The rate of increase of the conductance of activated carbon immersed in NaCl solutions was

found to be lOE.'er when the concentration of the salt was increased. This was probably due to the

increase in the viscosity of the solutions which prevented the filling of the pores to the same extent as in

the case of pure water. In addition, the final conductance of the sample was also lowered. The opposite

effect was however observed in HCl solution. Measurements in the presence of Li + and K + suggested

that the mobility of the ions determined the.final value

Awang , Harun

Badri, M.

Crouse, Karen Anne

Universiti Putra Malaysia Institutional Repository

Pertanika 8(3), 337 - 341 (1985)The Effects of Sorbed Alkali Metal Ions on theConductance of Activated CarbonM. BADRI, KAREN ANNE CROUSE and HARUN AvVANGDepartment of Chemistry,Faculty of Science and Environmental Studies,Universiti Pertanian Malaysia,43400 Serdang, Selangor, Malaysia.Key words: Conductance; activated carbon; pores; sorption; viscosity; mobility of alkali metalions.ABSTRAKKadar kenaikan kekonduksian karbon yang diaktifkan didapati lebih rendah apabila di-benamkan dalam larutan NaCl yang kepekatan garamnya ditambah. Kejadian ini telah disabit-kan sebagai bertambahnya kelikatan larutan yang menahan alz"ran ke segenap liang kepada tahapyang dicapai oleh air tulen. Nilai kekonduksian akhir pun lebih rendah juga. Kesan yang berlawanandidapati bagi larutan HCl. Pengukuran dalam larutan Li + dan K + membayangkan kelincahan ion-ion ini memainkan peranan penting dalam penentuan kerendahan kekonduksian itu.ABSTRACTThe rate of increase of the conductance of activated carbon immersed in NaCl solutions wasfound to be lOE.'er when the concentration of the salt was increased. This was probably due to theincrease in the viscosity of the solutions which prevented the filling of the pores to the same extent as inthe case ofpure water. In addition, the final conductance of the sample was also lowered. The oppo-site effect was however observed in HCl solution. Measurements in the presence ofLi + and K + suggest-ed that the mobility of the ions determined the.final value.INTRODUCTIONThe presence.of pores of different sizes inactivated carbon makes it an excellent adsorbentfor many chemical species both in the gaseousstate and in solution. This adsorbing power hasbeen attributed mainly to its ability to formbonds of varying strength with the adsorbates.There are numerous active sites on the walls ofthe pores where bonding can occur through theparticipation of the unsaturated orbitals of thecarbon atoms. Besides these neutral centres,there are also present various organic functionalgroups such as phenolic and carboxylic (Hassler,1984) that can attract the adsorbate strongly.When a piece of activated carbon is im-mersed in water, the filling of its internal cavitieswould be expected to be governed by the rate atwhich the water flows in through the pores thatare open to the bulk aqueous medium. As waterseeps into the pores, the various functionalgroups are ionized, giving hydrogen ions. It hasbeen suggested that these ions were responsiblefor the increase in the conductance of the carbon(Badri et al., 1984) because on formation theywould stay in the vicinity of the negatively charg-ed functional groups in the pores and thus act asthe charge carriers. As more pores were filled,the conductance would increase as a result of thegreater number of hydrogen ions fonned.The change in conductance of the activatedcarbon due to the sorption of water has beenM. BADRI, KAREN ANNE CROUSE AND HARUN AWANGThe conductance readings obtained usingNaCI solution of various concentrations aregiven in Fz'g. 1. Conductance was found toRESULTSment. The level of solution in the test tube waskept constant by dropwise addition of distilledwater as necessary. When the conductancereached a final constant value, a f' after fourdays, the conductor was thoroughly washed withdistilled water in a Soxhlet apparatus. It wasdried in an oven at HO°C overnight prior toevacuation at 70°C for three hours and wasstored in a desiccator.d30001000 2000TIME IN MINUTES1. Conductance of activated carbon immersedin NaCl at four different concentrations:a - 1 x 10 - 5M, b - 2.5 x 10 - 5M,c - 5 x 10 - 5M, d - 1 x 10 - 4M.Final conductance was read after 4320minutes.Fig.34woz«t-o::::>ozoowhere a 0' a f and a are the initial conduc-tance, the final conductance and the conduc-tance of the sample at time t respectively, Is' andTl are the surface tension and viscosity of water,c is a constant related to the size and shape of theconductor and r is the average radius of thepores.(a - a 0)2 = ,Is'T ctaf - aD TlMATERIALS AND METHODSfound to increase with time according to theequation:For a given piece of activated carbon, theconductance depends on the rate at which thewater flows into the pores. Since flow rate isdetermined by surface tension and viscosity, itcan be varied by addition of solutes. Higher sur-face tension and lower viscosity of the aqueousmedium woulg result in an increase in the rate offlow. This paper describes the effect of somemonovalent cations in solution upon the conduc-tance of activated carbon.A piece of activated carbon conductor wasprepared and assembled as previously described(Badri et al., 1984). Solutions of alkali metalchlorides, LiCI, NaCI and KCI, of 1.00 X 10 -4 1"M'were prepared from recrystallized reagents(Hopkin and Williams CPR) in distilled water.Solutions of NaCI of concentrations 5.00 X10 -5M~ 2.50 X 10 -5M and 1.00 X 10 -5Mwere prepared by dilution. HCI solutions ofvarious pH were prepared by diluting the A.R.grade supplied by Berk Spenser Acid Ltd ofLondon. pH measurements were done using aTOA HM 7A pH meter.The conductor assembly was placed in a testtube and allowed to equilibrate at the tempera-ture of the bath. The conductance of the dryconductor was measured using an· AutobalanceUniversal Bridge B642. When a constant valuewas obtained, the salt solution was quickly pour-ed in, completely immersing the conductor. Theconductance was immediately measured, a,and it was then followed to the end of the experi-338 PERTANIKA VOL. 8 NO.3, 1985CONDUCTANCE OF ACTIVATED CARBONFig. 2. The plot of ( a - a / a - a ) 2 foractivated carbon imm;rseJin NaCz offourdifferent concentrations.• 1 x 10 -5 M 0 1 x 10 -4 Mo 2.5 x 10 - 5 M pure distilled waterb. 5 x 10 -5 Mincrease with time in a manner similar to thatwhen only pure distilled water was used. In eachcase the initial slow increase of the conductancewas followed by an accelerated phase before afinal slow period as the maximum value, a f'was approached. The initial inductive periodseems to be lengthened with increasing NaCIconcentration. The rate of increase of the con-ductance and its final value were also smaller asNaCI concentration was increased. The maxi-inum increase in the conductance, ( a f - a),was found to be 4.35 X 10 -6, 3.727 X 10 -6,3.217 X 10 -6 and 2.459 X 10 -6 n -I respec-tively in increasing order of NaCI concentration.The results analysed according to the equa-tion given above, are plotted in F£g. 2. The limitof applicability of the equation has been foundto be up to about 73% of sorption fraction,( a - a / a - a ), similar to when only dis-o f 0tilled water was used. As is obvious from F£g. 2,deviation did occur at small sorption fractionsand became more significant as the concentra-tion of N aCI was increased. The slope of thecurve, which is proportional to ~/TJ ,decreasedas the concentration was increased.The final conductance of activated carbonimmersed in HCI of various concentrations, KCIand LiCI of 1.00 X 10 -!1M are given in Table 1.0·50-4N~I~I I0-0'--0·20,' /rP/p/S!'' •.t:J1000TIME IN2000MINUTESTABLE laConductance of activated carbon immersed in solutions of HCI (30°C) and NaCI (25°C)pH ( 0- a)Xl0 6 n- 1 NaClII0 -\1: ( a- a)x 10 6 n- 1'01.8 4.373 2.5 3.7273.1 2.546 5.0 3.1784.2 2.102 1.0 4.3455.9 1.920 10 2.4596.7 1.906TABLElbConductance of activated carbon immersed in HCI, LiCI, NaCI and KCI (1 X 10 -4 M) at 30°C( ) X 10 6n-1Of -00 wi.mobilityHCI2.398363LiCI1.98540.1NaCI2.01752.0KCI2.11776.2PERTANIKA VOL. 8 NO.3, 1985 33942M. BADRI, KAREN ANNE CROUSE AND HARUN AWANGA NaCI solution is more viscous than purewater and has a higher surface tension. Highviscosity has the effect of slowing down the flowas is evident from the longer time taken by thesample to attain its maximum conductance asthe concentration of the solution was increased,Fig. 1. The lower 0 f values showed by the con-ductor at higher NaCI concentrations wasprobably in part due to the reduced wettingcapability of the solutions. As its surface tensionwas increased, the ability of the solution to fillthe available space in the pores was reducedresulting in fewer ions present to carry thecharge across the walls of the pores and thereforelowering the final conductance.100 200 300v'CONCENTRATION- FAx 10-4Fig. 3. Final conductance difference, (Of - 0)'in n -] plotted against the square root ofconcentration in molar.