Degradation of sulfonamides and formation of trihalomethanes by chlorination after pre-oxidation with Fe(VI)

Mechanism of norfloxacin transformation by horseradish peroxidase and various redox mediated by humic acid and microplastics

The catalysis of HRP coupling with redox mediator was a feasible technology for the transformation of antibiotics. This work screened three effective redox mediators syringaldehyde (SYR), acetosyringone (AS) and p-coumaric acid (PCA) for the norfloxacin (NOR) transformation in HRP/redox mediator system. Then, compared their transformation characteristics under varying conditions. The molecular docking results indicated HRP catalytic mediator was spontaneous, and the absolute value order of free energy between three redox mediators and HRP was consistent with the order of NOR removal in experiment. The presence of humic acid (HA) and polystyrene (PS) microplastics could block the removal of NOR, and the inhibition effect was proportional to the level of HA and PS particles. Seven and six possible intermediate products were identified by using SYR/AS and PCA as redox mediators, respectively, and potential NOR transformation pathways were proposed. SYR and AS treatment had the same transformation products and pathways due to their similar structure, including defluorination, oxidation, cross-coupled with mediator, demethylation and dehydrogenation. While for the PCA group, NOR not only performed the above action (except defluorination), but also underwent decarbonylation. These findings may expand our knowledge of the conversion and fate of fluoroquinolones through HRP coupled with redox mediator in the environment.

Degradation of sulfamethoxazole by chlorination in water distribution systems: Kinetics, toxicity, and antibiotic resistance genes

Sulfamethoxazole (SMX) is one of veterinary drugs and food additives, which has been frequently detected in surface waters in recent years and will cause damage to organisms. Therefore, SMX was selected as a target to be investigated, including the degradation kinetics, evolution of toxicity, and antibiotic resistance genes (ARGs) of SMX during chlorination in batch reactors and water distribution systems (WDS), to determine the optimal factors for removing SMX. In the range of investigated pH (6.3-9.0), the SMX degradation had the fastest rate at close to neutral pH. The chlorination of SMX was affected by the initial total free chlorine concentration, and the degradation of SMX was consistent with second-order kinetics. The rate constants in batch reactors are (2.23 ± 0.07) × 10²  M^-1  s^-1 and (5.04 ± 0.30) × 10 M^-1  s^-1 for HClO and ClO^-1 , respectively. Moreover, the rate constants in WDS are (1.76 ± 0.07) × 10²  M^-1  s^-1 and (4.06 ± 0.62) × 10 M^-1  s^-1 , respectively. The degradation rate of SMX was also affected by pipe material, and the rate followed the following order: stainless-steel pipe (SS) > ductile iron pipe (DI) > polyethylene pipe (PE). The degradation rate of SMX in the DI increased with increasing flow rate, but the increase was limited. In addition, SMX could increase the toxicity of water initially, yet the toxicity reduced to the level of tap water after 2-h chlorination. And the relative abundance of ARGs (sul1 and sul2) of tap water samples was significantly increased under different chlorination conditions. PRACTITIONER POINTS: The degradation rate of SMX in batch reactor and WDS is different, and they could be described by first- or second-order kinetics. The degradation of SMX had the fastest rate at neutral pH. The degradation rate of SMX was also affected by pipe material and flow velocity. SMX increased the toxicity of water initially, yet the toxicity reduced after a 2-h chlorination. SMX increased the relative abundance of antibiotic resistance genes sul1 and sul2.

Indirect spectrophotometric determination of aqueous ferrate(VI) based on its reaction with iodide in acidic media

