1-s2.0-S0011916411005807-ECT CIANURADA.pdf

The influence of operational parameters on elimination of cyanide from wastewater using the electrocoagulation process Gholamreza Moussavi a, ⁎, Farzad Majidi a , Mahdi Farzadkia b a Department of Environmental Health Engineering, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran b Department of Environmental Health Engineering, Faculty of Public Health, Tehran University of Medical Sciences, Tehran, Iran a b s t r a c t a r t i c l e i n f o Article h
of 7
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
  The in 󿬂 uence of operational parameters on elimination of cyanide from wastewaterusing the electrocoagulation process Gholamreza Moussavi  a, ⁎ , Farzad Majidi  a , Mahdi Farzadkia  b a Department of Environmental Health Engineering, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran b Department of Environmental Health Engineering, Faculty of Public Health, Tehran University of Medical Sciences, Tehran, Iran a b s t r a c ta r t i c l e i n f o  Article history: Received 16 May 2011Received in revised form 26 June 2011Accepted 27 June 2011Available online 20 July 2011 Keywords: WastewaterCyanideEliminationElectrocoagulation process Theelectrocoagulationprocess(ECP)wasinvestigatedfortheremovalofcyanidefromwastewaterinbatchandcontinuous operation under different conditions. The batch experiments indicated that an iron – aluminumelectrode arrangement as anode – cathode attained the highest removal ef  󿬁 ciency. The increase of the currentdensitiesof2to15mA/cm 2 resultedinanincreaseofcyanideremovalfrom43%to91.8%after20 minofreactionin the absence of aeration. Under similar conditions, aeration of the reactor enhanced removal ef  󿬁 ciencies from45% to 98%. Continuous operation of the ECP reactor with various hydraulic retention times (HRT) led to anincreaseofcyanideremovalfrom57%atanHRTof15mintocompleteeliminationatanHRTof140 min.Thus,weconclude that electrocoagulation is a cost-effective promising process for ef  󿬁 cient treatment of cyanide-ladenwastewater.© 2011 Elsevier B.V. All rights reserved. 1. Introduction Cyanide is a carbon/nitrogen compound that is highly toxic andcauses several health problems in the people exposed [1]. Cyanide is usually present in various forms, such as ionic, molecular HCN, saltand metal-complexes, in the ef  󿬂 uent from different industriesincluding gold extraction, metal electroplating, metal processing,automobile manufacturing, steel tempering, mining, photography,pharmaceuticals, coal coking, ore leaching and plastics [2 – 4]. Topreserve human and environmental health, wastewater containingcyanide has to be treated in a viable process for removal of cyanidebeforebeingreleasedintotheenvironment.Differenttechniqueshavebeenevaluatedfortoremovecyanidefromcontaminatedwastewater,including oxidation/precipitation, adsorption, and biodegradation.Although biological processes are the most often applied techniquesdue to their cost-effectiveness for treating wastewater containingbiodegradable contaminants, they are to be not ef  󿬁 cient for treatingcyanide-laden wastewater because of the inhibitory and/or toxicity of cyanide on microbial metabolism [5].Several adsorbents have been investigated for their effectivenessin removing cyanide from contaminated solutions ([6] and referencestherein), and some of them had considerable ef  󿬁 ciency. Nonetheless,the main concern from the operational viewpoint is the separation of theadsorbentfromtheliquid.Infact,itisnecessaryfortheadsorptiontobefollowedwithasolid/liquidseparationunitsuchasa 󿬁 ltrationorcoagulation/ 󿬂 occulation system for the separation of the used-upadsorbent.Therefore, attention has been focused on the development of anef  󿬁 cient alternative treatment process. One of the recently consideredprocesses for treating contaminated water and wastewater is theelectrocoagulation process (ECP). In this technique, metal precipitatessuch as hydroxides, polyhydroxides and/or oxyhydroxides are gener-ated in situ [7,8] via electrochemical oxidation of different sacri 󿬁 cialanodessuchasironoraluminum,thusovercomingtherequirementforexternal coagulants [9]. This unique feature has made ECP a simple,reliable, and cost effective technique without