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An Integrated Electrocoagulation-Phytoremediation Process for theTreatment of Mixed IndustrialWastewaterClaudia Teodora Cano Rodríguez a , Araceli Amaya-Chávez a ,Gabriela Roa-Morales a , Carlos Eduardo Barrera-Díaz a & FernandoUreña-Núñez ba Universidad Autónoma del Estado de México, Facultad deQuímica , Toluca, Estado de México, Méxicob Instituto de Investigaciones Nucleares, México , México, D.F.Published online: 07 Oct 2010.

To cite this article: Claudia Teodora Cano Rodríguez , Araceli Amaya-Chávez , Gabriela Roa-Morales ,Carlos Eduardo Barrera-Díaz & Fernando Ureña-Núñez (2010) An Integrated Electrocoagulation-Phytoremediation Process for the Treatment of Mixed Industrial Wastewater, International Journal ofPhytoremediation, 12:8, 772-784, DOI: 10.1080/15226510903390429

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International Journal of Phytoremediation, 12:772–784, 2010Copyright C© Taylor & Francis Group, LLCISSN: 1522-6514 print / 1549-7879 onlineDOI: 10.1080/15226510903390429

AN INTEGRATED ELECTROCOAGULATION-PHYTOREMEDIATION PROCESS FOR THE TREATMENTOF MIXED INDUSTRIAL WASTEWATER

Claudia Teodora Cano Rodrıguez,1 Araceli Amaya-Chavez,1

Gabriela Roa-Morales,1 Carlos Eduardo Barrera-Dıaz,1

and Fernando Urena-Nunez2

1Universidad Autonoma del Estado de Mexico, Facultad de Quımica, Toluca,Estado de Mexico, Mexico2Instituto de Investigaciones Nucleares, Mexico, Mexico, D.F.

The elimination of organic contaminants in highly complex wastewater was tested usinga combination of the techniques: electrocoagulation with aluminum electrodes and phy-toremediation with Myriophyllum aquaticum. Under optimal operating conditions at a pHof 8 and a current density of 45.45 A m−2, the electrochemical method produces partialelimination of contaminants, which was improved using phytoremediation as a polishingtechnique. The combined treatment reduced chemical oxygen demand (COD) by 91%, colorby 97% and turbidity by 98%. Initial and final values of contaminants in wastewaters weremonitored using UV-vis spectrometry and cyclic voltammetry. Finally, the morphology andthe elemental composition of the biomass were characterized with using scanning electronmicroscopy (SEM) and energy dispersion spectroscopy (EDS). The presence of Al in theroots of plants in the system indicates that the aluminum present in the test solution couldbe absorbed.

KEY WORDS: Bioremediation, Myriophyllum aquaticum, superfaradaic efficiencies

1. INTRODUCTION

It has been observed that conventional treatment processes which operate with physic-ochemical (coagulation-flocculation) and biological (aerobic and anaerobic) systems arenot entirely efficient in the removal of industrial contaminants containing refractory organiccompounds, which hinder or inhibit such treatments. As a result, the physicochemical pa-rameters for environmental discharges are not always met (Barrera-Dıaz et al., 2006).

Efficient alternative treatment processes such as advanced oxidation processes (AOP)(ozone, Fenton’s reagent) and electrochemical processes (electrocoagulation) have recentlybeen developed which degrade and/or mineralize the recalcitrant pollutants (Barrera-Dıazet al., 2009). The process of electrocoagulation uses iron or aluminum electrodes (anodes

Address correspondence to Araceli Amaya-Chavez, Universidad Autonoma del Estado de Mexico, Facultadde Quımica, Paseo Colon interseccion Paseo Tollocan S/N, C.P. 50120, Toluca, Estado de Mexico, Mexico.E-mail: aamayac@uaemex.mx

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and cathodes). When the electric current is introduced, (Fe2+) or (Al3+) ions are electro-chemically generated near the anode creating good coagulants. The main reactions are:

Anode: Al → Al3+(aq) + 3e− (1)

Cathode: 3H2O + 3e− → 3/2H2 + 3OH− (2)

Hydrolysis: Al3 + (aq) + 3OH− → Al(OH)3 (3)

In general, three important processes occur during electrocoagulation: a) electrolytic reac-tions on the surface of the electrode; b) formation of a coagulant in the aqueous medium;and c) adsorption of soluble or colloidal pollutants by the coagulant (in this case Al(OH)3)and removal by sedimentation or flotation caused by small hydrogen bubbles generated bythe cathode, thus facilitating the separation of particles in wastewater (Chen, 2004; Holtet al. 2004, 2005).

