Raman spectroscopy of some complex arsenate minerals-implications for soil remediation

The application of spectroscopy to the study of contaminants in soils is important. Among the many contaminants is arsenic, which is highly labile and may leach to non-contaminated areas. Minerals of arsenate may form depending upon the availability of specific cations for example calcium and iron. Such minerals include carminite, pharmacosiderite and talmessite. Each of these arsenate minerals can be identified by its characteristic Raman spectrum enabling identification.     

Mass spectrometric evidence for different complexes of peptides and proteins with arsenic(III), arsenic(V), copper(II), and zinc(II) species

Trivalent and pentavalent arsenic were incubated with sulfur-containing amino acid, peptide and protein solutions both as organic compounds (phenylarsine oxide, phenylarsonic acid, dimethylarsinic acid, monomethylarsonic acid) and as inorganic compounds (arsenite, As(III), and arsenate, As(V)). After incubation of phenylarsine oxide solutions with cysteine and glutathione the mass spectra showed a covalent bond between arsenic and sulfur, which was stable at both acidic and neutral pH values. The mass spectra were dominated by monovalent ions at m/z 272 for cysteine samples and at m/z 458 for glutathione samples. Based on these masses the ionic structures could be ascribed to either fragment ions of the covalent arsenic-sulfur complexes or to other arsenic-bonding sites presumably at the amino group. Interestingly, under the same conditions no interactions of inorganic arsenite or arsenate could be measured. In the presence of added Cu(2+) ions all mass signals caused by a reaction of phenylarsine oxide with glutathione disappeared. In these mass spectra only the oxidised form of glutathione (GSSG) was found because of the redox activity of Cu(II). For the model protein lysozyme, no interactions with arsenic could be detected, whereas definite Cu- and Zn-lysozyme complexes with a stoichiometry of 1:1 and 2:1 for Zn(2+) ions and Cu(2+) ions, respectively, were observed. In contrast, for thioredoxin a bonding of As that depended on the concentration of the disulfide-reducing agent tris(2-carboxyethyl) phosphine was demonstrated.For three different phenylarsonic acids and for dimethylarsinic acid that all contain pentavalent arsenic, complexes with glutathione appeared in the mass spectra, which can be attributed to non-covalent interactions or to a covalent bond caused by an additive reaction.The optimisation of the experimental conditions necessary for the mass spectrometric analysis of the interactions of the arsenic species with peptides and proteins is described and the obtained mass spectra that provide information on the kinds of bonds are discussed.     

Analytical speciation as a tool to assess arsenic behaviour in soils polluted by mining

A study is performed to evaluate the occurrence of arsenic in polluted soils using acidic extractions and liquid chromatography–hydride generation–atomic fluorescence spectrometry (LC–HG–AFS) for speciation analysis. Seven soil samples were collected in an abandoned area polluted by mining in the Eastern Pyrenees (Spain), and two uncontaminated soils were taken for reference purposes. Moreover, the total arsenic content is evaluated in two different sieved fractions in order to obtain information on the possible particle-size-dependent association of arsenic with soil components. Soil samples were extracted with both phosphoric and ascorbic acids and the stabilities of the extracted species were studied. The arsenic species were determined by LC–HG–AFS. In addition, the ability of soil grinding to effect species change is also assessed. Arsenite and arsenate were found in the polluted soils, but only arsenate was found in the unpolluted soils. The quality of the results was assessed through a mass balance calculation and by analysing two soil Certified Reference Materials. Valuable information regarding arsenic occurrence in the studied soils is obtained from the speciation results. The presence of arsenite in the extracts can be attributed to arsenopyrite residues, whereas the presence of arsenate indicates release from weathered material. Figure Abandoned mining polluted area in Eastern Pyrenees     

Topical isotopic exchange and compartmental analysis approach for probing solute behaviour at the soil/arsenate solution interface

