CVEN 5424 Environmental Organic Chemistry Lecture 16 – Sorption to Mineral Surfaces

Announcements  Reading  Chapter 11, Sorption  Problem sets  PS 7 due next Thursday Tuesday, March 15 Office hours Tuesday 11:30 am-1 pm Wednesday 9-10 am Exams Exam 2: Tues March 15 to Thur March 17

Sorption by Ion Exchange  Overall sorption expression:

Sorption by Ion Exchange  Ion exchange reaction for a cation (e.g., a protonated organic base)  RNH Na:surf = RNH 3 :surf + Na +  surf is the ion exchange site  surf is negatively charged to adsorb the cation  Ion exchange reaction for an anion (e.g., a de-protonated organic acid)  RO - + Cl:surf = RO:surf + Cl -  surf is positively charged to adsorb the anion

Sorption by Ion Exchange  Ion exchange reaction for a cation  three kinds of equilibrium constants:

Sorption by Ion Exchange  Cation exchange capacity (CEC)  amount of negative charge per mass of solid  CEC =  surf ex × SA   surf ex is the surface charge density (mol m -2 or eq m -2 )  SA is the surface area (m 2 g -1 )  CEC (mol g -1 or eq g -1 )  (Table 11.3 confusion: shows  surf ex as “CEC”)  CEC, other units: eq g -1, eq (100 g) -1  Similar for anion exchange capacity (AEC)

Sorption by Ion Exchange  Amphoteric charge  gives rise to AEC or CEC depending on pH and pH pzc  AEC  surf ex = 1 to 10  eq m -2  CEC  surf ex = -1 to -10  eq m -2 M-OH M-O - M-OH 2 +

Sorption by Ion Exchange  Permanent charge  isomorphic substitution of Al 3+ for Si 4+ results in negative surface charge  CEC (only):  surf ex = 1-10  eq m -2 (SA = m 2 g -1 ) CEC = to eq g Na +

Sorption by Ion Exchange  Organic matter  ionized functional groups  mainly carboxyl and phenol  net negative charge  5-10  eq g - 1 COO - O-O- - OOC

Sorption by Ion Exchange  Features of K ex  preference of organic cation over competing cation  effect of charge and size of ions  K ex (RNH 3 + ) > K ex (Na + )  preference of organic cation over cation  hydrophobic interaction (e.g., surfactant)  K ex ( NH 3 + ) > K ex ( NH 3 + ) > K ex (NH 3 + )

Sorption by Ion Exchange  Estimating K ex for an organic base (or acid)  z is the charge of the counter-cation (e.g., 1 for Na + )  C w sat (L) (liquid or hypothetical liquid) is the solubility of the hydrophobic analogue of the base  e.g., decylamine, decane phenol, benzene K ex (RNH 3 + ) = 36

Sorption by Ion Exchange  Estimating K ex for an organic base (or acid)  z is the charge of the counter-cation (e.g., 1 for Na + )  C w sat (L) (liquid or hypothetical liquid) is the solubility of the hydrophobic analogue of the base  e.g., decylamine, decane phenol, benzene K ex (RNH 3 + ) = 36

Sorption by Ion Exchange  Estimating K ex for an organic base (or acid)  z is the charge of the counter-cation (e.g., 1 for Na + )  C w sat (L) (liquid or hypothetical liquid) is the solubility of the hydrophobic analogue of the base  e.g., decylamine, decane phenol, benzene K ex (RNH 3 + ) = 36

Sorption by Ion Exchange  Effect of high K ex  high K ex  very hydrophobic R group  e.g., decane on decylamine  at high K ex, RNH 3 + sorbed > Na + desorbed  …but surface charge cannot be out of balance  some co-ion (e.g., Cl - ) must come with RNH 3 + to surface  electrostatic work must be done for Cl -  electrostatic cost (Cl - ) = hydrophobic benefit of R

Sorption by Ion Exchange  Vicinal water volume V vic  product of surface area (m 2 ) and double layer thickness (m)  range of V vic  low surface area, high I: L w kg solid -1  medium surface area, medium I: 0.01 L w kg solid -1  high surface area, low I: 0.1 L w kg solid -1  qualitative choice -- a less-than-optimal choice will result in only a small error

Sorption by Ion Exchange  How much cation can be adsorbed?  if low K ex (R of RNH 3 + is fairly soluble), then  not much Cl - brought to surface by RNH 3 +  so CEC >> Cl - adsorbed  if high K ex (R is fairly insoluble), then  a lot of Cl - is adsorbed with RNH 3 +  and CEC is similar to the amount of Cl - adsorbed

Sorption by Ion Exchange  Solving for [RNH 3 :surf] for low K ex : for high K ex :  what is low, what is high?  just use high K ex equation

Sorption by Ion Exchange  Solving for K d ex  Solving for K d  for a base (cation)  for an acid (anion)

Sorption by Ion Exchange  Ion exchange calculations: procedure for a base; protonated fraction, e.g., RNH 3 + for an acid; deprotonated fraction, e.g., RO -

