Degradation of 1,2-dichloroethane using advanced reduction processes

https://doi.org/10.1016/j.jece.2013.11.013Get rights and content

Highlights

  • More reactivity of 1,2-DCA in output wavelength of UV-N than UV-M lamps.

  • Higher degradation of 1,2-DCA using S2O42− with UV-M in the range of pH studied.

  • Complete degradation of 1,2-DCA by S2O42−, SO32− and S2−/UV-M at basic pH.

Abstract

In this study, advanced reduction processes (ARPs) was applied for degradation of 1,2-dichloroethane (1,2-DCA). ARPs are based on combining reducing reagents and activating methods to produce highly reactive, reducing free radicals. In this study, a combination of different reducing agents (dithionite ions (S2O42−), sulfite ions (SO32−) and sulfide ions (S2−)) and two activating methods (medium pressure mercury (UV-M) lamp, narrow-banded mercury (UV-N) lamp) were evaluated for 1,2-DCA degradation. In the screening test, S2O42− was more effective than the other reducing agents during UV irradiation. Experimental results showed that the S2O42−/UV-M system degraded 1,2-DCA completely after 4 h of irradiation time over a wide range of pH values. In the alkaline solution, however, complete degradation of 1,2-DCA was also observed with all of three reducing agents under irradiation of UV-M lamp. UV-N/S2O42− gave slower degradation of 1,2-DCA, but complete removal was observed eventually except acidic pH condition. This work could provide information on a practical application of ARPs to treat chlorinated organics.

Introduction

Chlorinated aliphatic compounds found in various industrial wastes such as engine lead fouling, petrochemical plants, and materials composing PVC can cause potential risk to humans and ecosystems and the same is true for some of their intermediates [1]. For example, 1,2-dichloroethane (1,2-DCA) is one of the intermediates of degradation of hexachloroethane and it is a troublesome contaminant, because it can remain for several decades in a contaminated area without being degraded [2]. High concentrations of 1,2-DCA are found at many contaminated sites and available treatment processes are not efficient for destroying it [3].

1,2-DCA is a widely used as chlorinated C2 solvent and it can generate vinyl chloride as a toxic byproduct when it is dechlorinated [4]. Several studies have reported that 1,2-DCA is a problematic pollutant and is not completely transformed to nontoxic byproducts [3], [4]. A variety of treatment technologies such as chemical reduction, radiation, sol–gel catalysis, and biotransformation have been investigated for treatment of 1,2-DCA [5], [6]. Bio-dehalogenation uses a microbial community to remove 1,2-DCA, but it takes a longer time compared to other treatment methods [7]. For example, Marzorati et al. reported that 1,2-DCA was degraded within 11 days after incubating the microbial community for 15 days [8]. According to Pham et al., approximately 2 weeks was required to reach 85% removal of 1,2-DCA when anodophilic consortia was used in microbial fuel cells [9]. Duhamel and Edwards found that more than 50% of 1,2-DCA was removed by desired microbial cultures within 10 days [10]. Although the rate of 1,2-DCA biodegradation would be highly dependent on experimental conditions such as initial concentration, growth rate of 1,2-DCA enrichment culture and other solution compositions, most of the degradation rate was observed to be slow, as compared to the system using chemical treatment. The commonly observed long lag time for microbial growth may affect apparent degradation kinetics. However, the magnitude of the biodegradation level at last stage was greater than those of chemical methods [11].

During the past few years, advanced oxidation processes (AOP) have been widely employed to remove chlorinated organic compounds. There are many different types of AOP that use different combinations of oxidizing reagents (e.g. ozone, hydrogen peroxide, TiO2) and activating methods (e.g. ultraviolet (UV) light, ultrasonic, electron beam) to produce hydroxyl radicals, which are highly reactive oxidants. Vilhunen et al. investigated 1,2-DCA removal using AOP and they found that 70% of 1,2-DCA was removed by H2O2 with doses 24 times higher than 1,2-DCA initial concentration during 1 hour of UV irradiation [4]. However, application of the AOP technology that exhibited a good performance at aerobic condition into the subsurface environment contaminated with 1,2-DCA could not reach the treatment level of what we expect, because solution or environmental conditions are differ. Thus, the treatment system that can be applied into subsurface environment is required.

