Degradation of BTEX in groundwater by nano-CaO₂ particles activated with L-cysteine chelated Fe(III): enhancing or inhibiting hydroxyl radical generation

Mono-aromatic compounds (MACs), composed of one single benzene ring, exist in groundwater mainly as a result of accelerated industrialization (Dhanya 2019). Of these MACs, benzene, toluene, ethylbenzene, and xylenes (BTEX) are known to be relatively water-soluble and have carcinogenic and neurotoxic effects which can increase the risk of detrimental health effects (Stasik et al. 2015; Godin et al. 2020; Yang et al. 2020). It has been reported that BTEX are the most frequently detected aromatic hydrocarbons in contaminated groundwater in China, causing the rapid migration of BTEX pollution (Zhao et al. 2020). However, the groundwater in most MACs polluted sites is usually under anaerobic conditions where BTEX exhibits slower degradation kinetics and a more limited catabolic range in such environments (Stasik et al. 2015; Huang et al. 2017). Therefore, remarkable efforts are made toward in situ chemical oxidation (ISCO) to ensure BTEX degradation eventually to be broken off completely (Minetti et al. 2017; Ma et al. 2018; Xia et al. 2020).

However, Luo et al. (2016) reported that methylene blue removal increased with the concentration of L-cys up to 50 μM, and its removal decreased with further increase in L-cys concentration. Similarly, the degradation of sulfadiazine in an L-cys-Fenton system increased with the increase of L-cys usage at the beginning and decreased with much more L-cys addition (Lu et al. 2020). The consumption of HO• with excessive L-cys might be the primary reason. In order to clearly understand the performance of nCaO₂ in the L-cys-Fenton reaction on composite pollutant removal and to reveal the mechanism of the role of L-cys in this system, in this article, the evaluation of an L-cys chelating nCaO₂/Fe(III) system on BTEX removal was thoroughly investigated. The role of L-cys in the Fe(III)/Fe(II) cycle and the generation of HO• was confirmed. Finally, the application potential of the L-cys-Fenton system for BTEX removal in actual groundwater remediation implementation was demonstrated.

Benzene (C₆H₆, 99.0%), toluene (C₇H₈, 99.0%), ethylbenzene (C₈H₁₀, 99.0%), xylenes (C₈H₁₀, 99.0%), hydrogen peroxide (H₂O₂, 30%wt), ferric sulfate (Fe₂(SO₄)₃, 99.0%), calcium chloride (CaCl₂, 96%), polyethylene glycol 200 (PEG 200, 99.0%), ammonia (NH₃•H₂O, 28%), sulfuric acid (H₂SO₄, 98%) and sodium hydroxide (NaOH, 96.0%) were purchased from Aladdin Reagent Co. Ltd (Shanghai, China). Tert-butanol (C₄H₁₀O, TBA, 99.0%), benzoic acid (C₇H₆O₂, BA, 99.5%), 5,5-dimethyl-1-pyrroline N-oxide (C₆H₁₁NO, DMPO, 99.0%) and L-cysteine (C₃H₇NO₂S, L-cys, 99.0%) were supplied from Shanghai Lingfeng Reagent Co. Ltd (Shanghai, China). The ultrapure water was provided by a Milli-Q water purification system (Classic DI; ELGA, Marlow, UK). The actual groundwater from a well approximately 15 m deep below the surface (Songjiang, Shanghai, China) was used for preparing the actual BTEX contaminated solutions.

Briefly, CaCl₂ was utilized as the precursor and H₂O₂ was added drop-by-drop in the presence of the surface stabilizer PEG 200. The detailed preparation method of nCaO₂ can be found in the Supplementary Material, Text S1.

BTEX stock solutions (0.5 mM) were transferred into a 250 mL glass reactor. A magnetic stirrer was used to mix the solution homogeneously and the temperature was controlled at 20 °C. The test started immediately after introducing the predetermined dosages of Fe(III) and nCaO₂. At the desired intervals, 2.5 mL samples were withdrawn and transferred into headspace vials containing 1.0 mL methanol. The vials were sealed immediately and then analyzed by a gas chromatograph (GC) instrument.

