3.1 Adsorption research
To investigate the effects of the adsorbent dosage on the removal rate and adsorption amount of Pb2+, 5 mg, 10 mg, 15 mg, 20 mg, and 25 mg of 1:1 ZO and 1:2 ZO were added into 20 mL, 15 mg/L Pb2+ solution for adsorption observation. The obtained removal rate and adsorption amount are shown in the Fig. 1.
As shown in Fig. 1, with the increase of adsorbent dosage (5mg ~ 25mg), the removal rate of Pb2+ increases from 20.62–96.62% by 1:1 ZO and from 21.41–98.21% by 1:2 ZO. The Pb2+ adsorption amount of 1:2 ZO first increases from 12.8 mg/g to 16.76 mg/g, and then decreases to 11.79mg/g, while that of 1:1 ZO firstly increases from 12.37 mg/g to 13.36 mg/g and then decreases to 11.59 mg/g. Thus, 1:1 ZO and 1:2 ZO have the same trend in the removal rate and adsorption amount. Reaching the maximum adsorption amount, the dosage of 1:2 ZO (15 mg) is less than that of 1:1 ZO (20 mg). Under this condition, the removal rate of Pb2+ by both materials can reach more than 90%.
When the dosage is less than a certain value (15 mg of 1:2 ZO and 20 mg of 1:1 ZO), the active sites on the adsorbent are insufficient, the adsorption not reach a saturated state, and the removal rate is low. On the contrary, the adsorption is saturated and the adsorption amount decreases.
To research the effects of the initial concentration in Pb2+ solution on the removal rate and adsorption amount, Fig. 2 shows 1:1 ZO and 1:2 ZO adsorption following the concentrations of Pb2+ solution. The volume of the Pb2+ solution is 20 mL and the concentrations include 5 mg/L, 10 mg/L, 15 mg/L, 25 mg/L, and 35 mg/L. 20 mg of 1:1 ZO and 15 mg of 1:2 ZO were used as the adsorbent in the experiment.
As shown in Fig. 2, with the increase of the initial concentration of Pb2+ (5mg/L ~ 25mg/L), the adsorption amount of Pb2+ by 1:1 ZO (Fig. 2(a)) first increases from 4.81mg/g to 12.01mg/g. Then, it decreases to 7.17mg/g, and finally increases to 9.67mg/g. The removal rate continues to decrease (96.35%~27.62%). For comparison, the adsorption amount of Pb2+ by 1:2 ZO (Fig. 2(b)) first increases from 6.32 mg/g to 12.81 mg/g. Then, it decreases to 12.38 mg/g, and finally increases to 16.82 mg/g. The removal rate continues to decrease by 58.71%. Thus, the optimum initial concentration of Pb2+ to obtain the maximum adsorption amount and the peak removal rate removal rate is 15mg/L for 1:1 ZO and 1:2 ZO. Significantly, the adsorption amount and removal rate of 1:2 ZO are higher than those of 1:1 ZO at the same initial Pb2+ concentration. So, 1:2 ZO has a better removal effect of Pb2+.
For the same adsorbent, when the concentration of Pb2+ in the solution is less than 15 mg/L, it is easy to chelate with Pb2+ or produce electrostatic adsorption due to enough carboxyl groups. However, if the concentration of Pb2+ is greater than 15 mg/L, the carboxyl groups available for adsorption are insufficient, and sufficient active sites could not be provided for the adsorption of Pb2+, so the removal rate decreases.
To research the effects of the pH in Pb2+ solution on the removal rate and adsorption amount, 20 mg 1:1 ZO and 15 mg 1:2 ZO were added into 20 mL of simulated waste liquid with a Pb2+ concentration of 15 mg/L. 1 M NaOH and 1 M HCl were used to adjust the pH value containing 2, 3, 4, 5, 6, 7, and 8. The experiments results are shown in the Fig. 3.