Various studies have shown electrocoagulation to be an efficient technique for theelimination of pollutants in surface waters in lowlands (Jiang et al., 2002), of urban wastew-ater (Vik et al., 1984), restaurant effluent (Chen et al., 2000), chrome metal waste (Barrera-Dıaz et al., 2003a), and of industrial waste (Barrera-Dıaz, et al., 2003b, 2009). The resultshave consistently shown that this is one of the most promising techniques for the treatmentof waste waters, obtaining removal efficiencies of between 70 and 95% in terms of CODand an increase in biochemical oxygen demand (BOD) and lower production of sludge thanalternative procedures (Barrera-Dıaz et al., 2003a; Linares et al. 2007).

Biological treatments are often low cost, environmentally friendly alternatives. Someinnovative techniques have used coupled procedures where highly polluted wastewater hasundergone two-stage processing. The first stage is often carried out with activated sludges,obtaining an initial decomposition of compounds with high molecular weights. The lesspolluted water, which results from this first stage, is then polished using biosorption toattain higher quality water than if either process were used in isolation (Ibney et al., 2007).

Amongst the different biological treatments, one innovative technology is phytore-mediation which involves the use of plants to eliminate pollutants such as heavy metals,organic compounds, hydrocarbons and pesticides present surface and underground waterand soil (Wilson et al., 2000; USEPA 2001; Perez et al., 2002; Glick, 2003). This tech-nique affords many benefits including its being, cost effective, aesthetic and relativelymaintenance free (McKinlay and Kasperek, 1999), however, it is important to elucidate themetabolic pathway of the contaminants in plants in order to determine the toxicity of themetabolites formed and released into the environment.

A great variety of plants, including terrestrial plants (Thompson, 1998), axenic rootcultures (Bhadra et al., 1999a), aquatic and wetland species (Hughes et al., 1997), could beused in phytoremediation. Emergent macrophytes such as Myriophyllum aquaticum havebeen used in the degradation of 2,4,6-trinitrotoluene (TNT), perchlorates, organophosphatepesticides and organochloride compounds in liquid medium (Bhadra et al., 1999b; Susarlaet al., 1999; Gao et al., 2000a, 2000b; Turgut and Fomin, 2002; Garcıa, 2005; Turgut, 2007).

When wastewater has high concentrations of COD (between 1000 and 3000 mgL−1)even AOP does not result in reusable water, necessitating the use of integrated treatmentschemes (Ibney et al., 2007). A combined anaerobic-ozonation treatment applied to wastew-ater from manufacturing of olive oil achieved an 81% reduction in COD via the anaerobictreatment. Aromatic compounds and polyphenol content were eliminated by applying ozoneas a polishing technique (De Heredia and Garcıa, 2005).

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The treatment of highly contaminated distillery wastewater from cherry stillage hasbeen described in the literature using an integrated system, which combined aerobic bi-ological oxidation and ozonation. The major fraction of contaminants was eliminated bybiological oxidation (removal of COD and BOD above 95% and 80% respectively). Thebiological process was shown to be unsuccessful in the elimination of polyphenols andother compounds, which are absorbed by UV light. Ozonation proved to be successful inthe elimination of these compounds bringing the treatment up to environmental standards(Beltran et al., 2001).

The objective of this study was to evaluate the efficiency of a coupled process ofelectrocoagulation using aluminum electrodes with phytoremediation using M. aquaticumfor the treatment of industrial wastewater obtained from a the effluent from 137 mixedindustries.

2. MATERIALS AND METHODS

2.1. Wastewater Samples

The samples of wastewater were obtained from a treatment plant situated at the end ofan industrial park in the State of Mexico. Industrial effluent from 137 different companiesflows into this wastewater treatment facility. The treatment unit uses several treatmenttechniques, including fragmentation, silt separation, oil and grease separation, primaryclarifiers, biological reactors with activated sludge, secondary clarifiers and contact unitsfor disinfection by chlorination (Linares et al. 2007). The samples were collected fromthe effluent in the primary sedimentation tanks using plastic containers and cooled to 4◦C.These were transported to the laboratory for analysis and treatment with electrocoagulation-phytoremediation.

2.2. Electrochemical Reactor

An electrochemical reactor was constructed for the electrocoagulation process. Thecell contained a series of 10 Al electrodes in a parallel arrangement. The surface area ofeach electrode was 0.0132 m2 and a total surface area of 0.066 m2 Ae. The electrochemicalreactor capacity was 4 L. A dc source was used to supply the system at a rate of 1 to 4 Ato 13 V, corresponding to a current density of 15.15 to 60.6 Am−2.

