Fundamentos, Presente y Futuro de la EC

Journal of Hazardous Materials B114 (2004) 199210

Fundamentals, present and future perspectskybDaviaka, Dhiner, TXsity, PO

d Insti e V, Ca

ust 200ber 200

Abstract

Electroco that isconsiderable tter unparticularly those that could provide design parameters to optimize the performances of this relatively simple and inexpensive technique. Ina research programme to delineate the mechanisms of the fundamental processes involved in, the authors have realized that the technologyhas been insufficiently reviewed with emphasis on the fundamentals and their relationship to the performance of this technology. This paperpresents an in-depth discussion and consideration of the factors that need to be addressed for optimum performance of this technology. Recentimprovements of this technique and the theoretical model studies are also reviewed. 2004 Pub

Keywords: E

1. Introdu

Water atwo top chaavailable qtion from pagricultureThird-worlture nor capthough theengineeringcentralizednew challeoping counods of purto clean the

CorrespoE-mail a

0304-3894/$doi:10.1016/jlished by Elsevier B.V.

lectrocoagulation cells; Electrolysis; Bipolar electrodes; Wastewater treatment; Theoretical models

ction

nd energy have been identified by Smalley as thellenges for the 21st century [1]. Water quality anduantity are being challenged by increased pollu-oint and non-point sources such as industry and

, respectively, and the ever-increasing population.d countries generally have neither the infrastruc-ital to adequately address the water problem. Al-

wastewater treatment process is a well-establishedscience, the need for process incorporation, de-

treatment and simplified processes have openednges and new opportunities particularly in devel-tries [2,3]. Innovative, cheap and effective meth-ifying water for human consumption as well as

wastewater from industrial effluents before dis-

nding author. Tel.: +1 409 880 1862; fax: +1 409 880 8374.ddress: [email protected] (D.L. Cocke).

charging into any other water systems are needed. Electro-chemical techniques such as, electroflotation (EF), electrode-cantation, electrocoagulation (EC), electrokinetic remedia-tion (for contaminated soil) offer the possibility to be easilydistributed, require minimum amount and number of chem-icals. Robust and compact instrumentation is very easilyachievable in above mentioned electrochemical techniques,and hence, they will have the potential to replace sophisti-cated processes that require large volumes and/or number ofchemicals, massive containers that are present in a typicalwastewater treatment plant (WWTP), as shown in Fig. 1. Asa consequence, a very simple replacement will prevent eco-nomically unfeasible capital cost.

According to Rajeshwar et al. [4] benefits from using elec-trochemical techniques include: environmental compatibil-ity, versatility, energy efficiency, safety, selectivity, amenabil-ity to automation, and cost effectiveness. In addition tothese the following advantages can be added: electrochemicalbased systems allow controlled and rapid reactions, smaller

see front matter 2004 Published by Elsevier B.V..jhazmat.2004.08.009Mohammad Y.A. Mollaha, Paul MorkovMehmet Kesmezc, Jose Pargad,

a Department of Chemistry, University of Dhb Kaspar Electroplating Corporation, Sh

c Gill Chair of Chemistry and Chemical Engineering Lamar Univertute Technology of Saltillo, Department of Metallurgy and Materials Scienc

Received 8 April 2004; received in revised form 13 AugAvailable online 28 Septem

agulation is an electrochemical wastewater treatment technologytechnical improvements. There has been relatively little effort to beives of electrocoagulation

, Jewel A.G. Gomesc,d L. Cockec,

aka, Bangladesh77984, USABox 10022, Beaumont, TX 77710, USA

ranza 2400, Saltillo Coah, Mexico C.P. 25000, Mexico

4; accepted 18 August 20044

currently experiencing both increased popularity andderstand the fundamental mechanisms of the processes,

200 M.Y.A. Mollah et al. / Journal of Hazardous Materials B114 (2004) 199210

Fig. 1. Sc proces

systems bemicro-orgacilitate watniques, thefor treatmefoodstuff [8rine [18], pleachate [2chanical pofine particl[29], and rEDTA [30treatment [ater has befocused onof the perfoticle [36] wpertainingsenting anof this techture prospecontrol the

2. Theory

Electrocing many chable electroan EC procit involveslants by el(ii) destabision, and bdestabilize

of thef emummar

mpreargedidatioargestewa

l dissouce tht the vcoag

oc formates a

rticleshematic flow-diagram of a typical wastewater treatment plant and complex

come viable and, instead of using chemicals andnisms, the systems employ only electrons to fa-er treatment. Of the known electrochemical tech-re is much interest in using electrocoagulationnt of wastewater containing, heavy metals [57],,9], oil wastes [10,11], textile dyes [1217], fluo-

olymeric wastes [19], organic matter from landfill0], suspended particles [2124], chemical and me-lishing wastes [25], aqueous suspensions of ultra-es [26], nitrate [27], phenolic waste [28], arsenicefractory organic pollutants including lignin and]. Another approach of EC is for drinking water31,32]. The electrochemical treatment of wastew-en reported, but only a few authors [3335] havethe variables that are crucial to the improvementrmance of this application. In a recent review ar-

nisming obe su

Cochox

Chwa

ca

redthaing

Flcre

pa

e have highlighted some of the relevant issues

to this technique. In the present article we are pre-in-depth discussion of the fundamental principlesnique, its limitations, areas of improvement, fu-cts and surface and near-surface phenomena thatEC process.

of electrocoagulation

oagulation (EC) is a complicated process involv-emical and physical phenomena that use consum-des to supply ions into the wastewater stream. Iness the coagulating ions are produced in situ andthree successive stages: (i) formation of coagu-

ectrolytic oxidation of the sacrificial electrode,lization of the contaminants, particulate suspen-reaking of emulsions and (iii) aggregation of thed phases to form flocs. The destabilization mecha-

