Diopside dissolution kinetics as a function of pH, CO[sub 2], temperature, and time

Journal Article · Fri Jan 01 04:00:00 EST 1993 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(93)90431-U· OSTI ID:6951094

Knauss, K; Nguyen, S N; Weed, H C ^[1]

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1. Lawrence Livermore National Lab., CA (United States)

The authors have measured the dissolution rate of diopside in dilute solutions (far from equilibrium) at 25, 50, and 70[degrees]C from pH 2 through pH 12 using a flow-through reactor. Reducing the CO[sub 2] concentration tenfold produced little, if any, effect on dissolution rate at alkaline pH (pH 8 through pH 12) at 25 and 70[degrees]C. Linear dissolution kinetics (i.e., time-invariant rates) were eventually observed in all runs. The overall trend with increasing pH is decreasing diopside dissolution rate based on the release rate of all constituents. The authors fit these rates by regression to a general rate law of the form r = Ak[[alpha][sub H[sup +]]][sup n], where A is the surface area, k is the rate constant, and n is the order with respect to hydrogen ion activity. At 70[degrees]C over the range pH 2 through pH 10 in solutions equilibrated with atmospheric CO[sub 2], the rate of diopside dissolution based on Si release is rate (mol/cm[sup 2]-s) = 2.45 ([plus minus]0.96)[times]10[sup [minus]13] [alpha][sub H[sup +]][sup 0.18[plus minus]0.03]. At 50[degrees]C the rate based on Si release is rate (mol/cm[sup 2]-s) = 1.10([plus minus]0.61)[times]10[sup [minus]13] [alpha][sub H[sup +]][sup 0.21[plus minus]]0.04. At 25[degrees]C the rate based on Si release is rate (mol/cm[sup 2]-s) = 2.88([plus minus]1.33)[times]10[sup [minus]14] [alpha][sub H[sup +]][sup 0.18[plus minus]0.03]. Based on a regression of the rate constants, over the temperature interval 25 to 70[degrees]C, the activation energy for the dissolution of diopside is 9.7 [plus minus] 0.4 kcal/mol. This energy is indicative of a surface-reaction controlled dissolution process, as is the observation of crystallographically controlled etch pits. 31 refs., 5 figs., 5 tabs.

• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 6951094
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 57:2; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
58 GEOSCIENCES
580000 -- Geosciences
ACTIVATION ENERGY
DIOPSIDE
DISPERSIONS
DISSOLUTION
ENERGY
EQUILIBRIUM
KINETICS
MINERALS
MIXTURES
OXYGEN COMPOUNDS
PYROXENES
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SOLUTIONS
SURFACE AREA
SURFACE PROPERTIES