Prosun DU Short Course Part 1 2011

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LINNAEUS-PALME ACADEMIC EXCHANGE PROGRAMME

Chemical P rocesses and

SHORT COURSE

Department of GeologyUniversity of Dhaka

17-20 April, 2011

1

Prosun BhattacharyaProfessor

KTH-International Groundwater Arsenic Research GroupDepartment of Land and W ater Resources Engineering

Royal Institute of Technology (KTH)SE-100 44 Stockholm

SWEDENPart 1

Intended learning outcomes Identify and explain the basic principles and parameters

that govern the chemistry of groundwaters.

Identify and describe the geochemical processes in thegroundwater environment

Interpret groundwater data generated from field andlaboratory investigations

Use salient aspects of hydrogeochemical modeling todecipher the various hydrogeochemical processes

2

I ent y t e y rogeoc em ca processes ea ng to erentproblem of groundwater contamination with an emphasis onarsenic in different part of the world

Apply the overall knowledge to the investigation oncontaminated groundwater environments and development of methods for remediation.



Course structure• Groundwater as a eochemical s stem

Groundwater - geochemical system Principles of Thermodynamics Chemical kinetics Aqueous complexes Activity coefficients of the dissolved species Acids and Bases Carbonate geochemistry

3

Hydrological cycle and groundwater in the hydrosphere Chemical weathering

Controls of the composition of groundwater Quality and quantity of groundwater Adsorption and desorption

Course structure

• Redox rocesses Redox – theory and measurements Redox zones in natural systems Successions of redox reactions and zones Consumption of oxygen in the aquifers Biogeochemical restrictions Behaviour of Fe, As and S in groundwater

• Groundwater contamination and related

4

geoc em ca procesess

• Group assignments General hydrogeochemistry



5

Contents

• Groundwater Chemistry

Groundwater - geochemical system Principles of Thermodynamics Chemical kinetics Aqueous complexes Activity coefficients of the dissolved species

6

Acids and Bases Carbonate geochemistry



Groundwater as a geochemical

system

drinking, agriculture and industrial processes.

Traditionally, the geochemistry to groundwater was usedto classify different water types and to identify and quantifywater resources based on simple water quality parameters.

Generally, groundwater is considered as a l ow

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solid phases (minerals, crystalline and amorphoussolids, and organic material) dissolved gas phases an aqueous solution phase (water with dissolvedconstituents)

Groundwater - geochemicalsystem

Some of the typical reactions in groundwater systems:

CaCO3 (calcite) + CO2 (gas) + H2O ⇒ Ca2+ + 2HCO3-

2KFe3AlSi3O10 (OH)2 (biotite) + 14H+ + H2O ⇒

+ + 2+ + 0 +

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CH2O (organic material) + O2 (aq) ⇒ CO2 (gas) + H2O



Groundwater as a natural solution

comprising several dissolved ions and chemical species.

Groundwater is a primary medium of exchange andtransport.

Along the entire flow path from the surface to theaquifers, components of the solid phases may be dissolveinto the roundwater or reci itate from roundwater.

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The precipitation and/or dissolution of the solid phases

in the aquifers may facilitate or retard the movement of dissolved phases in groundwater.

Units of concentration

The results of groundwater analysis can be reported

in the following forms. Simply expressed in units of thequantity of the solute per unit quantity of the solvent .

A solute is for example sodium chloride (halite, NaCl),which dissolves in water which is a solvent.

10



Concentration can be expressed in terms of mass of the

Units of concentration

milligram / liter (mg/L)

microgram / liter (µg/L)

110

1

6

mg lkg

l solvente / =

−

110

1

9

μ g lkg

l solvente / =

−

solute / unit volume and mass of the solvent

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parts per million (1 ppm)

parts per billion (1 ppb)

11

10

6mg kg

g

g solvente

/ =

11

109

μ g kgg

g solvente / =

Concentration units

as moles of solute per unit volume and mass of thesolvent

n erms o p ys ca c em s ry, s necessary o express

concentrations in terms of the quantity of the molecules of acertain specie in water as mol/l (molarity) and mol/kg (molality).

