The construction of a carbon electrode and the measurement of the changes in the potential that developed

on its surface as a result of adsorption of ions are discussed as a function of the cation adsorption and ionization of the functional groups on the surface of carbon

Badri, M.

Crouse, Karen Anne

Universiti Putra Malaysia Institutional Repository

Pertanika 12(1), 65-70 (1989)Changes in Surface Potential of Activated CarbonDue to Adsorption of IonsM. BADRI and KAREN A. CROUSEDepartment of ChemistryFaculty of Science and Environmental StudiesUniversiti Pertanian Malaysia43400 UPM Serdang, Selangor Darul Ehsan, MalaysiaKey words: Activated carbon; pores; electrical double layer; adsorption; adsorption potential.ABSTRAKPernbinaan satu elektrod karbon dan sukatan ke atas pertukaran keupayaan di permukaan yang diakibatkandaripada penjerapan ion dibincangkan sebagai fungsi kepada penjerapan dan pengionan kumpulan-kumpulanberfung5i di permukaan karbon.ABSTRACTThe construction of a carbon electrode and the measurement of the changes in the potential that developedon its surface as a result ofadsorption ofions are discussed as a function ofthe cation adsorption and ionizationof the functional groups on the surface of carbon.INTRODUCTIONIt has been proposed that activated carbonpossesses many kinds of organic functionalgroups lined up along the wail of the pores(Hassler 1974). These functional groups reactwith water to produce hydrogen ions which canact as additional charge carriers in carbon underan applied voltage (Badri et at. 1984). As aresult of the reaction, the walls of the poreswould be lined with the negatively chargedconjugates of the acids. Since these conjugatesare immobile, the hydrogen ions would remainin their vicinity in the pores thereby establish-ing a charged double layer.Activated carbon is also known for itsability to adsorb cations. When cation adsorp-tion takes place, these adsorbed species willbe fixed at the active centres on the walls of thepores leaving the anions lined up alongsidethem in the solution. Thus another chargeddouble layer of opposite polarity to the onedescribed above will be formed.It was found possible to vary the degreesof ionization and adsorption by changing theconcentration of the H+ on the solution side ofthe pores (Badri and Crouse 1987). Thesechanges were reflected in the changes in thepotential developed across the double layer.In this paper the effect of the adsorption ofsome other cations will be described.MATERIALS AND METHODSAll reagents were of analytical grade unlessotherwise specified.Large pieces of activated carbon were pre-pared by a method described earlier (BritishPatent 1984). Samples thus prepared have verylow electrical conductivity, of the order of 1 x10-6 (ohm - cm)-l (Badri et at. 1987). Fairly flatpieces were chosen and were ground with finesand paper into discs of about 6 mm diameterand 1.5 mm thickness. These discs were re-fluxed in 1 M HCl for about one week then con-tinously washed with distilled water in a Soxhletapparatus for another week. Mter drying in anoven at 11 ODC for 12 hours, two discs were gluedtogether with silver-loaded epoxy resin andthen sealed in glass tubing of appropriatediameter with Araldite epoxy resin. Mter dryingat room temperature, the glass tube was filledM. BADRI AND KAREN A. CROUSEwith -0.5 ml 1M HCl. An Ag/AgCl electrode(prepared according to Shoemaker and Gar-land) was inserted into the tube. This assemblyis henceforth referred to as the carbon elec-trode. For the present experiments, three suchelectrodes were used.Solutions ofa number of chlorides ofvari-ous concentrations were prepared. These werekept in small, covered polyethylene bottles andwere equilibrated at 25 ± 0.2°C. The potentialof the double layer which developed when thecarbon electrode was dipped into the preparedsolution was measured against a Coleman calo-mel electrode using a dc microvoltmeter. Thevoltmeter reading was recorded when a cons-tant value was registered. After each measure-ment, both electrodes were rinsed with distilledwater and gently dried with soft tissue paperbefore proceeding to a solution of differentconcentration. In addition, the carbon elec-trodes were also soaked in distilled water for 16hours before being used again for measure-ments with a test solution containing a differ-ent cation.RESULTSThe potentials of the double layer measuredare given in Table 1 and are plotted as a functionof -log