Adsorption of Malachite Green Dye from Aqueous Solution Using Iron-Modified Biochar Synthesized from Municipal Sewage Sludge

This study investigated the adsorption of Malachite Green (MG), a toxic cationic dye, from aqueous solution using iron-modified biochar derived from municipal sewage sludge (Fe-MSS). Biochars were prepared from various industrial wastes (brewery spent grain (BSG), brewery sewage sludge (BSS), municipal sewage sludge (MSS), and metal-processing slag (MPS)). Preliminary studies showed that iron modification of MSS (Fe-MSS) significantly increased MG removal efficiency from 76.0% to 90.0%. Optimal Fe-MSS biochar preparation, achieved at a pyrolysis temperature of 500 °C for 3 h and a 2:1 Fe3+/Fe2+molar ratio, resulted in 97.6% MG removal. The prepared adsorbents were characterized using BET, FTIR, XRD, and SEM. Iron modification increased the surface area of MSS from 21.25 to 46.77 m2/g and roughly doubled the total pore volume. In contrast, the average pore size decreased from 8.11 to 6.46 nm. This increased surface area and mesoporous structure provide more active sites and improve MG diffusion, thereby enhancing the adsorption efficiency of Fe-MSS. Optimal MG removal was achieved at pH 10, with a 2 g/L adsorbent dose and a 3 h contact time. The adsorption process followed pseudo-second-order kinetic and the Langmuir isotherm models with a maximum capacity of 34.29 mg/g. The adsorbent demonstrated good selectivity in the presence of competing cations and maintained reasonable reusability over five cycles. The results highlight Fe-MSS biochar as an effective, low-cost adsorbent for MG removal, offering a sustainable approach for wastewater treatment and the valorization of sewage sludge.