3.3. Black Liquor Composition

The characterization of the obtained black liquors was determined using different laboratory analyses. The pH was analysed using a Crison GLP 21 pH meter (Crison Instruments, Barcelona, Spain). The density was determined by dividing their total mass by its total volume. The total dry solids (TDS) of these liquors was determined by drying samples in porcelain crucibles at 105 °C for 24 h. Afterwards, the samples were heated in a muffle at 575 °C for 3 h to measure the ash content [39]. A simple mass balance was performed to determine the lignin content of these spent liquors. Additionally, electrical conductivity was analysed with a Crison GLP 31 conductivity meter (Crison Instruments). All analytical measurements were performed in triplicate.

An aliquot of the different liquors was subjected to quantitative posthydrolysis with 4% H₂SO₄ at 121 °C for 60 min to determine its monosaccharides, acetic acid, furfural and hydroxymethylfurfural (HMF) content. Afterwards, the resulting supernatant was filtered through a 0.45-µm membrane filter before high-performance liquid chromatography (HPLC) analysis. HPLC analyses were performed using an Aminex HPX-87H column (Bio-Rad, Berkeley, CA, USA) at 30 °C eluted with 0.01 M H₂SO₄ at a flow rate of 0.6 mL∙min⁻¹ using a refractive index (RI) detector to quantify glucose, xylose, arabinose, acetic acid, HMF and furfural [49].