Green adsorbent from maize biomass for mercury capture: insights from sorption modeling and thermodynamic analysis

Adsorption isotherms and kinetics modeling, as well as thermodynamic analysis, are useful in providing insight into the nature and mechanisms of the adsorption process. The present study investigated the interactive behavior and mechanisms of mercury ions removal using activated carbon produced from maize biomass (bio-adsorbent). To determine the mechanism of mercury removal from the aqueous system using the activated carbon, the equilibrium adsorption isotherm, kinetics, and thermodynamic studies were performed using the batch technique. Among all the isotherm models analyzed, the Langmuir isotherm model best correlated with the equilibrium sorption data of Hg(II) attained by the bio-adsorbent with a high correlation coefficient of 0.9998. The Langmuir maximum monolayer sorption capacity attained by the bio-adsorbent was 112.46 mg/g, and the dimensionless separation factor (\({R}_{\text{L}})\) was in the range of \(0.00<{R}_{\text{L}}>1.00\) indicating favorable biosorption. The pseudo-second-order model well fitted the experimental data of Hg(II) better than the other kinetic models with a high correlation coefficient of 0.9712, which is close to unity with an uptake capacity of 82.10 mg/g. The negative values of \(\Delta G^{0}\) obtained from all the temperature ranges of 283–358 K indicate the spontaneous nature of Hg(II) ions removal from the adsorption system by the bio-adsorbent. The positive value of + 24.86 kJ/mol and + 8.13 kJ/mol attained for \(\Delta H^{0}\) and \(\Delta S^{0}\), respectively, indicates endothermic adsorption and an upsurge in disorder during the adsorptive removal of Hg(II) ions. Therefore, the study found that the activated carbon not only interacted well with the Hg(II) species in the aqueous solutions but also had a high uptake capacity. Hence, the bio-adsorbent is promising and could efficiently be applied for heavy metal remediation in aquatic environments.