A. Materials

CBZ (>98%), methanol (HPLC grade), and acetonitrile (LCMS grade) were obtained from Sigma-Aldrich. Formic acid (HCOOH), acetic acid (CH₃COOH), oxalic acid (H₂C₂O₄), NaCl, Na₂SO₄, NaHCO₃, CaCl₂, MgCl₂, Fe₂SO₄, Fe₂(SO₄)₃, FeCl₂ and FeCl₃ were purchased from Shanghai Chemical Reagent Co., Ltd. All chemicals were of analytical grade unless otherwise stated. The pure water (resistance >18.2 MΩ) was prepared by filtering through a Millipore Milli-Q system. The surface water was collected from a local river and the 0.22 μm filters was used to filter the degradation water. All experiments were performed at temperature of 20(2) ℃.

B. Photo irradiation procedure

A merry-go-round photochemical reactor (Sidongke Electric Plant, Nanjing, China) coupled with a max of 12 quartz tubes containing the reaction solution was used for the photodegradation experiments. The total volume of the tube was 100 mL and the liquid layer thickness was 35 mm. UV (ultraviolet) light irradiation from a 300 W high-pressure mercury lamp with 254 nm wavelength immersed in the circulated-water cooled quartz well were employed in the photolysis experiments. The light intensity was 9 W and the cell path length was 2 cm. The temperature inside the reactor was kept at 20(2) ℃ by a water-cooling jacket.

C. EB irradiation

The samples were irradiated to 0.5, 1, 2, 5, 10 and 20 kGy at ambient temperatures by 1.8 MeV electron beams of up to 10 mA from an electron accelerator. The samples were placed at 30 cm under the beam scanner.

D. Analytical methods

A high performance liquid chromatography (HPLC, Agilent 1200 series), consisted of C18 column (150 mm×4.6 mm) and an auto-sampler with 10 μL volume injection, was used to detect CBZ concentration at 230 nm by a VWD detector. The mobile phase was a mixture of methanol and water (55:45, v: v) at rate of 1.0 mL/min.

Organic acids, nitrate and nitrite ammonium ions produced by EB-radiolysis were detected by ICS1100 (Dionex). A hydrophilic anion exchange column was IonPac As22 (analytical, 4 mm×250 mm). The eluent mixed with 4.5 mM Na₂CO₃ and 1.4 mM NaHCO₃ at 1.20 mL/min flow rate and the injection volume was 25 μL. The suppressor was Anion Self-Regenerating Suppressor (ASRS 300 4 mm under AutoSuppression Recycle Mode at 31 mA. A hydrophilic cation exchange column was IonPac CS12A (analytical, 4 mm×250 mm). The eluent was methanesulfonic acid (20 mM) at 1 mL/min flow and the injection volume was 25 μL. The suppressor was Cation Self-Regenerating Suppressor (CSRS ULTRA II, 4 mm) under Auto-Suppression Recycle Mode at 59 mA.

Other by-products of CBZ by EB-radiolysis were monitored by LC/MS/MS using an Agilent 1260LC chromatograph coupled to an Agilent 6460 mass spectrometer with electron spray ionization (ESI) interface and a heated nebulizer. A Porshell 120 100 mm×3 mm EC-C18 end-capped column (2.7 μm particle size) was used at the flow rate of 0.4 mL/min. The injection volume was 10 μL. The mobile phase was a mixture of acetonitrile (A) and 0.1 % HCOOH in water (B). The gradient was operated from 5% to 95% A for 8 min from 95% to 100% A for 2 min, held at 100% for 2 min, and back to the initial conditions in 3.5 min. Mass spectrometry full scanning analysis was performed in the range of 50–500 m/z. For positive electron spray ionization, ESI (+), the operation conditions of the source were: capillary voltage, 4000 V; nebulizer pressure, 40 psi (0.28 MPa); drying gas flow, 8 L min^-1 at 300 ℃; and nozzle voltage, 0 V. For negative electron spray ionization, ESI (-), the operation conditions of the source were: capillary voltage, 3250 V; nebulizer pressure, 40 psi; drying gas flow, 7 L min^-1 at 350 ℃; and nozzle voltage, 500 V.

