Biosorption Process for Treatment of Electroplating Wastewater ContainingCr(VI): Laboratory-Scale Feasibility Test

Donghee Park,† Yeoung-Sang Yun,‡ Ji Hye Jo,† and Jong Moon Park*,†

AdVanced EnVironmental Biotechnology Research Center, Department of Chemical Engineering, School ofEnVironmental Science and Engineering, Pohang UniVersity of Science and Technology, San 31, Hyoja-dong,Pohang 790-784, South Korea, and DiVision of EnVironmental and Chemical Engineering, Research Institute ofIndustrial Technology, Chonbuk National UniVersity, 664-14ga, Duckjin-dong, Chonju 561-756, South Korea

Brown seaweedEcklonia biomass was used for the treatment of electroplating wastewater that containschromium and zinc ions. Batch experiments showed that Cr(VI) was removed from the wastewater throughreduction to Cr(III) by contact with the biomass, whereas Cr(III) and Zn(II) were removed through adsorptionto the binding sites of the biomass. Among various parameters, the solution pH most significantly affectedthe biosorptive capacity of the biomass. As the solution pH increased, the removal efficiency of Cr(VI)decreased, whereas that of Cr(III) and Zn(II) increased, for pH<5. This divergence of efficiency, because ofthe removal mechanisms of chromium and zinc ions, necessitated a two-stage biosorption process for thecomplete removal of both ions from the wastewater. The first stage comprises the removal of Cr(VI) byreduction into Cr(III) and of total chromium by partial adsorption at a low pH (1.5-2.5), and the secondstage the removal of residual total chromium and Zn(II) by adsorption at elevated pH (4-5). A series of twocolumns that contain theEcklonia biomass with a pH adjustment step between column operations wassuccessfully used as a feasibility test of the proposed process. In conclusion, the abundant and inexpensiveEckloniabiomass can be used in the two-stage biosorption process for the treatment of electroplating wastewaterthat contains Cr(VI) and other metal ions, because it shows the promise of being environmentally friendlierthan any existing chemical treatment process.

Introduction

The increase of industrial activities has intensified environ-mental pollution problems and the deterioration of severalecosystems with the accumulation of many pollutants, especiallyheavy metals. Among them, chromium in the aquatic environ-ment has been classified in the United States EnvironmentalProtection Agency (USEPA) Group A of human carcinogens.1

Nevertheless, the industrial use of chromium has increased,because of the extensive use of chromate and dichromate inelectroplating, leather tanning, metal finishing, nuclear powerplants, and textile industries.2 Among its several oxidation states(e.g., divalent, trivalent, pentavalent, and hexavalent), trivalent(Cr3+ and CrOH2+) and hexavalent (HCrO4- amd Cr2O7

2-)species of chromium are mainly found in these industrialeffluents. Particularly, effluents from electroplating and leathertanning facilities contain chromium at concentrations rangingfrom tenths to hundreds of milligrams per liter (mg/L).3 Cr(VI)is known to be very toxic to both plants and animals, becauseit is a strong oxidizing agent and a potential carcinogen,1

whereas Cr(III) is an essential nutrient for plant and animalmetabolism.4 Although Cr(III) is less toxic than Cr(VI) ornontoxic, long-term exposure to a high concentration of Cr(III)may cause poisoning symptoms, such as allergic skin reactions.5

Therefore, the discharge of Cr(VI) to surface water is regulatedto <0.05 mg/L, according to the USEPA, whereas the totalchromium (containing Cr(III), Cr(VI), and other forms ofchromium) is regulated to be discharged at<2 mg/L.6

To achieve these goals in pollution standards, many methodshave been used for the removal of chromium from wastewaters,

including chemical precipitation after reduction, adsorption usingion-exchange resins or activated carbons, and concentrationusing reverse osmosis or electrodialysis.6 However, thesemethods are not completely satisfactory and have followingdisadvantages: (i) generation of a large amount of secondarywaste products, because of various reagents used in a series oftreatments such as reduction of Cr(VI), neutralization of acidicsolution and precipitation; (ii) joint use of cationic and anionicresins to remove both cationic Cr(III) and anionic Cr(VI); (iii)the instability of resins or membranes, because of seriousoxidation by Cr(VI); and (iv) high costs of resins, activatedcarbons, and membranes.