Isotopic exchange based approaches have for many years been applied in soil and solute research. However, acquiring and elaboration of experimental data were not always straightforward and complete. A strict and correct use of combined isotopic exchange-compartmental analysis may widen the knowledge database and provide information not available as yet. The experiments were carried out with arsenic (arsenate) from IAEA-SOIL-5 in contact with water or phosphate solution in dynamic equilibrium. After contacting the soil suspension for 28 days, the amount of arsenate released is 2.8 and 6.3 % of arsenic (solutes) in the soil, respectively. Addition of a radioactive arsenate (73)As(V)-spike and following the distribution of this radiotracer from the aqueous to the solid phase in time shows that the accessible fraction, i.e. available for exchange, is in both cases 12%. This implies that the remainder of the arsenic is enclosed in the lattice of minerals and for that reason unavailable for exchange, at least on the time scale of the experiment (weeks). From deconvolution of compartmental analysis results the distribution of accessible arsenate in the soil could be attributed to sorption onto external surfaces (2.6 and 2.0% of total arsenic present for the water and phosphate system, respectively) and sorption onto internal surfaces after diffusion through soil particle pores (6.5 and 4.2% of total arsenic present for the water and phosphate system, respectively). The mean residence time in two out of three compartments was in the order of minutes for the external surfaces and in the order of days for the diffusion-controlled internal surfaces.     

Determination of urinary arsenic and impact of dietary arsenic intake

An analytical method based on microwave decomposition and flow injection analysis (FIA) coupled to hydride generation atomic absorption spectrometry (HGAAS) is described. This is used to differentiate arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) from organoarsenic compounds usually present in seafood. Without microwave digestion, direct analysis of urine by HGAAS gives the total concentration of As(III), As(V), MMA and DMA because organoarsenic compounds such as arsenobetaine, usually found in most seafood, are not reducible upon treatment with borohydride and therefore cannot be determined by using the hydride generation technique. The microwave oven digestion procedure with potassium persulfate and sodium hydroxide as decomposition reagents completely decomposes all arsenicals to arsenate and this can be measured by HGASS. Microwave decomposition parameters were studied to achieve efficient decomposition and quantitative recovery of arsenobetaine spiked into urine samples. The method is applied to the determination of urinary arsenic and is useful for the assessment of occupational exposure to arsenic without intereference from excess organoarsenicals due to the consumption of seafood. Analysis of urine samples collected from an individual who ingested some seafood revealed that organoarsenicals were rapidly excreted in urine. After the ingestion of a 500-g crab, a 10-fold increase of total urinary arsenic was observed, due to the excretion of organoarsenicals. The maximum arsenic concentration was found in the urine samples collected approximately between 4 to 17 hr after eating seafood. However, the ingestion of organoarsenic-containing seafoods such as crab, shrimp and salmon showed no effect on the urinary excretion of inorganic arsenic, MMA and DMA.     

Arsenic speciation in iron hydroxide precipitates

In this study a special sequential extraction method is proposed to discriminate between arsenic adsorbed and co-precipitated in precipitates arising mainly from iron hydroxides or bound in low solubility mineral phases. Synthetic iron hydroxide precipitates were prepared to investigate the influence of the amount of arsenate, of the manganese additionally added and of the valence state of arsenic on the remobilisation of arsenic. After preparing the precipitates with arsenate no arsenic could be detected in the supernatant solution. About 82% (w/w) of the arsenate is adsorbed to the precipitate and the remaining part can be dissolved by shaking with an oxalate buffer. A significant difference between the amount of arsenic added and the amount analysed in the two steps was not found. Consequently, compounds with a low solubility, such as scorodite, were not formed in the synthesized precipitates. The valence of the arsenic and addition of manganese influence significantly the uptake of arsenic by iron hydroxides. Natural precipitate samples from a percolate water of tin mill tailings were investigated using this method.     

