A COMPARATIVE REVIEW OF CLINOPTILOLITE (NATURAL ZEOLITE), MONTMORILLONITE (BENTONITE), AND CALCITE-BASED GREEN SORBENTS FOR HEAVY METAL REMOVAL FROM WATER

Source abstract

The continual contamination of aquatic (and terrestrial ecosystems) by heavy metals is a long-lasting, crucial problem that endangers both the environment and public health alike. Due to their toxic nature, non-degradable state, and the tendency to bio-accumulate in the food chain, heavy metals have rightfully earned their ominous reputation as “silent killers.” While traditional (conventional) treatment methods are generally effective in remediating these contaminants, unfortunately, their widespread application is often handicapped and challenged by their time-consuming and operational costs, urging the research community to explore cheaper and more eco-friendly (greener) methods. From this perspective, natural zeolites (clinoptilolite-dominant), bentonites (montmorillonite-rich) and prevalent carbonate minerals such as calcite, are some notable mentions. They present naturally abundant, affordable adsorbents with diverse structures and high pollutant affinity, used as remediation “agents”. However, despite their individual merits, systematic, side-by-side comparative assessments of their characteristics under variable conditions remain insufficient. Consequently, this review attempts to fill the gap in literature by comparing their structural and chemical properties, while dissecting their adsorption mechanisms across different conditions, simultaneously guiding their efficient and informed use in sustainable heavy metal removal, with a focus on water treatment/purification. This comparative evaluation of literature data, highlights that montmorillonite, exhibits the highest adsorption capacity, due to its high surface area and cation exchange capacity, clinoptilolite, provides a balanced combination of efficiency, selectivity and operational stability, owing to its zeolitic framework, structure and charge characteristics, whereas calcite, although influenced by solution chemistry, shows strong efficiency under alkaline conditions, emphasizing the importance on condition-specific adsorbent selection, while supporting the broader implantation of these benign minerals as trajectories toward sustainable heavy metal remediation, aligning both environmental protection objectives and public health safeguarding.