Dissolved organic nitrogen (DON) as precursors of nitrogenous disinfection byproducts (N-DBPs) has become a serious issue for drinking water treatment. Here, Fe3O4/peroxymonosulfate (PMS) system was used to examine the amino acid removal and formation of N-DBPs in the system and the corresponding mechanisms. Results showed a remarked variation in removal efficiency of three typical amino acids, i.e., glutamate (78%), histidine (53%) and phenylalanine (27%) in Fe3O4/PMS system at optimum conditions (0.1 g/L Fe3O4, 1.5 mM PMS, 1 h). Notably, Fe3O4/PMS treatment led to dichloroacetonitrile (DCAN) formation caused by the chlorination of glutamate, phenylalanine and histidine being reduced by 53.3%, 9.7% and 41.9%, respectively. The degradation and subsequent N-DBPs formation in the Fe3O4/PMS system mainly depended on the types and properties of the amino acids. The formation of dichloroacetamide (DCAcAm) exhibited different trends, which may be due to the different R group structure of the three amino acids and the special aromaticity of the imidazole ring in the histidine side chain that facilitates its quick electrophilic substitution and ring-opening reaction. This study highlights that the Fe3O4/PMS system is a promising strategy to remove DON and efficiently eliminate N-DBPs formation in the drinking water treatment process depending on the amino acid type.

  • Fe3O4 could effectively catalyze PMS for the degradation of three typical amino acids.

  • The formation potential of DCAN and DCAcAm by amino acids in the Fe3O4/PMS system was clarified.

  • Degradation pathways and intermediates of amino acids were also analyzed.

Dissolved organic nitrogen (DON), as the main component of the water nitrogen (N) pool, is widely distributed in urban water environments (Lusk & Toor 2016). DON usually occurs as amino acids, polypeptides and proteins and accounts for 0.5%–10% (by mass) of dissolved organic matter (DOM) (Tuschall & Brezonik 1980). Besides, DON in surface water is mainly derived from the discharge of urban sewage and agricultural water, soluble bacterial metabolites, metabolites of algae such as blue-green algae, and organic nitrogen present in the soil (Chu et al. 2012; Ostad-Ali-Askari et al. 2018). At present, more lakes are eutrophic and overbreeding of algae has become an increasing source of DON. The detrimental impact of DON has led policymakers to adopt effective management practices to control DON in water (Gkelis et al. 2014; Ostad-Ali-Askari et al. 2018; Feng et al. 2019).

The efficiency of the conventional water treatment process, which is being used for removal of algal-derived DON, is relatively low and residual DON in water will produce highly toxic nitrogenous disinfection by-products (N-DBPs) during chlorination (Westerhoff & Mash 2002; Bond et al. 2011; Bond et al. 2012). Yang et al. (Xin & Shang 2004) observed the formation of trihalomethanes, haloacetic acids, and cyanogen halides after chlorination of synthetic solutions containing humic acid, glutamate, ammonia, and bromide. The performance and mechanism of histidine as a heterocyclic amino acid in the UV/Cu–TiO2 system have been investigated in our previous study (Liu et al. 2016).

Free amino acids, which contain many low-molecular-weight organic nitrogen compounds, are also important DON compounds (Ittekkot et al. 1984). Amino acids can be classified by aliphatic, aromatic and heterocyclic amino acids according to their chemical structure and these amino acids have great potential to form N-DBPs during chlorination (Uyguner-Demirel & Bekbolet 2011; Chu et al. 2015a). Therefore, it is necessary to come up with a reliable and efficient technology to eliminate amino acids as precursors of N-DBPs in water.

