Changes in Isoelectric Point as Affected by Anion Adsorption on Two Brazilian Oxisols

Abstract

Iron and aluminum oxides are widespread in tropical environments and increase soil's isoelectric point (IP). Anion adsorption modifies the IP and the surface charge of oxidic materials. This study evaluated IP changes on A‐ and B‐horizon samples of two Brazilian Oxisols upon reaction with acetate, citrate, oxalate, phosphate and sulfate (1 mmol L⁻¹; soil–solution 1∶100; pH 5.5; ionic strength 30 mmol L⁻¹ as NaCl). Most anions seemed to be specifically adsorbed, because the IP decreased after the reaction with the ligands. The IP of natural and of the acetate‐, citrate‐, oxalate‐, phosphate‐, and sulfate‐treated samples followed the order: Red Latosol‐A: 2.89,2.86, <2.50, 2.63, 2.79, 2.86; Red Latosol‐B: 5.92, 5.89, 2.79, 3.42, 3.36, 5.84; Red‐yellow Latosol‐A: 2.89, 2.79, <2.50, <2.50, 2.66, 2.89; and Red‐yellow Latosol‐B: 6.38, 6.29, 3.06, 4.36, 3.69, 6.15. The decrease in IP caused by citrate, oxalate, and Phosphate is relevant because this interferes with charge‐dependent soil reactions.

Keywords:

• Isoelectric point
• pH
• specific adsorption
• surface charge
• variable‐charge soils