Abstract
Here, we propose an environmentally benign removal technique for heavy metal ions based on selective and spontaneous transfer to oleic acid. The ions can be removed via (1) the selective and rapid complexation with the carboxylic end of oleic acid at an oleic acid/water interface, and (2) the diffusion of such complex into the oleic acid layer. A wide variety of heavy metal ions such as Cu2+, Pb2+, Zn2+, and Ni2+ can be selectively removed over K+ and Na+. For example, the concentration of Cu2+ is reduced to below 1.3 ppm within 24 h, which corresponds to the level of Cu2+ permitted by the Environmental Protection Agency. The addition of ethylenediamine ligand to the metal ion solutions is also shown to enhance the phase transfer. The removal efficiency is increased by up to 6 times when compared with that in the absence of the ligand and follows the order, Cu2+ (99%) > Pb2+ (96%) > Zn2+ (95%) > Ni2+ (65%). Moreover, the removal time can be shortened from 24 h to 1 h. The effect of an emulsion induced by a mechanical agitation on the removal of heavy metal ion is also studied.
Highlights
To date, the removal of heavy metal ions in water has been performed by a variety of techniques[4,5,6,7,8,9]
The metal-carboxylate complex would diffuse from the interface into the oleic acid layer, resulting in the removal of the aqueous heavy metal ions (Fig. 1a)
The phase transfer of the metal ions could be chemically enhanced by the addition of ethylenediamine (EN) ligand to an aqueous phase (Fig. 1b) since more stable complex is known to be produced with the aid of the EN ligand (Fig. 1c)[29,30]
Summary
We propose an environmentally benign removal technique for heavy metal ions based on selective and spontaneous transfer to oleic acid. A few years ago, accidental lead (Pb2+) poisoning in drinking water in the city of Flint, Michigan, caused many people, especially children younger than 5 years, to suffer from skin rashes, as well as cognitive and behavioral disorders[1] This incident serves to demonstrate that the removal of heavy metal ions is still one of critical issues in modern public health[2,3]. Biocompatible oleic acid is chosen as the oil phase since its carboxylic end is known to form stable complexes with many heavy metal ions[25,26]. Our proposed method has several key advantages over chemical precipitation and adsorption It requires neither specially-designed adsorbent nor toxic precipitant. Our method does not require an additional filtration to remove insoluble precipitates or fine solid adsorbents
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