Mangrove studies: A source of tanning material

Abstract Mangroves are useful for tanning purposes, comprising a number of species of trees from various families. The commercially significant species, Ceriops tagal (perr.) and Rhizophora mucronata Lam. are characterised here. It has been found that bark of the Ceriops species contains a mean of 26.5% tannin while the bark of the Rhizaphora species contains a mean of 21.75% tannin. Chemically they are classified as catechol tannins. Introduction Mangroves are found in tropical and subtropical estuaries and sheltered coastal areas. Typical genera of these plants are Rhizophora, Ceriops and Avicenna. They tend to develop on mud flats and those that are technically eminent are described in detail. 1. Ceriops tagal: [^{2, 7}] A moderate sized tree, up to 8m tall with a grayish brown bark and adventitious roots from lower part of the stem. Leaves rarely broad, Flowers small 6-8mm long, Fruit ovoid, 1.5-2.5cm long. Botanical name - Ceriops tagal (perr.) Common name - Mangrove Family - Rhizophoraceae Distribution - Pakistan (Las Bella Dist, Miani Hor. and Kati Bander) India, Philippines, North Australia, Micronesia and African Coasts 2. Rhizophora mucronata Lam: [^{2, 8}] An evergreen tree up to 12m in height and 1.5m in girth. Bark is reddish to black with horizontal fissures encircling the stem. The tree possesses aerial and stilt roots. The leaves are broad coriaceous, 7-20 cm by  3-13 cm. Flowers yellowish-white, 4-8mm long. Fruit conical-ovoid, 4-5 cm. long, dull brownish green in colour. Botanical name - Rhizophora mucronata Lam. Common name - Mangrove Family - Rhizophoraceae Distribution - Pakistan (Las Bella Dist, Miani Hor. and Kati Bander) India, Philippines, North Australia, Micronesia and African Coasts Mangroves often occupy coastal locations. In Pakistan their forests are particularly thick and imposing from Ibrahim Hadri near Korangi creek for about 100 miles on the west southern coastline. Since these mangroves are the second largest in the world (in the arid zones), their potential is limitless. Since the turn of the century, mangroves have been recognized as a valuable resource. They are one of the major sources for charcoal, and mangrove wood is also used in shipbuilding because of its high termite and salt-water resistance. Method Various samples of Ceriops tagal (Perr.) and Rhizophora mucronata Lam. were collected from different areas, which represented the distributional range of mangroves from Pakistan. The air-dried portion of the bark from the respective species was used for characterisation to assess their potential use as tanning agents for the leather industry. Analytical solutions of each species were prepared using Koch's method6 of extraction for quantitative analysis. The tannin and non-tannin contents were determined by the hide powder or shaking method⁶. The details of data obtained are shown in Tables 1 and 2. The pH of the analytical infusion for each specimen was determined by means of a glass electrode pH meter (Model-pH HM-7E). In order to identify the nature of the tannin, the conventional methods⁴ of testing were performed on the extracted solutions. The observations noted against each test are given in Tables 3 and 4. Results and discussions The present work is carried out on various samples of bark from Ceriops tagal and Rhizophora mucronata species of mangrove to evaluate the tannin characteristics through different techniques. In qualitative tests, ferric chloride and iron alum tests give specific colour/precipitates showing that they contain catechol tannins. This is confirmed by the reaction with lead acetate, which gives no colour/no precipitate, while the addition of bromine water gives an immediate precipitate. Fehling's solution is also reduced by an aqueous solution of both Mangrove species, therefore, confirming the presence of free sugars in the extracted solution. From the leather manufacturing point-of-view, the catechol tannins are a more important tanning agent compared with the pyrogallol tannins. Of the total vegetable tannins consumed by the industry, 65-70% are catechol tannins³. The mean distribution of tannins in the bark of Ceriops tagal is 26.5% and in the bark of Rhizophora mucronata is 21.75%, which are quite satisfactory from a technical and commercial point-of-view, because these plant materials are abundantly available in the country. The materials that contain less than 10% of tannin cannot be economically extracted, and for comparison some important vegetable tanning materials are given in Table 5. An examination of Table 5 shows that the percentage of tannins in Ceriops and Rhizophora species are quite high as compared with the tannin of Avaram bark, Babul bark, Chestnut wood, Hemlock bark, Oak bark and Quebracho wood, which are commonly used throughout the world. As per tanning properties the Mangrove liquors are extremely stable, reddish brown in colour and weak acidic in nature (pH value 4.8 to 5.2). Furthermore they are very astringent tanning materials and when used alone impart a red colour to the leather. Mangrove is probably, therefore, most useful in the manufacture of heavy leathers. References: 1. E Kirk Raymond, F Othmer Donald et al, Encyclopedia of Chemical Technology (The inter Science Encyclopedic Inc, New York) Vol. 13, p579 (1994). 2. E Nasir, S I Ali et al, Flora of Pakistan - Family Rhizophoraceae, Fakhri Press, Karachi, p3 & 7 (1980). 3. G Thyagarajan, Indian Leather, Vol. 27, p20 (1994). 4. H J Emeleus, M A Whiteley et al, Thorpe's Dictionary of Applied Chemistry, Longmens, Green and Co, London, Vol. XI, p390 (1954). 5. V S Sundara Rao, H Purushotham et al, Pakistan Leather Trade Journal, Vol. 23, p80 (1996). 6. SLTC, Official Methods of Analysis, p48 (1981). 7. The wealth of India, Council of Scientific & Industrial Research, Delhi, Vol. II, p124 (1950). 8. The wealth of India, Council of Scientific & Industrial Research, Delhi, Vol. IX, p9 (1970). Acknowledgements The author wishes to express his special thanks to Fayaz Ahmed, Range Forest Officer, Malir Halt, Karachi, for providing the mangrove bark throughout the work. Thanks are also due to Dr Hasan Akhtar Khan, PSO and M Ishaq Subhopoto, SSO, Leather Research Centre, Karachi, for taking a keen interest and fruitful discussions.