A pyridinium-modified chitosan-based adsorbent for arsenic removal via a coagulation-like methodology

Abstract

The goal of this study was to synthesize a chitosan-derived adsorbent that can be used in a coagulation–flocculation (CF) process for facile integration into existing water treatment processes. Therefore, an insoluble pyridinium-modified chitosan (Chi-Py) was prepared. Structural characterization was achieved with spectroscopy (FT-IR, ¹³C solids NMR, and X-ray photoelectron) methods and thermogravimetric analysis. Approximately 7% di-nitrobenzene and ca. 30% pyridinium moieties were incorporated into the chitosan framework via an adapted, moderate-temperature, Zincke reaction. The arsenic removal efficiency was evaluated by a coagulation-inspired methodology at pH 7.5, where the results were compared against CF systems such as pristine chitosan, FeCl₃ and chitosan–FeCl₃. The kinetic and van't Hoff thermodynamic parameters for arsenic removal were calculated. Arsenic adsorption was shown to be a spontaneous and exothermic process (ΔG = −4.7 kJ mol⁻¹; ΔH = −75.6 kJ mol⁻¹) with a 76% arsenic removal efficiency at 23 °C and 96% at 5 °C with a maximum effective adsorbent dosage of Chi-Py of 300 mg L⁻¹. The adsorption process for Chi-Py followed pseudo-first order kinetics, where the pyridinium-modified chitosan adsorbent can be successfully employed similar to coagulant-like systems in conventional water treatment processes. In contrast to conventional adsorbents (1–2 g L⁻¹), a dosage of only 300 mg L⁻¹ was required for Chi-Py that offers greater sustainability and recycling of materials. This is contrasted with single-use conventional coagulants such as FeCl₃ or binary FeCl₃–chitosan CF systems.