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Retention of toxic pollutants by nanomagnetite aggregates

Periodic Reporting for period 1 - REPONANO (Retention of toxic pollutants by nanomagnetite aggregates)

Reporting period: 2021-03-01 to 2023-02-28

REPONANO addresses the fundamental scientific issue of the decontamination of drinking and waste water, focusing on the role of nanomagnetite in the immobilisation of contaminants that have started gaining a worldwide attention (i.e. As, Sb and U). Controlled experimental systems mimicking in-situ conditions are necessary to study the geochemical processes controlling Fe transformation and subsequent retention or release of contaminants. While the immobilisation of toxic elements by various Fe (hydr)oxides (e.g. ferrihydrite, hematite) has been widely studied, retention by magnetite is far less documented. The aim of the project is to study the combined interactions of physical and geochemical processes regulating the fate of most redox sensitive elements (focusing on As, Sb and U), in a controlled but complex setting, representative of natural systems, and to investigate the spatial distribution of the phases produced by nanomagnetite reduction. Apart from the conventional batch sorption experiments, a novel approach will be applied, using a new kind of submicron synthetic aggregate (i.e. PEGDA/nanomagnetite) in a microfluidic set-up. Alternative input solutions will be used, introducing for the first time a mixture of important contaminants to study their competitive behaviour, in addition to acidic, phosphate-rich waste leachates that pollute many coastal areas worldwide. Providing fundamental knowledge on the abiotic reduction mechanisms of trace elements, is crucial not only for setting new remediation strategies, but also for the human health, such as the development of therapeutic agents (e.g. Se nanoparticles tested to treat cancer cell spheroids, shown to behave amazing similarities to soil aggregates), contributing to the enhancement of the European competitiveness and excellence.
The work included a research program that brought together forefront knowledge on surface chemistry, mineral-water interaction, physical processes, environmental waste management, and waste/drinking water contamination. The activities carried out over the course of the project were directed along the three main objectives/work packages:
WP1. Study the retention potential of redox sensitive contaminants by (nano)magnetite, by performing a stepwise reduction of the ions of interest in order to develop a reactive barrier as a novel remediation technique. This work package was divided into two experimental parts:
a) We performed batch sorption experiments targeting toxic, acidic, and phosphate rich waste leachates. We focused on the effect of magnetite on U reduction, and As and Sb oxyanion removal, also, addressing the problems of high acidity, and high phosphate concentration, which are the major inhibitors of the treatments proposed so far. As such, we set the basis for an industrial process to decontaminate the aforementioned effluent waters by (i) first, using ZVI to increase the pH of the water while producing Fe2+, (ii) the precipitating phosphate ions as vivianite, and (iii) finally, using magnetite derived from steel industry wastes to immobilize toxic pollutants under different pH conditions and high phosphate concentrations. This three-step process provides new insights into a new ‘green’ route for the decontamination of drinking and waste waters.
b) We performed a series of flow-through controlled experiments through cylindrical aggregates in a microfluidic set up involving cylindrical structured PEGDA/nanomagnetite aggregates to obtain Break Through Curves (BTC) of the contaminants of interest. Antimony solution made with KSb(OH)6 at various pH values is used as the input solution. We showed the efficiency of the system and a significant retention of Sb depending on the initial concentration and the material of the aggregates.
WP2. Shed light into the driving processes during the microfluidics experiments and to investigate the contaminants’ uptake efficiency and mechanisms. We fully characterised the produced phases, following a multi-technique approach of synchrotron radiation based micro-probe techniques, such as X-ray absorption near-edge structure (XANES) and EXAFS (Extended X-Ray Absorption Fine Structure), combined with x-ray micro-fluorescence (µ-XRF) to map the diffusion of Sb within the PEGDA/nanomagnetite aggregates. We performed microfocus elemental and redox mapping of Sb on these aggregates, measuring its diffusion, and reduction particularly in their core to investigate the temporal distribution and the oxidation state of the reduced aqueous species of Sb produced by nanomagnetite reduction. We showed that Sb(V) reduction to Sb(III) is more efficient at low Sb initial concentrations, but the overall Sb removal from solution is increasing with the respective initial concentration.
WP3. Adaptation of a numerical (3D) reactive transport model to describe the μXAS and BTC measurements, in order to validate and compliment the experimental study. This model allows us to define the areas dominated by either diffusing or advective solute transport, identify the reaction zones and estimate the effect of kinetic parameters. The model will then be applied to the interpretation of natural system observations and to set up new water treatments based on such macroscopic devices.
During the course of the project, we extended our research network by collaborating with different European research groups in a national and international level. The results of the project were disseminated in various seminars organised by the host institution, in two workshops, and in four international conferences. One article is already accepted as a result of the work done for this project and enough material is produced for two more articles, already under preparation.
We have addressed the problematic of drinking and waste water contamination that has gained a worldwide attention, forcing the scientific community to focus on the search for innovative decontamination techniques. This project led to the development of novel remediation strategies of some of the most problematic, redox-sensitive contaminants worldwide, such as As, Sb and U. All three contaminants are included in the priority pollutants list of various environmental agencies worldwide, as excess intake by humans causes severe toxicity related health problems. In addition, we addressed the problems of high acidity, and high phosphate concentration, which are the major inhibitors of various treatments proposed so far, and we developed a novel three-step treatment process, providing new insights into a new ‘green’ route for the decontamination of drinking and waste waters, using materials from the recycling industry. The results of this project provided fundamental knowledge on the abiotic reduction mechanisms of trace elements, which is crucial not only for setting new remediation strategies for the drinking and wastewater, but also for the human health, such as the development of therapeutic agents (e.g. Se nanoparticles tested to treat cancer cell spheroids, shown to behave amazing similarities to soil aggregates), contributing to the enhancement of the European competitiveness and excellence.
A novel remediation strategy for the removal of As, Sb and U from acidic, phosphate-rich leachates
Microfluidic channel made of NOA and flow-through system