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The importance of marsh plants for pollutant removal in passive biological water treatment systems,
constructed wetlands, at two mining locations in Germany was investigated. The aim of this investigation
was to identify the fate of metals and radionuclides in eight small-scale, and one pilot-scale, constructed
wetlands on different uranium mine sites. The plant material (roots, shoots, and plant residues) was analyzed
at different times during the pilot run. Furthermore, the effect of plant fertilizer on contaminants
accumulation by plants and the release of contaminants from plant shoots and leaves, back into the mine
water, was investigated. Plants had a significant effect on arsenic, uranium and radium separation, as well
as on the dissolved oxygen content, redox potential and pH of the effluent of the pilot systems. Site-specific
differences in the efficiency of the tested helophyte species (Phragmites australis, Carex disticha, Typha
latifolia, and Juncus effusus) on contaminant removal were identified.
At one of the investigated mining sites, arsenic, uranium, radium, manganese and iron had to be removed
before discharge of the seepage into a nearby stream. The plant biomass was analyzed five times over a
period of 30 months. Phytoaccumulation resulted in the removal of 17,500 mg/kg of iron, 930 Bq/kg of
radium, 610 mg/kg of manganese, 360 mg/kg of arsenic and 30 mg/kg of uranium in dry plant matter,
especially roots, after this operation time.
At a second uranium mining site, an extensive research programme on the treatment efficiency, robustness
and long-term stability of a constructed wetland was carried out, focusing on the biological separation of
arsenic and uranium. A pilot-scale constructed wetland was designed and built with four serially-connected
ponds (total surface area of 1400 m²). For the effective separation of uranium and arsenic, different redox
conditions were needed. A carbon source was added to establish anaerobic conditions for the reduction of
uranium in the first two ponds. In the third pond, the water was enriched with oxygen to reach aerobic
conditions for the removal of arsenic. A concentration of 0.3 mg/L of uranium, and less than 50 µg/L of
arsenic in the effluent, could be reached. After two years of operation, samples were taken from the
helophytes in all ponds. The highest uranium concentrations (about 386 ppm of dry mass) were found in the
aerial plant parts of the plants in the first pond. Arsenic was accumulated in the roots of plants growing in
the last aerobic ponds, to a maximum concentration of 195 ppm of dry mass.
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