W. Davison

The kinetics of the oxidation of ferrous iron in synthetic and natural waters

Abstract The rate of oxidation of ferrous iron in a seasonally anoxic lake was measured on 39 occasions with respect to both depth and time. Sample disturbance was minimal as only oxygen had to be introduced to initiate the reaction. The data were consistent with the simple rate law for homogeneous chemical kinetics previously established for synthetic solutions. The rate constant for the oxidation reaction in lake water was indistinguishable from that measured in synthetic samples. It did not appear to be influenced by changes in the microbial populations or by changes in any particulate or soluble components in the water, including iron and manganese. Analysis of the errors inherent in the kinetic measurements showed that the estimation of pH was the major source of inaccuracy and that values of the rate constant determined by different workers could easily differ by a factor of six. The present data, together with a comprehensive survey of the literature, are used to suggest a ‘universal’ rate constant of ca. 2 × 1013 M−2 atm−1 min−1 (range 1.5–3 × 1013) in the rate law −d[Fe II] dt = k[Fe II]pO 2 (OH−) 2 for natural freshwaters in the pH range 6.5–7.4. Discrepancies in the effects of ionic strength and interfering substances reported in the literature are highlighted. Generally substances have only been found to interfere at concentrations which far exceed those in most natural waters.

Seasonal transformations and movements of iron in a productive English lake with deep-water anoxia

The seasonal and depth distributions of the concentrations of total and soluble iron in a seasonally anoxic lake basin are described for two years. Transport of iron is related to hydrodynamics and fluxes of iron both within and to and from the lake are calculated. The results enabled the construction of a schematic model for iron movement; it is critically dependent upon the many rate processes involved.

A critical comparison of the measured solubilities of ferrous sulphide in natural waters

Abstract A critical review of activity coefficients and calculation procedures appropriate to the numerical definition of the solubility of ferrous sulfide is presented. Past difficulties encountered when comparing results from waters of different ionic strengths are discussed and a new operational solubility product, which overcomes some of these problems, is defined. The available data for known anoxic basins are re-calculated and presented in this revised form. Analysis of the data suggests that the residence time in the water body of the particulate ferrous sulfide phase may influence the solubility. In a deep basin, such as the Black Sea, there may be a long residence time which allows the particulate matter to age and become less soluble. Very shallow basins do not allow substantial ageing to occur. It is probable that the anoxic waters of Lake Nitinat and the Cariaco Trench are not saturated with respect to ferrous sulfide. The inadequacy of using filtration for distinguishing soluble and hence ferrous iron is highlighted and polarographic or well defined colorimetric procedures are recommended for future work so that results may be unambiguously interpreted.

Manganese chemistry in rivers and streams

The speciation of Mn has been determined in 15 rivers and streams representing a wide variety of physico-chemical conditions. Using the technique of anodic stripping voltammetry (asv), specific for reduced Mn(II) species, it is found that a major part of the <0.015 μm Mn size fraction is present in a reduced Mn(II), asv-labile, form. In some waters there is also a significant asv inactive Mn fraction considered to be present as a ‘small colloidal’ species. The soluble (<0.015 μm) Mn fraction represents 15–95% of total Mn and does not appear to be dependent upon pH, alkalinity, specific conductance or humic substance concentration in the water. It is argued that under the dynamic, short residence time, conditions that apply in most rivers the paniculate and soluble Mn fractions are decoupled, their respective presence being dependent principally upon the catchment hydrogeological conditions. This contrasts with a previously held view that the paniculate phase is coupled to the dissolved phase by the pH dependent oxidation of dissolved Mn(II) to highly insoluble Mn(IV) species (Graham et al., 1976). Consideration of manganese speciation in waters which were incubated for five months showed that pH becomes the controlling factor when equilibrium is approached.

