Pentavalent State Research Articles

Adjustment of valence state of Pu and Np in nitric solution containing N,N-dimethylhydroxylamine and monomethylhydrazine by electrolysis

Abstract A study of the electrochemical behaviors of N,N-dimethylhydroxylamine (DMHAN) and monomethyl hydrazine (MMH) in nitric acid solution on Pt electrodes were carried out by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) methods. The diffusion coefficients of DMHAN and MMH were obtained by CV. The values are found to be 0.53 × 106 cm2/s D(DMHAN) and 0.88 × 105 cm2/s D(MMH). The equilibrium potentials (+0.47 V vs. SCE for DMHAN and +0.31 V vs. SCE for MMH) were also measured using Tafel curves. Various valence states of Pu and Np in HNO3 solution containing DMHAN and MMH in the electrolytic process were investigated by an electrolytic cell using a platinum as the anode and a titanium plate as the cathode. In this procedure, MMH was first electro-oxidized on the Pt anode and Np(V) was reduced to Np(IV) on the Ti cathode. After MMH was entirely consumed, the accumulation of HNO2 (due to the electrochemical reduction of nitric acid on the Ti cathode) caused a significantly fast catalytic reaction of DMHAN with HNO3 to form HNO2. HNO2 can oxidize Pu(III) to Pu(IV) quickly. As a result, both oxidation states of Pu and Np were found to be tetravalent in the post-electrolysis solution. A convenient method to keep the post-electrolysis solution at 70 ºC was used to adjust the oxidation state of Np in it's pentavalent state while retaining the tetravalent state of Pu. This study developed an electrolytic process for the preparation of 2AF feed (the feed of Pu purification cycle) in APOR (Advanced Purex Process Based on Organic Reductants) process by electrochemically oxidizing Pu(III) and selectively adjusting the valence state of neptunium to either Np(IV) or Np(V).

Microbial arsenic reduction in polluted and unpolluted soils from Attica, Greece

Indigenous soil microorganisms often affect the mobility of heavy metals and metalloids by altering their oxidation state. Under anaerobic conditions, the microbial transformation is usually reduction and may cause the mobilization of contaminants, as happens in the case of arsenic, which is much more stable in the pentavalent state compared to the reduced trivalent form. The aim of this work was to investigate the occurrence of such a microbial activity in representative Greek soils. Five soil samples, with As levels varying between 14 and 259mg/kg, were examined. The samples were artificially contaminated, by adding 750mg of As(V) per kg of soil. Initial sorption of As(V) ranged between 70 and 85%. Microbial reduction of arsenic was observed in three of the examined soils, without any obvious correlation with pre-existing levels of contamination. Reduction reached high percentages, i.e. up to 99%, and was accompanied by the corresponding release of reduced As in the aqueous solution. A simultaneous iron reducing activity was also observed in four of the five soil samples.

Mineralogic controls on aqueous neptunium(V) concentrations in silicate systems

The presence of radioactive neptunium in commercially spent nuclear fuel is problematic due to its mobility in environmental systems upon oxidation to the pentavalent state. As uranium is the major component of spent fuel, incorporation of neptunium into resulting U(VI) mineral phases would potentially influence its release into environmental systems. Alternatively, aqueous neptunium concentrations may be buffered by solid phase Np2O5. In this study, we investigate both of these controls on aqueous neptunium(V) concentrations. We synthesize two uranyl silicates, soddyite, (UO2)2SiO4·2H2O, and boltwoodite, (K, Na)(UO2)(SiO3OH)·1.5H2O, each in the presence of two concentrations of aqueous Np(V). Electron microscopy and electron diffraction analyses of the synthesized phases show that while significant neptunyl incorporation occurred into soddyite, the Np(V) in the boltwoodite systems largely precipitated as a secondary phase, Np2O5(s). The release of Np(V) from each system into aqueous solution was measured for several days, until steady-state concentrations were achieved. Using existing solubility constants (Ksp) for pure soddyite and boltwoodite, we compared predicted equilibrium aqueous U(VI) concentrations with the U(VI) concentrations released in the solubility experiments. Our experiments reveal that Np(V) incorporation into soddyite increases the concentration of aqueous U in equilibrium with the solid phase, perhaps via the formation of a metastable phase. In the mixed boltwoodite – Np2O5(s) system, the measured aqueous U(VI) activities are consistent with those predicted to be in equilibrium with boltwoodite under the experimental conditions, a result that is consistent with our conclusion that little Np(V) incorporation occurred into the boltwoodite. In the boltwoodite systems, the measured Np concentrations are likely controlled by the presence of Np2O5 nanoparticles, suggesting an additional potential mobility vector for Np in geologic systems. Our results demonstrate that in systems containing solid phases that cannot incorporate significant concentrations of Np(V), secondary precipitates such as Np2O5(s) are likely to control aqueous neptunium concentrations, and that uranium concentrations are buffered by the uranyl mineral assemblage present. For systems containing uranyl mineral phases such as soddyite, which can incorporate significant concentrations of Np(V), Np2O5(s) precipitation may be suppressed and the Np-bearing uranyl phase may act as a sink and a buffer for aqueous Np(V) in oxidizing environments.

