Eco-transportation
of PCB's - Degradation & Elimination in Environment
The PCB's are among the most ubiquitous and persistent pollutants in global system.
Because of their limited solubility in water, PCB's are usually found in only
trace concentration in surface water, while these are available in appreciable
concentration in body tissue of aquatic organisms from industrialized regions.
The most common pathway of transportation of PCB's is through the air. The open
burning or incomplete combustion of PCB's or waste containing PCB's, volatilization
and transport of aerosol followed by atmospheric deposition are probably responsible
for global dispersion of PCB's. The ability of PCB's to volatilize from landfills
into the atmosphere (adsorption to aerosols with particle size of less than 0.05-20
µm) and resist degradation at low increasing temperatures, makes atmospheric
transport the primary mode of distribution of PCB's. The half life of air borne
PCB's particles depend greatly on the size of particles and the extent of atmospheric
precipitation.
Duinker
& Bouchertall (1989) analyzed filtered air, particulates and rain, in the
city of Kiel, Federal Republic of Germany for fourteen different PCB congeners.
Their study indicated that PCB congeners with a low degree of chlorination were
dominant in filtered air, whereas, congeners with high degree of chlorination
dominated in aerosols and rainfall. The vapor phase represented upto 99% of more
volatile congeners. (The particulates were found to carry relatively more of the
less volatile congeners). Particle scavenging was the dominant source of PCB's
in rain water despite the small contribution of particulate PCB's to the overall
atmospheric concentration of PCB's (only 1 or 2%). Precipitation scavenging of
chlorinated hydrocarbons in the atmosphere is complex. Scavenging of particles
by cloud droplets and by rain drops in and below clouds and the scavenging of
the vapour phase occurs by rain. Thus, the chlorinated hydrocarbons are concentrated
in precipitation rather than in the atmosphere, resulting in their high rainfall
levels.
In water PCB's are adsorbed on sediments and other organic matter.
Strong adsorption on sediment, especially in the case of higher chlorinated PCB's
decreases the rate of volatilization. The transport of PCB's from sediment to
water occurs because of desorption, bioturbation, gas convection and erosion.
From the water, PCB's may be transported and enriched in the surface micro layer
or transported to air. The PCB's deposited in sediments may directly be taken
up from the sediment to the food chain by benthic organisms, while PCB's present
in water tend to bio-accumulate in food chain through phytoplankton, zooplankton
and other biota. On the basis of their water solubilities and n-octanol-water
partition coefficients, the lower chlorinated PCB's congeners are sorbed less
strongly than the higher chlorinated isomers. The low solubility and the strong
adsorption of PCB's on soil particles limits leaching in soil. Lower chlorinated
PCB's tend to leach more than highly chlorinated PCB's.
PCB's are highly
resistant to degradation, once these are in the environment, these are sorbed
onto the particles including sediments, suspended particulates and may bio-accumulate
in organisms. Degradation of PCB's in the environment is dependent on the degree
of chlorination of the biphenyl. The persistence of PCB's congeners increase as
the degree of chlorination sets. In the atmosphere, the vapor phase reaction of
PCB's with hydroxyl radicals (which are photochemically formed by sunlight) may
be the dominant transformation process. In the aquatic environment, hydrolysis
and oxidation do not significantly degrade PCB's. Photolysis appears to be the
only viable abiotic degradation process of PCB's in the water.
It
has been established that microorganisms degrade mono, di-, and tri-chlorinated
biphenyls relatively rapidly and tetra-chlorobiphenyls slowly, whilst higher chlorinated
biphenyls are resistant to biodegradation. Chlorine substitution positions on
the biphenyl ring play an important role in determining the biodegradation rate.
The PCB's containing chlorine atoms in the Para positions are preferentially biodegraded.
Higher chlorinated congeners are bio-transformed anaerobically, by reductive dechlorination,
to lower chlorinated PCB's, which may then be biodegradable by aerobic processes.
The first reaction step in the degradation of chlorobiphenyls in most cases
is de-oxygenation, eventually leading to the formation of chloro-benzoates. The
chlorine group in the ortho and meta positions offers steric hindrance to the
degradation reaction.
