Simultaneous mitigation of multiple pollutants in urban stormwater systems illicitly connected with wastewater systems by an Fe(VI)-based process

Abstract

The illicit connection of wastewater pipes to stormwater pipes might result in the direct discharge of wastewater into natural water and even drinking water sources. The multiple pollutants in untreated wastewater effluent, including organic matter, nutrients, emerging contaminants (ECs) and disinfection by-product (DBP) precursors, posed risks to ecological safety. Herein, Fe(VI) and Fe(VI)/Fe(III)-based processes were found to be effective in treating overflow wastewater as a combination of coagulation and oxidation. In the presence of Fe(VI) below 200 μM, the addition of Fe(III) could further improve the removal of COD (43.1%), TP (87.9%), and turbidity (95.3%) compared to that by Fe(VI) alone. With respect to ECs, the highly detected paracetamol (PCT) of 10 μM in wastewater can be efficiently degraded by Fe(VI) exceeding 300 μM, which reached approximately 97.2% removal within 22 min. The rapid consumption of Fe(VI) by other organics present in wastewater necessitates the addition of Fe(III) at a low [Fe(III)] : [Fe(VI)] ratio to expedite the oxidation of ECs. For DBPs, the Fe(VI)-based process decreased DBP formation and DBP-associated cytotoxicity by about 50–80% at optimal dosage (300 μM) and prioritize the removal of haloacetaldehyde and haloacetonitrile precursors. This may be attributed to the efficient removal of aromatic protein-like components. However, the addition of Fe(III) may deteriorate the DBP formation control due to interaction between Fe(III) and Fe(VI) reducing the Fe(VI) oxidation capacity on organics. This study demonstrated that Fe(VI) and Fe(VI)/Fe(III)-based processes might be a promising treatment process for simultaneous removal of multiple pollutants in wastewater from illicitly connected urban stormwater systems.

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Water impact

The illicit connection of wastewater pipes to stormwater pipes might result in wastewater containing multiple pollutants to be discharged into natural water and even drinking water sources. In this study, we use an Fe(VI)-based process to synchronously remove multiple pollutants in wastewater. The results indicate that the oxidation and coagulation effects of Fe(VI) can achieve simultaneous mitigation of conventional pollutants, partial ECs and DBP precursors and Fe(III) can enhance the coagulation performance at low dosage of Fe(VI).

1. Introduction

Urban drainage system pipes play a crucial role in the transportation of wastewater and rainwater, serving as an integral component of the urban water cycle. Currently, urban drainage systems in China are divided into two system types: combined sewer systems and separate sewer systems.¹ However, combined sewer overflow (CSO) from combined sewer systems contains a variety of complex mixtures of pollutants such as solids, organic matter, nutrients, metals, organic compounds and pathogenic microorganisms that can be charged into water bodies during heavy rainfall or snowmelt events.² Furthermore, recent studies have indicated that CSO can also serve as a potential source of emerging contaminants (ECs), such as pharmaceuticals, illicit drugs and personal care products.³ This may be attributed to the discharge of a mixture of untreated wastewater and rainwater. Therefore, the treatment of CSO has garnered attention from various countries. In addition, the construction cycle of drainage systems in developing countries is not synchronized, which causes the illegal connection of sewage pipes to stormwater pipes for separate sewer systems in developing counties.^4–7 This may pose another potential risk of untreated wastewater entering the natural waters and potentially contaminating drinking water sources through separate stormwater drainage systems.⁵ In addition to the large amount of organic pollutants introduced and potential ECs, the charge of dissolved organic matter (DOM) in the urban stormwater drainage system resulted in increased precursors of highly toxic haloacetaldehydes (HALs) and haloacetonitriles (HANs), which may have adverse effects on downstream water quality.^6,8 The current common treatment methods for CSO mainly involved some physical precipitation separation processes and disinfection treatment methods, which have limited treatment effects on ECs and DOM.^9,10 Many unfinished stormwater drainage systems lack proper treatment for the potential wastewater introduced. The prevalent employment of chlorine disinfection methods poses a chemical hazard in the form of disinfection by-products (DBPs).¹¹ Moreover, the current coagulation treatment of CSO and sewage mainly focused on the removal of conventional pollutants such as COD, TP, and turbidity, lacking treatment for micro-pollutants and DBP precursors.^12–14 Therefore, it is an urgent need to develop effective methods to remove ECs and reduce the chemical risk of DBPs produced by DOM in urban stormwater systems illicitly connected with wastewater systems.

Ferrate (Fe(VI)) has emerged as a highly effective oxidant in the field of wastewater treatment in recent years.^15,16 Fe(VI) has several advantages over other oxidants, including a wide range of oxidation potential (2.2–3.1 V), multiple oxidation states (Fe(VI), Fe(V), Fe(IV)), stability under alkaline conditions, and harmless by-products (Fe(III)) that can also act as coagulants or catalysts for further treatment.^17,18 Fe(VI) can oxidize various ECs by direct electron transfer or by generating Fe(V)/Fe(IV) intermediates that can further react with organic substrates or water molecules to produce ·OH, which have obvious advantages in selectivity and resistance to inorganic ions and DOM in the water matrix.^19,20 Fe(VI) has also been applied as a pre-oxidation method to remove DBP precursors in water, reducing the risk of DBP formation during subsequent disinfection processes.^21,22 Furthermore, due to the coagulation performance of subsequent Fe(III), Fe(VI) can also be used as a coagulant in the coagulation sedimentation process commonly used in CSO to meet the requirements for removing particles and chemical oxygen demand (COD).¹⁴ However, Fe(VI) has not been applied for the removal of common micro pollutants like paracetamol (PCT) and ibuprofen (IBP) in CSO and mixed wastewater.³ In addition, the treatment performance of DBP precursors in CSO and mixed wastewater has not been studied in detail.^6,8

The oxidation efficiency of Fe(VI) can be enhanced by different activation methods such as light irradiation,²³ reducing agents,²⁴ oxidants²⁵ or metal ion reactions²⁶ to produce ·OH or other free radicals. Among these methods, Fe(III) activation involves the reaction of Fe(III) with Fe(VI) to produce Fe(V) and Fe(IV) with stronger oxidation capacity as in eqn (1).²⁶ Fe(VI) can overcome the inhibitory effect of chloride ions on Fe(VI) and maintain the stability and activity of Fe(VI) in high salinity wastewater to promote the degradation of ECs with the assistance of Fe(III).¹⁸ Additionally, Fe(III) can improve the subsequent flocculation effect of Fe(VI) by forming larger particle precipitates with colloidal particles, facilitating their removal through downstream mechanical sieving-based filtration.²⁷ Studies have demonstrated the efficiency of Fe(III) coagulation treatment in controlling carbonaceous DBPs, including trihalomethanes (THMs) and haloacetic acids (HAAs),²² thus enhancing the application of Fe(VI) in wastewater treatment from multiple perspectives. However, the combined application of Fe(III) and Fe(VI) for controlling DOM and DBP precursors in wastewater has not been studied in detail.

