Elemental analysis versus species analysis (in example for arsenic and uranium) Doreen Ebert TU

Bergakademie

Freiberg

–

Department

for

Geology

Abstract.

Introduction Analytical methods may result in total concentration for a certain elements or result in a concentration for a certain species of an element. E.g. one might get the total arsenic concentration or the concentration for the arsenic species H2AsO4-. Species sensitive analytical methods are commonly more complex and thus time consuming and expensive. For some purposes, e.g. the check with respect to a maximum contamination level (MCL) or drinking water standard a total concentration is sufficient, since MCL´s and drinking water guidelines often use only total concentrations as criteria. However, since e.g. different arsenic species show differing toxicity and different mobility in the subsurface species sensitive analytic is necessary as well in some cases. The most toxic form of arsenic is gaseous arsine, followed by some ethylated As(III), inorganic As(III), inorganic As(V) and methylated As(V) compounds. On contrary arsenosugars are not toxic at all (Planer-Friedrich, 2004). The chemo toxic effect of uranium in humans is nephritis (kidney failure). This is caused by the precipitation of U(VI) in the proximal kidney tubules and the resulting tissue damage leads to kidney failure (Unsworth et al., 2005). Little is known about the toxicity of certain uranium(V)-species, however, the mobility of UO22+ is in particular different from an UO2SO40 species.

Analysis techniques Höll (2002) gives a good overview about analytical techniques. In the field of the elemental analysis, different techniques of atomic spectrometry are available. The important ones are atomic absorption spectrometry (AAS) and atomic emission spectrometry (AES). A combination of inductively coupled plasma (ICP) and

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mass spectrometry (MS) is excellent for higher requirements because it offers very good detection limits. Molecular spectrometry has been developed in ultraviolet and visible range (UV/VIS-Spectrometry). Fluorescence spectrometry and infrared spectrometry are the important ones. Spectrometry is based on interactions between electromagnetic radiation and matter at which only specific amounts of energy can absorb or emit. The result is an adsorption or emission spectra which can be measured with different detectors. In the field of chromatography, gas chromatography (GC), high-performance liquid chromatography (HPLC) and ion chromatography (IC) play important roles for elemental analysis. Common base of chromatography is the allocation from one mixture of materials to two different phases. One of the phases is immobile and the other one is mobile. The coefficient of allocation is a material specific factor. Differences of the coefficients of allocation influence the velocity of migration of components in the mobile phase causing a chromatographic separation. So the elements can be detected and quantified with specific detectors. The species of one element cannot be detected by means of atomic spectrometric methods. To sort out this problem, several techniques have to be combined. Mostly the combinations consists of a separation method (chromatographic or not chromatographic) and a spectrometric technique. With chromatographic separation methods (GC or HPLC) different species are separated from each other and then measured for example with AAS. With not chromatographic techniques e.g. the hydride techniques only one species or group of species is isolated from the sample and analysed apart from it.. Such combined methods have to satisfy high demands. Normally the total concentrations are in μg/l-rang, but after chromatographic separation the concentration from the single species are in the ng/l-range. The combined methods have to satisfy the following criteria: - Detection from all species of one element - no mutation of the species during analysis - high sensibility - simple treatment of the samples

Arsenic

Species of arsenic The most common detected arsenic species in environmental and biological samples are listed in Table1. In aquatic systems redox state of arsenic can be arsenite [As(III)] and arsenate [As(V)]. At low pE-values the As(III) species is predominant and at more higher pE-values As(V) is the dominant species (Fig.:1). Micro-

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organisms can methylate or dimethylate the dissolved inorganic arsenic (Table 1: green highlighted) over a wide range of pH and pE conditions (Table 1: red highlighted). For example they can produce monomethylarsonic acid, dimethylarsinic acid, trimethylarsine oxide, trimethylarsine and dimethylarsine [(CH3)2AsH]. In some geological systems, there are also volatile arsenic species (Table 1: yellow highlighted). Figure 1 shows also that in the pH range of natural waters (pH from 5 to 7), the predominant As(V) species are the anionic H2AsO4- and the HAsO42-. These species are often adsorbed by ferrihydrite. The predominant As(III) species in natural is the H3AsO30. The affinity of ferric hydroxide for dissolved As(III) is less than that for As(V). (Planer-Friedrich, 2004) Table 1. Arsenic species commonly detected in the environmental and biological system (after Gong et al., 2002) Name

