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8/6/2019 Prosun DU Short Course Part 1 2011
http://slidepdf.com/reader/full/prosun-du-short-course-part-1-2011 1/15
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.
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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
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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
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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
7
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 +
8
CH2O (organic material) + O2 (aq) ⇒ CO2 (gas) + H2O
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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.
9
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
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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
11
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
12
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 .
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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 =)(
13
molal units (mol/kg)
solvent kg
molmunit molal =)(
Concentration units
milliequivalent (meq / L)
speciestheof zmol
Lmeqalent milliequiv charge) / ( ×=
14
so ut on
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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.
15
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)
16
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
/
/ / =
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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
17
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
18
equilibrium between water and minerals
recrystalisation of minerals
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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-
19
- 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
20
The conversion of bicarbonate to carbonicacid. The conversion of carbonic acid to carbondioxide
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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:
21
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
22
ACB = the concentration of the productsα = the order of the reaction with respect to thereactantsβ = the order of the reaction with respect to theproducts
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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:
23
= 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.
24
that can combine with a cation to form acomplex.
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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-
25
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.
26
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.
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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
27
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
28
K PbCl
Pb Cleq =
+
+ −
[ ]
[ ][ ]2
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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.
29
. 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.