Degradation and dechlorination of trichloroacetic acid induced by an in situ 222 nm KrCl* excimer radiation

Abstract

Since the coronavirus disease 2019 (COVID-19) pandemic epidemic, the excessive usage of chlorinated disinfectants raised the substantial risks of disinfection by-products (DBPs) exposure. While several technologies may remove the typical carcinogenic DBPs, trichloroacetic acid (TCAA), their application for continuous treatment is limited due to their complexity and expensive or hazardous inputs. In this study, degradation and dechlorination of TCAA induced by an in situ 222 nm KrCl* excimer radiation as well as role of oxygen in the reaction pathway were investigated. Quantum chemical calculation methods were used to help predict the reaction mechanism. Experimental results showed that UV irradiance increased with increasing input power and decreased when the input power exceeded 60 W. Decomposition and dechlorination were simultaneously achieved, where around 78% of TCAA (0.62 mM) can be eliminated and 78% dechlorination within 200 min. Dissolved oxygen showed little effect on the TCAA degradation but greatly boosted the dechlorination as it can additionally generate hydroxyl radical (•OH) in the reaction process. Computational results showed that under 222 nm irradiation, TCAA was excited from S₀ to S₁ state and then decayed by internal crossing process to T₁ state, and a reaction without potential energy barrier followed, resulting in the breaking of C–Cl bond and finally returning to S₀ state. Subsequent C–Cl bond cleavage occurred by a barrierless •OH insertion and HCl elimination (27.9 kcal/mol). Finally, the •OH attacked (14.6 kcal/mol) the intermediate byproducts, leading to complete dechlorination and decomposition. The KrCl* excimer radiation has obvious advantages in terms of energy efficiency compared to other competitive methods. These results provide insight into the mechanisms of TCAA dechlorination and decomposition under KrCl* excimer radiation, as well as important information for guiding research toward direct and indirect photolysis of halogenated DBPs.

Graphical abstract

1. Introduction

Chlorination performs a critical role in preventing infectious illnesses from spreading among people through drinking water (Gilca et al., 2020). However, noxious chlorinated disinfection by-products (DBPs) were usually generated by the reaction of free chlorine (HOCl and OCl⁻) with natural organic matter (Liang et al., 2022). The use of chlorine for disinfection has grown since the COVID-19 pandemic epidemic, which is raising the hazards of chlorinated DBPs exposure, especially in raw water (Wang et al., 2020b; Cai et al., 2021). Due to widespread distribution, substantial quantities and considerable toxicity, trichloroacetic acid (TCAA) as one of the most common chlorinated DBPs, had drawn the public's attention (Humans et al., 2004; Hamidin et al., 2008; Varshney et al., 2014). Numerous studies have demonstrated the potential carcinogenicity of TCAA for human (Humans et al., 2004; Mazhar et al., 2020). World Health Organization, U.S., China, Japan, European Union, Australia and Canada have each published guidance to limit TCAA maximum concentration levels. Therefore, TCAA contaminated water must be effectively treated at the point-of-use (POU) before drinking (Sobsey et al., 2008).

