The Influence of Temperature on PbCO30 Formation in Seawater

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Carbonate, Chemical speciation, Complexation, Lead, Seawater, Spectrophotometry

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UV spectrophotometry was used to directly observe the partitioning of lead(II) between PbCO30 and the forms of lead in natural seawater (S = 35.4) that are dominant at low pH (Pb2+ and complexes with Cl and SO42−). Lead carbonate formation constants were determined in the form, β1 = MPbCO3 (MPbMCO3)− 1, where MPbCO3 represents the concentration (mol/kg) of PbCO30, MPb represents the sum concentrations of free lead (Pb2+) and its complexes with chloride and sulfate, and MCO3 represents the sum concentration of free and ion paired carbonate in seawater. Over a range of temperature between 15 and 35 °C, the reaction enthalpy appropriate to β1 was calculated as ΔH = − 1.4 ± 0.6 kJ/mol. This is sufficiently small that, in the context of the uncertainties in β1 measurements at each temperature, the PbCO30 formation constant over the 20 °C range in temperature is effectively constant (β1 = (1.27 ± 0.02) × 104 (mol/kg)− 1). For total dissolved inorganic carbon concentrations in the order of 2 mm (millimolal), PbCO30 is the dominant inorganic form of lead(II) when pH is greater than 7.6.

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Marine Chemistry, v. 110, issues 1-2, p. 1-6