Total organic carbon (TOC) was determined using a TOC analyzer (multi C/N 2100, Analytik Jena AG).

A. UV-photodegradation and EB-radiolysis of CBZ in pure-water

As shown in Fig. 1, only 15.74% CBZ (2 mg L^-1) was degraded after 180-min UV radiation; and 96%, under UV/H₂O₂; and 99.9% CBZ (75 mg L^-1), under 10-kGy EB irradiation. So, CBZ was refractory to degrade under UV irradiation only, and the existence of H₂O₂ in solution promoted the CBZ degradation; while EB-radiolysis is an efficient method to degrade CBZ, which was reported by Kwon et al [16], too.

Fig. 1.Full-screen View

Degradation of CBZ in pre-water by EB, UV and UV/H₂O₂ treatments (pH =6.3, no pH adjustment).

The EB-radiolysis enhancement is due to a large amount of free radicals of hydroxyl (H.) and hydrogen (OH.), and hydrated electrons (e_aq^-) generated during EB irradiation [20] at different G-values (μmol/J) (Reaction I). As well known, OH., e_aq^- and H. are high reactive radicals to react with organic compounds and cause degradation. The OH. is an active particle with oxidation potential (E₀ = 2.8 V), with strong oxidative ability to oxidize organic compounds in aqueous solutions, while e_aq^- and H. are of strong reductive ability to deoxidize the targeted organic substance, with the e_aq^- having a strong reductive ability (E₀ = -2.9 V) to effectively dehalogen the halogenated organic compounds [14].

The photooxidation of organic substances was using the line source spherical emission model which was proposed by previous authors in similar studies [22]. Similar to the above, when 90 % CBZ was degraded, the consumption energy Q₂ (J/mol) could be calculated by following Eq. (1).

where I_0,λ (Einstein/(L⋅s)) is the intensity of the incident light at 253.7: 9 w, T_0.9 is the time when 90% CBZ is degraded, M is molecular mass of carbamazepine (236.27 g/mol), ΔC is the concentration variation at 90 % CBZ degradation and L is the volume of CBZ solution (1.350 L).

The low percent of CBZ UV-photolysis in the UV treatment suggested that CBZ might degrade slowly. The numerous oxydic free radicals produced in the UV/H₂O₂ process (Reaction II– IV) react effectively with organic pollutants. [21] The results indicated that the removal of CBZ in UV/H₂O₂ system is probably controlled by the amount of hydroxyl radicals available. Therefore, EB irradiation and UV/H₂O₂ process are efficient methods to decompose the refractory pollutant organics, and EB irradiation is simple and convenient without adding any oxidizing agent.

B. Comparison of EB-radiolysis and UV/H₂O₂ photodegradation efficiency on CBZ degradation

For understanding energy consumption of the EB radiation and UV/H₂O₂ treatments, CBZ initial concentrations of 20 mg L^-1 and 25 mg L^-1 were used, with the initial pH of the solutions being 6.3. Degradation of CBZ at different initial concentrations by EB and UV/H₂O₂ treatments follows well the pseudo-first-order kinetics

where, C and C₀ are the residual and initial concentrations of CBZ in mg/L, respectively; k is the pseudo-first order rate constant for overall degradation rate of CBZ; and x is the absorbed dose (kGy) of EB irradiation or time (min) of UV/H₂O₂ process. Table 1 shows the fitted k of initial concentrations.

Generally, the D_0.90 (kGy) was used to represent the needed absorbed dose that the degradation efficiency was 90 % in EB degradation. Then, we defined the Q₁ (J mol) which the consumption energy of 90% CBZ was degraded. The equation was represented at Eq. (3):

where D_0.90 (kGy) is the degradation efficiency at the dose point where 90% CBZ is degraded, and M is molecular mass of carbamazepine (236.27 g/mol).