Ferrate(VI) (Fe(VI)) is utilized as an efficient and environmentally friendly water treatment agent that can be widely used for degradation of (in)organic pollutants in practical applications. However, only a few spectrophotometric methods for Fe(VI) determination were reported. In this study, a novel method for determining trace levels of aqueous Fe(VI) was developed based on the fact that Fe(VI) reacts with iodide at acidic pH to form iodine, which subsequently is treated with starch to yield the blue starch-iodine complex measured spectrophotometrically at 590 nm. The key measurement parameters, including acidic medium, starch dosages, temperature, time, and addition order were optimized to improve the sensitivity of detection. The increase in absorbance at 590 nm was linear with respect to Fe(VI) added (0.022-50 µM). Its sensitivity was determined as (4.61 ± 0.05) × 10⁴ M^-1 cm^-1, which was higher than that of existing spectrophotometric methods. The principle for Fe(VI) determination was studied by investigating stoichiometry, kinetics, and mechanism of Fe(VI) reaction with iodide. The molar stoichiometry of Fe(VI) with I₃^- species was determined to be 1:2. The reaction of Fe(VI) with iodide followed a second-order rate law with first order in each reactant and displayed apparent anti-Arrhenius kinetics, then its reaction pathway was proposed as well. Furthermore, the established method was successfully applied to measure Fe(VI) in various environmental water samples. The results show that the proposed approach is simple, convenient, highly reproducible and extremely sensitive, and is also expected to be of use for kinetic studies of Fe(VI) reaction with (in)organic compounds under acidic conditions.

Review of Advanced Oxidation Processes Based on Peracetic Acid for Organic Pollutants

In recent years, the removal of organic pollutants from water and wastewater has attracted more attention to different advanced oxidation processes (AOPs). There has been increasing interest in using peroxyacetic acid (PAA), an emerging oxidant with low or no toxic by-products, yet the promotion and application are limited by unclear activation mechanisms and complex preparation processes. This paper synthesized the related research results reported on the removal of organic pollutants by PAA-based AOPs. Based on the research of others, this paper not only introduced the preparation method and characteristics of PAA but also summarized the mechanism and reactivity of PAA activated by the free radical pathway and discussed the main influencing factors. Furthermore, the principle and application of the newly discovered methods of non-radical activation of PAA in recent years were also reviewed for the first time. Finally, the shortcomings and development of PAA-based AOPs were discussed and prospected. This review provides a reference for the development of activated PAA technology that can be practically applied to the treatment of organic pollutants in water.

Ferrate(VI) Oxidation Mechanism of Substituted Anilines: A Density Functional Theory Investigation

Ferrate(VI) (Fe(VI)) is a promising oxidant coagulant and disinfectant for the degradation of organic micropollutants. However, it is hard to elucidate the detailed oxidation mechanism through the current experimental approaches. Substituted anilines (SANs) are important chemical compounds that are widely used in many industries. This paper presents the use of density functional theory (DFT) to understand the oxidation mechanism of SANs by Fe(VI) and the effect of substituents. The calculation results revealed that the primary oxidations of SANs follow the hydrogen atom transfer (HAT) mechanism. Interestingly, the hydroxyl oxygen of HFeO₄ ^- is more reactive than the carbonyl oxygen when reacting with SANs. The formation of the SAN radical is crucial, and all of the products are formed from it. Azobenzene is more favorable to generate the above products. In addition, the obtained results indicate that this kind of substituent has a much greater influence on the reaction rather than the position. Thus, the present study provides a valuable insight into the transformation pathways of SANs in the Fe(VI) oxidation process and the effects of the substituent on oxidation. These results will advance the understanding of Fe(VI) involved in wastewater treatment.

Iodinated trihalomethanes formation in iopamidol-contained water during ferrate/chlor(am)ination treatment

Iopamidol is a commonly used iodinated X-ray contrast media in medical field, and its residue in water can react with disinfectants to form highly toxic iodinated disinfection by-products (I-DBPs). This study investigated the degradation of iopamidol and formation of DBPs, especially iodinated trihalomethanes (I-THMs), during ferrate (Fe(VI)) pre-oxidation and subsequent chlor(am)ination under raw water background. It was found that iopamidol degradation efficiency in raw water by Fe(VI) at pH 9 could reach about 80%, which was much higher than that at pH 5 and pH 7 (both about 25%). With Fe(VI) dose increasing, iopamidol removal efficiency increased obviously. During the iopamidol degradation by Fe(VI), IO₃^- was the dominant product among all the iodine species. After pre-treated by Fe(VI), yields of THM4 and I-THMs can be reduced in subsequent chlor(am)ination. Besides, pH was a crucial factor for Fe(VI) pre-oxidition controlling DBPs. With the pH increasing from 5 to 9, the yield of THM4 kept increasing in subsequent chlorination but showed the highest amount at pH 6 in subsequent chloramination. The yield of I-THMs increased first and then decreased with the increase of pH in both subsequent chlorination and chloramination. I-THM concentrations in chlorinated samples were lower than chloraminated ones under acidic conditions but became higher under neutral and alkaline conditions. The total CTI of THMs during Fe(VI)-chloramination was higher than that during Fe(VI)-chlorination under neutral condition, but sharply decreased under alkaline conditions. In summary, Fe(VI)-chloramination subsequent treatment under alkaline conditions should be an effective method for iopamidol removal and DBP control.

