Lizardite serpentine dissolution kinetics as a function of pH and temperature, including effects of elevated pCO2

Measurements of the dissolution rate of lizardite (r) were carried out as a function of pH (2.5 <= pH <= 6.7) and temperature (27 degrees C <= T <= 90 degrees C) in continuously stirred flow-through systems, either in liquid-filled reactors or in aqueous solutions equilibrated with a headspace of gaseous CO2 (4 MPa <= pCO(2) <= 6 MPa). For the whole dataset, the stoichiometty of the reaction was highly correlated with the extent of Mg release, and congruent reactions were observed for the whole pH range only at T = 90 degrees C. Far from equilibrium, dissolution rates in dilute HCl solutions based on Si release and normalized to BET surface area can be described by: r = k(0) exp(-E-a/RT)a(H+)(n) with k(0) = 10(-2.27 +/- 0.56) mol.m(-2).s(-1); E-a = 42.0 +/- 1.5 kJ.mol(-1); R is the gas constant, n = 0.53 +/- 0.08. Moreover, in the pH range 3.2-6.2, the concentration of protons at the lizardite surface is proportional to a(H+)(0.47), which suggests that the dissolution rate has a first-order dependence on the surface concentration of protons. When the reaction was initiated in solutions equilibrated with elevated pCO(2), a slight increase of the dissolution rate (up to a 5-fold factor at pH = 5.0) was observed with respect to CO2-free solutions at the same pH. This may be attributed to the rate enhancing effect of HCO3- ligands. For any single pH-T condition investigated in the present study, lizardite dissolution rates are orders of magnitude lower than those of other anhydrous basic silicates, such as olivine or pyroxenes. The sluggishness of the dissolution reaction probably explains the slow carbonation rates that have been measured in previous studies. Crown Copyright (c) 2013 Published by Elsevier B.V. All rights reserved.