Short-term and interannual variability of redox-sensitive chemical parameters in hypoxic/anoxic bottom waters of the Chesapeake Bay

A combination of CTD casts, discrete bottle sampling and in situ voltammetric microelectrode profiling was used to examine changing redox conditions in the water column at a single station south of the Bay Bridge in the upper Chesapeake Bay in late July/early August, 2002–2005. Short-term (2–4 h) fluctuations in the oxic/suboxic/anoxic interface were documented using in situ voltammetric solid-state electrodes. Profiles of dissolved oxygen and sulfide revealed tidally-driven vertical fluctuations of several meters in the depth and thickness of the suboxic zone. Bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺ also varied over the tidal cycle by approximately an order of magnitude. These data indicate that redox species concentrations at this site varied more due to physical processes than biogeochemical processes. Based on analysis of ADCP data, tidal currents at this station were strongly polarized, with the principal axis of tidal currents aligned with the mainstem channel. Together with the chemical data, the ADCP analysis suggests tidal flushing of anoxic bottom waters with suboxic water from north of the site. The present study is thus unique because while most previous studies have focused on processes across relatively stable redox interfaces, our data clearly demonstrate the influence of rapidly changing physical mixing processes on water column redox chemistry.Also noted during the study were interannual differences in maximum bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺. In 2003, for example, heavy spring rains resulted in severe hypoxia/anoxia in June and early July. While reported storm-induced mixing in late July/early August 2003 partially alleviated the low-oxygen conditions, bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺ were still much higher than in the previous year. The latter implies that the response time of the microbial community inhabiting the suboxic/anoxic bottom waters to changing redox conditions is slow compared to the time scale of episodic mixing events. Bottom water concentrations of the redox-sensitive chemical species should thus be useful as a tracer to infer prior hypoxic/anoxic conditions not apparent from ambient oxygen levels at the time of sampling.     

Modelling the equilibrium speciation of nickel in the Tweed Estuary, UK: Voltammetric determinations and simulations using WHAM

The complexation of dissolved Ni has been evaluated in a rapidly-flushed, rural estuary (Tweed, UK) by ligand exchange-adsorptive cathodic stripping voltammetry. Results suggest the presence of strongly binding ligands, L, throughout, with average stability constants of about 10¹⁹ and which are saturated by ambient Ni concentrations. Equilibrium speciation calculations incorporating these constants in WHAM, version 6, predict an increase in Ni complexation (as NiL) from about 50% of total dissolved Ni in fresh water to over 90% in sea water. Equivalent calculations using the default-mode fulvic and humic substances (FS and HS, respectively) encoded in the WHAM database predict a reduction in complexation (as NiFS + NiHS) from about 20% in fresh water to less than 1% in sea water. Discrepancies arising from the two approaches are largely attributed to the different analytical detection windows employed. Thus, a better representation of Ni complexation is derived from including both types of complexant in the speciation calculations, resulting in estimates of net complexation in excess of 60% of total dissolved Ni throughout the estuary. The uncertainties and assumptions inherent in all computations illustrate the difficulty in measuring or predicting metal complexation in estuaries.     

The speciation of dissolved copper,cadmium and zinc in Manila Bay,Philippines

At present, there is a very limited information on the levels and distribution of dissolved metals in Manila Bay. In this study, the horizontal and vertical distribution of operationally defined species (labile, bound and total) of dissolved copper (Cu), cadmium (Cd) and zinc (Zn) were determined using differential pulse anodic and cathodic stripping voltammetry in water samples obtained from 18 stations in November 1998. In addition, the 24-h variability in the concentrations of these species at different depths in the water column was determined. These measurements were complemented by the determination of temperature, salinity, dissolved oxygen, chlorophyll a, particulate organic carbon and nutrients. Results showed that more than 50% of total dissolved copper and cadmium were labile while 50% of total dissolved zinc was organically bound. Vertical profiles showed that Cu, Cd and Zn concentrations were generally high at the surface. Zinc and cadmium were characterised by the presence of a mid-depth minimum while copper did not show any clear vertical trend.

Dissolved Cu concentrations during the spatial and diurnal samplings ranged from 0.32 to 6.95 nM and 1.52 to 45.65 nM, respectively. For Cd, the concentrations in 18 stations ranged from 0.05 to 2.92 nM, and from 0.03 to 2.42 nM over a 24-h period. Zn concentrations ranged from 2.48 to 147.43 nM and 2.87 to 88.27 nM during the spatial and diurnal samplings, respectively. The large variation in the concentration of Cu, Cd and Zn in the bay was observed to be associated with the presence of a large vertical density gradient in the water column, which appeared to limit the exchange of materials between the surface and bottom waters. Elevated levels of these metals near point sources suggest anthropogenic inputs in the bay.     

