The addition of hydroxylamine hydrochloride (HAH), ascorbic acid (ASC), sodium ascorbate (SAS) to the OA-Fe(II)/SPC system could promote the generation of HO by accelerating Fe(II)/Fe(III) recycles and H2O2 decomposition. The enhancement of HAH on HO generation surpasses ASC and SAS in the OA-Fe(II)/SPC system. The generation of O2•− was also enhanced by HAH, ASC and SAS, and more significant promotion of O2•− generation was observed with ASC and SAS addition. More effective benzene removal was achieved in an OA-Fe(II)/SPC system with suitable HAH, ASC and SAS addition, compared to the parent system. Excessive HAH, ASC or SAS had a negative effect on benzene removal. Results of scavenger tests showed that HO is indeed the dominant free radical for benzene removal in every system, but the addition of HAH, ASC and SAS increased the contribution of O2•− to benzene degradation. HAH, ASC and SAS enhanced OA-Fe(II)/SPC systems could be well utilized to acidic and neutral conditions, while HCO3, high concentration of HA and alkaline conditions were not favorable to benzene removal. Moreover, the addition of HAH, ASC and SAS are conducive to benzene removal in actual groundwater, and HAH was the optimal reducing agent for the enhancement of the OA-Fe(II)/SPC system.

  • Benzene removal performance in an RA-OA-Fe(II)/SPC system was investigated.

  • HAH is more conducive to HO generation than ASC and SAS.

  • Significant promoted generation of O2•− was observed with ASC and SAS addition.

  • HAH, ASC and SAS weakened negative effects of the solution matrix on benzene removal.

  • HAH, ASC and SAS are conducive to benzene removal in actual groundwater.

Benzene, a toxic and hazardous compound of the BTEX family, has been listed as a USEPA priority pollutant and its maximum contaminant level was limited at 5 μg L−1 under the Safe Drinking Water Act (ATSDR 2004; USEPA 2009). This noxious compound can frequently be detected in contaminated groundwater and soils, and behaves as a significant menace to human health owing to its neurological damage and high carcinogenicity (Fu et al. 2015). In recent years, many effective methods have been applied for remediation of benzene and other organic pollutants from groundwater and soil (Johnson 1998; Siegrist et al. 2011; Muhammad et al. 2014; Neriah & Paster 2016). In situ chemical oxidation (ISCO) has been regarded as one of prominent and well-known technologies for contaminated groundwater and soils remediation because of its high cleanup efficiency. Several oxidants, such as Fenton's reagent (Ojinnaka et al. 2012), persulfate (Petri 2010), peroxymonosulfate (Ahn et al. 2016), permanganate (Mahmoodlu et al. 2014), ozone (Hu & Xia 2017), and percarbonate (Fu et al. 2016; Zang et al. 2017) have been applied in the ISCO process. Sodium percarbonate (2Na2CO3•3H2O2, SPC) can be dissociated to H2O2 and Na2CO3 when dissolved in water, providing a Fenton-like reaction. Besides, numerous advantages of SPC compared to H2O2, such as wider pH range applicability, effective contaminants degradation, and alkaline properties have attracted the attention of many researchers.

Effective benzene removal was achieved in the Fe(II)/SPC system; however, the rapid consumption of the soluble Fe ion was unfavorable to continuous degradation of benzene (Fu et al. 2015). Miao et al. and Fu et al. utilized chelating agents to keep the Fe ion in soluble form, thereby enhancing the degradation of organic contaminants (Fu et al. 2016; Miao et al. 2015a). Furthermore, it was reported that high reactivity of chelated-Fe(II) with SPC could improve the utilization of SPC, which eventually improved the contaminant's degradation (ElShafei et al. 2010). Fe(II) was also consumed rapidly within several minutes, and chelated-Fe(III) was the governing form of soluble Fe ion in the chelated-Fe(II)/SPC system (Fu et al. 2016; Zang et al. 2017). This impeded the further degradation of contaminants because of the low reactivity between chelated-Fe(III) and SPC. Thus, looking for reducing agents (RA) to enhanced Fe(II)/Fe(III) recycles is a favorable method for the promotion of contaminant degradation.

