Simultaneous extraction of Cr(VI) and Cu(II) from humic acid with new synthesized EDTA derivatives

Abstract

Soil washing is one of the few permanent treatment alternatives for removing metal contaminants. Ethylenediaminetetraacetic acid and its salts (EDTA) is very effective at removing cationic metals and has been utilized globally. However it is ineffective for anionic metal contaminants or metals bound to soil organic matter. The simultaneous removal of cationic and anionic metal contaminants by soil washing is difficult due to differences in their properties. The present study evaluated the potential of a washing process using two synthesized EDTA-derivatives, C6HEDTA (2,2′-((2-((carboxymethyl)(2-(hexanoyloxy)ethyl)amino)ethyl)azanediyl)diacetic acid) and C12HEDTA (2,2′-((2-((carboxymethyl) (2-(dodecanoyloxy)ethyl)amino)ethyl)azanediyl)diacetic acid), which consist of a hydrophilic polycarboxylic moiety and a hydrophobic moiety with a monoalkyl ester group. A series of equilibrium batch experiments at room temperature were conducted to investigate the efficacy of C6HEDTA and C12HEDTA as extractants for both oxyanion Cr(VI) and cationic Cu(II). Results showed that either C6HEDTA or C12HEDTA can extract both Cr(VI) and Cu(II) from humic acid simultaneously. However, C6HEDTA was less effective for Cr(VI) probably because it has no surface activities to increase solubility of humic acid, like C12HEDTA. Extraction of Cr(VI) was mainly attributed to the decreased surface tension and enhanced solubility of organic matter. Extraction of Cu(II) was attributed to both the Cu(II) chelation and enhanced solubility of humic acid. It was demonstrated that the hydrophilic polycarboxylic moiety of C12HEDTA chelates cations while the monoalkyl ester group produces surface active properties that enhance the solubility of humic acid.