Arsenic is a naturally occurring element, but it poses a significant health risk when present in drinking water, even in extremely low concentrations. Long exposure to arsenic leads to severe neurological, reproductive and carcinogenic health issues. The element exists in two forms, which are both toxic. As^III is neutral and extremely difficult to remove and As^V is an oxoanionic species easier to remove by electrostatic interactions. This research, published in Chemical Engineering Journal, offers a breakthrough platform to remove both species effectively, which current water treatment methods struggle to achieve.

The researchers used chitosan/pectin (CHIPEC) membranes functionalised with the porous and photo-active metal organic framework UiO-66-NH₂. The combination of biocompatibility and versatility of the CHIPEC biopolymers combined with the adsorptive capacity and photooxidation properties of UiO-66-NH₂ produces a composite membrane that efficiently photo-oxidizes As^III to As^V, while adsorbing As^V, using solar energy to drive the reaction.

The membrane's great optical properties (band gap) and the abundant presence of amino functional groups make it a perfect material for solar-driven photocatalytic and adsorptive processes. Notably, it performs well even in challenging environments, such as in surface- and groundwater. Integrating UiO-66-NH₂ within the biopolymer matrix increases the efficiency of arsenic transformation and adsorption by a factor of five when comparing to using the metal organic framework (MOF) itself. This allows the membrane to detoxify water more efficiently, reducing arsenic concentrations to safe levels.

​​(a) SANS curves and respective fittings for CHIPEC and CHIPEC@UiO-66-NH2 samples. (b) Schematic representation of the observed mass fractal organization of aggregates. (c) Schematic drawing of the photooxidation and adsorption mechanism of AsIII and AsV by the CHIPEC@UiO66-NH2 composite membrane.

“Small angle neutron scattering experiments performed at SANS2D at ISIS were pivotal for us to have insights into the internal nanostructure of the CHIPEC and CHIPEC@UiO-66-NH₂ porous membranes," explains Bruna Ferreira Gonçalves from BC Materials. “Our findings indicate the presence of polydisperse systems with large agglomerates of the order of hundreds of nm, with a mass fractal organisation for both systems."

The increase in the fractal dimension of CHIPEC@UiO-66-NH₂ when compared to CHIPEC indicates the generation of a denser structure of aggregates when the MOF is integrated into the polymer. This fractal nano-structuration is likely to be related to the organisation of the polymeric nanodomains and pores accommodating the MOF nanoparticles.

“This low-cost and environmentally friendly solution offers a promising future for water treatment technologies, making arsenic detoxification more accessible and effective on a global scale," adds Bruna. “With the combination of solar-driven photooxidation, adsorption efficiency and environmentally friendly materials, this work paves the way for developing similar solutions for other persistent contaminants, contributing to a cleaner and healthier planet."

The full paper can be found at: 10.1016/j.cej.2024.154417 ​