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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.50 n.4 Concepción dic. 2005

http://dx.doi.org/10.4067/S0717-97072005000400009 

 

J. Chil. Chem. Soc., 50, N° 4 (2005), págs: 691-696

 

RETENTION OF PB (II) ION FROM AQUEOUS SOLUTION BY NIPAH PALM (NYPA FRUTICANS WURMB) PETIOLE BIOMASS

 

1DONBEBE WANKASI, *2MICHAEL HORSFALL JNR AND 2AYEBAEMI I. SPIFF

1Department of Chemistry, Rivers State College of Education, Port Harcourt, Nigeria
2Department of Pure and Industrial Chemistry, University of Port Harcourt, Uniport P. O. Box 402, Choba, Port Harcourt, Nigeria.


ABSTRACT

The retention of Pb (II) from aqueous solution by pure (PB) biomass and thioglycolic acid (TGA) modified biomass (MB) of Nipah palm (Nypa fruticans Wurmb) petiole was studied using batch sorption technique. The equilibrium retention capacity of Pb (II) was determined from the Langmuir equation and found to be 15.06 mg g-1 and 17.60 mg g-1 for pure and modified biomass, indicating a small difference in Pb(II) retention after treatment with TGA. The data showed that chemisorption process could be the rate-limiting step in the retention mechanism. Studies on the retention of Pb (II) by Nipah palm petiole biomass are important because it may contribute in aiding the innovative removal and recovery of metal ions from contaminated industrial effluents.

Key words: Nipah palm, adsorption, waste management, heavy metals, bioremediation.


INTRODUCTION

This investigation is part of an on going extensive research works in our laboratories to study the feasibility of utilizing the abundant agricultural by-products and minimally used plant species that are causing environmental nuisance in Nigeria as adsorbents for the removal of toxic and valuable metals from single metal ion solutions and multiple metal ions solutions1, 2. Industrial and municipal wastewater frequently contains metal ions. These metal ions, when present in high quantity, can be harmful to aquatic life and human health. Therefore they must be removed before discharging effluents into the recipient ecosystem. A number of successful systems has been developed using sorption techniques3 -5. However, most of these techniques are expensive, selective to specific ions and sometimes ineffective, especially when the metals are present in solutions at very low concentrations6. Thus, the need for an economic and effective adsorbent for the removal of toxic and valuable metal ions from wastewater is highly imperative. Many studies have demonstrated that non-viable plant biomass can effectively remove toxic metals from solution7 -10. Nipah palm grows abundantly in the Niger Delta area of Nigeria and is presently causing great environmental nuisance by colonizing and destroying the more useful mangrove swamp forest. At the moment, no part of the Nipah palm has found any use in Nigeria. The principal aim of the present work is to report on the potential of Nipah palm (Nypa fruticans Wurmb) petiole and its derivative as low - cost adsorbent material for the sorption of valuable and toxic metal ions from aqueous system. This will add to the biosorption data bank, since at the moment, there is no information in this data bank on the adsorptive ability of Nipah palm (Nypa fruticans Wurmb) petiole.

EXPERIMENTAL

Biomass Preparation and Acid Treatment: Petioles from Nipah palm (Nypa fruticans Wurmb) were collected, cut into smaller bits, air-dried and oven dried at 1050C for 24h, ground and served through a 100µm mesh size to obtain finely divided biomass. The air - dried biomass was divided into two parts. The first portion was left untreated and designated the pure biomass (PB) while 1.00g of the other portion was acid treated with 250ml excess 1.00M thioglycolic acid solution and stirred for 4h at 300C while maintaining a pH of 7.1. This acid treated portion is designated modified biomass (MB). The degree of incorporation of thiol groups was further determined by reacting 0.5g of dried acid treated biomass with 20mL of iodine solution at pH 7.2 ± 0.2, followed by back titration of the unreacted iodine with standard thiosulphate solution and found to be 97.84%. For adequate comparison of the Pb(II) retention in both pure (PB) and acid treated biomass (MB), the amount of thiol group in PB was also determined and found to be significantly lower (12.14%) than that for MB.

pH effect determination: 200mg each of PB and MB of 100-µm mesh particle size samples were suspended in several flasks containing 50ml of 50 mg/l Pb(II) solution. The pH of these suspensions was adjusted to 2, 5, 8 and 10 in triplicate. The content of the flasks were reacted on a shaker for one hour at 30oC, centrifuged at 2800 rpm for 5 min and Pb(II) content determined in the supernatant. From contact time experiment, it was observed that one hour is adequate for effective sorption between biomass and metal ion in solution to occur.

