Mayenite-to-hydrocalumite transformation for the removal of chloride from salinized groundwater and the recycling potential of spent hydrocalumite for chromate removal

Abstract

This study explored the feasibility of exploiting the mayenite-to-hydrocalumite mineral phase transformation as a chloride (Cl−) removal process for salinized groundwater. We synthesized anhydrous calcium aluminate (Ca/Al) composites through the solid-state heat treatment of CaO and Al2O3 at different temperatures (300–1200 °C). The mayenite-enriched Ca/Al adsorbent synthesized at 1200 °C (Ca/Al-1200) was able to adsorb Cl− from water (1000–30,000 mg/L) through the mayenite-to-hydrocalumite transformation, resulting in significant Cl− removal (Langmuir qmax of up to 105.9 mg Cl−/g). Ca/Al adsorbents synthesized below 1100 °C were not effective at removing Cl− due to insufficient mayenite formation. Ca/Al-1200 reduced Cl− concentrations (630–2491 mg/L) below the drinking water standard (250 mg/L), even in the presence of natural competitive anions in potash brine-spiked groundwater (sulfate, nitrate, and bicarbonate). The morphological transformation from cage-like mayenite to layered hydrocalumite during chloride uptake was observed by scanning and transmission electron microscopy images (SEM and TEM). Adsorbed chloride in the layered hydrocalumite framework (Ca-Al-O) was also observed by scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS). The Cl−-bearing hydrocalumite was then recyclable for the removal of chromate (CrO42−) through anion exchange with Cl−, which was also visualized at the elemental level.