Enhanced remediation efficiency of Cr(VI)-contaminated heterogeneous aquifers: Improved sweeping efficiency using shear-thinning fluids.

Low permeability zones (LPZs) are typically bypassed when remedial reagents are injected into heterogeneous aquifers, which hinders the in situ remediation. Although shear-thinning polymers have emerged as promising tools to meet this challenge, their applicability in complex remedial systems remains unconfirmed. We investigated the sweeping efficiencies of calcium polysulfide (CPS) into Cr(VI)-contaminated LPZs using xanthan gum (XG) as the model shear-thinning polymer. Firstly, the compatibility of XG-CPS fluids and their reduction capacities toward Cr(VI) were demonstrated based on batch experiments. The removal rates of Cr(VI) exceeded 85% in the presence of 250-2000 mg/L of XG. Besides, XG-CPS fluids exhibited a greater impact on the permeability decrease of transmissive zones than that of LPZs as confirmed by sand column experiments. Furthermore, the sweeping efficiencies in LPZs during XG-CPS flooding were investigated by multiple sand tank experiments. The sweeping rate in LPZs (r) in Cr(VI)-contaminated aquifer (1.68 × 10/min) was found to be approximately 11% higher than that of uncontaminated system, and two possible reasons behind this phenomenon were proposed. The spatial distribution profiles of Cr under different XG-CPS flooding conditions were depicted based on 20 representative samples. The results indicated that all Cr(VI) in LPZs can be effectively removed either by displacement or immobilization as Cr(III). The percentages of displaced Cr(VI) and immobilized Cr(III) were calculated to be 65%-75% and 25-35%, respectively. This work demonstrates the applicability of XG-CPS fluids as remedial materials for Cr(VI)-contaminated heterogeneous aquifers and provides novel insights into the role of Cr(VI) in in situ remediation using shear-thinning polymers.