The soliactive surfa

Electrocmoving medyes, and oIn this procically fabritwo separa

Fe/Al ising metapolymerhydroxidable (saproduceode. Coabine witby electwastewases that can be replaced by EC (section with dashed box).

contaminants, particulate suspension, and break-lsions have been described in broad steps and mayized as follows:

ssion of the diffuse double layer around thespecies by the interactions of ions generated byn of the sacrificial anode.neutralization of the ionic species present inter by counter ions produced by the electrochemi-lution of the sacrificial anode. These counter ionse electrostatic interparticle repulsion to the extentan der Waals attraction predominates, thus caus-

ulation. A zero net charge results in the process.ation; the floc formed as a result of coagulation

sludge blanket that entraps and bridges colloidalstill remaining in the aqueous medium.d oxides, hydroxides and oxyhydroxides provideces for the adsorption of the polluting species.oagulation has been successfully employed in re-tals, suspended particles, clay minerals, organicil and greases from a variety of industrial effluents.ess, a potential is applied to the metal anodes, typ-cated from either iron or aluminum, which causeste reactions:

dissolved from the anode generating correspond-l ions, which almost immediately hydrolyze to

ic iron or aluminum hydroxide. These polymerices are excellent coagulating agents. The consum-

crificial) metal anodes are used to continuouslypolymeric hydroxides in the vicinity of the an-gulation occurs when these metal cations com-

h the negative particles carried toward the anoderophoretic motion. Contaminants present in theter stream are treated either by chemical reactions

M.Y.A. Mollah et al. / Journal of Hazardous Materials B114 (2004) 199210 201

and precipitation or physical and chemical attachment tocolloidal materials being generated by the electrode ero-sion. They are then removed by electroflotation, or sedi-mentatiolating chthese co

Water ising smaat the caticles anpollutan

In additalso take p

Cathodic Discharg Electrop Electrofl

bubbles Reductio Other el

In an ECis usually cof metal diof electriciple relationamount ofderived fro

w = itMnF

where w isM cm2), irelative moof electronconstant, 9

It is expthe calculapassing a damount de[33] althoucient attentassembly athe EC cellpotential oof three co

AP = +where APoverpotentiIR the ove(V).

The IR-delectrodes,conductivit

A) by the equation [33] shown below

= IdA

(3)

R-drobetwen of t

ion. Cass trage ine electtial is

entratice. Thn is suroactivation oant isass t

asinge surfahanciome bt a hi

tic oven in thns. Thtion o

entratiever, tecific

olutioe relely deliworkrodes.solutioearly

tech

eactio

simplnd onan exttion, wtive daction

ws:

the an

(s)

2O(l)n and filtration. Thus, rather than adding coagu-emicals as in conventional coagulation process,

agulating agents are generated in situ.also electrolyzed in a parallel reaction, produc-

ll bubbles of oxygen at the anode and hydrogenthode. These bubbles attract the flocculated par-d, through natural buoyancy, float the flocculatedts to the surface.

ion, the following physiochemical reactions maylace in the EC cell [37]:

reduction of impurities present in wastewater.e and coagulation of colloidal particles.horetic migration of the ions in solution.otation of the coagulated particles by O2 and H2produced at the electrodes.n of metal ions at the cathode.

ectrochemical and chemical processes.

experiment the electrode or electrode assemblyonnected to an external DC source. The amount

ssolved or deposited is dependent on the quantityty passed through the electrolytic solution. A sim-ship between current density (A cm2) and thesubstances (M) dissolved (g of M cm2) can bem Faradays law:

(1)

the quantity of electrode material dissolved (g ofthe current density (A cm2), t the time in s;M thelar mass of the electrode concerned, n the numbers in oxidation/reduction reaction, F the Faradays6,500 C mol1.ected that there should be an agreement betweented amount of substances dissolved as a result ofefinite quantity of electricity and the experimentaltermined. Usually a good agreement is obtainedgh significant error may be introduced if suffi-ion is not given to the geometry of the electrodes well as the optimum conditions of operation of. One area of uncertainty is in the measurement off the EC cell. The measured potential is the summponents:

Mt + IR (2)is the applied overpotential (V), the kinetical (V), Mt the concentration overpotential (V),rpotential caused by solution resistance or IR-drop

rop is related to the distance (d in cm) between thesurface area (A in m2) of the cathode and specificy of the solution ( in mS m1) and current (I in

IR

The Itancesectiosolutas m

chanof thpotenconc

surfaactioelectcentrconstThe mincreanodby enoverc

ode aKineorigiactioevoluconc

Howfor spous s

on thclearlittleelectbothbe clof EC

3. R

Aode afromoxidareduccal refollo

At

M

2Hp can be easily minimized by decreasing the dis-en the electrodes and increasing the area of cross-he electrodes and the specific conductivity of theoncentration overpotential (Mt, V), also knownnsfer or diffusion overpotential, is caused by theanalyte concentration occurring in the proximityrode surface due to electrode reaction. This over-caused by the differences in electroactive specieson between the bulk solution and the electrodeis condition occurs when the electrochemical re-fficiently rapid to lower surface concentration ofe species below that of the bulk solution. The con-verpotential is negligibly small when reaction ratemuch smaller than the mass transfer coefficient.

ransport overpotential (Mt, V) can be reduced bythe masses of the metal ions transported from thece to the bulk of the solution and can be achievedng the turbulence of the solution. It can also bey passing electrolyte solution from anode to cath-gher velocity by using some mechanical means.rpotential (also called activation potential) has itse activation energy barrier to electron transfer re-e activation overpotential is particularly high forf gases on certain electrodes. Both kinetic andon overpotential increase as the current increases.he effects of these changes need to be investigatedtypes of physical and chemical species in aque-

n. The detailed effects of the electric field gradientvant surface and solution reactions must also beneated. A survey of literature indicates that veryhas been done to characterize the deposits on theThe effect of pH and electrochemical potentials onn phase as well as interfacial reactions must also

understood for optimization of the performancesniques.

ns at the electrodes

e electrocoagulating reactor is made up of one an-e cathode (Fig. 2). When a potential is appliedernal power source, the anode material undergoeshile the cathode will be subjected to reduction or

eposition of elemental metals. The electrochemi-s with metal M as anode may be summarized as

ode:

M(aq)n+ + ne (4)

4H+(aq) + O2(g) + 4e (5)


Fig. 2. Schemtion cell.

