Enhanced removal of organics and ammonia by a composite anode in the electrochemically assisted ozonation (EAO) processes with reduced sludge and alleviated passivation

Treatment of organics and NH₄⁺-N containing wastewater has become a wide concern due to the difficulties for their removal in single technologies. In this study, a new composite anode (CA) was designed in which Fe powder was magnetically attracted by an internally inserted magnet onto a dimensionally stable anode (DSA) tube surface. The CA was utilized in the electrochemically assisted ozonation (EAO) process and the results were compared with the traditional Fe and DSA anode. Results showed that the CA-EAO process exhibited 9.46% and 53.40% higher efficiency for model organic oxalic acid (OA) removal compared to the Fe-EAO and DSA-EAO at optimal operating condition (current intensity 1 A, iron powder dosage 60 mg/cm², initial pH 3 and Cl⁻ concentration 50 mM). Besides, the removal efficiency of CA-EAO for NH₄⁺-N removal was 72.61% higher than Fe-EAO although slightly lower than DSA-EAO. A series of trapping experiments and electron paramagnetic resonance (EPR) tests were also conducted and results showed that higher ^[rad]OH and reactive chlorine species (RCS) were generated by the CA-EAO compared to Fe-EAO, which is favorable for organic and NH₄⁺-N removal, respectively. Corresponding mass transfer coefficients measurements demonstrated that attraction of iron powder on the DSA surface increased the mass transfer coefficients of the electrochemical system. Quantification of accumulated Fe illustrated that 68.99–76.64% reduction in the production of iron sludge was achieved by using CA in the EAO process compared with Fe anode. Moreover, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) mapping analysis proved that iron surface passivation was alleviated on the CA compared with Fe anode. The present study provides an implementable strategy for the treatment of wastewater containing organics and NH₄⁺-N with high efficiency and less by-product generation.