Fe(VI) + Fe(III) → Fe(V) + Fe(IV) (1)

Therefore, the treatment process based on Fe(VI) and Fe(VI)/Fe(III) as a combination of coagulation and oxidation may be a multifunctional approach to simultaneously remove conventional pollutants, ECs and DBP precursors. It is a simple and convenient way of pollution control for various pollutants. This study selected ibuprofen and paracetamol as typical ECs due to their widespread use and high concentrations in stormwater-affected areas. The objectives of this paper are to: 1) systematically compare the performance of Fe(VI), Fe(III) and Fe(VI)/Fe(III)-based processes for conventional pollution, EC and DBP control; 2) optimize the dosage of Fe(VI) and Fe(III) to improve the efficiency of oxidation and coagulation; 3) reveal the main mechanism of the oxidation effect and coagulation precipitation effect of Fe(VI) and Fe(III) on DBP control and EC degradation effect.

2. Materials and methods

2.1. Materials

All chemicals were purchased from commercial reagent companies and were of at least analytical grade. The specific information about these reagents is listed in Text S1 of the ESI.† Sodium hypochlorite solution (active Cl₂ > 10%, Sigma-Aldrich, St. Louis, USA) was diluted to approximately 7 g L⁻¹, as Cl₂, and stored in a foil-covered brown glass bottle at 4 °C for further use. The ultrapure water used to prepare the solution comes from a Millipore Milli-Q Gradient water purification system (18 MΩ cm, Billerica, MA, USA). Raw water samples were taken from a typical Chinese wastewater treatment plant (WWTP). The sample collection time was concentrated from May to September 2023 at 6 pm, during the peak period of residential water use. In addition, the samples were collected on sunny days to simulate the water quality of mixed wastewater entering the stormwater pipeline caused by illicit connection of wastewater pipes. The samples were collected at the inlet of the sewage treatment plant, with 20 L collected at a time and were stored in a refrigerator at 4 °C and used up within a week. The characteristics of raw samples are shown in Table S1.†

2.2. Experimental procedures

The treatment effect of potassium ferrate is analyzed by coagulation sedimentation experiments. Coagulation experiments were conducted using a 6-place programmable multiple stirrer (ZR4–6, Zhongrun Water Industry Technology Development Co., China). The raw water sample was treated by Fe(VI) and Fe(VI)/Fe(III)-based processes. Fe(VI) and Fe(III) were added to the water sample simultaneously during the Fe(VI)/Fe(III)-based process. The water samples were rapidly mixed for 1 min at 300 rpm at first, followed by medium speed mixing at 120 rpm for 1 min, slow mixing at 50 rpm for 20 min and settling for 30 min. Supernatants were collected for subsequent measurements after settling. The unfiltered supernatant was used for turbidity measurement and the supernatant filtered with 0.45 μm syringe filter membranes was used for fluorescence spectra, molecular weight, dissolved organic carbon (DOC) and UV₂₅₄ measurements.

The raw water and filtered water samples were filtered with 0.45 μm syringe filter membranes and chlorinated in sealed 40 mL headspace amber glass bottles in the dark at 25 ± 0.5 °C for 24 h, according to the methods developed in previous studies.^28,29 The chlorine/chloramine dosages were calculated according to DOC and ammonia nitrogen (NH₄⁺) by eqn (2).²⁹ Sufficient chlorine was added to break out any ammonia under these conditions.

Cl₂ dosage (mg L⁻¹) = 3 × DOC (mg_C L⁻¹) + 7.6 × NH₃ (mg_N L⁻¹) + 10 (mg L⁻¹) (2)

To calculate the oxidation efficiency of ECs under different ratios of Fe(VI) and Fe(III), 10 μM PCT and IBP were added into ultrapure water (200 mL) separately, then the certain volume of filtered stock Fe(VI) solution (200 μM) was added into the working solution containing the target organic pollutant to initiate the reaction. In addition, the different volume of Fe(III) stock solution was added simultaneously to explore the activation effect of Fe(III). Samples (1.5 mL) of the reaction solution were withdrawn at defined time intervals and immediately quenched with 20 μL of 200 mM Na₂S₂O₃. Each aliquot was filtered with a 0.45 μm polyethersulfone syringe filter before measurement. Along with the same time interval, samples (2.0 mL) were withdrawn and quenched with 2,2′-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) solution to measure the residual Fe(VI) concentration.^30,31 Concomitantly, another aliquot of 2.0 mL was sampled, and quenched with another ABTS solution for subsequent determination of potential hydrogen peroxide produced. The same experimental process was implemented in wastewater to explore the effects of other water matrices in wastewater.

To evaluate the removal efficiency of coagulation and oxidization during the actual coagulation sedimentation process to different ECs, 10 μM paracetamol (PCT) and ibuprofen (IBP) were also added into raw water separately and subjected to different coagulation processes. After mixing and settling, 10 mL samples were withdrawn and quenched with 150 μL of 200 mM Na₂S₂O₃ for measuring residual EC concentration.

2.3. Analytical methods

DOC was first diluted ten-fold and then measured using a TOC analyzer (Shimadzu TOC-VCPH, Japan). UV absorbance at 254 nm, which could reflect the aromatic structure in DOM, was measured by using a UV-vis spectrophotometer (UV–9000S, Metash Instrument, Shanghai). The fluorescence characteristics of DOM weer measured by three-dimensional spectrofluorometry (F-7100 Fluorescence, HITACHI, Japan). The molecular weight was analyzed by using an Agilent 1260 HPLC system equipped with a polymethacrylate packed column (250 mm × 20 mm, 3 μm) and a Sievers M9 SEC online DOC detector. The details of fluorescence and molecular weight measurements are described in Texts S2 and S3.† A Zetasizer Nano ZS 90 (Malvern, UK) was used to measure the zeta potential of water samples. Turbidity was measured by using a turbidimeter (HACH, 2100Q).

THMs, HNMs, HANs and HALs were detected by using gas chromatography/electron capture detection (QP2010plus, Shimadzu Corporation, Japan). Detailed information on the analytical methods of these DBPs is in previous studies³² and the instrument parameters of quantitation are listed in Table S2.†

The concentrations of EC residuals in the samples were measured with ultraperformance liquid chromatography (UPLC), and the details of UPLC analysis are summarized in Text S4.† The concentration of the Fe(VI) stock solution was determined by measuring its absorbance at 510 nm (ε = 1150 M⁻¹ cm⁻¹) with a UV-vis spectrophotometer (UV–9000S, Metash Instrument, Shanghai). The concentration of residual Fe(VI) at defined time intervals was determined by the ABTS method using absorbance at 645 nm (ε = 11 600 M⁻¹ cm⁻¹).³³ The details of the methods for quantifying Fe(VI) are provided in Texts S5.†

2.4. Calculation of DBP-associated cytotoxicity and bromine incorporation factor (BIF)

The overall toxicity of DBP generated by wastewater disinfection could be evaluated through cytotoxicity indexes (CTI), calculated by using eqn (3). The overall toxicity of DBP was derived from the addition of individual DBP toxicity gained by toxicity testing of Chinese hamster ovary (CHO) cells.^34–36where the % C_1/2 value (M⁻¹) is the molar concentration that induced 50% reduction of the CHO cell density compared to the control, and C_x is the average molar concentration of each DBP (nM). The % C_1/2 value of each DBP is present in Table S3.†

Widespread concern about bromine substitution of DBPs was prompted due to the higher toxicity of brominated DBPs. The proportion of brominated DBPs for an individual class of DBPs was expressed by the bromine substitution factor (BSF), calculated as follows:^37,38

where [RCl_aBr_b] is the molar concentration of a DBP with a and b moles of chlorine and bromine, respectively.

