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Lecture 13

By Lori Garcia,2014-05-06 12:23
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Lecture 13

    Lecture 13

    CE260/Spring 2000

    2+2+ Fe and Mn removal

    ? WHO standards for acceptable concentrations of iron and manganese are 0.3 and

    0.1 mg/L respectively

    ? Solubility is controlled by the oxidation state

    2??2?Fe?e?Fe 4??2?Mn?2e?Mn

    ? Iron oxide (rust) has reddish brown color and MnO has brown black color 2

    and they are both precipitates

    ? pH, temp, complexing agents and O effect oxidation 2

    ? If rxn rate of oxidation < input then iron and manganese will accumulate

    ? Can use Permanganate (MnO) to oxidize iron and manganese 4

    2+2+? Redox equations are below Mn likes high pH , while Fefavors low pH

    2???3Mn?2MnO?4OH?5MnO?2HO42(s)2 2???3?3Fe?MnO?4H?3Fe?MnO?2HO42(s)2

    ? Overall reaction for iron removal is favored under high pH conditions

    ???223Fe?MnO?5OH?2HO?3Fe(OH)?MnO 423(s)2(s)

    2+? The reactions for the oxidation of Fe with oxygen are:

    2??4Fe?O?10HO?4Fe(OH)?8H223 2??2Mn?O?2HO?2MnO?4H222(s)

    3+? Greensand - glauconite (K, Na, Ca)- (Fe, Al, Fe, Mg) SiAlO(OH) 1.2-2 47-7.61-1.424

    nHO 2

    2+2+? If MnO coats the surface then adsorbed Fe and Mn oxidizes completely to 24+3+3+Mn, Mn and Fe

    ? KMnOis used to regenerate the surface (HS and phenols are also removed) 4 2

     Phosphorus removal

    ? P is typically the limiting factor for freshwater systems

    .? Al(SO)18HO is alum 2432

    ? NaAlO is sodium alumnate 224

    ? Figures 15.4a nd b

     Activated carbon

    ? Removal of dissolved substances by adsorption onto surface of the carbon

? Carbon is the absorbent

    ? Solute is the absorbate

    ? Can remove SOCs (synthetic organic chemicals) also radium 222, Hg, and

    other metals

    --? 2Cl and chloroamines ? CO + Cl 2 2? Some organic chemicals that are not removed (e.g. THMs, methylene chloride,

    MTBE) ? Two types of activated carbon: granular GAC and powder PAC ? Can dechlorinate with GAC Cl? Activation process have wood or coal +oxidizing steam ~1700F (water-gas

    rxn)

    C?HO?H?CO (s)222(g)

    ? Loss of C in form of CO leaves a very porous structure to the char up to 1000 g2m/g carbon

    ? Adsorption capacity of activated carbon

    ? Adsorption isotherm describes relationship between the amount of adsorbate

    adsorbed and equilibrium adsorbate concentration in solution

    ? Bottle pt method

    ? Same conc. absorbate in solution

    ? Different concentration of carbon

    ? 10 bottles on shaker test table

Conc.

    x

     Time in hours

    ? run last for 1.5 x

    V(C?C)?M(q?q) oei

    ? Where:

    ? V = volume of sample

? C = initial adsorbate o

    ? C = final steady state concentration

    ? M = mass of carbon

    ? q = equilibrium conc. adsorbed/mass C e

    ? q = initial conc. adsorbed/mass C I

    (?)CCVo?q eM

    ? Langmuir Isotherm (Ex. 15.4)

    ? 1918 single layer adsorption model

    ? ? = fraction S.A. covered by adsorbate at equilibrium

    ? 1-? = bare fraction

    ? Since adsorption rate = desorption rate at SS

    ?)=k'? kC(1?

    kC????q?orq?k"eekC?k'

     kIfn?thenk'

    k"kCk"KCq??ekC?k'KC?k? If max amount of solute adsorbed is Q, Q = k" when ? = 1 oo? Substution of this into the above equation yields:

    QKCxab[C]oq?or? eKC?1mb[C[?1? Remember that C = C equil

    ? Can linearize the Langmuir three ways

    C1C??qQKQeoo

    111 ??qQKCQeoo

    q??qQoKC? Want to solve for K and Q o

    ? Use the equation that best fits the data range

? Can describe adsorption w/ Langmuir if you have a good linear fit. If not then

    maybe some other model will work

    1 Freundlich Isotherm - Heinrich Freundlich n q?KCeF? General exponential concentration fit ? K = specific capacity F

    ? N is a function of the energy of adsorption (linearize)

    logq?logK?nlogC eF

    ? General Freundlich isotherm lookalikes the following:

    n<1 favorable n=1 linear

     q en>1 unfavorable

    C e

     BET - Brunaver, Emmert, and Teller (1938)

    ? Extension of the Langmuir Isotherm to several layers of adsorption

    ? First layer is heat of adsorption the subsequent layers are condensation

    BCQoq?e??(1)B?C??1C?C?s??C s??

    dH??aB?C???RT??

