(31i) Effective Cationic and Anionic Dye Removal from Wastewater Using Multi-Functional Adsorbent

The global issue of water pollution is becoming increasingly severe, driven by rapid industrial development and significant population increases. A major contributor to this problem is the industrial sector, particularly textiles, paper, and paint manufacturers, which release effluents containing various artificial colorants. The released dyes, like methyl orange (MO) and crystal violet (CV), not only affect water aesthetics but also pose serious environmental risks. When these dyes enter aquatic ecosystems, they impede light transmission, disrupting the process of photosynthesis in aquatic plants and hindering the growth of marine life [1].

Moreover, these dyes possess complex molecular structures that make them resistant to photodegradation and oxidation, allowing them to remain active in the environment for extended periods of time. The impact of these pollutants extends beyond ecological concerns. For example, MO has been linked to genetic mutations and cancer development [2]. Similarly, CV, which is widely used across various industries, is recognized as a potential carcinogen and can cause irritation to both the skin and digestive system [3].

Numerous approaches have been explored to tackle the issue of dye-polluted water, spanning from chemical processes such as coagulation with various agents, and oxidation techniques, to more sophisticated methods like solvent extraction and reverse osmosis [4]. Among these diverse strategies, adsorption stands out as a particularly effective and adaptable solution. This method, which involves capturing dye molecules on a specific adsorbent, offers an efficient, environmentally friendly, and economical approach to water treatment.

Researchers have investigated a wide range of synthesized adsorbents, including zeolites, bentonites, and activated carbons, for their potential in dye removal. Scientific literature and research studies emphasize the effectiveness of the adsorption process, highlighting its widespread use in removing colored organic contaminants from wastewater using suitable adsorbents [5]. In the ongoing efforts to develop sustainable water treatment methods, adsorption presents a promising solution to the persistent problem of synthetic dye pollution, underscoring its importance in environmental conservation efforts.

Despite the abundance of adsorbent-related technologies, there remains a critical need to develop new adsorbent materials with improved characteristics. These include enhanced adsorption capacities, significant ion exchange capabilities, and the ability to be easily regenerated and reused. The continued pursuit of such advanced adsorbents is essential for further improving water treatment processes and addressing environmental challenges more effectively.

This study details the preparation of a novel nanocomposite, combining functionalized graphene oxide (FGO), bentonite (BT), and ternary layered double hydroxide (TLDH), referred to as FGO/BT/TLDH nanocomposite. This nanocomposite was, then, utilized to remove two harmful azo dyes, namely, crystal violet and methyl orange, from contaminated water samples.

The synthesis process began with the production of graphene oxide (GO) using the improved Hummers method. GO was then functionalized with polyethyleneimine (PEI) through a series of steps involving dispersion, sonication, and refluxing. The successful integration of PEI into GO was indicated by a color change from yellow-brown to black. The resulting FGO was purified, dried, and ground into a fine powder.

The FGO/BT/TLDH nanocomposite was then prepared using a co-precipitation technique. This involved preparing two solutions: one containing sodium carbonate and sodium hydroxide, and the other one containing various metal nitrates. Concurrently, an FGO/BT nanocomposite was prepared by dispersing bentonite in water, adjusting its pH, and combining it with FGO through sonication. The two prepared solutions were then carefully added to the FGO/BT mixture, maintaining specific pH levels and ratios. After extended stirring, washing, and drying processes, the final FGO/BT/TLDH nanocomposite was obtained.

For comparative purposes, TLDH was synthesized using the same method but without the inclusion of FGO and BT. The newly prepared nanocomposite and its constituents were thoroughly characterized using various analytical techniques, including BET surface area analysis, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). These analyses confirmed the successful synthesis of the FGO/BT/TLDH nanocomposite. After successfully synthesizing the FGO/BT/TLDH nanocomposite, its effectiveness in removing methyl MO and CV through adsorption was assessed and compared to the performance of its individual components: pristine GO, FGO, TLDH, and BT. As illustrated in Fig. 1, the FGO/BT/TLDH nanocomposite demonstrated superior adsorption capabilities for both CV and MO compared to its constituents.

