Aggregation kinetics of humic acid: Effects of Ca2+ concentration

Abstract

The aggregation kinetics of a humic acid (HA) sample as a function of Ca2+ concentration at pH 5, 7 and 9 was investigated. UV-visible spectroscopy was used to quantify the progress of the aggregation reaction, and zeta potential measurements were used to evaluate the charging state of the HA molecules. The aggregation rate increases by increasing Ca2+ concentration. The rate is also higher at pH 9 than at pH 5 and 7. The proposed reaction mechanism consists of two steps: a rapid binding of calcium to humic molecules followed by a slower step, where HA molecules approach each other and become aggregated. This aggregation is possible because calcium binding decreases the zeta potential of HA from -37 to -15 mV. The study also reveals that the resulting aggregates have a core with a composition very similar to the composition of the whole HA sample and an outermost surface enriched with larger or more condensed molecules.