key: cord-0702472-zs0swsyv
authors: Brünjes, Robert; Hofmann, Thilo
title: Anthropogenic gadolinium in freshwater and drinking water systems
date: 2020-05-29
journal: Water Res
DOI: 10.1016/j.watres.2020.115966
sha: 1e362b502884c2034c04c4756ea93d58a998e69e
doc_id: 702472
cord_uid: zs0swsyv

The increasing use of gadolinium-based contrast agents (GBCAs) for magnetic resonance imaging is leading to widespread contamination of freshwater and drinking water systems. Contrary to previous assumptions that GBCAs are stable throughout the water cycle, they can degrade. The stability of GBCAs depends largely on their organic ligands, but also on the physicochemical conditions. There is specific concern regarding UV end-of-pipe water treatments, which may degrade GBCAs. Degradation products in drinking water supplies can increase the risk of adverse health effects. This is of particular relevance where the raw water for drinking water production has a higher proportion of recycled wastewater. GBCAs concentrations in aquatic systems, often referred to as anthropogenic gadolinium, are determined using a variety of calculation methods. Where anthropogenic gadolinium concentrations are low, the inconsistent use of these methods results in high discrepancies and high levels of uncertainty. The current COVID-19 crisis will, in the short-term, drastically decrease the input of GBCAs to freshwater systems. Temporal variations in anthropogenic gadolinium concentrations in river water can be used to better understand river-aquifer interactions and groundwater flow velocities. Collecting urine from all patients following MRI examinations could be a way forward to halt the generally increasing concentrations of Gd in drinking water systems and recover this technologically critical element.

gadolinium concentrations are low, the inconsistent use of these methods results in high discrepancies and high levels of uncertainty. The current COVID-19 crisis will, 18 in the short-term, drastically decrease the input of GBCAs to freshwater systems. 19

Temporal variations in anthropogenic gadolinium concentrations in river water can be 20 used to better understand river-aquifer interactions and groundwater flow velocities. 21

Collecting urine from all patients following MRI examinations could be a way forward 22

to halt the generally increasing concentrations of Gd in drinking water systems and 23 recover this technologically critical element. 24 25

Magnetic resonance imaging (MRI) is a versatile radiological tool for generating 28 detailed images that can be used to assist in the diagnosis of a variety of diseases 29 affecting the brain, spinal cord, heart, blood vessels, bones, and joints; it can also be 30 used to check the health of organs such as e.g. breasts, kidneys, ovaries, the liver, 31 the pancreas, and the prostate. In contrast to X-rays and computerized tomography 32 (CT) scans, MRI does not employ cell-damaging ionizing radiation, but the relaxation 33 4 GBCAs are generally considered to be safe and adverse effects are rarely observed 66 (Kanda et al., 2016) . However, in 2006 first links were established between 67 nephrogenic systemic fibrosis (NSF) and GBCAs (Grobner, 2006; Marckmann, 68 2006) . Concerns have since increased, especially as Gd deposits have been 69 reported in brains, bone, skin, and other tissues following GBCA administration, even 70 in healthy patients (Kanda et al., 2016; Le Fur and Caravan, 2019; Montagne et al., 71 2016; Nehra et al., 2018; Xia et al., 2010) . The European Medicines Agency (EMA) 72 therefore released a warning and suspended the use within the EU of certain linear 73

GBCAs ( Table 1) (OECD, 2018) . Data from Austria, Great Britain, New Zealand, Switzerland, and Portugal are restricted to in-hospital data.