As shown in Table 1, the EB process was more efficient than the UV/H₂O₂ process, the latter has a greater Q values than the former. A smaller Q means better efficiency and EB irradiation is an efficient method for CBZ degradation.

C. Effect of ions on CBZ degradation

1. Effect of salts on CBZ EB-radiolysis

CBZ (75 mg L^-1) was dissolved in solutions with different cations (NaCl, CaCl₂, MgCl₂ and FeCl₃, 5 mM each) or different anions (NaCl, Na₂SO₄ and NaHCO₃, 5 mM each), and was EB-irradiated to check their effects on CBZ degradation. The experiment was performed three times and the coefficient of variation was within 5%. As shown in Fig. 2, the Na⁺, Mg²⁺ and Ca²⁺ cations, and Cl^-, SO₄^2- and HCO₃^- anions, had little impact on EB-radiolysis of CBZ, while Fe³⁺ promoted the degradation because of being as OH. promoter.

Fig. 2.Full-screen View

Effect of cations (a) and anions (b) in salts on EB-degradation of CBZ ([CBZ] =75.0 mg L^-1; pH =6.3, no pH adjustment).

2. Effect of salts on CBZ photodegradation

CBZ (2 mg L^-1) was dissolved in salt solution of NaCl, CaCl₂, Na₂SO₄ and NaHCO₃ (10 mM each). The samples were added with 20 mM H₂O₂ and irradiated under a 300 W mercury lamp. The sampling intervals were 0, 5, 10, 20, 40, 60, 90 and 120 min. The experiment was carried out three times and the variation was within 2%.

As shown in Fig. 3, NaHCO₃ (32.93%) inhibited the CBZ degradation more than Cl^- (78.60%) at 120 min. It could be that HCO₃^- might react with OH. to produce CO₃^.-, which is lower in activity than OH. [23, 24]. Being similar to HCO₃^-, NaCl and CaCl₂ had a little inhabitation on the CBZ degradation, because Cl^- could also eliminate OH. and competed with CBZ for OH.. [25]

Fig. 3.Full-screen View

Effect of salts on CBZ degradation in UV/[H₂O₂] process ([CBZ] =2.0 mg L^-1; [H₂O₂]=20 mM; pH =6.3, without pH adjustment).

3. Effect of Fe2+ and Fe3+ on CBZ photodegradation

The experiments were conducted by adding 2.0 mM Fe₂SO₄ and 1.0 mM Fe₂(SO₄)₃ into CBZ solution, respectively, and photodegraded under 300 W UV. Using an initial CBZ concentration of 2.0 mg L^-1 at pH=3, experiments were carried out three times and the variation of coefficients was within 5%.

As shown in Fig. 4, the CBZ-removal efficiency increased greatly by adding Fe²⁺ and Fe³⁺. Because the oxidation of Fe(II) to Fe(III) can produce H₂O₂ and OH. (Reaction V-VIII) in the presence of O₂. Fe(III) ions create hydroxyl radicals under UV irradiation (Reaction IX), and hydroxyl radicals react with organic substances and create organic radicals (Reaction X).

Fig. 4.Full-screen View

(Color online) Effects of Fe²⁺ and Fe³⁺ on CBZ degradation in UV process ([CBZ]=2.0 mg L^-1; pH=3).

