A novel sludge-based biochar composite adsorbent CS/SBC-H3PO4 for efficient Cr(VI) removal from aqueous solution.

In this paper, in order to solve the problem of more impurities and less functional groups on the surface of sludge-based biochar, phosphoric acid and chitosan were used to co-modify sludge-based biochar (CS/SBC-H₃PO₄) to enhance the removal of Cr(VI). CS/SBC-H₃PO₄ was characterized using SEM, XRD, FT-IR spectroscopy and XPS. When the initial concentration of Cr(VI) solution was 100 mg L⁻¹, the pH was 2.0, and the dosage of adsorbent was 50 mg, the Cr(VI) removal rate of CS/SBC-H₃PO₄ was 89.70%. The adsorption process of Cr(VI) by CS/SBC-H₃PO₄ was spontaneous and heat-absorbing, and was dominated by monolayer chemisorption, which followed the quasi-second-order kinetic model and Langmuir adsorption isotherm model, and its maximum adsorption capacity was 201.93 mg g⁻¹ at 298 K. The removal mechanism of Cr(VI) by CS/SBC-H₃PO₄ is electrostatic adsorption, reduction and complexation. In detail, CS/SBC-H₃PO₄ electrostatically adsorbs Cr(VI) through surface amino protonation, and then reduces Cr(VI) to Cr(III) through –NH₃⁺ and O–H groups to generate –NO₂, C–O and C＝O groups. Finally, Cr(III) is complexed with the C＝O group. [ABSTRACT FROM AUTHOR]