Valorization of Alum Sludge Waste through Zeolite Synthesis for Sustainable Fertilizer Production

Alum sludge (AS) from water treatment plants is an abundant, non-toxic residue with high Si and Al contents. These characteristics make it an ideal precursor for zeolite synthesis, not only reducing the environmental impacts from AS disposal but also adding value to the waste by promoting its reuse. Herein, we propose a route to prepare zeolitic materials from AS, aiming for its application as a component of slow-release fertilizers, using mild conditions and cost-effective hydrothermal synthesis. X-Ray Diffraction revealed the formation of sodalite (Na-SOD) in short times (5-10 h) and moderate temperatures (100-120 degrees C), with sodium silicate addition favoring Si-quartz consumption and zeolite crystallization, without a fusion step. The Na-SOD showed a desirable cation exchange capacity, reaching 117 mg g-1 for K+. A K-SOD fertilizer was then prepared with an average 490 nm size, 2 m2 g-1 BET surface area, and slow-release behavior. Incubation in soil confirmed that Al was inactive in K-SOD and did not immobilize phosphate, in addition to promoting high K+ availability. The results support the potential of AS reuse for application in agriculture as an efficient and sustainable fertilizer.