Marine Pollution Bulletin 107 (2016) 333–339

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Marine Pollution Bulletinjournal homepage: www.elsevier.com/locate/marpolbul

The potential of oceanic transport and onshore leaching ofadditive-derived lead by marine macro-plastic debris
Etsuko Nakashima ⁎,1, Atsuhiko Isobe 2, Shin'ichiro Kako 3, Takaaki Itai, Shin Takahashi 4, Xinyu Guo
Center for Marine Environmental Studies, Ehime University, Matsuyama, Ehime 790-8577, Japana r t i c l e

i n f oArticle history:
Received 29 November 2015
Received in revised form 14 March 2016
Accepted 16 March 2016
Available online 16 April 2016
Keywords:
Plastic additives
Lead
Leaching experimentLow-lead layera b s t r a c t
The long-distance transport potential of toxic lead (Pb) by plastic marine debris was examined by pure waterleaching experiments using plastic ﬁshery ﬂoats containing high level of additive-Pb such as 5100 ±74.3 mg kg−1. The leaching of Pb ended after sequential 480-h leaching experiments, and the total leachingamount is equivalent to approximately 0.1% of total Pb in a ﬂoat. But it recovered when the ﬂoat was scratchedusing sandpaper. We propose that a “low-Pb layer,” in which Pb concentration is negligibly small, be generated
on the ﬂoat surface by the initial leaching process. Thickness of the layer is estimated at 2.5 ± 1.2 μm, muchshallower than ﬂaws on ﬂoats scratched by sandpaper and ﬂoats littering beaches. The result suggests that the
low-Pb layer is broken by physical abrasion when ﬂoats are washed ashore, and that Pb inside the ﬂoats canthereafter leach into beaches.
© 2016 Elsevier Ltd. All rights reserved.

1. Introduction
Plastic products that reach the ocean as marine plastic debris cantravel great distances over long periods because of their light weightand durability. In East Asia, plastics are currently the most common
type of marine/beach debris (Zhou et al., 2011; Kusui and Noda, 2003;
Hong et al., 2014; Kuo and Huang, 2014). Thus, a large amount of plastic
marine debris originating from East Asian countries is even found onbeaches of remote islands such as the Goto Islands of Japan (Fig. 1a;
Kako et al., 2010; Nakashima et al., 2011, 2012). Plastic marine debris
washed ashore on beaches is not only an eyesore but can be a transport
vector of (i) chemicals absorbed on plastic debris surfaces from the surrounding seawater and (ii) additives originally incorporated in plastics(Teuten et al., 2009). Contaminants (i) are well known as hydrophobic
persistent organic pollutants (POPs) and polycyclic aromatic hydrocarbons that absorb on plastic particles from ambient seawater becauseof the hydrophobic nature of plastic surfaces (Teuten et al., 2009). Not
only POPs but also metals are detected in relatively high concentrations⁎ Corresponding author at: Tosa Food Business Creator Project Team, Kochi University,
B-200, Monobe, Nankoku, Kochi 783-8502, Japan.
E-mail address: nakashima@kochi-u.ac.jp (E. Nakashima).
1
Present afﬁliation: Tosa Food Business Creator Project Team, Kochi University,
Nankoku, Kochi 783–8502, Japan.
2
Present afﬁliation: Research Institute for Applied Mechanics, Kyushu University,
Kasuga 816–8580, Japan.
3
Present afﬁliation: Ocean Civil Engineering, Graduate School of Science andEngineering, Kagoshima University, Kagoshima 890–0065, Japan.
4
Present afﬁliation: Center of Advanced Technology for the Environment, Faculty ofAgriculture, Ehime University, Matsuyama, Ehime 790–8566, Japan.

http://dx.doi.org/10.1016/j.marpolbul.2016.03.0380025-326X/© 2016 Elsevier Ltd. All rights reserved.

