Journal of Hazardous Materials 271 (2014) 202–209
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Degradation of dimethyl phthalate in solutions and soil slurries by persulfate at ambient temperature Zhen Wang, Dayi Deng ∗ , Liling Yang School of Chemistry and Environment, South China Normal University, Guangzhou, Guangdong 510006, China
h i g h l i g h t s • • • •
Persulfate alone can efficiently degrade and mineralize dimethyl phthalate at ambient temperature. Effects of temperature, initial persulfate and DMP concentration, and initial pH were studied. Persulfate at 40 ◦ C is very effective for the remediation of DMP contaminated soil. The degradation intermediates of DMP were characterized and degradation pathways were proposed.
a r t i c l e
i n f o
Article history: Received 12 September 2013 Received in revised form 8 February 2014 Accepted 11 February 2014 Available online 28 February 2014 Keywords: Persulfate Dimethyl phthalate Ambient temperature Soil and groundwater remediation Degradation mechanism
a b s t r a c t The degradation of dimethyl phthalate (DMP) by persulfate at ambient temperature (T = 20–40 ◦ C) was investigated in aqueous solutions and soil slurries to assess the feasibility of using persulfate to remediate DMP contaminated soil and groundwater. First, the effects of temperature, initial oxidant concentration, initial DMP concentration and initial solution pH on the removal of DMP and TOC were studied in aqueous solutions. The results show that persulfate at 40 ◦ C can effectively mineralize DMP. Furthermore, dimethyl 4-hydroxyl phthalate, maleic acid and oxalic acid were identified as the degradation intermediates, and degradation pathways were proposed. Lastly, persulfate at 40 ◦ C was applied to remediate soil spiked with DMP at ∼600 mg/kg. The results show that persulfate at 40 ◦ C is highly effective for the remediation of DMP contaminated soil. Overall, this study provides fundamental and practical knowledge for the treatment of emerging phthalate esters (PAEs) contaminated soil and groundwater, as well as PAEs contaminated industrial wastewater, with persulfate at ambient temperature. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The contamination of phthalate esters (PAEs, Fig. 1) in groundwater and soil [1–3] is an emerging issue that will need to be addressed in the near future. Even though current public concern about PAEs focuses on the health impact of PAEs released from plastic consumer products, especially baby accessories in recent years, some recent reports have shown that soils near contaminated industrial sites, some E-waste recycle centers and landfills have high levels of PAEs contamination [4]. For example, >300 ppm dimethyl phthalate (DMP) has been found in chemical leachate [5], and PAEs contamination levels of several thousand ppm have been found near plastic manufactories [4]. These highly contaminated sites are serious point sources of PAEs, contaminating groundwater
∗ Corresponding author. Tel.: +86 20 39310213; fax: +86 20 39310213. E-mail address: [email protected] (D. Deng). http://dx.doi.org/10.1016/j.jhazmat.2014.02.027 0304-3894/© 2014 Elsevier B.V. All rights reserved.
[6,7], transferring into food chain and posing high exposure-risk [8] to the public. As PAEs are potential endocrine disrupters, environmental carcinogens, teratogens and mutagens [9–13], their existence in the environment has raised great concerns. For example, US EPA has listed six PAEs as Priority Pollutants [14]. Due to the low hydrolysis and biodegrading rate of PAEs in natural environment, especially in the subsurface [15], it is necessary to develop appropriate technologies for the remediation of PAEs contaminated soil and groundwater. Advanced oxidation methods, such as Fenton oxidation [16], photocatalytic oxidation [17], UV photolysis [18] and ozone oxidation [19], have been extensively tested for the destruction of PAEs from wastewaters in recent years. Even though these processes are effective for wastewater treatment, their potential application for soil and groundwater poses challenges. First, photocatalytic oxidation and UV photolysis are normally not applicable to in situ soil and groundwater remediation. While Fenton reagent has been used for in situ chemical oxidation of organic contaminants in soil
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identified and potential degradation pathways were then proposed.
O
Heat
OR OR'
O Fig. 1. Chemical structures of phthalate esters.
and groundwater, hydrogen peroxide suffers from rapid decomposition in subsurface, resulting in the generation of large amount of heat and gas, causing concerns on a series of issues, including safety, potential escape of VOCs, relatively short radius of influence and low remediation efficiency, etc. [20]. On the other hand, the limited aqueous solubility and inefficient dispersion of ozone gas in heterogeneous subsurface environment, as well as the requirement of on-site generation systems, also hinder the application of ozone remediation of organic contaminated soil and groundwater [21]. For the remediation of organic contaminated soil and groundwater, sodium persulfate is a newer but increasingly used oxidant for in situ chemical oxidation [22]. Persulfate is a stable oxidant under normal conditions. Therefore, it is relatively easy and safe to handle and transport to the field. Once on the site, it can be conveniently applied by mixing with soil or injecting to subsurface at high concentrations, as it is highly soluble in water. Normally, it tends to have a much longer life-time in the subsurface than hydrogen peroxide as well as relative larger radius of influence [22]. Through activation by heat, ferrous salt, Fe(III)-EDTA, NaOH, etc., reactive radical species, including sulfate radicals (E0 = 2.6 V) and hydroxyl radicals (E0 = 2.7 V), are generated and have been found to oxidize a variety of recalcitrant compounds, including BTEX, PAHs, PCBs, OCPs, etc. [22]. To the best of our knowledge, the interaction between persulfate and PAEs has been rarely studied. There are only a couple of reports, in which He and He [23] focused on UV-coupled persulfate treatment of dimethyl phthalate (DMP) contaminated wastewater and Sun et al. [24] studied microwave assisted Ag+ catalyzed persulfate oxidation of DMP in aqueous solution. However, it is difficult to apply either UV or microwave in the soil environment. Furthermore, the DMP degradation pathways were not characterized in these two studies. Therefore, it is crucial to both assess the feasibility of using persulfate to remediate PAEs contaminated soil and groundwater and elucidate the PAEs degradation pathways. Considering that DMP is one of the six PAEs on the US EPA Priority Pollutant list and that it tends to migrate with groundwater due to its high aqueous solubility, DMP is chosen as the model compound in the current study. Heat-activation is a frequently used field activation method for persulfate to generate sulfate radicals (reaction (1)) to oxidize organic contaminants. To reduce the high cost required to heat large amount of soil to the desired temperature and to resolve the potential issue of returning soil temperature back to the original one, an activation temperature around 40 ◦ C is commonly applied in the field [21]. Therefore, the current project first studied the effect of temperature on the oxidation of DMP in aqueous solutions at 20, 30, and 40 ◦ C, respectively. Furthermore, the effects of oxidant dosage, initial concentration of DMP, and initial solution pH on the removal of DMP and the mineralization of TOC were investigated. Degradation intermediates were
