Effect of NOM on arsenic adsorption by TiO2 in simulated As(III)-contaminated raw waters

The effect of natural organic matter (NOM) on arsenic adsorption by a commercial available TiO2 (Degussa P25) in various simulated As(III)-contaminated raw waters was examined. Five types of NOM that represent different environmental origins were tested. Batch adsorption experiments were conducted under anaerobic conditions and in the absence of light. Either with or without the presence of NOM, the arsenic adsorption reached steady-state within 1 h. The presence of 8 mg/L NOM as C in the simulated raw water, however, significantly reduced the amount of arsenic adsorbed at the steady-state. Without NOM, the arsenic adsorption increased with increasing solution pH within the pH range of 4.0–9.4. With four of the NOMs tested, the arsenic adsorption firstly increased with increasing pH and then decreased after the adsorption reached the maximum at pH 7.4–8.7. An appreciable amount of arsenate (As(V)) was detected in the filtrate after the TiO2 adsorption in the simulated raw waters that contained NOM. The absolute amount of As(V) in the filtrate after TiO2 adsorption was pH dependent: more As(V) was presented at pH>7 than that at pH<7. The arsenic adsorption in the simulated raw waters with and without NOM were modelled by both Langmuir and Frendlich adsorption equations, with Frendlich adsorption equation giving a better fit for the water without NOM and Langmuir adsorption equation giving a better fit for the waters with NOM. The modelling implies that NOM can occupy some available binding sites for arsenic adsorption on TiO2 surface. This study suggests that in an As(III)-contaminated raw water, NOM can hinder the uptake of arsenic by TiO2, but can facilitate the As(III) oxidation to As(V) at TiO2 surface under alkaline conditions and in the absence of O2 and light. TiO2 thus can be used in situ to convert As(III) to the less toxic As(V) in NOM-rich groundwaters.