Computer Science
Scientific paper
Oct 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011gecoa..75.5638e&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 75, Issue 19, p. 5638-5647.
Computer Science
Scientific paper
Lead speciation in many aqueous geochemical systems is dominated by carbonate complexation. However, direct observations of Pb 2 + complexation by carbonate ions are few in number. This work represents the first investigation of the equilibrium Pb 2 + + CO 3 2 - \rlhar PbCO 3 0 over a range of ionic strength. Through spectrophotometric observations of PbCO 3 0 formation at 25 °C in NaHCO 3 -NaClO 4 solutions, PbCO 3 0 formation constants of the form CO 3 β 1 = [ PbCO 3 0 ] / [ Pb 2 ] [ CO 3 2 - ] were determined between 0.001 and 5.0 molal ionic strength. Formation constant results were well represented by the equation: log β 1 = ( 6.789 ± 0.022 ) - 4.088 · I 0.5 1 + 1.5 I 0.5 + ( 0.244 ± 0.012 ) I . This result, combined with previous critical assessments of formation constants for the equilibrium PbCO 3 0 + CO 3 2 - \rlhar Pb ( CO 3 ) 2 2 - , was used to estimate the ionic strength dependence for the equilibrium Pb 2 + + 2 CO 3 2 - \rlhar Pb ( CO ) 2 2 - : log β 2 = ( 10.41 ± 0.18 ) - 4.088 · I 0.5 1 + 1.5 I 0.5 - ( 0.31 ± 0.33 ) I where CO 3 β 2 = [ Pb(CO ) 2 2 - ] / [ Pb 2 ] [ CO 3 2 - ] 2 . The carbonate complexation constants produced in this study, combined with previous complexation constants for formation of Pb II chloride and hydroxide species, were used to predict formation constants for mixed-ligand species Pb(CO 3 )Cl - , Pb(OH)Cl 0 , and Pb(CO 3 ) OH - . Formation constant estimates for the system Pb 2 + - HCO 3 - - Cl - - H + were then used to assess Pb II speciation in seawater. In the absence of complexation by organics, approximately 1.9% of the total lead in surface seawater ( S = 35, t = 25 °C, pH ˜8.2 (free H + concentration scale)) is present as free hydrated Pb 2 + . Carbonate complexes, PbCO 3 0 and Pb(CO 3 )Cl - , are predominant forms of Pb II in seawater at high pH, and lead chloride complexes are predominant species at low pH. For pH >7.7 the sum concentration of PbCO 3 0 , Pb(CO 3 )Cl - , PbOH + , and Pb(OH)Cl 0 in seawater exceeds the sum concentration of Pb 2 + , PbCl + , PbCl 2 0 , and PbCl 3 - .
Byrne Robert H.
Easley Regina A.
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