Visible Light Active Phosphorus-Doped TiO2 Nanoparticles: An EPR Evidence for the Enhanced Charge Separation

Phosphorus-doped TiO2 nanoparticles with visible light activity were prepared by sol–gel method by using Ti(IV) isopropoxide and phosphoric acid as precursors. As prepared samples were calcined at different temperatures, and the obtained samples were characterized by X-ray diffraction, UV–vis spectroscopy, Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy, scanning emission microscopy, electron paramagnetic resonance (EPR) spectroscopy, the photodegradation of methyleneblue (MB). The results indicate that phosphorus-doping into TiO2 lattice decreases the particle size, increases the thermal stability of titania, and retards the phase transition from anatase to rutile. UV–vis absorption of the P-doped samples shows the redshift in its absorption edge. Doped phosphorus exists in a pentavalent oxidation state by replacing part of lattice Ti4+ by the formation of Ti–O–P bonds. MB degradation profiles with visible light irradiation show that the photocatalytic activity of P-doped Titania is much enhanced and superior to undoped TiO2 and commercial Degussa P25. Low temperature EPR studies with in situ visible light irradiation on the samples heated at different temperatures clearly demonstrates that enhanced charge separation is the major reason for the enhanced photocatalytic activity.

Conserved phosphoryl transfer mechanisms within kinase families and the role of the C8 proton of ATP in the activation of phosphoryl transfer

BackgroundThe kinome is made up of a large number of functionally diverse enzymes, with the classification indicating very little about the extent of the conserved kinetic mechanisms associated with phosphoryl transfer. It has been demonstrated that C8-H of ATP plays a critical role in the activity of a range of kinase and synthetase enzymes.ResultsA number of conserved mechanisms within the prescribed kinase fold families have been identified directly utilizing the C8-H of ATP in the initiation of phosphoryl transfer. These mechanisms are based on structurally conserved amino acid residues that are within hydrogen bonding distance of a co-crystallized nucleotide. On the basis of these conserved mechanisms, the role of the nucleotide C8-H in initiating the formation of a pentavalent intermediate between the γ-phosphate of the ATP and the substrate nucleophile is defined. All reactions can be clustered into two mechanisms by which the C8-H is induced to be labile via the coordination of a backbone carbonyl to C6-NH2 of the adenyl moiety, namely a "push" mechanism, and a "pull" mechanism, based on the protonation of N7. Associated with the "push" mechanism and "pull" mechanisms are a series of proton transfer cascades, initiated from C8-H, via the tri-phosphate backbone, culminating in the formation of the pentavalent transition state between the γ-phosphate of the ATP and the substrate nucleophile.ConclusionsThe "push" mechanism and a "pull" mechanism are responsible for inducing the C8-H of adenyl moiety to become more labile. These mechanisms and the associated proton transfer cascades achieve the proton transfer via different family-specific conserved sets of amino acids. Each of these mechanisms would allow for the regulation of the rate of formation of the pentavalent intermediate between the ATP and the substrate nucleophile. Phosphoryl transfer within kinases is therefore a specific event mediated and regulated via the coordination of the adenyl moiety of ATP and the C8-H of the adenyl moiety.

Arsenic exposure and toxicology: a historical perspective.