concentration, pC, in Figs 1 to 4.rzg. 1 shows this relationship for the va-lues obtained from Electrode 1 in HCl and1 aCI solutions. Reproducibility was good withan average uncertainity of ± 4mV.The rate of change of the potential in thewhole range of pC from 0 to 7 was found to benot linear. To simplify discussion, each of thecurves given in rzg. 1 is separated into two re-gions. Region 1 refers to that for pC range fromoto 3 (or 4) where the rate of change was highand almost linear except for a slight shoulderat pC -1. Region 2 refers to that for higher pCrange where the rate of change was lower andin which maxima and minima were interspersed.Similar potential-concentration relation-ships were also found for other salts, Figs. 2 to4. The first set of values, Fig. 2(A), was obtainedfrom Elecu-ode 2 with solutions of HCl, NaCl,KCI and LiCI when measurements were donein that ordet.'The potentials obtained for KCland LiCl solutions in repeat measurementsafter the first set was completed are shown inFig. 2(B) to illustrate the possible error thatmight be introduced in a potential measure-ment if inadequate desorption of adsorbed spe-cies had not taken place. Curve 1 was obtainedimmediately after the first set while Curves 2were obtained after soaking Electrode 2 in dis-tilled water for approximately 12 hours. Elec-trode 2 was soaked in HCl (6M) for 3 days andthen in distilled water for three days in anattempt to restore it. The results which areshown in Fig. 3 seem to indicate some improve-ment. However, the original characteristics ofElectrode 2 were not fully restored. The linear-ity of the rate in the pC range of 0 to 1 was lostand the potential measured for 1M solutionwas found to have increased from the originalvalue of --15mV to -40mY. Data obtained forthe chloride of Ca2+(aq) are also included inFig. 3 for comparison.The potentials obtained from Electrode 3for the solutions of doubly- and triply -chargedcations are given in rzg. 4. The values obtainedfor KCl solutions are included for the purposeof comparison. The magnitude of chargepresent on the cations does not appear to affectthe poten tial characteristics of the double layer.DISCUSSIONActivated carbon is known to have a variety ofactive centres which are either basic or acidic.The acidic groups such as carboxylic and phe-nolic may dissociate in water to give H+ depe:1d-ing on the pH of the solution. The basic groupscan be carbonyl, ether, quinone, benzpyreneand so on. Both groups are responsible forthe adsorptive character of activated carbon.In these experiments, the opposite sur-faces of the carbon discs were in contact withsolutions of different concentrations. Since theconcentration of the HCl solution that was incontact with the internal surface of the disc waskept constant, the potential gradient developedacross the charged double layers due to ioniza-tion of the functional groups and adsorption ofH+ would remain constant as well. Thus changesin the potential observed as the external sur-face of the disc was dipped into the test solu-tions must be due to the changes in the poten-tial gradient across the charged double layerof the external surface alone.66 PERTANlKA VOL. J2 NO.1, 1989CHANGES IN SURFACE POTENTIAL OF ACTIVATED CARBON DUE TO ADSORPTION OF IONSTABLE 1Double layer potential* of activated carbonElectrode Solution - log Concentration, pC0 2 3 4 5 6 7HCI 20 57 87 113 103 110 129 110HCI 20 57 83 109 107 111 120 128NaCI 49 69 84 101 120 122 132 110NaCI 43 62 79 97 119 120 135 1002 HCI - 18 40 76 102 94 102 117 100(l st set) NaCI - 17 19 55 88 120 113 118 103KCI -9 27 67 101 110 123 120 110LiCI - 15 20 61 93 120 131 138 89KCI (Curve 1) 61 69 87 100 96 100 86 94KCI (Curve 2) 51 67 90 113 122 128 130 119LiCI 67 70 90 106 122 123 129 118(2nd set) NaCI 40 65 90 108 122 122 118 111NH4Cl 58 65 86 103 119 117 127 123KCI 51 67 90 113 122 128 130 119CaCI~ 44 65 90 108 121 118 139 120LiC] 67 70 90 106 122 123 129 1153 KCl - 27 23 56 73 69 74 76 75AICl~ -42 - 1 39 72 82 74 75 70MgC~ - 42 0 39 68 72 75 76 64CaCI~ - 42 -2 35 63 78 75 76 67'" All poteI1lials are expressed in mV.With respect to this, one might suggestthat the changes in the potential were due tothe ohmic drop across the electrode as thesolution filled the pores. We have discountedthis theory because according to our previouswork (Badri et at. 1985) the conductivity ofactivated carbon soaked in Hel followed theOnsager equation in the pH range of 1.6 to 6.7whereas the present