Study On The Degradation Of Gaseous Heterocyclic Compounds By Absorption Combined With Electrochemical Oxidation

Heterocyclic compounds and alkanethiols containing nitrogen, sulfur, or oxygen are widely used in chemical industries. These volatile organic compounds could easily become air pollutants and do harm to environment and human beings. Since their toxicity to microorganism,those compounds are recalcitrant to biodegradation, and were regarded as the most refractory compounds. This paper presents a hybrid process of absorption combined with electrochemical oxidation, which could effectively capture those air pollutants into liquid phase,then convert them into biodegradable organic acids due to the hydroxyl radical (·OH) produced in electrochemical reactions.In this paper, pyridine, tetrahydrofuran, thiophene and ethanethiol were selected as the typical heterocyclic compounds and alkanethiols which containing nitrogen,oxygen and sulfur. Water or the ionic liquid 1 -butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) was employed as the absorbents to capture those model pollutants from air stream. Then linear sweep voltammetry (LSV) was used out to investigate their electrochemical oxidation behaviors. A modified ?-PbO2 electrode was used as the working electrode while the counter electrode was platinum wire. The reference electrode (RE) was saturated calomel electrode (SCE) or non-aqueous Ag/Ag+ electrode which depends on the solvent. Electrochemical oxidation experiments were carried out using ?-PbO2 as anode and a stainless steel net as cathode. The selected pollutants and intermediates were monitored and quantified during electrochemical oxidation. According to the analysis results, the electrochemical oxidation mechanisms of heterocyclic compounds and alkanethiols were separately proposed. The kinetics of the electrochemical oxidation process was also analyzed. The main results were as follows:(1) Water was served as absorbents to absorb pyridine in the self-made absorption column. A high efficiency of 95% could be achieved while the inlet concentration was within 10000 mg/m3. LSV voltammograms showed that pyridine can be easily oxidized on the surface of P-PbO2 and the peak oxidation potential was 1.06 V vs. SCE at pH4 and 25 mV/s of scan rate. Pyridine of 100 mg/L could be efficiently oxidized by the OH generated on the surface of ?-PbO2 anode and completely eliminated after 90 min of reaction, under 150 mA/cm2 of current density,0.2 mol/L of Na2SO4 and pH4. Pyridine was converted into fumaric acid, oxalic acid and formic acid. Nitrogen element was probably changed into N2 because the Ion Chromatography could neither detected NH2OH nor nitrite ions. Kinetic model analysis showed that the electrochemical oxidation of pyridine obeyed the pseudo-first order kinetics. Higher current density benefited electrochemical oxidation,while the initial pyridine concentration affected the reaction insignificantly.(2) When water was used as absorbents to absorb tetrahydrofuran (THF) in the self-made absorption column, a high efficiency of 95% could be achieved while the inlet concentration was within 3000 mg/m3. LSV showed that THF could be easily oxidized on the surface of ?-PbO2 in NaCl and Na2SO4 medium, the oxidation peak potentials were 1.30 V and 1.20 V vs. SCE respectively, at pH5.5 and 25 mV/s of scan rate. The NaCl medium benefited the electrochemical oxidation of THF due to the indirect oxidation of chlorine producing on the surface of anode. THF of 200 mg/L could be efficiently oxidized by the ·OH generated on the surface of ?-PbO2 anode and completely eliminated within 180 min of reaction, under 50 mA/cm2 of current density, 10 g/L of NaCl and pH3. THF was converted into succinic acid.4-hydroxybutyrate and y-butyrolactone were not detected by gas chromatography mass spectrometry (GC-MS), implying that the oxidation pathway of THF under the attack of OH was starting via the cleavage of the ring. Kinetic model analysis showed that the electrochemical oxidation of THF obeyed the pseudo-first order kinetics. Higher current density benefited electrochemical oxidation, while the initial THF concentration affected the reaction insignificantly.(3) [BMIM]BF4 was also an absorbent to absorb ethanethiol from air stream. The max amount of ethanethiol absorbed by 10 g [BMIM]BF4 was 0.73 g, which was six times of water and twice of the organic solvent di (2-ethylhexyl) adipate (DEHA)LSV showed that ethanethiol could be easily oxidized on the surface of ?-PbO2 in[BMIM]BF4 without adding any other electrolytes or adjusting the pH value. The oxidation peak potentials were 1.34 V vs. RE(25 mV/s). Ethanethiol of 1000 mg/L could be efficiently oxidized by the ·OH generated on the surface of ?-PbO2 anode and completely eliminated within 90 min of reaction under 50 mA/cm2 of current density. Ethanethiol was converted into acetic acid while the S atom was changed into SO42-. Kinetic model analysis showed that the electrochemical oxidation of ethanethiol obeyed the pseudo-first order kinetics. Higher current density benefited electrochemical oxidation, while the initial ethanethiol concentration affected the reaction insignificantly.(4) [BMIM]BF4was used as absorbents to absorb thiophene from air stream. The max amount of thiophene absorbed by 10 g [BMIM]BF4 was 0.65 g, which was far more than water and DEHA. LSV showed that thiophene could be easily oxidized on the surface of ?-PbO2 in [BMIM]BF4 without adding any other electrolytes or adjusting the pH value. The oxidation peak potentials were 1.15 V vs. RE(25 mV/s).Thiophene of 1000 mg/L could be efficiently oxidized by the OH generated on the surface of P-PbO2 anode and completely eliminated within 60 min of reaction under 50 mA/cm2 of current density. Thiophene was converted into fumaric acid, maleic acid and oxalic acid, while the S atom was changed into SO42-. Kinetic model analysis showed that the electrochemical oxidation of thiophene obeyed the pseudo-first order kinetics. Higher current density benefited electrochemical oxidation, while the initial thiophene concentration affected the reaction insignificantly.In summary, the hybrid process which combines absorption with electrochemical oxidation, could effectively capture those air pollutants into liquid phase, then convert them into biodegradable organic acids due to the hydroxyl radical (·OH) produced in electrochemical reactions. This process offers a clean and environmental friendly technology for detoxification of heterocyclic copounds and deodorization of alkanethiol.