PHREEQC keywords

The input window consists of two windows. The left, initially blank window is the space to enter the chemical analysis to be modeled together with the commands to perform the particular modeling task. PHREEQC keywords and PHREEQC BASIC statements may be listed in the right window. A mouse click on the + symbol displays the list of keywords. The utilization of the BASIC commands is explained in chapter 2.2.2.22. 

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

Alternatively to the keyword SOLUTION, SOLUTIONSPREAD can be used for the input of solution. The input is transposed compared to the input for SOLUTION, i.e. the rows of input for SOLUTION become the columns of input for SOLUTION SPREAD. It is especially convenient to define more than one aqueous solution composition using this tab-limited format. Data obtained e g. from a laboratory spreadsheet format can be copied directly into the PHREEQC input file. SOLUTION SPREAD is compatible with the format of many spreadsheet programs, as e.g. Excel. The column headings are element names, element valence state names or isotope names. One subheading can be used to define speciation (e.g. as S04 , or as N03 ), specify element specific units, redox couples, phase names and saturation indices. All succeeding lines are the data values for each solution, with one solution defined on each line. 

To model balanced reactions, kinetics or reactive transports, more keywords besides TITLE, SOLUTION and END are needed, which will be listed in the following. Furthermore, the individual input parameters are described in detail in the PHREEQC manual. 

In comparison to this calculation, the dissolution of gypsum in distilled water shall now be modeled by means of PHREEQC The input is very simple as it concerns distilled water and thus, the SOLUTION block contains only pH = 7 and temperature = 20 °C. To force equilibrium with gypsum, the keyword EQUILIBRIUMPHASES and the saturation index of 0 are used. 

For kinetic modeling in PHREEQC two keywords are necessary KINETICS n (n = number of SOLUTION, for which the kinetics shall be calculated) and RATES. For both keywords, a rate name has to be entered, e g. calcite when the dissolution of calcite shall be kinetically modeled. The general syntax within the keyword KINETICS is as shown in Table 25. 

Table 25 Syntax within the keyword kinetics in PHREEQC...

As reaction rates can be fitted mathematically in very different manners, there is an option (and need) in PHREEQC to declare any mathematical term in the form of a little BASIC program within the keyword RATES as will be shown in the following example of a time-dependent calcite dissolution ... 

Further examples can be found as already quoted in chapter 2.1.4.2 for K-feldspar, albite, calcite, pyrite, organic carbon and pyrolusite in the data set PHREEQC.dat or WATEQ4F.dat with the keyword RATES. There, all parameters are marked as comments by means of the sign in the block KINETICS. 

Table 28 Syntax of the keyword TRANSPORT in PHREEQC (Parkhurst Appelo 1999)...

First of all both solutions, the acid mine water and the groundwater, are defined in the PHREEQC input file and mixed applying the keyword MIX. Then this solution is saved as solution 3 (SAVE SOLUTION) and the job is finished by END. A second job follows, which uses again SOLUTION 2 (groundwater) and SOLUTION 3 (1 1 diluted water) via the key word USE, once again mixes both solutions 1 1, and saves the result as SOLUTION 4, etc. SELECTED OUTPUT facilitates the further data processing in EXCEL by providing the pH values and the molalities of all uranium species. The key word itself has to be repeated for every job, as well as the definition of the desired parameters pH and molalities. 

Using the keyword TRANSPORT PHREEQC always expects an initial solution in the cells, defined as SOLUTION 1-n (here 1-40). For this exercise it is possible to... 

In phreeqc, a keyword signifies the beginning of a data block , which may contain several lines of data, and within which the ordering is important. After looking at a few examples, it is not hard to make such a file. As in most programs, there is a facility in the input file to change the data in the database, or to add new basis species, without actually editing the database itself. 

The phreeqc input file is shown in Table 8.1. Note that we use sulfate for charge balance, because it is the most abundant ion. The default definition of alkalinity in phreeqc is as calcite , which is what our analysis shows, so we simply enter that number (153 mg L-1). Under the keyword REACTION, a suitable combination of moles of HCl and number of steps is found with some calculation and a certain amount of trial and error. We specify the file name of the output file to ensure we get an output file with the essential data arranged for copying to a spreadsheet. The results are shown in Figure 8.2. 

KINETICS This keyword begins a block of data which identifies kinetic reactions and supplies parameters used in each reaction, such as activation energies, duration of reactions, number of steps, etc. In Table 11.2 only one kinetic reaction is specified, for albite dissolution. For comparison with the react example, we use the same amount of mineral, and the same kinetic parameters. The current amount (moles) of mineral, which will decrease during dissolution, is identified as -m, and the initial amount of mineral, in this case the same quantity, is identified as -mO. phreeqc measures... 

It is probably always best to begin by adding data to input files, rather than to the database, until we are quite sure of what we are doing, especially if we are not the only user of the database, phreeqc allows us to do this quite easily, using the special input file keywords SOLUTlON MASTER SPECIES, SOLUTlON SPEClES, and SURFACE SPECIES. 

Aqueous Species The keyword SOLUTION MASTER SPECIES allows us to define new basis species. To see how this is done, the easiest thing is to look into phreeqc.dat to see how others are defined. Since it has not previously been defined, we have a choice... 

Other aqueous arsenic species are then entered using the SOLUTION SPECIES keyword. Again copy the format given in phreeqc.dat. For As(OH>3, this is... 

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