| Arsenic is a toxic metalloid element in nature and arsenic (As) contamination is widely recognized as a global health problem. Arsenic mainly occurs in its inorganic form as As(Ⅲ) and As(V) oxo-anions in natural waters, and the distribution of the species depends on the redox potential and the pH of the water. The standard potential of the oxidation of As(Ⅲ) to As(V) is lower than the oxidation potential of Fe(II) to Fe(Ⅲ) and other species typically found in groundwater. However, it is known that although Fe(Ⅱ) oxidation occurs rapidly in the presence of air, the oxidation rate of As(Ⅲ) is extremely slow. Although As(V) is thermodynamically favored under oxidizing conditions, As(III) is only slowly oxidised by dissolved O2, and half-lives of around 9 d in low iron-content, air-saturated, pH 7.6-8.5 water have been reported. Due to this slow oxidation, arsenic is found as AsⅢ) in underground water. As(Ⅲ) is generally reported to have low affinity to the surface of various minerals compared with As(V) because As(Ⅲ) exists mainly as nonionic H3AsO3 in natural water when pH< 9. Nevertheless, arsenate adsorbs easily to solid surfaces, so it is easier to remove As(V). Since As (Ⅲ) is more toxic and more difficultly removed than As (V), a pre-oxidation technology by transforming As (Ⅲ) to As (V) is highly desirable to removal arsenic from water. But the cost of traditional chemical oxidation technology is high, and usually causes secondary pollution. Taking into consideration the problems existing in the process of chemical oxidation, this article focuses on As(Ⅲ) oxidation, researches the As(III) conversion mechanism under applied voltage and uses Pt/TiO2 nanotubes and Ag@AgCl core-shell nanowires to transform As(III) through photochemistry/photoelectrochemistry methods, and provides new technological means and research perspectives for the removal of arsenic (Ⅲ) in groundwater. In this paper, a separate system was established to study the redox process of As(III), especially exist form and reaction mechanism in cathodic cell. Pt/ TiO2 nanotubes materials were firstly tried to photoelectrocatalytic oxidation for As (III) in water driven by visible light; the obtained Pt/TiO2 nanotubes electrodes were fabricated by cathodic constant current deposition and characterized by FE-SEM, EDS, XPS and UV-vis spectrometer; the influence of Pt deposition current density, Pt deposition time, initial concentration of As(Ⅲ), applied bias potential and solution pH on the As(Ⅲ) photoelectrocatalytic oxidation efficiency was researched. Ag@AgCl core-shell nanowires with different ratio of Ag:AgCl were synthesized to realize the rapid oxidiation of As(III) under visible light irradiation were fabricated through the oxidation of Ag nanowires by moderate FeCl3 and characterized by FE-SEM, EDS, XRD and XPS; the effect of different molar ratios of Ag:FeCl3, solution pH, NaSO4 concentration, Fe3+ions and humic Acid on the As(III) photoelectrocatalytic oxidation efficiency were researched; the kinetics and mechanism of photocatalytic oxidation for As(III) were studied; stability and regeneration of Ag@AgCl core-shell nanowires were investigated. Details are presented in this thesis as follows:In order to provide theoretical support for the further photoelectrocatalytic oxidation of As(III), the anode/cathode seperated electrolytic cell was established to study the transform mechanism of As(Ⅲ).40.0V voltage was exerted to electrolyze As(Ⅲ) in the separation system. As the electrolysis went on, As(Ⅲ) (H3ASO3) in anode tank was gradually oxidized to As(V) (H3ASO4). The pH continued to drop down. In the cathode pool, within 27 min As(III) was reduced to As(0) or As(-III) which eventually converted to ASH3 and deviated from the liquid system. After 27 min, As(Ⅲ)(H3AsO4) left began to convert into As(V) (ASO43-), meanwhile pH jumped from acidic to alkaline.40.0V voltage was exerted to electrolyze the arsenic solution in the separation system as well, the pH in the cathode pool was adjusted to 1.50. According to the diagram of arsenic Eh-pH, H3AsO3 (arsenic trioxide) was the based form. As(III) in cathode pond was eventually converted to ASH3 and emitted into the air, causing the reduction of total arsenic. As(III)(H3AsO3) in anode pond was gradually oxidized to As(V)(H3AsO4). In the low 1.0 V voltage conditions, the electric potential of anode pool was around 1100mV, and the cathode pond 600 mV. As(III) in anode pool and cathode pool converted to As(V), but the converting rate of anode pool was faster.In order to reduce the energy consumption during the electrocatalytic oxidation process, the conversion of As (Ⅲ) in the mixed electrolytic cell, the anode cell/ cathode cell of the seperated electrolytic pond under the condition of low voltage 0-1.0V was further investigated. When the applied voltage is 0.1-0.8 V, As (Ⅲ) can all be converted to As (V) and effectively removed in mixed electrolytic cells and anode cells of divided systems. The change kinetics of As (Ⅲ) concentration conforms to the exponential decay equation Ct=C0 exp-kobs t, and the mathematical models have been established among the conversion rate of As (HI), the conversion rate of As (Ⅲ) and the voltage. When voltage is constant, the conversion rate of As (Ⅲ) is proportional to its concentration; when the concentration of As (Ⅲ) is constant, the conversion rate of As (Ⅲ) has an exponential growth relation with voltage. The conversion rate and removal rate of As (Ⅲ) in mixed electrolytic cells are higher than those in the anode cells of separated systems, for the reason of the high resistance of the salt bridge in the separated systems. Energy consumption analysis has shown that the consumed energy of the conversion and removal of As (Ⅲ) will reduce with the decrease of voltage, and the consumed energy of As removal in mixed systems is less than that in the separation systems, but the consumed energy of As(Ⅲ) conversion in mixed systems is lager than that in the separation systems.The modified TiO2 by noble metal exhibited strong photocatalytic ability under visible light, so researchers further researched the oxidatin of As(Ⅲ) using Pt/TiO2 nanotube arrays electrode at low voltage under visible light. A constant current deposition method was selected to load highly dispersed Pt nanoparticles on TiO2 nanotubes in this paper, to extend the excited spectrum range of TiO2-based photocatalysts to visible light. EDS analysis pr oved that Pt nanopaticles were existed and focused on the tube wall close to the nozzle. The UV-Vis diffuse reflectance spectra (DRS) showed that Pt/TiO2 nanotubes had narrower band gap and had a strong photoabsorption amount in visible light region. The photocatalytic and photoelectrocatalytic oxidation of As(Ⅲ) using a Pt/TiO2 nanotube arrays electrode under visible light (λ> 420 nm) irradiation were investigated in a divided anode/cathode electrolytic tank. Compared with pure TiO2 which had no As(Ⅲ) oxidation capacity under visible light, Pt/TiO2 nanotubes exhibited excellent visible-light photocatalytic performance toward As(Ⅲ). In anodic cell, As(Ⅲ) could be oxidized with high efficiently by photoelectrochemical process with only 1.2V positive bias. Experimental results showed that photoelectrocatalytic oxidation process of As(III) could be well described by pseudo-first-order kinetics model. Rate constants depended on initial concentration of As(Ⅲ), applied bias potential and solution pH. At the same time, it was interesting to find that in cathode cell, As(III) was also continuous oxidized to As(V). Furthermore, High arsenic ground water sample with 0.32 mg/L As(Ⅲ) and 0.35 mg/L As(V) collected from Datong basin, Northern China, could totally transform to As(V) under visible light in this system.It was found that there were plenty of visible light driven semiconductor photocatalytic materials with narrow band gap, which showed excellent ability to degradate organic pollutants under visible light without voltage. In order to promote the As(Ⅲ) photo-catalytic oxidation performance under visible light irradiation, the visible light driven photocatalysts Ag@AgCl core-shell nanowires were synthesized through the oxidation of Ag nanowires by moderate FeCl3. The photo-catalytic oxidation performance of Ag@AgCl core-shell nanowires with different ratios of Ag:AgCl for As(III) was investigated. Detailed studies revealed that Ag@AgCl core-shell nanowires synthesized with the optimum ratio of Ag nanowires: FeCl3 exhibited excellent photocatalytic activity for As(Ⅲ) oxidation and the photocatalytic oxidation efficiency was significantly dependent on the molar ratio of Ag:FeCl3. The oxidation rate rose as the Ag0 percentage decreased, up until the optimized ratio of Ag nanowires:FeCl3 values (2.3:2.2) with the oxidation rate of 0.023mg L-1 min-1, but the rate of Ag@AgCl synthesized with excess FeCl3 was slightly lower than the optimal ratio of Ag:AgCl. The trapping experiments using radical scavengers confirmed that h+ and ·O2- acted as the main active species during the photocatalytic process. Additionally, the recycling experiments validated that Ag@AgCl core-shell nanowires was a kind of active and stable photocatalyst for the oxidation of As(Ⅲ) under visible light irradiation. |
PDF Full Text Request