Microplastic pollution in benthic mid-stream sediments of the Rhine

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Microplastic pollution in benthic mid-stream sediments of the Rhine River Thomas Mani, Sebastian Primpke, Claudia Lorenz, Gunnar Gerdts, and Patricia Burkhardt-Holm Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01363 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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Environmental Science & Technology

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Microplastic pollution in benthic mid-

2

stream sediments of the Rhine River

3 4

Thomas Mani,† Sebastian Primpke,§ Claudia Lorenz,§ Gunnar Gerdts,*§ and

5

Patricia Burkhardt-Holm*†

6 7

†Department

8

University of Basel, Vesalgasse 1, 4051 Basel, Switzerland

9

§Department

of Environmental Sciences, The Man-Society-Environment Programme,

of Microbial Ecology, Biologische Anstalt Helgoland, Alfred-Wegener-Institut

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Helmholtz-Zentrum für Polar- und Meeresforschung, Kurpromenade, 27498 Helgoland,

11

Germany

12

1 ACS Paragon Plus Environment


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1.

ABSTRACT ART

14 15


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2.

ABSTRACT

18

Rivers are major transport vectors for microplastics (MP) towards the sea. However, there is

19

evidence that MP can be temporarily or permanently inhibited from migrating downstream by

20

sediment retention or ingestion by organisms. MP concentrations, compositions and fate within

21

the different compartments of the fluvial environment are poorly understood. Here, benthic,

22

mid-stream sediments of two undammed, open-flowing stretches were investigated in the

23

Rhine River, one of the world’s busiest inland waterways. Twenty-five samples were collected

24

at ten sites via riverbed access through a diving bell or dredging. We performed the first

25

comprehensive analysis of riverbed sediment aliquots that avoids visual selection bias using

26

state-of-the art automated micro-Fourier-transform infrared spectroscopy (μFTIR) imaging.

27

MP numbers ranged between 0.26±0.01–11.07±0.6 × 103 MP kg-1 while MP particles 1 g cm-3) dominated (85±18%), which contrasts

33

the large proportions of low-density polymers previously reported in near-surface

34

compartments of the Rhine.

35



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36

3.

INTRODUCTION

37

An estimated 4.8–12.7 million tonnes of plastic waste entered the oceans from land in 2010 1.

38

Rivers alone reportedly carry 0.47–2.75 million tonnes of plastic waste towards the seas every

39

year

40

world’s oceans today 4. MP emerge from varnish, paint, tire wear, textiles, agriculture, industry,

41

uncountable consumer products, packaging and atmospheric fallout

42

include municipal, agricultural, industrial and road run-off

43

plants (WWTP)

44

Although land-based sources are widely regarded as the main contributors to the marine plastic

45

load (~80%) 14, rivers cannot be simply regarded as linear vectors that transport all of the MP

46

they are polluted with

47

MP in lotic waters may be prevented from directly reaching downstream lentic waterbodies

48

due to ingestion by organisms 18,19 or retention in sediments 20–22.

49

The Rhine River catchment is home to ~60 million people 23, with over 10 million inhabitants

50

in the Rhine-Ruhr metropolitan area alone

51

approximately 10% of the world’s chemical industry

52

sailings carry ~150 million tonnes of goods through the Lower Rhine alone, 26 qualifying the

53

Rhine as one of the world’s most frequented inland waterways 27. The surface waters of the

54

Rhine, 21,28,29 as well as shoreline sediments 20 and estuary benthos, 21 have been investigated

55

for MP. The Rhine transports 10–30 tons of plastic litter towards the North Sea every year 28,30.

56

However, there is a knowledge gap regarding the role of benthic, mid-stream sediments as

57

potential temporary or permanent MP sinks in the Middle and Lower Rhine stretches. Until

58

now, it was unclear (i) which types and sizes of polymers may be retained in the riverbed, (ii)

59

if the retained polymers are complementary to those found in near-surface compartments, and

2,3

and it is suggested there are 4.85 trillion microplastic (MP, < 5 mm) particles in the

11,12

and (direct) industrial dischargers

15–17

9,13

8–10,

Diffuse sources

while wastewater treatment

represent potential point sources.

to downstream seas and oceans

24.

5,6,7.

2,7.

Indeed, there is evidence that

The Middle- and Lower Rhine host 25

and every year over 110,000 vessel


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60

(iii) if the “plastic footprint” could possibly indicate specific dominant emission sources (e.g.

61

shipping). Two sites where MP concentrations at the water surface notably increased and

62

peaked, respectively, were previously identified at Rhine kilometres (Rh-km) 640 and 837 28.

63

For the present study, two geographically proximal stretches (Koblenz, Rh-km 593.95–598.77

64

and Rees, 837.41–837.52) were chosen to investigate if these high surface water MP

65

concentrations are reflected in the mid-stream bed sediments 31. Until now, the interaction of

66

MP with benthic riverbed sediments in stretches of dynamic flow represented a highly

67

unknown factor in the mass balancing of MP downstream transportation 2. We hypothesised

68

that (hA) the sediments mainly contain MP polymers that are denser than freshwater

69

(>1 g cm-3) with small diameters ( 5 mm (fine–coarse pebbles) using stainless steel geological sieves

114

(see Table S1 and Figure S3 for the grain size ratios of each sample). Subsequently, the

115

replicate aliquots of the clay–silt–sand fraction (n = 25) from each of the ten sampling sites

116

were pooled, resulting in ten 60 g dw pools (five each for Koblenz and Rees). Aliquots of 30 g

117

were pooled in pairs for the five Koblenz sites (KB1–5) and aliquots of 20 g were pooled in

118

triplets for the five Rees sites (RB1–3 and RD1–2).

