Bioconcentration of Metals by the Aquatic Plant, Typha Latifolia, at Hwange Coal Mine, Zimbabwe

1. Introduction

Coal plays a crucial role in electricity generation as well as driving industrial processes globally. It contributes to the socio-economic development of many nations. As such, economic growth correlates with an increase in coal production [1]. However, coal mining puts enormous pressure on the natural environment resulting in the natural habitats and ecosystem being negatively affected. Coal is composed of some potentially harmful components that are released into the environment during its production. Coal mining uses copious amounts of water which is a major source of toxic elements in the environment when disposed of [2]. During the process of coal mining, toxic elements such as cadmium, lead, mercury, zinc, sulphur, and copper and iron are released into the environment and contribute to its pollution and degradation [3]. The dissemination of toxic elements into the environment is mainly caused by acid mine drainage which results in the dissolution of metal ions [4]. Toxic elements are a hazard to the environment particularly in the aquatic system where pollutants affect both terrestrial and aquatic organisms.

Aquatic plants take up toxic elements in the environment. These can either cause physiological and metabolic changes which can be lethal or can be bioaccumulated and tolerated. Hence their ability to take up toxic elements has been used as a bioindicator of pollution in aquatic environments [5]. The ability of aquatic plants to accumulate and tolerate toxic elements has made them a possible solution for mitigating environmental pollution. Literature has shown that different plant species have different bioaccumulation factors depending on the element(s) in question [5]. Most plants concentrate toxic elements in the roots and restrict movement to shoots, [3].

This study aimed to determine the effectiveness in bioaccumulation of different toxic elements of the aquatic plant Typha latifolia mostly found in the study area. Since many toxic elements are released into the environment, there is a need to remediate the area using plants endemic in the natural environment.

It was thought that determining the efficiency in bioaccumulation of metals in a plant native to the area would go a long way in finding a non-invasive and affordable solution.

2. Materials and Methods

2.1. Sampling and Physical Parameter Analysis

Water, sediment and plant samples were collected in triplicate from four sampling points during the wet and dry seasons and these were assigned as follows; Site A was effluent from an open cast mine, Site B was upstream of a coal processing plant, Site C was underground acid rock drainage point and Site D was effluent from a power production plant. One litre glass reagent bottles were used for water samples whilst polythene bags were used for sediment and plant samples. Sampling was carried out during the wet and dry seasons. Municipal water from Hwange was used as a control. Temperature, pH, conductivity and total dissolved solids parameters were measured at each site.

Figure 1. Map showing sampling collection sites in Hwange.

Figure 1. Map showing sampling collection sites in Hwange.

2.2. Heavy Metal Analysis

2.2.1. Plants

Two grams of dry plant leaf samples were weighed, ground and placed in 50 ml beakers. The samples were allowed to stand overnight in 5 ml of 55% HNO₃. The samples were heated carefully on a hot plate at 80^oC for an hour until the production of red NO₂ fumes had ceased. The temperature was increased to 120^oC and samples were heated for another hour. One millilitre of H₂O₂ was added and heated for an hour at 120^oC. Samples were allowed to cool and 1 ml of 32% HCl was added. The digests were filtered using a Whatman 40-micron filter paper and made up to 50 ml using distilled water [6]. The digests were then analysed using an atomic absorption spectrometer.

2.2.2. Water

The water samples were filtered using a Whatman 40-micron filter paper and then kept in the refrigerator for metal analysis.

2.2.3. Sediment

The sediment samples were oven dried at 100^oC for 12 hours. The dried sediment from each site was pulverized using pestle and mortar and 1 g was weighed and added to 100 ml beakers. The weighed samples were digested in 1 ml distilled water, 12 ml 32% m/m HCl, 4 ml 55% HNO₃ at 140^oC and left to reflux for about 20 minutes. Five millilitres of distilled water were then run down the side of the respective beakers and the solutions were refluxed for a further 10 minutes. The resultant solutions were transferred to 100 ml volumetric flasks and diluted to 100 ml with distilled water [7].

3. Results

3.1. Physico Chemical Analysis of Water Samples

The physico chemical results of water samples are shown in Table 1. Elevated conductivity and total dissolved solids levels were observed in water collected from Site C (underground acid rock drainage point), whilst low pH was observed when compared to other sites and the control during both the dry and wet season. No significant difference was observed in the pH values during the dry and wet season for Sites A, B, D and the control. A significant difference in conductivity and total dissolved values were observed between the control site and all sites for both the dry and wet seasons.

