Activated Carbon Fiber Catalyzed Peroxymonosulfate Oxidation Of Acid Orange7in Aqueous Solution

For refractory organic contaminants in wastewater such as azo dyes, advancedoxidation processes (AOPs) are effective treatment methods. In recent years, AOPsbased on peroxymonosulfate (HSO5, PMS), which involve certain means to activatePMS to produce strongly oxidizing sulfate radicals (SO4) for degradation of organicpollutants, have attracted broad attention of scientific researchers. The activationmethods of PMS focused on UV and cobalt, but either of them has its obviousshortcomings. Therefore, looking for more approaches for PMS activation isbecoming an academic concern.Carbon materials are not only good adsorbents, but green catalysts in catalyticoxidation of O3, H2O2, PS and PMS to degrade various refractory matters.This study investigated the feasibility of activated carbon fiber (ACF) as acatalyst activating PMS to eliminate azo dye AO7in aqueous solution, and verifiedthe following questions:(i) how does catalysis process occur;(ii) how do adsoptionand catalysis influence each other;(iii) how is AO7degraded. Specific studies are asfollows:(1) Results indicated that there exsited a remarkable synergistic effect inACF/PMS system. When ACF dosage was0.3g/L, PMS/AO7mole ratio was20/1,and the initial AO7concetration was100mg/L, complete decolorization can beachieved in2h. To some extent, the decoloring rate increased with the increase of thePMS concentration, and when the PMS dosage increased to a certain value, thedecoloring rate no longer changed. In addition, the decolorization of AO7increasedwith the increase of ACF dosage, and the decoloring rate and the dosage of ACF werelinearly dependent. When ACF dosage was0.3g/L, the concentration of PMS was20/1, the initial AO7concentration had little effect on the decolorization. Along withthe increase of repeated used times of ACF, AO7removal efficiency decreasedobviously. Inhibitors experiments showed that the main oxidizing species were SO4 and HO. In addition, it was found from analysis of surface properties of differentACF samples that basic sites were possible catalytic active sites of ACF.(2) In ACF/PMS or GAC/PMS system, the processes of adsorption andcatalysis influnced each other. On one hand, AO7or intermediate products adsorbedon the GAC or ACF surface can prevent the contact of PMS and catalytic sites, thusreducing the generation of free radicals and AO7degradation. This explained whyACF had faster adsorption rate of AO7and oppositely lower repeated-use efficiencythan GAC. On the other hand, the introduce of acid functional groups by PMSoxidation caused a huge decline in AO7adsorption by ACF, so many free radicalsgenerated were “wasted” because of there were few AO7moleculars near them, andthe degradation of AO7decreased.(3) The main intermediate products from AO7degradation were1,2-naphthalene diketone,2H-1-benzo pyran-2-ketone, indene three ketone,1,4-benzoquinone, formic acid and acetic acid. According to these intermediates, apossible degradation pathway of AO7was given: the rupture of azo bond causeddecolorization; the reaction products were oxidized to generate somenaphthalene-type and benzene-type compounds; a series of oxidation degradationsteps led to cleavage of aromatic ring, thus generating formic acid and acetic acid,which were finally transformed into CO2.(4) ACF/PMS system can degrade azo dyes including acid chrome blue K(ACBK), acid red249(AR249) and reactive black5(RB5) in various degrees.Although ACBK can be decolorized by PMS, ACF can significantly accelerate thedecolrization. It was difficult for AR249and RB5to be adsorbed by ACF or oxidizedby PMS directly, but the combined use of ACF and PMS obviously improved theirdegradation. The arrangement of several azo dyes studied in order of degradation ratesby ACF/PMS system was AO7> ACBK> AR249> RB5, and the order conformed toACF adsorption ability of them, which comfirmed that the adsorption process playedan important role in the catalytic degradation of contaminants.