Physical chemical properties of trace and minor components of natural waters: solubility, speciation, and density

The physical properties of minor/trace components of natural waters aren’t well known. Although these components have received a great deal of study recently only a small focus has been on properties such as speciation and solubility, which influence the behavior and fate in the environment. The pH has a large influence on speciation. This is of increasing importance due to ocean acidification from the anthropogenic input of CO2 into the environment and the resultant uptake by the oceans. Metals that form strong complexes with hydroxide and carbonate will see large changes speciation over the next few centuries. Metals with biological importance, either as a nutrient or toxin, are of most interest. The bioavailable form of most metals will increase; this can be potentially helpful or harmful depending on the metal. Knowledge of speciation is often limited, when measurements are lacking, correlations have been used to make reasonable estimates. The hydrolysis of Al(III) in NaCl is well known over a wide range of conditions. A near linear correlation between the hydrolysis constants of Al(III) and a variety of +2, +3, and +4 metals has been found. This provides estimates of hydrolysis constants when measurements are not available. Lead is extremely difficult to measure due to low solubility, but is important because of its toxicity. The formation constant (beta) of PbCO3 is not well known and most speciation calculations are done using correlations with Cd or Zn. The betaPbCO3 was measured in NaCl at 25°C from I= 0.05-3 m. This was then modeled using a Pitzer Model and combining the new measurements with all previously published data on PbCO3 and PbCln2-n. The Pitzer model can then be used for lead speciation in most natural waters including seawater. Calcite and Aragonite have been well studied due to their use by shell forming organisms. However, several lines of evidence show that 51-71% of the CaCO3 produced in the surface oceans is dissolving unexpectedly above the aragonite saturation horizon. The most likely explanation is a more soluble form of CaCO3, but no possible source was known. Then recently, it was discovered that telost fish produce a high magnesium calcite as a byproduct of osmoregulation. The solubility of fish produced high magnesium calcite was measured in Gulf Stream seawater at 25°C. The stoichiometric solubility product constant (K*sp) was determined to be 5.89, in agreement with Bahamas Banks high magnesium calcite and approximately twice as soluble as aragonite. This more soluble CaCO3 likely explains at least a portion of the CaCO3 dissolution above the aragonite saturation horizon. Minor components of seawater can also influence density, a highly used property; however most equations were determined on surfaces waters which have negligible concentrations of minor components. Deep waters can have significant amounts of silicate, nitrate and normalized total alkalinity. Using measurements of density and nutrient concentrations, semi-empirical equations have been determined for nitrate, silicate and normalized total alkalinity, and can increase density by up to ~20ppm. All of these measurements help to improve our understanding of the physical properties of minor components of seawater, and how they might change under future ocean conditions.

Woosley R. J., 2012. Physical chemical properties of trace and minor components of natural waters: solubility, speciation, and density. PhD thesis, University of Miami, 200 p. Thesis.

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