having to add externalchemicals [10] to remove contaminants from water and wastewater.Therefore, compared to conventional chemical coagulation, the mainadvantages of ECP include, (a) a lower amount of coagulant ionsrequired, (b) a higher rate of contaminant removal, (c) no need to addchemicals, thus preventing secondary pollution and reduction of amount of generated sludge needing disposal, (d) low reaction timeand thus small size of the reactor, and (e) simple operation andmaintenance [9,11 – 14]. Moreover, the hydrogen gas generated in thecathodecausesmixingofthesuspension in the reactor, thus enhancing 󿬂 occulationandthusoverallperformance[15].ThesefeaturesmakeECPtechnically and economically more ef  󿬁 cient than conventionalcoagulation.ThecapabilityofECPfortheremovalofseveralorganicandinorganiccontaminants including arsenic [9], dyes [16], and chromium (VI) [17] has been previously investigated with a high degree of ef  󿬁 cacy in mostofthestudies.DuetotheaforementionedmeritsofECP,andtoextenditsapplication for pollution control, it is very valuable to investigate theperformanceofECPineliminatingothercontaminants,suchascyanide,whichposesagreatenvironmentalandpublichealthrisk.Theliterature Desalination 280 (2011) 127 – 133 ⁎  Corresponding author. Tel.: +98 21 82883827; fax: +98 21 82883825. E-mail address: (G. Moussavi).0011-9164/$  –  see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.desal.2011.06.052 Contents lists available at ScienceDirect Desalination  journal homepage:  on cyanide removal from wastewater using electrocoagulation processis very rare. For instance, Kobya et al. [18] reported 99.7% cyanideremovalfromanelectroplatingrinsewaterinabatchelectrocoagulationprocess. However, to the best of our knowledge from reviewing theliterature, no report is available to date on the removal of cyanidefromwastewater using the continuous- 󿬂 ow electrocoagulation process.Accordingly, the present study was an evaluation of laboratory scaleECP for the removal of cyanide from wastewater. The effectiveness of iron and aluminum electrodes in monopolar and bipolar arrangementswas evaluated. Once the bestperforming electrodearraywas obtained,the in 󿬂 uence of aeration (presence/absence), current density(2 – 15 mA/cm 2 ), electrical conductivity (1.35 to 9.36 mS/cm) andreaction time (2 – 50 min) was studied with the performance of theselected ESP for the removal of cyanide. Finally, to examine thecapability of ECP for treating continuous  󿬂 ow cyanide-contaminatedstreams,theef  󿬁 cacyofECPfortheremovalofcyanidefromwastewaterwasinvestigatedin thecontinuously-operatedECP reactor athydraulicretention times (HRT) of 15 to 140 min. 2. Materials and ,methods  2.1. Materials Synthetic cyanide-laden wastewater was prepared by dissolvinganalytical grade (Merck Co.) NaCN into tap water; the desiredconcentration of cyanide was obtained by dissolving a speci 󿬁 c mass of NaCN in a known volume of water. All chemicals used in theexperiments were of analytical grade. Iron and aluminum metalsprepared locally were used as electrodes. The electrodes were thinplates(0.2cmthickness)ofaluminum(Al)and/oriron(Fe),eachwithawidth of 2 cm and a submerged length of 10 cm.  2.2. ECP reactor setup and procedure TheelectrocoagulationunitshowninFig.1consistedofacylindricalglass cell, in which the anode and cathode electrodes were  󿬁 xed at adistance of 3 cm from each other, a direct current power supply, anair-supply system,a magnetic stirrer, and a wastewaterfeedingsystemincluding a peristaltic pump (in the continuous operation mode).The ECP cell had a working volume of 250 mL. Considering thedimensions of the electrodes as well as their submergence depth(Section 2.1), this provides an electrode surface area to reactor volumeratio of 16 m 2 /m 3 in the reactor. The direct current was supplied by apower supply instrument (SANJESH TEK, 8051) in which the currentvoltage/density could be regulated at a given level via the switchesinstalled on the instrument.The ECP reactor was operated in both batch and continuous modes.For batch tests, 250 mL of the wastewater containing a knownconcentration of cyanide was poured into the reactor equipped