Zou et al. found hydroxylamine (HAH) could accelerate the recycling of Fe(III)/Fe(II) and then improve benzoic acid degradation in the Fe(II) catalyzed peroxymonosulfate system (Zou et al. 2013). Zhang et al. indicated that ascorbic acid (ASC) could not only enhance the reduction of Fe(III) to Fe(II) but also maintain the Fe ion in a soluble form (Zhang et al. 2017). This technique strengthened the generation of reactive oxygen species for further trichloroethylene removal in an Fe(III)-catalyzed calcium peroxide system. Fukuchi et al. reported that HAH, ASC, gallic acid, oxalic acid (OA), humic acid (HA), and p-hydroquinone could enhance the removal of 2,4,6-tribromophenol in a heterogeneous Fenton-like system catalyzed by iron-loaded natural zeolite (Fukuchi et al. 2014). The HAH had the most significant promoting effect of 2,4,6-tribromophenol removal among these several reducing agents. Enhanced effects of HAH, sodium ascorbate (SAS), sodium thiosulfate, ASC, and sodium sulfite (SS) on trichloroethylene removal were also observed in an Fe(II)-catalyzed persulfate system (Wu et al. 2015).

The above discussion concluded that reducing agents can be applied to alleviate the drawback of rapid Fe(II) consumption in a chelated-Fe(II)/SPC system, then improve the oxidation ability of the chelated-Fe(II)/SPC system for benzene removal. Therefore, we exploited the RA-OA-Fe(II)/SPC system to (i) investigate the effects of several reducing agents on the generation of free radicals by radical probe compounds and electron paramagnetic resonance (EPR) analysis, (ii) demonstrate the efficiency of benzene removal in RA-OA-Fe(II)/SPC systems, (iii) depict the contribution of free radicals to benzene removal by scavengers tests, (iv) valiate the effects of solution matrix on benzene removal and the applicability of RA-OA-Fe(II)/SPC systems for benzene removal in actual groundwater.

Materials

Benzene (>99.7%), ferrous sulfate heptahydrate (99.0%), oxalic acid (>99.0%), sodium sulphate (99.0%), sodium bicarbonate (99.5%), l-ascorbic acid (>99.0%), sodium nitrate (99.0%), methanol (99.8%), sodium chloride (99.5%), carbon tetrachloride (99.5%), nitrobenzene (99.0%), sodium sulfite (>99.0%), chloroform (99.0%), methanol (99.8%), hydroxylamine hydrochloride (>98.5%), isopropyl alcohol (99.5%) and sodium ascorbate (>99.0%) were achieved from Aladdin (Shanghai, China). Sodium percarbonate (98%) and 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) were purchased from Acros Organics (Shanghai, China) and Sigma (Shanghai, China), respectively. 1,10-phenanthroline monohydrate (98%), potassium biphthalate (99.0%) and humic acid (fulvic acid >90%) were achieved from Shanghai Jingchun Reagent Ltd Co. (Shanghai, China). H2SO4 (0.1 M) and NaOH (0.1 M) was applied to adjust initial solution pH. Ultrapure water was produced by a Milli-Q water process (Classic DI, ELGA, Marlow, U.K.). The actual groundwater was obtained from a 10 m deep well in Minhang, Shanghai, China. Its characteristics were as follows: pH: 7.5, total organic carbon: 11.5 mg/L, SO42−, Cl, NO3 and HCO3 concentrations (mg/L): 90.7, 53.6, 6.7 and 101.4, Fe(II) and total Fe ion concentration (mg/L): 1.0 and 1.4.

Experimental procedures

The stock solutions were prepared by dissolving pure non-aqueous phase liquid benzene, CT and NB in Milli-Q water or actual groundwater for 1 h. Then desired concentrations of benzene, CT and NB of 1.0, 0.15 and 1 mM respectively were obtained by diluting certain volume stock solutions into a 250 mL cylindrical glass reactor with a magnetic stirrer. The temperature during the reaction process was controlled at 20 °C by the flow of temperature-controlled water in the reactor's jacket. The experimental setup is shown in Supplementary Material (Fig. S1). Benzene, CT and NB were conducted in blank tests without addition of other reagents to validate the effect of volatization during degradation. Predetermined dosages of reducing agents, Fe(II), OA, and other demanded chemicals were added into the reactor for 10 min mixing prior to addition of SPC to initiate the reaction. 2.5 mL samples were taken periodically from the reactor into a headspace vial with 1.0 mL methanol for benzene analysis by headspace-gas chromatography (HS-GC). Finally, 1.0 mL DMPO solution (8.84 mM), a radical trapping agent was mixed with periodical samples (0.5 mL) for 1 min and placed in a capillary tube to detect the intensity of free radicals in EPR analysis.

Analytical methods

One headspace-gas chromatograph (Agilent 6890N, Palo Alto, CA) equipped with an auto-sampler (COMBI-PLA, CTC, Switzerland), an HP-5 column (30-m length, 0.32-mm I.D., 0.25-μm thickness) and a flame ionization detector (FID) were applied for benzene concentration analysis.