The metalloid arsenic is a natural environmental contaminant to which humans are routinely exposed in food, water, air, and soil. Arsenic has a long history of use as a homicidal agent, but in the past 100 years arsenic, has been used as a pesticide, a chemotherapeutic agent and a constituent of consumer products. In some areas of the world, high levels of arsenic are naturally present in drinking water and are a toxicological concern. There are several structural forms and oxidation states of arsenic because it forms alloys with metals and covalent bonds with hydrogen, oxygen, carbon, and other elements. Environmentally relevant forms of arsenic are inorganic and organic existing in the trivalent or pentavalent state. Metabolism of arsenic, catalyzed by arsenic (+3 oxidation state) methyltransferase, is a sequential process of reduction from pentavalency to trivalency followed by oxidative methylation back to pentavalency. Trivalent arsenic is generally more toxicologically potent than pentavalent arsenic. Acute effects of arsenic range from gastrointestinal distress to death. Depending on the dose, chronic arsenic exposure may affect several major organ systems. A major concern of ingested arsenic is cancer, primarily of skin, bladder, and lung. The mode of action of arsenic for its disease endpoints is currently under study. Two key areas are the interaction of trivalent arsenicals with sulfur in proteins and the ability of arsenic to generate oxidative stress. With advances in technology and the recent development of animal models for arsenic carcinogenicity, understanding of the toxicology of arsenic will continue to improve.

Nanocrystals of CeVO4 Doped by Metallic Heteroions

CeVO(4) nanocrystals doped by heteroions were prepared via a hydrothermal method without the presence of surfactants or templates. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), solid state (51)V NMR, and inductively coupled plasma (ICP) were used to characterize the morphology, structure, and compositions of the materials. X-ray photoelectron spectroscopy (XPS) results confirmed that there is a valence change from Ce(3+) to Ce(4+) for a fraction of cerium atoms whereas the vanadium atoms remain in the pentavalent state V(5+) upon the substitution of Ca(2+) into CeVO(4). Raman spectroscopy was used to monitor the effects of the doping ion on the CeVO(4) lattice contraction and distortion. The appearance of the shifted and broadened Raman peaks for the doped CeVO(4) was interpreted by theoretical calculations performed with Vienna ab initio simulation package. The redox properties and photocatalytic activities of the obtained nanocrystals were also investigated and discussed in detail.

Synthesis, Structural Characterization, and Electrical Properties of (Sr0.5Ca0.5)(Ca0.5Sb0.5)O3−δ Double Perovskites

Double perovskites which exhibit 1:2 B-site ordering are under focus because of their interesting structure−property relationships and high chemical versatility. Perovskites with a big cation, such as Sr2+ or Ca2+, occupying the octahedral B site are very scarce because the calculated Goldschmidt tolerance factor becomes quite small as these types of cations usually occupy the A-site. This work describes the synthesis and structural and microstructural characterization of a new calcium and strontium antimoniate which shows face-centered cubic symmetry and a rock-salt ordered distribution of Sb5+ and Sr2+. The ordered-cation distribution occurs due to the differences in both the ionic sizes and the bonding character of the two B-site cations. As a consequence of the cation stoichiometry of the title phase and assuming a pentavalent state for Sb, a large concentration of oxygen vacancies are created (y ∼ 0.25). Diffraction studies reveal that these vacancies tend to arrange in a short-ordered way and are re...

First-principles investigation of higher oxides of uranium and neptunium:U3O8and Np2O5

A computational study is presented of the structural, electronic, and magnetic properties of ${\mathrm{U}}_{3}$O${}_{8}$ and Np${}_{2}$O${}_{5}$, which are actinide oxides in a higher oxidation state than the tetravalent state of the common dioxide phases, UO${}_{2}$ and NpO${}_{2}$. The calculations are based on the density functional theory+$U$ approach, in which additional Coulomb correlations on the actinide atom are taken into account. The calculated properties of these two higher oxidized actinide oxides are analyzed and compared to those of their tetravalent analogs. The optimized structural parameters of these noncubic oxides are found to be in reasonable agreement with available experimental data. ${\mathrm{U}}_{3}$O${}_{8}$ is predicted to be a magnetic insulator, having one U atom in a hexavalent oxidation state and two U atoms in a pentavalent oxidation state. For Np${}_{2}$O${}_{5}$, which is also predicted to be an insulator, a complicated noncollinear magnetic structure is computed, leading to a nonzero overall magnetization with a slight antiferromagnetic canting. The calculated electronic structures are presented and the variation of the U $5f$ or Np $5f$--O $2p$ hybridization with the oxidation state is analyzed. With increasing oxygen content, the nearly localized 5$f$ electrons of the actinide elements are more positioned near the Fermi level and the hybridization between 5$f$ and 2$p$ states is markedly increased.

