Experimental investigation and thermodynamic modeling of extraction of heavy metal ions from aqueous solutions by chelation in supercritical carbon dioxide

Abstract

Wastewater streams containing heavy metals are common in industry. To prevent the contamination of clean water sources, the Clean Water Act specifies limits on the heavy metal concentrations of industrial waste water. This creates a strong need for developing cost effective and environmentally friendly metal removal technologies. Solvent extraction has been recognized as one of the best methods for removing metals from wastewater. Although the metals are easily removed, this process has two major disadvantages. First, the solvent/metal solution must be subsequently purified. Second, since the solvent may be miscible in the aqueous phase, the residual solvent must be removed from the water stream. These disadvantages can be eliminated by substituting conventional organic solvents with supercritical fluids. The main objective of this research has been to investigate the potential and feasibility of heavy metal ion extraction through chelation in supercritical CO2. Copper has been chosen as the model contaminant as it is frequently found in industrial waste streams. Different chelating agents have been tested to find the most appropriate one for copper. Analytical methods have been developed to quantify supercritical and aqueous phase compositions. Specifically, an Atomic Absorption Analyzer and a Gas Chromatograph have been employed. Copper ions have been successfully extracted up to 97% on different isotherms. Considering the phase equilibria and the thirteen reactions taking place in the system, a thermodynamic model has been developed. This model predicts the system pH which is a important factor in design of metal extraction units. With the model the efficiency of the extraction with different chelating agents at different temperatures and pressures is easily estimated. The model is also capable of calculating the concentrations of chemical species present in the system. This study proposed a novel and viable technique for the remediation of metal ions in waste water streams. In conjunction with the developed model the efficiency of this process for a specific industrial application can be accurately estimated. The results of this study demonstrate that this process is both environmentally friendly and economically feasible for wide spread industrial use.