Using Photocatalytic Oxidation and Analytic ... - ACS Publications

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX-XXX

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Using Photocatalytic Oxidation and Analytic Techniques To Remediate Lab Wastewater Containing Methanol Qing Xiong,† Mingliang Luo,*,† Xiaoming Bao,‡ Yurong Deng,† Song Qin,† and Xuemei Pu† †

School of Chemistry, Sichuan University, Chengdu 610064, P. R. China Shimadzu Co., Ltd., Chengdu 610064, P. R. China

‡

S Supporting Information *

ABSTRACT: This experiment is dedicated to second-year and above undergraduates who are in their experimental session of the analytical chemistry course. Grouped students are required to use a TiO2 photocatalytic oxidation process to treat the methanolcontaining wastewater that resulted from their previous HPLC experiments. Students learn to assemble a specified apparatus for a chemical reaction and analytical methods for the determination of environmental pollutants and reaction intermediates. The comprehensive experiment can give students a deep insight into application of several instruments (COD analyzer, UV−vis, GC-FID, and GC− MS) and the principle of photocatalytic oxidation reaction. Meanwhile, upon the completion of the experiment, students not only become aware of the wastes they generate but also understand how chemistry can be utilized for environmental remediation.

KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Hands-On Learning/Manipulatives, Environmental Chemistry, Analytical Chemistry, Catalysis, Photochemistry, Chromatography, Mass Spectrometry

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INTRODUCTION Facing the urgent need for new technologies to contain increasing environmental degradation, chemical educators have introduced to students the theory and application of semiconductor photocatalysis, in particular, the TiO2-mediated system, which is thought to be economically and environmentally benign.1−7 In this Journal, there are many reports of teaching experiments dedicated to help students gain insight into the chemistry of photocatalysis.8−19 Tatarko et al. introduced an inexpensive photocatalytic reactor, using a glass beaker and long durable fluorescent lamp, to illustrate the remediation of azo dye in water.8 Herrera-Melina et al. reported the photocatalytic remediation of lab wastewater containing pnitrophenol, which was referred to as a pedagogical use of lab waste.9 Ibanez et al. presented an experiment illustrating the simultaneous oxidation (of an organic species) and reduction (of metal ion Cu2+) process of photocatalysis.10 To make the experiments more spectacular, simulated wastewaters containing dyestuffs are often preferred because the color change is easily visible and thus few instruments are needed.8,11−13 In this article the lab waste containing methanol (ca. 60 wt %) was explored to develop a comprehensive experiment for second-year students and above. The waste arose from their previous experiment, analysis of caffeine in an APC tablet by HPLC. The use of this real wastewater can help students know © XXXX American Chemical Society and Division of Chemical Education, Inc.

that pollution may arise from their daily life, and therefore, every care should be taken for the sake of environment. The waste, after being properly diluted, undergoes TiO2-mediated photocatalytic oxidation in the presence of hydrogen peroxide. Both quantitative and qualitative analyses of samples are carried out to follow the progress of reaction. Several instruments were incorporated into the experiment including COD analyzer, UV−vis, GC-FID, and GC−MS, and thus supported well their ongoing course of analytical chemistry. In particular, mass spectral analysis of the intermediates arising from the photocatalytic reaction seems both challenging and appealing for the students who have relatively limited knowledge in chemistry. In general here we present students not only a live example of chemistry serving environment but also the employment of different instruments for chemical research.

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EXPERIMENTAL PROCEDURE

Equipment and Chemicals

The equipment required in this experiment includes the following: high-pressure mercury lamp (150 W, Shanghai Jiguang Light Source Company, China), GC-FID (SCION 456, Received: December 19, 2016 Revised: October 1, 2017

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DOI: 10.1021/acs.jchemed.6b00988 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Analytic Procedure

Techcomp, China), chemical oxygen demand analyzer (CTL12, Chengde Huatong Company, China), UV−vis (UV2450, Shimadzu, Japan), and GC−MS (QP2010, Shimadzu, Japan). The chemicals for this lab include the following: P25 TiO2 photocatalysts (Degussa, Germany), 30% H2O2 (Chengdu Jinshan Chemical Reagent Company, China), and n-butyl alcohol (Chengdu Jinshan Chemical Reagent Company, China).

COD analysis was carried out on the COD analyzer using a potassium dichromate method in accord with the national standard of China (HJ/T 399-2007).20 The concentration of methanol in the reaction solution was measured by GC-FID analysis using n-butyl alcohol as an internal standard. The intermediates of photodegradation were identified by GC−MS analysis. Details of the above analytical processes are provided in Supporting Information.

Wastewater Origin

The wastewater containing methanol is generated in the previous student experiment, HPLC analysis of caffeine in an APC tablet. It is transparent and colorless, consisting of ca. 60 wt % methanol and a trace amount of APC with pH around 7.0. The methanol wastewater is diluted using distilled water to around 0.1 wt % methanol in the following experiments.

Students Group

Students were grouped to carry out the experiment. Each group (of six students) was subdivided into three teams of two. Each team took one of three analytic tasks, namely, COD analysis, GC-FID, and GC−MS analysis.

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Experimental Apparatus

Figure 1 shows the apparatus of photocatalytic oxidation of methanol wastewater. The photoreactor is composed of a two-

HAZARDS

UV Light Exposure

The photoreactor should be carefully enveloped with aluminum foil to prevent operators from accidental exposure. Students were required to wear UV-resistant goggles while operating the photoreactor. Methanol Manipulation. It is toxic and may damage the kidney, liver, or eyes. A protective mask and gloves should be worn to avoid inhalation or absorption when dealing with methanol or the methanol wastewater. All operations related to methanol were carried out in a fume cupboard. COD Analysis

Students were instructed to take every care as the concentrated sulfuric acid was used. Eye protection, gloves, and clothing that completely covers the body should be worn.

