Strength of Humic Acid Aggregates: Effects of Divalent Cations and Solution pH

Aggregation-dispersion, charging, and aggregate strength of Leonardite humic acid (LHA) were investigated in CaCl₂ and MgCl₂ solutions as a function of pH and ionic strength (I). The strength or the withstanding force of aggregates of humic substances (HSs) against breakage is important because this force influences the transport and distribution of pollutants and nutrients along with HSs through the change in the size of HS aggregates as a transport unit. We observed the dominancy of aggregation of LHA at high pH than at low pH in every case of CaCl₂ and MgCl₂ solutions. This observation suggests the higher binding efficiency of these divalent ions at high pH, though there was no obvious relation with electrophoretic mobility and aggregation of LHA. Further, we first revealed the numerical value of the strength of HS aggregates by using a simple experimental setup of aggregate breakup under laminar converging flow through a capillary tube. The obtained values of the strength of LHA aggregates were higher in the presence of CaCl₂ solution than MgCl₂ solution, and the strength increased with pH. The highest strengths of LHA aggregates in 30 mM (I) CaCl₂ and MgCl₂ solutions were around 5.8 and 2.4 nN, respectively, at pH around 9.