Quantitative HPLC-ICP-MS analysis of antimonyredox speciation in complex sample matrices: new insights into the Sb-chemistry causing poor chromatographic recoveries

In solution antimony exists either in the pentavalent or trivalent oxidation state. As Sb(III) is more toxic than Sb(V), it is important to be able to perform a quantitative speciation analysis of Sb's oxidation state. The most commonly applied chromatographic methods used for this redox speciation analysis do, however, often show a low chromatographic Sb recovery when samples of environmental or biological origin are analysed. In this study we explored basal chemistry of antimony and found that formation of macromolecules, presumably oligomeric and polymeric Sb(V) species, is the primary cause of low chromatographic recoveries. A combination of HPLC-ICP-MS, AFFF-ICP-MS and spin-filtration was applied for analysis of model compounds and biological samples. Quantitative chromatographic Sb redox speciation analysis was possible by acidic hydrolysis of the antimony polymers prior to analysis. Sample treatment procedures were studied and the optimum solution was acidic hydrolysis by 1 M HCl in the presence of chelating ligands (EDTA, citrate), which stabilise the trivalent oxidation state of Sb.

Sensing properties of sprayed antimony doped tin oxide thin films: Solution molarity

Transparent conductive thin films of nanocrystalline Sb:SnO 2 have been deposited onto preheated glass substrates by using spray pyrolysis technique. The effect of the solution molarity on structural, morphological, optoelectronic properties of Sb:SnO 2 films has been investigated. XRD study reveals that films are polycrystalline with tetragonal crystal structure having average crystallite size about 20 nm. The compact and homogeneous grains are seen in FESEM images. The BEs of Sn 3d 5/2 for all samples show the Sn 4+ bonding state for SnO 2. The BEs of Sb 3d 5/2 are in the range of 530.6–530.9 eV, indicating that all antimony detected is in a pentavalent state (Sb 5+). Transparency of films in the visible region decreases with increase in precursor concentration. Photoluminescence study shows the strong violet and weak orange emission. The sensing properties of the Sb:SnO 2 films for acetone, ethanol and LPG with operating temperature and gas concentration have been investigated.

ChemInform Abstract: Potentiometric Determination of Stabilities of NiSb2O4 and NiSb2O6.

Abstract An isothermal section of the phase diagram of the system Ni–Sb–O at 873 K was established by isothermal equilibration and XRD analyses of quenched samples. Two ternary oxides, namely NiSb2O4 with Sb in trivalent and NiSb2O6 with Sb in pentavalent state, were identified. Making use of coexistence of NiSb2O4 with NiO and Sb, as well as NiSb2O6 with NiO and NiSb2O4, the following galvanic cells were designed to measure the Gibbs energies of formation of the ternary oxides: I Chromel, Mo, Sb, NiO, NiSb 2 O 4 ∣15 CSZ∣NiO, Ni, Mo, Chromel II Pt, NiO, NiSb 2 O 4 , NiSb 2 O 6 ∣15 CSZ∣air ( P O 2 =0.21 atm), Pt where 15 CSZ stands for ZrO2 stabilized by 15 mol% CaO The reversible emfs of the cells were measured over the temperature ranges 769–931 K and 894–1138 K respectively. The results could be represented by the following expressions: E±0.3 ( mV )=88.3–0.1043 T ( K ) E±0.3 ( mV )=1053.5–0.5544 T ( K ) The standard Gibbs energies of formation of NiSb2O4 and NiSb2O6 were computed from the emfs: Δ G° f (NiSb 2 O 4 )±4.2 (kJ mol −1 )=−988.3+0.3472 T (K) (769–931 K) Δ G° f (NiSb 2 O 6 )±4.3 (kJ mol −1 )=−1394.9+0.5472 T (K) (894–1138 K) The reasonability of the above data were assessed by computing the entropy change for the solid–solid reactions leading to the formation of ternary oxides from the respective pairs of constituent binary oxides. It was found that NiO forms a much more stable compound with Sb2O5 than with Sb2O3.

