Reuse of steel slag as a photocatalyst for tetracycline degradation: mechanism of oxygen vacancies

Abstract

Realizing the high value-added utilization of solid waste steel slag in environmental catalysis has importance for energy saving and environmental protection. In this article, steel slag was handled by an alkali activation process. The final preparation of alkali-activated steel slag (A-SS) mainly contained calcium silicate hydrate (CSH) and brownmillerite (C₄AF). The degradation of tetracycline (TC) was used as a model reaction to explore the potential of A-SS in photocatalysis. The effect of various parameters on the TC degradation was investigated in detail. Using the A-SS, the degradation rate of TC could reach 70% within 40 min under visible light irradiation. Moreover, the A-SS demonstrated superior long-term durability, and could retain more than 80% of the initial activity even after four cycles. The degradation of TC by A-SS was consistent with the pseudo-first-order kinetic model. According to the UV-Vis diffuse reflection spectra (DRS), X-ray photoelectron spectroscopy (XPS), active species trapping tests, and electron spin resonance (ESR) spectra results, the presence of oxygen vacancies was confirmed and the catalytic mechanism for A-SS could be ascribed to the synergistic effects of physical adsorption and ˙O₂⁻ radicals produced by oxygen vacancies. The degradation pathways of TC degradation could thus be inferred. A-SS also could be used as a promising photocatalyst for real-world water treatment. This work explored a feasible way to achieve the high value-added utilization of steel slag, which is in line with the concept of green development.