Adsorption of Inorganic and Organic Arsenic Compounds by Aluminium-loaded Coral Limestone

Coral limestones were treated with an aqueous solution of aluminium sulfate and thereby aluminium-loaded coral limestones (Al-CL) were prepared. By use of Al-CL as an adsorbent, the adsorption of inorganic arsenic compounds (arsenate [As(V)] and arsenite [As(III)] and of organic arsenic compounds (methylarsonic acid, dimethylarsinic acid, and arsenobetaine) was examined. The adsorption ability of Al-CL is superior to that of iron(III)-loaded coral limestone (Fe-CL) for As(V), As(III), methylarsonic acid and dimethylarsinic acid. The adsorption of As(V) and As(III) is almost independent of the initial pH over a wide range (2 or 3 to 11). The addition of other anions, such as chloride, nitrate, sulfate and acetate, in the solution does not affect the adsorption of As(V) and As(III), whereas the addition of phosphate greatly interferes with the adsorption. Arsenic adsorption is effectively applied to a column-type operation and the adsorption capability for As(V) is 150 μg/g coral limestone.     

Thermal decomposition of the vivianite arsenates—implications for soil remediation

The presence of arsenate compounds in soils and mineral dump leachates is common. One potential method for the removal of the arsenates from soils is through thermal treatment. High-resolution thermogravimetric analysis has been used to follow this thermal decomposition of selected vivianite arsenates. This decomposition occurs as a series of steps. The first two steps involve dehydration with 6 mol of water lost in the first step and two in the second. The third major weight loss step occurs in the 750-800 °C temperature range with de-arsenation. The application of infrared emission spectroscopy confirms the loss of water by around 250 °C and the loss of arsenic as arsenic pentoxide is observed by the loss of AsO stretching bands at around 826 cm⁻¹. Thermal activation of arsenic contaminated soils may provide a method of decontamination.     

白烟灰的物相分析及酸浸脱砷工艺

白烟灰因其含有砷等有毒化合物,其安全堆存是冶金企业面临的难题。 对白烟灰进行物相分析是处理该烟灰,使其达到环保要求堆存的重要依据。 采用X射线光电子能谱(XPS)和X射线粉末衍射(XRD)研究了河南某企业白烟灰的物相组成及化学结合状态,确定其所含化学成分及元素含量。 结果表明,样品中元素As相对含量高达20.99%,其中11%以游离态As₂O₃形式存在,其余以PbAs₂O₆形式存在。 样品中还含有少量的Cu、Pb、Zn等金属及其氧化物。 根据被测样品物相组成,提出了一种氧化酸浸脱砷的方法,并确定了最佳脱砷工艺,在最佳工艺条件下,砷的浸出率高达97.1%,高于文献报道的砷的浸出率。     

Adsorption of heavy metal ions on soils and soils constituents

The article focuses on adsorption of heavy metal ions on soils and soils constituents such as clay minerals, metal (hydr)oxides, and soil organic matter. Empirical and mechanistic model approaches for heavy metal adsorption and parameter determination in such models have been reviewed. Sorption mechanisms in soils, the influence of surface functional groups and surface complexation as well as parameters influencing adsorption are discussed. The individual adsorption behavior of Cd, Cr, Pb, Cu, Mn, Zn and Co on soils and soil constituents is reviewed.     

Can we trust mass spectrometry for determination of arsenic peptides in plants: comparison of LC–ICP–MS and LC–ES-MS/ICP–MS with XANES/EXAFS in analysis of <Emphasis Type="Italic">Thunbergia alata</Emphasis>

The weakest step in the analytical procedure for speciation analysis is extraction from a biological material into an aqueous solution which undergoes HPLC separation and then simultaneous online detection by elemental and molecular mass spectrometry (ICP-MS/ES-MS). This paper describes a study to determine the speciation of arsenic and, in particular, the arsenite phytochelatin complexes in the root from an ornamental garden plant Thunbergia alata exposed to 1 mg As L(-1) as arsenate. The approach of formic acid extraction followed by HPLC-ES-MS/ICP-MS identified different As(III)-PC complexes in the extract of this plant and made their quantification via sulfur (m/z 32) and arsenic (m/z 75) possible. Although sulfur sensitivity could be significantly increased when xenon was used as collision gas in ICP-qMS, or when HR-ICP-MS was used in medium resolution, the As:S ratio gave misleading results in the identification of As(III)-PC complexes due to the relatively low resolution of the chromatography system in relation to the variety of As-peptides in plants. Hence only the parallel use of ES-MS/ICP-MS was able to prove the occurrence of such arsenite phytochelatin complexes. Between 55 and 64% of the arsenic was bound to the sulfur of peptides mainly as As(III)(PC(2))(2), As(III)(PC(3)) and As(III)(PC(4)). XANES (X-ray absorption near-edge spectroscopy) measurement, using the freshly exposed plant root directly, confirmed that most of the arsenic is trivalent and binds to S of peptides (53% As-S) while 38% occurred as arsenite and only 9% unchanged as arsenate. EXAFS data confirmed that As-S and As-O bonds occur in the plants. This study confirms, for the first time, that As-peptides can be extracted by formic acid and chromatographically separated on a reversed-phase column without significant decomposition or de-novo synthesis during the extraction step.     