Oxidation method can be used to remove amino acids, among which potassium permanganate oxidation will bring the risk of manganese ion exceeding the standard, while chlorination will directly cause the generation of disinfection by-products, and when the water contains bromine ions, ozone oxidation will produce bromine disinfection by-products. Advanced oxidation processes (AOPs) based on hydroxyl radicals (•OH) and sulfate radical (SO4•) have received much attention for the degradation of environmental organic pollutants. Noticeably, SO4• has a higher redox potential (2.5–3.1 V) than •OH and a longer half-life (30–40 μs) for efficient degradation of pollutants (Qi et al. 2014; Luo et al. 2021). In the advanced persulfate oxidation process, persulfate (PS) or peroxymonosulfate (PMS) can be activated via UV (Yang et al. 2018), heat (Gao et al. 2018a) or transition mental ions (Gao et al. 2018b) to generate sulfate radicals (SO4•). However, activating PMS with homogeneous transition metal ions has some drawbacks, such as the release of metal ions, difficult recovery of catalyst from the reaction solution and high dependence on pH (Gao et al. 2018b). Therefore, heterogeneous iron-based catalysts have been widely studied and applied for persulfate activation due to their advantages of expanding the optimal pH range and avoiding the accumulation of iron oxide sludge (Cui et al. 2021). Fe3O4 is considered one of the most suitable transition metal catalysts due to its advantages of magnetism, accessibility, high catalysis efficiency, and low toxicity and price (Nfodzo et al. 2012; Malekzadeh et al. 2017).

Fe3O4/PMS system has been used successfully to oxidize contaminants in water such as acetaminophen, the azo dye orange G, and aniline (Xu & Li 2010; Zhang et al. 2010; Tan et al. 2014). A recent study showed the Fe3O4/PMS system could degrade the DON and histidine effectively and the removal rates were up to 45 and 57% within 1 h, respectively (Liu et al. 2017). Yan et al. (2013) investigated the oxidative degradation of organic pollutants by the Fe3O4/PS process and obtained a complete degradation of sulfamonomethoxine in 1 min. The above observations showed the potential of Fe3O4 in PMS activation and pollutant degradation. Previous studies tend to focus on a single pollutant without exploring different contaminants' removal efficiency and mechanism in the same system. Moreover, the removal performance, mechanism of different amino acids by the Fe3O4/persulfate system and the influence of amino acids' molecular structure on the oxidation effects are still unknown. Likewise, the impact of the amino acids' oxidation on the formation of nitrogenous disinfection byproducts (N-DBPs) also needs further research. Therefore, in the present study, glutamate, histidine and phenylalanine were used as model compounds due to their different self-characteristics, and high potential to form N-DBPs (Ueno et al. 1996; Dotson & Westerhoff 2009; Yang et al. 2012). The main purpose of this paper is to determine the potential of Fe3O4/PMS system in removal of major amino acids from water. The influencing factors of oxidation on amino acids and corresponding formation of two typical N-DBPs in aqueous solution are also investigated. In addition, the mechanisms for removing glutamate, histidine, and phenylalanine are discussed.

Materials

DCAcAm (98.5%) was purchased from Alfa Aesar (Karlsruhe, Germany). DCAN was purchased from Sigma-Aldrich (Oakville, ON, Canada). Sodium hypochlorite solution (active chlorine >5%) used to prepare free chlorine stock solutions was obtained from Sinopharm Chemical Reagent Co., Ltd, China. Potassium peroxymonosulfate (PMS, KHSO5⋅0.5KHSO4⋅0.5 K2SO4) and other materials were analytical grade and obtained from Sigma-Aldrich Chemical Co., Ltd (Shanghai, China).

Sampling

Three different amino acids (histidine, glutamate and phenylalanine) were chosen as target amino acids due to different self-characteristics and high potential to form N-DBPs. Histidine water was prepared with the concentration of 10 mg/L by diluting histidine with ultrapure water. Phenylalanine and glutamate were also prepared in the same way as histidine.

Fe3O4/PMS and chlorination treatment

For the Fe3O4/PMS system catalytic oxidation experiments, batch experiments were carried out in a 250 mL brown bottle by taking 200 mL amino acid solution of 10 mg/L concentration. 0.1 g/L Fe3O4 was added into the solution and the brown glass bottle evenly placed in a water bath shaker (190 rpm) at 25 °C in the removal performance experiment. At every turn, a known amount of PMS oxidant was added into each glass bottle and the onward reaction time was calculated. At the predetermined time point, which referred to every ten minutes from the reaction 60 minutes, excessive methanol quenching agent was added to quench any residual PMS-induced oxidation before analyzing the DON. Samples were filtered immediately using 0.45 μm acetate fiber membrane. After collection, samples were evaluated for water quality indexes. Three parallel samples were taken in the above tests to reduce the inescapable error.

