Removal of low-concentration nickel in electroplating wastewater via incomplete decomplexation by ozonation and subsequent resin adsorption

Effective removal of low-concentration chelated nickel in electroplating wastewater to meet the strictest discharge standard for nickel less than 0.1 mg/L has long been a challenging task. Herein, a method combining ozonation and resin adsorption was successfully used to remove the initial total Ni concentration of below 10 mg/L at ozone dosage of 8–38 mg/L. It is notable that the nickel complexation was destroyed by ozone while the co-existing organic matters were not completely mineralized. After ozonation, even the common cation-exchange resin can successfully remove the nickel. The fixed-bed adsorption experiment showed that the working capacity of cation-exchange resin for the ozonated wastewater was 1150 bed volume (BV) at the nickel breakthrough point of 0.1 mg/L, much higher than 5 BV without ozonation treatment. Three-dimensional fluorescence and HPLC-QTOF-MS were employed to characterize the change of Ni-complexes during the treatment process. The decomplexation mechanism of Ni-complexes by ozonation and removal mechanism by cation-exchange resins were proposed. After ozonation, the organic macromolecules were degraded into small molecular acids, which could chelate nickel with one carboxyl group and form positively charged complexes. These nickel complexes were readily adsorbed to the cation-exchange resin via cation exchange or complete complexation, enhancing the removal efficiency. Accordingly, ozone oxidation and subsequent resin adsorption are effective for the removal of low-concentration chelated nickel from electroplating wastewater.