Metalation-controlled charge-selective capture of dye pollutants and iodine by free-base and Zn-porphyrin-based ionic organic polymer adsorbents.

Removing charged organic dyes and neutral micropollutants selectively from water continues to be a main challenge in the design of adsorbents. Here, we demonstrate that the surface charge and hence the dye selectivity of a porphyrin adsorbent can be controlled by metalation. H2-ICOP is a free-base porphyrin with a positive surface charge (+25 mV) that rapidly adsorbs the anionic dyes, methyl orange (MO) and murexide (MRX), with 98-100% adsorption to the porphyrin at the tested concentration within 10-20 min; under stepwise loading, 262 mg g-1 MO could be uptaken. Zinc metalation inverts the surface charge (Zn-ICOP, -21.6 mV) and shifts selectivity toward the cationic dye methylene blue (MB), for which uptake follows a slow, pseudo-first-order, physisorption-controlled profile. Zeta-potential, FT-IR, and SEM-EDX analyses indicate that electrostatic attraction sets the dominant selectivity but cannot alone account for the behavior: neutral iodine is also captured from cyclohexane, implicating π-π stacking, hydrogen bonding, and coordination interactions. Iodine uptake by H2-ICOP reaches ≈ 205 mg g-1 (≈77% removal) and follows pseudo-second-order kinetics. Whereas MB uptake on Zn-ICOP follows pseudo-first-order kinetics, making it a physorption-controlled adsorption. The adsorption activity was maintained for three series of cycles (≈87% removal) without changing the framework features of H2-ICOP. The metalation-based charge switch can suggest a simple post-synthetic way to tune the dye selectivity based on one porphyrin platform.