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Food waste is a promising resource for the production of fuels and chemicals. However, increasing plastic contamination has a large impact on the efficiency of conversion for the more established biological routes such as anaerobic digestion or fermentation. Here, we assessed a novel route through the hydrothermal liquefaction (HTL) of a model waste (pistachio hulls) and polypropylene (PP). Pure pistachio hulls gave a biocrude yield of 34% (w/w), though this reduced to 16% (w/w) on the addition of 50% PP in the mixture. The crude composition was a complex blend of phenolics, alkanes, carboxylic acids, and other oxygenates, which did not change substantially on the addition of PP. Pure PP does not breakdown at all under HTL conditions (350 °C, 15% solids loading), and even with biomass, there is only a small synergistic effect resulting in a conversion of 19% PP. This conversion was enhanced through using typical HTL catalysts including Fe, FeSO 4 ·7H 2 O, MgSO 4 ·H 2 O, ZnSO 4 ·7H 2 O, ZSM-5, aluminosilicate, Y-zeolite, and Na 2 CO 3 ; the conversion of PP reached a maximum of 38% with the aluminosilicate, for example. However, the PP almost exclusively broke down into a solid-phase product, with no enhancement of the biocrude fraction. The mechanism was explored, and with the addition of the radical scavenger butylated hydroxytoluene (BHT), the conversion of plastic reduced substantially, demonstrating that radical formation is necessary. As a result, the plastic conversion was enhanced to over 50% through the addition of the co-solvent and hydrogen donor, formic acid, and the radical donor, hydrogen peroxide. The addition of formic acid also changed the crude composition, including more carboxylic acids and oxygenated species than the conversion of the biomass alone; however, the majority of the carbon distributed to the volatile organic gas fraction producing an array of short-chain volatile hydrocarbons, which potentially could be repolymerized as a polyolefin or combined with the biocrude for further processing. Catalytic HTL was therefore shown to be a promising method for the valorization of polyolefins with biomass under typical HTL conditions.

Marine macroalgae offers a promising third generation feedstock for the production of fuels and chemicals, avoiding competition with conventional agriculture and potentially helping to improve eutrophication in seas and oceans. However, an increasing amount of plastic is distributed into the oceans, and as such contaminating macroalgal beds. One of the major plastic contaminants is nylon 6 derived from discarded fishing gear, though an increasing amount of alternative nylon polymers, derived from fabrics, are also observed. This study aimed to assess the effect of these nylon contaminants on the hydrothermal liquefaction of Fucus serratus. The hydrothermal liquefaction (HTL) of macroalgae was undertaken at 350 °C for 10 min, with a range of nylon polymers (nylon 6, nylon 6/6, nylon 12 and nylon 6/12), in the blend of 5, 20 and 50 wt.% nylon to biomass; 17 wt.% biocrude was achieved from a 50% blend of nylon 6 with F. serratus. In addition, nylon 6 completely broke down in the system producing the monomer caprolactam. The suitability of converting fishing gear was further demonstrated by conversion of actual fishing line (nylon 6) with the macroalgae, producing an array of products. The alternative nylon polymer blends were less reactive, with only 54% of the nylon 6/6 breaking down under the HTL conditions, forming cyclopentanone which distributed into the biocrude phase. Nylon 6/12 and nylon 12 were even less reactive, and only traces of the monomer cyclododecanone were observed in the biocrude phase. This study demonstrates that while nylon 6 derived from fishing gear can be effectively integrated into a macroalgal biorefinery, alternative nylon polymers from other sectors are too stable to be converted under these conditions and present a real challenge to a macroalgal biorefinery.

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