−• S2 O2− 8 −→2SO4
(1) 40 ◦ C
After establishing that persulfate at can effectively mineralize DMP in aqueous solutions, the feasibility of using persulfate at 40 ◦ C to remediate DMP contaminated soil and groundwater was further investigated with soil containing DMP at ∼600 mg/kg, specifically assessing the potential of persulfate for the remediation of highly contaminated sites. The current study focused on the remediation of hot spots (source zones) because persulfate has been predominantly applied to treat highly contaminated (source zone) spots in hazardous sites and not commonly applied in the groundwater plume extending from the source zone, due to cost-effective considerations [21]. Overall, this study provides fundamental and practical knowledge for the treatment of emerging PAEs contaminated soil and groundwater, as well as PAEs contaminated industrial wastewater, with persulfate at ambient temperature. 2. Materials and methods 2.1. Materials Dimethyl phthalate (99%), benzyl benzoate (98%), sodium persulfate (98%), ceric sulfate (99.9%), ammonium iron sulfate hexahydrate (99.99%), anhydrous sodium sulfate (99.99%), anhydrous sodium sulfite (98%), sulfuric acid (98%) and sodium hydroxide (98%) were all analytical grade and used as received. Acetone and hexane were HPLC grade. N,Obis(trimethylsilyl)trifluoroacetamide/trimethylchlorosilane (BSTFA-TMCS; v/v, 99/1) from Regis Technologies, Inc., USA, was employed to derivatize potential DMP intermediates for GC–MS analysis. Milli-Q ultra pure water was used to prepare all aqueous solutions. 2.2. Persulfate oxidation of DMP in aqueous solutions Aqueous experiments were conducted in 1000-mL conical glass bottles with ground glass stoppers. Freshly prepared DMP stock solutions were diluted to the desired concentrations by the addition of ultra pure water and maintained at desired temperatures in temperature control incubators. Shortly afterwards, experiments were carried out by adding pre-determined amounts of sodium persulfate to DMP solutions and mixed rigorously until totally dissolved. NaOH or H2 SO4 solution (1 mol/L) was used for initial pH adjustment when studying the effect of initial pH on the degradation of DMP. Each time, 1000-mL solution was prepared, capped and placed into a temperature control incubator maintained at constant temperature (T = 20, 30, and 40 ◦ C, respectively) and kept in the dark without shaking. At pre-specified time intervals, 20 mL samples were retrieved from the bottles for the analysis of persulfate concentration, DMP concentration, TOC and pH, as well as the identification of potential degradation intermediates. All experiments were conducted in duplicates. Blank control experiments without the addition of oxidants were run at 40 ◦ C to monitor potential loss of DMP due to evaporation. 5 mL aqueous solutions were extracted by 5 mL hexane and analyzed by GC/FID for DMP with benzyl benzoate as the internal standard according to procedures in Section 2.4. The pHs were measured using a pH meter (Model PHSJ-3F from Shanghai INESA Scientific Instrument Co., Ltd). TOC analyses were carried out using a Shimadzu VCPN model carbon analyzer. Residual persulfate in the aqueous solution was quantified by reaction with ammonium iron sulfate and then back-titrate with ceric sulfate using an automatic
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potentiometric titrator (Model ZD-2, Shanghai INESA Scientific Instrument Co., Ltd) according to literature procedure [25]. 2.3. Persulfate oxidation of DMP in spiked soils The soil samples used were typical lateritic red soils obtained from Guangzhou, China at a depth of 20–40 cm, which contained approximately 66% well graded sand (61% DMP remained after 48 h with an initial persulfate concentration of 1.03 × 10−3 mol/L. The results show a pseudo-firs-order rate constant (kobs ) of 0.0159 h−1 (R2 = 0.99) for an initial persulfate concentration of 1.03 × 10−3 mol/L, 0.0495 h−1 (R2 = 0.98) at 5.15 × 10−3 mol/L, and 0.103 h−1 (R2 = 0.95) at 1.03 × 10−2 mol/L. The results imply that as initial concentrations of persulfate rose, more sulfate radicals were generated, resulting in more rapid removal of DMP. However, the increment of initial DMP removal rate was not proportional to the increase of persulfate concentration, possibly due to competition by organic intermediates generated during the oxidation of DMP and potential quenching
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Fig. 4. Effect of initial DMP concentrations on DMP (a) and TOC (b) removal by persulfate at 40 ◦ C ([S2 O8 2− ]0 = 5.15 × 10−3 mol/L).
of sulfate radicals by residual persulfate [29] and sulfate radicals themselves [30]. Fig. 3b indicates that a more rapid and more efficient mineralization of DMP was observed upon increasing the initial persulfate concentration. With an initial persulfate concentration of 1.03 × 10−2 mol/L, complete mineralization of DMP was achieved after 42 h, while there was only minor removal of TOC with persulfate at 1.03 × 10−3 mol/L at 48 h. 3.3. Effect of initial concentration of DMP The effect of initial DMP concentration on the removal of DMP and the abatement of TOC was investigated at various initial DMP concentrations (5.15 × 10−5 , 1.03 × 10−4 and 2.06 × 10−4 mol/L, equal to 10, 20, and 40 mg/L, respectively) at a persulfate dosage of 5.15 × 10−3 mol/L. Fig. 4a indicates that as DMP concentration increased, the removal rate dropped accordingly. The data were fitted with pseudo-first-order kinetics, and the rate constants (kobs ) were 0.0495 h−1 (R2 = 0.98) with an initial DMP concentration of 5.15 × 10−5 mol/L, 0.0201 h−1 (R2 = 0.97) at 1.03 × 10−4 mol/L and 0.0105 h−1 (R2 = 0.99) at 2.06 × 10−4 mol/L. Furthermore, increasing the initial DMP concentration had a negative impact on the abatement of TOC. Fig. 4b indicates that during the 48 h period study, the abatement of TOC was minor for DMP at 2.06 × 10−4 mol/L, even though ∼30% of DMP was oxidized by persulfate already, and the result reconfirmed the build-up of organic intermediates during the remediation process. 3.4. Effect of initial solution pH The solution pH plays an important role in the oxidation of organic contaminants by activated persulfate. Considering that it is hard to keep the solution pH at a fixed value in actual field applications, the removal of DMP and abatement of TOC by persulfate at 40 ◦ C were studied at various initial pHs (a pH
Fig. 5. Effect of initial solution pH on DMP (a) and TOC (b) removal by persulfate at 40 ◦ C ([DMP]0 = 5.15 × 10−5 mol/L; [S2 O8 2− ]0 = 5.15 × 10−3 mol/L).
