A comparative study of chemical precipitation and electrocoagulation for treatment of coal acid drainage wastewater

A comparative study of chemical precipitation and electrocoagulation for treatment of coal acid drainage wastewater M.S. Oncel a , A. Muhcu a , E. Demirbas b , M. Kobya a, * a Gebze Institute of Technology, Department of Environmental Engineering, 41400 Gebze, Turkey b Gebze Institute of Technology, Department of Chemistry, 41400 Gebze, Turkey Introduction Acid mine drainage (AMD) generated from active and abandoned mining is a serious
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  A comparative study of chemical precipitation and electrocoagulation fortreatment of coal acid drainage wastewater M.S. Oncel a , A. Muhcu a , E. Demirbas b , M. Kobya a, * a Gebze Institute of Technology, Department of Environmental Engineering, 41400 Gebze, Turkey b Gebze Institute of Technology, Department of Chemistry, 41400 Gebze, Turkey Introduction Acid mine drainage (AMD) generated from active andabandoned mining is a serious environmental problem with thepotential of severe contamination of surface and groundwater [1–3]. AMD most commonly initiates from coal mining as materialsexposed to water and atmospheric oxygen during the miningprocess and contains high concentration of FeS 2  [4]. In general,drainage from coal mines is not only of low pH but also includeshigh levels of sulphate and heavy metals such as Fe, Al, Mn, Ca andZn [5,6]. High content of toxic metals and high acidity in AMDadversely affects surface water, groundwater and soil. Thesepropertiesofminedrainagedisruptstreamecosystemsandfurtheraggravate the problem by creating yellow or white sediments [7].Moreover, AMD is a highly acidic aqueous solution and formedthrough the chemical reaction of surface and shallow subsurfacewater with rocks containing sulphur-bearing minerals to givesulphuric acid. Heavy metals are not biodegradable and tend toaccumulate in living organisms and many heavy metal ions areknown to be toxic or carcinogenic. Toxic heavy metals for thetreatment of industrial wastewaters include zinc, copper, nickel,mercury, cadmium, lead and chromium [8]. Therefore, thesewastewaters must be treated before being discharged into theenvironment.Passiveandactivetreatmentmethodshavebeenappliedforthetreatment of AMD [5,9]. Passive systems include using anoxiclimestone drains, aerobic wetlands, compost reactors, permeablereactive barriers and packed bed iron-oxidation bioreactors [10].Bioreactors represent a passive-treatment option for removal of sulphate and metals from AMD. However, their treatmentperformance can be quite variable depending on a number of factors including organic substrate sources and their degradation,mine-water chemistry, microbiological diversity and activities,reactor configuration and hydraulics [11]. Wetlands also areineffective in areas with rocky soils and steep slopes [12]. Closeproximityto floodsand large landrequirementsnegativelyimpactwetland use. The most widely used active treatment process forAMD is based on chemical neutralization and hydroxide precipi-tation of metals [13,14]. Most active treatment involves pHadjustment and removal by precipitation as a result of theformation of oxy/hydroxides. pH adjustment is needed fortreatment of large quantities of AMD. Active treatment enhancesthetreatment efficiencywithusingof chemicalsbut causes a largeeconomic burden owing to the high cost of maintenance andchemicals, and this process requires continuous operation [5]. Thedisadvantages of the traditional chemical treatment are high cost  Journal of Environmental Chemical Engineering 1 (2013) 989–995 A R T I C L E I N F O  Article history: Received 2 March 2013 Received in revised form 3 August 2013 Accepted 5 August 2013 