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The Evaluation of the Macrophyte Species in the Accumulation of Selected Elements from the Varkenslaagte Drainage Line in the West Wits, Johannesburg South Africa
Written By
Tinyiko Salome Mthombeni
Submitted: 26 April 2022Reviewed: 05 June 2022Published: 12 September 2022
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Although mining has over the centuries improved the livelihoods and economies of many countries, the results have not spared the environment’s luxurious legacy. Acid mine drainage contaminated sites with heavy metals that affect negatively and positively the macrophytes plants that grow on those sites. Accumulated elements by macrophytes planted on artificial wetlands portray the relative bioconcentration and translocation factors. Various elements were measured in the sediment, water, and macrophytes from the sampled sites and the results indicate that concentrations accumulated by plants play a significant role in biological and chemical processes in soil-water-plant relations. When comparing the drinking water quality standards by international organizations that were used as a guideline for the comparisons of elements concentration levels of elements found in water, Iron (Fe), Nickel (Ni), Manganese (Mn), and Copper (Cu) were found to be above the international water quality standards for drinking water and their average concentrations were 2230, 282, 5950, and 14,080 μg/l respectively. The sequence of elements accumulation by the macrophytes differed per plant and each of the three macrophytes plants was a hyperaccumulator of a certain element.
University of the Witwatersrand, Johannesburg, South Africa
*Address all correspondence to: tsmthombeni@gmail.com
1. Introduction
Although South Africa was the biggest producer of gold globally, the industry has been experiencing several drawbacks such as mine closure of older mines and shafts, declining mineral production, exhaustion of gold reserves, global low gold prices, the high energy requirement for deep-level mining, high wage demands and social unrests as well as the generation of acid mine drainage from the mines and tailings storage facilities [1]. The cessation of the large mining operations has detrimental effects as access to gold reserves are far underground, and mining operations resorted to dewatering activities to keep the groundwater level away from the mining operations [2]. Cessation of mining further resulted in flooding of the voids, a substantial cause of groundwater and surface water contamination by acidic water [3]. The acidic Sulfur rich wastewater or effluent from mining and industrial environments has greater consequences for Acid Mine Drainage in both actively operating and abandoned mines [4]. During the active mining process, the extraction of the gold-bearing conglomerate layer is crushed and gold become extracted [5]. Once the gold is extracted, the crushed rock is deposited on heaps known as slimes or tailings dumps, and generally, the gold-bearing conglomerates contain approximately 3% pyrite which gets deposited in slimes and tailings dumps. AMD is defined as a natural process (more correctly termed “acid rock drainage”, or ARD) that occurs when sulfur-containing minerals become exposed to water and oxygen, in the presence of bacteria known as the Acidithiobacillus and Ferrooxidans [6]. Sulfides in pyrite rock (Fool’s gold) then react with oxygen and water and leading to the production of sulfuric acid.
The sulfuric acid percolates through the slimes dam and dissolves some of the heavy metals. The resultant acidic, net acidic, and saline plume enters the surrounding soils, and eventually enters the groundwater and surface water bodies. AMD is a slow process characterized by low pH and high salinity levels with higher concentrations of sulfate, iron, aluminum, manganese, and the possibility of radionuclides [7]. The dark, reddish-brown and low pH water (often lower than 2.5) is difficult to rectify. The most important salts and heavy metals associated with AMD pose serious contamination threats to the environment and human health [8]. Metals such as Fe, Mn, Al, and other heavy metals as well as metalloids such as arsenic [9]. Heavy metals such as mercury and metalloid such as arsenic can become toxic and pose additional risks to the environment even when they are introduced to the water system in minute amounts [10].
Factors such as pH, redox potential, and soil types have a greater impact on the uptake of the element in the sediments. Soil types have a significant effect on the uptake of metals. In addition, clay soils, in particular, have higher sorption capacity which in turn reduces the availability of metals. Clay soils with high organic content enhance the conditions that favor successive precipitation of sulfides, and this can reduce the available elements at the lower depths. In addition, soils with less organic matter content tend to release elements from the sediments and improve metals uptake, a requirement of plant growth. Salinity levels also improve the rates of metal uptake especially Cr, Ni, and Zn [11]. According to [12], variations in water pH affect the ability of elements to be soluble. Furthermore, this also impacts the deposition ability of metals in the sediments as well as in the water column. The concentrations of Zn, Mn, and Ni in sediment have a direct bearing on the increased uptake of elements by plants [13].
