Coagulation Kinetics of Humic Aggregates in Mono- and Di-Valent Electrolyte Solutions

摘要

Coagulation behaviors of humic acids (HAs) aggregates inrnelectrolyte solutions at different pHs, valences and concentrations of electrolyterncations were investigated using dynamic light scattering technique inrncombination of other analytical tools. For monovalent electrolyte sodiumrnchloride (NaCl) solution, at its low concentrations the average hydrodynamicrnradius () of aggregates kept nearly constant. However, at high NaClrnconcentrations, could be scaled to the time t as α t~α, suggesting arndiffusion-limited colloid aggregation (DLCA). The coagulation value of NaCl inrna buffer at pH 7.1 was calculated to be in a range of 61.3-84.4 mM. Divalentrncation Mg~(2+) was far more effective in enhancing the HA coagulation, asrnevidenced by a lower coagulation value (between 1.0 and 1.7 mM) and a morernrapid coagulation rate. Such an enhancement could be explained by therncombined effects of electrostatic repulsion, complexation and bridging. Thernhighest coagulation rate (d/dt) and coagulation value at different pHs followed the order of: acidic ＞ neutral ＞ alkaline, andrnalkaline ＞ neutral ＞ acidic, respectively. Such a difference was associated with the extent of hydrogen bond and electrostaticrnrepulsion at different protonation/deprotonation states of carboryl and phenolic -OH groups. Transmission electronrnmicroscopic imaging reveals that HA was predominantly globular aggregates with a rough periphery at pH 5.26, and was changedrnto smooth spherical particles at pH 10.00. These results are useful for better understanding the coagulation behaviors of HAs inrnboth natural and engineered aqueous systems.