Olga M.S. Filipe

Cathodic stripping voltammetry of trace Mn(II) at carbon film electrodes

A sensitive voltammetric method is presented for the determination of tract levels of Mn (II) using carbon film electrodes fabricated from carbon resistors of 2 Omega. Determination of manganese was made by square wave cathodic stripping voltammetry (CSV), with deposition of manganese as manganese dioxide. Chronoamperometric experiments were made to study MnO(2) nucleation and growth. As a result, it was found to be necessary to perform electrode conditioning at a more positive potential to initiate MnO(2) nucleation. Under optimised conditions the detection limit obtained was 4 nM and the relative standard deviation for eight measurements of 0.22 nM was 5.3%. Interferences from various metal ions on the response CSV of Mn(II) were investigated, namely Cd(II), Ni(II), Cu(II), Cr(VI), Pb(II), Zn(II) and Fe(II). Application to environmental samples was demonstrated.

A solid-phase extraction procedure for the clean-up of thiram from aqueous solutions containing high concentrations of humic substances.

A simple solid-phase extraction (SPE) procedure with an octadecyl bonded phase silica (C(18)) was developed for clean-up of the fungicide thiram from aqueous solutions containing high concentrations of humic substances, for future studies of thiram adsorption onto solid humic substances or soils. Suspensions of humic acids and soil, in aqueous 0.01M CaCl(2) solution, were prepared and used as samples. These extracts were spiked with thiram and immediately applied to a C(18)-SPE cartridge. Thiram was eluted with chloroform and its concentration measured by spectrophotometry at 283nm. Non-spiked aqueous extracts (blanks) and a control sample of thiram in 0.01M CaCl(2) aqueous solution were also prepared and submitted to the same SPE procedure. The results show that humic substances are extensively retained by the C(18) cartridge but are not eluted with CHCl(3). Recoveries of 100-104% were obtained for thiram in the presence of humic substances. The SPE procedure described in this work is an efficient clean-up step to remove the interference of humic substances absorbance and to be coupled to any spectrophotometric or HPLC-UV method, usually used for thiram analysis in food extracts.

Fluorescence as a Tool for Tracing the Organic Contamination from Pulp Mill Effluents in Surface Waters

Three-dimensional fluorescence spectra of water samples from an eucalyptus bleached kraft pulp mill and from a river, upstream and downstream of the discharge of the effluent, revealed the existence of a peak at δexc = 280 nm and δem = 340 nm Δδ = 60 nm), characteristic of effluentπs organic matter. Humic substances were isolated from the effluent by sequential adsorption onto resins XAD-8 and XAD-4 in series. Their synchronous fluorescence spectra with Δδ = 60 nm do also exhibit an intense signal at δexc = 280 nm (≈ 300 nm in the humic acid fraction). The peak is absent in the spectra of humic substances isolated from a non-polluted site of the river, but it is clearly seen in the spectra of the humic substances from a site downstream of the discharge of the effluent. Synchronous fluorescence spectra (Δδ = 60 nm) of water samples from the river and its lagoon were recorded and revealed to be an easy and fast way of tracing the organic contamination from the effluent. Fluoreszenzspektroskopie zum Nachweis der organischen Kontamination von Oberflachengewassern durch Abwasser der Zellstoffindustrie Die dreidimensionalen Fluoreszenzspektren des Ablaufs einer Eukalyptus-verarbeitenden Zellstofffabrik sowie des Vorfluters oberhalb und unterhalb der Einleitungsstelle weisen einen Peak bei δex = 280 nm und δem = 340 nm (Δδ = 60 nm) auf, der fur die organischen Abwasserinhaltsstoffe der Zellstofffabrik charakteristisch ist. Durch sequentielle Adsorption an XAD-8- und XAD-4-Harz wurden Huminstoffe aus dem Ablauf isoliert. Ihre synchronen Fluoreszenzspektren mit Δδ = 60 nm zeigen ebenfalls ein intensives Signal bei δex = 280 nm bzw. bei ca. 300 nm fur die Huminsaurefraktion. Dieser Peak fehlt in Spektren von Huminstoffen, die aus einem nicht kontaminierten Abschnitt des Flusses isoliert wurden, ist aber in den Spektren von Huminstoffen, die aus einem Flussabschnitt unterhalb der Einleitungsstelle isoliert wurden, deutlich zu erkennen. Anhand der Synchronspektren (Δδ = 60 nm) von Wasserproben aus dem Fluss und der Lagune lasst sich somit die organische Kontamination durch das Abwasser leicht und schnell verfolgen.

