The capacity of Plectranthus sp. to perform phytoremediation on heavy metal mercury (Hg) in the tailing of gold processing industries

Sludge (tailing), a waste product of the amalgamation process used in the mining of gold, contains a high concentration of the heavy metal mercury (Hg). It is well known that mercury builds up in large quantities in Plectranthus sp. herbaceous plants in the Pancurendang Village Small-Scale Gold Mining area. The study intends to: 1) quantify the amounts of mercury present in plant organs and media tailing during phytoremediation treatment; 2) examine the mechanisms and capacities of plant phytoremediation using values of the Biological Accumulation Coefficient (BAC), Biological Concentration Factor (BCF), and Translocation Factor (TF); and 3) determine the impact of percentage variations in tailing and time on Plectranthus sp.’s capacity to accumulate mercury. On a lab scale, phytoremediation of tailings is done with concentrations of 0%, 30%, 50%, and 100%. Planting medium is combined with dried tailings. Pots are used for phytoremediation, and the residence times are 10, 20, and 30 days. With the use of a mercury analyzer, the amount of mercury present in plant media, roots, and shoots is determined. Using the BAC, BCF, and TF values, the mathematical calculation of phytoremediation ability is performed. Hg content ranged from 1.82 to 16.23 ppm in shoots and 5.15 to 37.19 ppm in roots. Throughout the course of treatment, the tailings Hg concentration varied between 13.18 and 220.05 ppm. Plectranthus sp. is categorized as a medium accumulator with a phytoextraction mechanism, according to the analysis’s findings. Hg phytomining is possible with Plectranthus sp., the TF value is 2.38. Within 10 days, Plectranthus sp. most effectively accumulates 50% Hg in tailings.


Introduction
Tailings, the waste from gold production, are characterized by fine granules measuring 0.001-0.600mm and taking the form of mud or slurry [1].Tailings are a mixture of crushed gold ore that is left over after extraction and water utilized in the process [2].Tailings from the refining of gold are thick, typically yellow, brown, or black, and include a lot of heavy metals, including mercury [3].Without any processing, the majority of Indonesia's small-scale gold miners release their tailings into the 1339 (2024) 012038 IOP Publishing doi:10.1088/1755-1315/1339/1/012038 2 environment [4], [5].Tailings have the potential to seriously harm the ecosystem, including soil, water, and air pollution [6], [7].Reducing the negative effects on the environment requires proper tailings management.
According to [8], mercury (Hg) is a heavy metal that is extremely harmful to both people and the environment.Various activities, including mining, burning coal, and industry, can release mercury into the environment [7].Waste from the refining of gold is one of the many ways that mercury can get into the environment [9].Indonesian waste from small-scale gold processing typically has high levels of mercury, which can pollute the environment and create health issues [8].Mercury exposure can lead to a number of health issues, including damage and diseases to the neurological system, brain, and kidneys in both adults and children [10].
This study looks for different ways to treat mercury-filled gold processing waste by utilizing a plentiful supply of local resources ground cover herbaceous plants, or cover crops.Herbs are plants that can be perennial, 2-season, or annual with soft, succulent stems that don't produce wood [11].Phytoremediation is the technique of employing plants to treat trash or contaminated land with heavy metals [12].Using plants to absorb, break down, convert, and immobilize contaminants is a technique known as phytoremediation [13].Phytoremediation of mercury has gained a lot of attention due to its many benefits over traditional remediation techniques.Specifically, it is effective in lowering mercury levels in soil, water, and air by a considerable margin.Well, it is eco-friendly because it doesn't require chemicals or unnecessary energy.Low cost due to its utilization of easily sourced and grown plants [14].Mercury phytoremediation is crucial for safeguarding both the environment and public health.
Prior studies discovered that Plectranthus sp were mercury heavy metal accumulators in Artisanal Small-Scale Gold Mining (ASGM) [15], [16], [17].Plectranthus sp. is herbaceous plant that economically valuable ornamental from Africa, enter to the family Lamiaceae [18], [19].In the Pancurendang Village gold mining area, the biological accumulation coefficient (BAC) of mercury in herbaceous plants ranges from 0.11 to 4.54, potentially indicating a medium to high category of mercury accumulator.According to this study, Plectranthus sp., which grows in regions with gold mining, has the greatest capacity to accumulate mercury, with a BAC value of 4.54 [17].The BAC value was obtained on Plectranthus sp. which grew in soil with a mercury concentration of 11.01 ppm and the mercury was measured at 49.96 ppm in 14.5 g dry weight of plant organs.Phytoremediation research is generally carried out on soil contaminated with low levels of mercury (<10 ppm) [15], [16].It is necessary to carry out more in-depth investigations into the ability of herbaceous plants, especially Plectranthus sp., to accumulate mercury if applied directly to tailings which have very high mercury concentrations (>80 ppm).The objectives of the study are as follows: 1) quantify the amounts of mercury present in plant organs and media tailing during phytoremediation treatment; 2) examine the mechanisms and capacities of plant phytoremediation using values of the Biological Accumulation Coefficient (BAC), Biological Concentration Factor (BCF), and Translocation Factor (TF); and 3) determine the impact of percentage variations in tailing and time on Plectranthus sp.'s capacity to accumulate mercury.

