David Read

Professor David Read

Professor of Radiochemistry
+44 (0)1483 683541
21 AZ 02

Academic and research departments

School of Chemistry and Chemical Engineering.


DAVID READ, EMMA BRAYSHER, Ben Russell, Franck Dal-Molin (2019)Production and characterisation of reference materials in support of naturally occurring radioactive material (NORM) industries, In: Environmental radiochemical analysis VIpp. 10-19 Royal Society of Chemistry

Naturally occurring radioactive material is a problematic by-product of a range of industries and needs to be handled, stored, processed and disposed of in a safe and economic manner. Accurate characterisation of such material should be underpinned by measurement of certified reference materials in order to validate the methods employed and ensure quality control. This work highlights the current shortage of suitable reference materials and the approach being followed to address this issue, initially for the steel and oil and gas industries.

Hanna Tuovinen, Daniela Vesterbacka, Kai Kaksonen, Juhani Virkanen, David Read, Dina Solatie, Jukka Lehto (2019)Release of Base and Potentially Harmful Metals from Waste Deposits at a Former Uranium Mine in Eastern Finland, In: International Journal of Environmental Monitoring and Analysis7(4)pp. 85-92 Science Publishing Group

Leaching of metals (Fe, Mn, Cu, Co, Ni, Zn, Cr, Cd, Pb) and arsenic from mine wastes and their accumulation in waters, soils and sediments have been determined at the former Paukkajanvaara uranium mine in Eastern Finland. A small-scale test mine, operating in 1959-1960, generated 7,300m3 of waste rock and 12,000m3 of mill tailings, which were covered with soil and till in the early 1990s. These two materials, waste rock and mill tailings, were studied as possible sources for metal contamination in surrounding water systems. Water samples from a nearby pond and streams flowing from the site into the pond were collected. In addition, soil, pond sediment and mill tailings samples were taken and metals were leached from them with strong acids. Metal concentrations in the water samples and acid leachates were measured with ICP-MS. Nickel and arsenic concentrations in stream water were seen slightly elevated when compared to natural Finnish waters and metal concentrations in lake sediments show an increasing trend with time, both indicating long term leaching of metals from the waste deposits. All other metal concentration measurements revealed no elevated concentrations. In conclusion, we may state that the site is not a big if any threat to the environment and humans.

Monica Felipe-Sotelo, J. Hinchliff, D. Drury, N.D.M. Evans, David Read (2022)Cellulose degradation products and high ionic strength enhance the retardation of strontium in hydrated cement paste, In: Frontiers in nuclear engineering

The present concept for the disposal of some low- and all intermediate-level radioactive waste in the United Kingdom involves grouting with cement in steel drums and placement in a geological facility. The vaults would then be backfilled with low strength, high porosity cement-based materials designed to promote a pervasive, alkaline environment in which many of the radioactive species present are sparingly soluble. This work investigates the interaction of strontium with such a backfill under both diffusive and advective conditions given the potential significance of the fission product 90Sr. An important characteristic of the United Kingdom waste inventory is the abundance of organic compounds, including cellulosic materials; consequently, the experiments were repeated with products of alkaline cellulose degradation. Additional experiments were performed at high ionic strength simulating anticipated changes in the salinity of the groundwater. The effective diffusivity (De) of strontium in the absence of the organic compounds ranged between 5.5 × 10−11 and 8.5 × 10−11 m2 s−1, with a distribution coefficient (Rd) of between 2.8 × 10−3 and 3.1 × 10−3 m3 kg−1. The presence of organic compounds and/or an increase of ionic strength enhanced the retardation of strontium. The results indicate that the retention of strontium by hydrated cement paste occurs via a reversible ion exchange mechanism rather than mineralisation and is promoted by decalcification of the calcium silicate hydrate phases.

Franck Dal-Molin, Adam Sutcliffe, David R Anderson, DAVID READ (2019)Determination of natural radioactivity in iron and steel materials, In: Environmental Radiochemical Analysis VIpp. 37-48 Royal Society of Chemistry

It has been known since the 1990s that two natural radioisotopes from the uranium-238 (238U) decay series, polonium-210 (210Po) and lead-210 (210Pb), originally present in trace amounts in raw materials, are volatilised and concentrate in the form of dusts during iron ore sintering. In the UK, most of the dust generated during this process is collected by means of electrostatic precipitators and recycled back into the production system using conveyor belts. Nevertheless, a small proportion passes into the atmosphere via stack emissions and some fugitive dusts can also escape into the workplace during maintenance operations. Tata Steel UK Ltd, a major European steel making company, has developed and validated in-house radioanalytical methods for the measurement of 210Po and 210Pb in a wide range of iron-making materials including raw feedstock, waste dusts, occupational and emission filter samples. The data gathered have enabled a better understanding of the fate of 210Po and 210Pb throughout the integrated steel making route, providing essential information to support environmental permits for discharges to the atmosphere and for confirming that chronic exposure to these two natural radioisotopes does not lead to significant radiological doses to the workforce. Additionally, since the implementation of the BSS Directive 2013/59 Euratom and the Construction Products Regulation (CPR), there is a need for the European steel industry to characterise the levels of radium-226 (226Ra), thorium-232 (232Th) and potassium-40 (40K) in slag materials and confirm that those materials do not pose a significant risk of internal and external exposure to radiation when reused or recycled in building materials. This paper highlights the technical challenges encountered when measuring those natural radioisotopes in various iron-making materials, including the difficulty of validating radioanalytical methods in the absence of suitable certified reference materials.

VICTORIA FRANKLAND, RACHIDA BANCE-SOUALHI, DAVID READ (2019)Raman Analysis of Meta-autunite, In: Environmental Radiochemical Analysis VI

A sample of meta-autunite (Ca(UO2)2(PO4)2·6-8(H2O)) from a national reference collection was characterised by Raman spectroscopy as a representation of a potential spent nuclear fuel corrosion product. Raman spectra were collected at 457, 532, 633 and 785 nm; all exhibited some fluorescence effects, though to a lesser extent at 785 nm. The phosphate (v2(PO4)3−, v3(PO4)3−, v4(PO4)3−) and uranyl (v1(UO2)2+ and v2(UO2)2+) features could be unambiguously assigned in the resolved 785 nm spectrum. The position of the v3(UO2)2+ mode was predicted but not observed. The uranyl bond lengths and force constants were determined from the v1(UO2)2+ dominant and shoulder peak, as 1.78±0.01 and 1.79±0.01 Å and 5.69±0.08 and 5.29±0.08 millidynes Å−1, respectively.

