December 28, 2007
Biosorption of Lead From Industrial Wastewater Using Chrysophyllum Albidum Seed Shell
By Amuda, Omotayo Sarafadeen Ojo, Olumuyiwa Idowu; Edewor, Theresa Ibibia
ABSTRACT The shell of the seed of Chrysophyllum albidum carbon was used to adsorb lead (Pb) from aqueous solution, the sorption process with respect to its equilibria and kinetics as well as the effects of pH, contact time, adsorbent mass, adsorbate concentration, and particle size on adsorption were also studied. The most effective pH range was found to be between 4.5 and 5 for the sorption of the metal ion. The first-order rate equation by Lagergren was tested on the kinetic data and the adsorption process followed first-order rate kinetics. Isotherm data were analyzed for possible agreement with the Langmuir and Freundlich adsorption isodierms; the Freundlich and Langmuir models for dynamics of metal ion uptake proposed in this work fitted the experimental data reasonably well. However, equilibrium sorption data were better represented by Langmuir model man Freundlich. The adsorption capacity calculated from Langmuir isotherm was 72.1 mg Pb (II) g^sup -1^ at initial pH of 5.0 at 30[degrees]C for the particle size of 1.00 to 1.25 mm with the use of 2.0 g/100 ml adsorbent mass. The structural features of the adsorbent were characterized by Fourier transform infrared (FTIR) spectrometry; the presence of hydroxyl, carbonyl, amide, and phosphate groups confirms the potential mechanism adsorption of the adsorbent. This readily available adsorbent is efficient in the uptake of Pb (II) ion in aqueous solution, thus, it could be an excellent alternative for the removal of heavy metals and organic matter from water and wastewater.
Industrial and mining wastewaters are major sources of pollution by heavy metals. These industrial activities can generate a considerable pollutional stress in the ecosystem. Industrial wastewaters in the developing countries, such as Nigeria, receive inadequate or, in some cases, no treatment before discharge into waterways. This is partly due to the nonexistence of stringent law as regards the discharge of such wastewater. It is only recendy that Nigerian government showed concern when Federal Environmental Protection Agency (FEPA) was established in 1989.
Heavy metals can pose health hazards to man and aquatic lives if their concentrations exceed allowable limits. Concentrations of heavy metals below these limits even have potential for long-term contamination, because heavy metals are known to be accumulative within biological systems . Lead contamination of the environment is primarily due to andiropogenic activities, making it the most ubiquitous toxic metal in the environment . Lead readily bioaccumulates in the humus-rich surface layer of the soils due to its complexation with organic matter and it was reported  to be the least mobile heavy metal in soils under reducing and nonreducing conditions. Lead has been implicated to be the most common heavy metal contaminants in urban soils due to atmospheric deposition from automobile emission and industries.
Adsorption has advantages over other methods of remediation of heavy metals from wastewater because its design is simple; it is sludge-free and can be of low capital intensive . Activated carbon has been reported to have high and fast adsorption capacities  due to its well-developed porous structure and tremendous surface area. The commonly used procedures for removing metal ions from effluents include chemical precipitation , coagulation/ flocculation [6-8], and ion exchange . These techniques, which have been reported to be expensive, also have disadvantages such as incomplete metal removal, high reagent and energy requirements, and generation of toxic sludge or odier waste products that require proper disposal. Thus, preparation and use of environmentally friendly materials for metal remediation is desirable.
Agricultural waste products such as the seed of Chrysopbyllum albidum (Star apple) originated from Brazil and are found in some parts of South America and Europe. The seed, when prepared into adsorbents, is one of the most effective and environment friendly low-cost materials for treatment of wastewater-containing heavy metals. Various agricultural products, such as coconut shell [10- 12], rice husk , groundnut husk , cassava peels , pecan shells , and tea wastes , have been reported to be effective in the remediation of heavy metals and organic matter in wastewaters. In this study, the shell of seeds of Chrysophyllum albidum was employed for the removal of Pb (II) ion from wastewater. The characteristics of the prepared adsorbent suggest its potential as a biosorbent. This paper reports for the first time, as confirmed by the results of thorough and comprehensive literature search, the preparation of the shell of seeds of Chrysophyllum albidum into activated carbon and its subsequent use for the removal of Pb ion from wastewater.
