Comparative study of the adsorption capacity of lead (II) ions onto bean husk and ﬁ sh scale from aqueous solution

The present study compared the adsorption capacity of Pb (II) ions from aqueous solution onto biopolymer materials (BPMs): (bean husk (BH) and ﬁ sh scale (FS)). Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were used to characterize the BPMs. The optimal conditions of the variables: pH, adsorbent dosage, initial metal concentration, contact time and temperature were ascertained. Experimental data were applied to the Langmuir, Freundlich and Temkin sorption isotherms using the linear equations form. The optimal removal of Pb (II) ions with BH and FS was obtained at pH 7.0 and 6.0, and 0.2 g adsorbent dosage each, respectively. The removal of Pb (II) ions ﬁ tted the pseudo-second order kinetic model well for the materials. Equilibrium Langmuir isotherm, which indicated a heterogeneous process, gave a better conformity than the other models used for BH while the Temkin isotherm gave better conformity for FS. The FS reached equilibrium faster (at about 30 min) compared to BH (at about 60 min); however, the processes are both spontaneous and endothermic. The BPMs gave about 90% removal of Pb (II) ions at the optimum dosage when used for textile wastewater. The BPMs, therefore, can be used as effective, low-cost and environmentally friendly adsorbents.


INTRODUCTION
Due to rapid industrialization and technological advancements, the natural environment suffers from the detrimental effects of pollution (Wahid et al. ). In particular, pollution of the environment through indiscriminate discharge of partially/untreated industrial effluents has been a serious concern in developing countries. Therefore, to protect our environment from these pollutants, this can be achieved either by minimizing the introduction of pollutants into the environment or by their removal from contaminated media (Baker & Khalili ). The industrial effluents contain pollutants in the form of organic and inorganic components.
An example of the inorganic component is potential toxic metals (PTMs) which found their way into the environment The health implications of acute lead poisoning in humans include effect on the nervous system, enzyme inhibitor, causing mental retardation and damage to the brain cells in young children (King et al. ; Nadeem et al. ).
Calcium in the bone is replaced by lead and subsequently released into the blood stream (King et al. ).
The conventional methods involved in the removal of these pollutants from wastewater include ion exchange, chemical precipitation, reverse osmosis and liquid-liquid extraction, resins, cementation and electrodialysis. However, these methods are often inefficient and very expensive. A promising method, termed adsorption, an economically feasible and environmentally friendly process which uses biological materials as a sorbent, is being looked into as an alternative treatment for removal of pollutants (Tan et al. ). Guyo & Moyo () studied the use of acid pre-treated cowpea pod (Vigna unguiculata) biomass for removal of Pb (II) ions from aqueous solution. The optimum conditions recorded for the biosorption of lead (II) ions were pH 6.0, contact time 30 min and dosage concentration of 3 g L À1 . The biosorption data were best described by the Langmuir isotherm model. The biosorption kinetics followed the pseudo-second order model. The thermodynamics data showed that the adsorption of lead (II) ions onto cowpea pod was endothermic.
Limited studies have used white croaker FS and bean husk (BH) for the adsorption of metals from wastewater.
These adsorbent materials are available in abundance as agricultural waste/agri-food by-products, hence the need for the beneficiation/valorization of such waste materials; also, the utilization of the different functional groups present on the surface of the biosorbent, such as amino, carboxylic, ester, hydroxyl, phenolic, phosphate and sulfhydryl to interact with the Pb (II) ions in aqueous solution. In this study, therefore, we set out to quantify and compare the adsorption

Preparation of biosorbent and the chemicals
The white croaker FS were obtained from a fish merchant while BH was obtained from a popular Alaba Rago market, both locations in Lagos, Nigeria. The fish scales were washed with distilled water and air dried for about 10 days, and stones, dirt and stalks were removed by hand from the BH. Both adsorbents were pulverized with a stainless steel grinder and sieved through a 2 mm mesh. They were separately stored in airtight containers.
The Pb (II) ions aqueous solution was prepared from Pb(NO 3 ) 2 salt. About 1.615 g of Pb(NO 3 ) 2 was used to prepare 1,000 mg/L lead stock solution while about two drops of concentrated nitric acid were added to the solution before making it up with distilled water in 1,000 mL volumetric flasks. The pH of solutions was 3.00. The reagents (lead nitrate, sodium hydroxide, hydrochloric acid and nitric acid) were purchased from Sigma Aldrich, South Africa.  (1) and the amount of Pb (II) ions adsorbed (mg/g biosorbent) was calculated using Equation (2): where C o and C e (mgL À1 ) are the initial and equilibrium concentration in the liquid-phase of the Pb (II) ions solution. V (mL) is the volume of the solution, W (g) is the mass of absorbent used and q e (mg g À1 ) is the amount adsorbed.

