Inhibition of Acidification of Kaolinite and an Alfisol Subsoil by Iron Oxides Through Electrical Double-Layer Interaction

Iron (Fe) oxides were found to inhibit natural acidification of highly weathered soils from tropical and subtropical regions. The mechanism for the inhibition of the acidification has not been well documented. Electrodialysis was used to simulate natural acidification process in soils under intensive leaching and to investigate mechanism of Fe oxides inhibiting acidification of kaolinite and an Alfisol subsoil. Results indicate that the electrodialysis treatment could release exchangeable base cations from kaolinite and phyllosilicate surfaces of the Alfisol subsoil gradually, produce exchangeable acidity, and ultimately result in strong acidification of the near-neutral kaolinite and Alfisol subsoil. Contrary to the traditional soil acidification principle, addition of Fe oxides to the kaolinite and Alfisol subsoil led to a decrease in effective cation exchange capacity (ECEC) and exchangeable acidity but inhibited the decrease in pH of the kaolinite and Alfisol subsoil during the electrodialysis process. Thus, the presence of Fe oxides could inhibit the acidification of the kaolinite and the Alfisol subsoil. The inhibition of acidification by Fe oxides was related to the amount of Fe oxides added and the surface charges per unit mass of negatively charged kaolinite or Alfisol subsoil and the positively charged Fe oxides carried. Higher content of Fe oxides and larger surface charges per unit mass resulted in a lower ECEC and exchangeable acidity but higher pH and thus weaker acidification of kaolinite or Alfisol subsoil. Changes in ECEC of kaolinite and Alfisol subsoil induced by Fe oxides were completely reversible with changes in ionic strength of the bathing solutions. These results confirm the occurrence of a diffuse layer overlapping on oppositely charged particles and support the hypothesis that Fe oxides inhibit the natural acidification process through an electric double layer overlapping in highly weathered soils.