Headspace conditions include incubation temperature (50 °C), agitator speed (500 rpm), incubation time (5 min) and syringe temperature (60 °C). The GC oven temperature follows the process: an initial 40 °C was held for 2 min, increased to 80 °C at the rate of 30 °C/min, and held for 1 min. The temperatures of the injector and detector were fixed at 150 and 250 °C, respectively. N2 is carrier gas with a flow rate of 3.0 mL/min. A 500 μL sample from the upper space was injected with splitless ratio. The concentrations of CT and NB were analyzed by a gas chromatograph (Miao et al. 2015a, 2015b). The intensity of free radicals was detected by EPR, more details of EPR analytical conditions are shown in the Supplementary Material. Fe(II) and total soluble Fe ions were determined with the application of the 1,10-phenanthroline method (Tamura et al. 1974). H2O2 was measured with the TiCl4 spectrophotometric method (Eisenberg 1943). The solution pH was measured by a pH meter (Mettler-Toledo DELTA 320, Greifensee, Switzerland).

Effects of reducing agent on the production of free radicals

Based on our previous work, both HO and O2•− exists in the OA-Fe(II)/SPC system (Fu et al. 2016). To explore the effect of reducing agent (RA) on HO and O2•− generation in the OA-Fe(II)/SPC system, nitrobenzene (NB) and carbon tetrachloride (CT) were applied as oxidant and reductant probes, respectively (Buxton et al. 1988). Identical initial concentrations of RA, Fe(II), OA, and NB were set at 1 mM, and 2 mM of SPC and 0.05 mM of CT were applied in the probe test.

Effects of reducing agent on HO generation

Figure 1 shows very little loss of NB due to volatilization in the blank test. But a significant removal of NB was noticed, showing the presence of HO in the OA-Fe(II)/SPC system, which is consistent with our previous work (Fu et al. 2016).

Figure 1

Effect of reducing agents on NB degradation in OA-Fe(II)/SPC system ([NB]0 = [RA]0 = [OA]0 = [Fe(II)]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

Figure 1

Effect of reducing agents on NB degradation in OA-Fe(II)/SPC system ([NB]0 = [RA]0 = [OA]0 = [Fe(II)]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

Compared to 13.6% final NB removal in the simple OA-Fe(II)/SPC system, 70, 36, and 29% of final NB removal were achieved with the addition of HAH, ASC, and SAS to the OA-Fe(II)/SPC system, respectively. Furthermore, as shown in Figure 2, HO intensity at 2 min detected by EPR technique were stronger individually with SAS, ASC, HAH addition to OA-Fe(II)/SPC system. These indicated that HAH, ASC and SAS could promote the production of HO, and HAH was the optimal reducing agent on HO generation. However, a decrease of NB removal rate to 16.3% was observed with SS addition, indicating that SS was not conducive to HO generation.

Figure 2

The intensity of HO in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC, SAS-OA-Fe(II)/SPC and OA-Fe(II)/SPC systems ([NB]0 = [Fe(II)]0 = [RA]0 = [OA]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

Figure 2

The intensity of HO in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC, SAS-OA-Fe(II)/SPC and OA-Fe(II)/SPC systems ([NB]0 = [Fe(II)]0 = [RA]0 = [OA]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

To investigate the mechanism of HAH, ASC, SAS and SS on the generation of HO in the OA-Fe(II)/SPC system, concentration changes of total soluble Fe ions, Fe(II) and H2O2 were measured in each system, as shown in Tables 1 and 2. The results elucidated that addition of HAH, ASC, and SAS to the OA-Fe(II)/SPC system, the total soluble Fe ion concentrations are higher than its concentration in the parent system, indicating that HAH, ASC and SAS is conducive to maintaining Fe in a soluble form. This improves the catalytic ability of the system, and enhances the generation of HO to a certain extent. NB removal in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems within 2 min were 48.6%, 19.8% and 14.3%, respectively, higher than its removal (10.7%) in the OA-Fe(II)/SPC system, indicating more HO generation within 2 min in RA-OA-Fe(II)/SPC systems. This is due to the higher Fe(II) concentration caused by RA within 2 min, which reacts with H2O2 rapidly in RA-OA-Fe(II)/SPC systems. The Fe(II) concentration in the HAH-OA-Fe(II)/SPC system decreases from 0.972 to 0.252 mM within 2 min, but is still higher than 0.167 mM in the OA-Fe(II)/SPC system. This resulted in the rapid consumption of H2O2, the concentration of which decreased to 1.497 mM from 2.959 mM, thereby producing more HO in the HAH-OA-Fe(II)/SPC system (Table 2). It is noted that the remaining H2O2 were 1.497, 1.192, and 0.857 mM, showing 1.462, 1.767, and 2.102 mM H2O2 were consumed in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems, respectively. Compared to its corresponding NB degradation (48.6%, 19.8% and 14.3%), a higher consumption of H2O2 can be deduced in ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems than in HAH-OA-Fe(II)/SPC system.

