1209_Modeling of pH and inorganic carbon speciation in estuaries using the composition...



Title and Author

Title: Modeling of pH and inorganic carbon speciation in estuaries using the composition of the river ad seawater end members

Authors: L.M. Mosley1, B.M. Peake2, K.A. Hunter3

1Water Quality Branch, Environment Protection Authority, GPO Box 2607, Adelaide, SA 5001, South Australia
2Chemistry Department, University of Otago, P.O. Box 56, Dunedin, New Zealand
3Chemistry Department, University of Otago, P.O. Box 56, Dunedin, New Zealand

    Summary of Paper
Hydrogen ion activity or pH in any natural water system is controlled by the acids and bases present in water system. Out of many acids and bases, carbonates and bicarbonates are of utmost importance because of their abundance due to exchange of CO2 with atmosphere, and the respiration process of aquatic organisms. In this paper, pH and inorganic carbon speciation are modeled and verified by laboratory as well as field experiments.

Initial concentration and alkalinity, which are represented by the following equations, are used to model the system.

C_T= [H_2 ?CO?_3^* ]+[?HCO?_3^- ]+[?CO?_3^(2-) ]

Alk=[?HCO?_3^- ]+2[?CO?_3^(2-) ]+[?OH?^- ]-[H^+ ]+[?B(OH)?_4^- ]+[?HPO?_4^(2-) ]+2[?PO?_4^(3-) ]+[?Si(OH)?_3^- ]-[?HSO?_4^- ]-[HF]-[H_3 ?PO?_4 ]+[Organic alkalinity]

Modeling was done for the complete salinity range. Field measurements as well as laboratory experiments agreed well with the developed model.

The field and the laboratory mixing experiments showed that pH decreased for low salinity range (S<2), it then increased for intermediate salinities (S ? 2 ? 15) whereas the pH remained nearly constant for higher salinities (S > 15). Slight deviations of the model from the experimental data may be due to dye addition as this addition of dye can alter seawater pH from 0.001 to 0.03 units even if the ratio of acids and bases in the dye is kept the same as in the samples. Dissolved organic matter which may be a possible source of alkalinity change was not taken into account in the model. Photosynthesis and respiration processes also affect pH which might be another source of deviation of the model from experimental data. It was found that ? HCO?_3^- was the dominant species at all salinities. It is due to the possible reaction of carbonic acid with carbonate ions formed due to initial dissociation. This reaction might have produced bicarbonates.

    Contribution to ESEL
pH measurements and inorganic carbonate speciation are, no doubt, very old topics of research. But, modeling of inorganic carbonate speciation and pH was a new work in modeling. So, it depends on thinking approach. A different thinking and look at an old topic may lead to something new.

By: Aamir Alaud-din

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