Many aquatic macrophytes are classified as heavy metal accumulators and they are known to accumulate metals to various degrees and store them in below-ground tissues (rhizomes, roots) or above-ground tissues (leaves, stems). In some cases, aquatic macrophytes have been found to absorb higher concentrations of metals than are found in the water [14]. Plant species differ in their ability and tolerance to metal uptake and accumulation. Some plant species can accumulate high concentrations of a single metal and translocate it to the roots, rhizomes, stems, and/or leaves, while others can accumulate more than one element in different parts. Another category of plants is known as the excluders and can tolerate metal-rich environments by reducing the number of elements translocated from the below-ground parts to the aboveground parts. For plants to survive, they must adapt to the chemical and physical characteristics of the soil, water, atmosphere, and climate. Plants that grow and survive in metal-contaminated environments (metallophytes) have the distinct characteristic of tolerance. Plants also may be categorized as metal excluders, indicators, accumulators, or even hyperaccumulators [15]. Hyperaccumulators can translocate metals to their above-ground organs, and thereby extract and accumulate quantities that exceed any other species in the same environment (Table 1) [25].
1.1 Study methods
The study area is found in the Gauteng Province, Southern Johannesburg, Under the Westonaria Municipality, Figure 1. The area is also called the West Wits, it is located in Carletonville, and it is situated along the 15 kilometers (km) Varkenslaagte drainage line, also known as the old canal of the West Wits Operations (AngloGold Ashanti). The area is divided by a rocky ridge called Gatsrand. It covers approximately 3785.5 ha and approximately 38.58% which equates to 14,604 ha under deep mining (Mponeng, Tautona, and Savuka mine). The rest of the area is occupied by mining-related infrastructures such as tailing dams, mining plants, shafts, related operations, and residential wells as excavations. The Northern part of the mining area has been converted into agricultural land and mining activities while only a small portion of the area is still in its natural state as shown in Figure 1.
Figure 1.
Map of South Africa showing the location of Western Deep Levels in Carletonville.
Figure 2.
Some P. communis species with AMD water flowing between the plants.
Figure 3.
Metal precipitation on the soil crusts with some metals sticking to the basal part of the S. corymbosus and P. communis species.
The study site consisted of 17 artificial wetlands and only five selected wetlands were studied these were artificial wetlands 1, 2, 4, 5, and 7, and regarded as site 1, site 2, site 3, site 4, and 5 in this study. The summary of the investigated sites is presented in Table 2. The selected five sites out of the seventeen, were due to clear observations made in terms of growth and development of the macrophytes and other physical characteristics (algae growth and tadpoles occurring) on these sites when compared to the rest of the other sites. Accessibility was also another factor considered when conducting sampling at those sites. The effectiveness of rehabilitation was much clearer on these sites. The wetlands were grown with macrophyte species of Typha Capensis, Schoenoplectus Corymbosus, and Phragmites Communis.
Site name
Site characteristics
GPS Location
Site 1
The first artificial wetland situated at the foothill of the mine dump. It received AMD water seeping from the tailings dam. The pH of this site was lower than one on many occasions when recorded. The soil was dominated by surface salt crusting with some visible salt crumbs attached to the base of the plant stem. Gravel was the dominating soil structure with visible red oxides on the soil surface. The plants at this site portrayed stressed conditions with inhibited growth and some dying off before the winter season. The flowing water into this site was through the canal but most of the inflows were through seepage from the tailings dam. As the sites and canal are situated on the foot of the mining dump, it was evident that A M D leaches from the mine dump to the canals and sites, and forms a lot of surface precipitation and salt crusting as shown in Figures 2 and 3.
S 26°25 48′ 96″ E 27° 22 16′ 46″
Site 2
The characteristics of this site were almost similar to those of the first wetland. The plants were also shorter with observable signs of stress from the elements at these sites as shown in Figures 4 and 5.
S26° 25 49 61″ E27° 25 15′ 92″
Site 3
This was the fourth wetland along the canal. The improvement in pH, temperature, and electrical conductivity was measured. Salt crusting and crumbling at this site were less reduced when compared to the first two sites. The growth of the macrophytes plants was also improving.
