Induced phytoextraction of mercury and gold from cyanidation tailings of small-scale gold mining area of West Lombok, Indonesia.
In 2010, there were approximately 900 artisanal and small-scale gold mining (ASGM) locations in Indonesia . At almost all of the ASGM, the amalgamation process followed by cyanidation process are common methods used to recover as much gold as possible from gold ore . Amalgamation with mercury is a simple process and requires only a small investment.
Mercury recovered from the amalgamation process is always re-used in subsequent amalgamation process. However, the binding ability of mercury on gold is highly dependent on the size and nature of the geochemical gold particles . In every gram of gold produced, there are about 1-3 grams of mercury are released into the environment from the amalgamation process , which is partially released into the water along with mud washing results. Results of studies in small-scale gold mining sites in the Philippines conducted by Hylander et al.  showed that only 5-10% of the gold was recovered through the mercury amalgamation process, whereas 90-95% can be recovered through the cyanidation process. Nevertheless, the cyanidation process requires more expertise and investment than the amalgamation process. It is known that mercury and cyanide can complex gold, it thus can be ascertained that the cyanidation process also transport and discharge mercury into the environment .
The remaining materials of the amalgamation and cyanidation processes that are in the form of sludge still contain mercury, gold and other heavy metals are generally disposed to agricultural lands and water bodies. Results of a survey at ASGM location of Sekotong District of West Lombok conducted by Krisnayanti et al.  indicated that the amalgamation and cyanidation tailings still contained 3,002 mg Hg /kg and 1,628 mg Hg/kg, respectively. The high concentration of Hg in the tailings caused the increase of Hg concentration in soil contaminated by the tailings. Results of analysis conducted by the University of Mataram showed that the levels of Hg in soils at the Sekotong District of West Lombok ranged from 25 to 40 mg/kg, while the Hg concentration in maize and rice seeds grown in the location contaminated by the tailings was around 0.20 mg/kg . In some other ASGM locations, the concentration of mercury in waters and sediments ranged from 0.6 to 4 mg/kg, which was much higher than the Indonesian Government standard of 0.002 mg/kg .
The discharge of amalgamation and cyanidation tailings containing 1628-3002 mg Hg / kg to agricultural land has seriously reduced crop growth and production in the area. Chlorosis is a major symptom of mercury toxicity, besides root damaged . One of remediation technology that is environmentally sound to apply is phytoextraction that can play a double role, i.e. phytoremediation and phytomining . Phytoremediation is the use of green plants to extract metals or eliminate pollutants and soil . Phytomining is the production of 'metal plants' by growing plants that can accumulate high concentrations of metals . The benefit of phytoremediation is the absorption of Hg by accumulator plants, while the benefit of phytomining is Au uptake by plants and then the content of Au in the accumulator plants can be harvested and processed into gold bio-ore . A study conducted by Krisnayanti et al.  indicated that gold mine tailings at various ASGM locations in the Sekotong District of West Lombok contained 1.2 to 6.28 mg Au/kg. The challenge is thus to find accumulator plants that are capable to accumulate Hg and Au at the same time.
Results of a study conducted by Handayanto et al.  showed that there were at least 37 plant species naturally grown in the ASGM area of Sekotong District. Among those, Cyperus kyllingia Endl., Lindernia crustacea (L.), F., and Paspalum conjugatum L. were found dominant and able to accumulate Hg that ranged from 7.83 to 12.6 mg / kg. It is thus necessary to test the potential of the three plant species for phytoextraction of Hg. Because Hg is widely used in the ASGM, then the plant is also expected to accumulate gold. Moreno et al.  reported that gold and mercury are found on the same plant grown in soil contaminated gold mine tailings in New Zealand. In addition to the selection of plant enable to accumulate Hg and Au, the solubility of Hg in the soil is generally very low, as Hg is generally retained by the soil solids through absorption in sulfide, clay particles and organic matter . According to Morel et al. , in soil mercury has a strong affinity with thiol groups. Therefore, application of sulfur-containing solution into the soil can stimulate the concentration of Hg in plant tissues . Au that is also known as a metal-ligand complex , Au has a very low solubility in the soil solution so that plants will be very difficult to accumulate Au without application of ligands. Anderson et al.  showed that application of thiosulfate or thiocyanate compounds increased the solubility of Au and thus increasing uptake of Au by plants. This objective of this study was to evaluate the potential of Lindernia crustacea (L.) F., Paspalum conjugatum L., and Cyperus kyllingia Endl. for phytoextraction of Hg and Au form gold cyanidation tailings.
