The role of metal complexation in the solubility and stability of humic acid

Humic acid (HA) obtained from a variety of sources displays somewhat varied chemical composition and residual metal content. In spite of extensive purification procedures as much as 0.5% metal (by weight) can remain in the humic acid. In some cases, particularly for Cu2+, the metal binding appears site specific, while in others the cupric ion is less strongly bound. Furthermore, the iron content of the samples appears to be dependent on the source and purification procedure. Quantitative removal or exchange of the residual metal is expected to alter the stability and solubility properties of the HA. The treatment of highly purfied HA with Chelex® resin has afforded a material in which the amount of the so-called residual metal has been reduced by 40% or more. This reduction in metal content appears to affect solution aggregation phenomena and consequently the rate of flocculation. Metal removal depends on pre-treatment conditions of the HA sample and is highly pH and temperature dependent. These observations suggest that chemical transformations of HA at high pH and moderate temperature may affect the equilibrium state between the bound metal in HA and the metal-Chelex® complex. The apparently selective removal of metal suggests a specific role of the metal.