Nickel accumulation and tolerance in Berkheya coddii and its application in phytoremediation.
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Date
1998
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Abstract
As pollution becomes an ever-increasing threat to the global environment pressure is being
placed upon industry to "clean-up" its act, both in terms of reducing the possibility of new
pollution and cleaning up already contaminated areas. It was with this in mind that Amplats
embarked on a phytoremediation project to decontaminate nickel-polluted soils at one of their
mine sites in Rustenburg, using the nickel hyperaccumulating plant, Berkheya coddii, which is
endemic to the serpentine areas near Barberton, Mpumalanga.
Besides the applied aspects pertaining to the development of the phytoremediation process we
were also interested in more academic aspects concerning the transport and storage of nickel
within the plant tissues. In order that the progress of nickel could be followed through the
plant, a radio-tracer of ⁶³nickel was placed in the soil and its movement within the plant
followed by analysing the plant material, at set intervals, using a liquid scintillation counter.
From these studies it was found that the nickel appeared to be transported from the roots to
the leaves of the plant via the xylem. It appeared that the nickel was not confined to the leaf to
which it was initially transported and so movement of nickel within the phloem also appears to occur in B. coddii. As nickel is generally toxic to most plants, hyperaccumulators contain
elements that nullify the toxic effect of nickel. In the case of Berkheya coddii it is thought that
the accumulated nickel is bound to malate to form a harmless nickel complex. With this in mind
an assay for L-malic acid was developed in order that any effect on L-malic acid, caused by
growing Berkheya coddii on soils containing various concentrations of nickel, could be
determined. This method also enabled comparisons of L-malic acid concentrations to be made
between hyperaccumulators and non-hyperaccumulators of various plant species. From the L-malic
acid comparisons it was found that the nickel concentration within soils affected the
levels of L-malic acid within B. coddii and that the levels of L-malic acid within B. coddii were
greater than that of a closely related non-hyperaccumulator, suggesting that L-malic acid is
indeed involved in the hyperaccumulation mechanism within B. coddii.
B. coddii was chosen as the tool in nickel phytoremediation at Rustenburg Base Metal
Refineries as it was found to accumulate up to 2.5% nickel in the dry biomass, it grows rapidly
and has a large above-ground biomass with a well developed root system, and it is perennial
and so does not need to be planted each season. Earlier work had shown that the nickel levels in the roots were comparatively low (up to 0.3% nickel in the dry material) and thus, for ease
of harvesting and to ensure the continued vegetative growth of the plant on the planted sites, it
was decided that the leaves and stems of the plants would be harvested at the end of each
growing season. The plant was also found to accumulate low levels (0.006 - 0.3 %) of
precious metals, including platinum, palladium and rhodium, within its above ground biomass,
making it attractive for the remediation of certain soils that contain low levels of these metals.
Before B. coddii could be introduced to the Rustenburg area a comparison of the climatic and
soil conditions of Barberton, the area to which B. coddii is endemic, and Rustenburg needed to
be made to ensure that the plant would be able to survive the new conditions. These
comparisons showed that Rustenburg receives on average, 484 mm less rain per year than
Barberton, indicating that irrigation was required when the Rustenburg sites were planted out
with B. coddii, in order to reduce water stress. Rustenburg was also found to be, on average,
4.6°C warmer than Barberton, but as B. coddii growth responds to wet/dry seasons, as
opposed to hot/cold seasons, it was not felt that this temperature difference would have a
negative effect on the growth of the plants. The soil comparisons showed the contaminated
Rustenburg sites to be serpentine-like in nature, with respect to Barberton, again giving
confidence that the plant would adapt to the conditions occurring at the contaminated sites.
However, to ensure optimal growth, nutrient experiments were also performed on B. coddii to
ascertain the ideal macronutrient concentrations required, without inhibiting nickel uptake.
These trials indicated that the individual addition of 250 mg/l ammonium nitrate, 600 mg/l
calcium phosphate, 2 000 mg/l calcium chloride, 600 mg/l potassium chloride and 250 mg/l
magnesium sulphate enhanced plant growth and nickel uptake, suggesting that, for
phytoremediation purposes, these nutrients should be added to the medium in which the plants
are growing.
The growth-cycle of naturally occurring B. coddii plants in Barberton was also studied in order
that seedlings could be germinated, in greenhouses, at the correct time of year so that the
plants could be sown as the naturally occurring plants were germinating. From this information
the seeds of the plants could be collected at the correct time of year and the above ground
biomass harvested when the nickel concentrations were at their highest. It was found that the
plants began to germinate as the first rains fell, which was generally at the beginning of September, and plant maturity was reached at about five months, after which flowers were
produced. Seeds were produced from the flowers and these matured and were wind-dispersed
one month to six weeks after full bloom, usually during February. The plants then started to
die back and dry out and dormancy was reached about nine months after germination,
generally in about mid- to late- May. It was found that the nickel concentration was at its
highest about one month after the plants had begun to dry out and thus it was decided that the
above ground biomass would usually be harvested at the end of April each season, in order to achieve maximum nickel recovery.
Finally, in order that the plant's potential for use in phytoremediation could be fully assessed,
field trials at the contaminated sites in Rustenburg were performed. Germination procedures
were developed for the mass production of B. coddii and it was found that, although fully
formed plants could be propagated in tissue culture, it was cheaper and faster to germinate the
seeds in speedling trays, containing a zeolite germination mix, in greenhouses. It was also
found that the seeds had a low germination rate, due to dehydration of the embryos and thus,
in order to obtain the number of plants required, four to five times the amount of seeds needed
to be sown. The two-month-old seedlings were transferred to potting bags, containing a
mixture of potting soil and RBMR soil, and grown up in the greenhouse for a further three
months. This growth period allowed B. coddii to adapt to the RBMR soil and also ensured that
the plants were relatively healthy when transplanted into three prepared sites at RBMR. The
plants were allowed to grow for the entire season after which the above ground biomass,
comprising the leaves and stems, was harvested, dried and then ashed in an ashing vessel
designed by the author, with the help of Mr K Ehlers. The ashed material was acid-leached
with aqua regia in order that the base metals (mainly nickel) and precious metals could be
removed from the silicates and carbonised material. The acid solution was then neutralised,
causing the base metals (mainly nickel) and precious metals to be precipitated. This precipitate
was then smelted with a flux in order that nickel buttons could be formed.
Thus, from all the phytoremediation trials it was found that this process is highly successful in
employing B. coddii for the clean-up of nickel-contaminated sites. This constitutes the first
time that such a complete phytoremediation process has ever been successfully developed with
B. coddii as the phytoremediation tool. It also appears to be the first time that phytoremediation has been performed "commercially" to produce a saleable metal product.
The success of this project has stimulated Amplats to continue with, and expand it, to include
more studies on phytoremediation as well as in the biomining of certain areas containing very
low levels of precious metals which, with conventional techniques, were previously not worth mining.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.
Keywords
Plants, Effect of heavy metals on., Berkheya coddii., Phytoremediation., Theses--Biochemistry., Nickel--Environmental aspects.