Diphenylarsinic acid contaminated soil remediation by titanium dioxide (P25) photocatalysis: Degradation pathway, optimization of operating parameters and effects of soil properties.

Diphenylarsinic acid (DPAA) is formed during the leakage of arsenic chemical weapons in sites and poses a high risk to biota. However, remediation methods for DPAA contaminated soils are rare. Here, the photocatalytic oxidation (PCO) process by nano-sized titanium dioxide (TiO2) was applied to degrade DPAA in soil. The degradation pathway was firstly studied, and arsenate was identified as the final product. Then, an orthogonal array experimental design of L9(3)(4), only 9 experiments were needed, instead of 81 experiments in a conventional one-factor-at-a-time, was used to optimize the operational parameters soil:water ratio, TiO2 dosage, irradiation time and light intensity to increase DPAA removal efficiency. Soil:water ratio was found to have a more significant effect on DPAA removal efficiency than other properties. The optimum conditions to treat 4 g soil with a DPAA concentration of 20 mg kg(-1) were found to be a 1:10 soil: water ratio, 40 mW cm(-2) light intensity, 5% TiO2 in soil, and a 3-hour irradiation time, with a removal efficiency of up to 82.7%. Furthermore, this method (except for a change in irradiation time from 3 to 1.5h) was validated in nine different soils and the removal efficiencies ranged from 57.0 to 78.6%. Removal efficiencies were found to be negatively correlated with soil electrical conductivity, organic matter content, pH and total phosphorus content. Finally, coupled with electron spin resonance (ESR) measurement, these soil properties affected the generation of OH• by TiO2 in soil slurry. This study suggests that TiO2 photocatalytic oxidation is a promising treatment for removing DPAA from soil.

[Photo-catalytical degradation of diphenylarsinic acid by TiO2 (P25)].

Diphenylarsinic acid (DPAA) is the major contaminant in environment polluted by abandoned chemical weapons. DPAA poses high risks to biota but remediation methods for this contaminant are rare. Previous research showed DPAA could be degraded within a short time by TiO2 (P25). Here the kinetics of DPAA degradation catalyzed by P25 was studied. Results showed the photo-catalytical degradation of DPAA by P25 consisted of two processes: adsorption and photo-reaction. The whole reaction could be fitted by Langmuir-Hinshelwood kinetics. Variation in pH and ionic strength caused change in adsorption of DPAA onto the TiO2 catalyst, which led to the change of reaction rate, showing a decreasing trend with the decreasing adsorption amount of DPAA. Dissolved oxygen promoted the catalytical degradation of DPAA by TiO2, and the hydroxyl free radical played the most important role in the photodegradation of DPAA, which was testified through quenching experiments with free radical scanvengers.

[Spectroscopic study of photocatalytic mechanism of methanol and CO2].

Ni-Ti-O/SiO2 catalyst was prepared by impregnation method, and its photocatalytic performance for carbonylation of methanol with CO2 was investigated under UV light. The in-situ IR, XPS and MS were carried out to analyze the possible photocatalytic reaction mechanism. Results indicated that the Ni-Ti-O/SiO2 exhibited good photocatalytic performance for carbonylation of methanol with CO2, the methanol conversion reached up to 24.9%, and the selectivity for the carbonylated products was more than 60% within 180 min reaction time. The catalyst characterization results showed that the O==C .--O- and CH3OC(O)* might be important intermediate in the carbonylation of methanol with CO2.