Contact time effect determination: 200mg each of PB and MB of 100-µm mesh particle size samples were suspended in several flasks containing 50ml of 50 mg/l Pb(II) solution. The pH of this suspension was adjusted to 5.0 and the temperature maintained at 30°C using a thermostated a water bath. Triplicates of this suspension were made in flasks for each time internal of 5, 10, 20, 30, 40, 50 and 60 min. The test tubes were centrifuged and the supernatants discarded. The content of the flasks were reacted on a shaker for one hour, centrifuged at 2800 rpm for 5min and Pb(II) content determined in the supernatant.

Analysis of metal content: The Pb(II) in each experiment was determined with a Buck Scientific Flame Atomic Absorption Spectrometer (FAAS) model 200A at an analytical wavelength of 283.3nm after centrifuging at 2800 rpm. Analytical grade standard chemicals were used to calibrate the instrument, which was checked periodically throughout the analysis for instrument response.

The amount of Pb(II) retention on the PB and MB during the series of batch investigations were determined using a mass balance equation expressed as:

Where qe = Pb(II) concentration on the biomass (mg/g) at equilibrium; Ce = Pb(II) concentration in solution (mg/l) at equilibrium; Co = initial Pb(II) concentration in solution (mg/l) Vi = Volume of initial Pb(II) solution used (l); Mb = mass of biomass used (g) .

RESULTS AND DISCUSSION

pH Effect and Sorption Isotherm

The retention of Pb(II) from aqueous solution was related to the pH of the solution, as the later affects the surface charge of the adsorbents, the degree of ionization and the species of the adsorbate. The result as shown in Fig 1, indicated that the Pb(II) retention by the Nipah palm (Nypa fruticans Wurmb) petiole biomass increased as the pH of the solution increased from 2 to 6 and thereafter remain steady. The extent of this increase is further enhanced by acid modification. As the pH of the solution increase from 2.0 to 6.0, Pb(II) showed an increase in retention by the biomass with optimum retention occurring between pH 4.0 and 6.0.


Fig 1. A plot of percent Pb(II) retention by pure (PB) and TGA treated (MB) Nipah palm petiole as a function.

This binding behaviour suggests that to some extent hydroxyl (-OH) and carboxyl (-COOH) groups may be responsible for the retention of Pb(II), since the ionization constant for a number of these groups range between 4 and 5. It can also be seen that at low pH values, the retention of Pb(II) was not efficient, especially for pure biomass where the Pb(II) ions removal was less than 10%. According to Low and co-workers5, 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 decreasing the percentage retention of Pb(II) at low pH. Again at the alkaline pH, hydrolysis of most divalent metal ions occurs11 forming aquo complex ions with the general formula M(H2O)2+n, where n is the number of water molecules complexing the metal ion.

Where Mz+ is metal ion

McKay and co-workers12 reported that biomass/copper reaction may be represented in two ways (scheme I) and it seems that other divalent metal ions could follow a similar mechanism:

where B- and HB are polar binding sites on the biomass surface.

The solubility of a metal ion is an important factor that enables metal ions to penetrate into the porous structure of the biomass. Metal ion speciation studies in aqueous systems13 revealed the presence of Pb2+, PbOH+, Pb2OH3+, Pb(OH)-3 and [Pb6O(OH)6]4+ species of metals in the solution during solvolysis and the solution equilibra for the fluted pumpkin waste biomass with divalent metal could be represented as in scheme II:

In solution, these metal species are all potential sorbates but are pH dependent. However, the concentration of the species Pb2+, PbOH+, Pb2OH3+, Pb(OH)-3, and [Pb6O(OH)6]4+ are too small to affect the concentration of Pb2+ under the experimental condition of pH < 6 [13]. Thus, the batch retention experiments in this study were carried out at pH 5. At this pH, the predominant species responsible for the retention on the biomass is the Pb2+.

