Efficiency and life cycle environmental impacts of ion-exchange regeneration using sodium, potassium, chloride, and bicarbonate salts

Abstract

The goal of this research was to investigate the efficiency and life cycle environmental impacts of ion exchange (IX) regeneration using conventional NaCl and the alternative salts KCl, NaHCO3, and KHCO3. The regeneration efficiency was evaluated by considering anion exchange resin (AER) exhausted with nitrate and cation exchange resin (CER) exhausted with calcium. The regeneration efficiency of chloride was higher than bicarbonate for polystyrene AER, while the regeneration efficiency of bicarbonate was higher than chloride for polyacrylic AER. The regeneration efficiency of potassium was higher than sodium for all CERs. Because all salts showed high regeneration efficiency, assessing the environmental impacts of the salts from upstream and downstream phases was deemed key to improving the sustainability of the IX unit process. The increasing environmental burden of the salts was NaCl < KCl < KHCO3, while the impacts of NaHCO3 were greater or less than KCl depending on the impact category. The environmental burden was based on a comparative life cycle assessment (LCA) of mining and processing, and did not include disposal of the waste regeneration solution. A literature study regarding the environmental impacts of the regeneration ions indicated the potential benefit of the alternative salts over NaCl was due to disposal applications. For instance, disposal of KCl waste regeneration solution to soils can offset the demand for potassium fertilizer inputs and is not harmful to soil structure, and disposal of KHCO3 and NaHCO3 waste regeneration solution can offset the chemicals needed during wastewater treatment when nitrification is limited by low alkalinity.