Geological Occurrence and Distribution of Heavy Metals in Nigerian Coal and Clay: Industrial, Environmental, and Health Implications: A Review
Nsikan J. Etukudo *
Department of Geology, Akwa Ibom State University, Ikot Akpaden, Nigeria.
Nsima A. Akpan
Department of Chemical Sciences, Ritman University, Ikot Ekpene, Akwa Ibom State, Nigeria.
Ifiok M. Ibanga
Department of Geology, Akwa Ibom State University, Ikot Akpaden, Nigeria.
Uwem B. Abia
Department of Geology, Akwa Ibom State University, Ikot Akpaden, Nigeria.
Aniekan E. Akpakpan
Department of Chemistry, Akwa Ibom State University, Ikot Akpaden, Nigeria.
*Author to whom correspondence should be addressed.
Abstract
Heavy metals are naturally occurring elements that can become environmentally hazardous when present at elevated concentrations. Coal and clay are among Nigeria's most important geological resources, contributing significantly to energy production, industrial development, construction, and ceramic manufacturing. However, these materials also contain varying amounts of heavy metals that may be released into the environment during mining, processing, utilisation, and disposal. This review examines the geological occurrence, distribution, and concentration of heavy metals in Nigerian coal and clay deposits and discusses their industrial, environmental, and health implications. The reviewed studies reveal considerable variations in heavy metal concentrations across different deposits due to differences in geological setting, mineral composition, and depositional history. Nigerian coal deposits generally contain higher concentrations of heavy metals than clay deposits. Manganese concentrations in coal range from 6.06 to 783.93 mg/kg, nickel from 39.69 to 85.38 mg/kg, chromium from 7.14 to 36.57 mg/kg, copper from 67.21 to 94.93 mg/kg, and strontium from 24.08 to 383 mg/kg. In contrast, clay deposits typically contain lower concentrations, with copper ranging from 0.113 to 1.228 mg/kg, zinc from 2.972 to 4.755 mg/kg, lead from 0.731 to 2.193 mg/kg, nickel from 0.712 to 5.25 mg/kg, and cadmium reaching 2.80 mg/kg in some locations. Elevated mercury and cadmium levels have also been reported in certain edible clays. The findings indicate that coal poses a greater risk of heavy metal release through mining, combustion, and fly ash generation, whereas clay-related concerns are mainly associated with industrial applications and geophagic consumption. Continuous monitoring, effective environmental management, and stricter regulatory measures are therefore essential for sustainable resource utilisation and the protection of public health.
Keywords: Heavy metals, coal, clay, environmental pollution, geochemistry, public health