To date, various removal methods for TCAA via POU systems have been reported, including heating/boiling (Chen et al., 2021), activated carbon (AC) adsorption (Wu et al., 2017) and membrane filtration (Pérez-Vidal et al., 2016), as well as electrolysis (Hill et al., 2022), ultraviolet (UV) based (Hejazi and Taghipour, 2022), and UV radiation (Ao et al., 2021) emerging technology. Boiling decreased poor levels of TCAA while increased the formation of dichloroacetic acid (DCAA), resulting in unchanged average levels of total risk (Levesque et al., 2006; Pan et al., 2014). TCAA removal via AC adsorption was relatively low due to its high hydrophilicity, and water quality may be deteriorated over time (Wang et al., 2018). AC cartridges also requires frequent replenishment or replacement (Xiao et al., 2014). High energy consumption and low water recovery limited the widespread applications of membrane filtration in large-scale engineering processes (Drioli et al., 2011). The addition of medication and oxygen, as well as material preparation and recycling, considerably raises the risk and cost of the UV/H₂O₂, UV/TiO₂, UV/Fenton (Liu et al., 2016) and electrolysis. UV rays can sanitize the water for POU without additional chemicals and has the potential to eliminate TCAA (Lui et al., 2014). Due to poor absorption capability in the UV domain (<240 nm), traditional mercury lamp (254 nm) and UV light-emitting diodes (UV-LEDs) (i.e., 255, 265, 280, and 310 nm) are difficult to degrade TCAA. Although vacuum UV (VUV) can radiate 185 nm and 254 nm light simultaneously, it suffers low specific energy and potential release of highly toxic mercury into the environment (Silva et al., 2021). Excimer molecules (rare gas, halogen) and exciplex molecules (rare gas halide RgX*) can be selected to provide narrow band radiation at different wavelengths. Low wavelength UV at 222 nm generated by a KrCl* excimer operated by a dielectric barrier discharge (DBD) has drawn interest as a novel POUs strategy (Oh et al., 2020). Its obvious advantages include instant-on capability, very quick warm-up time, flat geometry, dimmability (Raeiszadeh and Taghipour, 2019), environmental friendliness (mercury-free) (Hejazi and Taghipour, 2021) and without adding additional oxygen during usage. Recent studies have shown that irradiation with 222 nm UV might be an optimum choice for effective disinfection that is biologically safe for human cells (Narita et al., 2018; Bhardwaj et al., 2021). Vibrationally relaxed excimer molecules of KrCl* can decay through radiative processes via the following reactions (Eqs. (1), (2), (3), (4), (5), (6), (7), (8)) and generate UV radiation of 222 nm (Belasri et al., 2013).

$$\mathrm{e}+\text{Kr}\to {\text{Kr}}^{*}+\mathrm{e}$$ (1)

$$\mathrm{e}+\text{Kr}\to {\text{Kr}}^{+}+2\mathrm{e}$$ (2)

$${\mathrm{e}+\text{Cl}}_2\to {\text{Cl}+\text{Cl}}^{-}$$ (3)

$${\text{Kr}}^{+}{+\text{Cl}}^{-}+\mathrm{M}\to {\text{KrCl}}^{*}+\mathrm{M}$$ (4)

$$\begin{gathered} {\text{Kr}}^{*}+{\text{Cl}}_2\to {\text{KrCl}}^{*}+\text{Cl} \\ {k}_5=5.5\times {10}^{-10}{\text{cm}}^3\times {\mathrm{s}}^{-1} \end{gathered}$$ (5)

$$\begin{gathered} {\text{KrCl}}^{*}\to \text{Kr}+\text{Cl}+hv\left({\lambda }_{peak}=222\text{nm}\right) \\ {k}_6=4.5\times {10}^7{\mathrm{s}}^{-1} \end{gathered}$$ (6)

$$\begin{gathered} \mathrm{C}{{\mathrm{l}}_2}^{*}\to {\text{Cl}}_2+hv\left({\lambda }_{peak}=258\text{nm}\right) \\ {k}_7=1\times {10}^7{\mathrm{s}}^{-1} \end{gathered}$$ (7)

$$\begin{gathered} {\text{KrCl}}^{*}+{\text{Cl}}_2\to \text{Kr}+\text{Cl}+{\text{Cl}}_2 \\ {k}_8=6\times {10}^{-10}{\text{cm}}^3\times {\mathrm{s}}^{-1} \end{gathered}$$ (8)

There have been few mechanistic investigations on the use of KrCl* excimer radiation for both dechlorination and decomposition of halogenated organic pollutants, to the best of our knowledge. The purpose of this study was to assess the feasibility and analyze the reaction mechanisms of KrCl* excimer radiation for TCAA degradation and dechlorination. A series of operating variables and environmental conditions, such as input power (P _in), oxygen effect, UV irradiance value (I _UV), on the degradation and dechlorination were investigated. Quantum chemical calculation method was also utilized to help explore the reaction mechanisms, pathways, products, and driving forces (i.e., direct and indirect photolysis) potentially responsible for the photochemical process because some intermediate products are non-detectable (Yeung et al., 2021).