3. Results and discussion

3.1. Removal of conventional pollutants by Fe(VI)-based processes

The removal performance on different conventional pollutants during the separate Fe(VI) coagulation process is illustrated in Fig. 1. The removal efficiency of Fe(VI) on different pollutants was inconsistent. The maximum removal efficiency of DOC was only 20% at the Fe(VI) dose of 500 μM, which was consistent with the removal performance of Fe(VI) on DOC in drinking water.³⁹ It has been reported that the oxidation capacity of Fe(VI) was limited in mineralization of organic carbon, resulting in little impact on DOC removal regardless of the type of treated water.⁴⁰ Differently, Fe(VI) exhibited a more significant removal performance on UV₂₅₄ than DOC, which implied a priority removal of aromatic organic compounds. When the concentration of Fe(VI) added ranged from 100 to 300 μM, UV₂₅₄ was decreased by 50.9–53.6%. The weak effect of coagulation–precipitation on soluble NH₄⁺ and sluggish reactivity with NH₄⁺ for Fe(VI) resulted in a lower removal efficiency of 21.3%.⁴¹ Therefore, it is not a preferable choice for removing NH₄⁺ compared to the traditional folding point chlorination method.⁴² The removal efficiency of turbidity and COD was 93.4% and 42.6% at the Fe(VI) dose of 300 μM, respectively. The removal of COD and turbidity by Fe(VI) primarily occurred through coagulation, targeting particulate and colloidal pollutants. However, unlike the traditional coagulants, the pre-oxidation effect of Fe(VI) could disrupt the stability of colloids in water to enhance the subsequent coagulation performance.^39,43

Fig. 1 The removal performance on (a) DOC, (b) UV₂₅₄, (c) NH₄⁺, (d) COD, (e) TP and (f) turbidity by different Fe(VI) dosages under separate Fe(VI) coagulation processes.

The removal performance on different conventional pollutants by the Fe(VI)/Fe(III) coagulation process is illustrated in Fig. 2. The addition of Fe(III) facilitated the removal performance of Fe(VI) on DOC with the [Fe(III)] : [Fe(VI)] ratio [Fe(III)/Fe(VI)] below 2, but the effect was not significant and gradually decreased with the increase of Fe(III) dose. In addition, the positive effect of Fe(III) was more obvious when the dosage of Fe(VI) was lower than 200 μM. This may be more likely attributed to the enhanced coagulation performance of Fe(III) on Fe(VI). On the one hand, the number and growth rate of flocs generated by Fe(VI) were lower than those generated by Fe(III). On the other hand, the incorporation of the Fe(III)-related colloid can improve the zeta potential of the suspension.¹² Consequently, the assisted addition of Fe(III) promoted the quicker formation and settlement of flocs, leading to an improvement in DOC removal efficiency. However, as the Fe(VI) dose increased to 300 μM, the increase in Fe(III) dosage resulted in faster settling of flocs, causing the promotion effect of Fe(III) to weaken. The promotion effect of Fe(III) on the removal of UV₂₅₄ (30.1–60.8%) was also stronger than that of DOC (24.3–41.1%) in the wastewater, and the promotion effect will not gradually decrease with the increase of Fe(VI) dosage. This may be explained by two respects: 1) the Fe(VI)/Fe(III) process generated intermediates like Fe(V) and Fe(IV) with stronger oxidation ability, which could react with aromatic organic compounds and disrupt conjugated double bond systems without substantially mineralizing the carbon^19,39 2) The Fe(VI)/Fe(III) process exhibited stronger coagulation performance that could preferentially remove the fraction of DOM with rich aromatic structures.⁴⁴ NH₄⁺ removal exhibited little improvement when Fe(III) and Fe(VI) were used together. The addition of Fe(III) has a relatively small promoting effect on the removal of COD. The enhanced coagulation effect of Fe(III) promotes the removal of particulate COD, while the Fe(VI)-based process has achieved the removal of most of the particulate COD. Fe(III) exhibited the best promotion effect on the removal of turbidity and TP for different dosages of Fe(VI) added. Similar to the removal of DOC, the turbidity and TP removal efficiency gradually increased with Fe(III) dosage when the Fe(VI) dosage was below 200 μM, and the promotion effect of Fe(III)/Fe(VI) higher than 0.5 on the removal of TP and turbidity has not been significantly improved as the Fe(VI) dosage increased to 300 μM. Therefore, it is suggested that Fe(III)/Fe(VI) higher than 1 is suitable for Fe(VI) doses below 200 μM, while Fe(III)/Fe(VI) lower than 0.5 is recommended for Fe(VI) doses above 300 μM.

Fig. 2 The removal performance on (a) DOC, (b) UV₂₅₄, (c) NH₄⁺, (d) COD, (e) TP and (f) turbidity by different dosages under Fe(VI)/Fe(III) coagulation processes.

3.2. Removal of ECs by Fe(VI)-based processes

3.2.1. The degradation kinetics of different ECs by direct oxidation of Fe(VI). As shown in Fig. 3 and S1,† for ultrapure water, the removal efficiency of two typical ECs by Fe(VI) alone was relatively low at 80% for PCT and 1.3% for IBP within 30 min. However, the reaction rate of PCT with Fe(VI) was enhanced in wastewater compared to that in ultrapure water, which may be attributed to the influence of some salinity in wastewater according to previous research.¹⁸ Approximately 80% of PCT was degraded within 30 minutes in ultrapure water, but almost complete removal of PCT was achieved within 1 minute in wastewater. In contrast, the removal of IBP in wastewater did not exhibit any significant change. The presence of salt ions in wastewater facilitated the production of intermediate Fe(V)/Fe(IV), which further enhanced the oxidation efficiency of ECs. However, the insignificant reactivity of IBP towards these intermediates resulted in no obvious degradation.¹⁸

Fig. 3 The degradation processes of PCT (a and c) and IBP (b and d) by Fe(VI) and Fe(VI)/Fe(III) in ultrapure water (a and b) and wastewater (c and d). Conditions: [PCT]₀ = [IBP]₀ =10 μm, [Fe(VI)]₀ = 200 μm, 25 ± 1 °C.

Subsequently, Fe(III) was introduced to the system, and its impact on the degradation rates of PCT was investigated. The results demonstrated that the addition of Fe(III) significantly enhanced the degradation rates of PCT in ultrapure water, as evidenced by the gradual increase in the apparent second-order rate constants (K_app) with increasing Fe(III) dosage (Fig. 3). As the dose of Fe(III) gradually increased from 0 to 400 μM, the K_app between PCT and Fe(VI) increased from (11.77 ± 0.3) M⁻¹ s⁻¹ to (693.41 ± 4.21) M⁻¹ s⁻¹. However, the appropriate dosage of Fe(III) in wastewater differed from that in ultrapure water due to the effects of the other water matrix. The addition of excessive Fe(III) resulted in the rapid generation of active species like Fe(V)/Fe(IV), which were subsequently consumed by other organic substances. Consequently, the removal rate of target pollutants decreased, with a maximum removal efficiency of only 50% achieved with the addition of 200 μM Fe(III). In contrast, the addition of appropriate dosage of Fe(III) (100 μM Fe(III)) is suggested to ensure the removal of PCT in wastewater. For IBP with poor reactivity, the addition of 400 μM Fe(III) only resulted in lower removal efficiency of IBP in ultrapure water, with a maximum removal efficiency of 20% achieved within 10 minutes. Notably, the activation of Fe(III) had no significant effect on the removal of IBP in wastewater, primarily due to the preferential reaction between active intermediate iron species and other organic compounds present in wastewater. It can be summarized that the direct oxidation of Fe(VI) was effective in removing most of the PCT but had no effect on the removal of IBP in wastewater.

