Carbonates dissolution rate constants

The effect of organic matter driven dissolution is to raise the carbonate transition in sediments relative to the saturation horizon in the water column (Emerson and Archer, 1990 Figure 11), but there should be little change in the ACOa iys ccD necessary to create the transition in %CaCOa. Thus, the argument about the relationship between the dissolution rate constant and observed transition of %CaCOa in sediments (Appendix A) should not be affected. 

The efficiency of calcium carbonate dissolution by metabolic CO2 strongly depends on the organic carbon / calcium carbonate rain ratio at the sediment surface, the oxidation rate of organic matter (and the depth horizon, where oxidation occurs), as well as the saturation state of bottom water (Q) and the dissolution rate constant k. ... 

As outlined above for the generic precipitation and dissolution also for precipita-tion/dissolution reactions including acid-based chemistry the kinetic solid carbonate precipitation/dissolution rate constant is a critical and sensitive factor influencing the Fe and Mn distribution along a given flow path. Depending from actual pH conditions at stationary conditions saturation of the solid carbonates with saturation indices Q can be expected. 

The kinetic parameters for DOC biodegradation and solid carbonate precipita-tion/dissolution are given in Table 11.5. For calcite precipitation and dissolution rate constant were assigned for both solid carbonates. The simulation time was 100 years with a time step length of 10 d. 

Broecker and Peng, p. 59 dlsscon = 7 dissolution constant in DJSS pcpcon = / carbonate precipitation constant disfac -. 01 scaling factor in dissolution rate eole/e 3/y delcorg 10 Fractionation by photosynthetic organises dcse = 2 Delta 13C Isotope ratio for sea eater, per eil... 

At equilibrimn, the rate of calcium carbonate dissolution is equal to the rate of its precipitation. The concentrations of the reactants and products remain constant over time, so no further net dissolution occms. Since the solution can dissolve no more calcium carbonate, it is said to be saturated. The for this reaction is given by... 

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... 

Figure 2.15. Log rate constant, k, as a function of log reciprocal diameter for dissolution of various biogenic carbonates a comparison with the geometric model. (After Walter and Morse, 1984b.)...

Additional information on metal-carbonate dissolution kinetics could be obtained by evaluating dissolution in relatively weak concentrations of HC1 (Sajwan et al, 1991). A plot of pseudo first-order rate constants kf k - [HC1]) versus HC1 concentration would allow one to estimate first-order constants (k) as HC1 — 0 by extrapolating the line representing k to the y axis. Additional pseudo first-order dissolution examples are shown in Figure 7.9 where the linear form of the pseudo first-order acid dissolution of kaolinite in two different HC1 concentrations is shown. 

Morse (30) carried out an examination of the near-equilibrium dissolution kinetics of calcium carbonate-rich deep sea sediments. His results are summarized in Figure 14. The sediment samples from different ocean basins have distinctly different reaction orders and empirical rate constants. The dissolution rate equations for the different sediment samples are ... 

The result of the calculation (Appendix A) indicates that the rate constant necessary to create the AC03,iys ccD =19 pmolkg has to be at least 100 times less than the laboratory-determined values. This has been confirmed by in situ measurements (see later). Perhaps the most important lesson here is that models and laboratory experiments consider primarily pure phases whereas impurities and surface coatings may greatly influence dissolution rates in nature. Until there is better agreement between the laboratory and in situ model-determined values, the latter will have to be used in model reconstructions of the relationship between ocean chemistry and sedimentary carbonate content. 

TABLE 1. Comparison of Rate Constants of Dissolution and Precipitation of Simple Carbonates Determined in Various Experimental Studies(<1>... 

Steady states for flow and for chemistry are different concepts and are often confused steady-state flow means constant directions and magnitudes of velocities steady-state chemistry means constant concentration distributions. Local equilibrium can possibly maintain a chemical steady state for reactive constituents in a transient flow regime (e.g., Ca2+ and HCCTj" in a Karst aquifer). However, if kinetics plays a significant role, e.g., the dissolution rate of feldspars or oxidation of organic carbon, then the chemical state in a transient flow field depends on competing factors of kinetic rates and velocity changes. Thus, chemical steady state may be achieved for some constituents but not others. 

The rate constants and corrosion rates of the 1010 (carbon steel) coupons for five ammonium chelants tested at 320°F (160°C) are shown in Tabie 6. The dissoiution curves are seen in Figure 3. The EDTA was tested with two different cations -Na and NH j. The cation effect is shown in Table 7 and Figure 4. The effect of temperature is described in Figure 5 and Table 8. The corrosion rates of the 1010 CS coupons were very low, indicating that the iron detected was associated mostly with scale dissolution, not general solvent corrosion. 

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