Study On Enhanced Calcium Polysulfide Delivery By Xanthan Gum For Chromium (â…¥) Polluted Heterogeneous Aquifer

In our country, groundwater is seriously polluted by chromium(Ⅵ). In-situ chemical reduction method is one of the most efficient methods to treat chromium(Ⅵ). Calcium polysulfide has attracted much attention due to its low cost, the wide range of p H conditions and the good removal effect. However, heterogeneity is unavoidable in the field, especially the existence of the low permeable zones, which lead to the pass flow of remedial amendment and greatly reduce the effect of restoration. The study of the restoration of heterogeneous aquifer pollution is necessary.As for the pass flow, scholars have investigated that increasing the viscosity of reagent by adding organic polymer(such as Xanthan gum) could enhance the delivery of remedial amendment in heterogeneous media. Consequently, the contact efficiency of reagent and pollutant in low permeable media improves and the removal efficiency of pollutant increases.The chromium( Ⅵ) is target pollutant, calcium polysulfide is remedial amendment, xanthan gum is modifying agent of viscosity in this study. This paper discussed the application of xanthan gum, which was a shear thinning polymer in enhancing restoring aquifer pollution of chromium(Ⅵ). The concrete research content and results were as follows:(1)Xanthan gum had some inhibitory effect on the removal of chromium(Ⅵ) and the reaction fitted well with the second dynamics equation in 45 minutes. Ions in groundwater had different effects on the reduction of chromium(Ⅵ): HCO3- had an inhibitory effect, Mg2+ had a promoting effect, while NO3-、SO42-、Cl-、Ca2+ and K+ impacted a little.(2) Ion in groundwater exerted profound influence on solution viscosity: cationic ions decreased the solution viscosity. In addition, the CPS also could decrease the viscosity of XG solutions. The addition of media could greatly reduce the viscosity of XG solutions because of physical adsorption and biological degradation. The rheological properties of XG solutions could be described by a power law model(η(γ) = Kγ-n), the presented K and n empirical relations(logarithmic relationship) may also provide theoretical basis for field applications.(3)The delivery of CPS and XG was not complete simultaneous, while the delivery remained independent. The presence of XG could slow down the flow of the reagent and make it distribute uniformly. Too high or too low velocity of inject were both not beneficial to the delivery distribution of reagent. The variation rule of media permeability coefficient was constant stage, fast deepening stage and steady stage. Furthermore, the XG concentration and the water level difference fitted well with the linear relation when the permeability coefficient remained unchanged, i.e. the XG concentration fitted well with the loss of permeability coefficient of the media.(4)When the heterogeneous formation was lens, the permeability coefficient of media in lens had a prominent effect on displacement, chromium(Ⅲ) primarily distributed in lens zone and the concentration in fine media was obvious higher than that in medium media. The injecting velocity had little influence on displacement and the distribution of chromium(Ⅲ) in media. When the heterogeneous formation was layered, layer order of media(coarse and medium match) and xanthan concentration(200mg/L and 300mg/L in experiment) influenced little on displacement. The rule of distribution of chromium(Ⅲ) in media reduced gradually from low permeable zone to high permeable zone.