Degradation of citrate and reduction of Cu(II) in wastewater containing Cu(II)-citrate complex by UV irradiation

• 99.09% of Cu(II) and 97.34% of citrate were removed under UV irradiation . • Cu(II) was reduced to highly pure Cu(0) (purity > 99.5 wt%). • Citrate was degraded through a successive decarboxylation process. • H• and CO 2 •- radicals reduced Cu(II) to Cu(0). Advanced oxidation coupled with precipitation was employed to remove organic ligands (L) and Cu(II) from wastewater containing the Cu(II)-L complex. In this strategy, more than 99 % of Cu(II) and L were effectively removed. Cu(II) eventually entered the copper-containing sludge and cannot be recovered. In this study, the UV irradiation-induced degradation of citrate and simultaneous reduction of Cu(II) without any reagent was proposed. It was found that 97.34 % of citrate was degraded, simultaneously, 99.09% of Cu(II) were removed and recovered as high-purity Cu(0) (purity = 99.79%). The identification of free radicals and intermediates showed that the degradation of citrate was a successive decarboxylation process. H• and CO 2 •- formed during the successive decarboxylation and these two free radicals reduced Cu(II) to Cu(0). The pH values significantly affected the removal of citrate and Cu(II), and the optimal pH was 5.0 due to the strongest UV absorption by Cu(II)-citrate at pH 5.0. Dissolved O 2 , CO 3 2– and PO 4 3- slightly inhibited the citrate and Cu(II) removal, and Cl - , Ca 2+ and Mg 2+ exhibited negligible effects. This study provides a promising strategy for the degradation of citrate and simultaneous recovery of heavy metals from wastewater.