Oxidation of Sulfolane in Aqueous Systems by Chemical and Photochemical Processes

Abstract

In this research degradation of sulfolane in spiked water, contaminated ground water and soil wash water was investigated by several oxidative methods. Sulfolane is an organosulfur compound, which is commonly used for liquid-liquid aromatics extraction from mixtures containing aliphatic hydrocarbons and in Sulfinol® process for liquid natural gas treatment. Due to its large production, a significant amount of waste containing sulfolane is annually produced. Beyond this, over many years of operation, there have been some unpredicted or accidental spills, leaks from extraction units during processing as well as leachates from disposal areas from producing wells and unlined storage ponds, which have caused contamination of soil, ground water and wetland ecosystem around gas processing plants. The natural attenuation processes are not effective in sulfolane removal as they are quite slow. Based on the toxicological studies, sulfolane is considered as an emerging industrial contaminant, which should be removed from the environment. In this study, three different oxidative methods were evaluated for sulfolane degradation and possible adaptation for field application. This study builds on previous investigations conducted on application of oxidative methods and wherever relevant these have been referenced. The first oxidative method evaluated in this study was oxidation of sulfolane using ammonium persulfate (APS) along with ultraviolet light (UVC) and/ or bubbling ozone in spiked water with sulfolane as well as in ground water samples. To the best of our knowledge the synergistic effect of O3 and UV irradiation on activation of persulfate has not been investigated for degradation of any organic compounds prior to this study. This study demonstrated that persulfate along with UVC and UVC/O3 can efficiently degrade sulfolane in water. More than 90% removal was achieved after 35 min and 10 min respectively. Presence of 5 mg L-1 O3 in solution not only increased the rate of sulfolane removal (by up to three times) but also decreased at least 25% of the required dosage of persulfate. In general, at higher pHs than 6.9 the reactions were slower, and the quenching effect of carbonate seemed to be significant. Chloride at concentrations lower then 100 mg L-1 had no effect on reaction rate. The application of these methods was also tested for ground water samples collected from a sulfolane impacted site. For 90% sulfolane degradation in groundwater 60 and 22 min irradiation was required in presence of 3 g L-1 of APS for APS/UVC and APS/UVC/O3 systems respectively. The second oxidative method that was evaluated was the application of CaO2/O3 and CaO/O3 for mineralization of sulfolane in aqueous systems. This study demonstrated that the application of calcium peroxide or lime along with O3 is a viable and effective method for treatment of sulfolane in water and groundwater. If 1.6 g L-1 of oxidants (CaO2 and CaO) were used along with 0.5 L min-1 of O3 in a batch reactor, sulfolane and TOC were totally removed in less than 40 and 150 min, respectively. Once these conditions were established and optimised in the lab, field experiments were designed and evaluated to treat contaminated ground water samples. The field tests were successful in treating sulfolane with TOC removal within 150 min and after 4 h, respectively. Reduced treatment time compared to UV/O3 system, applicability of lime, which is readily available, negligible matrix effect and the potential for complete mineralization of sulfolane, make CaO/O3 or CaO2/O3 a practical method for sulfolane treatment. Among the methods tested for sulfolane remediation, this method is the only AOP method, which can be used in situ for treatment of groundwater contaminated with sulfolane. A mechanistic study was also performed for CaO2/O3 system compared to that of NaOH/O3. The proposed degradation pathways were quite similar. The more efficient TOC removal in case of CaO2/O3 was related to complexation of Ca2+ with oxidized sulfolane by-products. Involvement of CaCO3 as a solid formed during degradation of sulfolane, in a catalytic ozonation process, was not supported by the experimental results. Not only CaCO3 but also several other solids such as MgO, silica, zeolite and different types of activated carbons were inefficient in degradation of sulfolane along with O3. Only two types of carbon showed positive results, but the overall results were not consistent. Both these two oxidative methods (APS/UV/O3 and CaO/O3) were effective in treating sulfolane in soil washwater. Therefore, they can be combined with a soil washing/flushing process as a treatment method for contaminated soils. The third oxidative method tested in this study was based on application of aqueous chlorine (Cl2, OCl−, Cl−) along with UVC/UVB. Only hypochlorus acid (if it was added step-wise) along with UVC was effective in sulfolane degradation in water samples. This prevented the quenching effect of HOCl on reactive oxidative species. There is a possibility of formation of chlorinated by-products in this case and the presence of natural organic matter (NOM) might further complicate the application of this treatment method. Degradation of sulfolane under UVA and visible light irradiation was negligible. While aqueous chlorine in combination with longer wavelength ultraviolet light/visible light was not effective for sulfolane removal, it has been considered an Advanced Oxidation method for treatment of water and wastewater in the literature as a result of production of hydroxyl radicals. Proposed hydroxyl radical production requires efficient long wavelength UV/visible light absorption of chlorine species in presence of high concentration of coloured Natural Organic Matter (NOM); which creates some ambiguity on the reaction mechanism. Therefore, a mechanistic study was performed, and it was found that the chlorine species may be exploited to degrade many organic contaminants using UVA and visible light excitation. Hydroxyl radicals are not necessary for degradation of contaminants in presence of coloured compounds in water. Involvement of a photosensitization process for sulfolane degradation was not also evident as reactive oxygen species were not produced efficiently.