POLLUTANT ADSORPTION ONTO ACTIVATED CARBON MEMBRANES.pdf

Waf.Sci.Tech.Vol.23,Kyoto,pp.1659-1666,1991.Printed in Great Britain.All rights reserved.0273-1223/91$000+50 Copyright 1990 IAWPRC POLLUTANT ADSORPTION ONTO ACTIVATED CARBON MEMBRANES M.Baudu,P.Le Cloirec and G.Martin Laboratoire Chimie des Nuisances et Genie de tEnvironnement(CN.G.E.),Ecole Nationale Superieure de Chimie de Rennes(E.N.S.CR.),Avenue du General Leclerc,35700 Rennes Beaulieu,France ABSTRACT Filtration of polluted aqueous solutions is performed by activated carbon membranes.The adsorption capacities are determined in batch reactors.Classic models are applied and kinetic constants are calculated.A comparison with grains and powders of activated carbon is made.The performances of the membranes in continously fed reactors are also studied.The breakthrough curves are plotted for different concentrations of organic compounds and fluid velocities.A simple model of mass transfer into the porous volume of the membrane is put forward and used to predict the outlet concentrations as a function of time.The model parameters are correlated to the fluid velocities and the inlet concentrations.KEYWORDS Activated carbon;Membrane;Adsorption;Aromatic;Kinetic;Model.INTRODUCTION The removal of pollutants from diluted aqueous solution is often performed by means of granular activated carbon filters.A lot of publications deal with the modeling and performances of such adsorbing filters,removing organic substances,which consist of activated carbon in granular or powder form.The use of activated carbon in the form of membrane is relatively recent in the field of water and wastewater treatment.Thomy(1971)has developed a process for the removal of radioactive molecules.Recently,Sakoda and his coworkers(1987)published a study about the adsorption on activated carbon fibers of tri and tetra ethylene molecules from aqueous solutions.George and Davis(1988)showed the possibility of getting a biofilm on an activated charcoal cloth.The results from these stUdies seem to be very promising.According to studies already made on carbon grain filters(Schulhof,1979),the possibility of removing dissolved pollutants from water by physical adsorption or biodegradation using activated carbon membranes can be considered.From the engineering viewpoint,the using of activated carbon allows a simplification in the design of pollution control processes.Homogeneous pore size distribution and high surface area dictate an excellent adsorption capacity.The main objective of this study is to evaluate the performances of carbon fibers.These experiments are performed in batch reactors or in pilot units.The various adsorption of fibers,grains and powders are compared.1659 1660 M.BAUDU et al.MATERIAL AND METHODS The activated carbon The commercial activated carbon membranes are obtained by carbonization and activation of polyacrylonitriles.The characteristics of the different materials used in this study are presented in table 1.TABLE 1 Characteristics of Activated Carbons(PICA Company Levallois.France)Commercial name Origin Size(mm)Porosity Specific area(m 2g-1)Adsorption procedure Grains NC 60 coconut 1-1,5 microporous 1200 Powder Membrane NC 60 PAN coconut Polyacrilonitrile 0,1-0,2 microporous microporous 1200 1550 In batch reactors.Activated carbon in the form of grains,powder or as a membrane(0,5 to 4 g)is continously stirred with 1 liter of an aqueous solution at 25C.We have chosen aromatics because these molecules are commonly trapped with carbon.The concentration is about 500 mg/l.Samples are analysed at different stages of the adsorption reaction or when the equilibrium has been reached.This equilibrium is reached after a contact time of 48 hours for the grains and 3 hours for the powder or the membrane.An equilibrium between solid and liquid phases is observed.(Mattson,1971,Lafrance,1984,Baudu,1989).In the laboratory pilot unit.Aqueous solutions containing aromatic molecules are passed through a membrane of 12 cm diameter.Figure 1 shows the equipment used for these experiments which were performed for various outlet concentrations and fluid velocities.The inlet and outlet concentrations are measured and recorded against time.To the spectrophotometer Membrane Fig.1.Laboratory pilot unit Analysis The solutions containing aromatic molecules(phenol,p-nitrophenol,m-cresol,benzoic acid,benzaldehyde and phenylalanine)are analysed by U.V.radiation(254 nm)using a Philips spectrophotometer.Special quartz cells were used to monitor the concentration and the output recorded.RESULTS AND DISCUSSION Adsorption kinetics The kinetics of adsorption were performed in batch reactors.The adsorption isotherms for various aromatic adsorbed on 1 g of different materials(grains,powder or membrane)are presented in figures 2,3 and 4.Whatever the material used,a similar order of adsorption is observed:phenylalanine phenol m cresol benzaldehyde-benzoic acid membrane grain The velocity coefficient for powdered carbon is about ten times greater than the one for the membrane which itself is ten times greater than the one for the grains.Such results can be explained by the adsorption mechanisms such as transport of the solute across the boundary layer to the external surface of the adsorbing particle.The powder develops a larger external surface than the membrane which itself presents a larger external area than the grains.However,the industries making use of membrane and the grains are very similar.In this case the membrane seems to be the most interesting because of its rapid adsorption properties.TABLE 2 Comparison of the Adsorption Velocity Coefficient at t=Molecules Powder Grains Membrane Phenol 0,30 0,0023 0,0280 m-cresol 0,33 0,0028 0,0300 p-nitrophenol 1,10 0,0033 0,0290 Benzaldehyde 0,50 0,0035 0,0450 Benzoic acid 0,45 0,0022 0,0583 Phenylalanine 0,12 0,0012 0,0252 In order to specify the equilibrium between the membrane surface and the pollutant,the Freundlich equation is used:qe=Ke Ce l/n(4)with qe in mg/g and Ce in mg/l The parameters Ke and l/n are determined for the membrane and also for the grains and the powder.The results are presented in table 3.The comparison of these different materials shows a higher adsorption onto the membrane.This could be explained by the larger specific area of the membrane TABLE 3 Freundlich Equation Parameters Powder Grains Membrane Molecules Ke l/n Cor.Ke l/n Cor.Ke l/n Cor.phenol 4,0 0,545 0,980 39,1 0,304 0,940 27,6 0,390 0,992 m-cresol 11,4 0,451 0,957 46,5 0,289 0,981 34,1 0,387 0,988 P nitrophenol 16,6 0,629 0,977 43,8 0,346 0,961 77,5 0,283 0,974 benzaldehyde 7,4 0,605 0,980 48,0 0,350 0,992 43,4 0,363 0,983 benzoic acid 57,9 0,269 0,982 50,4 0,360 0,981 phenylalanine 0,72 0,944 0,944 11,4 0,325 0,975 22,1 0,365 0,990 Activated carbon membranes 1663 The breakthrough of a membrane adsorber:results and prediction The experiments are performed with phenol as a model compound.The breakthrough curves are plotted for different fluid concentrations and velocities.Examples are given in figures 4 and 5.A low velocity is required to obtain a breakthrough curve which presents itself as a curve with a plateau.This can be explained by the low thikness of the membrane(about 2-4 mn)and thus the adsorption front is difficult to obtain.Breakthrough curve shape is very similar to the curve obtained with grain microcolumns.In order to model these breakthrough curves,a simple model satisfying the adsorption conditions(plug flow reactor and external diffusion)is proposed.It is based on mass transfer equation and the Freundlichs relation applied to describe adsorption of the molecule in the case of batch reactors This model was previously used by Clarke(1987).dc u _=KT(C-Ce)(5)dz qe=Ke Cel/n(6)(5)and(6)give dc u The mass balance in the filter is uC=Lq(8)with L=transfer velocity of adsorbant to keep a constant adsorption.The migration velocity of the adsorption front in the reactor is dz v=By combining(7)et(8)and(9)we obtain:then dc C-(l/Ke)n(u/L)n Cn u 11K Cn-l=-1 1+(-1)ertb-rt KC n-l b with r=(KTI u)v and K=(1/Ke)n(u/L)n we know that Lim Cn-l=11K=Con-l-.