electrochemical degradation of geosmin using electrode of ti/iro2–pt
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Desalination 265 (2011) 135–139
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Electrochemical degradation of geosmin using electrode of Ti/IrO2–Pt
Qiang Xue a, Miao Li a,b, Kazuya Shimizu c, Motoo Utsumi a, Zhenya Zhang a,Chuanping Feng b, Yu Gao a, Norio Sugiura a,⁎a Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japanb School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, Chinac Faculty of Life Sciences, Toyo University, Gunma 374-0193, Japan
⁎ Corresponding author. Graduate School of LifeUniversity of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki29 853 4916.
E-mail address: [email protected] (N.
0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.07.043
a b s t r a c t
a r t i c l e i n f oArticle history:Received 19 February 2010Received in revised form 21 July 2010Accepted 22 July 2010Available online 21 August 2010
Keywords:Electrochemical degradationGeosminAnode Ti/IrO2–PtSodium chlorideHypochlorous acid
Electrochemical degradation of geosmin was assessed at a typical anode Ti/IrO2–Pt, as a treatment method inmusty odor compounds. The influences of dosage of NaCl, current density, initial geosmin concentration andinitial solution pH on electrochemical geosmin degradation were investigated. HOCl formed duringelectrolysis would play an important role on the oxidation of geosmin. For the Ti/IrO2–Pt anode, geosminconcentration decreased from around 600 to 8 ng/L after 60 min of electrolysis with 3.0 g/L NaCl assupporting electrolyte at the current density of 40 mA/cm2. Moreover, efficient degradation of geosminsolution with different initial pH was found and the electrochemical oxidation ability of Ti/IrO2–Pt anode willnot drop after serviced for long hours under the present experimental condition. These results suggested thatelectrochemical degradation would be a promising treatment for effective and rapid removal of musty odorcompounds.
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1. Introduction
Nowadays, the safety and quality of water supply are beingchallenged by the presence of various pollutants [1,2]. The taste andodor problem in drinking water are especially receiving widespreadattention in water industry, which has been reported in many areas ofthe world [3–6]. One of the typical musty odor compounds, geosmin(trans-1,10-dimethyl-trans-decalol), is able to be produced by manyspecies of cyanobacteria and actinomycetes in aquatic environment[7–9]. Even extremely low levels of geosmin can cause undesirabletastes and odors. This compound can be detected by humans at morethan 2 ng/L [10]. The concentration of 10 ng/L is the current nationalwater quality standard of geosmin for drinking water in Japan [11].Although the offensive odor compound is nontoxic for human beings,it is possible to cause psychosomatic effects, such as headaches, stress,or stomach upsets [10], which results in numerous consumercomplaints and a general perception that the water may not be safefor consumption [12].
Previous research showed that conventional water treatmentprocess cannot effectively decrease dissolved geosmin concentration[13]. Oxidant such as ozone is also not completely effective for theremoval [14–18]. Adsorption using powdered activated carbon (PAC)
or granular activated carbon (GAC) can effectively remove odorcompounds [19], but exhibits reduced efficiency in natural water dueto the competition of other organic materials, which can result inlarger doses of adsorbent being required for effective geosminremoval and increase the cost of construction and running [12]. Lotsof biological treatment studies for the removal of geosmin have beenreported, such as biofiltration [20–24]; however, researches indicatethat the biodegradation process requires a significant period of time tocommence due to acclimation, at the same time, greatly relies on thetypes of organisms present [25–27]. That means effective geosminremoval in a short time using biological method is not easy.
Hence, one approach for the rapid and effective removal ofgeosmin is necessary to be developed. Electrochemistry is a promisingmethod for the water treatment and has received considerableattention recently. Electrochemical degradation applied in watertreatment has been investigated by many researchers [28–33].Electrochemical reaction can effectively oxidize toxic organics[31,32,34–36], demonstrating that this approach may be feasible formusty odor compounds, such as geosmin. Due to simplicity androbustness in structure and operation and no addition of chemicals, itis possible that the electrochemical process can be developed as acost-effective technology for the treatment of aromatic pollutants,particularly for low volume applications [32]. Electrochemical watertreatment effect highly depends on the characters of the anodes thatare used during the process [35–37]. The current efficiency oftraditional electrodes is very low in organic degradation, such asusing graphite and nickel [30]. Electrode of Ti coated with IrO2 anddoped with Pt (Ti/IrO2–Pt), as one of the practical anodes, has been
Fig. 1. Performance of geosmin degradation with different NaCl dosages. Insetrepresents pseudo-first-order kinetic plot using data from the removal curve.
136 Q. Xue et al. / Desalination 265 (2011) 135–139
widely used as anode in electrochemical treatment of contaminatedwater, which has good performance [36,38–40].
