Uranium in phosphate rocks and mineral fertilizers applied to agricultural soils in East Africa

Abstract

Phosphate rock, pre-concentrated phosphate ore, is the primary raw material for the production of mineral phosphate fertilizer. Phosphate rock is among the fifth most mined materials on earth, and it is also mined and processed to fertilizers in East Africa. Phosphate ore can contain relevant heavy metal impurities such as toxic cadmium and radiotoxic uranium. Prolonged use of phosphate rock powder as a fertilizer and application of mineral fertilizers derived from phosphate rock on agricultural soils can lead to an accumulation of heavy metals that can then pose an environmental risk. This work assesses the uranium concentrations in four major phosphate rocks originating from East Africa and four mineral phosphate fertilizers commonly used in the region. The concentration measurements were performed using energy-dispersive X-ray fluorescence spectrometry. The results showed that the uranium concentration in phosphate rock ranged from as low as 10.7 mg kg−1 (Mrima Hill deposit, Kenya) to as high as 631.6 mg kg−1 (Matongo deposit, Burundi), while the concentrations in phosphate fertilizers ranged from 107.9 for an imported fertilizer to 281.0 mg kg−1 for a local fertilizer produced from Minjingu phosphate rock in Tanzania. In this context, it is noteworthy that the naturally occurring concentration of uranium in the earth crust is between 1.4 and 2.7 mg kg−1 and uranium mines in Namibia commercially process ores with uranium concentrations as low as 100–400 mg kg−1. This study thus confirms that East African phosphate rock, and as a result the phosphate fertilizer produced from it can contain relatively high uranium concentrations. Options to recover this uranium are discussed, and it is recommended that public–private partnerships are established that could develop economically competitive technologies to recover uranium during phosphate rock processing at the deposits with the highest uranium concentrations.

Influence of phosphate fertilizers on the radioactivity of agricultural soils and tobacco plants in Kenya, Tanzania, and Uganda

“…The results show that the GLT fertilizer had the highest 232 Th concentration (687.7 ± 1.5 Bq kg -1 ) and the highest 238 U concentration (3,216.3 ± 21.7 Bq kg -1 ). The high radioactivity of the GLT fertilizer is associated with the radioactive nature of the local phosphate ore, which serves as the primary raw material in fertilizer production (Mwalongo et al 2022 ). Potassium is one of the macronutrients present in the fertilizer.…”

“…The YaraMila Compound (YC) N 10 P 18 K 24 fertilizer contains three component nutrients: N, P, and K, while the YaraMila Blended (YB) N 10 P 18 K 24 fertilizer was blended mechanically to supply balanced nutrients of N, P, and K individually (Lisuma and Mbwambo 2022 ). In addition, the Golden Leaf Tobacco (GLT) N 10 P 18 K 24 fertilizer was blended from Minjingu organic Hyper-Phosphate (MoHP) rock containing significant concentrations of naturally occurring uranium (Mwalongo et al 2022 ). Therefore, these mineral fertilizers can contain elevated concentrations of naturally occurring radionuclides associated with the phosphate ore processed for its P content.…”

“…The fact that large populations in these countries are directly involved in agricultural production means that a large fraction of the population could potentially be exposed to radioactivity beyond natural background levels through the intensive application of phosphate fertilizers. However, systematic studies on natural radionuclides associated with fertilizer use in tobacco production in Kenya, Tanzania, and Uganda have to the best of our knowledge, not been performed yet, although different studies have determined elevated concentrations of radionuclides, mainly 238 U in regionally produced and used mineral fertilizers in those countries (Makweba and Holm 1993 ; Banzi et al 2000 ; Mwalongo et al 2022 ).…”

Uranium Dissemination with Phosphate Fertilizers Globally: A Systematic Review with Focus on East Africa

“…According to Banzi et al [46], Minjingu PR has an average U concentration of 390 mg kg −1 , which is significantly higher than that found in other PRs. Minjingu PR is nonetheless used to produce different types of fertilizer products (i.e., Fertilizer Powder, Fertilizer Granules, Nafaka Plus, Golden Leaf Tobacco, and Top Dressing), which are marketed and sold in the East African region [47].…”

Caractérisation de la cinétique des échanges isotopiques du phosphore et potentialités des roches naturelles phosphatées de la République Démocratique du Congo dans la fertilisation phosphorique des sols ferrallitiques

“…Notre étude se propose d'aborder la question sous l'angle de l'aptitude des sols et l'efficience des sources de phosphore moins onéreuses et plus accessibles aux agriculteurs telles que les agro-minéraux, matériaux géologiques naturels contenant un ou plusieurs éléments nutritifs pour les plantes telles que les RP (Appleton, 1990 ;Leonardos et al, 2000 ;Hinsinger et al, 2015) pouvant, selon leur composition et dans certains environnements, apporter en plus du P, des nutriments secondaires (Ca, Mg), et éventuellement certains oligoéléments (Zn, Mo) (Sinclair et al, 1990;Hu et al, 2004;Van-Straaten, 2007 ;Derungs, 2018 ;Mwalongo et al, 2022). L'objectif final est de caractériser RP de Kanzi et Fundu mais aussi des échantillons de sols de Mvuazi et de Kinshasa (N'djili) par la quantification du P biodisponible qu'elles contiennent.…”