Nitrate and phosphate removal from water using a novel cellulose-based anion exchange hydrogel

Abstract

Excessive nitrate and phosphate in freshwater threaten aquatic ecosystems and human health. Anion exchange resins commonly used to remove these nutrients are typically derived from synthetic polystyrene-based polymers and produced using toxic chemicals and flammable solvents. To address this challenge, this study developed and validated a safer, less toxic cellulose-based anion exchange hydrogel (CAH) for phosphate and nitrate removal from water. The CAH was synthesised using renewable cellulose as the backbone and functionalised via grafting with methylene bisacrylamide (MBA) as a crosslinker and (2-(methacryloyloxy) ethyl) trimethylammonium chloride (MTAC) as cationising agent. The impact of various factors on nutrient adsorption was examined. FTIR, and Raman characterisation confirmed successful functionalisation and nutrient adsorption. The pseudo-second-order kinetics and Sips isotherm models accurately described the experimental data. High adsorption capacities of up to 74.4 mg/g for nitrate and 50.3 mg/g for phosphate were observed, with stable performance across a wide pH range (4–9). Electrostatic and ion exchange interactions and CAH’s high swelling ratio contributed to efficient and rapid adsorption. The adsorbent exhibited good regeneration potential, with the nitrate adsorption capacity remaining stable over ten consecutive adsorption–desorption cycles. The results highlight CAH’s potential as a sustainable and scalable adsorbent for continuous water treatment systems. Considering its biobased composition, there is potential for the spent hydrogel adsorbent to be incorporated into compost for the recycling of nitrate and phosphate in agriculture. This would align with circular economy principles and support sustainable development goals, including clean water and sanitation, and responsible consumption and production.