Treatment of trichloroethene and hexavalent chromium by granular iron in the presence of dissolved CaCO3

Groundwater Cr(VI) contamination and remediation: A review from 1999 to 2022

Hexavalent chromium (Cr(VI)) contamination of groundwater has traditionally been an environmental issue of great concern due to its bioaccumulative and highly toxic nature. This paper presents a review and bibliometric analysis of the literature on the interest area "Cr(VI) in groundwater" published in the Web of Science Core Collection from 1999 to 2022. First, information on 203 actual Cr(VI)-contaminated groundwater sites around the world was summarized, and the basic characteristics of the sources and concentrations of contamination were derived. 68.95% of the sites were due to human causes and 56.43% of these sites had Cr(VI) concentrations in the range of 0-10 mg/L. At groundwater sites with high Cr(VI) contamination due to natural causes, 75.00% of the sites had Cr(VI) concentrations less than 0.2 mg/L. A total of 936 papers on "Cr(VI) in groundwater" were retrieved for bibliometric analysis: interest in research on Cr(VI) in groundwater has grown rapidly in recent years; 59.4% of the papers were published in the field of environmental sciences. A systematic review of the progress of studies on the Cr(VI) removal/remediation based on reduction, adsorption and biological processes is presented. Out of 666 papers on Cr(VI) removal/remediation, 512, 274, and 75 papers dealt with the topics of reduction, adsorption, and bioremediation, respectively. In addition, several studies have demonstrated the potential applicability of natural attenuation in the remediation of Cr(VI)-contaminated groundwater. This paper will help researchers to understand and investigate methodological strategies to remove Cr(VI) from groundwater in a more targeted and effective manner.

Application of zero-valent iron coupled with biochar for removal of perfluoroalkyl carboxylic and sulfonic acids from water under ambient environmental conditions

Advanced oxidation and reduction processes have been intensively investigated as potential methods to promote the decomposition of perfluoroalkyl substances (PFASs). However, extreme operational conditions such as highly acidic pH, high temperature, and high pressure are required to promote degradation reactions, which makes these technologies costly and less feasible for full-scale applications. The objective of this study was to evaluate the effectiveness of zero-valent iron (ZVI) alone and a mixture of ZVI and biochar (ZVI + BC) for removal of seven target PFASs from water under ambient environmental conditions. Target PFASs included three perfluoroalkyl carboxylic acids (PFCAs) [perfluorooctanoic acid (PFOA, C8-PFCA), perfluoroheptanoic acid (C7-PFCA), and perfluorohexanoic acid (C6-PFCA)] and four perfluoroalkyl sulfonic acids (PFSAs) [perfluorooctane sulfonic acid (PFOS, C8-PFSA), perfluoroheptane sulfonic acid (C7-PFSA), perfluorohexane sulfonic acid (C6-PFSA), and perfluorobutane sulfonic acid (C4-PFSA)]. Batch test results show that PFSAs (up to 94% removal) were more effectively removed than PFCAs (up to 60% removal) when utilizing either ZVI or (ZVI + BC). About 20-60% of input PFOA (~18,550 μg L^-1) and 90-94% of input PFOS (~18,580 μg L^-1) were removed by ZVI alone or the mixture of (ZVI + BC). The removal efficiencies of PFCAs and PFSAs by reactive media increased with increasing chain length, from 0 to 17% for short-chain PFCAs (C6-C7) and 20 to 70% for short-chain PFSAs (C4-C7). About 5-10% of input PFOA and PFOS was partially defluorinated by ZVI alone as indicated by F^- release; however, the defluorination efficiency may be underestimated due to the sorption of F^- by the reactive media. Overall, the reactive mixture (ZVI + BC) may be an effective and environmentally sustainable material for removing PFASs from water under ambient environmental conditions.

