Phenanthrene degradation using Fe(III)-EDDS photoactivation under simulated solar light: A model for soil washing effluent treatment

Efficient removal of polybrominated diphenyl ethers from soil washing effluent by dummy molecular imprinted adsorbents: Selectivity and mechanisms

Surfactant enhanced elution is an effective method for removing hydrophobic organic pollutants from soils. The key to the development of leaching technology is selective removal of targeted pollutants in soil washing effluent and recycling of surfactant solutions. In this study, a molecular imprinting technique was applied to selectively sorb polybrominated diphenyl ethers (PBDEs) in soil washing effluent. The novel molecular imprinted polymers (MIPs) using different template molecules were synthesized by precipitation polymerization. Adsorption behaviors and mechanisms of MIPs were studied through experiments and theoretical calculations. The results show that 4-bromo-4'-hydroxybiphenyl and toluene can be effective imprinting molecule for MIPs synthesis. The maximal adsorption capacity of selected dummy molecular imprinted polymer (D₁-MIP) was 1032.36 µmol/g, and that of part molecular imprinted polymer (P-MIP) was 981.13 µmol/g. Their imprinting factors in 5 PBDEs adsorption ranged from 2.13 to 5.88, the recovery percentage of Triton X-100 can reach 99.09%, confirming the feasibility of reusing surfactant. Various PBDEs could be removed by MIPs, and Quantitative Structure Property Relationship analysis revealed that PBDEs' molecular volume, planarity, polarity, and hydrophobicity have major influences on their adsorption performance. DFT calculation revealed that Van der Waals force and hydrogen bonding played important roles during selective adsorption. These results can provide effective theoretical guidance for surfactant enhanced soil elution in practical engineering applications.

Efficient degradation of trichloroethene with the existence of surfactants by peroxymonosulfate activated by nano-zero-valent iron: performance and mechanism investigation

In this study, the degradation of trichloroethylene (TCE) with the existence of tween-80 (TW-80) or sodium dodecyl sulfate (SDS) using peroxymonosulfate (PMS) activated by nano-zero-valent iron (nZVI) was investigated. Over 87.6% TCE (with 1.3 g L^-1 TW-80 presence) was degraded by 0.9 mM PMS and 0.12 g L^-1 nZVI, while 89.7% TCE (with 2.3 g L^-1 SDS presence) was degraded by 1.2 mM PMS and 0.20 g L^-1 nZVI, in which more than 71.9% TCE with TW-80 existence and 87.5% TCE with SDS existence were dechlorinated. Besides, the effects of some factors (i.e., PMS and nZVI dosages, initial solution pH, and inorganic anions) on TCE removal were evaluated. The degradation of TCE was restrained continuously with increasing surfactant concentration, and TW-80 was more easily decomposed than SDS in PMS/nZVI system. Furthermore, sulfate radical (SO₄^-•) and hydroxyl radical (HO•) were demonstrated the main reactive oxygen species (ROS) contributing to TCE degradation and SO₄^-• played a dominant role through EPR tests and ROS scavenging experiments. Finally, the results of TCE degradation in actual groundwater confirmed that PMS/nZVI process has great advantages and potential in remediation of actual TCE-contaminated groundwater with TW-80 or SDS existence.

Photolysis of Fe(III) complex with ethylenediamine-N,N'-disuccinic acid and its efficiency in generation of •OH radical

The mechanism of photolysis of the Fe(III) complex with ethylenediamine-N,N'-disuccinic acid ([FeEDDS]^-) was revealed using a combination of time resolved and stationary photochemical methods. Using laser flash photolysis (λ_ex = 355 nm), the formation of the primary intermediate, the radical complex of Fe(II) with quantum yield (φ₀ = 0.21) was detected for the first time. The lifetime (1.8 ms) and the spectral characteristics (λ_max = 520 nm, ε^{520 nm} = 160 M^-1cm^-1) of this intermediate were also determined. The dependence of the quantum yield of photolysis of the [FeEDDS]^- complex (φ_FeEDDS) and the hydroxyl radical quantum yield (φ_OH) on the excitation wavelength, pH, and concentrations of the ligand and iron ions were obtained for the first time. It has been established that under optimal conditions at neutral pH, the value of φ_FeEDDS is about 0.8, and φ_OH is about 0.15. It was found that φ_FeEDDS does not depend on the initial concentrations of Fe(III), EDDS, but depends on pH, the excitation wavelength and the presence of oxygen. φ_OH does not depend on the initial concentrations of Fe(III), EDDS, but depends on pH and the excitation wavelength. The high φ_OH values make the [FeEDDS]^- complex a suitable system for the generation of •OH radical at neutral pH under UV radiation.

