聚合物改質過硫酸鹽氧化抑制低滲透性土壤污染反向擴散之可行性評估

三氯乙烯（trichloroethylene, TCE）等含氯有機溶劑污染整治長久以來面臨許多困境與挑戰，受其物化及傳輸特性影響，這類污染物容易藉由「正向擴散」（forward-diffusion）進入低滲透性土壤層中，當整治工程積極地將高滲透性土壤層之污染物移除時，濃度梯度反轉便造成污染物從低滲透性土壤層中「反向擴散」（back-diffusion）至高滲透性土壤層，引起污染物濃度拖尾（tailing）及回升（rebound）等現象，導致污染整治期程大幅拉長。惟現有污染整治技術，例如：現地化學氧化法（in-situ chemical oxidation, ISCO），不易克服土壤異質性所導致的優勢流（preferential flow）現象，整治藥劑不易與低滲透性土壤層內之污染物接觸並反應降解。為了得將氧化劑輸送（delivery）至低滲透性區域（low permeability zone, LPZ），本研究嘗試改良傳統現地化學氧化技術，透過添加水溶性生物聚合物提高氧化劑溶液之黏度（viscosity），以強化錯流（cross-flow）潛勢，藉此改善其於異質性含水層之輸送效率及分布狀況。本研究選擇三種生物多醣聚合物作為增稠劑，包括：黃原膠（xanthan, Xan）、羥乙基纖維素（hydroxyethyl cellulose, HEC）及關華豆膠（guar gum, GG），分別探究三者與過硫酸鹽（persulfate, PS, S_2O_8^(2-)）之間的相容性，並評估其消弭低滲透性土壤內污染物反向擴散現象之可行性。相容性試驗結果顯示，黃原膠與過硫酸鹽具有較佳之相容性；而砂箱實驗結果顯示，聚合物改質型氧化劑（polymer-amended oxidant, PA-Oxidant）相比傳統氧化劑，處理異質性土壤污染系統後較不會出現污染物濃度回升及拖尾等現象，更有機會將污染物濃度降至法規標準以下。

關鍵字

反向擴散；低滲透性區域；異質性；現地化學氧化法；含水層污染整治

並列摘要

Subsurface remediation of chlorinated solvents, such as trichloroethylene (TCE), has always been a challenging environmental issue. Owing to the properties of TCE and its migration behavior in the subsurface, chlorinated solvents tend to forward-diffuse into low-permeability soils, such as silt and clay. When groundwater contaminants in high-permeability soils, such as gravel and sand, are treated by active remediation, the concentration gradient between high- and low-permeability zones inverts, and the contaminants back-diffuse from low-permeability soils to high-permeability soils. Hence, low-permeability soils can serve as long-term sources of groundwater contamination. Conventional in-situ chemical oxidation (ISCO) remediation technologies may not overcome the preferential flow caused by the geological heterogeneity of aquifers. Aquifer heterogeneity tends to drive the injected fluids, such as oxidants, to flow preferentially through the higher permeability zones, bypassing low-permeability zones. These conditions limit the contact between remedial agents and contaminants, leading to tailing and rebound in aquifer contamination. The main goal of this study was to improve the effectiveness of conventional ISCO and deliver oxidants to low-permeability zones, thereby inhibiting the back-diffusion of chlorinated solvents. Water-soluble biopolymers as shear-thinning viscosifiers were blended into sodium persulfate solutions. The polymer additives could increase the viscosity of the oxidant solutions, enhancing the cross-flow potential and improving sweep efficiencies within a heterogeneous aquifer contaminated with chlorinated solvents. Mixtures of biopolymers (xanthan, guar gum, and hydroxyethyl cellulose) and the oxidant (sodium persulfate) were investigated to determine their compatibility. In this study, we recorded the changes in oxidant demand and solution viscosity over time and aimed to find the most compatible oxidant-polymer combination for polymer-amended ISCO (PA-ISCO). The application of polymer-amended oxidants to eliminate contaminant back-diffusion in low-permeability soils was tested in a two-dimensional sand tank. The results of the compatibility experiments showed that xanthan gum had a higher resistance to persulfate than the other polymers. Compared to conventional ISCO, PA-ISCO exhibited minor contaminant rebound and tailing after treatment and was more promising for reducing contaminant concentrations to meet the strict TCE cleanup standard.

並列關鍵字

back-diffusion ； low-permeability zones ； heterogeneous ； in-situ chemical oxidation (ISCO) ； aquifer remediation

參考文獻

Adamson, D.T., de Blanc, P.C., Farhat, S.K., Newell, C.J., (2016) Implications of matrix diffusion on 1,4-dioxance persistence at contaminated groundwater sites. Science of the Total Environment, 562, 98-107. doi:10.1016/j.scitotenv.2016.03.211。

Akbari, S., Mahmood, S.M., Nasr, N.H., Al-Hajri, S., Sabet, M., (2019) A critical review of concept and methods related to accessible pore volume during polymer-enhanced oil recovery. Journal of Petroleum Science and Engineering, 182, 106263. doi:10.1016/j.petrol.2019.106263。

Amundarain, J.L., Castro, L.J., Rojas, M.R., Siquier, S., Ramírez, N., Müller, A.J., Sáez, A.E., (2009) Solutions of xanthan gum/guar gum mixtures: shear rheology, porous media flow, and solids transport in annular flow. Rheologica Acta, 48, 491-498. doi:10.1007/s00397-008-0337-5。

Aramideh, S., Vlachos, P.P., Ardekani, A.M., (2018) Unstable displacement of non-aqueous phase liquids with surfactant and polymer. Transport in Porous Media, 126, 455-474. doi:10.1007/s11242-018-1168-1。

Atteia, O., Andre, L., Dupuy, A., Franceschi, M., (2005) Contributions of diffusion, dissolution, ion exchange, and leakage from low-permeability layers to confined aquifers. Water Resources Research, 41, 9412-9426. doi:10.1029/2003WR002593。

主題瀏覽