Ben Webster, Peter Ivanov, Ben Russell, Sean Collins, Thierry Stora, Joao Pedro Ramos, Ulli Köster, Andrew Paul Robinson, David Read (2019)Chemical Purification of Terbium-155 from Pseudo-Isobaric Impurities in a Mass Separated Source Produced at CERN, In: Scientific Reports9(1)10884 Nature Publishing Group

Four terbium radioisotopes (149, 152, 155, 161Tb) constitute a potential theranostic quartet for cancer treatment but require any derived radiopharmaceutical to be essentially free of impurities. Terbium-155 prepared by proton irradiation and on-line mass separation at the CERN-ISOLDE and CERN-MEDICIS facilities contains radioactive 139Ce16O and also zinc or gold, depending on the catcher foil used. A method using ion-exchange and extraction chromatography resins in two column separation steps has been developed to isolate 155Tb with a chemical yield of ≥95% and radionuclidic purity ≥99.9%. Conversion of terbium into a form suitable for chelation to targeting molecules in diagnostic nuclear medicine is presented. The resulting 155Tb preparations are suitable for the determination of absolute activity, SPECT phantom imaging studies and pre-clinical trials.

Monica Felipe-Sotelo, J Hinchliff, LP Field, AE Milodowski, O Preedy, David Read (2017)Retardation of uranium and thorium by a cementitious backfill developed for radioactive waste disposal, In: Chemosphere179pp. 127-138 Elsevier

The solubility of uranium and thorium has been measured under the conditions anticipated in a cementitious, geological disposal facility for low and intermediate level radioactive waste. Similar solubilities were obtained for thorium in all media, comprising NaOH, Ca(OH)2 and water equilibrated with a cement designed as repository backfill (NRVB, Nirex Reference Vault Backfill). In contrast, the solubility of U(VI) was one order of magnitude higher in NaOH than in the remaining solutions. The presence of cellulose degradation products (CDP) results in a comparable solubility increase for both elements. Extended X-ray Absorption Fine Structure (EXAFS) data suggest that the solubility-limiting phase for uranium corresponds to a becquerelite-type solid whereas thermodynamic modelling predicts a poorly crystalline, hydrated calcium uranate phase. The solubility-limiting phase for thorium was ThO2 of intermediate crystallinity. No breakthrough of either uranium or thorium was observed in diffusion experiments involving NRVB after three years. Nevertheless, backscattering electron microscopy and microfocus X-ray fluorescence confirmed that uranium had penetrated about 40µm into the cement, implying active diffusion governed by slow dissolution-precipitation kinetics. Precise identification of the uranium solid proved difficult, displaying characteristics of both calcium uranate and becquerelite.

D Huxtable, David Read, G Shaw (2016)Measuring radon-222 in soil gas with high spatial and temporal resolution, In: Journal of Environmental Radioactivity167pp. 36-42 Elsevier

In order to exploit 222Rn as a naturally-occurring tracer in soils we need to sample and measure radon isotopes in soil gas with high spatial and temporal resolution, without disturbing in situ activity concentrations and fluxes. Minimization of sample volume is key to improving the resolution with which soil gas can be sampled; an analytical method is then needed which can measure radon with appropriate detection limits and precision for soil gas tracer studies. We have designed a soil gas probe with minimal internal dead volume to allow us to sample soil gas volumes of 45 cm3. Radon-222 is extracted from these samples into a mineral oil-based scintillation cocktail before counting on a conventional liquid scintillation counter. A detection limit of 320 Bq m-3 (in soil gas) is achievable with a one hour count. This could be further reduced but, in practice, is sufficient for our purpose since 222Rn in soil gas typically ranges from 2,000 - 50,000 Bq m-3. The method is simple and provides several advantages over commonly used field-portable instruments, including smaller sample volumes, speed of deployment and reliability under field conditions. The major limitation is the need to count samples in a liquid scintillation counter within 2 – 3 days of collection, due to the short (3.824 day) radioactive half-life of 222Rn. The method is not applicable to the very short-lived (55 second half-life) 220Rn.

Matthew Isaacs, John Hinchliff, Monica Felipe-Sotelo, David Read (2017)Factors affecting the suitability of superplasticiser-amended cement for the encapsulation of radioactive waste, In: Advances in Cement Research1700099pp. 1-15 Thomas Telford (ICE Publishing)

Organic superplasticisers improve the flow properties of cement, offering operational advantages for the disposal of radioactive wastes. However, there are concerns that they could increase the mobility of encapsulated contaminants significantly. The effect of polycarboxylate ether superplasticisers on the solubility of Ni(II), Am(III), Pu(IV) and U(VI) in two cement-equilibrated waters (ordinary Portland cement/pulverised fuel ash (OPC/PFA) and OPC/ground granulated blast-furnace slag (GGBS)) has been assessed. The study included four commercial superplasticisers, three adjuncts (a de-foaming agent, biocide and viscosity modifier) and a bespoke, synthesised superplasticiser from which residual monomer had been removed by dialysis. The commercial products (0·5% w/w dosage) had a much greater effect on metal solubility than the dialysed equivalent, increasing solubility by 2–3 orders of magnitude. As the adjuncts alone showed no effect, the difference between the commercial and synthesised superplasticisers is attributed to small molecules, primarily residual monomers, in the commercial formulations owing to incomplete polymerisation. The distribution of radionuclides in hardened cement pastes corresponds closely to the distribution of superplasticiser, as shown by 14C-labelling in combination with digital autoradiography and accounting for the bleed observed for certain slag-rich formulations.

E. Van Es, B.C. Russell, P. Ivanov, D. Read (2017)Development of a method for rapid analysis of Ra-226 in groundwater and discharge water samples by ICP-QQQ-MS, In: Applied Radiation and Isotope126pp. 31-34 Elsevier

A novel and rapid method has been developed for pre-concentration and measurement of 226Ra in groundwater and discharge water samples using the latest generation triple quadrupole inductively coupled plasma mass spectrometry (ICP-QQQ-MS). Cation exchange and extraction chromatography are capable of pre-concentration factors of ~200 based on 1 L samples. The sensitivity and interference removal capability of ICP-QQQ-MS was assessed from spiked groundwaters, with the introduction of He collision gas required to minimise instrument background in high-matrix samples. The technique developed is potentially capable of detecting 226Ra activities as low as 5 mBq L−1 when combined with pre-concentration prior to measurement.

