Enhancing the sustainability of geopolymer adsorbents for efficient ammonium removal using sludges from water potabilization and wastewater treatment plants

Aiming to “close the loop”, this research deals with the potential reuse of two types of sludge from water treatment plants as raw materials for the preparation of geopolymer sorbents designed for water treatment applications. The novelty of the study lies on the replacement (up to 50 %) of metakaolin (MK), the typical natural precursor used for the geopolymer synthesis, with water potabilization sludge and wastewater sludge, to reduce production costs. Furthermore, to reduce the environmental impact associated with sodium silicate solution (SS), two different preparation methods were adopted, namely by varying the amount of SS and using a fix SS content but adding silica fume as further source of SiO2. Results showed that geopolymers incorporating water potabilization sludge exhibit the most suitable properties using both types of preparation methods, even when replacing up to 50 % of MK. In contrast, suitable microstructural and mechanical properties in MK-wastewater sludge samples were achieved only for a 10 % MK substitution content. Compressive strengths of samples ranged between 1.5 MPa and 20 MPa. Ammonium adsorption tests confirmed that geopolymerization enhanced removal efficiency compared to raw precursors alone, with potabilization sludge-based geopolymers prepared via Method I achieving the best performance (up to 66.6 % of NH4+ removal and 18 mg/g adsorption capacity). These findings support the feasibility of using sludge-based geopolymers as effective and sustainable adsorbents for water treatment. This approach promotes circular use of local waste materials, helps to reduce transportation costs and CO2 emissions and support the development of innovative environmental technologies.