Determination of Chromium(VI) by Batch Injection Analysis and Adsorptive Stripping Voltammetry

Abstract A new sensitive voltammetric method is presented for the determination of trace levels of Cr(VI). The method is based on square wave adsorptive stripping voltammetry (AdSV) in conjunction with the electrochemical batch injection analysis technique at mercury thin-film electrodes. The determination of chromium is made in the presence of cupferron as ligand. Relevant experimental parameters are explored and optimized, such as injection mode, cupferron concentration, accumulation potential and time, and square wave parameters. A preconcentration time of 20 s at −0.8 V vs. SCE results in a detection limit of 32 nM. The relative standard deviation for 12 measurements of 0.12 µM was 6.5%. Possible interferences by other trace metals are considered.

Photodegradation of the fungicide thiram in aqueous solutions. Kinetic studies and identification of the photodegradation products by HPLC–MS/MS

In this study, the relevance of photodegradation processes on the persistence of the fungicide thiram in waters was investigated. The photodegradation of thiram in Milli-Q water and in aqueous solutions of humic and fulvic acids, as well as the photodegradation in spiked river water were studied. Both pure thiram and one of its commercial formulations were used to prepare the solutions which were irradiated in a solar light simulator. In general, thiram photodegradation follows pseudo-first order kinetics. The half-life time of thiram 2mgL(-1) in Milli-Q water was 28min. However, the degradation rate of thiram was significantly increased (p=0.02) by the inert components of the thiram commercial formulation as well as by commercial humic acids and by fulvic acids isolated from river water (p<0.004). Thus, the half-life time of thiram decreased to 24min in the presence of the inert formulation components, while, in the presence of both humic and fulvic acids (10mgL(-1)) it decreased to 22min. Furthermore, thiram photodegradation in natural river water showed that there is a significant enhancement of the degradation rate constant of thiram relatively to Milli-Q water, corresponding to a decrease of about 38% in its half-life time. This increase of the degradation rate in river water seems to be higher than that observed in the presence of FA, suggesting that beyond organic matter, other natural river components can increase the thiram photodegradation rate. These results allow us to conclude that photodegradation by solar radiation can be an important degradation pathway of thiram in natural waters. HPLC-MS/MS allowed to identify, for the first time, three products of the photodegradation of thiram in aqueous solution. Three compounds were identified and their structure was corroborated by the MS(n) spectra fragmentation profile. Pathways for the formation of the products from thiram photodegradation are proposed and discussed.

Comparison between DAX-8 and C-18 solid phase extraction of rainwater dissolved organic matter

Rainwater is a very low concentrated matrix and, for dissolved organic matter (DOM) characterization, an efficient extraction procedure is essential. Isolation procedures based on the adsorption onto XAD-8 and C-18 sorbents have been used in the literature for rainwater DOM isolation, but a comparison between these procedures is lacking. In this work, UV-visible and molecular fluorescence spectroscopies highlighted differences between rainwater DOM isolated by DAX-8 (replacement for XAD-8) and by C-18. It was possible to recover higher rainwater DOM percentage by the C-18 based procedure than by the DAX-8 one. Rainwater protein-like compounds were better concentrated by the C-18 procedure than by the DAX-8 one, while humic-like compounds were similarly concentrated by both procedures. Furthermore, rainwater DOM extracted by the C-18 procedure was more representative of the global matrix, while DAX-8 preferentially extracted humic-like compounds.

Effect of long term organic amendments on adsorption-desorption of thiram onto a luvisol soil derived from loess.

