Magnetic biochar catalyst derived from biological sludge and ferric sludge using hydrothermal carbonization: Preparation, characterization and its circulation in Fenton process for dyeing wastewater treatment

To solve sludge disposal and management problems during dyeing wastewater treatment, the produced excess biological sludge and ferric sludge were fabricated into a magnetic biochar composite (MBC) under the optimal hydrothermal carbonization (HTC) conditions. With ferric sludge mixing, the generated MBC contained paramagnetic Fe3O4, showed a smaller diameter of approximately 200 nm, a smaller pore size, a larger specific surface area and a higher carbonization degree than BC prepared using a single biological sludge process under the same HTC conditions. Additionally, biochar and Fe3O4 in the MBC were found to be tightly combined through chemical bonding, imparting MBC with its own property of magnetic recycling. The stable high Methylene Blue (MB) degradation performance in a Fenton reaction after recycling designated it as a good catalyst. The MB degradation pathway was proposed based on GC–MS results. When the MBC was used to treat actual dyeing wastewater through a Fenton process, the chemical oxygen demand (COD) and total organic carbon (TOC) removal efficiencies reached 47 ± 3.3% and 49 ± 2.7%, respectively. Therefore, MBC could be recycled as a catalyst in dyeing wastewater treatment. And a methodology is described that minimizes the produced sludge and enables sludge internal recycling in a dyeing wastewater treatment plant.