of ∼3.1, ∼7.0 and ∼9.0, respectively; [DMP]0 = 5.15 × 10−5 mol/L; [S2 O8 2− ]0 = 5.15 × 10−3 mol/L). During the experiments, the pHs of solutions dropped as the reaction progressed and all dropped to lower than 3.0 after 48 h, due to protons released from persulfate decomposition. Fig. 5 displays the changes in DMP concentration and TOC versus time under different initial pHs. Fig. 5a indicates that the degradation patterns of DMP were very similar under all three conditions, with complete removal of DMP after 36 h. Even though the solution pHs were continuously changing during the whole treatment period, DMP removal data fitted the pseudo-first-order kinetics very well under all conditions, and showed very similar rate constants (kobs ), with 0.0495 h−1 (R2 = 0.98) at an initial pH of ∼3.1, 0.0409 h−1 (R2 = 0.98) at ∼7.0 and 0.0441 h−1 (R2 = 0.97) at ∼9.0. The results indicate that initial pHs had minor effect on the removal of DMP in the pH range studied. Furthermore, Fig. 5b also indicates that initial solution pHs had negligible effects on the abatement of TOC, and the abatement patterns of TOC were very similar under all conditions. The effect of initial pH on the removal of DMP and TOC by persulfate at ambient temperature appears to be consistent with the observation of Huling et al. [31] that persulfate thermolysis is not affected by pH differences (pH between 3 and 12) and that it takes rather extreme pHs (12) in order to impact the persulfate activation significantly. 3.5. Degradation intermediates and degradation mechanism GC/MS analysis of hexane solution of the freeze-dried samples revealed dimethyl 4-hydroxyl phthalate as one main intermediate during persulfate oxidation of DMP. The identification of dimethyl 4-hydroxyl phthalate was performed by searching the NIST library with Agilent MassHunter Workstation software and comparing the mass spectrum observed in the current study with those reported in previous literature [16]. Another two intermediates, maleic acid and oxalic acid, were identified
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O
207
O
O
OCH3
SO4-. SO 24
OCH3
+ .
OCH3
H2O H+
OCH3
OCH3 . OCH3
HO H
O
O
O
oxidation O
O O
O
OCH3
OH
CO2
OCH3
OH HO
HO
OH
O
O Fig. 6. Potential DMP degradation pathways.
solution to totally eliminate the toxicity and hazardousness of PAES in industrial wastewater, contaminated soil and groundwater. 3.6. Oxidation of DMP in slurries Results from aqueous solutions indicate that persulfate at 40 ◦ C is effective for the mineralization of DMP. On the other hand, oxidation of organic contaminants in soil and groundwater is much more complicated than in aqueous solutions. The adsorption of organic contaminants, consumption of oxidants by natural organic matter and other reducible species, and limited mass transfer in the heterogeneous system, etc., are all important factors that impact the feasibility of applying persulfate for the remediation of PAEs contaminated soil and groundwater. Therefore, oxidation of DMP in spiked soil was carried out to further assess the feasibility of using persulfate for the remediation of DMP contaminated soil and groundwater. DMP profile (Fig. 7) of the blank control shows that DMP preferred to partition into the aqueous phase, with a ratio of aqueous concentrations to concentrations in soil of ∼3.8. Similar observation was also documented by Hunter and Uchrin [41]. The high aqueous solubility of DMP and its relatively small kow (log kow = 1.6), as well as the low carbon content of the studied soil, may contribute 120 0g/L
Soil Water
2d
4d
3.68 g/L
7d
18.4g g/L 36.80 g/L
0
18.4g g/L 36.80 g/L
20
18.4g g/L
36.80 g/L
40
36.80 g/L
60
18.4g g/L
80
0g/L
0g/L 3.68 g/L
3.68 g/L
0g/L 3.68 g/L
100
[DMP]/[DMP0] (%)
after the aqueous samples were freeze–dried and derivatized by BSTFA-TMCS and analyzed by GC/MS according to literature [26]. The degradation pathways of DMP were proposed based on the intermediate identification and previous mechanistic studies on the interaction of sulfate radicals with aromatic rings. Liang and Su [32] reported that sulfate radicals are the primary reactive species responsible for the destruction of organic contaminants in the heatactivated persulfate system. Furthermore, previous study indicates that sulfate radicals tend to react with aromatic rings through a oneelectron transfer process to generate radical cations [33], contrary to the direct addition to aromatic rings by hydroxyl radicals [34]. Thereafter, the radical cations generated would undergo hydration by water and further oxidation to generate phenolic product [35]. Accordingly, dimethyl 4-hydroxyl phthalate was proposed to be generated through the generation of a radical cation intermediate outlined in Fig. 6, and further oxidation would cleave the aromatic ring and yield maleic acid and oxalic acid as secondary intermediates, which would finally undergo complete mineralization to carbon dioxide. Another potential phenolic product, dimethyl 3-hydroxyl phthalate, was not observed in the current study. Potentially, the steric hindrance from the carboxyl groups would hinder the formation of dimethyl 3-hydroxyl phthalate and favor the formation of dimethyl 4-hydroxyl phthalate. On the contrary, both phenolic products had been observed in the Fenton oxidation of DMP through the direct addition of hydroxyl radicals to the aromatic rings [16], while hydroxyl radicals are normally considered more reactive and less selective than sulfate radicals [28], which may explain the different distribution of dimethyl 3-hydroxyl phthalate and dimethyl 4-hydroxyl phthalate for these two radical species. But the possible formation of dimethyl 3-hydroxyl phthalate in the persulfate process could not be totally excluded as it might either not be generated in enough amount or it might undergo further degradation. The ability of persulfate to fully mineralize DMP is very important, because the toxicity and hazardousness of PAEs and their daughter products are of great concern for soil and groundwater remediation, as well as for wastewater treatment. For example, phthalate monoesters are commonly found as the main metabolites of PAEs in the environment [36], and small to large portion of PAEs would remain untreated in the effluent of municipal wastewater treatment plants [37,38], while both PAEs and their monoesters are of great concerns due to their impact to the public health [39,40]. Therefore, persulfate at ambient temperature may offer a potential
14 d
Fig. 7. DMP decomposition profiles in the slurries with various levels of persulfate at 40 ◦ C ([S2 O8 2− ]0 = 0 g/L, 3.68 g/L, 18.40 g/L and 36.80 g/L, respectively; () DMP percentages remained in soil phase; () DMP percentages remained in aqueous phase).
