Sorption of Pb2+ and Mn2+ ions on modified bentonite

Abstract

The problem of environmental pollution is now becoming the most significant in the world. Heavy metal ions (HMI) are one of the most common waste water pollutants; they pose a threat to the environment. The problem of extracting HMI from waste water is important from a scientific and practical point of view. Adsorption extraction of HMI is one of the methods of waste water treatment. Recently, a special role in the creation of new sorbents with valuable sorption properties belongs to the modification of readily available, cheap, and promising natural aluminosilicates. Modified aluminosilicate minerals are the best adsorbents in relation to heavy metal ions and organic dyes. Montmorillonite is the main rock-forming clay mineral, it has a high specific surface area and cation exchange capacity; therefore, sorption processes on such a natural material are very effective. For the extraction of heavy metals, recently, monocation-substituted Na⁺, Ca²⁺, Ba²⁺, Fe³⁺ forms of montmorillonite and organobentonites, which adsorb Cu²⁺, Fe³⁺, Co²⁺, Cd²⁺, Mn²⁺, Pb²⁺, W³⁺, and Mo² well are used. The aim of the study was the investigation of the structure of natural and modified bentonite and the adsorption of heavy metal ions on modified bentonite.

In this study, octadecylamine bentonite (ODA-B) was used as an adsorbent for the sorption of Pb (II) and Mn (II) ions. Organobentonite (OB) was obtained on the basis of natural bentonite by ion exchange. The sorption capacity of organobentonite was determined under static conditions, at 20^oС. The adsorbates were lead (II) and manganese (II) salts as the most important pollutants of waste water. The experiments were carried out on model aqueous solutions prepared from nitrates and sulphates of the indicated metals Pb(NO₃)₂, MnSO₄∙4H₂O. The concentration of metal ions in the model solutions varied in the range of 0.1-0.5 mmol/dm³. The influence of the contact time, pH-medium on the adsorption of Pb²⁺ and Mn²⁺ions was studied. The kinetics of sorption of these ions was investigated for the determination of the time of chemical equilibria of sorption of Pb²⁺ and Mn²⁺ ions. The equilibrium states of sorption of Pb²⁺ and Mn²⁺ ions were archived after 2-3 h. In all cases, equilibrium concentrations were reached within 15-45 min.

The structure of natural and modified bentonite has been studied by scanning electron microscopy (SEM), infrared spectroscopy, and X-ray phase analysis. It was found that with an increase in the concentration of these ions in solutions within 0.1-0.5 mmol/dm³ the sorption capacity of the used sorbents increased. This finding indicates that organobentonite can serve as a selective adsorbent for the extraction of Pb²⁺ and Mn²⁺ ions from waste water and liquid waste. Based on the experimental results, it can be concluded that the process of sorption of these ions on aluminosilicate sorbents occurs mainly by the ion-exchange mechanism.