For diluted solutions, these two units are equal. Forconcentrated solutions, the density has to be considered toconvert the units.

Concentrations expressed in terms of molality is is considered to

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the solution as compared to the volume of the solution which isvariable with changes in the temperature.

Further, the unit expressed as miliequivalent / liter, is similar tothe molarity, only in this case it is multiplied by the charge of thespecies, z .



Concentration units

units expressed as number of moles of the solute perunit volume and mass of the solvent

molar units (mol/L)

solution L

mol M unit molar =)(

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molal units (mol/kg)

solvent kg

molmunit molal =)(

Concentration units

milliequivalent (meq / L)

speciestheof zmol

Lmeqalent milliequiv charge) / ( ×=

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so ut on



Conversion of the units

Concentrations can be expressed in unitsmg/L and mg/kg

In natural water, the mass of 1 L is equal tothe 1 kg, thus molar and molal concentrationsare same.

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1 mg/L = 1 mg/kg = 1 ppm 1 µg/L = 1 µg/kg = 1 ppb

The exceptions are waters which are highly

mineralized and saline. When the total dissolved

Conversion of the units

soilds (TDS) contents are more than 10 000 mg/L,the concentration is determined throughmultiplication of the density, ρ [kg/L, kg/m3] of thesolvent.

For conversion of an anal sis re orted in m L to

C [mg/L ] × 1/ρ [l/kg] = C [mg/kg] (= ppm)

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mol/L, it is required to divide the concentrationwith the atomic weight mm of the species:

[ ]

[ ][ ]

C mg l

m g molC mmol l

m

/

/ / =



The direction of a reaction is overned b the

Chemical kinetics

thermodynamic constraints. In general, data on the kinetics of the reactionsare scanty and mostly the existing informations arebased on the empirical results.

The study of the chemical kinetics is carried out in

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Measure quantitatively the rate of a reaction in

the laboratory and in the field. Interpret these quantitatve data with respectto the possible mechanisms of the reaction.

Chemical kinetics

→ time of reaction t 1/2

solute-solute

solute-water

gas

water

hydrolysis of multivalent ions (polimerisation)

adsorpsion - desorpsion

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equilibrium between water and minerals

recrystalisation of minerals



Chemical kineticsGeneral reaction types (at low temperatures) with examples and reaction half times

Reac t ion type and exam ple Hal f - t im e ( t1/ 2)Solute -solute H 2C O 3

o = H + + H C O 3- (ac id -base) ~10 -6 s

o u te -w a t er C O 2 (aq) + H 2O = H 2C O 3

o (hydrat ion /hydro lys is ) ~0 .1 sC u 2+ + H 2O = C u O H + + H + (hydrat ion /complexat ion) ~10 -1 0 sFe(H 2O )6

2+ = F e ( H 2O )52+ + H 2O (hydrat ion /complexa t ion) ~10 -7 s

A d s o r p t i o n - d e s o r p t i o n C d 2+ + C a X = C a 2+ + C d X (X -2 i s the sur face s i te ) ~s-hrG a s - w a t e r o r g a s s o l u t io n - e x s o l u t i o n C O 2 (g ) = CO 2 (aq) minO x i d a t i o n - r e d u c t i o n F e 2+ + ¼ O 2 + 5 /2 H 2O = F e ( O H )3 (ppt ) + 2 H + mi n - h rH y d r o l y s i s o f m u l t iv a l e n t i o n s A ln+ m ( O H )3 n + 2 m

+m + H 2O = ( n +m) A l ( O H ) 3 ( s ) + m H + m in-

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- C a 2+ + H C O 3

- = C a C O 3 + H + w e e k - yI s o t o p i c e x c h a n g e 34 S O 4

2- + H 32 S - = H 34 S - + 32 S O 42- y

Mine ra l recrys ta l l iza t ion C a 2+ + H C O 3

- = C a C O 3 + H + yR a d i o a c t i v e d e c a y 14 C 14 N + e - 5 5 7 0 y

In order to determine the kinetics of a specific

Chemical kinetics

reaction, for example dissolution of calciumcarbonate, the following sequence of chemicalreactions:

