 A study carried out by Dr Olaf Pollman of the
North-West University’s School of Environmental Science and
Develop-ment offers a new way of treating heavily polluted mine
water containing high levels of heavy metals and other undesirable
elements.
“The aim of the project was to use a totally different method
to clean the highly polluted acid mine drainage (AMD) in
underground water in an organically biochemical and mechanical
way,” Pollman says.
AMD results from mining activities exposing water to elements such
as iron, sulphates, manganese, mercury, zinc, copper and nickel.
Under normal circumstances, operational mines pump this water out
of the underground workings to dewater underground mining
operations. This water may then be released into the unpolluted
groundwater used, through boreholes, for agriculture and for human
and animal consumption. Polluted water from mines may also find its
way to rivers and streams.
The new technology proposed by Pollman uses only organic materials
and organic and biosystematic processes precipitate the heavy
metals out of the system.
Pollman explains that the method of deal- ing with AMD depends on
different cleaning steps. First the organic material intended to
clean AMD is moistened to open the plant cells in preparation for
the process. After the material has been left for a few days, the
AMD, which is highly polluted with trace elements, such as calcium,
magnesium, potash, sodium, iron and chloride, is run through the
different sacks filled with bark and needles.
The precipitation is then visible on top of the sacks filled with a
mixture of needles and bark from the Casuarina pine
species. In this way, the metals and salts removed from the water
may be reused in different processes.
“In the case of the laboratory experiment, three cylinders
were filled with different materials. “The first was filled
with a 50% mixture of the bark and needles of the pine species. The
materials contained organic composites, such as bacteria, fungi,
yeast and other micro organisms. The other 50% was filled with
sand. “The second cylinder only contained bark and sand and
the third contained needles and sand. This arrangement made it
possible to see which material combination was best suited to
precipitate the elements out of the AMD,” Pollman
explains.
To make the cleaning process even clearer, the polluted water was
recycled through the system more than ten times. After every cycle,
samples were taken from each cylinder to measure the pH and the
total iron (Fe) content.
After full recycling, the water was tested for macro-, micro, and
other elements by water and 1 to 2 extract analysis.
“The process of cleaning the AMD entails leading the water
through all the cylinders filled with the organic filters and
reaction materials. “In this case, 5 l of the AMD was filled
in the first cylinder and, after leaching through the material, the
AMD was pumped to the next cylinder. “After passing through
all three cylinders, the cleaned AMD was pumped back to the first
cylinder to pass through the system again. This was repeated more
than ten times,” Pollman says.
To test field conditions, the AMD was led over a testbed of
cascades, which was also filled up with different materials. The
first cascade was filled with needles and bark, the second with
bark and sand, and the third with only needles and sand. Finally
the AMD had to pass a cascade of the sedimentary rock,
dolomite.
The conditions in the laboratory were transferable to field
conditions and, consequently, the results were comparable to the
boundaries as limited by field conditions. The circulation of the
AMD water through the cylinders showed that the pH rose when
treated with semi- decomposed organic material and that the metals
were bound and precipitated to the surface. The treatment of the
AMD water in the cascades also showed that Fe can be bound and
removed from the water. As a result of oxidation and rain water,
the pH still remained low. The results of this test indicate that
the natural treatment of the heavy metals by the capillary pressure
of the pine needles and bark is working well.
The results attained in the laboratory experiment as well as the
AMD treatment after only 30 days shows that the reaction of the
semidecomposed organic material, named SupaZorb, binds and
immobilises the heavy metals and that the natural protection of the
pine needles and bark through capillary reaction can precipitate
the metals to the surface.
The lignin and tannin of the bark also raised the pH of the AMD and
supported the precipitation of metals.
“The removal of metals is not finally combined with the rise
of pH in the AMD water. The results of both experiments prove a
removal of metals of more than 80%. “In addition, the water
quality improved to such an extent that algae and zooplankton could
be detected in the treated AMD water,” Pollman says.
“To raise the pH of the system, the oxygen balance must be
corrected through a process such as ozonation, the system must be
covered from extreme rainfall and the sulphates have to be removed.
“To finalise the project, the cascades only have to be
stabilised and optimised to the ever-changing conditions on
site,” Pollman concludes.
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