To investigate whether hydrothermal liquefaction (HTL) could degrade perfluoroalkyl acids (PFAAs) accumulated in plant biomass, we first evaluated degradation of individual and mixed PFAAs in pure aqueous solutions. It was found that all five perfluorocarboxylic acids (PFCAs) were removed completely after 2 h at 300 °C. Three perfluorosulfonic acids (PFSAs) had removal efficiencies of less than 20%. With the amendment of KOH, however, the removal of PFSAs by HTL increased significantly to 85.9 ± 1.2%. HTL also removed PFAAs accumulated in common cattails (Typha latifolia). Regarding PFCAs, nearly 100% disappearance after HTL was observed. Specific to perfluorooctanesulfonic acid (PFOS), one of the PFSAs, the removal of this compound in roots was 98.4%. For shoots, it was 49.7%. These promising results point to the need for further investigation so that HTL can be optimized to handle biomass of plants used for phytoremediation of PFAS.

We conducted experiments for the hydrothermal liquefaction (HTL) of binary mixtures of biomass components at 300, 350, and 425 °C and then developed a component-additivity model that accounts for interactions among biomass components during HTL and predicts the oil yields for processing biomass mixtures in sub-, near-, and supercritical water. The experimental work provided new insights about the interactions between different biomass components during HTL. Specifically, the interaction and extent of synergy between soy protein and cellulose was a function of the relative amounts of the two materials. Moreover, alkaline lignin has a stronger synergistic effect when processed with cellulose and starch, whereas dealkaline lignin has a stronger synergistic effect with stearic acid. These differences could not be attributed solely to the influence of pH, so there must be other factors that influence interactions of lignin with other biomass components during HTL. The model predicted 70% of the 141 literature bio-oil yields considered to within 10 wt % and performed better by this metric than did prior component-additivity models. Parameterizing the model at different temperatures and including a composition-dependent interaction between cellulose and protein are at the heart of this improvement.

Three different potassium phosphates (KH 2 PO 4 , K 2 HPO 4 , and K 3 PO 4 ) were tested as potential homogeneous catalysts for the hydrothermal liquefaction of soybean oil, soy protein, potato starch, microcrystalline cellulose, and their mixture. Na 2 CO 3 and KOH, which have been widely applied in hydrothermal liquefaction (HTL) reactions, were also investigated for comparison to phosphates. The addition of K 2 HPO 4 and K 3 PO 4 , which form basic solutions, led to yields of biocrude from HTL of starch and cellulose that were sometimes 2−5 times higher than those without phosphates. Adding Na 2 CO 3 , which also forms a basic solution, for HTL of polysaccharides also generated higher biocrude yields but not as high as those obtained with added phosphates. The use of KOH (another base) resulted in the highest yield of biocrude from HTL of the mixture. These additives, along with Na 2 CO 3 , also resulted in less solid residue being produced. The additives had almost no positive effect, however, on biocrude production from HTL of soybean oil or soy protein. The biocrudes produced from polysaccharides with added K 2 HPO 4 , K 3 PO 4 , and Na 2 CO 3 have larger heating values and greater energy recoveries. The biocrudes mainly consist of acids/esters, alcohols, phenols, and ketones/aldehydes. The addition of phosphates led to a higher proportion of ketones/aldehydes at the expense of alcohols.

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Summary We produced oils via hydrothermal liquefaction (HTL) of binary mixtures of biomass components (e.g., lignin, cellulose, starch) with different plastics and binary mixtures of plastics themselves. Cellulose, starch, and lignin demonstrated synergistic interactions (i.e., enhanced oil yields) with the plastics tested (polypropylene, polycarbonate, polystyrene, and polyethylene terephthalate). Polystyrene exhibited synergy during HTL with the three other plastics as did polypropylene during HTL with PET or PC. We used the experimental results to develop the first component-additivity model that predicts the oil yields from HTL of biomass-plastic and plastic-plastic mixtures. The model accounts for interactions among and between biomass components and plastic components in sub-, near-, and supercritical water. The model predicts 88% of 48 published oil yields from HTL experiments with mixtures containing plastics to within 10 wt%.