Physical Chemistry CE 541. What is Physical Chemistry Is the science that deals with laws that is related (or that govern) chemical phenomena such as:

Physical ChemistryPhysical Chemistry

CE 541CE 541

What is Physical What is Physical ChemistryChemistry

Is the science that deals with laws Is the science that deals with laws that is related (or that govern) that is related (or that govern) chemical phenomena such as:chemical phenomena such as:

Gas lawsGas laws Oxidation-reduction reactionsOxidation-reduction reactions Equilibrium relationshipEquilibrium relationship

ThermodynamicsThermodynamics

Is the study of energy Is the study of energy changes accompanying changes accompanying physical or chemical physical or chemical processesprocesses


(a). Heat and Work(a). Heat and Work

HeatHeatIs a form of energy passing from one body to Is a form of energy passing from one body to another as a result of temperature differenceanother as a result of temperature difference

Heat UnitsHeat UnitsCalorieCalorie [ [heat required to raise the temperature of heat required to raise the temperature of one gram of water one degree Celsius, one gram of water one degree Celsius, C C]]British thermal unitBritish thermal unit (Btu) [ (Btu) [heat required to heat required to raise the temperature of one pound of water by one raise the temperature of one pound of water by one degree Fahrenheit, degree Fahrenheit, F F]]

Btu = 252 Calories = 1,054 Joules (J)Btu = 252 Calories = 1,054 Joules (J)

Specific heat of a substance [Specific heat of a substance [heat heat required to raise the temperature of 1 required to raise the temperature of 1 gram of the substance by one degree gram of the substance by one degree Celsius, Celsius, C C]]

WhereWhere C = specific heatC = specific heat q = heat added in Calories or Joulesq = heat added in Calories or Joules M = weight of the substance, gramsM = weight of the substance, grams T = raise in temperature, T = raise in temperature, C C

TM

qC

For water at 15 For water at 15 C C

C = 1.000 Cal or 4.184 J / gram-degree C = 1.000 Cal or 4.184 J / gram-degree CC

Heat of fusionHeat of fusion : heat required to melt a : heat required to melt a substance at its normal substance at its normal meltingmelting temperature.temperature.

Heat of vaporizationHeat of vaporization : heat required to : heat required to evaporate a substance at its normal evaporate a substance at its normal temperature of temperature of boilingboiling

WorkWork

Measured in:Measured in:

Force Force Distance Distance

Work (dw) is equivalent to:Work (dw) is equivalent to:

Pressure Pressure Volume Change Volume Change

Then,Then,

(dw) = P (dw) = P dV dV

Work UnitsWork Units foot – pounds (ft-lb)foot – pounds (ft-lb) Joules ( 1 Cal = 4.184 J)Joules ( 1 Cal = 4.184 J) Btu (1 Btu = 778 ft-lb)Btu (1 Btu = 778 ft-lb)

Work and Heat are forms of Energy. Work and Heat are forms of Energy. Therefore,Therefore,

Work = HeatWork = Heat


(b). Energy(b). EnergyConservation-of-energy LawConservation-of-energy Law"Any heat or work which flow into or "Any heat or work which flow into or out of the system must result in a out of the system must result in a change in the total energy stored in the change in the total energy stored in the system"system"

E = q - E = q - E = change in energy E = change in energy q = heat flowing into the systemq = heat flowing into the system = work done by the system= work done by the system

q is positive (+ve) if the system q is positive (+ve) if the system absorbs the heatabsorbs the heat

q is negative (-ve) if the system q is negative (-ve) if the system gives off the heatgives off the heat

is +ve if the system does the is +ve if the system does the workwork

is –ve if the surroundings do is –ve if the surroundings do the work on the systemthe work on the system

If the chemical system does not expand or If the chemical system does not expand or contract (volume is constant), then:contract (volume is constant), then:

E = qE = qvv

qqvv = heat absorbed in a constant-volume = heat absorbed in a constant-volume systemsystem

In Environmental Engineering Applications, In Environmental Engineering Applications, most of the systems are open, so they most of the systems are open, so they operate under constant pressure rather operate under constant pressure rather than constant volumethan constant volume

ThermodynamicsThermodynamics(c). Enthalpy (H)(c). Enthalpy (H)

H = E + PVH = E + PV

WhereWhere H = enthalpyH = enthalpy E = internal energy of the systemE = internal energy of the system P = pressure on the systemP = pressure on the system V = volume of the systemV = volume of the system

EnthalpyEnthalpy

A thermodynamic function of a A thermodynamic function of a system, equivalent to the sum of the system, equivalent to the sum of the internal energy of the system plus the internal energy of the system plus the product of its volume multiplied by the product of its volume multiplied by the pressure exerted on it by its pressure exerted on it by its surroundings.surroundings.

