Macroscopic and microscopic investigation of adsorption and precipitation of Zn on γ-alumina in the absence and presence of As.

Contaminants zinc (Zn) and arsenate (As) often coexist in soils. However, little is known concerning the impacts of coexisting As on Zn adsorption and precipitation on soil minerals. In the present study, adsorption and precipitation of Zn on γ-alumina in the absence and presence of arsenate was investigated employing batch experiments and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Results indicated that Zn formed edge-sharing tetrahedral surface complexes at pH 5.5 and Zn-Al LDH-like (layered double hydroxide) precipitates at pH 7.0 on the surface of γ-alumina. The presence of arsenate significantly enhanced Zn sorption densities, and remarkably changed its bonding environment. At pH 5.5, SR-XRD (Synchrotron Radiation-based X-ray Diffraction) and EXAFS showed that koettigite-like precipitate were formed in the cosorption of Zn and As on γ-alumina regardless of the addition sequence of As and Zn. At pH 7.0, when Zn was preequilibrated with γ-alumina prior to the As introduction, mixed Zn-Al LDH-like and amorphous adamite-like precipitates formed. However, when Zn and As were added simultaneously, only amorphous adamite-like precipitate was observed. Zn inner-sphere complexes and surface ternary complexes γ-alumina-As-Zn were the main outcome when As was preequilibrated firstly. Zn-arsenate precipitates could significantly decrease the concentration of Zn in aqueous solution and decrease the bioavailability and mobilization of Zn in soils.

Effect of aqueous Fe(II) on Sb(V) sorption on soil and goethite.

The effects of Fe(II) on the sorption and precipitation of Sb(V) on soils and goethite were investigated using batch experiments and X-ray photoelectron spectroscopy (XPS) in this study. The sorption capacity of Sb(V) were much higher in anoxic soil than oxic soil. Typically, dissolved Fe(II) concentration in anoxic soils decreased significantly with increasing Sb(V), which may be suggestive of Fe-Sb precipitation. The elevated concentrations of Fe(II) (1 mM) enhanced the sorption capacity of Sb(V) on goethite significantly. However, synchrotron radiation X-ray diffraction showed no new characteristic peak, indicating that this Fe-Sb precipitate might be poor crystallinity or amorphous. Moreover, Sb(III) was detected in anoxic soil, and the reduction of Sb(V) to Sb (III) improved the sorption capacity of Sb in anoxic soil because of the low solubility and migration of Sb(III). Nevertheless, Fe-Sb co-precipitation and Sb(V) reduction to Sb(III) might contribute simultaneously to the increased sorption capacity of Sb(V) on anoxic soils. This research could improve our current understanding of soil Sb chemistry in paddy and wetland soils.

Photo-induced oxidation of Sb(III) on goethite.

Goethite widely exists in soils and sediments, and plays a very important role in the environmental fate of toxic metal(loid)s. In the present study, photo-induced oxidation of antimonite [Sb(III)] on goethite was investigated with kinetic measurements and X-ray photoelectron spectroscopy (XPS) techniques. Effects of environmental factors including solution pH, the content of goethite as well as humic acid on the photo-induced oxidation of antimonite were tested. The results indicated that no oxidation of antimonite occurred in goethite suspension in the dark, but significant amounts of antimonite were transformed to antimonate when the suspension was exposed to light. Ferrous ions were found in the solution during the antimonite oxidation process, and its concentration decreased with increasing solution pH, which strongly affected the oxidation rate of antimonite. The initial solution pH has great impact on Sb oxidation. After 2h illumination, the highest oxidation rate was found at pH 3, while the initial oxidation rate was even higher at pH 9. In conclusion, the antimonite can be adsorbed and oxidized on goethite irradiated with light, which will greatly reduce its environmental risk.

Effect of iron oxide reductive dissolution on the transformation and immobilization of arsenic in soils: New insights from X-ray photoelectron and X-ray absorption spectroscopy.

The geochemical behavior and speciation of arsenic (As) in paddy soils is strongly controlled by soil redox conditions and the sequestration by soil iron oxyhydroxides. Hence, the effects of iron oxide reductive dissolution on the adsorption, transformation and precipitation of As(III) and As(V) in soils were investigated using batch experiments and synchrotron based techniques to gain a deeper understanding at both macroscopic and microscopic scales. The results of batch sorption experiments revealed that the sorption capacity of As(V) on anoxic soil was much higher than that on control soil. Synchrotron based X-ray fluorescence (µ-XRF) mapping studies indicated that As was heterogeneously distributed and was mainly associated with iron in the soil. X-ray absorption near edge structure (XANES), micro-X-ray absorption near edge structure (µ-XANES) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the primary speciation of As in the soil is As(V). These results further suggested that, when As(V) was introduced into the anoxic soil, the rapid coprecipitation of As(V) with ferric/ferrous ion prevented its reduction to As(III), and was the main mechanism controlling the immobilization of As. This research could improve the current understanding of soil As chemistry in paddy and wetland soils.

Transformation of polychlorinated biphenyls by persulfate at ambient temperature.

Increasing attention has been paid to persulfate due to its high efficiency in degrading organic pollutants. This paper investigated the transformation of a selected polychlorinated biphenyl (PCB) by sodium persulfate without activators at near ambient temperature (10-40°C). The results showed that 2,4,4'-CB was completely decomposed by persulfate at 30°C in 8 h. The products were identified by gas chromatography-mass spectrometry (GC-MS), and transformation pathways could be divided into two steps as dechlorination and hydroxylation. Electron paramagnetic resonance (EPR) technique was used to identify the generated radical species at different pH values at ambient temperature. The results showed that sulfate radicals (SO(4)(•-)) were predominant under acidic condition and hydroxyl radicals ((•)OH) were predominant under basic condition. This behavior was also confirmed by the quenching studies and kinetic model. Decreasing the solution pH resulted in increasing the degradation efficiency of 2,4,4'-CB. Moreover, the degradation of other PCBs such as 2-CB, 4-CB, 2,4-CB, 2,4'-CB, and 2,4,6-CB with persulfate was examined. The findings of this study can provide guidance in the remediation of PCBs contaminated soil and water with persulfate.