Keywords: Coal mine drainage wastewaterChemical precipitationElectrocoagulationIron electrodeOperating cost A B S T R A C T The present study provided a quantitative comparison between chemical precipitation andelectrocoagulation (EC) for removal of heavy metals such as Fe, Al, Ca, Mg, Mn, Zn, Si, Sr, B, Pb, CrandAs fromcoalminedrainagewastewater(CMDW)atalaboratoryscale. TheoptimumpHforremovalofmostofheavymetalsfromCMDWbythechemicalprecipitationusingsodiumhydroxidewas8exceptforCa, Srand B(pH10 orhigher).The removal efficienciesatthe optimumpHwerevaried from 28.4%to99.96%. Influence of current density and operating time in the EC process was explored on the removalefficiency and operating cost. Results from the EC process showed that the removal of metals present inCMDWincreasedwithincreasingcurrentdensityandoperatingtime.TheECprocesswasabletoachievehigher removal efficiencies( > 99.9%)atan electrocoagulationtimeof40 min,acurrentdensityof500 A/m 2 and pH of 2.5 as compared to the results obtained with the chemical precipitation at pH 8. Theoperating costs at the optimum operating conditions were also determined to be 1.98 s /m 3 for the ECand 4.53 s /m 3 for the chemical precipitation. The EC process was more effective than the chemicalprecipitation with respect to the removal efficiency, amount of sludge generated and operating cost.Electrocoagulation has the potential to extensively eliminate disadvantages of the classical treatmenttechniques to achieve a sustainable and economic treatment of polluted wastewater.   2013 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +90 262 6053214; fax: +90 262 6538490. E-mail address: (M. Kobya). Contents lists available at ScienceDirect  Journal of Environmental Chemical Engineering journal homepage: 2213-3437/$ – see front matter    2013 Elsevier Ltd. All rights reserved.  of the chemical reagents, inefficient removal of sulphate andproduction of a bulky sludge needed to be disposed of. Passivesystemsareeconomicalascomparedwiththeactivetreatmentbutrequire longer retention times and greater space. Therefore, theyare not appropriate for treatment of large-scale mine drainage.Although the passive treatment has been implemented on full-scale sites in several countries, treatment efficiency can beuncertain because of seasonal changes in flow rate and tempera-ture, and the systems are apt to fail during long-term operation.Current AMD treatment technologies are either inadequate ortoo expensive. Thus high cost of conventional treatment technol-ogieshasproducedeconomicpressureandhascausedengineerstosearch for cost effective and environmental friendly technologiesto treat AMD. In the past decades, research efforts have beendirectedtowardsadvancedtechniquesforremovalofheavymetalsfrom AMD as well as industrial wastewater. Several techniquessuch as chemical precipitation, oxidation, reduction, coagulation,solvent extraction and adsorption have been commonly employedfor the removal of heavy metal ions [8,15].Currently, there are a number of studies about hydroxideprecipitationprocessusinglime,Ca(OH) 2 andNaOHforremovalof heavy metals in the literature. Cu(II) and Cr(VI) ions fromwastewater were evaluated. Maximum precipitation of Cr(III)occurred at pH 8.7 with the addition of Ca(OH) 2  and theconcentration of chromate was reduced from 30 to 0.01 mg/L.The optimum pH for maximum copper precipitation was 12.0 forboth Ca(OH) 2  and NaOH and the concentration of copper wasreduced from 48.51 to 0.694 mg/L  [16]. Fly ash was used as a seedmaterial to enhance lime precipitation. The fly ash-lime carbon-ation treatment increased the particle size of the precipitate andsignificantly improved the efficiency of chromium, copper, leadand zinc removals. The concentrations of chromium, copper, leadand zinc