Spectroscopic (XAS, FTIR) investigations into arsenic adsorption onto TiO/FeO composites: Evaluation of the surface complexes, speciation and precipitation predicted by modelling

published in Results in Surfaces and Interfaces  

Photocatalyst-adsorbent composite materials are popularly developed for removing arsenic in a single-step water treatment. Here, As(III) is oxidised to As(V), which is subsequently removed via adsorption. We previously developed a component additive surface complexation model (CA-SCM) to predict the speciation of arsenic adsorbed onto TiO ₂/Fe₂O₃ under different environmental conditions, using surface complexes taken from studies of single-phase minerals. In this work, we critically evaluate this approach, using experimental observations of the surface structures of arsenic adsorbed onto TiO ₂/Fe₂O₃. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicates significant As(III) surface precipitation, and the possible formation of tridentate ³C complexes. EXAFS was unable to identify As binding modes for TiO ₂ and Fe₂O₃ surface complexes simultaneously, highlighting the challenge of analysing composite surfaces. FTIR and zeta potential analysis indicate that As(III)-Fe₂O₃ surface complexes are protonated at neutral pH, whilst As(III)-TiO ₂, As(V)-Fe₂O₃ and As(V)-TiO ₂ surface complexes are negatively charged. Our study confirms the speciation predicted by CA-SCM, particularly As(III) surface precipitation, but also introduces the possibility of tridentate As(III) at acidic pH. This study highlights how experiment and modelling can be combined to assess surface complexation on composite surfaces.