Natural attenuation of arsenic in the Tinto Santa Rosa acid stream (Iberian Pyritic Belt, SW Spain): The role of iron precipitates

Acid waters and sediments of the Tinto Santa Rosa acid stream (Iberian Pyritic Belt; SW, Spain) were analysed to determine the role of sedimentary phases in the behaviour of arsenic. Aqueous arsenic and iron concentrations decreased markedly from the adit mouth to 300 m downstream indicating iron minerals precipitation as well as arsenic sorption onto these newly-formed phases. This was confirmed by the high arsenic concentrations observed in bed-stream precipitates, which play a major role in controlling arsenic mobility. To unravel the complex nature of the AMD sediments a combination of techniques including X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), total solid digestions, X-Ray Fluorescence (XRF) and X-Ray Absorption Spectroscopy (XAS) were employed. Results showed that (1) arsenic was present predominantly in its pentavalent state; (2) upstream arsenic was sorbed onto the main phase, schwertmannite, whereas downstream it was chiefly associated with goethite and jarosite; and (3) changes in arsenic speciation with depth were observed in the consolidated terrace sediments, where arsenic appeared primarily associated with schwertmannite in the upper part of the terraces, but with goethite at depth. Arsenic mobilization was controlled by sorption onto newly formed precipitates (schwertmannite, goethite and jarosite), causing natural arsenic attenuation.

Mechanism of Am(III) oxidation with ozone in carbonate solutions

The reaction of ozone with Am(III) in bicarbonate and carbonate solutions was studied by spectrophotometry. On adding ozone-saturated water to a 2 × 10−4 M Am(III) solution in 1 M NaHCO3, about 1/3 of Am remains in the trivalent state and 2/3 is converted to Am(V), with no accumulation of Am(IV). The reaction of Am(III) with ozone involves replacement of H2O molecules in the coordination sphere of Am(III) by O3 molecule, followed by elimination of the O2 molecule. The third O atom remains bonded with Am, converting it to the pentavalent state.

Inside Cover: Stable Pentavalent Uranyl Species and Selective Assembly of a Polymetallic Mixed-Valent Uranyl Complex by Cation-Cation Interactions (Angew. Chem. Int. Ed. 45/2009)

Oligomeric cation–cation complexes of actinyls are highly relevant for nuclear waste reprocessing and the migration of actinides in the environment. In their Communication on page 8477 ff., M. Mazzanti et al. show that the N,N′-bis(salicylidene)ethylenediamine (salen) ligand can stabilize uranium in the pentavalent state. A tetrametallic cation–cation complex shows stability towards disproportionation and hydrolysis, and an unambiguous antiferromagnetic coupling between the four oxo-bridged uranium centers.

Biological Responses to Arsenic Compounds

Arsenic is a metalloid that generates various biological effects on cells and tissues. Depending on the specific tissue exposed and the time and degree of exposure, diverse responses can be observed. In humans, prolonged and/or high dose exposure to arsenic can have a variety of outcomes, including the development of malignancies, severe gastrointestinal toxicities, diabetes, cardiac arrhythmias, and death. On the other hand, one arsenic derivative, arsenic trioxide (As(2)O(3)), has important antitumor properties. This agent is a potent inducer of antileukemic responses, and it is now approved by the Food and Drug Administration for the treatment of acute promyelocytic leukemia in humans. The promise and therapeutic potential of arsenic and its various derivatives have been exploited for hundreds of years. Remarkably, research focused on the potential use of arsenic compounds in the treatment of human diseases remains highly promising, and it is an area of active investigation. An emerging approach of interest and therapeutic potential involves efforts to target and block cellular pathways activated in a negative feedback manner during treatment of cells with As(2)O(3). Such an approach may ultimately provide the means to selectively enhance the suppressive effects of this agent on malignant cells and render normally resistant tumors sensitive to its antineoplastic properties.

Violation of Vavilov’s law upon excitation of luminescence of uranyl solutions at the short-wavelength absorption band (200–240 nm)

It was found that in the UV spectral region (200–240 nm) where intense absorption bands of the UO22+ ion are located, excitation of its luminescence in solutions is not observed, thereby contradicting Vavilov’s law about independence of luminescence quantum yields from the excitation light wavelength. The violation of Vavilov’s law is explained in terms of nonradiative deactivation processes as the result of photoinduced electron transfer to the uranyl ion with its reduction to the pentavalent state and the subsequent disproportionation reaction to form uranium(IV). The presence of uranium(IV) ions during UV irradiation of uranyl solutions was proved by the chemiluminescent method.

