Magnetic hybrids synthesized from agroindustrial byproducts for highly efficient removal of total chromium from tannery effluent and catalytic reduction of 4-nitrophenol

The use of industrial waste to synthesize materials of technological interest is a rational way to minimize or solve environmental pollution problems. This work reports the removal of chromium ions from tannery effluent using magnetic hybrid adsorbents synthesized from waste biomass: coconut mesocarp (HCN), sugar cane bagasse (HBS), sawdust (HS), and termite nest (HT). The presence of organic matter and the cobalt ferrite phase in the structures of the materials were confirmed by XRD and FTIR analyses. Removal assays performed at different pH values showed the effectiveness of the adsorbents for removal at the natural pH of the effluent (pH 4.75), with adsorption capacity exceeding 6.6 mg g⁻¹ for all the hybrids studied. The adsorption processes showed fast kinetics, with the HS hybrid removing 5.1 mg g⁻¹ of the chromium species present in the effluent in the first 5 min. The HS hybrid presented the highest removal capacity, 6.7 mg g⁻¹, while HBS showed the lowest, 6.6 mg g⁻¹. The removal at equilibrium showed the following increasing order of efficiency of the hybrids: HBS < HCN < HT ≈ HS. The HCN and HT adsorbent matrices saturated with chromium ions (HCNCr and HTCr) showed excellent catalytic performance in the reduction of 4-nitrophenol, with 99.9% conversion within 180–240 s. The other hybrids did not present catalytic activity. The materials showed high capacities for reuse in two successive reduction cycles. The findings highlight the effectiveness of an industrial symbiosis approach to the development of new technologically important materials.