S 26° 25 5 l′ 37″ E 27° 22 14′ 51″
Site 4
The wetland consisted of tall green macrophytes plants with some growth of algae species observed. The pH at this site was in an improved range of 5–7. Salt crumbling and crusting were no longer visible at this site. The soil structure consisted of dark clay soils without gravel on the top layers.
S 26° 25 53′ 86″ E 27° 22 13′ 24″
Site 5
This was the biggest wetland of them all, it consisted of tall green macrophytes plants with deep waters. The pH condition at this site was 7. Some development of another aquatic biodiversity such as tadpoles was observed.
S 26° 25 54′ 78″ E 27° 22 12′ 78″
Table 1.
Descriptive summary of the study sites.
Botanical name
Common name
Process of metal accumulation and types of metals accumulated
Some herbaceous plants used in mine sited for phytoremediation.
Figure 4.
Some metal precipitates at the edge of the artificial wetland with some precipitates attached to the lower parts and leaves of the P. communis spp that fall off often become covered by the salts.
1.2 In situ physico-chemical measurements, water sampling, and analysis
In situ water measurements were done at each site. At each constructed wetland, the Physico-chemical measurements were done in a sequence of inflow, midflow, and outflow, and at each site, a total of three samples were collected, which amounts to the number of water samples 15, when collected from all the five sites. The parameters measured include pH, Conductivity, Dissolved Oxygen, and Redox Potential. Water measurements were done using ThermoScientific Orion Star A329 portable pH/ISE/Conductivity/RDO/DO Meter. The grab method was used to collect water from the study sites. Water samples collected in 500 ml water bottles were kept in a fridge on-site at a temperature of 5°C. Later in the lab, the water inside the bottles was filtered using 0.45 μm Millipore. The water samples were analyzed directly using ICP-OES after filtration. The concentration levels of these elements in water were compared to the international standards for drinking water and presented in Table 3.
Variable
Descriptive Statistics of Raw Water Data (n = 15)
Mean
p-value
Median
Min.
Max.
Range
Variance
Std. Dev
Temperature (°C)
23.63
0.999
23.40
20.20
27.40
7.20
5.01
2.2429
pH
6.15
1.000
6.06
5.69
6.70
1.01
0.0934
0.3056
EC (mS/cm)
6.925
1.045
4.54
1.49
41.40
39.91
99.163
9.958
Redox (mV)
99.91
2.1725
94.40
54.40
194.30
139.9
1517.20
38.95
DO (%)
14.63
1.5675
6.11
0.0235
0.1436
0.7234
570.62
23.88
Table 3.
Summary of descriptive statistics of summer in situ water measurements.
Figure 5.
Some AMD and metal precipitation of the floodplain of the right side adjacent to the canal.
1.3 Sediment sampling, preparation, and analysis
Sediment cores were extracted from the selected five sites mentioned above. PVC (Polyvinylchloride) pipe was inserted into the ground using a hammer and a plank. The sediment cores were pulled out using a long steel bar inserted into two holes made on the top sides of the pipe. The pulled-out cores were wrapped in refuse bags to prevent the sediments from falling out of the pipes. The pipes were taken to the laboratory and stored in the cold storage room at a temperature of 5°C before preparation and analysis. The pipe cores with sediment were cut vertically into two halves. Sediments inside the pipe were separated into three distinctive parts of 0–2, 2–10, and 10–30 cm, respectively using a plastic ruler.
A subsample of 20 g was extracted and preserved inside a plastic bag and freeze-dried for 3 days. The freeze-dried samples were milled using a mortar and pestle. The samples were further prepared and put inside the Spectroscout Geo + XRF Fluorescence small containers with a thin film inserted in the middle of the containers. The thin film served as the base of the container, and it was also polished before it was put into the Spectroscout Geo + XRF Analyzer Pro for analysis. The sediments were analyzed for the presence and concentration of Fe, S, Mg, Cr, Mn, Co, Ni, Cu, Zn, Mo, and Pb using the X-Ray Fluorescence.
