A single step water treatment for arsenic decontamination

The authors combined spectroscopic data obtained at Diamond Light Source (EXAFS) with FTIR and zeta potential measurements at Imperial College London to develop a picture of the dominant surface complexes formed when arsenic is adsorbed by TiO2/Fe2O3 composite nanomaterials. This was used to evaluate and verify the speciation of adsorbed arsenic predicted by the authors’ previous Surface Complexation Model (SCM), a theoretical model that is able to predict changes in adsorption as a function of environmental conditions. This model had been developed using surface complexes chosen based on data for pure titania and pure iron oxides, with experimental evidence for arsenic speciation on composite TiO2/Fe2O3 not being available prior to the Diamond Lightsource study.
Arrows indicate where the pH is increased from 5 to 9. Asterixes (*) and daggers † indicate evidence gained by EXAFS and FTIR/zeta potential analysis respectively. Image reused from DOI: 10.1016/j.rsurfi.2022.100084 under the CC BY 2.0 license.

Arsenic can be found in several forms in water including arsenite and arsenate. Arsenite (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 (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 maintenance 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 (XAS) 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).

Dr Bullen, first author of the publication, explains:

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.

Dr Bullen continues:

The team hopes 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.  The researchers already have a patent for this material for arsenic decontamination.

To find out more about the B18 beamline or discuss potential applications, please contact Principal Beamline Scientist Diego Gianolio: [email protected].