The Zambian Copperbelt is host to one of the world’s greatest copper and copper ore reserves and as a result, the Copperbelt is home to multiple large commercial mining operations. Many of these mining companies construct ponds called tailings ponds in which they dump the residue of the ore extraction process. Many of the metals and contaminants present in the ponds are toxic to most plants, however various plants have been identified as metallophytes and have the ability to thrive in toxic environs. Kříbek et al. determined the amount of arsenic, copper, cobalt and other metals in a Zambian tailings pond that Pteris vittata, or the Chinese brake fern— a well-known and studied metallophyte—could accumulate in its fronds (the leaves of a fern). The researchers compared the accumulation amounts of the Chinese fern to those of Cyperus involucratus, or the umbrella plant, in two distinct tailings: reddish-brown tailings with high amounts of arsenic, iron, and other metals, and grey-green tailings with a lower metallic and contaminant content. The authors found that the Chinese brake fern is a hyperaccumulator of arsenic and has adapted to high concentrations of copper, cobalt, and other metals present in Zambian tailings ponds. This conclusion points to the use of Chinese brake fern and other similar plants as tools for the remediation of contaminated soils and water in mining districts.—Monkgogi Otlhogile
Kříbek B., Mihaljevič M., Sracek O., Knésl I., Ettler V., Nyambe I., 2011. The extent of arsenic and of metal uptake by aboveground tissues of Pteris vittata and Cyperus involucratus growing in copper- and cobalt-rich tailings of the Zambian Copperbelt. Archives of Environmental Contamination and Toxicology 61, 228–242.
Kříbek et al. chose an old tailings pond located in Chambishi-North, Zambia and divided it into four vegetation zones: reed swamp, no vegetation, papyrus and fern, and semi deciduous tree forest zones. The researchers located four 1 m² sampling sites in the papyrus and fern zones and took numerous groundwater, tailings, and plant samples. The groundwater’s alkalinity, cations, and anions were determined by titration, coupled plasma-mass spectrometry, and liquid chromatography, respectively. To determine the total metal concentration in the tailing samples, the samples were digested with aqua regia—a corrosive mixture of acids—in an attempt to dissolve all of the metals in the samples. To determine the plant-available metals, the authors used two methods: DTPA and TEA extraction and sequential extraction. These methods both leach metals and contaminants out of soil, sediments, and sludge to mimic environmental process, such as the growth of a plant in contaminated soils. In addition to establishing the pH value of the tailings’ leachates, the authors used ELTRA CS 500 instruments to determine total carbon, carbonate carbon, organic carbon, and total sulphur present in the tailings samples. The plant samples were separated into shoot and roots and were oven-dried. The dried samples were then burned down in a muffle-oven and the remaining ashes were analysed for metals using hydrochloric acid and nitric acid. Iron and aluminium accumulation by plant roots were studied using an optical microscope and electron microprobe. Finally, the authors calculated bioaccumulation factors (BF) for each contaminate for each plant and tailings type. The BF is a measure of the ability of a plant to intake and transport metals to the rest of the plant tissues.
The groundwater samples showed that the dominant cation was calcium while sulphur was the dominant anion. The concentration of magnesium, copper, manganese, cobalt, and potassium were also significantly high. The other contaminants found in the tailings samples such as aluminium, iron, zinc, and arsenic were significantly lower in the groundwater. The groundwater concentrations were used in conjunction with the plant concentration to obtain an accumulation ratio which the authors used to evaluate the amount of contaminates the plants were accumulating from the pond water. The accumulation ratio of arsenic in both the umbrella plant and the Chinese brake fern was the highest of any of the contaminants. In the Chine brake fern, the arsenic accumulation ratio for the red-brownish tailings was 6, 275.9 while the ratio for the grey-green tailings was 10, 246.3. In comparison, the accumulation ratios for zinc were 0.92 and 1.31, respectively. This proves that the Chinese brake fern is a hyperaccumulator of arsenic and has tolerance for the other metals in the tailings.