Franck Louis Georges Dal Molin, P.E. Warwick, David Read (2017)Under-estimation of 210 Pb in industrial radioactive scales, In: Analytica Chimica Acta1000pp. 67-74 Elsevier Masson

Lead-210 (210Pb) can be present at high activity concentrations, in residues arising from the petroleum, mineral processing and chemical industries. Although 210Pb itself poses a low radiological risk, the nuclide decays via 210Bi to the alpha emitting and highly radiotoxic 210Po. Therefore, rapid, accurate determination of 210Pb is essential for assessing the radiological risk to plant operators and appropriate sentencing of waste. Unfortunately, direct measurement of 210Pb by gamma spectrometry is hindered by its weak gamma-ray emission at 46.5 keV, which is readily attenuated by mineral matrices. This paper demonstrates the extent to which 210Pb can be underestimated during routine analysis by an inter-laboratory exercise involving five accredited laboratories and a wide range of scales from diverse industrial sources. Two methods of addressing errors in 210Pb analysis are highlighted; the first, involving lithium tetraborate fusion prior to gamma spectrometry shows promise but is not suitable for all 210Pb-containing phases. The second method, requiring calculation of matrix attenuation factors for a representative fingerprint sample, was applied successfully to deposits from the steel and gas industries. However, its wider application depends on detailed chemical and mineralogical characterisation for each of the major categories of mineral scale found and at present, there is an acute lack of suitable certified reference materials.

Monica Felipe-Sotelo, J. Hinchliff, N.D.M. Evans, D. Read (2016)Solubility constraints affecting the migration of selenium through the cementitious backfill of a geological disposal facility, In: Journal of Hazardous Materials305pp. 21-29 Elsevier

This work presents the study of the solubility of selenium under cementitious conditions and its diffusion, as SeO32−, through monolithic cement samples. The solubility studies were carried out under alkaline conditions similar to those anticipated in the near-field of a cement-based repository for low- and intermediate-level radioactive waste. Experiments were conducted in NaOH solution, 95%-saturated Ca(OH)2, water equilibrated with a potential backfill material (Nirex reference vault backfill, NRVB) and in solutions containing cellulose degradation products, with and without reducing agents. The highest selenium concentrations were found in NaOH solution. In the calcium-containing solutions, analysis of the precipitates suggests that the solubility controlling phase is Ca2SeO3(OH)2·2H2O, which appears as euhedral rhombic crystals. The presence of cellulose degradation products caused an increase in selenium concentration, possibly due to competitive complexation, thereby, limiting the amount of calcium available for precipitation. Iron coupons had a minor effect on selenium solubility in contrast to Na2S2O4, suggesting that effective reduction of Se(IV) occurs only at Eh values below −300 mV. Radial through-diffusion experiments on NRVB and in a fly ash cement showed no evidence of selenium breakthrough after one year. However, autoradiography of the exposed surfaces indicated that some migration had occurred and that selenium was more mobile in the higher porosity backfill than in the fly ash cement.

Elsje Van Es, J. Hinchliff, Monica Felipe-Sotelo, A.E. Milodowski, L.P. Field, N.D.M. Evans, David Read (2015)Retention of chlorine-36 by a cementitious backfill, In: Mineralogical Magazine79(06)pp. 1297-1305 The Mineralogical Society of Great Britain and Ireland

Radial diffusion experiments have been carried out to assess the migration of 36Cl, as chloride, through a cementitious backfill material. Further experiments in the presence of cellulose degradation products were performed to assess the effect of organic ligands on the extent and rate of chloride diffusion. Results show that breakthrough of 36Cl is dependent on chloride concentration: as the carrier concentration increases, both breakthrough time and the quantity retained by the cement matrix decreases. Experiments in the presence of cellulose degradation products also show a decrease in time to initial breakthrough. However, uptake at various carrier concentrations in the presence of organic ligands converges at 45% of the initial concentration as equilibrium is reached. The results are consistent with organic ligands blocking sites on the cement that would otherwise be available for chloride binding, though further work is required to confirm that this is the case. Post-experimental digital autoradiographs of the cement cylinders, and elemental mapping showed evidence of increased 36Cl activity associated with black ash-like particles in the matrix, believed to correspond to partially hydrated glassy calcium-silicate-sulfate-rich clinker.

Joel Garner, James Cairns, David Read (2015)NORM in the East Midlands' oil and gas producing region of the UK, In: Journal of Environmental Radioactivity150pp. 49-56

Naturally occurring radioactive material (NORM) is a common feature in North Sea oil and gas production offshore but, to date, has been reported from only one production site onshore in the United Kingdom. The latter, Wytch Farm on the Dorset coast, revealed high activity concentrations of 210Pb in metallic form but little evidence of radium accumulation. NORM has now been discovered at two further onshore sites in the East Midlands region of the UK. The material has been characterized in terms of its mineralogy, bulk composition and disequilibrium in the natural uranium and thorium series decay chains. In contrast to Wytch Farm, scale and sludge samples from the East Midlands were found to contain elevated levels of radium and radioactive progeny associated with crystalline strontiobarite. The highest 226Ra and 228Ra activity concentrations found in scale samples were 132 and 60 Bq/g, with mean values of 86 and 40 Bq/g respectively; somewhat higher than the mean for the North Sea and well above national exemption levels for landfill disposal. The two East Midlands sites exhibited similar levels of radioactivity. Scanning electron microscope imaging shows the presence of tabular, idiomorphic and acicular strontiobarite crystals with elemental mapping confirming that barium and strontium are co-located throughout the scale. Bulk compositional data show a corresponding correlation between barium-strontium concentrations and radium activity. Scales and sludge were dated using the 226Ra/210Pb method giving mean ages of 2.2 and 3.7 years, respectively. The results demonstrate clearly that these NORM deposits, with significant radium activity, can form over a very short period of time. Although the production sites studied here are involved in conventional oil recovery, the findings have direct relevance should hydraulic fracturing for shale gas be pursued in the East Midlands oilfield.