3.2.2. The actual removal of ECs by oxidation and coagulation of Fe(VI)-based processes. Fig. S2† shows the removal efficiency of ECs by Fe(VI)/Fe(III)-based coagulation processes. Previous studies demonstrated that conventional Fe(III) coagulation only had limited efficacy in removing ECs, likely due to its poor ability to remove low molecular weight organics.^45–47 In contrast, Fe(VI) and Fe(VI)/Fe(III)-based processes exhibited effective removal of PCT due to the additional oxidation effect provided by Fe(VI). The majority of PCT was quickly eliminated during the mixing stage, and a small portion of remaining PCT then was slightly removed during the subsequent settling stage, which indicated the superior removal performance of PCT by oxidation compared with conventional coagulation. Furthermore, the removal efficiency of PCT increased with the increase of Fe(VI) dosage added when the Fe(VI) dosage was lower than 300 μM. When the Fe(VI) dosage exceeded 300 μM, the removal efficiency of PCT gradually reached a maximum of 97.2%, and there was no significant change with the increased Fe(VI) dosage. The difference in removal efficiency between the mixing stage and the settling stage also gradually disappeared with the increase of Fe(VI) dosage. However, consistent with the direct oxidation effect depicted in section 3.2.1, Fe(VI)-based coagulation had no obvious removal performance for IBP.

After adding Fe(III), the Fe(VI)/Fe(III)-based process only illustrated slight improvement in PCT removal compared to the Fe(VI)-based process. Similar to the results of direct oxidation of Fe(VI), PCT was quickly removed during the mixing stage, and then slightly decreased during the settling stage. Moreover, for the Fe(VI)/Fe(III)-based process, the addition of low doses of Fe(III) was beneficial for the removal of PCT and could slightly improve the removal efficiency of PCT in the mixing stage. However, when 400 μM Fe(III) was added, the removal efficiency of PCT actually decreased due to the consumption of intermediate iron species through reactions with other organics. Therefore, Fe(III)/Fe(VI) lower than 1 may be better choices to enhance Fe(VI) oxidation performance when using the Fe(VI)/Fe(III)-based process.

3.3. DBP control by Fe(VI)-based processes

3.3.1. DBP formation control. DBP formation during subsequent chlorination is illustrated in Fig. 4, and the concentrations of specific DBPs are available in Fig. S5 and S6.† Similar to surface water, carbonaceous DBPs (C-DBPs) like THMs and HALs occupied the majority of DBPs by weight in wastewater during chlorination. The separate Fe(VI)-based coagulation process could decrease the formation of most DBPs, especially for yields of THMs, HALs and HANs. Direct Fe(VI) oxidation could destroy THM precursors and result in the higher removal efficiency of THMFP with the increase of Fe(VI) dosage in most surface waters.³⁹ The DBP formation showed a trend of initially decreasing and then increasing with increasing Fe(VI) dosage by the Fe(VI)-based coagulation process. This may be attributed to various effects of Fe(VI)-based coagulation on DBP precursors, including direct oxidation and subsequent coagulation. Similar to ozonation, the strong oxidation of Fe(VI) may lead to decomposition of DOM, formation of polar byproducts, and accompanying increase of hydrophobic components in the partitioning behavior of NOM.^39,48 The decrease of overall molecular weight and increase of hydrophilicity might result in the subsequent poorer coagulation performance at the highest Fe(VI) dosage. In contrast to THMs, Fe(VI)-based coagulation was more effective and stable in controlling HALs. The concentrations of THMs decreased by 56.9–61.4% with the added Fe(VI) dosage below 200 μM, while HAL formation was reduced by 60.8–78.4% regardless of the Fe(VI) dosage change. This may be ascribed to different characteristics of C-DBP precursors. Hydrophilic DOM with low molecular weight tends to react with chlorine to produce HALs, which may have poor removal efficiency by coagulation.^46,49,50 However, the removal efficiency of HAL formation actually increased with higher dose of Fe(VI) (more than 500 μM), indicating that a clear increase of HAL formation might be attributed to the destruction of chlorine-reactive sites leading to the formation of HALs. HAN formation was significantly correlated with DON, originating from hydrophobic substances with amino acid moieties, and hydrophilic bases with amine moieties.^39,51 Therefore, Fe(VI)-based coagulation was effective in reducing the formation of HANs, resulting from the reactivity of Fe(VI) toward amino acids and amines and the control of organic matter by coagulation, which could be reflected by EEM spectra.^52,53 In contrast to HANs, HNM formation could only be controlled at low doses of Fe(VI) (less than 200 μM). This may be explained by 1) the oxidation of organic amines to nitro groups by high doses of Fe(VI), which contributed to the formation of new HNM precursors; 2) resistance of HNM precursors to coagulation treatment due to the primarily hydrophilic nature.^54,55 Generally, the removal efficiency of DOC was inconsistent with the removal efficiency of DBPs, especially at high doses of Fe(VI). This indicated that oxidation by Fe(VI) may change the DOM properties and further influence the transformation pathway during the reaction between chlorine and DOM and the effect dominated with the increase of oxidant dosage during Fe(VI)-based coagulation.

Fig. 4 Effect of Fe(III) and Fe(VI) dosage on DBP formation (a and d) and DBP toxicity (c and d) under the Fe(VI)-based process (a and c) and Fe(VI)/Fe(III)-based process (b and d).

As shown in Fig. 4 and S5,† adding Fe(III) during Fe(VI) coagulation could only inhibit DBP formation at low doses of Fe(VI) (100 μM) while the promotion effect of high Fe(III) dosage gradually disappeared as the Fe(III) dosage increased. Compared to Fe(VI) coagulation, the DBP control efficiency (the rate of decrease in DBP concentrations after coagulation) was elevated by 16.6–22.7% in the presence of Fe(III). However, the addition of Fe(III) actually impaired the control DBP formation by Fe(VI) at high dosage of Fe(VI) (more than 200 μM). Unlike the change of DOC and UV₂₅₄. Fe(III) could steadily improve the removal performance of Fe(VI) for DOC and UV₂₅₄. The addition of Fe(III) posed a dual effect on the Fe(VI)-based process. Fe(III) could improve Fe(VI) coagulation performance, but also accelerated the consumption of Fe(VI) in wastewater. The positive impact of enhanced coagulation performance could not offset the negative impact of the decreased Fe(VI) oxidation effect, which led to a decrease in DBP removal efficiency, especially at high dosage of Fe(VI). Therefore, the addition of Fe(III) had the greatest impact on the removal of HALs and HANs, which were mainly removed by oxidation instead of coagulation. HAL and HAN formation increased by 18.0–115.1% in wastewater compared with Fe(VI)-based coagulation. In contrast, at the Fe(VI) dose of 100 μM, THM and HNM formation decreased by 17.7–22.7% with the addition of 50–100 μM Fe(III). Therefore, Fe(VI)/Fe(III)-based coagulation was more suitable for the removal of DBP precursors at low dosage of Fe(VI).

