium.
Many techniques (e.g. reverse osmosis, nanofiltration, algal-bacterial) have been investigated to remove selenium from water. However, they are limited either by deficits in efficiency or by cost issues. Thus, adsorbent techniques remain the most promising method for water treatment due to their simple design and cost-effectiveness. So far, various adsorbents have been developed which include activated alumina, activated carbon and iron oxyhydroxides. Activated carbon is well known to be a good adsorbent for the removal of heavy metal cations, albeit, it is completely ineffective to remove both selenate and selenite oxyanions. Aluminum oxide has been reported to be effective for selenite removal only. Recently, schwertmannite, a ferric oxyhydroxide sulfate (Fe8O8(OH)6SO4) that is biogenically produced upon treatment of Fe(II) rich groundwater in opencast lignite mines, has been applied with success to remove the oxyanions chromate and arsenate (Regenspurg and Peiffer, 2005; Paikaray et al., 2011) . Therefore, schwertmannite is a suitable candidate to control the mobility of selenium oxyanions and to reduce their concentration in contaminated water.
The main purpose of NARS was to develop new ecological, low-cost and efficient filter materials based on schwertmannite to remove both selenite and selenate. The project aimed to fill the crucial knowledge gap that exists in understanding selenium oxyanions adsorption-desorption processes on schwertmannite surface sites, and that can be summarized by three key research objectives: i) determine and optimise the baseline process parameters needed to study the affinity of Se(IV) and Se(VI) to schwertmannite adsorbents, ii) study the efficiency of retention of Se species in column tests and, iii) investigate the regeneration and reuse of schwertmannite.
