Valorizing Plastic-Contaminated Waste Streams through the Catalytic Hydrothermal Processing of Polypropylene with Lignocellulose

Abstract

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.

Thermochemical Conversion of Plastic Waste into Fuels, Chemicals, and Value‐Added Materials: A Critical Review and Outlooks

“… Oil yield obtained with various catalysts used for converting plastics into fuels (aluminosilicate: Al 2 O 5 Si; bentonite clay: Al 2 H 2 Na 2 O 13 Si 4 ) [160c,162c,166,170] …”

Symbiotic Engineering: A Novel Approach for Environmental Remediation

“…Mariculture enterprises can be severely impacted by microplastics because farming sites are usually located in sheltered areas, where microplastics often accumulate . Microplastics are reported to adhere to external appendages of planktonic organisms and the gills of molluscs and shellfish. , Microplastics can be taken up directly via ingestion, either through filtration or scavenging. − Higher loads of microplastics cause low lysosomal stability, increased cellular demand, depleted energy storage, and increased mortality in oysters .…”

Hydrothermal Liquefaction of Food Waste: Effect of Process Parameters on Product Yields and Chemistry

“…The highest bio-crude oil yield from fast HTL was as high as the highest yields from isothermal HTL (28-30 wt.% at 300-400 • C). Recently, a study on co-HTL of pistachio hulls and polypropylene (PP) showed that HTL can be a promising method for valorization of food waste and associated plastic materials with the addition of hydrogen donor solvents and catalysts under typical conditions (<350 • C) (Hongthong et al, 2020).…”