Effect of fulvic acid on the crystallization of Fe(III) oxides

Abstract

Effects of increasing the pH and of adding increasing amounts of fulvic acid (FA) on the crystallization of goethite and hematite from aqueous suspensions of ferrihydrite were investigated. The crystallization of ferric oxides from Fe(III)—FA complexes with widely differing molar Fe(III)/FA ratios was also attempted. The most important factors governing the crystallization of FE(III) oxides are the form in which the Fe(III) occurs initially and how much FA is present. If Fe(III) is present as ferrihydrite and FA is added, the crystallization will depend on the FA concentration and the pH. FA concentrations of the order of o.5 g/l will favour the crystallization of hematite over that of goethite, but FA concentrations of 5.0 g/l will completely inhibit crystallization Increasing the pH from 4 to 10 tends to favour crystallization. If the Fe(III) is present as a Fe(III)FA complex, it is unlikely that Fe(III) oxides will crystallize. Failure to detect substantial concentrations of crystalline oxides in soils developed under cool temperate climates, although total Fe-concentrations are relatively high, is in part due to the adsorption of sufficient concentrations of FA and other organic materials on ferrihydrite surfaces. Also, the presence of relatively large amounts of FeFA and other Feorganic complexes can suppress and possibly inhibit the crystallization of Fe(III)-oxides.