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The synthetic arsenic contaminated water was prepared by adding the arsenic into Ultrapure water to produce required initial concentration of As. HCl and NaOH were used to adjust the pH to 7. Then, 50 mL of solution were poured into 100 mL shake flask before 10 mg of adsorbent was added. The shake flasks were agitated inside incubator shaker at room temperature (±25°C) at constant agitation speed of 150 rpm. The batch adsorption tests were done at various contact times and the adsorbents were filtered from water by using filter paper. Then the filtrates were analyzed using flame Atomic Absorption Spectrometer (AAS). The outcome from the AAS was the residual concentration of arsenic after the adsorption.
Adsorption isotherm is a mathematical expression that relates the concentration of the adsorbate in the interface to its equilibrium concentration in the liquid phase . Before calculating the isotherm, equilibrium sorption capacity, qe, was determined the using following equation:
where Ci and Ce are the initial and equilibrium concentration of metal element the solution (mg/L) respectively, V is volume of the solution (in Liter) and Ms is the weight of adsorbent (mg). The results from this computation were used in calculating the isotherm.
In this research, two types of isotherm were studied, namely Langmuir isotherm and Freundlich isotherm. For Langmuir isotherm, the equation is stated below (ii):
where b is the maximum adsorption capacity corresponding to complete monolayer coverage (mg of solute adsorbed per g of adsorbent) and Keq is the Langmuir constant related to energy of adsorption (l of adsorbent per mg of adsorbate). The linear form can be used for linearization of experimental data by plotting Ce/qe against Ce. The Langmuir constants b and K can be evaluated from the slope and intercept of linear equation.
The Freundlich equation as stated in equation (iii):
where K and n are Freundlich constant. A plot of log Ce against log Qe yields a straight line which indicates the confirmation of the Freundlich isotherm for adsorption. The constants can be determined from the slope and the intercept.
results and discussions
Effects of initial concentrations
Figure 1 to Figure 3 display the percentage removal of arsenic using both CNT and CNF at different concentrations. Initial concentrations of arsenic influence the adsorption process. Percentage removal of arsenic decreased with the increase in initial concentration of arsenic. For initial concentration of 0.5 mg/L, percentage removal was 75%, followed by 5 mg/L (73%) and finally initial concentration of 10 mg/L with 40% removal. However, for CNF, removal of arsenic at those concentrations were within a narrow band varying from 56%-59%. The theory behind this is, since the amount of adsorbent added is fixed, the adsorption sites are constant to absorb the arsenic ions. When the concentration increases, more ions present and need to be removed. With the inadequate surface area, the quantity of ions to be absorb limited and left over some of the arsenic ions. That shows from the result that the remaining arsenic ions in 0.5 mg/L was less than the initial concentration of 10 mg/L.
Figure 1: Percentage removal of CNT and CNF at initial concentration 0.5 mg/L (T=298oK, adsorbent dose=200 mg/L, pH=7)
Figure 2: Percentage removal of CNT and CNF at initial concentration 5 mg/L (T=298oK, adsorbent dose=200 mg/L, pH=7)
Figure 3: Percentage removal of CNT and CNF at initial concentration 10 mg/L (T=298oK, adsorbent dose=200 mg/L, pH=7)
Effects of contact time
The effects of contact times were studied too. It was observed that increasing the contact time has increased the adsorption of arsenic ions until it reached equilibrium. At equilibrium time, all adsorbent sites were occupied by the arsenic ions.
The times for adsorption to reach equilibrium for two adsorbents are shown in Table 1. It was observed that both adsorbents reached equilibrium at the same time of 90 minutes.
Table 1 also shows the adsorption capacity of both andosrbents at equilibrium at different initial concentrations. For low concentration, the adsorption capacity of CNT was better than that of CNF. However at 10 mg/L, CNF had higher adsortion capacity (27.97 mg/g), while the capacity of CNT was less (19.68 mg/g).
