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Dithiooxamide-Immobilized Microalgal Residue for theSelective Recovery of Pd(II) and Pt(IV)Kanjana Khunathai a , Katsutoshi Inoue a , Keisuke Ohto a , Hidetaka Kawakita a , MinoruKurata b , Kinya Atsumi b & Shafiq Alam ca Department of Chemistry and Applied Chemistry , Saga University , Saga , Japanb Research Laboratories, Denso Corporation , Komenoki, Nisshin , Aichi , Japanc Faculty of Engineering and Applied Science , Memorial University , St. John's , NL , CanadaAccepted author version posted online: 06 Mar 2012.Published online: 16 May 2012.
To cite this article: Kanjana Khunathai , Katsutoshi Inoue , Keisuke Ohto , Hidetaka Kawakita , Minoru Kurata , Kinya Atsumi& Shafiq Alam (2012) Dithiooxamide-Immobilized Microalgal Residue for the Selective Recovery of Pd(II) and Pt(IV), SeparationScience and Technology, 47:8, 1185-1193, DOI: 10.1080/01496395.2011.645384
To link to this article: http://dx.doi.org/10.1080/01496395.2011.645384
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Dithiooxamide-Immobilized Microalgal Residue for theSelective Recovery of Pd(II) and Pt(IV)
Kanjana Khunathai,1 Katsutoshi Inoue,1 Keisuke Ohto,1 Hidetaka Kawakita,1
Minoru Kurata,2 Kinya Atsumi,2 and Shafiq Alam3
1Department of Chemistry and Applied Chemistry, Saga University, Saga, Japan2Research Laboratories, Denso Corporation, Komenoki, Nisshin, Aichi, Japan3Faculty of Engineering and Applied Science, Memorial University, St. John’s, NL, Canada
Microalgal residue was chemically modified by immobilizing afunctional group of dithiooxamide to prepare a novel type of adsorb-ent. This adsorbent exhibited high adsorption affinity and selectivityfor Pd(II) and Pt(IV) whereas the adsorption of coexisting basemetal ions was negligible. From the adsorption isotherms, thisadsorbent was found to exhibit remarkably high adsorption capacity.The thermodynamic parameters indicated that the adsorption is gov-erned by an endothermic reaction. The effective separation of Pd(II)and Pt(IV) from Cu(II) was confirmed also by a dynamic adsorptiontest. The effectiveness of elution of adsorbed Pd(II) and Pt(IV) was85% and 96%, respectively.
Keywords adsorption; functional group immobilization; oil-richmicroalgae; precious metal recovery
INTRODUCTION
Nowadays, precious metals including gold, silver, andplatinum group metals are of the most important materialsin a variety of industries such as advanced electric andelectronic devices, catalysts, medical materials, etc. Conse-quently, wastes of household electric and electronic appli-ances are attracting big attention because they containsuch high value rare metals like gold, palladium, platinum,etc., in their components. Although, the portion of preciousmetals in one set of a device is very low compared to basemetals like copper, the amount of precious metals disposedin this form is much higher than that in copper and nickelores (1). The recovery of valuable metals from the wastesof electric and electronic appliances is crucially beneficialfrom both viewpoints of environmental and economicaspects since the recycling of these metals reduces the costof metallurgical steps associated with their mining activity,and sharply decreases the destruction to the environment
caused by excessive mining activity. In the recovery processof precious metals, feed materials like waste of printedcircuit boards are dissolved in aqua regia or chlorine con-taining hydrochloric acid for the total dissolution of metalcontents. Although solvent extraction and electrochemicalprocesses have been employed for their recovery in casetheir concentrations are high enough (2–4), ion exchangeand adsorption processes are more suitable in the case oflow or trace concentration of metals to be recovered as inthe case of the electric and electronic waste (5–8). However,the conventional ion exchange resins and other adsorbentshave suffered from various drawbacks such as low selec-tivity, low efficiency for dilute solution, and high cost.In order to effectively recover trace concentrations of valu-able metals, more effective processes are required to bedeveloped by improving their functional abilities such asselectivity, loading capacity, and kinetic behavior.
