Marta Plavšić

Determination of the apparent copper complexing capacity of seawater by anodic stripping voltammetry

Abstract A new experimental technique using differential pulse anodic stripping voltammetry with a thin-film mercury electrode and efficient mixing of the electrolyte for the apparent copper complexing capacity of seawater is proposed. The effects of some factors such as type of electrode and cell, pH of the solution, potential of deposition, time of equilibration, and influence of a non-ionic surface active agent on the measurements are examined. Experimental results for the apparent copper complexing capacity of seawater are calculated presuming 1:1 inert complex formation. By using EDTA as a model ligand, it is shown that the proposed procedure with efficient mixing of the solution excludes any appreciable kinetic influence upon the electroanalytical results.

Trace metal adsorption on inorganic solid phases under estuarine conditions

The adsorption of trace metals (Pb, Zn, Cu, Cd and Hg) on calcite, kaolinite and bentonite has been studied in Krka river water of various salinities (S = 3, 20 and 38‰). n nModel experiments were performed at pH 8 in natural water and in UV-irradiated water samples to determine the importance of natural soluble organic matter. Cold vapour atomic absorption spectrometry was used for the Hg determination, and a differential pulse anodic stripping voltammetry (DPASV) method for determination of other trace metals, using either a hanging mercury drop electrode or a mercury-coated glassy carbon working electrode. The following results have been obtained on calcite: n1. n(1) a powerful absorption of Pb and Zn was observed for both natural and UV-irradiated waters at S=20‰; n n2. n(2) a strong adsorption of Hg was observed in natural water, and significantly lower adsorption in UV-irradiated water at all three salinities, particularly at low salinity; n n3. n(3) Cd adsorption was of medium intensity in both waters at S=20‰; n n4. n(4) Cu was practically not adsorbable in both waters at S=20‰ if the total Cu ion concentration was lower than the copper complexing capacity value; n n5. n(5) Cu adsorption was of a medium intensity at S=38‰. It has not influenced by the EDTA model ligand. n n nTrace metals Pb, Zn and Cu are adsorbable on kaolinite and bentonite at S=38‰, whereas Cd is practically not adsorbed. The Krka River is a calcareous river, which supplies the sea with calcite and aluminosilicates. In the light of our model experiments, we suggest that the self-purification of the Krka River is remarkable in respect to Pb, Zn and Hg; this occurs to a lesser extent for Cu, and is negligible in the case of Cd.

The role of reduced sulfur species in the coalescence of polysaccharides in the Adriatic Sea

Abstract Massive mucilage events occur in the Northern Adriatic Sea presumably by cytoplasmic excretions from deteriorated diatoms. During three such events in the summer of 1991, 1997 and 1998 the presence of reduced sulfur species (RSS) was determined in samples of macroaggregate using electrochemical methods (in-phase alternating current (AC) and linear sweep voltammetry). The detected levels of sulfur, expressed as equivalent to sulfide concentrations, were about 200 nM. In the same mucilage samples, concentrations of organic matter were determined in the range from 60 to 600 mg/l of total organic carbon (TOC). The physico-chemical properties of organic matter in the macroaggregates correspond to those of polysaccharides of very high molecular mass. Scanning confocal laser microscopy (SCLM) and fluorescent molecular probes for sugars (the lectins concanvaline A (Con-A)) showed the transformation of polysaccharide polymer structure resulting in the formation of very stable filaments and layers after the treatment of mucilage samples with sodium sulfide. Commercial polysaccharides of bacterial and algal origin (xanthan, carrageenans types I and II, dextran-T-500) have been used to simulate macroaggregate formation under laboratory conditions after treatment with sodium sulfide. Raman spectroscopy indicated that for all model polysaccharides used, sulfide interaction occurred, as evidenced by visible change of the O–H stretching region in the vibration spectra of the water molecules. Our data suggest that the aggregated polysaccharides from the Adriatic Sea are: (1) structurally affected by addition of sulfide as was the case for most of the model polysaccharides, and (2) the stabilizing effect of sulfide on the aggregated polysaccharides is due to the formation of sulfur-organic compounds.

Mercury distribution in the water column of the stratified Krka river estuary (Croatia) : importance of natural organic matter and of strong winds

Abstract Estuarine water samples were collected at the vertical profile on one selected station in the Krka river estuary (Croatia) during the period from September 1995 to July 1998. The concentrations of reactive and total mercury (in the UV-irradiated samples) were measured in unfiltered and acidified samples by atomic absorption and show non-conservative behavior. The location of the mercury maximum found in the region, 2−12.5 ngl −1 , changes and is not always at the freshwater/seawater interface (FSI) as suggested in earlier estuary studies. The proportion of reactive to total mercury, two operational species, also changed, depending on salinity, seasonal and weather changes not only in fresh water and at the FSI but in some cases in seawater down to the bottom. Maximal apparent mercury complexing capacity was determined as 2.7 ngHgl −1 . Information about surfactant activity in the same vertical profile was obtained using a hanging mercury drop electrode (HMDE), by a.c. out of phase voltammetry at two fixed potentials (−0.2 and −0.6xa0V). Continuous variation of hydrophylic and hydrophobic substances was observed. No correlation with the mercury maximum was found. In samples showing the total mercury maximum, hydrophobic heterodispersed substances (>0.7xa0 μ m) were found, giving evidence that mercury is partly bound to particulate organic carbon having hydrophobic properties. Submicron colloids and aggregates (10xa0nm–1xa0 μ m) were directly proved using electronmicroscopy. Characteristic estimated time for the coagulation/sedimentation process in the Krka river estuary (about 8 days) can be influenced by the strong northerly Bura wind. Removal of mercury from the water column was evidenced in winter, when extremely low (less than 1 ngl −1 ) mercury concentrations were found in the water column after the very strong Bura wind. During the Jugo wind the outflow of brackish water decreases. The current going from sediment to FSI accumulates mercury at FSI (12.5 ngl −1 ), predominantly in inorganic form.