on surfaces of beached pellets, suggesting the accumulation of metals
in marine plastic debris from seawater (Aston et al., 2010; Holmes
et al., 2010). In addition, additive-derived metals are widely used in
manufacturing plastics. Some of these are known toxic substances, functioning as catalysts. Examples are antimony trioxide commonly incorporated into polyethylene terephthalate (Takahashi et al., 2008), andstabilizers like lead stearate that enhance smoothness and stability ofpolyvinyl chloride (PVC) polymer (Minagawa, 1966). Despite their
beneﬁts, these metals are known to be toxic to plants, animals and microorganisms (Fairbrother et al., 2007). The European Union (EU) has
therefore implemented a directive for “Restriction of Hazardous Substances” (RoHS) in electrical and electronic equipment for six toxic substances (including Pb) to reduce the risk to human health (European
Union, 2003a); Pb should not be contained with a concentration higher
than 0.1 wt.% (i.e., 1000 mg kg−1) in products. This regulation is applied
to mixed recycled plastic (European Union, 2003b), and there havebeen a number of reported violations of these regulatory standards forplastic products, such as paints or stabilizers used in children's toys
(Becker et al., 2010; Wäger et al., 2012).
In our earlier research, we detected high-concentration metals
contained in marine plastic debris washed ashore on Ookushi Beach inJapan (Fig. 1a; Nakashima et al., 2012). In particular, PVC ﬁshing ﬂoats
contain the greatest quantity of Pb such as 13,500 ± 8400 mg kg− 1(Fig. 1b and c). Of particular interest is that additive-derived toxic
metals leach into the beach environment after plastic marine debris
washes ashore. Therefore, we estimated the potential risk of toxic
metal that could leach into that environment from such debris. In
Nakashima et al. (2012), balloon aerial photography in conjunction
with a beach survey gave an estimated Pb mass derived from marine
plastic litter at 313 ± 247 g. Total Pb mass that could leach from PVC

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E. Nakashima et al. / Marine Pollution Bulletin 107 (2016) 333–339

Fig. 1. (a) Location of Ookushi and Inoda beaches (red squares). Polyvinyl chloride ﬁshing ﬂoats (PVC ﬂoat) were collected from Inoda Beach. (b) Photograph of PVC ﬂoats used in leachingexperiments. (c) PVC ﬂoats littering Inoda Beach (Ishigaki Island, Okinawa, Japan).

plastic debris over a year onto Ookushi Beach was estimated at 0.6 ±0.6 g year−1, from a leaching experiment together with a Fickian diffusion model analysis. This leaching experiment examined the solubilityof Pb into pure water because our attention was paid for how much
Pb in plastic debris littered on beaches may leach into the surrounding
water (such as rain water). The high variability of leaching Pb mass
was possibly due to different manufacturers of plastic debris in themarine environment.
Based on the aforementioned leaching experiment, it is suggested thatdurable plastic marine debris acts as a transport vector of toxic metals
such as Pb. However, one may question why additive-derived metals inplastic marine debris do not leach completely into seawater, particularly
if they have traveled long distances and possibly long periods in theocean, and why Pb are still extensively detected in plastic debris washedashore on beaches. The objective of this study is to answer these questions by focusing on additive-derived Pb leaching from ﬁshery ﬂoats
made from PVC, based on 5-day leaching experiments using newlypurchased ﬂoats in pure water (not seawater for ease of the experiments;
shown later in Section 2.2). To investigate the potential of long-distanceoceanic transport and leaching of additive-derived Pb from the PVC ﬁshing ﬂoats on beaches, we computed the amount of Pb leaching from a single new PVC ﬁshing ﬂoat to compare this amount with the total Pbamount in the ﬂoat. Although we start with these simple questions, we
forward a scenario in which the additive-derived metals are transported
onto beaches along with plastic marine debris.
2. Materials and Methods
2.1. Materials and Analytical Methods for Leaching Experiments
Fishery ﬂoats made from PVC (hereafter referred to as PVC ﬂoats;
Fig. 1b) were chosen for the experiments because of their high Pb concentration (e.g., 13,000 ± 8400 mg kg−1), as reported by Nakashima
et al. (2012). Such high concentrations are probably caused by lead
stearate, and greatly exceed the EU regulatory standard of Pb(1000 mg kg−1). Although the PVC ﬂoat is a speciﬁc marine plastic debris, it is also common debris that we can collect straightforwardly on
various beaches around Japan (Fig. 1c). The transport of additivederived Pb in the PVC ﬂoats was examined by the following leachingexperiment. Fundamental procedures of this experiment using PVC
ﬂoats are in Nakashima et al. (2012), so the description below focuseson adaptations of those procedures.