1.4 Plants sampling, preparation, and analysis
Samples of Typha capensis, Schoenoplectus corymbosus, and Phragmites australis species were collected from the five sampling sites. In each sampled site, the plants’ species were also sampled by using the sequence of inflow of the site, on the midflow, and also on the outflow of the constructed wetlands. Three samples of each of the plants were collected per site, and a total number of 45 samples were collected from all five sites in one season. The macrophyte species were uprooted using a spade. The plants were taken out of the sites and washed with local and rinsed with distilled water, for quantitative removal of soil and other foreign particles.
The plants were taken to the laboratory where they were thoroughly washed and rinsed with tap and distilled water. They were then separated into leaves, rhizomes, and roots. The stems were not included in the study. The plant leaves, roots, and rhizomes were also chopped into pieces of +/− two cm and homogenized and a subsample of +/− 20 g was extracted from all the samples and freeze-dried for two three days to ensure that the plants are much dry for easy grinding. The homogenized plants’ samples were milled using a Fritsch Pulverisette 6 Mill into pulverized powder before metals analysis.
1.5 Statistical analysis
Statistical analysis was done using the Statistica Analysis Package Version 10 computer package. Differences amongst the means were determined by analysis of variance. The Pearson correlation coefficients (r) were used to express the associations of quantitative variables. The basic descriptive statistics were performed to determine the Mean, P-value, Median, Minimum, Maximum, and standard deviation to indicate the relationship between the levels at which the elements were absorbed by the water and sediments as well as the level at which they were carried by the water and the relationship was again determined at p ≤ 0.05. The sample size (n = 15) makes statistical comparisons easier as it is not limited by the sample size.
The results for the Physico-chemical parameters are presented in Table 4. The average concentration for Zn accumulated by the plants in the autumn and summer seasons were in a range of 124.83 and 230.47 μg/g respectively, as shown in Figure 6. The result of the higher accumulation of Zn in summer could be that the new shoots are sprouting and as a result, more Zn is taken up by the plants as compared to autumn when the plants shed their leaves.
Variable
Plant species
Translocation factor (Autumn)
Bioconcentration factor (Autumn)
Translocation factor (Summer)
Bioconcentration factor (Summer)
Leaves
Rhizomes
Leaves
Rhizomes
Roots
Leaves
Rhizome s
Leave s
Rhizome s
Root s
Zinc
P. communis
0.35
0.32
26.64
24.66
76.14
0.3
0.41
59.28
82.8
200.8
S. corymbosus
0.97
0.72
40.1
29.86
41.36
0.48
0.78
65.58
106.27
135.6
T. capensis
0.49
0.81
22.91
38.02
46.75
Nickel
P. communis
0.29
0.25
027.8
3.88
17.76
0.1
0.27
3.96
10.5
39.3
S. corymbosus
0.35
0.45
3.95
5.05
11.35
0.22
0.81
4.27
15.9
19.6
T. capensis
0.24
0.17
3.86
2.79
16.21
Iron
P. communis
0.21
0.25
2.09
2.48
9.88
0.08
0.3
1.13
4.35
14.44
S. corymbosus
0.13
0.31
1.19
2.8
9.03
0.12
0.79
1.29
8.55
10.79
T. capensis
0.08
0.19
0.71
1.75
1.98
Manganese
P. communis
1.02
0.23
33.29
7.62
32.57
0.02
0.13
6.23
32.98
253.5
S. corymbosus
2.13
0.52
38.22
17.95
34.45
0.42
0.77
45.33
84.45
109
T. capensis
4.16
0.69
151.1
25.19
36.3
Copper
P. communis
0.26
0.28
8.1
8.44
30.65
0.14
0.26
12.86
24.28
92.4
S. corymbosus
0.35
0.64
8.02
14.63
22.86
0.24
0.76
11.72
36.34
48.14
T. capensis
0.36
0.54
7.1
10.68
19.94
Magnesium
P. communis
1.26
0.29
74.53
17.1
58.93
1.21
0.41
151.05
51.25
125.5
S. corymbosus
2.26
1.18
196.83
102.88
87.12
2.16
1.08
278.52
0.05
129.2
T. capensis
1.55
0.66
250.78
106.05
161.5
Cobalt
P. communis
0.17
0.18
5.67
5.76
32.51
0.06
0.18
7.05
33.4
55.14
S. corymbosus
0.28
0.33
6.8
7.96
23.91
0.13
0.61
5.66
16.63
