Assessment of heavy metal bioaccumulation capacities of some lowland plants in Yaounde (Cameroon)

Abstract

Lowlands are very important ecosystems, offering commendable goods and services to humans and the environment. However, they could be very fragile. Lowlands in Yaounde are subject to soil pollution due to various wastes, particularly heavy metals from anthropogenic activities. The aim of this study is to assess the bioaccumulation capacities of heavy metals of some plant species growing in lowlands of Yaounde in Cameroon. In this study, the identification of macrophytes that thrive on heavy metals polluted soils remains an essential step in the phytoremediation process. Floristic inventory was conducted using the quadrat method, in order to identify macrophyte species with phytoremediation capacities. Twelve lowlands were selected, of which eleven were polluted and one was the control (unpolluted area). Soil, water and plant samples were collected during the dry and rainy seasons in three potentially polluted lowlands of Yaounde namely: Mokolo-elobi (site 4), Mvan (site 9) and Etang Atemengue Obili (site 11), and physico-chemical parameters of soil and water samples were determined. The concentrations of Pb, Cd, Cu, Ni, Zn, Cr, As and Co were determined in three soil and water samples, and twenty-four plant organ samples to assess the level of metal pollution. Pollution and geoaccumulation indices as well as ecological risks were used to assess the level of soil contamination. Similarly, in water, the toxicity level, pollutant load and ecological risks of metals were assessed using the pollution, metal assessment and toxicity load indices. Principal component analyses (PCA) and cluster analyses (CA) were used to determine the metals sources in soil and water. The remediation performances of plants were determined using the mobility ratio, translocation (roots - aerial parts) and bioaccumulation factors. The metal accumulation and bioconcentration indices in plants were analysed. During the rainy season, 189 species belonging to 138 genera distributed in 63 families were identified in polluted sites, while 139 species belonging to 103 genera dispatched in 39 families were identified in the dry season. Macrophyte diversity was higher in each polluted site compared to the control. The Poaceae, Asteraceae, Fabaceae, Malvaceae, and Solanaceae families showed a higher taxonomic richness on polluted sites in the rainy season (23, 20, 14, 12 and 12 taxa respectively),compared to the Poaceae, Asteraceae, Cyperaceae, Convolvulaceae and Fabaceae families (19, 17, 9,8 and 8 species respectively) in the dry season. The results revealed a high diversity of species present in polluted lowlands, with Shannon's diversity index (H'=2.63) and Pielou's equitability index (J'=0.459-0.847), as compared to the control (H'=2.34 and J'=0.747) in the rainy season. The respective values were H'=2.61 and J'=0.692-0.819 as compared to the control (H'=2.45 and J'=0.866) in the dry season. Based on the characteristic criteria of accumulating species and their metal accumulation capacities, plants were classified into major relative frequence and relative abundance of the species (Fri and A> 10%) and intermediate (Fri > 10% and 2%≤ A< 10) categories. Thus, 15 assorted species had presented interesting characteristics that could be tested in preliminary trials to investigate their phytoremediation/cleaning-up capacities, among which Echinochloa pyramidalis, Pennisetum purpureum and Commelina benghalensis were chosen. Concerning soils, the mean concentrations (n=3) of Cr (202.01±83.81 μg/g) in three sites, Ni (80.29±24.88 μg/g) at site 11 and Co (8.17±0.6; 20.23±1.7 μg/g) at sites 11 and 4 of the study were high as compared to the threshold limits for soils used for irrigation agriculture. The geoaccumulation index (Igeo) values indicated that soils were heavily contaminated by Cr and moderately by Cu from anthropogenic sources. The Nemerow integrated pollution index (IPI) revealed the pollution of all 3 soils by heavy metals and classified it as follows: site 11 (8.06) > site 9 (5.79) > site 4 (3.41). The potential ecological risks (Eir) of toxic metals followed the order of Cr>Cu>Co>Pb>Ni>As>Zn>Cd and indicated a slight level of ecological risk, with Cr and Cu being the highest contributors to the increase of the ecological risk level in the lowlands. During the dry season, the mean concentrations (n=3) of Cd (0.336±0.235 mg/L), As (0.335±0.236 mg/L) and Co (0.34±0.235 mg/L) in water were higher than the standard used for irrigation agriculture. Total heavy metal toxicity load and heavy metal evaluation index values were found to be lower than the acceptable value. According to the ecological risk index classification, 100% of the total samples were found to pose low ecological risk during both seasons. The mean concentration of heavy metals in plants species (n=8) of Pb (9.67±6.05 μg/g), Cd(0.41±0.38 μg/g), Cr (22.36±17.09 μg/g), Ni (7.63±5.88 μg/g), Zn (252.62±65.71 μg/g), Cu (25.92±1.82 μg/g), As (0.00±0.00 μg/g) and Co (4.69±4.23 μg/g) were all above the norm except for As. C. benghalensis presented the highest accumulation of Zn. The metal accumulation gradient in plant organs followed a decreasing order, from roots>leaves>stems, with the exception Cd (stems>roots>leaves) and Zn (roots>stems>leaves), thus showing an antagonism in the uptake of these two metals by plants. C. benghalenis has the potential to be used as phytoextractor of Zn, Cu and Cd (translocation factor (TF) and bioaccumulation factor (BAF)>1) while E. pyramidalis can be used for Cd and Ni, and P. purpureum for Cd, but they are all phytostabilizers of Co and As. Thus,the heavy metals translocation from soil to plants characterized them as indicators of metal pollution. The present study revealed a high accumulation of metals in the Poaceae family due to their highly developed, fibrous, extensive and diversified root systems compared to the Commelinaceae,explainaing their rapid development in contaminated lowland areas