Novel speciation method based on Diffusive Gradients in Thin Films for in situ measurement of uranium in the vicinity of the former uranium mining sites

Environmental Pollution 214 (2016) 114e123

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Novel speciation method based on Diffusive Gradients in Thin Films for in situ measurement of uranium in the vicinity of the former uranium mining sites* Jagoda Drozdzak a, *, Martine Leermakers a, Yue Gao a, Vannapha Phrommavanh b, Michael Descostes b a b

Analytical, Environmental and Geochemistry (AMGC), Vrije Universiteit Brussels (VUB), Pleinlaan 2, 1050 Brussels, Belgium AREVA Mines, R&D Dpt., Tour AREVA, 1 Place Jean Millier, 92084 Paris La D
efense, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 January 2016 Received in revised form 23 March 2016 Accepted 1 April 2016

The Diffusive Gradients in Thin Films (DGT) technique using PIWBA resin (The Dow Chemical Company) was developed and validated for the measurement of uranium (U) concentration in natural and uranium mining influenced waters. The U uptake on the PIWBA resin gel was 97.3 ± 0.4% (batch method; Vsol ¼ 5 mL; [U] ¼ 20 mg L1; 0.01 M NaNO3; pH ¼ 7.0 ± 0.2). The optimal eluent was found to be HNO3conc/70  C with an elution efficiency of 88.9 ± 1.4%. The laboratory DGT investigation demonstrated that the PIWBA resin gel exhibits a very good performance across a wide range of pH (3e9) and ionic 4 M), strength (0.001e0.7 M NaNO3) at different time intervals. Neither effect of PO3 4 (up to 1.72  10 3 M) on the quantitative measurement of uranium by DGT-PIWBA method nor of HCO 3 (up to 8.20  10 4 M) concentration, the U were observed. Only at very high Ca2þ (2.66  104 M), and SO2 4 (5.55  10 uptake on DGT-PIWBA was appreciably lessened. In-situ DGT field evaluation was carried out in the vicinity of three former uranium mining sites in France (Loire-Atlantique and Herault departments), which employ different water treatment technologies and have different natural geochemical characteristics. There was a similar or inferior U uptake on DGT-Chelex®-100 in comparison with the U accumulation on a DGT-PIWBA sampler. Most likely, the performance of Chelex®-100 was negatively affected by a highly complex matrix of mining waters. The high concentration and identity of co-accumulating analytes, typical for the mining environment, did not have a substantial impact on the quantitative uptake of labile U species on DGT- PIWBA. The use of the polyphenol impregnated anion exchange resin leads to a significant advancement in the application and development of the DGT technique for determination of U in the vicinity of the former uranium mining sites. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Diffusive Gradients in Thin Films Uranium Former uranium mining sites Polyphenol impregnated anion exchange resin DGT

1. Introduction Uranium (U) is a naturally-occurring radioactive element, which is present in the aquatic systems at the wide range of concentration (0.002e20 mg L1) depending on the geological background, presence and type of anthropogenic activities (de Vos and Tarvainen, 2006; Vandenhove et al., 2010). The two dominant U aqueous redox states are U(IV) or U(VI), with the latter being mobile and

*

This paper has been recommended for acceptance by W. Wen-Xiong. * Corresponding author. E-mail address: [email protected] (J. Drozdzak).

http://dx.doi.org/10.1016/j.envpol.2016.04.004 0269-7491/© 2016 Elsevier Ltd. All rights reserved.

stable under oxidizing conditions (Günther et al., 2002; Ragnarsdottir and Charlet, 2000; Vandenhove et al., 2010). In the (þVI) oxidation state, the U speciation is controlled by pH, redox potential and the occurrence of the complexing agents. The free uranyl ion (UO2þ 2 ) governs dissolved U(VI) speciation at low pH, while at higher pH, hydroxy and carbonate complexes dominate. Uranyl hydroxy complexes such as UO2(OH)þ, play a significant role in uranyl aqueous speciation only under slightly acidic pH conditions. At near neutral and alkaline pH values, the dissolved U speciation is controlled by a series of a uranyl carbonates complexes 4  (i.e. UO2CO03(aq), UO2(CO3)2 2 , UO2(CO3)3 , (UO2)2(OH)3CO3 ) (Salbu et al., 2004; Wang et al., 2013). The uranium biogeochemical cycling is a research field of a