At the caM(aq)n+

2H2O(l)If iron

ated Fe(aq)further sping hydroAl3+ ionsAl(H2O)5Oproducts mspecies suAl(OH)4,Al13O4(OHrange. Simical oxidaions, Fe(namely: FeFe2(H2O)8on the pHides/polyhystrong affinto cause coimpinge on

The ECtroflotationduced at than EC procby the sizethe bubblessmaller bub

the particles in aqueous stream, resulting in better separationefficiency of the EF process [40].

To improve the performances of an EC it may be neces-o inteever, a

waterlutiond. PerC cel

s conntiallyeen twpolar

ternallectionpolara singxperimthe flos. Theon areificialnd mentalhe catrials.highew in aries hver, flrallel ae elect. The rfor disatic diagram of a bench-scale two-electrode electrocoagula-

thode:

+ ne M(s) (6)+ 2e H2(g) + 2OH (7)or aluminum electrodes are used, the gener-3+ or Al(aq)3+ ions will immediately undergoontaneous reactions to produce correspond-xides and/or polyhydroxides. For example,

on hydrolysis may generate Al(H2O)63+,

sary tHowwastedissoquireing Eserieessen

betwmono

is inconn

mono

lar toThe eulatevaluericatisacr

ode aelemand tmate

Ato floin sehowein paall thcells[41]H2+, Al(H2O)4(OH)2+ and the hydrolysisay form many monomeric and polymeric

ch as, Al(OH)2+, Al(OH)2+, Al2(OH)24+,Al6(OH)153+, Al7(OH)174+, Al8(OH)204+,

)247+, All3(OH)345+ [38] over a wide pHilarly, ferric ions generated by electrochem-

tion of iron electrode may form monomericOH)3 and polymeric hydroxy complexes,(H2O)63+, Fe(H2O)5(OH)2+, Fe(H2O)4(OH)2+,(OH)24+ and Fe2(H2O)6(OH)44+ depending

of the aqueous medium [39]. These hydrox-droxides/polyhydroxymetallic compounds haveity for dispersed particles as well as counter ions

agulation. The gases evolved at the electrodes mayand cause flotation of the coagulated materials.process is intrinsically associated with elec-

since bubbles of hydrogen and oxygen are pro-e cathode and anode, respectively. The success ofess, and for that matter EF process is determinedof the bubbles as well as by the proper mixing ofwith wastewater. It is generally believed that thebles provide more surface area for attachment of

monopolarBipolar

by some acial electrowithout anelectrodesinterconnecarrangemenmaintenancthrough thetive plate wopposite chsacrificial eelectrodes.

In additior other cofield, therefree-radicaical properthe reactiveleased from(non-sacrifirchange the polarity of the electrode intermittently.two-electrode EC cell (Fig. 2) is not suitable fortreatment, because for a workable rate of metalthe use of electrodes with large surface area is re-formance improvement has been achieved by us-ls with monopolar electrodes either in parallel orections (Figs. 3 and 4). The parallel arrangementconsists of pairs of conductive metal plates placedo parallel electrodes and a DC power source. In aarrangement, each pair of sacrificial electrodesy connected with each other, and has no inter-s with the outer electrodes. This arrangement ofelectrodes with cells in series is electrically simi-le cell with many electrodes and interconnections.

ental set-up also requires a resistance box to reg-w of current and a multimeter to read the currentconductive metal plates or rods used in EC fab-commonly known as sacrificial electrodes. Theanode lowers the dissolution potential of the an-inimizes the reduction or reductive deposition ofmetals on the cathode. The sacrificial electrodehode may be made up of the same or of different

r potential difference is required for a given currentseries arrangement, because the cells connected

ave higher resistance. The same current would,ow through all the electrodes. On the other hand,rrangement the electric current is divided betweenrodes in relation to the resistance of the individualeaders are referred to an article by Pretorius et al.cussion of the parallel and series arrangements ofcells.electrodes with cells in parallel have been useduthors (Fig. 5) [18,31]. In this case, the sacrifi-des are placed between the two parallel electrodesy electrical connection. Only the two monopolarare connected to the electric power source with notions between the sacrificial electrodes. This cellt provides a simple set-up, which facilitates easye during use. When an electric current is passedtwo electrodes, the neutral sides of the conduc-ill be transformed to charged sides, which havearge compared to the parallel side beside it. Thelectrodes in this case are also known as bipolar

on, as water containing colloidal particulates, oils,ntaminants moves through the applied electricmay be ionization, electrolysis, hydrolysis, and

l formation that may alter the physical and chem-ties of water and contaminants [42]. As a result,

and excited state causes contaminants to be re-water and destroyed or made less soluble. Inert

cial) electrodes, such as titanium and the passage


Fig. 3. Schematic diagram of a bench-scale EC reactor with monopolar electrodes in parallel connections (based on Ref. [41]).

of alternating current have been observed to remove metalions from solution and to initiate coagulation of suspendedsolids. Dirthe wastewsolids andeffective uelectrodes.these electr

more effective removal of the undesirable ions, wastewatercan be passed through a series of cells containing electrodes

up of various metals. In such cases, the contaminatedwater

rodesrical fiaracteonducect and indirect oxidation of organics present inater [43] and removal of oil, grease, suspendedCOD from restaurant water [44] were found verysing different metal/metal oxide-coated titaniumBut the high cost and the large consumption ofodes limit their wide application [32]. To ensure

madewasteelectelectter chand cFig. 4. Schematic diagram of a bench-scale EC reactor with monopolar electrodis passed through the annular spaces between theand is exposed to sequential positive and negativeelds. To optimize the removal efficiencies, the wa-ristics such as, pH, oxidationreduction potential,tivity can be adjusted for specific contaminants.es in series connections (based on Ref. [41]).