MATERIALS AND METHODS
Preparation of Adsorbent
The seeds of Chrysophylum albidum were picked at various locations in Ogbomoso town, Oyo State, Nigeria. Debris and stones were separated from the seeds by hand-picking. The seeds were washed and dried on laboratory bench for 72 h. Shells were removed, ground, and dried in an oven (Memmert Model OV-160, England) at 105[degrees]C to constant weight Chrysophyllum albidum seed shells were pyrolyzed in a furnace (Carbolite, CTE 12/75). During pyrolysis, nitrogen gas at a flow rate 0.1 m^sup 3^/h was used as purge gas. The furnace temperature of 500[degrees]C was maintained for 2 h. The weight before and after pyrolysis gave the weight loss of the sample. The pyrolyzed sample was crushed into powder form. The adsorbent was activated by soaking in excess 0.3 M HNO^sub 3^ for 24 h. This was followed by washing the activated adsorbent thoroughly with distilled-deionized water until the solution pH was stable at 7.02 +- 0.1. It was soaked in 27% NzHCO^sub 3^(w/v) until any residual acid has been removed. It was men dried overnight in an oven (Memmert, England) at 105[degrees]C, cooled at room temperature, and stored in desiccators until ready for use. The activation process removes any debris or soluble biomolecules that might interact with metal ions, and also, all biogenic metal ions in the adsorbent .
Surface Characterization of Adsorbent
The parameter that denote the accessibility of these pores is on the BET surface areas (SBET) of the adsorbent determined from N^sub 2^ adsorption isotherms, measured at 77 K with a Coulter Omnisorp 100 CX apparatus . The cation-exchange capacities (CEC) were determined by the Kjeldhal method. Table 1 shows some characteristics of the new adsorbent prepared from die shell of seed of Chrysophyllum albidum.
Different adsorbent dosages, ranging between 0 and 3 g having different particle sizes of 0.63, 0.80, 1.00, 1.25, 1.50 and 31.60 mm was added to 100 ml of aqueous solution containing Pb of varying concentrations, 100 to 1000 mg/L, in different 250-ml Erlenmeyer flasks capacity flasks. The pH of the solution in the flasks was varied between 1 and 10 using either 0.1 N NaOH and/or 0.1 N H^sub 2^SO^sub 4^.
The use of Fourier transform infrared (FTIR) spectrometer to determine the functional groups on adsorbents have been reported [5, 20]. In order to study the functional groups of the adsorbent, an IR analysis was performed with a FTIR spectrometer (Nicolet Avator 330, England). One milligram of dried, finely divided adsorbent was mixed with 200 mg of KBr and pressed using hydraulic press and mould. The mixture obtained was immediately analyzed with a spectrophotometer in the range 3500 to 500 cm^sup -1^ with a resolution of 1 cm^sup - 1^. The influence of atmospheric water and CO2 were always subtracted.
Batch Adsorption Studies
Batch experiments were conducted at ambient temperature using the optimum conditions of all factors that influence adsorption such as pH, adsorbent mass, contact time, and initial ion concentration. Wastewater containing varying concentrations of Pb prepared from the stock solution (1000 mg/L) was placed in series of 250-ml Erlenmeyer flasks. The pH of the solutions was adjusted by using 0.1 N NaOH and/ or 0.1 N H^sub 2^SO^sub 4^. The system was mechanically agitated at 150 rpm using a Teflon-coated half-inch bar on a Coming magnetic stirrer. The Pb (II) ion concentration of the treated wastewater was analyzed at time intervals between 0 and 100 min using standard methods recommended for examination of water and wastewater . Subsequent adsorption experiments were carried out by using optimized parameters. Adsorption isotherms were generated from reaction mixtures consisting of 2 g/100 ml of the adsorbents and 100 ml of Pb solution of varying concentrations (100 to 1000 mg/L). Concentration of the adsorbed Pb was determined spectrophotometrically by using standard mediods .
Two different controls were carried out, one is the control without adsorbent, which determined if the metal ion was adsorbed by the wall of the reaction flasks, and the other is the control without metal ion (distilled-deionized water was used instead of metal solution), which was to determine any leaching from adsorbents during the study period . Residual metal ion concentrations in the filtrate (after adsorption) was measured by atomic absorption spectrophotometry (Model 9100X, Philips, England) using an air- acetylene flame and single element hollow cathode lamp. The instrument was calibrated with standard solution of 1 to 20 mg/L Pb (II) ion. Lead absorbances were measured at 217.0 nm. The amount of Pb (II) ion adsorbed 'q'^sub t^ (mg/L) at time 't' was calculated by using the following equation:
where C^sub i^ and C^sub f^ are the Pb (II) concentrations in mg/ L initially and at given time, t, respectively.
