Molecular adsorption by biochar produced by eco-friendly low-temperature carbonation investigated using graphene structural reconfigurations

Biochar is a promising material for environmental amendment. However, the adsorption mechanism of nitrogen-based ionic species, particularly the pH dependence, is still under debate because of its high susceptibility to experimental conditions. In this study, the adsorption of ammonium sulfate by rice-husk biochar produced by low-temperature carbonation as an eco-friendly approach was studied using Fourier-transform infrared (FT-IR) spectroscopy, C-13 magic-angle spinning (MAS) nuclear magnetic resonance (NMR), and openspace analysis using positrons coupled with molecular simulations. In the aqueous solution with pH of 5.38, graphenes are isolated exposing fresh surfaces, which physisorb NH4+ via interaction with pi electrons rather than surface functional groups. Furthermore, the low-temperature carbonation leads to a lower degree of graphitization with grain boundary defects in graphene, triggering the fracture of graphene during the shaking process in an aqueous solution. SO42- physisorption occurs as an outer-sphere surface complex on the positive charge caused by a charge transfer of similar to 2.5% from the terminal hydrogen at the graphene edge. A decrease in aqueous pH by similar to 0.7% significantly changes the above adsorption properties both on the surfaces and at the edges: diminishment of physisorption and transition to chemisorption by the formation of an inner-sphere surface complex.