119

4.3. ZnCl2 density separation

120

Samples were density separated using a protocol inspired by the Munich Plastic Sediment

121

Separator

122

transferred into 100 mL Erlenmeyer flasks under a fume hood. The flasks were filled with

123

ZnCl2 solution ( ~1.7 g cm-3), leaving a 1.5 cm margin below the brim, and covered with

124

aluminium foil to prevent entry of airborne particles. To disaggregate clusters of sediment and

125

potential MP particles, the glass receptacles were submerged in an ultrasonic bath until the

126

mouth of the flask was 3 cm above the brim and sonicated for 15 min at 160 W/35 kHz

127

(Sonorex RK255 H; Bandelin Electronic GmbH & Co., Berlin, Germany). Subsequently,

128

samples were stirred for 1 h at 250 rpm using PTFE-coated magnetic stirrer bars to suspend

129

particles with  500 µm

141

fraction was retained for subsequent manual MP analysis (cf. section 4.7). Each disk was placed

142

into a dry 200 mL glass beaker in a 20 °C water bath, 10 mL FeSO4 solution (7.2 mM, pH 5)

143

was added, then 20 mL of H2O2 (30%) was added dropwise over 15 min to induce the

144

exothermic Fenton’s reaction while the temperature of the water bath was monitored (step A).

145

To remove residual particles from the steel meshes, the filters and beakers were placed into an

146

ultrasonic bath and sonicated at 215 W/35 kHz for 5 min (step B; Sonorex RK514). Steps (A)

147

and (B) were both repeated. Next, three 1 mL subsamples from every sample were analysed

148

using a FlowCam (Fluid Imaging Technologies, Inc., Scarborough, ME, USA) to calculate a

149

feasible suspension volume for filtration onto the Anodisc for FTIR imaging

150

Accordingly, the calculated volumes of suspension for each sample (between 7.5–100%; Table

151

S1) were filtered onto aluminium oxide filters (pore size: 0.2 m, diameter: 25 mm, Anodisc;

152

Whatman, Merck, Darmstadt, Germany). Sample pools KB3, KB5 and RB2 underwent

153

additional density separation using ZnCl2 and were passed through a separation funnel to

154

remove excess hydrated iron oxide particles that formed during the Fenton reaction.


36,45.

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4.5. Focal plane array µFTIR analysis

156

The sample fractions (< 500 μm) on Anodisc filters were analysed using a Hyperion 3000

157

μFTIR microscope equipped with a Focal Plane Array (FPA) detector (64 × 64 detector

158

elements) and TENSOR 27 spectrometer (Bruker Optics GmbH., Billerica, MA, USA).

159

Measurements were performed at wavenumbers from 3600–1250 cm−1 at a resolution of 8 cm−1

160

and binning factor of 4 with six scans per field (see 46 for details). Filters were dried for at least

161

2 days at 30 °C, placed onto a calcium fluoride window under the FTIR microscope, a visual

162

overview image was recorded (Figure S6), then the concentrates on the filter (166 mm2, 73 ×

163

73 FPA fields, 1.36 million spectra) were assessed (ca. 13 h per filter).

164

4.6. Automated analysis of μFTIR data

165

Each FTIR spectrum was automatically analysed using two library search methods to confirm

166

the identities of the polymers via automated analysis

167

adaptable design

168

quality and polymer type, and further investigated via image analysis based on Python 3.4

169

scripts and Simple ITK functions 38. This method excludes human selection bias, and enables

170

identification, quantification and size determination for all polymer particles

171

numbers were assessed on the aliquot filters with an accuracy of 5%; size classes were

172

introduced to reduce the complexity of the size distribution (e.g. 11 μm, 11–25 μm, etc., see 38

173

for details). Error calculation and values are provided in the SI and the Electronic SI (ESI).

174

4.7. Visual selection and ATR FTIR analysis of > 500 m fractions

175

Putative MP > 500 m in the < 2 mm aliquots were visually and tactually investigated on

176

cellulose filter paper (pore size: 0.45 µm, diameter: 47 mm; Whatmann ME25, mixed cellulose

177

ester, Merck) using ultra-fine forceps and a stereomicroscope equipped with a camera and

178

imaging software (Olympus SZ61, Olympus SC50, Tokyo, Japan; CellSens Entry Version

179

1.17.16030.0). Selection criteria (though not exclusive) were (i) homogenous texture with an

47.

38

combined with a database with an

Each identified pixel was recorded, together with its position, analysis


38.

MP particle


180

absence of cellular structure and absence of (ii) crushing or powdering upon applying force

181

with forceps, and (iii) conspicuous artificial colouring and (iv) shape (e.g. spherule or filament)

182

48,49.

