Twenty-three water samples and three “yellow boy” samples were obtained from different water bodies located at the foot of the Ngwenya Mountain on top of which the old Ngwenya Iron Ore Mine is located. The samples were analysed for pH, electrical conductivity (EC), redox potential (ORP) and temperature (T). The dominant Fe species was determined using a UV-VIS spectrophotometer. Selected anions namely: halogens (F-, Br-, Cl-), the nutrients (NO2-,NO3-,PO43-) and the best indicator for AMD pollution (SO42-) were analysed using Ion Chromatography (IC) while the selected heavy metals, namely: Cr, Mn, Fe, Ni, Co, Cu, Zn, Pb and Cd were analysed using Flame Atomic Absorption Spectrometry (FAAS). The physico-chemical parameters ranges obtained were pH (6.32 - 8.63), EC (11.00 - 585.33 μS/cm), ORP (-93.67 - 79.33 mV) and T (7.60°C - 18.57°C). The levels of the Fe species (ppm) in the water samples were Fe2+ (0.56 - 3.17) and Fe3+ (0.00 - 0.73). Measured mean anion ranges in ppm were F- (0.00 - 0.15), Cl- (1.5 - 11.19),
Acid Mine Drainage; Heavy Metal Pollution; Redox Potential; Physico-Chemical Properties; Iron Speciation1. Introduction
A previous study [1] revealed that the acid mine drainage (AMD) phenomenon occurs in the old Ngwenya Mine, Swaziland. Acid mine drainage is the acidic water that is produced in mining areas from the oxidation of pyrite. The formation of AMD is facilitated biochemically by bacteria called Thiobacillus ferrooxidans and Ferroplasma acidarmanus [2-4]. AMD is rife in abandoned mines and once produced, it can exist for ages thereby polluting the environment concerned in perpetuity [5]. AMD has detrimental effects on surface waters if existent [6,7]. It is therefore worthwhile that the surface water bodies around the mine should be assessed for acid mine drainage pollution. This would enhance the quality of the affected water bodies. Water is life and a safe drinking water is a human birthright [8]. Use of polluted water upsets the whole ecological system [9-12]. AMD pollution causes the release of heavy metals into the environment [10,13] rendering them available for intake by biological systems. High levels of heavy metals are a nuisance to the environment due to their adverse effects on both plants and animals including human beings [9,14]. They have low biodegradability in the body and accumulate in both soft and hard tissues in the human body [15].
It is hypothesized that acid mine drainage from the Ngwenya Mine ore and dumping sites deteriorates the quality of water in their vicinity [16]. Mobility of heavy metals is enhanced by acidic waters rendering them liable to contaminate the environment [17,18] and causing adverse effects on life generally. As a result, acidic waters are a menace to the environment as well as being incapable to carry out its functions well [2,19]. Such water is unsafe for drinking, irrigation, swimming or cooling purposes in industry. The increased acidity interferes with the characteristics of pure water rendering it a poor coolant in the industry and engines. Low-pH water is inhabitable to aquatic fauna and flora and animals in general [12]. The human population around Ngwenya Mine that can be potentially affected by the AMD stands at 11147 as per 2007 report [20].
AMD can develop throughout the mining process [4], that is, in underground workings, waste rock dumps, open pit mine fractures, tailing deposits, ore stockpiles, etc. Once it occurs, the adverse effects can be costly because of high clean-up costs and irreversibility because of lack of available technology [6,21]. Waters with low pH kill all forms of living organisms in them, save only the highly tolerant ones. This leads to poor water quality and release of bad odour due to the decomposing dead organisms. Moreover, in low pH water, heavy metals become soluble and as such are potential contaminants to soils, sediments and terrestrial plants that use such water for growth [9]. Aquatic organisms can also accumulate the dissolved metals in their bodies which eventually are consumed by higher organisms and sequentially causing pollution by biomagnifications. This continues up to the trophic ladder until these metals get into humans and on accumulating beyond maximum contamination limits (MCL) and cause detrimental effects on human health. Heavy metals are systemic toxins with specific carcinogenic, teratogenic, nephrotoxic, neurotoxic and fetotoxic effects [15]. Reproductivity in crops is hindered by watering with acidic water. Quite often, low pH water contains high concentrations of copper ions that interfere with photosynthesis, and thus resulting in poor yields. This eventually negatively affects food security [10].
Furthermore, acidic waters are unsuitable for tourism purposes. The water is corrosive; hence it is not good for swimming. It has bad odour due to the decomposition of dead organic matter which strips off the water of a substantial amount of dissolved oxygen, rendering the waters anoxic. The water is not fit for aesthetics as acidic waters are characterized by an orange-brown colour. This is the colour of ferric hydroxide which is a product of pyrite oxidation. The ferric hydroxide precipitate is called “yellow boy” [22].
Equations (1)-(3) summarize the oxidation of pyrite:
The formation of “yellow boy” removes dissolved oxygen from water [19]. The “yellow boy” is produced in the AMD receiving streams because there is no pyrite to be attacked by the ferric ion. The corresponding stepwise reactions are as follows:
Figure 1 is a typical picture of “yellow boy” produced in one of the tributaries to Mlondozi River which is very close to the mine.
In summary, the influx of acid mine drainage into streams can severely degrade both habitat and water quality. It often produces an environment devoid of most aquatic life and unfit for desired uses. The severity and extent of damage depend upon a variety of factors including the frequency, volume, and chemical nature of the drainage. The size and the buffering capacity of the
receiving stream are other vital factors. The transport route from the source to the receptor may equally be a determining factor of the extent of damage by the AMD.
2. Materials and Methods2.1. Description of the Study Area
The study area is the surface water bodies found around the Ngwenya Iron Ore Mine, Swaziland. The streams studied are used by the communities in the study area for domestic purposes, that is, they are the sole providers of potable water in this area. Some of these streams are feeders for the Hawane Dam which supplies water to the capital city of Swaziland, Mbabane. Figure 2 below shows the areas in the vicinity of the Mine and the streams that source their water from the Ngwenya Mountain where the AMD producing mine is located [1].
2.2. Experimental
Water samples were collected from streams and dams around the foot of the Ngwenya Mountain. The sampling points are shown in Figure 3 below. Water samples were collected into 1 L prewashed polyethylene bottles as described elsewhere [1]. They were thereafter placed in a cooler box containing ice blocks and transported to the laboratory.
2.2.1. Physico-Chemical Properties
Upon arrival in the laboratory, about 100 ml of each of the water samples were poured into a small beaker and then quickly analysed for the physico-chemical parameters, namely: pH, temperature (T), redox potential (ORP) and electrical conductivity (EC). These analyses were carried out using electrochemical methods during which WTW Multi 340i probes were used. The pH and the ORP were measured by one probe, the pH-Electrode Sen Tix 41-3, which can be switched from measuring pH to measuring ORP and vice versa. The EC and T were measured by another probe, the TetraConR 325. The ORP values were converted to Eh (-log of electron concentration) using Equation (7) below:
This is addition of the temperature-adjusted potential of the saturated calomel electrode, where T is the measured temperature of the sample in ˚C [23].
The water samples were taken out of the freezer, allowed to defrost and then filtered through 0.45 µm HVLP polymer membrane filter using a Nalgene filter funnel with a V-700 model vacuum pump. The filter funnel and the collecting flask were rinsed first with deionized water three times and then with the next sample to be filtered to avoid cross sample contamination.
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