Studies on the effect of pH on the sorption of Pb2+ and Cd2+ ions from aqueous solutions by Caladium bicolor (Wild Cocoyam) biomass
Michael Horsfall Jnr.*
Financial support: This
project was supported by a research grant from the International
Foundation for Science, IFS,
Keywords: adsorption, Cocoyam, heavy metals removal, phytoremediation, water treatment.
Environmental protection requires the use of natural products instead of chemicals to minimize pollution. This investigation studies the use of a non-useful plant material as naturally occurring biosorbents for the removal of cationic pollutants in wastewater. The effect of pH on the sorption of Pb2+ and Cd2+ ion onto Caladium bicolor corm biomass was investigated. The experimental results have been analysed in terms of Langmuir, Freundlich and Flory-Huggins isotherms. The data showed that the maximum pH (pHmax) for efficient sorption of Pb2+ was 7.0 and for Cd2+ 5.0. Evaluation using Langmuir equation gave the monolayer sorption capacity as 88.50 mg/g and 65.50 mg/g at the respective pHmax for Pb2+ and Cd2+. Surface characterization of acid and base treated C. bicolor biomass indicates a physiosorption as the predominant mechanism for the sorption process. The thermodynamic assessment of the metal ion - Caladium bicolor biomass system indicates the feasibility and spontaneous nature of the process.
The presence of Pb2+ and Cd2+ and other heavy metals in the environment has become a major threat to plant, animal and human life due to their bio accumulating tendency and toxicity and therefore must be removed from municipal and industrial effluents before discharge. Resultantly, it is necessary that there are technologies for controlling the concentrations of these metals in aqueous emissions.
The conventional technologies for effluent treatment are not economically feasible for small-scale industries prevalent in developing economies due to huge capital investment. It is therefore necessary to search for alternative adsorbents, which are low-cost, often naturally occurring products that have good sorbent properties and no value to the people. A range of products has been examined. Few of 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), Medicago sativa (Alfalfa) (Gardea-Torresdey, et al. 1998) and Spagnum Moss Peat, (Ho et al. 1995).
The adsorbent used in this study is Caladium bicolor (Wild Cocoyam) Biomass. The C. bicolor is a tuberous perennial plant with brightly coloured foliage in warm, shady areas. Eating of the corm produces an intense irritation in the throat, which makes the corm inedible. The gainful uses of the corm of this crop will not only bring about the practical exploitation of this inedible abundant natural resource, which is wild and available; but also will encourage local farmers and boost their economies. In addition, the use of the biomass as a biosorbent for trace metals in water and waste effluents will solve environmental problems.
The equilibrium distribution of metal ions between the sorbent and the solution is important in determining the maximum sorption capacity. Several isotherm models are available to describe this equilibrium sorption distribution. Three sorption models (the Langmuir, Freundlich and Flory-Huggins) will be used to assess the different isotherms and their ability to correlate experimental data. The Langmuir equation was chosen for the estimation of maximum adsorption capacity corresponding to complete monolayer coverage on the biomass surface. The Freundlich model was chosen to estimate the adsorption intensity of the sorbent towards the biomass. The Flory-Huggins model was chosen to estimate the degree of surface coverage characteristic of the biomass. The principal aim of the present work is to investigate the potential use of the biomass of Caladium bicolor (Wild Cocoyam) as a novel biosorbent for the sorption of valuable and toxic metal ions from aqueous media.
This paper reports the effect of pH on the ability of the biomass of the corm of Caladium bicolor (Wild Cocoyam) to remove Pb2+ and Cd2+ ions from single metal ion solutions. The effect of varying initial metal ion concentration, initial pH and equilibrium sorption on the potential ability of the C. bicolor in removing these two metal ions from solution was investigated.
