| Arsenic (As) is a ubiquitous metalloid in the environment with toxicity dependent on its various species. Among the four arsenic oxidation states (+?,+?,0,-?), the more toxic trivalent arsenic [As(?)] in multi-phases mediums has received more attention. This focused study on the effect of As(?) on algae is anticipated to provide valuable information about the risk of arsenic in aquatic ecosystem and the development of related arsenic criteria. The objective of this dissertation was to study the effects of arsenic on the several diatom species as the representative bio-indicator for their sorption and toxic effects of As(?) on algae. The sorption mechanism of As(?) on diatoms was examined by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The influence of dimethylarsinic acid (DMA) and polyunsaturated aldehydes (PUAs) on the As(?) behavior was also studied. Changes of algal ultrastructure were studied by a transmission electron microscopy (TEM), and the variations on diatom frustules were observed with an atomic force microscopy (AFM). The results are concluded as follows:(1) As(?) sorption on diatoms is mainly intracellular uptake, which is higher than the amount of the cell wall-bound. The absorption efficiency also changes with As(?) concentrations, which varies among diatom species. Toxicity and biosorption responses to As(?) were examined in a72-h exposure study using typical freshwater diatom Navicula sp. and marine diatom Phaeodactylum sp. Results indicated that in the range of0.05-20mg L-1As(?), two diatoms could reach their maximum absorption efficiency of55.6±4.5%and99.4±0.1%at5.0and10.0mg L-1As(?), respectively. With the increase of As(?) concentration, the absorption efficiency of freshwater diatom Navicula sp. increased at the low concentration range of As(?). At higher concentrations, absorption decreased with increasing As(?) concentration. The algal growth was completely inhibited at20mg L-1As(?) with the absorption efficiency decreasing to21.9±3.5%. The analysis of the intracellular and cell wall-bound arsenic content indicated that the intracellular uptake of freshwater diatom on As(?) was significant. (2) The mixed effects of As(III) and DMA on the growth of diatom Nitzschia sp. presented additive and antagonism effects at different arsenic concentrations. At the exposure of0.04-45.32mg L-1DMA and0.16-0.49mg L-1As(?), the combined effects were found to be additive. The growth rate of diatom decreased significantly, and the diatom frustule was destructed heavily. It was also verified by the analysis of the formation rate of frustules, which was faster in these treatments. However, with the As(?) concentration increased, the combined toxicity of DMA and0.58-1.26mg L-As(?) showed to be antagonistic, which the decrease of growth rate and the destruction of diatom frustule was improved. Results also indicated that the thiols increased in the treatment of0.58-1.26mg L-1As(?) and DMA, suggesting that the detoxification mechanism in diatom was activated to reduce the As(?) toxicity.(3) The efficiency of As(?) sorption by diatoms increased with the addition of polyunsaturated aldehydes (PUAs). The absorption efficiency of Cymbella sp. and Navicula sp. for As(?)(5mg L-1) was36.1±2.9%and12.4±2.7%, respectively. But after the addition of PUAs, they increased further to47.6±5.2%and23.7±1.3%. The modulation mechanism of PUAs on uptake of As(?) on diatoms may be caused by the reactions between PUAs and As(?) that reduced the As(?) toxicity. The analysis of PUAs adsorption for As(?) indicated that PUAs could adsorb20.0±2.5%As(?). The toxicity analysis also demonstrated that the growth rate of two diatoms increased49.0±10.5%and23.1±12.4%at0.1?M PUAs, respectively. The AFM observations suggested that PUAs could mitigate the destruction of diatom frustule by As(?). The adsorbed As(?) by PUAs was proved to enter into pores on diatom frustules, and thus the As(?) toxicity on intracellular organelles could be mitigated.(4) As(?) adsorption on freshwater diatom silica shells increased significantly following the surface modification through addition of amino and thiol groups. The adsorption of As(?) on raw and modified non-activated diatom was examined at varying pHs. Results indicated that the adsorption efficiency of As(?) on non-activated diatom was small, sorption reached only13.8±1.4%under the12mg L-1As(?) at pH7, and the highest sorption of20.5±0.6%occurred at pH4. However, after the introduction of amino and thiol groups onto diatom surface, the adsorption efficiency was improved by65.5±1.3%. The adsorption kinetics and isotherms could be described by psedo-second-order kinetic model and Langmuir-Freundlich model, respectively. Meanwhile, the negative values of Gibbs free energy A G0and enthalpy A H0indicate the spontaneous and exothermic nature of the adsorption. |
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