Reductive transformation of hexavalent chromium by ferrous ions in a frozen environment: Mechanism, kinetics, and environmental implications

Inorganic substrates in frozen solutions: Transformation mechanisms and interactions with organic compounds - A review

In cold environments, such as polar regions and high latitudes, the freezing of aqueous solutions plays a crucial role in releasing and transforming nutrients, organic compounds, and trace gases. Freezing processes typically affect biogeochemical cycles and environmental processes by reducing the rate of chemical reactions. However, substantial studies have found that some chemical reactions may accelerate unexpectedly under freezing conditions. These reactions include oxidation of nitrite, dissolution of metals/metal oxides, transformation of halogen species, etc. Although freezing process significantly affects the interaction between the inorganic substrate and coexisting organic compounds, there are few review articles on the behavior of the inorganic compound. Therefore, this review examines the transformation behavior of inorganic substrates and their interactions with organic compounds during freezing. The transformation behavior of inorganic substrates during freezing was comprehensively discussed, their underlying mechanisms were elucidated, and the interactions between inorganic substrates and coexisting organic compounds were highlighted. Meanwhile, key factors influencing the freeze-induced chemical processes were articulated. Furthermore, the potential application of freezing reactions in engineering processes is explored. This article aims to improve understanding of the important role of freezing processes in the recycling of substrates in the natural environment and supplement knowledge in the field of ice chemistry.

Visible Light Boosted the Reduction of Cr(VI) in the Absence of Conventional Reductants in Frozen Solutions: The Overlooked Role of Inorganic Anions

The conversion of Cr(VI) in frozen solutions is a mysterious process in the water environment. It has traditionally been believed that the reduction of Cr(VI) can only occur through interaction with traditional reducing agents like natural organic matters (NOMs) and Fe(II), with H2O merely acting as a solvent. However, this study reveals that visible light can induce the reduction of Cr(VI) by H2O even without conventional reducing agents. Specifically, within the narrow liquid-like layer between ice crystals, there is a significant increase in concentrations of protons, Cr(VI), and anions (Cl-, SO42-, and NO3-), which promotes the formation of various complexes between Cr(VI) and anions. DFT results reveal 11 types of stable Cr(VI)-anion complexes in solution, with six exhibiting visible light absorption properties. Quadrupole time-of-flight (QTOF) mass spectrometry confirms that the abundance of these six complexes (Cl-CrO2-Cl, Cr2O6-Cl, Cl-Cr2O5-Cl, Cr2O6-OSO3H, HO3SO-Cr2O5-OSO3H, and Cr2O6-ONO2) correlates with the extent to which visible light promotes Cr(VI) reduction, thus highlighting their crucial role as photoreactive intermediate complexes during this conversion process. These findings suggest that H2O in frozen solutions should no longer be regarded solely as a solvent but also as a reactant, thereby inspiring deeper insights into frozen solution chemistry.

Grinding siderite with ferric sulfate to generate an active ferrous source for Cr(VI) reduction

Activated siderite, endowed with excellent properties, was simply prepared by co-grinding with Fe sulfate to enhance its high reducing ability for Cr(VI). Batch experiments were conducted to investigate the main affecting parameters, such as material ratio, pH, temperature, etc. The removal of Cr(VI) by activated siderite was completed within 4 h of the reaction. The activated siderite maintained a high removal effect of Cr(VI) within a wide pH range (3-9). Various analytical methods, including XRD, SEM/EDS, XPS, etc., were employed to characterize the samples and discover variations before and after the reaction. The Fe (Ⅱ) in activated siderite becomes highly active, and it can even be released from the solid phase in the mildly acidic liquid phase to efficiently reduce Cr(VI) and mitigate its toxicity. These findings introduce an innovative approach for activating various minerals widely distributed in nature to promote the recovery of the ecological system.