In the present study, the PVC ﬂoats (13.5 cm × 3.3 cm × ϕ3.0 cm, average 39.5 g; Fig. 1b) used for the leaching experiments were not thoselittered on the actual beaches, but newly purchased directly from a single
Chinese manufactory to minimize variation of Pb leaching because ofboth ﬂaws on ﬂoat surface and different Pb concentrations in PVC ﬂoats
provided by various manufactories. Although several dozen ﬂoat manufacturers in China could be identiﬁed from textual information on theﬂoat surface, the purchased ﬂoats were similar to those littering beaches(Fig. 1b and c; samples corrected on Inoda Beach of Ishigaki Island,
Okinawa, Japan).
Properties of the purchased ﬂoats were examined as described
below before the leaching experiments. Surface areas of purchased
PVC ﬂoats were measured using a 3D laser scanner (DAVID 3D Solutions
GbR, Braunschweig, Germany). Pb concentrations in the ﬂoats were
measured using a handheld Energy Dispersive X-ray ﬂuorescence analyzer (Innov-X XRF Analyzer, Alpha-6500, Innov-X Systems, Inc.,
Woburn, Massachusetts, USA; hereafter referred to as XRF) equippedwith X-ray tube of tungsten anode, where the energy range is
10–40 kV. The XRF is an appropriate analyzer to subsequent leachingexperiments because it allows us to measure the concentrations of Pb
in the PVC ﬂoats with non-destructive analysis. Based on ten-time measurements of virgin pellets (Grand Polymer Co. Ltd., Tokyo, Japan), thelower limit of quantitation of Pb using the XRF was found to be
8.0 mg kg−1, which was calculated as ten times the standard deviationof 0.8 mg kg−1 obtained from these ten experiments. To improve theaccuracy of analysis, the XRF was calibrated beforehand against an
inductively coupled plasma mass spectrometry system (ICP-MS, Agilent
7500cx, Agilent Technologies, Santa Clara, California, USA) (Nakashima
et al., 2012). Microwave digestion using acids has been employed todecompose plastics before ICP-MS analysis. Prepared PVC plastics(n = 11) were cut into small fragments for the analyses. Every 0.2 g ofsamples was digested with polytetraﬂuoroethylene vessels using 3 ml
nitric acid and 5 ml sulfuric acid (for ultratrace analysis, Wako Pure
Chemical Industries, Ltd., Osaka, Japan). After microwave digestion,
samples were diluted to a volume of 50 ml with pure water (Milli-Q
water, Millipore, Merck KGaA, Darmstadt, Germany) and were injected
into ICP-MS. The accuracy of ICP-MS was calibrated by using EC680k
and EC681k (European Reference Materials) as reference materials(low density polyethylene). A regression curve was obtained as
y = 2724e10.2x, where the concentration of Pb measured using XRF(ICP-MS) is assigned to x (y); the correlation coefﬁcient was 0.98.
The concentrations of Pb measured using the XRF were therefore

E. Nakashima et al. / Marine Pollution Bulletin 107 (2016) 333–339

converted to those obtained from the corresponding ICP-MS values
on this regression curve. It is well known that, in quantifying using
XRF analysis, arsenic interferes with Pb due to the similarity ofeach main characteristic X-ray, and thus, we used the second X-ray(i.e., Lβ1, 12.612 keV) to avoid the interference. We also conﬁrmedthat no arsenic is in the PVC ﬂoats used for ICP-MS analyses.
2.2. Leaching Experiments Using New PVC Floats
In the leaching experiments, a single PVC ﬂoat was placed in a glassvial ﬁlled with 820 ml Elix pure water (Elix, EMD Millipore, Merck
KGaA, Darmstadt, Germany). Ten purchased PVC ﬂoats were used forthe following leaching experiments, and ten vials were prepared in addition to a control vial without a ﬂoat. A single-type vial was used tostandardize the volume ratio of water to a PVC ﬂoat in all experiments.
Each experimental vial was shaken continuously at 120 rpm using ashaking water bath (SWB-17/25, AS ONE Corp., Osaka, Japan) for
120 h, during which water temperature was maintained at 25.5 ±1.0 °C. During each 120-h experiment, 10 ml of water was sampledfrom the glass vials every 24 h, and the same volume of pure water
was added thereafter to each vial. A volume of 0.5 ml nitric acid (forultratrace analysis; Wako Pure Chemical Industries Ltd., Osaka, Japan)
was added to the 10-ml samples. The above set of experiments ishereafter referred to as the 120-h leaching experiment.
For ease of experiment, pure water instead of seawater was used in
the experiment; otherwise seawater contain an abundant variety of metallic salts (such as sodium salts, magnesium salts, and potassium salts),
which are known to interfere with measuring Pb using ICP-MS. For thisreason, we preliminarily analyzed Pb in seawater with a type of disk car-335

