Chinese Journal of Rare Metals

Industrial production wastewater has high chemical oxygen demand(COD) value and complex composition, which precludes effective treatment via a single unit operation. Direct discharge into the natural environment without treatment will cause environmental pollution, damage the ecosystem and threaten human health. Therefore, it is particularly important to seek an advanced and efficient process for the treatment of industrial wastewater.Heterogeneous catalytic ozonation, as a kind of advanced oxidation processes(AOPs), has attracted more and more attention from researchers due to its strong oxidizability and the ability to decompose ozone into reactive oxygen species to attack pollutants. However, the development of environmentally friendly and efficient cheap catalysts to promote ozone decomposition still faces enormous challenges. In this paper, actual high-COD flame retardant manufacturing wastewater collected from Xurui Industrial Wastewater Treatment Plant was used as the treatment target to investigate the COD removal performance of ozone catalytic oxidation over Fe-based amorphous alloy catalysts and its underlying reaction mechanism. Firstly, the master alloy rod was prepared by vacuum magnetron tungsten arc melting furnace, and then the master alloy rod was heated and spun into amorphous ribbon by high vacuum single roller-spinning and spray casting system to obtain the catalyst. X-ray diffraction(XRD), ultraviolet spectrophotometer, scanning electron microscopy(SEM) and electron spin resonance(ESR) were used to systematically analyze the structure, catalytic performance, surface morphology change, and the type and contribution of free radicals of the catalyst. In addition, the effects of ozone concentration and pH value on the efficiency of ozone catalytic oxidation were also discussed. The results showed that compared with the ozone oxidation method without obvious COD removal effect, the combination of amorphous alloy catalyst and ozone could effectively improve COD removal rate. Among Fe₇₈Si_9B₁₃,(CoFe)₈₀B₂₀,(CoFeNi)₈₀B₂₀,(CoFeMn)₈₀B₂₀ and zero-valent iron(ZVI) samples, Fe₇₈Si_9B₁₃ showed the best catalytic efficiency, and COD of wastewater could be removed by 15.28% within 2 h.(CoFe)₈₀B₂₀,(CoFeNi)₈₀B₂₀,(CoFeMn)₈₀B₂₀ and ZVI couldonly remove COD of wastewater by 11.61%, 7.9%, 4.19% and 3.23%, respectively. In particular, it was observed during the experiment that ZVI powder was easily adhered to the inner wall of the reactor, resulting in a poor catalytic effect.As the reaction progresses, a large number of holes and corrosion pits appeared on the surface of Fe₇₈Si_9B₁₃, constantly exposing new substrates, and gradually evolving into a three-dimensional nanoflower-like structure, increasing active sites, promoting ozone decomposition to produce free radicals to attack pollutants. Secondly, the optimal ozone concentration was explored. The experimental results showed that COD removal ratios at ozone concentrations of 4, 24, 34, 39, 42 and 44 mg·L^-1 were 17.95%, 22.09%, 30.95%, 31.58%, 32.09% and 32.19%, respectively. Among them, when O₂ flow rate was 1.5 L·min^-1, that was, the ozone concentration was 34.0 mg·L^-1, COD removal effect hadreached 30.95%.When the ozone concentration continued to increase, COD removal rate did not increase much. In view of the cost and environmental friendliness, 34.0 mg·L^-1 was selected as the optimal ozone concentration. Similarly, the reaction time was investigated, revealing a COD removal efficiency of 30.9% at 2 h. Prolonging the reaction time only marginally improved the removal efficiency. Therefore, 2 h was selected as the optimal reaction time to balance efficiency and cost-effectiveness.The lower pH, the better COD removal effect of ozone catalytic oxidation.When pH was 3, 5, 7, and 9, COD of wastewater could be removed by 35.67%, 31.02%, 30.58% and 15.51%, respectively. This might be due to the acidic environment to promote the consumption of the catalyst, thereby promoting the production of more free radicals and degrading pollutants. However, no acid-base adjustment treatment of neutral wastewater could avoid additional cost increase, and the removal efficiency at pH 7 was only about 5% different from that at pH3, which still had a good COD removal ratio.The removal effect of wastewater with different COD values was also studied. The production wastewater with COD values of 5600, 7500, 16000, 36000, 40000 and 44000 mg·L^-1 were selected as the degradation target. The experiment was carried out under the conditions of ozone concentration of 34.0 mg·L~-¹, catalyst dosage of 1 g·L~-¹, pH 7 and room temperature. COD removal ratios after 2 h were 45.21%, 31.04%, 16.86%, 10.47%, 17.63% and 9.09%, respectively. With the increase of COD value, the removal ratio of COD decreased as a whole, but the removal rate of wastewater with COD of 40000 mg·L^-1 increased instead, which might be due to the fact that the organic pollutants in this kind of wastewater were more easily degraded. In addition, although the removal ratiowas reduced, COD degradation amount increased with the increase of COD value of the raw water, indicating that the ozone catalytic oxidation had a good degradation effect on COD of the raw water, and could better adapt to different concentrations of production wastewater.Throughout nine consecutive cycling tests, Fe₇₈Si_9B₁₃ catalyst consistently maintained a removal efficiency exceeding 26%, demonstrating its excellent cycling stability. Electron paramagnetic resonance tests showed that a large amount of hydroxyl radical (·OH) radicals could be produced in the ozone catalytic oxidation system, but this result was not observed in the ozone oxidation system, which further emphasized the role of the catalyst. In addition, the active site Fe in Fe₇₈Si_9B₁₃ had a variety of valence states, which could further promote the electron transfer of O₃ molecules and promote the reaction.