One of the promising techniques to treat chromium-containingwastewaters is through the use of biomaterials as a low-costadsorbent for chromium. Over the past few decades, manyresearchers have examined various biomaterials, such as deadbiomass (of microalgae,7 seaweed,8-12 fungi,13 and bacteria),agricultural waste biomass,14 industrial waste biomass,15 andbiomaterial-based activated carbons,16,17 for the removal ofchromium from aqueous solutions or wastewaters in a batch orcolumn reactor system. Among them, some biomaterials10 andbiomaterial-based activated carbons17 have shown good per-formance as adsorbents for chromium removal. Under thepresent conditions, however, the practical application of abiosorption technique to an actual treatment process of chromium-containing wastewaters is not easy for the following reasons:(i) most researchers have made a principle mistake in analyzingchromium species in aqueous and solid phases, resulting inincorrect elucidation of Cr(VI) biosorption (i.e., they haveclaimed that Cr(VI) is removed from aqueous solution through“anionic adsorption”);18 (ii) they usually used synthetic Cr(III)or Cr(VI) solutions instead of actual wastewaters that containvarious oxidation states of chromium and other metal ions, andthus have not yet developed, to the best of our knowledge, a

* To whom all correspondence should be addressed. Tel.:+82-54-279-2275. Fax:+82-54-279-2699. E-mail: jmpark@postech.ac.kr.

† Pohang University of Science and Technology.‡ Chonbuk National University.

5059Ind. Eng. Chem. Res.2006,45, 5059-5065

10.1021/ie060002d CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 06/06/2006

biosorption process feasible for removing all metal ions; and(iii) because most of the biosorption experiments have beenexecuted in batch systems, there is a scarcity of informationneeded for scale-up of the biosorption process.

It was recently found that the protonatedEckloniabiomass,which is an abundant and inexpensive brown seaweed, canefficiently remove both Cr(III) and Cr(VI) from aqueoussolution.8-12 Cationic Cr(III) can bind to various functionalgroups of the seaweed biomass through an ion-exchangemechanism. The main functional group of theEckloniabiomassis known to be a carboxyl group, with a pKH value of 4.6.8

Thus, the removal rate and equilibrium uptake of Cr(III) increaseas the solution pH increases, which is the general pattern incationic metal biosorption.19-23 In addition, anionic Cr(VI) iseasily reduced to Cr(III) by being brought into contact with theEckloniabiomass, because of its high reduction potentials (above+1.3 V), and the converted Cr(III) appears in the aqueous phaseor is partly bound to the biomass.9-12 Because protons areconsumed during Cr(VI) reduction, the Cr(VI) removal ratedecreases as the solution pH increases. However, Cr(VI) canbe completely removed from aqueous solution, even at pH 5,if sufficient contact time is given. One gram of theEckloniabiomass can adsorb 34.1 mg of Cr(III) in equilibrium at pH 4,8

and reduce 233 mg of Cr(VI) to Cr(III) at pH 2.10

Generally, actual electroplating wastewaters contain manyions rather than just a single metal ion, and this variety mayaffect the biosorptive capacity of the biomass.12 Therefore, it isvery important to examine the removal performance of targetmetals in actual wastewaters and the effects of various opera-tional parameters on biosorption process. Table 1 shows theeffects of various parameters on Cr(III) or Cr(VI) biosorption(Ed - there is no respective comparison here), of which solutionpH may be the most important parameter to be considered indeveloping a biosorption process for the treatment of electro-plating wastewaters.

In this study, we conducted a feasibility test for the applicationof the Ecklonia biomass to the treatment of electroplatingwastewater that contains chromium and zinc ions. Initially, theeffects of solution pH on the removal of Cr(VI), total chromium,and Zn(II) were investigated in a batch system. A two-stagebiosorption process, for the sequential removal of Cr(VI), totalchromium, and Zn(II), then was proposed and tested in a columnsystem.

Materials and Methods

Preparation of the Biomass.The brown seaweedEckloniasp. was collected along the seashore of Pohang, Korea. Afterswelling and rinsing with deionized-distilled water, the sun-dried biomaterial was cut into∼0.5-cm-sized pieces. The cutbiomaterial was treated with a 1 M H2SO4 solution for 24 h,which replaced the natural mix of ionic species with protonsand sulfates. The acid-treated biomaterial was washed withdeionized-distilled water several times and thereafter dried at80 °C in an oven for 24 h. The resulting dried biomass waslater stored in a desiccator and used for the batch and columnexperiments.

Wastewater.The wastewater used in this study was collectedfrom an electroplating facility in Gwangyang, Korea. Thousandsof tons of electroplating wastewater are produced weekly fromthe rinsing of steel surfaces after electroplating. Table 2 showsthe chemical and physical characteristics of the electroplatingwastewater. The major pollutants in the wastewater werechromium and zinc ions, because the facility produces chro-mium- and zinc-plated steels, alternately. In addition to thesetwo metal ions, various ions existed at concentrations of<1mg/L. Notably, most of the chromium existed in the hexavalentstate, which is the most toxic form of chromium. To thecontrary, zinc is a well-known, essential micronutrient for livingorganisms and is, therefore, regulated to be discharged to surfacewater at a concentration of<5 mg/L. Therefore, the wastewatermay be reused as low-quality industrial water, if the chromiumand zinc ion concentrations are reduced to<1 mg/L. Thewastewater was stored in a refrigerator at a temperature of<5°C, until it was used in batch and column experiments.