tridge (Empore, 3M Science, St. Paul, MN, USA) whose surface adsorbs
and accumulates selectively aqueous Pb without metallic salts. To evaluate the effectiveness of the disk cartridge, three recovery tests had
been tried that standard Pb (100 μg l−1) was added into artiﬁcial seawater (Daigo's Artiﬁcial Seawater SP, Nihon Pharmaceutical Co., Ltd.,Tokyo, Japan), and Pb was measured by ICP-MS after removal of the metallic salts using disk cartridges. The recovery rates resulted in low
values ranging from 3 to 15% with considerable variation, which impliedthat Pb passed through disk cartridges due to its low adsorption. This
technical problem prevented us to measure Pb in seawater.
The usage of pure water is indeed a limitation of the present experimental design. For instance, we have to note that Pb leaching rate
(leaching speed) in the actual ocean is likely to differ from that estimated from the experimental results. In fact, the Pb leaching from PVC ﬂoats
must be accelerated because abundant aqueous ions (e.g., carbonate
2−(CO2−3 ) and sulfate (SO4 )) in seawater may immediately form leadsalts (lead sulfate (PbSO4) and lead stearate (Pb(C18H35O2)2)) fromlead carbonate (PbCO3) in PVC ﬂoats via a surface ion exchange mechanism. In the present study, it is assumed that the total amount of Pb
leaching into seawater becomes similar to that into pure water overthe course of the inﬁnitely long drift in the actual ocean, even if theleaching rate in pure water is different from that in seawater.
The 120-h leaching experiments were repeated four times (480 h
total). Immediately after each experiment, all pure water in the glass
vials was exchanged with new water to avoid equilibration of Pb concentration in the water. This water exchange allowed simulation of
the situation in which seawater surrounding the PVC ﬂoat continuously
varies during long-distance transport in the ocean. After the fourth exchange, the ﬂoats used in the 120-h leaching experiments were scariﬁed

Table 1
Experimental data used for estimating each leaching rate (Ri).

0-120 hours
Sample No.
R0-120 [μg hour-1]

240-360 hoursc∞[ μg L-1]

a [cm2]

Sample No.

R240-360 [μg hour-1]

c∞ [μg L ]

129
132
132
132
136
132
132
139
136
136
134

1
2
3
4
5
6
7
8
9
10

Average

24
15
14
22
23
23
21
26
20
19
21
3.5
2.7

1
2
3
4
5
6
7
8
9
10
Average

1.0
0.7
1.0
1.1
0.5
1.0
1.0
1.1
1.1
0.7
0.9

146
96
146
157
68
142
147
163
164
95
132

cf [mg cm-3]
6.3
6.4
6.4
6.3
5.8
6.5
6.5
6.4
6.3
6.4
6.3

Standard
Deviation

0.23

33

2.0

2.9

Standard Deviation

0.16
0.10
0.10
0.15
0.16
0.15
0.15
0.17
0.14
0.13
0.14
0.025

Error

0.17

25

1.5

2.2

Error

0.018

120-240 hours
R120-240
1
2
3
4
5
6
7
8
9
10

-1

360-480 hours
R360-480

Average

0.32
0.31
0.51
0.51
0.52
0.42
0.40
0.44
0.41
0.29
0.41

47
46
74
75
77
61
58
65
60
43
61

Standard
Deviation

0.086

12

Standard Deviation

Error

0.065

9.5

Error

Refer to text for meaning of each variable.