F. Dal Molin, R. Fisher, D. Frost, D.R. Anderson, D. Read (2017)Dose assessment from chronic exposure to industrial NORM in iron ore processing, In: Journal of Radiological Protection37(2)pp. 402-421 IOP Publishing

Radiological exposures due to naturally occurring radioactive material (NORM) can occur during a wide range of work-related activities in the mineral processing and chemical industries. However, evaluation of such exposures in industrial settings remains a difficult exercise owing inter alia to the large number of personnel, operations and plants affected; assumptions that often have to be made concerning the actual duration and frequency of exposures; the complex chemistry and radioactive disequilibria involved and typically, the paucity of historical data. In our study, the challenges associated with assessing chronic exposure to fugitive dust enriched in 210Pb and 210Po and the determination of the associated internal dose by inhalation and ingestion are described by reference to a case study undertaken at an iron ore sintering plant between June 2013 and July 2015. The applicability of default dose coefficients and biokinetic models provided by the International Commission for Radiological Protection (ICRP) was verified by combining air and dust monitoring with information on the characteristics of the aerosols and in-vitro solubility experiments. The disparity between particulate matter 100 microns or less in diameter (PM100), particulate matter 10 microns or less in diameter (PM10) and 210Pb/210Po activity concentrations observed over the different monitoring campaigns and sampling locations confirmed that use of positional short-term monitoring surveys to extrapolate intake over a year was not appropriate and could lead to unrealistic intake and dose figures. Personal air sampling is more appropriate for estimating the dose in such situations, though it is not always practical and may collect insufficient quantities of material for radiochemical analysis; this is an important constraint when dealing with low specific activity materials.

Monica Felipe-Sotelo, M. Edgar, T. Beattie, P. Warwick, N.D.M. Evans, David Read (2015)Effect of anthropogenic organic complexants on the solubility of Ni, Th, U(IV) and U(VI), In: Journal of Hazardous Materials300pp. 553-560

The influence of anthropogenic organic complexants (citrate, EDTA and DTPA from 0.005 to 0.1 M) on the solubility of nickel(II), thorium(IV) and uranium (U(IV) and U(VI)) has been studied. Experiments were carried out in 95%-saturated Ca(OH)2 solutions, representing the high pH conditions anticipated in the near field of a cementitious intermediate level radioactive waste repository. Results showed that Ni(II) solubility increased by 2–4 orders of magnitude in the presence of EDTA and DTPA and from 3 to 4 orders of magnitude in the case of citrate. Citrate had the greatest effect on the solubility of Th(IV) and U(IV)/(VI). XRD and SEM analyses indicate that the precipitates are largely amorphous; only in the case of Ni(II), is there some evidence of incipient crystallinity, in the form of Ni(OH)2 (theophrastite). A study of the effect of calcium suggests that U(VI) and Ni(II) may form metal-citrate-OH complexes stabilised by Ca2+. Thermodynamic modelling underestimates the concentrations in solution in the presence of the ligands for all the elements considered here. Further investigation of the behaviour of organic ligands under hyperalkaline conditions is important because of the use of the thermodynamic constants in preparing the safety case for the geological disposal of radioactive wastes.

Monica Felipe-Sotelo, E.R. Henshall-Bell, N.D.M. Evans, David Read (2015)Comparison of the chemical composition of British and Continental European bottled waters by multivariate analysis, In: Journal of Food Composition and Analysis39pp. 33-42 Elsevier

The elemental composition of 37 bottled waters from the UK and continental Europe has been determined. Ca, K, Mg, Na, Al, As, Ba, Cd, Cr, Co, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Sr, U, V and Zn were determined by ICP-OES and ICP-MS, in addition to inorganic and total organic carbon. The composition of all the waters analysed fell within the guideline values recommended by the World Health Organization. Na, Ca, Sr and Ba showed the widest variation in concentrations, ranging over two orders of magnitude. Levels of Fe were below the limit of detection (30 μg L−1) in all samples analysed. Waters produced in the UK generally showed lower levels of most major elements and trace metals, with the exception of Ba (up to 455 μg L−1). Italian waters showed the highest concentrations of Sr (3000–8000 μg L−1) and U (8–13 μg L−1), whereas waters produced in Slovakia and the Czech Republic showed the highest levels of Pb (0.7–4 μg L−1). The use of multivariate analysis reveals an association between high alkaline metal content and high concentrations of As and Cr. There also appears to be a correlation between high Ca and Sr content and high levels of U. Analysis of variance (ANOVA) indicates that the composition of bottled water can be distinguished primarily by the country of origin, over other factors including the geological environment of the source. This would suggest that composition reflects, and is biased towards, consumer preferences.

Hanna Tuovinen, Daniela Vesterbacka, Esa Pohjolainen, David Read, Dina Solatie, Jukka Lehto (2015)A comparison of analytical methods for determining uranium and thorium in ores and mill tailings, In: Journal of Geochemical Exploration148pp. 174-180 Elsevier

Three different methods for determining uranium and four for determining thorium in an apatite ore, black schist and mill tailings are compared. The aim of the comparative study is to obtain analytical verification of field results so that site characterization could be optimized without compromising reliability of the analysis. The samples were collected from a former pilot scale phosphate mine in Sokli, northern Finland and the Talvivaara nickel mine in Eastern Finland. A non-destructive, portable X-ray fluorescence (XRF) spectrometer was used on site to measure, semi-quantitatively, the composition of major elements, including thorium in each material. The samples were then analyzed by X-ray diffraction (XRD) to identify the main mineral components and electron probe micro-analysis (EPMA) used to identify the uranium and thorium-bearing minerals. Gamma spectrometry was used for direct determination of uranium and thorium isotopes in powdered samples. Thereafter, sample digestion experiments were performed with various acids to optimize the method for microwave digestion allowing determination of uranium and thorium by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and alpha spectrometry. For the latter method, uranium and thorium were separated using anion exchange resin filled in a column. The results show that overall a sample mass of 0.1–0.5 g with addition of 20 mL concentrated nitric acid is the most efficient means for extracting uranium and thorium simultaneously from apatite ore, black schist and mill tailings. The results for uranium and thorium concentrations determined by gamma spectrometry, alpha spectrometry and ICP-MS were in good agreement. Also the on-site measurements with portable XRF gave comparable results to other methods used in this study for thorium even without prior sample preparation.

