pollution and remediation

plsc204 final exam

soil contaminants BTEX (benzene, toluene, ethylbenzene, and xylene)

• Petroleum-related compounds

• Mobile in soil; often carcinogenic

soil contaminants MTBE (methyl tertiary-butyl ether)

• Replacement for lead in gasoline

• Very mobile in soil; very recalcitrant

soil contaminants PAHs (polycyclic aromatic hydrocarbons)

• Present in petroleum-based mixtures

• Less mobile and more recalcitrant than BTEX

soil contaminants PCBs (polychlorinated biphenyls)

• Developed as a coolant for electrical transformers (arochlor; banned for commercial use in the US)

• Highly recalcitrant, often carcinogenic

soil contaminants Chlorinated solvents (TCE, PCE)

• Used as cleaning solvents

• Often found in groundwaters; often carcinogenic

soil contaminants Energetics and explosives (TNT, RDX)

• Contaminate both soils and groundwaters

soil contaminants Metals (chromium, lead)

• Not metabolized but oxidation state may be changed

soil contaminants Radionuclides (plutonium, uranium)

• unstable, radioactive forms of elements that decay over time, emitting ionizing radiation

soil contaminants Inorganic compounds (P, N)

• Eutrophication of waters

small changes in molecular structure can greatly alter ____

biodegradability

in transfer process the ___ remain intact

organic chemicals

Anaerobic biological degradation 

the breakdown of organic matter by microorganisms in the absence of oxygen

Aerobic biological degradation

the natural process where microorganisms break down organic substances into simpler compounds in the presence of oxygen

Chemical decomposition

the process where a single compound breaks down into two or more simpler substances

Detoxication

the process of removing harmful contaminants to restore soil health

Photo-decomposition

the chemical process where a molecule breaks down into smaller parts due to absorbing light energy

____ is affected by contaminant properties

adsorption

adsorption is affect by type and amount of soil materials, especially ______

organic matter content and clay surface area

______ contaminants associate primarily with organic matter

hydrophobic

hydrophobic organic molecules tend to sorb onto humus more readily than clay because ____

mineral colloids in soils are nearly always surrounded by at least a thin film of water,

The decline in bioavailability in soil colloids can be due to _____

• irreversible sorption/trapping by mineral colloids/precipitates

• chemical complexation by humus, but some contaminant molecules may become physically isolated in nanopores that are too small to give access to bacteria or even their large, extracellular enzyme molecules.

Over time, chemical and physical ____interactions with soil colloids may slow microbial degradation of many organic contaminants, even while bioavailability continues to decline.

aging

bioremediation

A strategy or process that uses microorganisms, plants, or their enzymes to detoxify environmental contaminants

bioremediation common characteristics

• addresses a wide range of chemicals

• includes processes driven by many MOs that often act in concert and in sequence (“microbial consortia”)

• is applied under a variety of soil conditions

• encompasses numerous disciplines

outcomes of bioremediation

• Biotransformation – general term referring to some transformation producing daughter compounds

• Cometabolism – transformation which yields no benefit to the active microbes; “fortuitous degradation”

• Mineralization – complete biodegradation, generally to CO2 and water (desirable)

Biotransformation –

general term referring to some transformation producing daughter compounds

Cometabolism

– transformation which yields no benefit to the active microbes; “fortuitous degradation”

Mineralization

– complete biodegradation, generally to CO2 and water (desirable)

2 approaches to bioremediation

• biostimulation

• bioaugementation

biostimulation

• alteration of the environmental conditions (e.g., by nutrient addition) to stimulate indigenous microbes

bioaugmentation

• Addition of known degraders so as to enhance biodegradation

• Generally combined with biostimulation

ex situ

soil is excavated and treated off site

in situ

soil is treated in place

ex situ biostimulation example

employing composting

in situ biostimulation example

oil contaminating beach soils and needing to remediate the whole beach

advantages of bioremediation

• It’s generally less expensive than alternative technologies

• Positive public image (green technology)

• Offers the possibility of treating in situ (less disturbance)

disadvantages of bioremediation

• Usually takes more time and is less predictable (liability issues)

• Residual levels may be higher (what is clean enough?)

• Mixtures of contaminants may be problematic

phytoremediation

umbrella term describing the use of plants, often along with associated rhizosphere bacteria, to remove, contain, or transform contaminants.

hyperaccumulator phytoremediation

• plants take up and tolerate very high concentrations of an inorganic or organic contaminant.

• In the case of metal contaminants, the addition of chelating agents may increase the rate of metal uptake, but can add a major expense and may allow metals to migrate below the root zone.

enhanced rhizosphere phytoremediation

• the plants do not take up the contaminant. I

• nstead, the plant roots excrete substances that stimulate the microbes in the rhizosphere soil, speeding the degradation of organic contaminants (primarily via cometabolism)

remediation strategies for zinc smelting in palmerton, PA

• liming to raise the soil pH (metals become less mobile)

• addition of compost or similar material to complex the cationic pollutants

• planting hyperaccumulating plants

There are three limits* placed on the application of heavy metals to soils

• max concentration in sludge

• annual pollutant loading rates

• cumulative allowable pollutant loading

Increasing pH lowers the mobility of _____

heavy metals

Radioactive decay of uranium 238 in soils that results in the formation of inert but radioactive ___.

radon