Besides the conventional ion exchange resins, in therecent years, biomaterial and biomass based adsorbentsare expected as promising alternatives for the separationand recovery of precious metal ions from industrial solution(9). Among various kinds of biomass, microalgae are adiverse group of photosynthetic microorganisms that growrapidly due to their simple structure. It is estimated that thebiomass productivity of microalgae could be 50 times morethan that of the fastest growing terrestrial plant (10). Micro-algae have been initially examined as a potential fuel sourcealternative to fossil fuels; but its prohibitive productioncosts discouraged the commercial development of algae-based fuel production (11). In order to overcome this limi-tation of using such promising alternative fuel as well as tomeet the view point of full utilization aspect, we haveattempted to use the residual waste of microalgae which isgenerated following the extraction process of microalgae-based biofuel as the raw material for the preparation ofthe adsorbent. The prepared adsorbent was assigned forthe recovery of precious metals which would provide thecost optimization to microalgae-based fuel system.
Received 15 June 2011; accepted 23 November 2011.Address correspondence to Katsutoshi Inoue, Department of
Chemistry and Applied Chemistry, Saga University, 1-Honjo,Saga 840-8052, Japan. Tel.: þ81-952-28-8671; Fax: þ81-95- 28-8548. E-mail: [email protected]
Separation Science and Technology, 47: 1185–1193, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0149-6395 print=1520-5754 online
DOI: 10.1080/01496395.2011.645384
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Although various biomass- or biomaterial-based adsor-bents have been developed so far, they still suffer from somedisadvantages such as poor stability to use in the actualwaste stream, low adsorption capacity, and unsatisfactoryselectivity towards precious metals. Likewise, the residualwaste of microalgae as it is may not satisfactorily exhibitthe affinity towards metal ions. In order to improve thechemical properties of biomass based adsorbent such as sel-ectivity, adsorption capacity as well as chemical stability, avariety of chemical modification on the polymer matrices ofbiomass is required to establish the appropriate types ofadsorbent for target metals (12). A novel approach is totake advantage of the knowledge that the affinity and selec-tivity of the adsorbent can be enhanced by immobilizingligands that can coordinate target metals with high stability.According to the Hard-Soft Acid-Base (HSAB) theory (13),platinum group metals (PGMs) which are classified as softLewis acids preferentially give rise to stable complexes withsoft Lewis bases. According to this concept, the adsorbentswith nucleophilic ligands containing sulfur or nitrogendonor atoms exhibit strong interaction with platinum groupmetals than base metals (12,14,15).
In the present paper, we have investigated the prep-aration of an adsorbent from the residual waste of micro-alga (Pseudochoricystis ellipsoidea) generated in the biofuelextraction process to reclaim some values of microalgalwaste in the form of advanced functional material. Thisresidual waste mainly contains polypeptides and polysac-charides as its major components. A novel type of themicroalgae-based adsorbent containing sulfur and nitrogendonors was developed from this residual waste by means ofchemical modification based on the above-mentioned idea(12,14,15). That is, dithiooxamide was chosen as the func-tional group to be immobilized for this study because it con-tains both of sulfur and two nitrogen donor atoms which isconsidered to provide high selectivity towards preciousmetal ions like Pd(II) and Pt(IV). The adsorption behaviorof this prepared adsorbent was investigated in detail.
EXPERIMENTAL
Preparation of the Adsorbent
The raw material employed for the preparation of theadsorbent in this present study was residual waste of thePseudochoricystis ellipsoidea generated from biofuel extrac-tion and was kindly provided by DENSO CORPOR-ATION, Aichi, Japan. The adsorbent was preparedaccording to the synthetic route shown in Scheme 1; thatis, the raw material was interacted with thionyl chloride(SOCl2) to introduce chlorine atoms at alcoholic hydroxylgroups onto polymer matrices of cellulosic polysaccharidesin microalgal residue to be further interacted with dithioox-amide (DTO). Five gram of dry microalgal residue wastaken together with 200mL pyridine, which was kept in an
ice bath. Thirty milliliter of SOCl2 (Sigma-Aldrich ChemicalCo., USA) was added dropwise to the mixture under N2
atmosphere and the mixture was further stirred continuouslyat 348K for 5 h. The product was washed with distilled waterand dried in a convection oven at 343K for overnight. Toimmobilize DTO functional group onto polymer matricesof microalgal residue, 3 g of the SOCl2-treated microalgalresidue was then interacted with 0.2 g of DTO (Wako PureChemical Industries, Ltd., Japan) in the presence of sodiumcarbonate (1.2 g) in N,N-dimethylformamide (DMF) at343K for 48h by using an oil bath. After the mixture wasallowed to cool down at room temperature, the solid productwas separated by filtration and washed with diluted HCl andfurther washed with distilled water several times. Finally, thesolid product was dried in the convection oven at 343K for24h. The product was ground and sieved to uniform theparticle size prior to the adsorption study. The microalgalresidue-supported dithiooxamide thus prepared is abbrevi-ated as DTO-microalgae, hereafter.