The pH value affects the interaction between adsorbates and adsorbents by changing the charge distributing on the surface of the adsorbates and adsorbents [33]. When the pH ༜ 5, lead mainly exists in Pb2+ and Pb(OH)+, and when 5༜pH༜ 10, lead mainly exists in the form of Pb(OH)2, Pb(OH)42− and Pb(OH)3−.
As shown from Fig. 3, the adsorption amount and removal rate of 1:1 ZO and 1:2 ZO for Pb2+ are first increased and then decreased with the increase of pH. The Pb2+ adsorption amount (14.57 mg/g) and the Pb2+ removal rate (97.13%) of 1:1 ZO reach the peak at pH = 3, shown in Fig. 3(a). Because pHPZC of 1:1 ZO is 2.38 (shown in Fig S6), when pH ༜ 2.38, 1:1 ZO is positively charged, there are Pb2+ and Pb(OH)+ exist, and there is electrostatic repulsion between them. When 2.38 ༜ pH ༜5, the negative charge of 1:1 ZO is enhanced, and Pb2+ has a positive charge and attracts each other.
As shown from Fig. 3(b), both the Pb2+ adsorption amount (19.52 mg/g) and the Pb2+ removal rate (97.58%) of 1:2 ZO reach the peak at pH = 4. Since pHPZC of 1:2 ZO is 3.78 (shown in Fig S6), When pH ༜ 3.78, the 1:2 ZO is positively charged. At this time, lead mainly exists in Pb2+ and Pb(OH)+, while the Pb2+ and Pb(OH)+ are mutually repellent to the 1:2 ZO. When 3.78 ༜ pH༜4, 1:2 ZO is negatively charged, it electrostatically attracts with Pb2+ and Pb(OH)+. When 5༜pH, 1:2 ZO is negatively charged. The adsorption amount and the removal rate significantly decrease due to that lead mainly exists in the form of Pb(OH)2 and Pb(OH)42−, and mutual repulsion occurs between them.
To investigate the effects of the adsorption time on the Pb2+ removal rate and the Pb2+ adsorption amount, 20 mg 1:1 ZO and 15 mg 1:2 ZO were added into 20 mL of simulated waste liquid with a Pb2+ concentration of 15 mg/L, and the solutions were shaken at room temperature (200 rpm) in a constant temperature oscillator for 1h, 2h, 3h, 4h, 5h, 6h, 8h, 10h, 12h, 16h, 18h, 24h, 32h, and 36h. The experiment results are shown in Fig. 4.
As shown from Fig. 4, the Pb2+ removal rate of 1:1 ZO and 1:2 ZO gradually increase when increasing the adsorption time. The adsorption time of 1:1 ZO and 1:2 ZO to reach the saturation (13.81 mg/g for 1:1 ZO and 18.09 mg/g for 1:2 ZO) of Pb2+ adsorption is 16 h and 18 h, respectively. Significantly, at the same adsorption time, the adsorption amount of 1:2 ZO is much higher than that of 1:1 ZO, that is, the adsorption effect of 1:2 ZO is better.
At the beginning of adsorption, there are a large number of active sites on the surface of the adsorbent. So, 1:1 ZO and 1:2 ZO can well combine with Pb2+, resulting in the high removal rate. With the decrease of active sites, the adsorption rate gradually slows down, and the adsorption reaches saturation after a certain time.
3.3 Adsorption isotherm
The adsorption isotherm is employed to evaluate the adsorption characteristics of an absorbent. In this work, the Langmuir adsorption isotherm (5) and the Freundlich adsorption isotherm (6) were used to understand the Pb2+ adsorption behavior of 1:1 ZO and 1:2 ZO. The equations of the Langmuir model and the Freundlich model are as follows:
$$\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{{q}_{m}K}_{L}}+\frac{{C}_{e}}{{q}_{m}}$$
5
$${q}_{e}={K}_{f}{C}_{e}^{\frac{1}{n}}$$
6
Where qe (mg/g) is the equilibrium adsorption amount of 1:1 ZO and 1:2 ZO; Ce (mg/L) is the concentration of the Pb2+ when the adsorption is in equilibrium; qm (mg/g) is the maximum adsorption amount of 1:1 ZO and 1:2 ZO; KL is the Langmuir constant; Kf and n are the Freundlich constant, which is related to the adsorption strength of the adsorbent; C0 is the initial concentration of the Pb2+ (mg/L).