Since GBCA's use was first approved in 1988, more than 460 million doses have 83 been administered worldwide up to 2018 (Gibby et al., 2019) . In 2016, almost 84 200,000 MRI scans using GBCAs were conducted per day within the EU and the US 85 7 freshwater systems, and the importance of the method to differentiate between 121 naturally occurring and anthropogenic Gd. We conclude our review by suggestions 122 how to reduce GBCAs emissions. 123 124 Figure 2 . Due to COVID-19 and its implications, changing anthropogenic gadolinium concentrations in surface water and groundwater could be used to improve the between Gd and the GBCA ligand, they found that GBCAs were less stable in 179 humans than previously indicated by K cond . 180

The stability constants in Table 1 show that macrocyclic GBCAs are more stable than 181

linear GBCAs, and that the linear ionic GCBAs are more stable than linear non-ionic 182

GBCAs. The weakness in all stability concepts, however, is the assumption of 183 equilibrium conditions since environmentally relevant conditions are likely to be more 184 complex and not always in equilibrium (Prybylski et al., 2017) . The inversely 185

proportional relationship between the release of Gd from GBCAs and their stability is 186 generally accepted, but the extent to which GBCAs can be transformed within the 187 human body and the exact processes involved still remains unresolved. 188 (Bau and Dulski, 1996) . Since then GBCAs have been found in 196 the effluents from hospitals, sewage water treatment plants, as well as other surface 197

waters (Birka et al., 2013; Künnemeyer et al., 2009; Lindner et al., 2013) . Increased 198

Gd concentrations have even been reported in rural areas where there are no MRI 199 facilities in the catchments of the local sewage treatment plants 200 Rabiet et al., 2009 ). This is due to the large number of patients who receive MRI 201 scans as outpatients and are then sent home. Elevated Gd concentrations have 202 been found in drinking water and are of increasing concern to waterworks, as well as 203 to the general public (Kulaksiz and Bau, 2011a; Lindner et al., 2015; Morteani et al., 204 2006; Richardson and Kimura, 2016; Schmidt et al., 2019; Tepe et al., 2014) . 205

Gadolinium originating from GBCAs is one of a series of emerging contaminants 206 released from sewage systems, which makes it a suitable indicator for other sewage 207 water-borne xenobiotics (Reoyo-Prats et al., 2018) . 208

In general, most studies in freshwater systems pool GBCAs under the term 209 "anthropogenic Gd" (Gd anth ), which does not imply the transformation of GBCAs. We 210 hereafter refer to Gd anth when we do not distinguish between GBCAs and their 211 transformation products. Anthropogenic Gd has been globally detected, mainly in 212 densely populated areas with highly developed health systems; in Europe (Bau and 213 Dulski, 1996; Elbaz-Poulichet et al., 2002; Knappe et al., 2005; Möller et al., 2003 Möller et al., , 214 al., 2000 Ogata and Terakado, 2006; Song et al., 2017) . 219

Due to the widespread contamination of freshwater environments with Gd anth , a 220 variety of hydrogeological investigations have used it as an ideal tracer in different 221 hydrochemical settings, assuming that all types of GBCAs do not undergo 222 degradation once released from STPs (Barber et al., 2006; Bichler et al., 2016; 223 Klaver et al., 2014; Möller et al., 2011 Möller et al., , 2000 Petelet-Giraud et al., 2009) . Depending 224 on the operating mode of an STP, the effluent Gd concentration may show a 225 temporal variation (daily to weekly) because most MRI scans are carried out during 226 the daytime from Monday to Friday (Telgmann et al., 2012) . This transient signal can 227

propagate into the groundwater and can be used to calculate groundwater transit 228 times . based on this approach depend on the precise quantification of the hydrodynamic 234 dispersion and dilution, which can be susceptible to large errors. Further, the 235 degradation of the ligand DTPA and its inorganic complexes is in general thought to 236 be photochemical (Hinck et al., 1997; Means et al., 1980; Metsärinne et al., 2004) . the low concentrations involved, but it cannot be neglected over long time scales 242 (Möller et al., 2011; Möller and Dulski, 2010) . A modeling study by Schijf and Christy 243 (2018) has shown that elevated concentrations of calcium and magnesium can 244 destabilize Gd-DTPA in seawater. 245 246

