Partitioning of pollutants Sorption involving organic matter

Partitioning of pollutants

Sorption involving organic matter

Aims

• To address general aspects of solid-aqueous solution exchange involving natural sorbents

• To address aspects needed to quantify sorption equilibrium in natural environments and to predict partition coefficients

2Environmental processing / Partitioning of pollutants / Sorption involving organic matter

Outcomes

• Students will be able to evaluate compound partitioning between water, dissolved organic matter, and sediment organic matter based on physico-chemical properties of compounds

• Students will be able to estimate partition coefficients on the basis of compound's chemical structure and physico-chemical properties


• Sorption affects:

– Transport• generally, molecules which are sorbed are less mobile in the

environment• sorbed molecules are not available for phase transfer processes

(air-water exchange, etc.)– Degradation:

• sorbed molecules are not bioavailable• sorbed molecules usually shielded from UV light (less direct

photolysis)• sorbed molecules cannot come into contact with indirect

photoxidants such as OH• rates of other transformation reactions may be very different

for sorbed molecules4Environmental processing / Partitioning of pollutants / Sorption involving organic matter

5

• Sorption is complex phenomenon because sorbents in the natural environment are complex, and sorption may occur via several different mechanisms.

Environmental processing / Partitioning of pollutants / Sorption involving organic matter

Solid-water distribution coefficient

– Cis = mol/kg solid or mg/kg solid

– Ciw = mol/L water or mg/L solid

– Kid = L/kg

• This partitioning model assumes:

– All sorption sites have equal energy

– An infinite number of sorption sites exist

• The problem with sorption is that these two assumptions are generally not true!

6

iw

isid C

CK

equilibrium “constant” describing partitioning between solid and water phases


• Identical molecules behave very differently, depending on whether they are:

– in the gas phase (gas)

– surrounded by water molecules (dissolved)

– clinging onto the exterior of solids (adsorbed)

– buried within a solid matrix (absorbed)


Sorption isotherms

• Sorption takes place via many different mechanisms, even in the same system.

• The shape of the isotherm does not prove which sorption mechanism is operating.


Sorption models

• Freundlich model (b)

– ni – factor of nonlinearity (dimensionless)

– KiF – Freundlich constant or sorption capacity (units depend on units of Ciw and Cis)

• Due to the isotherm nonlinearity, Kd is not constant over the whole concentration range (unless n =1):

– multiple types of sorption sites, exhibiting a diversity of free energies, empirical

9

iniwiFis CKC

1 iniwiFid CKK


• n = 1

– all sites have equal energy at all sorbent concentrations

• n > 1

– more sorbate enhances the free energies of further sorption

• n < 1

– added sorbates are bound with weaker and weaker energies


• Langmuir isotherm (c)

– max – total number of available sites (usually depends on the sorbate)

– KiL – Langmuir constant

– KiL = KdCmax at low concentrations (linear region)

11

iwiL

iwiLis CK

CKC

1max

linear region (Ciw very small)

saturation (Ciw very big)

max


• In natural environments sorption takes place via many different mechanisms, even in the same system.

• Thus, a combination of isotherms may be necessary to adequately describe sorption behavior.

– Adsorption plus absorption:

– Example 1: Langmuir plus linear

– Example 2: Freundlich plus linear (sorption to sediments containing black carbon (important for PAHs))

12

iwiL

iwiLisiwipis CK

CKCCKC

1

max,

iniwiFiwipis CKCKC


• Dissolved fraction of a compound in a system:

– Vw – volume of water (out of total volume Vtot)

– Ms – mass of solids

13

siswiw

wiwiw MCVC

VCf

sidw

wiw MKV

Vf

idswidwsiw KrKVM

f

1

1

)/(1

1

rsw = solid/water ratio (kg/L)iW

if fR

1

Retardation factor:



• Porosity:

15

sswssw

w

sw

w

tot

w

rMV

V

VV

V

V

V

/1

1

/

s

ss

MV

s is usually about 2.5 kg/L

)1( stot

sb V

Mb – bulk density


• Example: 1,4-DMB (Kd = 1 L/kg)

