A low-cost system to simulate environmental microplastic weathering

Marine Pollution Bulletin 149 (2019) 110663

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A low-cost system to simulate environmental microplastic weathering ∗

T

J. Andrade , V. Fernández-González, P. López-Mahía, S. Muniategui Group of Applied Analytical Chemistry. University of A Coruña, Campus da Zapateira s/n, E-15071, A Coruña, Galicia, Spain

A R T I C LE I N FO

A B S T R A C T

Keywords: Microplastics Weathering Weathering system Polyamide 6.6 Polystyrene Polypropylene

Society concerns about the potential thread of microplastics into the environment call for detailed laboratory and field studies to assess their fate, in particular, their weathering. This can hardly be done in natural conditions and, so, a low-cost system (< EUR 1000) to accelerate photooxidative and hydrolytic weathering is presented in a way that standardizes major marine experimental conditions: incident radiation range, light intensity, temperature and mechanical stress. The system can be valid for many European countries, most US states, and other intermediate-latitude-countries; otherwise it can be scaled up easily. Validation was done by studying three different polymeric structures: polyamide 6.6, polystyrene and polypropylene. The results agreed nicely with previous reports derived from different working conditions. Therefore, this low-cost system would likely contribute to the standardization of microplastic marine weathering studies by, e.g., improving their intercomparability.

1. Introduction There is little doubt that plastic litter and microplastics (MPs), regardless of being primary or secondary, have become buzzwords in many societal, political and, even, economical discussions. Our Society is deeply concerned by clear evidences of ubiquitous large quantities of plastics everywhere and shocking photographs are disseminated fast by social media everywhere. MPs and nanoplastics are perhaps less well known by many citizens but people easily deduced that there should be an enormous amount of plastic fragments they cannot see and, thus, they could be the ‘hidden threat’ (Jahnke et al., 2017). Society demands actions but decisions are hugely difficult because decision-makers face many questions for which Science still has not definite answers. This is relevant because global economy and a huge amount of jobs might be affected by political decisions. An excellent source of ‘little knowns’ and ‘unknowns’ was presented recently within the EU Scientific Advice for Policy framework (SAPEA, 2018). By the end of 2016, GESAMP (GESAMP, 2016) established that there were no reliable estimates of the degradation of plastics exposed to weathering either on land or at sea under a specified set of conditions. Precisely, one of the main issues that has not still been addressed so far is the establishment of a minimum consensus on such a set of conditions (this paper can be seen as a step forward in this direction). GESAMP indicated also that research priorities (GESAMP, 2016) should consider developing better methods to age or date plastics, associated with weathering and fragmentation models. Similarly, SAPEA put

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forward the urgent need for harmonised methods to assess exposure, fates and effects of MPs (SAPEA, 2018). Further, it estimated that the main processes and timescales that cause plastic fragmentation are not well known in any environment and that a major shortcoming of many studies is that they are performed using unrealistic conditions (e.g., much higher/extreme concentrations/conditions than those currently found in the environment). The same opinion was hold very recently by the Group of Chief Scientific Advisors of the European Commission (SAM, 2019), who also pointed out that challenges remain for scientific measurement and test methods in order to better understand and monitor the origins, pathways, fate and behaviour of MPs. US-EPA kept also an eye on this issue after a high-level meeting where it was concluded that analytical characterization needs to include weathered microplastics and that the development of reliable, reproducible and high-quality methods for microplastics quantification and characterization is fundamental and of paramount importance for understanding microplastics risks (EPA, 2017). To further complicate the scientific studies (and, so, gather reliable data for political/management decision-making), the ‘life’ of MPs in the environment cannot strictly be monitored as such. It is not feasible to throw away a plastic item in a shoreline and follow its movements into the sea, monitor its degradation, measure its biofouling, the fragments it releases, where and when it breaks and, finally, where and how it beaches/sunk for a long time (where new degradation pathways might occur). This means that environmental scientists need frequently to design and deploy ‘systems’ (defined as ‘ … interacting or interdependent

Corresponding author. E-mail address: [email protected] (J. Andrade).

https://doi.org/10.1016/j.marpolbul.2019.110663 Received 24 July 2019; Received in revised form 10 October 2019; Accepted 10 October 2019 Available online 22 October 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved.

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for more comprehensive details): i) deployment of MPs in a container/ holder which is then submitted to the natural environment itself, with periodic monitoring; ii) use of commercial, artificial weathering chambers and iii) deployment of ad-hoc weathering systems (either in laboratories or other facilities). Option i) seems the simplest and, no doubt, resembles finely the local conditions (see e.g. (Arrieta et al., 2013)). However, it might be too local and, so, results could be difficult to extrapolate or compare. As a rule of thumb the conditions of extreme geographical locations (e.g., deserts or poles) might not be the best option to setup a general-purpose system (despite being intrinsically interesting by themselves), as it was noted when studying the fate of fishing nets after sun exposure in Europe and West Africa (where 80% more degradation was found than in Europe for the same exposition time) (Al-Oufi et al., 2004). A major drawback of natural weathering monitoring is that it is almost impossible to replicate studies, and results depend on totally uncontrollable conditions like series of unexpected hot/cold days, persistent rain, or strong winds/waves that might affect the weathering station. Biofouling is also an issue in several sites, noteworthy in highly productive and/or upwelling areas. Table 1 reviews some references using natural weathering, along with specific details on them. Option ii) appears as an excellent way to control and standardize various experimental parameters. However, it is very expensive and some drawbacks can be posed on it. Artificial weathering chambers were developed essentially to strongly stress industrial goods/materials in order to evaluate their ‘resistance’ and, so, control their quality (e.g., plastics for greenhouses, bags, building, shipping, and a huge number of daily commodities) and evaluate their duration and how they last/behave after use. It is, therefore, logical that they have day-night options, dry-humid cycles (Valadez-Gonzalez et al., 1999) and several radiation bulbs (Arias-Villamizar and Vazquez-Morillas, 2018). High temperatures and irradiations can be used there, as well as different levels of humidity and rain cycles. The objective here is to decline the polymer properties, while on the contrary environmentalists are more focused on the reactions that cause breakdown and the properties and hazards associated with released chemicals under natural conditions (Gewert et al., 2015). Some other applications to MPs have been presented, just for instance (Gijsman et al., 1999; Philip and Al-Azzawi, 2018; Salih et al., 2017). Table 1 presents a more comprehensive review, with some additional details. However, serious doubts appear on the suitability of such tough conditions to emulate the weathering processes underwent by MPs in environmental, natural compartments. Further, Petri dishes are used to place the MPs or plastic pieces on those devices because they have not much space and typical laboratory probes cannot be placed inside due to space limitations. This raises concerns on whether MPs or plastic pieces are ‘really’ submerged (when only a small layer of water is covering the test probe); no agitation is possible and aeration and the use of sand can be difficult (if possible at all). Also important, ISO 4892 clearly states that accelerated weathering can differ from real weathering due to the nature of the testing (ISO-4892-1, 2016). Therefore the scientific community has to be cautious with the conclusions derived from the use of these systems, especially when referring to MPs. The ad-hoc option is midway between the previous two. It can be customized to fit the user's needs as most devices are home-made, it is scalable and it can accurately and reproducibly control most experimental parameters. Therefore, in our view, this seems the most suitable strategy to develop a weathering system to study MPs aging, providing a proper objective justification of the experimental conditions is given. Despite this, there are not many reports on this topic (only a handful of applications to study MPs in the seawater ecosystem were found, see Table 1) and they present quite different experimental setups (various types of lamps, ranges of wavelengths and irradiation power), which make direct comparisons among results difficult, if possible at all. This is why –in our view- a sort of standardization for the experimental conditions of ad-hoc devices would be of use (without precluding

