Coatings of Fe/Al Hydroxides Inhibited Acidification of Kaolinite and an Alfisol Subsoil Through Electrical Double-Layer Interaction and Physical Blocking

Abstract

Variably charged soils confer a different pattern of soil chemical properties compared with a homogeneously charged colloidal system. The relative contents of soil mineralogical constituents and their interaction play important roles in the variation of charge characteristics and other chemical properties of the soils. In this study, coating and simple mixing of iron (Fe)/aluminum (Al) hydroxides with kaolinite and an Alfisol subsoil were made for comparative study of their effect on the electrochemical and acidity properties of kaolinite and Alfisol subsoil after electrodialysis. Transmission electron microscopy and X-ray diffraction analyses indicated that Fe hydroxide coating was more effective than their mixing in physically blocking the kaolinite surface and decreasing the peak intensity of XRD for kaolinite. Coated Fe hydroxides were also more effective than the mixed ones in shifting zeta potential in a positive value direction, decreasing effective cation exchange capacity, and inhibiting acidification of kaolinite and Alfisol subsoil, whereas there was little difference between coated and mixed amorphous aluminum hydroxide (Al(OH)₃). The effect of coated Fe/Al hydroxides on effective cation exchange capacity of electrodialyzed kaolinite and Alfisol subsoil varied irreversibly with ionic strength of the bathing solutions. All the results demonstrated that coatings of Fe/Al hydroxides could decrease effective negative charge and inhibit acidification of kaolinite and Alfisol subsoil mainly through two mechanisms: diffuse double-layer overlapping between oppositely charged particles and physical blocking of kaolinite and Alfisol subsoil during electrodialysis. Therefore, Fe/Al hydroxides can act as antiacidification agents in variably charged soils.