Table 1

Total Fe ion and Fe(II) concentrations in various RA-OA-Fe(II)/SPC systems

Time (min)Total Fe ion concentration (mM)
Fe(II) concentration (mM)
Without RAHAHASCSASSSwithout RAHAHASCSASSS
0.991 0.992 1.012 0.996 1.01 0.994 0.972 0.997 1.011 0.988 
0.900 0.923 0.926 0.909 0.418 0.167 0.252 0.224 0.177 0.092 
0.869 0.911 0.871 0.884 0.423 0.147 0.204 0.138 0.136 0.044 
10 0.856 0.921 0.903 0.829 0.383 0.103 0.217 0.146 0.143 0.053 
30 0.807 0.901 0.876 0.853 0.410 0.065 0.184 0.158 0.137 0.058 
60 0.872 0.948 0.930 0.905 0.327 0.072 0.227 0.145 0.138 0.035 
120 0.830 0.970 0.924 0.875 0.302 0.059 0.300 0.209 0.135 0.040 
Time (min)Total Fe ion concentration (mM)
Fe(II) concentration (mM)
Without RAHAHASCSASSSwithout RAHAHASCSASSS
0.991 0.992 1.012 0.996 1.01 0.994 0.972 0.997 1.011 0.988 
0.900 0.923 0.926 0.909 0.418 0.167 0.252 0.224 0.177 0.092 
0.869 0.911 0.871 0.884 0.423 0.147 0.204 0.138 0.136 0.044 
10 0.856 0.921 0.903 0.829 0.383 0.103 0.217 0.146 0.143 0.053 
30 0.807 0.901 0.876 0.853 0.410 0.065 0.184 0.158 0.137 0.058 
60 0.872 0.948 0.930 0.905 0.327 0.072 0.227 0.145 0.138 0.035 
120 0.830 0.970 0.924 0.875 0.302 0.059 0.300 0.209 0.135 0.040 

a [NB]0 = [Fe(II)]0 = [RA]0 = [OA]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C.

Table 2

H2O2 concentration in various RA-OA-Fe(II)/SPC systems

Time (min)BlankWithout RAHAHASCSASSS
0.009 0.019 0.019 0.021 0.017 0.005 
2.767 2.329 1.247 1.008 0.404 2.168 
2.959 2.306 1.497 1.192 0.857 2.002 
2.933 2.262 0.762 0.888 0.777 2.071 
2.932 2.205 0.360 0.788 0.642 2.051 
10 2.945 2.104 0.378 0.797 0.607 1.972 
30 2.922 1.821 0.198 0.413 0.274 1.854 
60 2.805 1.694 0.114 0.255 0.153 1.858 
120 2.638 1.600 0.064 0.174 0.130 1.732 
Time (min)BlankWithout RAHAHASCSASSS
0.009 0.019 0.019 0.021 0.017 0.005 
2.767 2.329 1.247 1.008 0.404 2.168 
2.959 2.306 1.497 1.192 0.857 2.002 
2.933 2.262 0.762 0.888 0.777 2.071 
2.932 2.205 0.360 0.788 0.642 2.051 
10 2.945 2.104 0.378 0.797 0.607 1.972 
30 2.922 1.821 0.198 0.413 0.274 1.854 
60 2.805 1.694 0.114 0.255 0.153 1.858 
120 2.638 1.600 0.064 0.174 0.130 1.732 

a [NB]0 = [Fe(II)]0 = [RA]0 = [OA]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C.

The H2O2 concentration decreased from 2.306 to 1.60 mM (120 min). Meanwhile, the Fe(II) concentration in the OA-Fe(II)/SPC system decreased to 0.059 mM continuously, in comparison to a total soluble Fe ion concentration of 0.83 mM. This indicated that Fe(III) was the main form of soluble Fe ion at this stage, which can react with H2O2 slowly, leading to the higher generation of HO after 2 minutes. Fe(II) concentration remained at a high level after 2 min in RA-OA-Fe(II)/SPC systems. Fe(II) concentrations at 120 min were 0.3, 0.209 and 0.135 mM with addition of HAH, ASC and SAS to the OA-Fe(II)/SPC system, respectively, significantly higher than that in the OA-Fe(II)/SPC system. This indicates that the addition of HAH, ASC and SAS, as strong reducing agents, accelerates Fe(II) regeneration from Fe(III), leading to the improvement of the catalytic ability of the system. The corresponding concentrations of H2O2 decreased to 0.064, 0.174, and 0.13 mM, respectively in all systems. This indicated that the enhanced regeneration of Fe(II) during this stage promotes the production of HO, which can be reflected by the continuous degradation of NB. It should be noted that the highest Fe(II) concentration and lowest concentration of remaining H2O2 were observed in HAH-OA-Fe(II)/SPC system. This validated the strongest reducing ability of HAH on Fe(II)/Fe(III) recycles, resulting in the optimal enhancement of HO generation.

