Degradation of chlorinated solvents with reactive iron minerals in subsurface sediments from redox transition zones

Reactive iron (Fe) mineral coatings found in subsurface reduction-oxidation transition zones (RTZs) contribute to the attenuation of contaminants. An 18.3-m anoxic core was collected from the site, where constituents of concern (COCs) in groundwater included chlorinated solvents. Reactive Fe mineral coatings were found to be abundant in the RTZs. This research focused on evaluating reaction kinetics with anoxic sediments bearing ferrous mineral nano-coatings spiked with either tetrachloroethylene (PCE), trichloroethylene (TCE), or 1,4-dichlorobenzene (1,4-DCB). Reaction kinetics with RTZ sediments followed pseudo-first-order reactions for the three contaminants with 90% degradation achieved in less than 39 days. The second-order rate constants for the three COCs ranged from 6.20 × 10^-4 to 1.73 × 10^-3 Lg^-1h^-1 with pyrite (FeS₂), 4.97 × 10^-5 to 1.24 × 10^-3 Lg^-1h^-1with mackinawite (FeS), 1.25 × 10^-4 to 1.89 × 10^-4 Lg^-1h^-1 with siderite (FeCO₃), and 1.79 × 10^-4 to 1.10 × 10^-3 Lg^-1h^-1 with magnetite (Fe₃O₄). For these three chlorinated solvents, the trend for the rate constants followed: Fe(II) sulfide minerals > magnetite > siderite. The high reactivity of Fe mineral coatings is hypothesized to be due to the large surface areas of the nano-mineral coatings. As a result, these surfaces are expected to play an important role in the attenuation of chlorinated solvents in contaminated subsurface environments.

Experimental study of viscosity modification coupled with phase transfer catalysis for enhanced remediation of non-aqueous phase trichloroethene polluted heterogeneous aquifer

The degradation of dense non-aqueous phase liquid trichloroethene in low permeability zone is a challenging issue due to limited mass transfer between water-soluble oxidants (i.e., MnO₄^-) and residual phase trichloroethene and the bypassing of amendments in low permeability zone. This work accomplished trichloroethene oxidation enhancement through coupling viscosity modification by using xanthan with phase transfer of MnO₄^- by using phase transfer catalyst (PTC). Experiments were conducted by sand columns and 2D-tanks, and results revealed that after ~11.7 g of trichloroethene was injected in each tank, the mass of trichloroethene degradation was 1.3, 5.9, 6.9 and 8.5 g in MnO₄^-, MnO₄^- + xanthan, MnO₄^- + PTC and MnO₄^- + PTC + xanthan reaction systems, respectively. Combining PTC and xanthan with MnO₄^- increased the rate of continuous formation of Cl^-, reflected in the acceleration of heterogeneous reactions and MnO₄^- transport enhancement in low permeability zone by PTC and xanthan. Moreover, PTC promoted dissolved Mn (Ⅱ) and Mn (Ⅲ) formation in the process of MnO₄^- reduction, and thus effectively inhibited MnO₂ generation. In conclusion, the results revealed that PTC and xanthan could perform their respective contributions to mass transfer and amendment transport for jointly enhanced the remediation of trichloroethene polluted heterogeneous aquifer.

Degradation of trichloroethene by citric acid chelated Fe(II) catalyzing sodium percarbonate in the environment of sodium dodecyl sulfate aqueous solution

In this study, the common chlorinated solvent trichloroethene (TCE) was selected as the target contaminant. The aqueous solution after solubilization treatment (containing TCE and sodium dodecyl sulfate (SDS)) was used as the research object to carry out the remediation technology research of citric acid (CA) enhanced Fe(II) activation in sodium percarbonate (SPC) system. In 0.15 mM TCE and 1 critical micelle concentration (CMC) SDS solution, CA chelating Fe(II) activated SPC could effectively promote 93.2% degradation of TCE when the dosages of SPC, Fe(II) and CA were 3.0, 6.0 and 3.0 mM, respectively. SDS had a significant inhibitory effect on the degradation of TCE, and the surface tension changed after the reaction. The addition of CA greatly increased the generation of hydroxyl radicals (HO) in the system, while the removal of TCE was mainly attributed to HO, and the removed TCE was almost completely dechlorinated. The pH range from 3 to 7 could keep the TCE degradation above 80.0%. In the actual groundwater remediation, this technique could also efficiently degrade TCE (including SDS), showing a great application potential and development prospective in practice.