J. Drozdzak et al. / Environmental Pollution 214 (2016) 114e123

growing interest due to the fact that U is known of its dual toxicity. Uranium is often called a nephrotoxic element, as its toxic effects are more likely due to its chemical properties than its radioactivity. Total U concentration and the isotopic composition are responsible for the radiological toxicity, while the chemical toxicity of U depends on the speciation as only the dissolved and labile uranium fraction is chemically toxic (Chapman, 2008; Goulet et al., 2011; Sheppard et al., 2005). In this context, the Diffusive Gradients in Thin Films (DGT) technique is a promising in-situ speciation tool, because it provides time-averaged concentration of labile metal species in water (Davison and Zhang, 1994; Zhang and Davison, 1995), sediment (Gao et al., 2006; Hooda et al., 1999) and soil ne et al., 2010). The DGT technique is (Degryse et al., 2009; Duque based on a simple device that accumulates solutes on a binding agent (i.e. resin/adsorbent immobilized in a thin layer of hydrogel) after a passage through a hydrogel. The key role of the hydrogel is a discrimination of metal species by their size, mobility and lability (so-called DGT labile metal species). Afterwards, the metal species are effectively immobilized and pre-concentrated on the binding phase gel. The DGT technique has been extensively applied for the range of the analytes over the wide pH and ionic strength ranges, however only few studies encompassed the DGT investigation under extreme field conditions, such as mining environments (Conesa et al., 2010; de Oliveira et al., 2013; Phrommavanh et al., 2013; Stockdale and Bryan, 2013). Mining environments are characterized by large variations in pH, ionic strength and high concentrations of competing and interfering ions, what might compromise the accuracy of the DGT measurement. Therefore, suitable resin for the DGT technique should be affected neither by the extreme pH and ionic strength values nor by high concentration of interferences such as Zn, Mn, Ca or SO2 4 . Up to date, several binding phases such as Chelex®-100, Metsorb™, Whatman DE 81, Dowex 2  8e400 and Diphonix® have been proposed for the assessment of labile U species in aquatic environments (Table 1). In the current study we propose a novel DGT technique with polyphenol impregnated weak base anion exchange resin (for the purpose of this study called PIWBA resin) for determining aqueous U species in the vicinity of the former uranium mining sites. Polyphenols are secondary metabolites of plants and can be found in significant amounts in fruits, vegetables, cereals and beverages (Handique and Baruah, 2002). Those substances are known for their antioxidant, anti-cancer and anti-aging properties (Pandey and Rizvi; Scalbert et al., 2005), but it has also been found that they possess the capability to remove uranium from the environment (Alexandratos, 2009; Nakajima and Sakaguchi, 2007). Liu et al. (J. Liu et al., 2013a) and Nakajima et al. (Nakajima and Sakaguchi, 2007) hypothesized that the binding mechanism occurs via polyhydroxybenzene groups, polyphenolic or catecholtype polyphenolic ligands, however the exact mode of action of polyphenols is still unknown. The PIWBA resin has been developed and patented by Rohm and Haas Company (a wholly owned subsidiary of The Dow Chemical Company), however it is not yet

115

commercially available (Rohm and Haas, 2014). Novel DGT method using PIWBA resin was comprehensively investigated under laboratory conditions. The PIWBA resin gel preparation and the elution procedure protocols were developed. The linear relationships between the mass of U accumulated on DGT-PIWBA within the deployment time were demonstrated, thereby validating that the novel DGT method operated in accordance with the assumptions of the DGT equation. The influence of a wide range of pHs (3e9) and ionic strengths (0.001e0.7 M NaNO3) on the performance of the DGT technique was also investigated. 2  Effects of ligands (Ca2þ, PO3 4 , SO4 and HCO3 ) on the U uptake on DGT-PIWBA were tested. The effective diffusion coefficients of U over the pH range were determined as well. Furthermore, the DGT field investigations in the vicinity of the former uranium mining sites were carried out. The DGT-PIWBA method was applied in natural and mining influenced waters and compared to the well-established DGT-Chelex®-100 method. 2. Materials and methods 2.1. General procedures All chemicals were of analytical reagent grade or greater. Milli-Q (ultra-pure) water (>18.2 MU cm, Millipore, USA) was used for the preparation of the solutions, gels and cleaning glassware and containers. All plastic equipment were pre-cleaned in 10% (v/v) HNO3 (pro analysis, Merck, Germany) for at least 24 h and rinsed thoroughly with Milli-Q water. Appropriate pH of all solutions was adjusted by using either 2% HNO3 or 2% NaOH. Temperature and pH measurements were performed using pH probe (WTW GmbH, Germany) and monitored during the experiment. The deployment solutions were well-mixed using a mechanic stirring system, so the diffusive boundary layer (DBL) of DGT was considered negligible (Zhang and Davison, 1999). 2.2. DGT preparation and assembly All experiments were carried out under laminar flow hood (class-100) in a clean room. All gels contained 15% (v/v) acrylamide solution (Merck, Germany) and 0.3% (v/v) patented agarose crosslinker (DGT Research Ltd., UK). N,N,N0 ,N0 -tetramethylenediamine (TEMED) (Acros Organics, Belgium) was used as a catalyst and daily prepared solution of 10% ammonium persulphate (Merck, Germany) was used as an initiator for polymerization. The polyacrylamide (PAM) diffusive gel (Dg ¼ 0.8 mm) was prepared according to the procedure described by Zhang and Davison (Zhang and Davison, 1995). The PIWBA resin gel (Dr ¼ 0.4 mm) was prepared by modifying the Chelex®-100 resin gel preparation protocol (Zhang and Davison, 1995). The PIWBA resin was first ground (Fritsch Pulverisette, Type 02.102) and sieved manually to obtain the particle size of 250 mm. Furthermore, an amount of 1.5e2 g of the resin was added to 10 mL of gel stock solution, and then the

Table 1 Summary of the DGT binding phases used for determination of labile U species in aquatic systems. Binding phase Chelex®-100

Reference

(Drozdzak et al., 2016, 2015; Garmo et al., 2003; Hutchins et al., 2012; Li et al., 2005; Phrommavanh et al., 2013; Stockdale and Bryan, 2013; Turner et al., 2012) Metsorb™ (Drozdzak et al., 2016, 2015; Hutchins et al., 2012; Turner et al., 2014, 2012) Whatman DE 81 (Li et al., 2007, 2006) Dowex 2  8(Li et al., 2007) 400 Diphonix® (Drozdzak et al., 2016, 2015; Turner et al., 2015) Spheron Oxin® (Gregusova and Docekal, 2011; Gregusova et al., 2008)

