Exploring the potential of horse amendment for the remediation of HCHs-polluted soils.

This study assessed for the first time the bioremediation potential of an organic horse amendment in soils contaminated with solid wastes of the obsolete pesticide lindane (α-hexachlorocyclohexane (α-HCH) = 80 mg kg-1, ß-HCH = 40 mg kg-1, γ,δ,ε-HCH≈10 mg kg-1) searching for a self-sufficient bio-based economy. Four treatments were implemented: polluted (PS, ΣHCHs = 130 mg kg-1) and control (CS, ΣHCHs = 1.24 mg kg-1) soils and the respective amended soils (APS and ACS). A commercial amendment, coming from organic wastes, was used for soil biostimulation (5% dry weight), and the temporal evolution of the enzymatic activity (dehydrogenase, ß-glucosidase activity, phenoloxidase, arylamidase, phosphatase, and urease) and HCHs concentration of the soils was evaluated over 55 days under controlled humidity and temperature conditions. The horse amendment positively influenced the physicochemical properties of the soil by reducing pH (from 8.3 to 8) and increasing the organic matter (TOC from 0.5 to 3.3%) and nutrient content (P and NH4+ from 24.1 to 13.7 to 142.1 and 41.2 mg kg-1, respectively). Consequently, there was a notable enhancement in the soil biological activity, specifically in the enzymatic activity of dehydrogenase, phenol-oxidase, phosphatase, and urease and, therefore, in HCH degradation, which increased from <1 to 75% after the incubation period. According to the chlorine position on the cyclohexane ring, the following ranking has been found for HCHs degradation: ß-HCH (46%) < Îµ-HCH (57%) < α-HCH (91%) ≈ Î´-HCH (91%) < Î³-HCH (100%). Pentachlorocyclohexene (PCCH) and 1,2,4-trichlorobenzene (1,2,4-TCB) were identified as HCHs degradation metabolites and disappeared at the end of the incubation time. Although further research is required, these preliminary findings suggest that organic amendments represent a sustainable, harmless, and cost-effective biostimulation approach for remediating soils contaminated with recalcitrant HCHs, boosting the circular economy.

Sustainable lindane waste remediation: Surfactant-driven residual DNAPL extraction and oxidation in a real landfill (LIFE SURFING).

The LIFE SURFING Project was carried out at the Bailin Landfill in Sabiñánigo, Spain (2020-2022), applying Surfactant Enhanced Aquifer Remediation (SEAR) and In Situ Chemical Oxidation (S-ISCO) in a 60-meter test cell beneath the old landfill, to remediate a contaminated aquifer with dense non-aqueous phase liquid (DNAPL) from nearby lindane production. The project overcame traditional extraction limitations, successfully preventing groundwater pollution from reaching the river. In spring 2022, two SEAR interventions involved the injection of 9.3 m3 (SEAR-1) and 6 m3 (SEAR-2) of aqueous solutions containing 20 g/L of the non-ionic surfactant E-Mulse 3®, with bromide (around 150 mg/L) serving as a conservative tracer. 7.1 and 6.0 m3 were extracted in SEAR-1 and SEAR-2, respectively, recovered 60-70 % of the injected bromide and 30-40 % of the surfactant, confirming surfactant adsorption by the soil. Approximately 130 kg of DNAPL were removed, with over 90 % mobilized and 10 % solubilized. A surfactant-to-DNAPL recovery mass ratio of 2.6 was obtained, a successful value for a fractured aquifer. In September 2022, the S-ISCO phase entailed injecting 22 m3 of a solution containing persulfate (40 g/L), E-Mulse 3® (4 g/L), and NaOH (8.75 g/L) in pulses over 48 h, oxidizing around 20 kg of DNAPL and ensuring low toxicity levels after that. Preceding the SEAR and S-ISCO trials, 2020 and 2021 were dedicated to detailed groundwater flow characterizations, including hydrological and tracer studies. These preliminary investigations allowed the design of a barrier zone between 317 and 557 m from the test cell and the river, situated 900 m away. This zone, integrating alkali dosing, aeration, vapor extraction, and oxidant injection, effectively prevented the escape of fluids to the river. Neither surfactants nor contaminants were detected in river waters post-treatment. The absence of residual phase in test cell wells and reduction of chlorinated compound levels in groundwater were noticed till one year after S-ISCO.

Remediation of groundwater polluted with lindane production wastes by conductive-diamond electrochemical oxidation.