Removal of pharmaceutical compounds, artificial sweeteners, and perfluoroalkyl substances from water using a passive treatment system containing zero-valent iron and biochar

Emerging contaminants are widely detected and persistent in environmental waters. Advanced oxidation processes are among the most effective methods for removing emerging contaminants from water; however, high energy consumption greatly increases the operating costs and limits large-scale applications. In this study, a passive treatment system consisting of four columns packed with mixtures of silica sand, zero-valent iron (ZVI), biochar (BC), and a mixture of (ZVI + BC) were evaluated for simultaneous removal of eight pharmaceuticals, four artificial sweeteners, and two perfluoroalkyl substances (PFASs) from water. Overall, the passive treatment system was more effective for removing target pharmaceuticals (almost complete removal) than artificial sweeteners and PFASs (partial removal). Columns ZVI, BC, and (ZVI + BC) exhibited similarly effective removal (>97%) of target pharmaceuticals, including carbamazepine, caffeine, sulfamethoxazole, 3,4-methylenedioxyamphetamine, 3,4-methylenedioxymethamphetamine, ibuprofen, gemfibrozil, and naproxen, from ~9 to <0.25 μg L^-1; pharmaceuticals were more rapidly removed by Columns ZVI and (ZVI + BC) than Column BC, except for ibuprofen. Column ZVI was more effective for removing artificial sweeteners acesulfame-K and sucralose than Columns BC and (ZVI + BC); however, BC exhibited relatively greater removal of saccharin than ZVI and (ZVI + BC). Acesulfame-K and saccharin (~110 μg L^-1) were partially removed in the treatment columns. Cyclamate was not removed in any of the columns. However, >76% of input sucralose (~110 μg L^-1) was removed in the three treatment columns. Reactive medium BC alone was more effective for removing target PFASs than ZVI and (ZVI + BC). Input perfluorooctanoic acid (PFOA) (~45 μg L^-1) was partially removed in the columns containing BC but not ZVI alone. Between 10 and 80% of input perfluorooctane sulfonic acid (PFOS) (24 ̶ 90 μg L^-1) was removed in Column ZVI; greater removals (57 ̶ 99%) were observed in Columns BC and (ZVI + BC).

Nitrate reduction and its effects on trichloroethylene degradation by granular iron

Laboratory column experiments and reactive transport modeling were performed to evaluate the reduction of nitrate and its effects on trichloroethylene (TCE) degradation by granular iron. In addition to determining degradation kinetics of TCE in the presence of nitrate, the columns used in this study were equipped with electrodes which allowed for in situ measurements of corrosion potentials of the iron material. Together with Raman spectroscopic measurements the mechanisms of decline in iron reactivity were examined. The experimental results showed that the presence of nitrate resulted in an increase in corrosion potential and the formation of thermodynamically stable passive films on the iron surface which impaired iron reactivity. The extent of the decline in iron reactivity was proportional to the nitrate concentration. Consequently, significant decreases in TCE and nitrate degradation rates and migration of degradation profiles for both compounds occurred. Furthermore, the TCE degradation kinetics deviated from the pseudo-first-order model. The results of reactive transport modeling, which related the amount of a passivating iron oxide, hematite (α-Fe₂O₃), to the reactivity of iron, were generally consistent with the patterns of migration of TCE and nitrate profiles observed in the column experiments. More encouragingly, the simulations successfully demonstrated the differences in performances of three columns without changing model parameters other than concentrations of nitrate in the influent. This study could be valuable in the design of iron permeable reactive barriers (PRBs) or in the development of effective maintenance procedures for PRBs treating TCE-contaminated groundwater with elevated nitrate concentrations.

Electrochemical dechlorination of trichloroethylene in the presence of natural organic matter, metal ions and nitrates in a simulated karst media

A small-scale flow-through limestone column was used to evaluate the effect of common coexisting organic and inorganic compounds on the electrochemical dechlorination of trichloroethylene (TCE) in karst media. Iron anode was used to produce ferrous ions and promote reducing conditions in the column. The reduction of TCE under 90 mA current, 1 mL min-1 flow rate, and 1 mg L-1 initial TCE concentration, was inhibited in the presence of humic acids due to competition for direct electron transfer and/or reaction with atomic hydrogen produced at the cathode surface by water electrolysis. Similarly, presence of 10 mg L-1 chromate decreased TCE reduction rate to 53%. The hexavalent chromium was completely reduced to trivalent chromium due to the ferrous species produced from iron anode. Presence of 5 mg L-1 selenate decreased the removal of TCE by 10%. Chromium and selenate complexation with dissolved iron results in formation of aggregates, which cover the electrodes surface and reduce TCE dechlorination rate. Presence of 40 mg L-1 nitrates caused reductive transformation of TCE up to 80%. Therefore, TCE removal is influenced by the presence of other contaminants that are present as a mixture in groundwater in the following order: humic acid, chromate, selenate, and nitrate.

