Oxidation of 2,4,6-trinitrotoluene (TNT) using simple ozonation and direct hydrogen peroxide was studied. An analytical method based on the HPLC (high performance liquid chromatography) system was used for TNT detection. The influence of parameters such as pH of the solution (3–10), initial TNT concentration (10–100 mg/L), dosage of ozone (0.2–1 g/hr), H2O2/TNT molar ratio (250–1,000), and reaction time (15–60 min) on TNT degradation ratio were investigated. Maximum TNT degradation efficiency occurred at pH = 10, initial TNT concentration = 20 mg/L, dosage of ozone = 1 g/hr in simple ozonation (90%), while this amount was obtained in H2O2 oxidation process with pH = 3, initial TNT concentration of 10 mg/L and H2O2/TNT molar ratio = 1,000/L. The results of kinetic experiments were shown to follow pseudo-second reaction. It can be recommended that simple ozonation seems to be the best at TNT degradation from aqueous solution. However, low TNT concentration could be reduced by performance of the hydrogen peroxide process, in which case, a longer reaction time is also required.
The most widely used explosive is 2,4,6-trinitrotoluene (TNT), also known as trinitrotoluol, trotyl, and tilite (Čėnas et al. 2001; Ayoub et al. 2010; Singh 2013). Several methods are available for TNT production. The most frequently used method for TNT production is nitration of toluene that leads to the production of a benzene ring with nitro groups on positions 2,4,6 and a methyl group. It has been estimated that during the First World War, the Germans produced 2,500 tons of TNT a week. TNT enters the environment through wastewater and solid waste from manufacture of the processing of compounds and destruction of bombs (King 2012). Inhalation of TNT in micro-levels can cause liver disease, anemia, and cataracts. TNT has potential impacts on the environment because of the adsorption to soil (Liu et al. 2008).
TNT moves from surface water and soil to groundwater. When TNT enters the human body, it spreads to the liver, breaks down and changes to several substances. Exposure to TNT in high concentrations has many effects, including such blood disorders as anemia and abnormal liver function. The United States Environmental Protection Agency (US EPA) has classified TNT under group C, a possible human carcinogen (Richter-Torres et al. 1995). TNT is a known mutagen and can cause pancytopenia as a result of bone marrow failure. Oral LD50 in rats is near 1 g/kg.day. The US EPA has regulated TNT contamination in soil to 17.2 ppm and in water to 2 ppb (King 2012). TNT has been discharged into the aqueous effluents of explosive manufacturing facilities assembling, packing, etc. Studies have reported TNT concentrations between 20 and 120 mg/L in manufacturing plant effluents (Richter-Torres et al. 1995). The Department of Defense (DoD) and Department of Energy (DoE) have many sites where the groundwater is contaminated with explosive compounds.
In the past, granular activated carbon (GAC) adsorption and incineration processes were used for the treatment of high explosives in water and soil, respectively (Alnaizy & Akgerman 1999). Recently, studies have shown that natural organic substances act as an absorbent for TNT degradation (Zhang et al. 2012). Also, some studies have reported the degradation of nitroaromatics using nanoparticles such as nZVI (Fu et al. 2014). However, advanced oxidation process is a new method employing reactive oxidizing agents such as hydrogen peroxide or ozone which generate highly reactive free radicals (Mokrini et al. 1997; Quiroz et al. 2011). This method has gained interest in the treatment of contaminated water to remove organic and inorganic contaminants, with or without the addition of catalysts or photolysis (Mokrini et al. 1997). Studies have shown that photo-Fenton process is a useful method for treatment of explosives and explosive removal efficiencies increase with an increase in Fe(II) and UV light (Liou et al. 2003). Son et al. (2004) reported that in the presence of a photocatalyst process TNT was more effectively degraded than with either UV or catalyst (TiO2) alone. Depending on the treatment method, different pathways are available for nitroaromatic degradation. For example, Sviatenko et al. (2014) reported that in alkaline hydrolysis, direct substitution of a nitro group by a hydroxide may be the most favorable pathway for all mentioned compounds (Sviatenko et al. 2014).
Nowadays, ozonation is considered as a process for oxidation of all organic contaminants in water (Hoigne & Bader 1976).
The main objective of this study was the investigation and comparison of TNT reduction by simple ozonation and direct H2O2 oxidation in different experimental conditions such as pH, contaminant initial concentration, dosage of ozone, various molar ratio of H2O2 to TNT, and reaction time.
MATERIAL AND METHODS
TNT was analyzed using a high performance liquid chromatography (HPLC) system, a Model 486 UV detector, and a Nova pak C18 guard column. The analytical column was an ODS2 optimal column (25 cm × 4.6 mm id, 5 μm) from Capital HPLC Ltd. The sample was injected into the HPLC system with the following condition: acetonitrile–water mixture (75:25 v/v) as the mobile phase at a flow rate of 1.0 mL/min (EPA 2007). Injection volume for all samples was 20 μl and a wavelength for the UV detector was 230 nm. Chemical oxygen demand (COD) was measured with HACH COD by the colorimetric method (APHA 1998). The sample was heated for about 2 hours in 150°C with potassium dichromate in the presence of silver sulfate and mercury sulfate. The absorbance was read at 620 nm by the spectrophotometric method. pH of the solution was adjusted by H2SO4 and NaOH (0.1 N) solutions. Initial pH solution was measured using a pH meter.
