Adsorption And Organic Chemical Structure




We have discussed adsorption of non-ionized contaminants in soil and determined that adsorption of a wide variety of organic molecules are governed by one soil property, that is, the soil organic matter content. Since water is the major carrier in soil, the advective transport is dependent on partitioning between the water phase and the soil organic phase. The relative adsorption of non ionized organics can be expressed in terms of soil/water partition coefficient Koc,
Koc = (Ksw)(foc)
where Ksw is the soil/water partition coefficient
foc is the parent organic carbon
The extent to which any organic chemical is adsorbed onto soil surface is directly affected by the chemical's structure. Structure determines the magnitude of organic chemical physiochemical properties:

€melting point€solubility
€vapor pressure€polarity
€acid/base characteristics€adsorption
€electromagnetic radiation
These, in turn, affect adsorption.
Adsorption is affected by the following 6 molecular properties:
Molecular Size
The overall size of the organic molecule has a significant effect on its adsorption potential. Within a homologous series, the larger the molecule, the greater its propensity to remain in the adsorbed state. One of the causes for this are the van der Waals' forces.
The larger the molecule, the higher the contribution of van der Waals' forces to the adsorption potential. Van der Waals' forces are relatively weak forces that result from individual electrostatic attractions between otherwise non-attracting atoms and molecules. The average distribution of charge on a neutral molecule is zero. But even these molecules experience van der Waals' forces. These are the weakest of all molecular interactions, and these interactions arise because, at any instant, all molecules have unsymmetric electronic distribution which creates small dipoles which can induce complementary dipoles in the neighboring surface. The van der Waals' force operates over short distances (only a few bond lengths).
Within a homologous series, the larger the molecular size, the larger the van der Waals' forces between the molecules and soil surfaces, consequently, the greater the extent of adsorption (higher Koc value); e.g., even water soluble polymers which possess large volumes such as polyethylene glycol, dextran and polyvinyl alcohol are adsorbed extensively on soil and are difficult to desorb.
A number of environmentally important organic molecules (PCBs, polyaromatic hydrocarbons, phthalates, DDT, etc.) are adsorbed on surfaces partly due to the weak van der Waals' forces.
Hydrophobicity
Another factor that contributes to chemical adsorption onto soil surface is the molecule's hydrophobicity (fear of water or repulsion from water - sometimes treated the same as lipophilicity). It refers to the preferential migration to and accumulation of organic chemical in hydrophobic solvents (water immiscible) or hydrophobic surfaces such as soil organic matter. The net hydrophobicity of any molecule is governed by the combined effects of hydrophic and hydrophobic fragments that comprise the molecule.
The preferential accumulation of an organic chemical on hydrophobic soil particles is sometimes referred to as "hydrophobic bonding" or hydrophobic partition and are most likely related to van der Waals' forces.
It should be pointed out that, even though soil organic matter is the major contributor to adsorption, other soil constituents (especially clay) can play a role in adsorption. The relative contribution is dependent on the extent of organic matter coverage on soil clay particles. In some cases, these interactions can be stronger than the contaminant organic matter interactions; e.g., adsorption of PCBs has been found to be stronger in high clay content low organic carbon subsurface soils than in certain high organic carbon top soils.
Molecular Charge
The third factor that affects organic chemical adsorption onto soil surfaces is the intrinsic positive or negative charge possessed by some organic molecules. A number of organic molecules such as the quaternary ammonium, RN+, diazonium R-N=N:+ and some metallo-organics possess functional groups that possess permanent, positive charges. These molecules are adsorbed onto the cation exchange sites on soil surfaces.
Some organic chemicals possess amine functional groups or phenolic groups that may or may not possess a charge, depending upon the acidity or the basicity of the system. If the soil is acidic pH ¾ 6.5, then H± concentration is high enough for a -NH2 group to acquire a positive charge.


Both reactions are reversible, and direction will depend on the pH. The extent is dependent on the difference between the pH and pKa values.

Ka = dissociation constant or ionization constant
[Ka]=\|F([Am][H+],[HAn])
[Am]=concentration of dissociated form
[H+]=concentration of H+
[HAn]=concentration of H+ and organic molecule complex (A)
The relationship between pH and pKa can be derived by rearrangement and converting parameters to a negative logarithm:
pH ­ pKa = log \|F(Am,HAn)
This relationship is useful because it permits one to determine the relative ratio of charged to uncharged molecules in soil/water systems.

