Averrhoa carambola: A Renewable Source of Oxalic Acid for the Facile and Green Synthesis of Divalent Metal (Fe, Co, Ni, Zn, and Cu) Oxalates and Oxide Nanoparticles

A green, simple, and environmentally benign synthetic approach has been utilised to obtain some bivalent metal oxalates from Averrhoa carambola juice extract, without any purification or special treatment of the juice. The main acid components (oxalic acid and ascorbic acid) of the juice were identified by HPLC technique. The effect of temperature on the purity of the product has been investigated. The as-synthesized metal oxalates were thermally decomposed at low temperatures to their respective metal oxide nanoparticles. The metal oxalates and their respective thermal decomposition products were characterized by Fourier Transform Infrared spectroscopy, X-ray diffraction analysis, and thermogravimetry. 1. Introduction Metal carboxylates have been widely investigated and have been used as precursors in the production of metal oxide nanomaterials [1–6]. Generally, these metal carboxylates are thermally decomposed at relatively low temperatures into the corresponding metal oxide nanomaterials. The precursors are usually obtained when the metal salt reacts with the free acid or any of its labile carboxylates. The reactants are generally synthetic, but many of these carboxylates are readily available in our environment (mostly in plants) and need only to be harnessed. They include, amongst others, citric acid, tartaric acid, and oxalic acid. The latter is known to be one of the major acids in many plants (e.g., Rhubarb and Averrhoa carambola). Averrhoa carambola, known as carambola or star fruit, is an attractive, exotic, tropical, and shrub-like ornamental tree of the Oxalidaceae family [7–10]. The ripe fruits can be eaten fresh or used to produce juice, jelly, or wine [11]. The fruit is also widely used in traditional medicine for the treatment of a wide range of ailments [10, 12]. It is also a potential source of pectin [12]. Carambola fruit juice has been shown [7, 13] to contain active constituents such as vitamins, amino acids, ascorbic acid, oxalic acid, tartaric acid, citric acid, carbohydrates, fats, and proteins. Assessment of the physicochemical properties of carambola fruit juice from ripe and unripe fruits showed that the pH of the fruits increased with advance in maturity, being 2.4 for green mature, 2.7 for half-ripe, and 3.44 for ripe fruits [7]. Averrhoa carambola is a very good source of oxalic acid, with oxalic acid contents that can be as high as 1？wt% of wet mass (representing about 74% of the total acid content depending on the level of maturity of the fruits) [7, 14, 15]. The high concentration of oxalic acid in this fruit is