2 It is needed in Environmental Engineering to understand: Electrochemistry is the relationship between Chemical Phenomena and Electrical PhenomenaIt is needed in Environmental Engineering to understand:CorrosionElectrochemical oxidation of wastesAnalytical proceduresAutomatic monitoring of waste streamsOxidation-reduction reactions
3 Current Flow in Solution Current can flow through:Solution of electrolyteMetallic conductorsCharacteristics of current flow through a metal:Chemical properties of metal are not changedCurrent is carried by electronsIncrease in temperature increases resistance
4 Characteristics of current flow through a solution: Chemical change occurs in solutionCurrent is carried by ionsIncrease in temperature decreases resistanceResistance is normally greater than that with metals
5 E = IR Conductivity of Solution "Is its ability to carry an electrical current"Conductivity can be measured by a conductivity meter and it is affected by:Number of ionsType of ionsE = IRWhere:E = electromotive force (volts)I = current (amperes)R = resistance (ohms)
6 AndWherel = length of conductorA = cross-sectional area of conductor = specific resistance of conductor (ohm-cm)k = specific conductance (1 / ohm-cm) or siemens (S)Specific conductance is conductance afforded by 1 cm3 of an electrolyte solution
7 Specific Conductance of a Solution = C / R Conductivity cells are calibrated by determining the resistance of a standard solution (Rs) and the cell constant (C) can be found.C = ksRsIn such cases N KCl is used in calibrating conductivity cells. For N KCl:ks = S = CSo,Specific Conductance of a Solution = C / RR needs to be determined
8 Equivalent Conductance () WhereN = normality of the solutionk = specific conductanceor,and are equivalent ionic conductance of cations and anions, respectively
9 Table 3-3 shows equivalent ionic conductance @ 25 C in S-cm3/equivalent. Only ions can carry current. Un-ionized species of weak acids or bases will not carry current. Also uncharged soluble organics (ethanol and glucose) can not carry current.Study Example page 80
10 What is the approximate specific conductance at 25 C of a solution containing 100 mg/l of CaCl2 and 75 mg/l of Na2SO4
11 Current and Chemical Change Chemical change depends on:Nature of solution (composition)Nature of electrodesMagnitude of electromotive force imposed
12 Applying a voltage of 1.3 v H2 evolves at Cathode Cl2 evolves at Anode HClH+e-Cathode (-)Cl-Anode (+)PlatinumElectrodesApplying a voltage of 1.3 v H2 evolves at Cathode Cl2 evolves at Anode
14 To bring about 1 equivalent of chemical change at an electrode: An Avogadro No of electrons must flow through the external circuitThis quantity of electrons is called the faraday (F)The rate of flow of electrons gives the current (I) in amperes1 F is equivalent to an ampere of current flowing for secondsAn ampere is defined as a Coulomb per second1 F = CoulombStudy Example page 78
15 What weight of silver will pass into solution from a silver anode by the passage of 0.02 A of current through the solution for 24 hours?
16 Electrochemical Celltwo electrodes will be connected by metallic conductorelectrons will flowchemical change beginselectromotive force (emf) will be generated by the cellthis emf is a measure of the driving force of the chemical reaction occurring in the solutionthe driving force represents the chemical potential or free energy of the reaction
17 Electrical Energy = EIt Based on that, a relationship between electrical potential and chemical free energy can be found:Electrical Energy = EItE = emf in voltsI = current in amperest = time in secondsElectrical energy is expressed in Volt-Coulomb or JouleElectrical energy required to produce one mole of chemical change = zEFWherez = number of electron-equivalent per moleF = faraday or coulombs per equivalent
18 If reaction proceeds (E is +ve), then: whereG is the free energy-zEF is the electrical energy
19 aA + bB cC + dD Consider the following reaction: Substituting in: we get:
20 The value of R in electrical units is: R = J / K-molAt 25 C and converting ln to log
21 If an electrochemical cell reaches the state of equilibrium, then: The emf can be found in Tables (Table 3-4) just like free-energy and enthalpy. The values in the table are for a reaction written for 1 mole of e- change, such as:If an electrochemical cell reaches the state of equilibrium, then:no current can flowemf is zero
22 In this case:sinceorlog K = 16.9zEStudy Examples page 85
23 Estimate the solubility-product constant for Mg(OH)2(s) at 25 C from standard electrode potential?
24 Chemical Kinetics then, the rate of reaction could be: Chemical kinetics deal with speed of reactions. Ifthen, the rate of reaction could be:first – order reaction (exponent 1)kCasecond – order reaction (exponent 2)kCa2kCaCbthird – order reaction (exponent 3)kCa3kCa2CbkCaCbCcCa, Cb, and Cc = concentrations of A, B, and C, respectivelyk = rate constant
25 The reaction rates are required in: These are simple reaction rates, but in reality there are more complex equations. The unit of k depends on the reaction order and units of concentration of A, B, and C.The reaction rates are required in:Microbial growthAerationDisinfectionRadioactive decay
26 Zero-Order Reactions They are independent of concentration Most of biological growth occur in linear relationship over a range of concentrations of substance (C).