in effluents were reduced from initial concentration of 100.0 mg/L to 0.08, 0.14, 0.03 and 0.45 mg/L  [17]. Chemicalcoagulation and precipitation by lime were employed to treatsynthetic wastewater consisting of Zn, Cd, Mn and Mg at theconcentration of 450, 150, 1085 and 3154 mg/L. It was found thatthe optimum pH was greater than 9.5 for treatment of thewastewater to meet the Wastewater Standard of the Ministry of Industry[18].Hydroxideprecipitationmayhavesomelimitations:(i) hydroxide precipitation produces secondary wastes such asmetal hydroxide sludge and gypsum which are highly regulatedand havecostlydisposalrequirements;(ii)somemetalhydroxidesare amphoteric, and the mixed metals create a problem usinghydroxide precipitation since the ideal pH for one metal may putanothermetalbackintosolution;(iii)whencomplexingagentsareinthewastewater,theyinhibitmetalhydroxideprecipitation[19].Removal efficiencies of Cu, Cr and Ni from metal platingwastewater using an Fe–Al electrode pair at a current density of 10 mA/cm 2 , pH 3.0 and an EC time of 20 min with energy andelectrode consumptions of 10.07 kWh/m 3 and 1.08 kg/m 3 wereachieved with electrocoagulation (EC) process as 100% [20].Optimum conditions for removal of Cr(VI) with a concentrationof1470 mg/Lweredeterminedtobe7.4 A,pH1.84and70 minandthe removal efficiency by the EC process was 100% [21]. Theperformance of simultaneous removal of Cu, Ni, Zn and Mn from amodel wastewater was investigated with the EC using ironelectrodes and removal efficiency of more than 96% was obtainedfor all metals with a total energy consumption of 49 kWh/m 3 at25 mA/cm 2 [22]. Treatment of spent final rinse water of zincphosphate from an automotive assembly plant was performed inan electrochemical cell equipped with aluminium or iron plateelectrodes. The highest phosphate and zinc removal efficiencies atoptimumconditionswere97.7%and97.8%forFeelectrode(60.0 A/m 2 ,pH3.0andoperatingtimeof15.0 min),and99.8%and96.7%forAl electrode (60.0 A/m 2 , pH 5.0 and operating time of 25.0 min)[23]. Removal efficiencies of arsenic in a batch EC reactor using Aland Fe electrodes with monopolar parallel electrode connectionmodewere93.5%forFeelectrodeat12.5 minandpH6.5and95.7%for Al electrode at 15 min, pH 7 and at 2.5 A/m 2 [24]. Ni and Znremovals from Ni and Zn plating processes by the EC usingstainless steel electrodes were achieved with 100% at 9 mA/cm 2 and pH 6 [25]. Separation of some heavy metal ions such as Fe, Ni,Cu, Zn, Pb and Cd with different initial concentrations in the rangeof 50–600 mg/L and initial pH between 7.5 and 7.8 was studiedwithelectrocoagulation–electroflotationprocess.Theremovalratewas 95% at 15 min [26]. The removal efficiency of Mn 2+ fromsynthetic wastewater containing 100 mgMn 2+ /L by the EC wasobtained for 78.2% at 6.25 mA/cm 2 and pH 7 [27].There has been no direct report as yet being published for thetreatment of coal mine drainage wastewater (CMDW) by the ECprocess, despite the considerable success of this process for thetreatment of industrial wastewater, groundwater and surfacewater containing dissolved metal ions in the literature [21–26,28,29,31]. Therefore, the present study was aimed to focus ontreatment of CMDW by the EC process using iron plate electrodes.Effects of current density and operating time for the removal of Fe,Al, Mn, Mg, Pb, Zn, Cr, As and Sr from CMDW at a laboratory scalewere investigated to determine the optimum operating conditionsin theECprocess. The chemicalprecipitationby sodiumhydroxide(NaOH) was used to treat CMDW. The operating costs for the ECand chemical precipitation processes at the optimum operatingconditions were calculated. Electrocoagulation and chemical precipitation processes Electrocoagulation process EC involves the generation of coagulants in situ by dissolvingsacrificial anodes such as aluminium or iron upon application of adirect current. Iron is oxidized in an EC reactor at anodic sites toFe 2+ ions whichdissolveto Fe 3+ . Thewastewatersolutionbecomesgreen and bubbles of gas at cathode are observed during the ECprocess. The effluent becomes clear and then a green and yellowsludge are formed which are attributed to Fe 2+ and Fe 3+ hydroxides. The following major reactions take place in the ECprocess [28–30]:Fe o !  