Effect of metal ion concentration on the biosorption of Pb2+ and Cd2+ by Caladium bicolor (wild cocoyam).pdf

African Journal of Biotechnology Vol.4(2),pp.191-196,February 2005 Available online at http:/www.academicjournals.org/AJB ISSN 16845315 2005 Academic Journals Full Length Research Paper Effect of metal ion concentration on the biosorption of Pb2+and Cd2+by Caladium bicolor(wild cocoyam)Michael Horsfall Jnr*and Ayebaemi I.Spiff Department of Pure and Industrial ChemistryUniversity of Port Harcourt,Uniport P.O.Box 402,Choba,Port Harcourt,Nigeria.Accepted 28 December,2004 The influence of initial metal ion concentration of the batch sorption of Pb2+and Cd2+onto a low-cost biosorbent was investigated.The experimental results were analysed in terms of Langmuir and Freundlich isotherms.According to the evaluation using Langmuir equation,the monolayer sorption capacity obtained were 49.53 and 65.50 mM/g for Pb2+and Cd2+,respectively The data further showed that,sorption of the two divalent metals onto the biomass increased with increase in initial metal ion concentration until monolayer coverage is attained.The thermodynamic assessment of the metal ion Caladium bicolor biomass system indicates the feasibility and spontaneous nature of the process and?Go was evaluated as ranging from 4.55 to 6.63 KJ mol-1 and 4.02 to 6.09 KJ mol-1 for Pb2+and Cd2+sorption,respectively.The order of magnitude of the?Go values indicates an ion-exchange physisorption process.Keywords:Phytoremediation,cocoyam,adsorption,water treatment,heavy metals removal.INTRODUCTION Environmental protection must require the use of natural products instead of chemicals to minimize pollution.Thus,this investigation studies the use of a non-useful plant material as naturally occurring biosorbents for the removal of lead and cadmium ions in aqueous solution.Lead is a toxic element,which is conservative and has cumulative characteristics(Ho et al.,1996).The majority of lead discharged into the environment comes from atmospheric and particulate sources.However,there are a range of industries which generate waste waters containing significant concentrations of lead;for example,the lead-acid battery industry,smelting,printed wiring board manufacture and paper mills.Mine drainage can also give rise to inputs of lead to the aquatic environment.Excessive amounts of lead in the human body can cause hypertension and brain damage.Cadmium is a highly toxic metal that is rather ubiquitous in its distribution in *Corresponding author.E-mail:.the ecosystem.There has been an increasing use of cadmium for electroplating,batteries,alloys,pigments,stabilizers for catalysts and in semiconductors and TV tube phosphors,(Kefala et al.,1999)over the years.Large dosage of cadmium in human can result in tubular and glomerula damage and causes proteinura and anaemia.The presence of Cd2+and Pb2+and other heavy metals in the environment has become a major threat to plant,animal and human life due to their bioaccumulating tendency and toxicity and therefore must be removed from municipal and industrial effluents before discharge.It is therefore necessary that there are technologies for controlling the concentrations of these metals in aqueous discharges/effluents.The conventional technologies,which have been used,ranged from granular activated carbon to reverse osmosis.However,these processes are not economically feasible for small-scale industries prevalent in developing economies due to huge capital investment.As such,it is necessary to search for alternative adsorbents,which are low-cost,often naturally occurring biodegradable products that have good 192 Afr.J.Biotechnol.adsorbent properties and low value to the inhabitants.A range of products has been examined.These include pillared clay(Vinod and Anirudhan,2001),Sago waste(Quek et al.,1998),Cassava Waste(Abia et al.,2003),banana pith(Low et al.,1995),peanut skins(Randall et al.,1974),Medicago sativa(Alfalfa)(Gardea-Torresdey et al.,1998)and Spagnum Moss Peat.(Ho et al.,1995),just to mention a few.The adsorbent used in the present study is Caladium bicolor(Wild Cocoyam)biomass.The plant is a tuberous perennial plant with brightly coloured foliage found in warm,shady areas.It grows to a height of 20 cm.The presence of calcium oxalate in the corm produces an intense irritation if eaten,and therefore makes the plant unedible.The gainful use of this crop will not only bring about the practical exploitation of this non-edible abundant natural resource because of their low cost and availability but would also encourage local farmers.In addition,the anticipated use of the biomass from this plant as a biosorbent for trace metals in water and waste effluents will solve environmental problems.The principal aim of the present work is to asses the potential use of the biomass of C.bicolor as a novel biosorbent for the sorption of valuable and toxic metal ions from aqueous media.The purpose of this paper is to report the effect of initial metal ion concentration and