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residual did not migrate too far away from the point of application [43]. 4. Conclusion
Fig. 8. Persulfate decomposition profiles (5.0 g DMP spiked soil; 20 mL persulfate solution; T = 40 ◦ C).
to DMP’s preferable partition into the aqueous phase. The results confirm that DMP can easily migrate with groundwater, posing potential threat to drinking water sources. Fig. 7 also shows minor loss of DMP (∼14%) for the blank control after 14 days, possibly due to evaporation at 40 ◦ C. The degradation profiles of DMP with persulfate oxidation are presented in Fig. 7. The results show that DMP was totally removed from the slurry system after 4 d at a persulfate dosage level of 36.80 g/L and after 14 d at a dosage level of 18.40 g/L. On the other hand, persulfate at 3.68 g/L was only able to remove ∼33.8% DMP after 14 d. The results confirm the feasibility of using persulfate to remediate soil contaminated by high levels of DMP. Furthermore, it is crucial to apply proper dosages of persulfate to achieve efficient removal of the target contaminant. Persulfate degradation profile (Fig. 8) indicates that the decomposition of persulfate was characterized by an initial fast stage in the first two days, when most DMP was removed. Thereafter, the persulfate decomposition leveled off. As the decomposition of persulfate also generated protons, pH of the slurries dropped rapidly, with pH 3.36, 2.33 and 1.94 at 2 d, and 3.19, 2.12 and 1.76 at 14 d, respectively, for persulfate dosage of 3.68, 18.40 and 36.80 g/L, while pH of the blank control remained at around ∼5.45 during the whole period. The results indicate that persulfate at 40 ◦ C is effective for the remediation of DMP contaminated soil and groundwater. While increasing the oxidant dosage results in more efficient and complete removal of DMP, cost should be an important considering factor for actual field projects. Furthermore, higher oxidant dosage will result in lower soil pH, which may be a potential issue. But natural soil buffering capacity may help to alleviate this phenomenon, and dilution through dispersion and diffusion also helps to reduce the impact of pH drop caused by the remediation process [20]. Another potential issue is the generation of sulfate, which may cause the sulfate concentration to increase above the regulatory standards. For example, the secondary maximum contaminant level of the US EPA for sulfate in drinking water is 250 mg/L. While the generation of sulfate is inevitable, sulfate is relatively non-toxic, and dilution and dispersion by incoming groundwater may lower the sulfate concentration. Furthermore, sulfate can stimulate the growth of sulfate-reducing bacteria to degrade residual organic contaminants, and subsequently transform sulfate to sulfide [42]. Possibly due to factors mentioned above, data from several field applications observed that sulfate level in groundwater dropped substantially in months after an initial rapid rise and that sulfate
This study illustrates that persulfate at ambient temperature can effectively decompose and mineralize DMP in aqueous solutions and slurries. From a perspective of wastewater treatment, persulfate at ambient temperature can provide an alternative for the treatment of DMP contaminated wastewater by avoiding the involvement of energy-intensive processes. From the perspective of groundwater and soil remediation, persulfate at around 40 ◦ C can remove high levels of DMP in days, offering an efficient remediation method for DMP contaminated sites. As indicated from the study, the destruction of DMP is highly dependent on the reaction temperature and oxidant dosage, and a treatability study is highly recommended to identify the optimal cost-effective conditions before the start of actual field projects. Currently, we are further exploring the feasibility of using persulfate for the remediation of other PAEs and will explore the impact of PAEs structure on the removal kinetics and degradation pathways of PAEs by persulfate. Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China (No. 41201304) and the National Natural Science Foundation of China-Guangdong Joint Fund (No. U12011244). References [1] P. Di Gennaro, E. Collina, A. Franzetti, M. Lasagni, A. Luridiana, D. Pitea, G. Bestetti, Bioremediation of diethylhexyl phthalate contaminated soil: a feasibility study in slurry- and solid-phase reactors, Environ. Sci. Technol. 39 (2005) 325–330. [2] W.L. Liu, C.F. Shen, Z. Zhang, C.B. Zhang, Distribution of phthalate esters in soil of e-waste recycling sites from Taizhou City in China, Bull. Environ. Contam. Toxicol. 82 (2009) 665–667. [3] F. Zeng, K.Y. Cui, Z.Y. Xie, L.N. Wu, D.L. Luo, L.X. Chen, Y.J. Lin, M. Liu, G.X. Sun, Distribution of phthalate esters in urban soils of subtropical city, Guangzhou, China, J. Hazard. Mater. 164 (2009) 1171–1178. [4] I.D. Ferreira, D.M. Morita, Ex-situ bioremediation of Brazilian soil contaminated with plasticizers process wastes, Braz. J. Chem. Eng. 29 (2012) 77–86. [5] I. Mersiowsky, Long-term fate of PVC products and their additives in landfills, Prog. Polym. Sci. 27 (2002) 2227–2277. [6] T. Schiedek, Impact of plasticizers (phthalic acid esters) on soil and groundwater quality, K. Kovar, J. Krasny (Eds.), Groundwater Quality: Remediation and Protection, IAHS, Press, Wallingford, 1995, pp. 149–156. [7] H. Liu, Y. Liang, D. Zhang, C. Wang, H.C. Liang, H.S. Cai, Impact of MSW landfill on the environmental contamination of phthalate esters, Waste Manag. 30 (2010) 1569–1576. [8] T.T. Ma, P. Christie, Y.M. Luo, Y. Teng, Phthalate esters contamination in soil and plants on agricultural land near an electronic waste recycling site, Environ. Geochem. Health 35 (2012) 1–12. [9] T. Lovekamp-Swan, B.J. Davis, Mechanisms of phthalate ester toxicity in the female reproductive system, Environ. Health Perspect. 