Efficient chromium remediation using eco-innovative biochar in a novel two-stage upflow fixed bed system

Chromium (Cr) pollution poses a significant environmental threat. This study addresses the highly toxic hexavalent form of chromium [Cr(VI)] by developing an innovative two-stage upflow fixed-bed system. The system uses nano iron sulfide-modified biochar (nFeS-BC), derived from spent substrates of Lentinus edodes, as the primary adsorbent. The study determines the removal mechanisms by nFeS-BC, including ion exchange, functional group interaction, complexation and co-precipitation. Fixed bed experimental results demonstrated a maximum adsorption capacity (q_e) of 2.751 mg·g⁻¹ for Cr(VI) with an initial concentration (C₀) of 20 mg·g⁻¹, bed height (Z) of 10 cm, flow rate (Q) of 2 mL·min⁻¹, and at pH 2. The adsorption process closely followed the Thomas and Yoon-Nelson models, confirming a high fitting accuracy (R² = 0.997), which underscores the predictability and stability of nFeS-BC in dynamic flow conditions. Subsequent treatment in a second fixed bed, filled with cetyltrimethyl ammonium bromide-modified biochar (CTAB-BC), reduced the total Cr concentration to 0 mg·L⁻¹. Furthermore, nFeS-BC retained 50 % of its initial adsorption capacity after five regeneration cycles. This study highlights the potential of nFeS-BC within a novel fixed-bed design for efficient and sustainable treatment of actual wastewater.