Abstract
This work explored the efficiency and mechanisms of phosphate (P) removal by Ca-impregnated biochar prepared from CaCl2-pretreated ramie stem (Ca-RSB) and ramie bark (Ca-RBB). The properties of Ca-modified biochar were analyzed using elemental analysis, scanning electron microscopy (SEM), BET specific surface analysis, energy-dispersive X-ray analysis (EDS), Fourier transform infrared (FTIR) and a zeta potential meter. The results of characterization suggested that the Ca-RSB had a much higher H/C ratio, total pore volume, BET surface area and more functional groups compared with pristine biochar (RSB). In addition, a higher yield of Ca-RSB (50.8%) than RSB (28.0%) was also observed. Comparison experiments suggested that Ca-RSB showed higher adsorption capacity than Ca-RBB and the adsorption amount of Ca-RSB was more than two-folds that of RSB. Adsorption experimental data fitted well with pseudo-second order kinetics and the Langmuir isotherm. The intra-particle diffusion and Boyd's film-diffusion models revealed that the rate-controlled step was controlled by film-diffusion initially and then followed by intra-particle diffusion. Electrostatic attraction served as the main force to adsorb phosphates at a lower pH, and the precipitation and surface deposition took over at higher pH. The results of this study indicated that Ca-RSB is a potential effective and low-cost adsorbent for phosphate removal from wastewater.