Pretreatment of trace element-enriched biomasses grown on phytomanaged soils for bioethanol production

Phytotechnologies are a set of sustainable, ecological options notably for alleviating pollutant linkages induced by contaminated soils. However, concerns exist regarding the processing of trace elements (TE)-enriched biomass and the fate of TEs in the end-products. The fractionation of phytoremediation TE-enriched non-woody lignocellulosic (tobacco) and woody (birch, willow) phytoremediation-borne biomasses was studied using ethanol organosolv, soda and dilute acid pretreatments. TE distribution in the process fractions (pulp, liquid effluents and lignin) was further examined. In dilute acid conditions, a wood pretreatment performed at 170 °C in the presence of 2% w/w of sulfuric acid allowed an efficient extraction of the metals in the water effluent (≈90% for Mn and Zn) producing a clean pulp. A soda pretreatment resulted in a low metal extractability, especially at high temperature. At 170 °C or above with 15% w/w of NaOH, metals were mostly recovered in the cellulosic pulp (70%- 98%). Using organosolv pulping, the TE contents were in the decreasing order: pulp ≫ water stream > lignin. The TE organosolv extraction in the liquid phase varied depending on the TE metal and on the species but was generally low. Metal extractability increased with the water content in the solvent of the organosolv pulping but decreased with temperature. Enzymatic hydrolyses of TE-enriched and metal-free pulps were investigated and it was concluded that the TE concentrations had little or no effect on polysaccharide enzymatic hydrolysability by cellulases. The final destination of the various fractions for valorization in a biorefinery setting is discussed, regarding their TE content.