The experimental data of the Pb(II) retention on pure and acid treated biomass was tested against the Langmuir adsorption isotherm model in order to evaluate the sorption capacity. The linear form of the Langmuir adsorption isotherm is expressed as

where qc is the adsorption density at equilibrium solute concentration Ce mg of adsorbate per g of adsorbent), Ce is the concentration of adsorbate in solution (mg/l), Xm is the maximum adsorption capacity corresponding to complete monolayer coverage (mg of solute sorbed per g of adsorbent). K is the Langmuir constant (l of solution per mg of adsorbate) related to energy of adsorption.

The potential capacity of Nipah palm (Nypa fruticans Wurmb) petiole biomass in the retention of Pb(II) was determined by plotting Ce/qe against Ce. The Xm parameter (monolayer capacity) and the Langmuir constant "K" were obtained from the slope and intercept of the plot. The plot of the isotherm as shown in Fig 2 are seen to be linear over the concentrations ranges considered.


Fig 2. Langmuir isotherm plot for the retention of Pb(II) on pure (PB) and modified (MB) Nipah palm petiole biomass.

This is because at lower concentrations, the biomass in its pure and treated form, quicKLy sorb all or a large proportion of the available metal ions. This is an indication that the biomass may function well only at lower concentrations and that high initial metal ion concentrations may be a limiting factor in the utilization of Nipah palm (Nypa fruticans Wurmb) petiole biomass as adsorbent and may require two or more recycles for complete reduction of metal ion concentrations in aqueous system.

The coefficients of determination (r2) values (Table 1) indicate that acid treated biomass (r2 = 0.995) had slightly enhanced retention potential over pure (r2 = 0.924) biomass. The Langmuir constant, KL, for pure biomass (K = 3.56 x 10-1 l g-1) was slightly greater than the modified biomass (KL = 2.34 x 10-2 l g-1). This could mean that the energy of retention is less favourable for pure biomass, indicative that not all active sites were available for Pb(II) retention. The data showed that the PB has an potential retention capacity of 15.06 mg/g while TGA treated is 17.60 mg g-1. The small differences in Pb(II) retention capacities between PB and MB of Nipah palm (Nypa fruticans Wurmb) petiole may be related to the nature of oxygen in pure biomass and sulphur in TGA acid treated biomass. Other workers using different biomasses have also made similar observations5,8,9,114. Oxygen is never more than divalent because the second shell is restricted to eight electrons (He: 2s2 sp4). However, sulphur has empty d - orbitals, which may be used for bonding, and they can form four or six bonds by unpairing electrons (Ne:3s2 3p4 3d0). This behaviour of sulphur in forming more bonds may have contributed to the small differences in the retention of Pb(II) between pure and TGA modified biomass of Nipah palm petiole.


Table 1. Langmuir adsorption isotherm parameter for the retention of Pb(II) on the pure (PB) and thioglycolic acid treated (MB) Nipah palm petiole biomass.

Biomass Langmuir Isotherm
sample
  Xm mg/g KL (l/mg) r2

PB 15.06 3.56 x 10-1 0.924
MB 17.60 2.34 x 10-2 0.995

Contact Time Effect

The effect of contact time on the retention of Pb(II) on pure (PB) and modified (MB) biomass was monitored and the data presented in Fig 3. There was no significant difference on the retention of Pb(II) between TGA treated biomass and the pure biomass. Percent retention increased markedly with increase in contact time for pure and TGA treated initially and became gradually at later time probably due to surface saturation. At 40 min of contact time, the retention of Pb(II) by pure biomass was 90% and the modified have a 93% of retention. The data showed that the retention process was rapid, being complete in 20 to 30 min. The rapid adherence of the metal ions to the biomass indicates that retention may be taking place on the cell wall pore of the biomass. However, after this initial period the rates and retention efficiency for Pb(II) by pure and TGA treated biomass became almost constant, probably due to surface saturation of the biomass. The rapid nature of the process showed that there is only a small effect on the contact time required to reach saturation due to a variation in TGA treatment. According to Gardea - Torresday and co-workers14 a long contact time necessary to reach equilibrium indicates that the predominant mechanism is physical adsorption, while short contact time indicates chemisorption. The relatively short contact times observed in this study indicates that chemisorption is probably important and that regeneration of spent biomass may be difficult. However, due to the cheapness of the biosorbent, combustion of the spent biomass for energy purpose and further extraction of metals on ash is feasible.


