Bioremediation

From Wikipedia, the free encyclopedia.

Jump to: navigation, search

Bioremediation can be defined as any process that uses microorganisms or their enzymes to return the environment altered by contaminants to its original condition. Bioremediation may be employed in order to attack specific contaminants, such as chlorinated pesticides that are degraded by bacteria, or a more general approach may be taken, such as oil spills that are broken down by the use of multiple techniques including the addition of nitrate and sulfate fertilizer to facilitate the decomposition of crude oil by indigenous or exogenous bacteria.

Not all contaminants are easily treated through the use of bioremediation; for example, heavy metals such as cadmium and lead are not readily absorbed or captured by organisms. The integration of metals such as mercury into the food chain may make things worse as organisms bioaccumulate these metals.

However, there are a number of advantages to bioremediation, which may be employed in areas that cannot be reached easily without excavation. For example, hydrocarbon spills (specifically, petrol spills) or certain chlorinated solvents may contaminate groundwater in the subsurface and injecting the appropriate organisms, in conjunction with electron acceptor or electron donor amendment, as appropriate, may significantly reduce contaminant concentrations after a period of time. This is typically much less expensive than excavation followed by disposal elsewhere, incineration or other ex situ treatment, and reduces or eliminates the need for pumping and treatment, which is a common practice at sites where hydrocarbons have contaminated groundwater.

Generally, bioremediation technologies can be classified as in situ or ex situ. In situ bioremediation involves treating the contaminated material at the site while ex situ involves the removal of the contaminated material to be treated elsewhere. Some examples of bioremediation technologies are bioventing, landfarming, bioreactor, composting, bioaugmentation and biostimulation.

[hide]

[edit]

Monitoring bioremediation

The process of bioremediation can be monitored indirectly by measuring the Oxidation Reduction Potential or redox in soil and groundwater, together with pH, temperature, oxygen content, electron acceptor/donor concentrations, and concentration of breakdown products (e.g. carbon dioxide). This table shows the (decreasing) biological breakdown rate as function of the redox potential.

Process	Reaction	Redox potential (E_h in mV)
aerobic:	O₂ + 4e^- + 4H⁺ → 2H₂O	600 — 400
anaerobic:
denitrification	2NO₃^- + 10e^- + 12H⁺ → N₂ + 6H₂O	500 — 200
manganese IV reduction	MnO₂ + 2e^- + 4H⁺ → Mn₂⁺ + 2H₂O	400 — 200
iron III reduction	Fe(OH)₃ + e^- + 3H⁺ → Fe²⁺ + 3H₂O	300 — 100
sulfate reduction	SO₄2^- +8e^- +10H⁺ → H₂S + 4H₂O	0 — -150
fermentation	2CH₂O → CO₂ + CH₄	-150 — -220

This, by itself and at a single site, gives little information about the process of remediation.

it is necessary to sample enough points on and around the contaminated site to be able to determine contours of equal redox potential. Contouring is usually done using specialised software, e.g. using Kriging interpolation.
if all the measurements of redox potential show is that electron acceptors have been used up, it is in effect an indicator for total microbial activity. Chemical analysis is also required in order to demonstrate that levels of hydrocarbons and breakdown products have been reduced by remediation to below regulatory limits.