Transformation of sulfadiazine in humic acid and polystyrene microplastics solution by horseradish peroxidase coupled with 1-hydroxybenzotriazole

Enzyme catalyzed coupling with redox mediators are considered as great interesting and viable technologies to transform antibiotics. This work demonstrated the horseradish peroxidase (HRP) was effective in transforming sulfadiazine (SDZ) transformation coupled with 1-hydroxybenzotriazole (HBT) at varying conditions. The removal of SDZ was independent of Na⁺ and its ionic strength, but Ca²⁺ could enhance transformation efficiency by increasing the enzyme activity of HRP. The presence of humic acid (HA) and polystyrene (PS) microplastics showed inhibition on the transformation of SDZ, and the transformation rate constants (k) decreased with the concentration of HA and PS particles increased. These primarily attributed to covalent coupling and electrostatic interaction between SDZ and HA, SDZ and PS, respectively, which reduced the concentration of free SDZ in the reaction solution. The presence of cation recovered the inhibition of SDZ transformation by HA and PS particles, which ascribed to compete between cation and SDZ. The divalent cations (Ca²⁺) showed more substantial competitiveness than mono (Na⁺) due to more carried charge. Eight possible transformation products were identified, and potential SDZ transformation pathways were proposed, which include δ-cleavage, γ-cleavage, carbonylation, hydroxylation, SO₂ extrusion and SO₃ extrusion. In addition, HA and PS particles couldn't affect the transformation pathways of SDZ. These findings provide novel understandings of the transformation and the fate of antibiotics in the natural environment by HRP coupled with redox mediators.

Removal of sulfamethoxazole, sulfadiazine, and sulfamethazine by UV radiation and HO• and SO4•- radicals using a response surface model and DFT calculations

In this work, the degradation of sulfamethazine (SMT), sulfadiazine (SMD), and sulfamethoxazole (SMX) by using UV light, UV/H₂O₂, and UV/S₂O₈^-2 was analyzed. Direct photolysis was studied by varying the lamp power and the solution pH. DFT calculations were carried out to corroborate the efficiency of the degradation as a function of the solution pH. The variation of the apparent rate constant, k_ap, was determined in the indirect photolysis by employing an experimental Box-Behnken-type response surface design. The results evidenced that SMX can be efficiently degraded by applying UV radiation independent of the operating conditions. Nevertheless, the quantum yields for SMT and SMD were close to zero, indicating a low energy efficiency for their photochemical transformation. The effect of the solution pH showed that the photodegradation of sulfonamides depends both on the amount of radiation absorbed as the electronic density. Calculations based on density functional theory and supported by the quantum theory of atoms in molecules allowed to describe fragmentation patterns in the systems under study, proving the lability of S₁₄-C₂, N₁₇-C₁₈, and N₂₂-O₂₂ bonds, for SMT, SMD, and SMX, respectively. From response surface methodology, four statistically reliable equations were obtained to determine the k_ap value as a function of the system operating conditions. Finally, SO₄^•- radicals proved to have a higher reactivity to degrade SMT and SMD compared with HO^• radicals regardless of the operating conditions of the system.

The degradation, biodegradability and toxicity evaluation of sulfamethazine antibiotics by gamma radiation

This study evaluated gamma radiation-enhanced sulfamethazine (SMT) degradation, which belongs to the heterocyclic sulfonamides, in different conditions in aqueous solution. The results showed that gamma irradiation could increase the SMT removal in aqueous solution, and the SMT degradation kinetic follow the modified pseudo-first-order kinetic. The degradation rate was nearly 95% at the absorbed dose of 4 kGy. The removal of total organic carbon (TOC) and total nitrogen (TN) could be also induced. When the radiation dose was 5 kGy, the removal rates were only about 6.8% and 10.5% for TOC and TN, respectively. Some ions including HCOO−, CH3COO− and SO42− released during SMT degradation were studied. The biodegradability and toxicity of intermediate products of the SMT degradation were also proposed in this experiment. The biodegradability of the SMT can be improved by the intermediate products of SMT degradation at the radiation dose of 1, 2 and 5 kGy. Gamma radiation could be used as a pretreatment technology before the biodegradation process of pharmaceutical waste water.