withthepreviouslydescribedelectrodes;thedensityoftheelectricalcurrentwasregulatedatthedesiredvalue,theelectricalcurrentwasestablishedbetween electrodes, and the reaction was started and allowed tocontinue up to the speci 󿬁 ed time. The suspension was then  󿬁 lteredthrougha0.45micronporesizepaper 󿬁 lter,andthe 󿬁 ltrateanalyzedforresidual cyanide. At the beginning of each test, the electrodes werewashed with 1 N HCl solution and then rinsed with distilledwater. Thecontent of the reactor was stirred either magnetically or by aeration,depending on the operational conditions. The lost amount of anodeweight, when required, was determined by subtracting the weight of the electrodes before and after the experiment. The second step of theexperiment was the operation of ECP in continuous mode. In this step,the cyanide-laden wastewater was continuously injected into thebottom of the reactor by a peristaltic pump (WATSON MARLOW101U/R).Theelectrocoagulatedsuspensionexitedthereactorthroughavalve located at a distance of 10 cm from the bottom (the workingvolume was 250 mL) and was collected in a container. The ECP wasoperated at different continuous  󿬂 ow rates ranging from 0.7 to6.7 mL/min, corresponding to hydraulic retention times (HRT) of 15 to140 min. The suspension aliquots at each HRT were taken from thecontainer and  󿬁 ltered; the  󿬁 ltrate was then analyzed for residualcyanide as stated above. All tests were carried out at a pH of 11.5 androom temperature (25±3 °C). The wastewater pH was adjusted at thedesired value using 1 N NaOH solution.The electrodes were selected from iron and aluminum materials.At the beginning of the batch experiments, several arrangements of electrodes were tested, including a monopolar iron array, a monopolar (1: ECP cell, 2: electrodes, 3: magnet, 4: stirrer, 5: power supply, 6: wires) 3226541 Fig. 1.  A schematic representation of the ECP experimental setup.128  G. Moussavi et al. / Desalination 280 (2011) 127  – 133  aluminum array, iron as the anode and aluminum as the cathode, andaluminum as the anode and iron as the cathode. The most effectivearrangement at which the highest cyanide removal under selectedconditions was attained was then selected for further experiments.Thereafter, the in 󿬂 uence of current density, ranging from 2 to15 mA/cm 2 , aeration, and reaction time (from 2 to 50 min) on theperformance of ECP for eliminating cyanide from the wastewater wasevaluated with the most ef  󿬁 cient electrode arrangement. During thesecond step of the reactor, the ef  󿬁 cacy of continuous ECP in removingcyanidefromrealwastewaterwasinvestigatedatHRTsbetween15and140 min. The experimental runs and conditions are given in Table 1.  2.3. Analysis The concentration of cyanide was measured by titration against astandardizedAgNO 3 solutionasdescribedintheStandardMethods[19].The weight loss of the electrodes was determined by weighing thembefore and after the reaction using a 5 digit electrical balance. Theamountofsludgegeneratedwasmeasuredby 󿬁 lteringthetotalvolumeof suspension after completing the desired test according to theproceduredetailedintheStandardMethods[19].ThesludgegeneratedintheECPwascharacterizedusingscanningelectronmicroscopy(SEM,Philips XL-30), and Fourier transform infrared (FTIR, Nicolet spectrom-eter) spectroscopy. 3. Results and discussion  3.1. Effect of electrode arrangement  Four electrode arrangements (based on Fe and Al electrodes) wereinvestigated for cyanide removal in the batchECP under the conditionsspeci 󿬁 ed in Table 1. Fig. 2 shows the effectiveness of the electrode arrangementsincyanideremovalunder similar operational conditions.According to Fig. 2, the arrangement of anode – cathode materials has apronouncedin 󿬂 uenceoncyanideremoval.Infact,ef  󿬁 cienciesofaround93%, 87%, 35% and 32% cyanide removal were obtained for the anode – cathodeelectrodearrays of Fe – Al, Fe – Fe, Al – Al, and Al – Fe, respectively.Therefore, it is observed that the sacri 󿬁 cial electrode of Fe in the Fe – Alarrangement attained the highest cyanide removal under the selectedconditions. The better performance of Fe than Al as the sacri 󿬁 cialelectrode is related to the higher oxidation potential of Fe ( − 0.447 V)compared to that of Al ( − 1.662 V) and consequently the higheroxidation rate for Fe than for Al [17]. The greater effectiveness of thearrangementofFe – Alcanbeattributedtothehigherelectrodepotentialgradient [20] and thus higher oxidation potential between anode – cathode compared to that of the other arrangements. These resulted inthe generation of higher coagulant agents and thus an improvement inthe cyanide removal percentage.ThesuperiorityofFecomparedtoAlasthesacri 󿬁 cialelectrodeintheelectrocoagulation process has been also reported by other researchersfor the removal of different organic compounds [14,16,21]. In contrast,El-Naasetal.