Abbreviation

Chemical formula

Arsenite (arsenous acid) Arsenate (arsenic acid) Monomethylarsonic aicd Monomethylarsonous acid Dimethylarsenic acid Dimethylersinous acid Dimethylarsinoyl Ethanol Trimethylarsine Oxide Tetramethylarsoniumion Arsenobetaine

AsIII AsV MMAV MMAIII DMAV DMAIII DMAE

As(OH)3 AsO(OH)3 CH3AsO(OH)2 CH3As(OH)2 (CH3)2AsO(OH) (CH3)AsOH (CH3)2AsOCH2 CH2OH (CH3)3AsO

Arsenobetaine 2

AsB-2

Arsenochline

AsC

Trimethylarsine Arsines

TMAIII AsH3, MeAsH2, Me2AsH

Ethylmethylarsines

EtxAsMe3-x

TMAO Me4As+ AsB

(CH3)4As+ (CH3)3As+CH2 COO(CH3)3As+CH2 CH2COO(CH3)3As+CH2 CH2OH (CH3)3As (CH3)xAsH3-x x = 0-3 (CH3CH2)xAs (CH3CH2)3-x (x = 0-3

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Fig.1. Eh-pH diagram for the As-H2O system at 25°C. The area with the vertical bars represents the common pE-pH domain in natural water (Le et al., 1999)

Arsenic analysis techniques A good overview on arsenic analytical determination techniques is given by Gong et al., 2002. The most general used speciation techniques are often a combination of chromatographic separation and spectrometric detection. Separation For arsenic speciation the high-performance liquid chromatography (HPCL) is the most commonly method used. Gas chromatography (GC), supercritical fluid

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chromatography (SFC) and capillary electrophoresis (CE) are also applied to arsenic speciation but with a lesser importance. HPCL separation for arsenic speciation analysis include ion-pairing, ion-exchange and size exclusion. 1. HPCL – Ion-pair chromatography This method has been developed for routine analysis of neutral and ionic arsenic species. For separation of arsenic species are anion-pairing and cation-pairing chromatography techniques commonly. Resolution depends on the ion-pair reagent, the flow rate, ionic strength and pH of mobile phase. For example the optimum range for separation the four negatively charged species AsIII, DMAV, MMAV and AsV with Tetrabutylammonium (TBA) is between 5 and 7 (= natural water). However at this range, As(III) and AsB co-eluate. If the pH-value from As(III)-species is above 9.2, this species becomes negatively charged and As(III) species can be separated from AsB, because As(III) is weakly retained. For this separation, a resin-based column is used. Tetraethylammonium hydroxide (TEAH) is also used for separation of AsB. The positive charged arsenic species can be determine with Pentanesulfonate, hexanesulfonate, heptanesulfonate, and dodecylsulfonate. There are also mixed ion-pair chromatography containing hexanesulfonate and TEAH for separation AsIII, DMAV, MMAV, AsV, AsB, AsC and Me4As+. 2. HPCL – Ion-exchange chromatography Both anion and cation-exchange chromatography techniques have been used for arsenic speciation analysis. Depending on the ionic characteristics of the arsenic compounds, anion exchange is used to analyze AsIII, DMAV, MMAV and AsV and cation exchange is used to separate AsB, AsC, TMAO and Me4As+ species. The application from polymeric anion-exchange columns is very useful because they are stable for a pH range from 1 to 13. However the problem is also that As(III) and AsB co-elute under neutral pH conditions. To solve this problem, tartaric acid (C4H6O6) is used as mobile phase to separate As(III) from AsB, for now As(III) could form an anionic complex species. So in the order of AsC, AsB, DMAV, MMAV, AsIII and AsV, these arsenic species could be separated within 15min. Above a pH-value of 9, ammonium carbonate ((NH4)2CO3) can also be used as the mobile phase, because AsB and As(III) separate. The whole separation require 20min. Recently, a cation-exchange method was developed for the determination of DMAIII and is use to separate arsenosugars and their metabolites. 3. HPCL – Ion-exclusion and size-exclusion chromatography Strong anion –or cation-exchange resins are used for ion-exclusion chromatography to separate weakly ionized of neutral compounds. The charge on the ionexchange resin is the same as that of the weakly ionized species. Ion-exclusion chromatography has three types of interactions, ion exclusion, ion exchange and hydrophobic interaction. Resin-containing anionic sulfonate functional groups separate anionic arsenic species and a carboxylated methacrylate resin separates excellent AsIII, DMAV, MMAV and AsV and AsB. The five species were analyzed in 13min. However TMAO and AsC were not resolved form each other.

