ANALYSIS OF THE ADSORPTION BEHAVIOUR OF LIMESTONE, LATERITE, VERMICULITE, AND ZEOLITE AS POTENTIAL REACTIVE MEDIA FOR PERMEABLE REACTIVE BARRIERS

This study evaluates the suitability of dolomitic limestone, ferricrete laterite, vermiculite, and clinoptilolite zeolite as natural reactive media for permeable reactive barriers (PRBs) in removing Cu²⁺, Zn²⁺, K⁺, Na⁺, and NH₄⁺ from groundwater. Batch adsorption tests were conducted at initial concentrations of 50, 100, 200, 1000, and 3000 g m⁻³, without pH adjustment. Limestone promoted the most pronounced modification of solution chemistry, raising the final pH to 6.96–7.95 and enhancing Cu²⁺ and Zn²⁺ removal through sorption, ion exchange, and precipitation. Laterite showed lower but chemically stable retention, controlled mainly by adsorption on Fe/Al oxide, hydroxide, and clay-mineral surfaces. Vermiculite and clinoptilolite exhibited the highest ion-exchange capacities, associated with their high cation exchange capacity and structural characteristics that favour cation retention. Equilibrium isotherms were predominantly type-L, and the Langmuir model provided the best overall fit, confirming finite adsorption capacity and preferential uptake of divalent metals. The Langmuir maximum adsorption capacities followed the sequences Cu²⁺ > Zn²⁺ > NH₄⁺ > K⁺ > Na⁺ for limestone, Zn²⁺ > Cu²⁺ > K⁺ > Na⁺ > NH₄⁺ for laterite, and Cu²⁺ > Zn²⁺ > K⁺ > NH₄⁺ > Na⁺ for vermiculite and clinoptilolite, with the highest qmax values recorded for clinoptilolite, followed by vermiculite, limestone, and laterite. Clinoptilolite and vermiculite are the most promising ion-exchange media, limestone is effective where pH buffering and metal immobilisation are desired, and laterite is better suited as a complementary component where moderate retention and geochemical stability are required.