Research methods
A lab-scale experiment was the research methodology used in this study.A factorial design with three replications for each treatment was employed in the lab-scale study.Plant-based treatments (Plectranthus sp.) are coded B, and plant-free treatments are coded F. Tailings concentration variations are designated with treatment codes P1, P2, P3, and P4.Variations in treatment duration are denoted by the codes T1, T2, and T3.Table 1 lists the many phytoremediation methods that use Plectranthus sp. to treat waste from gold processing.The concentration of mercury in the roots, shoots, and media of the replicate plants (R) 1, 2, 3, and 4 was analyzed and combined.The portion of the plant above the medium that is under consideration is the portion of the plant crown that consists of the stems, leaves, and flowers [20].

Tools and materials
For phytoremediation experiments, a straightforward bioreactor in the shape of a plastic pot with a 2 kg capacity is utilized.A mercury analyzer is the primary instrument used in the study to determine the amount of mercury present in plants and media [21].Tailing processing gold from small-scale gold mining in Pancurendang Village, Ajibarang District, Banyumas Regency, Central Java Province, Indonesia, is the primary source of material for this study.A ready-to-use blend of fertile soil, compost [22], husk charcoal, and fertilizer is used to make planting medium, such as the Aditama brand.The Sewon area of Bantul DIY provided the herbal plants employed in the research experiments.Hoes, buckets, plant shovels, scales, crushers, sieves, and ovens are examples of additional auxiliary equipment.

Method of operation 2.3.1. Measurement of field capacity and preparation of media.
At the Pancurendang Village gold processing facility, tailing is collected from multiple heaps of filled sacks.The tailings are ground till smooth and sieved after being air-dried for about ten days (Figure 1).100% tailings (P1), 30% tailings + 70% planting media (P2), 50% tailings + 50% planting media (P3), and 100% planting media (P4) were the different media mixtures that were used.For a duration of one month, P2 and P3 were incubated to facilitate interactions with microorganisms [23].Plants were acclimated for about a week in the greenhouse of the Faculty of Agriculture, Universitas Pembangunan Nasional (UPN) Veteran Yogyakarta, before being moved to media containing a mixture of tailings.Calculations of field capacity are used to determine the amount of water required for irrigation plant media with various of media mixtures.

Observation of plants, measurement of biomass, and Hg concentration in plants and media.
Every two days, along with watering, observations of the plants in the tailings media were made.A descriptive analysis was conducted on the anatomical conditions of the plants during the observations.At harvest days 10, 20, and 30 (T1, T2, and T3), measurements of plant biomass were made by covering the roots and shoots of the plants independently at 65°C for three to seven days, or until the weight remained steady and dry weight was achieved in gramme units [24].Using the Mercury Analyzer (Lab Analyzer 254 series) at LPPT UGM, the dried plant parts were ground into a powder in order to determine the content of mercury.At each T1, T2, and T3 harvest period, the tailings media are air-dried before being submitted to the lab for an examination of the content of mercury.