Voltammetric characterization of Boda Albitic Claystone. Comparison with previous Mössbauer spectroscopy results

We used ‘Voltammetry of MicroParticles’ to investigate electrochemically samples from the Boda Albitic Claystone, a potential site for deep underground disposal of nuclear waste in Hungary. The samples were collected near the surface and at a depth of 1050 m. The electrochemical characterization in acidic media allowed the identification of Fe²⁺ and Fe³⁺ ions associated with clays, hematite and pyrite. The cathodic detection of pyrite does not raise special difficulties, but the anodic one is complex and depends on the time of the samples exposure to the atmosphere. This work completes a previous Mössbauer spectroscopy study by clarifying the presence, or absence, of extremely small amounts of pyrite in some samples.     

碱性溶液中BH_4~-的电化学氧化机理及伏安法测定

对比Na BH4碱性溶液Cu、Ag、Ni、Pt和Au电极的循环伏安特性发现,BH-4在Au电极上活性最强,Pt有一定的活性,而Cu、Ag及Ni几乎没有活性。用三电极体系研究了BH-4在Au电极上的氧化机理,分析了碱性体系中不同浓度Na BH4溶液的线性循环伏安特性,讨论了扫描速度、OH-和B(OH)-4浓度等对BH-4氧化峰电位及峰电流的影响。结果表明,以Au电极为工作电极的峰值电流法是BH-4离子定性和定量检测的一种简单、快速、准确的方法。同时,溶液中较大浓度范围内变化的OH-和B(OH)-4对Na BH4的分析无明显影响。     

Zum Einfluß des pH-Wertes und des Calciumgehaltes auf Metall-Fulvinsäure-Bindungen — Inversvoltammetrische und chemometrische Untersuchungen

The Influence of pH and Calcium Concentration on Metal-Fulvic Acid Bonds – Stripping Voltammetric and Chemometric Investigations. Electrochemically available metal concentrations of Cd, Cu and Zn were analyzed in dependence of different concentrations of calcium, fulvic acids and pH by Differential Pulse Anodic Stripping Voltammetry (DPASV). A statistical experimental design was the base to minimize the number of experiments and to include the multifarious interactions between the independent variables (Ca concentration, concentration of fulvic acids and the pH value) and otherwise the electrochemically available metal concentrations of Cd, Cu and Zn. At first analysis of experimental data was carried out by multiple linear regression. The main influence on available metal concentrations is the pH value especially for Cu. The Ca ion influence has not competitive effect and is only significant for Cd and Cu. It has an effect on the metal-fulvic acid-bonds. Two factor interactions exist for pH/fulvic acids and for Ca/fulvic acids too. The partial least squares regression (PLS) model was used to include the interactions between the metals Cd, Cu and Zn. For proving these two models the three parameters pH, Ca and fulvic acid were varied within the calibration range of the models and predicted values were compared with the experimental values. The approach with the PLS model is better than the approach with the multiple linear regression (normally used mathematical method of analysis of a factorial plan) with a relative error of 9.7% for modelling of Cd, 6.0% for Zn and 58% for Cu in relation to multiple linear regression with errors of 12% for Cd, 8.6% for Zn and 65% for Cu. The PLS modelling is a suited tool for modelling chemical interactions also in simulated natural matrices.     

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

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.     

Use of voltammetric solid-state (micro)electrodes for studying biogeochemical processes: Laboratory measurements to real time measurements with an in situ electrochemical analyzer (ISEA)

