Optimisation and application of the voltammetric technique for speciation of chromium in the Patos Lagoon Estuary--Brazil

Labile metal assessment in water by diffusive gradients in thin films in shipyards on the Brazilian subtropical coast

Shipyards impact on estuarine environments because of the use of antifouling paints and petroleum products, which release trace metals that may remain in their bioavailable or labile form. Regardless of its importance, the relation between continuous input of trace metals (hotspot area) and their availability in the water column has been scarcely studied. This study evaluated seasonal variations in the concentrations of labile fractions of metals in shipyards located in estuarine areas on the Brazilian subtropical coast. These fractions were determined by the Diffuse Gradients in Thin Films (DGT) technique. Maximum labile fraction concentrations of Cr (0.3 μg L^-1), Ni (2.2 μg L^-1) and V (2.0 μg L^-1) are directly related to (i) their specific source: antifouling paints (for Cr), metal and steel alloys (for Cr and Ni) and petroleum products (for V), besides (ii) periods of intensive traffic and vessel repair. Additionally, variations in labile fractions of Ni and V in the Patos Lagoon estuary were influenced by salinity, which is known to affect metal desorption from surface sediments in resuspension events. Even though Cr is affected by the same processes, it is available as Cr(III) and does not represent any ecological risk in the study areas. Although the areas under study are affected by variations in physical and chemical conditions, shipyards were effectively hotspots of trace metals in their labile fraction in various estuarine systems in southeastern and southern Brazil. Thus, they represent areas where Ecological Risk Assessment, mainly of V, should be carried out.

Improved voltammetric methodology for chromium redox speciation in estuarine waters

Chromium is a toxic element naturally present in natural waters whose chemical speciation regulates its cycling, mobility and bioavailability. We present here: 1- an improved analytical method for chromium speciation (Cr(VI) vs Cr(III)) in estuarine samples by catalytic adsorptive cathodic stripping voltammetric (cat-AdCSV) and 2- a study highlighting a significant change of redox speciation during summer and winter. Initial measurements first revealed that surface-active substances (SAS) present in estuarine samples strongly influenced the analytical determination of Cr by partially masking the Cr peak through an increase of the background current. We found that the application of a low negative accumulation potential (-1.65 V) resulted in much better voltammograms compared to those obtained using the usual accumulation potential of -1.0 V. Using humic acid (HA) as a model SAS of natural origin, we show that this negative potential clearly prevents adsorption of SAS on the Hg-electrode surface, which in turns benefits the adsorption of the in-situ formed Cr(III)-DTPA complex and the resulting signal. The optimised method was applied to determine chromium redox speciation and distribution along the 23 km long salinity gradient, well oxygenated, Krka River estuary (Croatia). Cr(VI) was found to be the dominant redox species in both summer and winter, with Cr(III) contribution being lower in summer (up to ∼30%, average of ∼5%) than in winter (up to ∼50%, average of ∼30%). In summer, lower concentrations of Cr(VI) were found in the freshwater end-member (2.5 nM) than in the seawater end-member (4-5 nM), while the opposite trend was found in winter. Hexavalent chromium exhibited a non-conservative behaviour along the salinity gradient for both seasons. Chromium predominantly exists in dissolved phase, and contribution of particles reactive Cr(III) was minor.

Distribution, sources and consequences of nutrients, persistent organic pollutants, metals and microplastics in South American estuaries

Estuarine pollution imposes rapid, increasing and lasting environmental modifications. In the present review, especial attention is given to estuaries in South America (SA), where legislation, policies and actions to guarantee environmental quality remain ineffective. There, the majority of estuaries face uncontrolled occupation of its margins by urban and industrial centres, agriculture and aquaculture expansion, water extraction and flow control. The lack of basic sanitation and poor environmental management (including territories within Marine Protected Areas) often lead to hydrological alterations, high nutrient loads, and the presence and dynamics of pollutants (nutrient loads, persistent organic pollutants (POPs), metals and plastic debris) along the entire estuarine ecocline. Organic enrichment has increased dissolved oxygen consumption, with wide spatio-temporal variability along latitudes and estuarine gradients. The toxicity, biogeochemistry and availability of metals and POPs depend on the annual fluctuations of salinity, water renewal, dissolved oxygen levels, suspended particulate loads, sediment mobility, grain size and composition at the sink. Plastic debris from land sources are widespread in estuaries, where they continue to fragment into microplastics. River basins are the main contributors of plastics to estuaries, whose transportation and accumulation are subjected to interannual water flow variations. Although some systems seems to be in a better condition in relation to others around the world (e.g. Goiana and Negro estuaries), many others are among the most modified worldwide (e.g. Guanabara Bay and Estero Salado System). We propose that, estuarine conservation plans should consider year-round fluctuations of the ecocline and the resulting cycles of retention and flush of environmental signals and their influence on trophic webs over the whole extent of estuarine gradients.

Removal of traces of mercury from a carrier gas for analytical purpose

Background

The analysis of mercury by cold vapor requires a gas, usually argon or helium, to transport elementary mercury to the gold trap or directly to the detector. When analyzing mercury in environmental matrices, a gas with a metal concentration as low as a few picograms per cubic meter is needed. Different sorbents have been used to purify the gas for a long time, but little information is available about them, mainly considering the analytical purpose. This paper presents results of the absorption capacity for solids and hypochlorite solutions that usually are used as mercury sorbents, giving technical information to the analyst to decide the best gas cleaning process to be used.

Findings

The absorption capacities of different sorbents were tested using atomic fluorescence spectrometry. Among the tested solids, platinum presented the highest absorption capacity (13.04 pg Hg per gram of Pt). Interaction between sodium hypochlorite, sodium chloride, and EDTA in the absorption capacity was investigated by a 23 factorial design. Results showed a significant interaction between hypochlorite and chloride.

Conclusions

A solution of 1.26 mmol L−1 sodium hypochlorite, 0.48 mol L−1 sodium chloride, and 0.6 mmol L−1 EDTA shows the highest absorption capacity (167.3 pg Hg) among the tested compositions. That solution has eliminated even traces of mercury from gases, resulting in a carrier free of mercury, what cannot be achieved using the solid sorbents tested, despite the use of solutions which is more tedious than the use of solids to clean gas. Anyway, the hypochlorite solution shows to be a good option to clean gases that have to be used in the analysis of mercury in samples with very low concentration.