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Introduction

Metal pollution of soil dust and agricultural soils arising from industrial activities, vehicular emissions, and waste disposal sites are well documented [1-3]. The cement industry forms part of the industries that are well known to be problematic as regards the introduction of heavy metals from the dust emanating from their operations [1-3]. The deposition of these trace metals occurred at various distances around the cement factories and are influenced by wind velocity, particle size, and stack fumes [4]. Typical raw cement is made up of 25 mg/kg Cr, 21 mg/kg Cu, 20 mg/kg Pb, and 53 mg/kg Zn [5]. Further to this elemental composition, it was also reported that about 0.07 kg of dust is generated into the atmosphere when 1 kg of cement is manufactured [2]. Soil contamination by heavy metals can cause longterm problems on the biogeochemical cycle, which may affect soil functioning systems, leading to changes in soil fauna [6]. From previous studies in other countries, it has been established that dust containing elevated amounts of trace metals emanating from the vicinity of cement factories may adversely affect humans, plants, and soil composition within the vicinity [7]. Most cement factories have been noted as potential sources of metals such as Hg, Zn, Pb, Cr, and Cd [8-11]. The effects and concentrations of the dust containing trace metals as pollutants vary and depend largely on technology employed from the cement industries to ameliorate environmental degradation. In humans, trace metals such as Pb may affect the brain and cause retarded growth, especially in children [12]. In plants, excessive [Pb] alters normal metabolic pathways by disrupting specific cellular enzymes and may also inhibit the photosynthetic ability of plants [13]. On a general note, excessive levels of heavy metals may result in the induction of oxidation stress, damage to DNA, and disturbances in the biosynthetic pathways [14]. Quality of the environment is vital for sustainable development, especially in the face of rapid developmental programs from developing countries. The rapid economic developments in South Africa over the past few years have resulted in an increased demand for cement production [15], which stood at 14.9 million tons in 2012 and is expected to reach 18.1 million tons in 2018 owing to the emergence of new cement manufacturing plants in South Africa and neighbouring countries such as Lesotho, Botswana, and Swaziland [15]. Although several studies have noted the impact of the cement industry on the environment from developed countries, few studies have been conducted in South Africa [1, 5, 6, 10]. The present study was carried out to investigate the concentrations of heavy metals from soils and plants collected around the Hercules cement factory in Pretoria. The study also assessed the level of heavy metal contamination in the topsoil based on pollution index (PI).

Methodology

The study was carried out at about 50 m from a cement factory in Pretoria. The cement factory is situated just next to a very busy road (GPS: 25º۴۳’۲۱ S, 28º۱۰’۱۵ E). The area falls on the western part of Pretoria. There are two major seasons in the area (winter and summer), although the city usually witnesses a short period of spring and autumn. Sampling was done during the two major seasons. Sampling was carried out in the northeastern (NE), northwestern (NW), and southwestern (SW) areas of the cement company. Soil and plant samples were collected from these directions around the area: 30 soil samples from the topsoil (0-15 cm) and 30 soil samples from the sub soil (15-30 cm). Plants samples were collected from each of the directions where soil samples were collected and were identified up to the species. The soil samples were ground in the laboratory and airdried. From the ground soil samples, 0.5 g of the soil were added with 2.0 ml of HCl, 2.0 ml of HClO4, 2.0 ml of HF, and 8 ml of HNO3. The resulting solutions were then analyzed for trace metals contents using ICP-MS in order to determine the concentrations of trace metals from the soil samples. The plant samples were partitioned into three parts, namely for analyses: root, stem, and leaves. From these parts, 0.2 g of each of the different parts were acid-digested using 2 ml HCl, 1 ml HClO4, 2 ml of HF, and 5 ml of HNO3, and the resulting solutions were then analysed for trace metal contents using ICP-MS. Quality assurance was done using Certified Reference Materials for both soil and plant samples and the analysis was also carried out in triplicate. The ability of plants to uptake trace metals from the soil was determined using the transfer factor model [16]. The transfer factor is calculated as the concentration of heavy metals in plant parts to the concentration present in the soil. This is an index of soil-plant transfer. Values >1 indicate that plants are enriched in elements from soil (accumulator), ratios around 1 indicate that plants are not influenced by elements (indicator), and values <1 show that plants exclude the element from soil (excluder). Pollution Assessment Pollution assessment of the soil was calculated using the pollution index (Pi) method and the geo-accumulation index (Igeo). The pollution index was calculated using the formula:

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