Two novel applications of ion exchange fibers: Arsenic removal and chemical‐free softening of hard water

Abstract

Separation processes using ion exchange materials are ubiquitous regarding the selective removal of contaminants of environmental concern. Recently, highly regulated contaminants, such as arsenic, have gained notoriety on account of carcinogenic effects and subsequent promulgation of new drinking water standards by the United States Environmental Protection Agency. Other nonregulated compounds such as hardness (e.g., Ca2+, Mg2+), pose a particular concern to avoid scaling in heat‐transfer equipment, foulingz in membranes, and high consumption of detergents and sequestering chemicals in cooling and wash water. In general, ion exchange processes used to remove hardness generate concentrated brine or mineral acid as a waste regenerant stream. Residuals management and long‐term sustainability issues will continue to be major concerns with these processes. The subject paper reports and discusses the results and attributes of two new separation processes using ion exchange fibers (IX‐fibers). The first process involves the selective removal of arsenic using hybridized IX‐fibers that contain dispersed hydrated ferric oxide (HFO) nanoparticles. Anion‐exchanger‐supported HFO particles offered a high arsenic removal capacity; less than 10% of influent arsenic broke through after 30,000 bed volumes. Hybrid fibers were also amenable to efficient regeneration with 2% NaOH and 2% NaCl and capable of simultaneous removal of both arsenic(V) and arsenic(III). The second process involves the environmentally benign removal of hardness. Most importantly, this process uses harvested snowmelt (or rainwater) sparged with carbon dioxide as the regenerant. Consequently, the spent regenerant does not contain a high concentration of aggressive chemicals such as sodium chloride or acid like traditional ion‐exchange processes nor does the process produce voluminous sludges as does lime softening. The bulk of carbon dioxide consumed during regeneration remains sequestered in the aqueous phase as alkalinity. For both treatment strategies, IX‐fibers form the heart of the process. They are essentially thin cylindrical polymeric strands 10–20 μm in diameter with functional groups residing near to the fiber surface. Low intraparticle diffusional resistance is the underlying reason why calcium loaded weak acid IX‐fibers are amenable to efficient regeneration using either snowmelt or rainwater sparged with carbon dioxide. Additionally, for hybrid IX‐fibers, enhanced adsorption kinetics may offer the possibility for certain point of use applications, which are unavailable to their resin counterparts. © 2006 American Institute of Chemical Engineers Environ Prog, 2006

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