Characterization of the Adsorbent andMeasurement Methods
The elemental content of C, H, and N was evaluated byan elemental analyzer while sulfur content was quantitat-ively analyzed using a DIONEX model ICS-1500 ionchromatography equipped with an automated combustionsystem model AQF-100 and absorption unit modelGA-100. The FT=IR spectra of materials were recordedby a JASCO model FT=IR-410 Fourier transform infraredspectrophotometer. The morphological observation of theadsorbent surface was carried out by a JEOL modelJCM-5100 scanning electron microscope (SEM) at 20 kVof acceleration energy. In order to evaluate the aqueoussolubility of the adsorbent, the total organic carbon(TOC) concentration was measured by mixing 10mg ofthe adsorbent with 10mL of HCl solution ranged from0.1 to 5.0M (M¼mol=L). The mixture was shaken at303K for 24 h in a THOMAS model AT24R thermostatedshaking incubator. After filtration, the TOC concentrationof the filtrate was measured by using a Shimadzu modelTOC-VHS TOC analyzer. The metal ion concentrationsin the test solution in the adsorption tests were measuredby using a Shimadzu model ICPS-8100 inductively coupledplasma atomic emission spectrometer (ICP=AES).
Preparation of Metal Solutions
Analytical grade of H2PtCl6 � 6H2O, PdCl2, FeCl3 � 6H2O(Wako Pure Chemical Industries, Ltd., Japan), ZnCl2(Sigma-Aldrich Chemical Co., USA), CuCl2, and NiCl2 �6H2O (Katayama Chemical, Japan) were used to preparetest solutions of Pt(IV), Pd(II), Fe(III), Zn(II), Cu(II),and Ni(II), respectively. All other chemicals used for thepreparation of the adsorbent and for the adsorption tests
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were of analytical grade and were used without furtherpurification.
Batch Mode Adsorption Tests From Acidic ChlorideMedia
The adsorption tests for various metal ions, Pd(II),Pt(IV), Cu(II), Zn(II), Fe(III), and Ni(II), were carriedout using original microalgal residue, the feed material,and DTO-microalgae in batch mode to observe theiradsorption behavior in acidic chloride media at varyingHCl concentration. Ten milligrams of adsorbent and10mL of test solutions containing mixed metal ions of0.2mM of Pd(II), Pt(IV) together with 1.0mM of basemetal ions were mixed and shaken in the thermostatedshaking incubator (THOMAS AT24R) at 303K, at150 rpm for 48 h to attain equilibrium which was confirmedin the preliminary experiment. Under the same condition, aDIAION WA20, a commercially available anion-exchangeresin produced and marketed by Mitsubishi Chemical Cor-poration, Japan, containing secondary amine groups as wellas commercially available granular activated carbon (WakoPure Chemical Industries, Ltd., Japan) with high porositywere tested for comparison. After shaking, the mixtureswere filtered to separate the solid adsorbent. The filtrateswere collected for the further measurement.
The adsorption isotherms were measured to evaluate themaximum adsorption capacities for Pd(II) and Pt(IV), byshaking 10mL of test solutions ranged from 0.5 to 10mMof individual metal ions together with 10mg of adsorbentfor 96 h to attain adsorption equilibrium. The initial andremaining concentration of metal ions in the filtrate wasmeasured by the ICP=AES. The amount of adsorbed metal
ions was calculated from the decrease in metal concentrationin the filtrate and the dry weight of the adsorbent. Conse-quently, the results were further replotted according to theLangmuir adsorption model. From the linear relationshipof the Langmuir equation expressed by Eq. (1), the adsorp-tion capacity (qm) and Langmuir adsorption constant (KL)related to the energy of adsorption were evaluated fromthe slopes and intercepts of the straight lines.