Effect of pH
The study of the effect of pH on the biosorption was carried out at initial pH range between 1 and 8 using 50 mg/L of the Pb (II) ions initial concentration solution and a 0.5 g biosorbent dosage. The pH was adjusted with 0.1 M HCl or 0.1 M NaOH and measured using a pH meter. Shaking was performed at 25 C on an orbital shaker at 200 rpm for 120 min. The samples were filtered using 0.45 μm filter paper and the filtrate was analysed for residual Pb (II) ions.

Effect of biosorbent dosage
The effect of biosorbent dosages was investigated for the removal of Pb (II) ions from 50 mg/L solution at different biosorbent doses ranging from 0.1 to 2.0 g. The Erlenmeyer flasks containing the Pb (II) ions solutions at the optimum pH of the same initial concentration (50 mg/L) and temperature (25 C) but different biosorbent mass were placed on an orbital shaker at 200 rpm for 120 min. The samples were filtered using 0.45 μm filter paper and the filtrate was analysed for residual Pb (II) ions.

Effects of initial lead concentration and contact time
The effects of initial Pb (II) ions concentration and contact time on the biosorption process were studied with 50 mL Pb (II) ions solutions with initial concentrations ranging from 5 to 100 mg/L in a series of Erlenmeyer flasks with the optimal amount of both biosorbents (0.2 g) and pH (6.0 for FS and 7.0 for BH) on an orbital shaker at 200 rpm for 120 min. The contact time was investigated at varied times between 10 and 180 min at optimal pH, dosage and concentration. The samples were filtered using 0.45 μm filter paper and the filtrate was analysed for residual Pb (II) ions.

Effect of temperature
The effect of temperature was investigated at a range between 293 and 333 K at optimal pH, biosorbent dosage, concentration and contact time.

Adsorption isotherm models
The initial Pb (II) ions concentration provides information on either the adsorption of metal ions on the biosorbent surface that occurs through formation of a monolayer or multilayers, or gas adsorption. Langmuir isotherm type II (Langmuir ) was adopted, which describes the monolayer while multilayer adsorption is described by the Freundlich isotherm (Freundlich ). The Langmuir isotherm is based on the theoretical principle that only a single adsorption layer exists on an adsorbent. The linear form of the Langmuir model type II is presented in Equation (3): where C e is the equilibrium concentration of the Pb (II) solution (mg/L), q e is the amount of Pb (II) ions adsorbed per unit mass of the biosorbent (mg/g), q m is the Langmuir constant representing adsorption capacity (mg/g), and K L is the Langmuir constant representing energy of adsorption (L/mg). A plot of 1 q e versus 1 C e is linear for a sorption process obeying the basis of this equation with K L and q m obtained from the slope and intercept, respectively.
The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor R L that is given by Equation (4): where K L is the Langmuir constant and C o is the initial concentration of Pb (II) ions. The value of R L indicates the shape of the isotherm to be either unfavourable (R L > 1), The Freundlich isotherm assumes that the Pb (II) uptake occurs on a heterogeneous surface by multilayer adsorption and that the amount of the adsorbed metal increases infinitely with an increase in Pb (II) concentration. The linear form of the Freundlich equation is written as Equation (5): where C e is the equilibrium concentration of Pb (II) ions solution (mg/L), q e is the amount of Pb (II) adsorbed per unit mass of biosorbent (mg/g), n is the number of layers, and K F is the Freundlich constant. For a sorption process obeying the Freundlich isotherm, the plot of log 10 q e versus log 10 C e is linear with K F and n obtained from the intercept and slope, respectively.
The Temkin isotherm (Temkin & Pyzhev ) contains a factor that clearly taking into the account of adsorbentadsorbate interactions: Adsorption kinetics studies/models The mechanism of adsorption process was studied. The during the process.
The Lagergren pseudo-first order kinetic model is presented by Equation (7): where q e and q t are the amounts of the Pb (II) ions adsorbed (mg/g) at equilibrium and at time t (min), respectively, k 1 is the adsorption rate constant (1/min), and t is the time (min).
A linear plot of log 10 (q e À q t ) versus t gives the equilibrium adsorption capacity q e (mg/g) as intercept while the slope gives the pseudo-first order rate constant k 1 .
The linear form of the Lagergren pseudo-second order kinetic model as described by Ho & McKay () is given in Equation (8): where q e and q t are the sorption capacity (mg/g) at equilibrium and at time t, respectively and k 2 is the pseudosecond order rate constant (g/mg.min). A linear plot of t q t versus t gives the slope as equilibrium adsorption capacity, q e (mg/g), and the intercept pseudo-second order rate constant k 2 .

