Abstract
Elevated salinity levels in wastewater usually have negative consequences to soil quality and crop growth if used directly in irrigation without proper treatment. Though many techniques have been developed to solve the problem, high capital costs or energy consumption have hindered their application in wider regions. One economical solution of reusing the wastewater of high salinity is applying natural or synthetic zeolites as ion exchanger and adsorbent. Chemically modified natural zeolites or synthetic zeolites have been utilized to reduce Na+ in various kinds of saline/sodic waters. In this paper, we reviewed the recent technical improvements of using natural or modified zeolites in the field of water salinity reduction. The mechanisms governing the salt removal process by zeolites are mainly ion exchange, adsorption, and salt storage. Factors such as zeolite's geochemical properties, pH, co-existing anions, concentration, valency, surface charge, and experimental conditions all influence the ion exchange process. Adsorption isotherm of Na+ ions on zeolites, mostly reported to follow either Langmuir or Freundlich isotherm, has a composition-dependent behaviour. The adsorption kinetics of Na+ on zeolites is mostly a pseudo-second-order type with an exothermic nature. Sodium removal by zeolites appears to be an effective water treatment technology for maximizing the beneficial use of poor-quality saline/sodic wastewater. However, challenges still remain and further work is required in areas of lowering operational cost and improving zeolite's regenerability. To overcome these challenges, researchers could make more efforts in technical improvements, including alterable surface properties and the incorporation of other approaches to achieve better salt removal outcome.
•Few review articles focus on zeolites for salinity/sodicity reduction in waters.•Mechanisms, influencing factors, challenges and opportunities are discussed.•Zeolites present great potential for salinity reduction in wastewater.