INTRODUCTION
Irreversible reactions include mixing of solutions (MIX keyword), addition (or removal) of fixed amounts of specified reactants (REACTION keyword), and changing temperature (REACTION_TEMPERATURE keyword).
The same keyword may be used several times to define reactants; an integer number is used to identify each keyword data block. Thus, the user can define three different solutions and assign, for example, the integers 1, 10, and 20 to identify them--SOLUTION 1, SOLUTION 10, and SOLUTION 20. Integer identifiers also are used with EQUILIBRIUM_PHASES, EXCHANGE, SURFACE, GAS_PHASE, MIX, REACTION, and REACTION_TEMPERATURE data blocks.
Reaction-path modeling is used to simulate reactions that can be thought of as occurring in a beaker. A solution, or mixture of solutions, is placed in the beaker along with any other equilibrium reactants (phase assemblage, exchange assemblage, surface assemblage, or gas phase), irreversible reactions are applied (REACTION and REACTION_TEMPERATURE), and the new equilibrium state is calculated.
A key concept for PHREEQC reaction-path calculations is a "simulation." A simulation is defined to be a set of keyword data blocks terminated by the END keyword. Reversible and irreversible reactants are defined within a simulation either by keyword data blocks (first occurrence within a simulation of SOLUTION, EQUILIBRIUM_PHASES, REACTION, and other types of keyword data blocks) or by the USE keyword, which specifies the use of a reactant that has been defined in a previous simulation. New compositions of the solution, phase assemblage, exchange assemblage, surface assemblage, and gas phase that are calculated by a reaction simulation can be saved for use in subsequent simulations with the SAVE keyword.
Reaction-path modeling may include the use of any of the following keyword data blocks: SOLUTION, EQUILIBRIUM_PHASES, EXCHANGE, SURFACE, GAS_PHASE, MIX, REACTION, REACTION_TEMPERATURE, SAVE, and USE.