Comparative phytotoxicity of methylated and inorganic arsenic- and antimony species to Lemna minor, Wolffia arrhiza and Selenastrum capricornutum

The alkylation of metalloids through the transfer of methyl groups is an important factor in the biogeochemical cycling of elements like arsenic and antimony. In the environment, many different organic and inorganic forms of these elements can therefore be found in soils, sediments or organisms. Studies that compare the ecotoxicity of these different chemical species however are rare. Therefore, this study aimed to generate toxicity data on two scarcely studied organic compounds of arsenic and antimony, as well as to compare their toxicity to the inorganic species, which are studied so far to a higher extent, in order to improve the environmental effect assessment of these elements. To this purpose, bioassays were performed in which three different aquatic organisms (the floating water plants Lemna minor and Wolffia arrhiza and the green alga Selenastrum capricornutum) were exposed to a concentration series of 3 different arsenic species (sodium arsenite — As(III), sodium arsenate — As(V), and monomethylarsonous diiodide — MMAs(III)) and three different antimony species (antimony potassium tartrate hydrate — Sb(III), potassium hexahydroxoantimonate — Sb(V), trimethylantimony(V) bromide — TMSb(V). The observed effect concentrations demonstrated that the inorganic (III)- and (V)-valent species of arsenic were clearly more toxic than the corresponding antimony species. The highest overall toxicity has been shown by MMAs(III) followed by the inorganic As(III). The highest toxicity of the three tested antimony species has been observed for TMSb(V). The observed differences in effect levels stress the importance once more that speciation must not be ignored in toxicity studies.     

Sorption of As(V) on aluminosilicates treated with Fe(II) nanoparticles

Adsorption of arsenic on clay surfaces is important for the natural and simulated removal of arsenic species from aqueous environments. In this investigation, three samples of clay minerals (natural metakaoline, natural clinoptilolite-rich tuff, and synthetic zeolite) in both untreated and Fe-treated forms were used for the sorption of arsenate from model aqueous solution. The treatment of minerals consisted of exposing them to concentrated solution of Fe(II). Within this process the mineral surface has been laden with Fe(III) oxi(hydroxides) whose high affinity for the As(V) adsorption is well known. In all investigated systems the sorption capacity of Fe(II)-treated sorbents increased significantly in comparison to the untreated material (from about 0.5 to >20.0 mg/g, which represented more than 95% of the total As removal). The changes of Fe-bearing particles in the course of treating process and subsequent As sorption were investigated by the diffuse reflectance spectroscopy and the voltammetry of microparticles. IR spectra of treated and As(V)-saturated solids showed characteristic bands caused by Fe(III)SO(4), Fe(III)O, and AsO vibrations. In untreated As(V)-saturated solids no significant AsO vibrations were observed due to the negligible content of sorbed arsenate.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. III. Fixation of a Cr/As system on wood

The kinetic behavior of a Cr/As system in its reaction with wood is described, as are its reactions with lignin and carbohydrates of wood for Cr^VI alone. Chromium^III arsenate complexes with the guaiacyl units of lignin, as well as the usual chromic acid/guaiacyl unit complexes already characterized, was formed. The interference of As concentration on the various zones of fixation of chromium are identified. Complexes between guaiacol and chromium arsenates indicate that in this Cr/As system Cr^III can also be linked to the guaiacyl units of lignin when in the form of CrAsO₄·6H₂O contrary to what obtained with systems not containing As. An initial increase in the rate of reduction Cr^VI → Cr^III by the carbohydrates fraction in wood is ascribed to the catalytic effect of the arsenic. The percentages of chromium fixed onto lignin and cellulose of wood compare well with experimental values.     

