ZnO-based photocatalysis for the treatment of synthetic laundry wastewater derived from textiles: Performance optimization and ecotoxicological assessment

The increasing demand for sustainable water management calls for effective treatment technologies capable of addressing the high pollutant load and toxicity of textile- and laundry-derived effluents. In this study, the photocatalytic degradation of industrial textile inks and synthetic laundry wastewater (SLW) was investigated using commercial ZnO under solar-simulated irradiation. Process optimization was carried out by evaluating the effect of catalyst concentration (0.25–2.00 g/L), initial pH (3–10), and catalyst reuse over multiple cycles. Scavenger experiments were performed to elucidate the reaction mechanism, while a comparison with TiO₂ allowed benchmarking of photocatalytic performance. ZnO demonstrated excellent degradation capability toward both individual and mixed inks, achieving up to 100 % color removal but limited COD reduction ('25 %), indicating partial mineralization of residual organic matter. For SLW, ZnO reached a maximum degradation efficiency of 75 %, with 92 % color removal and 45 % COD reduction at an optimal concentration of 1 g/L and natural pH (6.7). Scavenger tests revealed that photogenerated holes (h⁺) were the dominant reactive species. ZnO outperformed TiO₂ under the same conditions, confirming its higher responsiveness to solar light. The catalyst maintained high efficiency ('75 %) over four consecutive reuse cycles. Ecotoxicity tests using Artemia franciscana showed that ZnO photocatalytic treatment reduced the intrinsic toxicity of ink mixtures and SLW; however, residual toxic effects were observed, mainly attributable to the presence of ZnO powders. Overall, the study presents a proof-of-concept for the use of ZnO as a reusable photocatalyst for solar-assisted treatment of colored effluents, showing optimal performance under near-neutral pH, while highlighting the need for catalyst immobilization and extended irradiation periods to enhance COD reduction and improve ecotoxicological compatibility.