Coupling surfactants with ISCO for remediating of NAPLs: Recent progress and application challenges

Non-aqueous phase liquids (NAPLs) pose a serious risk to the soil-groundwater environment. Coupling surfactants with in situ chemical oxidation (ISCO) technology is a promising strategy, which is attributed to the enhanced desorption and solubilization efficiency of NAPL contaminants. However, the complex interactions among surfactants, oxidation systems, and NAPL contaminants have not been fully revealed. This review provides a comprehensive overview on the development of surfactant-coupled ISCO technology focusing on the effects of surfactants on oxidation systems and NAPLs degradation behavior. Specifically, we discussed the compatibility between surfactants and oxidation systems, including the non-productive consumption of oxidants by surfactants, the role of surfactants in catalytic oxidation systems, and the loss of surfactants solubilization capacity during oxidation process. The effect of surfactants on the degradation behavior of NAPL contaminants is then thoroughly summarized in terms of degradation kinetics, byproducts and degradation mechanisms. This review demonstrates that it is crucial to minimize the negative effects of surfactants on NAPL contaminants oxidation process by fully understanding the interaction between surfactants and oxidation systems, which would promote the successful implementation of surfactant-coupled ISCO technology in remediation of NAPLs-contaminated sites.

Mechanism of trichloroethylene degradation in Fe(II)-activated peroxymonosulfate coupled with citric acid system in the presence of surfactants

This study demonstrated that peroxymonosulfate (PMS) activated by Fe(II)/citric acid (CA) could effectively degrade trichloroethylene (TCE) in the presence of Tween-80 (TW-80) or sodium dodecyl sulfate (SDS). Significant TCE removal of 91.6% (90.1%) with 1.3 g L^-1 TW-80 (2.3 g L^-1 SDS) were achieved at the PMS/Fe(II)/CA/TCE molar ratio of 4/4/4/1 (20/20/20/1). TCE degradation could be greatly elevated by Fe(II) and CA addition, while the existence of surfactants restrained TCE removal and the inhibitory effect increased with the higher surfactant concentration. The tests of the electron paramagnetic resonance (EPR) and reactive radicals scavenging experiments proved that sulfate radical (SO₄^-•), hydroxyl radical (HO•), and superoxide radical (O₂^-•) were responsible for TCE degradation and SO₄^-• acted as the major one. The influences of initial solution pH and inorganic anions k(Cl^- and HCO₃^-) on TCE removal were also investigated. Eventually, TCE removal in actual groundwater tests with surfactants confirmed that the PMS/Fe(II)/CA process has a huge potential of practical application in remediating the groundwater contaminated by TCE after the pretreatment by solubilization using surfactants.

Chelating Agents in Assisting Phytoremediation of Uranium-Contaminated Soils: A Review

Massive stockpiles of uranium (U) mine tailings have resulted in soil contamination with U. Plants for soil remediation have low extraction efficiency of U. Chelating agents can mobilize U in soils and, hence, enhance phytoextraction of U from the soil. However, the rapid mobilization rate of soil U by chelating agents in a short period than plant uptake rate could increase the risk of groundwater contamination with soluble U leaching down the soil profile. This review summarizes recent progresses in synthesis and application of chelating agents for assisting phytoremediation of U-contaminated soils. In detail, the interactions between chelating agents and U ions are initially elucidated. Subsequently, the mechanisms of phytoextraction and effectiveness of different chelating agents for phytoremediation of U-contaminated soils are given. Moreover, the potential risks associated with chelating agents are discussed. Finally, the synthesis and application of slow-release chelating agents for slowing down metal mobilization in soils are presented. The application of slow-release chelating agents for enhancing phytoextraction of soil U is still scarce. Hence, we propose the preparation of slow-release biodegradable chelating agents, which can control the release speed of chelating agent into the soil in order to match the mobilization rate of soil U with plant uptake rate, while diminishing the risk of residual chelating agent leaching to groundwater.

