Perturbation of taranakite formation by ferrous and ferric iron under acidic conditions
2000
Zhou, J.M. | Liu, C. | Huang, P.M.
Taranakite is an important reaction product of monoammonium phosphate fertilizer with soils. Its formation affects the transformation of nutrients in soils. The effect of different molar ratios of Fe(II)/Al and Fe(III)/Al on the formation of taranakite at pH 4.0 was investigated in this study. The results show that Fe(II) ion significantly perturbed the formation of taranakite at the Fe/Al molar ratio of 1.2 (Al = 1 x 10(-2) M and NH4H2PO4 = 1 M). When the Fe/Al molar ratio was increased to 2.5, the formation of taranakite was completely inhibited by Fe(II), whereas, under the same condition, some crystalline taranakite was still observed in the Fe(III) system. Although Fe(III) had less effect on the crystallization of taranakite than Fe(II) at lower Fe/Al molar ratios, it also completely inhibited the formation of taranakite at the molar ratio of Fe(III)/Al greater than or equal to 5. The solid products formed in the Fe(III) or Fe(II) system contained a substantial amount of Fe(III) and a much higher proportion of phosphate than that was required for the formation of NH4-taranakite. As indicated by the solution phase analysis at the end of the experiment, more Fe ions were present in the solution in the Fe(II) system, compared with the Fe(III) system, to perturb the nucleation and crystallization of taranakite. Since iron is a very common element in soil, taranakite formation may be perturbed in soils with high Fe content, especially under reduced and acidic conditions.
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