Fe 2 þ þ 2e  (1)Fe 2 þ !  Fe 3 þ þ e  (2)Fe o !  Fe 3 þ þ 3e  (3)2H 2 O ð l Þ  !  O 2 ð g Þ þ 4H þ þ 4e  (4)The metal ions can form wide ranges of coagulated species andmetal hydroxides, or precipitate and adsorb dissolved contami-nants at an appropriate pH value. When CMDW contains highconcentrations of different metals such as Fe, Al, Ca, Mg, Mn, Zn,several competitive reactions occur either at the cathode (metalsdeposition on the cathode electrodes) or in solution (precipitationand co-precipitation of metals with ferrous hydroxides). Anamount of metal(s) is removed by cathodic reduction (metal isformedanddepositedonthecathodeselectrodes)accordingtothefollowing reaction (5) [31]: Me n þ þ n e  !  Me o (5)2H 2 O  þ  2e  !  H 2 þ 2OH  (6)Several metals can be simultaneously or successively reduced oncathode electrodes. According the electrochemical motive forceseries the order of metals deposition should be as follows:Sr > Ca > Mg > Al > Mn > Zn > Cr > Fe > Pb. Furthermore, the M.S. Oncel et al./Journal of Environmental Chemical Engineering 1 (2013) 989–995 990  hydroxide ions formed at the cathode increase the pH of thewastewater thereby inducing the precipitation of metal ions ascorresponding hydroxides and co-precipitation with iron hydro-xides [29]:Me n þ þ n OH  !  Me ð OH Þ n ð s Þ  (7)4Fe 2 þ þ O 2 þ 4H þ !  4Fe 3 þ þ 2H 2 O (8)Fe 3 þ þ 3H 2 O  !  Fe ð OH Þ 3 þ 2H þ (9)M n + and OH  ions generated by electrode reactions (5) and (6)react to form various hydroxo monomeric and polymeric species,depending on pH range, which transform finally into M(OH) n (s) according to complex precipitation kinetics. Freshly formedamorphousM(OH) n (s)  (sweep flocs)have largesurface areaswhichare beneficial for a rapid adsorption of soluble organic compoundsand trapping of colloidal particles. These flocs polymerize furtheras M(OH) n (s)  and are removed easily from aqueous medium bysedimentation and flotation [32].One of the operational issues with the EC process is electrodepassivation. During the EC process using Fe electrodes, an oxidelayer is formed at the anode because the anode surfaces areoxidizedtoacoarse,rusty,brownish-redappearancewithincreaseusage and the electrode passivation causes Fe generation todecrease.Eliminatingtheoxideformationattheanodemayreducethis effect. Various methods were reported for preventing and/orreducingthepassivationproblemoftheelectrodesintheliterature[28,33–35] which were mechanical cleaning, adding chloride saltto the solution and using electrodes with reversible polarity.Mechanical cleaning and adding chloride salt to the solution (i.e.,limits the formation of the passivation layer and to increase theconductivity as to decrease the energy consumption) werepracticed in this study. For this reason, the electrodes are rinsedin the diluted HCl solution after the each experiment. Moreover,Cl  is discharged at the anode to generate Cl 2 , which is dissolvedimmediately in the solution, chemically converted to ClO  . ThenClO  oxidizes the pollutants effectively and increases removalefficiencies of metal ions (Eqs. (10)–(12)) [36] 2Cl  Ð Cl 2 þ 2e (10)Cl 2 þ H 2 O ! HClO þ H þ þ Cl  (11)HClO Ð ClO  þ H þ (12) Chemical precipitation process The chemical precipitation is a widely used for an