One-pot synthesis and characterization of FeII–FeIII hydroxide (green rust) intercalated with C9–C14 linear alkyl carboxylates

Layered FeII–FeIII hydroxides (green rusts, GR) intercalated with monovalent aliphatic carboxylate anions, called GR-carboxylates, possess hydrophobic interlayer regions which can adsorb non-polar organic compounds. GRs are well known reductants and hence GR-carboxylates may have the potential to detoxify adsorbed compounds, such as chlorinated solvents. GR-carboxylates, GRCX, for the first time, were precipitated directly by aerial oxidation of an ethanolic solution containing FeSO4 and linear alkyl carboxylate ions synthesis condition using monovalent carboxylic acids with X, the total number of carbon atoms in the acid, varying from 5 (pentanoic acid) to 14 (tetradecanoic acid), synthesis conditions were tested. GRCX formed for X ≥ 9. The syntheses were carried out at constant pH of 8 using 1 M NaOH as neutralizing agent, and with a maximum duration of 3.5 h. The basal spacings of GRCX increased with X and they were 2.68 nm (GRC9), 2.94 nm (GRC10), 3.59 nm (GRC12) and 4.16 nm (GRC14). The Mössbauer spectra measured at 80 K of different GRC12 synthesized under identical chemical conditions resulted in hyperfine parameters close to those of other GRs. The FeII/FeIII ratios of the final products (R) ranged between 1.4 and 2.8 demonstrating that FeII in GRCX can be partly oxidized without degradation of the structure. R at various stages of the synthesis decreased from 5.2 at the start of the reaction to 2.8 at the end of synthesis. In the specific case of GRC12, the optimum initial solution concentrations were [C12]0 = 25 mM, [FeII]0 = 30 mM and 40% (v/v) EtOH/H2O.

Research Highlights►Green rusts intercalated with linear carboxylic acids, GRCX, was for first time prepared directly from aerial oxidation of an ethanolic solution containing FeSO4 and linear alkyl carboxylate ions. ►The one-pot synthesis has shown to be not only more effective compare to direct ion-exchange but it is also faster with a reaction time of 3.5 h as maximum. ►Even though the syntheses were carried out in presence of sulphate, with stronger electrostatic attraction to the layers of GR, GR carboxylates were the major products with negligible formation of GRSO4 except if high ethanol contents were used (> 60 % v/v). ►The intercalation of of the long C12 molecules did not cause deviations in the bonding of Fe in the metal hydroxide layers compared with GRSO4 and ion exchanged GRC12 as seen from Mössbauer spectroscopy. ►The FeII/FeIII ration of GRC12 were more variable than typical R values for GR’s reflecting that the oxidation state of Fe in the GRCX can vary without degradation of the GR structure.