At constant pressure system,At constant pressure system,

Heat absorbed by the system = qHeat absorbed by the system = qpp

Work done by the system can be Work done by the system can be obtained by integrating obtained by integrating

dw = P dv dw = P dv = P (V = P (V22 – V – V11))

then, change in internal energy is:then, change in internal energy is:

E = EE = E22- E- E11 = q = qpp - - = q = qpp – [P (V – [P (V22 – V – V11)])]

OrOr

(E(E22 + PV + PV22) – (E) – (E11 + PV + PV11) = q) = qpp

(E(E22 + PV + PV22) is the final enthalpy) is the final enthalpy

(E(E11 + PV + PV11) is the initial enthalpy) is the initial enthalpy

So,So,

HH22 – H – H11 = q = qpp H = qH = qpp (T and P are constant)(T and P are constant)

+ve heat means endothermic reaction (absorbs +ve heat means endothermic reaction (absorbs heat)heat)

-ve heat means exothermic reaction (evolves -ve heat means exothermic reaction (evolves heat)heat)

Change of enthalpy or heat of a given Change of enthalpy or heat of a given reaction can be found in Tables such as reaction can be found in Tables such as (Table 3-1)(Table 3-1)

To calculate heat of a reactionTo calculate heat of a reaction:: Write a balanced equationWrite a balanced equation Find standard enthalpy of reactantsFind standard enthalpy of reactants Find standard enthalpy of productsFind standard enthalpy of products

Then, Then,

Heat = (products) – (reactants)Heat = (products) – (reactants)

Example (Problem 3.1)Example (Problem 3.1)

Determine the heat of combustion of ethane Determine the heat of combustion of ethane gas.gas.

The enthalpy of a chemical element (at its The enthalpy of a chemical element (at its standard state) at 25standard state) at 25 C and 1 atm is zero. For C and 1 atm is zero. For example, at standard states, Oexample, at standard states, O22 is gas, Mercury is gas, Mercury is liquid, Sulfur is crystal.is liquid, Sulfur is crystal.

Study Examples A and B on page 54Study Examples A and B on page 54

molkJcombustionforHthen

gOHofH

gCOofH

gOofH

gCHCHofH

Tablefrom

gOHgCOgOgCHCH

/.1428)68.84()8.241(3)5.393(2..,.

8.241)(....

5.393)(....

0)(....

68.84)(....

13..

)(3)(2)(2

13)(

298

2298

2298

2298

3298

22233

ThermodynamicsThermodynamics(d) Entropy(d) EntropyIs based on the second law of thermodynamics, which Is based on the second law of thermodynamics, which states "states "All systems tend to approach a state of All systems tend to approach a state of equilibriumequilibrium""In chemistry, we are interested in entropy to check the In chemistry, we are interested in entropy to check the position of the equilibrium of a chemical process.position of the equilibrium of a chemical process.

WhereWhere S = entropy of the systemS = entropy of the system T = absolute temperatureT = absolute temperature qqrevrev = amount of heat that the system absorbs if a = amount of heat that the system absorbs if a

chemical change is brought about in an infinitely slow chemical change is brought about in an infinitely slow reversible mannerreversible manner

T

dqdS rev

+ve +ve S indicates that change can occur S indicates that change can occur spontaneouslyspontaneously

-ve -ve S indicates that change tends to S indicates that change tends to occur in reverse directionoccur in reverse direction

Zero Zero S indicates that system is at S indicates that system is at equilibriumequilibrium

2

112 T

dqSSS rev

EntropyEntropy

For a closed thermodynamic For a closed thermodynamic system, entropy is a system, entropy is a quantitative measure of the quantitative measure of the amount of thermal energy not amount of thermal energy not available to do work. available to do work.