Caladium bicolor (Wild Cocoyam) plant used in this study was
harvested in Choba -
All reagents used were of analytical grade. The proximate composition of the C. bicolor biomass was determined using the Association of Official Analytical Chemist (AOAC) (1990).
obtain the pure cellulose, the non-cellulose materials in the C.
bicolor biomass were removed by extracting with an aqueous solution
of sodium hydroxide.
drying method was used in the determination of moisture content of
where Wo = loss in weight (g) on drying and Wi = initial weight of sample (g).
ash content was determined using the ignition method. The crucibles
used were thoroughly washed and pre-heated in a muffle furnace to
where Ma = Mass of ash (g) and Ms = Mass of sample used (g).
of crude protein was done by first all determining the total organic
nitrogen using the macro-Kjeldhal method. One gram of the finely divided
biomass was weighed in triplicate and placed in digestion flasks.
Few granules of anti-bumps and
where TV = Titre value, NE = mg nitrogen equivalent to molarity of acid, TVd = total volume to which digest was diluted, Ms = mass of sample (g) and Vd = volume of digest distilled.
% crude protein = % TON x 6.25 (4)
(6.25 is a general factor suitable for products in which the proportions of specific proteins are not well defined).
of crude lipid content of the biomass was done using the direct solvent
extraction method. The solvent used was petroleum ether (boiling range
where Mex = mass of extract (g) and Ms = Mass of sample used (g).
Total carbohydrate content of the biomass was estimated by 'difference'. In this method, the sum of the percentages of all the other proximate components was subtracted from 100 i.e. total carbohydrate (%) = 100 - (% moisture + % crude protein + % crude lipid + % ash).
method described by Munro and Bassir (1969) was
used for the determination of oxalate in the samples. In this method,
oxalate estimation, 5.0 cm3 of each extract was made alkaline
with 1.0 cm3 of 5.0 N NH4OH. This was then made
acid to phenolphthalein (2 or 3 drops of this indicator added, excess
decolourizes solution) by drop wise addition of glacial acetic acid.
1.0 cm3 of 5% calcium chloride was added and the mixture
allowed to stand for 3 hrs and centrifuged at 3000 x g for 15 min.
The supernatant was discarded and precipitates washed thrice with
hot water with thorough mixing and centrifuging each time. 2.0 cm3
where w = mass of oxalate in 100 cm3 of extract.
was estimated by the method of Burns (1971).
where C (mg) = concentration from standard curve; Vex = extract volume (cm3), A = aliquot (cm3) and Ms = mass of sample (mg).
surface properties were tested using three media with different pH
conditions to activate the biomass.
The surface properties of the activated and pure biomasses of the C. bicolor pellets were then determined.
surface characteristics of the C. bicolor biomass samples were
determined (Miguel et al. 2001; Horsfall
and Abia, 2003) as follows: The surface area of the biomass was
measured by the Brunauer-Emmett-Teller (BET) Nitrogen adsorption technique
using a Quantasorb surface area analyzer (Model - 05). The porosity
and particle density were determined by mercury intrusion porosimeter
(Micrometrics model-9310) and specific gravity bottle respectively.
Pore volume was obtained as the inverse relation of particle density,
while the cation exchange capacity (CEC) of the biomass samples were
determined by the ammonium acetate saturation procedure. In this method,
where CEC = cation exchange capacity and SA = surface area.
The Pb2+ content at each pH were determined with a Buck Scientific Flame Atomic Absorption Spectrometer (FAAS) model 300A. Analytical grade standards were used to calibrate the instrument, which was checked periodically throughout the analysis for instrument's response. The batch experiments were performed in triplicates and the averages were computed for each set of values.
where qe = metal concentration on the biomass (mg/g biomass) at equilibrium;
Ce = metal concentration in solution (mg/dm3) at equilibrium;
Co = initial metal concentration in solution (mg/ dm3);
ν = volume of initial metal solution used (dm3);
m = mass of biomass used (g).
The sorption data was tested against the Langmuir, Freundlich and Flory-Huggins adsorption isotherm models. The linear form of the Langmuir model as shown below was used.
where Xm (mg g-1) is the maximum sorption upon complete saturation of the biomass surface and K (dm3 g-1) is a constant related to the adsorption/desorption energy. The constants were obtained from the slope and intercepts of the plots of Ce/qe against Ce.
Plotting lnCe against lnqe using the linear equation below tested the Freundlich model:
where; qe = the adsorption density (mg/g ); Ce = Conc. of metal ion in solution at equilibrium (mg/ dm3); KL and n are the Freundlich constants determined from the slope and intercept. The value of n indicates the affinity of the sorbent towards the biomass.
