Enhanced adsorption of Cr(VI) under neutral conditions using a novel adsorbent with preorganized diquaternary ammonium structure

Abstract

In this study, polystyrene microspheres with diquaternary ammonium cation structure N+-C-C-N+ were designed and synthesized to realize the efficient removal of Cr(VI) in neutral wastewater. The key adsorption sites, N+-C-C-N+, can match the charge and the configuration of chromate anion, which greatly improves the ability of associating chromate anions under neutral conditions. Density functional theory (DFT) was used to compute the adsorption energies between Cr(VI) and N+-C-C-N+, which proved the structure's effectiveness. The adsorbent can work under a wide pH range and greatly improved the efficiency of removing Cr(VI) from wastewater under neutral and alkaline conditions. The adsorbent was characterized with Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Batch adsorption experiments show that the removal efficiency of Cr(VI) remains above 95% in pH values range from 2.0 to 12.0. The maximum capacity of the adsorption achieved within 40 min and conformed to the pseudo-second-order kinetic. According to the Langmuir model, the maximum capacity reached to 89.77 mg g−1 at pH 7.0. Regeneration tests show that adsorbent has good reusability and can be used in the repeated adsorption. FTIR and XPS analysis demonstrated the electrostatic interactions mechanism between the N+-C-C-N+ and Cr(VI) proposed for the adsorption process.