Data analysis
Plant Hg concentrations in the roots and shoots were examined during phytoremediation experiments.If a plant can absorb 1% of its dry weight or 10 ppm of its dry weight, it is considered a Hg hyperaccumulator [25].BAC, BCF, and TF calculations were then analyzed in order to identify the phytoremediation mechanism that took place.For phytoextraction, plants having BAC>1 and TF>1 can be employed.Phytostabilization is possible in plants with TF<1 and BCF>1 [26].Phytoextraction hyperaccumulators are fast-growing, high-biomass plants that may be employed in large-scale mercury phytoremediation or phytomining [27], [28].Based on the BAC or BCF value, the hyperaccumulator plant category is established in Table 2.

Hg concentration in tailings without plant
Table 3 displays the amount of mercury present in the tailings of every media mixture utilized in phytoremediation experiments conducted on a lab scale.The tailings sample comes from three different locations in ASGM Pancurendang Village where sacks containing tailings are stored.The Hg concentration dropped from 82.2 ppm in 50% of the tailings and 53.97 ppm in 30% of the tailings after they were pulverized, dried, and combined with planting material.The mixture was then incubated for 30 days.The 30% and 50% tailings media have comparable concentrations of mercury.Nevertheless, the phytoremediation experiment is still carried out in accordance with the planned experimental design because the results of the mercury test can be known one to two months after they are placed in the testing laboratory.The Hg content in tailings without plants was measured, and the findings showed that after 30 days, at 30% tailings, the Hg concentration decreased from 54.7 ppm to 19.71 ppm.In media containing 50% tailings, the opposite situation happened; the concentration of mercury rose from 53.97 to 61.51 ppm.Microbes such Mycolicibacterium peregrinum (bacteria) and Cladosporium halotolerans (mushrooms) in the tailings medium mixture can bind and breakdown mercury, which can cause a decrease or rise in the quantity of mercury in media without plants [2].The four processes that make up a plant's uptake of mercury are phytostabilization, phytoextraction, phytovolatilization, and rhizofiltration.Although four events may occur simultaneously in a single plant, existing references indicate that phytostabilization and phytoextraction are the most common outcomes [30].Plant regulating proteins will generate chelating substances known as phytochelatins in an environment containing mercury [31].After being created in the nucleus, this phytochelatin travels to the cell surface via the secretory vesicles, Golgi apparatus, and endoplasmic reticulum.Hg and phytochelatin combine to generate complex chemicals and sulfide linkages that are transferred to plant tissue [32], [33].Phytostabilization and phytoextraction are brought about by the osmotic diffusion event that results from variations in Hg concentrations in the media (waste) and plants [34], [35].
The phytoextraction mechanism refers to a plant's capacity to move metals from its roots to its shoots [26].Researchers studying the evolution of mercury phytoextraction have discovered that the mechanism underlying the accumulation of mercury in plants is related to the mobility of mercury ions (Hg 2+ ), which allows them to be easily translocated to the plant canopy and deposited in locations like subcellular vacuoles and leaf epidermal cells.In addition, some transporters, including glutathione conjugates, which pump Hg ions 2+ into the vacuole, are present [31], [32].
Within 30 days of treatment with 100% tailings media, the roots had the greatest levels of mercury (37.19 ppm).In comparison to the other treatments, shoots Hg had the lowest concentration under these circumstances, at just 1.82 ppm.A larger Hg buildup in the roots is referred to as a phytostabilization mechanism in phytoremediation technology [13].The primary idea behind phytostabilization is to reduce the mobility or bioavailability of metals by building up pollutants in the root tissue.The best methods for slowing the transport of pollutants, particularly heavy metals, are phytostabilization processes [36].

Hg concentration in tailings with Plectranthus sp.
At 10 and 20 days, there was an increase in the Hg concentration in the 50% and 100% tailings mixture in the planted tailings media Plectranthus sp.(Figure 4).While there was a drop in the mercury concentration on day 30, the value was remained greater than at day 0.This demonstrates that the plant Plectranthus sp.'s roots have the ability to draw or collect mercury from the media, concentrating and stabilizing it in the root zone [37].Inactivation or immobilization of the metals can be used to achieve heavy metal environmental cleanup.Either the rhizosphere (the media around the roots) or the roots themselves remove pollutants.According to Mocek-Płóci i k e .2023 , he stabilizing action of plant roots reduces the mobility and bioavailability of pollutants, which in turn lessens the harmful impacts of metals on the environment [38].8 by a value of 0.01-0.1,and non-accumulator plants are indicated by a value <0.01 [29], [39].BCF and BAC values of Plectranthus sp. at Figure 5 shows a value of 0.02-0.72,meaning it is included in the moderate accumulator and low accumulator category.

