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RSC Advances Review Table 1 (Contd.) Category of additives Example substances Polymeric applications BioPP; natural bre–PP composites Natural bre–PP composites PLA PLA Molecular weightb CASb (g mol1) 14720- 55-9 10043- 61.84 35-3 149- 170.12 91-7 59-02- 430.7 9 log Kow or log Kpb 0.175 0.7 12.2 at 25C Water Registered solubilityb in (mg L1) REACH?b Pre- registered 48.8–49.2 g Yes L1at20C 11.4gL1@ Yes 20C 1.3 mg L1 at Yes 20C Ready biodegradabilityb Readily biodegradable (100%) Inherently biodegradable (100%) Aquatic toxicityb PNEC 1⁄4 2.9 mg L1 in marine water EC10 or NOEC 1⁄4 1mgL1for freshwater algae PNEC 1⁄4 0.003 mg L1 in marine water Boric acid Antioxidants Gallic acid a-Tocopherol a BIOPP: biobased polypropylene; TPS: thermoplastic starch; BioPET: biobased polyethylene terephthalate; PLA: polylactic acid; PVA: poly vinyl alcohol; PHB: poly(3-hydroxybutyrate). b Values are selected from the European Chemical Agency (ECHA)'s REACH registered substance factsheets. Further information can be found at https://echa.europa.eu/information-on-chemicals/registered-substances.127 to the environment when leaching (see Section 4.2), if consid- ered as single substances (Table 1). However, the toxicity of leaching substances combined in a mixture is mostly unknown. 3.2 Leaching of additives from biocomposites Plastic additives are oen physically, rather than chemically, bonded to the main polymer matrix, and under stress condi- tions can be released, i.e., can leach, from the plastic applica- tions to the surrounding aqueous environment and organisms.110,111 The leaching potency of plastic additives is determined by their physical–chemical properties and by envi- ronmental conditions.110,112 Hence, leachates, i.e., additives released from the polymer matrix to the environment, with low hydrophobicity or octanol–water partition coefficient (Kow), and low molecular weight are more likely to migrate to the marine environment.113 Besides the inherent physical–chemical prop- erties of additives, environmental conditions, such as radiation and mechanical forces, play a major role in the migration of plastic additives to the aquatic environment. To date, additives of fossil fuel-based polymers (e.g., phthalates, bisphenol A, and brominated ame retardants) have already been reported to leach to the aqueous environment and to induce harmful effects on aquatic organisms.114 Luo et al.115 characterized the number of uorescent additives leached from new and aged poly- urethane (PU). The results showed that more additives were released from the aged PU, suggesting the stimulating effect of weathering on the leaching of additives. In addition, it has been reported that the amount of leachates is positively related with the salinity and turbulence of aqueous media.115–117 A rst step to assess the toxicity of leaching substances from fossil fuel-based plastic and biocomposite applications to the marine environment is to prepare environmentally relevant leachate solutions. Kocasoy et al.118 summarised a list of established standard leaching protocols (e.g., DIN 38414-S4 and Minnesota test) recommended by the European Union (EU) and United States Environmental Protection Agency (US EPA). Most of these tests are conducted in distilled water, and for short periods of time (<48 h), making them less representative for the leaching of substances that occurs in the marine environment. More environmentally relevant leaching tests under marine conditions with longer duration (>48 h) have been introduced in recent ecotoxicological studies.119,120 These studies provide a cost-effective way (e.g., not based on industrial bioreactors) of generating a large volume of leachate solution in various salt- water media which suits both ecotoxicological tests and chem- ical characterisation. Nevertheless, as demonstrated in Gunaalan et al.,112 these studies vary in several parameters like solid to liquid ratio, duration, hindering the comparisons in quantities of leachates (Table 2). Future work is encouraged to establish: (1) relationships between leachate concentrations and procedure parameters including solid to liquid ratios, duration and illumination; (2) guidelines to perform environ- mentally relevant leaching tests in the marine medium with both fossil fuel-based polymers and biocomposites. 3.3 Chemical characterisation of leaching substances A wide array of organic and inorganic compounds is added to polymer matrices, and their compositions and amounts in oil- and biobased polymers vary. In most cases, the information on the exact added portions and substances is unknown, especially in commercial products and end-of-life litter items, impeding the hazardous-substance screening in the leachates from plas- tics and further ecotoxicological studies. Most of the reports on hazardous substances leaching to the marine environment from fossil fuel-based polymers have focused on a single substance or a group of its congeners (e.g., bisphenol A;121 phthalates;122 brominated ame retardants;123 lead chromate pigment124). In addition, few studies have performed systematic 32928 | RSC Adv., 2021, 11, 32917–32941 © 2021 The Author(s). Published by the Royal Society of ChemistryPDF Image | Long-term durability and ecotoxicity of biocomposites in marine environments
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