Mobilisation of Heavy Metals from Natural Pozzolan by Humic Acid: Implications for Water and Environment

G. J. Kayem; R. Kamga; S. K. Ndi; B. S. Lartiges; J. M. Siéliéchi
Natural pozzolans and similar geomaterials derived from volcanic eruptions contain heavy metals in significant or trace amounts, viz Cr, Co, Pb, Cu, Ni, Zn. Rain water, being essentially soft water as is the case in tropical Africa, can dissolve these metals and thus facilitate their transport into surface waters or through soil into ground water. Soil humic substances produced principally by biogeochemical decomposition of plant residues are rich in humic and fulvic acids which are known to complex heavy metals. Mobilisation of heavy metals (Cr, Pb, Co, Cu, Ni, Zn) presented in a sample of black pozzolan from Djoungo, Cameroon, in the presence of humic acid extracted from river sediment was studied at 20.0±0.1°C, pH 6 and 8 in a Jar test (batch stirred reactor) for contact times of 5 min and 24 hr and variable humic acid concentration (0.5 to 75 mg/L). The heavy metals were analysed by plasma spectroscopy (ICP-AES). Results showed that generally, the presence of humic acid increases the release of these heavy metals by pozzolan and this depends on time and pH. At pH 6, humic acid mobilised more heavy metals than at pH 8. For some metals (Cr, Zn, Cu), the amount released in the presence of humic acid, increased and levelled off with increase in humic acid concentration. Whereas for others (Pb, Co), the amount increased continuously. This suggests that humic acid acts as a heavy metal sink in contact with pozzolan or volcanic scoria, immobilising them and preventing their transport to the water table.
Heavy Metals; Pozzolan; Humic Acid; Water
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