Green sustainable and highly efficient hematite nanoparticles modified biochar-clay granular composite for Cr(VI) removal and related mechanism

Agricultural straw has been used as the raw material for sewage treatment to achieve the objectives of agricultural waste utilization, ecological circle construction, and green sustainable development in recent years. Based on corn straw, attapulgite, zeolite, starch, and iron oxide, a series of hematite-modified biochar-clay granular composites (HGCs) were synthesized through a two-step pyrolysis process and were shown to be efficient materials for Cr(VI) removal from an aqueous solution. The physicochemical properties and morphologies of HGCs were characterized by SEM, FTIR, XRD, and XPS, which confirmed that hematite was successfully generated on the surface of HGCs. HGCs also showed a well-developed mesoporous structure as analyzed by N2 physisorption. The maximum adsorption capacity of Cr(VI) on the HGCs at pH = 5.0 was 19.51 mg/g as calculated by the Langmuir model. The experimental data revealed that both pseudo-first-order and pseudo-second-order models fitted well with adsorption kinetics of Cr(VI) on HGCs. The mechanisms of Cr(VI) removal by HGCs involved predominately with electrostatic interaction, hydrogen bond attraction, redox reaction, and coprecipitation reaction. Moreover, the strengthened electrostatic interaction between HGCs and Cr(VI) might explain the decreased adsorption capacities of Cr(VI) along with pH increasing from 1.0 to 10.0. HGCs are applied to wastewater treatment, which conforms to the concept of waste resource recovery and the topic of sustainable development. Since this novel material was proposed in this work, a new range of application is possible for water treatment processes.