Fractionation and valorisation of bark extractives from Eucalyptus species.

Abstract

The notion of zero-waste in industrial production processes of widely used materials has gained momentum, as industries aim to gain revenue from waste materials which were previously considered as waste. Tree bark from wood obtained from sustainably managed plantations used in the production of timber and pulp industries is an underutilised waste that is mainly used for energy production in mills or left on plantations after debarking. Eucalyptus tree species are commonly used as raw-material in the pulp and paper industry throughout the world. In this thesis, the potential beneficiation of bark from South African planted Eucalyptus tree species (Eucalyptus grandis, Eucalyptus dunnii, Eucalyptus smithii and Eucalyptus nitens) as a source of valuable materials is investigated. Secondary metabolites such as phenolic components, terpenes, steroids and alkaloids can extracted from Eucalyptus bark. In this study, accelerated solvent extraction was investigated for extraction of components in the bark and the components were characterised by a variety of analytical techniques. The investigation was undertaken by optimising the extraction process using the following parameters, temperature (80 to 160℃), number of static extraction cycles (1-3 cycles), solvent type (80% v/v of ethanol or 50% acetone v/v), Eucalyptus species, and particle size (850-500 μm, 500-375 μm and <375 μm). The extraction process was optimised and response surface methodology (RSM) was used to model experimental data for statistical analysis of the Box-Beheknen design of the extraction process. A quadratic model was fitted, and optimum extraction parameters were a temperature of 117℃ with greater than 2 number of static extraction cycles, and bark particles greater than 355 μm. The two target objectives were total phenolic content and total extractive content when using ethanol as the extraction solvent. The amount of phenolic components in the bark extracts was determined by the Folin–Ciocalteu method which using gallic acid as a calibration standard and detection on a UV-vis spectrophotometer. Amongst the four Eucalyptus bark species studied, Eucalyptus dunnii contained the highest amount of phenolic components (5.52g/100g GAE) In addition to using the Folin–Ciocalteu method, the chemical compositions of acetone and ethanol extracts of the bark samples were determined using Pyrolysis-Gas Chromography/Mass Spectrometry. The analysis showed that condensed tannins, with a building block of catechol units, were the most abundant phenolic components present in the bark extracts. Other components that were detected in high amounts were terpenes and terpenoids, and smaller amounts of monoterpenes and sesquiterpenes. Steroid components were also detected, with 𝛽-sitosterol being the most predominant one. The extractive-free bark samples, remaining after removal of solvent extracts, was analysed using high pressure liquid chromatography to determine their chemical content of cellulose, hemicellulose and lignin.