(140f) Fabrication of Novel ZIF-8/Layered Triple Hydroxides Composites for Efficient Adsorptive Removal of Azo Dyes from Wastewater

The rapidly escalating global freshwater crisis stemming from the rising global population, industrial growth and climate change exerts considerable pressure on the existing freshwater resources and adversely affect the water quality. Several industries including textiles, food processing, manufacturing, printing, pharmaceuticals and cosmetics industry generate vast quantities of wastewaters containing tons of dyes and other organic pollutants. The generated wastewaters are discharged into water bodies bringing and eventually accumulating these toxic compounds into the aquatic ecosystem. Therefore, such large quantities of wastewater streams must be efficiently treated to minimize the discharge of these organic pollutants below the acceptable limits.

Organic dyes form a major class of organic pollutants often found in water bodies. Commonly used synthetic dyes such as anthraquinone, xanthene, quinonimine and azo dyes have been identified as major pollutants in water bodies. Synthetic dyes are mostly non-biodegradable owing to their complex aromatic structures and can bioaccumulate and contaminate the aquatic food chain. Improper treatment of dye-contaminated wastewaters could adversely affect public health and the ecosystem. For instance, commercially utilized dye molecules like the anionic mono-azo dye (Acid Red 1) have been recognized as potential carcinogens due to the production of aniline in the human body upon consumption. Thus, the application of highly efficient dye removal techniques for treating wastewaters is paramount.

A wide range of conventional techniques have been implemented over to date in treating wastewaters containing high concentrations of synthetic dyes. These methods include sedimentation, flotation, coagulation-flocculation, filtration, adsorption and advanced oxidation processes. Among these techniques, adsorption has been widely applied for the removal of synthetic dyes from industrial wastewater streams due to the associated advantages with this technique such as ease of operation, versatility, high removal efficiency, inexpensiveness and scalability. Various adsorbents typically utilized for the efficient removal of organic dyes from wastewater effluents demonstrate low adsorptive uptakes, selectivity and reusability towards synthetic dyes. Therefore, the development of novel adsorbents presenting high selectivity and adsorption capacities for azo dyes alongside high regenerability and reusability is extremely significant in current research revolving around adsorption-based wastewater treatment.

Recently, the application of metal-organic frameworks (MOFs) as adsorbents has been gaining prominence in wastewater treatment. This can be attributed to their superior adsorption capacities, high selectivity and large specific surface area in their three-dimensional framework structures. Among the MOFs, zeolite imidazole frameworks (ZIFs) are prominently utilized in the adsorptive removal of heavy metals, dyes and other organic pollutants from wastewaters owing to their high structural and chemical stability and large surface area. However, pristine ZIFs exhibit a tendency to agglomerate in aqueous solutions, which hampers their adsorption efficiencies, limits their recovery from the aqueous solutions, and negatively impacts their regeneration and reusability. Therefore, ZIF-based composites are increasingly being investigated, which are anticipated to demonstrate improved structural and functional properties, thermal and chemical stability, specific surface area, porosity and dispersion in aqueous solutions.

Therefore, in the present work, ZIF-8 was combined with MnCuAl-layered triple hydroxide (MnCuAl-LTHs)/layered triple oxides (MnCuAl-LTOs) to fabricate novel composites exhibiting enhanced adsorption properties that can efficiently remove commonly occurring water pollutants like dyes from wastewaters. Herein, a series of novel adsorbents were fabricated by coupling ZIF-8 with MnCuAl-LTH/MnCuAl-LTO in different ways and characterized to examine their crystallinity, structural properties and the functional groups present on the adsorbents. These novel adsorbents were then applied to remove Acid Red 1 (AR1) azo dye from contaminated water samples. The adsorptive performance of the composites fabricated in this study, namely MnCuAl-LTH@ZIF-8, MnCuAl-LTO@ZIF-8, and ZIF-8@MnCuAl-LTH, were contrasted to that of the parental materials (i.e., MnCuAl-LTH, ZIF-8 and MnCuAl-LTO) towards the removal of AR1 azo dye from synthetic wastewater samples. Each of the composites outperformed its corresponding parental materials in terms of their adsorptive uptake of AR1 azo dye, as seen in Figure 1 (a). Among the composites, ZIF-8@MnCuAl-LTH demonstrated the most superior performance in the removal of AR1 azo dye from the wastewater samples, followed by MnCuAl-LTH@ZIF-8 and MnCuAl-LTO@ZIF-8. The excellent performance of ZIF-8@MnCuAl-LTH composite was primarily attributed to its high characteristic pore volume of 0.236 cm³/g in comparison to the MnCuAl-LTH@ZIF-8 and MnCuAl-LTO@ZIF-8 composites with respective pore volumes of 0.125 and 0.082 cm³/g.

Various adsorption mechanisms were proposed to play a role in the enhanced adsorption performance of each composite, especially the ZIF-8@MnCuAl-LTH composite. The presence of electrostatic attraction, pore filling, and π–π stacking are expected to be the dominant underlying adsorption mechanisms. However, other interactions such as van der Waals, hydrogen bonding and acid-base interactions were also predicted to potentially contribute to the adsorption performance of the composites. Figure 1 (b) illustrates these potential combinations of mechanisms that boost the adsorption performance of the fabricated ZIF-8@MnCuAl-LTH composite. It was also noted that the agglomeration of the composite adsorbents in the wastewater was minimum for ZIF-8@MnCuAl-LTH in contrast to the MnCuAl-LTH@ZIF-8 and MnCuAl-LTO@ZIF-8 composites, which further ensures a higher adsorption capacity of the composite adsorbent. Further, ZIF-8@MnCuAl-LTH exhibits a cage-type mesoporous structure that potentially facilitates greater adsorption of AR1 azo dye on both internal and external surfaces of the adsorbent. Other factors such as the impact of the wastewater pH on the adsorptive uptake of ZIF-8@MnCuAl-LTH was also determined in this study. The maximum adsorptive uptake of 664.5 mg/g was achieved at pH 8 and the adsorption performance was largely influenced by the medium pH.

Finally, the ZIF-8@MnCuAl-LTH composite could be suitably regenerated and reused up to five cycles with a much lower final drop in adsorption capacity of 22% for AR1 azo dye removal from wastewater samples compared to other absorbents examined in existing literature. In conclusion, the ZIF-8@MnCuAl-LTH composite is easy to synthesize, demonstrates excellent adsorption uptake of anionic AR1 azo dyes in wastewaters, and could be effectively reused over multiple adsorption-desorption cycles. Overall, the fabrication of ZIF-8/MnCuAl-layered triple hydroxide/oxide composites and their application as adsorbents in wastewaters for the removal of any type of pollutant has been conducted for the first time to the best of our knowledge, thereby making unique and significant contributions to existing literature pertaining to adsorption studies.