Accelerated chromate reduction by tea waste: Comparison of chromate reduction properties between water and ice...

The concentration of chromium (Cr) in natural water and soil environments has gradually increased in recent decades, owing to intensive use of Cr in industry and its subsequent disposal. In this study, we performed a comparison study on chromate (Cr⁶⁺) reduction by tea waste (green tea, black tea, red tea, and chamomile) in water (25 ^°C) and ice (-20 ^°C) to develop a new strategy for environmental-friendly stabilization of hazardous Cr⁶⁺ by freezing. This study shows that the freezing process can enhance the reduction of Cr⁶⁺ by tea waste. The residual Cr⁶⁺ concentration ratios (C/C₀, where C is the concentration of Cr⁶⁺ after the reaction (5 h) and C₀ is the initial concentration of Cr⁶⁺ (20 μM) in the system) by tea wastes in water were in the range of 0.71 (green tea) to 0.92 (chamomile); however, the ratios dramatically decreased under the freezing process (i.e., 0.06 by green tea, 0.13 by black tea, 0.18 by red tea, and 0.08 by chamomile). According to the results obtained from the fluorescent, chromatographic, and spectroscopic analyses, under the freezing process, the enhanced reduction of Cr⁶⁺ could be explained by the freeze concentration of Cr⁶⁺, phenolic components in tea extracts, and protons in small liquid pockets in liquid-like layers (LLLs). In addition, the proposed system can efficiently purify the real Cr⁶⁺-containing wastewater (i.e., electroplating wastewater), indicating that the system will be economically feasible in cold regions (i.e., polar regions).