Hanna Tuovinen, Mila Pelkonen, Janne Lempinen, Esa Pohjolainen, David Read, Dina Solatie, Jukka Lehto (2018)Behaviour of Metals during Bioheap Leaching at the Talvivaara Mine, Finland, In: Geosciences8(2)pp. 66-79 MDPI

The behaviour of base metals Ni, Zn, Cu, Co, Fe, and Mn, potentially toxic metals Pb, Cr, and Cd, and the radioactive elements, U and Th, in the Talvivaara mining process, Finland has been studied by tracing metal concentrations from the black schist ore, through ores subjected to bioheap leaching of varying duration, to pregnant leach solution (PLS), and solid process waste material deposited on site in gypsum waste ponds. It is apparent that Zn, Cu, Co, and Cd are leached from the ore in a similar manner and recovered efficiently in the PLS; however, Ni, though leached, was also found in the gypsum pond at relatively high concentrations. Relatively little Pb is released from the ore, but the small fraction that is mobilised accumulates in the gypsum pond. Of the radioactive constituents, Th is essentially immobile, whereas U is readily leached from the ore, again accumulating in gypsum pond waste. In addition, a laboratory-based sequential leach test was applied to assess the future leaching potential of metals from residual ore and process waste material under different environmental conditions.

Emma Braysher, B. Russell, S. Woods, M. García-Miranda, P. Ivanov, B. Bouchard, D. Read (2019)Complete dissolution of solid matrices using automated borate fusion in support of nuclear decommissioning and production of reference materials, In: Journal of Radioanalytical and Nuclear Chemistry321(1)pp. pp 183-196 Springer International Publishing

Accurate measurement of natural and anthropogenic radionuclide concentrations is of critical importance to end users in the nuclear sector to ensure correct classification prior to storage, recycling, reprocessing or disposal. Uncertainties in the characterisation of solid matrices and materials could lead to safety, quality and financial implications. Robust sample preparation methods are vital, in particular effective sample digestion, as under-estimated chemical yield recovery results in a corresponding under-estimation of activity levels. Borate fusion has been proven to effectively digest a range of complex sample matrices in the geosciences but is not used routinely elsewhere. In this study, we describe an automated procedure for borate fusion of multiple matrices encountered in nuclear decommissioning, containing diverse radionuclides over a range of activity concentrations. The impact of digestion flux, sample mass and sample to flux ratios are described, as well as the subsequent separation and measurement techniques. The results contribute to accurate and precise measurement of radionuclides in various matrices, as well as to characterisation of reference materials, providing greater confidence in nuclear industry programmes worldwide.

Hanna Tuovinen, Esa Pohjolainen, Janne Lempinen, Daniela Vesterbacka, David Read, Dina Solatie, Jukka Lehto (2016)Behaviour of radionuclides during microbially-induced mining of nickel at Talvivaara, Eastern Finland, In: Journal of Environmental Radioactivity151pp. 105-113 Elsevier

The Talvivaara mine in Eastern Finland utilizes microbe-induced heap leaching to recover nickel and other valuable metals (Zn, Cu, Co) from a black schist ore. In addition to the target metals, the ore contains uranium at a concentration of 17 mg/kg, incorporated as uraninite (UO2). Uranium oxidizes from the U(IV) to U(VI) state during leaching and dissolves as the uranyl ion (UO22+) in the acidic pregnant leach solution. Mobilisation of uranium has caused sufficient concern that plans have been developed for uranium recovery. The aim of this study is to generate new data leading to a better understanding of the fate of its radiotoxic daughter nuclides, primarily 226Ra, 210Pb and 210Po, in the mining process. It is shown that uranium daughters mostly remain in the heaps during the leaching process and are associated with secondary minerals, including jarosite, goethite and gypsum. Thorium and progeny (232Th plus 228Th, 228Ra) are also mainly retained. High sulphate concentrations in the acidic solutions limit the solubility of radium by incorporation in the crystal lattices of precipitated secondary sulphates. Electron probe microanalysis shows that goethite in the heaps is uraniferous, resulting from the adsorption of U(VI). After recovery of target metals, the pregnant leach solution is neutralized to further remove metal contaminants and the resulting slurries stored in a bunded tailings pond. The activity concentrations of thorium, radium, lead and polonium isotopes are generally low in the pond owing to prior retention by secondary minerals in the heaps. However, 238U activity concentrations range up to 3375 Bq/kg, which exceeds the permitted value (1000 Bq/kg) for natural radionuclides of the 238U series.

Matthew Isaacs, Martin Hayes, Steven Rawlinson, Michael Angus, Adam Qaisar, Steven Christie, Steven Edmondson, David Read (2017)Processing and product characteristics of a blended cement grout incorporating polycarboxylate ether superplasticiser, In: Advances in Cement Research30(4)pp. 148-158 Thomas Telford (ICE Publishing)

Superplasticisers improve the flow properties of cement and offer a number of operational advantages to producers of radioactive waste. Research is underway to clarify their mode of interaction using a bespoke, purified polycarboxylate ether (PCE) superplasticiser in controlled trials. A large-scale (200 dm3) product was prepared with a mix of ground granulated blast-furnace slag and ordinary Portland cement (9:1 mix) and tested using methods established by the UK nuclear industry to assess grout performance. The product met the essential criteria for a radioactive waste encapsulation grout. Laboratory-scale studies utilising a 14C-labelled superplasticiser and its components were employed to better understand the mechanisms involved and to determine the location and distribution of the superplasticiser within cured cement monoliths. The results suggest that a PCE superplasticiser may enable the use of cement industry standard powders for encapsulation processes, allowing lower water content matrices and mitigating the risk of future powder supply issues.

M. Felipe-Sotelo, J. Hinchliff, L.P. Field, A.E. Milodowski, J.D. Holt, S.E. Taylor, D. Read (2016)The solubility of nickel and its migration through the cementitious backfill of a geological disposal facility for nuclear waste, In: Journal of Hazardous Materials314pp. 211-219 Elsevier

This work describes the solubility of nickel under the alkaline conditions anticipated in the near field of a cementitious repository for intermediate level nuclear waste. The measured solubility of Ni in 95%-saturated Ca(OH)2 solution is similar to values obtained in water equilibrated with a bespoke cementitious backfill material, on the order of 5 × 10−7 M. Solubility in 0.02 M NaOH is one order of magnitude lower. For all solutions, the solubility limiting phase is Ni(OH)2; powder X-ray diffraction and scanning transmission electron microscopy indicate that differences in crystallinity are the likely cause of the lower solubility observed in NaOH. The presence of cellulose degradation products causes an increase in the solubility of Ni by approximately one order of magnitude. The organic compounds significantly increase the rate of Ni transport under advective conditions and show measurable diffusive transport through intact monoliths of the cementitious backfill material.