BTEX were analyzed by a GC coupling with a headspace auto-sampler, a flame ionization detector (FID), and an HP-5 column (30 m × 0.32 mm × 0.25 μm) as described in the Supplementary Material (Text S2). Benzoic acid (BA) was chosen as a probe to quantify the production of HO•. The calculated production of p-HBA by the reaction of BA and HO• has the conversion factor (5.87± 0.18) with the production of HO• in the system (Xue et al. 2018a, 2018b).

The samples were extracted at 3 min after the start of the reaction for the analysis of intermediates during BTEX degradation. An amount of 150 mL ethyl acetate was mixed with 150 mL of the reaction solution and rested for 5 min. The top layer ethyl acetate mixed solution was condensed to 1.5 mL by a rotary evaporator (N-1300D; Eyela, Japan). The condensed solution was filtered by an organic-phase filter and tested by liquid chromatography-mass spectrometry (LC-MS) (Q-Exactive plus; ThermoFisher, China) (Supplementary Material, Text S2). At 3 min after the start of the reaction, 1.0 mL samples were withdrawn from the reactor and mixed with 1.0 mL DMPO (20.0 mM) for 1 min, then the mixed samples were analyzed by electron paramagnetic resonance (EPR) (EMX-8/2.7; Bruker, USA) for the detection of reactive oxygen species. The DMPO-OH was monitored at the settings for the EPR spectrometer of center field (3,510.00 G), microwave frequency (9.79 GHz), and power (5.05 mW). Total organic carbon (TOC) was determined by using a TOC analyzer (LiquiTOC, Germany). The BTEX degradation test in the actual groundwater was carried out by replacing ultrapure water with actual groundwater. The natural organic matter (NOM) of the actual groundwater was determined by ion chromatography (ICS-1100; ThermoFisher, China).

A central composite design (CCD) based on the response surface methodology (RSM) was used for the analysis of BTEX degradation performance. The dosages of nCaO₂ and Fe(III) were coded as X₁ and X₂. The initial concentration of L-cys was considered as X₃, 21 groups of experiments were tested. The ranges of both X₁ and X₂ were set from 1.0 to 20.0 mM and the range of X₃ was set from 1.0 to 10.0 mM.

A series of comparative tests were carried out to estimate the performance of BTEX degradation in the nCaO₂/Fe(III)/L-cys system in order to investigate the L-cys chelating effect. The control tests for BTEX removal were conducted without nCaO₂ or Fe(III) under the same conditions and the results showed the BTEX volatilization rate was less than 5.0%. The removal of BTEX was below 8.0% when only nCaO₂ or Fe(III) existed. This means that nCaO₂ or Fe(III) alone was not available to degrade BTEX. In the nCaO₂/Fe(III)/L-cys system, rapid BTEX destruction was observed within 10 min, and the destruction became slower in the remaining reaction time. The reaction was terminated at 180 min in this test (Figure S1).

The performance of BTEX degradation in various systems is presented in Figure 1(a). As can be seen from Figure 1(a), the degradation efficiencies of BTEX were 19.7%, 17.6%, 21.6%, and 17.7%, respectively, when nCaO₂ and Fe(III) co-existed, but had a significant improvement (91.3%, 91.4%, 92.3%, and 86.6% degradation efficiencies) when 1.0 mM L-cys was introduced into the system. This means that nCaO₂ activated with Fe(III) could simultaneously degrade the combined BTEX pollutants, and L-cys could effectively improve the degradation capacity of the system on BTEX removal. The degradation efficiency of BTEX in the nCaO₂/Fe(II) system was higher than that in the nCaO₂/Fe(III) system. A possible explanation might be that Fe(III) reacted with H₂O₂ released from nCaO₂-producing HO• radicals and Fe(II), and then Fe(II) catalyzed H₂O₂ to complete the Fenton reaction (Munoz et al. 2015). Matta et al. (2007) reported that minerals containing both Fe(III) and Fe(II) were more effective than Fe(III) oxides in the degradation of 2,4,6-trinitrotoluene. Giannakis et al. (2017) reported that Fe(II) salt was more efficient than Fe(III) on removing viruses from wastewater in the photo-Fenton process.

