The sorption kinetics of Pb²⁺ and Cu²⁺ ions in aqueous solution by unmodified and mercaptoacetic acid modified biomass of Nypa fruticans shoot was investigated with a view to understanding the potential mechanism of the ability of Nipah palm shoot biomass as adsorbent for the removal of metal ions from aqueous solutions. Due to the increasing environmental concern regarding heavy metal contamination, there has been an abundance of interest in the removal of heavy metals from contaminated waste streams. Techniques presently in existence for removal of heavy metals from wastewater are relatively expensive involving either elaborate and costly equipment or high costs of operation with ultimate disposal problems (Cheng-Shlun and Shang-Da, 1994). In view of these reasons, development of a more cost effective remediation process using biological system for removal of heavy metal ions from wastewater is necessary. Biomaterials previously investigated include sago waste (Quek et al. 1998), Cyanidium caldarium (Lucido et al. 1990), sunflower (Sun and Weixing, 1998), Spagnum peat moss (Gardea-Torresdey et al. 1996), cassava waste (Horsfall et al. 2003), Fluted pumpkin waste (Horsfall and Spiff, 2005a) and Caladium bicolor (Horsfall and Spiff, 2005b) for the removal of metal ions from aqueous solutions, just to mention a few. However, the necessity of investigating more biomaterials is still important in order to obtain the best biomaterial for industrial application.

A brief of the methodology is out line as follows: The shoots of Nipah plant were obtained from the brackish waterfronts of the Niger Delta Area of Nigeria. They were processed, modified using 1.0 M mercaptoacetic acid (MAA) solution and used for batch adsorption study of Pb²⁺ and Cu². 10 ml of 37.5 mg/l Pb²⁺ (from Pb(NO₃)₂) and 40.0 mg/l Cu²⁺ (from CuSO₄) solutions were used to study the effect of time at a fixed pH of 5.5 and a constant temperature of 30ºC in a water bath. At the end of each time interval, the suspension was allowed to settle followed by centrifugation at 2500 rpm for 5 min and then decanted. The supernatants were analyzed for Pb²⁺ and Cu²⁺ using a Buck Scientific Atomic Absorption/Emission spectrophotometer 200A (AAES).

The result of the experimental data from the time course profile as percent sorption for the sorption of Pb²⁺ and Cu²⁺ from 37.5 mg/l Pb²⁺ and 40.0 mg/l Cu²⁺ solutions showed that as the contact time increased from 5 to 60 min, there was only a slight increase in the amount of metal ion removed by both unmodified and chemically modified biomass, until a contact time of ca. 20 min was reached. Within the first 5-10 min, both unmodified and chemically modified biomass was capable of removing over 55.6-92.2% of both metal ions. The relatively short contact times indicates that chemisorption is probably important.

The sorption data for Pb²⁺ and Cu²⁺ on Nipah was compared with other biomasses of previous workers. The comparative data indicates that the biomass of Nipah palm shoot is also a good source of adsorbent for remediation of metal ion polluted effluent and that was more effective for some of the inorganic materials.

The maximum sorption capacity for both metals on unmodified and chemically modified were found to be 15.59 mg/g and 21.85 mg/g respectively for unmodified biomass and 52.86 mg/g and 66.71 mg/g for modified biomass. The data revealed that the sorption capacity of the Nipah palm shoot biomass towards Cu²⁺ was greater than that of Pb²⁺. The differential sorption behaviour was attributed to ionic radii of the metal ions (Pb²⁺ = 1.20 Å; Cu²⁺ = 0.69 Å).

Two kinetic models were used in describing the sorption rate and confirm the reaction mechanism of Pb²⁺ and Cu²⁺ onto Nipah palm shoot biomass. The first model was the first order rate kinetics developed by Vinod and Anirudhan (2002). The equation is expressed as (equation 1):

where (1 - α) is the fraction of metal ion in solution at equilibrium and k is the overall rate constant. α is the fraction of metal ion adsorbed by the biomass at equilibrium, and it is the ratio of the amount of metal ion (q_t) removed from solution at time t  to that removed at equilibrium (q_e). This model was tried without success. Hence, the experimental data were further evaluated based on the pseudo-second order kinetic rate model proposed by Ho and co-workers in 1995. The pseudo-second order equation kinetic model is usually expressed as in equation 2:

where h = k₂q_e² can be described as the initial rate constant as t  approaches zero. q_t is the amount of metal ion on the biomass surface (mg/g) at any time t. q_e is the amount of metal ion sorbed at equilibrium (mg/g); k₂ is the pseudo-second order rate constant (g/mg min). If the pseudo-second-order kinetics is applicable, the plot of t / q_t vs. t will give a linear plot, which allows the computation of q_e, k₂ and h without having to know any parameter before hand.

According to Ho et al. (1995), if the plot is linear, then the sorption process may be described as chemisorption. The experimental data were fitted into equation 2 and a plot of t/q_t vs. t  made and a linear relationship was observed. The values of the sorption capacity of the biomass, q_e, the pseudo-second order rate constant, k₂, the initial sorption rate constant, h, and the coefficients of determination, r ², were evaluated from the plot (Figure not shown) and the data presented in Table 1.

The sorption rate constant was employed to evaluate the activation energy, using an Arrhenius form equation expressed by the following relationship (Ofomaja and Ho, 2005):

where k is the sorption equilibrium constant, k₂=g /mgmin, k₀ is initial rate constant, E_α is activation energy, R is the gas constant (8.314 J/k mol) and T is absolute temperature (K). In that equation, k₀ was be related to h, hence equation 3 may be rewritten as:

The linear form of the Arrhenius expression was applied to our experimental data (equation 5):

The parameters k and h were experimentally determined from the intercept and slope of a plot of t/q_t vs. t and the values of E_α computed from equation 5 at ca. 30ºC. The activation energies at 30ºC for the sorption of Pb²⁺ and Cu²⁺ onto the MAA modified (15.59 and 21.11, kJ/molK) and unmodified Nipah palm shoot biomass (13.83 and 19.98 kJ/molK) values are positive indicating endothermic reaction which is synonymous with chemisorption mechanism.

Conclusively, we can say that the kinetic data has provided information on the suitability of Nipah palm biomass as an excellent biosorbent for Pb²⁺ and Cu²⁺ in aqueous effluents, which is effective and environment friendly to remove and recover heavy metal ions from aqueous solution.

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