Cellulose-based nanostructured aerogels for leachate decontamination: Towards sustainable phosphorus recovery from sewage sludge ash

Recent EU data confirmed phosphorus as a critical raw material, with efficient recovery from sewage sludge ash presenting challenges, particularly due to the co-dissolution of heavy metals. This work investigated an innovative strategy for the removal of heavy metals from sewage sludge ash derived leachate by using sustainable cellulose nanosponges, synthetized by thermal cross-linking between branched polyethylenimine and 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-oxidized cellulose nanofibers. Lab-scale adsorption tests were conducted with varying sorbent amounts (1, 3 and 10 g/L) in absence or presence of citric acid (10 g/L) across pH values (1.8–10), to assess the removal of metals/metalloids (As, Cu, Cr, Ni, Zn, Fe, Al). P was subsequently precipitated from the decontaminated leachate by adding Ca(OH)2 till target pH. Results showed that cellulose nanosponges were able to adsorb considerable amounts of metallic ions, in particular Fe and Al, under strongly acidic conditions, reducing their percentage by 40% and 30%, respectively. Under optimized conditions (10 g/L of citric acid and 10 g/L of sorbent), the solid precipitate recovered at pH 8 from the decontaminated leachate was compliant with EU regulation 2019/1009 in terms of P2O5 and heavy metals content, suggesting its possible future use as raw material for inorganic fertilizers production.