Multicompound biorefinery based on combined acid/alkaline-oxidative treatment of olive stones

Olive stones (OS) constitute the main solid by-product of the olive oil industry, whose main application is direct burning. As an alternative, this work proposes an integrated multiproduct OS biorefinery, to produce furfural, lignin, antioxidants and sugars. A combination of pre-treatments, i.e. dilute acid pre-treatment followed by alkaline peroxide delignification, was assessed in order to improve on the previously limited results reported for the same raw materials. The operational conditions were optimised through experimental design and response surface methodology, to produce the highest recovery of sugars (xylose) from the liquid fraction plus glucose from the enzymatic hydrolysis (EH) of the pre-treated solids. This resulted in 70% and 41% recovery, respectively, while delignification yields reached 51% at 7% H2O2, increasing EH yields up to 70.3%. Phenolic compounds of potential interest were detected in the liquors, up to 25.3 mg gallic acid equivalent/g OS. Solubilised lignin was recovered by acid precipitation, with a yield of 76.3%, representing 11 g lignin/100 g OS, with a calorific value of 21,733 kJ/kg. 26.5 g/L of furfural was obtained from the xylose-rich hydrolysate with a yield of 45.7%. Consequently, OS can be considered to be a promising raw material for a multiproduct biorefinery.