Study of photophysical properties, photodegradation breakdown cost and recycling processes of Zn/SnO ₂ quantum dots via real industrial wastewater treatment

The present study investigates the synthesis, photophysical behavior, and photodegradation efficacy of SnO
2
and zinc-doped SnO
2
quantum dots (QDs) prepared via an ultrasonic-chemical method, applied to the degradation of industrial dye pollutants in real wastewater matrices. Two distinct calcination temperatures (275°C and 500°C) produced SnO₂ QDs with crystallite sizes averaging 3.80 nm (SnQD1) and 6.10 nm (SnQD2), respectively, confirmed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Zinc doping was performed at concentrations of 3% and 7% (SnZn1 and SnZn2), resulting in larger average particle sizes (9.3 nm and 10.2 nm). The observed quantum confinement effect is evidenced by energies band gap values ranging from 3.05 eV to 3.38 eV, as measured via UV-Vis diffuse reflectance spectroscopy (DRS) and Kubelka-Munk analysis, with Zn doping inducing mid-gap states that favor visible-light absorption and charge carrier separation. Photocatalytic degradation kinetics of Dianix blue dye revealed a pseudo-first-order reaction model with SnQD1 exhibiting a 43% higher rate constant (14.63 × 10⁻
3
s⁻
1
) than SnQD2 (10.22 × 10⁻
3
s⁻
1
), attributable to its larger Brunauer-Emmett-Teller (BET) surface area and smaller size (144 m
2
g
-1
vs. 106 m
2
g
-1
). Remarkably, SnZn1 outperformed all samples with a rate constant of 17.15 × 10⁻
3
s⁻
1
, demonstrating a 230% enhancement over SnZn2, underscoring the critical role of optimal doping in photocatalytic efficiency. Reactive species trapping identified hydroxyl radicals and photogenerated electrons as the principal oxidative agents, corroborating mechanistic pathways widely reported in semiconductor photocatalysis. The catalysts’ efficacy extended to simulated solar photodegradation of real industrial wastewater, where chemical oxygen demand (COD) reduction ranged from 55% to over 90%, surpassing regulatory thresholds for discharge. Recyclability tests over seven cycles confirmed the robust stability of SnQD1 and SnZn1, with minimal performance degradation and preserved structural integrity demonstrated by Fourier transform infrared (FTIR) analysis. An economic assessment revealed that SnZn1 and SnQD1 offered the most cost-effective treatment, reducing photodegradation operational costs by approximately 23% and 9%, respectively, relative to their higher-temperature or higher-doped counterparts. This comprehensive evaluation establishes the ultrasonic-chemical synthesis of SnO
2
-based QDs as a scalable, economically viable approach for sustainable wastewater treatment, combining superior photocatalytic performance, stability, and cost efficiency. The insights into particle size, surface area, doping concentration, and photophysical properties provide valuable design parameters for advancing metal oxide QD photocatalysts in environmental remediation applications.