On the contrary, total soluble Fe ion concentration is significantly lower in SS-OA-Fe(II)/SPC system. This indicates that the addition of SS is not conducive to the presence of the soluble Fe ion. In addition, the lowest Fe(II) concentration and largest H2O2 remaining amount showed the weak reducibility of SS on Fe(II)/Fe(III) recycles that further affected HO generation.

Effects of reducing agent on O2•− generation

Figure 3 showed the degradation of CT, which is a probe compound of O2•− in the RA-OA-Fe(II)/SPC and OA-Fe(II)/SPC systems. Compared to the negligible volatilization of CT in blank test, 14.6% CT removal indicates the presence of O2•− in OA-Fe(II)/SPC system. CT removal rate reached 22, 34, and 36% in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems, respectively, much greater than in OA-Fe(II)/SPC system. This indicates the addition of HAH, ASC and SAS can enhance O2•− generation. The effect on CT removal was partly observed with the addition of SS to OA-Fe(II)/SPC system.

Figure 3

Effect of reducing agents on CT degradation in OA-Fe(II)/SPC system ([RA]0 = [Fe(II)]0 = [OA]0 = 1 mM, [SPC]0 = 2 mM, [CT]0 = 0.05 mM, T = 20 ± 0.5 °C).

Figure 3

Effect of reducing agents on CT degradation in OA-Fe(II)/SPC system ([RA]0 = [Fe(II)]0 = [OA]0 = 1 mM, [SPC]0 = 2 mM, [CT]0 = 0.05 mM, T = 20 ± 0.5 °C).

In ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC system, ASC and SAS can dissociate into ascorbate anion (C6H7O6) when releasing one hydrogen ion and sodium ion, respectively. C6H7O6 can react with HO to yield oxidized dehydroascorbic acid radical (C6H6O6−•) which could react with dissolved oxygen (O2) to generate O2•− via one electron-transfer. In HAH-OA-Fe(II)/SPC system, NH2OH may reduce Fe(III) to Fe(II) via one-electron transfer, leading to the generation of H2NO radical. H2NO radical can further deprotonate to HNO−• radical (pKa = 12.6 ± 0.3). Then O2•− would also be generated via one electron-transfer between HNO−• radical and O2 at a rate constant of 2.2 × 108 M−1s−1. These may be the reason for the enhanced generation of O2•− in the RA-OA-Fe(II)/SPC systems (Lind & Merényi 2006; Lin & Liang 2013; Yang et al. 2015; Zhou et al. 2018).

In summary, the addition of HAH, ASC, and SAS can promote the generation of HO and O2•−. The enhanced effect of HAH on HO generation is significantly better than that of ASC and SAS, compard to on O2•− generation.

Benzene degradation performance in RA-OA-Fe(II)/SPC systems

Benzene degradation in various RA-OA-Fe(II)/SPC and OA-Fe(II)/SPC systems with different RA dosages were conducted with benzene, OA, Fe(II) and SPC concentrations of 1, 1, 1 and 2 mM, respectively. The results are shown in Figure 4. The optimal enhanced effect of RA on benzene removal was observed with HAH, ASC and SAS addition at the concentration of 1 mM. Compared to the 48.9% benzene removal rate in the OA-Fe(II)/SPC system, the addition of HAH, ASC, and SAS increased the benzene removal rate to 87.0%, 84.5%, and 81.9%, respectively. This indicated HAH, ASC and SAS can significantly enhance the ability of OA-Fe(II)/SPC system for benzene removal, and the enhanced effect of HAH, ASC, and SAS increased individually. This is due to the enhanced generation of HO and O2•− in RA-OA-Fe(II)/SPC systems, which was discussed in section 3.1. However, SS inhibited the degradation of benzene, which reduced benzene removal rate to 39.4% from 48.9%.