The Florry-Huggins model is represented by equation 12.
where θ is the degree of surface coverage, n is the number of metal ions occupying sorption sites, Ka is equilibrium constant of adsorption and C is equilibrium metal ion concentrations. A plot of against yielding a straight line was made to confirm the model.
Means and standard deviations were calculated for three independent determinations for each variable except for total carbohydrate, which was simply obtained by difference.
The proximate compositions of the corm of C. bicolor based on the chemical analysis are shown in Table 1. The data showed that, the corm of C. bicolor (Wild Cocoyam) is a complex material containing mainly organic residues made of several polar functional groups. These groups in biomaterials can be involved in chemical bonding and are responsible for the cation exchange capacity. Exchange sorption between the biomass and the metal ions reaction may be represented in two ways as shown in the following equation:
2B - + Mn2+ = Mn (B)2
2HB + Mn2+ = Mn(B)2 + 2H+
where B - and B are polar sites on the biomass surface.
Such reactions are highly pH-dependent. According to Gardea-Torresday (1996), pH-dependent binding suggests that metal ions are sorbed by biomass through carboxyl, carbonyl or hydroxyl ligands. The addition of NaOH to raise the pH before equilibration enhanced sorption of metal ions by the biomass, suggesting that metal ion binding was occurring as carboxyl or hydoxyl ligands.
The effect of surface characteristics of the sorption of Pb2+ and Cd2+ ions from aqueous solution was correlated to the pH of the solution, as this affects the surface charge of the adsorbents, the degree of ionization and the species of the adsorbate. The effects of pH treatment on the C. bicolor biomass's basic properties were determined to assess the extent of incorporation of surface charges on the biomass matrix and its attendant effect on the sorption of metal ions. The influence of pH treatment on the surface properties of the pure inactivated C. bicolor biomass was investigated using three pH treatment conditions. They are pH 7.0, pH 3 and pH 8. The influence on the surface properties was evaluated by measuring the surface area, particle density, porosity, pore volume, cation exchange capacity (CEC) and surface charge density (SCD) (Table 2). The results showed that treatment of the pure biomass with either acid or base presents the biomass with larger surface and enhanced the porosity on the biomass surface. However, the pore volume was observed to decrease when the biomass was treated with either acid or base, indicating that pH treatment enhances the occurrence of physisorption so that cationic reaction may take place on the biomass surface. pH treatment of the biomass increases the cation exchange capacity (CEC) of the biomass. This enhanced CEC indicates improvement of the biomass surface towards the sorption of cationic and anionic species in solution. Furthermore, the surface-charge density, (SCD), which gives an overall intensity of charges on the solid matrix surface was evaluated and was observed to increase with pH treatment. This is an indication of an increase in the number of surface charges as the biomass is being treated with acid or base. Increase in SCD is an indication of enhanced sorption sites as a result of greater surface charges, indicating ion exchange process between the biomass and the metal ion. Investigations (Bernal and Lopez-Real, 1993; Horsfall and Abia, 2003) have shown that ion - exchange mechanism prevails in high SCD materials during the sorption of cationic species on natural and acid modified biomaterials.
The data further showed that acid treated biomass had enhanced surface properties than base treated biomass. This differential behaviour in acidic and basic treatment may be an artefact of the presence of high affinity ligands in the biomass such as cyano and amino groups on the biomass surface. At high pH, metal ion may be forced to bind to low affinity ligands such as hydoxyl and carboxyl groups but at low pH, the binding may occur through high affinity ligands only. This is an indication that the degree of ionization on the biomass surface is pH dependent. Hence, complexation and ion exchange processes are both playing a part in the sorption mechanism.
The efficiency of metal ion sorption by the biomass is controlled by the initial pH of the reaction mixture. Measurements of the equilibrium pH showed that the reactions that occurred during the sorption process caused slight increases in the pH. Table 3 shows that as the initial metal ion concentration (Co) increases, the resultant equilibrium final pH (pHfin) decreases for all concentrations. The more metal ion added, the greater the effect.
