Liquid–Liquid Extraction of Phenolic Compounds from Spent Sulphite Liquor

The sulphite pulping process for cellulose extraction consists of lignin depolymerisation of wood by sulphonation and hydrolysis leading to the production of spent sulphite liquor (SSL) as waste stream enriched in hemicellulose sugars and lignin derivatives. In this work, liquid–liquid extraction was used in order to separate phenolics from the sugar-rich SSL fraction. The effects of type of solvent, settling time, pH value, number of extraction stages and sample-to-solvent ratio were investigated. Folin–Ciocalteu and UNE-EN 16109 methods were used for the analysis of total phenolic content and total phenolic hydroxyl groups, respectively. The DPPH· method was used to determine the radical scavenging capacity of the extracts. Individual phenolic compounds were determined via HPLC–DAD analysis. Sugars and other compounds (5-hydroxymethylfurfural, furfural and acetic acid) were also quantified. Effective extraction of phenolic compounds was achieved using diethyl ether in a single extraction stage, at room temperature, at pH value of 3.6 and a sample-to-solvent ratio of 1:3 (v/v). The diethyl ether extract contained 6.31 g gallic acid equivalents per L, indicating strong antioxidant activity index (4.9), as well as 0.024 g/L 5-hydroxymethylfurfural, 0.025 g/L furfural, 2.89 g/L acetic acid and 1.4 g/L of sugars.