Sink Processes of Organic Chemicals in Soil




An organic chemical in soil can degrade through abiotic or biotic reactions. The term abiotic refers to those reactions whch do not involve (a) microbial activity (b) extracellular enzymes, or (c) metabolic intermediates NADH, NADPH, flavins, hemoproteins, chlorophyls, etc. An understanding of these processes is important to assess persistence of environmental contaminants and selection of remediation technologies.
Five prominent abiotic reactions that an organic chemical can undergo are:

1.Hydrolysis
2.Substitution
3.Elimination
4.Oxidation
5.Reduction
Hydrolysis
In the simplest terms, hydrolysis can be defined as a reaction involving water or a hydroxide ion:
R ­ x + H2O Æ R ­ OH + H+ + x­
R ­ x + OH­ Æ R ­ OH + x­
In these reactions, H2O and OH­ act as nucleophiles. These interact with an electrophile and displace a leaving group x.
R ­ x , R ­ OH
The reaction involves cleavage of a R ­ x (where x is the leaving group) bond and formation of a new bond, R­OH bond.
The nucleophilic reactions are divided into two categories designated SN1 and SN2. SN1 (substitution, nucleophilic, unimolecular) reaction mechanism involves two separate reactions:
R ­ x ¤ R+ + x­
R+ + H2O Æ R ­ OH + H+
The molecule R ­ x must ionize to form a carbocation R+ and a leaving group (x). This is the slower, or the rate limiting, reaction. In the second step, the carbocation, R+, is rapidly attacked by the nucleophile, OH­, to form the reaction product R ­ OH.
The overall rate of the process is governed by the first reaction which is affected by electrons donating and withdrawing fragments of molecule R ­ x. For dissociation to occur, it must be assisted by the solvent. In soil systems, water acts as both the solvent and the nucleophile.
The SN2 (substitution, nucleophobic, biomolecular) reaction mechanism involves two molecules:
H2O + R ­ x Æ H2O º R ­ x Æ H+ + HO ­ R + x­
The nucleophile approaches R ­ x from a position opposite the leaving group x. The x group leaves as OH group moves in. The process is a single step with no intermediate chemical complexes. The energy required for the cleavage of the R ­ x bond is supplied by the simultaneous formation of the R ­ OH bond.
The processes can be differentiated through an examination of the energy level diagrams:

Kinetics of the reaction: SN1 reactions are unimolecular; consequently, the rate constant for SN1 (K1) is proportional to the concentration of R­x and rate constant for SN2 (K1) is proportional to concentration of R­x and Y­.

K1 a [R ­ x]
K2 a [R ­ x] [Y­]

Reaction Rate
In weak solution, the hydrolysis of organic chemicals follows a first order kinetics. The rate is dependent only on the concentration of the organic chemical.
We can thus obtain a linear relationship between time and concentration for SN1 reaction. The rate constant k can be determined through the following expression:
k=\|F(2.303,t) log \|F(ao,at)
k=rate constant moles/time
t=time (days)
ao=initial concentration (ppm or moles)
at=concentration at time t (ppm or moles)
If k is known, then half-life can be estimated from the expression:
t1/2 = \|F(0.693,k)
t1/2 = half life at = 0.5 ao (time at which concentration drops to half the initial concentration)
Hydrolysis Susceptability
The hydrolysis susceptability of organic chemicals is dependent on the structure, especially the presence of fragments which are good leaving groups. These include:
alkyl halides, (I>Br»cl>F)
carbamates
esters
epoxide
Some molecules are resistent to hydrolysis; these include alkanes, alkenes, alkynes and aldehydes.
The hydrolysis rate is dependent on a number of environmental factors such as temperature, acidity and basicity (pH), presence of metals and soil moisture of the soil system. e.g., hydrolysis of ester proceeds at a faster rate under acidic conditions.
OO
||||H
R1 ­ C ­ O ­ R + H+ Æ R1 ­ C+ ­ O ­ R
OO
||H||
R1 ­ C+ ­ O ­ R + OH­ Æ R1 ­ C ­ OH + HO ­ R

A more general form of the rate expression can be written as:
kt =kH (H+) + kN + kOH (OH­)
kH=rate constant for acid catalyzed reaction
kN=rate constant for neutral reaction
kOH=rate constant for base catalyzed reaction
H+=hydrogen in concentration
OH­=hydroxyl ion concentration

Since kt is dependent on kH, kN and kOH, a pH rate profile for susceptible organic molecules may take a U or V shape, depending on the magnitude of Kn:

pH Rate Profile for Organic Chemicals Undergoing

Acid and Base Catalyzed Hydrolysis

Hydrolysis of certain organic chemicals is catalyzed by metal ions. The metal ions can act as Lewis acid that directly or through a coordination complex polarize the organic molecule.
R ­ C = O + M+ Æ R ­ C ­ O Š M+
or
R ­ X + M+ Æ R+ + MX
R+ is susceptible to hydrolysis by H2O or OH­.
For example, organophosphates are an important group of pesticides. Their hydrolysis is catalyzed by Cu+2, which, in montmorilonite clays, has been found to be nearly as active as free Cu+2.
The nature of coordination legend is important; e.g., while Cu+2 in montmorilonite is active, Cu+2 adsorbed on ion-exchange sites or on soil organic matter is completely inactive.
The rate constant for metal catalyzed reaction can be written as:
kt = \|F(kN + kMkA(mt),(kA + [Ht]))
where kM= metal-ion catalysis constant
kA= equilibrium constant for hydrated ion complex
mt= total metal ion concentration
Conclusion:
The importance of hydrolysis from an environmental point of view is that the reaction introduces a hydroxyl group into the parent molecule. The resulting product is usually more susceptible to further attack through biodegradation and photolysis. Hydrolysis also reduces the bioaccumulation potential of the product.