This work studies the remediation of groundwater saturated with dense non-aqueous phase liquid (DNAPL) from lindane production wastes by electrochemical oxidation. DNAPL-saturated groundwater contains up to 26 chlorinated organic compounds (COCs), including different isomers of hexachlorocyclohexane (HCH). To do this, polluted groundwater was electrolysed using boron-doped diamond (BDD) and stainless steel (SS) as anode and cathode, respectively, and the influence of the current density on COCs removal was evaluated in the range from 5 to 50 mA cm-2. Results show that current densities higher than 25 mA cm-2 lead to the complete removal and mineralisation of all COCs identified in groundwater. The higher the current density, the higher the COCs removal rate. At lower current densities (5 mA cm-2), chlorobenzenes were completely removed, and degradations above 90 % were reached for COCs with more than five chlorine atoms in their molecules. The use of BDD anodes promotes the electrochemical generation of powerful reactive species, such as persulfate, hypochlorite or hydroxyl radicals, that contribute to the degradation and mineralisation of COCs. The applied current density also influences the generation of these species. Finally, no acute toxicity towards Vibrio fischeri was observed for the treated groundwater after the electrochemical oxidation performed at 5 and 10 mA cm-2. These findings demonstrate that electrochemical oxidation with BDD anodes at moderate current densities is a promising alternative for the remediation of actual groundwater contaminated with DNAPLs.

Sustainable application of surfactants in soil remediation: Selective pollutants adsorption and hydrogen peroxide-driven adsorbent regeneration.

The uncontrolled disposal of the liquid lindane wastes have led to the formation of dense non-aqueous phase liquids (DNAPL), consisting of 28 chlorinated organic compounds (COCs), contaminating soil and groundwater. Surfactant-enhanced aquifer remediation is proposed as technology to treat these sites. However, the polluted emulsion generated must be manged on-site. In this work a two-step process is applied to treat emulsion composed of E-Mulse® 3 (4 g·L-1) as surfactant and a DNAPL (2 gCOCs·L-1). In the first, the COCs were selectively adsorbed in a granular activated carbon (GAC) column with Fe (II) previously adsorbed (10-20mg·g-1) onto the carbon surface, recovering an aqueous phase with surfactant for their reuse. In the second step, the spent GAC was regenerated with a 40 g·L-1 solution of hydrogen peroxide fed to the column at 2 mL·min-1 to promote the oxidation of the COCs adsorbed in the GAC. The kinetic and adsorption model in a multisolute (surfactant and DNAPL) system has been proposed. Five successive cycles of regeneration/adsorption have been successfully applied in the column process. About 50 % of the COCs were retained from the emulsion, and more than 70 % of the surfactant was recovered. The consumption of unproductive oxidants decreased with the number of regeneration cycles. The water effluent obtained after regeneration of GAC did not present chlorinated compounds desorbed and nontoxic by-products generated, such as short-chain acids.

Solar-assisted oxidation of organochlorine pesticides in groundwater using persulfate and ferrioxalate.

The oxidation of hexachlorocyclohexane isomers in the aqueous phase (Milli-Q and groundwater) was studied using persulfate activated by ferrioxalate and solar light at circumneutral pH. The experiments were conducted in a solar simulator reactor with local radiation fluxes qw= 1.12·10-7 E cm-2s-1 and in compound parabolic collectors with solar light (qw≈10-7 E cm-2s-1) for 390 min. The effect of activator dosage (18-125 µM ferrioxalate) and persulfate concentration (520-2600 µM) on hexachlorocyclohexane conversion and oxalate and oxidant consumption was analyzed. Conversion of about 95% of ß isomer was achieved at 390 min using 1300 µM of initial persulfate and 63 µM of Fe3+ concentration despite this ß isomer being the most recalcitrant to oxidation (XHexachlorocyclohexanes=0.98). Dechlorination above 80% was achieved under these conditions, analyzing the chlorides released into the water. The influence of chloride and bicarbonate on hexachlorocyclohexanes degradation was analyzed in milli-Q water and in groundwater. Hexachlorocyclohexane conversion at 390 min decreases from 98% to 83, 75 and 65% in the presence of chloride, bicarbonate or groundwater, respectively. Results obtained with compound parabolic collectors and solar light using 2600 µM Na2S2O8 and 63 µM Fe for removing hexachlorocyclohexanes agreed with those from the solar simulator reactor, supporting using solar light to activate persulfate for sustainable abatement of persistent organic pollutants in aqueous matrixes.