3.3.2. DBP-associated cytotoxicity index (CTI) control. Fig. 4 shows the treatment effect of different coagulation processes on DBP-associated CTI. The decrease in DBP toxicity had a similar tendency to DBP concentration. However, different from THM occupying the majority of total DBP concentrations, the calculated DBP-associated CTI indicated that HANs and HALs were important cytotoxicity contributors.

Compared to DBP concentration, the Fe(VI)-based coagulation process could remove a higher proportion of CTI (68.0–94.3%), especially at low dosage of Fe(VI). It is worth noting that the decrease in DBP toxicity was mainly attributed to decreased HAN and HAL cytotoxicity, therefore, the priority control effect of HANs and HALs by high dosage Fe(VI) resulted in higher total DBP toxicity removal efficiency than total DBP concentration. Fig. S5 and S6† show the BSF of different DBPs affected by Fe(VI) and Fe(VI)/Fe(III)-based processes. The efficiency on controlling DBP toxicity at different dosages of Fe(VI) was superior to that of DBP concentration control, which could be explained by the markedly decreased BSF of DBPs after coagulation. Due to the stronger toxicity of brominated DBP, the preferential control effect of brominated DBPs contributed to the overall control of DBP toxicity during the Fe(VI)-based coagulation process. Even though HALs and HANs consisted of a small portion of total DBPs, they were major contributors to DBP-associated toxicity. Therefore, compared to traditional Fe(III) and Al(III)-type coagulants with poor removal efficiency for HAN and HAL precursors,^50,56 the Fe(VI)-based coagulation had stronger advantages in the control of HAN and HAL precursors. The toxicity of the chlorinated coagulated water tended to be underestimated due to THMs and HAAs as commonly applied DBP toxicity indicators. In contrast, Fe(VI) had a more comprehensive removal performance for DBP toxicity and a wider dosage range, which was a more suitable choice for coagulation.

Similar to the variation in DBP concentration, the addition of Fe(III) contributed to the control effect on overall DBP toxicity at low dosage of Fe(VI). Additionally, the promotion effect on controlling DBP-associated toxicity was inconsistent for different DBPs. When the Fe(III)/Fe(VI) was lower than 0.5 and the Fe(VI) dosage was 100 μM, the all DBP toxicity was reduced by 20.1–56.3% compared with Fe(VI)-based coagulation. In contrast, when the Fe(III)/Fe(VI) was higher than 0.5 and the Fe(VI) dosage was 100 μM, the toxicity of THM and HNM was further decreased but the efficiency on controlling HAL and HAN toxicity was decreased. The effect of added Fe(III) on DBP toxicity was greater than that of DBP concentration, which may be explained by 1) the priority control of HANs and HALs with stronger toxicity; 2) increased BSF of HANs and HALs after adding Fe(III). The reduction in BSF of DBPs may be attributed to the strong oxidation of Fe(VI), which converted some bromide into bromate, but Fe(III) will affect the oxidation of Fe(VI) to bromine ions, resulting in the increased BSF of DBPs in Fe(VI)/Fe(III)-based coagulation.

Generally, Fe(VI)-based coagulation was effective in DBP control compared with traditional coagulation due to the effect of oxidation on the characteristic of DOM, especially in wastewater with high bromide content. Fe(VI)/Fe(III)-based coagulation is more suitable for the use of low dosage of Fe(VI) to achieve higher removal efficiency.

3.4. Impact of the Fe(VI)-based process on DOM characteristics

3.4.1. Changes in fluorescence properties of DOM. The change of fluorescence properties of DOM after different coagulation processes in wastewater is illustrated in Fig. 5 and S7 and S8.† Different from NOM in surface water, the fluorescent substance of DOM in wastewater was mainly composed of aromatic protein-like components (region I and II) and SMP-like components (region IV), accounting for 70% of the total fluorescence intensity.⁵⁷ Previous studies have shown that aromatic protein-like fluorescent components were highly correlated with HAN and HNM precursors.^58,59 Therefore, the harmful N-DBP precursors in downstream drinking water sources may increase due to the discharge of wastewater into the water environment through stormwater pipes in the presence of illicit connections.⁶ Traditional coagulation mainly targeted humic-like components, but the removal efficiency of high hydrophilicity protein-like components was not satisfactory.⁴⁴ The coagulation process based on Fe(VI) could greatly promote the removal of aromatic protein-like components due to the high reactivity towards nitrogen-containing organic compounds such as amino acids.⁶⁰ As shown in the correlation heat map (Fig. S9†), the correlation (R² = 0.87, p < 0.001) between DOC and fulvic acid-like components was stronger than that of other types of fluorescent substances, indicating that fulvic acid-like components may be directly removed through coagulation–sedimentation. Meanwhile aromatic protein-like fluorescent substances may be mainly converted into non-fluorescent substances through the oxidation of Fe(VI), which demonstrated the advantage of removing fluorescent organic compounds by Fe(VI) compared to traditional coagulants. In addition, Fe(VI)-based coagulation could remove 62.0–81.0% of aromatic protein-like components in wastewater and the removal efficiency increased with the increased Fe(VI) dosage, which was consistent with the trend of changes in HAN and HAL formation. There was a significant correlation (R² = 0.61–0.75, p < 0.01) between HAN and HAL formation and aromatic protein-like components after different coagulation processes, which may explain the higher removal efficiency of HAN and HAL precursors. Since HANs and HALs contributed significantly to the cytotoxicity of DBPs, aromatic protein-like fluorescent substances could appropriately represent the toxicity risk of DBP generated by wastewater disinfection (R² = 0.73, p < 0.01). The removal efficiency (17.8–57.3%) of humic acid and fulvic acid-like substances was relatively low. Unlike other studies, there was no significant correlation between humic acid and fulvic acid-like substances and THM formation in this study, which may be attributed to the different properties of DOM in wastewater and the uncertain impact of Fe(VI) oxidation on humic-like components.

Fig. 5 Effect of Fe(III) and Fe(VI) dosage on region EEM intensity (a and b) and fluorescence region integration (FRI) distribution (c and d) of EEM under the Fe(VI)-based process (a and c) and Fe(VI)/Fe(III)-based process (b and d).

When Fe(III) was added, the removal of fluorescent substances was enhanced merely at high dosage of Fe(VI) (Fe(VI) dosage exceeding 200 μM), especially for humic-like components. This was similar to the removal performance of DOC but opposite to the change in DBP formation, which may be attributed to the enhanced coagulation performance with the addition of Fe(III). However, Fe(VI) mainly changed the properties of DOM by oxidation at high Fe(VI) dosage, thereby reducing the reaction rate with chlorine. The addition of Fe(III) inhibited this process, leading to a decrease in removal efficiency of DBP precursors.