Relationship Between pH, pKa and Ratio of \|F(Am,HAn)

pH ­ pKa
% Am
% HAn \|F(Am,HAn) 2 99 1 \|F(100,1) 1 91 9 \|F(10,1) 0 50 50 \|F(1,1) ­ 1 9 91 \|F(10,1) ­ 2 1 99 \|F(1,100)
Some anionic organics such as carboxylic acids and phenols dissociate at high pH and possess a negative charge. These may be adsorbed on positive charge centers on soil organic matter or silicate clay minerals. In addition, ion-exchange type interactions are also possible.
It should be pointed out that, in general, soils possess a higher number of negative surface charges than positive charges, which can lead to decreased adsorption of organic anions on soil surfaces due to charge repulsion between the contaminant anion and the soil surface.
Hydrogen Bonding
Hydrogen bonding is a very important type of charge dipole or dipole-dipole attraction. Hydrogen bonding occurs whenever a hydrogen atom serves as a bridge between two electronegative atoms; the hydrogen atom is limited to one electronegative atom by a covalent bond and to the other by an electrostatic bond.
Three distinct types of hydrogen bonding can occur between soil organic molecules and soil particle surfaces. The first is the formation of a hydrogen bond via the linking of a polar organic molecule at an adsorbed cation through a water molecule in the cation's primary hydrogen shell. This type of bond is associated with the montmorillonite systems. Aldehydes, ketones and carboxylic acids have been shown to be adsorbed through this interaction.


The second type of hydrogen bonding can occur between an adsorbed organic cation and another organic molecule.


The third type of hydrogen bonding can result from interactions between organic molecule fragments and oxygen or hydroxyl groups on soil surfaces. The interactions can again be charge-dipole type, e.g., metal-hydroxide (M-OH) can interact with


Stereochemical Effects
Molecular structures are three dimensional, and some of the properties of closely related molecules are dependent on the differences in the arrangement of atoms in space. The molecules are referred to as stereoisomers.
Stereoisomers can be defined as structures that differ only in the arrangement of atoms in space. Stereoisomers have the same molecular formula and the same linkages of bonded atoms.
Since organic matter and other interactive structures in soil also possess three dimensional structures, the degree of interaction (adsorption) between the contaminant molecule and the soil surface is dependent on the stereochemistry of the molecules; e.g., the results of adsorption studies show that two isomeric forms of benzene hexachloride (BHC) hexachlorocyclohexane, a common chlorinated hydrocarbon pesticide (known as lindane), exhibit significantly different partition co efficient values. The g somer (lindane) has greater adsorbability than the b somer . The difference is a result of spatial arrangement of Cl atoms in the molecule. The chair form structure of the two isomers are given below.


These molecules are not planar. In the g-BHC three chlorine atoms are equitorial, extending in the plane of the ring; the other three atoms are axial and extend above or below the plane of the ring. In the b isomer all chlorine atoms are equitorial, resulting in an equal pull on the electrons (dipole moment is near zero), whereas the g isomer has a dipole moment of 2.83 debye units ( calculated as 1018 times the product of distance and charge, both in cgs units).
As a result, one can say that the net adsorption surface is less than those between b isomer and soil surface.
Studies on adsorption of certain optical isomers have revealed that some stereoisomers exhibit adsorption behavior. L-leucine, L-aspartate and D-Glucose are adsorbed montmorillonite clays while D-leucine, D-aspartate and L-glucose are adsorbed only to a negligible extent.
The stereoisomers occur because of different molecular fragment about single bond; these cannot interconnect by internal rotation about single bonds. A pair of stereoisomers that are non superimposable mirror images are called enantiomers. D&L are enantiomer pairs that differ in spatial orientation of the molecular fragments.
In some cases, the substitution of molecular fragments can affect the planarity of the molecules; e.g., in case of polychlorinated biphenyls (PCBs), it is largely determined by degree of substitution on the ortho positions; i.e., 2,2' and 6,6':


In the absence of ortho substitution, the two phenyl rings are in more or less the same plane. The increasing substitution at the ortho position leads to increased crowding and results in twisting of the rings. Since the larger the planar surface, the larger the adsorption, the non-ortho substituted PCB isomers show greater adsorptivity; e.g., Koc value of 2,2' dichlorobiphenyl is 5 times less than Koc value for 2,4 dichlorobiphenyl.
Coordination
Another factor that can affect adsorption of organic chemicals onto soil surfaces is the formation of coordination complexes.
The coordination complexes result from weak bonds between an organic molecule which is capable of donating electrons and adsorbed cation which is capable of accepting electrons. The net result is partial overlap of orbitals and partial exchange of electron density.
The electron donors can be divided into two groups is composed of organic molecules that contain S, N and O atoms which contain lone pair electrons (R-NO2 nitro, R-OH alcohols, R-O-R ethers, R-SH thiols, R-S-R thioethers and R-NH2 amines; e.g., nitrobenzene has been shown to be adsorbed to soil surface through a coordination linkage with adsorbed K+ and NH4+ ions.