Nitrogen-doped Carbon Nanoporous Material A Catalyst For Direct Red23 Oxidation In Persulfate-containing Aqueous Solutions

The azo dyes-containing waste water presents a great challenge for its treatment because of the toxicity and poor biodegradability.Persulfate(PS)advanced oxidation technology is an effective approach for the treatment of dyeing waste water with high efficiency and good mineralization.Nevertheless,PS need to be actived with heavy metal catalysts or by heating or UV-irradiation.Compared with heavy metal catalysts,persulfate(PS)activated with metal-free catalyst is more economic,environmentfriendly and safe to human health.In this study,one type of nitrogen-doped carbon(NCN)material was evaluated to catalyze PS for the oxidation of Direct Red 23(DR23).Phosphoric acid,sucrose,and hexamethylenetetramine was used as carbon source and nitrogen source to compound the NCN.Then,various measures were conducted to definite the apparent and internal structures of NCN.The results show that N element was doped in the carbon structure successfully,and the mass fraction of N element is about 7.21%.Four kinds of nitrogen functional groups were found in NCN,including pyridinic-N(N6),pyrrolic-N(N5),quaternary-N(N-Q)and oxidized nitrogen(NOx),and the mass fraction of this four components are 1.86 at.%,1.01 at.%,3.50 at.% and 0.86 at.%,respectively.Results of batch experiments indicated that NCN could efficiently catalyze PS for degradation of DR23 and the reaction can be described by first-order kinetics and the observed first-order rate constant(Kobs)is 3.36 × 10-2 min-1,R2 = 0.99.Within 100 minutes,72% COD removal and nearly 100% DR23 and color removal were achieved,with the presence of NCN(typically 200 mg/L)and PS(5 m M).Byproduct analysis revealed that DR23 was completely degraded in the PS/NNC systemKey factors affecting the reaction were tested,including the PS concentration,NCN dosage,and pH.DR23 degrading rate increased remarkably with the increase of the PS concentration and NNC dosage.The observed first-order rate constants have a good linear relationship with the NCN dosage from 50 mg/L ~ 1000 mg/L.High PS concentration leading to a small increase of DR23 removal rate when the molar ratio of PS:DR23 over 300:1.The degradation was favored at low pH.For dyeing waste water with high pH(pH > 10),adding more PS is an effective choice.NCN can be easily and effectively reused with 78.4% recovery.A competitive radical experiment was conducted to evaluate the PS activation mechanism.The result indicates that the activation followed the non-radical mechanism,which was different with the most PS advanced oxidation processes.The catalytic mechanism of the NCN mediated DR23 degradation may be the function of enhancing electron transfer,on the basis of literature and observations made above.Specifically,in this study,the observed Kobs have a good linear relationship with the mass fraction of both N6 and N-Q,with R2 > 0.95,which indicate that N6 and G-N played a critical role in the catalytic oxidation of DR23.