Comparative Carbonate and Hydroxide Complexation of Cations in Seawater

Comparative concentrations of carbonate and hydroxide complexes in natural solutions can be expressed in terms of reactions with bicarbonate that have no explicit pH dependence (MOHnz-n+nHCO3-↔M(CO3)nz-2n+nH2O). Stability constants for this reaction with n = 1 were determined using conventional formation constant data expressed in terms of hydroxide and carbonate. Available data indicate that stability constants appropriate to seawater at 25 °C expressed in the form Φ1=[MCO3z-2][MOHz-1]-1[HCO3-]-1 are on the order of 104.2 for a wide range of cations (Mz+) with z = +1, +2 and +3. Φ1 is sufficiently large that MCO3z-2 species appear to substantially dominate MOHz−1 species in seawater. Evaluations of comparative stepwise carbonate and hydroxide stability constant behavior leading to the formation of n = 2 and n = 3 complexes suggest that carbonate complexes generally dominate hydroxide complexes in seawater, even for cations whose inorganic speciation schemes in seawater are currently presumed to be strongly dominated by hydrolyzed forms (M(OH)nz-n). Calculated stability constants, Φ2=[M(CO3)2z-4][M(OH)2z-2]-1[HCO3-]-2 and Φ3=[M(CO3)3z-6][M(OH)3z-3]-1[HCO3-]-3, indicate that the importance of carbonate complexation is sufficiently large that carbonate and hydroxide complexes would be generally comparable even if calculated Φ2 and Φ3 values are overestimated by two or more orders of magnitude. Inclusion of mixed ligand species in carbonate–hydroxide speciation models allows cation complexation intensities (MT/[Mz+]) to be expressed in the following form: MT/[Mz+]=(1+R1)×β1∗[H+]-1+(1+2R21/2+R2)×β2∗[H+]-2+(1+3R31/3+3R32/3+R3)×β3∗[H+]-3+C, where MT represents the concentration of total dissolved metal exclusive of organic complexes and R1=[MCO3z-2][MOHz-1]-1=Φ1[HCO3-],R2=[M(CO3)2z-4][M(OH)2z-2]-1=Φ2[HCO3-]2,R3=[M(CO3)3z-6][M(OH)3z-3]-1=Φ3[HCO3-]3,βn∗=[M(OH)nz-n][H+]n[Mz+]-1, and C is a constant term (independent of pH) that accounts for formation of chloride, fluoride and sulfate complexation. The terms for mixed ligand complexation in this equation demonstrate that carbonate-containing species, MCO3OHz−3, MCO3(OH)2z-4 and, to a lesser extent, M(CO3)2OHz-5 should be quite significant relative to M(OH)2z-2 and M(OH)3z-3 even when R2 = 0.01 and R3 = 0.001. The results in this work indicate that the general importance of carbonate complexation in environmental solutions is substantially underestimated. Further studies should be devoted to experimental examination of the carbonate complexation of strongly hydrolyzed cations.