| Hexavalent chromium is a kind of typical heavy metal pollutant in groundwater.it would cause serious contamination of surface water, groundwater and soil in local area. In-situ remediation technology is a promising remediation technology for heavy metal contaminated groundwater. Therefore, it is significant to search for economic,efficient and environmentally friendly in-situ remediation reagent. Industrial syrup,also known as molasses, is a sticky, dark brown, semifluid by-products produced by sugar industry, which possesses the advantages of economic and no secondary pollution. Yet little is known about its remediation mechanisms, related technical parameters and hydrogeochemical change in the process of in-situ remediation with syrup.The research discovered the feasible of Cr(VI) chemical reduction with syrup,and analyzed the reaction mechanism of in-situ remediation Cr(VI) contaminated aquifer with syrup by batch experiments. It clarified the mechanisms and kinetics of Cr(VI) reduction with syrup without adding an effective strain. Moreover, the effects of groundwater commons ions on Cr(VI) chemical reduction with syrup was evaluated in this work. Besides, this paper studied the migration of syrup in the aquifer, and analyzed the variation of water quality parameters and permeability in the process of running column experiments. Finally, this paper evaluated the efficiency of in-situ remediation Cr(VI) contaminated groundwater with syrup by column experiments, and analyzed the formation process of Cr(VI) reduction reaction zone and hydrogeochemical change by running the simulation tank experiment, providing theoretical and the related technical parameter support for the application of syrup in actual site. The concrete contents and results are as follows:1. Research on mechanism of in-situ remediation Cr(VI) contaminated aquifer with syrup(1) The mechanisms of in-situ remediation Cr(VI) contaminated aquifer with syrup are the reaction process of combination chemical reduction and biological reduction, in which functional microorganism divided into chromium reducing bacteria and iron reducing bacteria.(2) At p H<2.5, biological reduction is restrained, and the mechanism of in-situ remediation Cr(VI) contaminated aquifer with syrup mainly is chemical reduction. At p H>3.0, its mechanisms are the reaction process of combination chemical reduction and biological reduction. Cr(III) produced in the reaction system deposited on the surface of aquifer medium by Fe-Mn oxide combination form and organic matter-bound.2. Mechanisms and influence factor of Cr(VI) chemical reduction with syrup(1) Its mechanism is that reducing organic matter such as plant polyphenol in sugarcane syrup can reduce Cr(VI) to Cr(III), and in the process oxidizes reducing organic matter to an organic matter that has a benzoquinone structure or other oxidation products.(2) The reaction was pseudo-first order with respect to the Cr(VI) concentration. Decreasing the initial p H value, increasing the temperature, increasing the initial syrup concentration and decreasing the initial Cr(VI) concentration could promote Cr(VI) reduction and improves the removal efficiency and reaction rate..(3) Fe(III) and Al(III) could accelerate Cr(VI) reduction by syrup over a wide p H range. Ca(II) and Mg(II) showed inhibitory effects on Cr(VI) reduction by organic reduction substances from syrup under acidic conditions.3. Transport properties of syrup in saturated porous media(1) Syrup was easy to migrate in saturated porous media, and quickly stimulated indigenous microbial metabolism, forming a reducing environment. However, simulation experiment showed the blocking phenomenon when porous media was silt sand or fine sand.(2) At Brix 70%, it was difficult for injecting high concentration of syrup into fine sand porous media. And, high concentration of syrup was diluted during the migration process when porous media was coarse sand or medium sand.(3) The migration path of syrup presented a parabolic shape in two-dimensional tank. In the horizontal direction, the relationship between migration distance and time showed a linear function relation. In the vertical direction, the relationship between migration distance and time showed the nonlinear quadratic function equations.5. Research on efficiency of in-situ remediation of Cr(VI) contaminated groundwater with syrup by one-dimensional simulation experiment(1) When CCr(VI) was 100mg/L, the removal efficiency of in-situ remediation Cr(VI) contaminated groundwater with syrup was less than 2.5% by chemical reduction, and biological remediation was inhibited. When CCr(VI) was 10 and 50 mg/L, the removal efficiencies of Cr(VI) were 99.86% and 39.00%, its mechanisms were the reaction process of combination chemical reduction and biological reduction(2) Increasing syrup concentration, decreasing medium particle diameter and decreasing groundwater velocity could improve Cr(VI) removal efficiency.6. Efficiency analysis of in-situ remediation of Cr(VI) contaminated aquifer with syrup(1) The distribution characteristics of syrup in two-dimensional tank were that Brix where near injection well and at the bottom of tank was larger, and then that along the direction of groundwater flow and from bottom to up was gradually decreasing, and a regional where is the top right-hand corner of the groove was the blind spot of injection well. Cr(VI) was quickly reduced to Cr(III) in reaction zone,(2) Syrup built Cr(VI) reduction reaction zone successfully in the region of the pollution. In the process of experiment operation, OD600 was increased quickly in reaction zone, and its distribution characteristics were similar to that of syrup. DO was decreased from about 9.0mg/L to below 0.5mg/L, and ORP was decreased from about 290 m V to below-450 m V. p H was gradually decreased, and it was below 5.0 near injection well and at the bottom of tank. The permeability coefficient was decreased from 2.31×10-4m/s to 1.40×10-4~1.54×10-4m/s near injection well resulting from microorganisms multiply.(3) The maximum concentration that could be remediation by reduction reaction zone of industrial syrup was about 100 mg/L.(4) Aeration promoted the transfer of Cr(III) from the solution to the surface of the medium, and Cr(III) can stable in underground environment.The innovations of this research are:(1) On the basis of bioremediation, paper found the chemical way on Cr(VI) reduction with syrup, and illustrated the remediation mechanisms on in-situ remediation Cr(VI) contaminated groundwater with syrup. Determine the p H condition when the restoration mechanism is mainly based on the biological reduction and when the restoration mechanism is mainly based on the chemical reduction, making up for the inadequacy of the reaction mechanism for in-situ remediation of Cr(VI) contaminated groundwater with syrup.(2) Explored the migration characteristics of syrup in the aquifer, and determine the appropriate extent of Brix in the remediation process, providing theoretical basis reference for relevant technical parameters of in-situ remediation of Cr(VI) contaminated groundwater with syrup.(3) Discussed the effect of syrup concentration, Cr(VI) concentration and particle size on efficiency of in-situ remediation Cr(VI) contaminated groundwater with syrup. Analyzed the efficiency, water quality parameters and permeability coefficient of in-situ remediation of Cr(VI) contaminated groundwater with syrup, providing references for technical parameters of in-situ remediation of Cr(VI) contaminated groundwater with syrup. |
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