Tannin-based biosorbent encapsulated into calcium alginate beads for Cr(VI) removal

A composite biosorbent (AC-TFR) prepared by encapsulating tannin-formaldehyde resin (TFR) into calcium alginate (AC) beads was used to remove Cr(VI) from an aqueous solution. Various influencing factors, such as TFR dosage, pH, initial Cr(VI) concentration, contact time, temperature and presence of co-ions in the medium, were investigated. The structures and adsorption performances of the adsorbents were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Compared with other AC-TFR adsorbents, AC-TFR-2 (mass ratio of AC:TFR = 1:1) showed an excellent adsorption capacity based on the efficiency of Cr(VI) removal. The kinetic data fitted to pseudo-second-order and intra-particle diffusion models suggested that the adsorption process was subject to a rate-controlling step. The equilibrium adsorption data fitted well to the Langmuir isotherm model, and the maximum adsorption capacities of AC-TFR-2 were 145.99, 167.22 and 174.52 mg/g at 288, 298, and 308 K, respectively. The thermodynamic parameters revealed that Cr(VI) removal by AC-TFR-2 was endothermic and spontaneous, and the process was chemical adsorption. The mechanism of Cr(VI) removal consisted first of reduction to Cr(III), which has a low toxicity, and then chelation onto AC-TFR-2 via ion exchange.