111 (2003) 139–145. [10] A.J. Martino-Andrade, I. Chahoud, Reproductive toxicity of phthalate esters, Mol. Nutr. Food Res. 54 (2010) 148–157. [11] J.R. Warren, N.D. Lalwani, J.K. Reddy, Phthalate esters as peroxisome proliferator carcinogens, Environ. Health Perspect. 45 (1982) 35–40. [12] J.C. Caldwell, DEHP: genotoxicity and potential carcinogenic mechanisms—a review, Mutat. Res.: Rev Mutat. Res. 751 (2012) 82–157. [13] G. Lottrup, A.M. Andersson, H. Leffers, G.K. Mortensen, J. Toppari, N.E. Skakkebaek, K.M. Main, Possible impact of phthalates on infant reproductive health, Int. J. Androl. 29 (2006) 172–180. [14] National Primary Drinking Water Regulations, Federal Register; 40 CFR Chapter I, Part 141, US EPA, Washington, DC, 1991. [15] C.A. Stales, D.R. Peterson, T.F. Parkerton, W.J. Adams, The environmental fate of phthalate esters: a literature review, Chemosphere 35 (1997) 667–749. [16] K.S. Tay, N.A. Rahman, M.H.B. Abas, Fenton degradation of dialkylphthalates: products and mechanism, Environ. Chem. Lett. 9 (2011) 539–546. [17] B.L. Yuan, X.Z. Li, N. Graham, Reaction pathways of dimethyl phthalate degradation in TiO2 -UV-O2 and TiO2 -UV-Fe(VI) systems, Chemosphere 72 (2008) 197–204.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Degradation of dimethyl phthalate in solutions and soil slurries by persulfate at ambient temperature Zhen Wang, Dayi Deng ∗ , Liling Yang School of Chemistry and Environment, South China Normal University, Guangzhou, Guangdong 510006, China
h i g h l i g h t s • • • •
Persulfate alone can efficiently degrade and mineralize dimethyl phthalate at ambient temperature. Effects of temperature, initial persulfate and DMP concentration, and initial pH were studied. Persulfate at 40 ◦ C is very effective for the remediation of DMP contaminated soil. The degradation intermediates of DMP were characterized and degradation pathways were proposed.
a r t i c l e
i n f o
Article history: Received 12 September 2013 Received in revised form 8 February 2014 Accepted 11 February 2014 Available online 28 February 2014 Keywords: Persulfate Dimethyl phthalate Ambient temperature Soil and groundwater remediation Degradation mechanism
a b s t r a c t The degradation of dimethyl phthalate (DMP) by persulfate at ambient temperature (T = 20–40 ◦ C) was investigated in aqueous solutions and soil slurries to assess the feasibility of using persulfate to remediate DMP contaminated soil and groundwater. First, the effects of temperature, initial oxidant concentration, initial DMP concentration and initial solution pH on the removal of DMP and TOC were studied in aqueous solutions. The results show that persulfate at 40 ◦ C can effectively mineralize DMP. Furthermore, dimethyl 4-hydroxyl phthalate, maleic acid and oxalic acid were identified as the degradation intermediates, and degradation pathways were proposed. Lastly, persulfate at 40 ◦ C was applied to remediate soil spiked with DMP at ∼600 mg/kg. The results show that persulfate at 40 ◦ C is highly effective for the remediation of DMP contaminated soil. Overall, this study provides fundamental and practical knowledge for the treatment of emerging phthalate esters (PAEs) contaminated soil and groundwater, as well as PAEs contaminated industrial wastewater, with persulfate at ambient temperature. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The contamination of phthalate esters (PAEs, Fig. 1) in groundwater and soil [1–3] is an emerging issue that will need to be addressed in the near future. Even though current public concern about PAEs focuses on the health impact of PAEs released from plastic consumer products, especially baby accessories in recent years, some recent reports have shown that soils near contaminated industrial sites, some E-waste recycle centers and landfills have high levels of PAEs contamination [4]. For example, >300 ppm dimethyl phthalate (DMP) has been found in chemical leachate [5], and PAEs contamination levels of several thousand ppm have been found near plastic manufactories [4]. These highly contaminated sites are serious point sources of PAEs, contaminating groundwater
∗ Corresponding author. Tel.: +86 20 39310213; fax: +86 20 39310213. E-mail address: [email protected] (D. Deng). http://dx.doi.org/10.1016/j.jhazmat.2014.02.027 0304-3894/© 2014 Elsevier B.V. All rights reserved.
[6,7], transferring into food chain and posing high exposure-risk [8] to the public. As PAEs are potential endocrine disrupters, environmental carcinogens, teratogens and mutagens [9–13], their existence in the environment has raised great concerns. For example, US EPA has listed six PAEs as Priority Pollutants [14]. Due to the low hydrolysis and biodegrading rate of PAEs in natural environment, especially in the subsurface [15], it is necessary to develop appropriate technologies for the remediation of PAEs contaminated soil and groundwater. Advanced oxidation methods, such as Fenton oxidation [16], photocatalytic oxidation [17], UV photolysis [18] and ozone oxidation [19], have been extensively tested for the destruction of PAEs from wastewaters in recent years. Even though these processes are effective for wastewater treatment, their potential application for soil and groundwater poses challenges. First, photocatalytic oxidation and UV photolysis are normally not applicable to in situ soil and groundwater remediation. While Fenton reagent has been used for in situ chemical oxidation of organic contaminants in soil
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identified and potential degradation pathways were then proposed.