group of items forming a unified whole’, according to the MerrianWebster's Unabridged Dictionary) to mimic Nature (or part of it) in order to have a place where they can put the subject of their research, apply a set of ‘experimental events’ –controlled, as far as possible- and, finally, locate and collect the subject again to measure something on it. However trivial this statement might appear, the truth is that sometimes we fail in designing a ‘system’ that resembles acceptably well the part of the Nature we are studying. Further, there are doubts on the usefulness of the wealth of experimental data we are obtaining about MPs for correct decision-making and general governance (SAPEA, 2018). Field or closely-related-to-field experiments are needed to learn how polymers degrade due to natural photooxidative conditions, to identify aged polymers properly, to study their flotability, to monitor biofouling and colonizing species, and to assess their interaction with other chemical pollutants, among others. Such a wide range of different potential studies makes it hard to foresee an all-purpose system that can fit all needs and views. Probably this explains the very scarce number of reports dealing with the development of weathering systems. Only three exceptions were found: a pioneering study to evaluate biological degradation of plastics (being the first in its kind (Tosin et al., 2012)); an attempt to develop a system to leach plastic components (Gewert et al., 2018), although both performed only a partial study of the factors affecting weathering. Finally, a dedicated spectrograph was developed to expose samples to dispersed UV radiation and measure colour changes (the sample is contained within the spectrograph) (Vaskuri et al., 2017), although this system seems not adequate for usual environmental studies. Therefore, this paper focuses on physico-chemical polymer marine weathering and aims primarily to discuss the basic requirements that should be posed to develop a low-cost, laboratory-intended, general weathering system to monitor changes in MPs during oceanic ageing. To the best of our knowledge this is first time a work focuses on systematically discussing a device to simulate best the natural (cuasi-natural) major conditions that lead to photooxidative and hydrolytic weathering of plastics and MPs in the oceanic system. Besides, weathering under dry shoreline conditions is also considered briefly. Section 2 will review literature to look for the most relevant weathering options, Section 3 will briefly depict the low-cost system, Section 4 will discuss the design parameters, including a thorough review of irradiation values (ground level) –which so far had not been presented elsewhere for this purpose-. Finally, Section 5 will validate the low-cost system proposed here by considering three different types of pelletized plastic polymers (polyamide 6.6, polystyrene and polypropylene). Their weathering processes will be schematized briefly (a complete study of the weathering evolutions is out of scope here) and compared to previous reports. The low-cost weathering system has already been applied in the JPI/O-funded Baseman project, intended to look for consensus pathways to undergo MP research. 2. General designs Can consensus on the basic constituents of a marine weathering system for MPs be agreed upon? Further, is it possible to combine them in a low-cost device implementable in most research laboratories? Although it is difficult to answer definitely this question, we do think that it is possible to find out some basic characteristics/parameters that allow researchers to implement a system that can be accepted as a (minimum) workhorse to study MP weathering. If such sort of consensus is to be found, a trade-off between different views will be required because if a ‘perfect’ system is sought for, nothing resembles Nature better than Nature itself. So, why not simply deploying the polymers in a container in the ocean and monitor what happens there? This has indeed been a frequent option, although it has several drawbacks, as discussed next. Roughly, three main options were found in literature (see Table 1 2

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Table 1 Some selected reports to illustrate the three major models to study polymer weathering (natural weathering, artificial weathering chambers and ad-hoc systems). Polymers/Form

Weathering

Other Parameters

Reference

NATURAL WEATHERING PE, Microparticles extracted from toothpaste, particles ≈50 μm PE and PP pellets

PA6 and PET, Ropes and fibers

HDPE PP film

Approximately 3 g of particles weathered in closed 50 μm mesh bags, placed in an outdoor wave simulator with continuous intake of seawater (Drøbak, Norway) Three experimental treatments: dry/sunlight, seawater/sunlight, and seawater/darkness. For the dry/sunlight treatment, 250 mL of pellets placed in Pyrex glass trays at the roof of the building (La Jolla, California). For the seawater/darkness and seawater/sunlight treatments, 250 mL of pellets placed in 75.7 L aquaria with continuously flowing seawater (intake from the seaward end of a pier). The seawater/darkness treatment tanks were placed indoor. Sliced samples of PA6 and PET ropes ≈180 cm long were attached to a wooden frame located on the rooftop of a building (Montreal, Canada). Specimens were clamped to aluminium plates and put outdoor (Bandung and Tsukuba, India). Samples were mounted over glass plates and exposed directly to the natural environment on a exposure rack

PET pellets

Specimens were mounted on an outdoor frame (London).

Hemp fibers reinforced, PP biocomposites 100 × 100 × 2 mm specimens and ISO 1 dog-bone specimens LDPE films

Outdoor exposition corresponded to a decking use. Weathering conditions according with ISO 877–1:2011 (S.W. France) Outdoor weathering on the roof of a building. The sample size was 6x11 cm, and samples were assembled on steel cages facing South. (Riyadh, Saudi Arabia) Outdoor locations to compare terrestrial and marine and river conditions. Samples placed in floating traps for exposure. Experimental conditions followed ISO 15314. (Colombian Caribbean coast) The exposure suspension was prepared with 0.5 g of microplastics and 20 mL of freshwater in a glass vial and located at a building roof (Nanjing, China). Films (50 μm thick) exposed on the rood in Israel

HDPE, conventional and oxodegradable bags

PE and PS spheres

LDPE films

21 days

Brate et al. (2018)

36 months

Brandon et al. (2016)

Air temperatures between 5.9 and 30.4 °C and seawater temperatures between 14.6 and 18.3 °C

6 months

Arrieta et al. (2013)

4 years, 30 °C

Satoto et al. (1997)

2 months during both the tropical summer winter seasons, average temperature above 27.5 °C. 13,000 h 1 °C–32 °C Humidity: 100%–22% 12 months.

Rajakumar et al. (2009)

90 days 30–45 °C (day) and 25–31 °C (night). Humidity: 10–15% 180 days 26–35 °C

12 months 4–38 °C Irradiance (365 nm): 0.12–2.26 mW/cm2 4 months

Philip and Al-Azzawi (2018) Badji et al. (2018)

Tuasikal et al. (2014)

Arias-Villamizar and Vazquez-Morillas (2018)

Liu et al. (2019)

Hirsch et al. (2017)

ARTIFICIAL WEATHERING CHAMBERS PP, PE, PA6 and PBT films

PC films

Suntest® with a xenon lamp. Weather-O-Meter Ci 65 with a boron-borosilicate xenon lamp. Daily rain cycle of 102 min dry and 18 min wet. Samples were exposed in a QUV Panel with UVA-340 nm fluorescent lamps.

LDPE films LDPE films

QUV tester, UVA irradiance 1.55 W/m2 Ultraviolet A in an AW tester.