MATERIALS AND METHODS
A pot experiment was conducted from April to November 2013 in the green house (made of plastic) built at the field nearby gold cyanidation process at Sekotong District of West Lombok Regency ([115.sup.0].46' - [116.sup.0].20'E and [8.sup.0].25' - [8.sup.0].55'S). Materials used for this study were cyanidation tailings and three local plant species (L. crustacea, P. conjugatum, and C. kyllingia). The cyanidation tailings collected from the surface layer (0-20 cm) of a tailing disposal dam were then air-dried for 5 days. The tailings had been discharged and deposited at the tailing dam for about 14 days after gold cyanidation process. The air-dried tailing sample was sieved to pass through a 2 mm sieve for physical and chemical analyses.
The characterization of tailings that included texture, pH, organic C content, total-N, total P, exchangeable K, exchangeable Na, Ca available, exchangeable Mg, CEC, and Base Saturation was performed by standard laboratory methods of Soil Laboratory, University of Brawijaya. Hg concentration was determined using a F732-S Cold Atomic Absorption Mercury Vapor analyzer (Shanghai Huaguang Instrument Company) that works on the reduction of mercury by stannum chloride (Sn[Cl.sub.2]). Au analysis was carried out at the Centre for Coal and Mineral Research belonging to the Ministry of Energy and Mineral Resources of Indonesia in Bandung using a Graphite Furnace Atomic Adsorption coupled with Spectrophotometer (Perkin Elmer A Analyst 600). Results of analyses were as follows: sandy clay texture, pH 7.7, organic-C 1.19%, total N 0.001%, available P 2.9 mg/kg, exchangeable K 0.001 cmol / kg, 0.64 cmol / kg, exchangeable Na, exchangeable Ca 1.99 cmol / kg, exchangeable Mg 0.84 cmol / kg, CEC 11.57 mol / kg, 31% base saturation, Hg 1.090 mg /kg, and Au 1.68 mg/ kg.
Two seeds of each plant species which have been previously acclimatized for 2 weeks, were grown for 9 weeks on 5 kg growing media (cyanidation tailings) in a 10 kg plastic pot. To maximize plant growth, all tailings in pots were supplied with a combined NPK fertilizer with a rate of 100 kg / ha. At 8 weeks after planting, 2g/kg ammonium thiosulfate [[(N[H.sub.4]).sub.2][S.sub.2][O.sub.3]] or 1g/kg sodium cyanide (NaCN), each in 150 mL solution, was added to the three plant species separately. Nine treatments (three plant species with application of ammonium thiosulfate or sodium cyanide, and three plant species with no application of ammonium thiosulfate or sodium cyanide), were arranged in a randomized block design with three replicates. During the experiment, water was regularly supplied to maintain a sufficient supply of water for plant growth. One week after the application of ammonium thiosulfate or sodium cyanide, all plants were harvested (age 9 weeks).
At harvest, shoots and roots were separated, washed, weighed, and then oven dried for 48 hours at [60.sup.0]C for analysis of Hg and Au. Measurement of Hg content was performed according to the method Moreno et al. . A total of 0.2 g of sample plants that have been ground was incorporated into borosilicate digestion 50mL tube, added with 10 mL of nitric acid (HN[O.sub.3]) acid and then left to stand for 10-15 hours. The next day the mixture of plant samples and nitric acid was heated for 2 hours at a temperature of 120[degrees]C and then 20 mL distilled water was added. Determination of the standard 0.5, 1, 2, 4, 7.5 and 10 [micro]g Hg/kg, with reference to the standard of tomato leaves containing 34 [micro]g Hg/kg. Measurement of Hg content was carried out using a Cold Vapor Atomic Absorption Spectrometer F732--S (Shanghai Huaguang Instrument Company). For the analysis of Au, 0.2 g subsample of each ground plant sample was placed into a 10mL borosilicate test tube. The tube was then heated at 550[degrees]C for one night in a muffle furnace. On the following day the ash was dissolved in a water bath containing 5 mL of aqua regia (a mixture of nitric acid hydrochloric concentration with a proportion of 3:1), and then was made to 10 mL with distilled water. Au concentration in solution was measured by Graphite Furnace Atomic Adsorption Spectroscopy (Perkin Elmer A Analyst 600). For analysis with Graphite furnace, 5 mL aliquot was extracted to similar volume with methylisobutylketone. To maintain the accuracy of the analysis, the reference solution and the standard solution were diluted from 1000 mg / L.
