Fate of zinc in an electroplating sludge during electrokinetic treatments

Chemical structure of zinc in the electrokinetic treatments of an electroplating sludge has been studied by in situ extended X-ray absorption fine structural (EXAFS) and X-ray absorption near edge structural (XANES) spectroscopies in the present work. The least-square fitted XANES spectra indicate that the main zinc compounds in the sludge were ZnCO3 (75%), ZnOSiO2 (17%) and Zn(OH)2 (7%). Zinc in the sludge possessed a Zn–O bond distance of 2.07 Å with a coordination number (CN) of 5. In the second shells, the bond distance of Zn–(O)–Si was 3.05 Å (CN = 2). An increase of Zn–(O)–Si (0.05 Å) with a decease of its CN (from 5 to <1) was found in the early stage of the electrokinetic treatment. Prolong the electrokinetic treatment time to 180 min, about 34% of Zn(II) was dissolved into the aqueous phase and about 68% of Zn(II) in the sludge (or 23% of total zinc) was migrated to the cathode under the electric field (5 V cm−1). The dissolution and electromigration rates of Zn(II) in the sludge were 1.0 and 0.6 mmol h−1 g−1 sludge, respectively during the electrokinetic treatment. This work also exemplifies the utilization of in situ EXAFS and XANES for revealing speciation and possible reaction pathways during the course of zinc recycling from the sludge by electrokinetic treatments.