PP, pieces of plastic waste from coffee capsules

Accelerated weathering (model QUV), with a 12 h radiation and 12 h moisture cycle during 70 days according to ASTM G154-0. UV-340A accelerated weatherometer (GOTECH)

PP wood composites, powder

PP composites, rectangular foils

HDPE

Standard ISO 4892-2 A1 using the Atlas WeatherOmeter Ci4000, with a Xenon light and Boron/Boron (outer) filter. Continuous irradiation Weathering cycles included 18 min wet phases, followed by a 102 min dry phase. Accelerated test in a QUV/SE weathering chamber (QPanel Co.) with UVB-313 fluorescent lamps which have their maximum peak at 313 nm. An ASTM cycle (G5388) was used, consisting of 4 h of UV irradiation (with radiation level of 0.63 W/m2) at 50 °C, followed by 4 h of condensation at 60 °C.

Suntest: between 40 – 50 °C Irradiance (340 nm): 0.3 W/m2/nm WOM: 46 °C, Irradiance (340 nm): 0.35 W/ m2/nm Humitity: 55% 3000 h 40 ± 2 °C Humidity: either 0 or 42 ± 5% 60 °C 300 h 60 °C Humidity: 10% Irradiance: 1.55 W/m2 70 days

15 days 50 °C Irradiance: 0.83 W/m2 61 days Air chamber temperature and black panel thermometer temperature set at 38 °C and 65 °C, respectively. Humidity: 50% Irradiance (300–400 nm): 0.60 W/m2 1600 h 50 °C (4 h) and 60 °C (4 h)

Gijsman et al. (1999)

Tjandraatmadja et al. (1999) Hirsch et al. (2017) Hirsch et al. (2019)

de Bomfim et al. (2019)

Tian et al. (2019)

Niemczyk et al. (2018)

Valadez-Gonzalez et al. (1999)

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Table 1 (continued) Polymers/Form

Weathering

Other Parameters

Reference

NATURAL WEATHERING PE/lactic acid/chitosan, films

HDPE, conventional and oxodegradable bags PET yarn

PET pellets

PMMA nano composite materials and PMMA hybrid nano composite materials PP 2 mm plaques

PE films

HDPE and HDPE wood-flour composite, bars

Hemp fibers reinforced with PP biocomposites, 100 × 100 × 2 mm specimens and ISO 1 dog-bone ones

LDPE, LLDPE, HDPE pellets

PU coatings with different polyol compositions

PE, PP, PS and PET pellets

PE, PS and PP pellets

PS spheres

PP/PS blend films

Cellulose and synthetic cellulose, fibers

Q-Lab QUV/se equipment, using four UVA-340 lamps. Weathering cycles included 8 h of UV irradiance and a 4 h condensation period. Accelerated photo-aging chamber with eight Xenon arc lamps, according to ASTM D5071. Heraeus Xenotest 150S following ISO 105 B04, with a long arc, air cooled xenon lamp equipped with inner and outer filters to simulate sunlight. Rain and dry cycles selected as 1 min/29 min.

QUV tester (Q-Panel Lab Products), following BS EN ISO 4892–3. The UV-A lamp has a peak emission at 340 nm to simulate outdoor applications. Weathering cycles comprised 8 h of irradiation and 4 h of condensation. Weathering according to ISO 4892–3 (QUV accelerated weathering tester, Q-labs), with UV-A fluorescent lamps. The test specimen was exposed to heat, saturated moisture of air and water vapor. SEPAP 12–24 device, with 4 medium pressure mercury vapor lamps. Wavelengths below 295 nm were filtered.

200 h 40 °C–60 °C Irradiation: 0.89 W/m2 96 h 57 ± 1 °C 300 h Humidity: 7.3–30% Chamber temperature, black standard temperature, and white standard temperature were 18–31.6 °C, 50 °C, and 40 °C, respectively. Irradiation (300–400 nm): 105–140 W/m2 13,000 h 40–60 °C Irradiance (340 nm): 0.68 W/m2

Arias-Villamizar and Vazquez-Morillas (2018) Fashandi et al. (2008)

Philip and Al-Azzawi (2018)

120 h 55 °C Irradiance: 0.58 W/m2

Salih et al. (2017)

500 h 60 °C Irradiance (300–400 nm): 90 W/m2. 400 h 60 °C

Rouillon et al. (2016)

SEPAP 12.24 device, with 4 medium pressure mercury lamps of 400 W (borosilicate filters), samples were placed on a rotating carousel positioned in the centre of the lamps. 250, 500, 1,000, and 2000 h Samples were placed in a xenon arc-type light-exposure apparatus operated according to ASTM D2565. Samples were mounted in four rows on a drum that rotated around the xenon arc lamp at 1 rpm. The exposure cycle consisted of 108 min of light and 12 min of water spray and light. 1000 h QUV Spray apparatus (Q-Lab) with fluorescent UVA50 °C 340 lamps, following ISO 4892–3. Each 12 h Irradiance (340 nm): 0.76 W/m2 weathering cycle consisted of 8 h of dry exposure followed by 3.75 h of condensation exposure without irradiation and 0.25 h spray step. Weather_ometer (WOM): Xenon lamp 6500W, WOM: 63 °C, 60% humidity. 25, 50, 100, irradiance 0.35 ± 0.03 W/m2 (at 340 nm) 200, 400, 800 and 1600 h. Weathering tester (QUV): UVB 0.60 W/m2 irradiance Weathering tester (QUV): 50- 60 °C. 12, 24, (at 313 nm) 50, 100, 200, 400 and 800 h 55 °C and 75% humidity UV-weathering chamber NIST SPHERE 143 W/m2 in 28 days the wavelength of 295–400 nm and max irradiance around 0.85 W/m2 at 340 nm. AD-HOC WEATHERING SYSTEMS A high-intensity UV lamp (HTC 400-241 SUPRATEC HTC/HTT, purchased from OSRAM GmbH) with a spectral radiation distribution from 275 to 450 nm was used. The 5 day exposures correspond to approximately 510 days of sunlight exposure to European mean solar irradiance. An aluminium “weathering wheel” (ca. 1 rpm) holds six 350 mL quartz glass sample tubes horizontally. An electric motor rotated the tubes around the central UV lamp. Plastics were in Milli-Q water. Three environments (simulated seawater, ultrapure water, and air) were simulated using a UVA340 lamp. Pellets were in glass dishes, and then transferred into a homemade environmental chamber with a UV lamp 15 μm PS beads into a 1L glass bottle containing filtered seawater (610 μm) placed on a rotating plankton wheel in the dark A homemade equipment, using a 15 W High-borax ultraviolet TUV 15 W/G15 T8 light source (a fan renewed the atmosphere and removed the ozone produced). 0.5 mg of each material was put in a 30 mL glass bottle with 25 mL ultra-pure water, and exposed to 302 nm UV light.

Lizarraga-Laborin et al. (2018)

5 days

Gardette et al. (2013)

Stark and Matuana (2004)

Badji et al. (2018)

Gulmine et al. (2003)

Chang et al. (2018)

Gewert et al. (2018)

35 °C (strong air flow) Irradiance (at 8 cm): 702.8 W/m2.

