Precipitation of Humic Acid with Divalent Ions

Abstract

The aim of this study is to investigate precipitation properties of humic acid (HA). This is done by studying a commercial available humic acid salt (HA) from which a phase diagram is established by adding various amounts of BaCl2 to different concentrations of HA at pH 5.5. The phase diagram shows tree characteristic regions with markedly different precipitation courses: region I at HA concentration below 0.15g/1, region II at HA concentration between 0.15 and ~2g/1 and region IV at HA concentration above 3.5g/1. Furthermore, a forth intermediate region III is observed between ~2 and 3.5g/1.In order to understand these measurements and precipitation behaviour a phase diagram model is constructed using a simple thermodynamic approach. The result of this analysis shows that precipitation in region I and II can be explained by a simple equilibrium reaction between the HA and Ba2+ .The phase diagram model can not account for the observed precipitation in region III and IV. However, in region IV the HA appears to posses colloidal properties, which is supported by precipitation taking place at a constantξ-potential-21mV with the same amount of added barium ion regardless HA concentration. With respect to region ill, it is not exclusively determined whether precipitation is caused by HA behaving as a polyelectrolyte or possessing colloidal properties. The general observation throughout is that HA appears to behave as a polyelectrolyte at low concentrations and exhibits colloidal properties at high concentrations.