Research ArticleReceived: 11 October 2011 Revised: 22 December 2011 Accepted: 23 December 2011 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jctb.3742

Electrolysis of progesterone withconductive-diamond electrodesMarıa Jose Martın de Vidales, Cristina Saez, Pablo Canizaresand Manuel Andres Rodrigo∗

Abstract

BACKGROUND: Progesterone is considered an endocrine disruptor chemical. It can be found in industrial discharges, municipalwastewaters, and, in some instances, even in treated effluents at the level of ng dm−3.

RESULTS: Conductive diamond electrolysis can be used to remove progesterone from aqueous solutions. Increases in currentdensity lead to less efficient processes, indicating mass transfer control of the process rate. Occurrence of chlorides in theelectrolytic media favors the depletion of progesterone compared with sulphates, although it does not affect the mineralizationrate. Independently of the solubilizing agent used, the process behaves similarly during a first stage of the electrolysis (at thefour ranges of pollutant concentration studied). However, in a second stage, the rate changes abruptly due to reduced actionof hydroxyl radicals in methanol media.

CONCLUSIONS: Progesterone can be removed efficiently by conductive diamond electrolysis from aqueous solutions within therange of initial concentrations 10−2 to 102 mg dm−3. The process efficiency increases with the current density. Removal ratedoes not depend on the nature of the electrolyte, but this parameter affects the intermediates formed during the experiment.When pure methanol is used as solubilizing agent, only direct electro-oxidation takes place.c© 2012 Society of Chemical Industry

Keywords: conductive diamond; electrochemical oxidation; hormone; wastewater; progesterone

INTRODUCTIONProgesterone belongs to a class of hormones called progestogens,and it is the major naturally occurring human progestogen. Thistype of hormone is widely used in human and veterinary medicinebecause it is strongly related to the estrous cycle.

From the environmental point of view, the occurrence ofthis compound in water is undesirable because it is consideredan endocrine disruptor chemical (EDC) and, consequently, it isrelated to many potential hazards for aquatic organisms, includingfeminization, hermaphroditism and decrease in fertility. In general,EDCs are known to interfere with the mode of action of hormonesin the normal physiological system, and much evidence indicateslinks between compounds that act as EDCs and many sex hormonesensitive diseases/disorders. Occurrence of progesterone in waterused to be related to industrial discharges and to municipalwastewaters, in some instances even in treated effluents. Fromthese sources, it can be found in various aquatic environments,affecting surface and ground water reservoirs of drinking water.1 Atthis point, it is worth stating that progesterone has been detectedin low concentration (at the level of ng dm−3) in the effluents ofmunicipal wastewater treatment facilities2 and in drinking watersources such as river water.3

Its removal has been studied by ozonation,4,5 photocatalysis,6

or solar photo-Fenton,7 but to the authors knowledge no studiesinvolving the electrochemical treatment have been carried out.

Conductive-diamond electrochemical oxidation (CDEO) is con-sidered by many authors8 – 22 as a very robust and effective

technology to remove many types of organics. It can achievevery efficient removal of the organics contained in water or inwastewater, even at very different ranges of concentration. It pro-vides complete mineralization of the organic, usually in a verystrong oxidation process in which intermediates production is notpromoted and in which oxidation-refractory compounds are notproduced. This significant oxidation-potential is usually explainedin terms of the effect of hydroxyl radicals (and hence, it is consid-ered an advanced oxidation technology), but it is also explained interms of the synergistic effect of different oxidation mechanismsincluding mediated oxidation by electrogenerated oxidants suchas peroxosulphates, peroxophosphates, ozone, etc.

With this background, the goal of this work is to determine ifCDEO is a proper technology for the degradation of progesteronefrom waters and wastewaters at different concentration ranges,and also to study the main parameters influencing the degradationprocess in order to obtain information about the characteristics ofthe system.

∗ Correspondence to: Manuel Andres Rodrigo, Department of Chemical Engi-neering, Edificio Enrique Costa, Campus Universitario s/n. 13071 Ciudad Real,Spain. E-mail: manuel.rodrigo@uclm.es

Department of Chemical Engineering, Edificio Enrique Costa, CampusUniversitario s/n. 13071 Ciudad Real, Spain

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MATERIALS AND METHODSChemicalsTwo types of progesterone were used in this work: water-soluble progesterone (WSP) or progesterone/2-hydroxypropyl-β-cyclodextrin (7/93), where 2-hydroxypropyl-β-cyclodextrin is astabilizing agent,23 and pure progesterone (>99.6% purity). Bothreagents were supplied by Sigma-Aldrich Laborchemikalien GmbH(Steinheim, Germany). Anhydrous sodium sulphate and sodiumchloride used as supporting electrolytes were analytical gradepurchased from Fluka. All solutions were prepared with high-puritywater obtained from a Millipore Milli-Q system, with resistivity>18 M� cm at 25 ◦C. Sulphuric acid and sodium hydroxide usedto adjust the solution pH were analytical grade and supplied byPanreac Quımica S.A. (Barcelona, Spain).