183

attenuated total reflection (ATR) FTIR spectrometer (Bruker ALPHA with a Platinum

184

Diamond-ATR QuickSnap Sampling Module, Bruker). IR-Spectra were recorded over the

185

wavenumber range of 4,000–400 cm-1 at a resolution of 4 cm-1, applying 24 scans. Each

186

spectrum was compared against a reference spectra library using Opus 7.5 software (Bruker;

187

B-KIMW ATR-IR Polymers, Plastics and Additives, 898 entries, principle component

188

analysis). Particles with a synthetic polymer hit quality index (HQI) above 70% were

189

considered MP. The entire 2–5 mm and > 5 mm fractions were visually investigated on a

190

stainless steel tray and a stereomicroscope, in case of conspicuous particles.

191

4.8. Statistical analysis

192

MP concentrations were compared using unpaired t-tests with Welch’s correction; normality,

193

using the Shapiro-Wilk test (p > 0.05). Linear regression analysis of MP concentrations and

194

the proportion of sand in the samples was performed using Spearman’s rank correlation. All

195

statistical analyses were performed using GraphPad Prism version 7.03 for Windows

196

(GraphPad Software, La Jolla, CA, USA).

197

4.9. Quality assessment and quality control

198

Sampling, processing and analysis steps were performed using glass, stainless steel, wood or

199

PTFE materials instead of other plastics whenever possible; PTFE cannot be identified in the

200

spectral IR range between 3600–1250 cm-1 applied for automatic FTIR imaging 38. All items

201

were rinsed with Aq. dest. before use. The one exception was a PE squirt bottle used for Aq.

202

dest. rinsing; otherwise a PTFE squirt bottle was used. Samples were processed under a fume

203

hood whenever possible and white 100% cotton lab coats were worn at all times. Dustboxes

204

(DB1000, G4 prefiltration, HEPA-H14 final filtration, Q = 950 m3 h-1; Möcklinghoff

Particles meeting at least one of these four criteria (n = 126) were analysed using an


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Lufttechnik, Gelsenkirchen, Germany) that filter airborne particles were installed in the

206

laboratories for Fenton’s reagent purification, additional density separation (ZnCl2) after

207

Fenton’s and µFTIR imaging. Fenton’s reagent purification, additional density separation and

208

Anodisc filtration were performed in a laminar flow cabinet (Scanlaf Fortuna, Labogene,

209

Allerød, Denmark). The H2O2 and FeSO4 solutions were filtered through polycarbonate filters

210

(0.2 m, GTTP; Merck Millipore, Darmstadt, Germany) to remove particulate contaminants

211

before use. Sample receptacles were always covered or sealed as soon as possible after each

212

treatment step using glass lids, Parafilm or aluminium foil. Three procedural blanks were run

213

and analysed with automated µFTIR imaging to detect potential sample processing-introduced

214

MP in the samples (Figure S7); accordingly, all reported MP concentrations are blank corrected

215

with consideration to polymer types and size classes (SI).

216

5.

217

5.1. Microplastic concentrations and sizes

218

MP were identified at all sites from Koblenz and Rees at sizes ranging between 11–5033 μm

219

(orthogonal longest axis). Concentrations were 0.26 ± 0.01–11.07 0.6 × 103 MP particles kg-1

220

in the 11–500 μm size range, Figure 1). The MP concentrations for the five diving bell sites at

221

Koblenz

222

2.54 ± 0.14 × 103 kg-1 (KB4), 2.74 ± 0.15 × 103 kg-1 (KB2), and 5.22 ± 0.75 × 103 kg-1 (KB5,

223

Figure 1). At Rees, the three diving bell sites yielded 3.13 ± 0.44 × 103 kg-1 (RB1),

224

3.43 ± 0.19 × 103 kg-1 (RB3), and 11.07 ± 0.6 × 103 kg-1 (RB2). The two Rees bucket chain

225

dredge sites contained 1.79 ± 0.1 × 103 kg-1 (RD1) and 0.89 ± 0.05 × 103 kg-1 (RD2). Mean MP

226

concentrations were higher at Rees than Koblenz (only diving bell considered) and higher for

227

the diving bell sites than the bucket chain dredge sites (only Rees considered); however,

228

statistical comparisons yielded no significant differences, possibly due to the relatively low

229

sample pool numbers (N = 10, p > 0.05; Figure S8). The lowest MP concentrations at Rees

RESULTS

were:

0.26 ± 0.01 × 103 kg-1

(KB1),


2.05 ± 0.13 × 103 kg-1

(KB3),


230

were recorded in the bucket chain sites RD1 and RD2. The deeper dredge sediment sampling

231

depths (compared to the diving bell) may be responsible for “dilution” of MP concentrations

232

with clean, deeper lying sand, and the open dredging bucket possibly allowed for loss of smaller

233

MP during sample hauling. The highest MP pollution level in the central Rees site RB2 may

234

possibly be explained by its position within the highly frequented navigation channel, where

235

high shipping-related MP emissions may occur. Strikingly, MP concentrations at the five

236

Koblenz sites along the 4.82 km stretch between Rh-km 593.95–598.77 increased from

237

0.26 ± 0.01 × 103 kg-1 at the most upstream, central site (KB1) to 5.22 ± 0.75 × 103 kg-1 at the

238

most downstream site (KB5), located immediately downstream of the Rhine island Graswerth;

239

the three sites in-between KB2–4 yielded a mean MP concentration of 2.45 ± 0.26 × 103 kg-1.