2. Materials and methods

2.1. Chemicals and materials

AnalaR grade TCAA, 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) and methyl-tert-butyl-ether (MTBE) were purchased from the Aladdin Industrial Corporation, Shanghai, China. Chlorine and krypton were supplied from Hong Kong Specialty Gases Co., LTD. All other analytical-grade chemicals were obtained from National Medicines Corporation Ltd, China.

2.2. Experiment setup

Experimental device was composed of a power supply, a KrCl* excimer discharge tube, a reaction vessel and a series of analytical instruments, which were shown in Fig. 1 (a). The reaction vessel was a cylindrical quartz beaker (diameter: 4 cm, height: 42 cm) where the KrCl* excimer discharge tube was vertically positioned in the center. The KrCl* excimer discharge tube consisted of an inner discharge electrode and outer quartz tube, where the gap between the inner electrode and the inside surface of the outer tube was approximately 0.5 cm. The gap was filled with Kr and Cl₂ (V _Kr:V _Cl2 ≈ 200: 1) at a total pressure of about 200 Torr. At each run, 570 mL of TCAA solution was poured into the reaction vessel for treatment. The solution was earthed and as the outer electrode. DBD was initiated by switching on the AC high-voltage power supply (CORONA LAB. CTP-2000K). Light emission from the discharge tube was analyzed by using a spectrometer (Maya, 2000PRO, Ocean Optics, Dunedin, FL, USA). Voltage and current waveforms of the discharge were monitored using a digital oscilloscope (TBS 1102 B, Tektronix Technology Co., Ltd. USA). Typical emission spectra of the KrCl* excimer radiation and the molar absorption coefficients of TCAA were demonstrated in Fig. 1 (b). I _UV was determined according to the iodie/dodate chemical actinometry (S1). During deoxygenation experiments, dissolved oxygen (DO) levels were kept below 1.0 mg/L by purging the solutions with argon gas for 30 min, then maintaining a flow rate of 0.2 mL/min throughout the reaction procedure.

Fig. 1.

(a) Schematic of the experimental setup; (b) Molar absorption coefficient of TCAA and typical emission spectrum of the KrCl* excimer radiation; (c) equivalent circuit diagram of the KrCl* excimer discharge tube; (d) typical voltage and current waveforms of the discharge.

2.3. Analysis of TCAA and its degradation products

Concentrations of TCAA ([TCAA]₀ and [TCAA]) and chloride ion ([Cl⁻]₀ and [Cl⁻]) at the beginning and time t of the reaction in the solution were determined by Ion-chromatography (ICS-1100, Thermo Fisher, USA). Percentages of remaining (Eq. (9)) and dechlorination (Eq. (10)) were chosen as TCAA removal performance metrics in the photoreactor.

$$\text{Remaining}\text{TCAA}(\%)=\frac{\left[\text{TCAA}\right]}{{\left[\text{TCAA}\right]}_0}\times 100$$ (9)

$$\text{TCAA}\text{Dechlorination}(\%)=\left(\frac{\left[{\text{Cl}}^{-}\right]-{\left[{\text{Cl}}^{-}\right]}_0}{3\times {\left[\text{TCAA}\right]}_0}\right)\times 100$$ (10)

A liquid-liquid microextraction-acidic methanol derivatization together gas chromatography/mass spectrometry (GC/MS, Shimadzu GC-2010/QP2010, Japan) detection methods were developed for determining intermediate chloroacetic acids (CAAs) in solution (S2).Carbon dioxide (CO₂) was determined by the titrimetric method for free CO₂ (APHA, 1995). Amount of Total Organic Carbon (TOC) was measured by the high-temperature combustion method via a TOC analyzer (TOC-VCPH, Shimadzu, Japan). Short-lived species such as hydroxyl radicals (•OH) were trapped by DMPO and then were analyzed by Electron Paramagnetic Resonance spectrometer (EPR, Bruker E500, Germany) (Li et al., 2022b).

