Solventless synthesis of bixbyite (Mn2O3) and hausmannite (Mn3O4) nanoparticles for ammonia nitrogen removal

Excessive accumulation of ammonia nitrogen (AN) in water resources poses serious health effects to humans, and control of its discharge in wastewater has been a primary global concern. Therefore, it has to be removed by robust, eco-friendly, and cost-effective approaches. In this study, Mn2O3 and Mn3O4 nanoparticles (NPs) were facilely synthesized by solid-state thermolysis, and characterized by XRD, FTIR, SEM, and TEM techniques. XRD results revealed the formation of single-phase cubic bixbyite, Mn2O3 and, tetragonal hausmannite, Mn3O4 nanoparticles exhibiting crystallite size of 27.69 and 19.38 nm, respectively. Morphological analysis shows the formation of agglomerated nanoparticles with varying shapes. The experimental isotherm results indicated that AN sorption by Mn2O3 could be well described by Freundlich model, indicating multilayer adsorption on heterogeneous surface. Contrarily, AN adsorption by Mn3O4 was consistent with the Langmuir model, representing a monolayer physisorption process on a homogeneous surface. The pseudo-first-order kinetic model fitted well to the experimental data than the pseudo-second order kinetic model. The Langmuir sorption capacities (qm) for Mn2O3 and Mn3O4 were 23.642 mg/g and 52.779 mg/g, respectively. Thermodynamic experimental kinetic data demonstrated that AN adsorption on the two forms of manganese oxides was an exothermic and spontaneous process. Regeneration experiments inferred that both Mn2O3 and Mn3O4 adsorbents could be reused.