In order to determine the formation potential of N-DBPs from the chlorination of amino acids, two typical N-DBPs (DCAN and DCAcAm) were used as indicators to study the formation potential of N-DBPs from chlorination of amino acids in different reaction times after Fe3O4/PMS treatment. Chlorination treatment: The chlorine reaction was carried out at room temperature, and the amount of chlorine added was obtained by the following formula: Cl2 (mg/L) = 3 × DOC (mg/L) + 7.6 × NH3-N (mg/L) + 10 (Krasner et al. 2007; Tan et al. 2017), which was placed in a dark place for 24 h to react completely. After 24 h of chlorination, the disinfectant residual was quenched with 0.5 mL ascorbic acid and analyzed as soon as possible. All experiments were performed in triplicate. Data were reported as mean ± standard deviation (SD).

Analysis

NO3-N, NO2-N, and NH4+-N concentrations were measured by following the published method from Monitoring and Analysis Methods of Water and Wastewater 4th edition. Total dissolved nitrogen (TDN) was analyzed by the UV spectrophotometric method based on alkaline potassium persulfate. DON content was determined by subtracting the of measured concentrations of dissolved inorganic nitrogen species from TDN content as follows:
formula
(1)

Amino acids were analyzed by high-performance liquid chromatography (HPLC) using the derivatization of 6-aminoquinolyl-N-hydroxysuccinimidyl (AQC) (Díaz et al. 1996). In the present study, we established a method for the determination of amino acid oxidation products using HPLC-MS. GC experiments (Agilent, Waldbronn, Germany) were used to determine the formation potential of the N-DBPs (DCAN and DCAcAm). Specific experimental conditions for HPLC, HPLC-MS and GC are given in Supplementary Material, text S1.

Analysis of typical amino acids in natural water

Natural water has a wide variety of DON sources, including small molecules (amino acids, urea, purine, pyrimidine, amines, nucleotides) and macromolecules (proteins, nucleic acids and humus) (Lu et al. 2015). Algae is the main source of DON in natural waters, and amino acids are the main components of algal-derived DON (Liu et al. 2015). Lake Xuanwu (Nanjing, China) has been selected as a representative eutrophic lake for previous studies (Cheng et al. 2015) to determine the natural water concentration of 20 types of amino acids. The results are described in Table 1.

Table 1

The concentration of amino acids in Lake Xuanwu

Amino acidsConcentration (μmol/L)Amino acidsConcentration (μmol/L)
Alanine 3.452 Leucine 1.195 
Arginine 0.349 Lysine 0.818 
Asparagine N/A Methionine 0.077 
Aspartate 0.672 Phenylalanine 0.525 
Cysteine 0.631 Proline 0.451 
Glutamine N/A Serine 0.768 
Glutamate 1.633 Threonine 0.239 
Glycine 1.306 Tryptophan N/A 
Histidine 0.282 Tyrosine N/A 
Isoleucine 0.866 Norvaline 0.999 
Amino acidsConcentration (μmol/L)Amino acidsConcentration (μmol/L)
Alanine 3.452 Leucine 1.195 
Arginine 0.349 Lysine 0.818 
Asparagine N/A Methionine 0.077 
Aspartate 0.672 Phenylalanine 0.525 
Cysteine 0.631 Proline 0.451 
Glutamine N/A Serine 0.768 
Glutamate 1.633 Threonine 0.239 
Glycine 1.306 Tryptophan N/A 
Histidine 0.282 Tyrosine N/A 
Isoleucine 0.866 Norvaline 0.999 

It can be seen from Table 1 that 20 kinds of free amino acids detected in Xuanwu Lake covered the main categories of amino acids, showing substantial differences in their concentrations. Dissolved organic nitrogen (DON) such as amino acids (AAs) in source waters was the important halogenated N-DBP precursor (Bond et al. 2011; Zhang et al. 2019). Amino acids (AAs) were characterized by low molecular weights and high hydrophilicities with poor removal efficiencies in conventional water treatment plants (Dotson 2009). Amino acids could pass through the conventional water treatment processes and react with chlorine to form DBPs. The R bases in the structure of amino acids are the key factor affecting the generation of N-DBPs. It has been demonstrated that THMs could form from all AAs, although yields varied with AAs species and associated side chains (Hong et al. 2009).

