Toward Realizing Multifunctionality: Photoactive and Selective Adsorbents for the Removal of Inorganics in Water Treatment.

Persistent and potentially toxic inorganic oxoanions (e.g., arsenic and selenium) are one class of contaminants of concern in drinking water for which treatment technologies must be improved. Effective removal of these oxoanions is made difficult by the varying adsorption affinity of the different oxidation states, as well as the presence of background ions with similar chemical structure and behavior that strongly compete for adsorption sites, greatly reducing removal efficiencies. Recent studies pointing to the negative health effects of inorganic oxoanion contaminants have resulted or are expected to result in new regulations lowering their allowable maximum concentration level (MCL) in drinking water. While these regulations are intended to protect human and environmental health, they must also allow for balanced economic costs. As such, the MCLs are often set at levels that are not as health protective due to high treatment costs that continue to present a significant challenge for small (500-3300 people) to very small (25-500 people) communities. In this Account, we focus on the development of novel cost-effective, sustainable, and efficient multifunctional and selective adsorbents that offer solutions to the above challenges through two platforms: nanoenabled and transition-metal cross-linked chitosan (TMCC) and crystal facet engineered nanometal oxides (NMO). These complementary platforms offer treatment solutions at different scales and flow rates (e.g., in a point-of-use device versus a small-scale community system). Multifunctional adsorbents combine processes that traditionally require multiple steps offering the potential for reducing treatment time and costs. Development of selective adsorbents can greatly increase removal efficiencies of target contaminants by either promoting their adsorption or hindering adsorption of competitive ions. The following sections describe (1) synthesis of novel nanoenabled waste sourced bioadsorbents; (2) development of multifunctional adsorbents to simultaneously photo-oxidize arsenite and adsorb arsenate; (3) development of a selective adsorbent for removal of arsenate and selenite over phosphate; (4) investigations of the conventional wisdom that increased surface area yields increased oxoanion removal using selenium sorption on nanohematite as a case study; and (5) crystal engineering of nanohematite to promote selenite adsorption. The novel technologies developed through these research efforts can serve as templates for the creation of future adsorbents tailored for use targeting other oxoanion contaminants of interest.