Analytical proceduresThe total organic carbon (TOC) concentration was monitoredusing a Shimadzu TOC-5050 analyzer. Measurements of pH andconductivity were carried out with an InoLab WTW pH meter and aGLP 31 Crison conductimeter, respectively. The concentrations ofthe compounds were quantified by HPLC (Agilent 1100 series). Thedetection wavelength used to detect progesterone was 248 nm.The column temperature was 25 ◦C. Volume injection was setto 50 µL. The analytical column used was Phenomenex Gemini5 µm C18. Solvent A was composed of 25 mmol L−1 of formic acidwater solution, and Solvent B was acetonitrile. A linear gradientchromatographic elution was obtained by initially running 10% ofSolvent B ascending to 100% in 40 min. Samples extracted fromelectrolyzed solutions were filtered with 0.20 µm nylon filtersbefore analysis.

Electrochemical cellsElectrolyses were carried out in a single compartment elec-trochemical flow cell working in a batch-operation mode9. Aconductive-diamond electrode (p-Si–boron-doped diamond) wasused as anode and stainless steel (AISI 304) as cathode. Both elec-trodes were circular (100 mm diameter) with a geometric area of78 cm2 and an electrode gap of 9 mm. Boron-doped diamond filmswere provided by Adamant Technologies (Neuchatel, Switzerland)and synthesized by the hot filament chemical vapour depositiontechnique (HF CVD) on single-crystal p-type Si <100>; wafers(0.1 �cm, Siltronix). The boron content of the electrodes was500 ppm and the sp3/sp2 ratio was 194.

Experimental proceduresBench-scale electrolyses of 600 cm3 of wastewater were carriedout under galvanostatic conditions. The concentration of proges-terone was ranged from 0.1 to 100 ppm, and 0.035 mol L−1 Na2SO4

or NaCl was used as supporting electrolyte. The current densityemployed ranged from 15–100 mA cm−2. The cell voltage did notvary during each electrolysis, indicating that conductive-diamondlayers did not undergo appreciable deterioration or passivation.Prior to use in galvanostatic electrolysis assays, the electrode waspolarized for 10 min in a 0.035 mol L−1 Na2SO4 (pH = 2) solutionat 15 mA cm−2 to remove any kind of impurity from its surface.The wastewater was stored in a glass tank and circulated throughthe electrolytic cell by means of a centrifugal pump (flow rate 21.4dm3 h−1). A heat exchanger coupled with a controlled thermo-static bath (Digiterm 100, JP Selecta, Barcelona, Spain) was usedto maintain the temperature at the desired set point (25 ◦C).

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Figure 1. Changes in the concentration during the electrolysis of pro-gesterone at four different initial concentrations � 100 mg dm−3�10 mg dm−3� 1 mg dm−3� 0.1 mg dm−3; j = 15 mA cm−2; electrolyte:0.035 mol L−1 Na2SO4; solution medium: 2-hydroxypropyl-β-cyclodextrin.Inset: effect of the initial concentration of progesterone on the pseudo-first-order kinetic constant.

RESULTS AND DISCUSSIONFigure 1 shows the changes in the concentration of progesteroneduring electrolyses of synthetic wastes polluted with different con-centrations of this specie, covering a four order-of-magnitude con-centrations range: from a very high concentration, which can befound only in an industrial effluent of a pharmaceutical industry,24

down to the value in which it can be typically found in the effluentof a municipal wastewater treatment plant (some µg dm−3)1. Semi-logarithmic scales are used to compare clearly the different experi-ments, due the huge differences in the concentration ranges used.

At this point it is worth mentioning that solubility of proges-terone is very small. In order to get solutions in this wide rangeof concentrations, a special product named water soluble proges-terone (WSP) was used. As is known, this is not a pure product butit corresponds to a mixture (7/93) Progesterone/2-hydroxypropyl-β-cyclodextrin where 2-hydroxypropyl-β-cyclodextrin is astabilizing agent23 that increases solubility of progesterone. Thismixture behaves as a single product in aqueous solution (just oneHPLC peak is observed), and it permits concentrations as high as100 mg dm−3 of progesterone in aqueous solution.