Ce=qe ¼ Ce=qm þ 1=KLqm ð1Þ
The effect of temperature on the adsorption equilibriumconstants, KL, for Pd(II) and Pt(IV) on DTO-microalgaewas exploited according to the following thermodynamicrelationships:
G ¼ �RT lnKL ð2Þ
lnKL ¼ �DG=RT ¼ �DH=RT þ DS=R ð3Þ
where DG, DH, DS, are the free energy, enthalpy, andentropy changes, respectively, associated with the adsorp-tion process for Pd(II) and Pt(IV) on DTO-microalgae.KL, T, and R are the Langmuir adsorption constant, tem-perature in Kelvin, and universal gas constant, respectively.The enthalpy and entropy changes were evaluated from theslope and the intercept of Van’t Hoff plots of ln KL versusT�1, respectively.
Dynamic-Mode Adsorption and Subsequent Elutionusing a Packed Column
The dynamic-mode adsorption tests were carried outusing a transparent glass column of 0.8 cm inner diameter
SCH. 1. Preparation route of the DTO-microalgae from the residual waste of microalgae.
SELECTIVE RECOVERY OF Pd(II) AND Pt(IV) 1187
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and 20 cm high fitted with a glass support at the bottom.Before packing in the column, 0.1 g of the DTO-microalgaewith particle size ranged from 150 to 300 mm was soaked indeionized water to facilitate swelling. The test solutioncontaining 10mg=L of Pd(II) and Pt(IV) together with108mg=L of Cu(II) in 0.1M HCl solution was continu-ously percolated from the bottom of the column at a con-stant flow rate of 4.2mL=h by a peristaltic pump(PERISTA BIO-MINIPUMP ATTO, AC-2120). A frac-tion collector (Bio-Rad model 2110) was used to collectthe effluent at 1 h of time intervals for the further analysisof metal ions. Prior to the elution test, the column satu-rated with metal ions was fed with distilled water to expelany unbound metal ions. The elution was run by feedinga mixture of 0.5M thiourea and 0.5M HCl through thecolumn at the flow rate of 4.2mL=h.
RESULTS AND DISCUSSION
Characteristics of the Residual Waste of Microalgaeand DTO-Microalgae
The degree of dithiooxamide immobilization on microal-gal residue was examined by measuring its elemental con-tents. As shown in Table 1, the sulfur and nitrogencontents of the original microalgal residue were relativelylow, indicating the impractical use of this residual wastefor the recovery of Pd and Pt from acidic chloride media.In comparison with original microalgal residue, the decreasein carbon and hydrogen content was observed after thechemical modification whereas the nitrogen and sulfur con-tents of DTO-microalgae relatively increased up to 5.28 and18.7%, respectively, from which their elemental density wascalculated to be 3.8 and 5.8mmol=g, respectively. Thisresult confirmed the successful modification of microalgalresidue with dithiooxamide, in which the microalgal residuewas used as the supporting material. The high content ofnitrogen and sulfur thus introduced is expected to play animportant role on selective binding with Pd(II) and Pt(IV)according to the HSAB concept proposed by Pearson (13).
The morphological change by the chemical treatmentwas investigated by using the scanning electron microscope(SEM). Figure 1 shows the SEM images of the originalmicroalgal residue and DTO-microalgae. The shape ofmicroalgal residue was random and its surface was rather
rough due to the chemical treatment during the extractionprocess of biofuel. In comparison with this original micro-algal residue, it is clearly seen that the surface of DTO-microalgae was relatively much rougher with a high degreeof porosity developed by the chemical modification.
For the use of biomaterial as the adsorbent, their dissol-ution in aqueous media should be suppressed so as toimprove the stability and strength of the adsorbent. Thetotal organic carbon (TOC) concentration released from10mg of DTO-microalgae as well as from the same weightof original microalgal residue during contacting with HClsolution was quantitatively measured in order to examinetheir dissolution. It was found to gradually increase withincreasing HCl concentration from 124 up to 314mg=Lwith increasing acid concentration from 0.1 to 5.0M in caseof original microalgal residue whereas it was only from 6 upto 32mg=L in case of DTO-microalgae under the same con-ditions. This result indicates that one of the disadvantagesof using biomass as it is in such acidic solution can be over-come by chemical modification.