Mechanism of surfactant in trichloroethene degradation in aqueous solution by sodium persulfate activated with chelated-Fe(II)

The mechanism of surfactants in surfactant-in situ chemical oxidation (S-ISCO) coupled process for trichloroethene (TCE) degradation was firstly reported. The performance of TCE solubilization and inhibition of TCE degradation in three nonionic surfactants (TW-80, Brij-35, TX-100) in PS/Fe(II)/citric acid (CA) system was compared and TW-80 was evaluated to be the optimal surfactant in S-ISCO coupled process due to the best TCE solubilizing ability and minimal inhibition for TCE degradation (only 31.8% TCE inhibition in the presence of 1 g L^-1 TW-80 surfactant). The inhibition mechanism in TCE degradation was also demonstrated by comparing the strength of ROSs and PS utilization. In the presence of TW-80 (1 g L^-1), over 97.5% TCE was removed at the PS/Fe(II)/CA/TCE molar ratio of 30/4/4/1, in which more than 86.7% TCE was dechlorinated. The result of scavenger experiments revealed that the dominant radicals were HO• and SO₄^-• in PS/Fe(II)/CA system in TW-80 containing aqueous solution, among which SO₄^-• performed a greater role in TCE removal. Moreover, over 85.3% TCE degradation in actual groundwater revealed the potential of PS/Fe(II)/CA process for actual groundwater remediation in containing TW-80 of TCE contaminant. This research provided a novel alternative technology for groundwater remediation with TCE contaminant when containing surfactants.

Remediation of TCE-contaminated groundwater using KMnO4 oxidation: laboratory and field-scale studies

The objectives of this study were to (1) conduct laboratory bench and column experiments to determine the oxidation kinetics and optimal operational parameters for trichloroethene (TCE)-contaminated groundwater remediation using potassium permanganate (KMnO₄) as oxidant and (2) to conduct a pilot-scale study to assess the efficiency of TCE remediation by KMnO₄ oxidation. The controlling factors in laboratory studies included soil oxidant demand (SOD), molar ratios of KMnO₄ to TCE, KMnO₄ decay rate, and molar ratios of Na₂HPO₄ to KMnO₄ for manganese dioxide (MnO₂) production control. Results show that a significant amount of KMnO₄ was depleted when it was added in a soil/water system due to the existence of natural soil organic matters. The presence of natural organic material in soils can exert a significant oxidant demand thereby reducing the amount of KMnO₄ available for the destruction of TCE as well as the overall oxidation rate of TCE. Supplement of higher concentrations of KMnO₄ is required in the soil systems with high SOD values. Higher KMnO₄ application resulted in more significant H⁺ and subsequent pH drop. The addition of Na₂HPO₄ could minimize the amount of produced MnO₂ particles and prevent the clogging of soil pores, and TCE oxidation efficiency would not be affected by Na₂HPO₄. To obtain a complete TCE removal, the amount of KMnO₄ used to oxidize TCE needs to be higher than the theoretical molar ratio of KMnO₄ to TCE based on the stoichiometry equation. Relatively lower oxidation rates are obtained with lower initial TCE concentrations. The half-life of TCE decreased with increased KMnO₄ concentrations. Results from the pilot-scale study indicate that a significant KMnO₄ decay occurs after the injection due to the reaction of KMnO₄ with soil organic matters, and thus, the amount of KMnO₄, which could be transported from the injection point to the downgradient area, would be low. The effective influence zone of the KMnO₄ oxidation was limited to the KMnO₄ injection area (within a 3-m radius zone). Migration of KMnO₄ to farther downgradient area was limited due to the reaction of KMnO₄ to natural organic matters. To retain a higher TCE removal efficiency, continuous supplement of high concentrations of KMnO₄ is required. The findings would be useful in designing an in situ field-scale ISCO system for TCE-contaminated groundwater remediation using KMnO₄ as the oxidant.