Microcystins can be extracted from Microcystis aeruginosa using amino acid-derived biosurfactants

Microcystin, a cyanotoxin produced by Microcystis aeruginosa growing in eutrophic waters, can promote liver tumors in people ingesting contaminated water. To date, water treatment systems have not been effective in removing or degrading these cyanotoxins. In this work, we investigated the inhibitory activity of surfactants on the growth of M. aeruginosa and their application to extract the intracellular produced cyanotoxins. The experiments involving growth inhibition and extraction of cyanotoxins were carried out using the non-biodegradable surfactant cetyl trimethyl ammonium bromide (CTAB) in addition to other biodegradable surfactants. These were Tween 80 and surfactants derived from amino acids and peptides, respectively, from arginine, SDA, and hydrolyzed peptone, SDP. We demonstrated that the tested surfactants could be used to inhibit the growth of M. aeruginosa. At this point, CTAB and SDA proved to be the most competent surfactants in reducing cyanobacterial growth. Moreover, microcystins have been successfully removed from the water employing a cloud point extraction protocol based on the use of these surfactants and ammonium sulfate.

A novel transformation pathway of p-arsanilic acid in water by colloid ferric hydroxide under UVA light

Iron species that occur in natural surface water could affect the photochemical behavior of pollutants. Complexation between iron species and polycarboxylate or heavy metals has been widely reported, where the ligands could be oxidized via ligand-to-metal charge transfer (LMCT) by light inducement. Such complexation and photochemical reactions might also occur for low valance metal-containing organic compounds, which is worthy of investigation. This work studied the phototransformation of p-arsanilic acid (ASA), an organic arsenic compound that is widely used as a feed additive in the poultry industry, by colloidal ferric hydroxide (CFH) using black light lamps (λ = 365 nm) as the light source. The results revealed the contribution to ASA transformation at circumneutral conditions by CFH through an LMCT process, which is the same as that for As(III). The complexation between ASA and CFH was investigated using UV-vis spectroscopy. The estimated equilibrium constant for the CFH-ASA complex was log K_f271 = 4.22. The analysis of the photoproducts found the generation of both inorganic and organic arsenic. Our findings confirmed the similarities in the photochemical mechanisms of ASA and As(III) in the presence of CFH. The results help in further understanding the fate of organoarsenicals in the surface water environment.

Significant role of iron on the fate and photodegradation of enrofloxacin

Enrofloxacin (ENR) belongs to the fluoroquinolone (FQ) antibiotics family, which are contaminants of emerging concern frequently found in effluents. Although many works studying photo-Fenton process for FQ degradation have been reported, there are no reports analysing in deep the effect of iron complexation, as well as other metals, towards FQs' photolysis, which, evidently, also contributes in the overall degradation of the pollutant. Therefore, in this work, we report a comparative study between the photochemical fate of ENR and its complex with Fe(III) under simulated sunlight irradiation. In addition, the effect of dissolved oxygen, self-sensitization process, and H₂O₂ addition on the studied photochemical systems are also investigated. Results indicate that, for free and iron-complexed ENR, singlet oxygen (¹O₂) is generated from the interaction of its triplet state with ground state oxygen. Half-life time (t_1/2) of ENR under sun simulated conditions is estimated to be around 22 min, while complexation with iron enhances its photostability, leading to a t_1/2 of 2.1 h. Such finding indicates that at least the presence of iron, might notably increase the residence time of these pollutants in the environment. Eventually, only with the addition of H₂O₂, the FQ-iron complex is efficiently degraded due to photo-Fenton process even at circumneutral pH values due to the high stability of the formed complex. Finally, after LC/FT-ICR MS analysis, 39 photoproducts are detected, of which the 14 most abundant ones are identified. Results indicate that photoproducts formation is pH and iron dependent.