TNT crystalline form with purity more than 95% was obtained from a local industry producing explosive materials and was used without further purification. All the chemicals were purchased from Merck Company. TNT stock solution was prepared by dissolving an appropriate amount of TNT in 1 L deionized water (El Diwani et al. 2009). All solvents of HPLC grade were filtered through a 0.45 μm PTFE filter. Hydrogen peroxide solution was prepared from a solution grade 35wt%.
RESULTS AND DISCUSSION
Effect of pH solution on TNT degradation
Effect of initial concentration on TNT degradation
According to Figure 4, it can be observed that TNT degradation rate gradually decreases with an increase in TNT concentration (83% to 65%, respectively, corresponding to initial concentration of 10 to 100 mg/L) in simple ozonation and also rapidly decreases with an increase in TNT concentration (90% to 10%, respectively, corresponding to 10 to 100 mg/L) in the hydrogen peroxide process (Figure 5). A reason for this may be related to the generation of hydroxyl free radicals. The overall decomposition rate of TNT depends on the number of hydroxyl radicals in the solution. It is obviously found from Figures 4 and 5 that in high explosive concentrations, hydroxyl free radicals could act as a scavenger, especially OH generation by the H2O2 alone process, while in the simple ozonation process, OH generation and distribution are more efficient and also scavenger forming is considerably less than that in the hydrogen peroxide process. Thus, in comparison with the H2O2 process, the distribution of hydroxyl free radicals was better and the removal rate variation in O3 process was more significant. El Diwani et al. (2009) reported that higher initial concentration of TNT led to a slower degradation by the ozonation process.
Effect of dosage of ozone and hydrogen peroxide on TNT degradation
Ozone doses were changed between 0.25 and 1 g/hr each at a contact time of 30 min, respectively, performed at pH = 10 and pH = 3 for O3 and H2O2 experiments. The results showed an increase in ozone dose and H2O2/TNT molar ratio (250:1–1,000:1) had positive effects on TNT decomposition. This was related to increasing oxidant concentration, which resulted in an increase in formation of free radicals, causing an increase in TNT degradation rate. TNT removal up to 90% was observed by simple ozonation (ozone dosage = 1 g/hr) corresponding to TNT residual concentration of 2 mg/L and maximum removal around 85% by H2O2 alone oxidation (H2O2/TNT molar ratio = 1,000:1) corresponding to TNT residual concentration of 3 mg/L. El Diwani et al. (2009) reported that ozone dose parameter had a positive effect on TNT degradation by ozonation and multistage ozonation biological treatment. They reported that ozone dosage less than 0.177 g/L had a negative effect on TNT degradation.
Effect of reaction time on TNT removal rate
Figure 8 shows TNT degradation with simple ozonation. These runs were studied at optimum pH = 10 with initial TNT concentration of 40 mg/L. Due to the many TNT treatment studies that were achieved at high concentration (40 mg/L as initial concentration), the final stage of this experiment was performed with 40 mg/L as the initial concentration. It is apparent that more than 80% of TNT was oxidized in a reaction time of 60 min (TNT residual concentration = 8 mg/L). There is a linear relation between an increase of run time and TNT degradation rate in simple ozonation. We also studied TNT degradation rate with H2O2 alone at different reaction times at optimum pH of 3 and the same TNT concentration. It is learned from Figure 9 that although an increase in reaction time has a positive effect on TNT degradation rate, it is rather stable around 60% (TNT residual concentration = 17 mg/L) in reaction time of 45–60 min. It can be described that the destruction efficiency of TNT seems to be independent of the solubility of ozone (Chen et al. 2007). Therefore, in simple ozonation, TNT degradation rate increases as reaction time increases (Figure 8). In H2O2 alone oxidation, this phenomenon could be reversed; meanwhile, the TNT degradation rate depends on H2O2 solubility and distribution in solution. With the passage of time, hydroxyl free radicals, due to reaction with TNT contaminant, decrease and the degradation rate becomes constant (Figure 9).
Table 1 shows the kinetic results of this study. Table 1 makes it evident that TNT degradation by simple ozonation and direct hydroxide oxygen follows the pseudo-second reaction with R2 > 0.85 and 0.9, respectively.
|.||R2 .||K .||R2 .||K .|
|.||R2 .||K .||R2 .||K .|
Direct H2O2 and simple ozonation processes indicate the second-order behavior with rate constants of 0.019 and 0.033.
TNT degradation by direct H2O2 oxidation and ozonation was separately investigated. The conclusions are as follows:
H2O2 oxidation alone is not applicable for high TNT concentrations and could only be effective for low TNT concentrations.
Maximum TNT removal rate (90%) was obtained by the H2O2 oxidation process in the following conditions: pH = 3, initial TNT concentration 10 mg/L, H2O2/TNT molar ratio = 1,000:1; while in simple ozonation it was the same with the following conditions: pH = 10, initial TNT concentration 20 mg/L, ozone dosage of 1 g/hr with reaction time of 30 min.
The kinetic of two processes was studied through fitting data on TNT degradation. The comparison of reaction orders showed that TNT degradation was in accordance with the pseudo-second reaction.
Simple ozonation seems to be the best method for TNT degradation from aqueous solution.
Generally, using hydrogen peroxide in combination with ozonation may be efficient and there is no need to use hydrogen peroxide at a high dose.