Most municipal water treatments use biological processes to break down pollutants. But the size and low concentration of these MPs – µg or ng per litre – limits the effectiveness of these processes, meaning that MPs don’t degrade predictably, if at all. A promising alternative approach is to introduce a high surface area material that can ‘soak up’ the pollutants via adsorption. Granular activated carbon (GAC) is widely used in this way, but it too has limitations. For example, large molecules like fats and proteins that are often present in wastewater can clog its pores, inhibiting adsorption of MPs.
Researchers from Northwestern University say that they’ve found a better option: Crumpled graphene balls (CGBs), 1-5 µm in size, made from spray-dried graphene oxide sheets. In an upcoming paper in Carbon [DOI: 10.1016/j.carbon.2021.07.081], they report on the adsorption performance of these graphene balls to a range of micropollutants at different environmental conditions.
They chose eight micropollutants commonly found in waterways, representative of a number of different sectors. Some were pharmaceutical (e.g. carbamazepine and sulfamethoxazole), while others are common in foods (e.g. caffeine and sucralose). These were added, individually and in various mixtures, to water with properties (e.g. pH, hardness) typical of freshwater sources.
Despite having a specific surface area six times lower than GAC, the graphene balls had a greater adsorption capacity for seven of the eight MPs, when tested individually. One sucralose was absorbed more readily by the activated carbon. The authors suggest that this is partly explained by the fact that their CGBs – produced at a lower temperature than others reported in the literature – are particularly hydrophilic. This could mean that the “CGB surface resists aggregation, disperses better in water and hence, provides more interactions with MPs.” And the graphene balls displayed similarly high adsorption capacities when the MPs were present in a mixture.
In addition, this adsorption happened rapidly, and was largely undiminished under various environmental conditions. The team say that “on an equivalent surface area basis….six of eight MPs were fully removed in the first 15 minutes, Metolachlor [a herbicide] reached 90% removal and …sucralose reached 60% removal.” By comparison, GAC’s adsorption kinetics were relatively slow over the initial 15 minutes, “taking more than 24 hours to reach equilibrium.” The researchers then added macromolecules – e.g. lignin and degraded tannins – to the mix, in order to test the graphene balls in more realistic aqueous environments. They found that at high concentrations of macromolecules, the adsorption of four of the eight MPs was reduced by less than 20 %. There was no effect on the adsorption of the other four micropollutants. In contrast, they say “the maximum MP adsorption capacity by GAC was reduced from 35% to 80% with increasing [macromolecule] concentrations.”
So could this be a new super-adsorbent? We’ll have to wait and see.
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This story originally appeared in Materials Today: https://www.materialstoday.com/carbon/news/sucking-up-pollutants-with-crumpled-graphene/
Research paper: Han Fu, Jiaxing Huang, Kimberly Gray. “Crumpled Graphene Balls Adsorb Micropollutants from Water Selectively and Rapidly”, Carbon, In Press, 30 July 2021. DOI: 10.1016/j.carbon.2021.07.081