In every case, the supporting electrolyte consists of 5000 mgdm−3 of sodium sulphate. This electrolyte was used in orderto increase the ionic conductivity of the reaction media to thevalues required for an efficient electrolysis. It was chosen becauseof its assumed nil effect on the electrolyte product distribution(contrary to what it is expected in a chloride media), and becauseit is one of the salts typically contained in the effluents ofpharmaceutical processes.

Concerning Fig. 1, it can be clearly observed that progesteroneis removed efficiently in the four electrolyses. Data follow a lineartrend in semi-logarithmic scale, which initially may suggest a first-order kinetic with respect to the concentration of progesterone.This would be easily explained in terms of a mass transfer controlof the process, taking into account the small concentrations ofthe pollutant, even in the 100 mg dm−3 case which is many timesbelow the limit concentration at the current density applied.

However, the slope of this linear trend (kinetic constant) de-pends on the particular electrolysis and, consequently, on the initialconcentration of WSP, being smaller at larger concentration, as canbe observed in the inset of the figure. This behavior has previouslybeen observed for other species such as metoprolol and sulphame-

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Figure 2. Effect of the supporting electrolyte (0.035 mol L−1 NaCl orNa2SO4) on the removal of progesterone and TOC at high concentrationC0 = 100 ppm; j = 30 mA cm−2; solution medium: 2-hydroxypropyl-β-cyclodextrin. � progesterone Na2SO4 � progesterone NaCl � TOCNa2SO4◦ TOC NaCl.

toxazole in previous work. It may be explained assuming the ex-istence of indirect oxidation mechanisms with electro-generatedoxidants, and a second-order kinetic model k [oxidant][organic].At this point, it is known that many oxidants are produced dur-ing the electrolysis with conductive-diamond anodes of aqueoussolutions, including not only conventional oxidants such as per-sulphates, ozone or hydrogen peroxide but also powerful radicalssuch as the hydroxyl radical. The pseudo-stationary concentra-tion of these oxidants (difference at the steady-state between theoxidant produced electrolytically and the oxidant spent in the ox-idation of organics) decreases with the concentration of organics,explaining the decrease in the term k[oxidant] which behaves asthe observed pseudo-first-order kinetic constant.

Figure 2 shows the effect of the supporting electrolyte (sulphateor chloride) on the concentration of progesterone and on theTOC. It can be clearly observed that chloride anions favor the rapiddepletion of progesterone compared with sulphate, the processbeing more than ten times faster. However, the mineralization rateis nearly the same with both electrolytes as both concentrationcurves vs. Q overlap. This suggests that chloride only promotesthe production of intermediates, the complete removal of thepollutant by conversion into carbon dioxide being as efficient as inthe case of electrolysis with sulphate. Therefore, the mineralizationrate is similar in both media, and the oxidation of progesterone inchlorine media is not directly to carbon dioxide. In both media, theformation of oxidants from oxidation of the anions, hypochloritein the case of chloride25 and peroxosulphate in the case ofsulphates9 is well documented Hence, these oxidant speciesshould help to explain the results. Moreover, the productionof many chlorinated intermediates during chemical oxidationof organics with chlorine is well known, while, conversely, thedosing of persulphate leads to a more direct mineralization of theorganics and, consequently, to a smaller amount of intermediates.

Figure 3 shows the concentration of intermediates found in theelectrolyses of soluble progesterone in sulphate- and chloride-supporting electrolytes. HPLC detects only one intermediatein the case of electrolysis in sulphate media. This intermediateappears at 27.8 min, which it is the same time at which a peakappears for pure 2-hydroxypropyl-β-cyclodextrin. This peak isalso obtained in the case of chloride media, and supports the re-lease of some 2-hydroxypropyl-β-cyclodextrin (from the complex

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Figure 3. Intermediates found by HPLC during the electrolyses of ahighly concentration solution of soluble progesterone in 0.035 mol L−1

sodium sulphate (� i1(27.8 min)) and 0.035 mol L−1 sodium chloride (� i1(27.8 min) � i2 (31.0 min) ◦ i3 (36.7 min)) supporting electrolyte media. j =30 mA cm−2.

