Enhanced degradation of doxycycline by citric acid-functionalized graphitic carbon nitride decorated with MIL-88A and FeS: optimization, degradation mechanism, and degradation pathway

Abstract

This investigation provides a new Fenton-like heterogeneous catalyst construct, citric acid-functionalized graphitic carbon nitride decorated with MIL-88A and iron sulfide (FeS/MIL-88A@Cit–gCN). The characteristics of FeS_0.5/MIL-88A_0.5@Cit–gCN were scrutinized using different instruments to identify its surface charge, morphology, elemental and structural compositions, and crystallinity. The catalytic activity of FeS_0.5/MIL-88A_0.5@Cit–gCN was inspected by a series of adsorption/Fenton-like experiments, evaluating the best catalytic parameters for efficiently decomposing doxycycline (Dox). The maximum adsorption% and decomposition% of Dox were 48.78% and 99.40%, respectively, at H₂O₂ concentration = 100 mg L⁻¹, system temperature = 20 °C, pH = 5, and FeS_0.5/MIL-88A_0.5@Cit–gCN dose = 0.01 g. The second-order kinetic model best represented the Dox decomposition process by FeS_0.5/MIL-88A_0.5@Cit–gCN. The decomposition mechanism of Dox proceeded by a catalytic radical pathway, and most probably, ˙OH was the governing radical in the catalytic medium. The ˙OH radicals were produced through the contribution of the iron, sulfur, and electron-donor groups of FeS_0.5/MIL-88A_0.5@Cit–gCN to activate H₂O₂. The adsorption reaction played an excellent role in the decomposition capacity of Dox since the drug molecules were attached to the FeS_0.5/MIL-88A_0.5@Cit–gCN surface by n–pi interactions, coulombic interactions, and coordination bonds. The recycling study denoted the durability of FeS_0.5/MIL-88A_0.5@Cit–gCN after reusing for five times. These results render FeS_0.5/MIL-88A_0.5@Cit–gCN a premium heterogeneous catalyst that can be applied at an industrial scale.