SOIL-TO-PLANT TRANSFER FACTORS OF NATURAL RADIONUCLIDES OF THREE COMMON FOOD CROPS GROWN ON A TIN-MINING IMPACTED SOIL

Abstract

Tailings produced from tin-mining operations containing elevated levels of Natural Radionuclides (NRs; ) need to be disposed properly to prevent environmental contamination. The knowledge of the mobility of NRs in contaminated farmlands is important because of possible accumulation and their radiological implication in crops grown on such farmlands. The Transfer Factor (TF) of NRs is an important parameter for predicting migration and accumulation of radionuclides through the food chain. However, there is dearth of information on the TFs of NRs in the tropics. This study determined the TFs of NRs in three widely consumed food crops grown on tin mining-impacted soil and Committed Effective Dose (CED) to assess radiological hazards in Nigeria. Tin-tailings were collected from an abandoned tin-mining site in Alheri, Jos. Soil samples were collected from uncultivated, non-tin-mining site at the Botanical Garden of Redeemer’s University, along Lagos-Ibadan Expressway. Three soil sample groups were purposively formulated; group-A (non-tin-mining soil), group-B (tin-tailings) and group-C (tin-tailing and non-tin-mining soil). Cowpea (Vigna unguiculata) and maize (Zea mays) seeds were obtained from the Institute of Agricultural Research and Training and cassava stems (Manihot escalenta) from International Institute of Tropical Agriculture Ibadan. Ten planting pots were prepared for each plant per soil group. The seeds and stems were planted and harvested at their maturity periods. The activity concentrations (AC) of NRs in the formulated samples and plant compartments (seeds, tubers, stems, leaves and roots) were determined using sodium-iodide thallium activated gamma-detector. The TFs of the NRs and CEDs of the edible parts were evaluated using standard methods. Data were descriptive statistics and The mean AC (Bqkg-1) of 40K, 238U and 232Th obtained for the soil groups ranged from 179.65 ± 2.88 (group-A) to 3421.51 ± 3.64 (group-B); 90.35 ± 3.37 (group-A) to 1992.61 ± 1.55 (group-B), and 273.06 ± 5.37 (group-A) to 25232.30 ± 1.33 (group-B), respectively. The mean AC (Bqkg-1) of the plant compartments were 39.39 ± 26.67 (tuber; group-B) to 2400.17 ± 1791.18 (cowpea-leaf; group-A) for 40K; Below Detection Limit (BDL) (maize stems; group-A-B-C) to 717.90 ± 404.86 (cowpea-leaf; group-B) for 238U, and 89.05 ± 110.86 (tuber; group-C) to 15972.92 ± 453.97 (cowpea-seed; group-B) for 232Th. The calculated geometric mean of the TFs of 40K, 238U and 232Th, in cowpea ranged from 1.40 (1.15) (seed) – 9.67 (2.49)(leaf), 0.39 (1.18)(stem) – 2.34(1.23)(root) and 0.77(3.01)(seed) - 9.73(1.45)(leaf) for group-A; 0.05(1.88)(seed) - 0.18(3.09)(leaf), 0.02(3.31)(seed) – 0.33(1.53)(leaf) and 0.05(1.70)(stem) – 0.12(1.53)(root) for group-B; 0.21(2.37)(seed) – 2.56(1.38)(root); BDL(stem, leaf) – 0.21(1.18)(root) and 0.07(1.37)(stem) – 0.31(1.78)(root) for group-C. Those of maize were 0.65(1.31)(seed) – 3.61(1.38)(stem), BDL(seed, stem) – 1.00(5.18)(root) and 0.58(2.79)(seed) – 2.68(2.43)(root) for group-A; 0.03(2,14)(seed) – 0.07(1.88)(stem) – BDL(stem, leaf) – 0.20(2.45)(root) and 0.02(1.97)(stem) – 0.13(1.72)(root) for group-B; 0.23(3.31)(root) – 0.43(2.88)(stem), BDL(seed, stem) – 0.11(2.37)(root) and 0.015(4.00)(seed) – 0.12(2.59)(root) for group-C. And those of cassava were 0.74(1.75)(stem) – 1.46(2.64)(leaf), BDL(tuber) – 0.90(2.21)(leaf) and 0.49(1.87)(tuber) – 1.54(4.26)(leaf) for group-A; 0.01 (1.40)(stem) – 0.12(2.04)(leaf). BDL(tuber, stem, leaf) and 0.03(1.57)(stem) – 0.04(2.23)(tuber) for group-B; 0.11(1.50)(tuber)– 2.91(1.79)(leaf), 0.01(1.11)(stem) – 0.07(2.04)(leaf) and 0.006(2.51)(tuber) – 0.21(1.91)(leaf) for group-C. The transfer factors of the natural radionuclides were observed to be in the order cowpea  maize  cassava. Significant differences occurred in the TFs of theNRs among the soil groups. Cowpea exhibited the highest potential for possible phytoremediation of the natural radionuclides.Cassava tubers had the highest mean CEDs (mSv.y-1) (2.19 - 35.7) while cowpea seeds had the least (0.002 - 0.07). The CEDs of tuber and maize seeds exceeded the reference value (0.12 mSv.y-1) recommended by the United Nations. The transfer factor of the natural radionuclides varied across the food crops and soil groups and the cassava and maize grown on tin-mining impacted soil were of radiological concerns.