Effect of formaldehyde on Cu(II) removal from synthetic complexed solutions by ion exchange

The effect of formaldehyde (HCHO) on the ion exchange of Cu(II) from an equimolar EDTA (ethylenediaminetetraacetic acid, H4L) solution with a strong-base Amberlite IRA-400 resin was studied. Experiments were conducted as a function of the initial concentration of Cu(II) (0.5-10 mM), solution pH (1.0-6.0), HCHO concentration (0-6 vol%), and temperature (15-35 °C). It was shown that the amount of exchange of Cu(II), which exists in the form of complexed anions CuL2−, increased with increasing solution pH and reached a plateau at an equilibrium pH (pHe) of 3.5. However, the amount of exchange decreased with increasing HCHO concentration up to 3 vol% but then slightly decreased with a further increase in HCHO concentration. Such effect of added HCHO was determined by the following two factors: the competitive exchange of HCOO− anions and the enhanced exchange of Cu(I) in the form of complexed anions CuL3−. The exchange isotherm obtained at a fixed pHe could be well described by the Langmuir equation. The isosteric enthalpy change for the present ion exchange process was also evaluated and discussed.