Abstract
Ionic liquids (ILs) are per definition salts with melting points below 100°C and might be green alternatives for the extraction of heavy metals from aqueous solutions due to their favorable environmental and physico-chemical properties. Partial solution during extraction, so-called leaching, however, limits their applicability. The present study synthesizes three novel ammonium and phosphonium ILs based on 3-hydroxy-2-naphthoic acid—trihexyltetradecylphosphonium—([P66614]), methyltrioctylphosphonium—([P1888]), and methyltrioctylammonium 3-hydroxy-2-naphthoate ([N1888][HNA])—by a deprotonation-metathesis route. The aims were to improve stability during extraction while still achieving high selectivity toward heavy metal ions, as well as to study the impact of different alkyl chains and the central atom of the cation on physico-chemical properties, extraction efficacy, and leaching. Extraction capabilities for the seven heavy metals Ag, Cd, Co, Cu, Mn, Ni, and Pb were studied in pure water at pH 8.0. Further experiments were conducted in water containing 30 g L−1 NaCl to simulate a seawater matrix and/or 30 mg L−1 humic acids, as well as metal-spiked natural water samples. All three ILs showed extraction efficacies ≥90% for Cu and Pb after 24 h. Overall, extraction efficacies for Ag, Cd, Cu, and Pb were highest for drinking water samples. Ag and Cd extraction was increased by up to 41% in (hyper-) saline samples using IL [P66614][HNA] compared with pure water samples. Leaching values were reduced down to 0.07% loss of the applied IL, which can be attributed to the hydrophobic character of 3-hydroxy-2-naphthoate. Our results represent a positive development toward a greener extraction of heavy metals from natural waters.
Highlights
Heavy metals are pollutants of major concern in the aquatic environment globally
Ionic liquids (ILs) [P1888][HNA] and [N1888][HNA] differ only in the quaternary elements, while the alkyl chains are identical. This enables a direct comparison of the impact of the central atom on chelating properties between otherwise identical ILs. 3-Hydroxy-2-napthoate was chosen as the anion because it contains a salicylate functional group which is known for its complexation potential (Fischer et al, 2011), whereas its naphthalene backbone provides the needed hydrophobicity together with the respective cation
A low chloride content of 0.15 wt% was achieved (Table S1), which is in line with or significantly lower than the values of synthesized task-specific ionic liquids (TSILs) from chlorinated precursors, with contents up to 6 wt% (Mikkola et al, 2006; Kogelnig et al, 2008; Stojanovic et al, 2010; Fischer et al, 2011; Leyma et al, 2016)
Summary
Heavy metals are pollutants of major concern in the aquatic environment globally. They are toxic even at low concentrations, they are persistent and they tend to bio-accumulate (Pacyna et al, 2007; Khan et al, 2008; WHO, 2014). Efforts to reduce pollution in wastewaters and runoff have led to the development of several methods for metal removal and recovery. Amongst these are solvent extraction, adsorption, or precipitation. These methods have, besides their advantages, certain limitations such as lack of selectivity, expensive maintenance or the use of toxic, and/or flammable volatile organic compounds. Research toward more efficient and environmentally friendly alternatives for extracting metals from aqueous phases, including wastewaters and natural waters, has increased in the last years (Zhao et al, 2005)
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