Degradation of 2-mercaptobenzothiazole in aqueous solution by gamma irradiation

Highlights

• Radiation is an effective method to remove MBT from water.

• G-value and dose constant of MBT degradation under different conditions were discussed.

• Desulfuration of MBT molecule was observed during irradiation.

• γ-Irradiation can improve the biodegradability of MBT containing water.

Abstract

Industrial wastewaters containing 2-mercaptobenzothiazole (MBT), a widely used chemical additive, usually cannot be treated properly by conventional biological methods, thus cause an environmental risk. Ionizing radiation was proposed as a method for abatement of several refractory pollutants from water. The paper investigated MBT degradation using irradiation technology. The decomposition kinetics was described, and the transformation and the change of biodegradability were discussed. The results of gamma radiation experiments on MBT-containing aqueous solutions indicated that reactive radicals resulting from water radiolysis effectively degrade MBT and improve the biodegradability of the solutions. At a 20 mg/L MBT concentration, the removal of 82% was achieved at the absorbed dose of 1.2 kGy. The results of specific oxygen uptake rate (SOUR) test showed that MBT was decomposed into biodegradable products, after irradiation at 20 kGy. Radicals attacked the sulfur atoms of the studied molecule leading to the release of sulfate ions, but the mineralization of organic carbons was rather weak. Initial concentration significantly affected the degradation efficacy of MBT by gamma radiation.

Introduction

As an important chemical additive, 2-mercaptobenzothiazole (MBT) is widely used in synthesis of antibiotics, pesticides, rubber, leather, etc. According to EPA Toxic Release Inventory in 2010, total on-site and off-site disposals of MBT in USA was 190575.42 lb, of which chemical industry released 6071 lb and plastic/rubber industry released 60749.6 lb.

MBT is a condensed heterocyclic compound with a molecular weight of 167 amu (Fig. 1; Table 1). Because of its antimicrobial effect, wastewater containing MBT is usually resistant to conventional biological treatment.

For example, in the activated sludge system, MBT was found to be resistant and toxic towards activated sludges (Chudoba et al., 1977, Devos et al., 1993). Moreover, MBT even inhibited the degradation of the other heterocycles in the degradation test of heterocycle mixtures. It was reported that microorganisms composing the activated sludge from municipal wastewater treatment plant could be killed after exposure to MBT solutions for 2–3 days (Repkina et al., 1983). Dewever et al. (1994) found that 100 mg/L of MBT was enough to completely inhibit bacterial growth. In addition to the conventional activated sludge process, two-stage anaerobic and aerobic process (Reemtsma et al., 1995), the Bayer tower system (DeWever and Verachtert, 1997), etc. have also been studied. However, microbial tolerance to MBT is limited, therefore a large scale of dilution and pH adaptation were always required in order to achieve satisfactory treatment efficiency.

Recently developed advanced oxidation processes (AOPs) have been considered as an alternative for MBT removal from wastewater. Ozonation could remove MBT and its derivatives from wastewater and reduce the COD, but showed little effect on mineralization (Puig et al., 1996). Fiehn et al. (1998) found that ozonation of MBT was nearly independent of pH-values and proposed that the oxidation product was an organic sulfite. There have been several investigations on direct photolysis (Malouki et al., 2004) and TiO₂-based photocatalysis of MBT. Habibi et al. (2001) found that MBT could be degraded by photocatalytic oxidation, and alkaline medium was favorable. Under optimized condition, 98% removal could be achieved with 8 h reaction time. To enhance the photocatalytic activity for MBT degradation, Li et al. (2005) improved the catalyst performance by doping TiO₂ with Ce³⁺. In addition, peroxidase-catalyzed oxidation of MBT was recently reported (Al-Ansari et al., 2010).

Ionizing radiation, as a special kind of AOPs technology has demonstrated potential as a powerful method for degradation of recalcitrant pollutants (Kim et al., 2009, Sampa et al., 2007, Sun et al., 2013b, Vahdat et al., 2010) like halogenated organic compounds, pesticides, antibiotics, sulfonated aromatic compounds and azo dyes. This technology is also used for incomplete decomposition of the target pollutants into less toxic by-products in order to improve their biodegradability (Chmielewski, 2011, Sun et al., 2012, Sun et al., 2013a). The chemistry behind this technology is under extensive investigation (Al-Sheikhly et al., 2006, Homlok et al., 2011, Zacheis et al., 2000). But there is only one brief piece of report available regarding the ionizing radiation of MBT. Tolgyessy et al. (1986) carried out γ irradiation on 115 mg/L MBT solutions and found no significant change in COD, TOC or BOD₅ within the absorbed dose range of 0–16 kGy. However, neither MBT removal efficacy nor the influence of operational parameters was mentioned.

To our knowledge, this is the first study giving an insight into the radiolytic behavior of MBT in aqueous solution. The objectives of the present work were (1) to study the radiation-induced decomposition and transformation of MBT, and the change of its biodegradability; (2) to investigate the decomposition kinetics of this compound.