3.4.2. Changes in molecular weight distribution of DOM. The SEC-OCD chromatograms of raw water and samples after Fe(VI) and Fe(VI)/Fe(III)-based processes are illustrated in Fig. 6a and c. The MW of main components was divided into five parts, presenting biopolymers, humic substances, building blocks, low MW acids and low MW substances.⁶¹ The proportions of the five fractions in the molecular weight distribution are illustrated in Fig. 6b and d.

Fig. 6 SEC-OCD chromatogram (a and c) and molecular weight distribution (b and d) of raw water and treated water after under the Fe(VI)-based process (a and b) and Fe(VI)/Fe(III)-based process (c and d). LMW = low-molecular weight.

Compared to surface water and rainwater, higher proportions of biopolymers and low-molecular-weight (LMW) acids were observed in wastewater. Biopolymers with the molecular weight of 10 kDa or higher were considered to include polysaccharides and nitrogen-containing materials like proteins and amino sugars, serving as the important N-DBP precursors.^61,62 Zhang et al.⁶ also found that the illicit connections of wastewater pipes resulted in the input of LMW fraction of DOM and high-molecular-weight (HMW) fraction of DOM in the stormwater pipes. The Fe(VI)-based process could effectively remove humic substances and biopolymers and the removal efficiency increased with the increased Fe(VI) dosage, but exhibited limited removal efficiency of building blocks and LMW acids. The removal of the HMW fraction of DOM was attributed to the combined effect of oxidation and coagulation. Oxidation may decompose the HMW fraction of DOM into the LMW fraction of DOM, resulting in the slight increase of building blocks and LMW acids at high dosage of Fe(VI). Subsequent coagulation could remove HMW DOM by forming flocs to aggregate and precipitate the DOM in wastewater. Overall, Fe(VI)-based coagulation was more effective in removing the HMW fraction of DOM than traditional coagulants, which contributed to the reduction of subsequent DBP formation in disinfection.

In the Fe(VI)/Fe(III)-based coagulation process, the addition of Fe(III) had little effect on the removal efficiency of biopolymers at low dosage of Fe(VI) (100 μM). When Fe(III)/Fe(VI) was 0.5, the removal of humic substances, building blocks and low MW acids was enhanced. This may be attributed to the enhanced coagulation performance with the addition of low Fe(III) dosage. Different from the low dosage of Fe(VI), the addition of Fe(III) enhanced the removal of biopolymers but may lead to an increase in the proportion of LMW fraction of DOM at the high dosage of Fe(VI), which may explain the increase in DBP formation due to the stronger reactivity between the LWM fraction of DOM and chlorine. It also reflected that Fe(III) mainly enhanced the coagulation performance for humic substances at low dosage of Fe(VI). Therefore, the Fe(VI)/Fe(III)-based coagulation process was suggested for use at low dosage of Fe(VI).

4. Conclusions

This work investigated the control effect of Fe(VI) and Fe(VI)/Fe(III)-based coagulation processes on conventional pollutants, ECs and DBP formation in wastewater. Our findings offer the following insights:

(1) Fe(VI) could remove more than 90% of turbidity, 42% of COD and 88% of TP, and prioritize the removal of aromatic organics. The addition of Fe(III) facilitated the removal of turbidity, COD, and TP, and the promotion effect was better at low doses of Fe(VI).

(2) The other water matrix was beneficial to the removal of ECs by Fe(VI) in wastewater. The addition of Fe(III) could greatly enhance the oxidation rate of PCT in ultrapure water but the promotion effect of Fe(III) in wastewater was limited and was suggested to be used when the [Fe(III)] : [Fe(VI)] ratio was less than 0.5.

(3) The Fe(VI)-based coagulation process was more effective in controlling the precursors of HANs and HALs due to the removal of aromatic protein-like components and HMW biopolymers and was suitable for use in wastewater containing bromide. The addition of Fe(III) can only promote the control of DBP formation when adding low doses of Fe(VI).

(4) The Fe(VI)-based process mainly removed PCT and DBP precursors through oxidation and removed conventional pollutants through coagulation. Moreover, the addition of Fe(III) mainly enhanced the coagulation performance of Fe(VI), but might impair the oxidation effect of Fe(VI), especially at high doses of Fe(VI).

Fe(VI) and Fe(VI)/Fe(III)-based coagulation were more effective in controlling various pollutants and were more suitable for complex systems in wastewater to mitigate the hazards of overflow pollution and protect water sources than the traditional coagulation process. Therefore, it is necessary to further investigate the control effect of Fe(VI)-based coagulation on other pollutants and develop the application of other activation methods for Fe(VI).

Author contributions

Jinglong Hu: performing the experiment, writing, data curation. Ruihua Zhang: performing the experiment, writing – reviewing and editing. Zhengdi Wu: writing – reviewing & editing. Cheng Ye: writing – reviewing & editing. Wenyuan Yang: writing – reviewing & editing. Wenhai Chu: conceptualization, supervision, writing – reviewing & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 52325001, 52170009 and 52091542), National Key Research and Development Program of China (No. 2021YFC3200700 and 2021YFC3200702), and Program of Shanghai Academic Research Leader, China (No. 21XD1424000).