Plectranthus sp. plants' ability to act as a phytoremediation against mercury
To assess accumulator plants, use TF.According to [40], TF is the ratio of metal concentration in the shoot to metal concentration in the root.A plant's ability to acquire and store metals is demonstrated by BAC readings >1 and TF<1.More translocation occurs in the plant canopy when the TF value is greater than 1 [40].Studies pertaining to phytoextraction and phytomining.A plant may have the ability to engage in phytomining.It is best described as a metal hyperaccumulator with a high biomass, rapid growth, phytoextraction as the mechanism [41], [42], [43].Figure 5's TF value of >1 shows that Plectranthus sp. can accumulate Hg in the shoots in high concentrations (phytoextraction), specifically in plants with 50% tailings media treatment for 10 days and 30 days.Also, the same thing happened with 30% tailings media, 30 days.
Although hyperaccumulator limitations are still not common for all heavy metals, one limit applies if the accumulation of the metal exceeds 11% of the canopy's total dry weight or, depending on the metal type, is 100 times more than that of regular plants.Plants that can withstand at least 100 mg of Cd metal kg-1 biomass dry weight are known as Cd hyperaccumulators.On the other hand, according to [44], the standard values for the metal hyperaccumulators Pb and Hg are yet unknown [45].Lindernia crustacea plants had the most concentration of mercury accumulation in plants, at 89.13 ppm [46], [41].According to this investigation, Plectranthus sp.plant roots had a Hg value of 37.19 ppm (Figure 3).The biomass values of Plectranthus sp. and the types of treatments that have TF or BCF and BAC values >1 are summarized in Table 4.The accumulator properties of Hg in these 3 treatments, are classified as moderate, because BCF and BAC value is in range value 0.1-1 [29], [39].Table 4 shows the possible uses of Plectranthus sp. as a phytomining and phytoremediation agent for mercury.Treatments like BP3T1 (50% tailings, 10 days), BP2T3 (30% tailings, 30 days), and BP3T3 (50% tailings, 30 days) may be agents of phytomining and phytoremediation with moderate to medium accumulation capacities if their TF values are more than 1.One of research stated that the TF value of Plectranthus sp. in the Alacran gold mining area, Cordova, it was 1.73 [15].In research on Pancurendang gold mine tailings, the TF value of Plectranthus could reach 2.38 in the BP3T1 treatment.
Utilizing plants for remediation Due to Plectranthus sp.'s advantage over T1, a shorter phytoremediation duration (maximum 10 days) can be selected.Plants' capacity to take up heavy metals can be influenced by their growth phases, both generative and vegetative.Plants are better able to absorb metals during the vegetative phase.The phytoremediation test's plants are in the generative phase, or have flowered.Lowering the media's mercury concentration will improve the material's capacity as a metal accumulator.To maximize absorption, plant propagation might increase biomass.To maximize the effectiveness and efficiency of Plectranthus sp. in phytoremediation by phytoextraction, agronomic methods must be implemented to increase biomass production [22], [41].The parameters of root Hg, shoot Hg, media Hg, root biomass, and shoot biomass appear to be significantly impacted by the % of tailings and variations in treatment duration, as indicated by the results of Duncan's multivariate test in Figures 6 and 7.The amount of mercury that plant species absorb increases with both concentration and biomass.According to Duncan's data, Plectranthus sp.plants have the biggest impact on the rise in Hg title.While tailing 30% can effectively enhance root biomass, it has little effect on root Hg.The growth of mercury in the header and root biomass is positively impacted by tailing 50%.The amount of tailings increases root, shoot, and media Hg by 100%.Hg heading and root biomass are impacted by the ten-day treatment period.Root biomass can be increased with a 30-days treatment.
Hg-induced growth suppression is observed in Plectranthus sp.Chlorosis and necrosis on the tips and sides of the leaves, which turn yellow, are signs of heavy metal pollution [47].This demonstrates that plants are not producing as much chlorophyll.Because plants that appear healthy might contain more mercury than sick plants, the physical state of plant damage cannot accurately reflect the quantity of heavy metal ingested by plants [32].
Metal hyperaccumulation, according to [30], entails a number of physiological functions.The first is the rhizosphere-root zone interaction, wherein polluting metals are changed into forms that roots may readily absorb with the aid of root exudates.Metals can be more readily absorbed when they are in an easily soluble state, and hyperaccumulator plant roots can release chelates-which bind metals Hg_shoots Hg_roots

