De-min YANG*, Bing WANG Hong-yang REN Jian-mei YUAN

1. School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China

2. Chongqing Key Laboratory of Exogenic Minerallization and Mine Environment, Chongqing Institute of Geology and Mineral Resources, Chongqing 400042, P. R. China

3. Chongqing Research Center of State Key Laboratory of Coal Resources and Safe Mining, Chongqing 400042, P. R. China

Effects and mechanism of ozonation for degradation of sodium acetate in aqueous solution

De-min YANG*1,2,3, Bing WANG1, Hong-yang REN1, Jian-mei YUAN1

1. School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China

2. Chongqing Key Laboratory of Exogenic Minerallization and Mine Environment, Chongqing Institute of Geology and Mineral Resources, Chongqing 400042, P. R. China

3. Chongqing Research Center of State Key Laboratory of Coal Resources and Safe Mining, Chongqing 400042, P. R. China

The degradation efficiencies and mechanism of ozonation for the degradation of sodium acetate in aqueous solution were investigated under atmospheric pressure at room temperature (293 K). The effects of the initial pH value, reaction time, and concentrations, CaCl2, and Ca(OH)2on the removal rate of chemical oxygen demand (COD) were studied. The results indicated that ozonation obviously improved the degradation rate of sodium acetate when the pH value of the solution was not less than 8.5. A suitable long reaction time may be helpful in increasing the COD removal rate, and a removal rate of 36.36% can be obtained after a 30-minute treatment. The COD removal rate increased firstly and decreased subsequently with the increase of theconcentration (from 0 to 200 mg/L), and under the same experimental condition it reached the optimum 34.66% at theconcentration of 100 mg/L. The COD removal rate was 5.26% lower when the concentration ofwas 200 mg/L than when there was no. The COD removal rate decreased by 15.68% when theconcentration increased from 0 to 200 mg/L.has a more obvious scavenging effect in inhibiting the formation of hydroxyl radicals than. CaCl2and Ca(OH)2could increase the degradation efficiency of sodium acetate greatly, and the COD removal rates reached 65.73% and 83.46%, respectively, after a 30-minute treatment, 29.37% and 47.10% higher, respectively, than with single ozone oxidation. It was proved that the degradation of sodium acetate in the ozonation process followed the mechanism of oxidization with hydroxyl free radicals (·OH).

ozonation; sodium acetate; hydroxyl free radical; chemical oxygen demand (COD)

1 Introduction

Ozone generally exists as a colorless reactive gas and strong oxidizing agent at room temperature with a special smell. The reaction of ozone with organic or inorganic compounds in aqueous media has achieved a variety of treatment goals, such as bacterial disinfection andviral inactivation, decolorization, removal of taste and odor, organics control, and destruction of inorganics, and can improve the biodegradability of biorefractory and natural organic pollutants and flocculation. It has recently drawn much attention in water treatment technology (Hoigné 1998; Camel and Bermond 1998; Zhao et al. 2006).

Ozone has emerged as an environmentally safe oxidant and disinfector for water and wastewater treatment. The advantage of ozone over other oxidants is that the degraded products of ozonation are generally non-toxic: its final products are CO2and H2O, and the residual O3in the system transforms into O2in few minutes. Ozonation treatment of drinking water has a history of more than 100 years. Ozone was first used in 1893 for water sterilization at Oudshoorn, in the Netherlands (Guendy 2007).

It is well known that ozonation can act in two ways: direct oxidation (molecular ozone reaction) or indirect hydroxyl free radical (·OH) oxidation reactions (Staehelin and Hoigené 1982). The direct reaction is selective and generally incomplete. The indirect way consists of the generation of ·OH, which have a high oxidation potential (E0=2.80 V) and can promote the degradation of a large number of pollutants in a few minutes (Hoigné and Bader 1983a, 1983b). This latter action is not selective and can lead to complete mineralization of pollutants (Sarasa et al. 2002). The reaction rate of ·OH with organic molecules is usually in the magnitude of 106to 109L/(mol·s) (Amat et al. 2005). The pH value of the solution signi fi cantly in fl uences the decomposition of ozone in water. Basic pH level is favorable for ozone decomposition.