E. M. Van Es, B. C. Russell, P. Ivanov, M. García Miranda, D. Read, C. Dirks, S. Happel (2017)The behaviour of 226Ra in high-volume environmental water samples on TK100 resin, In: Journal of Radioanalytical and Nuclear Chemistry312(1)pp. 105-110 Springer Netherlands

Accurate, low-level measurement of 226Ra in high volume water samples requires rapid pre-concentration and robust separation techniques prior to measurement in order to comply with discharge limits and drinking water regulations. This study characterises the behaviour of 226Ra and interfering elements on recently developed TK100 (Triskem International) extraction chromatography resin. Distribution coefficients over a range of acid concentrations are given, along with an optimised procedure that shows rapid pre-concentration and separation of 226Ra on TK100 resin is achievable for high volume (1 L) water samples without the need for sample pre-treatment.

VICTORIA FRANKLAND, A.E. Milodowski, Robert A. Lawrence, Marco Sacchi, DAVID READ (2022)Characterisation of Vandenbrandeite: A Potential Alteration Product of Spent Nuclear Fuel, In: The American mineralogist Mineralogical Society of America

The use of copper canisters in the Swedish KBS-3 concept for spent nuclear fuel disposal could result in the formation of copper-bearing uranyl phases should a canister suffer from defects or if the containment were to fail before reducing conditions are established in the repository. Most uranyl species would be expected to display higher solubility than the original uranium(IV) dioxide fuel, leading to enhanced release, though this would depend on the phase and prevailing groundwater conditions. Secondary alteration products may also be poorly crystalline or even amorphous, making characterisation difficult during the pre-closure period owing to the high radiation field close to the canister. Vandenbrandeite, (CuUO2(OH)4), is a rare mineral in nature but known to form by alteration of primary uraninite through interaction with oxidising groundwater containing dissolved copper Consequently, an attempt has been made to characterise two vandenbrandeite specimens of varying crystallinity by luminescence and multiple-laser Raman spectroscopy; techniques amenable to remote, robotic deployment and which have proved useful in discriminating other uranyl oxy-hydroxides, silicates and phosphates. The first reported luminescence emission and excitation spectra for vandenbrandeite revealed near-negligible luminescence, with a slightly enhanced signal for the specimen displaying poorer crystallinity. This observation agrees well with density functional theory calculations. The simulated projected density of state and band structure show an unlikely transition from the U f-orbitals to Cu d-orbitals, or O states, would be required for luminescence to be detectable; this probably improves for poorly crystalline specimens as the spatial overlap between the orbitals increases. Further, negligible differences in the number of peaks and peak positions were detected in the laser wavelength-dependent Raman spectra although again, variation in background noise and peak shape was observed based on the degree of crystallinity. Good agreement was obtained between experimental and simulated Raman spectra, particularly with the environmentally sensitive axial uranyl stretching modes, validating the crystal system derived in this study. The findings of this study suggest luminescence spectroscopy, when combined with Raman spectroscopy, may be able to both identify vandenbrandeite and distinguish between crystalline and amorphous forms based on their relative luminescence intensity.

E Braysher, B Russell, S.M Collins, E.M van Es, R Shearman, F. Dal Molin, D Read, M Anagnostakis, R Arndt, A Bednár, T Bituh, J.P Bolivar, J Cobb, N Dehbi, S Di Pasquale, C Gascó, C Gilligan, P Jovanovič, A Lawton, A.M.J Lees, A Lencsés, L Mitchell, I Mitsios, B Petrinec, J Rawcliffe, M Shyti, J.A Suárez-Navarro, S Suursoo, E Tóth-Bodrogi, T Vaasma, L Verheyen, J Westmoreland, G de With (2021)Development of a reference material for analysing naturally occurring radioactive material from the steel industry, In: Analytica chimica acta1141pp. 221-229 Elsevier B.V

Accurate measurement of naturally occurring radionuclides in blast furnace slag, a by-product of the steel industry, is required for compliance with building regulations where it is often used as an ingredient in cement. A matrix reference blast furnace slag material has been developed to support traceability in these measurements. Raw material provided by a commercial producer underwent stability and homogeneity testing, as well as characterisation of matrix constituents, to provide a final candidate reference material. The radionuclide content was then determined during a comparison exercise that included 23 laboratories from 14 countries. Participants determined the activity per unit mass for 226Ra, 232Th and 40K using a range of techniques. The consensus values obtained from the power-moderated mean of the reported participant results were used as indicative activity per unit mass values for the three radionuclides: A0(226Ra) = 106.3 (34) Bq·kg−1, A0(232Th) = 130.0 (48) Bq·kg−1 and A0(40K) = 161 (11) Bq·kg−1 (where the number in parentheses is the numerical value of the combined standard uncertainty referred to the corresponding last digits of the quoted result). This exercise helps to address the current shortage of NORM industry reference materials, putting in place infrastructure for production of further reference materials. [Display omitted] •Current availability of reference materials for naturally occurring radioactive material has been expanded upon.•A blast furnace slag reference material has been developed to support the steel industry and regulations regarding its use in building materials.•A comparison exercise of 23 laboratories was used to determine consensus values for the activity per unit mass of selected radionuclides.•Infrastructure has been built for future development of reference materials in support of naturally occurring radioactive materials.

Tapio Vehmas, Vanessa Montoya, Maria Cruz Alonso, Radek Vašíček, Emily Rastrick, Stephane Gaboreau, Petr Večerník, Markku Leivo, Erika Holt, Nicolas Fink, Naila Ait Mouheb, Jiří Svoboda, David Read, Radek Červinka, Rita Vasconcelos, Claire Corkhill (2020)Characterization of Cebama low-pH reference concrete and assessment of its alteration with representative waters in radioactive waste repositories, In: Applied geochemistry121104703 Elsevier Ltd