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4. CE By using capillary zone electrophoresis (CZE) AsIII, DMAV, MMAV and AsV, AsB and AsC could separate. However problems occured with As(V). At alkaline pH it could not be analyzed, because its electrophoretic mobility was higher than the velocity of the electroosmatic flow, at acidic pH analysis time is long. 5. GC Gas chromatography is used for separation different volatile species by sequential release due to boiling point differences. Detection All of the following methods can detect the element arsenic. Also in combination with chromatographic and non chromatographic separation techniques it can analyze arsenic species. Atomic spectroscopy provides the best sensitivity for arsenic detection. 1. Atomic absorption and inductively coupled plasma optical emission spectrometry (AAS und ICPAES) Flame atomic absorption spectroscopy (FAAS) can used as a detector after the HPCL, but it has a low sensitivity and high background noise for arsenic determination. Most recent application is combined with HG. Some studies describe the use of graphite furnace atomic absorption spectrometry (GFAAS) for HPCL detection. However, a direct coupling of HPCL and GFAAS is difficult, because a long analytical cycle is typical. Other than AAS the inductively coupled plasma atomic emission spectrometry (ICP-AES) has been successfully coupled to the HPCL. However, this system is good for samples with high concentrations of arsenic, but not suitable for environmental samples with lower concentrations of arsenic 2. Inductively coupled plasma-mass spectrometry (ICP-MS) A combination between HPCL and ICPMS shows some advantages, because of the extremely high sensitivity, multi-element capability, large dynamic range and the isotope ratio measurement capability. HPCL-ICPMS is now the most common technique in the field of coupled approaches. Couplings ICP-MS with CE is another option. However, two major drawbacks are the mismatch of sample volumes between CE and ICPMS and the suction generated by nebulisation. 3. Hydride generation (HG) with spectrometry HG is a chemical derivatization process that produces volatile hydrides from a sample with a reducing agent. Typically, sodium borohydride is applied as reducing agent. This technique coupled with atomic adsorption, atomic emission, atomic fluorescence, and mass spectrometry, low concentrations of arsenic species can be determined. However, not all arsenic species form hydrides. 4. Mass spectrometry (MS) Electro spray ionization (ES) MS is suited for speciation alone or in combination with HPCL. In contrast to ICP-MS, ICP-AES, AAS and AFS where elemental arsenic is detected, ES-MS provides molecular information of arsenic compounds. The ES-MS can identify organ arsenicals, such as arsenosugars and unknwon ar-

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senic species (e.g. sulphur-arsenic species), because it delivers structural information of molecules. X-ray fluorescence (XRF), X-ray adsorption near edge structure (XANES), secondary electron microscopy (SEM), secondary ion mass spectrometry (SIMS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), extended X-ray adsorption fine structure (EXAFS) and wide-angle X-ray scattering a well as infrared spectroscopy and alpha proton X-ray spectrometry (APXS) are as well used for species analytics.

Uranium

Species of uranium Uranium occurs in 4+ and 6+ oxidation states in aqueous systems At low Eh, U(IV) and its aqueous complexes predominate (Fig.: 2). The U(IV) concentrations in groundwater at low Eh are usually low because of the extremely low solubility of its solids. In the U(V) oxidation state, uranium occurs predominantly as UO2+ ion (Uranyl). At high Eh-values, the highly soluble uranyl ion and its complexes predominate. Uranyl ion forms in particular stable complexes with carbonate ions. Other U(VI) complexes are formed with fluoride, phosphate, and sulphate ligands, for example.(Table 2) (Langmuir, 1997)

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Fig.2. Eh-pH diagram for aqueous species in the U-O2-H2O system in pure water at 25°C and 1 bar total pressure for ∑U = 10-8 M. The UO2 (c) solid/solution boundary for ∑U = 10-8 M is stippled (after Langmuir, 1997)

Table 2. Uranium species commonly detected in the environmental system (Merkel 2006) Name Uranylion Uranylhydrospecies Uranylcarbonate species Uranylsulfate species Uranylphosphate species Uranylfluoride species Uranylchloride species Uranylsilicate species

