Cheap Nano-Adsorbents Based on Zno/Mineral Nanocomposites for Removal of Chloroform from Water Solution

3.1. Materials

Zinc sulfate, sodium hydroxide, and hydrochloric acid were obtained from Merck, Germany. HPLC grade chloroform from Fluka (Switzerland) was used without further purification. Natural zeolite NPs were purchased from Tamad-Micro Inc. Ultrapure water (Millipore) was used through the work.

To prepare chloroform samples, a 500 mg. L^-1 chloroform stock solution was prepared by dissolving the appropriate volume of pure chloroform into 1L of ultrapure water in a volumetric flask. It was then used to prepare other diluter chloroform samples.

3.2. Synthesis

ZnO NPs were prepared as described in our previous work (9). The prepared ZnO NPs were used as a support for preparing the nanocomposites. ZnO/GO was prepared as described earlier (9, 38). For preparing ZnO/GO/Zeolite or GO/Zeolite nanocomposites, at first, the purchased zeolites were washed with plenty of ultra-pure water to remove ionic salts within the zeolite pores. So that 0.5 g of the zeolite weighted in a beaker, 0.5 g of ZnO NPs, and 0.1 g of GO was added to it, then 20 mL of ultrapure water was added to the mixture. The mixture was sonicated for 1 h. Afterward, it was placed over a magnetic stirrer for stirring overnight.

3.3. Experimental Procedure

Five different 100, 75, 50, 25, and 10 mg. L^-1 chloroform solutions were prepared, and then 20 mL of each of them was poured into four beakers containing 20 mg of the four nano-adsorbents, ZnO, ZnO/GO, GO/zeolite, and ZnO/GO/zeolite. The nano-adsorbents/chloroform mixtures were stirred over stirrers for 30 min. Then the mixtures were filtered. After that, 1 µL of each of the filtrates were presented to the GC to measure the equilibrium concentrations of chloroform and hence to obtain adsorption isotherm models.

50 mL of 50 mg. L^-1 solution was poured into the different beakers containing 20 mg of the different adsorbents. The mixtures were placed over the stirrer, and 5 mL of the samples were taken after each 10 min duration time. Each of the taken mixtures was centrifuged to remove the adsorbents, and then 1 µL of the clear solutions were presented to GC to measure the remained chloroform concentrations.

To study the effect of pH on the adsorption of chloroform, various 50 mg.dm^-3 chloroform solutions were prepared in different pHs. A metrohm 780 pH/ion-meter was used to monitor pH. Adjustment of pH was performed by adding appropriate amounts of sodium hydroxide salt and/or hydrochloric acid into the chloroform solutions.

3.4. Analysis

Concentrations of chloroform were measured using a gas chromatograph (GC), Varian CP-3800, Australia, equipped with a 300 mm, 0.32 mm in diameter column #cp 8752, flame ionization detector, 1177 split injector. Helium gas was used as carrier gas. The GC column worked over-temperature ranged from 35°C to 120°C BJH and BET models, based on N₂-adsorption-desorption at 77K, were performed with (BElSORP Mini (Microtrac Bel Corp CO)) to determine the pore diameter, pore volume, and active surface area of the nanocomposites. XRD patterns were obtained with X’Pert Pro (Panalytical Co.). The FESEM images were taken with Sigma VP, Zeiss, Germany.

Kinetic studies of adsorption phenomena are good keys to derive information about the adsorption mechanism. Different adsorption kinetic models, pseudo-first-order, pseudo-second-order, and inter-particle diffusion were introduced by the authors (40-44). Equilibrium adsorption capacity (q_e) and instantaneous adsorption capacity (q_t) were measured according to Equations 1 and 2.

Where C₀, and C_e are initial and equilibrium concentrations of chloroform, respectively. To fit the pseudo-first-order kinetic model of the adsorbents, the corresponding q_e and q_t were fitted in Equation 3. In the following Log (q_e- q_t) was plotted versus t (40, 41). So that intercept of the straight-line produced log q_e, and its slope presents the rate constant of the pseudo-first-order adsorption rate constant, K₁:

The pseudo-second-order kinetic model was obtained by regressing ${\displaystyle \raisebox{1ex}{$t$}\!\left/ \!\raisebox{-1ex}{$q_t$}\right.}$ versus t (Equation 4) (42). So that the pseudo-second order rate constant, K₂, and equilibrium adsorption capacity of the adsorbent were obtained from intercept and slope of the regression line, respectively. Squared correlation coefficient of the regression line gives the goodness of fit to the adsorption kinetic models:

Inter-particle diffusion model was obtained by regressing qt versus ${\displaystyle t^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}$ (44):

Where K_p is the inter-particle diffusion rate constant, which can be obtained from the slope of the inter-article diffusion model, and x_t is a constant related to the quantity of adsorption extent.