Concretes, mortars and grouts are used for structural and isolation purposes in radioactive and nuclear waste repositories. For example, concrete is used for deposition tunnel end plugs, engineered barriers, mortars for rock bolting and injection grouts for fissure sealing. Despite of the materials anticipated functionality, it is extremely important to understand the long-term material behaviour in repository environments. A reference concrete and mortar for the Cebama project based on a cement, silica and blast furnace slag ternary blend were designed and characterized in different laboratories with multiple experimental methods (XRD, XAS at the Fe and Cl K-edges, SEM-EDX, 29Si and 27Al MAS-NMR, TG-DSC, MIP and Kerosene porosimetry) and techniques (punch strength tests). The reference concrete enabled comparison of results from different institutes and experimental techniques, unifying the individual results to more comprehensive body. The Cebama reference concrete and mortar were designed to have high durability and compatible formulation with respect to an engineered barrier system in clay or crystalline host-rocks, having pore solution pH significantly lower than traditional concretes. This work presents main results regarding their characterization and alteration in contact with representative waters present in radioactive waste repositories. Pore solution pH of the matured reference concrete was 11.4–11.6. The main hydrated phases were C–S–H and C-A-S-H gels with a Ca:Si ratio between 0.5 and 0.7 and an Al:Si ratio of 0.05. Minor phases were ettringite and hydrotalcite. Iron(III) could be in the C–S–H phases and no Cl-bearing solid phases were identified. Connected porosity and pore size distribution was characterized by MIP observing that, as expected, the size of the pores in the hydrated cement phases varies from the micro-to the nanoscale. Connected porosity of both materials were low. Compressive strength of the concrete was 115 MPa, corresponding to traditional high-performance concrete. Degradation of these materials in contact with different waters mainly produce their decalcification and enrichment in Mg for waters containing high amount of this element, like the clay waters. •The performance of “low-pH” mix designs containing slag exceeds the performance of traditional Portland cement concretes.•That slag is able to replace fly ash when producing ”low-pH” mixtures, if needed due to material supply or quality problems.•Eight experimental techniques have provided mechanical, hydraulic and geochemical properties of new designed materials.•Micro-mortar in contact with clay and bentonite waters, shows a Mg enrichment and calcium depletion at the reaction front.

Matthew Isaacs, Steve Lange, Guido Deissmann, Dirk Bosbach, Antoni E Milodowski, David Read (2020)Retention of technetium-99 by grout and backfill cements: Implications for the safe disposal of radioactive waste, In: Applied geochemistry116104580 Elsevier Ltd

Technetium-99 (99Tc) is an important radionuclide when considering the disposal of nuclear wastes owing to its long half-life and environmental mobility in the pertechnetate (Tc(VII)) redox state. Its behaviour in a range of potential cement encapsulants and backfill materials has been studied by analysing uptake onto pure cement phases and hardened cement pastes. Preferential, but limited, uptake of pertechnetate was observed on iron-free, calcium silicate hydrates (C–S–H) and aluminate ferrite monosulphate (AFm) phases with no significant adsorption onto ettringite or calcium aluminates. Diffusion of 99Tc through cured monolithic samples, representative of cements being considered for use in geological disposal facilities across Europe, revealed markedly diverse migration behaviour, primarily due to chemical interactions with the cement matrix rather than differential permeability or other physical factors. A backfill cement, developed specifically for the purpose of radionuclide retention, gave the poorest performance of all formulations studied in terms of both transport rates and overall technetium retention. Two of the matrices, pulverised fuel ash: ordinary Portland cement (PFA:OPC) and a low-pH blend incorporating fly ash, effectively retarded 99Tc migration via precipitation in narrow, reactive zones. These findings have important implications when choosing cementitious grouts and/or backfill for Tc-containing radioactive wastes.

B Grambow, M López-García, J Olmeda, M Grivé, N.C.M Marty, S Grangeon, F Claret, S Lange, G Deissmann, M Klinkenberg, D Bosbach, C Bucur, I Florea, R Dobrin, M Isaacs, D Read, J Kittnerová, B Drtinová, D Vopálka, N Cevirim-Papaioannou, N Ait-Mouheb, X Gaona, M Altmaier, L Nedyalkova, B Lothenbach, J Tits, C Landesman, S Rasamimanana, S Ribet (2019)Retention and diffusion of radioactive and toxic species on cementitious systems: Main outcome of the CEBAMA project, In: Applied geochemistry112104480 Elsevier Ltd

Cement-based materials are key components in radioactive waste repository barrier systems. To improve the available knowledge base, the European CEBAMA (Cement-based materials) project aimed to provide insight on general processes and phenomena that can be easily transferred to different applications. A bottom up approach was used to study radionuclide retention by cementitious materials, encompassing both individual cement mineral phases and hardened cement pastes. Solubility experiments were conducted with Be, Mo and Se under high pH conditions to provide realistic solubility limits and radionuclide speciation schemes as a prerequisite for meaningful adsorption studies. A number of retention mechanisms were addressed including adsorption, solid solution formation and precipitation of radionuclides within new solid phases formed during cement hydration and evolution. Sorption/desorption experiments were carried out on several anionic radionuclides and/or toxic elements which have received less attention to date, namely: Be, Mo, Tc, I, Se, Cl, Ra and 14C. Solid solution formation between radionuclides in a range of oxidation states (Se, I and Mo) with the main aqueous components (OH−, SO4−2, Cl−) of cementitious systems on AFm phases were also investigated. •Understanding anion retention in cementitious materials.•Reducing uncertainties with respect to radionuclide behaviour in hardened cement pastes.•Justifying assumptions used when representing radionuclide migration in safety assessments.•Improving databases for assessing adsorption of radionuclides in fresh and degraded cement systems.

Victoria L Frankland, Antoni E Milodowski, David Read (2020)Characterisation of meta-autunite: Towards identifying potential alteration products of spent nuclear fuel, In: Applied geochemistry123 Elsevier Ltd

Laser-based spectroscopic techniques offer potential for characterising the alteration products of spent nuclear fuel in settings where the use of more traditional analytical methods is impracticable. Among these alteration products, uranyl phosphate phases have long attracted interest owing to their potential to form passivating surfaces on primary uranium phases inhibiting further uranium dissolution. Two strontium-rich meta-autunite ((Ca,Sr)(UO2)2(PO4)2·2–8(H2O)) samples from the Mount Spokane uranium deposit, Washington, USA were characterised by multiple laser wavelength Raman and time-resolved laser fluorescence spectroscopy. Well-defined Raman features were obtained, particularly at a laser wavelength of 785 nm, but partially hydrated meta-autunite phases could not be differentiated by Raman alone. However, subtle differences in three key modes were observed between meta-autunite and published data for fully hydrated autunite specimens enabling these minerals to be distinguished. Seven luminescence emission and several excitation features were resolved for the two samples, with the latter being the first reported excitation data for meta-autunite. The luminescence decay lifetime was found to be significantly longer than previously reported and sensitive to the meta-autunite dehydration phase. [Display omitted] •Alteration of spent nuclear fuel in the presence of phosphate may result in meta-autunite formation.•Type mineral specimens were characterised by SEM-EDXA, XRD, Raman and TRLFS.•Fluorescence excitation spectrum for meta-autunite is reported for the first time.•Fluorescence decay may be sensitive to meta-autunite dehydration.•Raman features are sufficient to discriminate the phase from fully hydrated autunite.

