Raspberry stalks-derived biochar, magnetic biochar and urea modified magnetic biochar - Synthesis, characterization and application for As(V) and Cr(VI) removal from river water

Raspberry stalks-derived biochar (BC), magnetic biochar-iron oxide composite (BC-Fe) and its derivative modified with urea (BC-Fe-U) were synthesized, characterized and tested as(V) and Cr(VI) ion sorbents. The surface area of BC, BC-Fe and BC-Fe-U was 259, 163 and 117 m2 g-1, respectively. The structure of BC was dominated by micropores, while in BC-Fe and BC-Fe-U mesopores predominated. Based on the XRD results, it was found that the magnetic properties of the biochar-iron oxide composites are due to the presence of ferrimagnetic magnetite (Fe3O4) and maghemite (Fe2O3). The optimal pH of As(V) and Cr(VI) adsorption onto the studied sorbents is in the range of 2.3-5.7. Pristine biochar (BC) does not adsorb As(V) ions; however, it enables rapid adsorption of Cr(VI) with the static adsorption capacity of 19.2 mg g-1. The maximum static adsorption capacities of As(V) and Cr(VI) ions onto BC-Fe and BC-Fe-U are within the range of 13.5-16.3 mg g-1. For most adsorption systems tested, adsorption equilibrium is reached within 4 h, though even a few minutes is enough to reach half of the adsorption static value. Phosphates over 0.005 mol L-1 hinder adsorption of As(V) and Cr(VI) ions. Application of at least 5 mol L-1 nitric acid allows about 95% of Cr(VI) and As(V) to be desorbed from adsorbate-loaded BC-Fe material. For other materials, the desorption efficiencies are significantly lower. BC-Fe and BC-Fe-U materials were successfully used for simultaneous Cr(VI) and As(V) removal from river water.