L-cys chelate-Fe(III) presented outstanding catalytic performance. Under the same molar ratio conditions, BTEX had higher removal in the nCaO₂/Fe(III)/L-cys system than those in the nCaO₂/Fe(III) and nCaO₂/Fe(II) systems (70.8%, 72.8%, 76.1%, and 61.5% removal of BTEX in the nCaO₂/Fe(II) system). However, with the addition of L-cys from 1.0 to 10.0 mM, the degradation efficiency of BTEX declined from 91.3%, 91.4%, 92.3%, and 86.6% to 41.4%, 40.7%, 45.8%, and 28.6%, respectively (Figure 1(b)). Therefore, it could be summarized that a proper amount of L-cys promoted the pollutant degradation ability of the nCaO₂/Fe(III) system, but excessive L-cys inhibited BTEX degradation. The excessive L-cys could compete for HO• with BTEX, leading to the decline of BTEX removal. This phenomenon of the consumption of HO• by excessive L-cys was also reported in a sulfadiazine degradation system (Lu et al. 2020).

To investigate the effect of the initial nCaO₂ or Fe(III) concentration on BTEX degradation, we established response surface methodology (RSM) models (Figure 2). When the initial L-cys concentration constant was made 1.0 mM, the degradation of BTEX increased along with the increase of nCaO₂ and Fe(III) concentrations. However, the increasing rates of xylenes by nCaO₂ and Fe(III) were higher than the others. This means that the degradation efficiency of xylenes was influenced by the initial concentration of nCaO₂ and Fe(III) more sensitively than benzene, toluene, or ethylbenzene. Overall, the influence of the BTEX degradation of factors X₁ and X₂ was the same, and a better degradation efficiency could be achieved at the same nCaO₂ and Fe(III) concentration. Making nCaO₂ or Fe(III) constant and increasing another concentration could not contribute to a better BTEX degradation. This result was also reported in the nCaO₂/Fe(II) system on BTEX removal (Sun et al. 2020). As the concentration of L-cys changed from 1.0 to 10.0 mM, more nCaO₂ and Fe(III) were needed to achieve the same BTEX removal (Figure S2 in the Supplementary Material). It also confirmed that excessive L-cys inhibited the system degradation ability for BTEX. The optimal molar ratio of nCaO₂/Fe(III)/L-cys was 10.5/20/1 based on the calculation by RSM. In subsequent studies, the molar ratios of 10/10/2/1 and 10/10/10/1 of nCaO₂/Fe(III)/L-cys were chosen to represent low and high L-cys concentrations to investigate the role of L-cys in the system.

In order to authenticate the occurrence and production of dominant radicals in the nCaO₂/Fe(III)/L-cys system, EPR tests were carried out using DMPO as the spin-trapping agent and the results are presented in Figure 3. In the nCaO₂/Fe(III) system, Fe(III) could not react directly with H₂O₂ to produce HO• and Fe(III) needs to be reduced to Fe(II) before reaction with H₂O₂ (Duesterberg et al. 2008; Fu et al. 2017). Therefore, the relative intensity of HO• in the nCaO₂/Fe(III) system was weak at 30 min of reaction. The concentration of Fe(II) increased significantly at 60 min, which strongly supported HO• production. However, the relative intensity of HO• in the nCaO₂/Fe(III)/L-cys system was kept strong from 5 to 60 min. This means that the Fe(II) concentration in the nCaO₂/Fe(III)/L-cys system was high, in which Fe(II) could react with H₂O₂ and produce a large amount of HO• in a short time. There is a close relationship between the change of HO• and Fe(II)/Fe(III) redox cycles (Huang et al. 2020). It is deduced that the participation of L-cys promoted the Fe(II)/Fe(III) redox cycles in the nCaO₂/Fe(III)/L-cys system.

