Adsorption of Cr(VI) and Phosphate on Mg-Al Hydrotalcite Supported Kaolin Clay Prepared by Ultrasound-Assisted Coprecipitation Method Using Batch and Fixed-Bed Systems

Modified kaolin clay (MKC) was synthesized through ultrasound-assisted coprecipitation method, and its adsorption behaviors to Cr(VI) and phosphate (P) from single and binary solutions were investigated by batch and column experiments. The adsorption capacities of Cr(VI) and P in single-ion solutions were found to be 309.60 and 605.75 mmol/kg, respectively. Because of competition adsorption, the adsorption capacities in binary solutions were reduced to 145.38 and 461.61 mmol/kg, respectively. The single and binary adsorption data fitted the pseudo-second-order kinetic model and the Langmuir isotherm well. The simultaneous addition of the two adsorbates led to a decrease and an increase in adsorption rate of Cr(VI) and P onto MKC, respectively. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy were performed to confirm the adsorption phenomenon. FTIR spectroscopy demonstrated that hydroxyl groups, NH4+, and NO3- ions on the adsorbent surface were responsible for Cr(VI) and P adsorption, and the adsorption mechanism involved anion exchange and surface complexation. Column experiments indicated that the breakthrough time and adsorption capacity of the fixed bed increased as the bed depth increased and decreased as the flow rate increased. The breakthrough curves were found to fit the Yoon-Nelson model well. Moreover, the feasibility of reusing MKC through three adsorption/desorption cycles in a fixed-bed column was studied. The column regeneration efficiency after the third cycle was 71.9% for Cr(VI) and 69.6% for P.