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sample was sonicated for at least 5 min. Further, 355 mL of 10% ammonium persulfate solution and 28 ml of TEMED were added to the resin gel solution stock. Afterwards, the resin gel solution was mixed well and casted gently between two glass plates. The assembly was placed in an oven at 45  C for 1 h. After that, the gel sheet was removed from the glass plates and hydrated in deionized water for at least 24 h. The resin gel discs were cut using a 2.5 cm diameter plastic cutter and stored in 0.01 M NaNO3 at 4 C prior to assembly. DGT samplers were supplied by DGT Research and assembled according to the protocol from Lancaster (www. dgtresearch.com). Concisely, the resin gel disc was placed on the bottom, with the resin side faced up; then a diffusive gel disc was overlying on it and finally on the top, a Millipore Durapore membrane filter (HVLP, 0.45 mm) was placed. Assembled DGT samplers were stored at 4  C in doubled zippered plastic bags, which contained 0.01 M NaNO3 to maintain the moisture. The preparation of DGT blanks went through the whole procedure as for DGT samplers. All DGTs values in this study were corrected for the blank values of each binding phase and each eluent used. 2.3. Sample analysis The sample analysis was performed using inductively coupled plasma sector field mass spectrometry (ICP-SF-MS, Element II, Thermo Fisher Scientific Bremen GmbH, Germany), equipped with ESI fast autosampler. Analytical standard solutions were prepared from a 1000 mg L1 in 2% HNO3 U stock solution (Johnson Matthey Materials Technology, UK) and a 1000 mg L1 in 2% HNO3 multielement solution (Merck, Germany). Calibration was performed by external calibration, using Indium (2.5 mg L1 in 2% HNO3) as internal standard and 2% HNO3 as carrier solution. Certified river water standard reference materials- 1640a (National Institute of Standards and Technology) and SLRS-5 (National Research Council Canada) were used to validate the precision and accuracy of the field analysis. An average instrumental blank of 0.6 ng U L1 was obtained, resulting in a detection limit of 1.3 ng U L1 (determined as the average blank þ 3 times standard deviation). The concentration of major cations and anions were measured by inductively coupled plasma atomic emission spectrometry (ICPAES, Optek Iris Advantage) and by ion chromatography (DIONEX DX 500), respectively. The concentrations of ammonium (NHþ 4 ), nitrite   3 and nitrate (NO 2 þ NO3 ), silicate (SiO4 ) and phosphate (PO4 ) were determined by automatic colorimetric methods (QuAAtro, Seal, Analytical). The concentrations of DOC were measured with a total organic carbon analyzer (HiPerTOC, Thermo) and DIC measurements were carried out using stable isotope ratio mass spectrometry (IRMS, Nu Perspective Instrument). The concentration of U measured by DGT samplers (CDGT, mg L1) were calculated using Eq. (1) (Zhang and Davison, 1995):

database was based on the LLNL database, modified to include the set of uranium thermodynamic data selected by NEA (Table S1 in SI) (Grenthe et al., 1992; Guillaumont et al., 2003).

2.5. The U uptake and the elution efficiency The U uptake and the elution efficiency of the binding phase gel were evaluated by immersing a single resin gel disc in 5 mL of 20 mg U L1 solution (0.01 M NaNO3) at pH 7.0 ± 0.2. After shaking for 48 h, the PIWBA gel disc was transferred into a new vial with the appropriate amounts of the eluents. To elute U from the PIWBA resin gel discs several elution procedures were investigated (1 M HNO3; 1 M NaOH; HNO3conc/70  C). Before analysis, the eluted solutions were diluted 10-times with Mili-Q water, and filtered through a 0.45 mm syringe filter if necessary. 2.6. Uranium capacity of the PIWBA resin gel A single gel disc of the PIWBA resin was immersed in 5 mL of solutions containing various concentrations of U (0.02e500 mg L1) at pH 5.0 ± 0.2 in order to assess the uptake capacity of U of the binding phase gel. After the vials were shaken for 48 h, the resin gel discs were transferred into the new vials and the elution procedure described in Section 2.5 were followed.

2.7. The effect of pH and ionic strength on DGT uptake The experiment solutions were stirred vigorously for at least 24 h before the DGT deployment in order to enable inorganic carbon in solution equilibrate with atmospheric CO2. Additionally, for the deployment solution at pH  6, a small quantity of NaHCO3 was added in order to buffer the solution and to maintain stable pH. The DGT samplers were deployed in 2 L well-mixed 20 mg L1 U solution (0.01 M NaNO3) at investigated pH (3e9) for a period of 2, 6, 10, 24 and 48 h, at room temperature. The effect of ionic strength on the DGT uptake was assessed by studying U accumulation on the binding phase gel disc in solutions containing 20 mg L1 U at pH 7.0 ± 0.2 with various concentrations of NaNO3 (0.001, 0.01, 0.1, 0.4 and 0.7 M). The ionic strengths in the testing solution were within the range of those encountered in the environment. The DGT samplers were deployed in duplicate for 10, 24 and 48 h and after that period, they were removed and the elution proceeded as per Section 2.5 was followed. One aliquot of 8 mL of the DGT deployment solution was taken and acidified with 1 M HNO3 at the beginning, at the time when the DGT samplers were retrieved from the solution and at the end of the experiment.

2.8. Effective diffusion coefficients of U species over the pH range CDGT ¼ (M Dg)/(D t A)

(1)

where M is accumulated mass of U on the binding layer (ng); D is the temperature-corrected diffusion coefficient for U (cm2 s1); Dg is the thickness of the diffusion layer (cm); t is the deployment time (s) and A is the exposure area of DGT sampler (cm2).

Diffusion coefficient (D, cm2 s1) of U at each pH was calculated using the slope of the linear regression of the measured mass accumulation of U as a function of time (Fig. S2). Following equation was used Eq. (2): D ¼ (a Dg)/(A C)

(2)

2.4. Speciation modelling The thermodynamic geochemical speciation software PhreeQC was used to predict the U speciation and to assist with the interpretation of the results due to the lack of up to date applicable techniques which are able to measure individual uranium chemical species in solution (Parkhurst and Appelo, 1999). A consistent

where a is the slope of the linear regression of U accumulation on a binding phase gel disc over time; C is the U concentration in the solution; Dg is the thickness of the diffusion layer (cm) and A is the exposure area of DGT sampler (cm2). All diffusion coefficients were corrected for the temperature (T) using Stokes-Einstein equation (Zhang and Davison, 1995).