Sustainable reuse of toxic spent granular activated carbon by heterogeneous fenton reaction intensified by temperature changes.

A common strategy for removing highly toxic organic compounds, such as chlorinated organic compounds, is their adsorption on granular activated carbon. Spent granular activated carbon results in a toxic residue to manage; therefore, the regeneration and reuse of granular activated carbon on the site would be advisable. This work studies the regeneration of a granular activated carbon saturated in 1,2,4-trichlorobenzene, chosen as the model chlorinated organic compounds, by heterogeneous Fenton, where iron was previously immobilised on the granular activated carbon surface. This methodology avoids the addition of iron to the aqueous phase at concentrations above the allowable limits and the need for acidification. Three successive cycles of adsorption-regeneration were carried out batchwise (5 gGAC·L-1) with a granular activated carbon saturated with 300 mg124-TCB·gGAC-1. The recovery of the adsorption capacity after regeneration was studied with H2O2 (166 mM, 1.5 the stoichiometric dosage), at different concentrations adsorbed with iron adsorbed concentrations (0-12 mgFe·gGAC-1) and temperatures (20-80 °C). Stable recovery of the adsorption capacity values of 65% were obtained at 180 min with 12 mgFe·gGAC-1 and 60 °C. The porosity and surface chemistry of the adsorbent remained very similar after different adsorption-regeneration cycles without iron leaching into the aqueous phase. The oxidant consumption was close to the stoichiometric value for the mineralization of 1,2,4-trichlorobenzene, with a low unproductive consumption of H2O2 with granular activated carbon. In addition, no aromatic or chlorinated by-products were detected in the aqueous solution obtained in the regeneration process. The negligible toxicity of the aqueous phase with the Microtox bioassay confirmed the absence of toxic oxidation by-products.

Treatment of a Complex Emulsion of a Surfactant with Chlorinated Organic Compounds from Lindane Wastes under Alkaline Conditions by Air Stripping.

Surfactant-enhanced aquifer remediation is commonly applied in polluted sites with dense non-aqueous phase liquids (DNAPLs). This technique transfers the contamination from subsoil to an extracted emulsion, which requires further treatment. This work investigated the treatment of a complex emulsion composed of a nonionic surfactant and real DNAPL formed of chlorinated organic compounds (COCs) and generated as a lindane production waste by air stripping under alkaline conditions. The influence of the surfactant (1.5-15 g·L-1), COC concentrations (2.3-46.9 mmol·L-1), and temperature (30-60 °C) on the COC volatilization was studied and modeled in terms of an apparent constant of Henry at pH > 12. In addition, the surfactant stability was studied as a function of temperature (20-60 °C) and surfactant (2-10 g·L-1), COC (0-70.3 mmol·L-1), and NaOH (0-4 g·L-1) concentrations. A kinetic model was successfully proposed to explain the loss of surfactant capacity (SCL). The results showed that alkali and temperature caused the SCL by hydrolysis of the surfactant molecule. The increasing surfactant concentration decreased the COC volatility, whereas the temperature improved the COC volatilization. Finally, the volatilization of COCs in alkaline emulsions by air stripping (3 L·h-1) was performed to evaluate the treatment of an emulsion composed of the COCs (17.6 mmol·kg-1) and surfactant (3.5 and 7 g·L-1). The air stripping was successfully applied to remove COCs (>90%), reaching an SCL of 80% at 60 °C after 8 h. Volatilization can remove COCs from emulsions and break them, enhancing their further disposal.

Comprehensive study of acute toxicity using Microtox® bioassay in soils contaminated by lindane wastes.

This research studies the acute toxicity of real contaminated soils (topsoil and subsoil) with hazardous chlorinated organic compounds (COCs) from lindane manufacturing wastes. The Microtox® bioassay was used to determine the toxicity of soils (modified Basic Solid Phase Test), soil elutriates (Basic Test), and organic extracts (adapted Organic Solvent Sample Solubilization Test), in which hydrophobic organic compounds are soluble. The acute toxicity of these persistent contaminants (hexachlorocyclohexanes, HCH isomers, as particulate matter in topsoil, and COCs, from dense non-aqueous phase liquid, DNAPL, in subsoil) and the commercial compounds were also measured. Soils tested showed different contaminant levels (topsoil: 0.9-1149 mg/kg and subsoil: 20-9528 mg/kg). Soil contaminants distribution, concentration and acute toxicity were highly related to the contamination source (HCHs or DNAPL). Soils, organic extracts, and subsoil elutriates presented high toxicity, highlighting the need for remediation of these sites. EC50 was calculated in the three-test applied for the soils tested. EC50 vs. COCs concentration in soils and soil elutriates showed an asymptotic trend, explained by the low pollutants solubility in the aqueous phase. Contrarily, EC50 vs. soil COCs concentration was more linear in the case of the organic extracts. This test was the most reliable from statistical analysis. The three methods reveal interesting and complementary information and are necessary for a complete overview of the acute toxicity of contaminated soils.

