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 1.5 h) 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. - Highlights: • DPAA was degraded into arsenate through TiO₂ (P25) photocatalytic oxidation. • Soil/water ratio was more influential on the removal of DPAA in soil by TiO₂ (P25). • Soil properties affected the adsorption of DPAA and the generation of OH• by TiO₂.