Optimization of natural zeolite powder milling parameters to enhance surface area and improve heavy metal adsorption from water

Milling parameters during the production of natural zeolite powder significantly contribute to achieving the desired surface area suitable for the adsorption of heavy metals. In this study, Response Surface Methodology (RSM) and Central Composite Design (CCD) were employed to optimize the uniformity coefficient and surface area of zeolite powder for the adsorption of heavy metals. To identify optimal processing conditions, the milling speed, milling time, and percentage of milling balls to zeolite were varied at 200–450 rpm, 12–36 min, and 45–60%, respectively. Zeolite powders produced under optimal conditions were subsequently studied for their efficacy in adsorbing heavy metals from water. Powders were characterized using SEM-EDX, XRD, BET surface area analysis. Zeolite pore structure was studied using Barrett-Joyner-Halenda (BJH) model. Analysis of variance (ANOVA) revealed that quadratic models best explained the responses of the uniformity coefficient and surface area. Particle sizes of the resulting powders ranged between 197.6 and 1635 nm, with optimized values of 5.2 and 147.4 m²/g for the uniformity coefficient and surface area, respectively. The BJH model results showed that an increase in relative pressure led to an increased uptake of nitrogen, which justified the porous nature of zeolites. Adsorption experiments on produced powders under optimal conditions demonstrated high affinity for Pb²⁺, Zn²⁺, and Cr⁶⁺, with Langmuir isotherm R² values of 0.992, 0.995, and 0.932, respectively, and corresponding adsorption capacities of 19.67, 18.35, and 15.02 mg/g. These results confirm that optimized natural zeolite powder is highly effective for removing heavy metal ions from aqueous solutions.