CVEN 5424 Environmental Organic Chemistry Lecture 15 – Sorption to Mineral Surfaces

Announcements  Reading  Chapter 11, Sorption  Problem sets  PS 6 due today  PS 7 out today; due next Thursday Office hours Tuesday 11:30 am-1 pm Wednesday 9-10 am Exams Exam 1: Q 3 re-do graded by tomorrow Exam 2: Tues, March 15, to Thur, March 17

Molecules you ought to be aware of…

Luthy et al. (1997, ES&T 31, ) ? Sorption to Mineral Surfaces

 Organic compounds  neutral organic compounds  “hydrophobic” sorption to bare mineral surfaces  from air to solid  from water to solid  electron donor-acceptor sorption  ionic compounds (next)  other mechanisms dominate  ion exchange  specific complexation

Sorption to Mineral Surfaces  Mineral surfaces  polar, charged (nearly all minerals)  important at low organic matter content  f oc <  of some concern in nature, but more in the laboratory

Sorption to Mineral Surfaces air  Sorption from air  partition between air and soil surface (by mass)  partition between air and soil surface (by surface area)  still considered linear adsorption

Sorption to Mineral Surfaces  What affects sorption from air (K a,surf )?  relative humidity  soil nature  compound nature  temperature

Sorption to Mineral Surfaces  Effect of relative humidity  dry air (low RH)  dry soil more sorption  monolayer water at 20% RH  wet air (high RH)  wet soil less sorption  multilayer coverage at >80% RH Unger et al. (1996). Predicting the effect of moisture on vapor-phase sorption of volatile organic compounds to soils. ES&T 30, dry wet

Sorption to Mineral Surfaces  Effect of relative humidity  “RS” (relative saturation)  more sorption to dry soil (0.0% RS) Unger et al. (1996). Predicting the effect of moisture on vapor-phase sorption of volatile organic compounds to soils. ES&T 30, TCE 1,1,1-TCA toluenebenzene

Sorption to Mineral Surfaces  Effect of soil type  toluene sorption to soils at 0% relative saturation  more organic matter, more sorption Unger et al., Predicting the effect of moisture on vapor-phase sorption of volatile organic compounds to soils. ES&T 30,

Sorption to Mineral Surfaces  Forces between organic compounds and mineral surfaces O-O- Si  + O-O- OH O-O- Si  + O-O- OH O-O- Si  + O-O- OH O-O- Si  + O-O- OH

Sorption to Mineral Surfaces  Forces between organic compounds and mineral surfaces O-O- Si  + O-O- OH van der Waals O-O- Si  + O-O- OH O-O- Si  + O-O- OH O-O- Si  + O-O- OH

Sorption to Mineral Surfaces  Forces between organic compounds and mineral surfaces O-O- Si  + O-O- OH van der Waals O-O- Si  + O-O- OH O-O- Si  + O-O- OH van der Waals H-bond acceptor O-O- Si  + O-O- OH

Sorption to Mineral Surfaces  Forces between organic compounds and mineral surfaces O-O- Si  + O-O- OH van der Waals O-O- Si  + O-O- OH van der Waals weak H-bond acceptor O-O- Si  + O-O- OH van der Waals H-bond acceptor O-O- Si  + O-O- OH

Sorption to Mineral Surfaces  Forces between organic compounds and mineral surfaces O-O- Si  + O-O- OH van der Waals O-O- Si  + O-O- OH van der Waals weak H-bond acceptor O-O- Si  + O-O- OH van der Waals H-bond acceptor O-O- Si  + O-O- OH van der Waals H-bond acceptor H-bond donor

Sorption to Mineral Surfaces  Effect of compound  sorption affinity depends (partially) on vapor pressure TCE (p L * bar) > 1,1,1-TCA ( ) > benzene ( ) > toluene ( )  incorporate other sorption interactions Unger et al. (1996). Predicting the effect of moisture on vapor-phase sorption of volatile organic compounds to soils. ES&T 30,

Sorption to Mineral Surfaces  K a,surf (m -1 ) prediction  dry surfaces (low RH); 15  C  apolar, monopolar compounds (Eqn. 11-8)

Sorption to Mineral Surfaces  K a,surf prediction: surface properties  vdW surf is van der Waals interaction of surface  about mJ 1/2 m -1 for water, ice, minerals  up to 11 mJ 1/2 m -1 for activated carbon, graphite  HA surf is hydrogen bonding acceptor interaction  HA surf = 1 for water; mineral surfaces similar  HA surf = 0 for organic surfaces  HD surf is hydrogen bonding donor interaction  HD surf = 1 for water; mineral surfaces similar  HD surf = 0 for organic surfaces

Sorption to Mineral Surfaces  K a,surf prediction: compound properties  p L * vapor pressure (units of Pa)  α (H-donor tendency) (Table 4.3)  0 for apolar compounds  0 to 0.6 for monopolar compounds  β (H-acceptor tendency)  0 for apolar compounds  ~0.01 to 0.6 for monopolar compounds

Sorption to Mineral Surfaces  Predict fraction of chloroform in air in dry soil dominated by quartz  RH = 0, f oc = 0,  s = 2.65 kg L -1, A surf = 100 m 2 kg -1,  = 0.40  fraction in the air of the soil:

Sorption to Mineral Surfaces  Predict fraction of chloroform in air in dry soil dominated by quartz  RH = 0, f oc = 0,  s = 2.65 kg L -1, A surf = 100 m 2 kg -1,  = 0.40  fraction in the air of the soil:

Sorption to Mineral Surfaces  Determine r as and K a,soil  r as depends on porosity, density of solids