Metal and non-metal micro-pollutants in cultivated soils of Lecce and Brindisi districts (Apulia, southern Italy)

To estimate the presence of metal and non-metal micro-pollutants in the soils of the Lecce and Brindisi districts, an analytical study has been carried out on samples of surface soils taken from agricultural areas. The research has concerned the determination of the following micro-pollutants: Cu, Ni, Cr, Zn, Pb, Be, Cd, As, Hg, Sb e Tl. Statistical techniques, such as Principal Component Analysis and Clustering Analysis, have been utilised to examine the correlations among the different parameters and to define contamination areas. The results show that the amount of micro-pollutants in the superficial stratum of the examined soils is in the range permitted by the regulations in force, with the exception of arsenic and thallium. Arsenic concentrations are near to the maximum admissible value, while thallium concentrations are in 56% of the samples higher than the admissible value both in the soil and underground. The most significant parameter from a toxicological point of view is thallium, which has its maximum concentration in the soils located near the industrial area of Brindisi.     

Simultaneous separation of 17 inorganic and organic arsenic compounds in marine biota by means of high-performance liquid chromatography/inductively coupled plasma mass spectrometry

A method using high-performance liquid chromatography/inductively coupled plasma mass spectrometry (HPLC/ICP-MS) has been developed to determine inorganic arsenic (arsenite, arsenate) along with organic arsenic compounds (monomethylarsonic acid, dimethylarsinic acid, arsenobetaine, arsenocholine, trimethylarsine oxide, tetramethylarsonium ion and several arsenosugars) in fish, mussel, oyster and marine algae samples. The species were extracted by means of a methanol/water mixture and a dispersion unit in 2 min, with extraction efficiencies ranging from 83 to 107% in the different organisms. Up to 17 different species were determined within 15 min on an anion-exchange column, using a nitric acid gradient and an ion-pairing reagent. As all species are shown in one chromatogram, a clear overview of arsenic distribution patterns in different marine organisms is given. Arsenobetaine is the major compound in marine animals whereas arsenosugars and arsenate are dominant in marine algae. The method was validated with CRM DORM-2 (dogfish muscle). Concentrations were within the certified limits and low detection limits of 8 ng g(-1) (arsenite) to 50 ng g(-1) (arsenate) were obtained.     

Retention behavior of arsenobetaine, arsenocholine, trimethylarsine oxide and tetramethylarsonium iodide on a styrene-divinylbenzene column with benzenesulfonates as ion-pairing reagents

The pH-dependent retention behavior of arsenobetaine, arsenocholine, trimethylarsine oxide, tetramethylarsonium iodide (cationic arsenic compounds), arsenite, arsenate, methylarsonic acid, and dimethylarsinic acid (anionic arsenic compounds) was studied on a Hamilton PRP-1 reversed-phase column (250×4.1 mm I.D.) with 10 mM aqueous solutions of benzensulfonic acids (X-C₆H₄SO₃⁻; X=H, 4-HO, 3-CO₂H; 4-HO-3-HO₂C-C₆H₃SO₃⁻) as ion-pairing reagents in the pH range 2–5 using flame atomic absorption spectrometry as the arsenic-specific detector. The dependencies of the k′-values of the ‘cationic’ arsenic compounds was rationalized on the basis of the protonation/deprotonation behavior of the arsenic compounds and of the four benzenesulfonates. The results provided evidence for the formation of a cationic species from trimethylarsine oxide below pH 3. Benzenesulfonate is the most hydrophobic ion-pairing reagent causing strong retention of the cationic arsenic compounds and consequently impeding their rapid separation. With the less hydrophobic, substituted benzenesulfonates the cationic arsenic compounds had retention times not exceeding 6 min. At a flow-rate of 1.5 cm³ min⁻¹ 10 mM aqueous 3-carboxy-4-hydroxybenzenesulfonate solution adjusted to pH 3.5 allowed the separation of arsenate, methylarsonic acid, arsenobetaine, trimethylarsine oxide, the tetramethylarsonium ion, and arsenocholine within 3 min. Dimethylarsinic acid coelutes with arsenobetaine at pH 3.5, but can be separated from arsenobetaine with the same mobile phase at pH 2.5. At pH 2.5 the signals for trimethylarsine oxide, the tetramethylarsonium ion, and arsenocholine are too broad to be useful for quantification. Arsenite and methylarsonic acid cannot be separated under these conditions.     

