Reaction kinetics in complex solutions

Mg2+ influences calcite dissolution rates the same way, but not to the same extent as Ca2+. The inhibition effects of Mg2+ can be described in terms of a Langmuir adsorption isotherm. Sjoberg (1978) found he could model results for the combined influences of Ca2+ and Mg2+ in terms of site competition consistent with ion exchange equilibrium. The inhibition effects of Mg2+ in calcite powder runs increase with increasing Mg2+ concentration and as equilibrium is approached. 

A major portion of the studies on calcium carbonate reaction kinetics has been done in seawater because of the many significant geochemical problems related to this system. Morse and Berner (1979) summarized the work on carbonate dissolution kinetics in seawater and their application to the oceanic carbonate system. The only major seawater component in addition to Mg2+ that has been identified as a dissolution inhibitor is SO42- (Sjoberg, 1978 Mucci et al., 1989). Sjoberg s studies of other major and minor components (Sr2+, H3BO3, F-) showed no measurable influence on dissolution rates. Morse and Berner (1979) and Sjoberg (1978) found that for near-equilibrium dissolution in phosphate-free seawater, the dissolution rate could be described as  

More recent studies have generally concluded that the inhibiting influence of Mg2+ results from difficulties in rapid dehydration of the Mg2+ ion, or from crystal poisoning by adsorption of Mg2+ at reactive sites. Mucci and Morse (1983) found that the log of the rate constant was a linear function of the solution Mg2+ to Ca2+ ratio and that the empirical reaction order increased from 3.07 to 3.70 as the Mg2+ to Ca2+ increased from 1 to 10.3. 

Phosphate has been found to be an extremely strong inhibitor of carbonate reaction kinetics, even at micromolar concentrations. This constituent has been of considerable interest in seawater because of its variability in concentration. It has been observed that phosphate changes the critical undersaturation necessary for the onset of rapid calcite dissolution (e.g., Berner and Morse, 1974), and alters the empirical reaction order by approximately a factor of 6 in going from 0 to 10 mM orthophosphate solutions. Less influence was found on the log of the rate constant. Walter and Burton (1986) observed a smaller influence of phosphate on calcite 

Studies of phosphate inhibition of calcite dissolution both in the presence and absence of Ca2+ and Mg2+ have shown that the inhibiting effect of phosphate increases with increasing Ca2+ concentration. Results in solutions with Mg2+ were less reproducible, indicating the same trend, but not as strong an inhibition relation (Sjoberg, 1978), and a decrease in apparent equilibrium with increasing phosphate, 

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