+co then(11)becomes 1 In-l C _ COn-l 1+Aert v dt(9)(10)(11)(12)(13)(14)(15)1664 M.BAUDU et al.with(16)The determination of the parameters A and r is made by a linear regression.The comparisons of the model curves and the experimental results are presented in figure 4 and 5.A good correlation can be observed.10 8.s c i 6 i c 4 8 2:Model III:Experiment 0 0 300 600 900 1200 Time(mn)Fig.4.Breakthrough curves of phenol.(Co=10 mg/l)100 c-80 r;,.s c.!60 e 10 III 40 I.)c 0(,)20 0 0 100:Model II:Experiment 200 Tim.(mn)300 400 Fig.5.Breakthrough curves of phenol(Co=100 mg/l)To be able to predict the breakthrough curves,we must correlate the constants of equations(15)and(16)with the operating parameters:the following relationships have been found:A=a exp(bu)and r=a1 u+a2Co+a3 with u in mlh and Co in mg/l(17)(18)The following parameters are obtained when phenol is removed by activated carbon membranes:and a=6474 b=-23,3 corr.=0,99 a1=0,0224 a2 1,79.10-4 a3=1,57.10-3 corr.=0,98 Figures 6 and 7 compare the experimental and the calculated results.These equations are therefore very useful to predict the breakthrough curve for a given molecule and then to design the activated carbon membrane filters.Activated carbon membranes 1665 4-3 2 2 3 4 log A experimental Fig.6.Comparison between the experimental and theoretical parameter A.0,03-r-0,02 0,01 0,00-.-,.-r-,.-.-_i 0,00 0,01 0,02 0,03 r experimental Fig.7.Comparison between the experimental and theoretical parameter r.CONCLUSION It can be concluded from this study that:-A very rapid adsorption is obtained on the activated carbon membranes.Its maximum adsorption capacity is slightly higher than that of the grains or of the powder.-After testing the operating parameters and determining the breakthrough curves in the case of the phenol adsorption onto activated carbon membrane,a simple model was obtained which can be used to derive the breakthrough curves.The velocity and the inlet concentration are parameters of the proposed model.-The utilization of the membrane in the field of water purification or wastewater treatment seems to be promising.For instance,the material could be used as a final stage after water previously treated by classical processes or ultrafiltration membranes.Small treatment units could therefore be designed for producing drinking water,treating waste or recycling water.1666 M.BAUDU et al.NOMENCLATURE A aIo a2,a3,b c:model parameter:constants:concentration at t time cb Ce Co Ke,n KT K1 K2 L m q qe qm t tb u v V z 6:breakthrough concentration:equilibrium concentration:initial concentration:Freundlichs equation constant:mass transfer coefficient:adsorption coefficient:desorption coefficient:transfert velocity of adsorbant:height of carbon:adsorption capacity at t time:adsorption capacity at the equilibrium:maximal adsorption capacity:time:breakthrough time:fluid velocity:adsorption velocity:volume of solution:height of carbon:coefficient of the equation(3)REFERENCES Abe,1.,Hayashik,K.,Kitagawa,M.,Hirashima,T.,(1983).Relationship between adsorption capacity and molecular structure b.!:.9!:.2:.Jpn.,1002-1009 Baudu,M.,Le Cloirec,P.,Martin,G.,(1989).Modelisations des isothermes dadsorption sur charbon actif de composes aromatiques en solution aqueuse.9!.Engn.J.,.!h.81-89 Clarke,R.M.(1987).Evaluating the cost and performance of field scale granular activated carbon systems.Environ.Sci.Technol.,ll.,!,573-580.George,N.,Davis,J.T.,(1988).Adsorption of microorganisms on activated charcoal cloth.9!:.Techn.Biotechnol.,.1b 117-129 Lafrance,P.,Mazet,M.,Villesot,D.(1984).Specific adsorption of organic micropollutants onto activated carbon:a study of electrokinetic phenomena due to multicoponent systems.In:Environmental Science,.!r Elsevier,Amsterdam,313-327 Le Cloirec,P.Guernion,C.,Benbarka,B.,Martin,G.,(1986).Modelling and prediction of adsorption on activated carbon.h Eau,ir 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