Valorisation of alkaline effluent derived from iron ore upgrading by Calcium-aluminium-silicate-hydrate (CASH) production and in its application as an adsorbent for heavy metals (Cu2+ and Zn2+) removal from aqueous medium

• A process proposed for the valorisation of iron ore alkaline effluent. • Calcium aluminium silicate hydrate (C-A-S-H) was produced using a precipitation route. • The produced C-A-S-H product was a mesoporous material and applied as an adsorbent. • The product showed excellent adsorption behaviour towards Cu 2+ and Zn 2+ uptake. • Kinetic modelling of the adsorption fitted a pseudo-first-order equation. This study presents a route for valorising highly alkaline effluents generated during caustic soda leaching of low-grade iron ore, where dissolved sodium alumino-silicate species are typically considered a processing liability. By converting this challenging effluent into a mixture of predominantly calcium–aluminium–silicate–hydrate (CASH) phases, which follows hydrothermal zeolite recovery reported elsewhere, the study demonstrates that a post-zeolite waste stream with low (<9 g/L) levels of Al 2 O 3 and SiO 2 can be directly transformed into a functional material. Very low (<0.009 g/L) levels of combined Al and Si remain in the post-CASH effluent stream. The uncalcined mixed CASH product demonstrated a strong affinity for heavy-metal removal, achieving nearly complete Cu²⁺ uptake and high Zn²⁺ removal under mildly acidic conditions required for wastewater treatment, indicating its potential use as a low-cost adsorbent. Upon thermal treatment, the material underwent progressive structural and phase changes: moderate heating increased porosity and surface area, thereby improving adsorption properties. At the same time, a higher temperature (800°C) led to densification and crystallisation into mainly a stable calcium aluminosilicate phase, anorthite. These transformations shift the potential application of the material from adsorption to construction-related uses such as cementitious or insulating components. Overall, the results establish that industrial alkaline effluents with very low levels of combined Al and Si, including effluent from current zeolite/CASH production processes, can be transformed into value-added materials with tuneable properties, supporting the circular economy principles, offering a sustainable route for waste minimisation, environmental remediation, and lower-cost materials production.