Batch Biosorption Experiment. The dynamic sorption ofchromium and zinc ions by theEckloniabiomass was investi-gated with the electroplating wastewater in batch experiments.Each trial was performed by contacting 1 g of thebiomass with200 mL of the wastewater in a 500-mL Erlenmeyer flask. Beforeand after the biomass addition, the pH of each test solution wasadjusted and maintained to the required value (pH 2.0, 2.5, 3.0,3.5, and 4.0) with a 1 M H2SO4 solution. The flasks wereagitated on a shaker at 200 rpm and 25°C. Samples wereintermittently removed from the flasks to analyze the Cr(VI),total chromium, and Zn(II) concentrations. The total volumeof withdrawn samples never exceeded 5% of the workingvolume. Time course experiments for chromium removal wereconducted until the Cr(VI) was completely removed from theaqueous phase. Depending on the solution pH, the contact time

Table 1. Various Parameters Affecting Cr(III) and Cr(VI) Biosorption

Cr(III) Biosorption Cr(VI) Biosorption

parameter (increasing) removal efficiency of Cr(III) removal rate of Cr(VI)a removal efficiency of total chromium

contact timev increase increase increasebiomass dosagev increase increase increaseinitial Cr(III) v decrease independent (<500 mg/L) not examinedinitial Cr(VI) v not examined increase decreasesolution pHv increase (pH<5) decrease optimum at pH 3-4temperaturev optimum at 25-35 °C increase not examinedother heavy metalsv decrease independent (<500 mg/L) decreaseionic strengthv decrease independent (<0.1 M) decrease

a Because Cr(VI) is completely reduced to Cr(III) with sufficient time, the removal rate of Cr(VI) is presented, instead of its removal efficiency.

Table 2. Characteristics of Chromium-Plating Wastewater

Species Content (mg/L)

Cr3+ Cr6+ Fe2+ Fe3+ Zn2+ Ni2+ Cl- NO3- SO4

3- PO43- pH conductivity (µS/cm)

1.5 35.4 NDa NDa 4.1 0.13 0.67 NDa 0.95 0.86 4.45 35.2

a Not detected.

5060 Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006

required for the complete removal of Cr(VI) ranged from hoursto days, whereas time course experiments for zinc removal wereconducted for 12 h, which was sufficient for the equilibriumstate to be reached. All batch experiments were conducted induplicate and were reproducible to within, at most, a 5% error.

Two-Stage Biosorption Process Experiment.Two columnswere used for the two-stage biosorption process. Each columnwas a 20-cm long and 4-cm internal diameter column of acrylyl,corresponding to∼250 mL of reactor volume. Each columnwas densely packed with 20 g of theEcklonia biomass. Theprocess was operated in an up-flow mode under temperaturecontrol at 25°C. To stabilize the process before each run,deionized-distilled water adjusted to pH 2.0 or 4.5 was passedthrough the first or second column, respectively, for 24 h at aflow rate of 2.0 mL/min. Then the electroplating wastewaterwas adjusted to pH 2.0 and passed through the first column ata flow rate of 2.0 mL/min. Effluent of the first column wasreadjusted to pH 4.5 using a 5 MNaOH solution and then passedthrough the second column. To measure pH, Cr(VI), totalchromium, and Zn(II) concentrations in the effluents of eachcolumn, 10-mL samples of the effluents were intermittentlycollected and analyzed immediately, according to the proceduredescribed below. The percolation of the wastewater into eachcolumn was stopped as soon as the Cr(VI) or total chromiumconcentration in the effluent exceeded the permissible limit of0.05 mg/L or 2.0 mg/L, respectively. The flow rate wasaccurately maintained using a pump (Model P-07523-37,MasterFlex) with a speed control of(0.25% and tubing (ModelP-06485-13, MasterFlex) with an inside diameter of 0.8 mm.A damper was also used to minimize the liquid fluctuation thatwas due to mechanical movement of the pump head.

Analysis of Metal Ions.A colorimetric method, as describedin the standard methods, was used to measure the concentrationsof the different species of chromium.24 The pink-coloredcomplex, formed from 1,5-diphenylcarbazide and Cr(VI) inacidic solution, was spectrophotometrically analyzed at 540 nm(model GENESYS 5, Spectronic Instruments). To estimate thetotal chromium concentration, Cr(III) was first converted to Cr-(VI), at high temperature (130-140 °C), by the addition ofexcess potassium permanganate prior to the 1,5-diphenylcar-bazide reaction. The Cr(III) concentration was then calculatedfrom the difference between the total chromium and Cr(VI)concentrations. The concentration of Zn(II) was determinedusing atomic absorption spectrophotometry (AAS) (modelSpectrAA.800, Varian) at a wavelength of 213.9 nm. Allchemicals used in this study were of analytical grade.

