New insight into the microstructure of natural calcined laterites and their performance as heterogeneous Fenton catalyst for methylene blue degradation

Abstract

In this work, natural laterites from Burkina Faso were calcined at different temperatures in the range 400–800 °C. XRD, EDS, N2 adsorption manometry and TGA–DSC analyses together with a thorough analysis of available literature data, support that the laterite sample consists mainly of goethite and hematite embedded in a framework of kaolinite and quartz with different interaction extents. During calcination up to 400 °C, the kaolinite structure is partially destroyed due to the loss of structural water and the goethite phase previously in association with the kaolinite lattice transforms into free hematite crystallites, resulting in a drop of the surface area of the composite material. At 600 °C, the kaolinite lattice is further deshydroxylated leading to an amorphous metakaolin matrix containing mainly surface hematite nanophases. Despite a substantial reduction of the surface area due to the calcination-induced shrinkage of alumino-silicate framework, the laterite calcined at 600 °C exhibits the highest performance in the methylene blue (MB) degradation by the Fenton process with a degradation percentage of 99% after 100 min of treatment at room temperature. The effects of solution pH, H2O2 concentration, initial MB concentration and catalyst dosage were investigated. A slight but significant visible-light-induced removal enhancement effect suggested a visible-light photocatalytic activity of the hematite phase. This calcined laterite demonstrates a strong catalytic stability over several utilizations therefore is worth to be seriously considered in the design of sustainable and readily affordable wastewater treatment solutions in developing tropical countries.