ctrodes

In the ECfor a shortintegrated cture separaand clear wdue to gravby convent

4. Design

It is imefficiency cfollowing f

the IR-d accumul

electrod impedim

the elect

The IR-

the cond the dista the elect

The mamized in th

use of hi decreasi devising

(feedbac

e masce lev

creasencreanear t

xygenathodeles aree elec

of thsed tothe acd theFig. 5. Schematic diagram of a bench-scale EC reactor with dipolar ele

process, an electric field is applied to the mediumtime, and the treated dispersion transferred to anlarifier system where the water-contaminant mix-tes into a floating layer, a mineral-rich sediment,ater. The aggregated mass eventually settles downitational force. The clear water can be extractedional methods.

of the EC cell

portant to design the EC cell so that maximuman be achieved. In dealing with this problem, the

Thbulenbe inThe ilayer

Othe cbubbat thtancebe umizearounactors must be taken into consideration [37]:

rop between the electrodes must be minimized;ation of O2 and H2 gas bubble nucleates at thee surfaces must be minimized;ent to mass transfer through the spaces betweenrodes must be minimum.

drop depends on [45]:

uctivity of the electrolyte solution;nce between the two electrodes;rode geometry.

gnitude of the error due to IR-drop can be mini-ree different ways:

ghly conducting solution;ng the distance between the electrodes;an electronic means to compensate for IR-drop

k action of a potentiostat).

bubbles.Electroc

configuratidisadvantaability. In thterms platcathodic suon a period

4.1. Tall ve

The tallmade of flcantly exceare typicallopen or clcontact abosubmergeda rate fasteinsulating min parallel connections (based on Ref. [41]).

s transport can be enhanced by increasing the tur-el in the flow through a EC reactor. Turbulence cand by increasing the flow rate inside the EC reactor.se in turbulence level also reduces the passivationhe electrode plates.and hydrogen gases are evolved at the anode andin the form of gas bubble nucleates. These gasinsulating spheres, and if allowed to accumulate

trode surfaces will increase the electrical resis-e cell and, as a result, more electric energy mustachieve optimum removal efficiency. To mini-

cumulation of the gas bubbles the electrolyte flowelectrodes must be increased to sweep out theoagulation reactors have been built in a number ofons. Each system has its own set of advantages andges, among which are varying degrees of treatmente following section, we discuss several types. The

es or tubes are used to refer to the anodic andrfaces, since the polarity is commonly reversedic basis in all reactors.

rtical-plate reactor

vertical-plate EC reactor [18] uses electrodesat steel plates whose vertical dimension signifi-eds the horizontal dimension (Fig. 6). The platesy arranged in a non-conductive case that may be

osed at the top. The open case allows electricalve the solution level, while a closed case requirescontacts. Submerged contacts typically erode atr than the plates unless they are coated with anaterial. This problem complicates the assembly.


Fig. 6. Schemtrodes.

A numconductiveThese reacall must benear the innthe flow inalso have lcenter of thened plate lbe from bothe solution1/8 to 1/4 inthe plates fiactor withiacid-washgenerating

Since plof bolts ordone by wtended dowactor is usereactor cellerosion, adtimes lockslots within

An openthe same diused. The p2 so that thplate groovsolution levThe reactortridge so this required.the plates toactor is cap

. Schemdes on

al-plaw raterpentdesig

e or casary fand cauid isor.

Long

ese rge (Fiondu

s are nhen tn the ssoon a

ontinuatic flow-diagram of EC reactor with tall, vertical-plate elec-

ber of attempts have been made using non-(typically PVC) pipe or tubing as the outer case.

tors use plates of various widths to fill the tube, asequally spaced and parallel. The narrower plateser tangent can offer more flow restriction, so thatthe reactor is not uniform. The narrower plates

ess surface area than adjacent plates closer to thee tubular case, causing uneven erosion and short-ife unless the plate thickness is varied. Flow mustttom to top to allow gas bubbles to move with. These reactors typically use a plate spacing of. and are subject to shorting as scale falling fromlls the void and collects at the bottom of the re-

n the plate field. The design usually includes anstep performed frequently to dissolve the scale,additional waste.ate spacing is not conducive to the additional bulkconnectors, assembly of the plates is typically

Fig. 7electro

verticof flo

Seplateanodneces

odicthe flreact

4.2.

Thon ednon-c

plateW

withiplateand celding. The welding operation results in an ex-

ntime during reactor rebuild unless a spare re-d. Some manufacturers have used a disposablethat is scrapped when the plates are exhausted by

ding to the cost of operation. Corrosion will some-the plates to the case if the plates are contained in

the case to maintain their spacing.reactor with a wider plate gap and with all plates

mensions within a rectangular case has also beenlates use a vertical to horizontal ratio of less than

e plates are removable even if corroded within thees in the case. The plates use contacts above theel so that welding and insulation are not required.itself contains the plates within a removable car-

at downtime is minimized when plate replacementThe reactor also has a large annular space belowprevent shorting if sheets of scale occur. This re-

able of treatment to a degree far exceeding other

oxygen, whscale falls,

When thphases of tso that onlybottom is sThe presenthe plates t

4.3. Short

These relel, horizonnon-conduthe solutionThe configlarity alongatic flow-diagram of EC reactor with long horizontal plateedge.

te reactors and can process water at a wide ranges.

