K+-Site Ce-Doped Jarosite for Phosphate Adsorption: a Mechanism Study

Jarosite is a common iron-containing mineral. Researchers have studied its application for removing aqueous pollutants, such as Cr(VI) and As(V). Surprisingly, it shows adsorption for arsenates, but little for the structurally similar phosphate ions. In this study, we prepare cerium doped jarosite and prove the successful doping of cerium at the K+ site by X-ray diffraction (XRD), inductively coupled plasma-optical emission spectrometry (ICP-OES), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Phosphorus adsorption experiments show that the small amount of cerium doping (Ce content: 8.75x10(-5) mol/g) significantly improves the phosphate adsorption of jarosite, from 1.69 mg/g to 29.33 mg/g (pH=7, 24 h). The phosphate adsorption of Ce-doped jarosite exhibits good pH stability (from pH=3 to pH=11) and excellent selectivity, which is capable of maintaining more than 91% of its adsorption capacity in the presence of various competing anions, such as HCO3-, CO32-, humic acid anion, SO42-, NO3-, and SiO32-. Further analysis reveals that the adsorption process obeys the pseudo-second order kinetic model while the adsorption isotherms represent the Freundlich isotherm. The analysis indicates that the adsorption may be a chemical adsorption process that is easy to proceed. To explore the mechanism of adsorption enhancement, we first characterize the Zeta-potential of the pure jarosite and Ce-doped jarosite. The result indicates similarity of the surface potential between the two samples, which rules out the electrostatic adsorption mechanism. Next, based on the result of anion exchange chromatography, we confirm that the cerium doping greatly increases the exchange between the sulfate groups in jarosite and the phosphate groups in solution, from 2.85 mg/g to 24.90 mg/g. Finally, XPS high-resolution spectroscopy reveals that the chemical environment of Ce changes after the phosphate adsorption, likely indicating the formation of Ce-O-P chemical bonds to achieve specific chemisorption. These results may provide insights for the modification and application of jarosite, as a new adsorbent material for treating phosphorus rich wastewater.