In our study, phenylalanine belongs to nonpolar amino acids (hydrophobic amino acids) with aromatic amino acid in R bases. Histidine and glutamate belong to polar amino acids. The isoelectric point of glutamate is 3.22, and the R group contains carboxyl group, which is an acidic amino acid; histidine isoelectric point is 7.59 and the R group contains amino group, which is the basic amino acid. Relevant properties of the three amino acids are shown in Supplementary Material, Table S1.

Degradation efficiency

We studied the feasibility of Fe3O4/PMS system for histidine degradation and found that a certain degree of degradation could be achieved previously (Liu et al. 2017). Therefore, different kinds of amino acids were selected for oxidative degradation under Fe3O4/PMS system in this study. Comparison of the degradation of three amino acids by Fe3O4/PMS system with separate Fe3O4 or PMS systems is shown in Figure 1(a)–1(c).

Figure 1

Changes in amino acids concentration ((a) glutamate, (b) histidine, (c) phenylalanine) in different reaction systems; (d)–(f) changes in DON concentration in different reaction systems with glutamate, histidine and phenylalanine respectively (0.1 g/L Fe3O4; 1 mM PMS; 190 r/min).

Figure 1

Changes in amino acids concentration ((a) glutamate, (b) histidine, (c) phenylalanine) in different reaction systems; (d)–(f) changes in DON concentration in different reaction systems with glutamate, histidine and phenylalanine respectively (0.1 g/L Fe3O4; 1 mM PMS; 190 r/min).

As can be seen from Figure 1(a)–1(c), there were significant differences in the removal effect of the three amino acids. The removal efficiency of glutamate was approximately 90%, while that of phenylalanine was only 19% in Fe3O4/PMS system. Compared with Fe3O4 or PMS alone, Fe3O4 catalyzed PMS oxidation system had a better removal effect on DON and amino acid. As shown in Figure 1(d)–1(f), the DON removal of glutamate, histidine and phenylalanine was about 69, 41 and 17% respectively in the Fe3O4/PMS system. Due to unavailable similar previous study, the amino acids attributed DON removal performance of the Fe3O4/PMS present study was compared to DON removal efficiency of other studies in different media. For instance, current DON removal in Fe3O4/PMS system was found to be higher than previously reported DON removal efficiency of O3/KMnO4/ClO2/NaClO oxidants (2–8%) in raw water (Liao 2014). Similarly, Guo (Guo 2015) reported 50% removal of DON produced by chlorination while DON removal of KMnO4 was insignificant. However, a similar phycocyanin derived DON removal efficiency (67%) was achieved using H2O2/mZVI system (Chen et al. 2020). The removal of glutamate by Fe3O4/PMS reaction system was better than histidine and phenylalanine, while the removal rate of phenylalanine was the lowest. This may be due to the difference of R bases of the three amino acids, as phenylalanine R base contained benzene ring and stable structure (Milić et al. 2015). On the other hand, the side chain of histidine contained an imidazole ring, which was difficult to open compared to the glutamate straight-chain structure. In the catalytic oxidation reaction, amino acids were degraded in two ways under the action of OH• and SO4•. When these amino acids exist simultaneously in the system, we could consider applying other oxidants, such as hydrogen peroxide and KMnO4 or increasing the activation pathway, and adopting UV coupled heterogeneous catalyst to activate PMS to enhance the removal efficiency of the system for amino acids.

Effect of various factors on the catalytic degradation

Fe3O4 dosage

In Figure 2(a)–2(c), the influence of Fe3O4 dosage on the amino acid catalytic reaction was studied. The dosage of Fe3O4 was 0.04, 0.1, 0.2, 0.3 and 0.5 g/L, respectively, while values for other parameters were kept constant (1 mM PMS, 10 mg/L amino acid, 25 °C).