Removal of disinfection by-products and their precursors during drinking water treatment processes

In this study, we investigated the control efficiency of a wide variety of disinfection by-products (DBPs) (including trihalomethanes [THMs], haloacetic acids [HAAs], haloacetonitiles [HANs], haloketones [HKs], haloaldehydes [Has], and trihalonitromethanes [THNMs]) with different drinking water treatment processes including pre-ozonation, coagulation-sedimentation, sand filtration, and ozone combined with biological activated carbon (O₃ -BAC) advanced treatment processes. The assessment of the treatment efficiency regarding the removal of organic matter was measured by the excitation emission matrix (EEM) spectra. There was a superior efficiency in reducing the formation of DBPs and their precursors by different drinking water treatment processes. Though some DBPs such as THMs could be promoted by ozonation, these by-products from ozonation could be degraded by the following BAC filtration process. In addition, the organic matter from the aromaticity, fulvic acid-like, protein, and soluble microbial by-products-like regions could be further degraded by the O₃ -BAC treatment. PRACTITIONER POINTS: A wide variety of DBPs in different drinking water treatment processes was investigated. The treatment efficiency regarding the removal of organic matter was measured. Some DBPs such as THMs and HAAs could be increased by ozonation. The removal percentage of nitrogen precursors and organic carbon would be increased by BAC filtration.

Oxidation of sulfonamides by ferrate(VI): Reaction kinetics, transformation byproducts and toxicity assesment

This study was aimed at the degradation of sulfonamides (SNs) via oxidation with Fe(VI). The reaction kinetics, identification of degradation byproducts and their toxicity were investigated. The pH solution and Fe(VI) loading had significant effects on the degradation of the sulfonamides. The maximum degradation rate occurred at pH 3.0 with a 6:1 ratio Fe(VI): sulfonamide, obtaining 100% degradation of 15 mg L^-1 SN within 5 min. Although Fe(VI) also showed an appreciable reactivity towards SNs (k_app = 9.85-19.63 × 10² M^-1 s^-1) at pH 7. The influence of solution pH on the values of k_app can be explained considering the specific reaction between Fe(VI) and SNs. Degradation rates are also influenced by the presence of inorganic ions in different water matrixes. For this reason, ions present in groundwater enhanced the SNs degradation through a synergistic effect among carbonates, sulfates and Fe(VI). Degradation byproducts identified, through UPLC analysis, allowed us to proposed three degradation pathways depending on pH. At acid pH there is a cleavage of C-S and S-N bonds. At neutral pH nitroso and nitro-derivates are formed. At basic pH hydroxylation is the main reaction. The cytotoxicity assay of HEK-293 and J774 cell lines exposed to Fe(VI) indicated that transformation byproducts had a lower toxicity than SNs as baseline products. Accordingly, this research suggests that Fe(VI) can act as a chemical oxidant to remove SNs antibiotics and it can be used to treat antibiotic pollution in wastewater.

Enhanced Degradation of Sulfamethoxazole (SMX) in Toilet Wastewater by Photo-Fenton Reactive Membrane Filtration

