Adsorption of Phosphorus in Aqueous Solution by Calcined Rare Earth Slag and Its Mechanism

Rare earth slag fromSouth Jiangxi,China, was prepared into a promising low-cost adsorbent (CRES) through carcination for phosphorus removal in phosphorus over-loaded wastewater. To characterize CRES systematically, a number of modern equipment, such as Scanning Electron Microscope (SEM), Infrared Spectroscopy (FTIR), X-ray Fluorescence (XRF), Specific Surface Area Analyze (BET-BJH) and Thermogravimetric Analysis (TGA) were used. Phosphorus adsorption and desorption processes of CRES were studies by means of adsorption isotherm and adsorption kinetics.Effect of pH of the solution on P adsorption by CRES was also explored. Results show that CRES has a well-developed porous structure, containing various metal elements and metalloid element compounds, such as Ba, V, Si, Y, Ca, Fe, Yb, etc., on its surface. Solution pH, ranging from 2.2 to 10.0, did not affect much on its P adsorption capacity. Additionally, desorption of phosphorus-loaded CRES was investigated in different aqueous solutions. It was found that the P desorption rate was much higher in HCl solution than in any other others.The Langmuir modelwas found to be good in describing the P adsorption isotherm (R² = 0.9325，n = 5) of CRES. Themaximum PO₄^3-adsorption capacityof CREScould be as high as 152 mg•g^-1, higher than any other adsorbents’ previously reported. P adsorption kinetics tests reveal that the P adsorption reached equilibrium pretty soon, only after 4 - 6 min, of which the process could be well described with Ho’s pseudo-second-order kinetic model, implying that chemisorption was the dominant mechanism of the P adsorption. Chemisorption was related to surface functional groups of CRES and coordination effect. Further, physisorption was also involved in this adsorption process, but mainly occurred at the initial stage of the adsorption process. Physisorption was related to various metal/metalloid elements contained in the surface as well as the well developed porous structure of CRES.