Pyroaurite-like phases (Mg-Fe3+ LDH) synthesis and solubility

Abstract

This study explores different methods for precipitating carbonated Mg-Fe³⁺ layered double hydroxides (LDHs) with Mg/Fe³⁺ ratios ranging from 2 to 2.5. Two synthesis approaches were investigated: a) CO₃-Mg-Fe³⁺-LDH co-precipitated directly from MgO, Fe(NO₃)₃ and Na₂CO₃ and b) NO₃-Mg-Fe³⁺-LDH precipitated by adding NaOH to Mg(NO₃)₂-Fe(NO₃)₃ solutions, followed by ion exchange with Na₂CO₃ to form CO₃-Mg-Fe³⁺-LDH. The solids were characterised using TGA, XRD and FTIR. For the co-precipitated samples in the presence of Na₂CO₃, the formation of carbonate-LDH was accompanied by ~10–30 wt% of hydromagnesite, while CO₃-Mg-Fe³⁺-LDH modified from the NO₃-Mg-Fe³⁺-LDH did not show any other Mg‑carbonates but contained Fe hydroxides, as evidenced by their brown coloration. All samples were re-equilibrated at 7, 20 and 50 °C. The solubility product (log Kso) was calculated from solution analysis, and preliminary thermodynamic data are presented. The findings provide insights into the stability and formation conditions of carbonated Mg-Fe³⁺ LDH phases in carbonate-rich environments.