The objective of this work was to assess the influence of soil organic amendments on the sorption properties of the fungicide thiram. The organic amendments studied were organic household compost (COM), sewage sludge from municipal water treatment facilities (SLU) and farmyard manure (FYM), which were compared to mineral fertilizer application (MIN). Sorption-desorption experiments were performed using the batch method and the results indicated that the adsorption isotherms were non-linear and were found to conform to the Brunauer-Emmett-Teller (BET) model, suggesting multilayer adsorption and adsorbate-adsorbate interactions after the saturation of the surface layer. In general, distribution coefficient values, K(D), are dependent on, but not proportional to, the initial concentration of thiram. For a fixed thiram initial concentration, a significant correlation (r(2)>0.851; p<0.001) between K(D) values and the soil organic carbon content (OC) was observed. The highest value of K(D) was observed for the soil amended with compost, which is the one with the highest organic carbon content. K(D) values were divided by the soil organic carbon contents in order to obtain organic carbon partition coefficients K(OC). Comparing K(OC) means from 3 (initial concentrations) x 4 (soil organic matter compositions) x 3 (replicates) factorial ANOVA allow us to conclude that there is a significant but not proportional influence of the initial concentration of thiram on those values, but changes in the soil organic matter composition, associated to different soil amendments, have no significant influence on adsorption of thiram. To evaluate the reversibility of thiram adsorption, two consecutive desorption cycles were performed with CaCl(2) 0.01 mol L(-1). The desorption K(D) values were consistently higher (approximately twice) than those for adsorption at the same equilibrium concentrations for all soil samples supporting the existence of hysteresis in the adsorption-desorption behavior of thiram. Despite the fact that the adsorption K(D) values were proportionally increased with increasing total organic carbon content, this was not the case for the desorption K(D) values.

Structural Characterisation of the Coloured Organic Matter from an Eucalyptus Bleached Kraft Pulp Mill Effluent

Abstract XAD-8 and XAD-4 resins in tandem were used to isolate and fractionate the coloured organic matter from an eucalyptus bleached Kraft pulp mill effluent which is discharged into river Vouga (Portugal) near the lagoon that exists in its terminal part. The composition of the isolated organic matter was investigated by FTIR and CPMAS-13C-NMR and compared to that of humic matter isolated before from the same river at a non-polluted site and a site very contaminated with the effluent of a sulphite pulp mill. The results put into evidence that the organic matter from both kinds of effluent is more aromatic and contains more lignin derived structural units than the humic matter from the unpolluted site. However, the organic matter from the sulphite pulp mill effluent is more soluble due to its content of sulphonic functional groups.

Influence of Fulvic Acids and Copper Ions on Thiram Determination in Water

The literature concerning the application of solid-phase extraction (SPE) to the concentration of thiram (bis(dimethyldithiocarbamoyl) disulfide) from natural waters is scarce, the available results being contradictory or with no analytical significance. To clarify these contradictory results, a C18-SPE procedure combined with HPLC-UV was applied to thiram analysis in river water, and the influence of several factors on recoveries was studied. This procedure gave thiram recoveries of about 100% when applied to thiram standard solutions. However, when the same procedure was applied to river water samples spiked with thiram, the recoveries depended on the equilibration time after spiking. The influence of river fulvic acids (FAs) and Cu(II) on thiram recoveries from standard solutions was studied as a possible interference for such a result. In the presence of FA, thiram recoveries were always higher than 85%. In the presence of Cu(II), thiram recoveries decreased significantly, due to complexation, but the addition of an excess of EDTA before C18-SPE eliminated that interference, and thiram was completely recovered. However, in river water samples the addition of EDTA had to be done before thiram spiking to obtain a recovery >90%. Thiram standard solutions containing both river FA and Cu(II) showed a behavior similar to the one observed in river water samples. On the basis of these results, the catalytic effect of Cu(II) on the degradation of thiram by FA, with formation of a Cu(II)-dimethyldithiocarbamate complex, was hypothesized.

Electrochemical studies of nitroprusside in the presence of copper(II): formation of Cu(I) reduced nitroprusside species

Abstract Cu 2+ is reduced in the presence of nitroprusside to form two Cu(I) reduced nitroprusside species at about +0.050 V (pH 7.6). These species are reduced further at about −0.60 V. The two species are formed by an EC mechanism, and the species are believed to be [Cu I Fe(CN) 4 NO] − , which predominates in acidic solution, and [Cu I Fe(CN) 5 NO] 2− , which predominates in alkaline solution. These conclusions are supported by cyclic voltammetric and bulk electrolysis/coulometric experiments.