One-pot synthesis of a magnetic Zn/iron-based sludge/biochar composite for aqueous Cr(VI) adsorption

Abstract

Using the traditional Chinese medicine residual (TCMR) as biomass, and the iron-based waterworks sludge (IBWS) and Zinc chloride (ZnCl 2 ) as modified materials, this study synthesized IBWS/ZnCl 2 /TCMR-derived biochar composite (Fe-Zn@BC) in one-pot technique for aqueous Cr(VI) adsorption. Batch experiments of Cr(VI) adsorption were carried out with the Fe-Zn@BC as an adsorbent. The pH research revealed that acidic conditions (pH < pHzpc) were in favor of Cr(VI) adsorption, with the highest removal observed at pH 2. The maximum adsorption capacity of Fe-Zn@BC was about 27.04 mg/g at 35 °C. Based on the examination of kinetics, isotherm, the adsorption process by Fe-Zn@BC was consistent with the equation of pseudo-second-order and the Langmuir model, which had a monolayer adsorption nature. Anions have negative effects on Cr(VI) reduction by Fe-Zn@BC, the negative effects order followed Cl − < SO 4 2 − < PO 4 3 − , which were positive correlated with the concentration and the charge of anions. Potential removal mechanism was investigated with various characterization techniques, indicating that the major mechanisms of Cr(VI) adsorption by Fe-Zn@BC were physical and chemical adsorption, including the reaction of Cr(VI) to Cr(III). IBWS was calcined to impart magnetism of composite, and ZnCl 2 generated magnetic zinc ferrite (ZnFe 2 O 4 ) while activating the composite, endowing the composite with enhanced Cr(VI) adsorption capability and stronger magnetism. Therefore, Fe-Zn@BC appears to be a viable material for removing Cr(VI) in wastewater. • A promising material was prepared in one-pot technique for Cr(VI) adsorption. • Synthesis and characterization of a new Zn/iron-based sludge/biochar composite. • A recycle method of Chinese medicine residual and iron-based waterworks sludge (IBWS). • IBWS provided great magnetism for composite through calcining. • The adsorption fitted the pseudo-second-order model and Langmuir isotherm model.