O
Heat
OR OR'
O Fig. 1. Chemical structures of phthalate esters.
and groundwater, hydrogen peroxide suffers from rapid decomposition in subsurface, resulting in the generation of large amount of heat and gas, causing concerns on a series of issues, including safety, potential escape of VOCs, relatively short radius of influence and low remediation efficiency, etc. [20]. On the other hand, the limited aqueous solubility and inefficient dispersion of ozone gas in heterogeneous subsurface environment, as well as the requirement of on-site generation systems, also hinder the application of ozone remediation of organic contaminated soil and groundwater [21]. For the remediation of organic contaminated soil and groundwater, sodium persulfate is a newer but increasingly used oxidant for in situ chemical oxidation [22]. Persulfate is a stable oxidant under normal conditions. Therefore, it is relatively easy and safe to handle and transport to the field. Once on the site, it can be conveniently applied by mixing with soil or injecting to subsurface at high concentrations, as it is highly soluble in water. Normally, it tends to have a much longer life-time in the subsurface than hydrogen peroxide as well as relative larger radius of influence [22]. Through activation by heat, ferrous salt, Fe(III)-EDTA, NaOH, etc., reactive radical species, including sulfate radicals (E0 = 2.6 V) and hydroxyl radicals (E0 = 2.7 V), are generated and have been found to oxidize a variety of recalcitrant compounds, including BTEX, PAHs, PCBs, OCPs, etc. [22]. To the best of our knowledge, the interaction between persulfate and PAEs has been rarely studied. There are only a couple of reports, in which He and He [23] focused on UV-coupled persulfate treatment of dimethyl phthalate (DMP) contaminated wastewater and Sun et al. [24] studied microwave assisted Ag+ catalyzed persulfate oxidation of DMP in aqueous solution. However, it is difficult to apply either UV or microwave in the soil environment. Furthermore, the DMP degradation pathways were not characterized in these two studies. Therefore, it is crucial to both assess the feasibility of using persulfate to remediate PAEs contaminated soil and groundwater and elucidate the PAEs degradation pathways. Considering that DMP is one of the six PAEs on the US EPA Priority Pollutant list and that it tends to migrate with groundwater due to its high aqueous solubility, DMP is chosen as the model compound in the current study. Heat-activation is a frequently used field activation method for persulfate to generate sulfate radicals (reaction (1)) to oxidize organic contaminants. To reduce the high cost required to heat large amount of soil to the desired temperature and to resolve the potential issue of returning soil temperature back to the original one, an activation temperature around 40 ◦ C is commonly applied in the field [21]. Therefore, the current project first studied the effect of temperature on the oxidation of DMP in aqueous solutions at 20, 30, and 40 ◦ C, respectively. Furthermore, the effects of oxidant dosage, initial concentration of DMP, and initial solution pH on the removal of DMP and the mineralization of TOC were investigated. Degradation intermediates were
−• S2 O2− 8 −→2SO4
(1) 40 ◦ C
After establishing that persulfate at can effectively mineralize DMP in aqueous solutions, the feasibility of using persulfate at 40 ◦ C to remediate DMP contaminated soil and groundwater was further investigated with soil containing DMP at ∼600 mg/kg, specifically assessing the potential of persulfate for the remediation of highly contaminated sites. The current study focused on the remediation of hot spots (source zones) because persulfate has been predominantly applied to treat highly contaminated (source zone) spots in hazardous sites and not commonly applied in the groundwater plume extending from the source zone, due to cost-effective considerations [21]. Overall, this study provides fundamental and practical knowledge for the treatment of emerging PAEs contaminated soil and groundwater, as well as PAEs contaminated industrial wastewater, with persulfate at ambient temperature. 2. Materials and methods 2.1. Materials Dimethyl phthalate (99%), benzyl benzoate (98%), sodium persulfate (98%), ceric sulfate (99.9%), ammonium iron sulfate hexahydrate (99.99%), anhydrous sodium sulfate (99.99%), anhydrous sodium sulfite (98%), sulfuric acid (98%) and sodium hydroxide (98%) were all analytical grade and used as received. Acetone and hexane were HPLC grade. N,Obis(trimethylsilyl)trifluoroacetamide/trimethylchlorosilane (BSTFA-TMCS; v/v, 99/1) from Regis Technologies, Inc., USA, was employed to derivatize potential DMP intermediates for GC–MS analysis. Milli-Q ultra pure water was used to prepare all aqueous solutions. 2.2. Persulfate oxidation of DMP in aqueous solutions Aqueous experiments were conducted in 1000-mL conical glass bottles with ground glass stoppers. Freshly prepared DMP stock solutions were diluted to the desired concentrations by the addition of ultra pure water and maintained at desired temperatures in temperature control incubators. Shortly afterwards, experiments were carried out by adding pre-determined amounts of sodium persulfate to DMP solutions and mixed rigorously until totally dissolved. NaOH or H2 SO4 solution (1 mol/L) was used for initial pH adjustment when studying the effect of initial pH on the degradation of DMP. Each time, 1000-mL solution was prepared, capped and placed into a temperature control incubator maintained at constant temperature (T = 20, 30, and 40 ◦ C, respectively) and kept in the dark without shaking. At pre-specified time intervals, 20 mL samples were retrieved from the bottles for the analysis of persulfate concentration, DMP concentration, TOC and pH, as well as the identification of potential degradation intermediates. All experiments were conducted in duplicates. Blank control experiments without the addition of oxidants were run at 40 ◦ C to monitor potential loss of DMP due to evaporation. 5 mL aqueous solutions were extracted by 5 mL hexane and analyzed by GC/FID for DMP with benzyl benzoate as the internal standard according to procedures in Section 2.4. The pHs were measured using a pH meter (Model PHSJ-3F from Shanghai INESA Scientific Instrument Co., Ltd). TOC analyses were carried out using a Shimadzu VCPN model carbon analyzer. Residual persulfate in the aqueous solution was quantified by reaction with ammonium iron sulfate and then back-titrate with ceric sulfate using an automatic
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potentiometric titrator (Model ZD-2, Shanghai INESA Scientific Instrument Co., Ltd) according to literature procedure [25]. 2.3. Persulfate oxidation of DMP in spiked soils The soil samples used were typical lateritic red soils obtained from Guangzhou, China at a depth of 20–40 cm, which contained approximately 66% well graded sand (61% DMP remained after 48 h with an initial persulfate concentration of 1.03 × 10−3 mol/L. The results show a pseudo-firs-order rate constant (kobs ) of 0.0159 h−1 (R2 = 0.99) for an initial persulfate concentration of 1.03 × 10−3 mol/L, 0.0495 h−1 (R2 = 0.98) at 5.15 × 10−3 mol/L, and 0.103 h−1 (R2 = 0.95) at 1.03 × 10−2 mol/L. The results imply that as initial concentrations of persulfate rose, more sulfate radicals were generated, resulting in more rapid removal of DMP. However, the increment of initial DMP removal rate was not proportional to the increase of persulfate concentration, possibly due to competition by organic intermediates generated during the oxidation of DMP and potential quenching
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Fig. 4. Effect of initial DMP concentrations on DMP (a) and TOC (b) removal by persulfate at 40 ◦ C ([S2 O8 2− ]0 = 5.15 × 10−3 mol/L).
