The influence of dissolved humic acids on the kinetics of calcite precipitation from seawater solutions

Abstract

The influence of dissolved organic matter on the complex mechanism of calcite crystal growth from seawater was evaluated by a set of experiments at different humic acid concentrations (i.e. [HA]=50, 500, 1000 μg/kg) in NaCl–CaCl 2 solutions at a total ionic strength of 0.7 mol/kg. The temperature and P CO 2 of the experiments were maintained at 298 K and 40 Pa, respectively. The constant addition technique was used in order to maintain [Ca 2+] at ≅10.5 mmol/kg, while the [CO 3 2−] was varied to isolate its role on the precipitation rate. Assuming that the calcite precipitation in this solution is dominated by the reaction: (A1) Ca 2++ CO 3 2− ↔ k b k f CaCO 3 where k f and k b are the forward and reverse reaction rate constants, respectively, the net precipitation rate, R, can be described at any dissolved organic matter content by the difference between the forward and reverse rates: (A2) R=k f (a Ca 2+ ) n 1 (a CO 3 2− ) n 2 −k b or, in its logarithmic form: (A3) log(R+k b )− logK f +n 2 log[ CO 3 2−] where n i are the partial reaction orders with respect to the participating ions, a and γ are the ion activities and activity coefficients, respectively, and, K f= k f( a Ca 2+ ) n 1 ( γ CO 3 2− ) n 2 , a constant. Results of this study indicate that, similarly to seawater and NaCl–CaCl 2 solutions at the same ionic strength, the partial reaction order with respect to the carbonate ion concentration is 3, while the forward reaction rate constant, K f, decreases by one order of magnitude when the dissolved organic matter concentration increased from 0 to 1000 μg/kg. This suggests that the mechanism of calcite precipitation is independent of the dissolved organic matter concentration even if such a component inhibits the calcite precipitation rate. Applying our model to previous rate measurements carried out in seawater solution under the compositional condition [Ca 2+]≫[CO 3 2−], we found that the rate of calcite precipitation from seawater solutions, a complex function of P CO 2 and seawater inorganic inhibitors, still decreases as a function of the [HA] by at least one order of magnitude. Finally, we propose that the dissolved organic matter under the form of HA inhibits the calcite precipitation rate from seawater by covering the active growth sites rather than by complexation of calcium in solution.