MERCURY QUANTIFICATION IN SOILS USING THERMAL DESORPTION AND ATOMIC ABSORPTION SPECTROMETRY: PROPOSAL FOR AN ALTERNATIVE METHOD OF ANALYSIS

Despite the considerable environmental importance of mercury (Hg), given its high toxicity and ability to contaminate large areas via atmospheric deposition, little is known about its activity in soils, especially tropical soils, in comparison with other heavy metals. This lack of information about Hg arises because analytical methods for determination of Hg are more laborious and expensive compared to methods for other heavy metals. The situation is even more precarious regarding speciation of Hg in soils since sequential extraction methods are also inefficient for this metal. The aim of this paper is to present a technique of thermal desorption associated with atomic absorption spectrometry, TDAAS, as an efficient tool for quantitative determination of Hg in soils. The method consists of the release of Hg by heating, followed by its quantification by atomic absorption spectrometry. It was developed by constructing calibration curves in different soil samples based on increasing volumes of standard Hg2+ solutions. Performance, accuracy, precision, and quantification and detection limit parameters were evaluated. No matrix interference was detected. Certified reference samples and comparison with a Direct Mercury Analyzer, DMA (another highly recognized technique), were used in validation of the method, which proved to be accurate and precise.

Eletroxidação do etanol em eletrodos de Ti/IrO2

It has been carried out an investigation of ethanol electro-oxidation on Ti/IrO2 electrodes. The experimental results show a high selectivity towards acetaldehyde formation thus, offering potential advantages in cost and availability of raw material. It has been observed that the electrode is partially blocked by a film formed after the oxidation of the starting material which can be removed by pulse technique between RDO and RDH onset. The mechanism and the selectivity of the product formed is presented.

Uma célula eletroquímica de fluxo para modificação permanente de tubo de grafite empregado em absorção atômica

A flow cell assembled on the original geometry of a graphite tube to achieve permanent chemical modifier is proposed. The graphite tube operates as the working electrode. A stainless steel tube, positioned downstream from the working electrode, was used as the auxiliary electrode. The potential value applied on the graphite electrode was measured against a micro reference electrode (Ag/AgCl) inserted into the auxiliary electrode. Palladium solutions in acetate buffer (100 mmol L-1, pH = 4.8), flowing at 0.5 mL min-1 for 60 min was used to perform the electrochemical modification. A mercury solution (1 ng) was used to evaluate the performance of the permanent palladium modifier.

Mechanism of 3,4-dihydroxybenzaldehyde electropolymerization at carbon paste electrodes: catalytic detection of NADH

Cyclic voltammetry was used to study 3,4-dihydroxybenzaldehyde (3,4-DHB) electropolymerization processes on carbon paste electrodes. The characteristics of the electropolymerized films were highly dependent on pH, anodic switching potential, scan rate, 3,4-DHB concentrations and number of cycles. Film stability was determined in citrate/phosphate buffer solutions at the same pH used during the electropolymerization process. The best conditions to prepare carbon paste modified electrodes were pH 7.8; 0.0 <= Eapl <= 0.25 V; 10 mV s-1; 0.25 mmol L-1 3,4-DHB and 10 scans. These carbon paste modified electrodes were used for NADH catalytic detection at 0.23 V in the range 0.015 <= [NADH] <= 0.21 mmol L-1. Experimental data were used to propose a mechanism for the 3,4--DHB electropolymerization processes, which involves initial phenoxyl radical formation.

Bastões de grafite reciclados de baterias comuns e seu uso como eletrodo modificado em hidrogenação eletrocatalítica de alguns substratos orgânicos

This paper presents some results on the employ of recycled graphite electrode obtained from used common 1.5 V batteries in the preparation of modified electrode and the electrocatalytical hydrogenation of benzaldehyde and of n-valeraldehyde. This inexpensive and easy to obtain electrode was prepared by coating it with a 1:1 mixed film of poly-(allylfenil ether): poly-[allyl p-(2-ethylammonium) benzene ether] and introduction of dispersed platinum particles by ion exchange and reduction of PtCl4-2. Electroreduction of H+ from aqueous H2SO4 using the proposed electrode hydrogenated the substrates in a way comparable with that of vitreous carbon electrode.

