Thermally derived sludge biochar as an efficient activator of persulfate for dye wastewater treatment

The expansion of urban sewage treatment plants has surged municipal sludge (MS) production in China, which has caused significant environmental impacts. As a result, the treatment and resource utilization of municipal sludge have attracted growing research attention. This study explored a thermochemical conversion method to transform municipal sludge from northern China into sludge-derived biochar (MS-BC) for dye wastewater remediation. The feasibility of MS-BC for dye wastewater treatment through adsorption and persulfate (PDS) activation was systematically investigated. Using the removal rate of Reactive Black 5 (RB5) as a benchmark, the operational conditions for PDS activation by MS-BC800 were optimized, and the activation and adsorption mechanisms were further elucidated. MS-BC1000 exhibited superior adsorption capabilities due to its high specific surface area and porous structure. In contrast, MS-BC800 demonstrated enhanced activation performance due to its relatively low charge transfer resistance and the presence of Fe₂O₃. Under optimal conditions (2 g/L PDS, 2 g/L MS-BC, 150 rpm shaking frequency, and pH 6.0), a removal efficiency of 99.52 % was achieved for 100 mg/L RB5 wastewater after 180 min of reaction. The MS-BC exhibited excellent activation performance even under a broad pH range of 2-10 and in the presence of coexisting substances (NaCl, MgCl₂). The pseudo-first-order kinetic model effectively described the RB5 removal process at 100 mg/L by the MS-BC/PDS system, wherein surface-adsorbed radicals (SO₄^-·, ·OH) and non-radical ¹O₂ collectively degraded the chromophoric groups of RB5, leading to decolorization and removal. The system demonstrated excellent repeatability, maintaining high removal efficiency even after five cycles. These results provide a theoretical basis for the resource utilization of municipal sludge and the efficient purification of dye wastewater.