Co-based Metal-organic Framework for High-efficiency Degradation of Methylene Blue in Water by Peroxymonosulfate Activation

In the field of water purification, the advanced oxidation process based on sulfate radical (SO4 center dot-) has great potential applications due to its high selectivity and oxidation advantages. The development of high-performance peroxymonosulfate (PMS) catalysts to produce SO4 center dot- remains a research hotspot for dye wastewater treatment. In this work, a cobalt based metal-organic framework compound with formula Co(mu(6)-odip)(0.5)(mu(2)-OH2)(0.5)(H2O)2 center dot 1.5H(2)O (1) was synthesized by solvothermal method using 5,5'-oxydiisophthalic acid (H4odip) ligand and Co2+ ions. The reaction solvent was acetonitrile and water, the reaction temperature was 135 degrees C. The obtained pink crystal compound has the good water stability and acid-base resistance. The structure and composition of 1 were characterized by X-ray single crystal and powder diffraction, thermogravimetric analysis, elemental analysis and infrared spectroscopy. Complex 1 belongs to monoclinic system, C2/c space group with cell parameters: a=1.6101(9) nm, b=1.5508(10) nm, c=0.9660(6) nm, alpha=90 degrees, beta=112.70(2)degrees,gamma=90 degrees. Moreover, the catalytic performance of 1 by peroxymonosulfate activation for degradation of methylene blue was tested in water by UV-Vis spectrophotometer. At the same time, the effects of 1 and peroxymonosulfate loading, reaction temperature and solution pH on dye degradation were systematically studied. The reaction mechanism of dye degradation was explored through free radical capture experiment. The test method is to take 30 mL of dye solution (20 mg/L) in a beaker and add 3 mg 1 and 0.5 mL PMS (100 mg/L). Keep stirring throughout the experiment. At a certain time interval, the supernatant of dye solution was taken and the absorption intensity at the maximum absorption wavelength (lambda= 665 nm) was measured by UV-Vis spectrophotometer to monitor the change of methylene blue (MB) concentration. The above method is also applicable to all comparison experiments. The pH of the dye solution was adjusted using hydrochloric acid and sodium hydroxide at a concentration of 0.1 mol/L. Radical scavenging agents were added to the reaction system in the free radical trapping experiment. The experimental results show that the 1 can generate reactive oxygen species (ROS) by activating peroxymonosulfate, and rapidly degrade MB in aqueous solution, the degradation rate can reach 97.4% within 8 min, and the catalytic activity is good in a wide range of pH. The results showed that 1 and PMS loading capacity were the key factors for the degradation of dyes by activated peroxymonosulfate, and the degradation rate could be improved by increasing the compound loading capacity, PMS concentration and temperature. After five cycles, the compound still retained a high degradation rate of 90.8%. A comprehensive analysis of the quenching experiment and electron paramagnetic resonance (EPR) test results confirmed that the ROS produced by the 1/PMS system include SO4 center dot-, center dot OH, O-2(center dot)- and O-1(2), among which SO4 center dot-, center dot OH and O-1(2) play a major role in the catalytic degradation of MB, but O-2(center dot-) can also promote the catalytic degradation of MB. The above research results indicate that 1 can be used as an effective and reusable heterogeneous catalyst for the treatment of dye wastewater by peroxymonosulfate activation.