Impacts of chloride-form anion exchange seawater regeneration performance

Seawater was investigated as an alternative regenerant source to conventional salt-imported brine solutions in an anion exchange process treating surficial Florida coastal groundwater for the removal of sulfate and organics. Bench-scale column testing revealed that filtered Sarasota Bay seawater efficiently regenerated the anion resin media; however, sulfate exchange capacity decreased by 8.42% compared with conventional 10% salt regeneration methods. Addition of 3% sodium chloride increased regeneration efficiency, reduced exchange capacity losses to 2.4% as compared to conventional 10% salt regeneration methods. Regeneration resulted in 2.13 mg/L of bromide leakage; however, addition of 3% sodium chloride to seawater reduced bromide leakage to 1.25 mg/L. A correlation between bromide exchange and the regenerant chloride-to-bromide molar ratio (CBMR) was observed, yielding less bromide exchange at higher CBMRs. Bromide adsorption followed pseudo 2nd order kinetics and chemisorption was the rate controlling step. Increasing the CBMR of the regenerant was found to shift adsorption behaviour, allowing intra-particle diffusion to occur sooner. Bromide equilibrium appeared to follow a logarithmic decay as the CBMR of the regenerant increased. Intra-particle and film diffusion mechanisms were evaluated that indicated the presence of diffusion-based processes and more than one rate controlling step. An empirical function was derived to approximate bromide equilibrium adsorption in relation to a regenerant's CBMR. Seawater as a regenerant when enhanced with sodium chloride shows promise as an anion exchange regenerant; additionally, classification of a seawater regenerant's CBMR can provide insight into the kinetic and equilibrium relationships of bromide exchange.