1
2
3
4
4
6
7
8
9
10

Average

0.02
0.01
0.01
0.01
0.02
0.07
0.02
0.11
0.08
0.02
0.04
0.035
0.026

3.4
1.4
1.8
2.1
2.5
9.6
3.1
17
11
3.3
5.5
5.1
3.8

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E. Nakashima et al. / Marine Pollution Bulletin 107 (2016) 333–339

with sandpaper (#120) and used again in the subsequent experimentwith pure water. The latter experiment was conducted to reproduce
the situation in which PVC ﬂoats washing ashore onto beaches are scariﬁed by the abrasive action of coastal sand. It was conﬁrmed that the
sandpaper contained no Pb by analysis of 10 replicate measurements
using XRF. As in the previous 120-h leaching experiments, 10 ml ofwater sampled every 24 h was injected into the ICP-MS to measure Pb
concentration. To ensure accurate estimation of Pb leaching in purewater, Pb loss to the glass vial surfaces was evaluated by a recovery
test (Nakashima et al., 2012). Analysis of data from this test indicatedthat an average 1.8% of Pb was lost on the glass surfaces, so this ﬁgure
was added to the Pb mass that could leach for each glass vial. Although
there were three concentration values that exceeded thrice the standard deviation from average in the ﬁrst 120-h leaching experiment(samples 2, 5 and 10; Table 1), there was little variation during thenext 120 to 480-h leaching experiments. Therefore, all data fromevery PVC ﬂoat (n = 10) are reported here.
2.3. Computation of Pb Leaching Amount From a Single PVC Float
As shown later (Section 3.1), the amount of Pb leaching from the PVC
ﬂoats decreased exponentially as the four 120-h leaching experiments
were repeated. This suggests that Pb leaching from the ﬂoats also
decreases during drift in the ocean although the leaching rate wasunderestimated in the experiments using pure water. Thus, we wereable to estimate the total amount of Pb leaching from the ﬂoats duringlong-term transport as follows. First, the amount of Pb leaching from aﬂoat per unit time during each 120-h leaching experiment (hereafterreferred to as “leaching rate (R)”) was estimated by
Ri ¼ C ∞ V=120 h;ð1Þ

where C∞ denotes the Pb concentration at the end of each 120-h experiment, V is the volume of Elix pure water (0.82 l), and subscript i (0–120,
120–240, 240–360 and 360–480 h) refers to periods of each experiment. Leaching rates in every experiment were calculated over theentire experimental duration of 480 h.
Each leaching rate Ri computed in the four 120-h leaching experiments was regarded as the value at the midpoint of each experimental
period (i.e., 60, 180, 300 and 420 h from the beginning of the ﬁrst
120-h experiment), and was approximated by the following exponential equation using a least squares method.
Rðt Þ ¼ R0 e−αt ;ð2Þwhere R(t) symbolizes the leaching rate as a function of time (t), R0 is
the initial leaching rate, and α is the leaching coefﬁcient. Temporally integrating Eq. (2) from 0 to inﬁnity yields the total amount of Pb that canleach from a single PVC ﬂoat (M; g, hereafter referred to as total leachingamount of Pb), as follows:
M¼

Z ∞R0 e−αt dt:

0

ð3Þ

As shown later (Section 3.1), we assumed that the PVC ﬂoat after theleaching experiments might be covered by a “low-Pb layer,” in which Pb
concentration became negligibly small because of diffusion from theﬂoat surface to the surrounding water as shown later in Section 3.1.
Thus, ﬁnally, the thickness (d; m) of that low-Pb layer, deﬁned as thatfrom which Pb leaches from a PVC ﬂoat, was simply estimated as 
d ¼ M= ac f ;ð4Þ