To the best of our knowledge, there is little information about thedegradation of geosmin using electrochemical method in previousstudies [13,14,19,24]. The main aim of this study is to evaluate thedegradation of geosmin using electrochemical method. Ti/IrO2–Ptanode was examined for the performance during electrochemicalgeosmin degradation. In order to investigate the geosmin degradationeffect and related reaction mechanisms, NaCl dosage, current density,initial geosmin concentration, initial solution pH, free radicals speciesand oxidizing substance and cyclic voltammograms were measured,and the role of hypochlorous acid formed during electrolysis was alsoanalyzed.
2. Materials and methods
2.1. Materials
Geosmin standard material used in this study was purchased fromWAKO Pure Chemicals Ltd. Osaka. Japan. The authentic sample of20 mg was dissolved in ethanol and diluted in Milli-Q water(resistivity 18.2 MΩ cm at 25 °C) prepared with a water purificationsystem (Purelite PRB-001A/002A) supplied by Organo, Japan. The finalstock solution of 40 mg/L was transferred into 500 mL brown glassbottlewith high gas tightness and stored in thedark at 4 °C prior to use.
A cylindrical electrochemical cell was designed with a networkingvolume of 200 mL. A DC potentiostat (Takasago, EX1500H2) with avoltage range of 0–240 V and a current range of 0–25 Awas employedas power supply. A Cu/Zn electrode of 75 cm2 (15×5 cm) was used asthe cathode and a Ti/IrO2–Pt electrode (TohoTech Company, Japan)with the same area was used as the anode, a distance of 8 mmbetween the two electrodes was set. The immersed areas of the anodeand cathode in the treated solution were the same at 40 cm2.
2.2. Experimental design
In the present study, geosmin solutions with different initialconcentrations of 60, 300, 600 and 1200 ng/L were prepared forelectrolysis experiments. During the electrolysis process 0.5 g/L Na2SO4
was added into all the experiments in order to enhance theconductivity. To investigate the effect of sodium chloride (NaCl) dosageon the geosmin degradation, the NaCl of 1.0, 3.0 and 5.0 g/L was addedinto the geosmin solutions as the supporting electrolyte, respectively.Electrolysis experiments were performed under galvanostatic controlat different current densities (I) of 20, 40 and 60 mA/cm2, respectively.Different initial pH of geosmin solutionwas also investigated during thedegradation process. A 200 mL of synthetic geosmin solution preparedas above was transferred into the electrochemical cell; the reactionstarted with the application of specified current density. Samples weretaken from the electrochemical cell at a regular interval (15 min) foranalysis. The electrolysis was ceased when 1 h elapsed. Time interval of15 min was chosen according to preliminary experiment; geosmin canbe almost degraded completely after 1 h. Three time intervals (5, 15,and 30min) were tried. Five minutes was too short, which wastes thetreated solution and greatly change the volume of the total treatedsolution. Thirty minutes was too long to well describe the process ofgeosmin removal. Finally, 15 min was chosen as the proper timeinterval.
To control for any losses of geosmin due to factors other thanelectrochemical degradation, extra cell was set in the same conditionexcept the existence of electrode.
2.3. Cyclic voltammetry
In order to investigate the behavior of Ti/IrO2–Pt anode during theelectrolysis of the geosmin solution, the cyclic voltammetry experi-
ment was operated by a computer controlled CS300 electrochemicalworkstation using a three-electrode cell (ALS Limited, Model 660); Ptwas chosen as the counter electrode, andAg/AgCl electrodewas servedas the reference electrode, and the working anodewas Ti/IrO2–Pt witha size of 1.0×1.0 cm. The electrolyte of 3.0 g/LNaCl solution containing1200 ng/L geosmin was used. The potential was scanned at a scan rateof 200 mV/s, starting from 0 V, and the scan range was run from 0 to1.6 V.
2.4. Detection of free radicals species and oxidizing substance
To measure the production of free radicals and oxidizing substanceformed during the electrochemical treatment, NaCl solution containing0.2 mmol/L RNO (p-nitrosodimethylaniline) was used because RNOreacts rapidly with hydroxyl radicals selectively. The bleaching of RNOsolution by hydroxyl radicals was measured by absorbance changes at440 nm [29]. Samples were taken at intervals of 1, 2, or 5 min andabsorbance of RNO was measured by 722 S spectrophotometer.
2.5. Analytical methods
Geosmin concentration in the samples was quantitatively analyzedby gas chromatography–mass spectrometry system (GC/MS system)equipped with a purge and trap apparatus (P&G: O.I. Analytical 4660,Auto Sampler: O.I. Analytical 4551A, GC: Agilent Technologies 6890 N,MS: Agilent Technologies 5973 inert). The detection limit of geosminwas 1.0 ng/L. Solution pH was measured by pH S-3C precision pH/mVmeter. Surfacemorphologyof anodewas characterized ex situbyatomicforce microscopy (AFM) (Digital Instruments, DimensionTM3000, USA).