240

The increase in MP concentrations along this stretch may possibly be explained by changes in

241

the riverbed’s flow velocity exposition moving downstream. Site KB1 contained a low MP

242

concentration, most probably due to its central location in the river cross-section upstream of

243

Niederwerth island, resulting in unsheltered exposure to higher flow velocity. In contrast, site

244

KB5 had the highest concentration of all Koblenz sites, presumably due to its location

245

downstream of Graswerth island that is sheltered from stronger flow dynamics. The sites in

246

between (KB2 to KB4) yielded a narrower range of MP concentrations, possibly as they are

247

exposed to similar water dynamics. However, linear regression analysis of MP concentrations

248

and fine-grain sediment proportions (< 2 mm) as a proxy for near-bed flow velocity at all ten

249

sites yielded no significant relationship (Figure S9). MP < 75 m accounted for 96.3 ± 5.7%

250

of all MP in the sample fractions analysed by µFTIR imaging from all ten sites; 68.2 ± 15.9%

251

of all MP were < 25 m (Figure S10).

252

5.2. Polymer composition

253

Eighteen polymer types were identified through µFTIR imaging in the 11–500 μm size fraction

254

(18 types in Rees and 14 in Koblenz, Figure 1). The acrylates/polyurethane/varnish cluster 12 ACS Paragon Plus Environment

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255

(containing 27 spectra) 47, henceforth termed APV, was found at all ten sites. The sites lying

256

within the shipping channel (all except RB1 and RB3) yielded a mean numerical proportion of

257

78 ± 6.4% APV MP compared to 39% and 33% at the two peripheral diving bell sites at Rees.

258

The next most widely distributed polymers were chlorinated polyethylene (CPE, 7 sites),

259

ethylene-propylene-diene rubber (EPDM, 7), polyester (PEST, 6), and polyethylene (PE, 6,

260

Table S3). The most polymer-diverse sites were RB3 (11 polymers), RB2 (10), KB4 (10), and

261

KB2 (9). RB3 lies at the slip-off slope of a sharp, left turning riverbend at Rees, where the

262

lowest flow velocity of all investigated sites was reflected by the highest fine-grain sediment

263

proportion (Figure S3). The low exposure to currents at RB3, resulting in sediment

264

accumulation, is a possible explanation for the settling of a wider density range of polymers,

265

manifested in the by far highest share of the low density PP for any site (57.2%). At RB2, with

266

the exception of APV (81.8%) and chemically modified cellulose (CMC, 12.2%), all other

267

polymer types accounted for less than 1.1% of particles. Hence, the high diversity is due to the

268

presence of a wide variety of scarcely represented polymers. Riverbed dredging does not take

269

place at RB2, potentially enabling this modest retention of a wide array of polymers. KB4 and

270

KB2 are similarly positioned on the left-hand border of the shipping channel downstream of

271

Koblenz. Their relatively high diversity is the result of the presence of several types of polymer

272

at proportions of 0.3–5.9% (apart from APV, 80.4 ± 6.7%). The lowest polymer diversities

273

were found at the Koblenz sites KB1 and KB5, which featured only three types each (Figure 1).

274

At Rees, the diving bell sampling sites yielded higher mean polymer diversities than the

275

dredged sites (9 ± 3.6 vs. 5.5 ± 0.7).

276

From the manually sorted > 500 m sediment fractions, eight of the total 126 putative

277

microplastic particles analysed by ATR-FTIR (6.3%) were confirmed as synthetic polymers

278

(HQI > 70%; cf. section 4.7): PTFE in KB1 (1), RB2 (2) and RB3 (2); urea formaldehyde (UF)

279

resin in KB5 (1) and RD1 (1); and PE in RB3 (1). PTFE was not further considered in this 13 ACS Paragon Plus Environment


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study, as it could not be detected by FTIR imaging 38 (cf. section 4.9). No putative MP were

281

detected in the 2–5 mm and > 5 mm fractions.

282

6.

283

6.1. MP sedimentation and retention in riverbed sediments

284

Aquatic suspended particulate matter, including MP, is expected to preferentially settle in low-

285

dynamic environments 50. Rivers are characterised by an elevation gradient resulting in time-

286

averaged motion of water in the longitudinal direction 51. As dynamics increase, the probability

287

of MP sedimentation, retention and burial decrease

288

moving waterbody counter the settling of MP 7; thus, among others, the factors relevant to mid-

289

stream riverbed MP sedimentation include (i) specific polymer density

290

hydrodynamics

291

efficiency of MP to ambient non-MP suspended solids

292

heteroaggregation with ambient particulate matter

293

substrates

294

polymer density, local hydrodynamics and potential heteroaggregation.

295

6.2. MP concentrations in riverbed sediments

296

Considering the preferable MP settling conditions of low water dynamics 7, the presence of

297

high MP concentrations in the mid-stream Rhine riverbed sediments from open-flowing,

298

dynamic stretches with mean flow velocities of 1.2–1.4 m s-1 62 is unexpected. The higher mean

299

MP concentrations at Rees compared to Koblenz may be due to general differences in natural

300

and artificial sedimentation and erosion regimes, as well as almost impossible to predict local

301

hydrodynamics. Downstream of Koblenz, the Rhine riverbed is subject to high erosion due to

302

inflow from sporadic rising of the Moselle river, which carries suspended solids but no bedload

303

63.

