Visible-Light photocatalytic degradation of tetracycline using optimized metal-free g-C ₃ N ₄ : Matrix effects, toxicity, and PMS-assisted intensification

In this study, g-C3N4 was synthesized from urea, selected using the TOPSIS framework, where urea ranked highest (score = 1). Further catalyst was optimized using the central composite design for the degradation of Tetracycline (TC), a representative Pharmaceutical and Personal Care Product (PPCP). Characterization confirmed a layered structure, surface functionalities, thermal stability, and a visible-light band gap of 2.74 eV. Under optimized conditions (180 min, 0.5 g L−1 catalyst dosage, pH 6.81, 10 mg L−1 TC), degradation efficiencies of 88.23% (Mercury 125 W), 85.93% (Tungsten 500 W), and 78.47% (LED 120 W) were achieved with pseudo-second-order kinetics. Reusability tests showed a ∼13% decline in performance after three cycles. Increasing background organic load (COD) from 0 to 768 mg L−1 prolonged the TC half-life from 87.72 to 144.37 min, indicating competitive inhibition. Scavenger studies identified superoxide radicals (O2•−) and holes (h+) as the dominant reactive species. In real-water matrices, TOC removal decreased due to matrix interference, with removal efficiencies of 68.6% (distilled water), 66.43% (surface water), and 37.17% (Treated Sewage water). LC–MS analysis detected low-molecular-weight by-products (m/z < 200), confirming TC degradation. After AOP treatment, GI values increased from 0.62 to 0.68 for Vigna radiata and 0.39 to 0.8 for Trigonella foenum-graecum, indicating reduced toxicity. PMS-assisted photocatalysis further enhanced TC removal to 94.42% within 2 h. Among the light sources evaluated, LED operation ($0.058 L−1) was the most economical option, supporting scalable PPCPs removal.