Synthesis of Cross-Linked Pyrazine-Based Polymers for Selective Removal of Mercury(II) Ions from Wastewater Solutions

The study represents the synthesis and characterization of two new pyrazine-based polymers to remove mercury(II) ions from wastewater solutions. The polymers were synthesized via the polycondensation of 2,6-diaminopyrazine (MXR-1) or 2,3-diaminopyrazine (MXR-2) with terepthaldehyde and glacial acetic acid as a catalyst. The polymerization reactions revealed the formation of a polyamine (MXR-1) and a polyimine (MXR-2). The SEM–EDX analysis showed a sphere-like structure for MXR-1 with a surface area of 56 m²/g, whereas MXR-2 showed a sheet-like structure with a surface area of 26 m²/g, the higher surface area of MXR-1 showed better adsorption capacity compared to MXR-2. The efficiency of the new polymers was tested to remove mercury ions at low concentrations from wastewater solutions. The effect of concentration on the adsorption capacity of MXR-1 to mercury ions showed that the experimental data fitted both Langmuir and Freundlich isotherm models, indicating homogeneous and heterogenous adsorption, respectively, whereas MXR-2 fitted the Freundlich isotherm model. The effect of time on the adsorption capacity study revealed that the adsorption is chemisorption in nature by pseudo-second-order kinetic model for both MXR-1 and MXR-2 polymers. The thermodynamic properties revealed that the adsorption process is endothermic in nature. The new polymers were tested on spiked wastewater samples and showed the superiority of MXR-1 polymer over MXR-2 with higher efficiency in the removal of mercury from wastewater solutions with a 95% removal efficiency. The study reveals the potency of MXR-1 polymer as an adsorbent for wastewater purification. Graphic Abstract: [Figure not available: see fulltext.].