Stability behavior of soil colloidal suspensions in relation to sequential reduction of soils

The cause of the decrease in the Fe²⁺ concentration of the soil solution in the later period of soil waterlogging was investigated. After 7-d incubation of the soil solutions separated from previously waterlogged soils (PWdS), a greyish precipitate (PPT) was observed in the soil solutions. The color of the PPT became reddish brown after separation from the solutions and freeze-drying. The PPT observed in 14-d-PWdS contained 352.6 g Fe kg^-1, 62.5 g C kg^-1, 22.6 g P kg^-1, 11.3 g Si kg^-1, 9.9 g N kg^-1, 0.7 g Al kg^-1 and a trace amount of Mn. However, Ca, Mg, K, and Na could not be detected. It was concluded that the separated PPT was dominated by amorphous ferric hydroxide based on the chemical analysis, broad IR absorption band at 585 cm^-1 and exothermic peak at 301°C. The data of chemical analysis and the characteristic IR bands of the PPT suggested that organic substances and presumably aluminosilicate anion were adsorbed onto the freshly-formed ferric hydroxide. The dominant phase of the greyish PPT in the reductive soil solution was considered to be ferrous PPT and was assumed to consist mainly of carbonate and/or hydroxide, and concomitantly of phosphate. The formation of the ferrous PPT in the soil solution in the later period of soil waterlogging was considered to (i) cause the decrease of concentration of Fe²⁺ ion and of other divalent cations such as Ca²⁺ due to the re-adsorption of Ca²⁺ on soil clays through the cation exchange reaction with Fe²⁺ ion, and consequently (ii) enhance the dispersion of the soil colloidal suspension.