3 months

Cai et al. (2018)

Ambient sea temperatures (6.4 °C) Three weeks

Vroom et al. (2017)

30 h Irradiance (254 nm): 4.0 ± 0.1 mW/cm2

Waldman and De Paoli (2008)

7 days

Cai et al. (2019)

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Table 1 (continued) Polymers/Form

Weathering

Other Parameters

Reference

NATURAL WEATHERING High impact PS with butadiene rubber

HDPE films of comercial bags

LDPE, HDPE, PS, PP; PET; PC, PMMA, PVC, PA

Multi-sample UV irradiator. Xe–Hg arc lamp (wavelength 280–450 nm). An optical fiber bundle was attached to the light source to irradiate each sample in the sample cuups simultaneously. Cups were placed in holes on a rotatable cup holder (rotation speed 0–8 rpm) Marine and dry conditions were simulated using aquarium tanks (both within a laboratory and outdoors) and sand beds (outdoors). Air pumps were used in the tanks. Marine weathering simulation: 750 mL of seawater into 1L pyrex laboratory cylinders + 100 mL of siliceous sand +10 g powder/20 g pellets. Constant aeration and agitation with air diffusors. Dry conditions: 10 g powder/20 g pellets in 12 cm diameter petri dishes. Both exposed to two metal-halide powerstar HQI-TS 2510W/NDL bulbs in zenital and ground level positions.

0,3,6,9 or 12 h 60 °C Irradiance 26.4 mW/cm2

Matsui et al. (2016)

6 months under outdoors natural conditions in Geece (February to August)

Kalogera-kis et al. (2017)

10 weeks

This work

Sea water temperature: 23–28 °C Illuminance: 12,200 lumen/m2 (ca. 24 lumen/cylinder). Irradiance on air 60 × 10−2 W/m2, on seawater 25 × 10−2 W/m2

Fig. 1. General appearance of the low-cost, ad-hoc weathering device during work.

3.2. Apparatus

scalability). As mentioned in the introduction, only a work was found which looks for a device specifically devoted to MP weathering (Gewert et al., 2018).

All measurements were made using a FTIR spectrometer (Spectrum 400, PerkinElmer), equipped with a horizontal single-reflection ATR (Attenuated Total Reflection) diamond, operating in the 4000600 cm−1 mid-IR region, 30 scans/sample, apodization Beer-Norton strong, 4 cm−1 nominal resolution. All spectra were corrected for lightreflectance penetration and baseline displacement. Hydrocarbons-free AC9908 Resun air Pumps (Xing Risheng Industrial co., Ltd, China), 20 L/min, were employed. Powerstar HQI-TS 250W/NDL Osram bulbs were used as light sources. A TES (TES electrical electronic Corp., Taipei, Taiwan) 1330A light meter, and a narrow-band UV113 Macam radiometer (formerly, Macam photometrics Ltd., nowadays Irradian Ltd., East Lothian, Scotland, UK) with an underwater PD135 detector with cosine response, were employed to measure light intensity. Temperature was measured with a 0–100 °C glass thermometer.

3. Experimental part 3.1. Samples The plastics employed in our studies belong to the so-called ‘Baseman's kit’. This is a set of nine types of MPs developed under the JPI-O Program's Baseman project. They were fabricated with the lowest possible amount of additives. Two forms were generated: powder (average size ca. 300 μm) and pellets (average size ca. 3 mm). For this work we selected three widely different types of polymers: a polyolefin (PP, polypropylene); an aromatic-based polymer (PS, polystyrene); and a polymer containing heteroatoms in the basic structure (PA 6.6 or Nylon, polyamide, poly(hexamethylene adipamide). PP was from Borealis (commercial name HL508FB), melting temperature 158 °C. PS was from INEOS Styrolution (commercial name, Styrolution PS 158 N/ L); density 1.04 g cm−3. PA6.6 was from BASF (commercial name, ‘Ultramid’), whose density and melting temperature were 1.13 g cm−3 and 260 °C. Control samples were also considered (pellets and powder submerged in seawater at dark).

3.3. Set-up for a low-cost ad-hoc system to weather MPs in laboratories A general picture of the whole system is presented in Fig. 1. Full details are given in this section and the next one. Seawater was collected at a shoreline clean site used frequently by the Faculty to fill tanks to grow plankton and algae. The water employed in the weathering setup was filtered through 10 μm sieves; 750 mL of seawater were 5

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hundreds of years) although, importantly enough, we should not modify the pathways.

poured into 1L Pyrex laboratory cylinders (borosilicated glass does not block UV radiation (Brandon et al., 2016; Gijsman et al., 1999)); 100 mL of siliceous sand (previously calcined at 400 °C, 12 h, to remove any organic material) were added. High quality siliceous sand (without carbonates or too fine powders, which might clog the systems) can be obtained easily from sellers of swimming pool accessories (in particular, filtering pumps; in our case Axton); the nominal size of the sand grains was 0.7 mm (range 0.4–0.8 mm). Each cylinder received a unique type of plastic, either 10 g of powder or 20 g of pellets. Constant aeration and agitation were kept at each cylinder using 5 mm, external diameter, silicon tubes equipped with a terminal PVC diffusor. The diffusor was placed as close as possible to the bottom so that the sand bed got disturbed and the grains interacted with the plastics. This is a refinement of a work where mechanical abrasion of pellets was evaluated after their UV weathering (Song et al, 2017). A similar strategy had been proposed to simulate the degradation of plastic bags on a coastal zone (Kalogerakis et al., 2017). To simulate weathering under dry conditions (i.e., at the highest top location of beaches) Petri dishes were employed (12 cm diameter), with 10 g of powder or 20 g of pellets. The content of each Petri dish was mixed every day and the locations of the dishes were interchanged each three days. Temperature in the water (as measured with a 0–100 °C range glass thermometer) was on average 25 °C, with a 23–28 °C variation range. It is important not to raise the temperature of water to unrealistic values because it was reported that high temperatures can lead to different chemical processes/reactions than those in Nature (Brandon et al., 2016). Hence, the system proposed here do not involved high temperatures, contrary to what was proposed sometimes (e.g. (ValadezGonzález e al., 1999; Gardette et al., 2013)). Further inclusion of a domestic cooling device can maintain room temperatures lower and more tightly controlled. It is worth noting that temperature and relative UV exposure can be recorded simultaneously (HOBO systems, and store many measurements in their data loggers), using rather cheap instruments (ca. EUR 70, accessories and software excluded), as one of the referees pointed out. Finally, the financial issue merits some attention because while a commercial chamber to carry out standard (industrial goods) weathering studies costs a minimum of EUR 50,000 and it is not too flexible to establish different setups that might be of interest, the low-cost system proposed here can be obtained by less than ca. EUR 1000 (light meter and radiometer excluded).

