Assessing the Conversion of Various Nylon Polymers in the Hydrothermal Liquefaction of Macroalgae

Sukanya Hongthong,Hannah S Leese,Christopher J Chuck,Michael J Allen

doi:10.3390/environments8040034

Abstract

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.

Highlights

Marine biorefineries, based around the valorization of salt water macroalgae, have been suggested as a promising improvement to terrestrial alternatives [1,2]
One of the main plastic contaminants found in macroalgal beds is nylon derived from One of the main plastic contaminants found in macroalgal beds is nylon derived from discarded or lost fishing gear (“ghost gear or lines”) and increasingly from fabrics
This discarded or lost fishing gear (“ghost gear or lines”) and increasingly from fabrics. This plastic waste presents a major challenge for a macroalgal biorefinery

Summary

Introduction

Marine biorefineries, based around the valorization of salt water macroalgae, have been suggested as a promising improvement to terrestrial alternatives [1,2]. Macroalgae are photosynthetically efficient, do not compete with agricultural land, do not contain lignin and represent a largely untapped bioresource To this end, a large body of research has been invested in the valorization of macroalgae, including pretreatments, fermentation and thermochemical conversion routes [3,4,5]. It seems likely that feedstock agnostic processes, which can handle a wide variability and produce an array of products, will be critical to further development in this field. One such processing methodology is hydrothermal liquefaction (HTL), a promising thermochemical pathway identified as a cost-competitive process for converting high-moisture biomass. HTL avoids energy losses associated with drying, which is needed for other thermochemical processes [11], delivers

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