| Molybdenum (Mo) is an essential trace element for plants and animals, as well as an important stratagic resource. However, some Mo pollution incidents have been produced in partial area, such as Colorado in the USA, British Columbia Province in Canada and Liaoning Province, due to the backwardness of Mo mining exploitation technology and the poor management. Rencently, the research on Mo was mainly focused on the effect and harm on animal for lacking Mo. Little research has been done on controlling Mo pollution, so treating this pollution is necessary.People of Huludao City had to find another riverhead for the Mo pollution of Wujintang reservoir in2005. Although the reservoir have supplied certain water for the local inhabitants now, the water supply is not enough. Therefore, based on Mo tailings pollution in Liaoning province and the aim of on-way control for Mo pollution, the removal mechanism of Mo was studied by the substrates and plants of constructed wetlands. The substrates were tested in static and dynamic experiments for further applied in wetlands. Moreover, the removal effect, mechanism and influence factor for mo by different constructed wetlands was studied. So it could provide some valuable reference to practice application of constructed wetlands for mo removal. The main contents are as follows:(1) Study on the adsorption mechanism of Mo(VI) by the substrates.In this study, the removal efficiencies of Mo(VI) from aqueous solutions by desulfurization steel slag (DSS), converter steel slag (CSS) and cinder (CI) were investigated and compared against that of loessial soil (LS). The results showed that the sorption isotherms fitted the Langmuir model well, and the Langmuir adsorption capacity (Qo) of the four sorption media generally complied with the following order:DSS>CSS>CI>LS. Adsorption reaction was found to follow the pseudo second-order rate,and the adsorption of Mo(Ⅵ) was sensitive to pH values.The four adsorbents exhibited a significant Mo(Ⅵ) removal at low pH values (e.g.,3<pH<4.5), but such adsorption decreased rapidly when pH was greater than5.Only little adsorption occurred when pH value was greater than8.The competitive effect of PO43-and SO42-with Mo(Ⅵ) for adsorption associated with the four sorption media followed the order: LS>CI>CSS>DSS,and moreover,the effect of PO43-on the adsorption of Mo(Ⅵ) was observed stronger than that of SO42-. The desorption capacity of the four sorption media generally complied with the following order:LS>CI>CSS>DSS.The value of E calculated by the isotherm model and the results of FTIR indicated that the iron exchange adsorption was the main mechanism.(2) The screening experiment of substrates for Mo(Ⅵ) removalMo(Ⅵ) removal by modified cinder and pyrite was investigated in static and dynamic experiments.The results showed that the surface pore of cinder was enhanced by the modification of sulphuric acid, and the adsorption amount of Mo(Ⅵ) was increased. Maximum adsorption of Mo(Ⅵ) by modified cinder occurred at pHs between4.0and6.0. Compared to magnetite,the pyrite was with large buffer capacity on alkali.When the pH was7.0,the removal rate was higher than60%.Kinetic studies showed that the adsorption generally obeyed a pseudo second-order model.The values of activation energy indicated the adsorption processes were governed by interactions of physical nature.Furthermore,Langmuir and Freundlich isotherms were used to model the adsorption equilibrium data and the adsorption behavior obeyed the Langmuir model.Finally, thermodynamic parameters such as△H0,△S0and△G0for the adsorption were also evaluated, which showed the adsorption of Mo(VI) on treated cinder and pyrite was endothermic,entropy increasing and spontaneous. The time of break-through was longer, when the particle size was1-2mm and2-4mm for modified cinder and pyrite, respectively. The maximum adsorption amount of Mo(Ⅵ) by modified cinder and pyrite in dynamic experiments was8.87g kg-1and5.0g kg-1,respectively. Moreover, the perfectly combination proportion of modified cinder/cinder and pyrite/cinder was1:2and1:1.(3) Study on the enrichment mechanism of Mo(Ⅵ) by reed and cattailThe adsorption characteristics of reed and cattail to Mo(Ⅵ) were studied. The Mo(Ⅵ) toxicity, removal rate, adsorption process and accumulation of Mo (Ⅵ) were investigated in the short-term indoor-culture experiment.The effects of Mo(Ⅵ) adsorbed by two plants in nutrition solution of different concentration were also studied. Due to the Mo(Ⅵ) toxicity, the color of stems and leafs of the two plants had become scorch and the transpiration was declined. The tolerance to Mo(Ⅵ) toxicity of cattail was better than reed at Mo(Ⅵ) concentration of2-20mg·L-1. The removal rate of Mo(Ⅵ) by cattail was87%, which was higher than reed (62%) at Mo(Ⅵ) concentration of2mg·L-1. The absorption process of Mo(Ⅵ) by two plants was homeostasis, and the passivity absorption was the main absorption mechanism. Mo(Ⅵ) enrichment ammount in cattail was higher than that in reed, and Mo concentrations in shoot were higher than that in roots. The results displayed that the two plants were with Mo(Ⅵ) hyperaccumulative characteristics. The absorption of Mo(Ⅵ) was not enhanced with the increase of nutrition solution concentration, owing to the competition of other ions.(4) Study on the removal of Mo(VI) by simulated constructed wetlandsThe efficacy and capacity of using vertical subsurface water constructed wetlands to removal Mo(VI) was examined, employing reeds and cattail as well as granular media of different adsorption capacities.Results showed that the performance of wetland V (combined with pyrite) was found to be more stable and excellent than that of other wetlands during14-week of intermittent influent,and2-d HRT is sufficient for Mo(VI) removal in wetland V. Most Mo(VI) was adsorbed by the pyrite,which retained primarily in water-soluble fraction. Cattail was more suitable for Mo(VI) absorption than reeds, but the bioaccumulation accounted for a very small portion of the total removal.
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