Preparation and use of low-cost magneto adsorbent for Cr (VI) removal from synthetic aqueous solutions

In this study, we prepared a low-cost adsorbent, magnetised activated carbon (MAC), from agricultural waste (Dodonaea Viscosa tree branches) and used it to adsorb hexavalent chromium (Cr (VI)) from aqueous solutions. The study includes a framework for material characterisation by (XRD, FTIR, SEM and BET). The major operating factors were tested, such as solution pH, adsorbent dose, initial Cr (VI) concentration, contact time and temperature, using a two-factor study method to determine optimal conditions. Adsorption isotherm models, kinetic investigations, and thermodynamic analyses were used to examine the adsorption performance and the processes involved. The FTIR spectra showed the presence of hydroxyl and carboxyl groups, and the XRD confirmed the formation of Fe 3 O 4 on the surface of the activated carbon. SEM images showed a rough heterogeneous morphology; BET analysis showed a porous structure with a surface area of 409.584 m 2 /g and a total pore volume of 0.55764 cc/g. The maximum removal efficiency (97.1%) was achieved at pH 4 and 60 min, with the optimum adsorbent dose, and the adsorption capacity increased at higher temperatures. Kinetic analysis indicated that the pseudo-second-order model best fit the adsorption data, suggesting that chemisorption was the main process. The Langmuir isotherm fitted well with the equilibrium data, with the maximum adsorption capacity of 15.89 mg/g at 60 °C with an adsorbent dose (1 g/L), indicating monolayer adsorption on a homogeneous surface. Thermodynamic data (ΔG, ΔH, ΔS) indicated that the process was spontaneous and endothermic, with increased adsorption at higher temperatures. The results showed that the prepared magnetic adsorbent performed well in Cr (VI) removal under controlled laboratory conditions. In contrast to prior studies, adsorbents efficiently remove Cr (VI) at low pH (1–2), but these conditions are impractical and expensive. In addition, the influence of surface functional groups on adsorption is poorly understood. Moreover, the adsorption mechanism remains unclear because the kinetic and thermodynamic analyses are incomplete.