The interaction between mercury(II) and sulfathiazole

The interaction of mercury(II) with sulfathiazole has been analyzed. IR and NMR spectral studies suggest a coordination of Hg(II) with the Nthiazolic atom, unlike related Hg-sulfadrugs compounds. The complex was screened for its activity against Escherichia coli, showing an appreciable antimicrobial activity compared with the ligand.

The attenuation of concentrations model: a new method for assessing mercury mobility in sediments

In this work we propose a new approach for the determination of the mobility of mercury in sediments based on spatial distribution of concentrations. We chose the Tainheiros Cove, located in the Todos os Santos Bay, Brazil, as the study area, for it has a history of mercury contamination due to a chloro-alkali plant that was active during 12 years. Twenty-six surface sediment samples were collected from the area and mercury concentrations were measured by cold vapour atomic absorption spectrophotometry. A contour map was constructed from the results, indicating that mercury accumulated in a "hot spot" where concentrations reach more than 1 µg g-1. The model is able to estimate mobility of mercury in the sediments based on the distances between iso-concentration contours that determines an attenuation of concentrations factor. Values of attenuation ranged between 0.0729 (East of the hot spot, indicating higher mobility) to 0.7727 (North of the hot spot, indicating lower mobility).

Eletrodo íon-seletivo para determinação potenciométrica de alumínio(III) em meio de fluoreto

The construction and analytical evaluation of a coated graphite Al(III) ion-selective electrode, based on the ionic pair formed between the Al(F)n3-n anion and tricaprylylmethylammonium cation (Aliquat 336S) incorporated on a poly(vinylchloride) (PVC) matrix membrane are described. A thin membrane film of this ionic pair and dibutylphthalate (DBPh) in PVC was deposited directly on a cylindric graphite rod (2 cm length x 0.5 cm diameter) attached to the end of a glass tube using epoxy resin. The membrane solution was prepared by dissolving 40% (m/m) of PVC in 10 mL of tetrahydrofuran following addition of 45% (m/m) of DBPh and 15% (m/m) of the ionic pair. The effect of membrane composition, fluoride concentration, and several concomitants as potential interferences on the electrode response were investigated. The aluminium(III) ion-selective electrode showed a linear response ranging from 1.4 x 10-4 to 1.0 x 10-2 mol L-1, a detection limit of 4.0 x 10-5 mol L-1, aslope of -54.3±0.2mV dec-1 and a lifetime of more than 1 year (over 3000 determinations for each membrane). The slope indicates that the ion-selective electrode responds preferentially to the Al(F)4- species. Application of this electrode for the aluminium(III) determination in stomach anti-acid samples is reported.

Utilização de um eletrodo de grafite-epóxi recoberto com [Zn(FEN)3][tetratris(4-clorofenil) borato]2 sensível a zinco(II) em meio 1,10-fenantrolina como eletrodo indicador em titulações potenciométricas de precipitação

The construction and analytical evaluation of a coated graphite-epoxy electrode sensitive to the zinc-1,10-phenantroline complex based on the [Zn(fen)3][tetrakis(4-chlorophenyl)borate]2 incorporated into a poly(vinylchloride) (PVC) matrix are described. A thin membrane film of this ion-pair, dibutylphthalate (DBPh) and PVC were deposited directly onto an electrically conductive graphite-epoxy support located inside a Perspex® tube. The best PVC polymeric membrane contains 65% (m/m) DBPh, 30% (m/m) PVC and 5% (m/m) of the ion-pair. This electrode shows a response of 19.5 mV dec-1 over the zinc(II) concentration range of 1.0 x 10-5 to 1.0 x 10-3 mol L-1 in 1,10-phenantroline medium, at pH 6.0. The response time was less than 20 seconds and the lifetime of this electrode was more than four months (over 1200 determinations by each polymeric membrane). It was successfully used as an indicator electrode in the potentiometric precipitation titration of zinc(II) ions.