where a is the average surface area of the ﬂoat and c f is the average Pb
mass per unit volume of the ﬂoat (measured by XRF). The units of c f areweight per unit volume (i.e., 6.3 × 10−3 g cm−3 (i.e., 6.3 mg cm−3) asshown in Table 1). Eq. (4) assumes that all Pb within the layer thickness(= dacf) leaches completely to the surrounding water, and the Pb concentration within the layer becomes negligibly small insufﬁcient for a
Fickian diffusion process (leaching) to be continued afterward.
2.4. Observation of Surface Scratches on PVC Float Surface Using a
MetallographOf particular interest is whether scratches observed on PVC ﬂoats
washed ashore on beaches are deeper than the thickness of the lowPb layer mentioned above. If this is the case, it is likely that Pb beneaththe low-Pb layer formed during oceanic transport would leach onto thebeaches through any physical damage, such as scratching by coastal
sand. We could simulate this “re-leaching” in the leaching experiments
using the PVC ﬂoat scratched by sandpaper.
We compared depths of surface scratches on three PVC ﬂoats thatwere purchased (new) and artiﬁcially scariﬁed, collected on Inoda
Beach (Fig. 1a) on 11 March 2011. Cross-sections of each PVC ﬂoat
were observed by a metallograph (VANOX-T, AHMT-513, Olympus
Corp., Tokyo, Japan) appropriate for measuring the layer thickness
computed in the study (Section 3.2). A new, unused PVC ﬂoat wasalso observed for reference. Before measurement, each ﬂoat was cut
into 2-cm squares, and each segment was ﬁxed in a cup ﬁlled withepoxy resin liquid composed of epoxy hardener and EpoxyCure®
epoxy resin (Buehler, Lake Bluff, Illinois, USA). The mixed epoxy resin
liquid was solidiﬁed at room temperature under atmospheric pressure
overnight. To observe cross-sections, each ﬁxed segment was groundunder 3 lb pressure by a grinder (Beta & Vector, Buehler) with a coarseﬁle (#280), second-cut ﬁle (#320), and smooth ﬁle (#600) for 3 mineach. The cross-sections of PVC ﬂoats were photographed using 100

Fig. 2. Results of leaching experiments using purchased PVC ﬂoats. Curves indicate Pb concentration (ordinate) in water versus time (abscissa).

E. Nakashima et al. / Marine Pollution Bulletin 107 (2016) 333–339

337

Table 2
Leaching rate (Ri) estimated at four 120-h leaching experiments.

Sample No.

R0-120 [μg hour-1]

R120-240

R240-360

1.0
0.7
1.0
1.1
0.5
1.0
1.0
1.1
1.1
0.7

0.32
0.31
0.51
0.51
0.52
0.42
0.40
0.44
0.41
0.29

0.16
0.10
0.10
0.15
0.16
0.15
0.15
0.17
0.14
0.13

1
2
3
4
5
6
7
8
9
10

R360-480
0.02
0.01
0.01
0.01
0.02
0.07
0.02
0.11
0.08
0.02

AverageStandard Deviation

Error

R0 [μg hour-1]
2.1
1.9
3.3
3.7
1.5
1.6
2.4
1.5
1.6
1.5

α hour-1]

2.1
0.8
0.6
R0

0.010
0.00230.00232.1 ± 0.6

0.010
0.012
0.013
0.013
0.00900.00800.011
0.00700.00700.010

α0.010 ± 0.002
3

These values are used for compute initial leaching rate ( R 0 ) and leaching coefficient (α) by least squares fit of Eq. (2).