\J Na+o H+H + gives a direct relationshzP whereasNa + gives an inverse relationshlp.Fig. 3 shows the change in ( 0 f- 0 0 ) with themobility of three alkali metal ions.DISCUSSIONThe conductance of activated carbon im-mersed in aqueous NaCI solution increased withtime in agreement with the results found whenonly distilled water was used. The increment wasattributed to the presence of hydrogen ions inthe pores resulting from the ionization of thefunctional groups as water seeped in. The rate ofincrease in the conductance was determined bythe rate of water entering the pores and its finalvalue was determined by the extent to whichthese pores were filled.In the plot of the square of sorption frac-tion, ( 0 - 0 / 0 f - 0 0> 2, against time, Fig.2, the two factors operated in opposition. Theslope of the curves is proportional to 1)' / 'T(. Thedata available on the surface tension of NaCIsolution (CRC Handbook, 1970 -71) indicatethe ever increasing value of t' with the increasein concentration. Thus the decrease in the slopeas N aCI concentration was increased indicatesthat the viscosity factor is more prominent,especially when the solution is very dilute.The increase in the conductance of activat-ed carbon immersed in distilled water at pH 6.7has been found as 1.906 X 10 -6 n -I 30a C(Table 1). Since there was no salt added, thefinal increase has been solely attributed to thepresence of H + in the pores due to the dissocia-tion of the functional groups. One would expectthat the change would be much greater if theconcentration of H + was increased. This hasbeen found to be so when measurements weredone in aqueous HCI of different concentrationsgiven in Table 1. A plot of ( 0 f - 0) againstthe square-root of H + concentration gave astraight line implying that the Onsager equationmay be applied to the system.A similar plot for the NaCI solutions pro-duced the opposite result, Fig. 3. For NaCI, thefinal change in the conductance ( 0 f -0 0>,decreased linearly as the square-root of concen-tration. The reason for this is still not very clear.340 PERTANIKA VOL. 8 NO.3, 1985CONDUCTANCE OF ACTIVATED CARBONIt might have been due to the increase in thesurface tension as discussed above but, partially,at least, it could have been due to reduced mobi-lity of the ions resulting from assymetric effectsas the concentration was increased. Alternative-2-42·0ly, as Na + ions seeped into the pores, they mayhave displaced some H + ions into the bulkthrough diffusion. This was possibie as there wasa concentration gradient 'of H + ion between thepores in the conductor and bulk solution outsideit. The result of this displacement would be alowering of the concentration of the more mobileH + ions in the pores thus reducing the conduc-tance.The effect of mobility of the ions on theconductivity has been dearly shown in Fig. 4where ( a f - a 0> for four different monovalentcations at the same concentration were measur-ed. This increasing order of mobility has beengiven as Li + < N a + < K + < H + • The plot of( a f - a 0> against the mobility for each speciesappears linear for the alkali metal ions. Thedeviation of H + in the series above could pro-bably be explained by the totally differentmechanism adopted by H + in its transport inaqueous solution.REFERENCES4. Final conductance ~ifference, ( ~f - a)in n - I plotted agaznst the mobzlzty of theions in cm 2 volt - I s - I. Deviation shown byH + probably due to the different mechanismadapted in its transport properties inaqueous solution.Fig.50 150MOBILITY250 350HASSLER, J.W. (1974): Purification with ActivatedCarbon, Industrial, Commercial, Environmental.Chemical Publ. Co., New York, p. 187.BADRI, M., CROUSE, K.A. and HARUN AWANG.(1984): Conductance Measurement of WaterSorbed activated Carbon Pertanika, 7(3):59 - 65.CRC HANDBOOK OF CHEMISTRY AND PHYSICS (1970-1971): Weast, R.C. (Ed.), The Chemical RubberCo., Ohio.(Recezved 26 April, 1985)PERTANIKA VOL. 8 NO.3, 1985 341

The Effects of Sorbed Alkali Metal Ions on the

Conductance of Activated Carbon

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The Effects of Sorbed Alkali Metal Ions on the Conductance of Activated Carbon

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