Biochar-Driven Activity Enhancement of Co3O4 Catalyst for 4-Nitrophenol Reduction: RSM-CCD Optimization, Kinetic Insights, and Recyclability

To decrease the environmental and health consequences associated with pollution, several cobalt oxides-decorated biochar substrates have been developed for effective catalytic reduction of 4-NP. The porous biochar was produced by pyrolysis of sewage sludge and acid demineralization, and subsequently, Co3O4 was loaded with varying amounts to synthesize Co3O4/biochar nanocomposites. The characterization of the materials included FTIR, XRD, SEM/EDX, TEM, BET, CHNS elemental analysis, TGA/DTA, and pHpzc analysis. The catalytic activity of synthesized composites was monitored by UV-Vis spectroscopy to measure the transformation of a priority toxic pollutant, 4-NP, to a desirable pharmaceutical intermediate, 4-AP, by NaBH4, considering it a reducing agent. The influence of corresponding parameters such as catalyst dosage, NaBH4 concentration, and Co3O4 content in the composite on the reduction time of 4-NP was modelled using central composite design (CCD) of RSM. The optimal conditions for the catalytic reduction of 0.3 mM 4-NP were found to be a catalyst dosage of 5 mg, a NaBH4 concentration of 26 mM, and a Co3O4 loading of 13%, resulting in complete reduction within 5 min. The optimized catalyst displayed a high porosity, well dispersion of Co3O4 nanoparticles, good catalytic stability, and cost-effectiveness, revealing new insights into application of cobalt oxide and sewage sludge for pollutant reduction.