The simple hydrolysis of most divalent metal ions especially Pb2+ and Cd2+ can be written as follows (Cotton and Wilkinson, 1979)
M2+ (aq) + H2O (l) M(OH)+ (aq) + H+ (aq)
where M2+ is either Pb2+ or Cd2+.
This reaction generates monovalent cations M(OH)+ and protons which contribute to the increased acidity of M2+ solutions. If M2+ is being taken up by the biomass, the reaction above shifts to the left, leading to the depletion of protons and hence a rise in pH. In contrast, if M(OH)+ species sorbs onto the biomass, the above reaction naturally shifts to the right and the solution becomes more acidic. The pHmax was used to predict the predominant metal ion species taken up by the biomass. The pHmax for lead ions was found to be 7.0, indicative that approximately 50% of the overall lead ion content in the system will be in the form Pb(OH)+ while the other 50% remains as Pb2+, indicating that the predominant sorbing form will be the two species Pb(OH)+ and Pb2+ present in solution. This is an indication of quantitative removal of lead ions by C. bicolor biomass. It also shows an interaction of lead ions with the biomass by complexation in addition to ion exchange. The pHmax for cadmium ion is on the acidic side (pH = 5.0), indicating that the predominant form adsorbing onto the biomass is Cd(OH)+ releasing more protons into the solution. It is thought that the presence of a relatively high concentration of hydrogen ions in the medium would influence the exchange of hydrogen ions on the substrate. The observed reduction in the levels of Cd2+ ions removed from solution by the biomass indicates that the interaction between the cadmium ion and the biomass is an ion exchange process.
dependence of sorption of Pb2+ and Cd2+ onto
C. bicolor biomass on pH is shown in Figure
1A and 1B respectively. Using 10.0 mg/L initial metal ion concentration,
the data revealed that at pH 2, no significant sorption of both metal
ions was observed. Only 10.1% of Pb2+ and 9.5% of Cd2+
were sorbed by the biomass. As pH increased to 3, the sorption
of Pb2+ and Cd2+ also increased to 19.4% and
11.9% respectively. The biomaterial sorbed approximately 48.3% and
40.5% of Pb2+ and Cd2+ at pH 4. At pH
This trend in pH-dependence suggests that the removal of both metal ions is favoured by high pH values and lower metal ion concentrations. With low pH, the metal ions bound to C. bicolor biomass could be desorbed and the spent biomass regenerated.
The binding behaviour suggests that to some extent carboxyl groups (-COOH) may be responsible for the binding of Pb2+ and Cd2+, since the ionization constants for a number of carboxyl groups range between 4 and 5. At lower pHs, the carboxyl groups retain their protons reducing the probability of binding to any positively charged ions. Whereas at higher pHs (above 4.0), the carboxylate (-COO-) ligands attract the positively charged Pb2+ and Cd2+ and binding occurs, indicating that the binding process is an ion-exchange mechanism that involves an electrostatic interaction between the negatively charged groups in the cell walls and the metallic cations. According to Low et al. (1995), at low pH values the surface of the adsorbent, would be closely associated with hydronium ions (H3O+) which hinder the access of the metal ions to the surface functional groups. Consequently, the percentage of metal ion removal may decrease at low pH.
The applicability of sorption processes as a unit operation can be evaluated using isotherm models. The equilibrium sorption data obtained were analysed in terms of the Langmuir, Freundlich and Florry-Huggins equations. Figure 2A and B shows the data linearised to fit the Langmuir equation and Table 4 presents the constants derived by regression analysis for the equation. A comparison of the Langmuir monolayer sorption capacity at various initial pH was made by plotting qmax (mg/g) against pHin (Figure 3). The results confirmed that at pH 2 and 3, the sorption of Pb2+ and Cd2+ on C. bicolor biomass was much less than at pH 4 and above, indicating that the C. bicolor biomass is a less effective sorbent at lower pH values. Hence, lowering the pH may aid the regeneration and recycling of spent biomass. The R2 values suggested that the Langmuir isotherm provides a good model of the sorption system. The sorption coefficient, KL, which is related to the apparent energy of sorption, was greater for Pb2+ than for Cd2+ at all pHin. The high values of KL for Pb2+ and Cd2+ at pH 7 and 5 respectively, indicate that the energy of sorption is more favourable at these pH maxima than other pH values.