magniﬁcations with a camera attached to the metallograph, allowing
the depth of ﬂaws on the ﬂoat surface to be observed directly.
3. Results and Discussion
3.1. Transport Potential of Additive-derived Pb in a PVC Float
The results obtained in leaching experiments are shown in Table 1
and Fig. 2. New PVC ﬂoats before the leaching experiment contained
high level of additive-Pb such as 6.3 ± 1.5 mg cm− 3 shown as cf inTable 1 (which is equivalent to 5100 ± 74.3 mg kg−1). This corresponds
approximately to 201 ± 2.9 mg per ﬂoat (39.5 g). Curves in Fig. 2
show that Pb concentrations in the water of the glass vials increased
with time in each 120-h leaching experiment, and appeared to reachan equilibrium at the end of those experiments. The maximum concentration (C∞) of Pb in water reached approximately 70 to 150 μg l−1 after
the ﬁrst 120-h experiment, but diminished to b20 μg l−1 after 480 h(Fig. 2). This indicates that leaching of additive-derived Pb from thePVC ﬂoats ended within a relatively short period (~480 h).
In the fourth experiment, Pb leaching to the surrounding water
largely disappeared. According to the Fickian diffusion, the leachingamount is proportional to κcf − c, where κ denotes the non-zero partition coefﬁcient and c the Pb concentration of the surrounding water(Nakashima et al., 2012). Note that κ is obtained as C∞/cf and thus, it isa dimensionless number. c in Elix pure water at the beginning of each120-h experiment was always zero. Thus, the disappearance of theleaching at the end of the experiment suggests that cf became negligibly
small at least on the ﬂoat surface, and that the ﬂoat might be covered by
the low-Pb layer described in Section 2.3.
We were able to conﬁrm that the low-Pb layer is very thin by scarifying the surface of PVC ﬂoats. That is, C∞ in the glass vials increased to
two-thirds of that found in the ﬁrst 120-h leaching experiment(Fig. 2). This result indicates that the low-Pb layer is probably sufﬁciently thin so as to be broken by the sandpaper abrasion. It is reasonable tobelieve that additive-derived Pb leaches only from the thin layer near
the ﬂoat surface into seawater. This is likely because water penetrabilityof hydrophobic plastic is low, and because the mass transport in the interior of the ﬂoats is much slower than that in the thin surface layer into
which seawater penetrates. It is therefore possible that the additivederived Pb contained inside the ﬂoats can be carried a long distance inseawater before leaching onto beaches.

3.2. Estimation of Low-Pb Layer Thickness
To prove that Pb contained in the PVC ﬂoats leaches after scratchingby beach sand, it was necessary to conﬁrm whether the depth of ﬂaws
on the ﬂoats littering the beaches was substantially deeper than theFig. 3. Leaching rates (Ri in Eq. (1); closed squares with bars show standard deviations) computed on the basis of four 120-h leaching experiments. Also shown is the curve ﬁt by an
exponential formula. In ﬁtting by least squares, leaching rate is at midpoint of each experimental period shown by steps.

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E. Nakashima et al. / Marine Pollution Bulletin 107 (2016) 333–339

Fig. 4. Photographs of cross-sections of (a) scratched PVC ﬂoat, (b) littered PVC ﬂoat, and (c) new PVC ﬂoat as reference. Visually substantial ﬂaws marked by inverted triangles. Reference
scale in the panels represents 100 μm.

thickness of the aforementioned low-Pb layer. We therefore estimatedthe thickness of that layer.
First, a discrete set of each Ri obtained in the four 120-h leachingexperiments was ﬁt by Eq. (2) with a least squares method. Ri of ten
samples was used to obtain the exponential function in that equation
independently, and R0 and α were then averaged (Table 2). Average
R0 and α were 2.1 μg h−1 and 0.010 h−1, respectively, with the marginpﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
of error deduced as 0:8  2:262= 10−1≈  0:6 μg h−1, where
2.262 is area of the t-distribution at 95% conﬁdence level with degreeof freedom 9. Error of α was estimated in the same manner as R0.
Thus, an exponential curve (Fig. 3) was approximated using R(t) =(2.1 ± 0.6)e−(0.010 ± 0.0023)t, where units of R(t) are μg h−1, and hours
for t. Temporal integration of the approximate leaching rate R(t) =R0e−αt from t = 0 to inﬁnity enables estimation of M by M = R0/α =2.1/0.010 = 210 μg. In addition, error propagation in this estimation
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
was computed by  ð0:6=2:1Þ2 þ ð0:0023=0:010Þ2  210 μg ≈  73 μg.
This total leaching amount is equivalent to approximately 0.1% of total
Pb (201 mg) per PVC ﬂoat.
Next, we estimated d of the low-Pb layer. Substituting M, a and c f(Table 1) into the right-hand side of Eq. (4), we obtained d = 2.5 μm.
In addition, error propagation in this estimation was computed
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
as  ð73=210Þ2 þ ð2:9=133Þ2 þ ð2:0=6:3Þ2  2:5 μm ≈  1:2 μm.
3.3. Measurement of Scratch Depths on PVC Float Surfaces Littering Beaches
To conﬁrm whether ﬂaws on the surface of PVC ﬂoats littering thebeaches are sufﬁciently deep for Pb to leach out, cross-sections of aﬂoat artiﬁcially scariﬁed by sandpaper, ﬂoats littering Inoda Beach,and a new ﬂoat purchased from a manufacturer were photographedwith a metallograph (Fig. 4a, b and c, respectively). Depths were visually
measured with each reference scale. Flaw depths on the artiﬁcially scariﬁed PVC ﬂoat were ~20 μm (Fig. 4a). The litter PVC ﬂoat was damaged,
with ﬂaws between 50 μm and 100 μm in depth (Fig. 4b). In contrast,