DISCUSSION

61.

52,55,

7,32,52.

Density and shear forces within a

53,54,

(ii) local

and (iii) formation of biofilms on MP 56–58 that alter (iv) the attachment

32,

59,60

and hence influence (v)

allowing retention, even in coarse

In this evaluation of the Rhine riverbed sediments, we focused on specific

Therefore, on average, every 1.5 years, 35,000 m3 of terrestrially quarried basalt rubble


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304

(grain size: < 100 mm) is artificially introduced into the Rhine immediately downstream of the

305

Moselle inflow

306

Rees, the slip-off slope on the inside of the relatively sharp river bend (samples RD1, RD2 and

307

RB3) is a net sedimentation zone, where periodic dredging is required to maintain the depth of

308

the shipping channel

309

basalt rubble downstream the Moselle-Rhine confluence at Koblenz is unlikely to be MP-

310

contaminated

311

compared to Rees where sediment and MP may accumulate on the riverbed over time.

312

The Rhine sediments at RB1–3 and KB5, sampled 163 and 402 km upstream of the estuary,

313

yielded MP concentrations of a comparable magnitude as findings in the downstream, tidal

314

harbour benthic sediments of Rotterdam (3.01–3.6 × 103 kg-1, > 10 μm) 21. Although the MP

315

concentrations at six sites (KB1–4 and RD1 and RD2) in the present study are lower than the

316

Rotterdam figures, the MP concentrations at KB5 and RB2 remarkably exceeded the

317

concentrations in Rotterdam harbour (while RB1 and RB3 are within the Rotterdam range).

318

This finding is remarkable as Rotterdam lies (a) at the mouth of the Rhine and is (b) home to

319

Europe’s busiest (and the World’s twelfth busiest) port

320

stronger flow dynamics, water dynamics are largely reduced to tidal, seiche and shipping

321

influences

322

anthropogenic pressure, receiving high pollution immissions 68, and is thus expected to retain

323

high amounts of MP in its sediments. Due to massive sedimentation at the mouth of the Rhine,

324

regular dredging is necessary at Rotterdam to maintain smooth shipping operations 69. Thereby,

325

anthropogenic pollutants, including MP, may be removed from the estuary

326

longitudinal study of the Rhine indicated decreasing water surface MP concentrations between

327

Rees and downstream Rotterdam which led to speculation that the “lost” MP could all be

328

retained in the estuarine sediments

67

63

63.

– just at the stretch where the Koblenz samples were taken. In contrast, at

64.

According to the responsible authorities, the artificially introduced

This possibly explains the generally lower MP concentrations at Koblenz,

which facilitates MP settling

28.

7

65,66.

Hence, (a’) in the absence of

and (b’) the estuary is under enormous

70.

A previous

If this were true, we would expect to observe lower 15

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21.

329

riverbed concentrations at upstream sites (this study) than in the estuary

330

assumption is not supported by our nor by other studies

331

important to state existing comparisons of upstream (present study) and estuary

332

riverbed MP concentrations are based on two individual studies, each with limited sample

333

numbers, highly variable concentrations (in this study), and different sampling, purification

334

and polymer analysis approaches and techniques. Furthermore, the depth, timing and season of

335

sampling, as well as previous dredging or artificial sediment supply events and the position of

336

the sites relative to the shipping channel may affect the resulting MP concentrations and

337

polymer compositions. Looking to other European waters, the MP concentrations of the present

338

study are comparable with those of canals in Amsterdam 21 and England 55, which yielded 0.07–

339

10.5 × 103 kg-1 (> 10 μm) and 0.3–4.8 × 103 kg-1 (> 63 μm), respectively. While Hurley et al.

340

(2018) 55 may have reported higher concentrations if smaller particles had been included (due

341

to the likely dominance of small MP

342

sediments contain similar concentrations of MP as the almost static downtown canals 21 of the

343

850,000-inhabitant Dutch capital 72. Generally, it seems that the local, small-scale variability

344

in MP pollution and retention is highly diverse

345

other, larger scale factors such as general suspended solid transport and sedimentation regimes.

346

The co-existence of hydrological, physical, hydraulic engineering and pollution influences

347

plays a crucial role in the outcome of MP investigations and will always lay the curse of high

348

uncertainty on interpretation of the data. However, putting the European figures into

349

perspective, the highest riverbed sediment MP concentrations reported to date were found in

350

the Wen-Rui Tang River at 18.69–74.8 × 103 kg-1, albeit very small MP particles > 0.4 μm

351

were included

352

0.05 m s-1 during sampling close to the > 3 million-citizen Wenzhou metropolis

353

Wen-Rui Tang canal system enters the Ouijang river slightly upstream of the East China Sea

73.

36),

31,55,71.

However, this

On a technical note, it is 21

Rhine

it is completely unforeseen that Rhine mid-stream

16

and that these effects may superimpose on

In that study, the low-elevation gradient dictated flow velocities below


74

where the

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354

73,

providing two highly favourable preconditions for MP in sediments: calm water 7 and a high

355

population 6.