Gadolinium is also naturally occurring in the environment as a result of the 249 dissolution of minerals. It is one of the elements in the lanthanide series. These 250 elements are frequently referred to as the rare earth elements (REE), a term that 251 usually also includes the elements scandium (Sc) and yttrium (Y). Except for two 252 elements, all lanthanides exist only in a trivalent oxidation state. Cerium (Ce) can 253 exist as both Ce 3+ and Ce 4+ and europium (Eu) as both Eu 2+ and Eu 3+ , which affects 254 their solubility and melting temperature. Due to their different redox behavior, these 255 two elements are decoupled from the coherent chemical behavior of the rest of the 256 lanthanides (Goldstein and Jacobsen, 1988; Möller et al., 2002) . The abundance of 257 lanthanides follows the Oddo-Harkins rule; elements with even atomic numbers are 258 naturally more abundant than those with odd atomic numbers. This leads to a typical 259 zig-zag shape on logarithmic plots of concentration versus atomic number. To avoid 260 this zig-zag shape, lanthanide concentrations are usually normalized against a 261 geological reference which allows anthropogenically induced anomalies to be 262 detected, as has been demonstrated for Gd, lanthanum (La) and samarium (Sm) in 263 aqueous samples (Kulaksiz and Bau, 2013, 2011b) . to ICP-MS systems for matrix removal and lanthanide preconcentration (Brünjes et 282 al., 2016; Hathorne et al., 2012; Lagerström et al., 2013) . Offline or online 283 preconcentration methods often use ion exchange resins with iminodiacetate 284 functional groups for the enrichment of lanthanides (Pyrzynska et al., 2016) . A crucial 285 step before any preconcentration by ion exchange resins is the degradation of the 286

GBCAs, e.g., by UV and H 2 O 2 . Due to their chemical properties, GBCAs are not 287 retained unless the chelate complexes are degraded and the Gd ions are released. 288

More specialized LC systems are equipped with hydrophilic interaction 289 chromatography (HILIC), size exclusion chromatography (SEC), or reversed-phase 290 chromatography (RPC) columns which can be coupled with MS-systems to enable 291 the direct measurement of specific GBCAs species and transformation products 292 (Birka et al., 2016; Clases et al., 2018b; Künnemeyer et al., 2009; Telgmann et al., 293 2012) . The location of Gd precipitated in brains, other human tissue, or other species 294 has commonly been determined by laser ablation coupled with ICP-MS (Clases et 295 al., 2018a (Clases et 295 al., , 2018b Fingerhut et al., 2018; Lingott et al., 2016) . 296

In many investigations total Gd (Gd total ) is measured. Because the anthropogenic 297 component Gd anth cannot be measured directly, the geogenic Gd (Gd*) background 298 needs to be subtracted (Equation 3). The advantage of this approach is that it has a 299 significant lower limit of detection than methods that measure individual species of 300

Equation 3:

There is no standard methodology on how to quantify Gd* in an aqueous sample, 303 which brought up a variety of different approaches. All calculations to determine 304 Gd anth are based on either interpolations or extrapolations of Gd* from the shale-305 normalized lanthanides (Ln SN ) by means of linear, logarithmic, geometric equations, 306 or third-order polynomial fit (Table 2 ). Slight differences, apart from the mathematical 307 methods, consist in the assumption of the specific behavior of Gd compared to the 308 other lanthanides. While Bau and Dulski (1996) and Kulaksiz and Bau (2011a) 309 assume that Gd behaves as a light lanthanide in freshwater systems and preferably 310 use the light lanthanides (praseodymium (Pr), neodymium (Nd), Sm and terbium 311 (Tb)) within their equations, Hissler et al. (2015) and Ogata and Terakado (2006) 312 include the heavier lanthanide dysprosium (Dy). The approach from Möller et al. 313 (2002) is the only one with no implicit assumption about the behavior of Gd 314 compared to that of lighter, middle or heavier lanthanides and includes all anomalies. As an exception, the method developed by Kulaksiz and Bau (2013) uses 319 the redox-sensitive element Eu excluding slightly acidic colloid-rich conditions, and 320 enrichments in the middle lanthanides. The use of lanthanides with any anomalous 321 concentrations, however, limits the applicability of the method. 322 