Freezing-enhanced chlorination of organic pollutants for water treatment

Freezing has been reported to accelerate chemical reactions and thus affect the fate of pollutants in the environment. However, little research has been conducted on the potential effects of freezing on the chlorination process. This study aimed to explore the freezing-enhanced chlorination process by comparing the oxidation of clofibric acid (CA) by chlorine in ice (at -20 °C) to the same reaction in water (at 25 °C). The degradation of CA, which was negligible in water, was significantly accelerated in ice. This acceleration can be attributed to the freeze concentration effect that occurs during freezing, which excludes solutes such as chlorine, CA and protons from the ice crystals, leading to their accumulated concentration in the liquid brine. The increased concentration of chlorine and protons in the liquid brine leads to higher rates of CA oxidation, supporting the freeze concentration effect as the underlying cause for the accelerated chlorination of CA in ice. Moreover, the chlorine/freezing system was also effective in the degradation of other organic pollutants. This highlights the environmental relevance and significance of freezing-enhanced chlorination in cold regions, particularly for the treatment of organic contaminants.

Detoxification of arsenite by iodide in frozen solution

The oxidation of arsenite (As(III)) to arsenate (As(V)) has received significant attention because it helps mitigate the hazardous and adverse effects of As(III) and subsequently improves the effectiveness of arsenic removal. This study developed an efficient freezing technology for the oxidative transformation of As(III) based on iodide (I-). For a sample containing a very low concentration of 20 μM As(III) and 200 μM I- frozen at -20 °C, approximately 19 μM As(V) was formed after reaction for 0.5 h at pH 3. This rapid conversion has never been achieved in previous studies. However, As(V) was not generated in water at 25 °C. The acceleration of the oxidation of As(III) by I- in ice may be attributed to the freeze-concentration effect. During freezing, all components (i.e., As(III), I-, and protons) are highly concentrated in the ice grain boundary regions, resulting in thermodynamically and kinetically favorable conditions for the redox reaction between As(III) and I-. The efficiency of the oxidation of As(III) using I- increased at high I- concentrations and low pH values. The low freezing temperature (below -20 °C) hindered the oxidative transformation of As(III) by I-. The efficiency of the oxidation of As(III) significantly increased using a fixed initial concentration of I- by subjecting the system to six freezing-melting cycles. The outcomes of this study suggest the possibility of the self-detoxification of As(III) in the natural environment, indicating the potential for developing an eco-friendly method for the treatment of As(III)-contaminated areas in regions with a cold climate. It also demonstrates radical remediation to almost completely remove a very small amount of As(III) that was input in As(III)-contaminated wastewater detoxification, a benchmark that existing methods have been unable to achieve.

Improved reduction efficiency, cycling performance, and removal rate of hexavalent chromium by adding water-soluble salts

Recently, the reaction speed and cycle performance of hexavalent chromium reduction over microsized zero-valent iron (ZVI) with an Fe0 core and iron oxide (FeOx) shell structure have been improved by activating the Fe0-core electrons through electromagnetic coupling between Fe0-core electrons and charges (hexavalent chromium in solution, double-charge layers of the ZVI/solution interface). Herein, the abovementioned electromagnetic coupling was greatly increased by adding salt (CH3COONa, NaCl, NaNO3, and Na2SO4) in the hexavalent chromium solution to increase the charge response. Adding salt greatly improved the reaction speed and cycle performance of hexavalent chromium reduction. It took 8 min to reduce hexavalent chromium with CH3COONa to below the discharge standard of wastewater in the first cycle and 20 min after reducing for 20 cycles. The best apparent rate of constant value (0.416 (min)-1) is nearly four times larger than those without salts. X-ray diffraction and X-ray photoelectron spectroscopy revealed the production of amorphous iron oxide shell with salt. The salt improves the hexavalent chromium reduction speed and cycle performance and impedes the Fe0-core-electron transfer via the produced Fe2O3, resulting in existence of an optimized salt dosage. This work aims to provide an effective route for enhancing the removal efficiency and cycle performance of heavy-metal-ion reduction via Fe0. And this work also proposes a novel viewpoint that adding salt in waste water would increase the electromagnetic coupling between the charges in solution and Fe0-core electrons which could finally activate the redox reaction.