However, by adding 1.0 mM L-cys into the nCaO₂/Fe(III) system, the concentration of Fe(II) increased to 1.15 mM instantly. During the reaction, the concentration of Fe(II) was changed within the range of 0.07 mM to 0.19 mM. When excessive L-cys (5.0 mM) was added to the system, the amount of Fe(II) increased to 3.73 mM (Figure 5(c)), and was still maintained high during the subsequent reactions. The L-cys could reduce Fe(III) to Fe(II), and Fe²⁺(L-cys) and Fe²⁺(L-cys)₂ complexes were generated simultaneously. The formations of Fe²⁺(L-cys) and Fe²⁺(L-cys)₂ complexes could increase the concentration of Fe(II) and strengthen the catalytic performance because the catalytic effect of Fe(II) was stronger than that of Fe(III) (Jiang et al. 2020). In addition, after 180 min reaction, the total soluble iron concentration in the nCaO₂/Fe(III) system was 2.53 mM (Figure 5(a)). However, the total iron concentration increased to 3.50 mM and 4.71 mM when the dosage of L-cys was 1.0 mM and 5.0 mM, respectively (Figure 5(b) and 5(c)), and this was simply due to less formation of precipitation after adding L-cys into the system (Ye et al. 2020).

Tert-butanol (TBA) was selected as HO• scavenger to evaluate the dominant radicals in the nCaO₂/Fe(III)/L-cys system due to TBA having a high reaction rate with HO• (k_HO• = 5.2 × 10⁸ M⁻¹s⁻¹) (Cai et al. 2020). The results showed that BTEX removal was significantly inhibited when TBA was added (Figure 6). BTEX removal at 180 min in the nCaO₂/Fe(III)/L-cys system decreased from 91.3%, 91.4%, 92.3%, and 86.6% to 9.5%, 9.7%, 11.8%, and 9.1%, respectively, which, along with the EPR test results, indicated that HO• was the dominant reactive species in the nCaO₂/Fe(III)/L-cys system.

The intermediates during BTEX degradation were analyzed by LC–MS and the possible BTEX destruction pathway is proposed in Figure 7. For the benzenoid compounds, the benzene-ring could be attacked by HO• to form various intermediate phenols (Gligorovski et al. 2015), such as C₆H₆O₃ (m/z = 126), C₇H₈O₃ (m/z = 140), and C₈H₉O₃ (m/z = 153). Further, HO• led to benzene-rings opening and formed short-chain alkanes and alkenes, such as C₅H₇O₄ (m/z = 131), C₄H₅O₃ (m/z = 101), and C₃H₅O₃ (m/z = 89). These intermediates and aromatic compounds were attacked by HO• and finally were completely mineralized. Since HO• are non-selective oxidation radicals, the reaction rate constants between organic compounds containing intermediates and HO• are in a range of 10⁹–10¹⁰ M⁻¹s⁻¹ (Xue et al. 2018a, 2018b). In this study, although BTEX removal exceeded 90% in the nCaO₂/Fe(III)/L-cys system, most intermediates in the reaction belonged to aromatic compounds. The toxicity during BTEX degradation and the related study need to be investigated in our future research.

To investigate the effect of the nCaO₂/Fe(III)/L-cys system on BTEX removal in the actual groundwater, the tests were carried out using the actual groundwater instead of ultrapure water (Figure 8). The characteristics of the actual groundwater used in the tests are listed in Table 1. As to the results, only 12.1%, 13.4%, 15.6%, and 14.0% BTEX had been removed when nCaO₂ and Fe(III) existed at the nCaO₂/Fe(III)/BTEX molar ratio of 10/10/1. When 1.0 mM L-cys was added into the system, the removal rate of BTEX increased to 56.3%, 57.2%, 59.3%, and 53.2%, which was about 60% of the degradation rate compared with ultrapure water. The main reason was that the initial pH of the actual groundwater was nearly neutral (7.31 ± 0.2) and it had a strong buffer capacity which could maintain the pH without large change during the reaction. As the Fenton reaction is affected by the pH of the solution (Sarmento et al. 2016), the effect of initial solution pH on BTEX degradation was investigated and the results are shown in the Supplementary Material (Figure S3a). At pH 6.2, the system had the highest BTEX degradation efficiency and the degradation ability was inhibited under acidic and/or alkaline conditions. Besides, the existence of Cl⁻, HCO₃⁻, SO₄²⁻ and the natural organic matter (NOM), humic acid (HA) being a typical NOM, were not conducive to the degradation of BTEX (Figure S3b). The results showed that HCO₃⁻ had the most significant effect on BTEX degradation, with a 52.7% reduction in degradation efficiency. The inhibitory effect of HCO₃⁻ on BTEX degradation in the nCaO₂/Fe(III)/L-cys system could be speculated as being for two reasons: the increasing of the solution pH and the scavenging effect of HO• (Zhang et al. 2015a, 2015b). HA had little effect on BTEX degradation. Therefore, under the actual groundwater conditions, the degradation ability of the nCaO₂/Fe(III)/L-cys system was inhibited. Encouragingly, the removal rates of BTEX were promoted to 94.0%, 96.2%, 97.1%, and 94.3%, respectively, when the molar ratio of nCaO₂/Fe(III)/L-cys/BTEX increased to 40/40/8/1. Increasing the molar ratio of nCaO₂/Fe(III)/L-cys/BTEX was an effective way to overcome the adverse effects caused by the complex of the actual groundwater. The degradation efficiencies of BTEX were 81.8%, 83.3%, 84.8%, and 75.6%, respectively, when the nCaO₂/Fe(III)/L-cys/BTEX molar ratio was 20/20/4/1. Increasing the molar ratio to 40/40/8/1, the degradation efficiencies of BETX increased to 94.0%, 96.2%, 96.9%, and 94.3%, respectively. Although the molar ratio of nCaO₂/Fe(III)/L-cys/BTEX doubled, the degradation efficiency only increased in the range of 12.1%–18.7%. Considering the remedial cost, the optimal ratio of nCaO₂/Fe(III)/L-cys/BTEX was set as 20/20/4/1 in the practical groundwater treatment. The agent cost of treating 1 m³ of groundwater was calculated to be $28.50 at the optimal molar ratio condition. The above results strongly demonstrated that the nCaO₂/Fe(III)/L-cys system has broad application prospects in the actual groundwater environment for BTEX removal.