The ability of plants to transport metals from the roots to the shoot was measured using TF (Translocation Factor) value, which is defined as the ratio of the concentration of metal in shoots and concentration of metal in the roots . A plant species that has a TF value of less than one is not suitable for phytoextraction (. TF > 1 indicates the effectiveness of plant to transport metals from the roots to the shoots . The means and standard deviations of data obtained were calculated by the Microsoft Office Excel 2007. One-way analysis of variance was carried out with SPSS19.0. When a significant (p<0.05) difference was observed between treatments, multiple comparisons were made by the LSD test.
RESULTS AND DISCUSSION
Results of tolerability test of three plant species showed that all plants were tolerant to gold mine cyanidation tailings containing mercury. Application of ammonium thiosulfate or sodium cyanide increased dry weight of the plant shoots at 6 and 9 weeks, with a sharp increase at 9 weeks (Fig. 1). At harvest (9 weeks), the highest shoot dry weight was found in P. conjugatum followed by C. kyllingia and L. crustacea. However, application of ligands did not show significant effects (p < 0.05) on the increase of root dry weight (Fig. 2). Only application of ammonium thiosulfate that markedly increased the dry weight of the plant shoots compared with the treatment without application of ligands. The increase in the shoot dry weight due to application of sodium cyanide was not significantly different (p < 0.05) with treatments without application of ligands. The increase in plant dry weight due to application of ammonium thiosulfate was probably due to additional supplies of nitrogen and sulphur contained in the ammonium thiosulfate applied at week 8. In addition to the supply of nutrients, the presence of S element in ammonium thiosulfate also potentially decreased pH of the tailings, which in turn increased the availability of other nutrients originally contained in the tailings. A plant species that can be classified as heavy metal accumulator ideally should also be able to produce high biomass in addition to having ability to survive at high metal concentrations in the soil . Fig. 1 and 2 show that at 9 weeks, P. conjugatum revealed the highest potential to produce biomass followed by L. crustacea and C. kyllingia. In terms of biomass production, P. conjugatum seems to be the best plant species for phytoremediation of mercury-contaminated soil.
Accumulation of Hg in Plants>
The highest concentration of mercury (22.69 mg/kg) was found in the shoot of P. conjugatum with application of ammonium thiosulfate, whereas the lowest (4.42 mg/kg) was observed in the shoot of L. crustacea without of ligands (Fig. 3). In the roots, the highest concentration of mercury (1.95 mg/kg) was found in the root of P. conjugatum without application of ligands, while the lowest (0.64 mg/kg) was found in the root of L. crustacea without application of ligands (Fig. 3). As observed in the root dry weight, application of sodium cyanide did not significantly influence the concentration of mercury (Fig. 3). These values exceeded the threshold value of mercury concentration of 10 mg/kg of total dry weight . Nagajyoti et al.  suggested that there is a relationship between the levels of heavy metal contamination in soils with uptake by plants.
The accumulation occurs because there is a tendency of forming heavy metal complexes with inorganic substances found in the body of soil organisms . Plants develop effective mechanisms to respond metal content in the soil . An accumulator plant does not inhibit metal to enter the roots but develop specific mechanisms for heavy metal detoxification in cells that allow bioaccumulation of metals in high concentrations . Plants can naturally accumulate metals exceeding the threshold value of 1% (Zn, Mn), 0.1% (Ni, Co, Cr, Pb and Al), 0.01% (Cd and Se), 0.001% (Hg) or 0, 0001% (Au) of dry biomass weight without showing symptoms of poisoning .
On average, application of ammonium thiosulfate or sodium cyanide increased the concentration of Hg in the shoots by 75% and 45% compared to treatments without application of ligands. Application of ammonium thiosulfate also increased 39% concentration of Hg in the roots, while application of sodium cyanide increased 26% concentration of Hg compared to treatment without application of ligands (Fig. 3). However, the increased accumulation of Hg in the roots due to application of sodium cyanide was not significantly different from treatments without application of ligands. According to Ghosh and Singh , phytoextraction is a process to remove contaminants from the soil without damaging the soil structure and fertility.
Accumulation of heavy metals can be associated with detoxification mechanisms based on the absorption of heavy metal ions in the vacuole, by binding them to the corresponding ligands such as organic acids, proteins and peptides in the presence of enzymes that can function at high levels of metal ions , and avoidance strategies of plants to absorb heavy metals .