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2.9. The effect of interferences on DGT uptake The interference effects of calcium (Ca2þ), phosphates (PO3 4 ), 2 bicarbonates (HCO 3 ) and sulphates (SO4 ) on the DGT uptake were investigated. Interference solution concentrations were selected for analytical and experimental convenience and to represent different water conditions, taking into account the extreme mining environments. The DGT samplers were deployed in triplicate in solutions containing 20 mg L1 U (0.01 M NaNO3) and various  2 concentrations of Ca2þ, PO3 4 , HCO3 at pH 6.4 ± 0.1 and SO4 at pH 0 4.0 ± 0.1 (Table S2 in SI). Piperazine-N, N -bis (2-ethanesulfonic acid) (PIPES; Sigma Aldrich) with pKa, 20⁰C ¼ 6.8 was used to make a buffer solution, which was added afterwards to the U solution to maintain the pH of 6.4 ± 0.1. PIPES is one of Good's 3 noncomplexing buffer and it has been chosen due to negligible metal
n binding abilities (Fredrickson et al., 2000; Good et al., 1966; Lyve et al., 2003; Vasconcelos et al., 1998). The DGT sampler was removed every 12 h and followed the same elution procedures as described above. 2.10. Field deployments The studied former uranium mines are located in western and re former mining southern regions of France. Chardon and L'Ecarpie sites are located in Pays de la Loire region (Loire-Atlantique ve mining site is situated in department) and Le Bosc-Lode Languedoc-Roussillon region (Herault department). The field sampling was performed at high flow regime of water systems in winter time (January 2014). The mining site of Chardon consists of a former open pit that has been filled up with drainage waters from the underground mine and it is characterized by high salinity (i.e. 1.8  102 M NaCl). In order to prevent the pit from overflowing, during the winter months a part of the water is pumped into the stream Margerie, which flows into the river Sevre Nantaise. The sampling was therefore performed at the former open pit (1), at the downstream of the stream Margerie (2) and at downstream of the river Sevre Nantaise (3) (Fig. S1). re has a water treatment plant, which tarThe site of L'Ecarpie gets particularly at the removal of radium and uranium by precipitation. This treatment involves the addition of BaCl2 to precipitate radium by co-precipitation of Ba(Ra)SO4 and the addition of lime to increase pH and to assist the precipitation of Fe(III) oxy-hydroxides, with co-precipitation of uranium and other mining related elements. The treated waters after passing several precipitation basins are discharged into the river Moine. The sampling was performed at the mining discharge water spot (4) and in the river Moine at the confluence zone after the mining discharge (5) (Fig. S2). ve can be characterized The former mining site of Le Bosc-Lode by high alkalinity (i.e. up to 6.7 meq L1) and high concentrations of arsenic (i.e. 5.1 mg L1) and molybdenum (i.e. 120 mg L1). The water treatment strategy involves U accumulation on ion exchange resins and the addition of a lime and FeCl2 to promote the precipitation of Fe(III)oxy-hydroxides, with co-precipitation of uranium, arsenic and molybdenum. The treated mining water is discarded into the river Lergue through its tributaries, the Mas d'Arly and the Rivieral streams. The sampling stations were located at the mining discharge water spot (6) and at the downstream of the river Lergue, approximately 4 km after the confluence (7) (Fig. 1). The DGT samplers with Chelex®-100 and PIWBA resin gel discs were deployed in 4 replicates at each sampling point. The Chelex100 was selected as a comparative DGT resin, because it has been demonstrated that DGT-Chelex-100 is reliable and robust DGT method for determination of labile U species (Table 1). The

117

Chelex®-100 resin gel (Dr ¼ 0.4 mm; Bio-Rad) were prepared according to the procedure described before (Zhang and Davison, 1995). The PIWBA resin gels ((Dr ¼ 0.4 mm) were prepared as described in Section 2.2. The DGT devices were affixed to several Perspex plates, which were specifically designed for deploying up to nine DGT devices. Then the DGT-Perspex units with the DGT devices facing towards the direction of the stream were attached to a rope and weighted to the river bed. Special caution was taken to prevent the DGT-Perspex units to be settled on the riverbed. The DGT deployment time was 24 h, to ensure that the saturation of either binding phase is not achieved and could negatively affect the U accumulation on each resin. Field DGT blanks (in triplicate) were prepared along with the deployed DGT samplers and exposed to the field environment during deployment and retrieval of the samplers. Batch water samples were taken at each sampling point. The water samples were filtered in the field through a 0.45 mm and 0.22 mm pore-size membrane syringe filters (Chromafil). The water samples for trace metals analysis were acidified on-site with ultrapure HNO3 and water samples for anions and major cations analysis were stored without treatment. Dissolved Organic Carbon (DOC) water samples were acidified with ultrapure H3PO4, Dissolved Inorganic Carbon (DIC) water samples were preserved using HgCl2 and no air space was left in the bottles. The field blanks were collected at the start and at the end of the sampling mission by passing MilliQ water through the membrane syringe filters. The temperature, pH, Eh, dissolved O2 and conductivity were measured in situ with VWR MU 6100H and WTW 3430 portable multi-parameter instruments and associated probes. Total alkalinity was measured titrimetrically using the Hach Digital Titrator and the measurements were done immediately on-site to prevent loss or gain of carbon dioxide or other gases when exposed to the atmosphere or to turbulence during the transport. 3. Results and discussion 3.1. Resin gel blanks and DGT detection limits The average DGT blank value based on 15 replicates measurement of the PIWBA resin gel was 0.16 ± 0.03 ng per resin gel disc. The method detection limit (MDL) of DGT-PIWBA was 0.005 mg U L1. The MDL was calculated for a 24 h deployment time, using diffusive thickness of 0.097 cm, sampling area of 3.14 cm2 and diffusion coefficient of uranium 4.40 ± 0.22  106 cm2 s1 (3s  standard deviation of the handling blank; n ¼ 15). This value is significantly higher than the blank values of other binding phases used previously for the U determination using the DGT technique (Drozdzak et al., 2015; Garmo et al., 2003; Hutchins et al., 2012; Turner et al., 2012). This is probably due to the fact that PIWBA resin had to be ground and sieved manually in order to be casted between the glass plates, therefore this step might be a source of a contamination affecting the blank values of the PIWBA resin. Precleaning the resin with HNO3 and NaOH prior the gel preparation might lower the detection limits considerably. However this was not performed in this study as the MDL value of DGT-PIWBA is much lower than the U concentration encountered under field conditions. 3.2. The U uptake and the elution efficiency The U uptake on the PIWBA resin was determined by taking into account the U mass balance before and after resin/adsorbent gel discs immersion (Zhang and Davison, 1995). The uptake of uranium on the PIWBA resin gel was 97.3 ± 0.4% (n ¼ 5). Control experiment showed no sorption of uranium into the walls of the vials. Several

