A single step water treatment for arsenic decontamination

A team of researchers from Imperial College London led by Prof. Dominik Weiss has been working with Diamond Light Source, the UK’s national synchrotron on a new material (TiO₂/Fe₂O₃ nanomaterial) combining photocatalytic oxidation with adsorption, which allows a one-step treatment of contaminated water.  Over 50 million people in South Asia are exposed to groundwater contaminated with carcinogenic arsenic.  The team’s results were recently published in Results in Surfaces and Interfaces outlining how the high versality of Diamond’s B18 beamline allowed them to perform X-ray Absorption Spectroscopy on their samples and compare the results with their predictive models.

The researchers already have a patent for this material for arsenic decontamination and they hope that  the new material might be incorporated into a filtration column for everyday use in affected households, as this cheap and efficient technology could improve the quality of water for millions of people. .

First author of the publication, Dr Jay Bullen explains; “These experiments allowed us to directly investigate the structure of the surface complexes formed when arsenic binds to  our TiO₂/Fe₂O₃ nanomaterial. We were able to conclude that the surface complexes we had chosen for our predictive surface complexation model, using pure titania and pure iron oxide reference materials, were appropriate. However, we also saw some tridentate As-Fe₂O₃ bonding in the EXAFS data that wasn’t included within our earlier model. We were also able to confirm surface precipitation of As(III) at low pH, which our model had predicted.”

Arsenic is a famously toxic element and long-term exposure to even trace amounts can lead to debilitating and potentially fatal diseases including skin cancer, lung cancer, keratosis, and neurological disorders. 100-200 million people globally are believed to be exposed to arsenic through contaminated groundwater drinking supplies. Water multi-step treatments to remove arsenic already exist, using filtration (adsorption), but this process requires pre-treatment (oxidation) to be efficient .

Dr Jay Bullen, “continues: We wanted to better understand the nature of arsenic adsorbed onto this composite TiO₂/Fe₂O₃ nanomaterial; to understand whether arsenic binds in the same way as onto pure TiO₂ and pure Fe₂O₃ minerals. X-ray Absorption Spectroscopy (XAS) is the perfect way to do this, with EXAFS revealing information about the local environment of arsenic atoms, e.g. how many covalent bonds arsenic forms with the TiO₂/Fe₂O₃ surface. The beamtime grant we received allowed us to realise these experiments at Diamond Lightsource.”

Spectroscopic observations to confirm a predictive model

Arsenic can be found in several forms in water including arsenite and arsenate. Arsenite (i.e. As(III)) ions are difficult to remove from contaminated water using conventional treatments such as adsorption or coagulation-flocculation due to its neutral charge (H₃AsO₃). In contrast, arsenate (i.e. As(V)) ions (HAsO₄^2- and H₂AsO₄^-) are both (i) more easily adsorbed and (ii) less toxic than H₃AsO₃. Consequently, the treatment of As(III)-contaminated water benefits from the oxidation of As(III) to As(V) prior to its removal via adsorption. Oxidation of As(III) can be achieved using heterogeneous photocatalysts, such as TiO₂, and ultraviolet (UV) radiation. However, other materials such as iron oxides (Fe₂O₃) are more efficient for adsorption. Therefore multiple-step treatment plants are currently used but their design and maintainance  is challenging.

This difficulty may be overcome by incorporating photocatalytic and adsorption capabilities into a single material. Previous studies by the same group showed that composite materials combining the excellent photocatalytic capabilities of TiO₂ with the high As(V) adsorption capacities of iron oxides (Fe₂O₃) were good candidates for efficient decontamination.  The authors subsequently developed a surface complexation model (SCM) to predict changes in the amount of arsenic adsorbed and its speciation as a function of experimental variables such as pH. However, the authors wanted to verify that the structures of the adsorbed arsenic surface complexes chosen for the model were realistic, so they used spectroscopic techniques. X-ray absorption spectroscopy played a key role thanks to its element selectivity, its capability to discriminate As oxidation state (XANES) and to identify the structure of the As complexes adsorbed on the TiO₂/Fe₂O₃ surface (EXAFS).

From a model to an application

The team used B18 to realise their experiments.  At the beamline, they were able to record EXAFS spectra of solid samples to determine how many covalent bonds arsenic forms with the surface of the materials, and XANES spectra of aqueous suspensions to measure photooxidation kinetics.

Related publication:

Bullen J.C, et al. Spectroscopic (XAS, FTIR) investigations into arsenic adsorption onto
TiO2/Fe2O3 composites: Evaluation of the surface complexes, speciation and precipitation predicted by modelling. DOI: https://doi.org/10.1016/j.rsurfi.2022.100084