Toxic heavy metals distribution in urban soils of Africa: a systematic review

Urban soils in Africa are increasingly contaminated by toxic heavy metals due to rapid urban expansion, industrialisation, traffic emissions, and inadequate waste management. Toxic heavy metals such as lead (Pb), mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), and nickel (Ni) are of major concern because of their persistence, bioaccumulation potential, and ecological risks for the environment and humans. This systematic review synthesises evidence from 38 peer-reviewed studies published between 2010 and 2024 on the concentrations and sources of toxic heavy metals in urban soils of Africa. The results demonstrated an uneven research distribution on toxic heavy metals across different countries of Africa, with Nigeria and Ghana most represented, while large regions, including countries such as Egypt and Ethiopia, remain poorly studied. Industrial activities (27%) and traffic emissions (26%) accounted for more than 50% of the reported contamination sources, followed by domestic waste disposal (18%) and agricultural inputs (10%). Weighted mean concentrations of Pb, Cd, and Cr in many countries exceeded FAO permissible limits, indicating a significant threat to humans and the environment. Microwave digestion was the most commonly used extraction method, while X-ray fluorescence has gained increasing application. The findings demonstrate widespread contamination from rapid urbanisation and industrialisation but highlight limited research done on toxic heavy metals in urban areas of most African countries. Future research should focus on quantifying the metal concentration in African countries, where limited work has been done; the bioavailable fractions of toxic heavy metals and possible remediation strategies to improve soil quality in urban areas.