Acute Toxicity Evaluation of Lindane-Waste Contaminated Soils Treated by Surfactant-Enhanced ISCO.

The discharge of lindane wastes in unlined landfills causes groundwater and soil pollution worldwide. The liquid waste generated (a mixture of 28 chlorinated organic compounds, COCs) constitutes a dense non-aqueous phase liquid (DNAPL) that is highly persistent. Although in situ chemical oxidation (ISCO) is effective for degrading organic pollutants, the low COCs solubility requires high reaction times. Simultaneous injection of surfactants and oxidants (S-ISCO) is a promising technology to solve the limitation of ISCO treatment. The current work studies the remediation of highly polluted soil (COCs = 3682 mg/kg) obtained at the Sardas landfill (Sabiñáñigo, Spain) by ISCO and S-ISCO treatments. Special attention is paid to acute soil toxicity before and after the soil treatment. Microtox®, modified Basic Solid-Phase Test (mBSPT) and adapted Organic Solvent Sample Solubilization Test (aOSSST) were used for this scope. Persulfate (PS, 210 mM) activated by alkali (NaOH, 210 mM) was used in both ISCO and S-ISCO runs. A non-ionic and biodegradable surfactant selected in previous work, Emulse®3 (E3, 5, and 10 g/L), was applied in S-ISCO experiments. Runs were performed in soil columns filled with 50 g of polluted soil, with eight pore volumes (Pvs) of the reagents injected and 96 h between successive Pv injections. The total treatment time was 32 days. The results were compared with those corresponding without surfactant (ISCO). After remediation treatments, soils were water-washed, simulating the conditions of groundwater flux in the subsoil. The treatments applied highly reduced soil toxicity (final soil toxicity equivalent to that obtained for non-contaminated soil, mBSPT) and organic extract toxicity (reduction > 95%, aOSSST). Surfactant application did not cause an increase in the toxicity of the treated soil, highlighting its suitability for full-scale applications.

Vis LED Photo-Fenton Degradation of 124-Trichlorobenzene at a Neutral pH Using Ferrioxalate as Catalyst.

Chlorinated organic compounds (COCs) are among the more toxic organic compounds frequently found in soil and groundwater. Among these, toxic and low-degradable chlorobenzenes are commonly found in the environment. In this work, an innovative process using hydrogen peroxide as the oxidant, ferrioxalate as the catalyst and a visible light-emitting diode lamp (Vis LED) were applied to successfully oxidize 124-trichlorobenzene (124-TCB) in a saturated aqueous solution of 124-TCB (28 mg L-1) at a neutral pH. The influence of a hydrogen peroxide (HP) concentration (61.5-612 mg L-1), Fe3+ (Fe) dosage (3-10 mg L-1), and irradiation level (Rad) (I = 0.12 W cm-2 and I = 0.18 W cm-2) on 124-TCB conversion and dechlorination was studied. A D-Optimal experimental design combined with response surface methodology (RSM) was implemented to maximize the quality of the information obtained. The ANOVA test was used to assess the significance of the model and its coefficients. The maximum pollutant conversion at 180 min (98.50%) was obtained with Fe = 7 mg L-1, HP = 305 mg L-1, and I = 0.12 W cm-2. The effect of two inorganic anions usually presents in real groundwater (bicarbonate and chloride, 600 mg L-1 each) was investigated under those optimized operating conditions. A slight reduction in the 124-TCB conversion after 180 min of reaction was noticed in the presence of bicarbonate (8.31%) and chloride (7.85%). Toxicity was studied with Microtox® (Azur Environmental, Carlsbad, CA, USA) bioassay, and a remarkable toxicity decrease was found in the treated samples, with the inhibition proportional to the remaining 124-TCB concentration. That means that nontoxic byproducts are produced in agreement with the high dechlorination degrees noticed.