Figure 4

Effect of reducing agent dosages on benzene degradation in various RA-OA-Fe(II)/SPC systems ([Fe(II)]0 = [OA]0 = [Benzene]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C). Inserted figure: Benzene degradation performance in various RA-OA-Fe(II)/SPC systems with 1 mM reducing agents addition ([RA]0 = [Fe(II)]0 = [OA]0 = [Benzene]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

Figure 4

Effect of reducing agent dosages on benzene degradation in various RA-OA-Fe(II)/SPC systems ([Fe(II)]0 = [OA]0 = [Benzene]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C). Inserted figure: Benzene degradation performance in various RA-OA-Fe(II)/SPC systems with 1 mM reducing agents addition ([RA]0 = [Fe(II)]0 = [OA]0 = [Benzene]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

It can be noted that benzene removal was inhibited to a certain extent with the increase in concentration of HAH, ASC, and SAS to 8 mM. This indicated excessive HAH, ASC, and SAS had a negative effect on benzene degradation. This is due to the scavenging effect of higher RA dosages which can react with HO at a high rate constant, leading to scavange HO. For example, HAH can react with HO at a rate of 5.0 × 108M−1s−1 when solution pH is 4, and large amount of Cl along with HAH addition can also consume HO via a series of propagation reactions (Yu & Barker 2003; Chen et al. 2011). This leads to the decrease of benzene removal to 61.9% at HAH concentration of 8 mM. Compared with HAH, a high concentrations of ASC and SAS have a more significant inhibitory effect on benzene degradation. ASC and SAS can react with HO at a rate of 1.0 × 1010 M−1s−1 and 1.3 × 1010 M−1s−1, respectively, which are significantly higher than the reaction rate between HAH and HO. The reaction products are more likely to result in further consumption of HO (Buxton et al. 1988). Besides, the increase of pH caused by high SAS concentration is also not conducive to the presence of the soluble Fe ion, weakening the catalytic ability of the system, and thus inhibiting benzene removal.

As shown in the inserted Figure of Figure 4, rapid benzene removal occurred in the first 2 min, and minor benzene removal was observed later in the OA-Fe(II)/SPC system. This is due to the rapid generation and consumption of HO in the OA-Fe(II)/SPC system at the start. The continuous generation of HO after 2 min in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems leads to the further degradation of benzene.

To clarify the role of HO and O2•− on benzene removal in RA-OA-Fe(II)/SPC systems at RA concentrations of 1 mM, scavenger tests were conducted in this study. 100 mM isopropanol (ISP) and 50 mM chloroform (CF) were employed as the scavengers of HO and O2•−, respectively. The contribution of HO and O2•− to benzene removal was analyzed by comparing the degradation of benzene in the system with or without scavengers. The results are shown in Figure 5. In OA-Fe(II)/SPC, HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems, 42.4, 73.3, 69.3 and 72.7% decrease of benzene removal were observed with the addition of ISP. This indicated that HO is indeed the dominant free radicals for benzene removal in every system. Even so, 6.5, 15.2, 14.6, and 6.5% of benzene were still degraded by OA-Fe(II)/SPC, HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems, respectively, showing that a non-HO oxidation pathway may exist. The addition of CF to OA-Fe(II)/SPC, HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems, benzene removal decreased to 3, 6.8, 6.7 and 5.2% respectively. This indicated that O2•− participated in the benzene removal, and HAH, ASC, and SAS increased the contribution of O2•− to benzene degradation.

Figure 5

Scavengers effect on benzene degradation in various RA-OA-Fe(II)/SPC systems: (a) without RA, (b) HAH, (c) ASC, (d) SAS ([RA]0 = [Fe(II)]0 = [OA]0 = [Benzene]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

Figure 5

Scavengers effect on benzene degradation in various RA-OA-Fe(II)/SPC systems: (a) without RA, (b) HAH, (c) ASC, (d) SAS ([RA]0 = [Fe(II)]0 = [OA]0 = [Benzene]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

Effects of solution matrix on benzene removal in RA-OA-Fe(II)/SPC systems

In this section, the effects of solution matrix, including SO42−, Cl, NO3, HCO3, initial solution pH and humic acid (HA) on benzene removal in RA-OA-Fe(II)/SPC systems were demonstrated thoroughly. The concentration of RA, benzene, OA, Fe(II), and SPC were fixed at 1, 1, 1, 1, and 2 mM, respectively. SO42−, Cl and NO3 had negligible effects on benzene removal with the tested concentration ranges of 1–100 mM (data not shown). The effects of HCO3, initial solution pH and HA on benzene removal are shown in Figure 6.

Figure 6

Effect of solution matrix conditions on benzene removal in various RA-OA-Fe(II)/SPC systems ([RA]0 = [Fe(II)]0 = [OA] = [Benzene]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

Figure 6

Effect of solution matrix conditions on benzene removal in various RA-OA-Fe(II)/SPC systems ([RA]0 = [Fe(II)]0 = [OA] = [Benzene]0 = 1 mM, [SPC]0 = 2 mM, T = 20 ± 0.5 °C).