Effect of HCl Concentration on the Adsorption Behaviorof Pd(II) and Pt(IV) on DTO-Microalgae
It is well known that Pd(II) and Pt(IV) exist as stableanionic chloro complexes of PdCl4
2� and PtCl62� as the
TABLE 1Elemental composition of microalgal residue and
DTO-microalgae
Material
Elemental content (%)
H C N S
Microalgal residue 6.33 43.6 4.04 1.34DTO-microalgae 3.86 42.3 5.28 18.7
FIG. 1. SEM images of (a) microalgal residue recorded at �200 magni-
fication and (b) DTO-microalgae recorded at �750 magnification.
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predominant species in acidic chloride media under the con-dition as in the present work. Figures 2(a) and (b) show theplots of %adsorption of various metal ions on the originalmicroalgal residue and DTO-microalgae, respectively,against HCl concentration. It is clearly seen from this figurethat the adsorption of Pd(II) on original microalgal residueis less than 40% at low concentration of HCl and sharplydecreased with increasing HCl concentration. The adsorp-tion of Pt(IV) and base metal ions like Zn(II), Cu(II), Ni(II),and Fe(III) was completely insignificant over the whole HClconcentration ranged from 0.1 to 5M. This is due to thelack of specific donor atoms like sulfur and nitrogen inthe original microalgal residue (Table 1). Under the sameexperimental conditions, DTO-microalgae exhibits effectiveand selective adsorption for Pd(II) and Pt(IV) over thewhole HCl concentration range and even the test solutioncontains higher concentration of coexisting base metal ions.However, the %adsorption of Pt(IV) on DTO-microalgaegradually decreased with increasing HCl concentration.
Figure 3 shows the comparative plots of the DIAIONWA20 resin and activated carbon for various metal ions.
The DIAION WA20 is a commercially available resincontaining a functional group of secondary amines incor-porated on the matrixes of polystyrene whereas the acti-vated carbon is well known to bear a large surface areawith its highly porous structure. Both adsorption materialsexhibited the sharp decrease in adsorption for Pd(II) andPt(IV) with increasing HCl concentration. However, thesimilar tendency of the order of selectivity among preciousmetal ions was observed as Pd(II)>Pt(IV) at only low HClconcentration, which is the same order with DTO-micro-algae. Because the DIAION W20 bears the secondaryamine groups, the ionic interaction of anionic chloro com-plexes of Pd(II) and Pt(IV) with protonated amines is apredominant mechanism (16,17). In the case of activatedcarbon, although the adsorption of organic compound ismainly controlled by physical adsorption due to the porouscharacteristics, the majority of adsorption of ionic specieslike metal ions is considered to be the roles of surface func-tional groups (18). In contrast to the DTO-microalgae, theconsiderable amount of Zn(II) adsorption on DIAIONWA20 resin and activated carbon was observed whereasno adsorption of Cu(II), Ni(II), and Fe(III) was observed
FIG. 2. Effect of HCl concentration on the adsorption behavior of
metal ions on (a) microalgal residue and (b) DTO-microalgae. Conditions:
initial concentration of metal ion (mM)¼Pd(0.2), Pt(0.2), Zn(1.0),
Cu(1.0), Ni(1.0), Fe(1.0), dry weight of adsorbent¼ 10mg, particle size¼75–150mm, volume of test solution¼ 10mL, shaking time¼ 48 h,
temperature¼ 303K.
FIG. 3. Effect of HCl concentration on the adsorption behavior of (a)
a commercially available ion exchange resin, DIAIONWA20 and (b) acti-
vated carbon. Conditions: initial concentration of metal ion
(mM)¼Pd(0.2), Pt(0.2), Zn(1.0), Cu(1.0), Ni(1.0), Fe(1.0), dry weight of
adsorbent¼ 10mg, volume of test solution¼ 10mL, shaking time¼ 48 h,
temperature¼ 303K.
SELECTIVE RECOVERY OF Pd(II) AND Pt(IV) 1189
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under this experimental condition. From these results, theresidual waste of microalgae exhibits a good property tobe used as the supporting material to prepare the adsor-bents for precious metals.