(e). Free Energy(e). Free EnergyIn environmental engineering processes, both In environmental engineering processes, both entropy and energy are needed in order to entropy and energy are needed in order to determine which processes will occur determine which processes will occur spontaneously.spontaneously.

G = H – TSG = H – TSWhereWhere

G = free energyG = free energy H = enthalpy (J)H = enthalpy (J) T = absolute temperature (T = absolute temperature ( K) [ K) [ K = K = C + C +

273]273] S = entropy (J / S = entropy (J / K) K)

At constant temperature and At constant temperature and pressure:pressure:

G = G = H - TH - TSS

Since Since

H = E + PVH = E + PV

ThenThen

H = HH = H22 – H – H11 = (E = (E22 + P + P22VV22) – (E) – (E11 + + PP22VV11))

At constant PAt constant P

H = H = E + PE + PVV

Since Since

E = q - E = q -

ThenThen

H = q - H = q - + P + PVV

FromFrom

At constant TAt constant T

TTS = qS = qrevrev

If the system change is very slow, then If the system change is very slow, then energy loss is energy loss is MINIMUMMINIMUM

q = qq = qrevrev

= = max max

2

112 T

dqSSS rev

In this case,In this case,

PPV represents the work that is V represents the work that is wasted during the expansion of the wasted during the expansion of the system. Therefore:-system. Therefore:-G is the G is the difference between the maximum difference between the maximum work and the wasted work, which can work and the wasted work, which can be described as the useful work be described as the useful work available from the system change. So:available from the system change. So:

VPG

qVPqG revrev

max

max

)tan...........( tconsarePandTG useful

If a system changes from a to b, then:If a system changes from a to b, then: --ve ve G means that the system or process G means that the system or process

can proceedcan proceed +ve +ve G means that the system or G means that the system or

process can proceed in the reverse process can proceed in the reverse direction (b to a)direction (b to a)

Zero Zero G means that the system or G means that the system or process is at equilibrium and can not process is at equilibrium and can not proceed in either direction.proceed in either direction.

At standard state of elements and at 25At standard state of elements and at 25 C C and 1 atm, the free energy ( ) is zero. and 1 atm, the free energy ( ) is zero.

For values of , see Table 3-1.For values of , see Table 3-1.

298G

298G

Consider the following reaction:Consider the following reaction:

aA + bB aA + bB cC + dD cC + dD

Taking into consideration the Taking into consideration the concentration of reactants and concentration of reactants and products:products:

ba

dc

BA

DCRTGG

][][

][][ln298

WhereWhere G = reaction free-energy change G = reaction free-energy change

(J)(J) = standard free-energy = standard free-energy

change (J)change (J) R = universal gas constant = 8.314 J R = universal gas constant = 8.314 J

/ K-mol = 1.99 cal / K-mol/ K-mol = 1.99 cal / K-mol T = absolute temperature in Kelvin T = absolute temperature in Kelvin

(( K) K) [ ] = activities of A, B, C, and D[ ] = activities of A, B, C, and D and and

298G

QquotientreactionBA

DCba

dc

,...][][

][][

At equilibriumAt equilibrium; ;

G = zeroG = zero

So,So,

At equilibriumAt equilibrium

K = equilibrium constantK = equilibrium constant

Therefore,Therefore,

mequilibriuba

dc

BA

DCRTG

][][

][][ln

KBA

DCba

dc

][][

][][

KRTG ln

Comparison between Q and KComparison between Q and K Q < K means the reaction proceeds Q < K means the reaction proceeds

from left to rightfrom left to right Q > K means the reaction proceeds Q > K means the reaction proceeds

from right to leftfrom right to left Q = K means the reaction is at Q = K means the reaction is at

equilibriumequilibrium

Study Examples A, B, C, and D Study Examples A, B, C, and D page 58-59.page 58-59.