Fig 3. Retention of Pb(II) by pure (PB) and TGA modified (MB) biomass of Nipah palm petiole.

The retention process was examined in terms of diffusion of metal ion in the solution in order to evaluate the influence of acid treatment on the rate-limiting step. It has been reported15 that, there are four consecutive steps in the metal - biomaterials retention process. They include (1) transport in the bulk of the solution (2) diffusion across the liquid film surrounding the sorbent particles (3) particle diffusion in the liquid contained in the pores and in the sorbate along the pore walls and (4) sorption and desorption within the particle and on the external surface. Any of the four steps mentioned above or any combination of them may be the rate-controlling factor. Many experimental designs including this investigation are designed to eliminate the effect of transport in the bulk solution by rapid mixing so that it does not become the rate-limiting step.

Several methods are available for evaluating the rate-limiting step of a retention process. In this investigation we have chosen the models of Chanda16 and that of Weber and Morris17. The Chanda model was chosen to estimate whether the retention process is particle diffusion controlled, while the Weber and Morris model was chosen to evaluate whether the retention process is a pore diffusion controlled. The Chanda equation for particle diffusion controlled retention process is expressed as:

If the plot of 1n (1-C/Ce) against time yields a straight-line, then the rate-limiting step is particle diffusion controlled.

The Weber and Morris equation is given as

If the plot of solute sorbed (qt) against square root of contact time, (t0,5 ), yielding a straight line will confirm the rate-limiting step as pore diffusion controlled. The experimental data were fitted into equations 6 and 7, which resulted in Figures 4 and 5. The figures revealed that the retention process is pore diffusion controlled mechanism, since its plot shows a better straight-line plot than the particle diffusion controlled mechanism. This is further confirmed by the coefficient of determination for both models for pure and treated biomass as shown in Table 3. This observation indicates and further confirms that the predominant interactive process in this investigation is chemisorption. However, it is possible in this system that there could be a contribution from particle diffusion controlled process, which is predominantly physisorption.


 
Fig 4. Particles diffusion controlled model plot for Pb(II) retention on pure (PB) and TGA treated (MB) of Nipah palm petiole biomass.   Fig 5. Pore diffusion controlled model plot for Pb(II) retention on pure (PB) and TGA treated (MB) of Nipah palm petiole biomass.


Table 3. Coefficient of determination (r2) for particle diffusion controlled and pore diffusion controlled mechanisms


Biomass

r2
sample
  Particle diffusion
controlled
Pore diffusion
controlled

PB 0.738 0.815
MB 0.943 0.982

CONCLUSION

The sorption of Pb(II) onto Nipah palm (Nypa fruticans Wurmb) petiole biomass is favoured by low metal ion concentration. The kinetics of the sorption process was found to follow a pseudo-second order rate law and the equilibrium data agrees well with the Langmuir isotherm. Small differences in Pb(II) retention after treatment with TGA were obtained. For both pure - metal or modified - metal biomass systems, chemical reactions are important and significant in the rate-controlling step. This investigation has revealed that Nipah palm (Nypa fruticans Wurmb) petiole, which is hitherto an environmental nuisance in the Niger Delta area of Nigeria could be converted to low-cost adsorbent for remediation of metal contaminated effluents.

ACKNOWLEDGEMENTS

The authors acknowledged with gratitude the University of Port-Harcourt for admitting Mr Wankasi, D for a PhD programme in Chemistry.

 

REFERENCES AND NOTES

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17. M. Chanda, K. F. O’Driscoll, G. M. Rempel, Reactive polymers. 1983, 1, 281         [ Links ]

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*Corresponding author

Dr Michael Horsfall Jnr
Department of Pure and Industrial Chemistry,
University of Port Harcourt,
Uniport P. O. Box 402, Choba,
Port Harcourt, Nigeria.

 

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