Fractionation of Selenium during Selenate Reduction by Granular Zerovalent Iron

Batch experiments were conducted using granular zerovalent iron (G-ZVI) with either ultrapure water or CaCO3 saturated simulated groundwater to assess the extent of Se isotope fractionation in solution under the anaerobic conditions characteristic of many aquifers. G-ZVI is a common remediation material in permeable reactive barriers (PRB) to treat Se-contaminated groundwater, and stable isotopes are a potential tool for assessing removal mechanisms. The solution composition, speciation of Se, and Se isotope ratios were determined during both sets of experiments. Dissolved Se concentrations decreased from 10 to <2 mg L(-1) after 3 d in the CaCO3 system and below 0.4 mg L(-1) after 2 d in the ultrapure water system. XANES analysis of the solid phase showed spectra consistent with the formation of Se(IV), Fe2(SeO3)3, FeSe, FeSe2, and Se(0) on the G-ZVI. Selenium isotope ratio measurements in solution in the CaCO3 and ultrapure water experiments showed enrichment of δ(82/76)Se values from -0.94 ± 0.07‰ and -1.93 ± 0.20‰ to maximum values of 6.85 ± 0.52‰ and 5.68 ± 0.20‰ over 72 and 36 h, respectively. The effective fractionations associated with the reduction of Se(VI) were 4.3‰ within the CaCO3 saturated water and 3.0‰ in ultrapure water.

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.

Dual Mechanism Conceptual Model for Cr Isotope Fractionation during Reduction by Zerovalent Iron under Saturated Flow Conditions

Chromium isotope analysis is rapidly becoming a valuable complementary tool for tracking Cr(VI) treatment in groundwater. Evaluation of various treatment materials has demonstrated that the degree of isotope fractionation is a function of the reaction mechanism, where reduction of Cr(VI) to Cr(III) induces the largest fractionation. However, it has also been observed that uniform flow conditions can contribute complexity to isotope measurements. Here, laboratory batch and column experiments were conducted to assess Cr isotope fractionation during Cr(VI) reduction by zerovalent iron under both static and saturated flow conditions. Isotope measurements were accompanied by traditional aqueous geochemical measurements (pH, Eh, concentrations) and solid-phase analysis by scanning electron microscopy and X-ray absorption spectroscopy. Increasing δ(53)Cr values were associated with decreasing Cr(VI) concentrations, which indicates reduction; solid-phase analysis showed an accumulation of Cr(III) on the iron. Reactive transport modeling implemented a dual mechanism approach to simulate the fractionation observed in the experiments. The faster heterogeneous reaction pathway was associated with minimal fractionation (ε=-0.2‰), while the slower homogeneous pathway exhibited a greater degree of fractionation (ε=-0.9‰ for the batch experiment, and ε=-1.5‰ for the column experiment).

Assessment of potential positive effects of nZVI surface modification and concentration levels on TCE dechlorination in the presence of competing strong oxidants, using an experimental design

Nanoscale zero-valent iron (nZVI) particles are efficient for the remediation of aquifers polluted by trichloroethylene (TCE). But for on-site applications, their reactivity can be affected by the presence of common inorganic co-pollutants, which are equally reduced by nZVI particles. The aim of this study was to assess the potential positive effects of nZVI surface modification and concentration level on TCE removal in the concomitant presence of two strong oxidants, i.e., Cr(VI) and NO3(-). A design of experiments, testing four factors (i.e. nZVI concentration, nZVI surface modification, Cr(VI) concentration and NO3(-) concentration), was used to select the best trials for the identification of the main effects of the factors and of the factors interactions. The effects of these factors were studied by measuring the following responses: TCE removal rates at different times, degradation kinetic rates, and the transformation products formed. As expected, TCE degradation was delayed or inhibited in most of the experiments, due to the presence of inorganics. The negative effects of co-pollutants can be palliated by combining surface modification with a slight increase in nZVI concentration. Encouragingly, complete TCE removal was achieved for some given experimental conditions. Noteworthily, nZVI surface modification was found to promote the efficient degradation of TCE. When degradation occurred, TCE was mainly transformed into innocuous non-chlorinated transformation products, while hazardous chlorinated transformation products accounted for a small percentage of the mass-balance.