Solid-state voltammetric (micro)electrodes have been used in a variety of environments to study biogeochemical processes. Here we show the wealth of information that has been obtained in the study of sediments, microbial mats, cultures and the water column including hydrothermal vents. Voltammetric analyzers have been developed to function with operator guidance and in unattended mode for temporal studies with an in situ electrochemical analyzer (ISEA). The electrodes can detect the presence (or absence) of a host of redox species and trace metals simultaneously. The multi-species capacity of the voltammetric electrode can be used to examine complex heterogeneous environments such as the root zone of salt marsh sediments. The data obtained with these systems clearly show that O₂ and Mn²⁺ profiles in marine sedimentary porewaters and in microbial biofilms on metal surfaces rarely overlap indicating that O₂ is not a direct oxidant for Mn²⁺. This lack of overlap was suggested originally by Joris Gieskes' group. In waters emanating from hydrothermal vents, Fe²⁺, H₂S and soluble molecular FeS clusters (FeS_aq) are detected indicating that the reactants for the pyrite formation reaction are H₂S and soluble molecular FeS clusters. Using the ISEA with electrodes at fixed positions, data collected continuously over three days near a Riftia pachyptila tubeworm field generally show that O₂ and H₂S anti-correlate and that H₂S and temperature generally correlate. Unlike sedimentary environments, the data clearly show that Riftia live in areas where both O₂ and H₂S co-exist so that its endosymbiont bacteria can perform chemosynthesis. However, physical mixing of diffuse flow vent waters with oceanic bottom waters above or to the side of the tubeworm field can dampen these correlations or even reverse them. Voltammetry is a powerful technique because it provides chemical speciation data (e.g.; oxidation state and different elemental compounds/ions) as well as quantitative data. Because (micro)organisms occupy environmental niches due to the system's chemistry, it is necessary to know chemical speciation. Voltammetric methods allow us to study how chemistry drives biology and how biology can affect chemistry for its own benefit.     

Interactions of cadmium(II) and protons with dead biomass of marine algae Fucus sp.

Samples of dead biomass from the marine brown algae Fucus ceranoides, Fucus vesiculosus and Fucus serratus were studied for their ability to remove cadmium from aqueous solutions. The metal sorption process is rapid, with 90% of the metal uptake completed within the first 25 min of contact. The kinetic data was described successfully by a pseudo second order chemical sorption process with rate constants of ca. 0.6 g mmol^− 1 min^− 1. At pH 4.5, the raw biomass of the three species exhibited equilibrium uptake capacities for Cd as high as 0.8 mmol g^− 1 (90 mg g^− 1), on a dry weight basis, without chemical pretreatment. These sorption capacities are much higher than those reported for activated carbon and chitin. The sorption of Cd was found to increase as pH increases, reaching a plateau at pH 5.Batch sorption experiments and continuous potentiometric titrations of acid-treated biomass samples in 0.05 M NaNO₃ were used to derive thermodynamic binding parameters according to the NICCA model. The total amount of acid sites was 2.4–2.9 mmol g^− 1, with median values of the affinity distribution for protons and cadmium ions, log K˜_H and log K˜_Cd, of 3.7 and 2.69, respectively (conditional values). The apparent heterogeneity of the sorbent was successfully taken into account by the empirical NICCA isotherm, which described very well the competition between protons and metal ions, in contrast with a simpler discrete competitive Langmuir model.The experimental results demonstrate that these seaweeds constitute a promising, efficient, cheap and biodegradable sorbent biomaterial for cadmium removal from wastewaters. This use would represent an example of exploitation of a renewable marine resource in water treatment technologies for the prevention of heavy metal pollution in the environment.     

Mobility of heavy metals from coal fly ash

The mobility of Cd, Co, Cu, Ni, Pb, Sb, and Zn from six different coal-fired power plant fly ashes that show a wide compositional range was examined using a sequential extraction procedure in order to assess their mobility when these wastes are ponded or landfilled. The extraction sequence was as follows: (1) water extractable, (2) cation exchangeable (CH₃COONH₄ at pH 7), (3) surface oxide-bound cations (CH₃COONH₄ at pH 5), (4) Fe oxide-bound cations (HONH₃Cl), and (5) residual (HF, HCl, HNO₃, 211). The heavy metal contents in the extraction solutions were determined by anodic (Cd, Cu, Pb, Sb, and Zn) and cathodic (Ni and Co) stripping voltammetry. The results reveal differences in the total contents of the selected trace elements among the fly ash samples, which must be related to differences in coal composition and combustion technology. The extractable fraction under natural conditions ranges from 1.5 to 36.4 percent of the total element content. Cadmium, Co, Cu, and Zn show the highest extractable fraction (10.8–18.9 percent on average). Cadmium is the most easily water-extractable element, while Co, Cu, and Zn increase their mobility as the severity of the extraction increases. Cobalt, Ni, Pb, and Zn are mainly associated with the surface oxide-bound and Fe oxide-bound fractions. Nickel, Pb, and Sb have low mobility potentials (5.3–6.6 percent as extractable fraction), but Sb presents a relatively high water-extractable fraction.