The percentage of Pb (II) removed (R^sub Pb(II)^ %) from solution was calculated by using the following equation:
All the experiments were duplicated to ensure accuracy, reliability and reproducibility of the collected data. Relative error did not exceed 1%.
RESULTS AND DISCUSSION Effects of pH
The experiments carried out at different pH (1 to 10) show that the removal of Pb (II) ion from aqueous solution by the adsorbent is dependent on the pH of solution. The pH is related to the surface charge of the adsorbent and the degree of ionization. Figure 1 shows the effect of pH as function of percent removal of the metal ion. The effect of pH was carried out at the predetermined initial ion concentration (800 mg/L), adsorbent dose (2 g/100 ml), contact time (60 min), and particles size (1.00 to 1.25 mm) and by varying pH values from 1 to 10 using dilute NaOH or H^sub 2^SO^sub 4^ solution as appropriate. The uptake of metal ions from aqueous solutions by cellulose materials is usually accompanied by reduction in the pH of the metal ion solution . This is generally believed to be due to the exchange of the hydrogen ion on the surface of the adsorbent by metal ions. The extent of hydrogen ion exchange would depend on the relative concentration of the exchangeable hydrogen and the hydrogen ion concentration (pH) of the medium.
It can be seen from Figure 1 that removal efficiency of Pb increases initially from pH 1 to 5. Above pH value of 5, the removal efficiency of the Pb decreased. This may be due to precipitation of lead at too high pH. As the pH increases mere is increasing trend in concentration of hydroxide ion (OH^sup -^) in solution, thereby causing shift in the equilibrium. Hence, the system adjusts to terminate mis effect (Le Chatelier principle) by precipitation of hydroxide out of the solution. At low pH value, there was reduction in the removal efficiency of the metal ion. This may be as a result of the presence of hydronium ions (H^sub 3^O^sup +^) on the surface of the adsorbent, which caused repulsion of metal ion to the surface functional groups and consequent reduction in the removal efficiency of the metal ion by the adsorbent.
Effects of Adsorbent Mass on Metal Sorption
The quantity of adsorbent is a significant factor to be considered for effective adsorption. The performance of the adsorbent was evaluated for the percentage removal of Pb. The dose of the adsorbent was varied between 1.0 and 3.0 g/100 mL. Percentage lead removal increased significantly as the amount of adsorbent added increased (Figure 2). Adsorbent dosage of (2.0 g/100 mL) was required to remove 99% of Pb in aqueous solution. This is expected because as the dose of adsorbent increased, there was increase in the available exchangeable sites for the Pb. At 2.0 g/100 mL dose of the adsorbent, the maximum adsorption set in and, hence, the concentration of free ions remained constant even with further increase in the dose of the adsorbent.
Effects of Contact Time on Metal Sorption by the Adsorbent
Equilibrium time is another important operational parameter for an economical wastewater treatment process. Figure 3 presents percentage removal of lead as function of contact time. The figure shows that the sorption process was rapid for the first 30 min. This was attributed to the instantaneous utilization of the most readily available sorbing sites on the adsorbent surface. Increased contact time increased percentage removal of lead until equilibrium adsorption was established.
Equilibrium adsorption was established within 60 min. The kinetic data was fitted to the Lagergren equation ,
X= the amount of solute, Pb (II) ion, (mg/L) removed at time t, X^sub e^ = amount removed at equilibrium and Kajs = the rate constant of adsorption (min^sup -1^). The linear plot of Log (X^sub e^-X) versus t shows the applicability of the above equation for metal sorption by me adsorbent (Figure 4). The rate constant value (K^sub ads^) calculated from the slope of the plot was 1.981 x 10^sup -2^ min^sub -1^. The regression coefficient (R^sup 2^) was 0.9875. Other parameters that determine the sorption rate, such as agitation rate in the aqueous phase, adsorbent structural properties, adsorbate properties (e.g. hydrated ionic radicals), initial concentration of ion, and chelate formation rate, may also have played a role in this study .
