Enhanced Removal of Heavy Metals from Water by Hydrous Ferric Oxide-Modified Biochar

Biochar has become an attractive adsorbent for heavy metal removal, but its application potential is very limited because of the relatively low adsorption capacity and poor selectivity. In the present study, we decorated the biochar (BC) by impregnating hydrous ferric oxide (HFO) within the pore of biochar and consequently obtained a new hybrid adsorbent denoted as HFO-BC. The results show HFO-BC exhibited excellent performance to two representative heavy metals, i.e., Cd(II) and Cu(II), with maximal experimental sorption capacities of 29.9 mg/g for Cd(II) and 34.1 mg/g for Cu(II). HFO-BC showed satisfactory anti-interference ability for Cd(II) and Cu(II) removal in the presence of high levels of Ca(II) and Mg(II) owing to the specific inner-sphere complexation between the immobilized HFO and Cd(II) and Cu(II), which was probed by XPS analysis. Cd(II) and Cu(II) removal onto HFO-BC experienced two distinct stages prior to be adsorbed, i.e., migration from solution to the outside surface of adsorbent and pore diffusion and approached equilibrium within 100 min. In the laboratory-scale small column adsorption experiment, HFO-BC can generate ∼129 and 300 BV effluents for Cd(II) and Cu(II), equivalent to 774- and 1854-fold of its own weight, to meet their treatment standards. Moreover, the exhausted HFO-BC can be effectively regenerated using HCl-CaCl₂ binary solution with a desorption rate more than 95%. All results validate that impregnating HFO inside the pores of BC is a promising approach to promote the practical applicability of BC for removing heavy metals from the polluted water.