Removal of fluoride by hydrous manganese oxide-coated alumina: Performance and mechanism

A novel hydrous-manganese-oxide-coated alumina (HMOCA) material was prepared through a redox process. The adsorbent was characterized by SEM, BET surface area measurement, XRD, pHPZC measurement, FTIR spectroscopy, and XPS. The manganese oxides were amorphous and manganese existed mainly in the +IV oxidation state. Batch and column experiments were carried out to investigate the adsorption potential of the adsorbent. Fluoride adsorption onto HMOCA followed the pseudo-second-order equation well with a correlation coefficient greater than 0.99. Both external and intraparticle diffusion contributed to the rate of transfer and removal. The adsorption of fluoride was thought to take place mainly by ion-exchange. Optimum removal of fluoride occurred in a pH range of 4.0-6.0. The maximum adsorption capacity calculated from the Langmuir model was 7.09 mg/g. The presence of HCO3−, SO42− and PO43− had negative effects on the adsorption of fluoride. The adsorbed fluoride can be released by alkali solution. Column studies were performed and 669 bed volumes were treated with the effluent fluoride under 1.0 mg/L at an influent F− concentration of 5.0 mg/L and flow rate of 2.39 m3/(m2 h) (empty bed contact time = 7.5 min).