4.1.1. Water So a first component of the weathering system should be water; here, seawater. Hydrolytic degradation is particularly important for some common plastics, as polyesters and polyamides (Brandon et al., 2016). When the laboratory is next to a shoreline, a river or a lake and it is possible to deploy a pipeline and a pump, a continuously feeded container in an open-loop design, would be a nice option (Brandon et al., 2016; Brate et al., 2018; Deroine et al., 2014). As this is not currently a viable option an alternative would be to locate MPs within containers and set a closed-loop circuit using a main tank to circulate seawater through the containers. However, this option would involve pumps and tubes whose deployment may be inconvenient. Thus, a common solution is to use a relatively small container filled with seawater. In this case seawater is not renewed and evaporation has to be taken into account and refill the containers to keep the same experimental conditions. When working with seawater excessive salt concentration has to be avoided (otherwise the MPs might float even when they should not according to their density (Frias et al., 2018)); hence, do not refill with seawater but with plain water. 4.1.2. Agitation Aeration is the next important point. Although oceanic anoxic conditions at high depths can be experienced by MPs when they sunk, almost all them are spilled at the water body surface and they stay/pass through the upper first water layers which are, essentially, oxygen-rich and, therefore, chemical reactions should not be limited by oxygen levels (Gijsman et al., 1999). So, air ventilation needs to be included in the weathering system. Closely related to aeration, agitation of the system seems also a must because water bodies move, more or less fiercely. MPs in the sea will definitely suffer strong waves, multiple collisions with other MPs, debris, sand, etc. Fragmentation has been widely associated not only to MPs formation from main plastic items but to their degradation to nanoplastics. To simulate the presence of particles in the sea and their abrasion/breakage effect on MPs a small amount of sand has to be added to the system (and incorporated to the overall movement). Whenever any fine powder was still at the water surface, a metallic spatula was used to incorporate the plastic grains back to the moving water. Therefore, aeration, agitation and fragmentation can be addressed quite easily with an air pump and they seem relevant to study MPs aging. To the best of our knowledge, this had not been considered in literature about simulation of MPs weathering before.

4. Results and discussion 4.1. Design parameters

4.1.3. Mesocosmos Biofouling and colonizing species can cover the surface of MPs and affect their flotability and, even subsequent weathering (O'Brine and Thompson, 2010). In our system biota was not avoided specifically and sterilized water was not used. As biofouling occurs, remove it before analytical FTIR polymer characterization (e.g. physically with a gentle washing and drying) because the biofouled surface might difficult a proper identification of the polymer by the IR databases.

To set the design parameters of a system, the scope of the research needs to be sharpen first. Probably, a system to study MPs uptake by biota will not be the same as that for studying photooxidation. Then, a primary goal that should be considered is to collect preliminary information on the main weathering processes that occur when MPs are exposed to natural environmental conditions. In this work photooxidative and hydrolytic processes undergone by MPs while floating in the oceanic system will be considered. There is ample consensus nowadays that physico-chemical polymer degradation occurs mainly through UV radiation-induced physical and chemical changes, with oxidative effects of oxygen (air and dissolved) and hydrolytic effects of water, plus the catalytic effects of metals dissolved in the water (Feldman, 2002). Thermal reactions are also possible although they are in general much less relevant (assuming typical ‘normal’ temperatures) and not easy to discern from photooxidation (Feldman, 2002). The common natural parameters determining them are considered below. For these studies, currently, an acceleration of the degradation pathways is sought for because the natural time scales are impractical (COM, 2018) (e.g. degradation of typical plastic bottles can last for

4.1.4. Radiation Finally, no doubt, the light to irradiate the system is a major issue and this is probably the most difficult issue to be addressed. For polychromatic lamps be useful, they must offer a range of wavelengths as similar as possible to ground-level sunlight, and with a similar distribution of their intensities. Hence, some brief notes on solar radiation seem in order. Its main composition (ground level) is: ca. 50% IR, ca. 44% VIS and ca. 3–4% UV, although these values vary slightly throughout literature (Fondriest_Environmental_Learning_Center, 2019; Institute_of_agriculture_University_of_Tennessee, 2019). The latter part is composed of the so-called UV-A and UV-B radiations. The A type 6

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halides offered spectra which highly resemble the solar spectrum (at ground level). They are small and easy to install and, so, were selected here. However, selecting the bulb is not the only problem. To simulate how weathering proceeds on a particular geographical area, the solar radiation reaching ground level has to be known approximately. Despite a wealth of physical units are used in literature to describe light (without easy transformation among them), irradiance is the most common one, defined as the amount of light incident on a unit area (units, W·m−2). The maximum irradiance that can be expected in a sunny, clear day is ca. 1000 W m−2 (Cognetti et al., 2001; OpenUniversity-Course-Team, 1997). Nevertheless, it is of utmost importance recalling that solar irradiation is absorbed strongly by water and clouds. So strongly that, with calm sea, irradiance will logarithmically decrease up to one-half within the first 10 cm and to one-fiftieth at 100 m (Open-University-Course-Team, 1997). Some authors state, simply, that sunlight penetrates less than 1 m (Karleskint et al., 2010), which corresponds to when infrared and visible red radiations are absorbed totally (Open-University-Course-Team, 1997). As a consequence, selecting the intensity to be set in a weathering system if a particular geographical region is to be simulated depends not only on its latitude but on its ‘typical’ weather (i.e., clouds, hours of daylight, etc.), the season and the quality of the water (sediments, agitation, biotic richness, upwelling months –like in Galicia, NW Spain-, etc.). Previous data, thus, would be required (and they are not always easy to gather). Finally, it also appears to be consensus in literature on considering prolonged irradiation times (although the number of hours vary enormously between authors, see Table 1) instead of radiation bursts during small periods of time. Day and night sequences can be applied by some commercial systems (see Table 1) but they are probably not a must because acceleration of the weathering processes requires radiation.

conforms ca. 95% of the UV radiation whereas the UVB radiation constitutes around 5% of the ground UV radiation (World-HealthOrganization, 2019) as part of it is absorbed by atmospheric ozone. The UV-C region (100–280 nm) should not be considered in our studies because it is absorbed by atmospheric oxygen and ozone and does not reach ground level. Besides, all lamps must have protective UV filters by law which block the UV-B region (and part of the UV-A one; be careful, some Hg-vapor lamps might emit discrete radiation there, which might be dangerous). Commercial UV-A emitting lamps (traditionally used to kill insects) are designed to emit intense radiation in very narrow ranges, mostly between 360 and a 380 nm and need to be deployed as traditional fluorescent bulbs (which are about 50 cm length) (Philips, 2018b). UV lamps to disinfect water would be another possibility but they need to be submerged into the water deposits and emit germicide UV-C radiation (which implies a risk during their use). Fluorescent lamps yielding radiation in the UV-B region alone (ca.340 nm) have been used frequently (Arias-Villamizar and VazquezMorillas, 2018; Cai et al., 2018; Tjandraatmadja et al., 1999; Rouillon et al., 2016; Salih et al., 2017; Valadez-Gonzalez et al., 1999). Although a combination of UV- and VIS-emitting lamps might appear a solution, it is not too convenient because several lamps and holders would be required in a limited space. Thanks to their chemical structures most ‘pure’ (i.e., no additives added during their formulation) polymers should be resistant to solar radiation once it was filtered by the Earth atmosphere. The wavelengths needed to scission C-C, C-O or C-H bonds lie in the UV-C (or B) ranges, essentially not available in environmental compartments. So why do they degrade? Because available UV energy can be absorbed by polymer impurities, excess of reagents in the polymer, additives or, even, thermal and oxidative (hydrolytic) degradations or charge transfer complexes that are induced during polymer fabrication (Aslanzadeh and Kish, 2010; Brandon et al., 2016; Brate et al., 2018; Valadez-Gonzalez et al., 1999). These effects create active functional groups or –more commonly- radicals which give rise to chain reactions and autocatalytic processes (Cerruti et al., 2005; Gijsman et al., 1999). Further, as many plastics and MPs that can be found in environmental compartments proceed from commercial items and possess chromophoric groups (e.g., C=C, N=O, C=S) and coloured pigments (formed by metals, metal complexes, diazonium salts, tertiary amines and/or organic conjugated systems), the visible radiation has to be considered as well because those bonds absorb it and can act as active sites for further degradation (Brate et al., 2018). With regards to the VIS region, traditional incandescent bulbs have been phased out in the European Union and, besides, they would release a lot of heat when high powers are required (like in this type of applications), therefore they were discarded. Xenon lamps might be a good option providing all the system (reflectors, filters and optics) shape the light beam to meet the required solar and spectrum irradiance (Chawla, 2019). A combination of fluorescent and xenon arc lamps is proposed in ISO 4892 to study accelerated degradation of ‘macro’ plastic commodity goods using weathering chambers (MPs were not considered at all) (ISO-4892-1, 2016). However, xenon lamps need to be refrigerated either with air or water, which make them to require auxiliary systems, relatively inconvenient for many environmental laboratory studies. Leds (light emitting diodes) offer by design very reduced ranges of VIS wavelengths, whose combination yields the visual white light but without covering the full spectral range and they were discarded. A search throughout different commercial options (including solar simulation equipment) lead us to metal halides lamps. In general, these are quartz discharge tubes containing high-pressure mercury and a mixture of metal halides (e.g., Dy, Ho, Y and Tm) with other metals (e.g., Na and Tl) added for colour correction and arc. Some vendors (not many, unfortunately) offer the standard emission spectrum of their bulbs and this is of great help to plan the design of the system. Among those that we could study, the Philips and Osram bulbs based on metal