Sorption  Effect of pH on sorption mechanisms  partition to octanol from water

Sorption  Effect of pH on sorption mechanisms  partition to octanol from water  sorption to organic matter in soil

Sorption  Effect of pH on sorption mechanisms  partition to octanol from water  sorption to organic matter in soil  sorption to mineral surfaces (hydrophobic)

Sorption  Effect of pH on sorption mechanisms  partition to octanol from water  sorption to organic matter in soil  sorption to mineral surfaces (hydrophobic)  ion exchange to mineral surfaces

resveratrol (anti-aging…?)

glutamic acid (an amino acid) resveratrol (anti-aging…?)

glutamic acid (an amino acid) resveratrol (anti-aging…?) (my occasional drug of choice)

glutamic acid (an amino acid) resveratrol (anti-aging?) (my occasional drug of choice) caffeine

Sorption by Ion Exchange  What is the fraction of aniline in the water in equilibrium with a sandy clay sediment?  aniline present at M (total)  groundwater composition  pH 6, 1 mM Na +, 0.1 mM K +, 1 mM Cl -, 0.32 mM HCO 3 -  sediment composition  85% quartz, 10% kaolinite, 4% iron oxide, 0.1% organic matter  solid density 2.6 kg L -1, porosity 0.4  V vic ~ 0.01 L kg -1 (medium ionic strength)

Sorption by Ion Exchange = H + + ArNH 3 + = H + + ArNH 2 AnH + = H + + An pK a = 4.63

Sorption by Ion Exchange  Aniline (An, AnH + ) is a weak base  pH 6, pK a = 4.63  neglect these sorption mechanisms in this example:  cation exchange to iron oxide  oxide is (+) at pH 6; no cation exchange  partition to organic matter and to bare mineral surface  aniline is polar, very low K ow ( )  specific complexation (save this for CVEN 6414)

Sorption by Ion Exchange  Assume sodium is major counter-ion

Sorption by Ion Exchange  Ion exchange to “surf” (= CEC) solid weight fraction quartz0.85 kaolinite0.10 organic matter0.001

Sorption by Ion Exchange  Ion exchange to “surf” (= CEC) solid weight fraction quartz0.85 kaolinite0.10 organic matter0.001 (Table 11.3)

Sorption by Ion Exchange  Ion exchange to “surf” (= CEC) solid weight fraction typical A (m 2 g -1 ) quartz kaolinite organic matter (Table 11.3)

Sorption by Ion Exchange  Ion exchange to “surf” (= CEC) solid weight fraction typical A (m 2 g -1 )  surf ex (mol m -2 ) quartz  kaolinite  organic matter  (Table 11.3)

Sorption by Ion Exchange  Ion exchange to “surf” (= CEC) solid weight fraction typical A (m 2 g -1 )  surf ex (mol m -2 ) quartz  kaolinite  organic matter  (Table 11.3)

Sorption by Ion Exchange  Ion exchange to “surf” (= CEC) solid weight fraction typical A (m 2 g -1 )  surf ex (mol m -2 ) CEC (mol g -1 ) quartz   kaolinite   organic matter   (Table 11.3)

Sorption by Ion Exchange  Ion exchange to “surf” (= CEC) solid weight fraction typical A (m 2 g -1 )  surf ex (mol m -2 ) CEC (mol g -1 ) quartz   kaolinite   organic matter    total CEC: mol kg -1 (Table 11.3)

Sorption by Ion Exchange  Ion exchange  estimate of K ex  aniline – hydrophobic equivalent is benzene  benzene C w sat (L) = M  easy – benzene is actually a liquid  z = 1 for Na +

Sorption by Ion Exchange  Ion exchange 3,4-dihydroxybenzoic acid pK a1 = 4.48 pK a2 = 8.83 pK a1 = aminonaphthalene (1-naphthylamine) pK a = 3.92

Sorption by Ion Exchange  Ion exchange ,4-dihydroxybenzoic acid pK a1 = 4.48 pK a2 = 8.83 pK a1 = aminonaphthalene (1-naphthylamine) pK a = 3.92

Sorption by Ion Exchange  Ion exchange ,4-dihydroxybenzoic acid pK a1 = 4.48 pK a2 = 8.83 pK a1 = aminonaphthalene (1-naphthylamine) pK a = 3.92

Sorption by Ion Exchange  Ion exchange ,2-dihydroxybenzene T m = 105  C naphthalene T m = 80.3  C

Sorption by Ion Exchange  Ion exchange ,2-dihydroxybenzene T m = 105  C naphthalene T m = 80.3  C

Sorption by Ion Exchange  Ion exchange ,2-dihydroxybenzene T m = 105  C naphthalene T m = 80.3  C Myrdal

Sorption by Ion Exchange  Estimating K d ex

Sorption by Ion Exchange  Calculating K d  Calculating f w

Next Lecture  Electron-donor acceptor sorption Dissolved organic matter