Ionic liquids as extracting agents for heavy metals

Ionic liquids are a family of organic salts with melting points lower than 100C. Many of these salts are molten at room temperature and even below. They show unique features like extremely low vapor pressure, and a large liquidus range, and their properties are tunable by chemical modifications of their cations and/or anions. Several working groups have studied the metal extraction behavior of hydrophobic ionic liquids. However, considering the state of the art of ionic liquids as metal extracting agents, their application in real water systems is still limited due to the lack of basic knowledge regarding their stability and metal-uptake mechanism. The hydrophobicity of the commonly used metal-extracting ionic liquids is given by fluorine containing anions. A disadvantage of these systems lies in their instability towards hydrolysis since the formation of hydrofluoric acid may pose severe environmental risks. Other approaches try to enhance the metal ion selectivity by adding a complexing agent to the ionic liquid phase. The addition of organic coordinating compounds like crown-ethers directly to the ionic liquid - phase significantly increases the distribution ratios of metal ions between the ionic liquid- and the aqueous phase, however, the extraction mechanism involves the exchange of a cationic metal complex with the cation of the ionic liquid, which would result in the destruction of the ionic liquid during the extraction process. In order to overcome these disadvantages, we envisage to apply an iterative synthetic approach by producing novel task-specific ionic liquids, which are not prone to hydrolysis, with suitable extracting entities grafted onto their cation and/or their anion. Our studies have recently shown that task specific ionic liquids bearing thiol, urea, or thiourea moieties allow the uptake of metal ions, such as Hg(II), Pt(II) and Cd(II), or metal oxide ions such as uranyl (UO 2 2+) from water, however, the dominant mechanism - which very likely depends on the ionic strength and pH in the water phase - is still unknown. We expect to uncover the mechanism of metal transfer processes for different ionic liquids and different real matrices (communal-, industrial-, hospital waste water as well as superficial and drinking water) based on liquid-liquid extraction experiments combined with extensive experimental investigations of the physico-chemical properties of the synthesized ionic liquids and their influence on the extraction process. The goal is to enhance fundamental understanding concerning ionic liquids potentially suitable for the extraction of toxic heavy-metal ions from wastewater and drinking water.

Heavy metal pollution in wastewaters and the receiving streams is a matter of great concern worldwide. Heavy metals (such as cadmium, mercury and lead) are of concern for human health due to their toxicity, their potential to cause cancer and their ability to cause harmful effects even at low concentrations. The 2015 Worlds Worst Pollution Problems report by Green Cross Switzerland and Pure Earth (formerly Blacksmith Institute) emphasized that the six pollutants that pose an outsized threat to human health are lead, radionuclides, mercury, hexavalent chromium, pesticides and cadmium. Please note that five out of the six top six rated pollutants are heavy metals or radionuclides, respectively. In order to achieve a good chemical status of surface waters as described in the European water frame directive (2000/60/EC), efforts for the reduction of toxic heavy metal concentrations in the water phase have to be enhanced. Unfortunately, in communal wastewater treatment plants, a complete retention of toxic heavy metals by conventional techniques is by far not attained. There is an appropriate demand for innovative techniques for an efficient elimination of toxic metals and their compounds to comply with legal requirements and to prevent environmental risks for biota and men. The aim of the present project was the development and evaluation of an efficient, commercially favorable method based on hydrophobic TSILs (task specific ionic liquids) for the removal of heavy metals from waste water effluents. The main focus of the project was the design, synthesis, characterization and evaluation of novel hydrophobic TSILs for the selective and efficient extraction of heavy metals and radionuclides from aqueous phases and the evaluation of their extracting behavior. Further important points were the development of suitable metal-stripping agents and the evaluation of the reusability of prepared TSILs. The leaching of the hydrophobic TSILs into the aqueous phase as well as their toxicity towards fresh water algae were extensively studied. The investigated ionic liquids revealed acute toxicity toward green freshwater algae even at low concentrations in water.