ine flow is not practical with any of these vertical-ns. Thus the fluid will be proximate to either anthode during most of its residence. The reactionsor treatment usually require exposure to both an-thodic conditions, so that with the typical designonly partially reacted in a single pass through the

horizontal plate reactor

eactors use horizontal plates lying either flat org. 7). The plates are mounted within a rectangularctive case with slots to maintain their spacing. Theearly impossible to remove once corrosion occurs.he plates are installed flat, the gas bubbles risetream causing resistance at the bottom of the upperfter the solution begins to move between the plateses down the long axis. The produced gases includeich causes rapid corrosion and scaling. When the

it causes short circuits between the plates.e plates are installed vertically (on edge), the threehe solution (solid, liquid, and gas) can separate,the central part of the plate is effective while the

horted with solids and the top is insulated by gas.ce of solids at the base edge of each plate lockso the case and complicates reactor service.

horizontal plate reactor

actors use plates that are roughly square, paral-tal, and set apart by spacers or grooves within a

ctive case (Fig. 8) [46]. Flow is serpentine, so thatmust pass between all spaces between the plates.

uration allows the use of multiple changes in po-the path. The water will have an opportunity to


Fig. 8. Schematic flow-diagram of EC reactor with short horizontal parallelplate electrod

approach bment in a srequiring ecase designand insulatthe sides ofplates. Reative and are

Some atvelocity tofluid velocremoves sc

This conattempts hato prevent scase.

4.4. Perfor

Even thcialized duto mentionparallel pla(Fig. 9). Trather thanto collect r

Fig. 9. Schematic diagram of EC reactor with perforated, flat and horizontall plate electrodes.

n adjaan on

erforgh thr is prrationSinceis no

ns. Thing wifficulh pre

Solid

succe

sacricondu

are hher, md, keee is nochargehe othdifferent surface areas, current reversal (switching an-es.

oth anode and cathode, allowing complete treat-ingle pass. The contacts are made within the case,ither difficult assembly or innovative methods of

that allow electrical contacts without weldingion. The short contact area between the plates andthe reactor case allows easy removal of corroded

ctors of this type are considered to be most effec-capable of treating high concentrations of waste.

tempts with these reactors have used a low flowmaximize residence time of the solution. A highity ensures that there is no phase separation andale in smaller pieces that do not foul the reactor.figuration can be difficult to stop fluid leaks. Someve used drip basins to capture leaks and shieldspray rather than attempting to thoroughly seal the

ated plate reactor

ough perforated plate reactor is not so commer-e to the drawbacks mentioned below, it is worthit here. Reactors with perforated, flat, horizontal,tes within a non-conductive case can also be usedhe flow of wastewater passes through the platesbetween them. This configuration allows solids

paralle

tweethat cThe pthroumatteperfoarea.

thereratiodrinkare dat hig

4.5.

Aas thenon-

tubesor otmizeThernon-

and thaveapidly between the plates in the dead space be-

Fig. 10. Schematic diagram of EC reactor with two concentric t

ode and cacent perforations, causing electrical short circuitsly be cleared with acid washing or disassembly.

ations erode with the plates, so that fluid velocitye perforations changes with time. If any foreignesent the perforations can be plugged. A pluggedwill accelerate fouling, as solids will build in thatthe many perforations amount to parallel flowway to ensure uniform flow between all perfo-

ese reactors are effective if they are processingater and do not use consumable plates, but theyt to assemble without leakage and usually operatessure.

tube reactor

ssful reactor with two concentric tubes employedficial surfaces are usually contained within a third,ctive tube can also be fabricated (Fig. 10). Theeld apart by wrapping with non-conductive cordore complex fixtures. The annular space is mini-ping reaction voltage low and fluid velocity high.opportunity to vary the current density by using

d surfaces, as one tube must always be the anodeer the cathode. Since the inner and outer tubesubes as sacrificial electrodes.

thode electrically) requires differing forward and


reverse times that vary with the amount of plate erosion. Thecurrent density at each reactive surface also varies dependingon the direction of the current.

These realso subjecuseful lifecur. A shortubes are uEC, the grethe pH is atbe considerset in a bathey are no

4.6. Perfor

The perin-tube tha(Fig. 10). Tas the soliovercome bmanifold foin attemptiresultant vaplugging aplate reacto

5. Benets

Electrocnomical anater and otefficient tecsurfaces areprecipitatedcoagulant.tually anystart-up andmaintenancchemical amum chancit can be rumills and ftively destaimal sludgenhance floefficientlybiological thave greatefield that se

Althougbacks as dbe replaceddependingtaining low

compounds, some toxic chlorinated organic compound maybe formed in situ if chlorides are also present. Wastewaterwith high humic and fluvic acid content may be amenable

e formbolic alead tobe forrmancreducegh in t

ecent

e ECchniqegreesned frdiscusrmancor to ute thess.a rec

stewausing

rent Eance areatmewater

lectrodonop

ectorson ofe Q (Farad3FV [

e F isconsu

ed cel

[kW

e n isinumation oontal flectrod

to bewith

ovatch5 Hz)proverodes.actors are difficult to seal from leakage. They aret to massive shorting near the end of the tubingsas stresses deform the tubes and short circuits oc-t circuit commonly results in external leakage. Thesually set in banks to increase residence time. Inatest erosion occurs near the solution inlet whereits lowest. With the tube-in-tube reactor there canable waste if all tubes are replaced when the first

nk suffers breakthrough or frequent downtime ift.

ated tube reactor

forated tube reactor [47] is a variation of the tube-t uses perforated inner or outer tubes or bothhese reactors suffer the same sealing problems

d tubes but have a shorter life. This is partiallyy placing all the tubes in a case that becomes ar the treated solution. This configuration results

ng parallel flow through a number of tubes withriations in flow and erosion. The tubes also suffernd other disadvantages common with perforatedrs.

and drawbacks of EC technique

oagulation has the potential to be the distinct eco-d environmental choice for treatment of wastew-her related water management issues. EC is anhnique since adsorption of hydroxide on minerala 100 times greater on in situ rather than on pre-hydroxides when metal hydroxides are used as

