Methylene Blue Adsorption Using Boric Acid Functionalized Activated Carbon: Kinetics, Isothermal, and Thermodynamic Studies

In this study, the activated carbon (AC) was prepared from Sesbania sesban plant stem. Boric acid (H₃BO₃) was used as an activating agent. During calcination, the optimized temperature was kept upto 500 °C for 2 h. The prepared adsorbents were characterized using various techniques such as FT-IR, scanning electron microscopy, energy dispersive x-ray spectroscopy, and thermogravimetric analyzer. The prepared adsorbents were used for the removal of methylene blue (MB) dye adsorption. The adsorption experiments were conducted at different pHs such as (2–11), doses (0.0025–0.020 mg), times (30–300 min), MB initial concentrations (100–600 mg L⁻¹), and temperatures (298–318 K), respectively. The maximum MB uptake capacity of the prepared adsorbent was 1380 mg g⁻¹ under optimized adsorption conditions. Furthermore, the kinetic study is well described by pseudo-second order, whereas the isothermal study showed the Freundlich isotherm was better followed by the equilibrium data. Based on thermodynamic studies, the negative values of ΔG° and ΔH° revealed that the MB adsorption process is spontaneous and exothermic in nature. However, the negative ∆Sxg° value indicates that solid-solute interaction decreased randomness in the adsorption systems. The overall studies of AC showed that it was better to remove MB from an aqueous solution.