356

6.3. Polymers in Rhine riverbed sediments

357

The high proportions of APV in the riverbed sediments are a novel finding for the entire Rhine

358

ecosystem. Two main members of the APV cluster, acrylates (ACR) and polyurethanes (PU,

359

both ~1.2 g cm-3, 75), are common components of antifouling paint used to protect and reduce

360

friction on ship hulls

361

2 g cm-3 (refs. 34,77) and substantial loads of antifouling paint particles reportedly erode into the

362

environment from vessel hulls as a result of chemical, physical or mechanical stress impact 34.

363

Given the high density of shipping traffic in the Rhine River, it is likely that considerable

364

proportions of the reported benthic APV particles stem from antifouling paint. The highest

365

APV concentration was detected at the centre-riverbed surface at RB2 (Table S3), which

366

spatially coincides with the navigation channel 78. However, ACR and PU polymers are also

367

used in other applications, including construction, automobiles, glass substitutes, textiles,

368

biomedicine and optics (ACR) 79,80 and construction, automobiles, refrigerator insulation and

369

furniture (PU) 81. The areas upstream of all sampling sites have high population densities as

370

well as high densities of construction and industrial activities 23,25, which may also contribute

371

to the frequent occurrence of APV in the sediments, as widely used APV materials are

372

potentially emitted to the environment due to riparian mismanagement 6,15. It is remarkable that

373

the highest APV concentration was identified at RB2 in the centre of the riverbed, which is

374

exposed to stronger water currents than peripheral sites closer to the riverbank, as confirmed

375

by the comparatively low clay–silt–sand proportion of only 8.4 ± 3.8% in these samples

376

(Figure S3 and Table S1). This finding possibly indicates that the concentrations of APV MP

377

in the central benthic Rhine sediment may at times be much higher or lower than reported, but

378

that these particles are continuously resuspended and washed downstream due to riverbed

76.

Antifouling paint formulae can reach specific densities of up to



55,82.

Page 18 of 39

379

erosion

The limited data on polymer types in benthic river sediments around the world

380

indicates low proportions of APV polymers. PU accounted for 6.5% of identified MP particles

381

(n = 293) in the Chinese Wen-Rui Tang watershed in Wenzhou

382

system does not facilitate high shipping activity and the high levels of heavy metal pollution in

383

sediments are attributed to industrial activities

384

Yangtze River upstream of the Three Gorges Dam (n = 174, > 48 μm) 31 despite this area seeing

385

an increase in shipping in the past few years 84. Similarly, benthic sediments from the Canadian

386

St. Lawrence River did not yield APV polymers; however, it should be noted that study only

387

included MP > 500 μm and focused on PE microbeads 22. It is important to state the Chinese

388

and Canadian studies 22,31 relied on visual preselection of potential MP particles, and employed

389

different polymer identification algorithms to the present study. Therefore, it remains unclear

390

what investigations at those sites could yield if conducted with the same methodology

391

employed in this paper. Turning to other studies of Rhine River near-surface compartments,

392

the proportion of APV-type polymers ranged from 1.5% 20 and 2.5% 29 to 18.6% 28. However,

393

the latter study

394

methacrylate (PMMA ~1.18 g cm-3) beads represented 9.3% of the reported APV polymers in

395

that study. The presence of these MP at the water surface is most probably due to the increased

396

buoyancy conferred by gas bubbles inside the PMMA beads 28. The non-selective, standardised

397

polymer analysis approach used in the present study enables a more objective assessment of

398

polymer types and proportions, as very small and even barely visible MP particles were

399

included automatically

400

visual MP selection techniques

401

use different polymer databases, HQI thresholds and polymer identification algorithms and

402

resulting clusters 38, it is possible that widespread APV pollution – potentially emitted from

403

erosion of antifouling paint – is widely overlooked 35.

28

83.

73;

where the narrow canal

No APV-relevant MP were found in the

relied on a visual pre-selection of only 118 MP particles; polymethyl

38.

Due to the fact many studies still rely on bias-afflicted, manual, 85,86,

coupled with the limitation that various research groups


Page 19 of 39


404

Polymers denser than freshwater (> 1 g cm-3) accounted for 85.1 ± 17.9% (mean ± SD) of the

405

numerical proportion of MP per site (Table S3). By contrast, only 5% 29 and 8% 20 of the MP

406

detected in near-surface compartments of the Rhine were > 1 g cm-3 (Figure 2). Note, PS was

407

considered denser than freshwater in this study, however, it frequently occurs as low-density

408

buoyant foam in near-surface freshwater compartments 87. The distinct polymer density/river

409

compartment allocation in the Rhine is not entirely corroborated by other studies, e.g. of the

410

Chinese Yangtze 31 and Wen-Rui Tang 73 rivers, where polymer particles > 1 g cm-3 accounted

411

for 39% and 41% of MP in the riverbed sediments, respectively. This variation may be the

412

result of different research methodologies and geographies 37,85, but could also suggest specific

413

pollution sources and pathways as each of these rivers endure distinct anthropogenic strains 6.

