Lignin-mediated AgCoOS bimetal oxysulfide catalysts for highly efficient catalytic reduction of organic and Cr(VI) pollutants in the dark.

Lignin, a negatively charged, three-dimensional natural biopolymer, serves as an ideal support for metal catalysts due to its abundant functional groups and tunable chemical properties, which enable strong metal coordination and effective immobilization. Herein, we demonstrate a lignin-mediated Co/O co-doped Ag2S, symbolized as L-AgCoOS, bimetal oxysulfide catalyst via a facile hydrolysis method for the efficient reduction of toxic phenolic compounds (4-nitrophenol, 4-NP), organic dyes (methyl orange (MO), methylene blue (MB), rhodamine B (RhB), and heavy metal ions Cr(VI)) under dark conditions. Lignin, used to immobilize catalysts, also contributes to increasing the number of active catalytic sites and enhancing catalytic activity. The co-doping of transition metal cations and anions was employed to effectively regulate the electron transfer property of the catalyst, with oxygen doping modifying the energy band structure and cobalt providing heterovalent Co3+/Co2+ states that facilitate rapid electron hopping between Co2+ and Co3+ (Co2+ ↔ Co3+) to transfer for catalytic reduction of pollutants. Furthermore, the hybridization of O 2p and S 3p orbitals improved the structural stability of the catalyst. L-AgCoOS with an appropriate level of cobalt doping exhibited significantly enhanced catalytic reduction efficiency. Specifically, 5 mg of L-AgCoOS-2 could completely reduce 100 ml of 20 ppm 4-NP within 8 min, with the reaction rate constant of 0.26 min-1. Additionally, it achieved a complete reduction of 100 ml of 50 ppm MB, MO, RhB, and Cr(VI) in 4, 3, 4, and 4 min, respectively, with the reaction rate constant of 0.49, 0.51, 0.57, and 0.54 min-1. L-AgCoOS-2 also demonstrated excellent performance in treating mixed pollutants, maintaining high activity across a wide pH range, and good reusability. These results highlight the significant potential of L-AgCoOS-2 for practical applications in wastewater treatment.