4. Results and discussion
4.5. Soil to plant transfer factors for heavy metals and radionuclides
4.5. Soil to plant transfer factors for heavy metals and radionuclides
Table 8 lists means, standard deviations and ranges of soil to plant transfer factors for metals at proposed uranium mining site in north-east Botswana. The transfer factors were calculated as a quotient of metal concentration in Colospermum mopane leaves (wet weight) and metal concentrations in substrate soil (dry weight). All the ranges of transfer factors were within a degree of magnitude; 0.00675 - 0.128, 0.00494 – 0.00780, 0.0193 – 0.0376, 0.0938 – 0.230, 0.407 – 0.720, 0.0840 – 0.270, 0.000754 - 0.00140, 0.00225 - 0.00609, 0.101 - 1.480 and 0.00225 - 0.00609 for Cr, Co, Ni, Cu, Zn, Cd, Th and Pb respectively. All the transfer factors were below unity because these metals exist in nature as solid materials in rocks and soils, metals ions are then released from the host material in response to change in ambient chemistry (Devkota & Schmidt, 2000). The chemical changes mobilise metal ions according to their solid to liquid partition ratio which means some ions will always remain in soil form;
hence remain unavailable to plant life. Less mobile metals like Pb have strong affinity for soil particles leading to reduced transfer factors, 00369 than the more mobile Zn which has a transfer factor that is two degrees of magnitude higher at 0.535 (Zhaung, Zou, & Shu, 2009;
Selvaraj & Agoramoorthy, 2006)
Table 8: Mean, standard deviation and range of soil to plant transfer factors for metals at proposed uranium mining site in north-east Botswana
No correlations were found between Fe soil concentration against Pb, Co, Cd and Ni transfer factors, meaning that these metals are not trapped in amorphous Fe (oxy) hydroxides known to scavenge metal ions and render them non-bioavailable. These findings were consistent
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with existing studies where treatment of soil with Fe oxide had no impact on wheat uptake of Pb and Cd (Chen, Lee, & Liu, 2000). However, Pb is known to have very strong affinity for Fe oxides which means that Pb could be irreversibly bonded to organic carbon (Stille, et al., 2011). The irreversible bonding of Pb makes it immobile and hence unavailable for plant uptake (Chen, Lee, & Liu, 2000) leading to observed conservative soil to plant transfer factors, 2.25 * 10-3 to 6.09 * 10-3 as depicted in Table 1
As transfer factor was a paltry 0.0172 ± 0.0230 implying that As is largely retained in soil rather than being availed to plant uptake through root absorption from soil pore water. From correlation data, As transfer factor has a strong correlation to soil pH and Mn concentration, R2 = -0.60 and 0.69 respectively. The opposing polarity of the correlation coefficient is in agreement with literature where by Mn exists in soil primarily as oxide, the oxides have negatively charged reactive sites at high pH (Meng, Zheng, Zhang, & He, 2009). The negatively charged reactive sites then bind As electrostatically, rendering As non-bioavailable to plants hence an increase in pH means reduced As transfer factors (Hennebel, De Gusseme , Boon, & Verstraete, 2009)
Co had one of the lowest transfer factors at (6.33 ± 1.11)10-3 because it exhibits limited mobility in soil due to its high affinity for organic ligands as well as oxides of Mn and Fe (Blanco Rodriguez, Vera Tome, & Lazano, 2002) (Johnson, et al., 2011). In this study, no correlation was found between Co and Mn but Fe/Co correlation was very strong, R2 = 0.82 while Co/pH had a weak R2 = -0.54 which implied that at weakly acidic to basic soils Co is bound to reactive sites of gelatinous Fe oxides hence immobilised resulting in low transfer factors.
The highest transfer factor was recorded for Zn, 0.535 ± 0.115, Table 8 which was at least a degree above the rest of the metals except Cu and Cd. Zn and Cu have relatively high concentrations in soil as shown in Table 2: Abundances of metals and radionuclides in soil from proposed uranium mining site in north-east Botswana and follow similar migration patterns in soil. Both Zn and Cu out-compete other metals at the rhizosphere due the two’s numeric advantage in terms of ions available at absorption site resulting in Zn and Cu having higher transfer factors (Ehlken & Kirchner, 2002).
Cr Th Pb Po S pH Fe Mn Co Ni Cu Zn As Th 0.44
Pb -0.12 0.52
Po -0.01 0.76 0.91
S -0.55 0.30 0.52 0.46
pH -0.51 0.07 0.83 0.58 0.71
Fe 0.81 0.35 -0.55 -0.30 -0.47 -0.83
Mn 0.27 0.33 -0.46 -0.05 -0.36 -0.78 0.64
Co 0.74 0.12 -0.49 -0.48 -0.29 -0.54 0.82 0.11
Ni -0.36 0.21 -0.32 -0.07 0.50 -0.22 0.20 0.52 0.01
Cu -0.74 -0.16 0.69 0.49 0.53 0.88 -0.98 -0.59 -0.86 -0.19
Zn -0.44 -0.64 0.20 -0.22 0.22 0.63 -0.69 -0.93 -0.18 -0.46 0.58
As -0.42 -0.56 0.03 -0.35 0.41 0.53 -0.50 -0.86 0.08 -0.14 0.37 0.91
Cd 0.40 -0.62 -0.65 -0.75 -0.89 -0.59 0.35 0.04 0.42 -0.51 -0.48 0.16 0.05
dissolves in acidic as well as basic soils alike (Fan, et al., Calcium water treatment residue reduces copper phytotoxicity in contaminated sandy soils, 2012). Mn and Fe concentration in soil were weakly, R2 = -0.59 and strongly, R2 = -0.98 to Cu transfer factor proving the theory that as the two known good scavengers of Cu in soil decrease, Cu transfer factor went up (Contin, Mondini, Leita, & De Nobili, 2007). However, a strong correlation, R2 = 0.88 was observe for pH/Cu transfer factor which implies that Mn and Fe oxide were not the only chemicals binding Cu but organic ligands were also present. The ligands are digested as the pH becomes more basic and mobilise Cu leading to an increase in Cu transfer factor (Contin, Mondini, Leita, & De Nobili, 2007).
Cd has a comparable transfer factor, 0.180 ± 0.088 to Zn and Cu even though it has trace concentration because it is more mobile than many other metals in soil rendering it bioavailable (Xhao, et al., 2013). The high mobility of Cd is attributed to lack of correlation between Cd and neither Mn nor Fe suggesting that Cd is not sequestered. Moreover, Cd has similar chemical properties to Zn (Devkota & Schmidt, 2000) hence follows similar migration patterns further increasing its uptake by plant roots. Pb had a low transfer factor, (3.69 ± 1.46)10-3 while showing weak correlations, R2 = -0.55 and -0.46 to Fe and Mn pointing to a reduced impact of these Mn and Fe on Pb sequestration. However, Pb/soil pH were strongly correlated, R2 = 0.83 implying that Pb was bonded to organic ligands which immobilised Pb leading to low transfer factors. Pb transfer factor increased with pH because organic matter disintegrates at high pH (Chen, Lee, & Liu, 2000; Stille, et al., 2011;
Jovanovic, et al., 2011)
4.6. Concentration of metals and radionuclides in Imbrasia belina larvae