Adsorptive removal of phosphate from aqueous solutions using different types of red mud

Red mud (RM) is an industrial waste generated during production of alumina from using the Bayer process or the sintering process. Four types of red mud from China were characterized for their diverse chemical and mineral compositions using inductively coupled plasma-atomic emission spectroscopy (ICP-AES), inductively coupled plasma-mass spectrometry (ICP-MS) and X-ray diffraction (XRD). Acid treatment was employed to obtain activated red mud (ARM), posing increased surface areas from 10-28 m²/g to 220-350 m²/g. RMs and ARMs were used to adsorb phosphate in solution to compare the adsorption capacity. Sample GZ3, a red mud from the sintering process, presented the highest adsorption capacity among the four raw RMs, posing an adsorption capacity of 0.37 mg P/g in the solution of 1 mg P/L with a solid/solution ratio of 0.5 g: 1 L. Whereas, activated GX (AGX), a high iron Bayer red mud from diaspore bauxite, showed the highest adsorption capacity of all the ARMs, with an adsorption capacity of 1.92 mg P/g in the same condition. The dynamic studies indicate that the adsorption mainly followed the pseudo second-order model. The models of Freundlich and Langmuir were used to simulate the sorption equilibrium on GZ3 and AGX. It suggests that the Freundlich model had a better correlation with GZ3 while the Langmuir model fitted well with AGX.