Cyanide contaminated water treatment by di-nuclear Cu(II)-cryptate: A supramolecular approach

A popular Cu(II) based metallo-cage cryptate, Complex 1 [LCu₂(ClO₄)₄], is synthesized by using a bicyclic cryptand, L (synthesized in large scale of ≈ 28 gm/batch) and Cu(ClO₄)₂ for its application towards the removal of toxic CN^¯ from cyanide contaminated water and cyanide containing industrial blast furnace (BF) wastewater. Complex 1, having very low solubility in aqueous medium, showed efficient removal of CN^¯ (initial CN^¯ conc. of 80 ppm to final conc. of 4.21 ppm; ≈ 95 % removal of CN^¯) from standard aqueous solution of CN^¯ (NaCN in water) within 10 min, through heterogeneous phase mixing as observed from Ion Selective Electrode (ISE) based measurements. Mechanistic investigation towards the binding of cyanide with complex 1 through spectroscopic studies indicated the initial formation of CN^¯ bridged green dimeric copper (II) complex upon addition of one equivalent of CN^¯ which subsequently decomposed upon addition of further equivalents of CN^¯ to produce a colourless solution. Finally, as a real-time application, complex 1 was treated with cyanide contaminated industrial wastewater in the presence of other interfering anions (e.g. Cl^¯, SO₄²⁻, NO₃^¯, NO₂^¯, SCN^¯, F^¯ etc.) which showed fast and efficient removal of cyanide (initial CN^¯ conc. of 6.7 ppm to final conc. of 1.3 ppm; ≈ 77 % removal of CN^¯ in 30 min) of BF wastewater from lab scale to pilot plant scale. Interestingly, the complex 1 showed recyclability up to six cycles in terms of removal of cyanide with high efficiency (≈ 72–74 % removal of CN^¯).