Direct ink writing of geopolymer adsorbents: A review

Geopolymers (GPs) are amorphous aluminosilicate binders valued as low-cost adsorbents for their cation-exchange capacity, tunable porosity, and chemical stability. Direct ink writing (DIW) has recently emerged as a processing route to shape these materials into structured lattice monoliths. This review critically examines whether the geometric control afforded by DIW offsets the inherent loss of specific surface area (SSA) and capacity, organized around ink rheology, microstructure, and fixed-bed performance. Across various adsorbates (including cesium, heavy metals, ammonium, dyes, and CO 2 ) the removal mechanism depends on the activation chemistry. Alkali-activated GPs enable broad multi-metal removal via neutralization–precipitation pathways. Conversely, recently developed acid-activated phosphate GPs act as Fe-selective, single-pass units driven by interfacial precipitation of strengite-like phases, though they face low intrinsic SSA and structural instability in wet environments. On a per-gram basis, ground powders still outperform printed lattices, because printing densifies the matrix struts. However, the monolithic form lowers hydraulic pressure drop by up to three orders of magnitude relative to powdered activated carbon (a smaller, roughly one-order-of-magnitude reduction relative to granular activated carbon), eases handling and regeneration, and avoids structural washouts under continuous flow. Real-effluent validation remains limited to acid mine drainage; alkaline, organic-laden, and saline matrices remain largely untested. Crucially, evaluating these structured adsorbents requires engineering process descriptors, such as column breakthrough, empty-bed contact time, and capacity normalized per bed volume. Ultimately, DIW does not enhance intrinsic capacity but converts high-capacity powders into robust, low-pressure-drop, reusable architectures with strong potential for industrial scale-up, pending pilot-scale, real-effluent validation.