State of Doped Phosphorus and Its Influence on the Physicochemical and Photocatalytic Properties of P-doped Titania

A series of phosphorus-doped titania samples with different phosphorus contents have been prepared by a simple, modified sol−gel method. The obtained samples were characterized by X-ray diffraction spectrometry, Brunauer−Emmett−Teller surface area, differential thermal analysis-thermogravimetry, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and UV−vis diffuse reflectance spectroscopy, and the photodegradation of methylene blue (MB) on the samples was tested under both UV and visible light irradiation. It was found that the doped phosphorus existed in a pentavalent oxidation state, replacing part of Ti4+ in the anatase lattice in the form of Ti−O−P bonds. Increased calcination temperature gave an enrichment of P species on the titania surface. The P species in the titania lattice should be the dominant group responsive to visible light, whereas the P species on the surface could retard the phase transition of anatase to rutile and increase the surface area and adsorption capability. The photocatalytic activity of P-doped titania with the optimal P content was 4-fold higher than that of pure TiO2 and was 41% higher than that of commercial Degussa P-25 under UV irradiation. Furthermore, P-doped titania presented significant photocatalytic activity under visible light (>400 nm) irradiation.

Arsenic neurotoxicity — A review

Arsenic (As) is one of the oldest poisons known to men. Its applications throughout history are wide and varied: murder, make-up, paint and even as a pesticide. Chronic As toxicity is a global environmental health problem, affecting millions of people in the USA and Germany to Bangladesh and Taiwan. Worldwide, As is released into the environment by smelting of various metals, combustion of fossil fuels, as herbicides and fungicides in agricultural products. The drinking water in many countries, which is tapped from natural geological resources, is also contaminated as a result of the high level of As in groundwater. The environmental fate of As is contamination of surface and groundwater with a contaminant level higher than 10 particle per billion (ppb) as set by World Health Organization (WHO). Arsenic exists in both organic and inorganic species and either form can also exist in a trivalent or pentavalent oxidation state. Long-term health effects of exposure to these As metabolites are severe and highly variable: skin and lung cancer, neurological effects, hypertension and cardiovascular diseases. Neurological effects of As may develop within a few hours after ingestion, but usually are seen in 2-8 weeks after exposure. It is usually a symmetrical sensorimotor neuropathy, often resembling the Guillain-Barré syndrome. The predominant clinical features of neuropathy are paresthesias, numbness and pain, particularly in the soles of the feet. Electrophysiological studies performed on patients with As neuropathy have revealed a reduced nerve conduction velocity, typical of those seen in axonal degeneration. Most of the adverse effects of As, are caused by inactivated enzymes in the cellular energy pathway, whereby As reacts with the thiol groups of proteins and enzymes and inhibits their catalytic activity. Furthermore, As-induced neurotoxicity, like many other neurodegenerative diseases, causes changes in cytoskeletal protein composition and hyperphosphorylation. These changes may lead to disorganization of the cytoskeletal framework, which is a potential mechanism of As-induced neurotoxicity.

Non-chromatographic hydride generation atomic spectrometric techniques for the speciation analysis of arsenic, antimony, selenium, and tellurium in water samples—a review

Hydride generation (HG) coupled with AAS, ICP–AES, and AFS techniques for the speciation analysis of As, Sb, Se, and Te in environmental water samples is reviewed. Careful control of experimental conditions, offline/online sample pretreatment methods employing batch, continuous and flow-injection techniques, and cryogenic trapping of hydrides enable the determination of various species of hydride-forming elements without the use of chromatographic separation. Other non-chromatographic approaches include solvent extraction, ion exchange, and selective retention by microorganisms. Sample pretreatment, pH dependency of HG, and control of NaBH4/HCl concentration facilitate the determination of As(III), As(V), monomethylarsonate (MMA), and dimethylarsinate (DMA) species. Inorganic species of arsenic are dominant in terrestrial waters, whereas inorganic and methylated species are reported in seawater. Selenium and tellurium speciation analysis is based on the hydrides generation only from the tetravalent state. Se(IV) and Se(VI) are the inorganic selenium species mostly reported in environmental samples, whereas speciation of tellurium is rarely reported. Antimony speciation analysis is based on the slow kinetics of hydride formation from the pentavalent state and is mainly reported in seawater samples.