3.2. Heavy Metal Analysis of Water

Table 2 shows heavy metal concentration in water samples. Site C (underground acid rock drainage point) exhibited high levels of zinc (540 mg/ml) during the wet season and 535 mg/ml during the dry season, whilst Site B (upstream of a coal processing plant) had elevated levels of copper (270 mg/ml) and cadmium (72 mg/ml) during the dry season. Site B had statistically significant higher concentrations for all metals in the dry season compared to the wet season.

3.3. Heavy Metal Analysis of Sediment

Heavy metal concentration levels in sediment are shown in Table 3. High concentrations of zinc were observed at all sites for both seasons and only Site D (underground acid rock drainage point) exhibited high levels of lead during both seasons. Statistically significant differences were observed for zinc levels among all sites, whilst lead concentrations for Site D were significantly different compared to Sites A, B and C.

3.4. Heavy Metal Concentration and Bioconcentration Factors of Aquatic Plants

Concentrations of heavy metals in plants and their bioconcentration factors are shown in Table 4. High bioaccumulation factors were observed for cadmium for all sites and lead for Sites A, B and C.

4. Discussion

Aquatic plants can take up large amounts of metals from water and/or sediment through active and passive absorption, through different organs such as roots, stems, and leaves. Metals like lead are introduced in aquatic ecosystems as effluent from mining and industrial activities. Literature shows that metals induce oxidative stress in cells of living organisms which results from an imbalance between the generation and elimination of reactive oxygen species [8,9].

The order of metal concentration in aquatic plants was Zn > Pb > Cu > Cd with plants collected from Site D (effluent from a power production plan) exhibiting the highest metal concentrations (Table 2). Plant samples collected during the wet season showed elevated copper and zinc concentrations compared to those collected during the dry season. A study by D’ Souza et al., (2013) showed that the uptake of metals as well as the degree of tolerance by plants is dependent on the bioavailability of the metals, plant species and their metabolism. In another study Amin et al., (2018) concluded that high concentrations of lead in plant tissue correlate to the concentration in the growing environment. The ability of aquatic plants to take up heavy metals is also dependent on whether it is submerged, floating or emerging, with submerged plants bioaccumulating higher concentrations of metals [12]. According to Baker and Walker (1990), plants are classified into three groups of heavy metal accumulation; metal excluders, indicators and accumulators or hyperaccumulators. Metal excluders limit the levels of metals taken up by the plant and maintain low concentrations in the shoots whilst metal indicators take up metal concentrations in plant tissue above the ground that reflect the metal levels in the soil. Metal accumulators are plants that bioaccumulate metals in the above ground tissue resulting in levels that exceed the concentration levels currently in the soil [14]. The classification of the plants in the current study differs with different metals. They may be classified as hyperaccumulators as cadmium had high bioaccumulation factors compared to other metals followed by Pb (Table 4). This is in agreement with the study by Mojiri and colleagues (2013) who observed Typha domingensis as an effective accumulator of Pb, Cd and Ni. This may be a result of the mobility of the metals in soil, hence it is readily available for uptake by plants [16]. The plant may be further classified as a metal excluder as the bioaccumulation factor of copper observed for all sites except for Sites A and C during the wet season is below one. This may be as a result of regulation of uptake of copper by plants as it is toxic in high concentrations [16]. Although the plants showed high concentration of zinc, they had low bioaccumulation factors compared to other metals with the exception of plants collected from Site D in the wet season.

The diversity, population and productivity of aquatic organisms is affected by the abiotic factors of an aquatic body [12]. In the current study, water collected from Site C was acidic (pH of 1.96) with high levels of total dissolved solids and electrical conductivity. This site consequently had elevated levels of zinc. However, the plant used in the study had a low bioaccumulation factor for zinc. A study by Li et al., 2015 concluded that uptake of metals by plants is facilitated by slightly acidic water with a pH range of 5.6 – 6.5. The low bioaccumulation factor may be due to a pH lower than the optimum required for metal availability to plants [17].

Further studies need to be carried out to determine whether the bioaccumulation factor for Zn and Cu can be increased by altering environmental conditions.