2.4. Analysis of discharge indices

The P _in was directly displayed from the power supply. The Lissajous figure assumed that the KrCl* excimer discharge tube was regarded as a capacitor C _t, which constituted by the equivalent capacitance of the quartz dielectric layer (C _d) and the discharge gap (C _g) in series relationship (Wang et al., 2020a). The equivalent circuit model is shown in Fig. 1 (c). The quantitative connection between the three capacitors is given in Eq. (11). Under low-frequency sinusoidal voltage driving, the voltage across the discharge tube (U _KrCl*) and charge quantity (Q _KrCl*) in the equivalent capacitance will appear on the digital oscilloscope as an approximate parallelogram figure (Fang et al., 2008).

The Q _KrCl* was measured using a capacitor (C_x) of 0.47 μF in series with the KrCl* excimer discharge tube, where U _x is the voltage across the C_x at time t. When the charges deposited on the dielectric surfaces are zero, the gas gap voltage (U _gap) in Eq. (12) equals the applied voltage (U _a and U _b), which is the distance between the two points on the parallelogram intersects the x-axis of the Lissajous figure. The average power consumption (${\bar{P}}_{\text{KrCl}*}$) of KrCl* excimer discharge tube could be calculated in Eqs. (13), (14), (15), where area inside the closed Lissajous curve (S _lisa) divided by the AC cycle period was equal to ${\bar{P}}_{\text{KrCl}*}$ (Wang et al., 2021b).

$$C_{\mathrm{g}}=\frac{C_{\mathrm{d}}·C_{\mathrm{t}}}{C_{\mathrm{d}}-C_{\mathrm{t}}}$$ (11)

$$U_{\text{gap}}=\frac{\left|U_{\mathrm{a}}\right|+\left|U_{\mathrm{b}}\right|}{2}$$ (12)

$$W_{\text{KrCl}*}={\int }_{-\frac{T}{2}}^{\frac{T}{2}}{U}_{\text{KrCl}*}·I_{\text{KrCl}*}dt$$ (13)

$$I{}_{\text{KrCl}*}=\frac{dq}{dt}=C_x\frac{d{U}_x}{dt}$$ (14)

$${\bar{P}}_{\text{KrCl}*}=\frac{1}{T}{\int }_{-\frac{T}{2}}^{\frac{T}{2}}{U}_{\text{KrCl}*}·C_x\frac{d{U}_x}{dt}dt=\frac{1}{T}{\int }_{-\frac{T}{2}}^{\frac{T}{2}}{U}_{\text{KrCl}*}·C_{x}d{U}_x=\frac{1}{T}{\int }_{-\frac{T}{2}}^{\frac{T}{2}}{U}_{\text{KrCl}*}d{Q}_{\text{KrCl}*}=f·S_{lisa}$$ (15)

The voltage and current waveforms in Fig. 1 (d) had a sinusoidal shape at 19 kHz. The current waveform featured a sequence of high-amplitude spikes caused by each micro-discharge, resulting in a significant electrical impedance fluctuation within the gas chamber (Li et al., 2020).

2.5. Quantum calculation

Quantum mechanical calculations of TCAA were performed using Gaussian 16 (Frisch et al., 2016). Geometry optimizations and vibrational frequencies of TCAA were calculated using the density functional theory (DFT) at M06–2x/6-311 + G** basis set. Their vertical excitation properties were computed by time-dependent density functional theory (TDDFT) calculations with the function mentioned above. No imaginary frequency modes were observed at the stationary states of the optimized structures shown in this study. To mimic the water environment, solvent effect was an integral equation formalism polarizable continuum model (IEF-PCM) (Klamt et al., 2015). Besides, potential energy curves were obtained from relaxed scans. Electrophilic and nucleophilic interactions were predicted from the Fukui function (Yang and Parr, 1985) calculations.

