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From a sustainability viewpoint, sodium exchange softening, although used widely, is under scrutiny due to its production of excess Na-laden spent regenerant and subsequent discharge to the environment. Many arid regions are introducing regulations disallowing dumping of concentrated sodium salts, the residuals from popular Na-exchange softening. The sodium content of the softened water is, also, always higher than in the feed, which poses a dietary health concern when used for drinking or cooking. An efficient, easy-to-operate hardness removal process with reduced sodium in both the treated water and in the spent regenerant is an unmet global need. Use of a cation exchange resin in Al-form for hardness removal, that is, exchange of divalent Ca or Mg with trivalent Al, is counterintuitive, and this is particularly so, because the aluminum ion to be exchanged has higher affinity than calcium. Nevertheless, ion exchange accompanied by precipitation of aluminum hydroxide allows progress of the cation exchange reaction leading to hardness removal. Experimental results demonstrated that calcium can be consistently removed for multiple cycles using a stoichiometric amount of AlCl as the regenerant. The process essentially operates at the maximum possible thermodynamic efficiency: removal of one equivalent of Ca corresponds to use of one equivalent of Al as a regenerant. During the Al-cycle process there is no increase in Na concentration and partial reduction in the total dissolved solids (TDS) of the treated water. It is noteworthy that the ion-exchange resin used, components of the fixed-bed column and operational protocol are nearly the same as traditional softening processes on Na-cycle. Thus, existing Na-cycle systems can be retrofitted into Al-cycle operation without major difficulty.

Metal oxide nanoparticles like hydrated ferric oxide (HFO) or hydrated zirconium oxide (HZrO) are excellent sorbents for environmentally significant ligands like phosphate, arsenic, or fluoride, present at trace concentrations. Since the sorption capacity is surface dependent for HFO and HZrO, nanoscale sizes offer significant enhancement in performance. However, due to their miniscule sizes, low attrition resistance, and poor durability they are unable to be used in typical plug-flow column setups. Meanwhile ion exchange resins, which have no specific affinity toward anionic ligands, are durable and chemically stable. By impregnating metal oxide nanoparticles inside a polymer support, with or without functional groups, a hybrid nanosorbent material (HNM) can be prepared. A HNM is durable, mechanically strong, and chemically stable. The functional groups of the polymeric support will affect the overall removal efficiency of the ligands exerted by the Donnan Membrane Effect. For example, the removal of arsenic by HFO or the removal of fluoride by HZrO is enhanced by using anion exchange resins. The HNM can be precisely tuned to remove one type of contaminant over another type. Also, the physical morphology of the support material, spherical bead versus ion exchange fiber, has a significant effect on kinetics of sorption and desorption. HNMs also possess dual sorption sites and are capable of removing multiple contaminants, namely, arsenate and perchlorate, concurrently.

The advent of horizontal hydraulic fracturing and its implementation for unconventional shale gas development in the Marcellus Shale region in the northeastern United States have created a uniquely challenging wastewater stream, termed produced (or flowback) water. The salinity or the total dissolved solids (TDS) of this wastewater stream often exceeds that of seawater by 3−4-fold (i.e., >150000 mg/L), and in addition, the produced water stream also contains very high concentrations of dissolved barium and strontium and elevated levels of radioactivity due to the presence of radium ( 228 Ra and 226 Ra). Both deep well injection and evaporative crystallization with waste heat are major avenues for produced water disposal. However, the presence of radioactivity and a high concentration of barium render the disposal unacceptable from an environmental impact viewpoint. In the same region, naturally produced acid mine drainage (AMD) is pervasive and this impaired water stream is relatively high in sulfate. Previous studies used AMD to precipitate divalent cations in produced water as insoluble sulfates. Due to dilution, however, the disposable volume of the treated water is greatly increased, thus causing transport to be more involved and expensive. In this study, we present a novel selfregenerating anion exchange process for treating the produced water using AMD but without any increase in the treated water volume. No other chemical is needed in the process, and most importantly, the efficiency of the proposed process is not influenced by the sulfate content of the AMD. During our laboratory investigation, nearly complete removal of radioactivity and >95% removal of barium and strontium were achieved at a Marcellus site in Pennsylvania using only a local source of AMD water.