    ? C = saturation concentration of solute in HO s2

    ? B and Q are constants o

    ? H is the enthalpy of adsorption a

    ? Linearize the BET

    C1B?1C?? ??C?CqBQBQCseoos

     qeQ o

     C C s? GAC adsorbers (Figure 15.9)

    ? Look at adsorption zone progression in fixed bed adsorbers (Figure 15.11)

    ? EBCT = empty bed contact time = ?

     = t= V/Q ~ 7 to 20 min Hd

     Rate of adsorption zone formation

    *1dmC?C*'()N???k?kC?Cadtx

    **ka?()() r?C?C?kC?CV

    a??V

    ? N = mass flux

    ? a= adsorption area

    ? m = mass of adsorbate

    ? k' = mass transfer constant

    ? C = conc. in the liquid

    ? C* = conc. in equilibrium with the adsorbate

    ? k = k'/x

    ? r = rate of mass removal Q, C

     exhausted

     Dist

    x ?x

     available

     C Q, C + dC/dx ?x

? Doing a mass balance on the elemental volume yields:

    dCdC??QC?QC??x?r?V?e?V?? dxdt??

    In?Out?generation?accumulation

    ? Since A = x-c of bed and A ?x = ?V

    QdCdC*?V?k?(C?C) Adxdx

    ? V = nominal velocity through the bed, consider

    ? F = mass flux of the liquid m

    ?QlF?mA

    dCQdC??* F??k?k?(C?C)??mlddxAdx??

    k?k?dl

    ? If m = mass adsorbent in the bed

    ???m?V?ADordm?bAdDbb dC*?FA?k?(C?C)pmddm

    ? Describes adsorption in the adsorption zone

    ? Capacity in adsorption zone

    ?VVVebe??? texAQ

    ? V = volume of liquid passed through the bed e

    ? V = volume of liquid passed at breakthrough b

    ? t = time to bed exhaustion ex

    ? t = time for adsorption zone to be moved ?

    ?

    ?t?V

     t?t?tefD

    ?tt????tDt?tDef? t = time to form 1 ? f

    ? t = time to travel bed distance D D

    ? V = velocity

    ? Total mass of a contaminant adsorbed (m) T

    m?C(V?V)Toeb ?m?(C?C)?Vo? ?m = is the incremental mass adsorbed

    ? Define the fractional capacity of the adsorption zone

    eV

    (C?C)dV?obV f?C(V?V)oeb

    C o Ce

    C

     CD

     V V eb

     .

    ? Extreme value of f = 0 and 1

    t?f?1?orf?(1?f)tf?t??t??Dt?t(f?1) eVV?Veeb?t?t?EQQ

    ?V?Veb?DV?f(V?V)beb? We can also define

    ? M = amount of adsorbate that will accumulate at complete saturation ?

    ? q = amount of adsorbate at the initial concentration/unit mass of ?

    adsorbate

    ? M = amount of solute accumulated at point of breakthrough b

    M??M?ADqor?Dq??b?b?A

    M???? b?q(D?)?q(1?f)b?p?A

    Mb????q(D??)??(1?f)b?A

    ? Thus the % saturation of the whole column at breakthrough (S) is: b

    q(D?)?q(1?f)????M/Abp??bS?*100?*100bMAqD/?b??

    D?f???S?*100??bD??

    ? Since QC over a time (t) gives the mass needed to be removed can calculate o

    V needed to achieve this Column

    ? So shape of the breakthrough is critical

    ? From the isotherm data we can get sets of data of C and q oo

     C o

     C1 Conc. in

    equil w/ C adsorbent qq1* ? q 1

    Mass adsorbed

? Can define superficial mass rate of saturation (M

    ) s

    massofadsorbentsaturatedM? stime*Acolumn

    ? For any point on above figure for a C and a q FC = Mq ms? We can either graphically or numerically integrate this equation

    *? Need relationship between C-C and C

    ? Method

    ? Draw best fit isotherm and plot isotherm (Figure above)

    ? Draw operating line

    *? Find C and corresponding q from op. line now find corresponding C 111

    that would be in equilibrium

    *,s? Repeat to get C-C for q's and C's then integrate

    C13?CdCi??A?A?A?123**?C?C(C?C)?i1iCb

    Cen?CdCi ??A?**?()C?CC?C?i1iCb

    ???VVAAAxx123xb??or?V?V?Aeb

    ? Then plot C/C versus (V - V)/(V - V) oxbeb

    ? Bed depth service time method (BDST)

    ? Based on lab and pilot column tests

    ? 2 design constraints

    ? minimum contact time must be provided

    ? carbon must be supplied at rate exhausted

    ? example of a wastewater at same concentration for 3 columns of different

    depths (Figure 15.22)

    ? draw a line at a specific C

    , the t is determined for each DD? t < tbecause bed needs to be filled sb

    ? Re = the rate of carbon exhaustion

    eADt?t?Qsb ?ADpRe?tb? The EBLT = empty bed contact time is calc.

    VDb t?EBLT??dQQ/A? Plot bed depth versus t (Figure 15.23) s

    ? Find the D required to meet effluent criteria (or min t) then plot Re versus mind

    EBDT (Figure 15.24)

    QCoReq?QC?Re? minomin?q?? If some fraction of wastewater is not adsorbable then:

Q(C?C)onRe? minq?

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