Many physical, chemical, and biological mechanisms are
known to occur within passive treatment systems to reduce the metal
concentrations and neutralize the acidity of the incoming flow streams.
Notable mechanisms include the following:
- Sulfide and carbonate precipitation catalyzed by sulfate reducing
bacteria (SRB) in anaerobic zones.
- Hydroxide and oxide precipitation catalyzed by bacteria in aerobic
zones.
- Filtering of suspended material.
- Metal uptake into live roots and leaves.
- Adsorption and exchange with plant, soil and other biological materials.
Remarkably, some studies have shown that plant uptake does not contribute
significantly to water quality improvements in passive treatment systems
(Wildeman, et al., 1993). However, plants can replenish systems with
organic material and add aesthetic appeal. In aerobic systems, plant
assisted reactions appear to aid overall metal-removal performance,
perhaps by increasing oxygen and hydroxide concentrations in the surrounding
water through photosynthesis related reactions and respiration in the
plant root zone. Plants also appear to provide attachment sites for
oxidizing bacteria/algae. Research has shown that microbial processes
are a dominant removal mechanism in passive treatment systems (Wildeman,
et al., 1993). One anonymous researcher considered a passive treatment
system as a "bioreactor with a green toupee," referring to
the substrate where most of the bioreactions occur and the collection
of plants that grow on top of the treatment cells.
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