Figure 6 shows the Langmuir adsorption isotherm equation and the Freundlich adsorption isotherm equation. As shown from Table 3, Langmuir's fitting coefficients (1:1 ZO R2 = 0.95058 1:2 ZO R2 = 0.97488) are better than Freundlich's (1:1 ZO R2 = 0.91288 1:2 ZO R2 = 0.82627). Thus Langmuir is more suitable to describe the adsorption of 1:1 ZO and 1:2 ZO for Pb2+, indicating that the adsorption of 1:1 ZO and 1:2 ZO for Pb2+ belongs to monolayer adsorption.
Table 3
Adsorption isotherm parameters of 1:1 ZO and 1:2 ZO adsorption for Pb2+
material | temperature | Langmuir adsorption isothermal equation | Freundlich adsorption isothermal equation |
| (K) | qm (mg/g) | KL (L/m) | R2 | 1/n | KF (L/mg) | R2 |
1:1 ZO | 298 | 15.22 | 2.5575 | 0.95058 | 0.5650 | 2.5346 | 0.91288 |
1:2 ZO | 298 | 17.80 | 2.0271 | 0.97488 | 1.3480 | 0.9792 | 0.82627 |
Adsorption thermodynamics can be used to calculate the driving force of the adsorption process. The enthalpy change (ΔH0) and the entropy change (ΔS0) can be obtained by multiplying the slope to the intercept of the lnK versus 1/T fitted curve (Equations 7 and 9) with the gas molar constant R. The value of ΔG0 can be calculated from ΔH0 and ΔS0 at a temperature (Eq. 8). The specific equations involved in the fitting curve are as follows:
$$\text{ln}{K}_{d}=\frac{\varDelta {S}^{0}}{R}-\frac{\varDelta {H}^{0}}{RT}$$
7
$$\varDelta {G}^{0}=\varDelta {H}^{0}-T\varDelta {S}^{0}$$
8
$${K}_{d}=\frac{{q}_{e}}{{C}_{e}}$$
9
Where Kd is the adsorption equilibrium constant (L/kg); R is the gas molar constant (8.314 J/mol•K); T is the absolute temperature (K); ΔS0 (kJ/mol), ΔH0 (kJ/mol) and ΔG0 (kJ/mol) are entropy change, enthalpy change and Gibbs free energy, respectively.
According to the thermodynamic analysis in the temperature range (298 K, 308 K, 318 K), the adsorption amount data at each concentration can be calculated, and a scatter plot of lnK vs. 1/T is fitted shown in Fig. 7. From the slope and the intercept of the fitted curve to the gas mole constant, the value of ΔH0 and ΔS0 can be obtained, as shown in Table 4. It can be seen from Fig. 7 that as the temperature of the system increases, the adsorption amount of Pb2+ by 1:1 ZO and 1:2 ZO both decrease, and the △H0 is a negative value in Table 4, indicating that the adsorption of Pb2+ by 1:1 ZO and 1:2 ZO is an exothermic process. In addition, △G0 is a negative value, indicating that the adsorption process of Pb2+ by 1:1 ZO and 1:2 ZO is spontaneous. A negative value of ΔS0 indicates that the disorder of the solid-liquid interface is reduced during the adsorption process.
Table 4
Thermodynamic parameters of adsorption of Pb2+ by 1:1 ZO and 1:2 ZO
Material | T (K) | △G0 (kJ/mol) | △H0 (kJ/mol) | △S0 (kJ/mol) |
1:1 ZO | 298 | -6.1741 | -15.1439 | -0.0301 |
308 | -5.8731 |
318 | -0.5721 |
1:2 ZO | 298 | -6.26578 | -11.7311 | -0.01834 |
308 | -6.0824 |
318 | -5.8990 |