Effects of Initial Ion Concentration on Sorption by the Adsorbent
The percentage removal of Pb (II) ion by the adsorbent initially increased rapidly with increasing Pb concentration and slowed down when Pb concentrations reached 800 mg L^sup -1^ (Figure 5). At lower concentrations, Pb ions in the solution would interact with the binding sites and thus facilitated 100% adsorption. At higher concentrations, more Pb ions are left unadsorbed in solution due to the saturation of binding sites. Maximum removal (97%) of Pb ion was achieved when 800 mg L^sup -1^ Pb (II) ion concentrations was adsorbed with 2.0 g/100 ml. In this study, the concentration of Pb that will attain high percent removal of Pb was 800 mg L^sup -1^ at 60 min of treatment.
Effects of Particle Size of the Adsorbent on Sorption of Metal
The adsorption experiments were carried out using various particle sizes of the adsorbent such as 0.63,0.80, 1.00,125,1.50,1.60 mm. The percentage removal ofPb ion at different particle sizes showed diat die percentage removal rate increased with a decrease in particle size (Figure 6). The higher adsorption level achieved by smaller particle size of the adsorbent may not be unconnected to the fact that smaller particles give large surface areas. There is a tendency that smaller particles produce shorter time to equilibrate. Particle sizes of 1.00 and 1.25 were found to yield highest percentage removal of Pb (II) ion.
Adsorption isotherm models are widely employed to present the amount of solute adsorbed per unit of adsorbent, as a function of equilibrium concentration in bulk solution at constant temperature. The equilibrium data obtained from Pb sorption capacity of the adsorbent were fitted to Langmuir  and Freundlich  isotherms.
The Langmuir Isotherm
The Langmuir isotherm represents the equilibrium distribution of metal ions between the solid and liquid phases. The following equation can be used to model the adsorption isotherm.
where q is milligrams of metal accumulated per gram of the adsorbent material; C^sub eq^ is the metal residue concentration in solution; q^sub max^ is the maximum specific uptake corresponding to the site saturation; and b is the ratio of adsorption and adsorption rates. The Langmuir isotherm is based on the following assumptions: (a) metal ions are chemically adsorbed at a fixed number of well defined sites; (b) each site can hold one ion; (c) all sites are energetically equivalent and; there is no interaction between the ions .
When the initial metal concentration rises, adsorption increases while the binding sites are not saturated. The linearized Langmuir isotherm allows the calculation of adsorption capacities and the Langmuir constants. This is given by the following equation:
The linear plots of C^sup eq^/q versus C^sub eq^ show that adsorption followed the Langmuir model (Figure 7). The correlation coefficient is 0.997. q^sub max^ and b were determined from the slope and intercept of the plot, and were found to be 103.42 mg/g and 0.009 L/mg respectively (Table 2). The essential characteristics of the Langmuir isotherms can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, R^sub L^, which is defined as
Where b is the Langmuir constant and Co is the initial concentration of Pb (II) ion. The R^sup L^ value indicates the shape of isodierm, in which case R^sup L^ value is unity. According to Ahalya et al. , R^sub L^ values between 0 and 1 indicate favorable adsorption. In this study, the R^sup L^ values were found to be 0.9248 to 0.1483 for concentrations of 100 to 1000 mg/L Pb.
Effect of Temperature
The adsorption study was conducted on Pb at ambient temperature. The results were analyzed and it was found that the adsorption process was endothermic and best fitted to Langmuir model. From the linear plots of C^sup eq^/q versus C^sub eq^, q^sub max^b, and R^sup L^, values for different particle sizes of the adsorbent were calculated, and R^sup L^ values were between 0 and 1. This confirms that adsorption process fits Langmuir model.
The Gibbs free energy (DeltaG^sup 0^) for the adsorption process for the aqueous solution containing Pb was obtained from the following equation.