EC needs simple equipments, designable for vir-size. It is cost-effective, and easily operable. The

operating costs are relatively low. It requires lowe cost with no moving parts. Since practically no

ddition is required in this process, it brings mini-e of secondary pollution. It needs low current, andn even by green processes, such as, solar, wind-uel cells. Electrocoagulation process can effec-bilize small colloidal particles. It produces min-

e. Gas bubbles produced during electrolysis cantation. It removes the smallest colloidal particles

in compared with the conventional chemical andechniques, because the smallest charged particlesr probability of being coagulated by the electricts them in motion.h EC has numerous advantages, it has some draw-iscussed below. The sacrificial anodes need toperiodically. It requires a minimum conductivity

on reactor design, limiting its use with water con-dissolved solids. In case of the removal of organic

to thmetamaymayperfohelpbe hi

6. R

Thcal teing dobtailittleperfotionlineaproce

Inof waniquediffeformthe twithlar ethe mconn

ductichargusing

=

wherergyappli

SEEC

wheralumfigurhorizlar elfoundlation

K(257to imelectation of trihalomethanes. If phenols and algalnd decomposition products are present, chlorinebad taste and odor. An impermeable oxide film

med on the cathode that may interfere with thee of the EC cell. However, changing polarity maythis interference. The cost of operating EC may

hose are as where the cost of electricity is high.

improvements of electrocoagulation

phenomenon has been known as an electrochemi-ue for the last century. It has been used with vary-of success for treatment of water and wastewater

om various sources. However, there has been verysion regarding the improvement of the design ande of EC reactors with respect to power consump-nderstanding the actual mechanisms to help de-

physical and chemical parameters involved in EC

ent article Jiang et al. [35] reported the treatmentter by a hybrid electrocoagulationflotation tech-aluminum electrodes. The authors designed two

C reactors to evaluate their water treatment per-nd found out the best configurations to maximizent efficiency. The EC reactors were configuredup-flow or cross-flow arrangements. The bipo-es were connected in series via the water, and

olar electrodes connected in parallel via copper. The current efficiency () required for the pro-dissolved Al3+ due to the passage of electricalC) in a volume V (m3) of water was calculatedays law:Al(III)]Q

(8)

the Faraday constant. The specific electrical en-mption (SEEC) was calculated as a function of thel voltage (U):

h (kg Al3+)1] = nFU3.6 103MAl

(9)

the number of moles of electrons per mole of thedissolved. It was concluded that an up-flow con-f the electrocoagulator performed better than aow configuration, with both bipolar and monopo-es. Also, the performance of the EC process wassuperior to that of conventional chemical coagu-aluminum sulphate.eva and Parlapanski [48] applied low frequencysonic field at current densities of 0.10.4 A m2the performance of the EC cell with carbon steelIt has been reported to increase the quantity of iron


hydroxides, and thereby enhance coagulation. It was assumedthat the applied sonic field:

Enhance Decrease

electrod Activate

crystal la Activate

ultra-sou Increase

friction b

Howeveof applyingwaves maywell as theparticles.

One of tcost of thecurrent effisents an unall current ethe choicetential for thigh overpowhich are nthe oxidatioefficiency.have usedcarbon elective oxideswastewaternols, chloro

Mattesoaqueous sustirred cellconsistingdescribed b

dcdt= k1c

where c isvelocity oframeter ofBoltzmannmedium). Telectrophorof dischargthis equatiocoagulationtential of thtial concenThe authortrafine kaolin parallel cwith the th

Chen et al. [34] reported a theoretical model involvingthe electrolysis voltage required in an EC process, the cur-rent density, conductivity of the water/wastewater, the inter-

rode dts weronfirms usedble tossivatetally ve theoassesthe eathod

electrofore, b

Eeq +

e Eeqlectroda,c thve ove

l, |c,oncen

ity (Aand ever, te, redediumequatpropolation

A+

e U0 icurre

lectrothe c

passivcted.

anged

A+

e K2ionedxperimrimentt on thodels

ge reqan ea

ng terl, andd the rates of mass transport.d the thickness of the diffuse double layer at the

e surface.d the electrode surface by generating defects at thettices of the electrodes.

d the ions in the reaction zone at the electrodes bynd waves.d the temperature at the electrode surface due toetween the liquid and the surfaces.

r, the authors also mentioned the disadvantagesthe sonic field. Among other things the acousticdestroy part of the colloidal iron hydroxides asadsorption layer at the surface of the colloidal

he most important considerations in lowering theEC process is to minimize the IR-drop to enhanceciency. Evolution of oxygen at the anode repre-wanted leakage of current, which reduces the over-fficiency. This problem can be minimized through

of an anodic material that presents a high overpo-he oxygen evolution reaction near the anode. Thistential permits the realization of anodic reactions,ot possible with other materials, and in particularn of organic compounds with a very high current

In order to achieve this goal, a number of authorsbinary [4951] and ternary [52] alloys as well astrodes and titanium sheets coated with certain ac-[53] as anodes for electrochemical treatment of

, in particular those containing organic dyes, phe--substituted phenols, and tannery wastes.

n et al. [26] investigated the electrocoagulation ofspensions of ultrafine kaoline particles in a singleapparatus and in a continuously flowing system

of three cells in series. The coagulation rate wasy a second-order rate equation:

2(1 ek2t)2 (10)

the concentration at time t, u the electrophoreticthe particles, k2 = u/x (x being the geometric pa-the electrode size), and k1 = 4kT/3 (k is the

s constant and the viscosity of the continuoushis second-order rate equation incorporates the

etic migration of particles, and subsequent releaseed particles into the bulk. The constant terms inn were derived from the basic electrophoretic andphenomenon, which is dependent on the zeta po-

e suspended particles, the applied voltage, the ini-tration of the particles and the electrode geometry.s conducted a number of EC experiments with ul-ine particles in a single stirred cell with electrodesonnections and found reasonably good agreement

eoretically predicted fraction coagulated.