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ve (www.maps.google.com). Fig. 1. Sampling stations at former mining site of Le Bosc-Lode

elution procedures for the PIWBA resin gel were examined, however the elution efficiencies did not yield a satisfactory level when using either 1 M HNO3 or 1 M NaOH. In order to gain higher elution efficiency, concentrated HNO3 was used and the elution process was taking place at 70  C for at least 48 h. In this study, the implementation of HNO3conc/70  C as the elution procedure gave the efficiencies of 88.9 ± 1.4% for the PIWBA resin gel disc (n ¼ 5). 3.3. Uranium capacity of the PIWBA resin gel It is essential for any DGT method validation to estimate the maximum capacity of the binding phase to enable an appropriate estimation of the DGT field deployment. This is especially crucial in mining environments where the capacity of the binding phase might be seriously affected by the elevated levels of several metals and metalloids. The maximum U binding capacity of the PIWBA resin gel under non-competitive conditions was 0.42 mmol U per resin gel disc (Fig. S3). This is appreciably lower than the U capacity of Chelex®-100, Metsorb™ and Diphonix® binding phases reported previously (Drozdzak et al., 2015). Most likely, the PIWBA functional groups and their properties have been altered during grinding of the resin beads. Nevertheless, the DGT samplers packed with PIWBA resin gel still can be deployed in natural environments for a conventional DGT deployment time without the risk that the saturation is attained (i.e. ~16 days1). 3.4. The effect of pH and ionic strength on DGT uptake The mass of U accumulation on the PIWBA resin gel showed a good correlation with the deployment time (R2 ¼ 0.995) over the range of pH 3e9 (Fig. S4). This implies that the PIWBA resin fulfills the criteria and assumptions of the DGT equation over a time scale (48 h) across the wide pH range of 3e9 in simple matrices. In order to study the effect of pH and ionic strength on the DGT

1 The maximum DGT deployment time in natural waters ([U] ¼ 0.5 mg L1; A ¼ 3.14 cm2; Dg ¼ 0.097 cm; DDGT ¼ 4.40  106 cm2 s1).

uptake, the DGT measured concentrations of uranium (CDGT) were compared to the U concentration in the deployment solution (Csol). The results obtained in this study fell within the ratio CDGT/Csol of 1.0 ± 0.1 (Fig. S5). This study demonstrated that there was no appreciable dependency of the U accumulation on DGT-PIWBA over the pH range investigated. This is specifically important as U speciation is highly pH-dependent, so at pH 3e4 the dominant U species is UO2þ 2 , at pH 5e7 neutral and anionic U species (i.e. UO2CO3 and UO2(CO3)2 2 ) play a leading role, while under alkaline conditions the major U species are negatively charged uranyl car3 bonates (i.e. UO2(CO3)2 2 and UO2(CO3)4 ) (Cotton, 2006). Several authors reported a diminished analyte uptake on DGT-Chelex®-100 or on DGT- Metsorb™ due to the distribution of analyte species and character of an adsorbent at specific pH (Panther et al., 2013, 2012; Price et al., 2013). Nevertheless, this study showed that the U uptake on DGT-PIWBA was not disrupted by the U species distribution and the PIWBA resin was capable of the quantitative measurement of U species over a wide pH range (3e9). The PIWBA resin gel exhibited very high affinity towards U and the U uptake was quantitative and rapid irrespectively of the charge of U species and/ or the mechanism of the binding of the PIWBA resin. There was a good agreement between the U concentrations measured by DGT-PIWBA and the one in the DGT deployment solutions over the ionic strength range examined (0.0001e0.7 M NaNO3) at pH 7.0 ± 0.2 (Fig. S6). These results indicated that U uptake efficiency by DGT-PIWBA is independent of ionic strength range studied.