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Figure 4. Effect of current density on the removal of a highly loadedsolution of water soluble progesterone (100 mg dm−3), with sodiumsulphate (0.035 mol L−1) as supporting electrolyte. Current densities: �15 mA cm−2; • 30 mA cm−2; � 100 mA cm−2.

progesterone/2-hydroxypropyl-β-cyclodextrin) with the oxidationof progesterone. This species clearly behaves as an intermediate,and it is removed during electrolysis. In the case of the electrolysesin the chloride media, two additional intermediates (three reactionintermediates in total with cyclodextrin) are found using HPLC at31.0 and 36.7 min. It is possible that these compounds are formedby the attack of ClO−, generated electrochemically by oxidationof Cl−25 to progesterone and 2-hidroxypropyl-β-cyclodextrin,compound that accompanies progesterone. In order to check thisfact, a solution of 0.035 mol L−1 of Ca(ClO)2 was added to twosolutions with 100 mg dm−3 of progesterone and 100 mg dm−3

of 2-hidroxypropyl-β-cyclodextrin, respectively. After 2 h, thefinal samples were analyzed by HPLC. Two peaks with the sameretention time that the intermediates formed in the electrolyseswith NaCl as electrolyte were found. Therefore, these compoundsmay be explained in terms of the result of chlorination ofprogesterone and 2-hidroxypropyl-β-cyclodextrin molecules byattack of the anodically electrogenerated ClO−.

Figure 4 shows the effect of current density on the removal ofprogesterone and on the mineralization of the synthetic waste.Within the range 15 to 100 mA cm−2, progesterone is completely

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depleted and mineralization is almost complete. The higher thecurrent density, the lower the efficiency of the process. This canbe clearly observed in the graph: lower current densities lead tothe same removal for smaller current charges applied. On thecontrary, the kinetic constant increases with the current densityalmost linearly, as is shown in the inset of the figure. This behavioris characteristic of a process in which mass transfer limitations playa key role. At this point, it is worth stating that the current densityapplied is significantly over the limit current density calculated forthe hydrodynamic conditions of the electrochemical cell.

The product used in the previous studies to evaluate theoxidation of progesterone was a mixture of progesterone and2-hidroxypropyl-β-cyclodextrin, because the solubility of pureprogesterone in water is very low, unless this species is combinedwith a solubilizing agent. Like progesterone, 2-hidroxypropyl-β-cyclodextrin was also found to be removed during electrolysisover the complete range of concentrations applied, and in fact,it was detected separately in the electrolytic solution during theprocess behaving as an intermediate.

To study the influence of the solubilizing agent on theremoval of progesterone, pure progesterone was dissolvedin methanol and in water – methanol solutions. Solubility ofprogesterone in methanol is very high and over the 100 mg dm−3

concentration used in this work. From a practical point of view, theratio methanol/progesterone to assure complete dissolution ofprogesterone in an aqueous solution is around (250 cm3)/(100 mgdm−3) (determined experimentally). This means that to get anaqueous solution with 100 mg dm−3 of progesterone it is necessarythat this solution will be 25% (v/v) in methanol.

Figure 5 compares the electrolysis of solutions with highconcentrations of progesterone (100 mg dm−3) in aqueousmedia using 2-hidroxypropyl-β-cyclodextrin and methanol assolubilizing agents. In addition, this figure also reports the resultsof the electrolysis of progesterone in non-aqueous media (puremethanol) for comparison purposes. Electrolyses in aqueous mediashows no differences for the removal of progesterone whenusing methanol or 2-hidroxypropyl-β-cyclodextrin as solubilizingagents during a first stage of the electrolyses in which the samerate is observed for both electrolysis. Then, this rate decreasessignificantly for the electrolysis with methanol as solubilizing

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Figure 5. Effect of the solubilizing agent of progesterone and of thereaction media on the electrolysis of highly loaded progesterone solutions.� water soluble progesterone in water ◦ progesterone in methanol/water25% (v/v); � progesterone in methanol. Current density 15 mA cm−2.

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Figure 6. Influence of concentration on the effect of the solubilizingagent, of progesterone, and of the reaction media on the electrolysisof progesterone solutions. Current density: 15 mA cm−2. Part a: initialconcentration of progesterone 10 mg dm−3; � water soluble progesteronein water; ◦ progesterone in methanol/water 2.5%(v/v); � progesteronein methanol. Part b: initial concentration of progesterone 1 mg dm−3; �water soluble progesterone in water; ◦ progesterone in methanol/water0.25 %(v/v); � progesterone in methanol.

agent, but it is still maintained in the case of the WSP, which stilldecreases at the high rate two orders of magnitude more.

