humic substances on surfaces of natural clays. Humic sub-Time-resolved dynamic light scattering was used to investigate stances are present in practically all surface-near soil and the influence of pH and adsorbed humic acid on the coagulation aquatic environments. They are negatively charged polykinetics of kaolinite clay colloids. Stability ratios, defined as the electrolytes of varying molecular weight and can strongly experimental fast coagulation rate divided by the coagulation rate sorb to surfaces of oxides and clay minerals. Adsorption of the sample under consideration, were determined from the iniof humic substances has been shown to increase negative tial increases in average hydrodynamic radius with time. Pure
surface charge and colloidal stability of iron oxide ( 10,11 ) kaolinite exhibited fast coagulation (stability ratio W Å 1) at all ionic strengths (0.001 to 1 M NaClO 4 ) when suspension pH was and clay colloids ( 5, 9 ) . In agreement with these studies, below 5.8. Above pH 5.8, coagulation rates were significantly reit has been shown that removal of adsorbed organic matter duced (W ú 1) and W exhibited pronounced ionic strength depenfrom natural soil clays reduces colloidal stability (6-8). dence. The observed behavior is explained by edge-to-face hetero-It has been speculated that a combination of electrostatic coagulation by electrostatic attractive forces at low pH and electroand steric stabilization is responsible for the increased colstatic stabilization at high pH. The transition in coagulation loidal stability of clay-humic complexes ( 6,8,9 ) . Howbehavior occurs at the point of zero charge (PZC) of the kaolinite ever, unequivocal evidence for steric stabilization of clay edge surfaces near pH 5.8. Small additions of humic acid at pH mineral colloids by adsorbed humic substances is lacking 4 resulted in edge charge reversal from positive to negative and and is difficult to obtain without direct measurements of substantially reduced coagulation rates (W ú 1); similar behavior was observed at pH 6. Relating the observed stability ratios W to coagulation kinetics (9).
zeta potentials suggests that at low to medium ionic strength Published studies on coagulation behavior of clay colloids (°0.01 M NaClO 4 ) the stabilizing effect of adsorbed humic acid are almost exclusively based on viscosity measurements usis purely electrostatic. However, at high ionic strength ( ¢0.1 M) ing concentrated suspensions (1, 3) or traditional sedimentasteric stabilization may also play a role. ᭧ 1998 Academic Press tion experiments with more dilute suspensions (2, 4-7, 9).