3.5. Effective diffusion coefficients of U species over the pH range The diffusion coefficients of U species over the pH range 3e9 based on the calculations of U mass accumulation on the DGTPIWBA sampler (DDGT) are shown in Table 2. The DDGT values are compared to the U diffusion coefficients reported by other authors and the U species distribution in the experiment solution is displayed as well. The average effective U diffusion coefficient obtained in this study at 25  C was 4.93 ± 0.32  106 cm2 s1 and no

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Table 2 The effective diffusion coefficients (  106 cm2 s1, at 25  C) determined by a DGT-time series experiment over a wide pH range (3e9) (DDGT) (mean ± standard deviation). The uncertainties of the diffusion coefficient values are combination of the uncertainties of the slope of the plots (95% confidence interval of regression line), the thickness of the diffusive gel and U concentration of the exposure solution. pH (±0.1)

Major U species distribution

This study

3.0 4.0 5.0 6.0 7.0 8.0 9.0

UO2þ 2 (100%) UO2þ 2 (96%) þ 0 UO2þ 2 (51%); UO2(OH) (21%); UO2CO3(aq) (28%) UO2CO3(aq) (66%); UO2(CO3)2 (22%) 2 UO2(CO3)2 2 (83%) 2 UO2(CO3)2 (59%); UO2(CO3)4 3 (40%) 4 UO2(CO3)2 2 (13%); UO2(CO3)3 (86%)

5.02 5.37 4.63 4.05 4.95 5.13 5.15

DDGT

a b c d

Literature D values DDGT

± ± ± ± ± ± ±

0.41 0.35 0.12 0.14 0.35 0.13 0.78

2.67 3.14 4.53 3.61 3.03 4.32 4.13

Dcell ± ± ± ± ± ± ±

0.14a; 4.07 ± 0.09b; 4.97 ± 0.24b; 0.13a; 4.42 ± 0.15b; 4.65 ± 0.13b; 0.27a; 3.89 ± 0.29b; 4.13 ± 0.10b; 0.11a; 4.25 ± 0.21b; 4.81 ± 0.17b; 0.27a; 4.34 ± 0.44b; 5.03 ± 0.38b; 0.21a; 4.63 ± 0.28b; 4.82 ± 0.25b; 0.79b; 4.35 ± 0.49b; 4.22 ± 0.32b

4.55 4.66 3.90 4.56 3.83 4.19

± ± ± ± ± ±

0.26b 0.27b 0.69b; 4.7c 0.82b; 3.4c; 0.13b 0.12b

6.13d; 7.20 ± 0.26b 5.81d; 7.37 ± 0.47b 4.96d;4.88 ± 0.19b 3.27d; 3.88 ± 0.17b 2.53d; 2.99 ± 0.09b 2.01d; 2.44 ± 0.09b 1.80d; 1.90 ± 0.06b

(Hutchins et al., 2012). (Drozdzak et al., 2015). (Garmo et al., 2003). (Li et al., 2006).

trend with increasing pH could be observed. These results demonstrated that U species are accumulated on the PIWBA resin irrespectively of pH and the U binding into the PIWBA resin occurred in a rapid and irreversible fashion (Zhang and Davison, 1995). In general, there was a good agreement between the effective U diffusion coefficients estimated in this study and the ones reported in the literature over the pH range investigated (Table 2). Most likely, small divergences between the values obtained here and the ones reported previously, have arisen from different matrix composition, pH variation over the DGT deployment and/or differences in the mass of U accumulated on a specific DGT binding layer. Nevertheless, there was poor correlation between the DDGT and the diffusion U coefficients measured by the diffusion cell experiment (Dcell), what was reported previously as well (Drozdzak et al., 2015). Further investigation on the diffusion coefficient of U species is needed in order to rationalize the discrepancies between DDGT and Dcell and their impact on the DGT data interpretation.

Acid mine drainage (AMD), which is a metalerich waste disposed in the areas surrounding the mines is characterized by very high SO2 4 concentrations and low pH (Johnson and Hallberg, 2005). Therefore it was crucial to mimic, to some extent, the AMD conditions and to investigate its effect on the U-DGT uptake prior a DGT field deployment. Quantitative U uptake was obtained for DGT-PIWBA at SO2 concentration of 0.11e1.11  104 M (log 4 1.04log 2.04). The U uptake on DGT-PIWBA dropped significantly to 74.0 ± 8.3% and 70.7 ± 1.6% at 5.55  104 M and 4.44  103 M of SO2 4 , respectively (Fig. 2). Most probably, the observed diminished U uptake on DGT-PIWBA is correlated to U species distribution, more precisely, to the decrease in UO2OHþ species and concurrent increase in UO2SO4.

3.6. The effect of interferences on DGT uptake It was of a great relevance for this study to investigate the performance of the DGT technique in presence of calcium (Ca2þ),  2 phosphates (PO3 4 ), bicarbonates (HCO3 ) and sulphates (SO4 ) as those interferences are present in mining environments sometimes in very high concentrations (Balistrieri et al., 2007; Bernhard et al., 1998, 1996; Merkel and Schipek, 2011). DGT-PIWBA produced quantitative measurements at 0.27e2.66  105 M Ca2þ concentrations (log 1.42 and log 2.42 respectively shown in Fig. S7). However, at very high Ca2þ concentration (log 4.12), the U uptake on DGT-PIWBA decreased to 80.5 ± 0.5%. This is assumedly associated with the competition for the binding sites between uranium species and calcium ions. Under those conditions, approximately 100% of the U in the deployment solution occurs as CaUO2(CO3)2 3 and 97% of the Ca is expected to be present as the free metal ion Ca2þ. Over the PO3 4 5 3 (3.4  1051.7  104 M) and HCO M) 3 (1.6  10 8.2  10 concentration range investigated, there was a quantitative adsorption of uranium species on DGT-PIWBA (Figs. S8 and S9 in SI, respectively). No effect of increasing concentration on either phosphates or carbonates was observed, suggesting that the binding affinity of the PIWBA resin towards U is strong enough to suppress the influence of the elevated levels of phosphates and bicarbonates. Therefore those findings are extremely important, because it was reported previously that the U uptake on DGTChelex®-100 (Gregusova and Docekal, 2011; Turner et al., 2012) and DGT-Metsorb™(Turner et al., 2012; Wazne et al., 2006) was nega3 tively affected by increasing HCO 3 and PO4 concentration.

