Development of a bioresource-based adsorbent from activated coconut shell and chitosan-oxalic acid for safranin O dye removal: RSM optimization and circular resource management

This work demonstrates the design of a bioresource-based adsorbent for the effective removal of organic dyes, embodying the principles of circular resource management by transforming waste into a water-treatment resource. Thus, the current study aims to valorize activated lignocellulosic biomass (coconut shell) combined with chitosan-oxalic acid to develop an eco-friendly adsorbent of cross-linked chitosan-oxalic acid/activated coconut shell (CTS-OX/CS-H) for the uptake of safranin O (SFO) dye from water systems. Several methods, including pHpzc, XRD, CHNS-O, FTIR, BET, and FESEM, were used to study the physicochemical properties of CTS-OX/CS-H. The adsorption performance of CTS-OX/CS-H was optimized by the use of response surface methodology (RSM), taking into account variables such as CTS-OX/CS-H dose (0.02–0.08 g), pH (4–10), and time (10–40 min). The BBD model findings showed that a CTS-OX/CS-H dosage of 0.059 g, a solution pH of 9, and a removal time of 24.7 min were the ideal values for maximal SFO uptake (94.7%). The adsorption of SFO onto CTS-OX/CS-H is well described by the Freundlich isotherm and pseudo-first-order model, indicating multilayer adsorption on a heterogeneous surface governed mainly by physisorption. The CTS-OX/CS-H adsorbent exhibited a high adsorption capacity of 496.19 mg/g at an adsorbent dosage of 0.059 g, pH 9, and a temperature of 25 °C. Electrostatic forces between the positive group of SFO and the acidic groups of CTS-OX/CS-H are principally responsible for the remarkable adsorption of SFO onto CTS-OX/CS-H. This work supports environmental sustainability by integrating water treatment with circular waste management principles.