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

Abstract

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).