Fig. 2. (A) The effect of SO2 4 on U uptake on DGT-PIWBA. The solid and dotted horizontal lines represent target values 1 ± 0.1. (B) Speciation of U(VI) in the deployment solution modelled with PhreeQC program. Only major U species displayed. 5 3 [U] ¼ 20 mg L1; [SO2 M; [NaNO3] ¼ 0.01 M; 4 ] ¼ 1.11  10 e4.44  10 pH ¼ 4.0 ± 0.1; deployment time ¼ 48 h; DDGT ¼ 4.40 ± 0.22  106 cm2 s1 (25  C); mean ± standard deviation; n ¼ 3.

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3.7. Field deployments The average U concentrations in water samples and measured by different types of the DGT samplers are presented in Figs. 3e5. Detailed information about major cations, anions, trace metals and DOC concentrations at each sampling point is presented in Supplementary Information (Table S3). At all sampling spots of the three mining sites the membrane filtered (<0.45 mm and <0.2 mm) U concentration markedly exceeded the DGT measured U concentration, except at the mining discharge point of mining site of re (4). Most likely, the U was bound to colloidal fraction, L'Ecarpie which was probably a mixture of Fe(III)oxy-hydroxides and aluminosilicates (Dahlqvist et al., 2007; R. Liu et al., 2013b; Phrommavanh et al., 2013). The lack of a significant difference in terms of U concentration between the dissolved 0.45 mm fraction and the DGT measured one re may be attributed to at the mining discharge point of L'Ecarpie very low total dissolved Fe concentration (i.e. 21.3 mg L1) encountered at this sampling spot. It is noteworthy that the removal of Fe in the precipitation ponds is one of the objectives of re mining site. Moreover, the water treatment strategy at L'Ecarpie uranium concentrations at the discharge point may vary during the day, making it difficult to compare the 0.45 mm spot water sampling to the DGT integrated values. The geochemical modelling speciation indicated that the inorganic U species predominate at all three mining sites. The prevailing U species (approximately 100%) at all re and Le sampling points of mining sites of Chardon, L'Ecarpie ve was CaUO2(CO3)2 Bosc-Lode . 3 The repeatability of the DGT method was expressed as the relative standard deviation of the CDGT values at each sampling point (n ¼ 8). The repeatability of DGT-Chelex®-100 was estimated at 9.5% (with 5.4% min, and 13.0% max) and DGT-PIWBA at 6.4% (with 2.9% min, and 11.8% max). The artifacts influencing the precision of the DGT technique were described previously in detail (Buzier et al., 2014; Kreuzeder et al., 2015), however it is possible that the spatial and temporal heterogeneity of the mining influenced waters have deteriorating effects upon the DGT repeatability. The repeatability of the DGT technique should be precisely estimated per binding phase and per analyte of interest, but in general the precision of 10e15% is considered satisfactory (Buzier et al., 2014; Kreuzeder et al., 2015). The analysis of variance using Holm-Sidak method for multiple pairwise comparisons was applied to assess binding phases' performance. The Grubbs test was applied to detect the presence of outliers. At least one of the replicates of Chelex®-100 at each

sampling spot has been identified as an outlier and excluded from the DGT data interpretation. At all sampling points of three different mining sites, there was similar or inferior U uptake on Chelex®-100 in comparison to DGT-PIWBA. It has been found that there is a statistically significant difference between different DGT samplers at (1), (5), (6) and (7) the sampling spots, where the U uptake on DGT-Chelex®-100 was appreciably inferior. Presumably, the U uptake DGT-Chelex®-100 is hampered by very high concentrations of major cations, anions and other metals typical for mining environments, such as Zn, Mn, Ba or Sr, which can compete individually and/or collectively for the binding sites of an adsorbent. It has been previously pointed out that the selectivity order of specific binding phase towards the U species might be altered under field conditions, due to the distribution of U species and highly complex chemistry of the aquatic environment (Conesa et al., 2010; de Oliveira et al., 2013). Most likely, much lower U accumulation on ve was caused by a synergistic DGT-Chelex®-100 at Le Bosc-Lode effect of the alkaline pH and high concentration of other cove accumulating solutes. The discharge waters of Le Bosc-Lode mining site had neutral pH value (7.4) and the on-site alkalinity measurement was 6.7 meq L1. However, at the Lergue river downstream sampling spot (7) the surface water was alkaline (pH ¼ 8.3) and the alkalinity was 4.6 meq L1. Other studies have shown that the performance of Chelex®-100 might be negatively affected by the increasing carbonate concentrations and high pH values (Drozdzak et al., 2015; Gregusova and Docekal, 2011; Turner et al., 2012). 4. Conclusions and perspectives In this study the Diffusive Gradients in Thin Films technique with the PIWBA resin was successfully developed, applied and validated for the U speciation in the vicinity of former uranium mining sites. This is the first study that verified the feasibility of the DGT-PIWBA method as a water monitoring and speciation tool for U determination in uranium mining influenced environments. A good linearity of the U accumulated mass on the PIWBA resin gel as a function of the exposure time intervals (2, 6, 10, 24 and 48 h) was obtained. This study demonstrated that the PIWBA resin exhibits very good performance across a wide pH range 3e9 and across an environmentally relevant ionic strength range. Possible interferences with Ca2þ (up to 1.33  102 M), PO3 (up to 4 3 1.72  104 M), SO2 M) and HCO 4 (up to 4.44  10 3 (up to 8.20  103 M) on U-DGT uptake ([U] ¼ 20 mg L1) were

Fig. 3. The total dissolved U concentration (0.45 mm, 0.2 mm) and U-DGT labile concentration (DGT-PIWBA, DGT-Chelex®-100) at Chardon mining site. Uncertainty bars of DGT concentration represent standard deviation of 4 replicates. Uncertainty bars of 0.45 mm and 0.2 mm total concentrations represent standard deviation of duplicate measurements at the beginning and at the end of the DGT deployment.

