Valorization of paper mill sludge using protic ionic liquids and deep eutectic solvents as a potential feedstock for biorefineries

In this article, protic ionic liquids (PILs) and deep eutectic solvent (DESs) were explored to enrich the cellulose content of a paper mill sludge (PMS) received from a sanitary paper manufacturing plant. Compositional, morphological, physico-chemical, and thermal decomposition features of PMS were monitored using XRF, SEM, FTIR, XRD, and TGA. Triethylammonium hydrogen sulfate (TEAHSO₄), which functioned selectively for the elimination of mineralogical content and cellulose enrichment of PMS, enhanced the cellulose content of the material from 45% to 74% following the pretreatment at 150 °C. Though DESs such as choline chloride-urea behaved poorly toward demineralization relative to PILs, they preserved cellulose effectively in the structure and gave satisfactory saccharification yields that reached over 90%. In addition to integrating the sustainable benefits of PILs and DESs into the valorization of PMS for the first time in literature, this work highlighted the competency of PMS as a cellulose-rich feedstock for biorefinery applications.