Yu-Chen Changa,
Ting-Yu Chenb,
Yung-Pin Tsaia and
Ku-Fan Chen*a
aDepartment of Civil Engineering, National Chi Nan University, 1 Univ. Rd., Puli, Nantou 54561, Taiwan. E-mail: kfchen@ncnu.edu.tw; Tel: +886-49-2910960 ext. 4983
bDepartment of Landscape Architecture, National Chin-Yi University of Technology, Taiping, Taichung 41170, Taiwan
First published on 10th January 2018
This study uses a trichloroethene (TCE)-contaminated site to determine the efficacy of persulfate oxidation for the treatment of TCE-contaminated groundwater. The main objectives of this study are: (1) to evaluate the efficacy of TCE treatment using persulfate with different injection strategies; (2) to determine the persistence of persulfate in the aquifer; (3) to determine the radius of influence (ROI) and transport distance of persulfate and (4) to determine the impact of persulfate on indigenous microorganisms during remediation. TCE concentrations are 0.26 mg L−1 in P143 and 0.361 mg L−1 in P146 and the microbial numbers are 6.1 × 103 CFU mL−1 in P143 and 4.4 × 104 CFU mL−1 in P146, before persulfate is injected. The results of the pilot study show that persulfate eliminates TCE. 100% of TCE is removed in P146 and 95% in P143. Single injection of a total amount of 275 kg of 5% persulfate produces better TCE removal than two half persulfate injections in sequence. The transport distance of persulfate ranges from 3.6 to 4.5 m. Persulfate also persists for 14 days in the aquifer. After persulfate is injected, the total bacterial counts decrease slightly to 2.4 × 103 CFU mL−1 in P143 and 1.8 × 103 CFU mL−1 in P146. When persulfate is consumed, the total bacterial counts increase but there is no recovery of the microbial community. The results show that sequential injections of a large amount of persulfate are suggested to maintain good long-term performance for TCE treatment.
In situ chemical oxidation (ISCO) is a widely used technology for the remediation of contaminated soil and groundwater. Conventional ISCO oxidants that are used for the treatment of contaminated sites include hydrogen peroxide (H2O2) solutions, such as Fenton's reagent and Fenton-like reaction, ozone (O3), and permanganate (MnO4−).4 Recently, persulfate has been applied to remediate soil and groundwater contamination because of its high redox potential (reaction (1)).
S2O82− + 2e− → 2SO42−, E0 = 2.01 V | (1) |
Persulfate is activated by activators such as heat or ferrous ions to generate powerful sulfate free radicals (SO4−˙) (reaction (2)):1,5
S2O82− + activators → 2SO4−˙ or SO4−˙ + SO42−, E0 = 2.7 V | (2) |
Persulfate oxidation is used to degrade various contaminants, such as chlorinated organic compounds and petroleum hydrocarbons.6,7 Because persulfate is a stable oxidant that persists in the subsurface for a long time, it is a useful oxidant for soil and groundwater remediation.1,8,9 Many studies of persulfate oxidation focus on the development of activation methods,2,10–12 the feasibility of contaminant removal,5,6 the effect of pH on pollutant degradation13–15 and the mechanism and kinetics of contaminant removal.12,16,17 Studies involving the application of persulfate are fewer. Although the results of some field studies and site remediation that use persulfate oxidation have been published, these reports mainly focus on the efficiency with which contaminants are removed.18 However, other practical parameters and information, such as injection strategies, the radius of influence (ROI), the transport distance, the persistence of persulfate, persulfate dosage and the damage that persulfate does to indigenous microorganisms are not widely studied in the field.
This study uses a TCE-contaminated site for a field-scale study, to collect practical information about in situ persulfate oxidation. The main objectives of this study are to determine: (1) the efficiency of TCE treatment using persulfate oxidation; (2) the persistence of persulfate in the aquifer; (3) the ROI and transport distance of persulfate and (4) the impact of persulfate on indigenous microorganisms during remediation.
Item | Measured value |
---|---|
pH | 8.42 |
ORP (mV) | 188 |
Total iron (mg kg−1) | 40![]() |
Soil organic matter (%) | 2.52 |
Total bacterial count (CFU g−1 soil) | 7.75 × 104 |
Texture classification | Loamy sand |
Sand (%) | 85 |
Silt (%) | 12 |
Clay (%) | 3 |
A plastic manufacturing factory in the industrial park with an area of 8750 m2 was selected for the field-scale study. An estimated TCE plume of approximately 5200 m2 was present in the subsurface. An area of 16 m × 9 m was selected for the pilot study. Seven 1-inch wells were installed at this site. Fig. 1 shows the site map, showing the groundwater flow direction, two injection wells (P143 and P146), a background well (P147) and four monitoring wells (P142, P144, P145, and TW38) at the test site. All wells were approximately 10 m in depth, with screen intervals from 6 to 10 m bgs.
