Development of Hybrid Polymeric/ Inorganic Ion Exchanger: Preparation, Characterization, and Environmental Applications

Abstract The study involves the development of two environmental applications using hybrid polymeric/ inorganic ion exchangers. First, the hybrid polymeric ion exchangers supported hydrated zirconium oxide (HZO) nanoparticles, referred to as HIX-Zr were developed for selective removal of heavy metals and anionic ligands. Second, salt-free water softening processes were presented using three different heterogeneous cation exchanger resins along with novel regeneration schemes. The HIX-Zr nanosorbents have been prepared, characterized, and extensively studied in relation to heavy metal (i.e., zinc) and anionic ligand (i.e., arsenate and fluoride) removal in fixed-bed processes at trace concentrations with the presence of high concentrations of innocuous competing ions. The HIX-Zr adsorbents are essentially nanoparticles of HZO irreversibly dispersed onto the polymeric phase of either anion exchangers containing quaternary ammonium functional (R4N+) groups or cation exchangers containing sulfonate (SO3-) functional groups which are referred to as HAIX-Zr and HCIX-Zr, respectively. The new class of hybrid nanosorbents provides a synergy unattainable separately by either inorganic metal oxide nanoparticles or polymeric ion exchangers alone. HZO particles have long been known for their high chemical stability under varying conditions of pH and redox and exhibit amphoteric sorption properties near neutral pH. Besides providing high durability, the Donnan membrane effect from the polymeric ion exchanger plays an important role to enhance the permeation of target contaminants. HIX-Zr nano-adsorbents were characterized by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/ EDX), high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD). The HZO nanoparticles were uniformly distributed throughout the polymeric ion exchanger phases at an approximately 12% (w/w) with sizes well below 50 nm, while the material's high surface area from the amorphous structure of HZO still remains even after 5 cycles of sorption-desorption as confirmed by XRD. From the equilibrium batch isothermal test, the arsenic and fluoride sorption behaviors follow the Langmuir isotherm with the maximum sorption capacity of 20 mg As(V)/g at pH 7 and 35 mg F/g at pH 5, respectively. The sorption capacity of the HAIX-Zr for both arsenic and fluoride is three times higher than the most commonly used activated alumina (AA). Kinetic studies on arsenate and fluoride adsorption onto the HAIX-Zr confirmed that intraparticle diffusion was the rate limiting step. The HAIX-Zr nanosorbents are amendable to efficient regeneration with more than 90% recovery within 15 bed volumes and can be reused for many cycles of sorption-desorption. The regenerable nature of HAIX-Zr reduces the volume of disposable waste more than 100-fold versus the commercially available granulated metal oxide adsorbents. Due to the high chemical stability of HZO nanoparticles, the HAIX-Zr can be disposed of safely in a landfill without risk of toxic leaching. In general water softening processes, lime soda and ion exchange are the most widely used techniques for removal of hardness (e.g., Ca2+, Mg2+, etc.) from hard water, however, these technologies generate voluminous sludge and concentrated brine/mineral acid as waste stream, respectively. Residual management and long-term sustainability issues will continue to be major concerns with these processes. In this study, three different salt-free water softening processes using different cation exchangers and regeneration schemes are developed. First, the shallow shell technology (SST) resins with dry-ice regeneration; second, a weak acid cation exchanger (WAC) with biodegradable organic acid regeneration (i.e., diluted acetic acid); and third, a strong acid cation exchanger in the aluminum form (SAC-Al) and stoichiometric of aluminium salt regeneration are evaluated for simultaneous softening of hard water and removal of fluoride at high pH. For the commercially available SST resins, the process takes advantage of the shorter diffusion path length due to the inert core of the resin similar to the ion exchange fibers used previously and the high preference of hydrogen ions from the weak acid cation (WAC) exchanger. From the experimental hardness removal column runs and solid CO2 (dry ice) regeneration study, we found that the solid CO2 sparged in DI water was not effective for desorption of hardness (i.e., Ca2+) from the SST resins as expected. Although, the solid CO2 (dry ice) is available, there are some difficulties to control the flow rate of CO2 dissolved in water as regenerant solution at high pressure including the CO2 gas in solution tend to disturb the resin bed. For the second salt-free water softening scheme, we decided to use the traditional spherical weak acid cation (WAC) exchange resins which have high affinity toward hydrogen ions and using the diluted biodegradable organic acid such as 2% acetic acid instead of aggressive 5% inorganic HCl acid as hydrogen source as a regenerant. From the hardness regeneration studies, we found that the calcium recoveries as high as 98% were achieved with only 10 bed volumes by using stoichiometric amounts of dilute acetic acid. The novel water softening process by using WAC resins and stoichiometric amounts of dilute biodegradable organic acids (i.e., acetic acid) results in two main attributes; first the WAC have higher capacity than the traditional SAC resins, and the second benefit is that the waste acid generated from the process is much less (near stoichiometric efficiency) than the brine solution (only 30% efficiency). Moreover the organic acids are biodegradable while the brine solution is permanently present in the environment. The third salt-free water softening process uses polymeric cation exchangers pre-loaded with aluminum (SAC-Al) or other polyvalent cations (i.e., SAC-Fe). The process uses close to stoichiometric amounts of aluminum salts (i.e., Al2(SO4)3, AlCl3 ) for regeneration and significantly less volume of waste brine is generated compared to traditional brine regeneration for strong acid cation exchange processes. Since no NaCl is added during regeneration, sodium is virtually absent in the disposable waste regenerant. The spent regenerant essentially contains only salts of hardness (e.g. Ca2+, Mg2+) removed during the regeneration cycle. Also, no mineral acid is needed for regeneration. Along with hardness, the process also removes fluoride when the bed is initially in the Al3+ form or contains precipitated aluminum (hydr) oxide.