Formation of pyrite in freshwater sediments: Implications for C/S ratios

Abstract The vertical distribution of pyrite, acid volatile sulphide (AVS), carbon, and total S (St) were determined directly in the sediments of three lakes of different trophic status. The results showed that freshwater pyrite formation reflects the redox status of the sediment or overlying waters. It appears to form chiefly in reducing sediments which are subject to oxidizing influences, by either a low turnover of organic carbon or periodic incursions of oxygen. Although there are high concentrations of AVS in the near-surface sediments of productive lakes, very little is diagenetically converted to pyrite. The feasibility of using sulphur ratios to diagnose whether rocks were formed in marine or freshwater environments is assessed. New values for FeS 2 /FeS of 0.5-5 show that this ratio does not provide a reliable test. Values of C/Sp, where Sp represents pyrite sulphur, lie within the range of 160–700 and are much higher than previously measured ratios of C/S t of 1–50. These new determinations show that, if pyrite sulphur is unequivocally measured, C/S ratios may be a more sensitive indicator of salinity than had been previously thought.

Soluble inorganic ferrous complexes in natural waters

Abstract Recent stability constant data for ferrous complexation by chloride, hydroxide and sulphate ions are used to recalculate the equilibrium distribution of soluble ferrous species at pH 7 and 8 in fresh- and seawater. Contrary to previous calculations hydroxy complexes are negligible and the aquated Fe 2+ ion is the dominant species for all conditions. In seawater up to 15% of the iron might be present as chloride complexes, and 9% as sulfate complexes. The implications of these results to mineral formation are discussed.

Improvements in the precision of pH measurements a laboratory reference electrode with renewable free-diffusion liquid junction

Abstract Deficiencies in the performance of commercial reference electrodes with liquid junctions have been demonstrated in pH measurements on diluted standard buffers and fresh water samples. These problems can be overcome with renewable liquid junctions. A novel reference electrode is described, and its advantages demonstrated in titrimetric determination of the apparent second acidity constant of phosphoric acid. The new system suffers from no stirring, bias or memory errors.

Internal elemental cycles affecting the long-term alkalinity status of lakes: implications for lake restoration

Although the major processes affecting the acid-base balance of a lake are directly linked to reactions involving nitrogen and sulphur they are also influenced by the carbon cycle which is regulated by the supply of phosphorus. Changes in the long-term alkalinity of a lake are brought about by redox reactions in solution and through interactions with the atmosphere and sediments. Adding organic material or nutrients may help to restore acidified lakes, by generating base and by the increased carbon turnover creating a self-regulating ecosystem which is resistant to change.

Interlaboratory comparisons of the determination of pH in poorly buffered fresh waters

Abstract Ten participants gathered at one location to compare measurements, made in the field and laboratory, of the pH of quiescent solutions of natural waters and dilute acids. The total error associated with a single routine measurement in the laboratory can be controlled to within ±0.2 pH, comprising a standard deviation of 0.05 pH and a bias of 0.1 pH. Inaccurate preparation of standard solutions is an appreciable source of bias. Larger errors observed for field measurements made with commercial equipment arise from poorly defined analytical procedures, the use of inappropriate electrodes, and probably dampness affecting the electronics. Stirring dilute solutions depresses the pH. This bias error, which is similar for all dilute solutions, depends on the type of liquid junction used in the reference electrode. Combination electrodes produce the largest stirring shifts (>0.2 pH) and separate electrodes, especially those with free-flowing junctions, the smallest. Correct choice of equipment and use of well-defined proven procedures are essential for the accurate and precise determination of pH.

pH and ionic strength dependence of the ASV response of cadmium, lead and zinc in solutions which simulate natural waters☆

Anodic stripping voltammetry (ASV) peak currents of Pb, Zn and Cd (5 × 10−8M) in low ionic strength (1 mM), bicarbonate-buffered solutions were suppressed by increasing the pH (range 4–8). Increasing the ionic strength (1–500mM) of Pb solutions reduced the pH-dependent response, especially when NaClO, rather than KCl was used as the supporting electrolyte. At low ionic strengths and pH > 6, Pb solutions showed a broadened or double peak indicating an alternative oxidation mechanism. Use of NaCl as a supporting electrolyte showed that the cation controlled the current. This and other evidence, such as the time-dependent response, indicates that low currents were due to adsorption of trace metal onto the cell assembly. The inability to predict pH and ionic strength dependent changes of stripping currents measured in simple inorganic solutions illustrates the many problems associated with using ASV for speciation studies on untreated natural waters which contain extremely low concentrations of trace metals.