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Fig. 1 Site map showing the groundwater flow direction, the injection wells, the background well and other monitor wells at the test site (●: injection well; ○: monitor well; ◇: background well). |
a Injection well.b Monitoring well.c Background well.d Not detectable.e Total bacterial count. | |||||||
---|---|---|---|---|---|---|---|
Wells | P142 | P143 | P144 | P145 | P146 | P147 | TW-38 |
Well function | MWb | IWa | MWb | MWb | IWa | BWc | MWb |
TCE (mg L−1) | 0.151 | 0.264 | 0.245 | 0.384 | 0.361 | 0.245 | 0.040 |
1,1-DCE (mg L−1) | 0.008 | 0.011 | 0.007 | 0.013 | 0.017 | 0.006 | 0.016 |
cis-DCE (mg L−1) | 0.003 | 0.006 | 0.003 | 0.006 | NDd | NDd | 0.008 |
trans-DCE (mg L−1) | NDd | NDd | NDd | NDd | NDd | NDd | NDd |
VC | NDd | NDd | NDd | NDd | NDd | NDd | NDd |
Temperature (°C) | 31.3 | 30.4 | 31.2 | 31.6 | 30.1 | 30.4 | 30.5 |
pH | 6.70 | 7.06 | 7.07 | 7.22 | 7.22 | 7.12 | 7.00 |
ORP (mV) | 275.2 | 250.2 | 321.3 | 265.3 | 261 | 275.1 | 115 |
DO (mg L−1) | 2.2 | 1.93 | 1.44 | 1.35 | 1.35 | 1.28 | 1.38 |
Total iron (mg L−1) | 26.5 | 28.9 | 23.2 | 16.5 | 34 | 18.5 | 64.3 |
Ferrous iron (mg L−1) | 0.220 | 0.355 | 0.065 | 0.345 | 0.050 | 0.105 | 1.605 |
CO2 (mg L−1) | 250 | 140 | 172 | 76 | 52 | 32 | 183 |
Alk (mg L−1 as CaCO3) | 341 | 294 | 268 | 312 | 312 | 293 | 315 |
TBCe (CFU mL−1) | 2.8 × 103 | 6.1 × 103 | 3.13 × 105 | 8.9 × 104 | 4.4 × 104 | 5.1 × 104 | 3.6 × 104 |
In a persulfate system, pH is changeable due to the generation of bisulfate (HSO4−) during the decomposition of persulfate.24 Decreases of 2.5 to 6.0 pH units caused by persulfate have been observed in aqueous and soil slurry systems.1 In general, pH plays an important role in affecting the performance for the degradation of contaminants in persulfate systems. Hussain et al.24 found that acid conditions (pH 2.0 and pH 4.0) are favorable to the degradation of p-chloroaniline (PCA) in a zero-valent iron-activated persulfate system. Additionally, PCA degradation at pH 7 is higher than that at pH 9 and pH 11. Liang et al.14 indicated that maximum TCE degradation occurs at pH 7 in persulfate systems, which are activated by ambient temperatures (10 to 30 °C). It has also been demonstrated that high pH conditions may cause a decrease in the oxidation of contaminants.15,24 Therefore, acid and neutral conditions seem to be advantageous to the degradation of pollutants in persulfate systems according to the above reports. In this study, pH decreases to approximately 5.80 after persulfate is injected and then pH in the injection wells remains near neutral because of the buffering capacity of groundwater. The results show that pH conditions at this site are favorable to the degradation of TCE during the field study.
Liang et al.14 and Fang et al.25 noted that sulfate radicals are produced at ambient temperature (10–30 °C) by thermal activation, which are predominant under acid and neutral conditions. In this study, the temperature and pH in groundwater are between 30.4 to 31.6 °C and 6.70 to 7.22, respectively (Table 2). pH slightly decreases to around 5.80 after persulfate is injected to the wells and then increases to above 6.00. This indicates that sulfate radicals are dominant, which contribute the removal of TCE during the field test. Waldemer et al.26 also reported that the rate-determining step for chlorinated ethane oxidation by heat activated persulfate involved the reaction with sulfate radicals but not the formation of sulfate radicals. In addition, sulfate radicals can react with water to produce hydroxyl radicals under all pH conditions, as shown in reaction (3).26 Thus, hydroxyl radicals may also be responsible for the degradation of TCE in groundwater.
