PHREEQC

The program PHREEQC dates back to 1980 (Parkhurst et al. 1980), at that time written in FORTRAN and named PHREEQE. The option of the program comprised  

In 1988, a version of PHREEQE was written including PITZER equations for ionic strengths greater 1 mol/L thus applicable for brines or highly concentrated electrolytic solutions (PHRQPITZ, Plummer et al. 1988). PHREEQM (Appelo Postma 1994) included all options of PHREEQE and additionally a one- 

In 1995 PHREEQC (Parkhurst 1995) was completely rewritten using the C programming language. This version removed nearly all limits regarding number of elements, aquatic species, solutions, phases, exchangers and surface complexes and caused the abolition of Fortran formats in the input files. Additionally, the equation solver was revised (more robust now) and several other options were added. With the 1995 version to the present, the following options have been possible  

The most recent version, PHREEQC in the version 2 (Parkhurst Appelo 1999), additionally allows for the following simulations  

Furthermore, it is possible to shorten the data output user-defined and to export it in a spreadsheet compatible data format. A BASIC interpreter program is implemented for programming user specific questions concerning kinetics and output formats. The BASIC interpreter also supports direct graphic output in 

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Parkhurst DL.andAppeloCAJ. User s guide to PHREEQC (version 2)-A comuter program for speciation, batch-reaction, one-dimentional transport and inverse geochemical calculations. 1999 US Geological Survey Water-Resources Investigations Report 99-4259. 

PHREEQC origin David Parkhurst US Geological Survey Lakewood, Colorado USA... 

Dissolved arsenic is correlated with ammonia (Fig. 4), consistent with a release mechanism associated with the oxidation of organic carbon. Other chemical data not shown here provide clear evidence of iron, manganese and sulfate reduction and abundant methane in some samples indicates that methanogenesis is also occurring. It is not clear however if arsenic is released primarily by a desorption process associated with reduction of sorbed arsenic or by release after the reductive dissolution of the iron oxide sorbent. Phreeqc analysis shows PC02 between 10"12 and 10"° bars and that high arsenic waters are supersaturated with both siderite and vivianite. 

KEYWORDS Kinetic Testing, AMD, Tailings, Aqueous Speciation, PHREEQC, Nickel Mining. 

PHREEQC version 2.15 was used to calculate equilibrium concentrations of ion pairs from the leachate chemistry. PHREEQCi was initially developed by the United States Geological Survey and a substantial library of thermodynamic constants has built up over the ongoing development period (Appelo Postma 2005). 

The distributions of CuS04 and NiS04 molality (m), determined using PHREEQC, are presented as Figure 1. NiS04 m is maximum at week 15 and decreases thereafter with peaks at week 34 and around week 60. 

The negative sign refers to mineral precipitation instead of dissolution. Such computations are done with PHREEQC (Parkhurst Appelo 1999). An inescapable conclusion of mass balances is that during the weathering of pyritiferous hydrothermally altered rock, iron and silica are precipitated. 

Aqueous geochemical modeling was performed using the program PHREEQC... 

Interactive Version 2.15.0 (Parkhurst Appelo 1999). The llnl.dat database distributed with PHREEQC was used for all calculations. Geochemical modeling was used to speciate the components of the tailings pore-water at discrete depths and to investigate the thermodynamic stability of mineral phases through out the tailings. 

Fig. 3. Pore-water mineral saturation indices calculated with PHREEQC (Parkhurst Appelo 1999) using the llnl.dat database.

Parkhurst, D.L. Appelo, C.A.J. 1999. User s Guide to PHREEQC (Version 2.15.0)-A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport and Inverse Geochemical Calculations. U.S. Geological Survey, Water Resources Investigation Report 99-4259. 

The analysis that follows has been developed using the inverse modelling capabilities of PHREEQC, in which we account for the chemical changes that occur as a water evolves along a flow path [as] one aqueous solution is assumed to mix with [another], and [/or] to react with minerals and gases to produce the observed... 

Fig. 3. Simulations calculated with the PHREEQC geochemical code (Parkhust Appelo 1999) (a) time-dependent diagram for the pH evolution of the Aspo ground water/bentonite interaction (b) time-dependent diagram for the pe evolution of the Aspo groundwater/bentonite interaction. Curves correspond to different initial partial oxygen pressures. Initial calcite and pyrite contents are 0.3 wt% and 0.01 wt% respectively, except for the curve of log/02 = —0.22 where calcite and pyrite contents are 1.4 wt% and 0.3 wt%, respectively, pe calculated stands for the cases where the oxygen fugacity is obtained from the groundwater redox potential (Bruno et at. 1999).

Fig. 4. Simulations calculated with the PHREEQC geochemical code (Parkhust Appelo 1999) (a) time-dependent evolution of Eh (mV) for a Spanish granite groundwater in contact with FEBEX bentonite (b) time-dependent evolution of pH for a Spanish granite groundwater in contact with FEBEX bentonite (Arcos et al. 2000a).

Initially, Oz diffuses through the bentonite and granitic domains, controlling the redox state of the system. Once 02 is exhausted, granitic groundwater controls the redox state of the system. The results of these calculations performed with the PHREEQC geochemical code (Parkhust Appelo 1999) clearly indicate that there is a substantial variability in pH/pe space along the temporal and spatial evolution of the near field of a repositoiy. This has clear consequences for the subsequent interactions with the Fe canister material and finally with the spent fuel matrix. 

Fig. 9. Experimental solubilities as total uranium concentration in solution for experiments on dissolution of uraninite samples from Oklo and Cigar Lake. Solid lines correspond to the calculated solubilities. Calculations performed with PHREEQC geochemical code (Parkhust Appelo 1999) and uranium database taken from Grenthe et al. (1992) and Bruno Puigdomenech (1989).

Application of the collected thermodynamic data to model the oxidative alteration pathway of U02 under repositoiy conditions by using the PHREEQC code (Parkhurst Appelo 1999) is given in Fig. 1 la and b. Once the thermodynamic framework is set for the geochemical evolution of the repositoiy system, we have to take into consideration that for many of the processes involved, there will be some kinetic constraints. This is illustrated by Table 2, where a comparison of the expected lifetime for some of the phases expected in the repositoiy system is made. 

Fig. 11. (a) Thermodynamic reaction pathway for the initial oxidative alteration of the spent fuel matrix at pH 8, calculated by using the PHREEQC code (adapted from Bruno etaL 1995). (b) Thermodynamic reaction pathway for the alteration of schoepite in granitic/bentonite groundwater at pH 8, calculated by using the PHREEQC code (adapted from Bruno et al. 1995 with permission). 

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