In this study, BTEX removal in the nCaO₂/Fe(III)/L-cys system was performed and the results showed that the nCaO₂/Fe(III)/L-cys technique could be efficient in destruction of BTEX in both ultrapure water and actual groundwater. A proper amount of L-cys could promote the degradation ability of the nCaO₂/Fe(III) system because the participation of L-cys accelerated the Fe(III)/Fe(II) redox cycles, but excessive L-cys inhibited them. After adding 1.0 mM L-cys into the nCaO₂/Fe(III) system, the concentration of Fe(II) increased to 1.15 mM instantly. Simultaneously, the proper concentration of L-cys was conducive to the accumulation of HO•. The yield of HO• produced in the 1.0 mM L-cys-containing system was 0.066 mM, higher than that without L-cys (0.049 mM) at 180 min. When excessive L-cys (5.0 mM) was added to the system, the amount of Fe(II) increased to 3.73 mM, however, the yield of HO• decreased to 0.043 mM. Excessive L-cys caused a large amount of Fe(III) in the system to be reduced, and excessive Fe(II) could in turn scavenge HO• to produce Fe(III). The optimal molar ratio of nCaO₂/Fe(III)/L-cys was 10.5/20/1 based on calculation by RSM. EPR tests and quenching results indicated that HO• was the dominant reactive species in the nCaO₂/Fe(III)/L-cys system. According to the detected intermediates by LC-MS, the BTEX destruction pathway was proposed. For these benzenoid compounds, the benzene-ring was attacked by HO• to form various intermediate phenols, and further HO• led to benzene-rings opening to form short-chain alkanes and alkenes and finally being completely mineralized. Under the actual groundwater conditions, the nCaO₂/Fe(III)/L-cys system has broad application prospects in the actual groundwater environment for BTEX removal. Considering the remedial cost, the optimal molar ratio of nCaO₂/Fe(III)/L-cys/BTEX was set as 20/20/4/1.

This study was financially supported by the National Key R&D Program of China (No. 2018YFC1803304) and the National Natural Science Foundation of China (No. 41977164).

The authors confirm that this manuscript has not been previously published as a whole or part and it is not under consideration by any other journal.

All authors have approved the content and consent to submit it.

All authors have approved the content and consent to publish it.

Xuecheng Sun: Conceptualization, Methodology, Investigation, Writing - Original Draft, Meesam Ali: Validation, Formal analysis, Writing – Review & Editing, Changzheng Cui: Resources, Visualization, Shuguang Lyu: Conceptualization, Writing – Review & Editing, Supervision.

All relevant data are included in the paper or its Supplementary Information.