results do not.As the surface of the disc comes intocontact with a solution, adsorption occurs atthe basic active centres, since these electron-rich groups would share the electron cloudswith the adsorbed species. As a result of this, ashift of the electron cloud from the graphiterings of the carbon crystallite to the surfacewould occur. Taking the carbonyl as an exam-ple, the reaction could be written as:Since the adsorbed ions were immobilized,their negatively charged conjugates, X, wouldbe lined up along the surface but on the solu-tion side of the pores. An array of these oppos-ing charges would give rise to an electricaldouble layer on each of the opposite surfacesof the disc. We denote the respectve potentialsas a for the internal surface and a for the, eexternal surface.The acidic functional groups on thesurface could also undergo ionization when thepores are flooded with the solutions. Takingthe phenolic group as an example, the ioniza-tion reaction of these acidic groups could bewritten as(solid) (:o,urface) (~o]ution) (solio) (surface) (solution)r;c--OH + oq 1>=--0- + H+(oq)(:oolj.o) (surfnce) (301ution) (solid) (m..lrface ) (::oltr';ion) It is assumed that most of these acidicgroups were weakly ionized and their popula-tion was relatively small compared to the totalnumber of adsorption sites. However, whenPERTANlKA VOL. J2 NO.1, 1989 67140120' DNoCIoHCI100·>E..i 80<t~Zwf-ea.M. BADRI AND KAREN A. CROUSE120 oHelANcCI0KCI100CLiel80'>E..J 60<l....Zw....00..1----r---2..----'3=---'4--=-5--6::"·-----;7 2 4 5-LOG CONCENTRATION. pCFig. J. Double layer potential oj activated carbon Electrode 1 inHCI and NaCI solutions-LOG CONCE T RATION. pC.Fig. 2. Adsorption potential oj activated carbonElectrode 2. (A) 1st set oj readings(B) 2nd set oj readings130110~" 90..J<tf-ZLJJ.....oCl.5o HCIDNoCIOKCI'VNH4CIOUCIOCoCI2806040>E..J"<t 20f-ZWf-ea.-40O~12OAICI 3OCaC'2OKCI-LOG CONCENTRATION, pCFig. 4. Adsorption potential oj activated carbonElectrode 3 in solutions oj doubly-and t'riply"charged cations2 3 4 5 6-LOG CONCENTRATION, pCFig. 3. Adsorption potential oj activated carbonElectrode 2 aJter partial regeneration oj active sites7 2 3 4 5 6 768 PERTANIKA VOL. 12 NO. I, 1989CHANGES IN SURFACE POTENTIAL OF ACTIVATED CARBON DUE TO ADSORPTION OF IONSIVand at the same time reduce a to increase V.cThe fact that there were three minima ina V-pC curve (including the shoulder in Region1) migh t indicate that there were at least threeacidic groups, each comprising members hav-qHi(ypo,),-' --.J'0;IIIwhich contribute to the development of ac' Atlower concentration of acids, the charged spe-cies would revert to its neutral form accordingto scheme IVIIThe protonated species contribute to ac'whereas the unprotonated species is neutral.As the concentration was reduced, dissociationof the protonated species took place hencereducing ac'Similarly, if a benzpyrene group which isfound in many natural products is present(Tsuchida and Muir 1986), the surface ofactivated carbon would also have species suchas III.On ionization, the activity of other electron-rich functional groups was enhanced accordingto I above, thus increasing the adsorption. Thisresulted in greater aewhich in turn reduced V.The subsequent increase in V could per-haps be explained as the direct result of com-pletion of ionization of these weak acids. Onceionization was complete, there would be nomore less-active sites to be promoted accord-ing to Scheme I and therefore a decrease in theconcentration of the action would decrease a ,and hence V would increase again. The subs~­quent increase of V in region 2 could also bedue to the dissociation of a protonated pyronetype structure which was formed according toscheme II (Parentich and Kinsella, 1984).V = a - (a - A)I a JJeIn region 1, where the concentrations ofthe cations were large, extensive adsorptionoccured resulting in a large acand small V. Atlower concentrations of the cation, V increasedbecause a decreased. The trend continued untilca maximum was reached at pC = -3 or 4 wherethe effect of the reversal potential due toionization took place. It is assumed that at thesepCs, some of the acidic groups began to ionize.ionization took place, an array of negativelycharged conjugates would line up the surfaceof the pores with protons staying near themon the solution side. This situation would alsogive rise to an electrical double layer but thepotential gradient, ~, across the layers would besmall and opposite to that discussed above forthe adsorption.However, such ionization of the acidicgroups might further increase the activity ofother electron-rich groups at least by inductionif not by outright transfer of the electron cloudthrough the graphite rings of the carbon crys-tallite. In this way the less active groups such asperoxides or etheral oxygen which may bepresent on different rings of the same graphitelayer would be transformed into more activesites and hence increase the adsorptive capacityof the surface. An example of such a mecha-nism could be given as follows:HO,~/...!,y ~-"o o~-~T'/~l -!""r'" ~ .......