Iron oxides serve as natural anti-acidification agents in highly weathered soils

The effect of Fe oxides on the natural acidification of highly weathered soils was investigated to explore the natural acidification process in variable charge soils A variety of highly weathered soils with different Fe oxide contents were collected from the tropical and subtropical regions of southern China to investigate the soil acidity status. Electrodialysis experiments were conducted to simulate natural acidification process and promote accelerated acidification in a variety of systems such as relatively less weathered soils, mixtures of goethite with montmorillonite or kaolinite, an Alfisol, a limed Ultisol, and Fe oxides coated montmorillonite. The objective was to gather evidence for the occurrence of Fe oxide inhibited natural acidification in highly weathered soils. Highly weathered soils with free Fe2O3 < 100 g/kg (17 soils) had an average pH = 4.64 +/- 0.06, while the soils with free Fe2O3 > 100 g/kg (49 soils) had an average pH = 5.25 +/- 0.04. A significant linear relationship was found between the soil pH and Fe oxide content of these soils. Similar results were obtained in electrodialysis experiments, i.e., in soils that underwent accelerated acidification. A negative correlation was found between the Fe oxide content and exchangeable acidity or effective cation exchange capacity, respectively. In another set of experiments, goethite slowed down acidification in experiments conducted with this Fe oxide and montmorillonite, or kaolinite, or an Alfisol, or a limed Ultisol. The overlapping of the electrical double layers on the positively charged Fe oxide particles and negatively charged minerals may have caused the release and subsequent leaching of the base cations, but inhibited the production of exchangeable acidity cations. In addition, when montmorillonite or Fe oxide-coated montmorillonite were electrodialyzed in another set of experiments, exchangeable acidity of the former was much greater than that of the latter, suggesting that the positively charged Fe oxide coatings on montmorillonite have partially neutralized the permanent negative charge on montmorillonite surfaces, decreasing exchangeable acidity. Fe oxides may function as natural anti-acidification agents through electric double-layer overlapping and coating of phylliosilicates in highly weathered soils.