Aggregation kinetics of humic acids in the presence of calcium ions

NANCI KLOSTER; MAXIMILIANO BRIGANTE; GRACIELA P. ZANINI; MARCELO J. AVENA

COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2013 vol. 427 p. 76 - 82

0927-7757

The aggregation kinetics of a humic acid (HA) sample as a function of Ca 2+ concentration at pH 5, 7 and 9 was investigated. UV–VIS spectroscopy was employed to quantify the progress of the aggregation reaction, and electrophoresis was used to evaluate the zeta potential of the HA molecules. The aggregation rate increases significantly by increasing Ca 2+ concentration at all investigated pH, being higher at pH 9 than at pH 5 and 7. Ab aggregation mechanism is proposed, which consists of at least two steps: a rapid binding of Ca2+ to humic molecules, followed by a slower process where HA molecules approach each other and become aggregate. This aggregation is possible because Ca2+ binding decreases the zeta potentioal of HA from -37 mV to -15 mV and because it is able to form bridges between HA molecules. There is no molecular fractionation up to 60% of aggregation. Above this value, HA aggregates became enriched with more aromatic HA molecules. The fast aggregation of HA in the presence of calcium ions implies that aggregates may temporarily trap, protect and transport pollutants in the environment.