Sorption to Mineral Surfaces  Determine r as and K a,soil  r as depends on porosity, density of solids  K a,soil is the air-soil partition coefficient (kg L -1 ) SA is surface area (m 2 kg -1 )

Sorption to Mineral Surfaces  Estimate K a,surf  quartz: vdW surf = 6.8; HA surf = 0.89; HD surf = 1.06  chloroform: p L * = Pa,  = 0.15,  = 0.02

Sorption to Mineral Surfaces  Estimate K a,surf  quartz: vdW surf = 6.8; HA surf = 0.89; HD surf = 1.06  chloroform: p L * = Pa,  = 0.15,  = 0.02

Sorption to Mineral Surfaces  Estimate K a,surf  quartz: vdW surf = 6.8; HA surf = 0.89; HD surf = 1.06  chloroform: p L * = Pa,  = 0.15,  = 0.02

Sorption to Mineral Surfaces  Calculate f a for chloroform in dry quartz soil  r as = 0.25 L kg -1  K a,soil = 7.5 kg L -1

Sorption to Mineral Surfaces  Calculate f a for chloroform in dry quartz soil  r as = 0.25 L kg -1  K a,soil = 7.5 kg L -1

Luthy et al. (1997, ES&T 31, ) ? Sorption to Mineral Surfaces

 Overall sorption expression:

Sorption to Mineral Surfaces water  Sorption from water  linear, reversible equilibrium  normalized to surface area, not mass

Sorption to Minerals  Only at very low f oc  Example: PAH sorption to aluminum and iron oxides, varying organic matter (f oc ) as humic acid adsorbed to mineral surfaces: K min significant for f oc < log f oc < -3 Mader et al. (1997, ES&T 31, ) Al 2 O 3 Fe 2 O 3

Sorption to Minerals  Mineral surface charge  metal oxides  e.g., FeOOH, AlOOH, SiO 2  amphoteric (pH-dependent)  surface hydroxyl groups (>M- OH)  clay minerals  e.g., kaolinite, montmorillonite  faces: permanent  edges: amphoteric  organic matter  ionized functional groups

Sorption to Mineral Surfaces  Amphoteric charge  surface hydroxyls (>M-OH or  M-OH)  proton exchange  pK a values depend on metal oxide  gives rise to AEC or CEC  Anion Exchange Capacity: adsorption of anions (positive charge on the surface)  Cation Exchange Capacity: adsorption of cations (negative charge on the surface) M-OH M-O - M-OH 2 +

Sorption  Amphoteric charge  effect of pH  effect of electrostatic interactions  effect of ionic strength pH pzc

Sorption to Mineral Surfaces  Permanent charge  Al 2 O 3 and SiO 2 layers  isomorphic substitution: Al 3+ for Si 4+  results in charge imbalance: SiO 4 4- becomes AlO 4 5-  gives rise to CEC (cation exchange capacity; ability to adsorb cations to negatively charged surface) Na +

Sorption to Minerals  Effect of mineral surface charge  sorption of PAHs to aluminum oxides  varying pH – alters surface charge  I = 5.9 mM  no significant difference in K d Mader et al. (1997, ES&T 31, ) Al 2 O 3 Fe 2 O 3 + -

Sorption to Minerals  Effect of ionic strength  varying ionic strength – alters effect of surface charge on double layer distance  pH = 7  no significant difference in K d Mader et al. (1997, ES&T 31, ) Al 2 O 3 Fe 2 O 3

Sorption to Minerals  Effect of temperature  varying temperature – a measure of the enthalpy of sorption  sorption increases with decreasing T (opposite of solubility)   min H about the same as -  w H  similar enthalpies for different surfaces Mader et al. (1997, ES&T 31, )

Sorption to Minerals  K min and solubility  K min increases as solubility decreases  and as  w increases  slopes are significantly different – why? Mader et al. (1997, ES&T 31, ) Backhus (1990); silica, kaolinite log K min (mL cm -2 ) = 1.7 log  w sat – 14.8

Sorption to Mineral Surfaces  Why is K min related to  w ?  enthalpy?  from water to vicinal water  little enthalpy change  entropy!  from partially organized water to fully organized water  no need to pay entropy cost of organizing water,  S ice

Sorption to Mineral Surfaces  Example  Effect of sorption to glass on the sorption of hexachlorobenzene (  w sat = 9.8  10 8, K ow = ) to soil (f oc = 0.02) at 25  C  first, the control experiment (no soil): V w = 15 mL SA glass = 8 cm 2

Sorption to Mineral Surfaces  Solving for f w in control experiment:

Sorption to Mineral Surfaces  Solving for f w in control experiment:

Sorption to Mineral Surfaces  Solving for f w in control experiment:

Sorption to Mineral Surfaces  Now, solving for f w with soil present: [soil] = 1 g L -1 V w = 15 mL SA glass = 8 cm 2

Sorption to Mineral Surfaces  Now, solving for f w with soil present: [soil] = 1 g L -1 V w = 15 mL SA glass = 8 cm 2

Sorption to Mineral Surfaces  With the soil present (Eqn. 9-26a):

Sorption to Mineral Surfaces  With the soil present (Eqn. 9-26a):

Sorption to Mineral Surfaces  With the soil present (Eqn. 9-26a):

Sorption to Mineral Surfaces  With the soil present (Eqn. 9-26a):

Sorption to Mineral Surfaces  What fraction is in the water if you don’t account for the glass?

Luthy et al. (1997, ES&T 31, ) ? Sorption to Mineral Surfaces

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  Solve for [RNH 3 :surf]  Langmuir form (limited number of “surf” sites)

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

Next Lecture  Sorption to mineral surfaces