When activated carbon (AC) is modified with zirconium(IV) by impregnation or precipitation, the fluoride adsorption capacity is typically improved. There is significant potential to improve these hybrid sorbent by controlling the impregnation conditions, which determine the assembly and dispersion of the Zr phases on carbon surfaces. Here, commercial activated carbon was modified with Zr(IV) together with oxalic acid (OA) used to maximize the zirconium dispersion and enhance fluoride adsorption. Adsorption experiments were carried out at pH 7 and 25 °C with a fluoride concentration of 40 mg L−1. The OA/Zr ratio was varied to determine the optimal conditions for subsequent fluoride adsorption. The data was analyzed using the Langmuir and Freundlich isotherm models. FTIR, XPS and the surface charge distribution were performed to elucidate the adsorption mechanism. Potentiometric titrations showed that the modified activated carbon (ZrOx-AC) possesses positive charge at pH lower than 7, and FTIR analysis demonstrated that zirconium ions interact mainly with carboxylic groups on the activated carbon surfaces. Moreover, XPS analysis demonstrated that Zr(IV) interacts with oxalate ions, and the fluoride adsorption mechanism is likely to involve –OH− exchange from zirconyl oxalate complexes.

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The sorption of Cd2+ onto nonliving Typha latifolia roots was investigated using carbon paste electrodes modified with a powder formed from the ground dried root. The equilibria, kinetics, and proton exchange processes associated with the biosorption process were determined. Cd2+ was found to accumulate on the root‐modified electrode under open circuit conditions, and the electrode was transferred to an electrochemical cell for square wave anodic stripping voltammetry measurements. The measured currents were proportional to the amount of Cd2+ adsorbed onto the root, which permitted the biosorption process to be measured as a function of cadmium solution concentration, time, and pH. The equilibrium values were fit to the Langmuir or Freundlich equations, and that the Langmuir isotherm model was found to provide a better fit than the Freundlich model. The pH‐dependent sorption curves showed that the accumulation of cadmium proceeded with proton exchange, and each adsorbed cadmium ion resulted in expulsion of two protons. The sorption kinetics were studied, the data obtained were fit to two models: a pseudo‐second‐order model and an intraparticle diffusion model. The pseudo‐second‐order model provided the best description of the bioadsorption data. These results indicated that these methodologies have clear advantages over existing methods, because the amount of reagents and sorbent used for the experiments are much less than are required for batch experiments.

Agave salmiana bagasse has proved to be a good biosorbent for heavy metal ions in aqueous solutions. However, more information about its chemical structure and the interaction with metal ions remains to be understood. Raw bagasse and its fractions hemicellulose, cellulose, and lignin were characterized by acid–base titrations, Attentuated Total Reflectance Fourier Transform Infrared Spectroscopy and thermogravimetric analysis (TGA), before and after they were tested to remove Cd­(II), Pb­(II), and Zn­(II) from aqueous solution. Oxygen, sulfur, and nitrogen containing groups were identified in both raw bagasse and its fractions. TGA of the exhausted biosorbent demonstrated that Cd­(II) and Pb­(II) ions shifted the pyrolisis temperature of the neutral detergent fiber (NDF) fraction while Zn­(II) decreased it. Carboxylic and strong acid groups were mainly responsible for metal removal. Lignin contributes the most to metal uptake in the NDF adsorption process. Finally, the higher or lower pyrolisis temperature of exhausted NDF fraction was related to decarboxylation reactions and elimination of hydroxyl groups.