ZnO quantum dots for wastewater Remediation: Phytotoxicity, photocatalytic performance, and economic assessment

Addressing persistent organic pollutants like Brilliant Blue R (BBR) dye in industrial effluents necessitates efficient photocatalysts. This work details zinc oxide quantum dot (ZnQD) synthesis via a modified wet chemical route, using controlled aging to induce quantum confinement. Comprehensive characterization confirmed crystalline wurtzite ZnQDs. Two batches, ZnQD1 and ZnQD2, yielded distinct average crystallite sizes (∼3. 9 nm and ∼7. 8 nm via TEM), influencing optoelectronic properties. ZnQD1 displayed a wider band gap (3. 16 eV vs. 3. 10 eV for ZnQD2) and blue-shifted optical spectra, confirming quantum confinement. Correspondingly, ZnQD1 exhibited a vastly larger specific surface area (358. 4 m 2 /g vs. 46. 6 m 2 /g for ZnQD2). Photocatalytic degradation of BBR was assessed under xenon and solar light. ZnQD1 demonstrated superior performance, yielding 17. 4×10 -3 min −1 more than twice of BBR degradation efficiency via ZnQD2 (7. 5×10 -3 min −1), directly linking enhanced activity to smaller size, wider band gap, and larger surface area. While degradation occurred under both light sources, xenon irradiation provided faster kinetics. The catalyst showed excellent operational stability, retaining high efficiency over 12 recycling tests reached to 66-71% from the 1 st recycling process which reached to12. 5 x 10 -3 and 10. 8 x 10 -3 for ZnQD1 and ZnQD2, respectively. Crucially, the study extended to practical considerations, including preliminary phytotoxicity assessments to evaluate environmental safety via germination rates for viable tomato seeds under optimal conditions which are typically over 90%, confirming the strong inhibitory effect of the untreated dye. Also, an initial economic analysis focused on mineralization costs, likely using metrics like electrical energy per order (E E/O) which the overall treatment cost per cubic meter is 27. 68 for ZnQD1 and 33. 10 for ZnQD2. This research highlights the efficacy of tuning ZnQD properties through size control via a facile synthesis, establishes a clear structure-activity relationship for BBR photodegradation, and incorporates vital assessments of stability, environmental impact, and economic feasibility relevant for potential wastewater treatment applications.