Chemical formula UO22+ UOH3+, U(OH)22+, U(OH)3+, U(OH)40, U(OH)5-, U6(OH)159+, UO2OH+, (UO2)2(OH)22+, (UO2)3(OH)5+ UO2CO30, UO2(CO3)22-, UO2(CO3)34USO42+, UO2SO40, UO2(SO4)22-, U(SO4)20 UHPO42+, U(HPO4)20, U(HPO4)32-, U(HPO4)44-, UO2(HPO4)22-, UO2H2PO4+, UO2HPO40, UO2(H2PO4)20, UO2(H2PO4)3UF3+, UF22+, UF40, UF5-, UF62-, UO2F+, UO2F20, UO2F3-, UO2F42UO2Cl+, UCl3+ UO2H3SiO4+

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Uranium analysis techniques

Elemental analysis techniques Elemental analysis techniques for uranium are Neutron Activation Analysis (NAA), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Proton Induced X-ray Emission (PIXE) and X-ray Absorption Fine Structure (XAFS) (www.elementalanalysis.com).

Speciation techniques The most commonly used uranium speciation techniques are laser-induced spectroscopy and x-ray adsorption spectroscopy; less often used are vibrational spectroscopy, nuclear magnetic resonance, and UV-Vis adsorption. vibrational spectroscopy The linear dioxo cation O=U=O gives both IR and Raman bands. These bands are sensitive to coordination of U(VI) and have been applied for speciation. Applicability of Raman and IR spectroscopy to speciation of U(VI) in solution is limited, because relatively high concentrations in the mM range are needed. Hence, with respect to hydrolysis and coordination by carbonate, only Raman shifts for the species (UO2)2(OH)22+, (UO2)3(OH)5+ and UO2(CO3)34- could be determine. (Meinrath, 1997; Cornelis 2005) Nuclear magnetic resonance The UO2+ compound is only weakly paramagnetic and hence allows for nuclear magnetic resonance in aqueous solutions. Like the vibrational spectroscopies, nuclear magnetic resonance spectroscopy needs rather high U(VI) concentrations above 1mM. (Meinrath, 1997) For example by use of O-NMR spectroscopy and potentiometric titration, (UO2)2(OH)2(SO4)24-, (UO2)3(OH)4(SO4)34-, (UO2)4(OH)7(SO4)47and 10(UO2)5(OH)8(SO4)6 can be detected (Cornelis, 2005). UV-Vis absorption The UV-Vis adsorption technique is used for detection of the U(VI) and the U(VI) species in solution. It was shown that the single component spectra of the species UO2CO30, UO2(CO3)22-, UO2(CO3)34- (UO2)2(OH)22+ were derived from measurements with the UV-VIS absorption spectra (Cornelis, 2005).. Laser-induced spectroscopy Laser-induced studies of U(IV) show fluorescence properties of the element in this oxidation state. However, there are low detection limits of photon counting methods and despite the short fluorescence lifetime. Also it is possible to detect directly the speciation of the uranium(VI). Single fluorescence spectra of the hydrolysis species (UO2)2(OH)22+ and (UO2)3(OH)5+ are described as well. With LPAS, a coupled tunable laser, the complex formation of uranium with carbonate could be detect. The stopped flow spectrometry technique could affect the formation of uranium(VI)-diphosphonic acid species.(Cornelis, 2005)

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X-ray absorption spectroscopy EXAFS assign hydration number of 10 plus minus 1 for U(IV) considering structure information. It is often used for measurements of uranium complexes with carboxyl acid or humic acid (Nitsche et al. 1999)

Speciation with the help of thermodynamic models Hydrogeochemical modelling is based on complete and correct water chemical analysis It may be based on the assumption of a thermodynamically equilibrium or a kinetically controlled approach. For example, different species affect complexation and redox reactions that shows pronounced kinetic and so there is likely disequilibrium for long space of time. Another but also important source of error is located is the thermodynamic datasets. These datasets give basic geochemical information about the individual species. Different modeling programs may use different thermodynamic datasets with different solubility products, different species, minerals and chemical equations. For some species doubtful complexation constants are published and in some cases even the existence of species publishes is not verified. Grenthe et al. 1992 [NEA] and Fuge et al. 1992 [IAEA]) give e.g. different numbers for U (VI)hydroxospecies (Table 3). This differences have a significant impact on the speciation calculation from total uranium concentration. Table 3. Dissoziationconstants for U (VI)-hydroxospecies Species

NEA (92) log (K)

IAEA (92) log (K)

UO2OH+ UO(OH)20 (UO2)2(OH)22+ (UO2)3(OH)5+ (UO2)3(OH)2+ (UO2)2(OH)3+ (UO2)4(OH)7+ UO2(OH)3(UO2)3(OH)7UO2(OH)42-

-5,2