92.3
T. capensis
0.15
0.1
8.35
5.88
57.18
Sulfur
P. communis
1.29
0.34
135.54
46.62
175.3
0.95
0.53
274.72
223.1
166.6
S. corymbosus
1.68
1
216.11
128.84
128.9
1.65
1.34
228.58
128.08
0.01
T. capensis
0.78
0.27
145.35
50.71
185.3
Phosphorus
P. communis
1.8
0.01
45.53
0.27
81.89
1.32
0.52
0.06
59.93
0.06
S. corymbosus
1.37
3.49
105.8
269.32
77.22
1.67
2.42
0.11
231.14
95.55
T. capensis
1.93
0.89
131.58
60.95
105.8
Chromium
P. communis
0.44
0.32
4.42
3.27
10.09
0.16
0.35
2
4.23
12.18
S. corymbosus
0.09
0.42
0.94
4.22
10.01
0.15
1.43
1.29
12.44
12.44
T. capensis
0.07
0.15
0.53
1.21
7.9
Molybdenum
P. communis
0
0
1.94
0.06
0
0
0
1.09
0.24
0
S. corymbosus
0
0
2.36
0
0
0
0
2.67
1.03
0
T. capensis
0
0
1.03
0.73
0
Lead (Pb)
P. communis
0.37
0.2
3.98
2.21
10.64
0.16
0.33
2.73
5.77
17.55
S. corymbosus
0.03
0.04
2.35
3.16
7.93
0.32
0.96
2.96
8.84
9.23
T. capensis
0.16
0.16
11.98
1.96
1.78
Table 4.
Translocation and bioconcentration factors for autumn and summer seasons.
Figure 6.
Variations in the average Zinc concentration accumulated by plant in summer and autumn.
2.2 Nickel (Ni)
When plant samples were analyzed for Ni content, the higher average concentration for Ni was measured in summer. The average concentration of Ni accumulated by the plants during the autumn and summer seasons were 36.36 and 49.55 μg/g, respectively as shown in Figure 7. Ni accumulation in the plants in the two seasons follows that of Zn.
Figure 7.
Variation in the average Nickel concentration accumulated by plant in summer and autumn.
2.3 Iron (Fe)
During the investigation period, it was observed that Fe was accumulated by the plants as FeO. A higher amount of Fe in the plants was accumulated in the summer season. The average concentration for Fe accumulated by the plants during the summer and autumn seasons was 25657.92 and 24807.34 μg/g, as shown in Figure 8. Unlike Zn, Fe accumulation in the two seasons did not differ much.
Figure 8.
Variation in the average Iron concentration accumulated by plant in summer and autumn.
2.4 Manganese (Mn)
Plants accumulated Mn in the form of MnO. Mn was highly accumulated by the plants during the summer season and this also follows the trends of Zn and Ni. The average concentration for Mn accumulated by the plants were 5984.96 and 3950.78 μg/g in the summer and autumn seasons respectively as shown in Figure 9.
Figure 9.
Variation in the average Manganese concentration accumulated by plant in summer and autumn.
2.5 Copper (Cu)
In all the sampled plants, Cu was more highly accumulated by the plants in summer than in autumn in the same way as Zn, Ni, and Mn. The average concentration of Cu accumulated in the plants sampled during the summer and autumn months was 78.30 and 45.63 μg/g as shown in Figure 10.
Figure 10.
Variation in the average Copper concentration accumulated by plant in summer and autumn.
2.6 Magnesium (mg)
During the autumn and summer seasons, the average concentration for Mg accumulated in the plants was 18561.31 and 14976.21 μg/g respectively, as shown in Figure 11. A higher concentration of magnesium was accumulated by the plant during autumn. When the plants were measured for Mg, seasonal differences in the accumulation of Mg were not very pronounced.
Figure 11.
Variation in the average Magnesium concentration accumulated by plant in summer and autumn.
2.7 Cobalt (Co)
During the sampling periods, harvested plants were measured for the presence of Co. After the plant has been measured for the metals accumulated by the plant, it was observed that the plant harvested in summer has accumulated and stored in higher amounts of Co than in autumn, and this also follows the trends like that of Zn, Ni, Cu, and Mg. The average concentration for Co measured in summer was 58.13 μg/g, whilst the average concentration of Co in autumn was 43.24 μg/g as shown in Figure 12.
Figure 12.
Variation in the average Cobalt concentration accumulated by plant in summer and autumn.
