Access to high quality water is one of the greatest global problems. More than two billion people are exposed to water scarcity and lack of access to drinking water. In addition to growing demand and scarcity exacerbated by climate change, the pollution of available freshwater sources, which has increased over the past two decades, is one of the major challenges to be faced. According to the United Nations, around 400 million tons of heavy metals, solvents, toxic sludge and other industrial waste enter our planet’s waters every year.
New pollutants that are classified as new are not removed by existing water treatment technologies. Because of this, researchers around the world have been looking for more efficient alternatives that use new materials. In Brazil, different research groups work with a variety of techniques. On this World Water Day (3/22) I would like to highlight two works that, although very different, have the adsorption process in common (adhesion of molecules in a liquid to a) solid surface).
Elias Paiva Ferreira Neto has been searching for nanomaterials that can break down pollutants through photocatalysis (photocatalysts) since completing his doctorate at the group for hybrid and inorganic materials at the São Carlos Institute of Chemistry at the University of São Paulo (USP). In photocatalysis, light from a natural (sunlight) or artificial source, when absorbed by the catalyst material, triggers chemical reactions that can convert organic pollutants – such as the methylene blue dye tested by Ferreira Neto in his most recent research – and inorganic pollutants. as heavy metals – in harmless substances or at least much less toxic to the human organism.
The researcher examined the molybdenum disulfide (MoS2) photocatalyst. For use in real situations, however, it was necessary to build macroscopic objects with the properties of nanomaterial. After completing her doctorate in the laboratory for photonic materials at the Chemistry Institute of the Universidade Estadual Paulista (Unesp), Ferreira Neto found the solution in one of the research group’s specialties, bacterial cellulose.
By combining the two approaches – by coating the bacterial cellulose with a layer of the photocatalyst – the researcher came up with a membrane that could decontaminate the water flowing through it and remove organic and inorganic pollutants through filtration and degradation. One of the main advantages of the new material is the possibility of reuse as many of the existing alternatives have to be applied as a powder or suspension, which prevents recovery after use.
The bacterial cellulose produced by some types of bacteria forms a hydrogel made of 99% water, which after a controlled drying process becomes an airgel. In the airgel, water is replaced by air, which leads to a very porous structure, which at the same time allows water to pass through and holds back pollutants through adsorption. When trapped in the membrane, the pollutants are broken down by the photocatalyst.
In the tests already carried out, the membrane removed 96% methylene blue and 88% of the carcinogenic metal chromium VI (chromium in oxidation state VI, based on its electrical charge) from the water during a two-hour experiment. both are common, for example, in industrial wastewater from textile and leather production. Work continues in the search for greater efficiency – including using other photocatalysts and testing other impurities – and also characterizing the products that result from degradation.
“Our results show the importance of scientific collaboration, as they would not have come about if we had not combined the specialties of two different groups, working with photocatalysts in the case of USP and bacterial cellulose at Unesp,” emphasizes Ferreira Neto.
The second research was also carried out in partnership between the Laboratory for Polymeric and Biosorbing Materials at the Federal University of São Carlos (UFSCar) and the Laboratory for Integrated Sciences at the Federal University of São Paulo (Unifesp).
The adsorbent material used was sugar cane bagasse, one of the main remnants of Brazilian agribusiness, which resulted from activity in the ethanol and sugar factories. The bagasse – a biosorbent, adsorbent of biological origin – was used in a composite material with synthetic magnetite nanoparticles, whereby the properties of the bagasse adsorbent were combined with the magnetic properties of the nanoparticles.
The bagasse retains the pollutants through adsorption – in the case of this study, copper and the same chromium VI were tested – and is then removed from the water of the composite by the action of a magnet that attracts the magnetic nanoparticles. In this case, too, the research group continues to test this and other biosorbent composites with regard to the retention capacity of other organic and inorganic molecules. Research has also shown the potential to remove oils from the surface of the water in the event of a spill.