Sequestration of Pb(II) and Cr(VI) from aqueous environment using low-cost immobilised tannin resin

The present study was conducted to optimise the process parameters for the extraction of tannin (adsorbent raw material) while minimising waste with the aid of response surface methodology and the application of the immobilised tannin as a mob for metal ions. Based on the reduced second-order polynomial quadratic model, the optimised conditions for the maximum response (tannin): a material mass of 6 g, pH 7, 37.5 °C, 22.5 min and 50 mL of water is required. The immobilised tannin adsorbents; magnetite tannin (MTR) and sulphur doped magnetite tannin resins (S-MTR), were characterised by XRD, FT-IR, TGA, SEM, EDS, BET and an elemental analyser. The influences of pH, contact time, temperature and initial metal ion concentration on adsorption performance of MTR and S-MTR were evaluated. Equilibrium adsorption data were assessed for fitness into four adsorption isotherms and kinetic models. The maximum adsorption capacities of MTR and S-MTR for Pb(II) were observed at optimum pH 4 with adsorption capacities of 534.8 mg/g and 15.1 mg/g, respectively. However, the capacities of MTR and S-MTR for Cr(VI) were 813.0 mg/g, and 709.2 mg/g observed at optimum pH 4 and 2, respectively. Equilibrium data obtained from the uptake of Pb(II) and Cr(VI) ions by MTR and S-MTR best fit into the Langmuir and Freundlich isotherms. Whereas, the sorption of Pb(II) and Cr(VI) on MTR fits into the pseudo-first order while the sorption on S-MTR fits into a pseudo-second-order model. Thermodynamic parameters revealed a feasible, spontaneous and endothermic process for the adsorption of Pb(II) on MTR; however, incorporation of thiourea improves the uptake capacities of both ions from solution but reduces feasibility and spontaneity of the process.