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Sustainable granular materials improve removal of natural organic matter, turbidity and microplastics during adsorption, ballasted flocculation and granular filtration
Summary
Scientists coated everyday materials like sand, recycled glass, and treatment plant grit with iron to create a cheap, reusable way to clean drinking water, and it worked impressively well, removing up to 92% of microplastics along with other contaminants and organic matter. This matters because it could make water treatment plants both more effective at filtering out microplastics (which we're increasingly exposed to and still learning about the health effects of) and more environmentally friendly, since it reduces reliance on costly, single-use chemicals and creates less toxic sludge waste.
Single-use metal-based coagulants, high molecular weight flocculants, engineered adsorbents, and porous media are key materials for several processes in water treatment. However, many of these materials are often expensive, unsustainable, and can increase the amount of sludge produced during the water treatment process. Moreover, the accumulation of toxic metals and synthetic flocculants limits sludge reusability as a fertilizer in agriculture. Herein, we propose reusable, low-cost and sustainable modified grains that can be simultaneously used as positively charged adsorbents to improve removal of soluble matter (natural organic matter), and ballast media to improve removal of particulate matter (turbidity or total suspended solids). Three different starting materials were used to synthesize these modified grains: pristine sand, recycled crushed glass, and grit extracted from a wastewater treatment plant, all of which were grafted with iron (hydr)oxides (Fe surface coverage of 13 and 81%, synthesized with 0.02 and 2 M Fe, respectively). The modified grains were then employed as (i) adsorbents to remove natural organic matter, (ii) ballast media during flocculation to increase floc size and density, and (iii) filtration media for simultaneous removal of natural organic matter and turbidity. Compared to conventional treatment alone (coagulation and flocculation), the incorporation of modified grains simultaneously used as adsorbents and ballast media increased removal of natural organic matter (up to 16%), microplastics (up to 92%), and turbidity (< 1 NTU, with settling rate 18 times faster). Ultimately, modified grains could be used in existing ballasted flocculation processes – replacing conventional sand – to provide an additional NOM removal. Iron-grafted sand, glass, and grit – simultaneously used as adsorbent and ballast media – reduced the settling duration by 18 times compared to conventional treatment (without iron-grafted media) and increased removal of natural organic matter (up to 23%) and microplastics (up to 92%). • Three different starting materials were used to synthesize Fe-coated grains. • Fe surface area coverage of 81% was obtained. • Modified grains were employed as adsorbents and ballast media • Modified grains media increased removal of microplastics (up to 92%). • Modified grains could be used in flocculation processes to provide NOM removal.