414

This is the first quantitative evaluation of polymer concentrations down to a particle size of

415

11 μm in benthic riverbed sediments based on a comprehensive sample aliquot analysis. The

416

very low proportion of particles > 500 μm positively confirmed as polymers in manual analysis

417

by ATR-FTIR (6.3% cf. section 5.2) corroborates the frequently described high bias-affliction

418

of manual MP identification 88–90, emphasizing the importance of standardised, bias-mitigating

419

analysis techniques 38,91,92.

420

At this point we are able to ascertain that all three hypothesises stated in the introduction can

421

be confirmed: (hA) that benthic Rhine sediments mainly contain small MP of polymers denser

422

than freshwater; (hB) that benthic MP polymer densities complement near-water surface MP

423

polymers and (hC), that the Rhine River benthic sediments are possibly strongly polluted by

424

antifouling ship paint erosion (cf. also section 5.2.).

425

6.4. Dominance of smallest size MP in the Rhine riverbed

426

As plastics fragment into ever-smaller pieces in aquatic media

427

distribution of particles is naturally expected to shift and skew towards the smallest detectable


93,

the size frequency


428

size classes 94. The MP data from the Rhine riverbed dramatically corroborates this notion, as

429

a mean (± SD) of 96.3 ± 5.7% MP particles per site were < 75 m in diameter (Figure S10).

430

The striking prevalence of the smallest particle sizes in the Rhine riverbed sediments may be

431

explained by biofouling and additional grinding up of particles on the riverbed. Biofouling on

432

MP (i) leads to relative increases in the specific density 32, and (ii) at the same time enhances

433

the attachment efficiency of small MP which – coupled with the high abundance of small MP

434

94

435

therewith increase the probability of sedimentation 32. In a lotic waterbody, settling of MP in

436

mid-stream bed sediments more strongly depends on the particles’ sedimentation potential –

437

due to the higher dynamic forces – than in a lentic water body, where all particles, homo- and

438

heteroaggregates >1 g cm-3 will eventually sink to the ground 7. Once at the riverbed, MP may

439

be continuously ground up into ever finer particles in the moving bedload, which acts as a mill

440

33.

441

sediments 77.

442

6.5. Sampling method and seasonal influences on sediment MP abundance

443

The three Rees sites in the centre of the cross-section, RB2, RD1 and RD2, yielded decreasing

444

MP concentrations with increasing sediment depth: 11.07 ± 0.6 × 103 kg-1, 7 cm (RB2),

445

1.79 ± 0.1 × 103 kg-1, 42 cm (RD1) and 0.89 ± 0.05 × 103 kg-1, 111 cm, (RD2). Considering

446

that the upper sediment layers usually display the highest levels of MP pollution 95,96, the lower

447

mean MP concentrations in the bucket chain dredge samples could be explained by “dilution”

448

of the samples with deeper, less-contaminated sand

449

dynamic and subject to regular dredging and deposition of artificial sediments along the

450

shipping routes, the stratification of these sediments does not necessarily represent historical

451

timelines

– may increase the ability of small MP to form heteroaggregates with natural colloids, and

Similarly, ship paint particles are speculated to undergo attrition in dynamic marine

98.

97.

As Rhine riverbed sediments are

Nevertheless, at any point in time, the sediment-water interface is immediately


Page 20 of 39

Page 21 of 39


452

exposed to water pollution and is therefore the sphere of latest potential MP retention, possibly

453

explaining the higher MP concentrations in the upper sediment layers.

454

Significant flushing of MP out of riverbed sediments during high-discharge periods has

455

recently been described 55. At Rees, the bucket chain dredge samples (RD1–2) were collected

456

on the 2nd of February, 2016, during river discharge (Q) of 3070 m3 s-1, following a 20-day

457

period of Q as low as 2020 m3 s-1. The Rees diving bell samples (RB1–3) were taken on the

458

15th of March, 2016, at Q 2670 m3 s-1 following a 40-day period of higher discharges ranging

459

up to Q 5350 m3 s-1 (Figure S11). Despite the period of high discharge between the two

460

sampling dates, the diving bell samples taken on the later date yielded clearly higher MP

461

concentrations, underlining the possibly strong effects of the sampling method and potentially

462

sediment depth and other factors, such as local hydrodynamics (0.89 ± 0.05 and

463

1.79 ± 0.1 × 103 kg-1 dredge vs. 3.13 ± 0.44–11.07 ± 0.6 × 103 kg-1 diving bell). However, the

464

relatively low sample size compounds conclusive comparisons at this stage.

465

6.6. Appraisal of methodology

466

This is the first study to investigate riverbed MP using automated (FPA) μFTIR microscopy

467

and image analysis (for the 11–500 μm size range)

468

demonstrated on samples from wastewater treatment plant in- and effluents

469

Arctic deep sea sediments

470

exceptional degree of standardisation, which circumvents human bias by (i) avoiding visual

471

pre-selection of potential MP and (ii) automatic comparison of up to 3 million IR spectra

472

against reference libraries

473

accurately reproduce MP particle morphology as (i) only the two-dimensional shape is captured

474

38

475

weathering

476

representation 38. Like most other MP polymer analysis techniques, FPA μFTIR-spectroscopy

36

47

and sea ice

99.

38,46.

This method has successfully been 11,44,

as well as

The greatest advantage of this approach is its

in each mapping

38.