3. Results and discussion

3.1. Effect of input power on the KrCl* excimer radiation

Functionality of the designed KrCl* excimer radiation was validated by analyzing the Lissajous figures in Fig. 2 (a). The curves were standard parallelograms (Rodrigues et al., 2018) with the same slopes and intercepts at different P _in, C _g and U _gap with the P _in increasing were similar values calculated by Eqs. (11), (12) in Table S1, it demonstrated discharge uniformity and stable transition of the discharge mode between positive and negative half-cycles. The possible reason was that P _in only affects the reaction of the gas in the discharge gap, as the insulating medium layer (Zhang et al., 2017), but the materials, configurations of high-voltage pole (Li et al., 2019) and the distance of the discharge gap (Uytdenhouwen et al., 2018) determined the uniformity and stability of the discharge.

Fig. 2.

(a) Lissajous-figures of the KrCl* excimer radiation at different P_in; (b) ${\bar{P}}_{\text{KrCl}*}$ and I_UV of the KrCl* excimer radiation under different P_in. ([TCAA]₀ = 0.62 mM; DO = 9.62 mg/L; initial solution pH 7.0).

Both ${\bar{P}}_{\text{KrCl}*}$ and I _UV were measured to obtain the optimal P _in, as I _UV plays a vital role in TCAA removal. Fig. 2 (b) shows values of ${\bar{P}}_{\text{KrCl}*}$ and I _UV under different P _in, which were calculated by Eqs. (13), (14), (15). It can be observed that both ${\bar{P}}_{\text{KrCl}*}$ and I _UV increased with increasing P _in from 30 W to 60 W. However, when P _in exceeded 60 W, P _in dropped while ${\bar{P}}_{\text{KrCl}*}$ increased as usual. This may be explained by the fact that when the reaction rate increases, the dominating mechanism for exciplex production shifts from harpooning to three-body recombination, resulting in enhanced radiant power of the major bands (Eq. (6)) (Zhuang et al., 2010) with increasing ${\bar{P}}_{\text{KrCl}*}$. But when P _in further increased from 60 W to 70 W, I _UV decreased from 0.12 mW/cm² to 0.08 mW/cm², because the dissociation and quenching processes become more dramatic at higher P _in, owing to the frequent collision of excited molecules with other particles as a result of the high concentration in the following reaction (Eq. (8)) (M. and I., 2004), making I _UV dramatically reduces. Therefore, 60 W was selected as the optimum P _in in this study.

3.2. Degradation and dechlorination of TCAA

The trend of TOC, TCAA, and Cl⁻ during the irradiation treatment is depicted in Fig. 3 (a). As demonstrated in Fig. 3 (a), the remaining TCAA concentrations and TOC declined while the concentration of Cl⁻ rose with increasing irradiation period. Approximately 78% of the TOC was eliminated, 79% of the TCAA was removed, and 1.48 mM Cl⁻ was produced after 200 min treatment. Additionally, the elimination of TOC was extremely close to that of TCAA, implying that litter organic intermediates generated during the irradiation. The bulk of the carbon atoms in TCAA were clearly mineralized into inorganic carbon, specifically carbon dioxide.

Fig. 3.

(a) Degradation of TCAA and generation of Cl⁻ in the photoreactor as a function of irradiation time; (b) Variations of [Cl⁻]/([TCAA]₀-[TCAA]) and ([Cl⁻] + 3 × [TCAA]) during UV irradiation treatment of TCAA ([TCAA]₀ = 0.62 mM; P_in = 60 W; DO = 9.62 mg/L; initial solution pH 7.0).

The variation of [Cl⁻]/([TCAA]₀-[TCAA]) with irradiation time estimated from Fig. 3 (a) was displayed in Fig. 3 (b) to clarify the chlorine balance. As demonstrated in Fig. 3 (b), the ratio between the amount of Cl⁻ produced and the quantity of TCAA removed increased with irradiation time and finally to ca. 3:1 (2.4 ∼ 2.9:1 in the figure). This means that during irradiation, the majority of the chlorine atoms of TCAA were converted to Cl⁻, progressively to total dechlorination. The possible reason is that TCAA undergoes efficient C–Cl bond cleavage from a repulsive surface (Saha et al., 2013). Sections 3.4 and 3.5 will go into detail on the formation of intermediate byproducts and the dechlorination procedure.