2.8 Sulfur (S)
In summer, the plants have accumulated higher concentrations of S. The average concentration of S in the plants was 22826.03 μg/g in summer whilst in autumn it was 22749.46 μg/g as presented in Figure 13. The uptake of S is also not very pronounced and the average values, do not differ much between the plants measured from the sites as shown in Figure 13.
Figure 13.
Variation in the average S concentration accumulated by plant in summer and autumn.
2.9 Phosphorous (P)
Phosphorous was highly accumulated by the plants in autumn. The average concentration for P accumulated by the plants in autumn was 3759.61 μg/g whilst in summer it was 3487.44 μg/g as presented in Figure 14.
Figure 14.
Variation in the average P concentration accumulated by plant in summer and autumn.
2.10 Chromium (Cr)
Higher amounts of Cr were measured in the plants in autumn. The average concentration for Cr in autumn was 68.76 μg/g and in summer it was 64.82 μg/g as shown in Figure 15.
Figure 15.
Variation in the average Cr concentration accumulated by plant in summer and autumn.
2.11 Molybdenum (Mo)
In the plants sampled measured for Mo in autumn and summer, it was found that Mo was highly accumulated by the plants in autumn. The average concentration for Mo measured in autumn was 0.09 μg/g and in summer was 0.07 μg/g, as portrayed in Figure 16.
Figure 16.
Variation in the average Mo concentration accumulated by plant in summer and autumn.
2.12 Lead (Pb)
The average concentration for Pb in the summer and autumn months was 12.082 and 12.084 μg/g respectively as shown in Figure 17. Higher concentrations for Pb were measured in the plants in summer.
Figure 17.
Variation in the average Pb concentration accumulated by plant in summer and autumn.
2.13 Statistical significance of elements between water, sediments, and plants
The correlation coefficients were performed to determine the relationship between the levels at which the elements were absorbed by the three species of Schoenoplectus, Phragmites, and the Typha spp. The concentrations of elements in the water, sediments, and plants were compared by the use of the significant variance at p = 0.0500. When the significant correlations coefficient was performed, several significant correlations were found to take place between the plants, water, and sediment. A correlation was found between sediments and plants, where Magnesium was negatively correlating with Cr and Fe and positively correlating with P, S, Mn, Co, and Ni. Their associated r values were (Cr) r = −0.06143 and (P) r = −0.6232. The r values for the positive correlations were (P) r = 0.7598, (S) r = 0.6406, (Mn) r = 0.8714, (Co) r = 0.8021 and (Ni) r = 0.6704. The p values were significant at p = 0.0500 and all the p values were in arrange of p = 0.000 and 0.0015.
P was found to be having both negative and positive correlations with the elements in water, sediments, and plants. The negative correlations were found between the concentrations of Cr, Fe, and Pb while the positive correlations were obtained between the concentrations of Mn and Co. The r values of the elements with negative concentration correlations were (Cr) r = −0.6488, (Fe) r = −0.7973 and (Pb) r = −6581, while the correlation for the elements with positive concentrations of elements were (Mn) r = 0.07636 and (Co) r = 0.7841. Their p-values were significant at p = 0.0500 and their p values were in arrange of p = 0.000 and p = 0.009 in water, sediments, and also in plants. Cr was also found to have a negative correlation with Mn and positive correlations with Fe and Pb.
The correlations were significant at p = 0.0500 and the p values were in a range of p = 0.000 and p = 0.038. The correlations were (Mn) r = −0.5389, (Fe) r = 0.8005 and (Pb) r = 0.8584 respectively in water, sediments and plants respectively as shown in Appendix 1.
2.14 Translocation and bioconcentration factors of sediments and plants
Bioconcentration factor is defined as the ratio of metal concentration in plant aboveground part to the total metal concentration in the soil. The translocation factor is the ratio of metal concentration in the shoots to the metal concentration in the shoots. Bioconcentration of elements of the plants’ species between the seasons. Since the amounts of elements enter the aquatic ecosystem after being washed from the mine dump, the elements (some of which are toxic) become accumulated in the water column, in the sediments, and also uptake by the plants which then pose some health threats when accumulated in higher amounts. Bioconcentration factor (BCF) is described as the measure of the amount of an element accumulated in the plants from their surrounding environment that is in contact with it [26]. It can be obtained by dividing the trace element concentration in plant tissues harvested by the initial concentration of the element in the external nutrient solution. Translocation factor (TF) on the other hand is defined as the ratio of element concentration in the root to shoot (Table 4) [27, 28].