The influence of organics on the adsorption of copper (II) on γ-Al2O3 in seawater, model studies with EDTA

Abstract The natural “complexing capacity” of aquatic systems was determined using titration with cupric ions. It was compared with “complexing capacity” determined in the presence of both EDTA, used as a model for organic substances, and of EDTA and γ-Al 2 O 3 . The γ-Al 2 O 3 was used as a model for solid particles of hydrous oxide in natural waters. The adsorption isotherm of copper in seawater on γ-Al 2 O 3 particles was determined with and without EDTA. For EDTA alone and for the Cu—EDTA complex, it was found that they are not adsorbed on γ-Al 2 O 3 in the natural seawater. However, by adsorption on solid particles, the labile ionic Cu-complex can be removed from the seawater.

Adsorption Properties of Different Polysaccharides on Mercury in Sodium Chloride Solutions

The electrochemical method based on the measurement of alternating current (a.c.) in out of phase mode (expressed also as a capacitance ) at the hanging mercury drop electrode has been applied for the study of polysaccharides from different sources (microbial, algal). The difference in the adsorption behaviour is discussed in the relation to polysaccharides structural characteristics, as well as their molecular mass. Some of selected polysaccharides as well as natural seawater mucillage sample exibit also in phase - reduction wave at a potential of ~ - 1.35 V .

Voltammetric study of adsorption of cu(II) species on solid particles added to seawater

The adsorption on solid particles of natural organic ligand in seawater of Cu(II) ions, and of the inert Cu(II) complexes has been studied. Model solids, γ-Al2O3, Na+-0.392-γ-Al2O3, ‘Aerosil 200’, chrysotile, northupite and CaCO3 were added to seawater. It was observed that at pH 8 natural organic matter was strongly adsorbed on chrysotile and was not adsorbed on Na+ -0.392-γ-Al2O3; it was also adsorbed on γ-Al2O3 over the range of 3 < pH < 7. In this pH range, the complexing capacity and adsorption of Cu is at a minimum because Cu(II) is not adsorbed on γ-Al2O3 and natural organic matter is adsorbed. Inert CuL complexes were adsorbed at pH 8.0 on γ-Al2O3, ‘Aerosil 200’, CaCO3, and chrysotile but they were not adsorbed on northupite. The voltammetric method can be recommended for use in natural waters for distinguishing between metal ionic and metal inert complex species which are adsorbed from solution onto various solid particles.

Characterization of surface-active substances during a semi-field experiment on a phytoplankton bloom

Abstract The determination of surface-active substances (SAS) during the evolution of a phytoplankton bloom in a model plankton ecosystem was done by electrochemical methods. The separation and characterization of the organic material present were also done on XAD-8 resin columns. It was found that large amounts of the SAS, predominantly of the humic (fulvic) type, were present from the beginning of the experiment and partly masked the effects of organic substances released by phytoplankton. Although the transformation of the organic matter (consumption and/or excretion) was obvious, no general trend in the amount or type of surface-active matter was observed during the experiment.

Voltammetric study of the role of organic acids on the sorption of Cd and Cu ions by alumina particles

Abstract The adsorption of Cd and Cu ions was studied on bare γ-Al2O3 particles and on γ-Al2O3 particles pre-equilibrated with humic acid (HA), ι-carrageenan or polyacrylic acid (PAA) in 0.55M NaCl solution, similar to natural seawater. The adsorption of the mentioned organic substances on alumina particles was studied simultaneously. It was found that the interaction of copper ions with alumina particles was enhanced in the presence of HA, and was not significantly changed in the presence of PAA or ι-carrageenan when compared to the interaction with the bare alumina surface. For cadmium ions, which interact to a very small extent with the HA in the solution as well as with mineral particles, the interaction observed on the γ-Al2O3 particles pre-equilibrated with HA was also negligible. There was no indication of the change in adsorption of organic matter on the alumina particles in the presence of metal ions.

Adsorption of acrylic and polyacrylic acids on the mercury electrode/sodium chloride solution interface

Abstract In this paper the adsorption of acrylic acid, glutaric acid and polyacrylic acids (PAAs) (molecular weight (MW) 2000, 5000 and 90 000) on the mercury surface by phase selective a.c. voltammetry at a potential of - 0.6 V vs. Ag/AgCl reference electrode was investigated. The adsorption of these compounds was determined in the pH range 2–8, which is important, because by changing the pH values the degree of neutralization of PAAs changes and influences adsorption. The adsorption of PAAs is pronounced at low pH values (pH 2.0), while at higher pH values (pH 7) they are not adsorbed at all, regardless of the degree of polymerization. No effect of ionic strength (0.01 M and 0.55 M NaCl) was detected for adsorption of PAA (MW 2000 at pH 1.8 and MW 5000 in the pH range 2–8) under the experimental conditions. The study of adsorption of acrylic acid and glutaric acid as possible monomers of PAAs at the mercury electrode has shown that even at low pH glutaric acid is poorly adsorbed, while acrylic acid is adsorbed more strongly than predicted from its hydrophobic properties defined through the octanol/water distribution coefficient.