Figure 2

(a)–(c) Effect of Fe3O4 dosage in the Fe3O4/PMS system with glutamate, phenylalanine and histidine respectively (0.04–0.5 g/L Fe3O4 (if any); 1 mM PMS; 190 r/min); (d)–(f) effect of PMS dosage in the Fe3O4/PMS system with glutamate, phenylalanine and histidine respectively (0.1 g/L Fe3O4; 0.2–2.0 mM PMS (if any); 190 r/min); (g)–(i) effect of pH value in the Fe3O4/PMS system with glutamate, phenylalanine and histidine respectively (0.1 g/L Fe3O4; 1 mM PMS; 190 r/min).

Figure 2

(a)–(c) Effect of Fe3O4 dosage in the Fe3O4/PMS system with glutamate, phenylalanine and histidine respectively (0.04–0.5 g/L Fe3O4 (if any); 1 mM PMS; 190 r/min); (d)–(f) effect of PMS dosage in the Fe3O4/PMS system with glutamate, phenylalanine and histidine respectively (0.1 g/L Fe3O4; 0.2–2.0 mM PMS (if any); 190 r/min); (g)–(i) effect of pH value in the Fe3O4/PMS system with glutamate, phenylalanine and histidine respectively (0.1 g/L Fe3O4; 1 mM PMS; 190 r/min).

As shown in Figure 2(a)–2(c), with an increase in Fe3O4 dosage, the removal of DON increased at first and then began to decrease steadily. It was shown that removal of DON was maximum at 0.1 g/L dosages of Fe3O4; that is, 71% (glutamate), 48% (histidine) and 21% (phenylalanine), after 1 hour. The results could be explained by the number of active sites for PMS activation being proportional to the dose of Fe3O4 (Cai et al. 2020). Thus, too low and too high dosages of Fe3O4 were not effective. However, when the dosage of catalyst was further increased from 0.1 g/L, the removal efficiency of DON decreased significantly. When the dose of Fe3O4 was 0.1 g/L, the active sites on Fe3O4 were sufficient for activation of 1.0 mM PMS. Besides, when the catalyst was overdosed, the agglomeration effect of catalyst could occur, which led to a decrease in the contact between active sites of catalyst and PMS.

PMS dosage

As shown in Figure 2(d)–2(f), different levels of PMS (0.2, 0.5, 1.0, 1.5 and 2.0 mM) were applied to evaluate system performance while keeping other factors constant (0.1 g/L Fe3O4, 10 mg/L amino acid, 190 r/min).

With the increase of the concentration of PMS, the removal efficiency of DON showed a trend of increasing and then decreasing. When the concentration of PMS increased from 0.2 mM to 1.5 mM, the DON concentration of three amino acids decreased from 0.81 mg/L to 0.2 mg/L (glutamate); 1.65 mg/L to 0.83 mg/L (histidine); 0.45 mg/L to 0.32 mg/L (phenylalanine) respectively. A further increase of PMS to a level of 2 mM increased DON levels, which proved that the further increase of PMS concentration inhibited the degradation of three amino acids. It was explained that when the PMS concentration was low, the active sites on the catalyst were sufficient. An increase in the PMS concentration up to a certain level in the reaction system could generate more SO4• to improve the removal efficiency of DON. However, excessive PMS could not participate in the reaction to produce more SO4• as the active site of the Fe3O4 surface was saturated. Similar inhibition was described as follows (Chen et al. 2008). Equation (2)-(4) show that the system would produce SO5• with weak reactivity compared to SO4•. Moreover, the self-quenching of SO4• would lead to the consumption of free radicals, as shown in Equation (5). Therefore, the best dosage of PMS was 1.5 mM.
formula
(2)
formula
(3)
formula
(4)
formula
(5)

Initial pH

The pH value of water is an important factor that not only affects the activity of the catalyst surface but also changes the electrification of the amino acid itself. The pH of the system was adjusted to 4–9 by sodium hydroxide and hydrochloric acid. In order to clarify the influence of pH value on the degradation of amino acids in the reaction system, the system performance was evaluated at different pH values while the other preliminary test conditions (0.1 g/L Fe3O4; 1.0 mM PMS; 190 r/min) were kept same. The results were illustrated in Figure 2(g)–2(i).