Joel Garner, David Read (2020)Optimisation of Radium Removal from Saline Produced Waters during Oil and Gas Extraction, In: Minerals (Basel)10(3)pp. 278-289 MDPI

Unconventional shale gas exploitation presents complex problems in terms of radioactive waste disposal. Large volumes of saline produced water resulting from hydraulic fracturing are typically enriched in radium isotopes, up to several hundred Bq/dm3, orders of magnitude above national discharge limits. There is a need, therefore, to decontaminate the fluid prior to discharge, preferably by creating a less problematic radium-containing, solid waste form. Barite (barium sulphate) co-precipitation is a cost-effective method for achieving these objectives, provided the process can be controlled. In this work, radium recovery of ~90% has been achieved for simulant produced waters containing 100 Bq/dm3, using a single, optimised co-precipitation step. However, salinity has a significant effect on the efficiency of the process; higher salinity solutions requiring substantially more reagent to achieve the same recovery. If >90% radium removal is sought, multiple co-precipitation steps provide a much faster alternative than post-precipitation recrystallization of the barite solid phase, albeit at higher cost. The resulting solid waste has a relatively high specific radium activity but a much smaller volume, which presents a less intractable disposal problem for site operators than large volumes of radium-contaminated fluid.

H. Tuovinen, E. Pohjolainen, D. Vesterbacka, K. Kaksonen, J. Virkanen, D. Solatie, J. Lehto, D. Read (2016)Release of radionuclides from waste rock and mill tailings at a former pilot uranium mine in eastern Finland, In: Boreal Environment Research21(5-6)pp. 471-480 Boreal Environment Research Publishing Board

The Paukkajanvaara mine in Eno, eastern Finland, started to operate in 1959. The mine was a test site for assessing the feasibility of larger-scale uranium extraction. Pilot-scale mining and milling were carried out between 1958 and 1961, and the mine site was rehabilitated in the early 1990s. The aim of this study was to examine the potential for further mobilization of radionuclides after remediation. There are two primary sources of contamination at the site, the waste-rock pile and the tailings. Our study indicates that 226Ra leached from the waste-rock pile and accumulated in surrounding soil. In run-off sediment samples collected from a dry stream bed near the waste-rock pile, the activity concentrations of 226Ra and 238U were higher than in soil samples. From the tailings, radionuclides can leach directly to the lake and to another small stream, which flows to the east of the waste-rock pile. In the water samples collected from this stream, uranium concentrations increased at the outlet to the lake. The results from the soil samples collected between the tailings area and the stream indicate leaching of 238U and 226Ra with the surface flow. Sediment samples collected from the bottom of the lake displayed pronounced uranium series disequilibrium with fractionations of 210Pb and 226Ra relative to the parent 238U. The results therefore indicate that leaching and accumulation of at least 226Ra from the waste-rock pile and possibly tailings is still ongoing.

David Read, F. Dal-Molin, D. R. Anderson (2015)Determination of Polonium-210 and Lead-210 in Iron- and Steel-Making Materials, In: Environmental radiochemical analysis Vpp. 175-184 RSC Publishing

This paper presents details of the challenges encountered when measuring 210Po and 210Pb, originally present in trace amounts in raw materials and then concentrates in the form of dusts during iron ore sintering, and validating radioanalytical methods in the absence of suitable reference materials. In-house radioanalytical methods for the measurement of these two radionuclides in a wide range of iron and steel-making materials including raw materials, waste dusts and emission samples have been developed. The methods have been validated and accredited by the United Kingdom Accreditation Service (UKAS) according to the International Standard Organisation (ISO) 17025.

Ben Russell, Elsje Van Es, Glenn Woods, David Read (2017)Rapid analysis of radium in water samples by ICP-QQQ: Agilent Technical Note (Nuclear and Environmental) Agilent Technologies
David Read (2015)Analytical Challenges Posed by Norm Contaminated Sites, In: Environmental radiochemical analysis Vpp. 81-91 RSC Publishing

This paper describes some of the challenges faced by testing laboratories when conducting compliance measurements for NORM wastes. Focus is placed on solid materials as these comprise by far the most common samples of this type sent for commercial analysis. The importance of understanding the industrial processes giving rise to the waste and the need for representative sampling are emphasised. The market for analysis of NORM will undoubtedly grow, driven by more rigorous, modern legislation; however, existing capacity is limited. Future needs are discussed with reference to hydraulic fracturing for shale gas, already a mature technology in North America and one that is set to play an increasing role in the UK and continental Europe.

E. van Es, P. Ivanov, D. Read (2017)The adsorption of radium on borosilicate glass and the implications for the pharmaceutical industry, In: Journal of Labelled Compounds and Radiopharmaceuticals Wiley
Hanna Tuovinen, Daniela Vesterbacka, Janne Lempinen, Esa Pohjolainen, David Read, Dina Solatie, Jukka Lehto (2015)Radionuclide and heavy metal redistribution at the former pilot-scale apatite plant in Sokli, northern Finland, In: Boreal Environment Research20(3)pp. 350-362 Boreal Environment Research Publishing Board.

A new phosphate mine is being planned at Sokli in northern Finland. In the late 1970s, pilot-scale mining and mineral processing took place at the site. The mobilisation of radionuclides and heavy metals from the mill tailings was examined in order to assess the potential environmental impact of past and future mining activities. Given the considerable amount of apatite still present, the waste material does not represent true tailings. Variations in abundance probably represent material discharges to the tailings rather than mobilization of the elements from the tailings themselves. No indication of heavy-metal migration was found. Extraction results suggest that only a small proportion of cadmium is in exchangeable form. Elements that are partly soluble under weakly-acidic conditions include copper, zinc, cadmium, uranium and lead. However, most of the elements are tightly bound to the sample matrix and therefore not easily released to the environment.