Retention of Co(II), Ni(II), and Cu(II) on a purified brown humic acid. Modeling and characterization of the sorption process.

Brown humic acids (BHAs) constitute the most polar and soluble fraction of humic acids. Their colloidal character and their high number of functional surface groups justify their higher reactivity as against metallic cations with respect to other humic fractions (i.e., gray humic acids and humins). The aim of this work is to study the retention mechanisms of Cu(II), Ni(II), and Co(II) on a BHA by means of a proper combination of physical and chemical techniques: sorption isotherms, mathematical modeling of these isotherms, molecular modeling, FTIR, and N2 (77 K) and CO2 (273 K) adsorption. Electrostatic retention for the three cations is an important mechanism at very low concentrations. Its magnitude is higher than that of the specific retention in the initial stages of the retention but it decreases progressively with respect to the former as the initial metal concentration increases. The BHA surface area varies with the amount of retained metal. When the initial amount of added metal is low (n0 < 80 mmol kg(-1)), the cations form 2:1 complexes, which are energetically favored due to the chelate effect. To obtain this coordination, the BHA slightly modifies its conformation by decreasing its area. When the initial amount of added metal is sufficiently high to occupy most of the surface functional groups (n0 > 1280 mmol kg(-1)), the cations are heterogeneously retained over the whole surface, thus preventing the available groups at low n0 from giving place to the 2:1 complexes due to the fact that they are already occupied.