Estimation and photocatalytic reduction of toxic chromium metal ions from environmental samples by zinc-based nanocomposite

Abstract

Hexavalent chromium (Cr(VI)) is a major water pollutant and suspected carcinogen with high persistence. Therefore, advanced and fast processes based on low-cost and highly proficient nanomaterials are required for its reduction. Ecological concern about this emerging contaminant calls for low-cost and efficient removal practices. Recently, doped or coupled nanomaterials with advanced characteristics have fascinated researchers. Therefore, we have synthesized a crystalline ZnO@ZnHCC nanocomposite using a plant extract of A. indica and subsequently used it for the reduction of Cr(VI). The reduction of Cr(VI) in simulated water (94%) and soil (86%) samples was then accomplished using the nanocomposite. Diffusion is slowed when the organic matter in soil and sediment interacts with Cr(VI), which may explain why these contaminants degrade more slowly. Under sunlight, a rapid initial exponential decrease in the concentration of Cr(VI) over time revealed the high photoactivity of doped ZnO@ZnHCC because of its remarkably improved surface area (118.15 m² g⁻¹) and bandgap energy (1.7 eV). At the optimum catalyst dose (25 mg) and neutral pH, photoreduction of Cr(VI) followed first-order kinetics and Langmuir adsorption isotherms. Moreover, the t_1/2 value of Cr(VI) (0.9 h) was reduced greatly compared to that of ZnO (t_1/2 = 1.3 h) and ZnHCC (t_1/2 = 1.1 h) nanoparticles. The photocatalytic reduction was probed using radical-scavenger analysis. A charge separation mechanism was supported by photoluminescence and UV reflectance studies. Overall, due to its greater surface activity, stability, reusability for up to ten cycles, and charge separation (e⁻/h⁺ pairs), which led to the promotion of large amounts of charge carriers, ZnO@ZnHCC might be expected to represent a promising photocatalyst for industrial applications with a bright future. To summarize, the ZnO@ZnHCC nanocomposite is believed to be a promising photocatalyst for environmental protection by virtue of its greater amount of active sites, high surface activity, low bandgap with charge separation, and semiconducting nature.