This resulted in BCFs for the different types of plant organs (Table 4). The BCF for the shoots, rhizomes, and leaves was calculated from the elements accumulated by the plants in both the summer and autumn seasons.
Table 4 below shows the TF and BCF of quantified elements in the study. In both seasons, elements that were mostly taken with high BCF were S, Mg, Zn, Mn, P, Cu, Ni, and Co, and elements that were mostly taken with the highest TF were P, Mg, S, Cr, Zn, Pb, Cu, and Mn. In autumn, the plants’ organs that were found to have the highest BCF were the leaves of T. capensis and P. communis as well as the roots of Scirpus corymbosus. In summer, the highest BCF was obtained in the roots and rhizomes of P. communis and S. corymbosus. In summer, the plant species with the highest BCF and TF was S. corymbosus; and in autumn, T. capensis was the plant species with the highest BCF while S. corymbosus was the plant species with the highest TF. It was observed that both the TF and the BCF are affected by seasonality. The TF was higher in the autumn season than in the summer season, and the BCF was higher in the summer season than in the autumn season. It became evident that BCF active growth of the plants in summer as most of the elements are used by the plants during processes such as photosynthesis which is active in green leaves compared to when the plants’ leaves start drying up and photosynthesis ceases and most element losses occur, as the leaves die off and become brittle.
When making comparisons of the results of the translocation, Bioconcentration factors for plants and sediments, it was found that P, S, Mn, Mo, and Pb were lower in all three plant species and higher in the sediments. On the other hand, elements such as Mg, Cr, Fe, Co, Ni, Cu, and Zn were accumulated in higher concentrations by the plants and lower in the plants.
The results for TF and BCF indicate that the investigated plants accumulate in higher concentrations of certain elements and some in smaller concentrations. The elements that were found to be accumulated in lower concentrations by the plants were on the other hand found to be accumulated at higher concentrations by the sediments. This could be the case where the plants release the elements back into the substratum when they die off. This was observed in the accumulation of Molybdenum, where the measured Mo concentration was below the detection level by the plants. The plants with high BCF were regarded as suitable to be used to decontaminate soils. Although the plants showed high BCF, they still do not meet the criteria of being hyperaccumulators. The plants accumulated levels of elements such as Cu, Zn, and Pb in amounts with higher BCF (92.40, 200.79, and 17.55 in summer and 30.65, 76.14, and 11.98 in autumn) but the concentration of these elements was not greater than 1000 mg/kg to be regarded as hyperaccumulators. The plants were regarded as moderate accumulators [26]. The plants were suitable to be applied in contaminated soils for phytoremediation processes [29].
2.15 Comparison of elements in water with the international organizations
Table 5 illustrate the current drinking water quality guidelines by international organizations, and for the basis of this study, the levels of elements in water were compared with the water quality guidelines to indicate whether the level of elements in water was either above or below the required or acceptable levels. The last column indicates the concentration levels of elements measured in the water sample sites of this study.
Drinking water quality guidelines (μg/L–1) for elements in water in this study.
World Health Organization (WHO 2011).
United States Environmental Protection Agency (USEPA, 2011).
European Commission Environment (ECE, 1998).
Federal-Provincial-Territorial Committee on Drinking water (CDW), Health Canada (FTP-CDW, 2010).
Pakistan Council of Research in Water (PCRWR, 2008).
Australian Drinking Water Guidelines (DDWG, 2011).
Norma Official Mexicana NOM-127-SSA1–1994 (DOF, 1994).
