Self-Competitive Adsorption Behavior of Arsenic on the TiO2 Surface.

TiO2 is a commonly used material to remove arsenic from drinking water by adsorption as well as photocatalytic oxidation (PCO). In the present paper, arsenic adsorption and PCO at different pH environments are studied on the (1 1 0) facet of rutile TiO2 (r-TiO2). A self-competitive adsorption (SCA) behavior of arsenic is observed; i.e., arsenic species compete to adsorb on the surface. Related DFT calculations are carried out to simulate adsorption. SCA behavior is the key to connecting calculation results with experimental results. Furthermore, PCO of arsenite is performed at different pH values. Of note, PCO is related to adsorption; namely, the adsorption process determines the whole PCO reaction speed. Therefore, SCA is also helpful for the PCO reaction. The SCA behavior is useful not only for arsenic on r-TiO2 but also for arsenic on anatase TiO2 (a-TiO2). It may be helpful to further study arsenic adsorption and PCO on other materials such as Fe2O3 and MnO2. The SCA behavior extends our understanding of arsenic and provides new insights into arsenic removal and its cycle in nature.

Preparation of CeO2-doped carbon nanotubes cathode and its mechanism for advanced treatment of pig farm wastewater.

The effluent from conventional treatment process (including anaerobic digestion and anoxic-oxic treatment) for pig farm wastewater was difficult to treat due to its low ratio of biochemical oxygen demand to chemical oxygen demand (BOD5/CODCr) (<0.1). In the present study, electro-Fenton (EF) was used to improve the biodegradability of the mentioned effluent and the properties of self-prepared CeO2-doped multi-wall carbon nanotubes (MWCNTs) electrodes were also studied. An excellent H2O2 production (165 mg L-1) was recorded, after an 80-min electrolysis, when the mass ratio of MWCNTs, CeO2 and pore-forming agent (NH4HCO3) was 6:1:1. Results of scanning electron microscopy (SEM), transmission electron microscope (TEM) and x-ray photoelectron spectroscopy (XPS) showed that addition of NH4HCO3 and the doping of CeO2 could increase the superficial area of the electrode as well as the oxygen reduction reaction (ORR) electro-catalytic performance. The BOD5/CODCr of the wastewater from the first stage AO process increased from 0.08 to 0.45 and CODCr reduced 71.5% after an 80-min electrolysis, with 0.3 mM Fe2+ solution. The non-biodegradable chemical pollutants from the first stage AO process were degraded by EF. The non-biodegradable pollutants identified by LC-MS/MS in the effluent from AO process including aminopyrine, oxadixyl and 3-methyl-2-quinoxalinecarboxylic acid could be degraded by EF process, with the removal rates of 81.86%, 34.39% and 7.13% in 80 min, and oxytetracycline with the removal rate of 100% in 20 min. Therefore, electro-Fenton with the new CeO2-doped MWCNTs cathode electrode will be a promising supplement for advanced treatment of pig farm wastewater.

Kinetics and mechanisms of oxytetracycline degradation in an electro-Fenton system with a modified graphite felt cathode.

The removal of trace antibiotics from the aquatic environment has received great interest. In this investigation, NaOH activated graphite felt (NaOH-GF) was characterized by multiple-methods, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle, linear sweep voltammetry (LSV) and electron paramagnetic resonance (EPR). The NaOH-GF was then used as the cathode in the electro-Fenton process for oxytetracycline (OTC) degradation, the experiment was carried out in an undivided and light-proof beaker with a Pt anode and a NaOH-GF cathode at pH 3. The results showed that the modification with NaOH enhanced the antibiotics degradation efficiency of graphite felt by increasing the oxygen reduction capacity and hydroxyl radicals yielding rate. Complete OTC removal was achieved at 5.17 mA cm-2 after 40, 60 and 90 s with initial OTC concentration of 22, 44, and 66 µM, respectively. With an initial OTC concentration of 44 µM, after 30 min the removal rates of chemical oxygen demand (COD) by Raw-GF and NaOH-GF were 59.18% and 83.75%, respectively. The proposed degradation mechanism of OTC was an EF process, which consisted of hydroxylation, secondary alcohol oxidation, demethylation, decarbonylation, dehydration and deamination. This study demonstrates that NaOH activated GF cathode possesses high degradation capacity and good stability. It provides insight into the removal of non-biodegradable antibiotics and may shed light on future to its practical application.

