Researchers at Hebei University of Technology, Nankai University, and the Oil & Gas Technology Research Institute of Huabei Oilfield Company have developed a millimeter-scale, magnetically recoverable carbon bead catalyst that achieves complete removal of the antibiotic sulfamethoxazole (SMX) from water within 20 minutes, while largely avoiding the metal leaching that has limited earlier cobalt-based catalysts for this kind of water treatment.
Sulfamethoxazole is a widely used antibiotic that has been accumulating in water systems, and cobalt-based catalysts are among the most effective materials for breaking it down via peroxymonosulfate (PMS) activation, but conventional powdered cobalt catalysts tend to leach metal into the water over time, degrading their performance and creating a secondary pollution problem of their own. To get around this, the team built millimeter-sized beads (named CoNC@cPAN/rGO-800) by combining a cobalt-based metal-organic framework precursor (ZIF-67) with polyacrylonitrile (PAN) and graphene oxide, then calcining the mixture into a porous carbon structure with cobalt species locked inside. The graphene oxide is reduced to rGO during this process and lowers the material's electronic impedance and helps disperse the cobalt more evenly through the carbon framework. The primary catalytic site is the embedded cobalt nanoparticles and cobalt oxide.
The beads' porous, honeycomb-like internal structure turned out to matter for more than surface area. The team traced the reaction's kinetics to two distinct phases: cobalt metal near the bead's outer shell directly activates PMS early on, while oxygen vacancies concentrated in cobalt oxide toward the bead's core convert dissolved oxygen into superoxide radicals (O2·−) as the reaction progresses, and this superoxide production measurably accelerates the later stage of the reaction - quenching experiments and electron paramagnetic resonance measurements were used to confirm the role of each reactive species involved (hydroxyl and sulfate radicals, superoxide, and singlet oxygen).
Under standard test conditions (pH 7), the catalyst degraded 10 mg/L of SMX completely within 20 minutes, and remained effective across a broad pH range (2–11) and in real river and lake water samples. Across eight reuse cycles it retained 82.4% of its initial degradation efficiency, and cobalt leaching stayed well below China's industrial wastewater discharge standard - a sharp contrast with a powdered cobalt catalyst made from the same precursor, which the team tested for comparison and found leached over 30 times more cobalt. The beads are also magnetically recoverable, and toxicity modeling of the breakdown products indicated most of them are less toxic than SMX itself.