~, I' !) - , ,+ H+L ;l' : - J.,; L... j~ ",~/........ / ~'~i· /~- '-IA strong acid, such as HCI, at a concen-tration of 1M, would supress the ionization ofthe acidic groups. Thus, being weak and rela-tively small in number, the acidic groups wouldcontribute very little to the development of thepotential at the internal surface of the disc.Hence, the potential gradient at the internalsurface of the disc could be safely assumed tobe due to adsorption? a i . However, at the exter-nal surface of the disc this would be true onlyat low pC, and therefore, the potential gradientwas determined by both acand ~c' the potentialdue to the ionization, ~" being very smallcompared to ac' Thus the net potential, V, thatwas measured was the sum of ai' a c and ~c:PERTANIKA VOL. 12 NO. I, 1989 69M. BADRI AND KAREN A. CROUSEing similar ionization constants. The identifica-tion of these species may be determined onlythrough further experiments.There seems to be very little difference inthe effect of adsorption of various cations onthe adsorption potential of the carbon surface.Adsorption of cations carrying higher chargeswould be expected to suppress the activity ofthe adjacent sites more than singly-chargedcations. If this did not occur, equal number ofCa2+adsorbed, for example, would decrease Vby 2-fold over that due to the adsorption ofK+. But the fact that at pC = 0, V wasjust slightlysmaller for the doubly-charged cations indicatesthat the number of occupied sites was onlyfractionally greater that one-half of the totalsites occupied by the singly-charged cations.After adsorption, the characteristics of thecarbon electrode could be reestablished simplyby soaking it in distilled water except for thecase where the adsorbed cation was Li+. Li+ wasmore strongly adsorbed compared to othercations including Aj3+. This is probably due toits ability to enter the micropores of the acti-vated carbon. This further substantiates the con-clusion that the magnitude of the charge on acation did not playa main role in determiningthe strength of the bond formed at the adsorp-tion sites. It is interesting to note, however, thata surface contaminated with some preadsorbedspecies gave higher values of V, Fig. 2. It seemsthat pre-adsorption did not shift the entireordinate of Fig. 2 towards a higher value butaffected only the adsorption side of the V-pCcurve, i.e. Region 1. This could only mean that,if the solution were to contain various cations,these cations would compete for the same ad-sorption sites.REFERENCESBADRi, M., CROUSE, KA. and HARUN AWANG. 1984.Conductance Measurement of Water-SorbedActivated Carbon. Pertanika 7 (3): 59-65.BADRI, M., CROUSE, KA. and HARUN AWANG. 1985.The Effects of Sorbed Alkali Metal Ions on theConductance ofActivated Carbon. Pertanika8 (3):337-34l.BADRI, M. and CROUSE, KA. 1987. AdsorptionPotential ofActivated Carbon. In Pmc. Asian Science& Tech. Congress '87, Kuala Lumpur, Malaysia, 14-17 Oct. 1987. (In press).HASSLER, KW. 1974. Pwification with Activated Car-bon. Industrial, commercial and Environmental. U.S.A.:Int. Textbook Co.PARENTICH, A. and KINSELLA, B. 1984. TitrimetricStudies on Activated Carbons. In Aus. Inst. ofMining and Metallurgy Regional Conference.SHOEMAKER D.P. and GARLAND, c.w. 1962. t;xpeli-ments in Physical Chemistry. p. 445. lew York:McGraw-Hill.TSUCHIDA, 1 . and MUIR, D.M. 1986. Studieson Role of Oxygen in the Adsorption of Au(CNf, and Ag(CN)~ onto Activated Carbon.Metaflurgical Trans. B 17B 529-533.UNIVERSITI PERTAJ'JIAN MALAySIA. 1984. British Pa-tent o. 2086864.(Received 17 September, J988)70 PERTANlKA VOL. 12 NO. I, 1989

Changes in Surface Potential of Activated Carbon Due to Adsorption of Ions

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Changes in Surface Potential of Activated Carbon Due to Adsorption of Ions

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