Salinity Mitigation for Potash Mine Sites: Synergistic Cation and Anion Removal Using a Dual-Adsorbent

Abstract

Potash (KCl) mining generates large quantities of tailings and brine which can impact proximate aquifers, elevating the concentrations of Na+, K+, and Cl-. To desalinate brine-impacted groundwater near potash mines and other inland locations, two adsorbents were sequentially applied: (1) calcined layered double hydroxide (CLDH), to adsorb anions, divalent cations, and transiently raise the pH; and (2) acid-treated clinoptilolite zeolite, to adsorb monovalent cations and neutralize the effluent pH. To evaluate this dual-adsorbent process, equilibrium adsorption experiments were conducted and the adsorbents were characterized through X-ray diffraction, X-ray florescence, porosimetry, scanning electron microscopy, and synchrotron-based scanning transmission X-ray microscopy.

Using synthetic NaCl solution, the Langmuir maximum adsorption capacity for Cl- onto CLDH and Na+ onto acid-treated zeolite was 116.3 and 28.4 mg/g, respectively. The Na+ uptake was greatly enhanced by solution pre-treatment (dechlorination) using CLDH, and also by zeolite acid treatment (with 1M = 2 M > 0.1 M > untreated, irrespective of acid type). Pre-conditioning the zeolite with Na+ prior to acid treatment also improved the adsorption capacity and promoted crystallinity.

Desalination of potash brine-impacted groundwater was systematically investigated. Under the optimal conditions, the dual-adsorbent reduced the concentration of Cl- by 95.8%, Ca2+ by 89.8%, Mg2+ by 92.3%, Na+ by 91.9%, K+ by 96.5%, preserved a neutral pH (7.72), and lowered the sodium adsorption ratio (36.5 to 12.5) and the hardness (574 to 56.3 mg/L as CaCO3). In contrast, natural zeolite alone only removed 51.2% of the Na+ and 79.6% of the K+, and also generated extremely hard water (3620 mg/L as CaCO3) due to Ca2+ and Mg2+ exchange.

Finally, zeolite regeneration studies were conducted using 0.1 M HCl. The Na+ was efficiently desorbed, but K+ remained. Over four consecutive adsorption–desorption cycles, the net K+ loading increased from 4.8 to 21.2 mg/g. This K-form zeolite could potentially be applied as a slow-release fertilizer, thereby transforming a potash mining waste material into a valuable resource.