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Novel ecological adsorbent using Schwertmannite material for Removal of Selenate and Selenite from contaminated water

Periodic Reporting for period 1 - NARS (Novel ecological adsorbent using Schwertmannite material for Removal of Selenate and Selenite from contaminated water)

Reporting period: 2016-05-01 to 2018-04-30

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.
To achieve the overall objective of NARS, the project provided an interdisciplinary approach combining hydrogeochemistry and technical development that takes on consideration the physicochemical characterisation of schwertmannite, the adsorption kinetics and the spectroscopic analyses. In a first step, we have performed batch experiments regarding the adsorption of selenite and selenate on schwertmannite. The ability to predict the fate and transport of selenium in aquatic environments requires an understanding of the sorption processes that occur at the mineral-water interface. The mechanism of adsorption on contaminant mobility is mainly impacted by three factors which are pH, ionic strength and temperature. The concentration of selenium, sulphate and iron was analysed, and the relation between these elements and the factors was defined. In the next step the mechanism of adsorption of both selenite and selenate on schwertmannite was identified and the stability of the redox states of the two anions during the adsorption process was determined. For this purpose, a combination of synchrotron methods at different resolution, mainly, EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure) have been applied. The studies were performed at the Canadian Light Source – Saskatoon. The obtained results helped us to learn about the bond lengths and the type of the chemical bonding between selenite, iron and oxygen, and between selenate, sulphate, iron and oxygen. To explore the technical aspects of selenium water contamination, a secondment phase by GEOS Ingenieurgesellschaft mbH has been completed. During that period, column experiments have been conducted to assess the fate and transport of selenite in the presence of competitive anions as well as the re-use and the regeneration of schwertmannite materials.
The main results are:
The adsorption kinetic studies of selenite and selenate on schwertmannite show that selenite has a stronger affinity and quicker adsorption to schwertmannite at acidic pH. Synchrotron results indicate that time process has impact to the partitioning between the absorbed oxyanions and the sorbent.
Contamination by the toxic element selenium has become an increasing threat to aquatic environments and human health. To remove selenium oxyanions from water, schwertmannite adsorbents are advantageous, due to their availability and their promising adsorption behaviour towards other oxyanions. Relevant results have been found in kinetic mechanism and sorption rate of selenite and selenate into schwertmannite. Moreover, needed information regarding the long-term loading capacity of schwertmannite has been gained. As a waste product of mine drainage treatment, schwertmannite could be used as sustainable and eco-friendly resource for an innovative, clean and economically efficient selenium water clean-up technology.
Understanding the sorption mechanisms of Se oxoanions with schwertmannite