Co-liquefaction of Macroalgae with Common Marine Plastic Pollutants

Macroalgal blooms are environmentally problematic and costly to remediate, but they also represent a vast untapped resource for the production of renewable chemicals and fuels. The responsible exploitation of such marine resources will become increasingly prominent in the transition away from the crude oil economy that currently dominates global productivity. However, crude oil-derived plastic pollution is now a ubiquitous presence in the marine environment, which hampers the effective conversion of marine feedstocks. If the full potential of macroalgae is to be realized, any large-scale industrial process will need to accommodate the presence of this plastic. This study, for the first time, aimed to assess the effect of several common marine plastic pollutants on the hydrothermal liquefaction (HTL) of four UK macroalgae species and determine the impact on the major HTL products and biocrude oil quality. Co-liquefaction of polyethylene and polypropylene with L. digitata, U. lactuca, F. serratus, and S. muticum led to modest synergistic effects for plastic conversion. Under hydrothermal conditions, polyethylene underwent fragmentation to olefinic species, as well as oxidative depolymerization to form ketones. Modest synergistic effects on biocrude production were also observed for polypropylene, which depolymerized more readily in the presence of biomass to form gaseous propylene as well as oil-phase products. In both cases, the presence of plastics increased total biocrude carbon content, decreased nitrogen, and boosted higher heating value (HHV), constituting an overall improvement in biocrude fuel properties. Alternatively, nylon-6, typically originating from fisheries debris, depolymerized almost entirely under HTL conditions to form caprolactam, which partitioned mainly to the aqueous phase. While this is not favorable for biocrude production, the reclamation of marine nylon debris for hydrothermal processing to monomers may present a promising revenue stream in future biorefineries. The results demonstrate that plastic contaminants may well represent an opportunity, rather than a threat, to the successful development of an HTL macroalgal biorefinery.