In order to compare and evaluate the different approaches, we used published data 327 on lanthanides in freshwater systems under a variety of hydrochemical conditions. 328

We compared the Gd anomaly, defined as the ratio between the shale-normalized 329 Table S1 ). The comparison also elucidates that the smaller the Gd 335 anomaly, the less important is the choice of geological reference. Nevertheless, the 336 different approaches resulted in a broad range of amplitudes for Gd anomalies 337 In contrast to all other methods, the calculations based on the two methods by 354

Kulaksiz and Bau resulted in Gd anomalies closest to 1.0, in agreement with the 355 assumptions by the authors that no anthropogenic Gd was present in the samples 356 ( Figure 3b ; Table S1 ). The largest Gd anomalies were obtained using the lanthanides Tb and Dy are more abundant compared to the lighter lanthanides Nd 361 and Sm (Figure 3a, Table S1 ). While the influence of Tb and Dy can be decisive 362 using approaches from Hissler et al. (2015) or Ogata and Terakado (2006) , the 363 advantage of the third-grade polynomial fit approach by Möller et al. (2002) is the 364 larger number of elements included. This reduces the influence of minor anomalies 365 and element-specific measurement errors on the interpolated geogenic Gd. 366 

Once GBCAs reach the aquatic environment, they are diluted to ng/L or to µg/L 372 levels, which is at least an order of magnitude below the free Gd toxicity level for 373 humans (Merbach et al., 2013) . However, the potential toxicity of Gd needs to be 374 considered together with that of the entire lanthanum series, as they form a uniform 375 group of elements (Blinova et al., 2018) . Anthropogenically elevated concentrations 376 of other lanthanides further increase the risks to aquatic ecosystems, because 377 aquatic organisms absorb lanthanides through their skin, gills, and digestive systems 378 (Kulaksiz, 2012; Kulaksiz and Bau, 2013; Lingott et al., 2016) . 379

Unaltered GBCAs have been shown not to sorb to materials such as activated 380 carbon (contrary to the findings of Elizalde-González et al., 2017), but once acidified, 381 they release Gd 3+ with a high sorption affinity to many adsorbers (Anastopoulos et 382 al., 2016; Elizalde-González et al., 2017; Kovalova et al., 2013; Patra et al., 2017) . 383

The sorption of Gd 3+ onto activated carbon strongly depends on the pH of the 384 solution (Pourret and Houben, 2018) . The environmental mobility of Gd 3+ is similar to 385 that of the other lanthanides and mainly involves complexation with dissolved 386 organic carbon (DOC) under slightly acidic or neutral conditions (Pédrot et al., 2010) . 387

This complexation is based on electrostatic interactions and multidentate bonding 388 between positively charged Gd 3+ and inorganic anions, or negatively charged 389 organic ligands (Byrne and Kim, 1990; Davranche et al., 2015; Luo and Byrne, 390 2004) . 391

In contrast to natural lanthanides, GBCAs have so far not been reported in sewage 392 sludge, even though coagulation-flocculation is reported to remove at least some of the linear GBCAs Neubert, 2008; Telgmann et al., 2012; 394 Verplanck et al., 2010) . Most of the commercially used coagulants are based on 395 ferric or aluminum salts (e.g. FeCl 3 and Al 2 (SO 4 ) 3 ), which are Lewis acids and form 396 acidic microenvironments (Lee et al., 2014) . Under these conditions, linear GBCAs 397 are destabilized resulting in rapid transmetallation (Rabiet et al., 2014) . The 398 formation of Fe-oxide flocs further assists in the removal of Gd 3+ from solution due to 399 the high sorption affinity of lanthanides towards iron phases (Davranche et al., 2008 (Davranche et al., , 400 2004 . Once Gd 3+ is released from GBCA complexes, this sorption affinity could be 401 utilized to recover Gd from sewage water (Nassar et al., 2015; Stepka et al., 2018) . 402