Amino acids were amphoteric electrolytes, showing negative charges in alkaline solutions and positive charges in acidic solutions. In a given pH solution, the pH at which the positive and negative charges of an amino acid were equal is called the isoelectric point of the amino acid. The isoelectric point of amino acids and pH would affect their electricity and subsequent reactivity in water. The charge on a solid surface could be zero at a certain pH, known as the zero-charge point. Figure 2(g)–2(i) showed a difference in removal efficiency between the three amino acids during the change in pH value. The removal efficiency of glutamate increased and then began to decline and the removal efficiency was the highest, up to 79% at the pH value of 5. This was because the zero-point charge of the nano Fe3O4 catalyst was about 5 and the isoelectric point of the glutamate was 3.22. Due to the surface properties of Fe3O4, a net positive charge would be shown on the surface of Fe3O4 when the solution pH was lower than the zero-point charge. Amino acids became negatively charged in solution when the pH was higher than the isoelectric point of the amino acid. The surface of the catalyst and glutamate were negatively charged when the pH value was higher than 5, which reduced the probability of collision due to electrostatic repulsion. Therefore, the surface of the catalyst was positively charged, while the glutamic acid was negatively charged at pH 5, greatly increasing the degradation efficiency via electrostatic interaction and mutual collision. Similarly, the zero-point charge of the nano Fe3O4 catalyst was about 5 and the isoelectric point of phenylalanine and histidine was 5.48 and 7.59, respectively. Therefore, the optimum pH of phenylalanine in Fe3O4/PMS was 5 and that of histidine was approximately 7, which was in accordance with the experimental results shown in Figure 2(g)–2(i). Generally speaking, the optimum pH value was approximately 6 ∼ 7 when the raw water contained the three amino acids simultaneously.

Reusability of Fe3O4

Figure 3 showed the reusability potential of Fe3O4 catalysts during four recycling. As seen in Figure 3, DON removal gradually decreased with the increase of cycle times. The removal efficiency within 60 min was 47%, 40, 28 and 25% for the first, second, third and fourth runs, respectively. The regeneration rate of Fe3O4/PMS in the present study (25%) was similar to those reported by Tan et al. (2019) for ascorbic acid-modified Fe3O4 (26.2%) in treatment of sulfadiazine after three cycles. It was observed that Fe3O4 catalysts agglomerated during the reaction, and the surface area of the materials decreased substantially, leading to a reduction of degradation efficiency. Moreover, the amount of Fe2+ decreased and converted into Fe3+ after 4 cycles of recycling. Thus, the catalyst efficiency decreased upon reuse. As shown in Figure 3, the DON removal rate by catalyst declined from 47% to 25% after three cycles. However, the removal effect of the catalyst on DON did not significantly decrease after reusing the catalyst for the first time, suggesting that the catalyst could be used effectively for up to two cycles. Nevertheless, the reusability of catalyst needs to be further strengthened.

Figure 3

The effect of reusability of Fe3O4 on the DON degradation of histidine (0.1 g/L Fe3O4; 1.5 mM PMS; 190 r/min).

Figure 3

The effect of reusability of Fe3O4 on the DON degradation of histidine (0.1 g/L Fe3O4; 1.5 mM PMS; 190 r/min).

Analysis of free radicals and degradation pathways

Behaviors of radicals during oxidation

Radical scavengers were used to identify the main functional groups in the system. Methanol (MeOH) is an alcohol-containing α-H, which can quickly quench OH• and SO4• by reacting with them. Tert-butyl alcohol (TBA) does not contain α-H, and it can react rapidly with OH• only, while its reaction with SO4• is much slower (Zhao et al. 2010).

The result of the quenching of free radicals is shown in Figure 4(a). It was generally known that both MeOH and TBA could inhibit DON removal to a certain extent. The removal rate of DON of histidine was about 52% after 1 h in the Fe3O4/PMS system without quenching agents. When adding 0.02 M TBA and 0.02 M MeOH respectively, the removal rate of DON decreased from 1.65 mg/L by 13 and 29% respectively after 1 h reaction. It could be seen that compared with TBA, the presence of MeOH quencher had great influences on the degradation of DON in Fe3O4/PMS system. Therefore, the above results indicated that the reactive radicals were OH• and SO4• in the Fe3O4/PMS system, in which SO4• played a more important role.