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 soils contaminated by hydrophobic organic compounds: How to recover extracting agents from soil washing solutions?

A lot of soil (particularly, former industrial and military sites) has been contaminated by various highly toxic contaminants such as petroleum hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenyls (PCBs) or chlorinated solvents. Soil remediation is now required for their promotion into new industrial or real estate activities. Therefore, the soil washing (SW) process enhanced by the use of extracting agents (EAs) such as surfactants or cyclodextrins (CDs) has been developed for the removal of hydrophobic organic compounds (HOCs) from contaminated soils. The use of extracting agents allows improving the transfer of HOCs from the soil-sorbed fraction to the washing solution. However, using large amount of extracting agents is also a critical drawback for cost-effectiveness of the SW process. The aim of this review is to examine how extracting agents might be recovered from SW solutions for reuse. Various separation processes are able to recover large amounts of extracting agents according to the physicochemical characteristics of target pollutants and extracting agents. However, an additional treatment step is required for the degradation of recovered pollutants. SW solutions may also undergo degradation processes such as advanced oxidation processes (AOPs) with in situ production of oxidants. Partial recovery of extracting agents can be achieved according to operating conditions and reaction kinetics between organic compounds and oxidant species. The suitability of each process is discussed according to the various physicochemical characteristics of SW solutions. A particular attention is paid to the anodic oxidation process, which allows either a selective degradation of the target pollutants or a complete removal of the organic load depending on the operating conditions.

What is the role of light in persulfate-based advanced oxidation for water treatment?

Persulfate-based advanced oxidation processes (PS-based AOPs) under UV, visible, or solar irradiation are being intensively investigated for water treatment. Tremendous advances have been made for enhancing the performance towards the destruction of target pollutants, but a deeper understanding of the role of light in different photo-activated PS-based AOPs is still needed as a basis for improving the efficiency. This paper intends to provide an in-depth review of the underlying photo-activation mechanisms and recent progress in various common photo-activated PS-based AOPs reported over the last decade. Based on a comprehensive survey of previous studies, we categorize these processes according to their reaction mechanisms, including activation by direct UV radiation, processes based on dye-photosensitization, activation through ligand-to-metal charge transfer (LMCT), and photocatalytic processes. Moreover, the improvement in performance of contaminant degradation in these processes compared with those in the absence of light are summarized. Finally, we conclude this review by proposing critical challenges and future perspectives for developing efficient photo-activated PS-based AOPs toward improvement in water treatment and remediation.

Phenanthrene decomposition in soil washing effluents using UVB activation of hydrogen peroxide and peroxydisulfate

In this work, the decomposition of phenanthrene (PHE) in mimic and real soil washing (SW) effluents was investigated using UVB light assisted activation of hydrogen peroxide (H₂O₂) and peroxydisulfate (PDS) oxidation processes. The impact of oxidant concentration, initial pH, and coexisting inorganic anions (Cl^-, HCO₃^- and NO₃^-) on PHE removal was evaluated. PHE degradation efficiency under UVB irradiation followed the order of UVB/PDS > UVB/H₂O₂ > UVB. The increase of PHE decomposition efficiency was observed with increasing oxidant dose in the range of 2-30 mM upon the two processes. It was found Cl^- played different roles in the two activation systems depending on the solution pH and Cl^- concentration. The influence of HCO₃^- on PHE elimination was negligible in the UVB/PDS process, while an inhibitory effect was observed in the UVB/H₂O₂ system. Nitrate inhibited the PHE decay in both UVB/H₂O₂ and UVB/PDS processes at the investigated pH 3.3, 7.1 and 8.6. Finally, the application of the two activation processes to the treatment of real SW effluents indicated that up to 85.0% of PHE degradation could be reached under 6 h UVB irradiation with PDS, indicating UVB/PDS process is a promising alternative for SW effluent treatment.