Water suppliers and STPs seek to eliminate GBCAs (Lingott et al., 2016; Rogowska 403 et al., 2018; Thomsen, 2017) . Promising results have been achieved using advanced 404 oxidation processes based on in situ formation of • OH radicals by means of various 405 chemical, photochemical, sonochemical, or electrochemical reactions (Oturan and 406 Aaron, 2014) . While direct reactions between GBCAs and ozone are insignificant, 407 the • OH radicals they produce can result in degradation of GBCAs (Cyris et al., 408 2013) . Treatment with a combination of UV-C irradiation and hydrogen peroxide 409 (H 2 O 2 ) effectively degrades GBCAs within 24 hours . Low 410 energy (15 W) irradiation with UV-C over long periods of time (>12 hours) has been 411 shown to degrade all types of GBCAs, but these conditions cannot be met by most 412 commercial water sanitation facilities (Brünjes et al., 2017) . Birka et al. (2016) 413 reported a series of degradation products from the linear Gd-BOPTA complexes after 414 applying UV-irradiation, while other complexes such as the macrocyclic Gd-DOTA 415

and Gd-BT-DO3A, and the linear Gd-DTPA showed no degradation even after 300 416 minutes. Because of this duration of exposure to UV, the study does not simulate drinking water purification (Birka et al., 2016; Thomsen, 2017) . So far, there is no 419 systematic study on the stability of GBCAs regarding end-of-pipe UV treatments. 420

Since the human uptake of potential harmful GBCA transformation products is of 421 special concern, further studies are recommended. 422

To reduce GBCAs contamination of drinking water, a few starting points can be 423 considered. During drinking water production, improved water purification using 424 expensive reverse osmosis would be required as this is the only efficient way to fully 425 remove GBCAs . Reverse osmosis can be applied as drinking 426 water treatment or as the last step of sewage water treatment to prevent Gd 427 emissions into the aquatic environment. Because of the high percentage of 428 outpatients undergoing MRI scans, treating only hospital effluents would not prevent 429 the increasing input of GBCAs into freshwater water resources. 430

The simplest way to reduce the input of Gd into the aquatic environment and its 431 potential health risk would be to collect urine from patients for at least 24 hours 432 following the administration of GBCAs. This would require urine collection not only in 433 hospitals, but also at the patients' homes. Urine collection bags can be made leak-434 proof by including super absorbent polymers. A trial in Germany found that although 435 medical staff were skeptical about integrating these bags into their existing routines, 436

there was a high level of acceptance by patients (Niederste-Hollenberg et al., 2018) . 437

Collecting urine would also allow to recover and recycle Gd from GBCAs and 438 prevent the technologically critical element Gd from being lost into aquatic 439 environments (Cobelo-García et al., 2015) . Extraction procedures from polymer 440 matrices would need to involve the degradation of GBCAs but benefit from the high 441 concentrations of the recycling target. For example, GBCAs could be degraded by a 442 combination of H 2 O 2 and UV radiation and recovered by using a variety of existing 443 green technology methods such as biosorption that are already used for lanthanides 444 from wastewater (Pereao et al. and references therein, 2018) . In contrast, recovering 445 diluted Gd in sewage systems or after STPs would be economically less attractive 446 for recycling. 447 448

Due to their ability to significantly enhance the contrast of MRI scans there is 450 currently no viable alternative to GBCAs for medical use. MRI will develop towards 451 higher resolutions, but the use of Gd for MRI contrast agents remains necessary. In 452 general, GBCAs are renally excreted with plasma elimination half-lives of 1.5 to 34 h, 453 however, there is increasing evidence that some of the Gd may be retained in the 454 human body, but so far, the exact processes and amounts remain unknown. 455

As conventional sewage water treatment cannot eliminate GBCAs, they contaminate 456 freshwater systems which provide our drinking water resources. GBCA 457 concentrations in drinking water, the main pathway for unintentional human intake, 458 are still at the nanogram per liter levels but will further increase. As soon as they 459 enter the stomach their stability is reduced by the acidity of the gastrointestinal fluids. The amount of GBCA contamination in freshwater environments, also referred to as 469 anthropogenic gadolinium, is determined using a variety of calculation methods. In 

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