In this study, sodium acetate was selected as the model pollutant in water and the efficiency and mechanism of degradation of sodium acetate in aqueous solution with ozonation were investigated. The effects of initial pH value, reaction time, and the concentrations of,, CaCl2, and Ca(OH)2on the removal rate of COD were studied.

2 Materials and methods

2.1 Materials and reagents

The raw water in this study was prepared with sodium acetate, which was an analytical grade reagent with a purity of 99% and was produced by Chengdu Kelong Chemical Reagent Factory, in China, in tap water, and the COD value of the standard solution was about 250.00 mg/L. Other chemicals used in the experiments, anhydrous sodium carbonate, sodium bicarbonate, calcium chloride hexahydrate, calcium hydroxide, hydrochloric acid, potassium iodide, and sodium hydroxide, were also analytical grade reagents produced by the ChengduKelong Chemical Reagent Factory of China.

All glass wares were soaked in chromic acid, and then rinsed with tap water and distilled water.

2.2 Ozonation procedure

The experiments were carried out in an ozonation reactor (with a height of 600 mm, an inside diameter of 50 mm, and a volume of 500 mL) made of stainless steel (shown in Fig. 1). Before the experimental operation, the ozonation reactor was pre-ozonated for three minutes and washed five times with deionized water to minimize the potential side effects of the residues in the reactor. Ozone was produced from pure oxygen (produced by Chengdu Xinju Chemical Co., Ltd., in China, with a purity of 99%) through an CFJ-5 ozone generator (manufactured by Chengdu Starry Observation and Control Engineering Co., Ltd., in China, with a maximum ozone generation capacity of 2 mg/min) at a power of 50 W, and was subsequently fed into the ozonation reactor to have thorough contact with water samples through microporous diffuser at the bottom of the reactor. The ozone exhaust was absorbed with a KI solution. In the ozonation experiments, the raw water (500 mL) with sodium acetate was all fed into the reactor at one time. The ozonation time was controlled at five minutes for all the samples. Water samples (each is 20 mL) were taken from the reactor at various reaction time to analyze the COD value. The oxidative reaction was stopped with the addition of a small amount (1 mL) of sodium thiosulphate solution with a molar concentration of 0.025 mol/L.

Fig. 1 Schematic diagram of ozonation setup

2.3 Analytical methods

The concentration of ozone in the gas was measured with the iodometric titration method (Rakness et al. 1996). The concentration of the residual ozone in aqueous solution was measured with a spectrophotometer using the indigo method (Bader and Hoigné 1981). The pH value was recorded with a pHS-2 precision pH meter (manufactured by Leici Instrumental Factory in Shanghai, China). The COD value was determined through the potassium dichromate method (EPBC 2002).

3 Results and discussion

3.1 Effect of initial pH value on COD removal rate

In water, the decomposition rate and oxidation capacity of ozone are highly dependent on the pH value. High pH values lead to the formation of ·OH through an indirect route, which is a source of ·OH.

Experiments were carried out at various pH values in order to verify the effect of the initial pH value of the solution on the COD removal rate. The pH values of the solution were adjusted to 7.5, 8.5, 9.5, and 10.5 using sodium hydroxide with a concentration of 200 mg/L. After each experiment, a portion of the treated effluent was taken out and filtered. The clear liquid was separated and its COD value was measured. The results are shown in Fig. 2.

Fig. 2 Effect of pH value on COD removal rate

From Fig. 2 it can be seen that the increase of the pH value has obviously improved the removal efficiency of COD. At a pH value of 7.5, only 12.30% of COD was removed after a reaction time of 30 minutes. The removal rates of COD were about 26.61%, 31.37%, and 36.36% at pH values of 8.5, 9.5, and 10.5, respectively, after a reaction time of 30 minutes. The best degradation efficiency of sodium acetate was obtained when the pH value was 10.5. When the pH value increased from 7.5 to 10.5, the removal rate of COD increased from 12.30% to 36.36% after a reaction time of 30 minutes. This indicates that the initial pH value is the main factor affecting the removal rate of COD. A possible explanation of this phenomenon is that the concentration of OH-ions increased with the pH value of the solution. Higher pH value promoted the decomposition of ozone, producing more ·OH radicals for the effective oxidization or degradation of organic acids, thus a higher COD removal rate was obtained.

3.2 Effect of reaction time on COD removal rate

Fig. 3 shows the effect of reaction time on the degradation efficiency of sodium acetate with the ozonation process at a pH value of 10.5 and at an ozone rate of 8.00 mg/min. It shows that a suitable long reaction time may be helpful in increasing the COD removal rate. It was also observed from the experiment that the reaction rate of sodium acetate with ozoneincreased slightly with the reaction time, and the COD value decreased from 250.00 mg/L to 101.15 mg/L at a reaction time of 60 minutes: the removal rate was about 59.54%, 23.18% higher than that at the reaction time of 30 minutes.

Fig. 3 Effect of reaction time on COD removal rate at pH value of 10.5

In natural waters, there exists a balance betweenand. Inorganic carbon exists mainly in the form ofunder natural pH conditions (with a pH value from 6 to 10.3) and in the form ofunder high-alkalinity conditions (with a pH value larger than 10.3). Carbonate mainly exists in surface and ground waters with a typical concentration range of 50 to 200 mg/L (Zhao et al. 2006). Bicarbonate ions and carbonate ions react with ·OH in competition with organic pollutants, which have a high reaction activity. For instance, the rate constants were 7.9×107L/(mol·s) and 4.2×108L/(mol·s), respectively, for bicarbonate ion and carbonate ion in Acero and Gunten (2000).

In order to verify the degradation efficiency of sodium acetate with the ozonation process, the two ·OH inhibitorsandwere added into a kind of molecular ozone that produces ·OH under alkaline conditions, and the effects ofand(with same concentrations of 0 mg/L, 50 mg/L, 100 mg/L, 150 mg/L, and 200 mg/L), on the degradation efficiency of sodium acetate with the ozonation process were investigated. The results are shown in Fig. 4.

Fig. 4(a) shows the effect of theconcentration on the COD removal rate. The results indicate that the COD removal rate both increased firstly and decreased subsequently with the increase of theconcentration from 0 to 200 mg/L, and under the same experimental conditions it reached the optimum 34.66% at theconcentration of 100 mg/L. The removal rates of COD were about 26.61%, 30.79%, 34.66%, 24.12%, and 21.35% at theconcentrations of 0 mg/L, 50 mg/L, 100 mg/L, 150 mg/L, and 200 mg/L, respectively, after a reaction time of 30 minutes. The maximum COD removal rate decreased by 5.26% at the concentration ofof 200 mg/L compared to that without, indicating that at a low concentration of, the degradation of sodium acetate is promoted by ozone, however, at a high concentration of, which acts as an ·OHscavenger, the reaction of ·OH with the organic matter is inhibited. It also indirectly proves the studies about the effect of bicarbonate concentration on the trace nitrobenzene in aqueous solution with ceramic honeycomb-catalytic ozonation (Zhao et al. 2007).

Fig. 4 Effects ofandconcentrations on COD removal rate

Fig. 4(b) shows the effect of theconcentration on the COD removal rate. It can be seen from Fig. 4(b) that the existence ofreduced the COD removal rate greatly, and the COD removal rate decreased with the increase of theconcentration. The data shows that the existence ofaffected the degradation efficiencies of COD, and the COD removal rate reached 31.17% at aconcentration of 50 mg/L. When theconcentrations were 100 mg/L and 150 mg/L, the COD removal rates were about 28.78% and 26.44%, respectively. Increasingconcentration reduced the COD removal rate from 36.36% at aconcentration of 0 mg/L to 20.68% at 200 mg/L: the removal rate decreased by 15.68% at theconcentration of 200 mg/L compared to that without. This can be explained in thatinhibits the production of ·OH, thus inhibiting the oxidation process of sodium acetate. From Figs. 4(a) and (b), it can also be observed thathas a more obvious scavenging effect in inhibiting the formation of ·OH than(Yang et al. 2006).

The above-mentioned results further proved that the degradation efficiencies of sodium acetate with the ozonation process, except for the direct oxidization of ozone molecules, were greatly dependent on the decomposition of ozone in the alkali solution, which produced ·OH. ·OH further reacted with sodium acetate, resulting in a high COD removal rate. The results also proved that the ozonation process followed the mechanism of oxidization by ·OH to a certain extent.

3.4 Effects of CaCl2and Ca(OH)2on COD removal rate

It has already been reported thatandare the most important ·OH inhibitors in natural waters, and they can compete with organics to react with ·OH in water and react with ·OH at a high rate, reducing ·OH. This reaction cannot reproduce new radicals topromote the chain reaction, so the whole radical chain reaction was interrupted and the oxidation reaction process terminated (Hoigné et al. 1985; Acero and Gunten 2000; Hua et al. 2006).

Adjustment of the pH value and addition of Ca2+to shield the inhibition ofandon ·OH were investigated in this study, and two group of experiments were carried out. Under basic conditions of the ozonation process (the temperature was 293 K, the ozone concentration was 100 mg/L, the reaction time was 30 minutes, and the pH value was 10.5), the first experimental group adjusted the pH value to 10.5 using sodium hydroxide (the concentration is 200 g/L), and then added excess CaCl2solution (the mass concentration was about 50 g/L); the second experimental group adjusted the pH value to 10.5 only using Ca(OH)2(the concentration of Ca(OH)2was about 100 g/L). The results are shown in Fig. 5.

Fig. 5 Effects of CaCl2and Ca(OH)2on COD removal rate

Fig. 5 shows that the added CaCl2and Ca(OH)2have a remarkable promoting effect on the degradation efficiencies of sodium acetate with ozone. The COD removal rate was improved, COD removal rates of 65.73% and 83.46% were obtained, respectively, with the additions of CaCl2and Ca(OH)2at the reaction time of 30 minutes, which were, respectively, 29.37% and 47.10% higher than the results of oxidation only with ozone, and there still existed a rising tendency. This proves that the added Ca2+has effectively shielded the production ofandcaused by the complete oxidation of organic pollutants, and also that it promoted the progress of the ozonation reaction and increased both the utilization ratio of ozone and the removal rate of organic pollutants. At the same time, the shielding efficiency of Ca(OH)2was better than that of CaCl2, and the COD removal rate with Ca(OH)2was 17.73% higher than that with CaCl2. Since the raw materials (CaO) of Ca(OH)2are very cheap, the CaO powders are recommended to adjust the pH value in industrial processes, and to reduce the costs of wastewater treatment.

4 Conclusions

In this study, the ozonation process was applied to the degradation of sodium acetate in aqueous solution. The effects of the initial pH value, reaction time, and concentrations ofand Ca(OH)2on the removal rate of COD were studied; the degradation efficiencies and mechanism of sodium acetate in aqueous solution with the ozonation process were investigated. The following conclusions can be made based on the present investigation:

(1) A removal rate of 36.36% can be achieved at a pH value of 10.5 after a 30-minute treatment. The optimum reaction time is 30 minutes taking into consideration the removal rate and treatment cost. The COD removal rate increased firstly and decreased subsequently with the increase of the concentration offrom 0 to 200 mg/L, and it reached the optimum 34.66% at theconcentration of 100 mg/L; the COD removal rate decreased by 5.26% when theconcentration was 200 mg/L compared to that when there was no. The existence ofdecreased the COD removal rate greatly, and the COD removal rate decreased with the increase of theconcentration. When theconcentration was 200 mg/L, the removal rate of COD reached 20.68%, 15.68% lower than when there was nowas obviously more effective in restraining the formation of ·OH than. The ozonation process followed the mechanism of oxidization by ·OH to a certain extent.

(2) Additions of CaCl2and CaO have a remarkable promoting effect on the degradation efficiencies of sodium acetate by ozonation. COD removal rates of 65.73% and 83.46% were obtained at a reaction time of 30 minutes with the additions of CaCl2and Ca(OH)2, respectively, which were, respectively, 29.37% and 47.10% higher than the result of oxidation only with ozone, and the removal rates were still rising. CaO powders are recommended to adjust the pH value in industrial processes, and can reduce the costs of wastewater treatment.

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(Edited by Yun-li YU)

This work was supported by the Key Projects in the National Science and Technology Pillar Program during the Twelfth Five-Year Plan Period (Grant No. 2011BAC06B05) and the Open Fund of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Grants No. PLN1126 and PLN1127).

*Corresponding author (e-mail:yangdemin8628@163.com)

Received May 6, 2011; accepted Feb. 27, 2012