ThermodynamicsThermodynamics(f). Temperature Dependence of K(f). Temperature Dependence of KFrom relationship between From relationship between

G and KG and K G and HG and H

In environmental engineering In environmental engineering practices, the temperature range is practices, the temperature range is limited and, therefore, limited and, therefore, HH is constant. is constant. So, So,

Study Example page 60Study Example page 60

2

ln

RT

H

dT

Kd

21

21

1

2lnTT

TT

R

H

K

K

T

T

OsmosisOsmosis

Flow direction from dilute solution to Flow direction from dilute solution to concentrated solution is more rapidly than concentrated solution is more rapidly than the other direction (concentrated the other direction (concentrated diluted) diluted)

In order to oppose that flow, pressure to the salt solution can be In order to oppose that flow, pressure to the salt solution can be applied to produce equilibrium. That pressure is called osmotic applied to produce equilibrium. That pressure is called osmotic pressure (pressure ())

= osmotic pressure, atm= osmotic pressure, atm R = 0.0882 l-atm / mol-KR = 0.0882 l-atm / mol-K T = absolute temperature, T = absolute temperature, KK VVAA = volume per mole of solvent = 0.018 liter ( for water) = volume per mole of solvent = 0.018 liter ( for water) PPAA and P and PAA = vapor pressure of solvent in the dilute and = vapor pressure of solvent in the dilute and

concentrated solutions, respectivelyconcentrated solutions, respectively

A

A

A P

P

V

RT

ln

For dilute solutions, the reduction in vapor For dilute solutions, the reduction in vapor pressure of a solvent is directly proportional to pressure of a solvent is directly proportional to the concentration of particles in solution. So, the concentration of particles in solution. So,

c = molar concentration of particlesc = molar concentration of particles

In environmental engineering Reverse Osmosis In environmental engineering Reverse Osmosis is used to demineralized brackish watersis used to demineralized brackish waters

cRT

Dialysis and Electro-Dialysis Dialysis and Electro-Dialysis

Dialysis is a phenomena that is related to the Dialysis is a phenomena that is related to the principle of OSMOSIS principle of OSMOSIS

Main Membrane ProcessesMain Membrane Processes

Are used to separate substances (solutes) from Are used to separate substances (solutes) from a solution (solvent)a solution (solvent)

The main membrane processes areThe main membrane processes are DialysisDialysis Electro-dialysisElectro-dialysis Reverse osmosisReverse osmosis

Driving forces that cause mass transfer of Driving forces that cause mass transfer of solutes are:solutes are:

Difference in concentration (dialysis)Difference in concentration (dialysis) Difference in electric potential (electro-dialysis)Difference in electric potential (electro-dialysis) Difference in pressure (reverse osmosis) Difference in pressure (reverse osmosis)

DialysisDialysis

Consists of :Consists of : Separating solutes of different ionic or molecular sizeSeparating solutes of different ionic or molecular size SolutionSolution Selectively permeable membraneSelectively permeable membrane

The driving force is the difference in the solute The driving force is the difference in the solute concentration across the membraneconcentration across the membrane

Batch Dialysis CellBatch Dialysis Cell Solution to be dialyzed is separated from solvent by a Solution to be dialyzed is separated from solvent by a

semi-permeable membranesemi-permeable membrane Small ions and molecules pass from solution to solventSmall ions and molecules pass from solution to solvent Large ions and molecules do not pass due to relative size of Large ions and molecules do not pass due to relative size of

membrane poremembrane pore

The mass transfer of solute through the membrane is The mass transfer of solute through the membrane is given bygiven by

M = mass transferred per unit time (gram/hour)M = mass transferred per unit time (gram/hour) K = mass transfer coefficient [gram/(hr-cmK = mass transfer coefficient [gram/(hr-cm22)(gram/cm)(gram/cm33)])] A = membrane area (cmA = membrane area (cm22)) C = difference in concentration of solute passing through the C = difference in concentration of solute passing through the

membrane (gram/cmmembrane (gram/cm33))

CKAM

Applications of DialysisApplications of Dialysis

Sodium hydroxide was recovered from textile Sodium hydroxide was recovered from textile wastewater at:wastewater at:

Flowrate = 420 – 475 gal/dayFlowrate = 420 – 475 gal/day Recovery of 87.3 to 94.6% Recovery of 87.3 to 94.6%

Dialysis is limited to small flows due to small Dialysis is limited to small flows due to small mass transfer coefficient (K)mass transfer coefficient (K)

Electro-DialysisElectro-Dialysis

The driving force is an electromotive forceThe driving force is an electromotive force If electromotive force is applied across the If electromotive force is applied across the

permeable membrane:permeable membrane: An increased rate of ion transfer will occurAn increased rate of ion transfer will occur This results in decrease in the salt concentration of the This results in decrease in the salt concentration of the

treated solutiontreated solution

The process demineralizes The process demineralizes Brackish water and seawater to produce fresh waterBrackish water and seawater to produce fresh water Tertiary effuentsTertiary effuents

How it Works?How it Works?