2.3. Phytoremediation

Myriophyllum aquaticum (Vell.) Vercourt, also known as parrotfeather, is a memberof the dicotyledons class. It is a submersed macrophyte with roots.

2.3.1. Plant Collection and Climatization. The sample of M. aquaticum wascarried out at Cerrillo Piedras Blancas, State of Mexico (an elevation of 2 624 msnm,19◦24′23.5′′N y 99◦41′28.8′′ W). The plants were taken to the laboratory and placed inplastic tanks with drinking water and Hoagland nutrient solution which was changed everyweek (Wilson et al., 2000). The specimens were kept at room temperature (20 + 5◦C), witha pH of 6.5 to 7.5 and exposed to natural periods of light and dark. The collection tookplace 15–20 days before the test.

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2.3.2. Phytoremediation Systems. Five systems were set up to eliminate thecontaminants from the electrocoagulated wastewater. The systems consisted of glass con-tainers with a capacity of 12L. The first system (plant control) contained only drinkingwater, the other three were test systems containing electrochemically treated wastewaterat the following concentrations: 50, 75, and 100% and the fifth contained only wastewater(water control). Six plants with an average weight of 69,7 ± 1,8 g were placed in eachsystem. In order to monitor photosynthetic potential and state of health, measurements weretaken of basal chlorophyll, a and b chlorophyll, total chlorophyll and the chlorophyll a/bratio before and after treatment (12 days) (USEPA 1994; Delgado 1993). Values for pH ineach system were monitored daily and the amount of water lost via evapotranspiration wasrecharged. The experiment was performed in triplicate.

2.4. Methods of Analysis

The efficiency of removal of contaminants from wastewater using electrocoagulationand phytoremediation was determined by periodically analyzing COD and color (Pt/Coscale) on a regular time scale. Once the optimal processing conditions were found, thequality of the raw and treated wastewater was analyzed by determining COD, aluminumcontent and color as outlined in standard methods procedure (APHA, AWWA, 1998).

2.5. UV-Vis Spectrometry

UV-vis spectra were obtained for the samples of raw wastewater and those treatedwith electrocoagulation and phytoremediation using a double beam Perkin-Elmer 25 spec-trophotometer. The scan rate was 960 nm s−1 at an interval of 900 to 200 nm wavelength.Samples were scanned in quartz cells with a 1 cm optical path.

2.6. Cyclic Voltammetry

Cyclic voltammetry was performed for all the samples of raw and treated wastewaterusing a standard three-electrode cell, a reference electrode of Ag/AgCl saturated with KCl,an auxiliary electrode of platinum wire and a working carbon paste electrode (CPE). Thevoltammograms were obtained using a BAS100W model potentiostat with a scan rate of100 mV s−1. The CPE was circular with a geometric surface area of 3.5 mm2 and wasprepared with a 1:1 mixture of graphite powder (Alfa AESAR) and mineral oil (Fluka).The resulting paste was inserted into a PVC tube and compacted to eliminate air bubbles.Then a copper conductor was inserted to take the readings. The electrode surface wasrenewed after each potential scan.

2.7. Characterization of the Biomass

Samples were taken of the plant roots before and after treatment and analyzed usingscan electron microscopy (SEM) and X-ray microanalysis, using a Phillips XL-30 mi-croscope to observe the composition and morphology of the tissue. SEM images had aresolution of one micrometer and energy dispersive X-ray spectroscopy (EDS) providedin situ elemental analysis.

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3. RESULTS AND DISCUSSION

3.1. Preliminary Effects on the Electrocoagulation of Wastewater with

Different pH Conditions

Samples were taken of industrial wastewater from the effluent of primary sedimen-tation tanks and were treated electrochemically. The treatments were carried out at varyingpH values: 2, 4, 6, 8, 10, and 12 (adjustments were made using NaOH or H2SO4) with a3A current during 40 minutes. The efficiency of the treatment was evaluated by measuringCOD. It was observed that for pH 2 and 8 removal values were 65 and 75% respectively.The greatest reduction in COD was observed at a pH between 8 and 10.

These results coincide with those found in other studies where maximal removal ofCOD in wastewater has been reported as around pH 7, the effect of pH on COD removalis not very significant at the 3 and 10 intervals. COD removal was observed to dropsignificantly at pH levels above 8 (Canizares et al., 2005; Barrera-Dıaz et al., 2009).

