Perturbation of taranakite formation by ferrous and ferric iron under acidic conditions

Taranakite is an important reaction product of monoammonium phosphate ferti lizer with soils. Its formation affects the transformation of nutrients in soils. The effect of different molar ratios of Fe(II)/AI and Fe(III)/AI on the formation of taranakite at pH 4.0 was investigated in this study. The r esults show that Fe(II) ion significantly perturbed the formation of tarana kite at the Fe/AI molar ratio of 1.2 (AI = 1 X 10(-2) M and NH4H2PO4 = 1 M) . When the Fe/AI molar ratio was increased to 2.5, the formation of taranak ite was completely inhibited by Fe(II), whereas, under the same rendition, some crystalline taranakite was still observed in the Fe(III) system. Altho ugh Fe(III) had less effect on the crystallization of taranakite than Fe(II ) at lower Fe/AI molar ratios, it also completely inhibited the formation o f taranakite at the molar ratio of Fe(III)IAI greater than or equal to 5. T he solid products formed in the Fe(III) or Fe(II) system contained a substa ntial 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 solu tion 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.