electimenand ctermplicato paimen

Thrent pcome

tial, cThethere

U0 =

wherthe eode,passitentiathe cdens(m),Howanodthe mTafelnallycalcu

U0 =

wherj thethe eand non-

neglerearr

U0 =

whermentbe eexpeeffecthe mvolta

Involvitentiaistance, and the surface of the electrodes. Exper-e carried out with aluminum electrodes to verifythe theoretical model and determine the constant

in the model. Two mathematical models, one ap-non-passivated aluminum electrode and the otherd aluminum electrodes were proposed and exper-erified.retical model is based on the fact that when cur-through the electrochemical reactor, it must over-quilibrium potential difference, anode overpoten-e overpotential and IR-drop of the solution [54].lysis voltage U0 (V) between electrodes may,e considered to consists of the following terms:

a,a + a,c + a,p + |c,a| + |c,a| + dj (11)

is the equilibrium potential difference betweenes (V), a,a the activation overpotential at the an-

e concentration overpotential at the anode, a,p therpotential at the anode, a,a the activation overpo-

a| the activation overpotential at the cathode, |c,c|tration overpotential at the cathode, j the currentm2), d the net distance between the electrodesthe conductivity of the wastewater (1 m1).

he authors then considered the oxidation at theuction at the cathode and hydrolysis reaction in

to evaluate different potential terms by usingion [45] and NernstPlanck equation [55] and fi-se the following mathematical relationships for

s of U0 values:

a,p + dj +K1 ln j (12)

s the potential, A and K1 are the constant terms,nt density (A m2), d the net distance betweendes (m), a,p the anode passive potential (V)onductivity of the wastewater (1 m1). For a

ated electrode, the passive overpotential can beFor a passivated electrode the relationship can beto

d

j +K1 ln j + K2j

n

m(13)

is the constant term. It should, however, bethat the values of A, K1, K2, m and n need toentally determined. Theoretical analysis and

s demonstrated that pH and flow rate had littlee electrolysis voltage of the EC process. However,

can be used to calculate the total electrolysisuired for an EC process.rlier article, Vik et al. [33] proposed a model in-ms of, activation potential, concentration overpo-IR-drop due to solution resistance. However, this


model cannot directly predict the electrolysis voltage since itcontains unknown terms, which include the activation over-potential and the concentration overpotential.

7. Conclu

The fullment alternically optimderstandingprises commany surfaa paucity owhich limimum perfonovative teconfiguratizontal flowstirrer andand providcomprehenical factorsmake inroamerous advwater need

Acknowled

The authby the Uni(USAID) uFoundationthe Nationaacknowledgies, LLC,port and sk

Reference

[1] R.E. SPresenta3:45 p.m

[2] C. Lindep.stateForum/Inat 3:47 p

[3] A. Black[4] K. Raje

(1994) 1[5] N.P. Gn

Appl. Ch[6] V.L. Pog

N.P. Pro[7] N.P. Gn

Chem. T[8] X. Chen

[9] E.C. Beck, A.P. Giannini, E.R. Ramirez, Food Technol. 22 (1974)18.

[10] J. Lawrence, L. Knieper, Ind. Wastewater 12 (2000) 20.. Bisw. Vlyssas, Was.-S. Do.G. Iba040.. Vlysater. 7urses,

. Xiong17) (20. Mamariou,. Pani

601.. Tsai,.C. Don1994) 6.S. Ab(2002).R. Re

. Szynk2 (1994. Belo

ochem..J. Maaits II

ects 10. Kopa. Phut

ci. 16 (. Balas55..C. Ch

1997) 1.F. Po. Chen. Vik,

11) (19. Chen

.-Q. Jiaes. 36.Y.A.ater. 8.B. Pauality. Rebh

.N. Joh. Chen.A. Pr

1991) 1. Jofferg/Memt 9:30 a. Naum4 (11). Chenaboratoel DekkASELC:30 p.m. Mills.K. Kong. Assions

potential of EC technique as a wastewater treat-ative is yet to be fully realized. It is still an empir-ized process that requires more fundamental un-for improved engineering design. The EC com-

plex chemical and physical processes involvingce and interfacial phenomena. There is, however,f scientific understanding of these phenomena,

ts the engineering design of EC reactor for opti-rmance and future progress of this novel and in-chnology. Literature survey indicates that up-flowon of the electrocoagulator is better than a hori-configuration. In addition, the use of sonic field,

hybrid EC-EF technique increases the efficiencyes better control of this process. In this review, asive discussion of the fundamental electrochem-is presented. This technology will continue to

ds into the water treatment arena because of nu-antages and the nature of the changing strategic

s in the world.

gements

ors are grateful for financial support of this projectted States Agency for International Developmentnder project no. 631562 and the Robert Welch(grant no. V-1103). The financial support froml Science Foundation (grant no. 0116153) is also

ged. The authors also thank Poleware Technolo-for their great help on computerized graphics sup-etching images.

s

malley, Our energy challenge. http://smalley.rice.edu/tions/columbia09232003.ppt (visited on March 18, 2004 at. Central Time).

dell, Decentralized wastewater treatment. http://www..pa.us/dep/deputate/watermgt/WSM/WSM TAO/InnovTech-novTechForum-IIB-Lindell.pdf (visited on March 18, 2004.m. Central Time).man, Resour. Future 141 (2000) 9.

shwar, J. Ibanez, G.M. Swain, J. Appl. Electrochem. 24077.usin, N.V. Vitulskaya, L.I. Zabolotskaya, I.A. Belobrov, J.em. (USSR) 50 (1997) 1663.rebnaya, A.A. Klimenko, T.N. Bokovikova, E.P. Tsymbal,

nina, Chem. Petrol. Eng. 31 (1995) 280.usin, L.I. Zabolotskaya, N.V. Vitulskaya, Sov. J. Waterechnol. 7 (4) (1985) 32., G. Chen, P.L. Yue, Sep. Purif. Technol. 19 (2000) 65.