$\begin{array}{l}{\text{Fe}}^{\text{2+}}{\text{+O}}_{\text{2}}\text{+}h\nu \to {\text{Fe}}^{\text{3+}}{\text{+O}}_2^{\cdot \text{-}}\text{(V)}\hfill \\ {\text{Fe}}^{\text{2+}}{\text{+O}}_2^{\cdot \text{-}}{\text{+2H}}^{\text{+}}\to {\text{Fe}}^{\text{3+}}{\text{+H}}_{\text{2}}{\text{O}}_{\text{2}}\text{(VI)}\hfill \\ {\text{Fe}}^{\text{2+}}{\text{+H}}_{\text{2}}{\text{O}}_{\text{2}}\to {\text{Fe}}^{\text{3+}}{\text{+OH}}^{\text{-}}\text{+OH}\cdot \text{(VII)}\hfill \\ {\text{Fe}}^{\text{2+}}\text{+OH}\cdot \to {\text{Fe}}^{\text{3+}}{\text{+OH}}^{\text{-}}\text{(VIII)}\hfill \\ {\text{Fe}}^{\text{3+}}{\text{+H}}_{\text{2}}\text{O+}h\nu \to {\text{Fe}}^{\text{2+}}{\text{+H}}^{\text{+}}\text{+OH}\cdot \text{(IX)}\hfill \\ \text{OH}\cdot \text{+RH}\to {\text{H}}_2\text{O+R}\cdot \text{(X)}\hfill \\ \hfill \end{array}$

Therefore, the anions existed in natural water affected enormously UV-degradation of CBZ, while they had little effect on EB-radiolysis of CBZ. And the alkali metal cations had little influence on both UV- and EB-degradation of CBZ. The existence of Fe³⁺ could enhance the CBZ degradation. So, EB irradiation was a perfect method to degrade the contaminants in solution with various salts.

D. Degradation process of CBZ EB irradiation in solution with ions

The intermediates of CBZ dissolved in solution with all ions were detected by IC and LC/MS/MS. The organic acids were formic, acetic, oxalic, malonic and succinic acid, and the inorganic ions were nitrite and nitrate ions detected by IC under anion mode and the ammonium ion under the cation mode. Nitrite and nitrate ions were disappeared after 15 kGy, therefore the organic nitrogen was mainly transformed to ammonium ion (NH₄⁺) or N₂. Other complex transformations were showed in Table 2.

TABLE 2.Full-screen View

Intermediates and products detected by LC-MS/MS during EB-radiolysis of CBZ (25 mg L^-1)

Code	Structure	ESI(+) Precursor ion m/za	aESI(+) MS2	Remarks
CBZ		237[M+H]+, 259[M+Na]+, 275[M+K]+,	237, 220, 194b	Detected
I		239[M+H]+, 261[M+Na]+, 277[M+K]+	239, 194b	Detected
II		253[M+H]+, 275[M+Na]+, 291[M+K]+	253, 236, 210b, 180	Detected
III		239[M+H]+, 180[M+H]+	180, 152b	Detected

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a probable fragment ions from the precursor ions;

b the most abundant fragment ion.

Figure 5 shows the dose effects on the removal efficiency of TOC and the concentration evolution of NH₄⁺ for CBZ solutions. At 20 kGy, the residual TOC was about 20%, and the NH₄⁺ concentration was about 2.01 mg L^-1. The calculated total organic nitrogen was 3.81 mg L^-1, suggesting that 52.8% organic nitrogen might be changed into ammonia nitrogen and about 25% of it might be translated into N₂.

Fig. 5.Full-screen View

The evolution of TOC and NH₄⁺ concentration in EB-irradiation of CBZ (CBZ 25 mg L^-1;■, TOC in the left-Y-coordinate; ∘, NH₄⁺ in the right-Y-coordinate; [TOC], the residual TOC concentration; [TOC]₀, the initial TOC concentration).

On basis of the intermediates’ appearance detected by LC/MS/MS and the variation of ions detected by IC in the radiolysis, a main degradation pathway for the mineralization of CBZ was proposed, as shown in Fig. 6.

Fig. 6.Full-screen View

Proposed radiolysis process of CBZ (25 mg L^-1) in ion water under EB irradiation.

A 20 kGy EB-irradiation led to almost mineralized CBZ into CO₂, H₂O and mineral nitrogen, while it took a much longer time to mineralize CBZ by UV/H₂O₂-photodegradation than by the EB irradiation.