Improving the treatment of non-aqueous phase TCE in low permeability zones with permanganate
Graphical abstract
Introduction
The release of dense non-aqueous liquids (DNAPL) into the subsurface typically results in the formation of disconnected blobs and ganglia depending on the soil's physical heterogeneity [1], [2]. Dissolved contaminants migrating from DNAPL source zones will often diffuse from transmissive regions into low permeable zones (LPZs). The residual build up of contaminants in LPZs over time becomes particularly challenging for injection-based remedial treatments because chemical oxidants typically bypass low porosity zones. Given that the mass of contaminant stored in low permeability soils can be substantial and that these LPZs can serve as a long-term source of contamination, removing chlorinated solvents like trichloroethene (TCE) from low permeablity zones is recognized as one of the most difficult problems associated with groundwater pollution [3].
Although permanganate is extremely efficient in oxidizing TCE [4], [5], treating non-aqueous phase TCE trapped in low permeable zones has significant challenges. When permanganate (MnO4−) is used, the three hurdles to successfully treating non-aqueous phase TCE in LPZs include: (i) getting the MnO4− to penetrate and not bypass lower porosity zones where the contaminant is located, (ii) minimizing MnO2 rind formation at the MnO4−–DNAPL interface, which can block or inhibit the DNAPL from further oxidant contact, and (iii) overcoming the kinetic constraints of treating a sparingly soluble DNAPL with an aqueous-phase oxidant. While significant progress has been made in combating these challenges, improving the treatment of chlorinated solvents in LPZs is still an active area of research.
To get remedial fluids to penetrate LPZs, shear-thinning polymers like xanthan have been used as a co-injected remedial agent to increase the viscosity of the displacing fluid [6], [7], [8], [9], [10], [11], [12], [13] and stimulate cross-flow between layers that differ in permeability [14], [15]. Smith et al. [16] provided the first evidence that xanthan was compatible with MnO4− and could be used as a polymer-enhanced chemical oxidation treatment for perchloroethylene (PCE). Their work provided important groundwork for further studies aimed at determining what polymer and oxidant concentrations were needed to effectively oxidize chlorinated solvents in low permeability zones during transport. McCray et al. [17] showed that use of xanthan increased the sweeping efficiency of MnO4− into LPZs (containing non-aqueous phase PCE) and improved the percentage PCE oxidized. Recently, Chokejaroenrat [18] found that adding xanthan enhanced MnO4− delivery into LPZs to treat aqueous-phase TCE and minimized the potential for rebound.
Treating non-aqueous phase TCE with MnO4− is more challenging than treating aqueous phase TCE due to the higher propensity for manganese dioxide (MnO2) to form. Many researchers who have treated non-aqueous phase DNAPL with MnO4− have reported the formation of distinct MnO2 rinds, which can protect the contaminant from further contact with the oxidant e.g., [19], [20], [21], [22], [23], [24]. Moreover, substantial MnO2 deposits have the potential to alter the advective flow of the oxidant from the target zone [23], [25], [26], [27]. One way researchers have combated this problem is by recognizing that soluble Mn(IV) and colloidal Mn(IV) precede the aggregation and formation of the insoluble MnO2 product. This has given rise to the use of stabilization aids. Mata-Perez and Perez-Benito [28] found that the conversion rate of soluble Mn(IV) to MnO2(s) could be delayed when phosphate was present. Kao et al. [29] found that ∼82% of MnO2 production could be inhibited by including Na2HPO4 with MnO4− without affecting TCE loss. Crimi and Ko [30] tested a variety of stabilization aids and reported that sodium hexametaphosphate (SHMP) was superior in minimizing MnO2 formation.
Phase transfer catalysts are agents that facilitate reactions between two or more phases (i.e., polar, non-polar) and allow reactions to occur that otherwise might be inhibited [31]. The idea for using phase-transfer catalysts would be to allow some of the oxidant (i.e., MnO4−) to partition into the non-aqueous phase DNAPL so that oxidation can occur in both the organic and bulk aqueous phases and reduce the time needed to remove the non-aqueous phase product. Seol and Schwartz [32] and Seol et al. [33] used the phase transfer catalyst, pentyltriphenylphosphonium bromide (PTPP) in bench studies and reported increased dechlorination of both TCE and PCE. Although the initial results were promising, reports of using phase-transfer catalysts under transport conditions have not yet been reported.
In this study, our objective was to improve the sweeping efficiency of MnO4− into LPZ and increase the percentage of 14C-TCE oxidized by MnO4−. This was accomplished by creating a LPZ that had a high concentration of TCE and then treating with a variety of solutions that paired permanganate with: (i) a shear-thinning polymer; (ii) stabilization aids that minimized MnO2 formation and (iii) a phase-transfer catalyst. Both transport and batch experiments were performed to quantify the efficacy of these chemical additives to improve TCE oxidation.
Section snippets
Chemicals and soils
Trichloroethene (TCE; C2HCl3; ACS reagent, ≥99.5%), Oil-Red-O (an organic-soluble dye, C26H24N4O), hydrazine hydrate (35 wt% in H2O), xanthan gum (CAS-11138-66-2) and ethyl acetate were obtained from Sigma-Aldrich (St. Louis, MO). Potassium permanganate (KMnO4) was obtained from Fisher Scientific (Pittsburgh, PA). Additional chemicals included: acetonitrile (Midland Scientific), nitric acid (J.T. Baker, Phillipsburgh, NJ), and sodium hydroxide (Fisher Scientific, Pittsburgh, PA).
Stabilization
Transport experiments
Multiple transport experiments were performed and photographed to systematically evaluate the ability of MnO4− to penetrate the LPZ and react with the non-aqueous phase TCE, with and without chemical additives (Fig. 3). Using only MnO4−, the injection front moved quickly through the transmissive zone and eventually into the LPZ (Fig. 3, Exp. A). Within 1.5 PV, a distinct rind of precipitated MnO2 began to form around the non-aqueous phase TCE. Eventually, the rind engulfed much of the LPZ and
Discussion
SHMP and TKPP are considered dispersants or stabilization aids, which mean they stabilize colloids by inhibiting particle aggregation, which leads to precipitation. The multiple mechanisms by which this occurs with MnO4− have been detailed elsewhere [30], [43], [44], [45], [46] but in brief the colloidal stability of hydrous oxides is strongly dependent on their net charge. The higher the net charge of the oxide surface, either positive or negative, the greater the repulsive forces of the
Conclusion
Improvements in the treatment of high concentrations of TCE in low permeability zones were accomplished by pairing permanganate with xanthan and stabilization aids. Using xanthan with MnO4− improved the sweeping efficiency of permanganate into LPZs but MnO2 rinds prevented good contact between the oxidant and the DNAPL. By including sodium hexametaphosphate (SHMP) with xanthan, MnO4− covered all of LPZ and no MnO2 rinds were observed. Batch experiments with LPZ cylinders also demonstrated that
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