A recyclable adsorbent for salinized groundwater: Dual-adsorbent desalination and potassium-exchanged zeolite production

This study focused on advancing the dual-adsorbent desalination technique that sequentially combines calcined layered double hydroxide (CLDH) and acid-treated zeolites (H-form zeolite) using groundwater spiked with potash mining effluent (brine). In sequential batch experiments, the CLDH adsorbent first reduced the high Cl− concentration (4600 mg/L) of saline groundwater by 96%, the Ca2+ by 90%, and the Mg2+ by 92%, while transiently raising the pH to 12.80. H-form zeolites preconditioned with Na+ then removed 92% of the Na+ (2010 mg/L), while neutralizing the adsorption effluent pH to 7.7 and lowering the sodium adsorption ratio (SAR; 139.6 to 6.6) and the hardness (574 to 48.4 mg/L). In comparison, an equivalent amount of unmodified zeolite removed only 51% of the Na+ and generated extremely hard water due to Ca2+ and Mg2+ release (1519 mg/L). Na+-conditioning the zeolites prior to acid treatment enhances native cation removal, forming H-form zeolites. Synchrotron-based X-ray scanning transmission microscopy (STXM) showed the occurrence of dealumination and visualized the sodium distribution associated with Si and Al sites in the H-form zeolites. Four consecutive desalination cycles were feasible for Na+ and K+ adsorption by regenerating the H-form zeolite. During regeneration, the Na+ desorbed while the K+ remained in the regenerated zeolites. Cumulative K+ loading in the regenerated zeolites increased from 4.8 to 21.2 mg/g, producing K-form zeolites. These K-form zeolites released K+ (2.15 mg/L for 24 h) in a leaching test and could potentially be considered as nutrient-supply media in other applications, thereby recycling the spent zeolites after multiple desalination treatments.