3.2. Kinetics of Pollutant Removal

The COD of wastewater fell as a function of contact time. After 40 minutes ofexposure, the value of COD achieved a maximum reduction of 75%. For longer periodsof contact time (60 minutes), increases in removal were not significant. The kinetics ofremoval of COD fit to a first and second order process using 40 minutes as the final time ofelectrolysis. The values obtained for the first order equations are: k1 = 0.034 min−1 withr2 = 0.95; for the second order equation, the following was obtained k2 = 4 × 105 L mgmin−1 with r2 = 0.99. The model which best fit the reduction in COD concentration acrosstime was:

dCCOD

dt= −k[CCOD]2.

where CCOD is COD [mgL−1]; t is time; and k is the velocity constant (L mg min−1). (Koybaet al., 2006) have shown that this model fits the application of the kinetics of COD removalby electrocoagulation.

The results obtained by electrocoagulation for initial concentrations the CODof wastewater ranging from 2000 to 4000 mgL−1 showed a diminished only 75%.For this reason, it is necessary to coupled it with another treatment process, such asphytoremediation.

3.3. Electrochemical + Phytoremediation Treatment

Once the electrochemical process had produced wastewater with a final COD of1000 mgL−1 and 425 mgL−1, phytoremediation was applied. Results obtained for CODremoval as a function of time are presented in Figures 1 and 2.

As can be observed in figures 1 and 2, the wastewater treated with electrocoagu-lation, in both concentrations (1000 mgL−1 y 425 mgL−1) but without the application ofM. aquaticum (water control) did not present variation in COD as a function of time.However, the quality of wastewater when M. aquaticum was applied improved in the caseof 75% and 50% concentrations (test system). It can also be observed that after 8 daysof contact with the plant there are no longer significant values for COD removal, whichindicates that maximum removal is obtained during this period. In the samples with initial

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Figure 1 COD removal as a function of time. � Raw wastewater without treatment, � electrochemically treatedwater, � electrochemically treated water diluted at 75%, ◦ electrochemically treated water diluted at 50%.

COD of 1000 mg/L the maximum reduction in the wastewater was 50%, while for thesample with initial COD 425 mgL−1, 52% of COD was eliminated. This can be explainedby the characteristics of the wastewater samples, even though these were collected fromthe same treatment plant and from the same sampling site, they were not collected on thesame day, and thus, they had different constituents.

Figure 2 COD removal as a function of time. � Raw wastewater without treatment, � electrochemically treatedwater, � electrochemically treated water diluted at 75%, ◦ electrochemically treated water diluted at 50%.

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The wastewater used in this study is a mixture of industrial wastes, the removalefficiency obtained for contaminants was similar to that found for the removal of 10 mgL−1

of demeton-S-methyl and cruphomate (75 and 58%, respectively) present in liquid solutionsusing M. aquaticum (Gao et al., 2000b).

It is interesting to observe that in spite of concentrations of 50 to 75% the plants weretolerant to the quality of the wastewater to which they were exposed, indicating that they arecapable of resisting exposure to different contaminants. The results show that the toxicityof wastewater on M. aquaticum is relatively low in relation to biological parameters. Thebasal average of total chlorophyll was 44.52 ± 1.61 mg mL−1, and the chlorophyll a/b ratiowas 3.09 ± 0.21. After 12 days of contact with the wastewater, no significant differencesin the total chlorophyll content or the chlorophyll a/b ratio were observed between thedifferent concentrations (50% and 75%). The plants only died in the system with an initialconcentration of COD of 1000 mgL−1. In vascular plants, like M. aquaticum, chlorophyll ais found in concentrations between 2.5 and 3.5 times higher than chlorophyll b. Changes intotal chlorophyll and the chlorophyll a/b ratio may directly affect CO2 adsorption and thephotosynthetic process, which makes their measurement a good indicator of plant health(Delgado, 1993; Gao, et al., 2000b; Turgut and Fomin, 2002; Sharma et al., 2003).

The wastewater treated by electrocoagulation-phytoremediation was characterizedwith UV-vis and cyclic voltammetry to corroborate the changes in the quality of wastewater.The results of this analysis are described in the following sections.

3.4. UV-Vis Spectra

The UV-vis spectra of raw, electrocoagulated and treated electrocoagulation-phytoremediation wastewater are shown in Figure 3. The spectra which correspond to the

Figure 3 UV-vis spectra. �Raw wastewater without treatment with dilution factor of seven, � electrochemicallytreated water with dilution factor of three, � electrochemically-phytoremediation treated water with dilution factorof two.

