PART-2 Geochemical Equilibrium Models

CASE STUDY: MINEQL+

An interactive data management system for chemical equilibrium modeling

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 -distribution 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 calculation5. Viewing and extracting output data

PROBLEM SOLVING

View/Extract Output Data

Run Calculation

Set Total Concentrations

Create Chemical Species from ComponentsSearch Thermodynamic Database

Create New Species

Select Chemical Components

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 TABLEAUThe “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+ LogK

HL 1 1 K1

H2L+ 2 1 K2

MOH+ -1 1 K3

Tot. Conc (M) HT LT MT

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

TYPES OF CHEMICAL SPECIES THAT CAN BE FORMED IN MINEQL+