Another important observation, it is the behaviour of theelectrolysis of progesterone in pure methanol. A linear trendis obtained, the reaction rate being smaller than in the othertwo cases studied. This means that although progesteronecompetes with methanol during oxidation, this oxidation occursat a high efficiency even in pure methanol. This also suggeststhat competition between methanol and progesterone oxidationis not responsible for the change of slope observed duringthe electrolysis of progesterone in methanol–water solutions,although it should play an important role in the oxidation process.

The same behavior can be observed in the electrolysis ofprogesterone at smaller concentrations, as can clearly be seenin Fig. 6, in which results of electrolysis of solutions with 10and 1 mg dm−3 of progesterone are shown. The first zone withsimilar rates for the removal of progesterone in water/methanolsolutions and with 2-hidroxypropyl-β-cyclodextrin as solubilizingagent can be clearly discerned in both graphs, although theycorrespond to different orders magnitude orders. The change inthe rate in the case of the use of methanol as solubilizing agent inaqueous solutions can also be seen, which should be interpreted

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Figure 7. First-order kinetic constant of the electrochemical degradationof progesterone in aqueous media (� first stage of methanol and WSP; �second stage of methanol) and in pure methanol media (�).

in terms of an inhibiting effect of methanol on the action ofhydroxyl radicals, once a particular ratio progesterone/methanol isreached during the electrolyses. Thus, the main difference betweenthe experiments with pure methanol and with methanol/wateris the potential formation of hydroxyl radicals in this lastcase.

Figure 7 shows the kinetic constants (first-order approach)obtained by mathematical fitting during the first and secondstage of the electrolysis in methanol/water (first stage matchesaccording to the previously described behavior of WSP) andin pure methanol. This last constant does not depend on theprogesterone concentration, indicating a real first-order process.Alternatively it could be explained in terms of an order lower thanfirst (moving towards zeroth) simply because oxidizing agentsbecome the limiting reactant at high progesterone concentrations.However, the other two fitting kinetic-constants decrease withprogesterone concentration, suggesting pseudo-first-order kineticprocesses instead of a real first-order process. Differences betweenthe rate of electrolysis in pure methanol and the oxidationrate found in the electrolysis in aqueous media become moresignificant at smaller initial progesterone concentrations for thefirst stage, as can be observed in Fig. 7. This can be easilyexplained in terms of the smaller concentration of methanolrequired to solubilize progesterone at smaller concentrations, andconfirms the existence of competition between methanol andprogesterone to be oxidized at these conditions. Concerningthe second stage rate in the electrolysis in methanol/watermedium, it is clearly observed that oxidation rate is below thevalue obtained in pure methanol, and also that the differencesare higher for higher progesterone concentration, which alsomeans higher concentration of methanol. This may suggestsome sort of protective effect against oxidation of methanolat these particular conditions. As this effect is not observed inpure methanol electrolysis, an inhibitory effect of methanol onthe hydroxyl radicals effect (which are not produced in puremethanol) is the lone reasonable explanation for this observation,although more work has to be done in order to clarify thispoint.

CONCLUSIONSFrom this work the following conclusions can be drawn:

– Progesterone can be removed effciently from aqueoussolutions within the range of initial concentrations 10−2 to102 mg dm−3. Decay follows a pseudo-first-order kinetics(linear fitting in semilogarithmic plot) during each electrolysiswith a kinetic constant that decreases with the initialconcentration of progesterone.

– Increases in current density lead to less efficient processes,indicating mass transfer control of the process rate. However,mediated electro-oxidation plays an important role not inthe mineralization rate but in the mechanisms. Comparedwith sulphates, chlorides promote the rapid depletion ofprogesterone and the formation of the same intermediatesoccurring during the chemical dosing of hypochlorite.

– During a first stage of the electrolyses (at the different rangesof pollutant concentration studied), there are no differenceswhen methanol is used as solubilizing agent instead of2-hidroxypropyl-β-cyclodextrin but then, the rate changesabruptly down to a value that it is even lower than the rateobtained in pure methanol electrolysis. This observation canbe explained in terms of the inhibiting effect of methanolon hydroxyl radicals action, which it is not observed in puremethanol electrolysis because in those electrolyses only directelectro-oxidation occurs.

ACKNOWLEDGEMENTSThe authors acknowledge funding support from the nationalSpanish Ministry of Education and Science (Project CSD2006-00044CONSOLIDER INGENIO 2010 ‘TRAGUA project’).

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