Solution pH affects both aqueous chemistry and surface binding sites of the adsorbents. The effect of initial pH on adsorption of AB129 was studied from pH 2 to 12 at 25°C, initial dye concentration of 40 mg/L, adsorbent dosage of 0.4 g and contact time of 10 min. The maximum adsorption of the AB129 is obtained at pH = 2 (Figure 1). The acidity constant value of the most acidic group of the AB 129 molecule is 1.6. This functional group can be easily dissociated and thus, the AB 129 molecule has net negative charges in the working experimental conditions [30, 31].
At acidic pH the H+ ion concentration in the system increased and the surface of the AS acquires positive charge by absorbing H+ ions. As the pH of the system increases, the number of negatively charged sites increases and the number of positively charged sites decreases. Negatively charged surface sites on the AS do not favor the adsorption of AB129 anions due to the electrostatic repulsion. Also lower adsorption of AB129 at alkaline pH is due to the presence of excess OH¯ ions, which destabilize anionic AB129 and compete with the AB129 anions for the adsorption sites. The most effective pH was 2.0 and it was used in further studies.
The particle size distribution of AS determined by sieving the samples manually shaking with stainless steel mesh screens of standard (international ASTM with meshes 40, 60 and 100). For batch adsorption experiments, three different particle sizes viz. 40 and 60–100 AS mesh were selected and difference in the amount adsorbed was noticed by using different mesh sizes. Effect of sieve size of adsorbent on the adsorption was studied at 25°C, 0.4 g of AS, pH = 2 and 40 mg/L of AB129. It was observed that adsorption was found to increase with the 60–100 mesh sizes in Figure 2. This is due to increase in the surface area of the adsorbent and accessibility of the adsorbent pores towards the AB129 [32].
Figure 3 demonstrates the effect of adsorbent dosages for removal of the AB129 from aqueous solution. It was observed that highest amount of AB129 removal was attained for adsorbent mass of at least adsorbent.
For adsorbent dosage higher than 0.4 g the AB129 removal remained almost constant. Increases in the percentage of AB129 removal with adsorbent dosage could be attributed to increases in the adsorbent surface areas, augmenting its number of adsorption sites available for adsorption [33, 34]. In order to continue this work, the adsorbent dosage was fixed at 0.4 g, since this adsorbent dosage correspond to the minimum amount of adsorbent which lead to a constant and maximum removal of AB129.
Studies for investigation of the effect of dye concentration on its removal carried out by adding of 0.4 g prepared AS in 25 mL of solution at different concentration of 10–70 mg/L, pH = 2 and room temperature (25°C). As shown in Figure 4, by increasing of initial dye concentration, removal of dye was decreased.
The adsorption rate, obtained for AB129 adsorption on AS was observed by decrease of the concentration of AB129 within the adsorption medium with contact time. As shown in Figure 5, it can be concluded that maximum dye removal could be achieved when the sonication time was above 14 min. After equilibrium, the amount of adsorbed dye did not change significantly with time.
At 40 mg/L of AB129, the removal rate in the first varies from 58.5% to 94.24% of the maximum removal onto AS. For instance, the adsorbents exhibited three stages, which can be attributed to each linear portion of the figure. The first linear portion was attributed to the diffusion process of AB129 to the adsorbent surfaces [35, 36], hence, was the fastest adsorption stage.
This result is corroborated by the factionary-order kinetic model. The second linear portion was attributed to intra-particle diffusion, which was delayed process. The third stage may be regarded as the diffusion through smaller pores, which is followed by the establishment of equilibrium. The surface of AS may contain a large number of active sites and the solute adsorption can be related to the active sites on equilibrium time. Also up to 90% of the total amount of AB129 adsorption was found to occur in the first rapid phase (10 min) and thereafter the adsorption rate was found to decrease. The higher adsorption rate at the initial period (first 10 min) may be due to too number of vacant sites available at the initial stage. As a result there exist too concentration gradients between adsorbate in solution and onto adsorbent surface. This increased in concentration gradients tends to increase in AB129 adsorption at the initial stages.
Adsorption kinetic studies
The mechanism of adsorption was investigated by pseudo first order and pseudo second order models.
Based on the pseudo first-order expression (Lagergren model) by plotting the values of log (q
e
− q
t
) against t give a linear relationship that q
e
and k1values can be determined from the intercept and slope of the obtained line, respectively (Figure 6 and Table 2). The parameters qe and qt are amounts of dye adsorbed (mg/g) on adsorbent at equilibrium and at time t, respectively and k1 is rate constant of pseudo first order adsorption (1/min).
Table 2
Kinetic parameters for the adsorption of 40 mg/L AB129 onto AS
Figure 6 was used to determine pseudo first order rate constant (k1) and theoretical amount of dye adsorbed per unit mass of adsorbent qe(the). Distance of qe(the) from qe(exp) value indicate that this model was not fit well with the experimental data (Table 2) [37].