of sulfate radicals by residual persulfate [29] and sulfate radicals themselves [30]. Fig. 3b indicates that a more rapid and more efficient mineralization of DMP was observed upon increasing the initial persulfate concentration. With an initial persulfate concentration of 1.03 × 10−2 mol/L, complete mineralization of DMP was achieved after 42 h, while there was only minor removal of TOC with persulfate at 1.03 × 10−3 mol/L at 48 h. 3.3. Effect of initial concentration of DMP The effect of initial DMP concentration on the removal of DMP and the abatement of TOC was investigated at various initial DMP concentrations (5.15 × 10−5 , 1.03 × 10−4 and 2.06 × 10−4 mol/L, equal to 10, 20, and 40 mg/L, respectively) at a persulfate dosage of 5.15 × 10−3 mol/L. Fig. 4a indicates that as DMP concentration increased, the removal rate dropped accordingly. The data were fitted with pseudo-first-order kinetics, and the rate constants (kobs ) were 0.0495 h−1 (R2 = 0.98) with an initial DMP concentration of 5.15 × 10−5 mol/L, 0.0201 h−1 (R2 = 0.97) at 1.03 × 10−4 mol/L and 0.0105 h−1 (R2 = 0.99) at 2.06 × 10−4 mol/L. Furthermore, increasing the initial DMP concentration had a negative impact on the abatement of TOC. Fig. 4b indicates that during the 48 h period study, the abatement of TOC was minor for DMP at 2.06 × 10−4 mol/L, even though ∼30% of DMP was oxidized by persulfate already, and the result reconfirmed the build-up of organic intermediates during the remediation process. 3.4. Effect of initial solution pH The solution pH plays an important role in the oxidation of organic contaminants by activated persulfate. Considering that it is hard to keep the solution pH at a fixed value in actual field applications, the removal of DMP and abatement of TOC by persulfate at 40 ◦ C were studied at various initial pHs (a pH
Fig. 5. Effect of initial solution pH on DMP (a) and TOC (b) removal by persulfate at 40 ◦ C ([DMP]0 = 5.15 × 10−5 mol/L; [S2 O8 2− ]0 = 5.15 × 10−3 mol/L).
of ∼3.1, ∼7.0 and ∼9.0, respectively; [DMP]0 = 5.15 × 10−5 mol/L; [S2 O8 2− ]0 = 5.15 × 10−3 mol/L). During the experiments, the pHs of solutions dropped as the reaction progressed and all dropped to lower than 3.0 after 48 h, due to protons released from persulfate decomposition. Fig. 5 displays the changes in DMP concentration and TOC versus time under different initial pHs. Fig. 5a indicates that the degradation patterns of DMP were very similar under all three conditions, with complete removal of DMP after 36 h. Even though the solution pHs were continuously changing during the whole treatment period, DMP removal data fitted the pseudo-first-order kinetics very well under all conditions, and showed very similar rate constants (kobs ), with 0.0495 h−1 (R2 = 0.98) at an initial pH of ∼3.1, 0.0409 h−1 (R2 = 0.98) at ∼7.0 and 0.0441 h−1 (R2 = 0.97) at ∼9.0. The results indicate that initial pHs had minor effect on the removal of DMP in the pH range studied. Furthermore, Fig. 5b also indicates that initial solution pHs had negligible effects on the abatement of TOC, and the abatement patterns of TOC were very similar under all conditions. The effect of initial pH on the removal of DMP and TOC by persulfate at ambient temperature appears to be consistent with the observation of Huling et al. [31] that persulfate thermolysis is not affected by pH differences (pH between 3 and 12) and that it takes rather extreme pHs (12) in order to impact the persulfate activation significantly. 3.5. Degradation intermediates and degradation mechanism GC/MS analysis of hexane solution of the freeze-dried samples revealed dimethyl 4-hydroxyl phthalate as one main intermediate during persulfate oxidation of DMP. The identification of dimethyl 4-hydroxyl phthalate was performed by searching the NIST library with Agilent MassHunter Workstation software and comparing the mass spectrum observed in the current study with those reported in previous literature [16]. Another two intermediates, maleic acid and oxalic acid, were identified
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O
207
O
O
OCH3
SO4-. SO 24
OCH3
+ .