When direct current is applied to electrodes:When direct current is applied to electrodes: All cations (+vely charged) migrate towards cathodeAll cations (+vely charged) migrate towards cathode All anions (-vely charged) migrate towards anodeAll anions (-vely charged) migrate towards anode Cations can pass through the cation-permeable Cations can pass through the cation-permeable

membrane (C) but can not pass through (A)membrane (C) but can not pass through (A) Anions can pass through the anions-permeable Anions can pass through the anions-permeable

membrane (A) but can not pass through (C)membrane (A) but can not pass through (C)

Alternate compartments are formedAlternate compartments are formed Ionic concentration in compartments is less Ionic concentration in compartments is less

than or greater than that in the feed solutionthan or greater than that in the feed solution

The MembraneThe Membrane

Membranes used in electro-dialysis are:Membranes used in electro-dialysis are: PorousPorous Sheet-likeSheet-like Its structural matrix is made of synthetic ion exchange Its structural matrix is made of synthetic ion exchange

resinresin

Current RequirementCurrent Requirement Can be calculated from Faraday’s laws of electrolysis:Can be calculated from Faraday’s laws of electrolysis:

One Faraday (F) of electricity (96,500 ampere-seconds or coulombs) cause one gram One Faraday (F) of electricity (96,500 ampere-seconds or coulombs) cause one gram equivalent weight of a substance to migrate from one electrode to anotherequivalent weight of a substance to migrate from one electrode to another

I = current in amperesI = current in amperes F = Faraday’s constant (96,500 ampere-seconds per gram equivalent weight F = Faraday’s constant (96,500 ampere-seconds per gram equivalent weight

removed)removed) Q = solution flowrate (liters/second)Q = solution flowrate (liters/second) N = normality of the solution (gram eq weight per liter)N = normality of the solution (gram eq weight per liter) Er = electrolyte removal as a fractionEr = electrolyte removal as a fraction Ec = current efficiency as a fractionEc = current efficiency as a fraction

c

r

E

FQNEI

Current RequirementCurrent Requirement

If the number of cells in a stack = n, thenIf the number of cells in a stack = n, then

Electro-dialysis stack usually have 100 to 250 cells Electro-dialysis stack usually have 100 to 250 cells (200 to 500 membranes)(200 to 500 membranes)

EEcc for a electro-dialysis stack and feed water must be for a electro-dialysis stack and feed water must be determined experimentallydetermined experimentally

Ec is 0.90 or moreEc is 0.90 or more Er is usually 0.25 to 0.50Er is usually 0.25 to 0.50

c

r

nE

FQNEI

Cell CapacityCell Capacity

The capacity of the cell to pass an electric The capacity of the cell to pass an electric current depends on:current depends on:

Current density [ = current / membrane area (ma/cm2)]Current density [ = current / membrane area (ma/cm2)] Normality of the feed (number of gram equivalent Normality of the feed (number of gram equivalent

weight per liter of solution)weight per liter of solution) Current density / normality ratioCurrent density / normality ratio This ratio may vary from 400 to 700 This ratio may vary from 400 to 700

Power RequirementPower Requirement

The resistance (R) of an electro-dialysis stack The resistance (R) of an electro-dialysis stack treating a particular feed must be determined treating a particular feed must be determined experimentallyexperimentally

If resistance (R) and current (I) are known:If resistance (R) and current (I) are known: Required Voltage, E = RIRequired Voltage, E = RI Required Power, P = RIRequired Power, P = RI22

R = ohms; I = amperes; E = volts; and P = wattsR = ohms; I = amperes; E = volts; and P = watts

ApplicationsApplications

Electrical energy requirement is directly Electrical energy requirement is directly proportional to the amount of salt removedproportional to the amount of salt removed

So, electrical cost is governed bySo, electrical cost is governed by Dissolved salt content of the feed waterDissolved salt content of the feed water The desired dissolved solids content of the product waterThe desired dissolved solids content of the product water

Energy consumption increases with deposition Energy consumption increases with deposition of scale upon the membraneof scale upon the membrane