3.3. Effects of dissolved oxygen on decomposition and dechlorination

Oxygen often played important roles in water treatment (Cao et al., 2023). Degradation and dechlorination of TCAA in oxygenated and deoxygenated water samples were shown in Fig. 4 (a) and (b), respectively. It can be seen that degradation of TCAA in deoxygenated water was close to, but dechlorination was much lower than those in oxygenated solution. This finding means that oxygen directly or indirectly participates in the dechlorination of TCAA. A prior study discovered •OH was generated under 254 nm UV irradiation of halogoacetic acids due to oxygen bubbling (Wang et al., 2021a). We employed DMPO as a trapping reagent to further explore the mechanism of the oxygen effect. In the absence of irradiation (control), no signals were captured, but a four-line signal with a 1:2:2:1 intensity ratio DMPO-•OH symbol was observed in the irradiated water (Fig. 4 (c)). The signal strength increased as oxygen content rose. This suggests that O₂ was a necessary element in the formation of •OH. In general, water molecules only generated •OH under VUV (<200 nm) conditions (Li et al., 2021; Wang et al., 2022) or •OH was generated by an excited state halogoacetic acids electron transfer to O₂. However, O₂ could be decomposed in the UV at a wavelength less than 240 nm (Farooq et al., 2014). According to the results, a possible pathway for •OH production was proposed in this study as follows (Eqs. (17), (18), (19), (20), (21)) (Xu et al., 2020).

$${\mathrm{O}}_2+hv\stackrel{{\mathrm{\lambda }}_{\text{peak}}=222\text{nm}}{\to }2\mathrm{O}({}^3\mathrm{P})$$ (17)

$${\mathrm{O}(}^3\mathrm{P}{)+\mathrm{O}}_2\to {\mathrm{O}}_3{k}_{18}=4\times {10}^9{\mathrm{s}}^{-1}$$ (18)

$${\mathrm{O}}_3+hv\stackrel{{\mathrm{\lambda }}_{\text{peak}}=222\text{nm}\text{or}258\text{nm}}{\to }{\mathrm{O}(}^1\mathrm{D}{)+\mathrm{O}}_2\left({}^1\Delta _{\mathrm{g}}\right)$$ (19)

$$\mathrm{O}({}^1\mathrm{D}{)+\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{O}}_2$$ (20)

$${\mathrm{H}}_2{\mathrm{O}}_2+hv\stackrel{{\mathrm{\lambda }}_{\text{peak}}=222\text{nm}\text{or}258\text{nm}}{\to }·\text{OH}$$ (21)

Fig. 4.

Degradation (a) and dechlorination (b) of TCAA as a function of irradiation time in oxygenated (DO = 9.62 mg/L) and deoxygenated (DO < 1 mg/L) water; (c) EPR spectra of DMPO spin adducts recorded under different conditions: blue line: oxygenated water without irradiation; red line: oxygenated water with 2 min irradiation; black line: deoxygenated water with 2 min irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Moreover, the intensity of DMPO-•OH spin was reduced compared to water with the same oxygen concentration when TCAA was present in the solution, further indicating that dissolved oxygen can additionally generate •OH in the irradiation process. The roles of •OH in the degradation and dechlorination will be discussed in section 3.5.

3.4. Theoretical prediction of the reaction mechanism

The TCAA potential energy curves of the S₀, S₁, and T₁ (Lee et al., 2021) states were explored to further investigate the mechanism of dechlorination in the irradiation process and understand the optical characteristics. The curves were drawn by increasing the bond length with fixed step sizes as well as revealed stable structures and reactive potential barriers. Firstly, the potential energy curve of the C–Cl cleavage in the S₀ state was investigated. As illustrated in Fig. 5 (black), barrier-free C–Cl recombination will result in the production of TCAA. Even where the species are 4.0 Å apart, there is no barrier to recombination, implying that TCAA dechlorination was hard to carry out, even when passing an energy barrier of around 55 kcal/mol.

Fig. 5.