SO4˙− + H2O → SO42− + ˙OH + H+ | (3) |
Although it has been reported that persulfate might be activated by native soil minerals, such as iron to enhance contaminant degradation rates in a slurry system,27 more recent studies indicated that major subsurface minerals do not activate persulfate.28,29 Ahmad et al.28 found that synthetic iron and manganese oxides are able to activate persulfate. However, the masses of iron and manganese oxides typically found in natural soil are not sufficient to activate persulfate. TCE oxidation by sulfate free radicals can be simply described as reaction (4):14
SO4˙− + C2HCl3 + 4H2O → 2CO2 + 9H+ + 3Cl− + 6SO42− | (4) |
Yan et al.30 indicated that hydroxyl radicals converted TCE to dichloroacetic acid via the electrophilic addition reaction of TCE:
˙OH + C2HCl3 → CCl2CHOOH + Cl− | (5) |
According to the above discussion, sulfate radicals produced by thermal activation and hydroxyl radicals derived from sulfate radicals play a major role in the destruction of TCE during the field test.
Oxidation intermediates of TCE are not identified during remediation. Although the issue of determining the pathway of TCE degradation by persulfate has been mentioned, the information regarding intermediates of TCE oxidation by persulfate is limited.14 Yan et al.30 recently reported that formic acid and dichloroacetic acid were detected during TCE oxidation by siderite-catalyzed hydrogen peroxide coupled with persulfate. Therefore, more studies on this issue are needed in the future.
The injection of persulfate into the groundwater may reduce the concentrations of pollutants via dilution.4,11 It should be noted that CO2 concentrations increase from the background level of 140 mg L−1 to 835 mg L−1 in P143 and from 52 mg L−1 to 584 mg L−1 in P146 after 2 days of persulfate injection. Although it is difficult to distinguish the effect of oxidation and dilution on the decrease in TCE concentrations by persulfate, the increase in CO2 concentrations in the injection wells provide strong evidence that an extent of TCE and aquifer organics are certainly mineralized by the injected persulfate. Additionally, the TCE that is adsorbed onto the sediment may be a source for the continuous release of TCE to the groundwater. The injected persulfate treats the soluble TCE in the groundwater and degrades the adsorbed TCE in the aquifer sediment. Contaminants in soil and groundwater are effectively removed by persulfate,11,30 so although TCE in groundwater may be diluted by the injected persulfate, persulfate still contributes to the removal of TCE in both aqueous and soil-sorbed phases. Groundwater flow may also affect the elimination of TCE. Although TCE in the pilot study area may be removed by groundwater flow, TCE is transported to the treatment zone from the upgradient area continuously. Since oxidation also contributes TCE degradation, it is difficult to determine the contribution of hydraulic dynamics to the removal of TCE at this site.
The single injection process maintains the concentration of TCE in P146 at less than 0.05 mg L−1 for 14 days. However, the TCE concentration increases to 0.138 mg L−1 on day 28. The TCE concentration in P143 is reduced only for 5 days and then rapidly increases to 0.153 mg L−1 on day 7. These results show that the injection of a large amount of persulfate results in more stable TCE remediation. When the second injection is applied to P143 on day 28, TCE concentration decreases immediately and then increases to more than the groundwater control standard after 7 days of reaction. The TCE concentrations in P144 and P145, the downgradient monitoring wells of P146, decrease significantly at day 2 and then increase again at day 7 (Fig. 2(b)). Groundwater was not sampled in P144 after day 28 because a bailer was accidently dropped into the well, causing a blockage of the well. In TW-38, a decrease in TCE concentrations is observed after 2 days and then the concentrations of TCE increase with time. The decrease in the TCE concentrations in P144, P145, and TW-38 may be due to persulfate seepage from P146 (Fig. 3(a)). Although there is a slight decrease in the TCE concentrations in P142, which is the downgradient monitoring well of P143, during the first 2 days of persulfate injection, the variation may be caused by the heterogeneity of the subsurface, because no persulfate is detected in P142. The TCE concentration in the background well P147 is stable during the field study.