4.1.5. Irradiance of the set-up The weathering system was deployed around two metal-halide Powerstar HQI-TS 250W/NDL Osram bulbs; in zenital and ground level positions, each (Fig. 1). To assure that all probes received the same irradiation at horizontal and zenital levels the positions of the cylinders were recorded and interchanged each three days. The emission spectra of the bulbs –as declared by the manufacturer (Osram, 2018) - employed in this setup was compared to other commercial brand (Philips, 2018a) (no technical information was found for other companies) and with the standard solar irradiation at ground level (as recorded previously (Behar-Cohen et al., 2014)), see Fig. 2. Although the units given by the manufacturers are different (a very common situation in literature), the relevant fact is that the shape of the emission bulbs is highly similar among them and, importantly, they emulate quite well the solar irradiance at ground level. The most remarkable difference is due to two emissions spikes at ca. 540 and 590 nm, probably due to the metal halides or ions of the metals that are used in the lamps (likely, Na –characteristic D-lines (doublet) at 589 nm- and Tl –the characteristic green line at 535 nm-). The UV region is simulated pretty fine as the bulbs start to emit radiation after ca. 350 nm; this is correct because, as mentioned above, solar UV-C and UV-B radiations are absorbed in the atmosphere. The next relevant question is to measure the irradiance the deployed system offers. Recall that scalability and customization is a relevant advantage of the ad-hoc system under description. This means that, in principle, as much bulbs as required can be added to carry out a study. However, as discussed in the previous sections, we were concerned by simulating natural weathering conditions that avoid modifying the major processes ruling natural aging. In this work, illuminance was measured on air (the TES 1330A light meter cannot be submerged). On average, 12,200 lumen·m−2 (lux) were measured (ca. 24 lumen/cylinder). Considering a simplified 7

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during 5 days. However, such irradiation falls totally out of common solar values. As two examples, in South Mediterranean regions (like Andalucía –Spain-, one of the sunniest European regions) and North of Africa, maximum irradiance values ca. 375 W m−2 were averaged last summer (2018) (Deutscher-Wetterdienst, 2019); which agree very well with a ca. 330 (W·m−2) summer long-term (1990–2005) average (EUMETSAT, 2019). If intense radiation ‘bursts’ are applied to the MPs one has to be careful about whether some sort of plastic ‘burning’ might happen. The experimental average UV-B (310–315 nm) irradiance (using the UV113 Macam radiometer) yielded by the low-cost system on air was found to be 60·10−2 W m−2. The seawater volume used in the cylinders yielded a water column of 30 cm (5 cm internal diameter) and irradiance was measured at its lowest depth (just above the bottom sand deposit), which was 25·10−2 W m−2. Once more, this value agrees very well with reported irradiances of clean seawater in the winter season in the island of Gran Canaria (Canary Islands) (Hader et al., 2001). We have not found values for Galicia, and despite the difference in latitude, it is possible to accept this value as a reliable starting guideline because the winter irradiance in Canary Islands is similar to the summer irradiance in Galicia (as verified using historical data from the Deutscher Wetterdienst, Germany, available at www.dwd.de) and it was reported that UV radiation is attenuated much less than traditionally thought in clear water (Whitehead et al., 2000), up to 12 m in clear coastal waters and 60 m in oceanic seawater (Llabres et al., 2013). To estimate to which oceanic depth this UV-B irradiance corresponds is more cumbersome. Unfortunately, PAR (photosynthetically active radiation, 400–700 nm) values for Galicia are not available and, so, we could not ascertain the decay of visible light with depth and carry out our own depth estimation. Therefore, we had to indirectly evaluate it. First, a maximum value of 25·10−2 W m−2 UV-B irradiance was reported at noon, February, for clean oceanic water in Gran Canaria (4–5 m depths) (Hader et al., 2001). Second: 10% Z-UV-B values (i.e., depth at which 10% of UV-B radiation remains) were found to be 4–6.7 m for Cabo de Gata (SE. Spain), and around 1 m for fiords (N. Europe) and estuaries (in Canada). Therefore, our system seems good enough to simulate the irradiance of Galician rias (they are a particular geological form of sunk valleys, with the relevant North Atlantic upwelling system, from May–June to October, which raises biological productivity) and the first layer of oceanic waters. Note also that the irradiance decay obtained in the proposed system agrees very well with the theoretical logarithmic trend of the penetration of solar light in seawater (Open-University-Course-Team, 1997). It is a bit higher than expected because a figure on the order of 10·10−2 W m−2 would be expected at 30 cm depth. However, the lamp on the ground gives a lateral component to irradiation which is not present in Nature. In that way we can weather the MPs throughout all their movement in the cylinder. As the final radiation obtained with this design does not increase disproportionally, we can be confident that the simulation device does not produce an extreme situation. It is also worth recalling that it was considered important to move the MPs in the water column in the cylinders so that they are exposed to a range of irradiances (and mechanical stress). This is an advantage of this system because typical oceanic sampling devices (e.g., manta trawls) can only focus on floating plastics and most studies so far focused on them (Eriksen et al., 2014; Ryan, 2015; SAPEA, 2018; van Sebille et al., 2015). However, it is known that many plastics might be at intermediate depths either because of waves, biofouling and colonization (Kooi et al., 2017), or resuspension and they are becoming to be searched for (Kooi et al., 2017; Reisser et al., 2015). Further, some models (Kooi et al., 2017) predict that a maximum concentration of MPs could occur at intermediate depths and, so, relatively low abundances of small particles are predicted at the uppermost ocean surface, while at the same time these small particles may never reach the ocean floor. This is relevant for weathering studies.

