Dehalogenation of α-hexachlorocyclohexane by iron sulfide nanoparticles: Study of reaction mechanism with stable carbon isotopes and pH variations

Natural attenuation of contamination by hexachlorocyclohexanes in the soils around the Bailín landfill site (Sabiñánigo, Huesca province, NE Spain)

INQUINOSA company dumped waste from lindane production in Sabiñánigo (Huesca, Spain). Lindane is the γ-isomer of hexachlorocyclohexane (γ-HCH), a persistent organic pollutant listed in the Stockholm Convention in 2009. The uncontrolled dumping at two poorly managed landfills (Sardas and Bailín) has become one of the most serious contamination cases in Europe. In 2014, after discovering the underground migration of liquid HCH towards the Gállego River, more than 400,000 tonnes of contaminated materials were relocated from the old Bailín landfill to a new security cell. During the transfer operations, a small fraction of ∼100-200 kg HCH was dispersed in the area. The aim of this study is to assess the spatio-temporal dynamics of HCH-isomers around the Bailín landfill. Data from five soil sampling campaigns have been used, spanning the period from 2014 to 2022. Notably, HCH concentrations decreased from 170 kg in 2014-2.4 kg 2022, with the highest contamination levels concentrated near the road used by trucks during the removal operations. Several potential sinks for topsoil pollution were identified and their contribution quantified. For instance, the amount of HCH stored in vegetation (< 1 kg) is negligible, as is the downward movement of HCH through the vadose zone into the water table. Surface runoff does not appear to be a significant mode of HCH loss, as it would have occurred rapidly during the relocation operation, likely reaching the Gállego River. Therefore, volatilisation of HCH into the atmosphere and biological degradation are the two primary sinks for HCH in the area.

Physical-chemical transformations for the remediation and valorization of hexachlorocyclohexanes (HCHs) including lindane: A review

The production of the former insecticide lindane (γ-HCH) resulted in the generation of vast quantities of hexachlorocyclohexanes (HCH) residues, creating one of the most significant environmental challenges related to persistent organic pollutants in the world. This contamination is present today in different scenarios, including stockpiles and highly concentrated mixed waste, contaminated surface soils, subsoil, and waters. In particular, Dense Non-Aqueous Phase Liquids (DNAPLs) represent challenging subsurface and groundwater contamination. This review provides a comprehensive and critical overview of the physical-chemical methodologies and remediation projects reported in the literature for addressing lindane contamination through separation, transformation, disposal, and valorization approaches. The available physicochemical techniques include dehydrochlorination, oxidation, reduction, substitution, isomerization, as well as electrochemical, photochemical, sonochemical, plasma, and other high energy treatments. Key aspects, such as advantages and limitations, remediation effectiveness, technological maturity, scalability, estimated costs, and applicability to different contamination scenarios are thoroughly analyzed for each method. The review culminates in a detailed comparison of these methodologies for various contamination contexts, providing valuable insights for the identification of optimal solutions to this global environmental challenge. In addition, the review assesses, for the first time, the potential for valorization of the products formed during HCH treatment or remediation. This aspect highlights the opportunity to transform HCH residues into higher value-added chemicals, thereby enhancing the circular economy of the remediation process. Finally, the integration of physicochemical methods with separation and biological tools offers a holistic perspective that underscores the importance of comprehensive strategies for addressing HCH contamination effectively and sustainably.

Degradation study of δ-hexachlorocyclohexane by iron sulfide nanoparticles: Elucidation of reaction pathway using compound specific isotope analysis and pH variation

pH influences the reactivity of iron (II) minerals towards halogenated pollutants like hexachlorocyclohexanes (HCHs). To explore these incompletely understood interactions, we investigated the carbon isotope fractionation of the δ-HCH isomer during dehalogenation by iron sulfide at pHs spanning a pH range across slightly acidic to alkaline domains (5.8-9.6). The δ-1,3,4,5,6-pentachlorocyclohex-1-ene (δ-PCCH) was the intermediate degradation product, while benzene, monochlorobenzene (MCB), but especially 1,2-dichlorobenzene (1,2-DCB), and 1,2,4-trichlorobenzene (1,2,4-TCB), were the main degradation products of δ-HCH. These degradation products suggested dehydrochlorination as the main degradation pathway of δ-HCH by iron sulfide. Different kinetic experiments indicate that the rate constants (ka) during dechlorination of δ-HCH by iron sulfide rose with pH: 0.003 d-1 (pH 5.8) < 0.034 d-1 (pH 8) < 0.085 (pH 9.3) < 0.286 d-1 (pH 9.6). Upon Rayleigh model calculations, an enrichment factor (εC) of -7.8 ± 1.0 ‰ was calculated for δ-HCH dehalogenation by FeS at pH 8.0. This suggests an apparent kinetic isotope effect (AKIEC) value of 1.049 ± 0.006 for dehydrohalogenation. The magnitude of the isotope effect from this paper furthermore supports dehydrohalogenation and opens the possibility to study the degradation of HCHs by iron (II) minerals containing FeS as mackinawite in oxygen-deprived environments.

