Preparation of red mud-modified sludge char through microwave-assisted one-step pyrolysis and steam activation and its adsorption properties for hydrogen sulfide

To improve the hydrogen sulfide (H₂S) adsorption performance of sludge-derived char, a type of red mud-modified sludge char (RSC) was prepared through microwave-assisted one-step pyrolysis and steam activation of sludge and red mud (RM). The effects of pyrolysis temperature, RM mass percentage, and steam flow rate on the cumulative adsorption capacity of H₂S were systematically investigated using response surface method. The results indicated that the sludge char showed a significant increase in cumulative adsorption capacity from 1.47 mg/g to 22.83 mg/g when it was modified with RM at a pyrolysis temperature of 625 °C, a mass percentage of RM of 20%, and a steam flow rate of 0.46 mmol/min. The XRD and XPS analysis results indicated that the RM doping generated abundant iron oxides on the surface of RSC, which is beneficial for the adsorption of H₂S. Adsorption thermodynamics, isotherm fitting and thermodynamic calculations indicate that the adsorption mechanism of H₂S on the RSC surface was attributed to the combined effects of physisorption and chemisorption. Additionally, the material exhibited reliable reusability, retaining more than 80% of its initial breakthrough capacity after three adsorption-regeneration cycles. Therefore, the RSC prepared in this study can be regarded as a promising adsorbent due to its low cost, effective adsorption capabilities, and reusability. The developed method is promising as it achieves environmental remediation through the utilization of waste sludge and RM.