4.1. Characterization of Nanocomposites

To obtain structural information of the composites, XRD patterns of the nanocomposites were taken. The results are depicted in Figure 1. Figure 1A - D indicate the XRD patterns of the pristine ZnO, ZnO/GO, ZnO/GO/zeolite, and GO/zeolite, respectively. As shown in Figure 1A, the appeared sharp peaks are attributed to ZnO NPs (JCPDS Card No. 036-1451). Comparing Figure 1A and B reveals the effect of the presence of GO on the lattice structure of the pristine ZnO NPs, and also the presence and absence of new small peaks may be due to the presence of GO. Figure 1C is related to ZnO/GO/Zeolite. It preserves the characteristic peaks related to pristine ZnO. The remained peaks are attributed to zeolite and GO. Figure 1D is related to GO/zeolite that its peaks were at 2tetah 9.89, 11.22, 13.05, 16.96, 17.36, 20.95, 21.94, 22.45, 22.8, 26.04, 26.64, 30.03, and 32.05, attributed to both of GO and zeolite. In addition, the morphology of the nanocomposites was also studied (Figure 2). As shown in Figure 2A - C, ZnO NPs revealed potato-like particles, which decorated over GO or zeolite in the case of ZnO/GO. ZnO/GO/Zeolite (Figure 2B and C). Finally, Figure 2D showed zeolite NPs that were well dispersed within the GO sheets. In order to obtain surface characteristics and porosity of nanocomposites, N₂- adsorption-desorption on the nanocomposites was studied. The BET and BJH models of the nanocomposites were obtained, and then the active surface area, pore diameters, and pore volume of the adsorbents were calculated (45-47). The results of the study are depicted in Figure 3A and B as well as Table 1. As shown in Figure 3A, a hysteresis loop shape revealed that the samples had mesostructured. In addition, a greater extent of adsorption was achieved with ZnO/nanocomposites compared to the pristine ZnO NPs. This may be due to the creation of the core-shell of ZnO-NPs. with zeolite and GO-NPs, so it can cause ZnO lattice layers to be separated from each other, which in turn causes improved porosity of the nanocomposites. On the other hand, active surface areas for the ZnO/GO are extremely greater than the other nanocomposites. Based on the BJH model, pore volumes of ZnO/GO were extremely greater than pure ZnO NPs (Figure 3B and Table 1). Mean pore diameters of ZnO NPs were increased from 116.5 nm to 140.2 nm, due to the presence of GO. On the other hand, mean pore diameters were decreased in the case of ZnO/GO/zeolite and GO/zeolite, it may be due to the penetration of ZnO NPs and GO NPs into the zeolite tetrahedral frameworks. Hence, the decrease in the mean pores diameter can be justified. Meanwhile, the greater pore volume and surface area of ZnO/GO/zeolite and GO/zeolite, compared to that of ZnO NPs, can be explained by the presence of zeolite and GO sheets, which causes the spread of ZnO NPs over GO sheets to produce an extensive surface. Zeolite could not help ZnO NPs to produce more extensive surfaces than GO, because of penetration of some of ZnO NPs into the zeolite frameworks, as discussed earlier.

Figure 1.

XRD patterns of ZnO NPs (A), ZnO/GO (B), ZnO/GO/zeolite (C), and GO/zeolite (D)

Figure 2.

FESEM images of ZnO NPs (A), ZnO/GO (B), and ZnO/GO/zeolite (C), and GO/zeolite (D)

Figure 3.

A, BET isotherm based on Nitrogen-adsorptiondesorption on the different adsorbents; B, Pore size distribution of the different nanocomposites

4.2. Effect of Adsorbent Dosage

Different amounts of adsorbents were examined to investigate the effect of the adsorbent dosage on the removal of chloroform (Figure 4). As shown in Figure 4, about 0.3 g of the adsorbents was enough to reach the maximum chloroform removal for each of the nano-adsorbents. The figure also shows that ZnO/GO could remove a greater extent of chloroform compared to the other nanocomposites.

Figure 4.

Effect of the different nanocomposite amount on the removal percent of chloroform, (a) ZnO/GO, (b) GO/Zeolite, (c) ZnO/GO/Zeolite, and (d) ZnO NPs

4.3. Kinetic Study of Chloroform Removal

Figure 5 shows the fitted data, based on the above-mentioned adsorption kinetic models for the different adsorbents. As obvious in the figure, almost all of the adsorbents followed the inter-particle diffusion kinetic model. All of the corresponding kinetic constants of the adsorbents are summarized in Table 2. As shown in the table, ZnO/GO and GO/Zeolite showed a greater adsorption rate constant compared to the other two adsorbents. However, the results of the isotherm study, which are provided in the following sections, confirm the superiority of ZnO/GO to GO/Zeolite, because of the greater maximum capacity of the former compared to the latter.