Results and Discussion

Removal Mechanisms of Chromium and Zinc Ions fromthe Electroplating Wastewater.Time variations of chromiumremoval from the electroplating wastewater by theEckloniabiomass are illustrated in Figure 1a, b, and c for Cr(VI), Cr-(III), and total chromium, respectively. At all pH levels, theCr(VI) concentration sharply decreased and was completelyremoved from the aqueous phase, and the removal rate increasedunder more-acidic conditions. Meanwhile, the Cr(III) concentra-tion, which was initially 1.5 mg/L, increased in the aqueousphase at solution pH of<3.0. After the complete removal ofCr(VI), the final concentration of total chromium, i.e., Cr(III),remained constant in the range of 1 mg/L. A previous studyhas proved that the chromium bound on the biomass was in atrivalent state.10 Thus, it can be concluded that Cr(VI) wascompletely reduced to Cr(III) by contact with theEckloniabiomass, with sufficient contact time. It is interesting to note

that other researchers conducted batch equilibrium experimentsfor <12 h and analyzed only Cr(VI) or total chromium in theaqueous phase.18

Since protons are consumed during Cr(VI) removal by thebiomass,10 the removal rate of Cr(VI) increased as the solutionpH decreased, and the contact time required for the complete

Figure 1. Dynamics of chromium removal from the electroplatingwastewater by theEckloniabiomass at various pHs, in terms of (a) Cr(VI),(b) Cr(III), and (c) total chromium.

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removal of Cr(VI) increased from hours to days as the solutionpH increased. Cr(VI) was completely removed in 6 h at pH2.0, whereas 124 h were required at pH 4.0 (Figure 1a). Thedifference between initial and final Cr(III) concentrationsincreased as the solution pH decreased (Figure 1b), because ofthe interference of protons in the binding of Cr(III) to thebiomass at low pH.8

As expected, the removal efficiency of total chromiumincreased as the contact time increased at all pH levels (Figure1c). However, it seems to be the existence of optimum pH fortotal chromium removal at a given contact time. Figure 2 showsthe removal efficiencies of total chromium at various pH levelswith different contact times, which were calculated from themass balance for total chromium in the aqueous phase. Anoptimum pH existed for the removal efficiency of total chro-mium at any given contact time. This optimum pH increasedas the contact time increased, eventually reaching∼3.5 at 124h of contact time, within which time Cr(VI) was completelyremoved from the aqueous phase in all the experiments. Theoptimum pH increased because, at a high pH, Cr(VI) is veryslowly reduced to Cr(III), and Cr(III) is easily bound to thebiomass.10 Thus, as the pH increases, the reduction rate of Cr-(VI) is rate-limited; i.e., as soon as Cr(III) is formed from Cr-(VI) reduction, it is bound to the biomass. However, becausethe Cr(VI) reduction is irreversible, although slowly at a highpH, as the contact time increases, Cr(VI) can be completelyreduced to Cr(III), which is then removed by adsorption. Theoptimum pH of∼3.5 at 124 h probably resulted from the releaseor destruction of Cr(III)-binding sites during the oxidation ofthe biomass by Cr(VI). The optimum pH for total chromiumremoval was pH∼3.0 in the case of a high concentration ofCr(VI) of 500 mg/L (data not shown). The release of organiccompounds from the seaweed biomass is known to increase atelevated pH.23

Meanwhile, Figure 3 shows the time variations over the pHrange of zinc removal from the electroplating wastewater bythe Ecklonia biomass. In contrast to Cr(VI) biosorption, theequilibrium state of Zn(II) biosorption was achieved in a contacttime of 6 h. Zn(II) could not bind to the biomass at pH 2, andthe removal rate and efficiency increased with increasing solu-tion pH. Interestingly, Zn(II) could be removed to a residualconcentration of<0.1 mg/L at solution pH of>3.5. It has beenproved that seaweed biomass contains a sufficient amount of

carboxyl groups to be capable of binding metal ions throughcation-exchange.19-23 In the case of theEckloniabiomass, thefunctional group related to the interaction between protons andmetal ions is a carboxyl group with a pKH of 4.6 ( 0.1.8

Development of Novel Biosorption Process.As seen inTable 1, various biosorption parameters, such as contact time,pH, temperature, ionic strength, biomass, and metal ion con-centrations, influence the biosorptive capacity of the used bio-mass. Therefore, it is very important to develop a proper bio-sorption process for the actual treatment of industrial electro-plating wastewater, with consideration for the biosorption mech-anisms of the target metals to be removed. As mentioned pre-viously, the anionic Cr(VI) ion is removed through the reductionto Cr(III) by contact with theEckloniabiomass, whereas cationicCr(III) and Zn(II) ions are removed through adsorption to thebinding sites of the biomass (i.e., carboxyl group). Note thatthe existence of other metals such as Zn(II) competitivelyinhibits the binding of Cr(III) to the biomass.12 Among thevarious parameters, the solution pH is the most importantparameter affecting the removal efficiencies of Cr(VI), totalchromium, and Zn(II) from the wastewater. As the solution pHincreases, the removal efficiency of Cr(VI) decreases, whereasthat of total chromium and Zn(II) increases, for pH<5.