[11] N[12] G

p[13] J[14] J

1[15] G

M[16] G[17] Y

([18] N

B[19] M

2[20] T[21] J

([22] N

6[23] R[24] J

4[25] M

tr[26] M

Wp

[27] S[28] W

S[29] N

8[30] L

([31] M[32] G[33] A

([34] X[35] J

R[36] M

M[37] A

Q[38] M[39] P[40] G[41] W

([42] L

o

a

[43] J3

[44] G[45] L

c

[46] K3

[47] D[48] V

Eas, G. Lazarescu, Int. J. Environ. Stud. 38 (1991) 65.ides, D. Papaioannou, M. Loizidou, P.K. Karlis, A.A. Zor-te Manage. 20 (2000) 569.

, M.-L. Chen, J. Appl. Electrochem. 24 (1994) 785.nez, M.M. Singh, Z. Szafran, J. Chem. Educ. 75 (1998)

sides, M. Loizidou, P.K. Karlis, A.A. Zorpas, J. Hazard.0 (12) (1999) 41.M. Yalcin, C. Dogar, Waste Manage. 22 (2002) 491., P.J. Strunk, H. Xia, X. Zhu, H.T. Karlsson, Water Res. 35

01) 4226.eri, A.R. Yeddou, H. Lounici, D. Belhocine, H. Grib, B.Water Res. 32 (1998) 1604.zza, C. Bocca, G. Cerisola, Water Res. 34 (9) (2000)

S.T. Lin, Y.C. Shue, P.L. Su, Water Res. 31 (1997) 3073.nini, J. Kan, T.A. Hassan, K.L. Kar, Can. J. Chem. Eng. 7267.uzaid, A.A. Bukhari, Z.M. Al-Hamouz, Adv. Environ. Res.

325.nk, Energy Prog. 8 (1988) 205.arczuk, J. Kan, T.A. Hassan, J.C. Donnini, Clays Clay Miner.) 667.ngia, P.D. Harworth, J.C. Baygents, S. Raghavan, J. Elec-Soc. 146 (1999) 4124.

tteson, R.L. Dobson, R.W. Glenn Jr., N.S. Kukunoor, W.H.I, E.R. Clayfield, Colloids Surf. A: Physicochem. Eng. As-4 (1995) 101.ral, U.B. Ogutveren, J. Hazard. Mater. 89 (1) (2002) 83.dhawong, S. Chowwanapoonpohn, D. Buddhasukh, Anal.2000) 1083.ubramanian, K. Madhavan, Chem. Eng. Technol. 24 (2001)

iang, J.E. Chang, S.C. Tseng, Water Sci. Technol. 36 (23)23.uet, A. Grasmick, Water Sci. Technol. 31 (1995) 275., Sep. Purif. Technol. 38 (2004) 11.D.A. Carlson, A.S. Eikum, E.T. Gjessing, Water Res. 1884) 1355., G. Chen, P.L. Yue, Chem. Eng. Sci. 57 (2002) 2449.ng, N. Graham, C. Andre, G.H. Kelsall, N. Brandon, Water(2002) 4064.Mollah, R. Schennach, J.R. Parga, D.L. Cocke, J. Hazard.4 (1) (2001) 29.ul, Proceedings of the 22nd WEDC Conference on Waterand Supply, New Delhi, India, 1996, p. 286.un, M. Lurie, Water Sci. Technol. 27 (1993) 1.nson, A. Amirtharajah, J. AWWA 5 (1983) 232., X. Chen, P.L. Yue, J. Environ. Eng. 9 (2000) 858.etorius, W.G. Johannes, G.G. Lampert, Water S. Afr. 17 (2)33., L. Knieper, Ind. Wastewater 20 (2000). http://www.wef.berZone/pdffiles/iww1Jan00.pdf (visited on March 18, 2004.m. Central Time).czyk, L. Szypyrkowicz, F. Zilio-Grandi, Water Sci. Technol.(1996) 17., X. Chen, P.L. Yue, J. Environ. Eng. 126 (9) (2000) 858.ry Techniques in Electroanalytical Chemistry, 2nd ed., Mar-er, New York, 1996.O. http://www.kaselco.com/ (visited on August 18, 2004 at

. Central Time)., Am. Water Works Assoc. 92 (6) (2000) 34.vatcheva, M.D. Parlapanski, Colloid Interf. A: Physicochem.pects 149 (1999) 603.


[49] G. Vlyssides, P.K. Karlis, N. Rori, A.A. Zorpas, J. Hazard. Mater.95 (12) (2002) 215.

[50] G. Vlyssides, M. Loizidou, P.K. Karlis, A.A. Zorpas, D. Papaioan-nou, J. Hazard. Mater. 70 (12) (1999) 41.

[51] G. Vlyssides, C.J. Israilides, J. Environ. Sci. Health 33 (5) (1998)847.

[52] L. Szpyrkowicz, J. Naumczyk, F. Zilio-Grandi, Water Res. 29 (2)(1995) 517.

[53] R. Cossu, A.M. Polcaro, M.C. Lavagnolo, S. Palmas, F. Renold,Environ. Sci. Technol. 32 (1998) 357.

[54] K. Scott, Electrochemical processes for clean technology, R. Soc.Chem. (UK) (1995) 1262.

[55] J. Bard, L.R. Faulkner, Electrochemical MethodsFundamental andApplications, 2nd ed., Wiley, New York, 2001, pp. 2829.

Fundamentals, present and future perspectives of electrocoagulationIntroductionTheory of electrocoagulationReactions at the electrodesDesign of the EC cellTall vertical-plate reactorLong horizontal plate reactorShort horizontal plate reactorPerforated plate reactorSolid tube reactorPerforated tube reactor

Benefits and drawbacks of EC techniqueRecent improvements of electrocoagulationConclusionsAcknowledgementsReferences

Fundamentos, Presente y Futuro de la EC

Documents

Transcript of Fundamentos, Presente y Futuro de la EC