TCAA potential energy curves: the black, red, and blue lines depict energies versus C–Cl bond lengths in the S₀, S₁, and T₁ states, respectively. The minimal energy crossover point (MECP) and its associated structure have been shown. The numerical value on the graph represents the reaction's energy barrier. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Secondly, we studied the potential energy curves in the S₁ state. The TDDFT vertical transition and oscillator strength for TCAA was summarized in Table S2. The oscillator strength of TCAA in the S₁ state is 0.0035, implying that TCAA in the S₁ state exists for a short time. Therefore, The potential energy curves of the T₁ state were further calculated which had a longer lifetime than S₁ (Lower and El-Sayed, 1966). Fig. 5 demonstrated that the energy level difference between S₁ and T₁ was small enough. The energy level difference between the S₁ and T₁ states was just 0.43 eV, implying that the intersystem crossing (ISC) mechanism would take place quickly (Li et al., 2022a). In addition, as the bond length of C–Cl increased, the energy decreased rapidly, and the C–Cl bond length was increased to 2.9 Å, minimizing the energy, indicating that the bond breaking occurred spontaneously in the T₁ state. The TCAA structure was then thought to undergo a non-radiative transition from the T₁ to the S₀ states. As a result, herein searching for the MECP has become a major task for us. In this investigation, the sobMECP suite (Lu, 2016) was utilized, and the MECP structure is depicted in Fig. 5. The C–Cl bond length was around 2.9 Å. The ISC process may be the dominant channel in the T₁ → S₀ process at this stage. Based on the foregoing, the photodissociation could be obtained from the radiation less decay process, •CCl₂COOH and •Cl were the intermediated products.

3.5. Subsequent •OH attack reaction

To understand the roles of •OH on the degradation and dechlorination, Fukui function was calculated to predict the electrophilic and nucleophilic sites for a molecule, which was shown in Fig. 6 (a). The dark blue indicated that it was easily attacked by free radicals, including oxygen, C–C bonds, and Cl of •CCl₂COOH. The potential energy curves of the radical attack were further calculated, and the radical recombination between •CCl₂COOH and •OH occurs barrierless forming the intermediate CCl₂OHCOOH (Li et al., 2006).

Fig. 6.

Fukui function descriptor surfaces (a) and (c). Free radical parts are marked in dark blue. Optimized geometries for CCl₂OHCOOH, TS₁, TS₂, ${}_{•}{}^{•}\mathrm{C}\text{OClCOOH}$, ${}_{•}{}^{•}\mathrm{C}\text{OOHCOOH}$ and schematic potential energy surface produced by the M06–2x/6-311 + G** calculations for (b) Second step dechlorination and (d) the third step dechlorination. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

The second step dechlorination as demonstrated in Fig. 6 (b), the process of HCl-elimination requires the energy of about 27.9 kcal/mol, which means CCl₂OHCOOH can be decomposed in water and subsequently release HCl molecules. As shown in Fig. 6 (c), the C–C bond of ${}_{•}{}^{•}\mathrm{C}\text{OClCOOH}$ was easily attacked. The free energy profile for the product continues to react with the •OH as shown in Fig. 6 (d), which represent the third step dechlorination. We proposed that the optimum structures of this process are a one-step reaction in which the C–Cl bond broken and is followed by the •OH insertion. The energy barrier reaches 14.6 kcal/mol, suggesting that it could carry out reactions at 222 nm. Finally, the overall decomposition pathway (Fig. S1) for TCAA would lead to HCl and CO₂ effectively.

3.6. Energy efficiency

When comparing different techniques and assessing its potential applications, energy efficiency is a crucial ingredient to consider. In this study, the energy efficiency of TCAA degradation (J _TCAA) is defined as the ratio of decomposing half of the initial TCAA molecules to the necessary energy. J _TCAA can be formulated as follows (Eq. (22)):

$$J_{\text{TCAA}}=\frac{\left({\left[\text{TCAA}\right]}_0-{\left[\text{TCAA}\right]}_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}\right)\times Vol}{W\times t_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}=\frac{\frac{1}{2}\times {\left[\text{TCAA}\right]}_0\times Vol}{W\times t_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}$$ (22)

where Vol is the volume of the TCAA solution in L, ${\left[\text{TCAA}\right]}_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$ represents half of the initial TCAA concentration (mM), W denotes the treatment technology's overall power consumption in W, and t _1/2 represents the time necessary to decompose half of the initial TCAA molecules (s). The current work's energy efficiencies as well as some common competitive techniques are shown in Table 1 .

