Comparative Adsorption of Mono-, Di-, and Tricarboxylic Acids on Magnetically Modified Graphene Oxide: Influence of Molecular Structure on Adsorption Behavior

Abstract This study investigates the efficient adsorption and recovery of biotechnologically significant organic acids (lactic, succinic, and citric acids) using a magnetically modified graphene oxide (MGO) composite. The MGO was synthesized via chemical coprecipitation, and its structural, functional, and textural properties were comprehensively characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) analysis. Nitrogen adsorption–desorption measurements revealed a mesoporous structure with a specific surface area of 37.94 m2/g, a total pore volume of 0.078 cm3/g, and an average pore radius of 4.12 nm, which facilitate effective molecular diffusion into the internal pore framework. Batch adsorption experiments were conducted at 25 °C using an adsorbent dosage of 10 mg in a 20 mL solution volume across a concentration range of 10–100 g/L to evaluate equilibrium performance in comparison with a magnetically modified activated carbon (AC) reference. The equilibrium times were established as 50–80 min depending on the specific organic acid. Adsorption kinetics were accurately described by the pseudo-second-order (PSO) model (R2 > 0.99), confirming a chemisorption-controlled mechanism for all investigated acids. To ensure physical consistency and rigorous mass balance reconciliation, adsorption capacities were evaluated using mass-normalized (mg/g) units. The equilibrium data were analyzed using Langmuir, Freundlich, and Temkin isotherm models. While the results showed excellent agreement with the Langmuir model, the calculated dimensionless separation factor (RL) values (ranging from 2.3 × 10–6 to 2.8 × 10–3) further confirmed that the adsorption process is highly favorable for all investigated acids across the concentration range. Lactic acid exhibited the highest maximum adsorption capacity (qmax = 334.45 mg/g), which is attributed to its smaller molecular size and the specific interaction of its hydroxyl (−OH) group, enabling enhanced hydrogen bonding and electrostatic interactions with the oxygen-rich surface sites of the MGO. Furthermore, regeneration studies demonstrated that the MGO composite maintains a high capacity retention rate of 91.4% after five consecutive cycles, confirming its excellent structural stability and long-term industrial feasibility. The results demonstrate that the MGO composite provides superior adsorption performance compared to AC and allows for rapid magnetic separation from aqueous solutions. This study underscores the potential of functionalized graphene-based materials for the sustainable separation and purification of platform chemicals in the biotechnology industry.