The increased accumulation of Hg, especially for the ammonium thiosulfate treatments, occurred because mercury has a strong affinity with thiol groups, especially complex sulphide and bisulphide . Bracia juncea has been shown to accumulate Hg to 40 mg / kg in plant tissue after application of ammonium thiosulfate in mining waste contaminated with 2.8 mg Hg / kg .
All plant species investigated had TF values of more than one in all treatments (Fig. 4). The TF value of P. conjugatum with application of ammonium thiosulfate was the largest, followed by L. crustacea and C. kyllingia. The TF value differences for the three plant species show the different effectiveness of each type of plant in transporting mercury from the roots to the shoots .
Fig. 4: TF (translocation factor) values of three species grown for 9 weeks.
Accumulation of Au in plants:
The highest concentration of Au (601.9 [micro]g/kg) was found in the shoot of P. conjugatum with application of ammonium thiosulfate, while the lowest (58.9 [micro]g/kg) was observed in the shoot of L. crustacea without application of ligands (Fig. 5). In the roots, the highest concentration of Au (45.3 [micro]g/kg) was also observed in P. conjugatum with application of ammonium thiosulfate, while the lowest (7.0 [micro]g/kg) was found in C. kyllingia. without application of ligands (Fig. 5).
Compared with the results of several other studies, the accumulation of Au observed in these plants was relatively small and less than 1 mg/kg. This value was much lower than the results of the study reported by Msuya et al.  that Raphanus sativus L. Allium cepa L., Beta vulgaris L. and Daucus carota L. grown on media containing silica sand containing 3.8 mg Au/kg produced Au concentrations greater than 200 mg / kg of plant dry weight. The results of the study conducted by Piccinin et al.  also showed that the addition of sodium cyanide improved Au accumulation by 27 mg / kg of dry weight of Trifolium repens.
Total accumulation of Au recorded in this study appeared to be related to the low biomass production of the plants, especially for L. crustacea. However, on average, application of ammonium thiosulfate or sodium cyanide increased the concentration of Au in the shoot by 106% and 30%, respectively, compared to treatments without application of ligands. The difference of the results of this study with that reported by other researchers is obviously due to differences in plant species and environmental growing conditions .
The higher influence of ammonium thiosulfate on concentration of Au compared to sodium cyanide indicated that thiosulfate further increased the solubility of Au . Application of ammonium thiosulfate also increased 79% Au concentration in roots, and application of sodium cyanide increased 62% Au concentration compared to treatment without application of ligands (Fig. 5). The difference influence between ammonium thiosulfate and sodium cyanide to the concentration of Au is because in ammoniacal thiosulfate solution, both ammonia and thiosulfate can form complexes with gold . Thiosulfate is also an alternative lixiviant to cyanide for leaching gold under low acidic or low basic conditions (pH 5-9), while cyanide is more suitable under basic conditions for pH > 10 . Because gold cyanidation tailings used in this study had a pH value of 7.7 (Table 1), the uptake of Au was driven by thiosulfate .
Application of ammonium thiosulfate [[(N[H.sub.4]).sub.2][S.sub.2][O.sub.3]] or sodium cyanide (NaCN) as a ligand on gold cyanidation tailings of small-scale gold mining area in West Lombok increased accumulation of Hg and Au by Paspalum conjugatum, Lindernia crustacea and Cyperus kyllingia. Ammonium thiosulfate was a better ligand than sodium cyanide for enhancing Hg and Au accumulation in plants.
Received 14 Feb 2014
Received in revised form 24 February 2014
Accepted 29 March 2014
Available online 14 April 2014
Authors thank to the University of Brawijaya and Directorate General for Higher Education for financially supporting this study with contract no: 023.04.2.414989/2013 and 295/SK/2013; June 12, 2013. Many thanks are also due to Head of Sekotong District of West Lombok for providing unlimited access to the ASGM area of the district.
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(1) Eko Handayanto, (2) Nurul Muddarisna, (1,3) Baiq Dewi Krisnayanti,
(1) IRC-MEDMIND, University of Brawijaya, Jl. Veteran No 1, Malang 65145, Indonesia
(2) Wisnuwardhana University, Jl. Danau Setani No 99, Malang 65139, Indonesia
(3) Department of Soil Science, University of Mataram, Jl. Pendidikan 37, Mataram 83125, Indonesia
Corresponding Author: Nurul Muddarisna, Wisnuwardhana University, Jl. Danau Setani No 99, Malang 65139, Indonesia.
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|Author:||Handayanto, Eko; Muddarisna, Nurul; Krisnayanti, Baiq Dewi|
|Publication:||Advances in Environmental Biology|
|Date:||Apr 1, 2014|
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