Low-Cost and Eco-Friendly Fabrication of Diphenylamine-Based Hyper-Cross-Linked Polymer for Water Softening

Pure, clean, and safe drinking water availability has become a burning challenge globally, especially considering that many people are consuming hard water, leading to several health concerns. This study focused on developing an efficient, cost-effective, and eco-friendly adsorbent, DPA-HCP, to remove hard water ions and improve water quality for drinking. The synthesis of DPA-HCP utilized an external cross-linking strategy, resulting in a high specific surface area (<i>S</i> _BET) of 611.99 m²/g, indicating a significant number of available adsorption sites. Comprehensive characterization techniques such as Fourier transform infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), and X-ray diffraction (XRD) were employed for the analysis of the structure, as well as the morphology of the adsorbent. There was a reduction up to 84% in water hardness by DPA-HCP under operating conditions of pH 8, with a contact time of 100 min and an adsorbent concentration of 1.2 g/L. A maximum adsorption capacity of 100.51 mg/g was identified for hard water ions, whereas kinetic analysis reveals that this adsorption process is pseudo-second order (<i>R</i> ² = 0.9981), showing that the process is an exothermic and spontaneous process. The experimental data showed a very good fit with the Langmuir isotherm model with an <i>R</i> ² value of about 0.984. Thermodynamic analysis gave an enthalpy change (Δ<i>H</i> ^o) of -21.52 kJ/mol, an entropy change (Δ<i>S</i> ^o) of -59.89 J/mol·K, and a Gibbs free energy change (Δ<i>G</i> ^o) of -4.306 kJ/mol. Such results accentuate DPA-HCP as a promising candidate for addressing hard-water-related problems, thus providing hope for improving access to safe and clean drinking water all over the globe. Furthermore, its eco-friendliness, easy synthesis, and low cost make it a strong candidate to replace costly adsorbents such as ion-exchange resins.