– In a lake, rsw = 1 mg/L = 10-6 kg/L

• Dissolved

– In an aquifer, rsw = 10 kg/L

• one molecule in 11 dissolved

16

11101

1

1

16

idsw

iw Krf

09.01101

1

1

1

idswiw Kr

f


The complex nature of Kd

17

ioniwneutiw

surfisurfisurfiociocid

CC

ACACACfCK

,,

rxn surfrxnex surfexmin

total amount in dissolved phase consists of neutral and ionized forms

sorption to organic carbon

adsorption to mineral surface

exchangeable adsorption of ionized form to charged surface

covalently bonded adsorption of ionized form to mineral surface

refers to c of suitable sites (mol/m2)


18

ocioc fC

surfi AC rxn surfrxn

surfi AC min

surfi AC ex surfex

both adsorption and absorption to different types of OC

adsorption to many different types of minerals (each with different K and different concentrations)

adsorption to many different types of minerals (each with different surface charge)

reaction (adsorption) to many different types of reactive sites


Sorption of hydrophobic (neutral) organics to natural organic matter (NOM)

• foc = fraction of organic carbon in solid

• Even at foc = 0.0001, sorption to OC may still dominate

19

iw

ociocid C

fCK

oc

idioc f

KK

ococswiw fKr

f

1

1


• Not only quantity but also quality of OC matters!


LFERs for Koc

• The examples of compound class-specific LFERs:


Sorption of hydrophobic (neutral) organics to dissolved organic matter (DOC)

• Effects of DOC:

– increases apparent solubility

– decreases air/water distribution ratio

– may decrease bioavailability

– may affect interactions of compounds with light

• KDOC is hard to measure because it is difficult to separate the dissolved and sorbed phases of organic compounds


LFERs relating KDOC to Kow


Sorption of acids and bases to NOM

• Acids and bases may partially or fully ionized at ambient pH

• When considering sorption of neutral species, must consider:

– Van der Waals interactions

– polarity

– H-bonding

• When considering sorption of charged species, must ALSO consider electrostatic interactions and formation of covalent bonds with the NOM

• D instead of K for distribution ratio


• For weak acids with only one acidic group:

• Usually:

– thus if pH < 2 + pKa then sorption of ionized species is usually negligible

25

ww

ococioc AHA

AHAD

][][

][][

iapKpHia

101

1

Aiocia

HAiociaioc KKD )1(

Aioc

HAioc KK


• Sorption of bases:

– sorption of the cationic form to negatively charged sites in the NOM may dominate the overall sorption of the compound:

• therefore the sorption isotherm is non-linear and

• competition with other cations can occur

26

quinoline pKa = 4.9

sorption max at this pH


References• “Environmental Chemistry (a global perspective)” Gary W.

vanLoon, Stephen J. Duffy; Oxford University Press, New York (2nd edition 2005), ISBN 978-0-19-927499-4

• “Environmental Soil Chemistry” Donald L. Sparks; Academic Press, Published 1995. ISBN 0-12-656445-0

• “Environmental Organic Chemistry ” ; Rene P. Schwarzenbach, Philip M. Gschwend and Dieter M. Imboden; 2nd Edition, John Wiley &L Sons, Inc. Copyright 2003. ISBN: 0-471-35750-2

• Schwarzenbach, R.P., Gschwend, P.M., Imboden, D.M. (2003). 2nd Edition John Wiley and Sons, New Jersey, ISBN 0-471-35750-2

Environmental processing / Partitioning of pollutants / Sorption involving organic matter 27

References

• Aboul-Kassim, T.A.T., Simoneit, B.R.T., 2001. Chemistry and Modeling. Springer-Verlag, Berlin. P107-167, ISBN: 3-540-41650-1

• Allen-King, R.M., Grathwohl, P., Ball, W.P., 2002. Advances in Water Resources 25, 985-1016.

Environmental processing / Partitioning of pollutants / Sorption involving organic matter 28

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Partitioning of pollutants Sorption involving organic matter