The plot of t/q
t
versus t for the pseudo-second-order kinetic model gives a straight line that k2 and equilibrium adsorption capacity (q
e
) were calculated from the intercept and slope of this line, respectively (Figure 7). Where k2 is rate constant of second order adsorption (g/(mg min)). The high value of R2 (0.994) and closeness of experimental and theoretical adsorption capacity (qe) value show the applicability of this model to explain the experimental data (Table 2).
Adsorption equilibrium study
Adsorption isotherms are prerequisites to understand the nature of the interaction between adsorbate and the adsorbent used for the removal of organic pollutants [38, 39]. The equation parameters of various adsorption isotherm equations such as Langmuir, Freundlich, Temkin, Dubinin-Radushkevich (D-R) and Harkins-Jura isotherms provide some insight into the adsorption mechanism, the surface properties and affinity of the adsorbent for adsorbate [40, 41].
The Langmuir isotherm is based on the assumption that the adsorption process takes place at specific homogeneous sites within the adsorbent surface and that once a dye molecule occupies a site, no further adsorption can take place at that site, which concluded that the adsorption process is monolayer in nature.
Langmuir isotherm is based on the assumption that:
-
a)
Maximum adsorption corresponds to a saturated monolayer of adsorbate molecules on the adsorbate surface.
-
b)
The energy of the adsorption is constant.
-
c)
There is no transmigration of adsorbate molecules in the plane of adsorbent surface [42]. Based on the linear form of Langmuir isotherm model (according to Table 3), the values of K
a
(the Langmuir adsorption constant (L/mg)) and Q
m
(theoretical maximum adsorption capacity (mg/g)) were obtained from the intercept and slope of the plot of Ce/qe vs Ce, respectively (Figure 8). The values of Qm and K
a
are 11.95 m/g and 0.902 L/mg, respectively shown in Table 3. The high correlation coefficient (0.994) shows the applicability of Langmuir model for interpretation of the experimental data.
Table 3
Isotherm constant parameters and correlation coefficients calculated for the adsorption of AB129 onto AS
The data was analyzed by the linearized form of Freundlich isotherm model. In this model, qe is the amount of adsorption, kf is the Freundlich constant related to sorption capacity and 1/n is a constant related to energy or intensity of adsorption. This gives an expression encompassing the surface heterogeneity and the exponential distribution of activated sites and their energies. This isotherm dose not predicts any saturation of the adsorbent surface. The Freundlich exponents kF and 1/n can be determined from the linear plot of log qe vs. log Ce is shown in Table 3. The values of the Freundlich constants KF and 1/n are 0.508 respectively shown in Table 3. The slope 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneous, becoming more heterogeneous as its value gets closer to zero [43].
Heat of adsorption and the adsorbent-adsorbate interaction on adsorption isotherms were studied by Tempkin [44]. The constants obtained for Tempkin isotherm are shown in Table 3. The linear form of Dubinin-Radushkevich isotherm equation was applied to estimate the porosity apparent free energy and the characteristic of adsorption [45]. The constant obtained for D–R isotherms are shown in Table 3. The mean adsorption energy (E) gives information about chemical and physical nature of adsorption.
As seen from Table 3, the Langmuir model yields a somewhat better fit (R2 = 0.994), Tempkin isotherm (R2 = 0.993) than the Freundlich model (R2 = 0.929) and Dubinin-Radushkevich model (R2 = 0.948). Equilibrium data fitted well with the Langmuir and Tempkin model.
Thermodynamic study
Thermodynamic parameters such as change in free energy (∆G°) (J/mole), enthalpy (∆H°) (J/mole) and entropy (∆S°) (J/(K mole)) were determined using following equations
(1)
(2)
(3)
Where Ko is equilibrium constant, Csolid is solid phase concentration at equilibrium (mg/L), Cliquid is liquid phase concentration at equilibrium (mg/L), T is absolute temperature in Kelvin and R is gas constant. ∆G° values obtained from equation (2), ∆H° and ∆S values obtained from the slope and intercept of plot lnKo against 1/T (Figure 9). The negative value of ∆G° indicates the adsorption is favorable and spontaneous (Table 4). ∆H◦ and ∆G◦ can be obtained from the slope and intercept of Van’t Hoff plot of ln K c vs. 1/T.
Table 4
Thermodynamic parameters for adsorption of AB129
The positive values of ∆H◦ further confirm the endothermic nature of the adsorption process and the positive ∆S◦ values suggest the increase in adsorbate concentration in solid–liquid interface indicating thereby the increase in adsorbate concentration onto the solid phase. It also confirms the increased randomness at the solid–liquid interface during adsorption. This is the normal consequence of the physical adsorption phenomenon, which takes place through electrostatic interactions.
Comparison with literature
The performance of the proposed method has been compared with other adsorbents. As is seen in Table 5 the contact time for proposed method in comparison with all of the adsorbents are preferable and superior to the literature which show satisfactory removal performance for AB129 as compared to other reported adsorbents [46, 47].
Table 5
Comparison for the removal of AB129 by different adsorbents