Dissociation of the ions of calcium andcarbonate del mineral The conversion of carbonate ions tobicarbonate

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The conversion of bicarbonate to carbonicacid. The conversion of carbonic acid to carbondioxide



The rate of a reaction can be dependent on the

Chemical kinetics

Rate of the reaction

concentration of the reactants, the products orboth. For example, in the reaction:

A ( reactants) = B (products)

the r a t e o f t h e r e a c t i on can be described in thefollowing form:

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k dC dt

dC dt

A B= − =

The rate of the reaction is expressed as:

Chemical kinetics

Rate of the reaction

where:k = the rate constant of the reaction [in mol/L .s]

=

= k C C A Bα β

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ACB = the concentration of the productsα = the order of the reaction with respect to thereactantsβ = the order of the reaction with respect to theproducts



Chemical kinetics

Order of the reaction

The order of the reaction (n ) in general is thesum of the orders of the reaction with respect tothe reactives and the products:

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= α

Aqueous complexes

os race me a s an many o e ma orelements are transported in surface andgroundwaters cheifly in complex form. A com pl ex is a dissolved species thatexists because of the association of a cationand an anion or neutral molecule.

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that can combine with a cation to form acomplex.



Aqueous complexes are important in natural

Aqueous complexes

geoc em ca processes ecause o our as c reasons: Complexing of dissolved species that occurs in agiven mineral tends to increase the solubility. Forexample if we consider that the concentration of ΣmCa is controlled by calcite solubility

ΣmCa = mCa2+ = Ksp /m CO32-

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enough, significant amounts of complexes such asCaHCO

3

+ and CaSO4

0 are formed and the total Caconcentration in equilibrium is represented as

ΣmCa = mCa2+ + mCaHCO3+ + mCaSO4

0

Some elements occur in solution more often

Aqueous complexes

as comp exes an as ree ons. For examp e

Cu2+, Hg2+, Pb2+, Fe3+, U4+ etc.

The oxycation UO22+ and oxyanions such as

AsO42- and SO4

2- are complexes in which U, Asand S are always complexed with oxygen.

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favoured and/or inhibited when they occur ascomplexes as compared to the free ions.

The toxicity and bioavailability of metals innatural waters are dependent upon theaqueous speciation.



The complexation of each ions is based on the law of mass action. For exam le in natural environmental

Aqueous complexes

systems, lead (Pb) may exist in several forms. Theseinclude its complexation with different anions suchas Cl-, HCO3

-, CO3-, SO4

2-, OH-. In this case, theconcentration of total Pb in the system can bewritten as:

= 2+ + o

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Tota 2

(PbHCO3+) + (PbCO3

o) + (PbSO4o)+ (Pb(OH)+)

+ ....

where the total concentration of each species isdependent on the equillibrium constants.

For example, the formation of the complex PbCl+ can

Aqueous complexes

be expressed by the reaction

Pb+2 + Cl- = PbCl+

and the equillibrum constant can be written as

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K PbCl

Pb Cleq =

+

+ −

[ ]

[ ][ ]2



Specific problems to solve1. A sample of water from groundwater source analyses:

150 mg/L of calcium ions and 10 mg/L of sodium ions.Express these concentrations in mmol/L and meq/L. Whatis the concentration of sodium is required to completelyreplace the calcium ions during an ion exchange process?Express the results in mg/L, mmol/L and meq/L.

2. A solid solution of Ca-MgCO3 contains 5 percent of Mg.Calculate the fraction of moles of MgCO3 in this solidsolution.

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. e ens y o a sa ne wa er s . g a . swater contains 100 000 ppm of sodium. Express thisconcentration in mg/L. Now the saline water is heated

to100 °C. At this temperature one kg of the water has avolume of 0,84 L. Calculate the concentration of sodiumat this temperature.

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Prosun DU Short Course Part 1 2011