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re mining site. Uncertainty bars of Fig. 4. The total dissolved U concentration (0.45 mm, 0.2 mm) and U-DGT labile concentration (DGT-PIWBA, DGT-Chelex®-100) at L'Ecarpie DGT concentration represent standard deviation of 4 replicates. Uncertainty bars of 0.45 mm and 0.2 mm total concentrations represent standard deviation of duplicate measurements at the beginning and at the end of the DGT deployment.

ve mining site. Uncertainty bars Fig. 5. The total dissolved U concentration (0.45 mm, 0.2 mm) and U-DGT labile concentration (DGT-PIWBA, DGT-Chelex®-100) at Le Bosc-Lode of DGT concentration represent standard deviation of 4 replicates. Uncertainty bars of 0.45 mm and 0.2 mm total concentrations represent standard deviation of duplicate measurements at the beginning and at the end of the DGT deployment.

 investigated. Neither effect of PO3 4 , nor of HCO3 on the quantitative measurement of U by DGT-PIWBA method were observed. Only at very high Ca2þ and SO2 4 concentration, the U uptake on DGTPIWBA was appreciably lessened. The field DGT investigations showed that the high concentration of the analytes that bind with high affinity to Chelex®-100 will have adverse effect on the quantitative uptake of U species on DGTChelex®-100 in the mining influenced environments. The PIWBA resin exhibits very high selectivity towards uranium and the performance of DGT-PIWBA was not affected even by the elevated levels of heavy metals typical for mining waters. DGT-PIWBA overpowered DGT-Chelex®-100 in terms of U accumulation and precision in mining influenced environments. The application of

the PIWBA resin leads to a significant advance in the development of the DGT technique for determination of labile U species in uranium mining environments. This study underlined that it is a prerequisite to perform an exhaustive laboratory characterization of the DGT binding phase layer prior to undertaking an in situ fieldwork. The comprehension of the optimal working parameters of the DGT technique under specific field conditions ensures a reliable and precise metal speciation measurement. The comprehensive laboratory characterization of a specific DGT method facilitates its use and widespread application. Furthermore, such a holistic approach, which combines an extensive DGT laboratory experiment, in-situ field studies and geochemical speciation modelling, contributes towards

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the applicability of DGT as a water monitoring and speciation tool in mining environments. Future work should be performed to assess the performance of the PIWBA resin for DGT measurements of U in natural aquatic systems and mining influenced environments with widely different physicochemical characteristics (fresh water, estuarine water, organic rich waters…). Acknowledgments We acknowledge AREVA Mines France for funding this project. We thank the Hercules Foundation for financing the ICP-SF-MS instrument (UABR/11/010). The authors thank The Dow Chemical Company for the provision of the PIWBA resin and the assistance in the preparation of this manuscript. Furthermore, the authors are grateful to two anonymous reviewers for their helpful comments and suggestions. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2016.04.004. References Alexandratos, S.D., 2009. Ion-exchange resins: a retrospective from industrial and engineering chemistry research. Ind. Eng. Chem. Res. 48, 388e398. http:// dx.doi.org/10.1021/ie801242v. Balistrieri, L.S., Seal, R.R., Piatak, N.M., Paul, B., 2007. Assessing the concentration, speciation, and toxicity of dissolved metals during mixing of acid-mine drainage and ambient river water downstream of the Elizabeth Copper Mine, Vermont, USA. Appl. Geochem. 22, 930e952. http://dx.doi.org/10.1016/ j.apgeochem.2007.02.005. Bernhard, G., Geipel, G., Brendler, V., Nitsche, H., 1998. Uranium speciation in waters of different uranium mining areas. J. Alloys Compd. 271e273, 201e205. http:// dx.doi.org/10.1016/S0925-8388(98)00054-1. Bernhard, G., Geipel, G., Brendler, V., Nitsche, H., 1996. Speciation of uranium in Seepage waters of a mine tailing pile studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Radiochim. Acta 74. http://dx.doi.org/ 10.1524/ract.1996.74.special-issue.87. che, P., Joussein, E., Buzier, R., Charriau, A., Corona, D., Lenain, J.F., Fondane Poulier, G., Lissalde, S., Mazzella, N., Guibaud, G., 2014. DGT-labile As, Cd, Cu and Ni monitoring in freshwater: toward a framework for interpretation of in situ deployment. Environ. Pollut. 192, 52e58. http://dx.doi.org/10.1016/ j.envpol.2014.05.017. Chapman, P.M., 2008. Environmental risks of inorganic metals and metalloids: a continuing, evolving scientific odyssey. Hum. Ecol. Risk Assess. An Int. J. 14, 5e40. Conesa, H.M., Schulin, R., Nowack, B., 2010. Suitability of using diffusive gradients in thin films (DGT) to study metal bioavailability in mine tailings: possibilities and constraints. Environ. Sci. Pollut. Res. Int. 17, 657e664. http://dx.doi.org/10.1007/ s11356-009-0254-x. Cotton, S., 2006. Lanthanide and Actinide Chemistry. John Wiley & Sons, Ltd, West Sussex, England. Dahlqvist, R., Andersson, K., Ingri, J., Larsson, T., Stolpe, B., Turner, D., 2007. Temporal variations of colloidal carrier phases and associated trace elements in a boreal river. Geochim. Cosmochim. Acta 71, 5339e5354. http://dx.doi.org/10.1016/ j.gca.2007.09.016. Davison, W., Zhang, H., 1994. In situ speciation measurements of trace components in natural waters using thin-film gels. Nature 367, 546e548. http://dx.doi.org/ 10.1038/367546a0.
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