Studies of Rare Earth Elements and Associated Radioactivity in the Coastal Sand of the Central and Western Regions of Ghana

Abstract

Beach sands are known host of different heavy minerals, which occur mostly as placer deposits and are of great importance to the electronics, metallurgy, medical, defense, and automobile industries. Typical examples of such heavy minerals are the rare earth-associated minerals (e.g. monazite, bastnaesite, xenotime and euxenite). Some of these rare earth elements-containing ores are known to occur in association with naturally occurring radioactive uranium (U-238) and thorium (Th-232). Limited studies are available on identification of rare earth element fingerprints in the coastal sands of Ghana. Also, the relationship between the Naturally Occurring Radioactive Materials (NORMs) and the Rare Earth Elements (REEs) in Ghanaian coastal sands has not been well studied. The study developed an analytical procedure for mapping out the composition of rare earth elements in beach sands via the pathfinder role of naturally occurring radioactivity along the coast of the Central and Western regions of Ghana. This was achieved through: (i) assessment of naturally occurring radionuclides (238Th, 232Th and 40K) using Gamma Spectrometry and ascertaining the presence of REEs-associated minerals using bromoform (density = 2.89 g/cm³) prior to petrography of the heavy mineral concentrates; (ii) investigation of REEs in coarse, medium and fine fractions using Lithium Metaborate Fusion Method using Inductively Coupled Plasma Mass Spectrometry (ICP-MS); (iii) development of chemical method in the separation of selected REEs (Pr, Nd, Sm, Eu); and (iv) establishment of geospatial distribution pattern to aid exploration of REEs minerals. Beach sand samples were collected from 15 locations in the Central region (Gomoa Fetteh, Senya Beraku, Winneba, Mankwadze, Apam, Mumford, Dago, Akra, Ekumpoano, Edumafa, Anomabu, Cape Coast, Elmina, Dutch Komenda and Kafodzizi) and 10 locations in the Western region (Shama, Abuesi, Sekondi, Takoradi, Cape Three Points, Egyembra, Axim, Esiama and Sanzule) along the coastline of Southwestern Ghana. The average concentrations of 238U, 232Th and 40K in the beach sands of the Central and Western regions of Ghana were found to contain the United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR) recommended permissible levels (35; 30; and 400 Bq/Kg respectively) for radionuclides; with mean activity concentrations (ranges) of 1.3 ± 0.47 to 31.50± 3.31 Bq/Kg (238U); 0.7± 0.04 to 71.70± 4.55 Bq/Kg (232Th); 73.9± 6.72 to 1775.5± 28.35 (40K) for the Central region. In the Western region, the mean activity concentration ranged fom 1.0 ± 0.03 to 5.6± 0.24 Bq/Kg (238U); 0.8± 0.04 to 3.8± 0.14 Bq/Kg (232Th); and 18.6± 0.23 to 343.2± 18.35 (40K). Beach sand dose rate in the Central region and Western region ranged from 4.13 to 132.39 (nGy/hr) and 1.78 to 19.32 (nGy/hr) respectively; with a total average across the two regions being 11.40 (nGy/hr). The annual effective dose in the Central region ranged from 0.0051 to 0.1624, while that for the Western region ranged from 0.002 to 0.024; total average for the two regions was 0.014 mSv/Yr. High radioactivity levels (Bq/Kg) (238U; 232Th; 40K; Raeq) observed in the beach sands of Dago (31.5±3.31; 71.7±4.55; 1775.5±28.35; 258.21); Akra (4.1±1.13; 2.0±0.51; 81.6±6.39; 12.66); and Ekumpoano (27.2±5.8; 6.2±1.20; 69.7±8.12; 33.67) of the Central region exceeded the individual radioactivity levels of the Western region. Heavy minerals such as Zircon, Rutile and Amphibole were identified in the beach sands of the Central and Western regions. The minerals found are known to concentrate REEs and are indicative of shore-derived minerals. Rare earth elements were found in beach sands at both regions. Total Rare Earth Elements (TREEs) distribution in the beach sands of the Central region ranged as coarse fraction (6.33 to 13.30 ppm); medium fraction (8.56 to 53.15 ppm) and fine fraction (16.67 to 795.01 ppm). The sum of Light Rare Earth Elements (LREEs); Heavy Rare Earths (HREEs); and ratio of light to heavy rare earth (LREE/HREE) distribution in the Central region were: coarse fraction (5.56 to 11.77 ppm; 0.64 to 1.53 ppm; 5.81to 9.89); medium fraction (7.72 to 48.13 pm; 0.84 to 5.02 ppm; 5.83 to 9.95); and fine fraction (14.98 to 727 ppm; 1.69 to 74.53 ppm; 4.43 to 10.79). The TREE distribution in the beach sands of the Western region varies as follows; coarse (5.69 to 29.78 ppm); medium (9.51 to 85.58 ppm) and fine fraction (24.3 to 86.28). The sum of light LREEs, HREEs and ratio of light to heavy rare earth (LREE/HREE) distribution in the Western beach sands were: coarse fraction (5.07 to 26.08 ppm; 0.62 to 3.70 ppm; 6.67 to 10.11); medium fraction (8.47 to 76.8ppm; 1.04 to 8.78 ppm; 3.97 to 9.56) and fine fraction (21.6 to 77.46 ppm; 2.7 to 17.63 ppm; 2.76 to 8.78). Despite the prominence of REE- fingerprints in the beach sands of Dago, Akra and Ekumpoano in the Central region, the corresponding increase in radioactivity concentrations at these locations (Dago, Akra and Ekumpoano) suggest strong influence of the geology of these areas. The sharp decrease in the concentration of total REEs in the beach sands of Akra shows an anomaly despite the fact that the sampling points are along the same trend. The geospatial observation of the coastline along the Central region showed that the sampling location at Dago and Ekumpoano are on probable geological faults and have differing geology. The study has also revealed that the radioactivity distribution in the beach sands serves as pathfinders to potential rare earth elements deposits in the Central region. Although heavy REEs were found in relatively higher concentrations in the beach sands of the Western region, the REEs in the fine fractions of the Central region exceeded that of the Western region. Consequently, the renewable energy target which relies on selected rare earth elements is achievable if more resources are committed towards potential sources of the REEs in-land.