Application of the Taguchi Experimental Design to the Removal of Toxic Metals From Waste Waters by Precipitation as Magnetic Ferrites

Abstract

Waste waters polluted with toxic metals can be successfully purified by precipitation of the metals from an alkalinised solution containing iron(II) as ferrite sludge with magnetic properties. The magnetic ferrites obtained by this treatment can be disposed or reutilised with smaller risk for the environment since metals contained in the ferrite are less available. A Taguchi experimental design has been applied to the optimisation of the ferrite precipitation process, the aim being to achieve the maximum purification of the solution and the best magnetic characteristics of the ferrite. Four control factors at three levels were explored. (Fe(II) concentration)/(total metal concentration) ratio, temperature, treatment duration and pH, and assigned to the columns of a L₉(3⁴) orthogonal array. A noise factor (potassium permanganate) at three concentration levels was also introduced to simulate the uncontrollable variability of the waste water composition. Optimal heavy metal removal (>99%) and magnetic susceptibility of the ferrite were attained when waste water samples where treated for 2 hours at 50 °C and pH 10 in the presence of iron(II) sulphate in a ratio [Fe(II)] / [total metal] of 15. Ferrites obtained by the optimised procedure have been characterised by X-Ray Diffractometry and X-Ray Fluorescence Spectroscopy, confirming that the solid has a magnetite structure and contains all the metals initially in solution.