effectivetreatmentprocessforremovalofdissolvedmetalsfromwastewatersolution containing toxic metals. Effectiveness of the chemicalprecipitation process is dependent on several factors, includingconcentrationofionicmetalspresentinthesolution,theprecipitantused and the presence of other constituents that may inhibit theprecipitation reaction. The chemical precipitation in water treat-mentinvolvesthe additionof chemicaltoalterthe physicalstateof dissolved and suspended solids and to facilitate their removal bysedimentation or filtration. A commonly used chemical as thereagentintheprecipitationprocessissodiumhydroxideorcalciumhydroxidetocreatesolidmetalhydroxides.Thehydroxideionsreactwithmetalstoforminsolublemetalhydroxides(Eqs.(13)and(14))Me 2 þ þ 2OH  !  Me ð OH Þ 2 ð s Þ  (13)Me 3 þ þ 3OH  !  Me ð OH Þ 3 ð s Þ  (14)The insolublemetal hydroxidessuch asMe(OH) 2(s)  and Me(OH) 3(s) are precipitated. The treated water is then decanted andappropriately discharged or reused [19,37].When solution pH becomes acidic, the oxidation of ferrous iron(Fe 2+ ) to ferric iron (Fe 3+ ) diminishes and therefore the metalremoval decreases. Alkaline pH, however, tends to favours Fe 2+ toFe 3+ oxidationaswellascomplexpolymerization.Theformationof the metal hydroxides with increase in pH accompanied bycoprecipitation and/or adsorption of metal hydroxides for themajority of cases gives a mixed precipitate [38]. Materials and methods Characterizations of CMDW  CMDWwas collected from different parts of an abandoned coalmine disposal area in Turkey and stored in high-densitypolyethylene containers. The main contaminants of CMDW andcoal mine drainage (CMD) in literature were presented in Table 1. Chemical precipitation studies CMDW was treated with 1 N NaOH (Merck). The chemicalprecipitationwascarriedoutina1 LPyrexglasscontaining0.5 Lof wastewater which was mixed by a magnetic stirrer (Heidolp MR 3000D).NaOHwasaddedintoCMDWuntiladesiredpHvaluewasreached. After the pH adjustment, the mixture was mixed for20 min at 400 rpm and allowed to settle for 30 min. Each samplewas filtered through 0.45 m m Millipore membrane filter andconcentrations of samples were determined from an inductivelycoupled plasma atomic emission spectrometry (ICP-OES, Perki-nElmer Optima 7000 DV model). The operating conditions for ICP-OES were shown in Table 2. Electrocoagulation studies The EC study was performed in a batch mode using verticallypositioned iron electrodes with dimensions of 50 mm  73 mm  3 mm in a 1 L Plexiglas reactor(120 mm  110 mm  110 mm) at a constant temperature of 20  8 C. The experimental setup was shown in Fig. 1. Two ironplate anodes and two iron plate cathodes ( > 99.5% purity) wereconnected to a digital DC power supply (Agilent 6675A; 120 V and18 A) in a monopolar parallel connectionmode and equipped withgalvanostaticoperationaloption.Totaleffectiveelectrodeareawas  Table 1 Characterizations of CMDW sample and typical CMD.Parameters Units Values for CMDW (this study) Values for CMD a pH – 2.43 2.71–3.85Eh mV 420 372–493Conductivity mS/cm 2.35 1.69–3.96Total Fe mg/L 743.5 1297–1703Fe 2+ mg/L 501.7 654–986Fe 3+ mg/L 241.8 206–875Ca mg/L 259.6 165–242Al mg/L 64.1 60–82Mg mg/L 73.7 53–97Mn mg/L 40.6 30–42Zn mg/L 19.3 9–18Si mg/L 17.3 3–14Na mg/L 34.2 3–10Sr mg/L 9.1 2–5B mg/L 15.3 0.7–1.4Cr mg/L 0.20 0.1–0.5As  m g/L 0.015 90–215Pb  m g/L 27.6 11–36SO 2  4  mg/L 2650.8 2100–3240NO  3  mg/L 32.2 21–86Cl  mg/L 201 175–236 a Values taken from [1]. M.S. Oncel et al./Journal of Environmental Chemical Engineering 1 (2013) 989–995  991  219 cm 2 and the electrodes were spaced by 10 mm. Organicimpuritiesandoxidelayerontheelectrodesurfaceswereremovedby dipping in a solution freshly prepared by mixing HCl solution(35%) and hexamethylenetetramine aqueous solution (2.80%) for2 min [39].0.85 L of CMDW was placed into the batch EC reactor for eachexperimental run. The reactor wasmixed at 400 rpm to reduce themass transport over potential of the EC reactor. Current andvoltage were held constant at desired values for each run and theexperiment was started. The samples taken at pre-determinedintervals from the EC reactor were filtered using a 0.45 m mMillipore membrane filter. At the end of the run, the electrodeswere washed thoroughly with water to remove any solid residueson the surfaces, dried and reweighed.  