Mineralogy and bioavailability of Selected Heavy Metals in Agricultural Soils

Abstract

Contamination of soils with heavy metals from agrochemicals presents a threat to food security and safety in communities depending on these soils for subsistence. Knowledge of the various factors and properties dictating metal behavior in agricultural soils is of paramount importance in the management of such contamination. This study considered the relationship between soil properties and metal mobility in three agricultural soil types from Botswana and appraised the role of clay mineralogy in heavy metal (HM) mobility in these soils. The mobility factor (MF) of metals in the three soils was determined from results of sequential extraction of HM in the soils. Mean concentrations of Cr, Co, Cu, Mn, Pb and Zn were highest in the vertisol with values of 127.7 mg/kg, 45.3 mg/kg, 119.4 mg/kg, 476.9 mg/kg, 52.2 mg/kg and 113.0 mg/kg respectively. Though the concentrations of HM in the vertisol were highest, the availability of these HM as indicated by values for MF of the different metals in the vertisol was lowest (0.02 - 1.13) whereas the arenosol with the lowest concentration of HM had the highest values for MF of most of the metals (0.54 - 13.38). These MF values were higher than those obtained for the luvisol which ranged from 0.05 to 1.07. The trends in MF did not match the trends observed in metal uptake by spinach grown on these soils. The observed patterns in HM mobility in the soils were attributed to the clay content and mineralogical assemblage of the soils.