Influence of microwave-assisted CTAC modification of montmorillonite on nitrate adsorption behavior

Nitrate (NO<sub>3</sub><sup>-</sup>) contamination remains a challenge for water treatment, whereas pristine montmorillonite (Mt) exhibits limited NO<sub>3</sub><sup>-</sup> uptake due to its permanently negative layer charge. In this study, Mt was sequentially preconditioned by microwave-assisted Li-exchange (100–1000 W) and then modified with cetyltrimethylammonium chloride (CTAC) to prepare CTAC/Li-Mt(px) composites for nitrate removal. XRD/FTIR and zeta potential measurements confirmed successful CTAC modification and surface charge reversal. Increasing microwave power promoted Li<sup>+</sup> migration/fixation and reduced layer charge density and cation exchange capacity (CEC), which decreased CTAC loading and surface positive charge, while increasing textural accessibility (BET surface area increased from 11.33 to 22.65 mA<sup>2</sup>/g from p100 to p1000). As a result, nitrate adsorption showed a non-monotonic dependence on microwave power. Langmuir analysis indicated that CTAC/Li-Mt(p800) achieved the highest capacity (Q = 0.41 ± 0.08 mmol/g), exceeding that of the non-microwave sample CTAC/Li-Mt (Q = 0.35 ± 0.02 mmol/g). Competitive-anion tests indicated preferential uptake of NO<sub>3</sub><sup>-</sup> relative to SO<sub>4</sub><sup>2-</sup> and PO<sub>4</sub><sup>3-</sup>. These results highlight that microwave-power preconditioning is an effective route to tune CTAC loading and optimize Mt-based adsorbents for nitrate removal.