Advanced reduction processes (ARPs) for environmental application has been proposed for destroying chlorinated or other chemical compounds by our research group or others [12], [13], [14], [15], [16], [17]. ARPs combine a reducing agent with an activating method to produce highly reactive reductant radicals. Such combinations will produce electron-rich radicals that can reduce electron-deficient (oxidized) target contaminants. Here, each of three reagents (dithionite (S2O42−), sulfite (SO32−), and sulfide (S2−)) were used as the reducing agents and combined with ultraviolet light produced by two types of lamps (medium pressure mercury UV, or UV-M; narrow-banded UV, or UV-N) as the activating methods.

Dithionite could be the most promising of these reductants for application in ARPs, because it is known to form two reductive free radicals called sulfur dioxide radical anions (SO2radical dot) (Scheme 1a) [18]. These radicals can be produced by various methods, because the chemical structure of dithionite has a weak Ssingle bondS bond [19]. With these chemical properties, dithionite has been commercially used for bleaching wood pulp [19] and it has been shown to be able to dechlorinate organic compounds such as carbon tetrachloride [20]. Industrial use of dithionite makes it a high production volume chemical and commercially available with low cost.

Sulfite can also be used as reducing agent in an ARP because it can form the sulfite radical anion (SO3•−) and hydrated anion (eaq), or hydrogen radical (Hradical dot) if HSO3 specie is predominant (Scheme 1b) [21]. However, in the presence of oxygen, sulfite is rapidly consumed, which would inhibit its ability to treat target compounds [16]. Sulfide could be also an effective candidate for being a reductant in an ARP because its reduction-oxidation potential (ROP) relative to S2O42− is low [19], [22] and it can absorb ultraviolet (UV) light, which can induce free reactive radicals through a photochemical reaction [21], [23], [24].

The purpose of this study was to evaluate the effectiveness of using ARP for destroying 1,2-DCA from water. Different combinations of reagents and activating methods were investigated in order to identify the most promising ARP for efficient removal of 1,2-DCA.

Section snippets

Chemicals and sample preparation

1,2-Dichloroethane (C2H4Cl2, ACS reagent, ≥99%, Sigma–Aldrich) was used as a target compound. n-Hexane (CH3(CH2)4CH3, HPLC grade, ≥95%, VWR) was used as a solvent to extract all organic compounds from aqueous phase samples. Sodium hydrosulfite (Na2S2O4, Sigma–Aldrich), sodium sulfite (Na2SO3, Mallinckrodt Chemicals), and sodium sulfide nonhydrate (Na2S 9H2O, Sigma–Aldrich) were used as reducing agents. The pH of aqueous-phase samples was adjusted by acetate buffer solution (pH 4.5, HACH) and

UV absorbance spectra of reducing agents

UV absorption spectra of the three sulfur-based reducing agents were measured to evaluate both their abilities to absorb light and to develop a method for measuring their concentrations in order to determine their sensitivities to exposure to oxygen. Fig. 1 shows that among the three reducing agents, dithionite was highly sensitive to oxygen compared to other reducing agents. When the dithionite solution was placed in contact with air, its appearance was changed from clear to milky. However,

Conclusions

In the study, the reactivity of three ARP using sulfur-based reducing agents activated by UV light was strongly dependent on pH and varied with the different sources of UV light. The ARP that combines dithionite with UV-M achieved complete degradation of 1,2-DCA after 300 min of irradiation time at all pH values tested. The ARP with dithionite and UV-N irradiation gave slower degradation of 1,2-DCA, but complete removal was observed eventually except at the lowest pH. Although the ARP using

Acknowledgements

This research was made possible by National Priorities Research Program (NPRP) grant (project number 08-172-2-049) from Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely by the responsibility of the authors.

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