Reductive dechlorination of trichloroethene under different sulfate-reducing and electron donor conditions

The effect of sulfate presence on reductive dechlorination of chlorinated ethenes has been a matter of conflict among the limited reports found in literature. This paper aims to clarify the misconceptions regarding the performance of trichloroethene biotransformation under sulfate reducing conditions by evaluating the effect of different sulfate concentrations on reductive dechlorination and to assess the influence of electron donor dose on dechlorination rate. To this end, batch experiments containing different sulfate and butyrate concentrations were conducted using trichloroethene-dechlorinating and sulfate-reducing parent cultures. Results demonstrated that if sufficient time and electron donor is provided, complete dechlorination can be achieved, even at up to 400 mg/L initial sulfate concentration. However, the rate of dichloroethene and vinyl chloride degradation is reduced as sulfide concentration increases. Moreover, the excess electron donor dose induced a slightly slower dechlorination rate. The findings of this paper present an explanatory framework for the dechlorination of TCE under sulfate reducing conditions and can contribute to the state-of-art bioremediation of contaminated sites.

An efficient catalytic degradation of trichloroethene in a percarbonate system catalyzed by ultra-fine heterogeneous zeolite supported zero valent iron-nickel bimetallic composite

Zeolite supported nano iron-nickel bimetallic composite (Z-nZVI-Ni) was prepared using a liquid-phase reduction process. The corresponding surface morphologies and physico-chemical properties of the Z-nZVI-Ni composite were determined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Energy dispersive X-ray spectra (EDS), Brunauer Emmett Teller (BET) adsorption, wide angle X-ray diffractometry (WA-XRD), and Fourier transform infrared spectroscopy (FTIR). The results indicated high dispersion of iron and nickel nano particles on the zeolite sheet with an enhanced surface area. Complete destruction of trichloroethene (TCE) and efficient removal of total organic carbon (TOC) were observed by using Z-nZVI-Ni as a heterogeneous catalyst for a Fenton-like oxidation process employing sodium percarbonate (SPC) as an oxidant. The electron spin resonance (ESR) of Z-nZVI-Ni verified the generation and intensity of hydroxyl radicals (OH•). The quantification of OH• elucidated by using p-chlorobenzoic acid, a probe indicator, confirmed the higher intensity of OH•. The transformation products were identified using GC-MS. The slow iron and nickel leaching offered higher stability and better catalytic activity of Z-nZVI-Ni, demonstrating its prospective long term applications in groundwater for TCE degradation.

Enhanced degradation of trichloroethene by calcium peroxide activated with Fe(III) in the presence of citric acid

Trichloroethene (TCE) degradation by Fe(III)-activated calcium peroxide (CP) in the presence of citric acid (CA) in aqueous solution was investigated. The results demonstrated that the presence of CA enhanced TCE degradation significantly by increasing the concentration of soluble Fe(III) and promoting H₂O₂ generation. The generation of HO• and O₂^-• in both the CP/Fe(III) and CP/Fe(III)/CA systems was confirmed with chemical probes. The results of radical scavenging tests showed that TCE degradation was due predominantly o direct oxidation by HO•, while O₂^-• strengthened the generation of HO• by promoting Fe(III) transformation in the CP/Fe(III)/CA system. Acidic pH conditions were favorable for TCE degradation, and the TCE degradation rate decreased with increasing pH. The presence of Cl^-, HCO₃^-, and humic acid (HA) inhibited TCE degradation to different extents for the CP/Fe(III)/CA system. Analysis of Cl^- production suggested that TCE degradation in the CP/Fe(III)/CA system occurred through a dechlorination process. In summary, this study provided detailed information for the application of CA-enhanced Fe(III)-activated calcium peroxide for treating TCE contaminated groundwater.