Fig. 2. Emission profiles of two characteristic bulbs employed for weathering (Osram Powerstar HQI-TS 250W/NDL and Philips MHN-TD 150W/730 RX7s) compared to the solar irradiance at ground level –upper subplot- (adapted from Behar-Cohen et al., 2014).

overall factor to transform sunlight illuminance to irradiance (Hossain et al., 2011; Singh and Kumar, 2018), the bulbs yielded 96.4 W m−2 (i.e., 0.19 W m−2 per cylinder). This value resembles very well the solar irradiance for Galicia in sunny winter days, reported to be on average less than 100 W m−2. That value is also true for many summer days because Galicia –NW Iberian Peninsula- (in particular the Northern coastal region) has many cloudy and misty days in summer because of prevailing Northern winds; with sunny days, summer solar irradiance can be around 300 W m−2 (Deutscher-Wetterdienst, 2019; Rosón et al., 2008). These values correspond to a range from less than 1 kWh·m−2 (winter) to 5.5 kWh·m−2 (summer) (Instituto_Geográfico_Nacional, 2018; Pettazzi and Salsón-Casado, 2011). All these values suggest that the device under study might be useful for many Central and Northern European countries and, also, for most US states (Solar-Maps, 2019) (certainly for the East coast and Central States, although not for the more irradiated ones at the SouthWestern region, whose insolations are similar to the Mediterranean region). For the more irradiated locations the very simple scalability of the system allows introduction of more bulbs, if local conditions are searched for. These realistic values contrast with a device developed to study scission products released by plastics exposed to UV light (Gewert et al., 2018). There, huge irradiation values (ca. 703 W m−2) were applied 8

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Of widespread use when studying polymers (see, e.g. (Brandon et al., 2016)), the carbonyl index (A1640/A2914 integrated areas ratio) was monitored (see inset in Fig. 3). Despite a general increasing trend, three stages can be seen, and they agree with previous studies (Bieniek et al., 2009; Cerruti and Carfagna, 2010). An initial increase suggests oxidative degradation, a subsequent plateau because the oxidative process is slowed down and finally a continuous increase because a degradation in the crystalline phase (Cerruti and Carfagna, 2010). The process is more intense for pellets submerged in seawater than for those weathered at dry conditions (see inset in Fig. 3), which agrees with previous studies (Goodridge et al., 2010). Fig. 4 presents the general pattern obtained when PP pellets were studied. This polymer degrades more easily than other olefins (e.g. PE) because every other carbon atom in the backbone is a tertiary carbon that is prone to be attacked. The formation of new functional groups, especially carbonyl and hydroperoxides is very relevant here (Gewert et al., 2015). The IR spectrum changes substantially with weathering due to the appearance of bands related to those groups: 3370 and 3240 cm−1 (hydroxyl groups), 1640 cm−1 (C=O and double bonds), 1530 cm−1 (C=O ketones), 1440 cm−1 (carboxylic acids) 1140 cm−1 (alkanes), 1100 cm−1 (C-O bond) and 720 cm−1 (CH2). The inset in Fig. 4 presents the evolution of the typical C = X index (defined as the ratio of the band areas at 1600–1680 cm−1 and 2920 cm−1). The band at 1600 cm−1 does overlap the stretching of the C=C and the C=O bonds (the latter being more characteristic of photooxidation) and it is not possible to differentiate them. This is why they were considered in the same index. The behaviour observed here shows a remarkable increase in the signal just after some days (ca. 15 days, second bar from the left at the inset of Fig. 4), a slight decrease (likely due to chain scission and dissolution into the water –observe the huge difference with what was measured for pellets kept dry and the control samples-), and a new increment in the ratio due to further degradation of the polymer (characterized by the double bonds embedded within this band, see inset in Fig. 4). These general pattern agrees with previous reports (Brandon et al., 2016; Cai et al., 2018; Rouillon et al., 2016) and resemble well the two-stage weathering process of PP (Aslanzadeh and Kish, 2010). The aromatic nature of PS causes it to present slightly different weathering mechanisms than other polymers (not aromatic-based). This is because its larger UV absorption (thanks to the aromatic ring) which yields to the formation of more stable tertiary carbon radicals (Waldman and De Paoli, 2008). The FTIR spectra changes significantly (Fig. 5), which can be monitored straightforwardly using the system presented here. Major spectral changes correspond to the formation of

4.2. Evaluation of the performance of the low-cost ad-hoc weathering system A possible way to validate a weathering system is to compare the aging pattern it induces on the plastics with already published results using natural or similar weathering conditions. It is worth noting that one has to look for general patterns and behaviours, not minor details, because one has to keep in mind that: first, natural weathering studies were done in quite different conditions, and, second, different brands fabricate the ‘same’ polymer with slightly different formulations (e.g., types and quantities of additives, according to the various intended uses of the polymer). Therefore, an exact match between different studies when evaluating the fate of ‘the same’ polymer under non standardized experimental conditions cannot be expected. It seems more reasonable to examine whether similar overall patterns for polymer degradation can be observed, but for different time scales or ‘intensity’ of weathering. For the present work, the three types of MPs indicated in section 3.1 were considered. Please, note that a detailed study/comparison of each polymer and comprehensive details on their evolutions are out of the scope of this paper. The aim here is to demonstrate that the weathering trends obtained with the system above resemble the main patterns put forward in different studies and various natural conditions (weathering chambers or very extreme situations were not considered for the discussions here). This will demonstrate that the system is not producing new weathering patterns and that it can be considered as a reliable system to imitate natural weathering, although in a more systematic and controllable way than off-laboratory studies made at coastal or offshore locations (which, of course still play a relevant role in MP studies). Fig. 3 shows the spectra of pristine (as received) and weathered PA 6.6 pellets (ca. 3 months in the system described above). The characteristic spectral bands of PA are recorded fine: CH–related spectral bands (stretching, at 2863 and 2934 cm−1; and bending, 1450 and 1375 cm−1). The major amide bands at 3400 cm−1 (NH stretch-free motion), at ca. 3300 cm−1 (NH stretch H-bonded), 1633 cm−1 (C=O stretching) and at 1537 cm−1 (NH mono-substituted amide bending plus C-N stretching) (Fernández-González et al., 2018). With regards to the weathering itself, the increasing intensity of the 1150 cm−1 band is compatible with different end products for photooxidation and/or thermo-oxidation of PA (El-Mazry et al., 2013; Gijsman et al., 1999) (tertiary or secondary alcohols, ketones, ethers (symmetric and asymmetric stretching of C-O-C), esters (C-O-R bending) and/or the typical combination band of the carboxylic acids).

Fig. 3. FTIR-ATR spectra of pristine and weathered (10 weeks) PA 6.6 pellets obtained after weathering by the proposed system. The inset shows the evolution of the typical carbonyl band in different weathering conditions (the x-axis denotes weathering: 1 = original, 2 = 1 week, 3 = 3 weeks, 4 = 5 weeks, 5 = 7 weeks and 6 = 9 weeks; the y-axis denotes the spectral ratio, see text for details).