OCH3
H2O H+
OCH3
OCH3 . OCH3
HO H
O
O
O
oxidation O
O O
O
OCH3
OH
CO2
OCH3
OH HO
HO
OH
O
O Fig. 6. Potential DMP degradation pathways.
solution to totally eliminate the toxicity and hazardousness of PAES in industrial wastewater, contaminated soil and groundwater. 3.6. Oxidation of DMP in slurries Results from aqueous solutions indicate that persulfate at 40 ◦ C is effective for the mineralization of DMP. On the other hand, oxidation of organic contaminants in soil and groundwater is much more complicated than in aqueous solutions. The adsorption of organic contaminants, consumption of oxidants by natural organic matter and other reducible species, and limited mass transfer in the heterogeneous system, etc., are all important factors that impact the feasibility of applying persulfate for the remediation of PAEs contaminated soil and groundwater. Therefore, oxidation of DMP in spiked soil was carried out to further assess the feasibility of using persulfate for the remediation of DMP contaminated soil and groundwater. DMP profile (Fig. 7) of the blank control shows that DMP preferred to partition into the aqueous phase, with a ratio of aqueous concentrations to concentrations in soil of ∼3.8. Similar observation was also documented by Hunter and Uchrin [41]. The high aqueous solubility of DMP and its relatively small kow (log kow = 1.6), as well as the low carbon content of the studied soil, may contribute 120 0g/L
Soil Water
2d
4d
3.68 g/L
7d
18.4g g/L 36.80 g/L
0
18.4g g/L 36.80 g/L
20
18.4g g/L
36.80 g/L
40
36.80 g/L
60
18.4g g/L
80
0g/L
0g/L 3.68 g/L
3.68 g/L
0g/L 3.68 g/L
100
[DMP]/[DMP0] (%)
after the aqueous samples were freeze–dried and derivatized by BSTFA-TMCS and analyzed by GC/MS according to literature [26]. The degradation pathways of DMP were proposed based on the intermediate identification and previous mechanistic studies on the interaction of sulfate radicals with aromatic rings. Liang and Su [32] reported that sulfate radicals are the primary reactive species responsible for the destruction of organic contaminants in the heatactivated persulfate system. Furthermore, previous study indicates that sulfate radicals tend to react with aromatic rings through a oneelectron transfer process to generate radical cations [33], contrary to the direct addition to aromatic rings by hydroxyl radicals [34]. Thereafter, the radical cations generated would undergo hydration by water and further oxidation to generate phenolic product [35]. Accordingly, dimethyl 4-hydroxyl phthalate was proposed to be generated through the generation of a radical cation intermediate outlined in Fig. 6, and further oxidation would cleave the aromatic ring and yield maleic acid and oxalic acid as secondary intermediates, which would finally undergo complete mineralization to carbon dioxide. Another potential phenolic product, dimethyl 3-hydroxyl phthalate, was not observed in the current study. Potentially, the steric hindrance from the carboxyl groups would hinder the formation of dimethyl 3-hydroxyl phthalate and favor the formation of dimethyl 4-hydroxyl phthalate. On the contrary, both phenolic products had been observed in the Fenton oxidation of DMP through the direct addition of hydroxyl radicals to the aromatic rings [16], while hydroxyl radicals are normally considered more reactive and less selective than sulfate radicals [28], which may explain the different distribution of dimethyl 3-hydroxyl phthalate and dimethyl 4-hydroxyl phthalate for these two radical species. But the possible formation of dimethyl 3-hydroxyl phthalate in the persulfate process could not be totally excluded as it might either not be generated in enough amount or it might undergo further degradation. The ability of persulfate to fully mineralize DMP is very important, because the toxicity and hazardousness of PAEs and their daughter products are of great concern for soil and groundwater remediation, as well as for wastewater treatment. For example, phthalate monoesters are commonly found as the main metabolites of PAEs in the environment [36], and small to large portion of PAEs would remain untreated in the effluent of municipal wastewater treatment plants [37,38], while both PAEs and their monoesters are of great concerns due to their impact to the public health [39,40]. Therefore, persulfate at ambient temperature may offer a potential
14 d
Fig. 7. DMP decomposition profiles in the slurries with various levels of persulfate at 40 ◦ C ([S2 O8 2− ]0 = 0 g/L, 3.68 g/L, 18.40 g/L and 36.80 g/L, respectively; () DMP percentages remained in soil phase; () DMP percentages remained in aqueous phase).
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residual did not migrate too far away from the point of application [43]. 4. Conclusion
Fig. 8. Persulfate decomposition profiles (5.0 g DMP spiked soil; 20 mL persulfate solution; T = 40 ◦ C).
to DMP’s preferable partition into the aqueous phase. The results confirm that DMP can easily migrate with groundwater, posing potential threat to drinking water sources. Fig. 7 also shows minor loss of DMP (∼14%) for the blank control after 14 days, possibly due to evaporation at 40 ◦ C. The degradation profiles of DMP with persulfate oxidation are presented in Fig. 7. The results show that DMP was totally removed from the slurry system after 4 d at a persulfate dosage level of 36.80 g/L and after 14 d at a dosage level of 18.40 g/L. On the other hand, persulfate at 3.68 g/L was only able to remove ∼33.8% DMP after 14 d. The results confirm the feasibility of using persulfate to remediate soil contaminated by high levels of DMP. Furthermore, it is crucial to apply proper dosages of persulfate to achieve efficient removal of the target contaminant. Persulfate degradation profile (Fig. 8) indicates that the decomposition of persulfate was characterized by an initial fast stage in the first two days, when most DMP was removed. Thereafter, the persulfate decomposition leveled off. As the decomposition of persulfate also generated protons, pH of the slurries dropped rapidly, with pH 3.36, 2.33 and 1.94 at 2 d, and 3.19, 2.12 and 1.76 at 14 d, respectively, for persulfate dosage of 3.68, 18.40 and 36.80 g/L, while pH of the blank control remained at around ∼5.45 during the whole period. The results indicate that persulfate at 40 ◦ C is effective for the remediation of DMP contaminated soil and groundwater. While increasing the oxidant dosage results in more efficient and complete removal of DMP, cost should be an important considering factor for actual field projects. Furthermore, higher oxidant dosage will result in lower soil pH, which may be a potential issue. But natural soil buffering capacity may help to alleviate this phenomenon, and dilution through dispersion and diffusion also helps to reduce the impact of pH drop caused by the remediation process [20]. Another potential issue is the generation of sulfate, which may cause the sulfate concentration to increase above the regulatory standards. For example, the secondary maximum contaminant level of the US EPA for sulfate in drinking water is 250 mg/L. While the generation of sulfate is inevitable, sulfate is relatively non-toxic, and dilution and dispersion by incoming groundwater may lower the sulfate concentration. Furthermore, sulfate can stimulate the growth of sulfate-reducing bacteria to degrade residual organic contaminants, and subsequently transform sulfate to sulfide [42]. Possibly due to factors mentioned above, data from several field applications observed that sulfate level in groundwater dropped substantially in months after an initial rapid rise and that sulfate
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