The linear Freundlich isotherm plots for the sorption of the two divalent metals (Pb2+ and Cd2+) onto the C. bicolor biomass are presented in Figure 4A and 4B. Examination of the plot suggests that the Freundlich isotherm is also an appropriate model for the sorption study of Pb2+ and Cd2+. Table 5 shows the linear Freundlich sorption isotherm constants and the coefficients of determination (R2). Values of 1/n for Pb2+ and Cd2+ were found to be less than unity at initial pH of 5, 6, 7, 8 and 4, 5, 6 respectively, indicating that significant sorption could take place at these initial pH values. Confirmation of the pHmax was made by plotting the Freundlich equation parameter, , which is a measure of the sorption intensity against pHin (Figure 5). The plot further confirmed that, the pH maxima for the sorption of Pb2+ and Cd2+ onto C. bicolor biomass are 7.0 and 5.0 respectively. Based on the coefficient of determination (R2) values, the linear form of the Freundlich isotherm appears to produce a reasonable model for the sorption of the two metals, with Pb2+ fitting the data better than Cd2+. The K-values suggests that Pb2+ has greater sorption tendency towards the C. bicolor biomass in neutral solutions. However, for Cd2+ the greater affinity towards active groups on the biomaterials was observed at pH 5.0.
To account for the sorption surface behaviour of the metal ions on the C. bicolor biomass, the fraction of biomass surface covered by metal ion was also studied at the different pH values using the Flory-Huggins isotherm. The plot of against was made (Figure 6A and B) and regression lines were obtained. The data shows that, Flory-Huggins adsorption is obeyed at higher pH. The Flory-Huggins model was chosen to estimate the degree of surface coverage on the C. bicolor biomass. The number of metal ions occupying sorption sites, n, was plotted against the pHin (Figure 7). The overall coverage process indicate that, increase in initial pH increases the surface coverage on the biomass until the surface is nearly fully covered with a monomolecular layer at pH 5.0 for Cd2+ and 7.0 for Pb2+.
Furthermore, the equilibrium constant, Ka, obtained from the Flory-Huggins isotherm was used to compute the apparent Gibbs free energy of change. The apparent Gibbs free energy of sorption, ΔGo, is the fundamental criterion of spontaneity. Reactions occur spontaneously at a given temperature if ΔGo is negative quantity. ΔGo (KJ mol-1 K-1) was evaluated using the following equation:
where R is the universal gas constant, 8.314 J/mol K and T is absolute temperature.
The negative values of ΔGo (Table 6) confirm the feasibility of the process and the spontaneous nature of sorption with high preference at pH values of 7.0 and 5.0 for Pb2+ and Cd2+ respectively.
In conclusion, the data has shown that, the sorption process of Pb2+ and Cd2+ ions on to C. bicolor biomass is feasible and spontaneous in nature. The metal ions binding capacity of the biomass was shown as a function of initial pH of the aqueous solution. The equilibrium data fitted the Langmuir, Freundlich and the Flory-Huggins isotherms very well. The three models confirmed that maximum sorption capacity onto the biomass occurred at pH 7.0 for Pb2+ and pH 5.0 for Cd2+. Thus efficient removal of the two metal ions in an effluent using C. bicolor biomass may require pH adjustment between 7.0 and 5.0. The equilibrium sorption of the two metal ions at their pHmax was determined from the Langmuir equation and found to be 88.50 mg/g and 65.50 mg/g for Pb2+ and Cd2+ respectively. The data showed that, C. bicolor is a successful biosorbent for treating heavy metal contaminated wastewater and may serve as an alternative adsorbent to conventional means. Hence, not only is C. bicolor readily available, it also has the potential for metal removal and recovery from contaminated waters. This process will be environment friendly and convert the non-useful plant into an economic crop for local farmers. It may also provide an affordable technology for small and medium-scale industries. The pH study is particularly important for determining the ligands that may be involved in the reaction for optimization in the design of sorption process units.
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