Consequently, electro-dialysis is not used to deionize Consequently, electro-dialysis is not used to deionize seawater seawater

ApplicationsApplications Electro-dialysis is used in demineralization of brackish Electro-dialysis is used in demineralization of brackish

waterwater Brackish water having TDS concentration of 500 mg/l can be de-Brackish water having TDS concentration of 500 mg/l can be de-

mineralized using electro-dialysis to produce a product water of 500 mineralized using electro-dialysis to produce a product water of 500 mg/l TDSmg/l TDS

Membrane replacement and power costs are about 40% of total costMembrane replacement and power costs are about 40% of total cost Electro-dialysis have been used to de-mineralize Electro-dialysis have been used to de-mineralize

secondary effluents secondary effluents Scale formationScale formation Organic foulingOrganic fouling 25 to 50% TDS can be removed in single pass25 to 50% TDS can be removed in single pass Coagulation, settling, filtration and activated carbon adsorption can Coagulation, settling, filtration and activated carbon adsorption can

used as pre-treatment processes to reduce organic fouling OR by used as pre-treatment processes to reduce organic fouling OR by cleaning the membrane using an enzyme detergent solutioncleaning the membrane using an enzyme detergent solution

Scale formation can be reduced by adding small amount of acid to Scale formation can be reduced by adding small amount of acid to the feedthe feed

Electro-Dialysis InstallationsElectro-Dialysis Installations

Reverse OsmosisReverse Osmosis

Diffusion is the movement of molecules from a region of Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. higher concentration to a region of lower concentration. Osmosis is a special case of diffusion in which the Osmosis is a special case of diffusion in which the molecules are water and the concentration gradient occurs molecules are water and the concentration gradient occurs across a semipermeable membrane. The semipermeable across a semipermeable membrane. The semipermeable membrane allows the passage of water, but not ions (e.g., membrane allows the passage of water, but not ions (e.g., Na+, Ca2+, Cl-) or larger molecules (e.g., glucose, urea, Na+, Ca2+, Cl-) or larger molecules (e.g., glucose, urea, bacteria). Diffusion and osmosis are thermodynamically bacteria). Diffusion and osmosis are thermodynamically favorable and will continue until equilibrium is reached. favorable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the sufficient pressure is applied to the membrane from the 'concentrated' side of the membrane. 'concentrated' side of the membrane.


Reverse osmosis occurs when the water is moved across Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from the membrane against the concentration gradient, from lower concentration to higher concentration. To lower concentration to higher concentration. To illustrate, imagine a semipermeable membrane with illustrate, imagine a semipermeable membrane with fresh water on one side and a concentrated aqueous fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to is exerted on the side with the concentrated solution to force the water molecules across the membrane to the force the water molecules across the membrane to the fresh water side. fresh water side.


Reverse osmosis is often used in commercial and Reverse osmosis is often used in commercial and residential water filtration. It is also one of the residential water filtration. It is also one of the methods used to desalinate seawater. Sometimes methods used to desalinate seawater. Sometimes reverse osmosis is used to purify liquids in reverse osmosis is used to purify liquids in which water is an undesirable impurity (e.g., which water is an undesirable impurity (e.g., ethanol). ethanol).

Reverse Osmosis - Pros and ConsReverse Osmosis - Pros and Cons

The semi-permeable membrane used in reverse osmosis The semi-permeable membrane used in reverse osmosis contains tiny pores through which water can flow. The contains tiny pores through which water can flow. The small pores of this membrane are restrictive to such small pores of this membrane are restrictive to such organic compounds as salt and other natural minerals, organic compounds as salt and other natural minerals, which generally have a larger molecular composition which generally have a larger molecular composition than water. These pores are also restrictive to bacteria than water. These pores are also restrictive to bacteria and disease-causing pathogens. Thus, reverse osmosis is and disease-causing pathogens. Thus, reverse osmosis is incredibly effective at desalinating water and providing incredibly effective at desalinating water and providing mineral-free water for use in photo or print shops. It is mineral-free water for use in photo or print shops. It is also effective at providing pathogen-free water. In areas also effective at providing pathogen-free water. In areas not receiving municipally treated water or at particular not receiving municipally treated water or at particular risk of waterborne diseases, reverse osmosis is an ideal risk of waterborne diseases, reverse osmosis is an ideal process of contaminant removal. process of contaminant removal.