Reductive Transformation of Hexavalent Chromium in Ice Decreases Chromium Toxicity in Aquatic Animals

In this study, the toxicity of hexavalent chromium [Cr(VI)] reduced by citric acid in ice was measured using representative aquatic model invertebrates (i.e., rotifer, water flea, amphipod, and polychaete) and a vertebrate (zebrafish) by analyzing short- and/or long-term endpoints that are frequently applied to each animal. Cr(VI) reduction in the presence of citric acid was markedly enhanced in the ice phase compared to that in an aqueous solution through the freeze concentration effect. The highly concentrated Cr(VI) and citric acid in ice grain boundaries were also confirmed using in situ cryogenic confocal Raman spectroscopy. Overall, exposure to Cr(VI) resulted in higher acute and/or chronic effects on aquatic animals, such as drastic mortality, growth inhibition, and decrease in offspring number, whereas the animals were increasingly tolerant to Cr(VI) that was reduced in the ice phase. Sublethal concentrations of Cr(VI) significantly decreased the antioxidant capacity in the aquatic animals. However, when the same concentrations of Cr(VI) were reduced in ice, these treatments showed no modulation or increase in the antioxidant defense system. Taken together, our results suggest that Cr(VI) reduction into Cr(III) was successfully achieved in ice and that this methodology can decrease the actual toxicity of Cr(VI) in aquatic animals.

Chromium Distribution, Leachability and Speciation in a Chrome Plating Site

Hexavalent chromium (Cr(VI)) waste produced by chrome plating activities pollutes the surrounding environment and harms human health. However, information about the chromium (Cr) pollution characteristics of actual electroplating sites is still lacking. In this study, the concentration, leachability and speciation of Cr in soils from a typical chrome plating site were analyzed. Our results showed that this site was severely contaminated by Cr (7.2 to 7735.2 mg/kg) and Cr(VI) reached the mean concentration of 138.7 mg/kg. The spatial distribution of Cr(VI) was related to the plating processes. Chrome plating and sewage treatment areas could be considered as the hot spots of contaminated sites. The vertical distribution of Cr(VI) was mainly affected by soil properties, where the loam layer retained and reduced a large amount of Cr(VI) due to its high content of iron minerals and finer particle fractions. Additionally, the chemical extraction results showed that Cr was mainly in non-residual fractions and the existence of Cr(VI) led to a high leaching toxicity based on the toxicity characteristic leaching procedure (TCLP) results. Moreover, X-ray photoelectron spectroscopy (XPS) results revealed the speciation of Cr in the long-term contaminated soils. A large amount of Cr(VI) was reduced into Cr(III) and mainly existed as Cr(OH)3 and Cr2O3. Furthermore, Cr(VI) tended to precipitate as CaCrO4 and persisted in soils. Therefore, it is necessary to find appropriate strategies to remediate these contaminated soils. Overall, these findings strengthen our understanding of Cr(VI) behaviors and lay a foundation for the future pollution investigation, ecological remediation and risk assessment of sites contaminated by electroplating.

Ti3C2Tx MXene sensor for rapid Hg2+ analysis in high salinity environment

Mercury is one of the leading chemicals of concern and receives much attention in environmental safety. It is of great necessity to develop advanced Hg²⁺ analysis method for rapid detection and monitoring. Field-effect transistor (FET) sensor, an emerging electronic sensor, has received great attention in environmental analysis since it has unique advantages in achieving rapid analysis of chemicals. Herein, an FET sensor is constructed with Ti₃C₂T_x MXene as the channel material to detect Hg²⁺ in water. The sensor displays rapid and selective response to Hg²⁺. Moreover, the sensor achieves satisfactory performance in Hg²⁺ detection in high salinity environment (1 M NaCl), which benefits its applications in real water analysis. Based on the investigation of sensing mechanism, the strong response of Ti₃C₂T_x MXene FET sensor to Hg²⁺ is due to the adsorption and reduction of Hg²⁺ to Hg⁺ on the Ti₃C₂T_x surface. This reported label-free Ti₃C₂T_x MXene platform can detect Hg²⁺ in high salinity environment with high specificity, which has significant application potential for on-site monitoring and risk assessment of Hg²⁺ in aqueous systems.

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

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).