Removal of arsenic(III) from aqueous solution using a low-cost by-product in Fe-removal plants—Fe-based backwashing sludge

摘要

This study investigated the elemental composition and distribution of the Fe-based backwashing sludge (FBBS), and studied its adsorption behaviors and mechanisms towards arsenite [As(III)]. The characterization results of EDS, XPS, and XRF corroborated that the valuable constituents within FBBS are ferric oxhy-droxide (γ-FeOOH) and sulfate inter-layered Fe hydroxide [Fe(SO4)OH]. The zeta'potential results indicated a pH_(ZPC) value of 7.7. The adsorption equilibrium could be reached within 18 h, and the kinetics data were well described by the Elovich and Power models due to the heterogeneous surfaces of FBBS. The isotherm experimental results suggested that the maximum adsorption amount of As(III) was around 59.7 mg/g (initial As(III) = 1-120 mg/L, pH = 7.0, T= 25 °C), which is higher than most of other low-cost adsorbents, The uptake of As(III) onto FBBS would increase with an increase in temperatures, inferring that it is an endothermic process. The optimal initial solution pH for As(III) removal was around pH 8.0. The release of sulfate from FBBS after As(III) adsorption implied the occurrence of ligand exchanges, while the mechanism of Fe(III) precipitation might be also involved. The spectra of FTIR and XPS revealed that the surface hydroxyl groups played an important role in the adsorption of As(III), and the oxidation state of As(III) was not changed. Moreover, phosphates (>1 mM) could strongly inhibit the removal of As(III). The desorption results indicated that the release of As caused by alkali or phosphate eluent should be avoided for the scrutiny of waste landfill.