Thermodynamics of the adsorption process
The thermodynamics provide general information about the influence of temperature on adsorption and is, importantly, useful to predict the feasibility of the adsorption process.  (9)- (12): where C o À C e (mg/L) is the concentration after Pb (II) ions adsorption and C e (mg/g) is the equilibrium concentration The real textile effluent was digested without using the adsorbent to ascertain the actual level of the Pb (II) ions.

Characterization of the adsorbent
The presence of functional groups on the surface of the biosorbents was confirmed using FTIR, as shown in  The amide I and amide II stretches at 1,631 and 1,536 cm À1 are present in the FS spectra but not in the BH spectra.
The TGA and DTG graphs for the FS and BH are shown in Figure 2. The FS is thermally stable at 330 C, compared to BH at 270 C. The DTG graph of FS shows two endothermic peaks at 95 C and 340 C, however, BH shows three endothermic peaks at 95 C, 230 C and 320 C.
The SEM images of the FS and BH are shown in

Effects of initial lead concentration and contact time
The adsorbate concentration and the contact time between adsorbent and adsorbate species play an important role in the removal of pollutants from water and wastewater. The effect of initial Pb (II) ions concentration on the adsorption process is shown in Figure 7. It was observed that as the concentration increases the percentage adsorption increased.

Effect of temperature
The effect of temperature on the adsorption of Pb (II) ions onto FS and BH adsorbents was studied within the range    for FS and BH, respectively, after which, equilibrium was achieved. The results were applied in the thermodynamic study.

Adsorption isotherm models and kinetic studies
The equilibrium data were analysed using Langmuir,

Freundlich and Temkin equilibrium models (Equations
(3)- (6)) in order to obtain the best fitting isotherm. The isotherms are modelled graphically in Figures 10 and 11    Various studies have investigated Pb (II) adsorption by different low-cost adsorbents as shown in Table 2.
By comparison, it is found that FS and BH are also efficient adsorbents for the removal of Pb (II) ions from water.
The adsorption kinetics were investigated for the contact time of 180 min. The kinetic parameter details of the first order and second order rates are shown in Table 3 and the  pseudo-second order graph is shown in Figure 12.

Adsorption thermodynamic
The effect of heat of adsorption of the adsorbate onto the adsorbent material was determined using the thermodynamic parameters: free energy change (ΔG o ), enthalpy change (ΔH o ) and entropy change (ΔS o ), and was calculated using the Van Hoff's Equations (9)-(12).
The Van't Hoff plot for the adsorption of Pb (II) ions onto FS and BH is shown in Figure 13. The standard free energy (ΔG o ), enthalpy (ΔH o ) and entropy (ΔS o ) changes for BH and FS were obtained using equilibrium constant K and the data are given in Table 4.
The positive values of ΔH show that the adsorption process is endothermic, while a positive ΔS indicates increased randomness in the interface between adsorbent and adsorbate (solid-solution). Moreover, the negative value of free energy change (ΔG o ) shows that the adsorption process is spontaneous at high temperature, and has good feasibility for Pb (II) ions adsorption onto FS and BH.

Treatment of real textile effluent with the adsorbents
The adsorbents were used to treat textile wastewater with effluents, before being discharged into the environment.

ACKNOWLEDGEMENTS
The work was supported by grants from NRF/TWAS Postdoctoral Fellowship grant no. 99680, awarded to