In order to understand the mechanism by which theDTO-microalgae interacts with precious metal ions, theFT=IR spectra of DTO-microalgae before and after theadsorption of Pd(II) and Pt(IV) were observed as shown inFig. 4 for comparison. From these FT=IR fingerprints, thebands of the secondary amine moiety are assigned at 1625and 1577 cm�1. Bands at 1142, 1066, and 1038cm�1 areassigned to the general stretching of thiocarbonyl groups(vC¼S). The appearance of these bands provides evidence ofthe immobilization of dithiooxamide moieties on microalgaematrices. In the same figure, the spectra of DTO-microalgaeloaded with Pd(II) and Pt(IV) shows that the frequency ofvC¼S slightly shifted to 1035cm�1 and the vibration at 1066and 1048cm�1 is relatively weaker than that before adsorp-tion, which indicates the involvement of coordination ofC=S groups onto metal ions (19). For the bands of secondaryamine moiety, the significant shift from 1625cm�1 to a lowerfrequency at 1618cm�1 is observed after Pd(II) and Pt(IV)adsorption. Also, the band at 1577 cm�1 becomes weaker,indicating that Pd(II) and Pt(IV) bound on DTO-microalgaevia the electrostatic interaction between protonated aminemoieties (16,17) and anionic chloro complexes of PdCl4
2-
and PtCl62-. Based on this information, it is considered that
metal chelation and electrostatic interaction are the dominant
driving forces for Pd(II) and Pt(IV) adsorption fromHCl sol-ution on DTO-microalgae.
Adsorption Isotherms and Thermodynamic Parameters
Figure 5(a) and (b) show the adsorption isotherms Pd(II)and Pt(IV) on DTO-microalgae from 0.1M HCl at tem-peratures of 298, 303, 313, and 323K. As shown in these fig-ures, the adsorption of metal ions increased with increasingmetal concentration and tended to approach the plateauregion at high concentration, which appears to be in accord-ance with the Langmuir adsorption model. Figures 6(a) and(b) show the linear relationship of the Langmuir adsorptionmodel for Pd(II) and Pt(IV) adsorption, respectively. Theadsorption capacity (qm) and Langmuir adsorption con-stant (KL) evaluated from the straight lines (Fig. 6(a) and(b)) are listed in Table 2. From this table, it is seen thatthe maximum adsorption capacities of both Pd(II) andPt(IV) is increased with increasing temperature, which indi-cates that the adsorption of Pd(II) and Pt(IV) on DTO-microalgae was governed by the endothermic reaction.
FIG. 4. FT=IR spectra of original DTO-microalgae taken before
adsorption and DTO-microalgae exposed with Pd(II) and Pt(IV).
FIG. 5. Adsorption isotherms of (a) Pd(II) and (b) Pt(IV) on DTO-
microalgae at varying temperature. Conditions: dry weight of adsorbent¼10mg, particle size¼ 75–150mm, volume of test solution¼ 10mL, HCl
concentration¼ 0.1M, temperature¼ 303K, shaking time¼ 96 h.
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The values for Pd(II) are greater than Pt(IV) for alltemperatures studied.
Figure 7 shows the Van’t Hoff plots in which theenthalpy and entropy changes were evaluated. These evalu-ated values of thermodynamic parameters are summarizedin Table 3. The positive values of enthalpy change indicatethe endothermic nature for both Pd(II) and Pt(IV) adsorp-
tion on DTO-microalgae. The positive value of entropychange indicates the increasing randomness at the interfacebetween the dithiooxamide moieties and Pd(II) and Pt(IV)ions. The free energy changes during the adsorption processare all negative values and decreased with increasing tem-perature as shown in Table 3, which suggests that theadsorption process of Pd(II) and Pt(IV) is spontaneousand more favorable at higher temperature.
Continuous-Mode Adsorption Followed by Elution TestUsing a Packed Column
On the basis of the high selectivity of DTO-microalgae forPd(II) and Pt(IV) over base metal ions observed in the batchwise adsorption tests (Fig. 2(b)), the continuous flow adsorp-tion test was carried out using model solution. Figure 8shows the breakthrough profiles of metal ions tested. Asdepicted in this figure, although the feed concentration of
FIG. 6. Langmuir model plots of (a) Pd(II) and (b) Pt(IV) adsorption on
DTO-microalgae at varying temperature.