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Fig. 3 (a) The variation in persulfate concentrations in P143 to P146 and TW-38 and (b) the distance between each well. |
Fig. 3(a) shows that on day 14, persulfate concentration in P146 decreases from an initial value of about 46000 to 3500 mg L−1 and the TCE concentration is still less than 0.05 mg L−1 (Fig. 2(a)). This indicates that the presence of persulfate inhibits any further increase in TCE. Similar results are also observed for P143 on day 5. Do et al.31 reported that sequential injections of peroxymonosulfate (PMS)/Co(II) improves the degradation of diesel in soils. When the concentration of PMS increases, fewer injections are needed to achieve remediation. The results of this pilot study suggest that sequential injections of a large amount of persulfate allow good long-term TCE treatment. The TCE concentrations increase to approximately 0.300 mg L−1 in P146 and 0.200 mg L−1 in P143 at the end of the experiment (Fig. 2(a)), probably because TCE is transported from the upgradient areas.
Since no persulfate is detected in other monitoring wells after all persulfate is injected to P143 and P146, the ROI for persulfate injection is not determined in this study. Therefore, more monitoring wells closer to the injection wells are needed in the future to obtain the ROI for persulfate delivery. Persulfate is detected in P144 (464 mg L−1), P145 (1292 mg L−1) and TW-38 (98 mg L−1) after 5 days of operation. The distance between each well at this site (Fig. 3(b)) gives an estimated value for the transport distance of persulfate of 3.6 to 4.5 m. The transport distance of persulfate depends on the rate of decomposition and the velocity. Tsitonaki et al.18 found that transport distances of persulfate ranged from 0.1 to 23.0 m with persulfate decomposition rates between 0.01 to 2.39 1 per day at a seepage velocity of 0.1 m per day. However, the values are based on very limited data. In this study, the transport distance of persulfate was 3.6 to 4.5 m at a groundwater velocity of 0.5 m per day. The transport distance of persulfate obtained from this field test provides additional practical information for in situ persulfate oxidation. The results for pH monitoring show that the groundwater in each injection well remains near neutral pH during the field test (Fig. 4). Therefore, the presence of bicarbonate does not significantly elevate the concentration of CO2.
Oxidants can change the community and the predominant species of in situ microorganisms, so DGGE analysis was used to monitor the variation in the major microbes during persulfate oxidation. Fig. 6 shows the DGGE profiles for the PCR-amplified 16S rDNA during the pilot-scale study. The green bars signify bacteria that appear in the background before persulfate injection. The yellow bars signify bacteria that do not appear in the background. The red bars signify bacteria that match the bacteria in the background. In well P146, the microbial community changes significantly and the microbial diversity decreases after persulfate is injected. These results are in agreement with the results of other studies. Richardson et al.32 reported that microbial diversity decreases after exposure to persulfate, for a column study. The recovery of specific contaminant degraders also takes longer. Chen et al.9 showed that persulfate causes more significant destruction in a microbial community than hydrogen peroxide or permanganate. It should be noted that although the microbial diversity for this study is still low on day 28 (Fig. (6)), the number of bacteria in P146 recovers (Fig. 5). The results show that only some species of microbes that are resistant to persulfate contribute to the total number of bacteria. In P144, the downgradient well of P146, the microbial diversity also decreases during 5 days of operation. However, the diversity of the microbial community recovers at the end of the test. During the pilot study, low concentrations of persulfate (464 mg L−1) are observed in P144 on Day 5 because there is transport from P146. Although low concentrations of persulfate also have an adverse effect on the microbial community, the damage to the indigenous microbes is not significant.
(1) TCE is removed effectively by the injection of 5% persulfate.
(2) Persulfate persists for 14 days in the aquifer. The transport distances for persulfate injection at this site range from 3.6 to 4.5 m.
(3) It is necessary to monitor persulfate concentrations to ensure effective removal of the contaminant.
(4) Although the numbers of total bacterial counts recover after persulfate is depleted, there is no recovery of the microbial community in the injection well P146. Therefore, only some species of microbes that are resistant to persulfate contribute to the total number of bacteria.
(5) Sequential injections of a large amount of persulfate are suggested to give good long-term performance for TCE treatment.
The results of this study will provide useful information for the application of persulfate in the field. More injection protocols and down-gradient wells could be used for this site in the future, to evaluate the effect of the injection processes and ROI for persulfate injection in more detail.
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