Figure 4

(a) The effect of different free radical quenching agents on DON degradation of histidine; (b) changes of TDN, NO3, NH4+ in the DON degradation in the Fe3O4/PMS system (0.1 g/L Fe3O4; 1.5 mM PMS; 190 r/min).

Figure 4

(a) The effect of different free radical quenching agents on DON degradation of histidine; (b) changes of TDN, NO3, NH4+ in the DON degradation in the Fe3O4/PMS system (0.1 g/L Fe3O4; 1.5 mM PMS; 190 r/min).

Degradation pathway

The changes of TDN, NO3, NH4+ in DON degradation in the Fe3O4/PMS system are presented in Figure 4(b). The main forms of nitrogen showed different variation tendencies in the Fe3O4/PMS system. It was obvious that the concentration of DON decreased from 1.65 mg/L to 0.87 mg/L within 60 minutes, while the concentration of NH4+ increased from 0.043 mg/L to 0.58 mg/L. The content of NO3-N remained unchanged during the entire oxidation process, while NO2-N was not produced in the system. There was a slight decrease in the concentration of total nitrogen, which may be due to the adsorption of the catalyst on DON. According to the variation of DON and NH4+-N, it could be seen that the increasing concentration of NH4+-N was caused by the deamination of histidine in the catalytic oxidation process, while NH4+-N was not converted into N2.

Figure 5

Model of degradation mechanism of amino acid in Fe3O4/PMS system.

Figure 5

Model of degradation mechanism of amino acid in Fe3O4/PMS system.

The mechanism of amino acid (AA) degradation in Fe3O4/PMS system can be described in Figure 5:
formula
(6)
formula
(7)
formula
(8)
In addition, HPLC-MS technology was used to identify the intermediates and final products of amino acid oxidation. The mass spectrum of three amino acids after the catalytic treatment within 60 min is shown in Supplementary Material, Fig. S1(a–c). The mass ratio of the main intermediate products was different in three amino acids after Fe3O4/PMS system treatment. Due to the difference in the initial molecular formula of the three amino acids, the mass ratio of intermediate products produced in the degradation process was also different. According to the analysis result, the molecular structure of the intermediate product was speculated and confirmed in Supplementary Material, Table S2. The histidine transformation process followed the presumed rules in Supplementary Material, Fig. S2.

Although the intermediate products of different amino acid degradation are different, there is a common trend in the persulfate catalytic oxidation system. In general, the amino acid deamination reaction occurred in the system, which showed that the increase of NH4+-N led to the decrease of DON, but NH4+-N was not further oxidized to NO3-N.

Analysis of N-DBPs formation and intermediate products

Although DON was effectively degraded in Fe3O4/PMS system, it was necessary to explore the generation potential of N-DBPs of degradation products in the system because amino acids were precursors of N-DBPs in drinking water treatment. Figure 6 reveals the formation of N-DBPs during chlorination of three typical amino acids oxidized by persulfate at different reaction times in the Fe3O4/PMS system. The concentration of DCAN decreased in varying degrees due to the increase of PMS oxidation time. Compared with glutamate and histidine, DCAN, as one of the chlorination products of phenylalanine, had no obvious downward trend, its production was relatively stable, and the concentration of DCAN was higher than the other two amino acids. This was due to the fact that the R group of phenylalanine contained benzene ring and its structure was relatively stable (Milić et al. 2015).

Figure 6

Formation of N-DBPs during chlorination of amino acids at different reaction times in the Fe3O4/PMS system with (a) glutamate; (b) phenylalanine; (c) histidine (0.1 g/L Fe3O4; 1.5 mM PMS; 190 r/min).

Figure 6

Formation of N-DBPs during chlorination of amino acids at different reaction times in the Fe3O4/PMS system with (a) glutamate; (b) phenylalanine; (c) histidine (0.1 g/L Fe3O4; 1.5 mM PMS; 190 r/min).