Table 1.

Energy efficiency of KrCl* excimer radiation compared to other competing TCAA removal technologies.

[TCAA]0 (mM)	Method	JTCAA (10−9 mmol/J)	References
0.62	KrCl* excimer radiation, 60 W, DO 9.62 mg/L, pH0 7.0	613.54	This work
1.00	Glow discharge plasma, 50 W, pH0 6.5	162.76	Wang et al. (2014)
0.13	UV high pressure mercury lamp, 250 W, DO 5 mg/L, pH0 7.0	8.67	Li et al. (2018)
0.62	Ultrasonic/UV lamps, 98 W, pH0 7.0, TiO2 62.5 μg/mL	11.72	Hu et al. (2014)
1.00	Pd/Ni electrode glow discharge electrolysis, 35 W	198.4	Zhao et al. (2019)

As can be shown, the energy efficiency of TCAA degradation by the KrCl* excimer radiation is not only than those of photocatalysis and electrolysis, but also only requires electric energy to drive, without the use of additional substances, oxygen or materials. Besides, it should be emphasized that the only byproducts of the TCAA decomposition are HCl and CO₂ that are neither poisonous or detrimental. These results show that KrCl* excimer radiation is a viable and dependable technique to eliminate TCAA.

4. Conclusions

In this work, our study focused on the applicability of in situ KrCl* excimer radiation for TCAA degradation and dichlorination. as well as role of oxygen in the pathway. The following conclusions can be drawn.

• i)In general, ${\bar{P}}_{\text{KrCl}*}$ and I _UV increased with increasing P _in. However, when P _in exceeded 60 W, quenching and dissociation processes resulted in a rapid fall in I _UV. Therefore, 60 W was the optimum P _in in this study. The energy efficiency of KrCl* excimer radiation has obvious energy consumption advantages compared to other methods.
• ii)TCAA can be efficiently decomposed and dechlorinated under the KrCl* excimer irradiation. The majority of carbon and chlorine atoms were mineralized to inorganic carbon and chloride ion, respectively.
• iii)Dissolved oxygen shows little effect on the degradation but greatly enhances the dechlorination because it can additionally generate •OH in the irradiation process.
• iV)Under 222 nm irradiation, TCAA was excited from S₀ state to S₁ state and then by ICS process to T₁ state, and a reaction without potential energy barrier occurs, resulting in the breaking of C–Cl bond and finally returning to S₀ state. •OH reacts with the resulting •CCl₂COOH, forming the intermediate product CCl₂OHCOOH. Subsequently, C–Cl bond cleavage resulted in the formation of ${}_{•}{}^{•}\mathrm{C}\text{OClCOOH}$ and the release of HCl. •OH attacked on C–Cl, causing the breakage of the C–Cl bond as well as disintegration of ${}_{•}{}^{•}\mathrm{C}\text{OClCOOH}$, producing ${}_{•}{}^{•}\mathrm{C}\text{OOHCOOH}$ and •Cl. Finally, the ${}_{•}{}^{•}\mathrm{C}\text{OOHCOOH}$ was completely decomposed to the final product CO₂.

Credit author statement

Jiaming Gan and Lei Wang conceived the ideas and designed the methodology; Jiaming Gan, Yizhan Zhang, Ting Li, and Min Zhao carried out the experiments and analyzed the data; Ting Zhu, Dailin Li, Jiaming Gan, and ZengXia Zhao provided guidance for model calculations; Jiaming Gan, Ting Zhu, and Lei Wang contributed to the revision of this manuscript; Dailin Li and Lei Wang supervised the project, provided critical feedback, and helped shape the research and manuscript.

Declaration of competing 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.

Handling Editor: Junfeng Niu

Appendix A. Supplementary data

The following is the Supplementary data to this article:

Data availability

Data will be made available on request.