References

1. J. Wang, G. H. Liu, J. Y. Wang, X. L. Xu, Y. T. Shao, Q. Zhang, Y. C. Liu, L. Qi and H. C. Wang, Current status, existent problems, and coping strategy of urban drainage pipeline network in China, Environ. Sci. Pollut. Res., 2021, 28, 43035–43049.
2. A. Montserrat, O. Gutierrez, M. Poch and L. Corominas, Field validation of a new low-cost method for determining occurrence and duration of combined sewer overflows, Sci. Total Environ., 2013, 463, 904–912.
3. B. Petrie, A review of combined sewer overflows as a source of wastewater-derived emerging contaminants in the environment and their management, Environ. Sci. Pollut. Res., 2021, 28, 32095–32110.
4. Z. X. Xu, Y. Qu, S. Y. Wang and W. H. Chu, Diagnosis of pipe illicit connections and damaged points in urban stormwater system using an inversed optimization model, J. Cleaner Prod., 2021, 292, 126011.
5. Z. X. Xu, J. Xu, H. L. Yin, W. Jin, H. Z. Li and Z. He, Urban river pollution control in developing countries, Nat. Sustain., 2019, 2, 158–160.
6. R. H. Zhang, R. Xiao, F. F. Wang, W. H. Chu, J. L. Hu, Y. Zhang, W. Jin, J. P. Hoek and Z. X. Xu, Direct discharge of sewage to natural water through illicitly connected urban stormwater systems: An overlooked source of dissolved organic matter, Sci. Total Environ., 2023, 890, 164248.
7. T. Li, W. Zhang, C. Feng and J. Shen, Performance assessment of separate and combined sewer systems in metropolitan areas in southern China, Water Sci. Technol., 2014, 69, 422–429.
8. W. Yang, C. Fang, T. Bond, X. Luan, R. Xiao, Z. Xu and W. Chu, Stormwater discharge: An overlooked source of disinfection byproduct precursors, J. Hazard. Mater., 2023, 132720,  DOI:10.1016/j.jhazmat.2023.132720.
9. A. Botturi, E. G. Ozbayram, K. Tondera, N. I. Gilbert, P. Rouault, N. Caradot, O. Gutierrez, S. Daneshgar, N. Frison, C. Akyol, A. Foglia, A. L. Eusebi and F. Fatone, Combined sewer overflows: A critical review on best practice and innovative solutions to mitigate impacts on environment and human health, Crit. Rev. Environ. Sci. Technol., 2021, 51, 1585–1618.
10. A. F. Brunsch, P. Z. Florez, A. A. M. Langenhoff, T. L. ter Laak and H. H. M. Rijnaarts, Retention soil filters for the treatment of sewage treatment plant effluent and combined sewer overflow, Sci. Total Environ., 2020, 699, 134426.
11. P. E. Peters and D. H. Zitomer, Current and future approaches to wet weather flow management: A review, Water Environ. Res., 2021, 93, 1179–1193.
12. L. Zheng and Y. Deng, Settleability and characteristics of ferrate(VI)-induced particles in advanced wastewater treatment, Water Res., 2016, 93, 172–178.
13. R. Sakrabani, J. Vollertsen, R. M. Ashley and T. Hvitved-Jacobsen, Biodegradability of organic matter associated with sewer sediments during first flush, Sci. Total Environ., 2009, 407, 2989–2995.
14. R. Gandhi, A. K. Ray, V. K. Sharma and G. Nakhla, Treatment of Combined Sewer Overflows Using Ferrate (VI), Water Environ. Res., 2014, 86, 2202–2211.
15. A. Talaiekhozani, M. R. Talaei and S. Rezania, An overview on production and application of ferrate (VI) for chemical oxidation, coagulation and disinfection of water and wastewater, J. Environ. Chem. Eng., 2017, 5, 1828–1842.
16. S. C. Wang, B. B. Shao, J. L. Qiao and X. H. Guan, Application of Fe(VI) in abating contaminants in water: State of art and knowledge gaps, Front. Environ. Sci. Eng., 2021, 15, 80.
17. R. Ezhov, A. K. Ravari and Y. Pushkar, Characterization of the Fe-V=O Complex in the Pathway of Water Oxidation, Angew. Chem., Int. Ed., 2020, 59, 13502–13505.
18. Y. X. Mao, J. L. Liang, L. Jiang, Q. S. Shen, Q. Zhang, C. C. Liu, H. Zheng, Y. Liao, X. K. Cao, H. Y. Dong and F. Y. Ji, Removal of micro organic pollutants in high salinity wastewater by comproportionation system of Fe(VI)/Fe(III): Enhancement of chloride and bicarbonate, Water Res., 2022, 214, 118182.
19. S. C. Wang, Y. Deng, B. B. Shao, J. H. Zhu, Z. X. Hu and X. H. Guan, Three Kinetic Patterns for the Oxidation of Emerging Organic Contaminants by Fe(VI): The Critical Roles of Fe(V) and Fe(IV), Environ. Sci. Technol., 2021, 55, 11338–11347.
20. X. G. Duan, H. Q. Sun, Z. P. Shao and S. B. Wang, Nonradical reactions in environmental remediation processes: Uncertainty and challenges, Appl. Catal., B, 2018, 224, 973–982.
21. X. Yang, W. H. Guo, X. Zhang, F. Chen, T. J. Ye and W. Liu, Formation of disinfection by-products after pre-oxidation with chlorine dioxide or ferrate, Water Res., 2013, 47, 5856–5864.
22. J. L. Hu, W. H. Chu, M. H. Sui, B. Xu, N. Y. Gao and S. K. Ding, Comparison of drinking water treatment processes combinations for the minimization of subsequent disinfection by-products formation during chlorination and chloramination, Chem. Eng. J., 2018, 335, 352–361.
23. S. H. Wu, H. Y. Liu, Y. Lin, C. P. Yang, W. Lou, J. T. Sun, C. Du, D. M. Zhang, L. J. Nie, K. Yin and Y. Y. Zhong, Insights into mechanisms of UV/ferrate oxidation for degradation of phenolic pollutants: Role of superoxide radicals, Chemosphere, 2020, 244, 125490.
24. S. F. Sun, S. Y. Pang, J. Jiang, J. Ma, Z. S. Huang, J. M. Zhang, Y. L. Liu, C. B. Xu, Q. L. Liu and Y. X. Yuan, The combination of ferrate(VI) and sulfite as a novel advanced oxidation process for enhanced degradation of organic contaminants, Chem. Eng. J., 2018, 333, 11–19.
25. F. Widhiastuti, L. H. Fan, J. Paz-Ferreiro and K. Chiang, Oxidative degradation of bisphenol A in municipal wastewater reverse osmosis concentrate (ROC) using ferrate(VI)/hydrogen peroxide, Process Saf. Environ. Prot., 2022, 163, 58–67.
26. X. B. Zhang, M. B. Feng, C. Luo, N. Nesnas, C. H. Huang and V. K. Sharma, Effect of Metal Ions on Oxidation of Micropollutants by Ferrate(VI): Enhancing Role of Fe-IV Species, Environ. Sci. Technol., 2021, 55, 623–633.
27. L. Zheng, J. K. Cui and Y. Deng, Emergency water treatment with combined ferrate(vi) and ferric salts for disasters and disease outbreaks, Environ. Sci.: Water Res. Technol., 2020, 6, 2816–2831.
28. W. H. Chu, N. Y. Gao, D. Q. Yin and S. W. Krasner, Formation and speciation of nine haloacetamides, an emerging class of nitrogenous DBPs, during chlorination or chloramination, J. Hazard. Mater., 2013, 260, 806–812.
29. S. W. Krasner, M. J. Sclimenti, W. Mitch, P. Westerhoff and A. Dotson, Using formation potential tests to elucidate the reactivity of DBP precursors with chlorine versus with chloramines, in AWWA Water Quality Technology Conference, AWWA, Publishing, Denver, CO, 2007.
30. Y. Lee, J. Yoon and U. von Gunten, Spectrophotometric determination of ferrate (Fe(Vl)) in water by ABTS, Water Res., 2005, 39, 1946–1953.
31. K. Manoli, R. Maffettone, V. K. Sharma, D. Santoro, A. K. Ray, K. D. Passalacqua, K. E. Carnahan, C. E. Wobus and S. Sarathy, Inactivation of Murine Norovirus and Fecal Coliforms by Ferrate(VI) in Secondary Effluent Wastewater, Environ. Sci. Technol., 2020, 54, 1878–1888.