Field Enhanced Sample Injection for the CE Determination of Arsenic Compounds Using Successive Multiple Ionic Polymer Layer Coated Capillaries

A capillary electrophoresis method using indirect UV detection has been applied to the determination of arsenate [As(V)], arsenite [As(III)], monomethylarsonic acid and dimethylarsinic acid. The arsenic species were successfully separated in a successive multiple ionic polymer layer coated capillary. On-line sample preconcentration of arsenic compounds were performed by employing field enhanced sample injection. A baseline separation was achieved in a basic background solution of 10 mM 2,6-pyridinedicarboxylic acid at pH 10.3. The precision of migration time was 1.2–2.4% RSD and peak height was 8.1–12.9% RSD. The limits of detection at a S/N ratio of 3 for the four arsenic compounds were found to be 20–70 ppb, which are comparable to other on-line preconcentration techniques. The enhancement factor was improved by 230–1,500-fold.     

Field Enhanced Sample Injection for the CE Determination of Arsenic Compounds Using Successive Multiple Ionic Polymer Layer Coated Capillaries

A capillary electrophoresis method using indirect UV detection has been applied to the determination of arsenate [As(V)], arsenite [As(III)], monomethylarsonic acid and dimethylarsinic acid. The arsenic species were successfully separated in a successive multiple ionic polymer layer coated capillary. On-line sample preconcentration of arsenic compounds were performed by employing field enhanced sample injection. A baseline separation was achieved in a basic background solution of 10 mM 2,6-pyridinedicarboxylic acid at pH 10.3. The precision of migration time was 1.2–2.4% RSD and peak height was 8.1–12.9% RSD. The limits of detection at a S/N ratio of 3 for the four arsenic compounds were found to be 20–70 ppb, which are comparable to other on-line preconcentration techniques. The enhancement factor was improved by 230–1,500-fold.

     

Speciation of arsenic compounds by HPLC with hydride generation atomic absorption spectrometry and inductively coupled plasma mass spectrometry detection

An arsenic specific detection system utilizing on-line microwave digestion and hydride generation atomic absorption spectrometry (MD/HGAAS) is described for arsenic speciation by using high performance liquid chromatography (HPLC). Both ion exchange chromatography and ion pair chromatography have been studied for the separation of arsenite, arsenate, monomethylarsonic acid (MMAA), dimethylarsinic acid (DMAA), and arsenobetaine (AB). When the commonly used mobile phases, phosphate and carbonate buffers at pH 7.5, are used on an anion exchange column, arsenite and AB co-elute. However, selective determination of these two arsenic compounds can be achieved by using the new detection system. Partial separation between arsenite and AB can be achieved by increasing the mobile phase pH to 10.3 and by using a polymer based anion exchange column. The detection limit obtained by using anion exchange chromatography with MD/HGAAS detection is approximately 10 ng/ml (or 200 pg for a 20-mul sample injection) for arsenite, DMAA and AB, 15 ng/ml (or 300 pg) for MMAA, and 20 ng/ml (or 400 pg) for arsenate. Complete separation of the five arsenic compounds is achieved on a reversed phase C18 column by using sodium heptanesulfonate as ion pair reagent. Comparable resolution between chromatographic peaks is obtained by using MD/HGAAS detection and inductively coupled plasma mass spectrometry (ICPMS) detection.