This necessitates the proposal of a two-stage biosorptionprocess for the complete removal of chromium and other metalions from electroplating wastewaters that contain Cr(VI). In firststage, toxic Cr(VI) can be completely reduced to less toxic ornontoxic Cr(III) through a redox reaction with theEckloniabiomass at lower pH (pH 1.5-2.5), and a portion of the totalchromium can be removed through adsorption to the bindingsites of the biomass. In second stage, the residual total chromiumand other metal ions can be then completely removed throughadsorption to the biomass at higher pH (pH 4-5). Finally, thetreated water is virtually free of metal ions.

To apply this novel process, a packed-bed column may bethe most effective apparatus for continuous operation, verysimilar to that used for ion exchange, because it has manyprocess engineering advantages, including high yield operationsand relatively easy scale-up from a laboratory-scale procedure.The stages in the separation protocol can also be automated,with high degrees of purification often achieved in a single-step process. A large volume of wastewater can be continuouslytreated using a defined quantity of biomass in the column. After

Figure 2. Removal efficiencies of total chromium at various pH withdifferent contact times. In all the experiments, Cr(VI) was completelyremoved from the wastewater within 124 h.

Figure 3. Dynamics of Zn(II) removal from the electroplating wastewaterby theEckloniabiomass at various pH.

5062 Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006

loading, the metal may be concentrated in a small volume ofsolid material or desorbed into a small volume of eluent forrecovery, disposal, or containment. Because of these advantages,many researchers have used a column packed reactor withvarious biomasses capable of removing heavy metals.25,26Figure4 shows a diagram of the “two-stage biosorption process” thathas been proposed in this study. The process is a series of twocolumns that contain biomass with a pH adjustment step betweencolumn operations. To increase the economic value of thecommercial biosorption process, waste acid or alkali solutions,which are easily supplied from facilities near the electroplatingfacility, may be used to adjust the wastewater pH to desiredvalues in this process.

Feasibility Test of the Two-Stage Biosorption Process.Thefeasibility of the two-stage biosorption process for treating theelectroplating wastewater was examined in a packed-bed reactorsystem consisting of two columns. After adjusting to pH 2.0,the wastewater was passed through the first column at a flowrate of 2.0 mL/min. The effluent was then readjusted to pH 4.5and passed through the second column. Generally, the perfor-mance of the column reactor is described through the conceptof the breakthrough curve. The time for breakthrough appear-ance and the shape of the breakthrough curve are very importantcharacteristics for determining the operation and dynamicresponse of the column. Figure 5 shows breakthrough curvesof the first and second columns, in terms of effluent pH andCr(VI), total chromium, and Zn(II) concentrations. By passingthrough the first column (Figure 5a), 35.4 mg/L of Cr(VI) wascompletely removed until 170 h, thereafter was slightly released,and finally exceeded the limit of its discharge level, i.e., 0.05mg/L. Although the effluent concentration of total chromiumexceeded the discharge limit of 2.0 mg/L from the beginning,and reached∼73% of the influent value at 224 h, almost noZn(II) was removed in the first column, probably because ofthe low affinity of cationic zinc ions for the biomass at the lowpH values studied. This is quite typical for biosorption behaviorthat is based on carboxylic sites.19-23 Because the Cr(VI)reduction into Cr(III) occurs with proton consumption, the pHof the effluent was higher than the influent pH, which was 2.0.

Meanwhile, the breakthrough curves of the second column(Figure 5b) indicate that the residual total chromium and Zn-(II) were removed in the second column. The effluent concen-trations of total chromium and Zn(II) were<1 mg/L until 120and 200 h, respectively. The effluent pH of the second columnwas 3.3-3.4 as protons were released during adsorption of totalchromium and Zn(II) ions to the binding sites of the biomass(i.e., carboxyl group). The amounts of chromium and zinc loadedon the biomass packed in each column were calculated fromthe mass balance for each metal in the effluents. The uptake of

chromium by the first column was estimated at 24.0 mg/g,whereas that of chromium and zinc by the second column wasestimated at 22.6 mg/g and 5.0 mg/g, respectively.