the new PVC ﬂoat had no damage on its surface (Fig. 4c). The observations of the artiﬁcially scariﬁed ﬂoat indicate that ﬂaw depths on its surface scratched by sandpaper were an order of magnitude larger than thelayer thickness 2.5 ± 1.2 μm. Thus, it is reasonable to conclude that the
low-Pb layer was broken by sandpaper scarifying before the 480–600-h
experiment shown in Fig. 2, and that Pb contained inside the ﬂoat
re-leached in that experiment. It was also found that the PVC ﬂoats scariﬁed on the beaches had ﬂaws signiﬁcantly deeper than the low-Pb
layer. The suggestion here is that Pb preserved in PVC ﬂoats duringlong-distance transport over the sea will again leach Pb upon destruction of the thin low-Pb layer washing ashore. Hence, plastic PVC ﬂoats
can carry potentially toxic additive Pb over long distances, releasingthat Pb as a pollutant into the marine/beach environment by breakingof the low-Pb layer on beaches.
Nevertheless, there remains a question regarding the leaching ratein actual conditions. In the present study, pure water was used for allleaching experiments for ease of experimentation and analysis. In reality, there is leaching of metals from marine plastic debris in seawater.
The seawater may accelerate leaching to a rate faster than in ourstudy. Further, the leaching of metals from marine plastic debris into
the surrounding seawater may be inhibited by microbial bioﬁlms covering the debris (Lobelle and Cunliffe, 2011), hydrophobic compounds
such as POPs adsorbed on the surface (Bakir et al., 2012), and aquatic
ion metals repeating an adsorption–desorption cycle on marine plastic
litter (Holmes et al., 2010). Thus, the leaching rate, total amount of Pb
leaching from PVC ﬂoats, and low-Pb layer thickness are quantiﬁed
more precisely in the actual oceans.
4. Conclusions
The processes uncovered in the present study are as follows. A single
plastic ﬁshery ﬂoat contains additive-Pb with the high concentration of
5100 ± 74.3 mg kg− 1. After sequential 480-h leaching experiments
using pure water, the total leaching amount is approximately 0.1% of
total Pb in a single ﬂoat. Therefore, additive Pb within the PVC ﬂoats

E. Nakashima et al. / Marine Pollution Bulletin 107 (2016) 333–339

leached only from a thin layer on the ﬂoat surface during long-distancedrift on the sea. PVC ﬂoats are covered by a low-Pb layer generated bydiffusion control, and Pb within the ﬂoats is prevented from leaching
into seawater during transport. Pb leaches out when the ﬂoats are
washed ashore on beaches, because the aforesaid layer is broken byphysical damage such as scratching by coastal sand. Thus, additive Pb
potentially travels long distances in the ocean because the PVC ﬂoats
act as an effective transport vector.
Finally, although not a subject in the present work, the speed of Pb
leaching from microplastics may be much higher than that computed
herein, because of their large surface area to volume ratio. Hopefully,
the present study is a step toward evaluating the environmental riskof marine/beach plastic debris to the marine ecosystem.
Acknowledgments
This research was supported by the Environment Research andTechnology Development Fund (B-1007 and 4-1502) of the Ministryof the Environment (Japan), Global Center of Excellence Program of
the Ministry of Education, Culture, Sports, Science, and Technology(MEXT) of Japan (E07), and Research Fellow of the Japan Society for
the Promotion of Science (Japan) (23-55433). We are all grateful to Professor Takada (Tokyo University of Agriculture and Technology) for theoretical guidance and technical advice. We also appreciate the Ehime
Institute of Industrial Technology for excellent technical assistance andexpertise with a metallograph.
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