27 First-Order Reactions The rate is directly proportional to the concentrationif we are dealing with a decay or decomposition reaction, then the rate can be expressed asUnit of k is (1/time) and the –ve sign indicates the loss of material with time.
28 Integrating the above equation: Converting to log10
29 k = - slope of line [t versus ln(C/C0)] k = -slope [t versus log10(C/C0)]Half-life (t1/2)In this case,t = t1/2 and C = (1/2)C0ThenApplications of 1st order reactions in Environmental Engineering:Dissolution of gases in waterRemoval of gases from waterRate of death of microorganismsDecomposition of organic matters (BOD5 test)Study Example page 89
30 The radioactive nuclide P32 has a half-life of 14. 3 days The radioactive nuclide P32 has a half-life of 14.3 days. How long would a waste containing 10 mg/l of this nuclide have to be stored in order to reduce the concentration to 0.3 mg/l?
31 Second-Order Reactions The rate of reaction is proportional to the square of the concentration of one of the reactants or to the product of concentrations of two different reactants.
32 Ca and Cb are concentrations of A and B, respectively Ca and Cb are concentrations of A and B, respectively. Integrating (1) and (2), we obtain:
33 Consecutive Reactions If rates of reactions are 1st order, then:
37 Total enzyme concentration in the system = E = [Ef] + [EcS] Enzyme ReactionsAre used to describe the rate of biological waste treatment. The relationship between Substrate (S) and the rate of utilization per unit mass of enzyme or bacteria (V/E)Michaelis – Menton RelationshipEf = free enzymeS = substrateEcS = enzyme-substrate complexTotal enzyme concentration in the system = E = [Ef] + [EcS]
38 The rate of formation of enzyme-substrate complex is: The rate of complex formation rate of overall reaction. Therefore, d[EcS] / dt can be considered as ZERO when overall reaction rate is required to be determined. So:
39 Rate of product formation V = k[EcS] The rate of product formation = overall rate of reactionRate of product formation V = k[EcS]
40 Study Example page 95 Then Where k is the maximum rateKs is the substrate concentration at arte = (1/2)kKs is called the "half velocity" constantV/E k'S when S<< Ks (1st order with respect to S)V/E k when S >> Ks (zero order with respect to S)Study Example page 95
42 Temperature Dependence of Reaction Rates "rates increase with increase in temperature"Rate doubles for each 10 C rise.Using Arrehenius equation:WhereT = temperature, KR = universal gas constantEa = constant
43 Integratingk2 and k1 are rate constants at T2 and T1. In environmental engineering processes, the range of temperature is small. So T2T1 can be assumed constant.
45 Adsorption"sorption is the concentration or movement of contaminants from one place to another""adsorption involves partitioning of contaminants from one phase to another""adsorption is the process by which ions or molecules present in one phase tend to condense and concentrate on the surface of another phase"
46 Adsorption Physical Chemical Ion exchange Weak multi-layers free movingreversibleChemicalStrongmono-layerno movementnon-reversible in most casesIon exchangeelectrical attractionsmaller particleshave stronger attractiontrivalent have stronger attraction than monovalent ions
47 Adsorption depends on: Activated Carbon1 gram has surface area of 1000 m2Pore size ranges between 10 to 1000 AAdsorption of gas increases wit the increase I pressureAdsorption depends on:Nature of materialsConcentrationTemperature
50 Freundlich Isotherm Freundlich found that: Whereq = mass of contaminant per unit weight of the adsorbentC = concentration of solute after adsorptionK and n = constants (they should be evaluated for each soluble and temperature)Freundlich isotherm can be expressed as:
55 BET IsothermA third isotherm is BET (Brunauer, Emmett, and Teller) which can be used to describe multi-layer adsorptionAssumptionsMulti-layers of adsorbent accumulate at the surface of adsorbentEach layer can be described by Langmuir isothermCs = saturation concentration for the adsorbate in solution
56 If C > Cs then the solute precipitates or condenses from solution as solid or liquid and concentrates on the surfaceBET equation can be put in this form:If we have data, then we have to find the best isotherm that can be used to describe the data (get straight line)