There are some reports similar to our study. Ajmal et al.27

reported that phosphate-treated sawdust could completelyremove Cr(VI) from electroplating wastewater (50 mg/L Cr-(VI), 56 mg/L Zn(II), 20 mg/L Ni(II), 20 mg/L Cu(II), pH 5.6)in batch experiments as well as column experiments. However,they analyzed only total chromium in the aqueous phase, usingAAS, and thereby obtained incorrect data for Cr(VI) removal.Pandey et al.28 used calcium alginate beads containing humicacid to reduce the toxicity of tannery effluent (2 mg/L Cr(VI),12 mg/L Cr(III)). However, they also interpreted the columndata in terms of the total chromium concentration, and theremoval efficiency of total chromium was∼80%. Selvakumariet al.16 used carbonized maize cob as an adsorbent for chromium,nickel, and iron that were contained in the electroplatingwastewater (no information about metal concentrations wasgiven) in both batch and column systems. However, they alsofailed to determine the chromium removal rate, because theyanalyzed only total chromium. Zhao and Duncan29 used driedaquatic fern,Azolla filiculoides, in column operations for theremoval of Cr(VI) from synthetic solution and electroplatingwastewater (18 mg/L Cr(VI)) at acidic pHs. However, they onlyinvestigated the removals of Cr(VI) and total chromium, eventhough the wastewater is very likely to contain other metal ions

Figure 4. Diagram of the “two-stage biosorption process”. Legend: 1,chromium wastewater storage tank; 2, acid storage tank; 3, pH control tankfor adjusting to pH 1.5-2.5; 4, first column packed with biomass; 5, alkalistorage tank; 6, pH control tank for adjusting to 4-5; 7, second columnpacked with biomass; and 8, pump.

Figure 5. Breakthrough curves of the (a) first and (b) second columns, interms of effluent pH, and Cr(VI), total chromium, and Zn(II) concentrations.

Ind. Eng. Chem. Res., Vol. 45, No. 14, 20065063

(no information about other metal ions was given). The mostcomprehensive study, which is also similar to our own, wasreported by Muthukumarn et al.17 They used chemicallyactivated coconut shell carbons to treat electroplating wastewater(300 mg/L Cr(VI), 3 mg/L Cr(III), 5 mg/L total iron, pH 3.5).To remove chromium completely, they used a series of twocolumns that contained different carbon types with a pHadjustment step between column operations. One was the high-temperature activated carbon for adsorbing Cr(VI), and the otherwas low-temperature activated carbon for adsorbing Cr(III).However, they only investigated the chromium removal fromthe wastewater, and they failed to elucidate the removalmechanism of Cr(VI) by the biomaterial-based activated carbonsstudied.

Meanwhile, various chemical and physical pretreatments canbe used to enhance the biosorptive capacity of biomass. Thermaltreatment (at<100 °C) decreased the content of the carboxylgroup present in theEcklonia biomass, thus reducing itsadsorption capacity, while strengthening the biomass as a Cr-(VI) reductant, and thereby enhancing the Cr(VI) removalefficiency.9 Acid treatment improved the removal rate of Cr-(VI) from the aqueous phase, and organic-solvent treatmentimproved the removal efficiency of total chromium in equilib-rium.11 In addition, amination of the carboxyl group present inthe biomass significantly enhanced its removal rate of Cr(VI).11

Therefore, these treatments can be used to prepare an efficientbiomass to be used in the two-stage biosorption process. Thethermally treated, acid-treated, and/or aminated biomass isappropriate for use in the first column for removing Cr(VI), asis organic-solvent-treated and/or carboxylated biomass in thesecond column for the removal of residual total chromium andother metal ions.

For the practical application of the biosorption process, amathematical model that is capable of predicting its performancemust be developed before its scale-up. In fact, many mathemati-cal models have been used to study biosorption column systems.These models have mainly originated from research on activatedcarbon sorption, ion exchange, or chromatographic applications,and some of them accurately predicted the effluent concentrationof target metals, even in multimetal biosorption systems.25,26

Thus, it is possible to develop a theoretical model that is capableof predicting effluent dynamics of the second stage in ourproposed biosorption process. However, none of the mathemati-cal models reported to date could describe the dynamic behaviorof effluent Cr(VI) in the first stage, because the removalmechanism of Cr(VI) is different from that of cationic heavymetals, such as lead and zinc. To the best of our knowledge,no theoretical model has been developed for Cr(VI) removal incolumn reactor systems. Therefore, a future research directionprogressing from this study will be to develop a theoreticalmodel capable of predicting effluent dynamics of the first statein our proposed biosorption process.

Chromium Recovery and Regeneration of Columns.Asuitable method for the disposal of the metal-laden biomass isjust as important as its accumulation from wastewaters. Theremoval of the metal from the biomass is required for saferbiomass disposal, unless the metal-laden biomass is to be utilizedas an alternative product. For desorption/recovery of chromiumand zinc ions bound on the biomass, 1 L of 1 M H2SO4 solutionwas circulated through each column for 48 h. The desorbedchromium was in the trivalent form, and the desorptionefficiency of chromium only reached to 67% and 76% in thefirst and second columns, respectively. These results imply thatchromium could not be completely desorbed or recovered from

the biomass by flushing with acids. However, zinc was desorbedonly from the second column to the extent of 98%, becausezinc was not bound to the biomass in the first column. Generally,the rate and efficiency of chromium desorption by acids wereeach lower than those for divalent metal ions such as lead andzinc.30 The poor rate and efficiency of chromium desorptionby acids might be due to the physical or chemical properties ofchromium binding; however, little information on this issue ispresently available. Thus, there is a need for detailed study onchromium desorption from biomass.

Meanwhile, the column regenerated by acid flushing wassubsequently operated with the wastewater (data not shown).However, the first column showed poor removal efficiency ofCr(VI), because of the remarkable oxidation of the biomass byCr(VI), and the second column also showed poor removalefficiency of total chromium, because of the remaining Cr(VI).This result indicates that the regeneration of columns using acidflushing may be insufficient for practical reuse.

As an alternative for treating the chromium-laden seaweedbiomass, Aravindhan and co-workers31,32 tested its applicationas a reductant in the preparation of a basic chromium sulfate(BCS) tanning agent. The seaweed contains organic compoundsand is rich in carbohydrates and proteins. Therefore, it shouldact as an excellent reductant. Furthermore, the chromium presentin the seaweed biomass does not require separation, becausethe end-product is in the trivalent form, and the produced BCSwas successfully used as a tanning agent. Meanwhile, thechromium-laden biomass may be burned, as the incineration ofthe seaweed biomass is energetically and economically attrac-tive. As a result, the polluted wastes may be obtained in a highlyconcentrated form, thereby greatly reducing the costs of finalstorage.

Conclusions

In this study, the brown seaweedEckloniabiomass was usedfor the treatment of electroplating wastewater that containschromium and zinc ions. Batch experiments showed that thesolution pH strongly influenced the biosorptive capacity of thebiomass. As the solution pH increased, the removal efficiencyof Cr(VI) decreased, whereas that of total chromium and Zn-(II) increased, for pH<5. Based on the removal mechanismsof chromium and zinc ions, which produced a diverging effectof solution pH on the removal efficiency, a “two-stage bio-sorption process” was proposed for the treatment of electroplat-ing wastewaters that contain Cr(VI) and other metal ions: afirst stage for the removal of Cr(VI) by reduction into Cr(III)and of total chromium by partial adsorption to the binding sitesof the biomass at low pH, and a second stage for the removalof residual total chromium and other metal ions by adsorptionat elevated pH. A series of two columns containing theEckloniabiomass with a pH adjustment step between column operationswas successfully used as a feasibility test of the proposedprocess. Although it was impossible to regenerate the biosorptivecapacity of the biomass completely by acid flushing, severalalternatives for treating the chromium-laden biomass weresuggested with reliable references. Above all, application of theabundant and inexpensiveEcklonia biomass in the proposedprocess to actual electroplating wastewaters promises to beenvironmentally friendlier than any existing chemical treatmentprocess.

Acknowledgment

This work was financially supported by KOSEF throughAEBRC at POSTECH and, in part, by Grant No. R08-2003-000-10987-0 from the Basic Research Program of the KOSEF.

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Literature Cited

(1) Costa, M. Potential hazards of hexavalent chromate in our drinkingwater.Toxicol. Appl. Pharmacol.2003, 188, 1-5.

(2) Barnhart, J. Occurrences, uses, and properties of chromium.Regul.Toxicol. Pharmacol.1997, 26, S3-S7.

(3) Xi, D.-L.; Sun, Y.-S.; Liu, X.-Y.EnVironment Monitor; AdvancedEducation Press: Beijing, 1996; pp 2-3.

(4) Anderson, R. A. Chromium as an essential nutrient for humans.Regul. Toxicol. Pharmacol.1997, 26, S35-S41.

(5) Rudolf, E.; Cervinka, M. The role of biomembranes in chromium-(III)-induced toxicity in vitro. ATLA, Altern. Lab. Anim.2005, 33, 249-259.

(6) Baral, A.; Engelken, R. D. Chromium-based regulations and greeningin metal finishing industries in the USA.EnViron. Sci. Policy2005, 5, 121-133.

(7) Gupta, V. K.; Shrivastava, A. K.; Jain, N. Biosorption of chromium-(VI) from aqueous solutions by green algaeSpirogyraspecies.Water Res.2001, 35, 4075-4085.

(8) Yun, Y.-S.; Park, D.; Park, J. M.; Volesky, B. Biosorption of trivalentchromium on the brown seaweed biomass.EnViron. Sci. Technol.2001,35, 4353-4358.

(9) Park, D.; Yun, Y.-S.; Cho, H. Y.; Park, J. M. Chromium biosorptionby thermally treated biomass of the brown seaweed,Eckloniasp.Ind. Eng.Chem. Res.2004, 43, 8226-8232.

(10) Park, D.; Yun, Y.-S.; Park, J. M. Reduction of hexavalent chromiumwith the brown seaweedEckloniabiomass.EnViron. Sci. Technol.2004,38, 4860-4864.

(11) Park, D.; Yun, Y.-S.; Park, J. M. Studies on hexavalent chromiumbiosorption by chemically treated biomass ofEcklonia sp. Chemosphere2005, 60, 1356-1364.

(12) Park, D.; Yun, Y.-S.; Jo, J. H.; Park, J. M. Effects of ionic strength,background electrolytes, heavy metals and redox-active species on thereduction of hexavalent chromium byEcklonia biomass.J. Microbiol.Biotechnol.2005, 15, 780-786.

(13) Park, D.; Yun, Y.-S.; Jo, J. H.; Park, J. M. Mechanism of hexavalentchromium removal by dead fungal biomass ofAspergillus niger. WaterRes.2005, 39, 533-540.

(14) Chun, L.; Hongzhang, C.; Zuohu, L. Adsorptive removal of Cr-(VI) by Fe-modified steam exploded wheat straw.Process Biochem.2004,39, 1-5.

(15) Selvaraj, K.; Manonmani, S.; Pattabhi, S. Removal of hexavalentchromium using distillery sludge.Bioresour. Technol.2003, 89, 207-211.

(16) Selvakumari, G.; Murugesan, M.; Pattabi, S.; Sathishkumar, M.Treatment of electroplating industry effluent using maize cob carbon.Bull.EnViron. Contam. Toxicol.2002, 69, 195-202.

(17) Muthukumaran, K.; Balasubramanian, N.; Ramakrishna, T. V.Removal and recovery of chromium from plating waste using chemicallyactivated carbon.Met. Finish.1995, 93, 46-53.

(18) Park, D.; Yun, Y.-S.; Park, J. M. Comment on the removalmechanism of hexavalent chromium by biomaterials or biomaterial-basedactivated carbons.Ind. Eng. Chem. Res.2006, 45, 2405-2407.

(19) Davis, T. A.; Volesky, B.; Mucci, A. A review of the biochemistryof heavy metal biosorption by brown algae.Water Res.2003, 37, 4311-4330.

(20) Volesky, B.; Holan, Z. Biosorption of heavy metals.Biotechnol.Prog. 1995, 11, 235-250.

(21) Yun, Y.-S. Characterization of functional groups of protonatedSargassum polycystumbiomass capable of binding protons and metal ions.J. Microbiol. Biotechnol.2004, 14, 29-34.

(22) Yun, Y.-S.; Volesky, B. Modeling of lithium interference incadmium biosorption.EnViron. Sci. Technol.2003, 37, 3601-3608.

(23) Fourest, E.; Volesky, B. Alginate properties and heavy metalbiosorption by marine algae.Appl. Biochem. Biotechnol.1997, 67, 215-226.

(24) Clesceri, L. S.; Greenberg, A. E.; Eaton, A. D.Standard Methodsfor the Examination of Water and Wastewater, 20th Edition; AmericanPublic Health Association, American Water Work Association and WaterEnvironment Federation: Washington, DC, 1998; pp 366-368.

(25) Kratochvil, D.; Volesky, B. Multicomponent biosorption in fixedbeds.Water Res.2000, 34, 3186-3196.

(26) Figueira, M. M.; Volesky, B.; Azarian, K.; Ciminelli, V. S. T.Biosorption column performance with a metal mixture.EnViron. Sci.Technol.2000, 34, 4320-4326.

(27) Ajmal, M.; Rao, R. A. K.; Siddiqui, B. A. Studies on removal andrecovery of Cr(VI) from electroplating wastes.Water Res.1996, 30, 1478-1482.

(28) Pandey, A. K.; Pandey, S. D.; Misra V.; Srimal, A. K. Removal ofchromium and reduction of toxicity to microtox system from tannery effluentby the use of calcium alginate beads containing humic acid.Chemosphere2003, 51, 329-333.

(29) Zhao, M.; Duncan, J. R. Column sorption and desorption ofhexavalent chromium from aqueous solution and electroplating effluentusing Azolla filiculoides. Resour. EnViron. Biotechnol. 1997, 2, 51-64.

(30) Kratochvil, D.; Pimentel, P.; Volesky, B. Removal of trivalent andhexavalent chromium by seaweed biosorbent.EnViron. Sci. Technol.1998,32, 2693-2698.

(31) Aravindhan, R.; Madhan, B.; Rao, J. R.; Nair, B. U.; Ramasami,T. Bioaccumulation of chromium from tannery wastewater: an approachfor chrome recovery and reuse.EnViron. Sci. Technol.2004, 38, 300-306.

(32) Aravindhan, R.; Madhan, B.; Rao, J. R.; Nair, B. U. Recovery andreuse of chromium from tannery wastewaters usingTurbinaria ornateseaweed.J. Chem. Technol. Biotechnol.2004, 79, 1251-1258.

ReceiVed for reView January 2, 2006ReVised manuscript receiVed May 2, 2006

AcceptedMay 8, 2006

IE060002D

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