thermodynamics on the sorption of Pb2+and Cd2+ions from aqueous effluents by C.bicolor biomass.MATERIALS AND METHODS Materials The C.bicolor corm used in this study was harvested in Choba Port Harcourt,Nigeria,were it grows as weed in cassava farms.The corm was washed with deionized water,air-dried,cut into 5 cm pieces and ground using a food processor(Magimix Cuisine System 5000),dried in an oven(GallenKamp,model OV-160,England)at 1050C for 24 h and then screened through a 100-?m mesh Tyler screen to obtain a fine biomass.The finely divided biomass was activated by soaking 500 g biomass in excess 0.3 M HNO3 for 24 h,followed by washing thoroughly with deionized water until a pH of 7.1 was attained and then air-dried.The air-dried activated biomass was then washed with deionized water and re-suspended in 1.0 M hydroxylamine to remove all Oacetyl groups.To remove all other soluble materials,the biomass was further washed with deionized water and centrifuged at 3000 x g for 5 min using a Portable Refrigerated test tube centrifuge model PR 2 with 20”diameter stainless solid basket 3/4HP 1/60/115 volt motor with temperature indicator,timer and speed controls.The supernatants obtained were discarded and the paste air dried and grounded to a finely divided powdered biomass.Batch sorption experiment The batch experimental procedure to determine the effect of metal ion concentration is described below.An equilibrium contact time of 2 h was used for metal ion-C.bicolor biomass.A 10 mg of the biomass samples was weighed and placed in pre-cleaned test tubes in triplicates.Several metal ion solutions with standard conc-entrations of 1,2,3,4,5,6,7,8,9,and 10 mM were made from HPLC analytical grade standards of Pb2+(from Pb(NO3)2 and Cd2+(from Cd(NO3)2.4H2O).The two sets of metal solutions made separately were adjusted to pH 5.0 with concentrated HCl.2 mL of each metal solution were added to each tube containing the biomass and equilibrated for 2 h by shaking at 29C.The supernatants were analyzed for Pb2+and Cd2+using flame atomic absorption spectrometer model 300A.RESULTS AND DISCUSSION The percent removal of metal ion by C.bicolor biomass was calculated using the equation:100%xCCCRowo=(1)where R is removal and Co and Cw are the initial and equilibrium metal ion concentrations in solution.The maximum adsorption efficiency of the C.bicolor was 96.4 and 92.4%for Pb2+and Cd2+,respectively.The apparent capacity of the biomass for metal ions was further determined at the different concentrations.The capacity in m mole/g was calculated from the following relationship:mv x C x 100R%(mM/g)o=Capacity (2)where,Co=initial concentration of metal ion solution(M),v=volume of metal ion solution used(L),and m=weight of biomass used(g).The plot of initial metal ion concentration and the capacity of the biomass were then made.Figure 1 clarifies the relation between capacities and metal ion concentrations,which shows that as the metal ion concentration increases,the capacity increases for both metal ions until 5.2 x 10-3 M and 6.5 x 10-3 M Pb2+and Cd2+,respectively,and it remained nearly constant after this value.The selectivity order for metal ion towards the studied biomass matrices is Pb2+Cd2+.For a given initial metal ion concentration,the amount of Pb2+ions sorbed by the biomass was greater than the corresponding amount of Cd2+ions.This differential soption of the two ions may be ascribed to the difference in their ionic radii.The ionic radius of Pb2+is 1.20 while that of Cd2+is 0.97 .The smaller the ionic radius,the greater its tendency to hydrolysed leading to reduced sorption.The biomass exhibited adsorption isotherms of mixed type I type IV,which is a characteristic of the biomass substrate containing both micropores and mesopores(Vinod and Anirudhan,2001).00.511.522.53024681012Concentration(mM)Capacity(mM/g)Capacity for PbCapacity fot Cd Figure 1.The effect of metal ion concentration on the capacity of Cladium bicolor biomass.Figure 2.The Langmuir plot.In general,the data indicates that sorption capacity increased with increase in initial metal ion concentration for both metals on the biomass.This sorption characteristic indicates that surface saturation is dependent on the initial metal ion concentrations.At low concentrations,adsorption sites took up the available metal more quickly.However,at higher concentrations,Horsfall and Spiff 193 metals need to diffuse to the biomass surface by intra-particle diffusion and greatly hydrolyzed ions will diffuse at a slower rate.The maximum metal ion sorption of an adsorbent may be determined from column experiments,by the use of a large excess of the adsorbate.If,however,the metal ions are taken up independently on a single type of binding site in such a way that the uptake of the first metal ion does not affect the sorption of the next ion,then the sorption process would follow the Langmuir adsorption isotherm equation,which was linealized to the form:meLmeeXCKXqC+=1 (3)where Xm and KL are the Langmuir constants.Table 1.Langmuir