Effects of phosphorus concentration on Cr(VI) sorption onto phosphorus-rich sludge biochar

To investigate effects of phosphorus content on Cr(VI) sorption onto phosphorus-rich biochar, sewage sludge of different phosphorus concentrations from 4 to 60 mg center dot g(-1) by dry weight were prepared and carbonized to make biochar for batch sorption experiments. Test results revealed that different phosphorous concentration of raw sludge had respective impacts on surface area, pore surface area, average pore diameter and pH value of derived biochar. The adsorption kinetics of phosphorus-rich biochar could be described by the pseudo-second-order model. The sorption isotherm data followed Langmiur model better than Freundlich model. Biochar produced from sludge with phosphorus concentration of 20 mg center dot g(-1) gave the largest chromium sorption capacity, which could be attributed to its largest surface area and pores surface area comparing with those of biochars from sludge with other phosphorus concentrations. The chromium loaded biochar was analyzed using Fourier Transform Infrared Spectroscopy and X-ray Diffraction measurement. The results indicated that chemical functional groups hydroxyl and methyl on surface of biochar were involved in Cr(VI) binding and its reducing to Cr(III). Then, a portion of Cr(III) in form of various phosphate precipitates was bound onto biochar surface and the rest was released into the solution. The experimental results suggested that phosphorus played an important role in pore and surface area development of sludge biochar during pyrolytic process. It also could react with Cr(III) on the biochar surface that impacted on capacity of Cr(VI) removal from solution by sludge biochar. Therefore, phosphorus concentration in sludge should be considered when sludge pyrolytic residue would be reused for heavy metals sorbing.