Degradation Of 4-NP By Sequential Electrolysis And Visible-light-driven Photocatalysis

Nitrophenols are widely used in producing pesticides, dyes, explosives and other chemical products. The contamination of nitrophenols and their products have attracted more concerns for the stability to biodegradation. Therefore, it is necessary to find new method to dicompose nitrophenols and their products. In this dissertation,4-nitrophenol (4-NP) was chosen as the model pollutant and its degradation by sequential electrolysis and visible-light-driven photocatalysis processes were investigated. The contents are described as follow:(1) Analogue 4-NP wastewater was electrolyzed in non-membrane electrolytic reactor with Ti/Pt anode and stainless steel cathode. At the optimized condition of pH=2.0,300 mA/cm2 and 100 mg/L initial concentration,4-NP removal and COD removal were 94% and 51%, respectively. Cyclic voltammetry study indicated Ti/Pt belonged to electro-conversion anode which could not mineralize organics completely.4-NP could be reduced by steps to 4-aminophenol (4-AP) which was more easily to be oxidized than 4-NP. Intermediate products were analyzed by high performance liquid chromatography (HPLC) and the mechanism of 4-NP degradation was proposed.4-NP was oxidized at anode to hydroquinone and benzoquinone. And then the benzene ring was decomposed into fatty compounds by further oxidation. At cathode,4-NP was reduced to 4-AP which could transfer to anode and was oxidized to hydroquinone. Nitro group was oxidized to nitrite radical at last. The function of anode was removing COD but less responsible for decomposing 4-NP.(2) The electrochemical degradation of 4-NP was operated in membrane electrolytic reactor by sequential electrolysis process. It was proved that at the conditions of unadjusted pH,300 mA/cm2 and 100 mg/L initial concentration, the best 4-NP removal and COD removal were 89.3% and 75.5% with the sequential order of reduction for 240 min and then oxidation for 60 min. The COD removal was raised by 24.5% compared with that in none-membrane electrolytic reactor. The brown deposit found in electrolytic reactor was analyzed and confirmed by fourier transformed infrared (IR) as the polymer of 4-AP. The mechanism of polymerization was also proposed as the polymerization of quinoneimine oxidized from 4-AP by 1,4-additoin reaction.(3) Three novel photocatalysts Zn-Cr layered double hydroxide (LDH), Zn-Ti LDH and were fabricated and their photocatalytic activities were investigated. Scanning electron microscope (SEM), ultroviolet-visible spectrophotometer (UV-vis) and diffused X-ray diffractometry (XRD) analysis indicated Zn-Cr LDH could absorb visible light with two absorbance peaks at 410 nm and 560 nm while Ag3PO4 could absorb light less than 460 nm, but Zn-Ti showed no absorbance above 390 nm. The experiments of water splitting and decomposing methylene blue (MB) and 4-NP under visible light irradiation (≥420 nm) proved that the activity of the three semiconductors was Ag3PO4> Zn-Cr LDH> Zn-Ti LDH. However, Ag3PO4 is easily to be oxidized in air as the color turning to brown, and could decompose itself without electron accepter in photocatalysis experiments. Zn-Cr LDH is not stable in the experiments for leasing Cr3+ ion into solution. Zn-Ti LDH does not have photocatalytic activity under visible light illumination.(4) Three complex metal oxide photocatalysts Bi-V (Bi:V=6:4), Fe-Sn (Fe:Sn=99:1) and Bi-V-W (Bi:V:W=6:2:2) were fabricated and screened with dispenser and scanning electrochemistry mircroscope (SECM). With SEM, UV-vis and XRD analysis of film samples, it was confirmed that doping W to Bi-V photocatalyst could make the absorbance edge shifting from 550 nm to 635 nm. Bi-V-W could decompose 4-NP in 180 min completely under visible light irradiation (≥420 nm), while Fe-Sn and Bi-V could decompose 48% and 15% 4-NP, respectively.