Effects of initial solution pH

Figure 6(a) shows the effect of various initial solution pH on benzene removal in RA-OA-Fe(II)/SPC systems to illustrate the applicable pH range of RA-OA-Fe(II)/SPC systems. Compared to the control test, which was not adjusted the initial solution pH, benzene removal slightly increased to 90.9% from 87.0% when adjusted the initial solution pH to 3 in the HAH-OA-Fe(II)/SPC system. In addition, benzene removal of 90.9% was achieved within 10 min when the initial solution pH is 3 (data not shown), much faster than 69.9% in the HAH-OA-Fe(II)/SPC system without adjusting the initial solution pH. This indicates acidic condition benefits to benzene removal in the HAH-OA-Fe(II)/SPC system. This may be due to the increase of catalyst concentration caused by acidic conditions, which can retard the precipitation of the Fe ion. Furthermore, when the pH is lower than 5.96, NH3OH+ is the main form of hydroxylamine, and the reaction rate between NH3OH+and HO is less than 5.0 × 108 M−1s−1. This can reduce the invalid consumption of HO (Buxton et al. 1988; Zou et al. 2013).

Less than 2% inhibition of benzene removal was observed at pH = 7, indicated the applicability of HAH-OA-Fe(II)/SPC system at neutral conditions. However, benzene removal was inhibited when the initial solution pH was adjusted to 8.5 and 10.5, showing more significant effect at higher pH. Benzene removal decreased to 44.4% from 87.0% at pH = 10.5, leading to the inhibition rate of 49% ((87.0 − 44.4)/87.0). High solution pH can promote the precipitation of the Fe ion, reducing the catalytic ability of the system. The higher decomposition of H2O2 to H2O and O2 under high pH conditions can also lessen the generation of HO. Besides, the major form of hydroxylamine under high pH conditions is NH2OH, which can react with HO faster, leading to more invalid consumption of HO (Chen et al. 2011; Zou et al. 2013).

Slight effects of initial solution pH from 3.0 to 7.0 were observed in ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems, indicating these two systems are suitable for acidic and near neutral conditions. But with the increase of pH to 10.5, benzene removal was inhibited significantly and its removal rate decreased to 83.9% and 79.2% from 50.5% and 47.1% in ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems, respectively. This indicated that high pH conditions also adversely affect benzene removal in ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems. Nevertheless, the inhibition rates are 40% ((84.5 − 50.5)/84.5) and 42.5% ((81.9 − 47.1)/81.9) in ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems, respectively, which are significantly lower than 49% in HAH-OA-Fe(II)/SPC system. It is speculated that ASC and SAS can not only reduce Fe(III) to Fe(II), but also act as a chelating agent to maintain Fe ion in a soluble form. This can weaken the effect of the initial pH on Fe ions, thereby lowering the negative effect of pH on benzene removal.

In summary, HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems can be well applied to acidic and neutral conditions for benzene removal, even though strong alkaline conditions can weaken their applicability to a certain extent.

Effects of HCO3

It is reported that HCO3 is adverse to organic contaminants removal in multi-chemical oxidation systems because of the following reasons: (a) the increase of pH with HCO3 addition; (b) the formation of solid precipitates between soluble metal ions and HCO3; (c) scavenging effect on HO (Liang et al. 2006; Gu et al. 2011; Fu et al. 2015). Therefore, effects of HCO3 on benzene removal were studied in RA-OA-Fe(II)/SPC systems at various concentration levels of 1.0, 10 and 100 mM.

As shown in Figure 6(b), 1 mM of HCO3 could inhibit the degradation of benzene in every RA-OA-Fe(II)/SPC system. The removal rate of benzene decreased from 87.0%, 84.5% and 81.9% to 67.7%, 73.9% and 64.8%, with the inhibition rate of 22.2%, 12.5% and 20.9% in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems, respectively. The increase in the concentration of HCO3, a significant inhibition of benzene removal was observed in every RA-OA-Fe(II)/SPC systems. When the HCO3 concentration was increased to 100 mM, the benzene removal rate dropped to 10.6, 17.7 and 11.3%, and the inhibition rate reached to 87.8, 79.0 and 86.2%, respectively. This indicated HCO3 had an adverse effect on RA-OA-Fe(II)/SPC systems for benzene removal, and the adverse effect enhanced with the increasing concentration of HCO3. It is noted that the inhibition rate of HCO3 decreased sequentially in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems with the same concentration of HCO3, indicating that the ability of HAH, SAS and ASC to overcome the adverse effects of HCO3 increased in succession.