[15]observedagreaterperformanceforAlcomparedtoFeas an electrode for ECP in treating re 󿬁 nery wastewater. Thus, thesuperioranodematerialandarrangementdependsstronglyonthetypeof contaminant(s) and on the operational conditions. The reactionoccurringinthe ECPcellwith thebestelectrodearraycanbewritten asfollows: ã  Anodic reactions: Fe ð s Þ → Fe 2 þð aq Þ  þ  2e − ð 1 Þ H 2 O → 2H þ þ  1 = 2O 2  þ  2e − ð 2 Þ ã  Cathodic reaction: 2H 2 O  þ  2e − → H 2 ð g Þ  þ  2OH – ð 3 Þ ã  Liquid bulk reactions: 2Fe 2 þð aq Þ  þ  3 = 2O 2  þ  3H 2 O → 2Fe ð OH Þ 3 ð S Þ  ð 6 b  pH b 10 Þ ð 4 Þ n Fe ð OH Þ 3 → Fe n ð OH Þ 3 n  ð 5 Þ ã  Cyanide removal CN − ð aq Þ  þ  Iron hydroxide floc → CN – Fe precipitate complex :  ð 6 Þ Accordingly, the main mechanism involved in the removal of cyanide from the wastewater by ECP might be (a) Fe oxidation intoferrous ions (Eq. 2) and concurrent water electrolysis on the surface of anode, resulting in the generation of oxygen (Eq. (3)); (b) oxidation of ferrous ions into ferric ones through reaction with oxygen moleculesand subsequent formation of iron hydroxide/polyhydroxide/polyhy-droxyoxideprecipitates(Eqs.(5)and(6))dependingonthesolutionpH[7]; and (c) interaction of cyanide ions with iron precipitates (Eq. (6)), thereby eliminating cyanide from the solution.Based on the reactions, and taking into consideration the pH of thesolution in the reactor (ca. pH=9.5), the predominant coagulantspecies formed in the ECP cell is deduced to be monomeric [Fe(OH) 3 ] ? and/or polymeric  n Fe(OH) 3 → Fe n (OH) 3 n ?  iron hydroxide precipitates[20]. Therefore, the surface complexation of cyanide ions with ironprecipitates formed and cyanide enmeshment in the porous hydroxideprecipitates [21] might be the predominant mechanisms of cyanideremoval in the ECP under these conditions.  3.2. Effect of aeration Fig. 3 compares the effect of the presence or absence of aeration oncyanide removal ef  󿬁 ciency in ECP using Fe – Al metal plates as anode – cathode electrodes under the conditions speci 󿬁 ed in Table 1. Fig. 3 shows that the cyanide removal percentage was higher in ECP withaerationthanthatwithoutaerationundertheconditionsevaluated.Thein 󿬂 uence of aeration on the cyanide removal was more pronounced inthe initial period of the reaction. Based on data shown in Fig. 3, thecyanideremovalinECPwithoutaerationincreasedtoaround94%whenthe reaction time was increased to 30 min. However, when the reactorwasaerated,completecyanideremovalwasaccomplishedatareaction  Table 1 Experimental runs and conditions.Run Experiment Operational conditionsMode Current (mA/cm 2 ) Electrode (anode – cathode) Aeration Reaction time (min)1 Effect of type of electrode Batch 15 Fe – Fe, Fe – Al, Al – Al, Al – Fe No 202 Effect of aeration Batch 15 Fe – Al No/Yes 203 Effect of current density and reaction time Batch 2 – 15 Fe – Al Yes 2 – 504 Sludge analysis  –  15 Fe – Al Yes 305 Effect of HRT Continuous 15 Fe – Al Yes 15 – 140 (HRT)129 G. Moussavi et al. / Desalination 280 (2011) 127  – 133  time of 30 min. This  󿬁 nding suggests a signi 󿬁 cant contribution of aeration on the performance of ECP in the elimination of cyanide.Todeterminethecontribution oftheaeration, theamountof sludgegenerated in the reaction with and without aeration (at a currentdensity of 15 mA/cm 2 and a reaction time of 20 min) was quanti 󿬁 ed(datanotshown). Itwasobserved that theamountofsludgegeneratedin the presence of aeration wasaround twice asmuchasthatwhen theaeration pump was off. Therefore, the positive in 󿬂 uence of aeration onthe performance of ECP can be explained to the effect that aerationoxidized the ferrous ions released from the sacri 󿬁 cial anode into thesolution bulk