Enthalpy change (DeltaH^sup 0^) can be calculated by using the diermodynamic equation,
Equation 7 can be rewritten as:
The results of DeltaH^sup 0^, DeltaH^sup 0^, and DeltaS^sup 0^ are presented in Table 3. The negative values of DeltaG^sup 0^ at all temperatures indicate the spontaneous nature of the adsorption of Pb (II) ion on the adsorbent. The positive values of DeltaH^sup 0^ indicate that the adsorption is involved with weak forces of attraction. It was observed that the DeltaH^sup 0^ values increased with decrease of particle size, also the adsorption was found to be endodiermic. The DeltaS^sup 0^ values were positive, this shows the increased randomness at the solid/solution interface during the adsorption process, thus suggesting that Pb (II) ions replace some water molecules from the solution previously adsorbed on the surface of the adsorbent. The DeltaS^sup 0^ values increased as particle size decrease. This describes the new adsorbent as effective towards sorption of Pb. The Freundlich Isotherm
The Freundlich isotherm is represented by the equation:
where C^sub a^ is the amount adsorbed (mg/g) C^sub e^ is the equilibrium concentration and k and 1/n are empirical constants incorporating all parameters affecting the adsorption process, such as adsorption capacity and intensity respectively. The linearized form of Freundlich adsorption isotherm was used to evaluate the relationship between the concentration of Pb adsorbed by the adsorbent and Pb equilibrium concentration in wastewater, and is given as
Parameters k and 1/n are respectively equal to the intercept and slope of the plot of ln C^sub a^ versus In C^sub e^, and were found to be 98.16 and 0.53, respectively (Table 3). According to Amuda and Ibrahim , a larger value of k indicates good adsorption efficiency for a particular adsorbent, while a larger value of 1/n indicates a larger change in effectiveness of adsorbent over different equilibrium concentrations. The correlation coefficient is 0.943 (Figure 8). In this study, Langmuir isotherm had a better fitting model than Freundlich because the former has higher correlation coefficient man the later (Langmuir, R^sup 2^ = 0.997; Freundlich, R^sup 2^ = 0.943). Table 4 shows a comparison of the Langmuir sorption capacities of Pb to some agricultural by-products carbons and adsorbent employed in this study.
Infrared Spectral Analysis
FTIR analysis of the new adsorbent was performed, and the percentage transmission for various wave numbers is presented in Figure 9. The FTIR spectra of the adsorbent, showed the presence of amine R-NH2 (amino acids, protein, glycoprotein etc), carboxylic acids (fatty acids, lipopoly saccharides, etc.), and phosphates. The characteristic absorption bands of hydroxyl and amine were identified at 3427.61 cm^sup -1^, indicating the presence of exchangeable OH^sup -^ and NH^sup +^ group, respectively; alkyl chain at 2926.06 cm^sup -1^; amide and phosphate groups at 1637.14 cm^sup -1^ and between 1197.59 and 1124.75 cm^sup -1^, respectively; and P-O vibration of C-PO^sup 2-^^sub 3^ moiety at 1041.32 cm^sup - 1^. The absorption bands identified in the spectra and their assignment to the corresponding functional groups in the adsorbent could enhance the surfaces on which adsorption would take place. Table 5 listed die absorption bands identified in the spectra and dieir assignment to the corresponding functional groups in the adsorbent.
Based on die present study, it can be concluded diat die use of chemically activated low-cost agricultural products, such as shells of die seed of ChrysophyUum albidum have potential application in the sequester of Pb (II) ion in wastewater. The optimum conditions of Pb (II) ion uptake by die adsorbent were: initial concentration of 800mg/L, pH value of 5, adsorbent mass of 2.0 g/100 mL, contact time of 60 min, and particle size of 1.00 to \25 mm. Langmuir isotherm has better fitting model dian Freundlich because die former has a higher correlation coefficient man the latter.
Langmuir capacity was used to compare the efficiency of the new adsorbent with odier adsorbents reported elsewhere for die adsorption of lead ion; die adsorbent derived from the shell of die seed of ChrysophyUum albidum had a capacity than other adsorbents. These preliminary studies suggest diat adsorbent prepared from shell of ChrysophyUum albidum seed can be used effectively for the adsorption of heavy metals. Cost analysis for die preparation of activated carbon of shells of die seed of ChrysophyUum albidum has not been performed. The seed of ChrysophyUum albidum is available abundandy and can be obtained for nominal price as agricultural by- product in the country.
The authors acknowledged Salami Taofik of the Central University Research Laboratory for the use of FTIR and AAS and Dr. I. O. Oladosu of Department of Chemistry, University of Ibadan, for the interpretation of IR spectra.
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Chemistry Unit, Department
of Pure and Applied Chemistry,
Ladoke Akintola University
Olumuyiwa Idowu Ojo
Department of Agricultural
Engineering, Ladoke Akintola
University of Technology,
Theresa Ibibia Edewor
Industrial Chemistry Unit,
Department of Pure
and Applied Chemistry,
Ladoke Akintola University
Address correspondence to Omotayo Sarafadeen Amuda, Analytical/ Environmental Chemistry Unit, Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology, Ogbomoso, 210001, Nigeria. E-mail: [email protected]; [email protected]
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