Effects of HA

Natural organic matters (NOM) exists ubiquitously in soil and groundwater, which can affect contaminants degradation. NOM was substituted by HA which is a main fraction of NOM in this study. Figure 6(c) shows the effects of HA with different concentrations (0–400 mg/L) on benzene removal in every RA-OA-Fe(II)/SPC system. Low concentrations of HA (0–40 mg/L) had a slight effect on benzene removal in every system, while high concentrations of HA (100 and 400 mg/L) inhibited benzene removal. When the concentration of HA is 400 mg/L, the benzene removal reduced to 64.6, 67.1, and 59.4% in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC, and SAS-OA-Fe(II)/SPC systems, respectively.

The inhibition of HA for benzene degradation in these systems could be explained from the following two aspects. In one hand, the presence of HA had a scavenging effect for HO via Equation (1) and competed with benzene to consume HO (Bogan & Trbovic 2003; Westerhoff et al. 2007; Khan et al. 2016). On the other hand, HA could react with dissolved Fe to generate a stable complex that furtherly inhibited the regeneration of Fe(II), thus decreasing the removal of benzene (Wang & Lemley 2004).
(1)

Benzene removal performance by RA-OA-Fe(II)/SPC systems in actual groundwater

The above results indicate that solution matrix has a significant effect on RA-OA-Fe(II)/SPC systems for benzene removal. In view of the complexity of actual groundwater, benzene degradation in actual groundwater by RA-OA-Fe(II)/SPC systems was investigated to evaluate the applicability of every system in situ remediation process. The concentrations of RA, benzene, Fe(II), SPC and OA were 1, 1, 2, 2, and 4 mM, respectively.

Figure 7 showed that benzene removal rate of OA-Fe(II)/SPC system in ultrapure water and actual groundwater were 83.6% and 44.2%, respectively. This indicates actual groundwater significantly affects benzene removal in the OA-Fe(II)/SPC system. Compared to 90.4, 89.4, and 90.0% benzene removal of HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC system in ultrapure water, benzene removal rate decreased to 73.0, 64.0, and 63.2% in actual groundwater, respectively. This indicated the actual groundwater had adverse effects on RA-OA-Fe(II)/SPC systems for benzene removal. However, benzene removal of every RA-OA-Fe(II)/SPC system is still higher than 44.2% in OA-Fe(II)/SPC system, indicating the addition of HAH, ASC and SAS benefits to benzene removal in actual groundwater. The highest benzene removal in HAH-OA-Fe(II)/SPC system showed HAH was the optimal reducing agents for the enhancement of OA-Fe(II)/SPC system.

Figure 7

Benzene degradation performance in actual groundwater by various RA-OA-Fe(II)/SPC systems ([RA]0 = [Benzene]0 = 1 mM, [Fe(II)]0 = [SPC]0 = 2 mM, [OA]0 = 4 mM, T = 20 ± 0.5 °C).

Figure 7

Benzene degradation performance in actual groundwater by various RA-OA-Fe(II)/SPC systems ([RA]0 = [Benzene]0 = 1 mM, [Fe(II)]0 = [SPC]0 = 2 mM, [OA]0 = 4 mM, T = 20 ± 0.5 °C).

In this study, the application of OA-Fe(II)/SPC systems enhanced by RA for benzene removal in groundwater was investigated. The addition of HAH, ASC and SAS could enhance the generation of HO and O2•−. The enhanced effect of HAH on HO generation is superior to that of ASC and SAS, while ASC and SAS showed more significant promotion of O2•− generation than HAH. Benzene removal rate increased with suitable addition of HAH, ASC and SAS, while excessive addition of HAH, ASC and SAS was harmful to further improvement of benzene removal. SS was not conducive to HO and O2•− generation leads to inhibition of benzene degradation. Scavenging studies showed that HO were the principal radicals for benzene removal in HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems. Meanwhile, the addition of HAH, ASC and SAS increased the role of O2•− in benzene removal. The effects of SO42−, NO3, Cl and low concentration of HA on benzene removal were negligible. The effective benzene removal by HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems were observed in acidic and neutral conditions. On the other hand, the alkaline conditions and the presence of HCO3 and high concentration of HA were not beneficial to benzene removal in these systems. Furthermore, effective benzene removal by HAH-OA-Fe(II)/SPC, ASC-OA-Fe(II)/SPC and SAS-OA-Fe(II)/SPC systems was obtained in actual groundwater, indicating HAH, ASC and SAS enhanced OA-Fe(II)/SPC systems are applicable for benzene-contaminated groundwater remediation.

This study was financially supported by the grant from the National Key R&D Program of China (No. 2018YFC1803304).

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

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Supplementary data