Specifically, the adsorption capacity of CV onto the nanocomposite is about 8.5, 1.5, 1.3, and 1.2 times higher than those obtained using FGO, TLDH, BT and GO, respectively, while the adsorption capacity of MO onto the nanocomposite is about 2.2, 1.3, 1.09, and 1.07 times higher than those obtained using FGO, GO, TLDH, and BT, respectively. The enhanced performance of the FGO/BT/TLDH nanocomposite can be attributed to a combination of adsorption mechanisms, including electrostatic interactions, π−π stacking, hydrogen bonding, and pore filling. These diverse mechanisms likely contribute to the nanocomposite enhanced efficiency in removing both CV and MO from aqueous solutions.

Following the demonstration of the FGO/BT/TLDH nanocomposite superior performance, further studies were conducted to explore the effects of process parameters. These investigations examined the impact of contact time (adsorption kinetics), initial concentrations of CV and MO (adsorption isotherm), as well as the influences of pH and temperature. Additionally, the nanocomposite ability to be regenerated and reused was assessed. The adsorption kinetics data for MO and CV onto the FGO/BT/TLDH nanocomposite were analyzed using two models: the pseudo-first order (PFO) and the pseudo-second order (PSO). The results indicated that the PSO model provided a better fit to the experimental data, with a coefficient of determination (R²) of 0.9901 and 0.9419, compared to 0.9865 and 0.8598 for the PFO model, respectively.

For the adsorption isotherm analysis, both the Langmuir and Freundlich models were applied to the experimental data. The Langmuir model was found to offer a more accurate replication of the adsorption process. Using this model, the maximum adsorption capacities (q_max) of CV and MO under adsorbent saturation conditions were predicted to be 1172 and 1705 mg/g.

The study also examined the influence of pH on the adsorption of CV and MO by the FGO/BT/TLDH nanocomposite; the results of the pH effect are shown in Fig. 2. For CV, the optimal pH was found to be 6, at which the adsorption capacity reached 623 mg/g from a 100 ppm dye solution. CV adsorption decreased at pH levels both below and above 6, with capacities of 47 mg/g at pH 2 and 439 mg/g at pH 8. For MO, the adsorption was most effective at pH 2, with the nanocomposite achieving an adsorption capacity of 1084 mg/g from a 100 ppm dye solution. The lowest adsorption capacity for MO, 106 mg/g, was observed at pH 12.

Moreover, thermodynamic analysis revealed different behaviors for the two dyes. MO adsorption onto the FGO/BT/TLDH nanocomposite was found to be exothermic, while CV adsorption exhibited endothermic nature. The study also included an evaluation of the nanocomposite reusability, highlighting the comprehensive nature of the investigation into its adsorptive properties and potential practical applications.

In conclusion, the FGO/BT/TLDH nanocomposite demonstrates significant potential as an effective adsorbent for water purification. However, its full capabilities have yet to be fully explored. Further research is necessary to assess the nanocomposite efficiency in removing a broader spectrum of water contaminants, including both organic and inorganic substances.

Of particular importance is the need for comprehensive evaluation of the FGO/BT/TLDH nanocomposite performance in treating real wastewater samples. This crucial step would provide valuable insights into its practical applicability in real-world water treatment scenarios. Such studies would help bridge the gap between laboratory findings and industrial applications, potentially paving the way for innovative solutions in water decontamination technology.

Fig. 1: Benchmarking the adsorption of CV and MO onto the FGO/BT/TLDH nanocomposite to their adsorption onto TLDH, BT, FGO, and GO. These adsorption experiments were conducted at C_o = 100 mg/L, T= 25 ℃, t = 20 h, adsorbent dose = 50 mg/L, and pH = 6.

Fig. 2: pH effect on the CV and MO adsorption onto the FGO/BT/TLDH nanocomposite. These adsorption experiments were conducted at C_o = 100 mg/L, T= 25 ℃, t = 20 h, and adsorbent dose = 50 mg/L.

References

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