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Fig. 4. FTIR-ATR spectra of pristine and weathered (10 weeks) PP pellets obtained after weathering by the proposed system. The inset shows the evolution of the typical C = X bond (with X being C or O) in different weathering conditions (the x-axis denotes weathering: 1 = original, 2 = 1 week, 3 = 3 weeks, 4 = 5 weeks, 5 = 7 weeks and 6 = 9 weeks; the y-axis denotes the spectral ratio, see text for details).

new bands at 3360-3240 cm−1 (hydroxyl group), 1640 cm−1 (double bonds or C=O groups) and ≈1100 cm−1 (C-O bonds). About this latter band, the C-O index representing it was calculated (defined as the A1000-1200/A2914 ratio) and it was observed that after a strong increment of the band (strong oxidation) within the first fortnight (second bar from the left at the inset of Fig. 5), a smooth decrease indicates most probably the loss of most volatile/soluble reaction products (phenol, benzyl alcohol, ketones, etc), as reported elsewhere (Gewert et al., 2015; Yousif and Haddad, 2013).

latitude), light penetration in clean seawater, wave agitation, mechanical stress and temperature. According to our review on solar irradiation figures reported by meteorological services worldwide, the system can be applied directly to many European countries, US Eastern and Central States and other countries at intermediate latitude. The system is easily scalable and costs less than EUR 1,000, as opposed to more than EUR 50,000 that cost commercial weathering chambers (which in turn had not been developed to carry out environmental weathering studies, but to stress plastic goods intended for commercial use). In addition, the system can also be employed to simulate dry shoreline conditions (microplastics deposited at the highest tide line). Validation was done by considering three different pelletized polymers (polyamide 6.6, polystyrene and polypropylene), with weathering patterns essentially coincident with other reports obtained in other weathering conditions. This is very advantageous because we could not

5. Conclusions A low-cost system to monitor plastic/microplastic weathering under accelerated cuasi-natural oceanic conditions has been presented. It takes into account the solar irradiation at ground level (intermediate

Fig. 5. FTIR-ATR spectra of pristine and weathered (10 weeks) PS pellets obtained after weathering by the proposed system. The inset shows the evolution of the typical C-O bond in different weathering conditions (the x-axis denotes weathering: 1 = original, 2 = 1 week, 3 = 3 weeks, 4 = 5 weeks, 5 = 7 weeks and 6 = 9 weeks; the yaxis denotes the spectral ratio, see text for details).

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find unexpected weathering patterns when compared to other previous reports. As a final conclusion, it is the authors’ opinion that this lowcost system allows for a standardization of the seawater weathering studies and, so, it would contribute to the intercomparability of weathering results to ascertain the fate of microplastics into the environment.

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Acknowledgements This work was supported through the JPI-Oceans BASEMAN project, by the Spanish Ministry of Economy and Competitiveness -partially financed by the European Regional Development Fund program- (Grants: PCIN-2015-170-C02-01 and CTM2016-77945-C3-3-R, ARPA-ACUA). The Program ‘Consolidación e Estructuración de Unidades de Investigación Competitiva’ of the Galician Government (Xunta de Galicia) is also acknowledged (Grant: ED431C 2017/28). Thanks are also given to Miguel Caetano (Instituto Português do Mar e da Atmosfera), and Jose Antonio Bouzas, Service for Risk Prevention of the University of A Coruña, for their help with the irradiance and illuminance measurements. References Al-Oufi, H., McLean, E., Kumar, A.S., Claereboudt, M., Al-Habsi, M., 2004. The effects of solar radiation upon breaking strength and elongation of fishing nets. Fish. Res. 66, 115–119. Arias-Villamizar, C.A., Vazquez-Morillas, A., 2018. Degradation of conventional and oxodegradable high density polyethylene in tropical aqueous and outdoor environments. Rev. Int. Contam. Ambient. 34, 137–147. Arrieta, C., Dong, Y.Y., Lan, A., Vu-Khanh, T., 2013. Outdoor weathering of polyamide and polyester ropes used in fall arrest equipment. J. Appl. Polym. Sci. 130, 3058–3065. Aslanzadeh, S., Kish, M.H., 2010. Photo-oxidation of polypropylene fibers exposed to short wavelength UV radiations. Fibers Polym. 11, 710–718. Badji, C., Beigbeder, J., Garay, H., Bergeret, A., Benezet, J.C., Desauziers, V., 2018. Correlation between artificial and natural weathering of hemp fibers reinforced polypropylene biocomposites. Polym. Degrad. Stab. 148, 117–131. Behar-Cohen, F., Baillet, G., de Ayguavives, T., Garcia, P.O., Krutmann, J., Pena-Garcia, P., Reme, C., Wolffsohn, J.S., 2014. Ultraviolet damage to the eye revisited: eye-sun protection factor (E-SPF), a new ultraviolet protection label for eyewear. Clin. Ophthalmol. 8, 87–104. Bieniek, A., Lipp-Symonowicz, B., Sztajnowski, S., 2009. Influence of the structures of polyamide 6 fibers on their ageing under intensive insolation conditions. Polimery 54, 840–844. Brandon, J., Goldstein, M., Ohman, M.D., 2016. Long-term aging and degradation of microplastic particles: comparing in situ oceanic and experimental weathering patterns. Mar. Pollut. Bull. 110, 299–308. Brate, I.L.N., Blazquez, M., Brooks, S.J., Thomas, K.V., 2018. Weathering impacts the uptake of polyethylene microparticles from toothpaste in Mediterranean mussels (Mgalloprovincialis). Sci. Total Environ. 626, 1310–1318. Cai, H.W., Du, F.N., Li, L.Y., Li, B.W., Li, J.N., Shi, H.H., 2019. A practical approach based on FT-IR spectroscopy for identification of semi-synthetic and natural celluloses in microplastic investigation. Sci. Total Environ. 669, 692–701. Cai, L.Q., Wang, J.D., Peng, J.P., Wu, Z.Q., Tan, X.L., 2018. Observation of the degradation of three types of plastic pellets exposed to UV irradiation in three different environments. Sci. Total Environ. 628–629, 740–747. Cerruti, P., Carfagna, C., 2010. Thermal-oxidative degradation of polyamide 6,6 containing metal salts. Polym. Degrad. Stab. 95, 2405–2412. Cerruti, P., Lavorgna, M., Carfagna, C., Nicolais, L., 2005. Comparison of photo-oxidative degradation of polyamide 6,6 films stabilized with HALS and CuCl2+KI mixtures. Polymer 46, 4571–4583. Cognetti, G., Sarà, M., Magazzù, G., 2001. Biologia Marina. Ariel Ciencia, Barcelona, Spain. COM, 2018. Final Report from the Commission to the European Parliament and the Council on the Impact of the Use of Oxo-Degradable Plastic, Including OxoDegradable Plastic Carrier Bags, on the Environment. Chang, C.H., Tien, C.C., Hsueh, H.C., Sung, L.P., 2018. A macroscopically nondestructive method for characterizing surface mechanical properties of polymeric coatings under accelerated weathering. J. Coat. Technol. Res. 15, 913–922. Chawla, M.K., 2019. A Step by Step Guide to Select the ‘right’ Solar Simulator for Your Solar Cell Testing Application. Photo Emission Tech, Inc, Moorpark, CA, US 2015. Available at: http://www.photoemission.com/techpapers.html. de Bomfim, A.S.C., Maciel, M., Voorwald, H.J.C., Benini, K., de Oliveira, D.M., Cioffi, M.O.H., 2019. Effect of different degradation types on properties of plastic waste obtained from espresso coffee capsules. J. Waste Manag. 83, 123–130. Deroine, M., Le Duigou, A., Corre, Y.-M., Le Gac, P.-Y., Davie, P., César, G., Bruzaud, S., 2014. Seawater accelerated ageing of poly(3-hydroxybutyrate-co-3-hydroxyvalerate). Polym. Degrad. Stab. 105, 237–247. Deutscher, Wetterdienst, 2019. https://www.dwd.de/EN/ourservices/rcccm/int/rcccm_

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