Zn concentration in water was within the acceptable range of 267 μg/l, and when compared with the international guidelines for water quality standards which were above 500 μg/l. Fe, Ni, Mn, and Cu were found to be highly concentrated above the acceptable levels in water when compared to the international water quality guidelines, with the concentration levels of 2230, 282, 5900, and 14,080 μg/l respectively. The units for the elements concentrated in water were illustrated in μg/l in this section to easily compare with international guidelines as the standards were expressed in μg/l rather than in mg/l as shown in Table 6. To evaluate the effectiveness of these macrophytes in the accumulation of selected elements, a study of the whole life cycle of the plants has to be conducted, to capture all the processes that the plants undergo to survive the heavy metal contaminated environment. Amongst the three macrophytes investigated, there is no single plant that accumulates more than four elements than the other plants. In this study, it was observed that there are elements that are highly accumulated by either the roots, and less by the rhizomes, and more by the leaves and less by the roots. The rate of metal accumulation between the plants as well as the plant parts varies between the two seasons of autumn and summer investigated. This indicates that this could be a result of the functionality of the elements during the period of maximum absorption.
Variable
Descriptive statistics of elements accumulated by sediments n = 15
Mean
p-value
Median
Min.
Max.
Range
Variance
Std. Dev
Mg (μg/g)
0.1887
1.000
0.2027
0.039
0.321
0.282
0.0072
0.09
P (μg/g)
487
2.679
526.33
145.37
649
503.63
17973.47
147.39
S (μg/g)
1891.46
0.000
1813.67
519.67
3927
3407.33
629351.4
793.32
Cr (μg/g)
192.03
1.371
167.25
97.13
282.67
185.53
3450.42
58.74
Mn (μg/g)
1081.76
0.000
873.33
222.17
2807
2584.83
628712.2
792.91
Fe (μg/g)
7.23
0.9455
7.01
3.16
11.59
8.43
3.48
1.93
Co (μg/g)
29.51
1.6718
27.47
9.53
51.83
42.3
112.41
10.60
Ni (μg/g)
59.23
0.0001
60.33
20.67
75.67
55
175.61
13.25
Cu (μg/g)
38.37
0.1889
39.43
16.00
41.60
30.1
50.44
7.10
Zn (μg/g)
39.11
0.0022
40.23
10.5
50.27
39.77
94.29
9.71
Mo (μg/g)
1.73
0.6704
1.6
0.000
3.5
3.5
1.380
1.175
Pb (μg/g)
29.20
0.0210
29.67
8.53
41.3
32.77
55.71
7.464
Table 6.
Summary of descriptive statistics of elements accumulated by sediments.
Although the plants were found to accumulate the elements investigated, it is clear that some factors lead to the plants not uptake some elements in higher concentrations than the others. This creates a knowledge gap in this study, and as a result, further studies should be undertaken to investigate the reason the plants accumulate certain elements in lower concentrations. Another factor to be investigated is the overall performance of the plants in elements uptake throughout the whole growth cycle, and the results of such a study could be able to fill the gap in this study.
Rehabilitation of acid mine drainage contaminated sites requires an in-depth understanding of the appropriate macrophyte plants that could be used effectively in the accumulation of various elements from the sediments, and water column. Harvesting of the plants’ biomass before the winter season when the plants’ growth becomes inactive and dies off should be prioritized as more elements accumulated in the different plants’ organs are released back into the soils and this adds more elements to the substratum. Harvesting of the plants’ biomass for elements extraction for various metallurgical purposes could also be viewed as a value-adding initiative as this will be reducing the concentration amounts of these elements in the environment and also give more room for other less hardy plants to grow and thrive in those contaminated sites. The accumulation of elements in the sediment especially of most toxic elements enhances the release of other reactive ions into the environment, in addition, the reduction of such toxic elements also improved the pH conditions of the soil and water. In addition, there was no single plant amongst the three investigated species that was found to accumulate a higher amount of certain elements in all the plants’ organs of leaves, rhizomes, and roots. The investigated macrophytes species have demonstrated merit results in decontaminating AMD in the soil and further portrayed the potential of being used for phytostabilisation and phytoextraction as they are fast and easily spreading on contaminated sites, and can withstand high metal toxicity.
I would like to express my gratitude to Prof Luke Chimuka of the University of the Witwatersrand, Isabel Weiersbye, Innocent Rabohale, Sashnee Raja, Ike Rampedi, Maxine Joubert, and the entire Ecological Engineering and Phytoremediation Programme, for the tireless effort efforts and contributions that they have made in this study. I also send my sincere gratitude to Ms. Isabel Weiersbye for assisting with funding from DTI-THRIP.
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Written By
Tinyiko Salome Mthombeni
Submitted: 26 April 2022Reviewed: 05 June 2022Published: 12 September 2022
Open access peer-reviewed chapter
