SERS analysis and titanium dioxide treatment of arsenic in water

This dissertation investigated arsenic analysis using surface-enhanced Raman scattering (SERS) technology, size effects of TiO2 nanoparticles on arsenic adsorption and photooxidation, and photodegradation mechanisms of two organic arsenic compounds using TiO2 nanoparticles assisted by UV irradiation.;A modified mirror reaction was developed to prepare a sensitive and reproducible Ag nanofilm substrate for the surface-enhanced Raman scattering (SERS) analysis of arsenate (As(V)) and arsenite (As(III)). A good linear relationship between the SERS intensity of As(V) and As(III) and their concentrations in the range from 10 to 500 microg-As/L was achieved using the SERS substrate. As(V) and As(III) appear to be adsorbed on the Ag nanofilm through formation of surface complexes with Ag, based on the comparison of Raman spectra of the arsenic species in solutions, on the SERS substrate, and in silver arsenate and arsenite solids. As(V) and As(III) species on the SERS substrate and in the solids had the same Raman band positions at 780 and 721 cm-1 respectively. The effect of eight ions in natural waters on the SERS analysis of As(V) was studied. K+, Na+, SO4 2-, CO32- and NO3- in the range of 0.1-100 mg/L did not interfere with the SERS detection of As(V) for a As(V) concentration greater than 100 microg-As/L. Cl- (50 mg/L), Mg2+ (10 mg/L) and Ca2+ (1 mg/L) were found to quench the SERS intensity of 100 microg/L As(V). Cl- (at concentration >10 mg/L) formed silver chloride with the adsorbed Ag+ and decreased the SERS detection limits for arsenic species. The mechanism of the Ca2+ effect on the SERS analysis of As(V) was through the formation of surface complexes with As(V) in competition with adsorbed Ag +. When Ca2+ concentration increased from 0 to 100 mg/L, the amount of As(V) adsorbed in Ag nanoparticles was reduced from 38.9 to 11.0 microg/mg-Ag. When the Ca2+ concentration increased to values higher than 1 mg/L in the As(V) solution, the As(V) peak height was decreased in the corresponding SERS spectra and the peak position shifted from 780 to 800 cm-1. The fundamental findings obtained in this research are especially valuable for the development of sensitive and reliable SERS methods for rapid analysis of arsenic in contaminated water.;The physicochemical properties of TiO2 particles in the diameter range between 6.6 and 30.1 nm and the effect of the crystalline size on arsenic adsorption and photocatalytical oxidation were investigated. TiO2 nanoparticles of different sizes were single-phase anatase. The adsorption capacity of the TiO2 for As(III) and As(V) increased linearly with the N2 Brunauer-Emmett-Teller surface area (SBET) of the particles. There was not much difference in the rate of As(III) photooxidation when the diameter of the TiO2 nanoparticles was between 6.6 and 14.8 nm. However, the As(III) photooxidation rate clearly decreased when the particle size increased to 30.1 nm. Arsenite photooxidation data could be fitted with a first-order kinetics equation.;Photodegradation mechanisms of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) with nanocrystalline titanium dioxide under UV irradiation were investigated. In the presence of UV irradiation and 0.02 g/L TiO2, 93% MMA (initial concentration=10 mg-As/L) was transformed into inorganic arsenate, [As(V)], after 72 h of batch reaction. The mineralization of DMA to As(V) occurred in two steps with MMA as an intermediate product. The photodegradation rate of MMA and DMA could be described using first-order kinetics, where the apparent rate constant is 0.033/h and 0.013/h for MMA and DMA, respectively. Radical scavengers, including superoxide dimutase (SOD), sodium bicarbonate, tert-butanol and sodium azide, were used to study the photodegradation mechanisms of MMA and DMA. The results showed that hydroxyl radicals (HO*) was the primary reactive oxygen species for the photodegradation of MMA and DMA. The methyl groups in MMA and DMA were transformed into organic carbon, including formic acid and possibly methanol, also through photochemical reactions. The results showed that nanocrystalline TiO2 can be used for the photocatalytical degradation of MMA and DMA and subsequent removal of the converted As(V), since the high adsorption capacity of the material for inorganic arsenic species has been demonstrated in previous studies.