Radiolytic degradation of 1,2,4-trichlorobenzene (TCB) in some organic solvents by gamma rays: The kinetic properties of complete dechlorination of TCB and its pathway

This study investigates the degradation of TCB in methanol, ethanol, hexane, and benzene solutions using gamma radiolysis. Kinetic properties of TCB dechlorination and its pathway are examined, with TCB selected as a representative chlorinated organic compound. Chromatograms of irradiated samples and mass spectra of liquid-phase products are presented. The change in concentration of TCB, dichlorobenzenes (DCB), chlorobenzene (MCB), and benzene with absorbed doses are observed. The radiation-chemical yield (G values) of TCB in the solvents are calculated as 1.83, 2.56, 1.93, and 1.84 100eV-1 in methanol, ethanol, hexane, and benzene solutions, respectively. 100 % degradation of TCB by gamma irradiation is found to be efficient in polar solvents but leads to a wide variety of byproducts in low polar solvents, particularly the formation of polychlorinated biphenyls in TCB + benzene solutions, making benzene an incompatible medium. The main dechlorination pathway of TCB involves the formation of 1,4-DCB, MCB, and benzene. Environmental Implication. The gamma irradiation of chlorinated organic compounds, focusing on TCB as a model compound, was investigated due to its status as a hazardous material for the environment and living organisms. TCB is a byproduct of the dechlorination of certain chlorinated pesticides listed under the Stockholm Convention's Persistent Organic Pollutants (POPs) list, which prohibits their production and use. Gamma irradiation was found to be an effective method for the degradation of chlorinated compounds, achieving 100 % degradation during irradiation. The study underscores the potential of gamma irradiation as a viable approach for the treatment of chlorinated compounds, particularly in addressing environmental and health concerns associated with TCB and related compounds.

Recent progresses in compound specific isotope analysis of halogenated persistent organic pollutants. Assessing the transformation of halogenated persistent organic pollutants at contaminated sites

Compound specific isotope analysis was extensively used to characterise the environmental processes associated with the abiotic and biotic transformation of persistent halogenated organic pollutants including those of contaminants of emerging concern (CECs). In the last years, the compound specific isotope analysis was applied as tool to evaluate the environmental fate and was expanded to larger molecules like brominated flame retardants and polychlorinated biphenyls. Multi-element (C, H, Cl, Br) CSIA methods have been also employed both in laboratory and field experiments. Nevertheless, despite the instrumental advances of isotope ratio mass spectrometers systems, the instrumental detection limit for gas chromatography-combustion-isotope ratio mass spectrometer (GC-C-IRMS) systems is challenging, especially when it is utilized to δ13C analysis. Liquid chromatography-combustion isotope ratio mass spectrometry methods are challenging, taking into consideration the chromatographic resolution required when analysing complex mixtures. For chiral contaminants, enantioselective stable isotope analysis (ESIA) has turned up as alternative approach but, up to now, it has been used for a limited number of compounds. Taking into consideration the occurrence of new emerging halogenated organic contaminants, new GC and LC methods for non-target screening using high resolution mass spectrometry are needed to be developed prior to the compound specific isotope analysis (CSIA) methods.

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.

Removal of lindane using electrokinetic soil flushing coupled with air stripping

This paper evaluates the remediation of soil spiked with lindane using a combined treatment consisting of electrokinetic soil flushing (EKSF) with air stripping to elucidate the main processes occurring in the soil when electric fields of 0.75 V cm−1 and 1.50 V cm−1 are applied. The results demonstrate that lindane is efficiently transported to the anodic and cathodic wells using flushing fluids containing sodium dodecyl sulfate (SDS). Additionally, an important amount is volatilized and stripped with the injected air. In the cathodic well, lindane is rapidly transformed into other species because of the strongly alkaline media. These other species are also found in the portions of soil next to this well, confirming the efficient transport of chlorinated organics with SDS. After 14 days of operation, nearly 50% of the spiked lindane can be removed from the soil. Operation with large electric fields does not improve the performance of the treatment technology and results in lower current intensities and electro-osmotic fluxes and in higher evaporated water, despite the water content in the soil matrix, indicating the coexistence of multiple inputs in these processes.

Graphical abstract

Stable Carbon Isotope Analysis of Hexachlorocyclohexanes by Liquid–Liquid Extraction Gas Chromatography Isotope Ratio Mass Spectrometry: Method Evaluation and Applications

Compound specific isotope analysis (CSIA) and enantiomer specific isotope analysis (ESIA) are powerful tools for assessing the fate of hexachlorocyclohexanes (HCHs) in the environment. However, there is no systematic study on the CSIA and ESIA analysis test methods of the carbon isotopes of HCHs in water and soil environments, in particular the isotope fractionation in the pre-concentration process. We endeavored to test the compatibility of CSIA and ESIA with the liquid–liquid extraction method of HCHs in water. The results showed that there were negligible changes in the δ13C of HCHs after extraction, indicating that liquid–liquid extraction can be used as a pre-concentration method for the determination of δ13C of HCHs in water. The optimized method was validated and then applied to differentiate three HCHs from different manufacturers, to identify in situ degradation of HCHs of groundwater from a contaminated site and to resolve the carbon isotope fractionation occurring in the α-HCH oxidation by CaO2/Fe(II) Fenton system. The results showed that the same reagents from different manufacturers have different carbon isotope compositions, and different isomers from the same manufacturer also have different isotope compositions, showing useful evidence in identifying the source of HCHs. The more enriched δ13C in the down-gradient wells indicated that HCHs have undergone biodegradation or/and chemical reactions in the groundwater system of the site. Carbon isotopic enrichment factors (εC) of −1.90 ± 0.10‰ were obtained in the oxidation process. Hence, the method validated in this study has great potential as a method for identifying the degradation of HCHs in a water environment.