Adsorption of flavonoids using Zinc Aluminium Layered Double Hydroxides-kinetic, and isotherm studies

Flavonoids as a class of natural compounds with medicinal properties possess high pharmacological potential such as anticancer, antioxidant, antiviral, and anti-inflammatory properties. they are highly present in natural resources such as fruits (apples, grapes, and berries), and vegetables (onion, garlic, and cruciferous vegetables). In this regard finding affordable and accessible resources to fulfill of finding flavonoids is important. Onion peels are waste materials and are also rich in flavonoid compounds moreover, red onion peels are selected based on having the most flavonoids. To make an extract consisting of flavonoids from red onion peels first step is to synthesize an aqueous mixture of ethanol as a green solvent and the second step is to immerse onion skin in ethanol/water for a specific period of time. There are a wide variety of parameters that are involved in boosting adsorption efficiency such as using ultrasound wavelength, increasing temperature, and changing in water and ethanol ratio as solvent mixtures. the extraction process is a crucial way to adsorb flavonoids from natural resources and one of the proposed factors to improve the yield of extraction is adsorbent selection. Besides traditional extraction methods such as Soxhlet extraction and maceration, nowadays using novel extraction methods plays a critical role in improving extraction yield. In this work, zinc-aluminum layered double hydroxide as 2D nanosheets which is synthesized by co-precipitation method, is used as an adsorbent. LDH is effectively used as a desired solution for improving extraction yields. Layered double hydroxides are two-dimensional materials belonging to the anionic clay family which is reputed for adsorbent-specific behavior, anion exchange properties, and biocompatibility. Adsorption occurred from the onion peel solution in contact with zinc-aluminum LDH with different molar ratios from 1:1 to 1:3, looking out over the influence of the onion peel extract purity defined as distilled water/ethanol ratio, the temperature of the adsorption media, and the Zn2+/Al3+ ratio. In this regard, different concentrations of ZnAl-LDH for removing flavonoid compounds from onion peels extraction were used. The results of XRD patterns and FTIR for LDHs prove layered double hydroxides structure which is synthesized. For the determination of total flavonoids, the addition of AlCl3 finally results in the forming of yellow-colored Al(3)-flavonoid complexes. subsequently, absorbance was measured after half an hour at a specific wavelength in the range 410 – 440 nm. Finally, results show ZnAl-12-LDH had maximum adsorption capacity for flavonoids in comparison to other ratios of Zn and Al in LDHs structures and adsorption process is determined by the kinetic model of pseudo-second-order (R2 9891) and the model of Langmuir isotherm(R2 0.9792) showed better adsorption isotherm compared to Freundlich isotherm(R2 0.8592).