TIME, CONCENTRATION AND CAPACITY AT EQUILIBRIUM
Isotherm is a basic requirement for adsorption study to know the relationship between adsorbates and adsorbents. Such information is also required to design any adsorption system. Discussion on the isotherm studies are given below.
The linear Langmuir equation expressed in equation (iv):
The linear form can be used for linearization of experimental data by plotting Ce/qe against Ce. The linearized plot of Langmuir isotherm for CNF and CNT are given in Figure 4 and Figure 5, respectively.
Figure 4: Langmuir plot for CNF
Figure 5: Langmuir plot for CNT
The slope and the intercepts were used to calculate the Langmuir adsorption constants stated in Table 2. Correlation coefficient (R2) of the CNT was higher (0.967) than that of CNF indicating better fit of Langmuir model by CNT. As for CNF, the correlation coefficient was only 0.471, which was far less than the desired value of 1, indicating that CNF is not suitable for Langmuir model to represent the adsorption process for As removal. Maximum arsenic sorption capacity for CNT and CNF are 23.26 mg/g and 333.33 mg/g, respectively.
The Langmuir adsorption model is the most common model used to quantify the amount of adsorbate adsorbed on an adsorbent as a function of partial pressure or concentration at a given temperature. It considers adsorption of an ideal gas onto an idealized surface.
LANGMUIR AND FREUNDLICH CONSTANTS
Inherent within this model, the following assumptions are valid specifically for the simplest case: the adsorption of a single adsorbate onto a series of equivalent sites on the surface of the solid.
The surface containing the adsorbing sites is perfectly flat plane with no corrugations.
The adsorbing gas adsorbs into an immobile state.
All sites are equivalent.
Each site can hold at most one molecule.
There are no interactions between adsorbate molecules on adjacent sites.
When there are two distinct adsorbates present in the system (e.g. in this study PAC and CNT or CNF), the following assumptions can be applied:
All the sites are equivalent.
Each site can hold at most one molecule.
There is no interaction between adsorbate molecules on adjacent sites.
Linear equation for freundlich stated in equation (v):
COMPARISON OF MAXIMUM SORPTION CAPACITY FOR ARSENIC BY DIFFERENT ADSORBENT MATERIALS
Maximum Capacity (mg/g)
Iron (III) oxide / silica
Freundlich plot for CNF and CNT are shows in Figure 6 and Figure 7. The slope and intercepts from the plot were used to determine the n and K values. Unlike Langmuir isotherm, correlation coefficient for CNF was near 1 (0.999) which indicated that this adsorbent fits better for Freundlich model. The n charge determines the adsorption intensity. The favorable n value for Freundlich isotherm design must be between 1 and 10. The n value of CNF was 1.038 and that of CNT was 1.553; indicating that both of the materials fit the Freundlich model satisfactorily. Table 3 compared the adsorption capacity of other adsorbent material with those of CNT and CNF. It can be observed that the capacity of the CNF is relatively high.
Figure 6: Freundlich plot for CNF
Figure 7: Freundlich plot for CNT
A new composite was produced with the use of PAC as a cheap substrate. The idea of growing CNM on substrate gave the advantage of clean production of CNM. The method was less hazardous compared to the floating catalyst CVD reactor. Adsorption tests showed that the CNT was suitable for low concentration while CNF was better adsorbent when deals with high concentration of As. Equilibrium capacity for CNT were 1.855 mg/g (Ci=0.5 mg/L), 18.44 mg/g (5 mg/L) and 19.68 mg/g (10 mg/L), while for CNF were 1.46 mg/g (0.5 mg/L), 13.78 mg/g (5 mg/L) and 27.97 mg/g (10 mg/L). Increasing of contact time increased the adsorption capacity until it reaches equilibrium. The adsorption process of CNT was a better fit for Langmuir isotherm model with the correlation coeficinte of 0.967, while Freundlich fit better for CNF (R2 = 0.999).