A Co2+ ion was reduced by NaBH4 to form cobalt nanoparticles of 10–20 nm in size. The as-obtained Co nanoparticles were stable in air. Congo red was used to evaluate the Co nanoparticles' wastewater treatment capability. It was found that 150 ppm of Congo red could be removed from aqueous solution within 2 min with the Co nanoparticles, and the adsorption maximum is 800 mg g−1.
