Sequential precipitation of a new goethite–calcite nanocomposite and its possible application in the removal of toxic ions from polluted water

► A simple and innovative synthesis route for goethite–calcite nanocomposite. ► Nanosized goethite particles adhered onto sub-micrometric calcite. ► Goethite–calcite composite has a well sequestration capacity for toxic ions. This study proposes a simple and innovative synthesis route for a goethite–calcite nanocomposite. This synthesis is summarised by three sequential precipitation reactions: (1) precipitation of nanosized acicular goethite (α-FeOOH) using a high OH/Fe molar ratio (=5); (2) instantaneous precipitation of portlandite (Ca(OH)2) by adding CaCl2 salt to a goethite alkaline suspension (2NaOH+CaCl2→Ca(OH)2+2NaCl) and; (3) sub-micrometric calcite precipitation by injection of CO2 into a goethite–portlandite alkaline suspension (Ca(OH)2+CO2→CaCO3+H2O). The XRD patterns have confirmed the goethite and calcite mineral composition in the composite precipitated at 30 and 70°C. FESEM and TEM observations have revealed the formation of nanosized goethite particles well dispersed with sub-micrometric calcite particles, leading to an orange–brown colour nanocomposite with high specific surface area of around 92m2/g for a composite synthesized at 30°C and 45m2/g for a composite synthesized at 70°C. Both values were determined using the conventional BET method on N2 sorption isotherms. Finally, a goethite/calcite weight ratio equal to 0.8 in the composite was determined by Thermogravimetric Analysis (TGA). Additionally, some adsorption experiments carried out at two different pH values revealed that the goethite–calcite composite has a good sequestration capacity for Cu>Cd>As(III)>Se(IV)>As(V). Conversely, the Se(VI) did not show any chemical affinity with the goethite–calcite composite under the physico-chemical conditions studied. In practice, the goethite–calcite composite can neutralise acidic wastewater by slight calcite dissolution, enhancing the removal of heavy metals (e.g. Cu and Cd) at the calcite–solution interfaces.