Toward Sustainable Water Treatment: Use of Biomaterials in Water Purification.

摘要

Climate change, demographic and land-use changes, urbanization, and consumption are interconnected global stressors that affect the quality, quantity, and availability of water. Demand for clean water continues to grow rapidly. As we seek to develop sustainable water systems, we must also consider the life cycle of materials these systems depend upon. Understanding how renewable, non-toxic, biodegradable materials can be used to provide safe, high quality water will enhance the sustainability of water treatment systems, making them more versatile and robust.;Over 120 million people in India and Bangladesh are exposed to toxic levels of arsenic in their drinking water because of a geogenic arsenic contamination of the groundwater in this region. As community scale infrastructure for water treatment is not likely in the near future, point-of-use technologies, most relying on adsorption, have been advocated for arsenic removal. Despite promising lab results for many of these technologies, none has adequately demonstrated long-term effectiveness and sustainability in the field.;Arsenic in groundwater primarily exists as arsenite (As(III)) and arsenate (As(V)). Arsenite is up to 60 times more toxic than arsenate. Because arsenite is uncharged at environmentally relevant pH, it is also more difficult to remove than arsenate, which is negatively charged at environmentally relevant pH. Arsenic removal technologies that rely on ion exchange require a pre-oxidation step, converting all arsenic to arsenate, to achieve total arsenic removal.;This research has pursued targeted design of biomaterials for arsenic sorption, incorporating green design principles throughout. The result is a novel arsenic sorbent with unique composition and functionality -- TiO2-impregnated chitosan bead (TICB). TICB is composed of TiO 2, a nanopowder ubiquitous in consumer products, and chitosan, a renewable, biodegradable biopolymer. Chitosan is the deacetylated form of chitin, the second most abundant biopolymer in the biosphere, and is produced as a waste byproduct in the shellfish processing industry. Chitosan is an attractive material for water treatment because, in aqueous solution, it forms hydrogels that can interact with dissolved materials, including arsenic, without dissolution.;TICB can perform simultaneous sorption and oxidation and has demonstrated the ability to 1) remove arsenate, 2) remove arsenite, and 3) oxidize arsenite to arsenate in the presence of UV light, including sunlight. The removal capacity of TICB is equivalent to the removal achieved by the mass of TiO2 nanopowder impregnated in the chitosan matrix, regardless of bead diameter. However, unlike TiO2 nanopowder, TICB self-separates according to density, obviating the need for post-treatment filtration, and resulting in energy and cost savings.;The mechanism of TICB sorption has been characterized, whereby the TiO 2 within the bead forms bidentate, binuclear chemical complexes with the arsenic oxyanions. A model to predict TICB capacity, based on TiO 2 loading and solution pH, is presented for arsenite, arsenate, and total arsenic in the presence of UV light. The rate of removal is increased with reductions in bead size and with exposure to UV light. Phosphate is found to be a direct competitor with arsenate for adsorption sites on TICB, but other relevant common background groundwater ions do not compete with arsenate for adsorption sites. TICB can be regenerated with weak NaOH and maintain full adsorption capacity for at least three adsorption/desorption cycles.