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Changes of COD and Methanol Content over the Photocatalytic Oxidation Process

Figure 1. Experimental apparatus used in the research.

The changes of COD and methanol concentrations are shown in Figure 2. COD measurement indicates the total amount of aqueous organic pollutants in wastewater. The decrease of COD represents a slow conversion of methanol into inorganic products (CO2 and H2O), which is echoed by the coincidental decrease of methanol concentration. After 4 h of photodegradation, COD declined from ∼1180 to ∼170 mg/L (removal ratio ∼85%) and methanol from ∼770 to ∼10 mg/L

layer glass vessel and a high-pressure mercury lamp (UV lamp) enveloped in a quartz tube. Cooling water circulates in the outer layer to keep the temperature of the reaction solution at 22 ± 1 °C over the experimental period. The irradiation of the UV lamp has a center wavelength of 365 nm according to the specification provided by its manufacturer. The emission spectrum is attached in the Supporting Information.

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RESULTS AND DISCUSSION

EXPERIMENTAL SECTION

Photocatalysis Procedure

The photoreactor was charged with about 1 L of the diluted wastewater. Fine particle TiO2 (0.2 g) was added in, and a milklike suspension is obtained with vigorous stirring. The photoreactor was then wrapped in aluminum foil to avoid escape of UV irradiation. After 30 min in dark conditions, allowing absorption to reach equilibrium, H2O2 (1.5 mL) was added into the suspension, and the UV lamp was turned on. An aliquot (10 mL) of the reaction solution was taken every hour and filtered through a 0.22 μm polytetrafluoroethylene (PTFE) filter prior to analysis. During the reaction, H2O2 (1.5 mL) was added repeatedly every hour of reaction. In total, 6 mL of H2O2 is consumed.

Figure 2. Change of COD and methanol during the photoreaction process. B




Figure 3. Change of GC−MS profile during the photoreaction process.

Figure 4. Mass spectrum of the compounds eluting at 2.2 min (a), 8.2 min (b), 8.5 min (c), and 9.0 min (d).

(removal ratio ∼99%). Compared to the amount of methanol removed, the remaining COD is attributed mainly to the organic intermediates that resulted from the photocatalytic conversion of methanol.

spectral analysis of each peak in the TIC yields molecular ion and fragment ion, which are listed in Figure 4a−d. Figure 4a exhibits significant mass spectral abundance at m/z 29, 31, and 32. The weight at m/z 32 is consistent with the molecular ion of methanol. In Figure 4b, the weight at m/z 60 is assigned to the molecular ion of acetic acid. The ion fragments at m/z 43 and 45 resulted from the loss of OH and CH3 from the molecular ion, respectively. Formic acid (Figure 4c) is eluted at 8.5 min with ion fragments at m/z 29 and 45, which are attributed to the loss of OH and H from the molecular ion m/z 46, respectively. Finally, Figure 4d shows a weak molecular ion m/z 62 of ethylene glycol and a strong fragment ion [CH2−OH]+ m/z 31, suggesting the C−C bond

GC−MS Analysis of Intermediate Products

GC−MS was employed to identify the intermediates that were formed as the photodegradation proceeded. The change of total ion chromatogram (TIC) was shown in Figure 3. There are four significant peaks occurring at 2.2, 8.2, 8.5, and 9.0 min, respectively. According to a mass spectral search of the 2014 NIST library, those products are (a) methanol, (b) acetic acid, (c) formic acid, and (d) ethylene glycol, respectively. Mass C




sampling and instrumental analysis, and data interpretation and drawing conclusions. A teacher may accompany the students to provide assistance when needed.

of the compound is readily broken. We present in Figure 5 the fragmentation process of formic acid as an example to illustrate how a molecule is fragmentized.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00988. Instructor notes, students’ experimental procedure, and questions for students (PDF) (DOC)

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Figure 5. Fragmentation process of formic acid molecule.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

As shown in Figure 3, the changes of each product suggest that (1) methanol is gradually diminished, (2) both formic acid and ethylene glycol first evolved and then diminished, and (3) acetic acid is gradually developed and reaches a summit at the end of the reaction. This observation agrees well with our common knowledge that acetic acid is a relatively stable compound and is often the final product of an oxidation process of organic compounds. The remaining COD (∼170 mg/L in Figure 2) is thus ascribed largely to acetic acid. Figure 6 shows a possible scheme of how formic acid, acetic acid, and ethylene glycol are formed.21,22 It demonstrates the

ORCID

Mingliang Luo: 0000-0002-5017-4506 Xuemei Pu: 0000-0002-5519-4258 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the students who took part in the experiment session and Experimental Technology Program of Sichuan University for financial aid.

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REFERENCES

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Figure 6. Formation pathways of formic acid, acetic acid, and ethylene glycol.

central role of free radicals, including (i) H-abstraction, (ii) joining of two radicals, and (iii) oxidation processes promoted by hydroxyl radicals. The experiment presented here takes one teaching day. The photocatalytic reaction process may take 4.5 h, and the analysis of samples using different instruments takes about 2 h. This experiment is recommended to second-year students of chemistry, environmental sciences, and chemical engineering, and above. After one year of training in fundamental chemistry experiments, the students’ lab skills may be considered medium or high. The students need to carry out a more complicated experiment which includes assembling a complex apparatus, D




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Using Photocatalytic Oxidation and Analytic ... - ACS Publications

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