Application of graphene-based materials for removal of tetracyclines using adsorption and photocatalytic-degradation: A review

Tetracyclines are extensively used to treat human and animal infectious diseases due to its effective antimicrobial activities. About 70-90% of its parent materials are released into the environment through urine and feces, implying they are the most frequently detected antibiotics in the environment with high ecological risks. Adsorption and photocatalysis have been promising techniques for the removal of tetracyclines due to effectiveness and efficiency. Graphene-based materials provide promising platforms for adsorptive and photocatalytic removal of tetracyclines from aqueous environment owning to distinctive remarkable physicochemical, optical, and electrical characteristics. Herein, we intensively reviewed the available literatures in order to provide comprehensive insight about the applications and mechanisms of graphene-based materials for removal of tetracyclines via adsorption and phototocatalysis. The synthesis methods of graphene-based materials, the tetracycline adsorption and photocatalytic-degradation conditions, and removal mechanisms have been extensively discussed. Finally concluding remarks and future perspectives have been deduced and recommended to stimulate further researches in the subject. The review study can be used as theoretical guideline for further researchers to improve the current approaches of material synthesis and application towards tetracyclines removal.

Remediation of Cu-phenanthrene co-contaminated soil by soil washing and subsequent photoelectrochemical process in presence of persulfate

A promising technology was proposed for the remediation of Cu and phenanthrene (PHE) co-contaminated soil. Firstly, Cu and PHE were simultaneously removed from soil using an extraction agent containing ethylenediaminetetraacetic acid (EDTA) and Tween 80 (TW80). Specifically, Cu was mobilized by the chelation of EDTA while PHE was desorpted from soil by TW80 micelles. At 20 mL g^-1 of liquid to soil ratio, 10 g L^-1 TW80 and 1 mM EDTA, 73.5% Cu and 68.1% PHE were removed by soil washing. The extraction agent was then recovered from effluent by photoelectro-persulfate (PE/PS) process after soil washing. Under the conditions of 10 mM PS, 4 mA cm^-2 current density, natural pH of 3.5 and 150 min treatment, 83.6% Cu was reduced to elemental copper but only 36.8% EDTA was recovered, while 99.6% PHE was eliminated and 94.0% TW80 was recovered. The result of quenching experiments indicated PHE was predominantly oxidized by sulfate radical, though hydroxyl radical and singlet oxygen were also identified by electron paramagnetic resonance technique. The recovered extracting agent could be reused for further three washing cycles. The results demonstrated that soil washing followed by PE/PS process could be a cost-effective remediation technology for heavy metal and organic co-contaminated soil.

Mechanism and stability of an Fe-based 2D MOF during the photoelectro-Fenton treatment of organic micropollutants under UVA and visible light irradiation

This work reports the novel application of an Fe-based 2D metal-organic framework (MOF), prepared with 2,2'-bipyridine-5,5'-dicarboxylate (bpydc) as organic linker, as highly active catalyst for heterogeneous photoelectro-Fenton (PEF) treatment of the lipid regulator bezafibrate in a model matrix and urban wastewater. Well-dispersed 2D structures were successfully synthesized and their morphological, physicochemical and photocatalytic properties were assessed. UV/Vis PEF using an IrO₂/air-diffusion cell with an extremely low catalyst concentration (0.05 g L^-1, tenfold lower than reported 3D MOFs) outperformed electro-oxidation with electrogenerated H₂O₂, electro-Fenton and visible-light PEF. Its excellent performance was explained by: (i) the enhanced mass transport of H₂O₂ (and organic molecules) at the 2D structure, providing active sites for heterogeneous Fenton's reaction and in-situ Fe(II) regeneration; (ii) the ability of photoinduced electrons to reduce H₂O₂ to ^•OH, and Fe(III) to Fe(II); and (iii) the enhanced charge transfer and excitation of Fe-O clusters, which increased the number of electron-hole pairs. LC-QToF-MS and GC-MS allowed the identification of 16 aromatic products of bezafibrate. The complete removal of four micropollutants mixed in urban wastewater at pH 7.4 revealed the great potential of (Fe-bpydc)-catalyzed PEF process.