TABLE 2Effect of temperature on Langmuir parameters for theadsorption of Pd(II) and Pt(IV) on DTO-microalgae
Metal T (K)qm
(mmol=g)KL� 10�3
(L=mol) R2
Pd(II) 298 3.44 11.7 0.9976303 3.63 13.2 0.9976313 4.20 14.7 0.9969323 4.82 15.2 0.9977
Pt(IV) 298 1.32 5.62 0.9895303 1.48 6.18 0.9905313 1.86 9.85 0.9988323 2.11 10.4 0.9980
FIG. 7. Van’t Hoff plots for the adsorption of Pd(II) and Pt(IV) on
DTO-microalgae.
TABLE 3Thermodynamic parameters for the adsorption of Pd(II)
and Pt(IV) on DTO-microalgae
Metalion T (K)
DG(kJ=mol)
DH(kJ=mol)
DS(J=molK)
Pd(II) 298 �23.2 þ21.7 þ145303 �23.9313 �25.0323 �25.9
Pt(IV) 298 �21.4 þ8.26 þ106303 �22.0313 �23.9323 �24.8
SELECTIVE RECOVERY OF Pd(II) AND Pt(IV) 1191
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Cu(II) was several times higher than Pd(II) and Pt(IV), thebreakthrough of Cu(II) took place immediately just afterthe running operation. This result confirmed the negligibleuptake of Cu(II) ion on this adsorbent which was in goodagreement with the result observed in batch mode study.From the same figure, the breakthrough point of Pt(IV)was subsequently observed at around of 1067 B.V. (80 h)and became saturated at 3533 B.V. (265h). The breakthroughpoint of Pd(II) was obtained at the bed volume of 2400 B.V.(180h) and saturated at 8000 B.V. (600h), suggesting highloading capacity for Pd(II). These breakthrough profiles sug-gest that the separation of Pt(IV) from Pd(II) in the effluentwas easily achieved during the period from 1067 to 2400B.V. The loading capacity of packed DTO-microalgae forPd(II) was evaluated from the area of the breakthroughcurves as 16mg (1.5mmol=g of dry adsorbent) whereas thatof Pt(IV) was only 6mg (0.3mmol=g of dry adsorbent). Thesevalues are much different from those evaluated in the batchmode study which were 3.63 and 1.48mmol=g for Pd(II)and Pt(IV), respectively. One of the factors is possibly dueto the short contact time in continuous flow because of theslow adsorption kinetics of Pd(II) and Pt(IV).
It is well known that a mixture of thiourea and HCl sol-ution is an efficient eluent for Pd(II) and Pt(IV). The elutiontakes place in cases where the thiourea forms stable com-plexes with Pd(II) and Pt(IV) in acidic condition, resultingin their desorption from solid adsorbents. Figure 9 showsthe elution profiles of Pd(II) and Pt(IV) by aqueous mixtureof 0.5M thiourea and 0.5M HCl. As shown in this figure,both metal ions were successfully eluted from the fixedbed with a precencentration factor (Ct=Ci) greater than100 for Pd(II) and 50 for Pt(IV). In the first 4 h of elution,about 75% of Pd(II) and 90% of Pt(IV) were eluted from
the fixed bed. The elution run for 24 h allowed more than85% of Pd(II) and 96% of Pt(IV) to be eluted and recovered.
CONCLUSIONS
The microalgal residue-supported dithiooxamide (DTO-microalgae) was successfully developed by means ofchemical modification. The sulfur and nitrogen contentswere found to be relatively high as much as 5.28% N(3.8mmol=g) and 18.7% S (5.8mmol=g) compared with theoriginal microalgal residue. The DTO-microalgae exhibitedhigh selectivity for Pd(II) and Pt(IV) over base metals inacidic chloride media. The adsorption mechanisms of Pd(II)and Pt(IV) on DTO-microalgae was discussed on the basis ofFT=IR spectra, indicating that coordination and electro-static interaction took place via thiocarbonyl groups andamine moieties, respectively. The obtained thermodynamicparameters demonstrated endothermic adsorption reaction.The column adsorption and subsequent elution of adsorbedmetal ions using acidic thiourea revealed the effective pre-concentration, separation, and recovery of Pd(II) and Pt(IV).
ACKNOWLEDGEMENTS
The present work was financially supported by theMinistry of Agriculture, Forestry, and Fisheries, Japan,for the research project on Development of InnovativeTotal Technologies for Carbon Sequestration, BiomassProduction, and Biomass Utilization.
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FIG. 9. Metal concentration profile during desorption of DTO-
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