to ferric ions thus forming more gelatinous hydroxideprecipitates. This in turn improved cyanide removal through increasedadsorption onto the formed precipitates. Moreover, aeration improvedmixing of the reactor's content, which resulted in the increased size of  󿬂 ocs and in improved contact between precipitates and the targetcontaminant, leading to an enhancement of the adsorption of contaminants onto the  󿬂 ocs. Moreover, aeration has additionaladvantagesincludingcreatingturbulenceinthereactor,thuspreventingthe deposition of precipitates on the anode and preventing anodedeactivation, and limiting the release of ferrous irons to the environ-ment through the ef  󿬂 uent. Both of these features resulted inimprovement of the performance of ECP.  3.3. Effect of current density and reaction time The amount of coagulating ions released from the sacri 󿬁 cial anodeinto the solution in an electrocoagulation reactor is affected by thecurrent density applied to the electrodes. Therefore, the in 󿬂 uence of current density between 2 and 15 mA/cm 2 on the performance of ECPforremovalofcyanideversusreactiontimewasinvestigated,undertheconditions given in Table 1. The results are depicted in Fig. 4, which shows an increase in the cyanide removal ef  󿬁 ciency over time withincreasing current density. Based on the plots shown in Fig. 4, thein 󿬂 uenceofthecurrentdensityismorepronouncedintheinitialperiodsof the reaction. According to the results depicted in Fig. 4, the cyanideremoval increased from 59% to around 90% after 50 min of reactionwhen the current density was elevated from 2 to 10mA/cm 2 . Furtherincrease of current density to 15 mA/cm 2 resulted in the completeremoval of cyanide after 30 min of reaction. Therefore, ECP is a veryef  󿬁 cientand promising process for thetreatmentof high concentrationcyanide-containing wastewater in a relatively short reaction time.The improvement of ECP performance with an increase in currentvoltagecanbeexplainedbytakingintoconsiderationFaraday'slawandthe fact that anode (Fe) sacri 󿬁 ce, hence coagulant generation, wasincreased with the increase of the current density applied betweenelectrodes [16,22,23]. This explanation was experimentally con 󿬁 rmedbymeasuring the loss in anode (Fe) weight as a function of the appliedcurrent density ata given reaction time. Fig. 5 indicates both the anode(Fe) weight loss and cyanide removal versus the applied currentdensities for the 30 min reaction time, whereby complete cyanideremovalwasattainedundertheselectedconditions.AsseeninFig.5,theanode weight loss, i.e. Fe sacri 󿬁 ce, increased almost linearly with theincrease in current density, corresponding to an improvement of cyanideremovalef  󿬁 ciency.Itisthereforecon 󿬁 rmedthatironhydroxide 02040608010005101520253035    C  y  a  n   i   d  e  r  e  m  o  v  a   l   (   %   ) Reaction time (min) with aerationwithout aeration Fig. 3.  EffectofaerationoncyanideremovalintheECP(cyanideconcentration=300 mg/L; current density=15 mA/cm 2 ; Fe-Al electrode; reaction time=20 min). 02040608010001020304050    C  y  a  n   i   d  e  r  e  m  o  v  a   l   (   %   ) Reaction time (min) 15 mA/cm2 (30 V)10 mA/cm2 (20 V)4 mA/cm2 (10 V)2 mA/cm2 (5 V) Fig. 4.  Effect of current density on cyanide removal in the ECP (cyanide concentration=300mg/L; current density=2-15mA/cm 2 ; Fe-Al electrode; reaction time=2 – 50min).    A  n  o   d  e   (   F  e   )  w  e   i  g   h   t   l  o  s  s   (  g   )   C  y  a  n   i   d  e  r  e  m  o  v  a   l   (   %   ) Current density (mA/cm 2 ) cyanide removal (%)anode experimental weight loss (g)anode theoretical weight loss (g) Fig. 5.  Cyanide removal and anode (Fe) weight loss as a function of current densitiesapplied in the ECP (cyanide concentration=300 mg/L; current density=15 mA/cm 2 ;reaction time=30 min; Fe-Al electrode; aeration). 020406080100Fe-AlFe-FeAl-AlAl-Fe    C  y  a  n   i   d  e  r  e  m  o  v  a   l   (   %   ) Electrode arrangement (anode-cathode) cyanide removal Fig. 2.  EffectofelectrodematerialsoncyanideremovalintheECP(cyanideconcentration=300mg/L; current density=15mA/cm 2 ; reaction time=20min; no aeration).130  G. Moussavi et al. / Desalination 280 (2011) 127  – 133
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks