디젤로 오염된 지하수의 오존산화처리에 대한 연구

권충일, 공성호 김무훈*

한양대학교 화학공학과, 삼성엔지니어링 기술연구소*

A study on the Ozone oxidation of diesel-contaminated Groundwater

Choong-Il Kwon,  Sung-Ho Kong, Moo-Hoon Kim*

Dept. of Chemical Eng., Hanyang Univ. Samsung Engineering R&D Center*

Introduction

Remediation of contaminated soils and groundwater has gained more attention as a result of the discovery of thousands of contaminated sites.  The US EPA reported that many volatile compounds (VOCs) were existed in many groundwater wells. In Korea, numerous subsurface contamination by leakage from underground storage tanks (USTs) was also reported, recently.

To remediate contaminated soils and groundwater various advanced oxidation processes (AOPs) have been developed.  The most common chemical oxidation arrangements are ozone, ozone/UV, ozone/H2O2, Fenton reaction, photooxidation etc..

Especially ozone is considered to be an attractive method in chemical oxidation processes due to its high oxidant capacity (E0=2.08V) and electrophilic character.  Hydroxyl radicals were also produced during ozonation process that has even higher oxidation potential (E0=3.06V).

Hydroxyl radical(OH·) is a highly reactive transient that can rapidly oxidize most organic substances, a number of studies have focused on its role as an oxidant in the environment.

However few studies have been performed to estimate the hydroxyl radical production rate in ozone oxidation though it has an important effect in remediation process.

In this study, we investigated the diesel decomposition in groundwater by ozonation process for further pilot-scale application at site and quantified hydroxyl radical generation rates at various conditions.(Ozone feed rate, the effect of hydroxyl radical scavenger, pH, addition of hydrogen peroxide)

Experiment

1) Diesel decomposition by ozonation

The reactor vessel was a cylindrical and ozone-rich air was introduced into the bottom of the reactor via diffuser. Ozone gas was provided by an ozone generator(OZATR CFS-2A,Ozonia Co.). Gas phase ozone measurements, off gas as well as input gas, were made by an in-line ozone monitor(Afx series H1-S) and aqueous phase ozone concentration was followed by indigo procedure.  To determine changes of diesel decomposition in aqueous solution, TOC(TOC analyzer, Sievers.Co) was measured with varying ozone dosage.

Diesel was purchased from L & S Co. and each was mixed.  Sodium bicarbonate as a hydroxyl radical scavenger was supplied by Junsei Co..

Simulated groundwater(SG) was prepared based on the major components of J site groundwater that was contaminated with diesel. All experiments were performed with SG and the pH was adjusted with hydrogen chloride(HCl) and sodium hydroxide(NaOH).

2) Estimation of hydroxyl radical production rate

Nitrobenzene(Junsei Co.) was selected as a probe compound because it has high reactivity(kOH=3.9*109M-1s-1) with hydroxyl radical, but little reacts with ozone(6M-1s-1). Tert-butyl alcohol, a hydroxyl radical scavenger, was obtained from Aldrich. Nitrobenzene was determined by gas chromatography(HP 6890) with electron capture detector after liquid-liquid extraction with n-hexane.

As for oxidation test, residual ozone and radicals were quenched with H2SO4 after sampling.

Results and discussion

Figure 1 shows the remaining of TOC in diesel-saturated SG. Air purging itself could not lower TOC values. TOC values were only lowered by ozonation process and the higher ozone concentration, the higher removal of TOC.

Figure 2 presented the effect of hydroxyl radical scavenger in SG. As a result of groundwater analysis, bicarbonate which has high reactivity with hydroxyl radical, was major component in groundwater as a scavenger. In figure 2, the more bicarbonate concentration, the lesser TOC removal.

The removal rate was lesser in SG than in deionized water, which was not recorded in this paper, these results indicated that the hydroxyl radical was quenched by scavengers such as carbonate and bicarbonate.

Figure 1. Diesel decomposition by ozonation in Simulated groundwater

Figure 2. The effect of hydroxyl radical scavenger on diesel decomposition

Figure 3 shows the results of TOC removals at different pH. It indicated that the pH was not effected on diesel decomposition below neutral pH, but removal efficiency was high at alkaline conditions. These results were due to hydroxyl radical formation through ozone decomposition by hydroxide ions(OH-). Figure 4 presents the results of addition of H2O2 in ozonation system. As a result of experiment in SG, the H2O2/O3 system was far more effective than ozone alone and the optimal H2O2/O3 molar ratio was 1.0. Comparing with figure 3 and 4, addition of hydrogen peroxide was better results than pH adjustments indicating that hydroperoxide ion(HO2-) was superior to hydroxide ion(OH-) in hydroxyl radical formation.

Figure 3. Diesel decomposition by ozonation at different pH solution

Figure 4. Diesel decomposition in O3/H2O2 system

Figure 5 shows the nitrobenzene decomposition by varying ozone flow rate for estimating the hydroxyl radical generation rate.  In this study, where air at 60Lhr-1 was passed through the solution, shown no evidence that volatilization takes place. Also as previously mentioned, only hydroxyl radical decomposes the nitrobenzene.  In figure 5, nitrobenzene decomposition rate was pseudo-first order(r2>0.95), and it was increased as the ozone concentration was increased. Using Zepp's equation, hydroxyl radical generation rate was calculated with above results. It was proportional to ozone concentration linearly, and it was generated 0.24mol to 1mol ozone.

Figure 5 Influence of ozone flow rate on nitrobenzene decomposition

Figure 6. Hydroxyl radical generation rates with ozone flow rates

References

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3. F.J. Beltran, G. Ovejew, H.M. Encinar, and J. Rivas, □□Oxidation of polynuclear aromatic hydrocarbons in water. 1. Ozonation□□, Ind. Eng. Chem. Res., 34, 1995

4. R.G. Zepp, B.C. Faust, and J. Hoigne, □□Hydroxyl radical formation in aqueous reactions (pH 3-8) of iron (Ⅱ) with hydrogen peroxide: The photo-Fenton reaction□□, Environ. Sci. Technol., 26, 1992