Multi-elemental analysis of heavy metals present in dust emitted from cement plants located in Pretoria and Thabazimbi, South Africa

Abstract

Increasing health and environmental concern about the effects of most toxic heavy metals emitted from cement plants in developing countries, which are going through rapid development, has led to this study. Cement industry in South Africa has been the primary industry over the years contributing immensely to infrastructure development and economic growth. Cement has been used to build many large cities, industries, homes, bridges and shopping malls around the country and still continue to be used by constructors. At this point, there has been no other substitute for cement and it will continue to be produced for decades to come. Unfortunately, this industry is now known to be amongst the major environmental polluters. Less has been done to address the adverse effects that comes with the production of cement, especially in the developing countries where there is huge demand for cement. This study focusses on dust emanating from production processes especially cement manufacturing from rotary kiln stage during production of cement and cement bricks. The production of cement and cement bricks generate dust, which is distributed over large areas of the environment. In South Africa, there are a number of factories in operation without proper planning of pollution prevention and compliance to environmental legislature. Since the production of cement is associated with the release of dust containing heavy metals, the dust is atmospherically deposited on the land, water surfaces and residential areas. The soil, street pavements, wetlands and water surfaces have become the sinks of heavy metals. Heavy metals that are being deposited include arsenic, cadmium, chromium, manganese, cobalt, copper, barium, antimony, selenium, vanadium, nickel and lead. Such metals pose health threat to the animals, plants and human beings living around the cement factories. These metals can easily be leached out from the soil and washed to the water bodies causing water pollution. Old processing techniques have been found to be inefficient to prevent emission of dust to the atmosphere. Hence, the emission of the toxic heavy metals to the environment was uncontrollable. Since cement is used to produce cement bricks, the whole process is subjected to heavy metals being discharged with dust from the factory to the surrounding environment. Four papers (I, II, III and V) were written to assess the level of heavy metals. In paper I, water and plants samples (Bidens Pilosa, Phragimites Australis and Xanthium Strumarium) were collected in the Mvudi River nearby a cement factory. Sampling was done before, within and after the wetland. Samples were digested with nitric acid for analysis. The concentration of zinc, chromium and lead were determined in the samples using a graphite furnace atomic absorption spectromentry. Results showed that the concentrations of zinc, chromium and lead were above the permissible limits in different parts of the plants analysed and water. The pH of water samples were below the threshold recommended by Department of water affairs and forestry (DWAF) and World health organisation (WHO). In paper II, seven soils at different distance, seven soils below soil surface at seven different layers and a bulk were sampled nearest to the cement brick making factory. Bulk sample was separated into five particle sizes (2 - 3 mm, 1 - 2 mm, 0.5 - 1 mm, 0.5 mm). Five sediments samples were also collected before, within and after the wetland along Mvudi river. Modified three step BCR sequential extraction was applied to the 23 samples in order to obtain the metal distribution in the samples. Heavy metal concentrations of nickel and chromium were determined using graphite atomic absorption spectrometry. Results showed that the levels of nickel and chromium exceeded the permissible limits recommended by WHO. Elevated concentrations Ni and Cr in soil and sediments also showed that the cement brick making factory is the main source of pollution in the area. To assess the contribution of cement dust to heavy metal pollution from the cement plants to the surrounding environment, two studies were carried out in the vicinity of two cement plants one in Thabazimbi and the other in Pretoria. Two papers (III and IV) were written from the studies and were summarised as follows: In paper III, dust samples were collected along the road leading to and passing by the cement plant in Thabazimbi, South Africa. The samples were collected using a brush and pan into sampling bags. After sampling dust samples were sieved into three particle size fractions (PM125, PM75, and PM32). A bulk and five samples were collected beneath the soil at different depth for depth analysis nearest to the cement plant. Water samples were collected along the Crocodile River before and after the cement plant site. The samples were digested using aqua ragia and extracted using Modified BCR sequential extraction. The samples were analysed using inductive coupled plasma optical emission spectrometry (ICP-OES) for concentration of platinum group metals and x-ray fluorescence for elementary analysis (XRF). Analysis of samples included characterisation of the dust samples using x-ray diffraction (XRD). The vi concentrations were also compared to that of the control study (blank) area to find out if the metals were discharged from the cement factories of interest. In paper IV, street dust samples were collected randomly on the paved surfaces, on the streets and accessible residential and roadsides on locations close to the cement plant in Pretoria. Some samples were collected along the road leading to the gate of the factory and also on the road near the cement plant. The samples were collected into sampling bags using a brush and pan. After sampling dust samples were sieved into three particle size fractions (PM125, PM75 and PM32). A bulk and five samples were collected beneath the soil at different depth for depth analysis nearest to the cement plant. Water samples were collected along the Apies River before and after the cement plant. All samples were kept in a cooler box with ice bags to keep them in good condition. The samples were digested using aqua ragia and extracted using Modified BCR sequential extraction. Results were used to establish spatial distribution of the heavy metals around the urban streets. The samples were analysed using ICP-OES for concentration of heavy metals and XRF. Analysis of samples included characterisation of the dust samples using XRD. The concentrations were also compared to that of the control study (blank) area to find out if the metals were discharged from the cement factories of interest. In paper V, seven different vegetables (spinach/Spinacia oleracea, Chinese cabbage/Brassica rapa, onion/Allium cepa, beetroot/Beta vulgaris, sweet potatoes/Ipomoea batatas, tomatoes/ Lycopersicon esculentum and cabbage/Brassica pekinensis), fruits (bananas/Musa acuminate) and their soils taken after uprooting them were sampled in farming area close to Thohoyandou town and the cement factory. The concentrations of cadmium, nickel and manganese were measured using the graphite atomic absorption spectrometry (GFAAS). Cadmium, nickel and manganese levels were found above permissible limits proposed by Food agricultural organisation (FAO) and WHO in edible parts of vegetables, fruits and soils and hence, may pose a health risk to consumers. Similarly the results from XRF also showed high concentration of the heavy metals in soil analysed. The aim of this project is to determine the levels of toxic heavy metals carried with dust emanating from cement factories. This assessment is meant to identify and highlight the levels of heavy metals in areas that are close to cement factories. The study will develop a database of heavy metals in affected areas and the pollution impact to the affected environments.