The reddish-brown tailings, which dominate most of the fern and papyrus zone, had a slightly alkaline pH of 8.19 while the grey-green tailings had a pH of 7.23. The concentration of organic carbon, carbonate carbon, and total sulphur were much higher in the contaminant-rich reddish-brown tailings than in the grey-green tailings. The reddish-brown tailings are rich in iron but also contain high concentrations of copper and cobalt which suggests that the reddish-brown tailings were probably formed by the dumping of ore processing waste. In contrast, the low concentration of contaminants in the grey-green tailings suggests that it is formed by the reduction of the reddish-brown tailing during the rainy season. With the use of the DTPA and TEA extraction method, the amount of plant-available metals and arsenic showed similar trends in both tailings types. For the reddish-brown tailings, the ascending order of plant-available contaminants were iron, arsenic, copper, cobalt, manganese and zinc. While in the grey-green tailings the order varied slightly with iron, arsenic, cobalt, copper, manganese, and zinc. The researchers suggested that the concentration of plant-available arsenic, copper, cobalt, manganese, and zinc increases proportionally with the total concentrations in the tailings. However, the authors asserted that the DTPA and TEA extraction method was designed for the determination of metals and therefore may not have been efficient in the determination of arsenic. They therefore used sequential extraction and found the arsenic extraction levels to be 10–15% while the DTPA and TEA only established them at 1.0–1.5%.
The mean concentration of arsenic in the Chinese brake fern was found to be high in the reddish-brown tailings and even higher in the grey-green tailings. This may be as a result of the linear uptake of arsenic at lower concentrations and then the levelling off of uptake at higher concentrations. This suggests that the Chinese brake fern has an avoidance mechanism which protects its system from high concentrations of arsenic. Previous investigations suggest that high levels of arsenic inhibit the phosphate pathway used to take up arsenic and may explain the results. The optimum pH for arsenic uptake by the Chinese brake fern is 6.5 and decreases rapidly with alkalinity which suggests that the slight alkalinity of the reddish-brown tailings may have also accounted for the higher arsenic concentration of plants in the grey-green tailings. The arsenic may have also been competing for uptake with the higher concentration of phosphate in the reddish-brown tailing.
The higher concentration of arsenic in the reddish-brown tailings resulted in a bioaccumulation factor of 18 compared to that of 184 of the grey-green tailings. Although there was a vast difference in bioaccumulation factors, both the tailings had similar concentrations of arsenic in their plants, which suggests that the BF does not account for actual accumulation but rather reflects the concentration of arsenic in the tailings. Concentrations of metals in the fronds in both types of tailings decreased in the following order: iron, copper, manganese, cobalt, and zinc. The BF of each metal negatively correlated with its concentration in the plants. Much like the BF of arsenic, the BFs of the above mentioned metals were higher in the grey-green tailings with the exception of manganese. The high concentration of copper and cobalt found in the tailings pond may have been preventing the uptake of the other metals because the two types of tailings were similar despite different values of contaminants. The plaques or hydroxide and carbonate accumulation on the roots of the Chinese brake fern may have also decreased the amount of metals taken up by the plants. Compared to the concentration obtained from the fronds, the concentrations established in the leaves of the umbrella plant were substantially lower. Much like the BFs of the Chinese brake fern, there was no positive correlation between BF and the concentration of the contaminant in the plant.
The results of Kříbek et al. show that the Chinese brake fern is a hyperaccumalator of arsenic and is highly tolerant of the many metals contained in the Zambian Copperbelt tailings. The Chinese brake fern may also have a true tolerance to arsenic which means arsenic has no effect on the internal functions of the plant. Unlike many wetland plants, the Chinese brake fern does not trap metals and contaminants in its roots but rather in its fronds which suggests that it could be harvested and continuously grown to decontaminate mining areas. This suggests that the fern could be used for the remediation of tailings ponds across Zambia and in other mining countries. The industrial waste dumping of one element is rare. Rather, dumped industrial waste usually contains a range of heavy contaminants, so plants such as the Chinese brake fern which can accumulate and tolerate the wider ranges of contaminants will be the most efficient at remediating them. The authors emphasize the need for further investigation of the long-term effects that the contaminants have on growth, germination, and other plant functions of the Chinese brake fern. The value of hyperaccumulating plants for mining countries means that these further investigations should be given high priority as the plants could decontaminate mining areas for many other organisms including humans.