<p>The conversion of industrial biowaste into low-cost and environmentally friendly valuable materials without harm to the environment is very important from an economic perspective. The present work investigated the sorptive remediation performance of methylene blue (MB) dye-contaminated water from a novel magnetic carbon nanocomposite (MnF@BAC) material synthesized by loading manganese ferrite (MnF) nanoparticles onto black cumin (<i>Nigella sativa</i> L.) industrial processing solid waste-based activated carbon (BAC) in a microwave irradiation medium. Batch sorption tests were performed to investigate the effects of process factors such as the sorbent dose, initial dye concentration, contact time, ionic strength, and system temperature. The kinetic data were well fitted to the pseudo-second-order (PSO) model. Sorption occurred through a complex mechanism that included both an intraparticle diffusion step and other steps. The Langmuir isotherm model best describes the equilibrium isotherm data. The maximum MB sorption quantity was 588.2&#xa0;mg/g. The thermodynamic parameters revealed that sorption was spontaneous and endothermic. Reusability studies revealed that it could be a high-performance sorbent for MB sorption for up to six cycles. In conclusion, the experimental findings emphasize that it can be implemented as a promising, eco-friendly sorbent for treating MB-containing wastewater without causing secondary pollution to the medium.</p> Graphical abstract <p></p>