Abstract
In this study, a series of synthetic ferrihydrite samples of Si/Fe molar ratios ranging from zero to 1.5 were heated up to 1000 °C using simultaneous TG–DTA equipment. The XRD, FTIR, SEM–EDS and magnetic susceptibility measurements were carried out prior to and after heating. It has been found that silicate retards ferrihydrite transformation to hematite and affects crystallinity of the product. Low Si admixture in the precursor reduces hematite crystal size severely, but the increase in average crystal dimensions with increasing Si/Fe molar ratio was observed. High Si content results in the formation of hematite which exhibits a wide range of crystal habits. The conversion of pure ferrihydrite to hematite proceeds without any intermediate phase, whereas the increasing silicate content in the pristine oxyhydroxide strongly affects the transformation pathway. During annealing of high-Si ferrihydrites, the presence of two or three intermediate Fe2O3 polymorphs (gamma, epsilon and beta) was demonstrated prior to the crystallization of final α-Fe2O3. The conditions favoring crystallization of intermediate phases result from progressive silica polymerization which forms separate matrix-type phase and impedes the aggregation of iron oxide nanoparticles.
Highlights
Ferrihydrite (Fe5HO8Á4H2O) is a reddish-brown, poorly ordered iron oxyhydroxide, with variable and non-stoichiometric composition [1–3 and references therein]
The samples were divided into three groups: pure ferrihydrite (PFh), low-Si ferrihydrites (LSFh; samples FHYD-005, FHYD-010 and FHYD-020) and high-Si ferrihydrites (HSFh, samples FHYD-050, FHYD-075, FHYD-100 and FHYD-150)
A boundary between the groups of Si-ferrihydrites is 0.2 Si/Fe molar ratio, which is equal to the number of ferrihydrite adsorption sites, estimated to be 0.2 mol mol-1 Fe [19]
Summary
Ferrihydrite (Fe5HO8Á4H2O) is a reddish-brown, poorly ordered iron oxyhydroxide, with variable and non-stoichiometric composition [1–3 and references therein]. The most common and well-documented impurities include silicate, phosphate, arsenate, sulfate, calcium, aluminum and organic compounds [e.g., 1–3, 14–16] These ions affect ferrihydrite composition, surface chemistry, reactivity and sorption properties [14, 17,18,19,20,21,22]. The opposite observation [27] suggested that enhanced adsorption of silicate ions to the ferrihydrite surface caused the increase in the nanoparticle dimensions These ambiguous results might result from the character of the observed objects—Cismasu et al [17] noted the decrease in individual ferrihydrite particle sizes, but the oxyhydroxide aggregates appeared more compact. It may be explained by the increasing oxyhydroxide aggregation and the enhanced compactness of the aggregates, which permitted the penetration of nitrogen molecules during BET measurements
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