32. C. Fang, S. K. Ding, S. B. Gai, R. Xiao, Y. N. Wu, B. Geng and W. H. Chu, Effect of oxoanions on oxidant decay, bromate and brominated disinfection by-product formation during chlorination in the presence of copper corrosion products, Water Res., 2019, 166, 115087.
33. J. H. Zhu, F. L. Yu, J. R. Meng, B. B. Shao, H. Y. Dong, W. H. Chu, T. C. Cao, G. F. Wei, H. J. Wang and X. H. Guan, Overlooked Role of Fe(IV) and Fe(V) in Organic Contaminant Oxidation by Fe(VI), Environ. Sci. Technol., 2020, 54, 9702–9710.
34. M. J. Plewa, M. G. Muellner, S. D. Richardson, F. Fasanot, K. M. Buettner, Y. T. Woo, A. B. McKague and E. D. Wagner, Occurrence, synthesis, and mammalian cell cytotoxicity and genotoxicity of haloacetamides: An emerging class of nitrogenous drinking water disinfection byproducts, Environ. Sci. Technol., 2008, 42, 955–961.
35. E. D. Wagner and M. J. Plewa, CHO cell cytotoxicity and genotoxicity analyses of disinfection by-products: An updated review, J. Environ. Sci., 2017, 58, 64–76.
36. X. Wei, M. T. Yang, Q. Y. Zhu, E. D. Wagner and M. J. Plewa, Comparative Quantitative Toxicology and QSAR Modeling of the Haloacetonitriles: Forcing Agents of Water Disinfection Byproduct Toxicity, Environ. Sci. Technol., 2020, 54, 8909–8918.
37. W. H. Chu, N. Y. Gao, D. Q. Yin, S. W. Krasner and W. A. Mitch, Impact of UV/H2O2 Pre-Oxidation on the Formation of Haloacetamides and Other Nitrogenous Disinfection Byproducts during Chlorination, Environ. Sci. Technol., 2014, 48, 12190–12198.
38. C. Fang, S. W. Krasner, W. H. Chu, S. K. Ding, T. T. Zhao and N. Y. Gao, Formation and speciation of chlorinated, brominated, and iodinated haloacetamides in chloraminated iodide-containing waters, Water Res., 2018, 145, 103–112.
39. Y. J. Jiang, J. E. Goodwill, J. E. Tobiason and D. A. Reckhow, Impacts of ferrate oxidation on natural organic matter and disinfection byproduct precursors, Water Res., 2016, 96, 114–125.
40. J. J. Yu, H. Xu, D. S. Wang, H. Y. Sun, R. Y. Jiao, Y. Liu, Z. Y. Jin and S. Zhang, Variations in NOM during floc aging: Effect of typical Al-based coagulants and different particle sizes, Water Res., 2022, 218, 118486.
41. V. K. Sharma, Oxidation of nitrogen-containing pollutants by novel ferrate(VI) technology: A review, J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng., 2010, 45, 645–667.
42. S. Y. Xiang, Y. H. Liu, G. M. Zhang, R. Ruan, Y. P. Wang, X. D. Wu, H. L. Zheng, Q. Zhang and L. P. Cao, New progress of ammonia recovery during ammonia nitrogen removal from various wastewaters, World J. Microbiol. Biotechnol., 2020, 36, 144.
43. M. Amano, J. Lohwacharin, A. Dubechot and S. Takizawa, Performance of integrated ferrate-polyaluminum chloride coagulation as a treatment technology for removing freshwater humic substances, J. Environ. Manage., 2018, 212, 323–331.
44. J. K. Edzwald, Coagulation in drinking-water treatment - particles, organics and coagulants, Water Sci. Technol., 1993, 27, 21–35.
45. L. Li, S. H. Yang, S. L. Yu and Y. A. Zhang, Variation and removal of 2-MIB in full-scale treatment plants with source water from Lake Tai, China, Water Res., 2019, 162, 180–189.
46. X. Y. Liu, S. K. Ding, P. Wang, Y. T. Hong, H. Y. Zhao and W. H. Chu, Simultaneous mitigation of disinfection by-product formation and odor compounds by peroxide/Fe(II)-based process: Combination of oxidation and coagulation, Water Res., 2021, 201, 117327.
47. Z. Q. Chen, B. X. Yang, Q. X. Wen and C. X. Chen, Evaluation of enhanced coagulation combined with densadeg-ultrafiltration process in treating secondary effluent: Organic micro-pollutants removal, genotoxicity reduction, and membrane fouling alleviation, J. Hazard. Mater., 2020, 396, 122697.
48. Y. S. Ma, Reaction mechanisms for DBPS reduction in humic acid ozonation, Ozone: Sci. Eng., 2004, 26, 153–164.
49. A. Dabrowska and J. Nawrocki, Controversies about the occurrence of chloral hydrate in drinking water, Water Res., 2009, 43, 2201–2208.
50. Y. Q. Mao, X. M. Wang, X. F. Guo, H. W. Yang and Y. F. F. Xie, Characterization of haloacetaldehyde and trihalomethane formation potentials during drinking water treatment, Chemosphere, 2016, 159, 378–384.
51. W. Lee, P. Westerhoff and J. P. Croue, Dissolved organic nitrogen as a precursor for chloroform, dichloroacetonitrile, N-Nitrosodimethylamine, and trichloronitromethane, Environ. Sci. Technol., 2007, 41, 5485–5490.
52. N. Noorhasan, B. Patel and V. K. Sharma, Ferrate(VI) oxidation of glycine and glycylglycine: Kinetics and products, Water Res., 2010, 44, 927–935.
53. V. K. Sharma, Oxidation of Inorganic Compounds by Ferrate(VI) and Ferrate(V): One-Electron and Two-Electron Transfer Steps, Environ. Sci. Technol., 2010, 44, 5148–5152.
54. A. D. Shah and W. A. Mitch, Halonitroalkanes, Halonitriles, Haloamides, and N-Nitrosamines: A Critical Review of Nitrogenous Disinfection Byproduct Formation Pathways, Environ. Sci. Technol., 2012, 46, 119–131.
55. J. Hu, H. Song, J. W. Addison and T. Karanfil, Halonitromethane formation potentials in drinking waters, Water Res., 2010, 44, 105–114.
56. Y. Wang, L. Li, Z. Sun, H. Dong, J. Yu and Z. Qiang, Removal of disinfection by-product precursors in drinking water treatment processes: Is fluorescence parallel factor analysis a promising indicator?, J. Hazard. Mater., 2021, 418, 126298.
57. W. Chen, P. Westerhoff, J. A. Leenheer and K. Booksh, Fluorescence excitation - Emission matrix regional integration to quantify spectra for dissolved organic matter, Environ. Sci. Technol., 2003, 37, 5701–5710.
58. P. Jutaporn, M. D. Armstrong and O. Coronell, Assessment of C-DBP and N-DBP formation potential and its reduction by MIEX (R) DOC and MIEX (R) GOLD resins using fluorescence spectroscopy and parallel factor analysis, Water Res., 2020, 172, 115460.
59. P. Kiattisaksiri, E. Khan, P. Punyapalakul, C. Musikavong, D. C. W. Tsang and T. Ratpukdi, Vacuum ultraviolet irradiation for mitigating dissolved organic nitrogen and formation of haloacetonitriles, Environ. Res., 2020, 185, 109454.
60. S. C. Wang, Y. Deng, B. B. Shao, J. H. Zhu and X. H. Guan, Reinvestigation of the oxidation of organic contaminants by Fe(VI): Kinetics and effects of water matrix constituents, J. Hazard. Mater., 2022, 430, 128421.
61. S. A. Huber, A. Balz, M. Abert and W. Pronk, Characterisation of aquatic humic and non-humic matter with size-exclusion chromatography - organic carbon detection - organic nitrogen detection (LC-OCD-OND), Water Res., 2011, 45, 879–885.
62. A. H. Zhang, F. F. Wang, W. H. Chu, X. Yang, Y. Pan and H. F. Zhu, Integrated control of CX3R-type DBP formation by coupling thermally activated persulfate pre-oxidation and chloramination, Water Res., 2019, 160, 304–312.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ew00770g

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