Rational design of groundwater treatment sludge into visible-light-driven photocatalyst with carbon nitride

For the first time, groundwater treatment sludge was integrated with g-C₃N₄ towards highly efficient and cost-effective visible-light-initiated catalysts for organic removal. The optimized sample of g-C₃N₄/GWS-M(2.5 %), which was synthesized using the sludge rich in Al and Fe, was explored with improved photocatalytic activity. Its photocatalytic performance was ∼6, 4, and 7 times that of g-C₃N₄ in terms of removal of methyl orange, cephalexin, and ketoprofen, respectively. The observed greater photocatalytic activity was attributed to its upgraded physicochemical properties, including specific surface area, porous structure, visible light absorption, charge separation and transfer. In particular, the co-existence of dominant Al and Fe dopants in g-C₃N₄/GWS-M(2.5 %) aided abstraction of photogenerated charge carriers. After photocatalytic reaction, only 0.02 % and 0.01 % loss of Al and Fe was observed from the catalyst, respectively. A superior organic removal (∼92 %) was still observed by using g-C₃N₄/GWS-M(2.5 %) with no change in its crystal and chemical structures at the 5^th cycle of photocatalytic degradation. The primary reactive species responsible for the reaction were inferred to be the superoxide and singlet oxygen radicals.