Extraction and characterization of lignin from olive pomace: a comparison study among ionic liquid, sulfuric acid, and alkaline treatments

Olive pomace is a phytotoxic by-product in the olive oil production. Lignin is a biopolymer present in the olive pomace in remarkable percentages, which has a great variety of potential industrial uses. The extraction of lignin using the ionic liquid triethylammonium hydrogen sulfate resulted in recovery yields as high as 40% of the available lignin in the dry olive pomace. This percentage was obtained after optimizing conditions such as temperature, extraction time, and water content in the ionic liquid. This is the first time such a high percentage of extraction has been achieved when evaluating this type of feedstock. For comparison, two other extraction methods (sulfuric acid and alkaline treatments) were used to assess their extracting performances. Lignin was quantified after developing a rapid, robust, and reliable method by Fourier transform infrared spectroscopy (purity of 101 ± 16%) and characterized by proton nuclear magnetic resonance and gel permeation chromatography. The assay total phenolic content (TPC) revealed high content of phenolic groups (212 ± 26.9 mg of gallic acid equivalents per g of lignin). The high purity and TPC conferred on the extracted lignin a potentially high antioxidant activity. In addition, a 67-fold scale-up extraction of initial mass loading was performed obtaining same results as in the lower scale. Thus, the extraction of lignin using this methodology is expected to mitigate the disposal of the olive pomace and provide certain revenue to the oil mill.