Structural stability and selective iron immobilization of 3D-printed phosphoric acid-based geopolymer composites under acidic conditions

The development of architected adsorbents for water treatment is essential to overcome the operational limitations of powdered materials, such as high pressure drops and difficult recovery. This study investigates the printability, structural stability, and ion retention/release behavior of 3D-printed phosphoric acid-activated geopolymer (PAGP) lattices modified with Pluronic F127 (PLU) or activated carbon (AC) under acidic conditions relevant to acid mine drainage (AMD, pH ≈ 3). Rheological analyses showed that both additives improved ink elasticity and structural stability (G' > G”), with PAGP-AC exhibiting faster thixotropic recovery (66 s) and PAGP-PLU achieving the highest compressive strength (13.7 ± 0.3 MPa). Self-supporting lattices were successfully fabricated by DIW printing; however, BET analysis revealed limited surface area enhancement (5.70 and 4.89 m 2 g −1 for PAGP-AC and PAGP-PLU), indicating encapsulation of AC microporosity by the dense phosphate-aluminosilicate gel and predominance of PLU-generated macropores inaccessible to N 2 adsorption. Under acidic batch conditions, Fe was selectively retained, reaching 95.2% net removal and qe ≈ 1.03 mg g −1 for PAGP-PLU, mainly due to phosphate-mediated precipitation of strengite-like phases rather than classical adsorption. In contrast, significant Al release was observed, increasing from 0.67 mg L −1 to 56.0 and 67.8 mg L −1 for PAGP-PLU and PAGP-AC, respectively, consistent with H + -driven dealumination of aluminosilicate frameworks. Under dynamic acidic conditions, PAGP-AC structures collapsed, whereas PAGP-PLU retained macroscopic integrity, demonstrating superior chemical durability. These findings provide insights into the formulation-dependent performance of 3D-printed PAGP composites for acidic-stream remediation applications.