Gypsum Dissolution

CaSOi ZHjO (gypsum) = Ca + SO + 2H2O follows first-order kinetics as expressed by the equation 

James and Lupton (1978) found that gypsum solution rate increased with ionic strength. Their rate data, measured at a flow velocity of 0.15 m/s in up to 1.7 molal NaCl solutions, fit the equation A/ u = 1 + 2.2V7. The rate constant for gypsum dissolution increases with flow velocity v, at 25°C according to 

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Fig. 9 TDS derived from carbonate weathering (TDScarb) and from evaporite (gypsum) dissolution ("l DSevaplGY,.,) versus total TDS for all the monitoring stations. All expressed in mg L 1...

Raines MA, Dewers TA. Mixed transport/reaction control of gypsum dissolution kinetics in aqueous solutions and initiation of gypsum karst. Chem Geol 1997 140( 1—2) 29—48. 

Another key process involves gypsum dissolution gypsum is found in the subsurface as a natural constituent or as an added reclamation material. In cases where human actions enhance subsurface alkalinity, as a result of irrigation with alkali... 

The FREZCHEM model is a chemical equilibrium model. For a reaction such as gypsum dissolution... 

In another example, five test cases were computed by PHREEQE and EQ3/6 and the same thermodynamic database was run for each program (INTERA, 1983) to test for any code differences. The five examples were speciation of seawater with major ions, speciation of seawater with complete analysis, dissolution of microchne in dilute HCl, reduction of hematite and calcite by titration with methane, and dedolomitization with gypsum dissolution and increasing temperature. The results were nearly identical for each test case. Test cases need to become standard practice when using geochemical codes so that the results will have better credibility. A comparison of code computations with experimental data on activity coefficients and mineral solubilities over a range of conditions also will improve credibility (Nordstrom, 1994). 

In the Floridan aquifer, traces of gypsum are present in the carbonate rock. The high calcium concentration from gypsum dissolution exceeds its value at saturation with calcite, leading to precipitation of the carbonate and the production of additional CO2 (reaction 8). Concurrently, anaerobic decay of buried organic matter (reaction 3) and sulfate reduction (reaction 6) take place. The combination of these processes has caused an increase in the CO2 pressure of the groundwater from 10" bar in the recharge zone to 10" bar downdip as the pH decreases from 8.0 to 7.4 over a map distance of 115 km (Fig. 5.3) (cf. Back and Hanshaw 1970 Plummer et al. 1983). 

Increases in the concentrations of calciunj, bicarbonate or carbonate from sources other than the dissolution of calcite may supersaturate a water with respect to calcite, causing it to precipitate. In the Floridan carbonate aquifer, groundwaters attain calcite saturation by contact with limestone, but then may become supersaturated with respect to calcite because of gypsum dissolution (Back and Han-shaw 1970 Wicks and Herman 1996). Calcium from the gypsum, which is far more soluble than calcite, drives the common ion effect reaction... 

The same result can be obtained after gypsum dissolution (through the effect of cation in common with the carbonate), which sometimes occurs in the southern areas. 

During the flushing period, the uncontaminated upgradient groundwater dissolves gypsum. Dissolution of gypsum causes precipitation of a small amount of calcite and that in turn causes a drop of pH. The drop of pH causes dissolution of small amounts of Al(OH)3(fl) andFe(OH)3(fl). 

To explore the differences between the methods, we use REACT to calculate at 25°C the solubility of gypsum (CaSO4-2H2O) as a function of NaCl concentration. We use two datasets thermo. data, which invokes the B-dot equation, and thermo. hmw, based on the HMW model. The log K values for the gypsum dissolution reaction vary slightly between the datasets. To limit our... 

Gypsum has structure of alternating layers of electrostatically associated ions Ca and and layers of dipoles H O. H O bonds are very weak and are easily destroyed. That is why gypsum dissolution mechanism is controlled mostly by the dissociation reaction... 

Example 7.3. Gypsum dissolution into a static fluid... 

This means that for a 1 mm (0.001 m) wide fracture, the rate of gypsum dissolution nearly equals the rate of diffusive transport away from the dissolving surface. For narrower fractures, Da < 1, which means that the rate of diffusive transfer is fast compared to the dissolution rate. Dan 1 for wider fractures, which means that the reaction rate is faster than the diffusive transfer rate. 

Example 7.4. Gypsum dissolution as a function of fluid velocity... 

Example 7.5. Gypsum dissolution flux as a function of temperature... 

Modeling the effect of temperature on gypsum dissolution requires temperature functions for the variables in Eq. (7.49). 

This model assumes laminar flow past a flat plate with L = 0.001 m and q = 0.001 m/sec (Table 7.3). Figure 7.7 shows that the rate of gypsum dissolution changes from reaction limited at T < 0°C to transport limited for T > 100° C. The apparent activation energy for the mixed kinetics model ranges from 22 kJ/ mol at 0°C to 5 kJ/mol at 100°C. 

Most kinetics data in the literature suggest that gypsum dissolution is diffusion transport controlled. However, the work by Raines and Dewers (1997) shows that a mixed suiface reaction/transport control mechanism can operate over a range of hydrodynamic conditions and chemical saturation states for gypsum dissolution. Additionally, very little work can be found on gypsum dissolution kinetics at near equilibrium conditions where surface reaction controlled dissolution could be the dominated mechanism. Research on surface behavior of gypsum during dissolution is consistent with the conclusion that dissolution on the 010 surface is a lay-by-lay process and is not characterized by the formation of deep etch pits, even at conditions far fi om equilibrium. 

Hall, C. Cullen, D. C. (1996). Scanning force microscopy of gypsum dissolution and crystal growth. American Institute of Chemical Engineers Journal, 42,232-238. 

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