Assessing the Conversion of Various Nylon Polymers in the Hydrothermal Liquefaction of Macroalgae
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environments Article Assessing the Conversion of Various Nylon Polymers in the Hydrothermal Liquefaction of Macroalgae Sukanya Hongthong 1,2,*, Hannah S. Leese 2, Michael J. Allen 3,4 and Christopher J. Chuck 2 1 Department of Production Engineering, Chaiyaphum Rajabhat University, Chaiyaphum 36000, Thailand 2 Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK; [email protected] (H.S.L.); [email protected] (C.J.C.) 3 Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK; [email protected] 4 College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4QD, UK * Correspondence: [email protected]; Tel.: +66-840029670 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 Citation: Hongthong, S.; Leese, H.S.; gear was further demonstrated by conversion of actual fishing line (nylon 6) with the macroalgae, Allen, M.J.; Chuck, C.J. Assessing the Conversion of Various Nylon producing an array of products. The alternative nylon polymer blends were less reactive, with only Polymers in the Hydrothermal 54% of the nylon 6/6 breaking down under the HTL conditions, forming cyclopentanone which Liquefaction of Macroalgae. distributed into the biocrude phase. Nylon 6/12 and nylon 12 were even less reactive, and only traces Environments 2021, 8, 34. https:// of the monomer cyclododecanone were observed in the biocrude phase. This study demonstrates that doi.org/10.3390/environments8040034 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 Academic Editors: Teresa A. and present a real challenge to a macroalgal biorefinery. P. Rocha-Santos and Joana C. Prata Keywords: plastic; HTL; macroalgae; seaweed; nylon; biofuel Received: 17 March 2021 Accepted: 13 April 2021 Published: 15 April 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Marine biorefineries, based around the valorization of salt water macroalgae, have published maps and institutional affil- been suggested as a promising improvement to terrestrial alternatives [1,2]. Macroalgae are iations. 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 [3–5]. A considerable number of challenges remain however, including having the ability to convert multiple species with highly variable Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. composition [6,7], to handle salt water as part of the process [8] and to be able to cope with This article is an open access article either heavy metal [9] or plastic contamination [10]. As such, it seems likely that feedstock distributed under the terms and agnostic processes, which can handle a wide variability and produce an array of products, conditions of the Creative Commons will be critical to further development in this field. One such processing methodology is Attribution (CC BY) license (https:// hydrothermal liquefaction (HTL), a promising thermochemical pathway identified as a creativecommons.org/licenses/by/ cost-competitive process for converting high-moisture biomass. HTL avoids energy losses 4.0/). associated with drying, which is needed for other thermochemical processes [11], delivers Environments 2021, 8, 34. https://doi.org/10.3390/environments8040034 https://www.mdpi.com/journal/environments Environments 2021, 8, x FOR PEER REVIEW 2 of 18 Environments 2021, 8, 34 2 of 17 avoids energy losses associated with drying, which is needed for other thermochemical processes [11], delivers a high-energy biocrude (30–40 MJ kg−1) [11] and produces a more a high-energy biocrude (30–40 MJ kg−1)[11] and produces a more stable crude product stable crude product than pyrolysis [12–14]. HTL generally uses temperatures between than pyrolysis [12–14]. HTL generally uses temperatures between 280 and 370 ◦C and 280 and 370 °C and pressures between 10 and 25 MPa to maintain water in the liquid state pressures between 10 and 25 MPa to maintain water in the liquid state [15]. Water acts [15]. Water acts as an important reactant and the addition of catalyst leads to several op- as an important reactant and the addition of catalyst leads to several opportunities for portunities for separations and further chemical reactions [16,17]. The reactions that take separations and further chemical reactions [16,17]. The reactions that take place during place during HTL are decomposition and repolymerization to form biocrudes with high HTL are decomposition and repolymerization to form biocrudes with high heating values, heating values, a solid residue containing the inorganic fraction and a water-soluble frac- a solid residue containing the inorganic fraction and a water-soluble fraction that can be tion that can be used as a fertilizer [18]. used as a fertilizer [18]. Contamination of oceans by microplastic and macroplastic debris has become one of Contamination of oceans by microplastic and macroplastic debris has become one the most publicized marine environmental issues of recent years, affecting all of the of the most publicized marine environmental issues of recent years, affecting all of the world’sworld’s oceans.oceans. For instance,instance, microplasticsmicroplastics have beenbeen demonstrateddemonstrated toto affectaffect thethe growthgrowth of macroalgae. Alarmingly, Alarmingly, ingestion ingestion of of plastic plastic debris debris has has been been recorded recorded in in44–50% 44–50% of all of allseabirds. seabirds. This This leads leads to blocking to blocking of the of digestive the digestive tract, damage tract, damage stomach stomach lining, less lining, feeding less feedingand starvation and starvation [19]. Plastics [19]. Plasticshave been have accumu been accumulatinglating in substantial in substantial densities densities in the marine in the marineenvironment, environment, from the from sea thesurface sea surface down to down deep-sea to deep-sea sediments sediments [20,21]. [20 In,21 a ].marine In a marine biore- biorefinery,finery, macroalgae macroalgae could could possibly possibly be washed be washed in clean in clean water water to remove to remove microplastics, microplastics, but butresidual residual microplastics microplastics can canbe absorbed be absorbed onto onto the surface the surface of marine of marine macroalgae, macroalgae, or be or em- be embeddedbedded in inthe the structure structure [22]. [22 ].Macroplastic, Macroplastic, on on the the other other hand, hand, would would need need to be removedremoved manually. Marine plastics originate mainly from many types ofof plasticplastic debris,debris, suchsuch asas fishingfishing nets, ropesropes andand plasticplastic bags.bags. While most of the focusfocus hashas beenbeen onon terrestrialterrestrial PETPET and polyolefinpolyolefin litter,litter, overover 20%20% ofof marinemarine plasticplastic debrisdebris foundfound inin thethe oceanocean isis estimatedestimated toto come fromfrom commercialcommercial fishingfishing activityactivity [[23].23]. ForFor example,example, anan estimatedestimated 640,000640,000 tonnestonnes ofof nylon fishingfishing gear enters the oceansoceans everyevery year,year, whichwhich amountsamounts toto approximatelyapproximately 10%10% ofof thethe totaltotal marinemarine debrisdebris [[24].24]. TheseThese discardeddiscarded fishingfishing items,items, includingincluding monofilamentmonofilament lineslines and nylon netting used used in in fishing fishing activity, activity, have have contributed contributed to to the the considerable considerable growth growth in inthe the marine marine plastic plastic contamination contamination that thatis having is having global global impact impact on the on entanglement the entanglement of ma- ofrine marine life [23]. life Furthermore, [23]. Furthermore, due to due the tonature the nature of the offibers, the fibers,nylon nylonfishing fishing lines are lines one are of onethe main of the contaminants main contaminants found in found macroalgal in macroalgal beds and beds seaweed and seaweed farms [10]. farms Nylon [10]. refers Nylon to referssimple to polyamides simple polyamides (Figure 1). (Figure For example,1). For example, nylon 6 nylonis synthesized 6 is synthesized by the ring by theopening