Persimmon Tannin Gel: Formation by Autoxidation and Caffeine Adsorption Properties

Abstract

Water-insoluble gel was prepared from persimmon tannin (PT) solution by the autoxidation process. The gelation reaction proceeded by applying oxygen gas and natural light to the aqueous PT solution, without using harmful reagents or catalysts. Adsorption behavior of the PT gel was assessed using caffeine. Batch adsorption studies were conducted to evaluate the influence of experimental parameters such as contact time, initial concentration of adsorbates, adsorbent dose and temperature. The equilibrium data were analyzed using Freundlich and Langmuir isotherm models. Thermodynamic parameters such as Gibbs free energy, enthalpy, and entropy changes were also calculated, showing that caffeine adsorption on the PT gel is an exothermic process and feasible at lower temperatures. The present results suggest that the PT gel can be used as an effective adsorbent for removing caffeine from solutions.Keywords: adsorption, autoxidation, caffeine, gel, isotherm, persimmon tannin *To whom correspondence should be addressed. E-mail: loahmad@stu.kanazawa-u.ac.jp (L.O. Ahmad) kunimoto@se.kanazawa-u.ac.jp (K. Kimoto) IntroductionTannins are a group of polyphenol compounds that are distributed widely in many plant species, where they serve as defense mechanisms against predators. Persimmon fruits (Diospyros kaki L.) are rich in soluble and non-soluble condensed tannins (proanthocyanidins). Low molecular weight soluble tannins are believed to be responsible for the astringency of the persimmon fruit (Matsuo and Ito, 1978). Tannins are also known to form complexes and precipitates with macromolecules, such as proteins and polysaccharides, alkaloids, and heavy metals (Haslam, 1989).Chemical characterization of persimmon tannin (PT) has long been hampered because of its heterogeneous character and high molecular weight. Matsuo and Ito (1978) first reported on the composition and structure of persimmon condensed tannin. Their proposed structure consists of coupled flavan-3-ols, catechin, catechin-3-O-gallate, gallocatechin, and gallocatechin-3-O-gallate residues as the repeating units, with a molar ratio of 1:1:2:2, respectively. They also showed that the PTs are large molecular weight (ca. 13.8 kDa) polymers belonging to the proanthocyanidin B group, with carbon-carbon interflavan linkages between C-4 of one unit and C-8 (or C-6) of another. According to Xu et al. (2012), the stereochemistry of the flavan-3-ol unit of the PT is mainly 2,3-cis, which corresponds to epi-type catechins as shown in Fig. 1.Kaki-shibu, a fermented product made from the juice of immature astringent persimmons, is an important source of condensed tannins traditionally used in tanning paper (fiber hardening) and fishing nets (corrosion prevention), and for removing protein during the brewing of rice wine "sake" and soy sauce in Japan (Sugiura and Subhadrabandhu, 1996). PT gel was used in all adsorption experiments.A stock solution of caffeine (1,000 mg/L) was prepared by dissolving the required amount of caffeine in ultrapure water. The stock solution was ...

Immobilization of Persimmon Tannin on Cellulose Viscopearls and Removal of Caffeine from Aqueous Solution

“…This is due to the increase in the number of caffeine molecules competing for available binding sites on the PTIC gel. Thus, the available active sites of the PTIC gel become saturated at higher concentration of caffeine [10,23]. …”

“…However, tannins are water-soluble compounds, and non-practical on aqueous systems, which restrict their application as an adsorbent. This disadvantage can be overcome by crosslinking gelation or immobilization of tannins into a water insoluble matrix [7,13] and autoxidation [10]. Therefore, various methods have been reported for gelation, using formaldehyde or other aldehydes in a basic or acidic medium [14,15], and by acidic gelation [16,17].…”

“…Persimmon fruit (Diospyros kaki L.) are rich in soluble and non-soluble condensed tannins (proanthocyanidin) [10]. It is a fermented product (kaki-shibu) made from the juice of immature persimmons, and an important source of condensed tannins.…”

“…Kunimoto [18] reported a method to prepare non-water-soluble tannins by autoxidation and tested for adsorption of green tea components in which the gel was found to be selective for caffeine. Caffeine is responsible for the stimulating effect of tea, while the health benefits are mainly due to catechins [10]. In recent years, there has been increasing interest for developing an environment-friendly method for removing caffeine from the food industry and great efforts have been made by immobilizing tannins onto various water-insoluble matrices such as agarose, viscose rayon fiber [19], cellulose [14], and other matrices [20].…”

Study of Equilibrium and Kinetics of Pb(II) in Solution Using Persimmon Tannin Gel as an Adsorbent

“…The resulting filtrate was analyzed using an atomic absorption spectrophotometer to determine the metal ions that are absorbed by the adsorbent. Several methods have been reported in the preparation of tannin gels, mostly using formaldehyde or aldehydes in acid or base media and autoxidation [12]. Furthermore, the gel has been made from persimmon tannins using formaldehyde cross-linking agents and shows a high affinity for methylene blue dyes [ 5 ].…”