isotherm parameters.Metal ions Xm mM g-1 KL R2 Pb2+49.53 2.09 x 10-3 0.975 Cd2+48.20 5.7 x 10-3 0.995 The capacity of C.bicolor biomass in binding with Pb2+and Cd2+was determined by plotting Ce/qe against Ce,using the Langmuir equation.Figure 2 shows the data linearised to fit the Langmuir equation.The plots of specific sorption(Ce/qe)against equilibrium concentration(Ce)gave the linear isotherm parameters of Xm,KL and the coefficient of determination and these are presented in Table 1.The R2 values suggested that the Langmuir isotherm provides a good model of the sorption system.The sorption capacity,Xm,which is a measure of the maximum adsorption capacity corresponding to complete monolayer coverage showed that the C.bicolor biomass had a slightly higher mass capacity for Pb2+(49.53 mM g-1)than for Cd2+(48.20 mM g-1).The adsorption coefficient,KL,which is related to the apparent energy of adsorption for Cd2+(5.7 x 10-3 dm3 g-1),was greater than that of Pb2+(2.07 x 10-3)and this could mean that the energy of adsorption is more feavourable for Pb2+than Cd2+.This indicates that not all binding sites may be available for Cd2+binding due to its relatively larger hydration energy.The shape of the Langmuir isotherm can be used to predict whether a sorption system is favourable or unfavourable in a batch adsorption process(Poots et al.,1978).Accordingly,the essential features of the Langmuir isotherm was expressed in terms of a dimensionless constant called the equilibrium parameter,Ep,which is defined by the following relationship:00.20.40.60.811.21.41.600.511.5Ce,mMCe/qeCdPb194 Afr.J.Biotechnol.Ep =11OLCK+(4)Where Ep=is a dimensionless equilibrium parameter or separation factor KL =constant from Langmuir equation Co =initial metal ion concentration of 10mg/l.The parameter,Ep,indicates the shape of the isotherm and nature of the sorption process as given below:Figure 3.The calculated separation factor profile for the two metal ions as a function of metal ion concentration.The values of Ep for Pb2+and Cd2+were calculated from equation 4 and plotted against initial metal ion concentration.The data(Figure 3)showed that,the sorption of Pb2+and Cd2+on the C.bicolor biomass increased as the initial metal ion concentration increased from 1.0 to 10 mM,indicating that adsorption is even favourable for the higher initial metal ion concentrations.The sorption process was favourable for Pb2+and Cd2+removal at all concentrations investigated.According to this classification,removal ability tends to be in the order Pb Cd and that the C.bicolor biomass is an excellent adsorbent for the two divalent metal ions.Adsorption-partition constants were determined for Pb2+and Cd2+using the following log form of the Freundlich isotherm:KCnNeloglog1log+=(5)Figure 4.Freudlich isotherm plot.where N is metal ion sorbed(mM/g),Ce is the equilibrium concentration of metal ion solution,mM,K is a constant and 1/n is a constant.The constants K and 1/n were determined by linear regression from the plot of log N against log Ce(Figure 4).K is a measure of the degree or strength of adsorption,while 1/n is used as an indication of whether adsorption remains constant(at 1/n=1)or decreases with increasing adsorbate concentrations(with 1/n?1).The Freundlich isotherm data as in Table 2 suggest that the biomass sorbed increasing concentrations of the metal ions as 1/n values approach unity in a linear fashion.However,the small K values for Cd2+indicate minimal sorption while more sorption was observed for Pb2+because of larger K values.To account for the adsorption behaviour of the metal ions on the C.bicolor biomass,the Langmuir type equation related to surface coverage was used.The equation is expressed as follows:KC=1 (6)Values of Ep Type of isotherm Ep 1 Unfavourable isotherm Ep=1 Linear isotherm Ep=0 Irreversible Isotherm 0 Ep 1 Favourable isotherm 0.940.950.960.970.980.9911.01024681012Metal ion Conc.(mM)Separation factor,EpEp for PbEp for Cd00.20.40.60.811.21.400.511.5log (N)PbCdlog Ce,mM Where K is the adsorption coefficient and?is surface coverage.Table 2.Freudlich isotherm parameters.Metal ions 1/n K R2 Pb2+0.973 0.838 0.998 Cd2+0.990 0.663 0.996 Figure 5.A plot of surface coverage(?)against concentration(mM).The fraction of biomass surface covered by metal ion was studied by plotting the surface coverage values(?)against metal ions concentration.The data is presented in Figure 5.The figure shows that,increase in initial metal ion concentration for both metals increases the surface coverage on the biomass until the surface is nearly fully covered with a monomolecular layer.Further examination of Figure 5 reveals that the surface coverage ceases to vary significantly with concentration of metal ion at high concentrations and the reaction rate becomes independent of the metal ion concentration.The overall adsorption process indicate that,the reaction is first order at lower metal ion concentration and zero order at higher concentration,indicative that the biomass will be highly ef