Treatment of acidic mine drainage in an adsorption process using calcium silicate modified with Fe(III)

Two compounds based on calcium silicate hydrate and modified with Fe(III), using FeCl3 and Fe(OH)3 as iron sources, were synthesized, characterized and evaluated as adsorbents for removing ionic species from an arsenic-containing acidic aqueous mine solution. The adsorbents were prepared by contacting an aqueous solution of sodium silicate with calcium hydroxide and Fe(III) compounds. The structure of these adsorbents was determined by X-ray diffraction, FTIR, DSC, BET porosimetry analysis, mean particle size measurements and chemical analysis. Both adsorbents are amorphous, consisting of large agglomerates of particles whose mean particle size averages 358nm with a surface area variable between 80 and 150m2/g. Both adsorbents showed very similar fast adsorption behavior, achieving an almost quantitative and simultaneous uptake of arsenate, phosphate, Cu(II), Zn(II) and Cd(II) ions. In the pH range between 2 and 4, it was measured a maximum arsenic and phosphate loading capacities close to 55mgAs(V)/g of adsorbent and 81mg phosphate/g adsorbent. Adsorption efficiencies over 99.94% were determined for the three metallic ions studied. The latter allowed the raffinate to comply with the limits accepted by environmental national regulation for ions discharging in natural water bodies. Particularly, As(V) equilibrium and kinetics were established in adsorption experiments. The hybrid Redlich-Peterson adsorption isotherm model explained the experimental results and a pseudo-second-order kinetic model provided a good fit to the experimental data. The modification with Fe(III) of the nano-structured calcium silicate generated a suitable adsorbent for removing arsenic species by forming highly insoluble and very stable double iron and calcium arsenate salts, keeping its ability to uptake other contaminants commonly present in acidic mine waters.