Transcript USE OF GEOCHEMICAL EQUILIBRIUM

PART-2
Geochemical Equilibrium Models
CASE STUDY: MINEQL+
objectives
• PREDICTION OF METAL SPECIATION IN AQUEOUS
PHASE BASED ON:
– USE OF GEOCHEMICAL EQUILIBRIUM
MODELING
– CALCULATIONS BASED ON SEVERAL WATER
QUALITY MASTER VARIABLES
MINEQL+ LEARNING PLAN
•
1. BASIC CALCULATIONS
•
2. PERFORMING A SERIES OF CALCULATIONS
•
3. SURFACE ADSORPTION MODELING
•
4. OXIDATION-REDUCTION
•
5. SOLVING REAL WORLD EXAMPLE
PROBLEMS
WHY USE CHEMICAL
EQUILIBRIUM METHODS?
• The chemistry of water, including soil water,
is very complicated.
• Chemical constituents that are dissolved in
water may form chemical complexes,
precipitates as solid phases, de-gas from the
system or adsorb onto particulate surfaces
• All of these reaction-pathways are affected
by, and will affect, water quality parameters
such as pH, alkalinity, or ionic strength.
• This approach offers a way to understand
these chemical interactions in a straight
forward, unified manner.
THE CHEMICAL EQUILLIBRIUM IS OF INTEREST TO THE
FOLLOWING RESEARCH FIELDS
• Engineers and scientists studying the fate and transport of
chemicals in surface and ground waters
• Environmental Engineers examining drinking water
• Geologists and engineers looking at solute interactions
with minerals
• Toxicologists interested in the interactions of chemical
species with organisms
• Risk Assessors trying to improve the accuracy of models
• Ecologists looking at the dynamics of chemicals in the food
chain
WHAT CAN MINEQL+ DO?
• Investigate the speciation of any type of
aqueous chemical system.
• Key Features are:
» Compose a system from a choice of over 145 chemical
components
» Create new chemical components
» Scan a database of thermodynamic data for over 2300
complexes, solids and gases
» Review, edit, create new thermodynamic data
» Calculate the equilibrium conditions for any system
Key Features cont’d
» Perform synthetic titrations and sensitivity analysis
» Automatically process an unlimited number of field
data samples
» Calculate pH from electroneutrality or total proton
concentration
» Manage/view/extract output data from any perspective
» Graph any output data versus any independent
variable; for instance: titration curves or logC-pH or adistribution diagrams
» Extract the total dissolved concentration for any
component; optionally take surface complexes into
consideration.
Key Features cont’d
» Extract the pH and alkalinity of the
system for any number of calculations
» Extract the ion balance for an unlimited
number of runs
» Extract the saturation indexes (SI) for all
solids and see what solids control the
system
» Compile all species output into a single
report for easy comparison
» Save extracted output views as Text,
Lotus 1-2-3, etc.
MINEQL+ AND TYPICAL TYPES OF
CALCULATIONS
• Modeling the interaction of soil water with
mineral phases
• Simulating surface adsorption behavior
• Modeling the removal of metals from
drinking water
• Modeling the mobility of metals (e.g. Al
mobility in soils exposed to acid rain)
MINEQL+ AND TYPICAL TYPES OF
CALCULATIONS(cont’d)
• Assessing laboratory quality assurance,
quality control data
• Determining the predominant form of a
chemical constituent to asses toxicological
mechanisms
• Simulating acid-base titrations
• Assessing the environmental impact of
chemical additions
• Simulating the mixture of two waters
FIVE BASIC STEPS IN CHEMICAL
EQUILIBRIUM
1. Selection of the components that will
define the system
2. Creation of chemical species from these
components (could include scanning a
database of thermodynamic constants or
creating new species)
3. Setting the total concentration of individual
components
4. Running the calculation
5. Viewing and extracting output data
PROBLEM SOLVING
Select Chemical Components
Create Chemical Species from Components
Search Thermodynamic Database
Create New Species
Set Total Concentrations
Run Calculation
View/Extract Output Data
DEFINING THE CHEMICAL
SYSTEM
• Chemical Components = basic building
blocks of the system or chemical entities
used to create every reaction that you define
• Chemical components can combine in any
number of ratios, therefore, there can be a
high number of chemical species formed
from even a small number of components
Creating new Components
• Allows the creation of components for specialized use
• 55 NULL components are available for making your own
components
• For a given problem, a max of 25 components can be selected
• To create a new component, click the EDIT MODE button and
then select a NULL species that you would like to define. The
component information tool will appear. In this window, replace
NULL by the name of your component of interest, and in the
ionic charge box, insert the electrical charge of the new
component.
SCANNING THE THERMODYNAMIC
DATABASE
• Click the SCAN THERMO button
• MINEQL+ will then scan the default database
and/or the personal database if any
• During this scanning process, the chemical
equilibrium problem is automatically reset.
TAKING CONTROL OF CHEMICAL
REACTION DATA
 THE TABLEAU
The “Tableau” is a table of stoichiometric coefficients.
The columns of the table represent chemical
components
The rows of the table represent chemical species
A tableau will also include
Thermodynamic data that is associated with each
individual chemical species and
Total concentrations associated with each chemical
component
EXAMPLE: FOR THE FOLLOWING SET
OF REACTIONS
H+ + L- = HL
2H+ + L- = H2L+
M2+ + H2O = MOH + + H+
THE TABLEAU OF THESE REACTIONS WOULD BE WRITTEN AS FOLLOWS
Species
H+
L-
M2+
HL
1
1
K1
H 2 L+
2
1
K2
MOH+
-1
Tot. Conc (M)
HT
1
LT
MT
LogK
K3
TYPES OF CHEMICAL SPECIES THAT CAN BE FORMED IN
MINEQL+
Species TYPE
Category
Accessed by
Tableau Tool
1 (I)
Free, Aquo, or unassociated
NO
2 (II)
Dissolved complexes
YES
3 (III)
Fixed entities (e.g. solids,
gases, pH)
YES
4 (IV)
Precipitated solids
NO
5 (V)
Dissolved solids: solids w/ the
potential to precipitate
YES
6 (VI)
Species not considered
YES