Ferric Chloride Complexes in Aqueous Solution: An EXAFS Study

Ingmar Persson

doi:10.1007/s10953-018-0756-6

Abstract

A series of concentrated aqueous solutions of ferric chloride with different chloride:iron(III) ratios has been studied by means of EXAFS to determine the structure around the iron(III) ion of the dominating species in such solutions. The dominating species in dilute acidic aqueous solution of ferric chloride, at less than 1 mmol·dm−3, are the hydrated iron(III) and chloride ions, while in concentrated aqueous solution and in solutions with an excess of chloride ions, up to 1.0 mol·dm−3, it is the trans-[FeCl2(H2O)4]+ complex. Possible higher chloroferrate(III) or dimeric [Fe2Cl6] complexes at room temperature, as proposed in the literature, were not observed in any of the studied solutions in spite of an excess of chloride ions of 1 mol·dm−3.

Highlights

Ferric chloride, or iron(III) chloride, is a very important chemical used in several branches of industry [1,2,3,4]
Including industrial water treatment applications and its use in pre-treatment of seawater prior to desalination, the total water treatment accounts for approximately 80% of the total demand of ferric chloride globally [1]
Other uses of ferric chloride are as a leaching agent in chloride hydrometallurgy [2], for example in the production of silicon from FeSi [3], and as catalyst for the reaction of ethylene with chlorine forming ethylene dichloride (1,2-dichloroethane) [4], an important commodity chemical mainly used for the industrial production of vinyl chloride

Summary

Introduction

Iron(III) chloride, is a very important chemical used in several branches of industry [1,2,3,4]. Wertz and co-workers reported that different species are acquired, even for solutions having the same concentration, if prepared from different starting materials, i.e. anhydrous ferric chloride and ferric chloride hexahydrate, respectively [13, 14] They proposed that hydrated iron(III) ions, [Fe(H2O)6]3?, are present in a freshly prepared solution of FeCl3Á6H2O, without any complex formation, but octahedral chloro complexes [FeClx(H2O)6-x](3-x)? The literature survey of the speciation of the ferric chloride system in aqueous solution, summarized above, shows clearly a scattered picture of the speciation and the structure of the dominating species in this system This is only the second EXAFS (extended X-ray absorption fine structure) spectroscopy study applied on this system. Fe K-edge EXAFS spectra have been collected in order to improve the knowledge of the structure of the chemical species present in these aqueous solutions of iron(III) chloride with different concentrations and iron(III):chloride ratios, that are widely used industrially

Preparation of Samples

Results and Discussion

Conclusions