Adsorption and oxidation of arsenic by two kinds of ß-MnO2.

MnO2 is one of the most widespread and cheapest materials in nature that can both adsorb arsenic and oxidize arsenite [As(III)] to arsenate [As(V)]. In this study, column ß-MnO2 [CM] with the main facet of {110} and pincer ß-MnO2 [PM] with the facets of {110} and {101} are synthesized and used to remove arsenic in water under different conditions. For the adsorption process, the experimental data are fitted well with the pseudo second-order kinetic model; the Langmuir model is better than the Freundlich model to describe the adsorption equilibrium isotherms. Furthermore, the As(III) oxidation rate can be denoted by the pseudo zero-order kinetic model and is related to the O2 concentration, the pH value, the light source and the initial concentration of As(III). Finally, the oxidation mechanism is investigated, and the oxidant should be related to O2. It is interesting to find that these two kinds of ß-MnO2 exhibit different pH effects for both adsorption and oxidation. For As(III), the adsorption and oxidation abilities of CM follow the order pH 9 > pH 7 > pH 4, whereas the adsorption and oxidation orders of PM are pH 4 > pH 7 > pH 9.

pH effects of the arsenite photocatalytic oxidation reaction on different anatase TiO2 facets.

TiO2 is one of the most cheap materials which can both adsorb arsenic and oxidize arsenite [As(III)] to arsenate [As(V)]. In this study, anatase TiO2 crystals with different main facets such as {101}, {001} and {100} are synthesized and used to investigate arsenic adsorption kinetics, adsorption isotherms, photocatalytic oxidation (PCO) process and the pH effects. The adsorption kinetics of arsenic on TiO2 crystals can be described by the pseudo second-order kinetic model. For the adsorption isotherms, the Langmuir model is better than the Freundlich model for arsenic on these TiO2 crystals. For the PCO process, the rate of As(III) oxidation can be denoted by the pseudo first-order kinetic model. It should be noted that at neutral condition the adsorption and PCO rates of the three kinds of TiO2 crystals follow the order of {101} > {001} > {100}. The pH effect is above all important for both the arsenic adsorption and its PCO. The highest PCO speed appears at high pH values such as at pH 11 or 12.

The effect of pH on the adsorption of arsenic(III) and arsenic(V) at the TiO2 anatase [101] surface.

Octahedral TiO2 nanocrystals (OTNs) have been prepared by a hydrothermal method with the main surface of (101). Then the arsenic adsorption behavior on OTNs is investigated in a broad experimental pH range from 1.0 to 13.5. The maximum adsorptions of arsenite (As(III)) and arsenate (As(V)) appear at pH values 8 and 4, respectively. It is interesting to see that the minimum adsorptions of As(III) and As(V) are both at pH 12 and then their adsorptions increase again at higher pH values such as 13.0 and 13.5. To our best knowledge, it is quite new to report the arsenic adsorption on the controlled TiO2 surface especially at very high pH values. These results might be helpful to understand the adsorption mechanism. On the other hand, periodic slab models of TiO2 anatase (101) surface with some H(+) cations, some water molecules or some OH(-) ions are suggested to simulate the pH effect. Using these models, the adsorptions of As(III) and As(V) are simulated by the density functional theory (DFT) method. Qualitatively, the adsorption abilities of arsenic species, water and OH(-) follow the order of AsO3(3-)>OH(-)>HAsO3(2-)>H2AsO3(-)>H2O>H3AsO3 for As(III) and AsO4(3-)>OH(-)>HAsO4(2-)>H2AsO4(-)>H3AsO4>H2O for As(V). It implies that H2AsO3(-) should be the major As(III) species at pH 8 and H2AsO4(-) should be the major As(V) species at pH 4, and the most negative charged ions AsO3(3-) and AsO4(3-) should correspond to the adsorptions at the high pH values 13 and 13.5.