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components of the wastewater are present on a wavelength interval of 200–400 nm. Itshould be mentioned that the water samples were diluted in order to obtain these spectra.The intensity of adsorption fell after treatment. An adsorption band with a maximumabsorbency of 214 nm can be seen, which decreased 15% after the electrochemicaltreatment (absorbency dropped from 4.0 to 3.4) and a further 33% after phytoremediation.The results indicate a significant reduction in the coloring of raw wastewater after thecombined treatment process.

3.5. Cyclic Voltammetry

To determine the electrochemical characteristics of the raw and treated wastewatersand the processes, which occurred at the electrodes, a series of experiments were carriedout using cyclic voltammetry. The results in the voltammograms show a peak in irreversibleoxidation in the wastewater which is detected at a lower potential than those of oxygenevolution as can be seen in Figure 4. This peak corresponds to the electrochemical oxi-dation of the contaminants present in the wastewater. It should be noted that when cyclicvoltammetry is applied to wastewaters which have been treated with electrocoagulation andphytoremediation the oxidation peak falls in the voltammogram showing that the dissolvedcontaminants have been removed. This clearly indicates that there are processes which canbe attributed to the direct oxidation of the contaminants and which contribute to the destruc-tion of the organic matter present in the solution. Finally this study also confirms that thequality of wastewater is improved using the combined electrochemical-phytoremediationtreatment.

Figure 4 Cyclic voltammogram. � raw wastewater without treatment, � electrochemically treated water,� electrochemical-phytoremediation treated water.

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3.6. Biomass Characterization

The morphology of the roots is shown in Figure 5. Three representative micrograph-ics are presented: the control system (Figure 5a) and the test systems at 50% and 75%dilutions of the electrochemically treated wastewaters (Figures 5b and 5c, respectively).The micrography of the roots in the control system show long wrinkled structures, how-ever, in comparison with the roots, which were in contact with different concentrationsof wastewater post-treatment with the electrochemical method the roots show more pro-nounced wrinkles and compacting of their structures.

In Figure 6 the EDS of the previous micrographics (see Figure 5) are depicted,obtaining the elemental composition of the plant roots as follows: C, O, P, Fe as principalcomponents; Mg, K, Na, Ca, Mn, and Si as oligoelements, and Al as a contaminant (Gil2002). Amongst the elements that undergo significant changes in the contents of the plantroots tested were Si and Al. The control system presented 0.81% of Si and 0.26% of Al inits elemental composition and both showed an increase in an order of magnitude of 5 and8.5, respectively, in the test systems. Si was found to be present in the systems in an intervalof 0.84 to 3.98%, this is probably due to the reparation mechanism in the vegetable fiber ofthe plant since is essential to its structure.

However, the elemental composition interval of Al was found to be between 0.42 to2.22% in the systems. It should be noted that this element could be present in concentrationsof around 50 mgL−1 in the treated wastewater, since the cell used for the electrochemicalwas aluminum. This indicates that the plant absorbs important quantities of this metal

Figure 5 Root Micrography a) blank; wastewater at a dilution of b) 50% and c) 75%. The photograph wasrecorded at 100X and the marker is 100 µm.

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like has showed other investigations (Gardea-Torresdey et al., 2005), which enhances itscapacity for contaminant removal.

4. CONCLUSIONS

The electrocoagulation treatment obtained a 75% removal of COD from wastewater,after phytoremediation a total removal of 91% was obtained. UV-vis spectrophotometryand cyclic voltammetry confirmed the improvement of the quality of the wastewater.

M. aquaticum was tolerant to contact with the contaminants present in the wastewaterthat had been treated with electrocoagulation using Al electrodes, and in the elementalanalysis, an increase in the Al in the plant roots was observed contributing to its removalfrom the solution.

In conclusion, the combination of the electrochemical process and phytoremedi-ation is a viable for the treatment of industrial wastewater with a mixture of differentcontaminants.

ACKNOWLEDGMENTS

The CONACyT project No. 62000 has supported this research work. Claudia CanoRodrıguez gratefully acknowledges the scholarship from CONACyT to pursue her post-graduate studies.

NOMENCLATURE

A m−2 Current densitym2 SurfaceL VolumeA AmpereV Volts◦C Temperatureg GramsmgL−1 per for millionnm s−1 Wavelength Scan ratenm Wavelengthcm Lengthmm2 Surface area100 mV s−1 Potential scan ratemin Timemin−1 First order constantL mg min−1 Second order constant

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