Analytical method The chemical analysis of CMDW was carried out according tostandard methods [40]. pH and conductivity of solutions beforeand after the EC process were measured by a pH meter (MettlerToledo 2050e) and a conductivity meter (Mettler Toledo 7100e).The total concentrations of metals were determined by ICP-OES.Ferrous ions were determined colorimetrically using 1,10-phe-nanthroline with a UV-spectrophotometer (PerkinElmer, Lambda35) according to procedure in the standard methods (3500-Fe.B).Theexperimentswererepeatedtwice. Theexperimentalerrorwasbelow 2% and the average data were reported. Results and discussion Chemical precipitation of CMDW  The removal of the metal ions from CMDWdepended primarilyon the solubility of the various complexes formed in water. Forexample, heavy metals formed hydroxide solid forms which hadlow solubility limits in water. Thus, the metals were precipitatedout of solutionas a result of the formationof insolublehydroxides,i.e.thevaluesoftheconcentrationofcationsandanionsweresuchthat their product exceeded the solubility product,  K  sp  (Table 3).The solubility of the metal compounds thus formed was pHdependent; most tended to be least soluble in alkaline solutionssince the optimal pH for precipitationdepended both on the metalto be removed and the counter ion used. Solubility of metalhydroxides as a function of pH for several metal hydroxides wasshown in Fig. 2. As seen in this figure, the total residualconcentration of Zn(II), as an example, increased when the pHvalue increased above 9 by adding NaOH because of an increase inthe concentration of the negatively charged hydroxide metalcomplex [41].ChemicalprecipitationexperimentwasconductedatpHvaluesbetween 4 and 10. The optimumpH for maximumFe, Al, Zn, Si, Pb,Cr and As precipitations was 8 and removal efficiencies at theoptimum pH were 99.96% for Fe, 99.66% for Al, 99.59% for Zn,96.99% for Si, 99.96% for Pb, 99.50% for Cr and 99.87% for As,respectively (Table 4). Maximum precipitation of Ca, Sr and Boccurred at pH 10 and the concentrations were reduced from  Table 2 Operating conditions for ICP-OES.ICP-OESRf power (W) 1450Plasma Ar flow (L/min) 15Auxiliary Ar flow (L/min) 0.2Nebuliser Ar flow (L/min) 0.7Delay time (s) 60Integration time Automatic (15s)Measurement mode Axial (Al, Cr, Pb, B, Zn, Sr, As) andradial (Fe, Ca, Mg, Mn)Analytical lines (nm) 267.22 (Cr), 257.61 (Mn), 249.68 (B),220.35 (Zn), 285.21 (Mg), 206.20 (Zn),407.77 (Sr), 193.70 (As) a , 238.20 (Fe),396.15 (Al), 317.93 (Ca) a Concentrations were determined with hydride generation. [ Fig. 1.  Experimental setup used in the EC process.  Table 3 Solubility product constants for metal hydroxides at room temperature [38].Substance Formula  pK  s Aluminium hydroxide Al(OH) 3  33.5Calcium hydroxide Ca(OH) 2  5.3Chromium(III) hydroxide Cr(OH) 3  30.2Iron(II) hydroxide Fe(OH) 2  15.1Iron(III) hydroxide Fe(OH) 3  37.4Manganese(II) hydroxide Mn(OH) 2  12.7Magnesium hydroxide Mg(OH) 2  10.8Lead hydroxide Pb(OH) 2  16.1Zinc hydroxide Zn(OH) 2  15.7Strontium hydroxide Sr(OH) 2  2.64 [ Fig. 2.  Solubilities of metal hydroxides as a function of pH. M.S. Oncel et al./Journal of Environmental Chemical Engineering 1 (2013) 989–995 992
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