On the downside, this method does not

and (ii) some pixels may be unassigned for identified MP particles as a consequence of 100

or insufficient removal of biofilms

57,

which would alter true particle



Page 22 of 39

44,92.

477

relies on effective sample purification to remove refractory content

478

heteroaggregation of MP with natural suspended solids – which may possibly yield important

479

knowledge of MP sedimentation behaviour 32 – cannot be assessed after sample preparation.

480

MP loss during laboratory processing cannot be ruled out 92. However, we assessed our adapted

481

ZnCl2 protocol

482

fragments (~1.18 g cm-3; 62–125 μm; 125–250 μm; 250–500 μm and 500–700 μm) in

483

triplicate. Mean recovery rates ranged from 55 to 100%, with the lowest mean retrieval of

484

55 ± 8.7% for the smallest size class (Figure S5). Considering that 96  6% of MP particles

485

from the Rhine River sediments were < 75 μm, the total MP load could be substantially

486

underrepresented. On the other hand, use of an ultrasonic bath for sample cleansing and

487

separation

488

potentially brittle ship paint fragments.

37

43

Therefore,

in a pilot spike-recovery test using four particle size classes of PMMA

may result in additional fragmentation of MP

489


92,

especially considering the

Page 23 of 39

490


7.

FIGURES

491 492 493 494 495 496 497 498 499

Figure 1: Microplastics concentrations at Rees and Koblenz in the size range 11–500 μm. Location of the sampling sites at Rees and Koblenz (a–c); MP concentrations correspond to the area of the circles (d); microplastics particle concentrations in MP kg-1 and polymer numerical proportions at Rees (e) and Koblenz (f). APV: acrylates/polyurethanes/varnish cluster; PP: polypropylene; PS: polystyrene; CMC: chemically modified cellulose; EPDM: ethylene-propylene-diene rubber; PEST: polyester; PE: polyethylene; EVA: ethylene-vinyl-acetate; CPE: chlorinated polyethylene; PA: polyamide; PLA: polylactide acid; OPE: oxidised polyethylene; PCP: polychloroprene; NR: nitrile rubber; PC: polycarbonate; ACNB: acrylonitrile-butadiene; PVC: polyvinylchloride.



500

501 502 503 504

Figure 2: Mean microplastic polymer cluster proportions reported for three Rhine River compartments in five studies. APV: acrylates/polyurethanes/varnish cluster; PEST: polyester; PVC: polyvinylchloride; PA: polyamide; PS: polystyrene; PP: polypropylene; PE: polyethylene.

505


Page 24 of 39

Page 25 of 39


506

8.

ASSOCIATED CONTENT

507

8.1. Supporting Information

508

The Supporting Information is available free of charge on the ACS Publications website at

509

DOI: …

510

Additional sections on ZnCl2 density separation and MP recovery experiment; eleven figures

511

depicting the sampling and separation methods, sediment grain size classes, visual aliquot

512

example with a µFTIR false colour image, blank-correction values, comparison of mean MP

513

concentrations by location and sampling method, regression analysis of fine-grain sediment

514

proportion vs. MP concentration, MP size class distribution and river discharge timelines.

515

Three tables listing sample metadata, literature overview of MP in lotic sediments and

516

polymer type proportions per site (PDF). Electronic Supporting Information (ESI) is provided

517

for uncertainty analysis of reported MP concentrations, comprehensive polymer and particle

518

size class distribution (XLS).

519

9.

520

9.1. Corresponding Authors

521

*Phone: +41 61 207 04 02; e-mail: [email protected].

522

*Phone: +49 4725 819 3245; e-mail: [email protected].

523

9.2. ORCID

524

Thomas Mani: 0000-0002-5746-9972

525

Sebastian Primpke: 0000-0001-7633-8524

526

Claudia Lorenz: 0000-0002-7898-7728

527

Gunnar Gerdts: 0000-0003-0872-3927

AUTHOR INFORMATION



528

Patricia Burkhardt-Holm: 0000-0001-5396-6405

529

9.3. Author Contributions

530

T.M., P.H. and G.G. designed the study; T.M. performed the field work; T.M. and C.L.

531

processed and prepared the samples for µFTIR imaging; S.P. conducted the polymer

532

identification and analysis; T.M. and P.H. analysed the data and wrote the manuscript with

533

substantial contributions from and final approval of all authors.

534

9.4. Notes

535

The authors declare no competing financial interests.

536

10. ACKNOWLEDGEMENTS

537

We thank the waterways and shipping administrations (WSA) Duisburg-Rhein and Bingen,

538

Germany for vessel access and technical support. We also thank Nicole Seiler-Kurth, Heidi

539

Schiffer and Hedwig Maria Scharlipp, MGU University of Basel, as well as Vanessa

540

Wirzberger, AWI Helgoland, for laboratory assistance. Special thanks go to Andrea Devlin,

541

chief editor of Science Editing Experts for proof reading and language support. This work was

542

supported by the World Wide Fund for Nature (WWF) Switzerland and the German Federal

543

Ministry of Education and Research (Project BASEMAN - Defining the baselines and

544

standards for microplastics analyses in European waters; BMBF grant 03F0734A). C.L. thanks

545

the Deutsche Bundesstiftung Umwelt (DBU) for financial support.

546


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Page 27 of 39


547

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