3. Results and discussion
3.1. Effect of NaCl dosages
The presence of NaCl was significant for enhancing the degrada-tion efficiency of some organicmaterials [36]. In the experiment, threedifferent NaCl dosages of 1.0, 3.0, 5.0 g/L were used as supportingelectrolyte for geosmin electrochemical degradation. It is obviousfrom Fig. 1 that at a current density of 40 mA/cm2, geosminconcentration decreased from around 600 ng/L to 60, 8, and 4 ng/Lrespectively after 60 min. However, no obvious reduction of geosminwas observed without NaCl addition (data not shown). Sharplyreduced geosmin concentration suggested that NaCl, which could be
137Q. Xue et al. / Desalination 265 (2011) 135–139
oxidized to form a strong oxidant of HOCl, could play an importantrole on degradation of geosmin. The possible process was listed below[33]:
Anode reaction 2Cl−→Cl2 þ 2e
− ð1Þ
Hydrolysis reaction Cl2 þ H2O→HOCl þ Hþ þ Cl
− ð2Þ
As shown in Fig. 1, with 1.0 g/L NaCl as supporting electrolyte, thefinal concentration of geosmin solution is much higher than 10 ng/L,which is the current water quality standard in Japan, after 60 minelectrolysis, while the phenomenon was not observed with 3.0 or5.0 g/LNaCl as supporting electrolyte. This ismay due to the fact of thatHOCl could not be produced enough for complete geosmin degrada-tion with 1.0 g/L NaCl as supporting electrolyte. At the current densityof 40 mA/cm2 with 3.0 or 5.0 g/L NaCl as supporting electrolyte, thefinal concentration of geosmin can meet the current national waterquality standard of geosmin for drinking water in Japan. Theelectrochemical degradation of geosmin at different NaCl dosagesoccurred via a pseudo-first-order mechanism (Fig. 1, inset), withrate constants of 0.0391 min-1 (R2=0.98) at 1.0 g/LNaCl, 0.0701 min-1
(R2= 0.95) at 3.0 g/L NaCl and 0.0813 min-1 (R2=0.97) at 5.0 g/L NaCl.Loss rate caused by volatilization and other factors was not found
during the degradation process in the present study (data not shown).Therefore, the control curves were not shown in all the figures. Duringthe electrochemical degradation of geosmin, CO2 was assumed to bethe final product. However, as the by-products produced during theelectrolysis were not detected, in the further research, it is still neededto be studied to confirm whether toxic by-products exist. At the sametime, further research on reducing the amount of chloride ion usedduring the electrolysis will be needed, so that the concentration ofchloride ion could fall below the risk criteria.
3.2. Voltammetric and electrode surface investigations
According to the cyclic voltammograms obtained for the reductionof geosmin with Pt used as the counter electrode and Ti/IrO2–Ptelectrode as working electrode (Fig. 2), an obvious peak was observedat around E=1.2 V. The peak may be attributed to the oxidation ofmusty odor compound. The electric potential difference was a directproof that a prominent redox reaction existed during the electro-chemical degradation of geosmin. The result agrees with previousfindings reported in the literature for the same electrode on phenoldegradation [36].
Fig. 2. Cyclic voltammograms of the anode Ti/IrO2–Pt obtained at a scan rate of 200 mV/s.Dash line: 3.0 g/L NaCl+0.5 g/LNa2SO4; solid line: 3.0 g/L NaCl+0.5 g/L Na2SO4+600 ng/L geosmin.
It can be seen from Fig. 3 that the surface of the anode appeared toremain unchanged after electrolysis, implying that the electrochem-ical oxidation performance of Ti/IrO2–Pt anode may not decrease afterrepeated use under the present experimental condition, and Ti/IrO2–
Pt anode was suitable for geosmin reduction.
3.3. Influences of current density and initial geosmin concentration
At different current densities of 20, 40 and 60 mA/cm2, geosminconcentration decreased from around 600 ng/L to 18, 8, and 6 ng/L,respectively after 60 min in the presence of a selecting NaClconcentration. It is clear that the geosmin concentration wassignificantly decreased during the first 30 min (Fig. 4). The mecha-nism of geosmin degradation at different current densities wasdemonstrated to be of pseudo-first-order (Fig. 4, inset). The datarevealed that the speed of reduction of geosmin increased with theincreasing of current density. The rate constants are 0.0576 min-1
(R2=0.95) at 20 mA/cm2, 0.0701 min-1 (R2=0.95) at 40 mA/cm2 and0.0755 min-1 (R2=0.98) at 60 mA/cm2, respectively. In the presentexperiment, under the condition that current density was 40 or60 mA/cm2, the final concentration of geosmin couldmeet the currentnational water quality standard of geosmin for drinking water inJapan. Current density of 40 mA/cm2 was the optimal choice if theeconomy factor was taken into consideration.