Determination of rate constants and branching ratios for TCE degradation by zero-valent iron using a chain decay multispecies model

The applicability of a newly-developed chain-decay multispecies model (CMM) was validated by obtaining kinetic rate constants and branching ratios along the reaction pathways of trichloroethene (TCE) reduction by zero-valent iron (ZVI) from column experiments. Changes in rate constants and branching ratios for individual reactions for degradation products over time for two columns under different geochemical conditions were examined to provide ranges of those parameters expected over the long-term. As compared to the column receiving deionized water, the column receiving dissolved CaCO3 showed higher mean degradation rates for TCE and all of its degradation products. However, the column experienced faster reactivity loss toward TCE degradation due to precipitation of secondary carbonate minerals, as indicated by a higher value for the ratio of maximum to minimum TCE degradation rate observed over time. From the calculated branching ratios, it was found that TCE and cis-dichloroethene (cis-DCE) were dominantly dechlorinated to chloroacetylene and acetylene, respectively, through reductive elimination for both columns. The CMM model, validated by the column test data in this study, provides a convenient tool to determine simultaneously the critical design parameters for permeable reactive barriers and natural attenuation such as rate constants and branching ratios.

Heterogeneous hyporheic zone dechlorination of a TCE groundwater plume discharging to an urban river reach

The typically elevated natural attenuation capacity of riverbed-hyporheic zones is expected to decrease chlorinated hydrocarbon (CHC) groundwater plume discharges to river receptors through dechlorination reactions. The aim of this study was to assess physico-chemical processes controlling field-scale variation in riverbed-hyporheic zone dechlorination of a TCE groundwater plume discharge to an urban river reach. The 50-m long pool-riffle-glide reach of the River Tame in Birmingham (UK) studied is a heterogeneous high energy river environment. The shallow riverbed was instrumented with a detailed network of multilevel samplers. Freeze coring revealed a geologically heterogeneous and poorly sorted riverbed. A chlorine number reduction approach provided a quantitative indicator of CHC dechlorination. Three sub-reaches of contrasting behaviour were identified. Greatest dechlorination occurred in the riffle sub-reach that was characterised by hyporheic zone flows, moderate sulphate concentrations and pH, anaerobic conditions, low iron, but elevated manganese concentrations with evidence of sulphate reduction. Transient hyporheic zone flows allowing input to varying riverbed depths of organic matter are anticipated to be a key control. The glide sub-reach displayed negligible dechlorination attributed to the predominant groundwater baseflow discharge condition, absence of hyporheic zone, transition to more oxic conditions and elevated sulphate concentrations expected to locally inhibit dechlorination. The tail-of-pool-riffle sub-reach exhibited patchy dechlorination that was attributed to sub-reach complexities including significant flow bypass of a low permeability, high organic matter, silty unit of high dechlorination potential. A process-based conceptual model of reach-scale dechlorination variability was developed. Key findings of practitioner relevance were: riverbed-hyporheic zone CHC dechlorination may provide only a partial, somewhat patchy barrier to CHC groundwater plume discharges to a surface water receptor; and, monitoring requirements to assess the variability in CHC attenuation within a reach are expected to be onerous. Further research on transient hyporheic zone dechlorination is recommended.

Analysis of sources of bulk conductivity change in saturated silica sand after unbuffered TCE oxidation by permanganate

Time lapse resistivity surveys could potentially improve monitoring of permanganate-based in situ chemical oxidation (ISCO) of organic contaminants such as trichloroethene (TCE) by tracking changes in subsurface conductivity that result from injection of permanganate and oxidation of the contaminant. Bulk conductivity and pore fluid conductivity changes during unbuffered TCE oxidation using permanganate are examined through laboratory measurements and conductivity modeling using PHREEQC in fluid samples and porous media samples containing silica sand. In fluid samples, oxidation of one TCE molecule produces three chloride ions and one proton, resulting in an increase in fluid electrical conductivity despite the loss of two permanganate ions in the reaction. However, in saturated sand samples in which up to 8mM TCE was oxidized, at least 94% of the fluid conductivity associated with the presence of protons was removed within 3h of sand contact, most likely through protonation of silanol groups found on the surface of the sand grains. Minor conductivity effects most likely associated with pH-dependent reductive dissolution of manganese dioxide were also observed but not accounted for in pore-fluid conductivity modeling. Unaccounted conductivity effects resulted in an under-calculation of post-reaction pore fluid conductivity of 2.1% to 5.5%. Although small increases in the porous media formation factor resulting from precipitation of manganese dioxide were detected (about 3%), these increases could not be confirmed to be statistically significant. Both injection of permanganate and oxidation of TCE cause increases in bulk conductivity that would be detectable through time-lapse resistivity surveys in field conditions.