Mercury in soils and sediments from gold mining liabilities in Southern Amazonia

The 1980-1990 Amazonian gold rush left an enormous liability that increasingly has been substituted by developing fish aquaculture. This work aimed at the identification of the mercury levels in the environment, associated with fish farms located in the North of Mato Grosso State, Southern Amazon. Sediment and soil samples were analyzed for total organic carbon and total mercury. Results indicate that the chemical characteristics of the sediment largely depend on the management procedures of the fish pond (liming, fish food used and fish population). The soils presented relatively low concentrations when compared with other data from the literature.

Metodologias eletroanalíticas para a determinação de herbicidas triazínicos por voltametria de onda quadrada e técnicas de deconvolução

This work describes the development of electroanalytical methodologies for the determination of atrazine, ametrine and simazine by square wave voltammetry on a mercury electrode. For pure atrazine and pure ametrine, the detection limits (DL) were 3.7 and 4.3 µg L-1, respectively, while they increased to 4.8 and 6.5 µg L-1 in the presence of 3.0 x 10-6 mol L-1 of the other component (a mathematical deconvolution program was used in the mixture cases). The voltammetric response for simazine could not be separated from that of atrazine and measurements were carried out only in pure simazine solutions (DL: 7.5 µg L-1).

Titulações potenciométricas de cátions metálicos tendo como eletrodo indicador o sistema Cu/Cu(II)-EDTA

In potentiometric titrations of metal cations with EDTA the Hg/HgY2- system is usually used to detect the end point. However, the use of mercury has been discouraged in analytical procedures due to its toxicity. In this work the Cu/CuY2- system was used as indicator electrode for potentiometric titrations of some metal cations with EDTA. The solutions of Cu2+, Cd2+, Mn2+, Co2+ and Zn2+ were titrated with Na2EDTA solution in the presence of a small concentration of the CuY2- complex using a copper wire as indicator electrode. The potentiometric titrations with the Cu/CuY2- system showed good correlation when compared with an Hg/HgY2- system.

Comportamento eletroquímico da N-nitrosotiazolidina ácido carboxílico sobre eletrodos de ouro e mercúrio

The electrochemical behavior of N-nitrosothiazolidine carboxylic acid (NTAC) on gold and hanging mercury electrodes, using the cyclic and square wave voltammetries, was studied. Whereas NTAC suffer reduction in a single step on the mercury electrode, two peaks appears on the cyclic voltammograms on the gold electrode, one anodic peak overlaying the gold oxide process at 1.2 V and one cathodic peak at -0.41 V vs Ag/AgCl, KCl 3.0 mol L-1. The cathodic peak depends on the previous oxidation of NTAC at the electrode surface, presents irreversible and adsorption controlled characteristics and it is suitable for quantitative purposes.

Highly selective transport of mercury(II) ion through a bulk liquid membrane

In this work carrier-facilitated transport of mercury(II) against its concentration gradient from aqueous 0.04 M hydrochloric acid solution across a liquid membrane containing isopropyl 2-[(isopropoxycarbothiolyl)disulfanyl]ethane thioate (IIDE) as the mobile carrier in chloroform has been investigated. Sodium thiocyanate solution (1.6 M) was the most efficient receiving phase agent among several aqueous reagents tested. Various parameters such as investigated. Under optimum conditions the transport of Hg(II) across the liquid membrane is more than 97% after 2.5 h. The carrier, IIDE, selectively and efficiently could able to transport Hg (II) ions in the presence of other associated metal ions in binary systems.

Kinetic measurements during transient film growth on zinc

The electrochemical behaviour of zinc has been extensively studied in alkaline and acid media, but only a few studies have been reported in neutral solutions, particularly in deaerated media. Zinc passivation in neutral medium and the effect of the ClO4- ion on the nucleation and growth of the passive layer is studied in this paper by a transient technique at different electrolyte concentrations and applied potentials. ZnO growth rate was shown to decrease with increasing electrolyte concentration. Moreover, passive layer growth occurred followed by pitting nucleation and growth. Film growth and pit nucleation are explained by means of the Macdonald and Engell-Stolica models.