Biomass_roots Biomass_shoots Biomass_roots Biomass_shoots
Hg_roots Hg_shoots into metal-chelate bonds-that facilitate easy absorption and internal plant translocation.The permeability, transpiration, root pressure, and presence of an increased metal uptake system are the factors that govern how much metal is absorbed by roots.Public disclosure of this method is still pending.Metal is then moved from the roots to the shoots in the following procedure.Ion transport to the xylem and flux within the xylem are the two primary mechanisms governing this translocation.One hyperaccumulation mechanism that determines the type of metal bonds and the location of tissue deposition is called sequestration, followed by complexation [30], [33].

Conclusion
Hg concentration in the plant shoot of Plectranthus sp. is higher than the roots, meaning this plant has good translocation abilities.The Hg concentration in tailings fluctuates, going up and down at certain times and with different patterns at each percentage.The Hg of 100% tailings continued to experience a much higher increase from the initial concentration to the end of treatment due to the process of metal immobilization by the root zone.Judging from the plant's ability to accumulate mercury, the results of the phytoremediation test show that Plectranthus sp. is classified as a medium accumulator with a phytoextraction mechanism and has the potential for phytomining Hg; the TF value reached 2.38.Plectranthus sp.most efficiently accumulates Hg in a 50% tailings media mixture within 10 days, even though under these conditions there is an increase in Hg in the media.An increase in Hg concentration in the media indicates that one of the goals of phytoremediation to collect heavy metals has been achieved.Phytoremediation capabilities can be increased by reducing the Hg concentration in the media and multiplying plants to increase biomass.

Figure 3 .
Figure 3. Hg concentration in roots and shoots Plectranthus sp.during phytoremediation

Figure 4 .
Figure 4. Hg concentration in tailings after treatment with Plectranthus sp.

3 .
The effect of differences in tailings percentage and time on the ability to accumulate Hg in Plectranthus sp.

Figure 6 .
Figure 6.Results of statistical analysis of the effect of differences in tailings concentrations on Hg concentrations and plant organ biomass

Figure 7 .
Figure 7. Results of statistical analysis of the effect of different treatment times on Hg concentration and plant organ biomass

Table 1 .
Treatment of gold processing tailings using phytoremediation

Table 2 .
[29]accumulator plant category is based on the BAC or BCF value[29] Multivariate statistical study has determined the impact of variations in tailings percentage and duration on Plectranthus sp.'s ability to absorb Hg.

Table 3 .
Hg test results on media used for phytoremediation BP3T1, BP2T3, and BP3T3, there were higher quantities of mercury in the shoots.Positively, this high concentration of Hg in the canopy suggests that Hg translocation takes place in the stems, leaves, and flowers.Plectranthus sp. can translocate mercury to the shoots at 16.23 ppm in 10 days with 50% medium.
source: Results of analysis by LPPT UGM and BPTP Yogyakarta laboratory3.1.2.Hg concentration in plant organs of Plectranthus sp.Figure3displays the amount of mercury present in plant organs.The range of roots of Plectranthus sp. was 5.17-37.19ppm, whereas the range in the shoots was 1.82-16.23ppm.The majority of treatments demonstrated that the roots had a higher percentage of mercury than the shoots did.Under treatments 6

Table 4 .
Potency of Plectranthus sp. as a hyperaccumulator and agent phytomining Hg based on TF, BCF, BAC, and biomass values in gold processing tailings