Chloride Ion Adsorption by Modified Pisha Sandstone-Based Cementitious Materials

Pisha sandstone (PS) has potential as a low-cost adsorbent due to its abundant surface-active adsorption sites. In this work, mechanical grinding coupled with high-temperature calcination was employed to activate and modify raw PS for improved chloride ion adsorption performance and efficient resource utilization. Adsorption kinetic experiments demonstrated that the PS modified via 15 min of mechanical grinding (PSM15) exhibited the optimal chloride adsorption performance and achieved adsorption equilibrium within 240 min. The adsorption kinetics data were well fitted by the pseudo-second-order model, indicating that chemisorption dominates the chloride adsorption process. The chloride removal efficiency of PSM15 reached a maximum value of 33.3%, which was superior to that of calcined PS (30.1%) and raw PS (23.6%). Combined characterization results from XRD, FTIR, and SEM-EDS revealed that mechanochemical activation does not alter the main crystalline phases of the material. Instead, it significantly enhances chloride adsorption capacity by refining crystallite size, exfoliating layered microstructure, and exposing surface active sites. Moreover, the in situ formation of C–S–H gel reinforces chloride immobilization via physical encapsulation and electrostatic attraction. Collectively, the enhanced chloride adsorption by modified PS can be attributed to synergistic mechanochemical activation, surface and interlayer retention, and gel-mediated immobilization. As a low-cost and eco-friendly adsorbent, the PS-based cementitious material shows promising application potential in chloride-containing wastewater purification.