Pharmaceutical residuals are increasingly detected in natural waters, which made great threat to the health of the public. This study evaluated the utility of the photo-Fenton ceramic membrane filtration toward the removal and degradation of sulfamethoxazole (SMX) as a model recalcitrant micropollutant. The photo-Fenton catalyst Goethite (α-FeOOH) was coated on planar ceramic membranes as we reported previously. The removal of SMX in both simulated and real toilet wastewater were assessed by filtering the feed solutions with/without H₂O₂ and UV irradiation. The SMX degradation rate reached 87% and 92% respectively in the presence of UV/H₂O₂ for the original toilet wastewater (0.8 ± 0.05 ppb) and toilet wastewater with a spiked SMX concentration of 100 ppb. The mineralization and degradation by-products were both assessed under different degradation conditions to achieve deeper insight into the degradation mechanisms during this photo-Fenton reactive membrane filtration. Results showed that a negligible removal rate (e.g., 3%) of SMX was obtained when only filtering the feed solution through uncoated or catalyst-coated membranes. However, the removal rates of SMX were significantly increased to 67% (no H₂O₂) and 90% (with H₂O₂) under UV irradiation, respectively, confirming that photo-Fenton reactions played the key role in the degradation/mineralization process. The highest apparent quantum yield (AQY) reached up to approximately 27% when the H₂O₂ was 10 mmol·L⁻¹ and UV254 intensity was 100 μW·cm⁻². This study lays the groundwork for reactive membrane filtration to tackle the issues from micropollution.

Evaluation of disinfection byproduct formation from extra- and intra-cellular algal organic matters during chlorination after Fe(vi) oxidation

Blue-green algae commonly bloom in fresh water in summer, producing extra- and intra-cellular algal organic matters, which are important precursors for disinfection byproduct (DBP) formation. In this study, we evaluated the effect of pre-oxidation with ferrate(vi) (FeO4 2-, Fe(vi)) on the characterization of intracellular organic matter (IOM) and extracellular organic matter (EOM). The results indicated that soluble microbial-like products of EOM and IOM decreased and humic acid-like products of IOM increased, which would influence the DBP formation in the subsequent chlorination step. Therefore, in this study, the effect of Fe(vi) pre-oxidation on the DBP formation from IOM and EOM with subsequent chlorination was also investigated. For Chlorella sp., EOM presented no significant change, and IOM presented a reduction of THMs (8.2%) after Fe(vi) oxidation at a dosage of 16 mg L-1. For P. limnetica, EOM and IOM both exhibited reduction of trihalomethanes (THMs) and chloral hydrate (CH) after Fe(vi) oxidation. Besides, THMs had the lowest concentration at pH 8.0 in all four solutions. Haloacetonitriles (HANs) and haloketones (HKs) showed slight changes with increasing pH values. Due to the frequent detection of bromide (Br-) in surface water, the effect of bromide existence on THM formation was also investigated. The results indicated that all brominated DBPs increased, and chlorinated DBPs decreased with the increase in bromide concentration. In addition, the bromine substitution factor (BSF) of Chlorella sp. and P. limnetica both increased with the increase in Br- concentration.

Comparative Study of the Oxidative Degradation of Different 4-Aminobenzene Sulfonamides in Aqueous Solution by Sulfite Activation in the Presence of Fe(0), Fe(II), Fe(III) Or Fe(VI)

This study is focused on advanced oxidation technologies (AOTs) using the combined effect of Fe(0–VI)/sulfite systems, that produce mainly SO4•− radicals, to remove different 4-aminobenzene sulfonamides (SAs), namely sulfamethazine, sulfadiazine, sulfamethizole, from aqueous solutions. Results obtained showed that neither sulfite nor iron alone is able to degrade SAs; however, the combined effect depends on the oxidation state of iron species whose effectiveness to activate sulfite to promote the degradation of SAs increased following this order: Fe(III) < Fe(II) < Fe(0) < Fe(VI). Using Fe(VI)/sulfite, the complete removal of SAs was obtained in 5 min largely surpassing the effectiveness of the other three systems. The sulfonamides’ removal percentage was markedly influenced by sulfite concentration and dissolved oxygen, which improved the generation of oxidant radicals. Response surface methodology was applied, and a quadratic polynomial model was obtained, which allowed us to determine the percentage of SAs degradation as a function of both the iron species and sulfite concentrations. The study of the influence of the water matrix on these AOTs revealed an inhibition of SAs’ removal percentage when using ground water. This is probably due to the presence of different anions, such as HCO3−, Cl−, and SO42− in relatively high concentrations. According to the byproducts identified, the proposed degradation pathways include hydroxylation, SO2 extrusion, and different bond-cleavage processes. Cytotoxicity of degradation byproducts, using MTS assay with HEK 293 and J774 cell lines for the first time, did not show an inhibition in cell proliferation, sustaining the safety of the process.