From red to green: Smart thiol-porphyrin cryogels combining high mercury efficiency removal and visual material saturation alert

• A new thiol-functionalized cryogel for mercury (II) removal was designed. • High Hg(II) adsorption capacity of 1229 mg/g and selectivity > 95 % are achieved. • Porphyrin (0.25%w/w) causes red-to-green color shift close to material saturation. • Real-time material saturation monitoring can simplify water treatment procedures. • Material shows reusability for up to five cycles. A novel and smart methacrylic acid (MAA) based macroporous material functionalized with thiol groups (−SH) was developed to achieve highly efficient and selective removal of toxic Hg(II) ions from water. With the aim of visualizing the saturation of the active sites of the material in real-time, a porphyrin-based co-monomer was synthesized and included (only 0.25 % w/w) as a part of the co-polymeric structure. The porphyrin ring can capture Hg(II) ions, causing a color change from red to green. However, due to the higher affinity of Hg(II) towards thiol groups, the porphyrin will be able to interact with free Hg(II) only when the adjacent thiols are no longer available and, therefore, close to the complete saturation of −SH sites. The color shift alerts that the material is approaching saturation, hence, a regeneration step is necessary for subsequent adsorption cycles. The typical interconnected macroporous (3–23 µm) network of the cryogel allows fast water diffusion and easy access to the −SH and PORPH sites. Such design achieves an exceptional Hg(II) adsorption capacity (Q max > 1200 mg/g), calculated from the Langmuir isotherm model. The synthesized material shows high selectivity towards Hg(II) (S% > 95 %) in a solution with the simultaneous presence of other metal cation species. The kinetics of Hg(II) capture, pH behavior, material dosage, and regeneration cycles were tested, highlighting its potential applicability across a broad pH range and its reusability for at least five cycles. These combined features underscore the superior performance of this advanced material compared to current state-of-the-art competitors.