Figure 5.

Kinetic models for the adsorption of chloroform on ZnO/GO, (a1-a4) concentration-time profile of chloroform removal, (b1-b4) pseudo-first-order model, (c1-c4) pseudo-second-order model, and (d1-d4) inter-particle diffusion model for ZnO/GO, ZnO, ZnO/GO/Zeolite, and GO/Zeolite, respectively

4.4. Maximum Capacity of the Adsorbents

The maximum capacity of an adsorbent is the maximum amounts (mg) of adsorbate that can be adsorbed on one gram of the material. Different sorbents have various adsorption mechanism or adsorption routes. Different adsorption isotherms are introduced by authors (48-51). In this work, two famous adsorption models, Langmuir (Equation 6) and Freundlich (Equation 7), were studied for the different adsorbents (48, 49). Adsorption models of the adsorbents are shown in Figure 6. As this figure shows, almost all of the nanocomposites follow the Langmuir adsorption model, which predicts a monolayer physical adsorption of the chloroform on the sorbents. This is more rational, because of the hydrophobic character of chloroform, which makes it possible to be adsorbed on the nanocomposites. Different isotherm parameters were also computed using the different isotherm models of interest (Table 3). As indicated by the results, ZnO/GO showed a greater adsorption capacity compared to the other nanocomposites. As evident from Table 3, the order of the maximum adsorption capacity of the nanocomposites was predicted as follows: (ZnO/GO) > (ZnO/GO/Zeolite) > (GO/Zeolite) > (ZnO NPs.). These results are consistent with those earlier obtained with BET and BJH models (Table 1).

Figure 6.

Different isotherm models, (a1-a4) Langmuir models, and (b1-b4) Freundlich models for ZnO/GO, ZnO NPs, ZnO/GO/Zeolite, and GO/Zeolite, respectively

4.5. Effect of the pH on the Removal of Chloroform

pH is a crucial condition for all aquatic environments. It also can affect chemical, physical, and microbial properties. For this reason, the pH of water samples was changed from 3 to 10. The different adsorbents showed different trends toward the removal of chloroform in a different pH. As indicated in the inset of Figure 7, about all of the adsorbents had better adsorption of chloroform in the pH range between 7 to 8 compared to the other pHs. This is due to the solubility of ZnO in acidic and alkaline media. ZnO/GO and GO/Zeolite even had shown remarkable adsorption in acidic regions because of the presence of GO in these harsh conditions. On the other hand, in alkaline regions, the removal of chloroform was diminished remarkably for all adsorbents. This may be due to the negative charge of the nanocomposites resulted from the adsorption of hydroxide ions in high concentrations in these regions. Hence optimal pH range for the best removal percentage for all of the adsorbents was concluded to be 7 to 8.

4.6. Regenerability of the Nanocomposites

From a commercial viewpoint, using regenerable adsorbents is necessary to reduce the cost of water treatment. Hence, the regeneration of the adsorbents is highly important. For this reason, four removal cycles were performed using sixteen industrial wastewater samples that were spiked with 50 mg.dm^-3 of chloroform. Afterward, four aliquots of 50 mL of the wastewater samples, containing chloroform, were treated with the different adsorbents in each removal cycle. After each chloroform-removal cycle, the nano adsorbents were regenerated by incubating in an electric oven at 150°C for 1 hour, during which the adsorbents were degassed. Then, the removal tests were performed with the rest of the wastewater samples in the same way. The results of the study are depicted in the inset of Figure 7. As shown in the figure, the approximately adsorptive character of the nano adsorbents remained intact after their repetitive usages. Indeed, the porosity of the nano adsorbents did not change during several usages, which confirms their stability and applicability.

Figure 7.

Regenerability of the nano adsorbents for removal of chloroform from 50 mg.dm-3 chloroform solutions after four times use (a) ZnO/GO, (b) ZnO/GO/Zeolite, (c) GO/Zeolite,(d) Zn NPs, Inset: Effect of pH on the removal of chloroform by using the different adsorbents: (a) ZnO/GO, (b) GO/Zeolite, (c) ZnO NPs and (d) ZnO/GO/Zeolite

4.7. Comparing ZnO/GO Nanocomposites with other Adsorbents

Different characteristics of the ZnO/GO nanocomposite were compared with reported adsorbents used for the removal of chloroform, and the results are summarized in Table 4 (23, 50-52). As seen, the proposed ZnO/GO had a greater maximum adsorption capacity and surface characteristics compared to the other adsorbents. The ZnO/GO showed a greater chloroform removal efficiency than the reported adsorbents. These results can be explained by the fact that both GO and ZnO have adsorption capacity toward chloroform, and GO acts as good support for dispersing ZnO NPs. Hence, the prepared nanocomposite indicated better surface characteristics and removal efficiency compared to the other adsorbents.