Humic acids coagulation: influence of divalent cations

The effects of the ionic strength (maintained by LiCl, NaCl or KCl) and Ca²⁺ and Mg²⁺ concentration on the coagulation of purified humic acids (HA) was studied. Solutions of known ionic strengths, pcH, Ca²⁺ and Mg²⁺ concentrations were prepared with HA and filtered to obtain the fraction with a size smaller than 100 kD. After a 50 day storage, samples of these solutions were filtered again (100 kD) and the total organic C (TOC) of the filtered solutions measured. The HA coagulation increased with salt concentration, with the cationic charge, and for cations of the same charge, with the cationic charge density. The coagulation decreased for pcH values of 4 to 7–8 in the absence of and presence of Mg²⁺ and Ca²⁺. In the absence of the divalent cations, the coagulation has a constant value for pcH>8, but, in the presence of Mg²⁺ and Ca²⁺, increases at pcH values greater than 9. The coagulation of humic materials occurs whether the samples are exposed to light or kept in the dark, although the coagulation kinetics are slower for the samples kept in the dark. The size distribution of size-fractionated humic solutions changes over time to a size distribution similar to that of the original humic solution before it was size-fractionated. The results are explained by the DLVO theory.