Noticeably, the formation of DCAcAm during the chlorination of three amino acids emerged with different variation tendencies by increasing reaction time. When glutamate was treated with Fe3O4/PMS system for 1 h, the formation potential of DCAcAm decreased; after degradation of histidine under the same condition, the formation potential of DCAcAm increased first and then decreased; after phenylalanine treatment, the formation potential of DCAcAm was unchanged. On the one hand, it was due to the different removal rates of DON. Along with the DON removal rate of different amino acids, the results of the previous experiments showed that the removal rate of glutamate was higher than that of histidine and phenylalanine in the Fe3O4/PMS system, which also led to a decline in the concentration of DCAcAm than the other two amino acids. As we all known, amino acids were the precursors of N-DBPs (Bond et al. 2011; Zhang et al. 2019) and the high removal of glutamate implied that the generation potential of disinfection by-products would be relatively low, which was also consistent with the phenomenon on the formation potential of DCAcAm.

On the other hand, this was principally owing to the varying structure of R base. The side chain of glutamate and phenylalanine did not contain nitrogen, while the histidine side chain contained imidazole ring, which had some aromatic property, and was easier to have electrophilic substitution reaction and ring-opening reaction (Yadav et al. 2018). There were two N atoms on the imidazole heterocycle, which may be further converted to DCAN by ring opening reaction. DCAN hydrolyzed to produce DCAcAm (Chu et al. 2015b; Chu et al. 2016) and the precursor of DCAcAm decreased gradually with reaction time. Finally, the reaction of DCAcAm formation was complete, and because of its own instability, it could be seen from Figure 6(c) that the formation of DCAcAm during chlorination of histidine appeared to increase firstly and then decreased. The concentrations of nitrogenous disinfection byproducts of the three amino acids were all in the order of 0–3 μg/L, and the trend of degradation of glutamic acid (R group without heterocyclic ring) was consistent with the N-DBPs. While for amino acids containing nitrogen heterocyclic rings, the change of disinfection by-products produced under the Fe3O4/PMS system had no obvious rule. In general, the Fe3O4/PMS system had a certain inhibitory effect on the formation of DCAN, but the effect on the formation potential of DCAcAm was not stable.

Twenty free amino acids were detected from the water of Lake Xuanwu. Three of these amino acids; that is, glutamate, histidine and phenylalanine, were chosen to treat in Fe3O4/PMS system due to their distinct characteristics and high potential to form N-DBPs. Fe3O4 showed promising catalytic performance for PMS and led to significant removal of glutamate (78%), histidine (53%) and phenylalanine (27%) in Fe3O4/PMS system. The active radicals in the Fe3O4/PMS system were SO4−• and OH, while SO4−• performed dominantly in this system. The Fe3O4/PMS system performed better at optimum doses of Fe3O4 (0.1 g/L) and PMS (1.5 mM) at 6–7 pH values. The formation and removal of N-BBPs trends showed that the effectiveness of the Fe3O4/PMS system is highly associated with the nature of amino acid, particularly the structure of R group of amino acid. The Fe3O4/PMS system efficiently reduced the DCAN formation from glutamate and histidine. In contrast, the N-DBPs formation from phenylalanine remained relatively stable in Fe3O4/PMS, mainly due to its R group that contained a comparatively stable benzene ring. Hence, the formation and removal of N-DBPs trends showed that the effectiveness of the Fe3O4/PMS system is highly associated with the nature of amino acid, particularly the structure of the R group of amino acids. The irregular DCAcAm formation trend attributed to histidine could be due to the presence of the imidazole ring, which may affect the formation potential of DCAcAm. In the present study, the N-DBPs production by three amino acids; that is, glutamate, phenylalanine and histidine, were 2.93, 2.98 and 2.28 μg/L and the corresponding removal rate was 53.3%, 9.7% and 41.9%, respectively. Noticeably, the present study only evaluates the three amino acids that have a high potential to form N-DBPs. However, different results are expected while treating other types of amino acids Fe3O4/PMS system. Therefore, more studies are warranted to identify more general mechanisms involved in forming and removing amino acids and associated N-DBPs in Fe3O4/PMS system. Moreover, other optimization such as catalyst modification, combined with UV-activated persulfate methods could be considered to enhance the mineralized removal of amino acids and prevent the increase of disinfection by-products due to incomplete mineralization of amino acids.

This work was supported by the National Natural Science Foundation of China (51378174, 51438006) and the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. We are also thankful to the anonymous reviewers and editors for their invaluable suggestions, which would beneft the improvement of this paper and our future research.

Data cannot be made publicly available; readers should contact the corresponding author for details.

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