Abstract
Shear-thinning polymers have been introduced to contaminant remediation in the subsurface as a mobility control method applied to mitigate the inefficient delivery of remedial agents caused by geological heterogeneity. Laboratory experiments have been conducted to assess the compatibility of polymers (xanthan and hydrolyzed polyacrylamide (HPAM)) and oxidants (KMnO4 and Na2S2O8) through quantitative evaluation of the viscosity maintenance, shear-thinning performance, and oxidant consumption. The mechanism that causes viscosity loss and the influence of the groundwater environment on the mixture viscosity were also explored. The xanthan–KMnO4 mixture exhibited the best performance in both viscosity retention and shear-thinning behavior with retention rates higher than 75% and 73.5%, respectively. Furthermore, the results indicated that xanthan gum has a high resistance to MnO4− and that K+ plays a leading role in its viscosity reduction, while HPAM is much more sensitive to MnO4−. The viscosity responses of the two polymers to Na2S2O8 and NaCl were almost consistent with that of KMnO4; salt ions displayed an instantaneous effect on the solution’s viscosity, while the oxide ions could cause the solution’s viscosity to decrease continuously with time. Since xanthan exhibited acceptable oxidant consumption as well, xanthan–KMnO4 is considered to be the optimal combination. In addition, the results implied that the effects of salt ions and the water pH on the mixture solution could be acceptable. In the 2D tank test, it was found that when xanthan gum was introduced, the sweeping efficiency of the oxidant in the low-permeability zone was increased from 28.2% to 100%. These findings demonstrated the feasibility of using a xanthan–KMnO4 mixture for actual site remediation.
Highlights
In situ chemical oxidation (ISCO) is a chemical approach applied to treat organic contaminants in the subsurface, which can be traced back to the early 1990s [1,2,3,4,5,6], when groundwater contaminated by chlorinated solvents was widespread [7]
This paper focuses on assessing the compatibility of polymer–oxidant mixtures through quantitative evaluation of viscosity maintenance, shear-thinning performance, and oxidant consumption, as well as exploring the mechanism that causes viscosity loss
HPAM, xanthan gum showed higher viscosities under the same conditions polymer solutions exhibited shear-thinning behavior; that is, the viscosity decreased with increasing shear rate for both polymers
Summary
In situ chemical oxidation (ISCO) is a chemical approach applied to treat organic contaminants in the subsurface, which can be traced back to the early 1990s [1,2,3,4,5,6], when groundwater contaminated by chlorinated solvents was widespread [7] Among these compounds, chloroethenes such as perchloroethene (PCE) most commonly exist in the subsurface [8,9] due to their wide use as degreasing solvents and dry cleaners in industry and to poor emission management afterward. The oxidizing process of MnO4 − is achieved by electron transfer, which is relatively slow and appears to be more suitable for subsurface treatment [8] Another test oxidant is sodium persulfate (Na2 S2 O8 ), which has received growing attention in the field of ISCO in recent years, and its oxidation reaction is completed by free radical production, a mechanism quite different from that of KMnO4. Considering the actual site conditions, the influences of the groundwater environment, including the groundwater pH, the main salt ions, and their relative effect strength on the viscosity and the rheological behavior of the polymer–oxidant mixture were evaluated to assess the feasibility of this method
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