Green synthesis, kinetics and photoactivity of novel nickel oxide-decorated zinc hexacyanocobaltate catalyst for efficient removal of toxic Cr(VI)

Abstract Chromium (VI) is a major water pollutant and suspected carcinogen with high persistence. Therefore, advance and fast processes based on low-cost and highly proficient nanomaterials are required for its elimination. Herein, nickel oxide-decorated zinc hexacyanocobaltate framework (NiO@ZnHCC) was synthesized by green method and subsequently, evaluated as photocatalyst for removal of Cr (VI) from synthetic wastewater. Various parameters were optimized in simulated water consisting of variable amounts of Cr(VI) (50–250 mg L−1) and catalyst dose (5–25 mg) at different pH (4–9)under sunlight exposure for 5 h. Highly crystalline nanocomposite (particle size range: 50–100 nm) consisting of NiO wrapped ZnHCC cubes piled together was confirmed by spectroscopic and microscopic analysis. At optimum catalytic dose (15 mg) and neutral pH, sharp decline in 50 mg L−1of Cr (VI) to Cr (III)was visually confirmed by colour change from orange to green. Highest removal (92%) of Cr (VI) by NiO@ZnHCCnanocomposite (Langmuir Xm= 39 mg g−1) followed first order kinetics and indicated its greater efficiency as compared to individuals (NiO: 79% and ZnHCC: 86%). This might be due to improved surface area (78.9 m2 g−1), low band energy (2.1 eV) and semiconducting nature resulted from synergism of NiO (32.4 m2 g−1; 3.6 eV) and ZnHCC (38.9 m2 g−1; 2.3 eV). Moreover, NiO@ZnHCC reduced the half-life of Cr(VI) up to 1.1 h than that with ZnHCC (2.3 h) and NiO (3 h). Photo-catalytic reduction was probed using radical-scavenger analysis. Charge separation mechanism was supported by photoluminescence and UV reflectance studies. Overall, due to greater surface activity, stability, reusability up to ten-cycles and charge separation (e-+h+ pairs) led to promotion of huge free radicals, NiO@ZnHCC might be supposed as promising photocatalyst for industrial applications with bright future.