In order to investigate the treatment efficiency on different initialconcentrations of geosmin, the experiments of electrochemicaldegradation of 1200, 600, 300 and 60 ng/L geosmin solutions werecarried out with a selecting current density and NaCl concentration. It
Fig. 3. AFM photograph of (A) unused and (B) used for electrolysis Ti/IrO2–Pt anode.
Fig. 4. Performance of geosmin degradation with different current densities. Insetrepresents pseudo-first-order kinetic plot using data from the removal curve.
Fig. 6. Performance of geosmin degradation under different initial solution pH.
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is clear from Fig. 5 that at a current density of 40 mA/cm2 with 3.0 g/LNaCl as supporting electrolyte, the geosmin concentration decreasedfrom around 1200, 600, 300 and 60 ng/L to 15, 8, 6, and 3 ng/L,respectively after 60 min, which exhibited high removal efficiency ofgeosmin under both high and low initial concentrations. In addition,when the initial concentration of geosmin was 600, 300 and 60 ng/L,the final concentration of geosmin could meet the current nationalwater quality standard of geosmin for drinking water in Japan.
The rate constant data indicated that the higher initial geosminconcentration resulted in a more rapid degradation of the compound.The most rapid rate of geosmin degradation occurred at a initialconcentration of 1200 ng/L, followed by 600, 300 and then 60 ng/L,with rate constants of 0.0722 min-1 (R2=0.96), 0.0701 min-1
(R2=0.95), 0.0642 min-1 (R2=0.98) and 0.0509 min-1 (R2=0.99)respectively (Fig. 5, inset). In conclusion, the Ti/IrO2–Pt anodeperformed well for electrochemical degradation with appropriatecurrent density and NaCl as supporting electrolyte.
3.4. Performance of geosmin degradation under different initial solutionpH
Three different initial pH of geosmin solution were set toinvestigate the influence on geosmin degradation, and pH 3, 7, and11 represented the acid environment, neutral environment andalkaline environment, respectively. As shown in Fig. 6, the similartrend of the geosmin degradation was observed at the current densityof 40 mA/cm2 with 3.0 g/L NaCl as supporting electrolyte. Geosmin
Fig. 5. Effect of initial concentrations of geosmin on the degradation efficiency. Insetrepresents pseudo-first-order kinetic plot using data from the removal curve.
concentration decreased from around 600 ng /L to less than 10 ng /Lafter 60 min, which revealed high efficiency of degradation, and thedegradation rates did not display notable differences during theelectrochemical degradation process. It means that electrochemicaltreatment can be used to degrade geosmin solution with a wide rangeof initial pH.
3.5. Free radicals production
Fig. 7 revealed that the absorbance of RNO sharply decreased in theinitial 5 min at 40 mA/cm2 with 3.0 g/L NaCl addition, and thebleaching ratio was up to 87%. It was clear that bleaching rate wasfast at the current density of 40 mA/cm2. However, it showed that theabsorbance had almost no change without NaCl addition, indicatingthat formation of hypochlorous acidwas an important bleaching factorduring the electrolysis. Comninellis [29] suggested that hydroxylradicals reacted selectively with RNO, but hypochlorous acid played avery important role in the RNO bleaching from the present study,which would enhance the geosmin degradation. Therefore, NaClexistence is necessary for efficient removal of geosmin.
4. Conclusions
The electrochemical approach is a novel process for effectiveremoval of geosmin. In the present work, effects of dosage of NaCl,current density, initial geosmin concentration and initial solution pHon the performance of geosmin electrolysis were investigated usingTi/IrO2–Pt anode.
Fig. 7. Electrochemical bleaching of 0.2 mmol/L in 3.0 g/L NaCl solution as a function oftreatment time at Ti/IrO2–Pt anode; I=40 mA/cm2.
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It was found that HOCl formed quickly with sodium chloride assupporting electrolyte and that would play an important part in thedegradation of geosmin. For the Ti/IrO2–Pt anode, geosmin concen-tration decreased from around 600 ng/L to less than 10 ng/L after60 min of electrolysis with 3.0 g/L NaCl as supporting electrolyte atthe current density of 10 mA/cm2. The removal ability fluctuatedslightly over the pH range of 3 to 11, which means that Ti/IrO2–Ptelectrode can adapt to a wide range of pH for geosmin degradation.The electrochemical oxidation performance of Ti/IrO2–Pt anode willnot decrease after serviced for long hours at the current density of40 mA/cm2 with 3.0 g/L NaCl as supporting electrolyte.
Acknowledgements
We would like to thank Mito City Waterworks Department for thehelp and the financial support of the Japan Science and TechnologyAgency (JST No. ADD20057) is gratefully appreciated.
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