Acid hydrolysis of compost humic acid functional groups heterogeniety in lead (II) complexes

Abstract

An acid hydrolysis of compost humic acid functional groups heterogeneity in lead (II)-humate complexes was carried out. Humic acid from compost was isolated and its functional groups were characterized by Fourier transformed infrared (FTIR). The dissociation constants (pKa), stepwise stability constants (Log K) and formation constants, (K_f), of the hydrolysed and unhydrolysed humic acids were determined and compared. A potentiometric titration – technique was used to determine the dissociation constants (pKa) and the stepwise stability constants (Log K) of hydrolysed and unhydrolysed humic acids in aqueous solution at carboxyl functional group. The FTIR spectra of isolated humic acid from compost had distinct clear absorption bands indicating the presence of OH (3620cm^-1), C=O of carbonyl (1704 cm^-1), functional groups aliphatic components CH2 (2922 cm^-1) and C-O of polysaccharide (1023 cm^-1). The pKa values of the hydrolysed and unhydrolysed compost humic acids were found to be 5.0107 and 5.4743 at carboxyl groups respectively. The values of the step stability constants were found to be Log K₁ = 11.4772, Log K₂ = 10.3058 and Log K₃ = 9.7251 for lead (II) hydrolysed compost humic acid and Log K₁ = 11.0111 and Log K₂ = 10.0161 for lead (II) unhydrolysed compost humic acid at carboxyl group respectively. The formation constants of the lead (II) complexes were found to be 1.15× 10³ and 1.102× 10² at carboxyl group respectively. Acid hydrolysis causes change of dissociation constants (pKa) at carboxy group of hydrolysed humic acid which was probably the main factor that gave rise to the changes in stepwise stability constants and formation constants of the complexes.