The reverse osmosis process contains several downsides The reverse osmosis process contains several downsides which make it an inefficient and ineffective means of which make it an inefficient and ineffective means of purifying drinking water. The small pores in the purifying drinking water. The small pores in the membrane block particles of large molecular structure membrane block particles of large molecular structure like salt, but more dangerous chemicals like pesticides, like salt, but more dangerous chemicals like pesticides, herbicides, and chlorine are molecularly smaller than herbicides, and chlorine are molecularly smaller than water (Binnie et al, 2002). These chemicals can freely water (Binnie et al, 2002). These chemicals can freely pass through the porous membrane. For this reason, a pass through the porous membrane. For this reason, a carbon filter must be used as a complimentary measure carbon filter must be used as a complimentary measure to provide safe drinking water from the reverse osmosis to provide safe drinking water from the reverse osmosis process. Such chemicals are the major contaminants of process. Such chemicals are the major contaminants of drinking water after municipal treatment.drinking water after municipal treatment.


Another downside to reverse osmosis as a Another downside to reverse osmosis as a method of purifying drinking water is the method of purifying drinking water is the removal of healthy, naturally occurring minerals removal of healthy, naturally occurring minerals in water. The membrane of a reverse osmosis in water. The membrane of a reverse osmosis system is impermeable to natural trace minerals. system is impermeable to natural trace minerals. These minerals not only provide a good taste to These minerals not only provide a good taste to water, but they also serve a vital function in the water, but they also serve a vital function in the body’s system. Water, when stripped of these body’s system. Water, when stripped of these trace minerals, can actually be unhealthy for the trace minerals, can actually be unhealthy for the body. body.


Reverse osmosis also wastes a large portion of Reverse osmosis also wastes a large portion of the water that runs through its system. It the water that runs through its system. It generally wastes two to three gallons of water generally wastes two to three gallons of water for every gallon of purified water it produces. for every gallon of purified water it produces. Reverse osmosis is also an incredibly slow Reverse osmosis is also an incredibly slow process when compared to other water treatment process when compared to other water treatment alternatives.alternatives.

Module TypesModule Types

Spiral WoundSpiral Wound Hollow FiberHollow Fiber TubularTubular

Membrane InstallationsMembrane Installations

CostsCosts

Principles of Solvent Extraction Principles of Solvent Extraction

Objectives:Objectives: To recover valuable constituentTo recover valuable constituent For analytical purposesFor analytical purposes

Solvents are used (immiscible)Solvents are used (immiscible) Petroleum etherPetroleum ether Diethyl etherDiethyl ether BenzeneBenzene HexaneHexane DichloromethaneDichloromethane OthersOthers

Solute will be distributed based on their solubilities in Solute will be distributed based on their solubilities in water and solvent water and solvent

K = distribution coefficientK = distribution coefficient

Usually a solvent is selected so that K is greater than 1Usually a solvent is selected so that K is greater than 1

().(

water

immisciblesolventSolute

KC

C

C

C

w

s

water

solvent

If K = 9 and volumes of water and solvent are If K = 9 and volumes of water and solvent are equal, then:equal, then:

In the first extraction step 90% will be extractedIn the first extraction step 90% will be extracted After 3 extraction steps with fresh solvent, 99.9% After 3 extraction steps with fresh solvent, 99.9%

of the solute will be extractedof the solute will be extracted

The mathematics are simpleThe mathematics are simple

In practice, it may be unjustified to use equal In practice, it may be unjustified to use equal volumes of water and solvent. In such cases:volumes of water and solvent. In such cases:

WW00 = weight of solute in water (originally) = weight of solute in water (originally)

WW11 = weight of solute in water (remained after one = weight of solute in water (remained after one

extraction)extraction) VVss = volume of solvent = volume of solvent

VVww = volume of water = volume of water

)/(

/)(

1

10

w

s

w

s

WW

VWW

C

CK

ws

w

VKV

VWW

01

In the second step of extractionIn the second step of extraction

In terms of the original sample In terms of the original sample

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12

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02

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After n extractionsAfter n extractions

WWnn = remaining weight of solute in water after n = remaining weight of solute in water after n

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n

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Physical Chemistry CE 541. What is Physical Chemistry Is the science that deals with laws that is related (or that govern) chemical phenomena such as: