Facile synthesis, characterization and application of heterogeneous Al@Si materials for adsorptive mitigation of fluoride: Optimization and cost analysis

Abstract

The present study reports on the investigation on the facile synthesis of Al-Si based low cost heterogeneous adsorbent from the naturally available laterite soil and clay. The soil-clay mixture was blended at a ratio of 3:1, which was thermochemically activated and was applied for fluoride separation from synthetic groundwater. The developed material was validated through some characterization techniques, such as SEM-EDX, FTIR, XRD and BET surface area measurement. Optimization on various operating parameters such as contact time (0.5–10 h), pH (4.5–9.5), adsorbent dose (10–60gL−1), stirring speed (20–120 rpm) and initial fluoride concentration (2–15 mgL−1), for fluoride separation were performed in batch operation, which was further optimized with Central Composite Design approach of Response Surface Methodology in Design-Expert software where Analysis of Variance suggested the validity and success of the model with its high R2 value (>0.97). Experimental validation with RSM software prediction delivered the optimum parameter values. At pH 6.5, initial fluoride concentration of 10 mgL−1, adsorbent dose of 60 gL−1, contact time of 10 h and stirred speed of 120 rpm, the activated adsorbent material showed > 96% fluoride removal efficiency. Extensive kinetic, isotherm and thermodynamic studies were formulated and validated with experimental data. Pseudo 2nd order model and Langmuir isotherm model were the best fits for kinetics and for adsorption equilibrium. It was found that the adsorbent could be used up to 3 cycles of operations and regeneration while retaining up to 86% rejection efficiency to fluoride. For the treatment of fluoride rejects for safe disposal, concrete blocks of 20 MPa compressive strength were developed where the adsorbed fluoride was stabilized and fixed. After TCLP test, fluoride concentration of 0.16 mgL−1 was obtained in the extraction fluid which was well below the permissible limit. Fluoride free drinking water at low cost (0.17 $/m3) confirms high sustainability to the complete process.