| Chromium is a dangerous environmental pollutant.Water chromium pollution is not only a serious environmental problem to be solved,but also a social security problem which concerns public health.Leersia hexandra Swartz,a hygrophyte Cr-hyperaccumulator plant first discovered in China,has a high tolerance and absorption capacity of Cr(Ⅵ).For the treatment of Cr(Ⅵ)polluted water with a low Cr(Ⅵ)concentration and a large area,Leersia hexandra Swartz constructed wetland can be a promising and cost-effective method.The substrate of constructed wetlands plays an important role in the decontamination of heavy metals.Improvement of the substrate is therefore important for improving the Cr(Ⅵ)decontamination capacity of the constructed wetland.Bagasse,which is produced by the cane sugar industry that is the pillar industry in Guangxi,was used as raw material to make the biochar in this study.Bagasse biochar was used to improve the substrate of Leersia hexandra Swartz constructed wetland.This study shows that bagasse biochar can improve the decontamination performance of Leersia hexandra Swartz constructed wetland.It also shows an effective method to reuse bagasse.The toxicity of chromium is closely related to its valence state.Cr(Ⅵ)is far more toxic than Cr(Ⅲ).In this study we used electron paramagnetic resonance(EPR),X-ray photoelectron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FTIR),in order to explore the distribution and transformation of Cr in the wetland,as well as the transportation and transformation of Cr(Ⅵ)after it enters the wetland.Furthemore,we studied the litters of L.hexandra in the wetland system,such as the adsorption and release of Cr(Ⅵ)by litters,which provides bases for the scientific management of the wetland.Microorganism is an important component of the ecological system.We systematically studied the community structure and succession characteristics of the microorganism in the wetland.And we discussed the mechanism of microbial ecology in the wetland.A Cr(Ⅵ)reduction bacteria strain,whose optimum environmental condition was explored,was selected and separated.Our study can provide a cost-effective method for the treatment of Cr(Ⅶ)polluted water and the reuse of bagasse.The main results of this study are:1.Under greenhouse condition,the removal rate of Cr(Ⅵ)polluted water(5.00 mg/L、7.50 mg/L and 10.0 mg/L)by Leersia hexandra Swartz constructed wetland adding 5%of bagasse biochar was 99.52%,99.48%and 98.43%,respectively.Under winter condition,the removal rate of Cr(Ⅵ)polluted water(5.00 mg/L、7.50 mg/L and 10.0 mg/L)by Leersia hexandra Swartz constructed wetland adding 5%of bagasse biochar was 99.36%,98.96%and 96.40%,respectively.It indicates that Leersia hexandra Swartz constructed wetland adding bagasse biochar can effectively remove Cr(Ⅵ)from the water.Besides,its performance keeps excellent under winter condition,which shows a good adaptability for seasons.2.Under greenhouse condition,the removal rate of Cr(Ⅵ)polluted water(5.00 mg/L、7.50 mg/L and 10.0 mg/L)by Leersia hexandra Swartz constructed wetland adding bagasse biochar soil substrate was 9.38%,11.51%and 13.78%higher than that of the construted wetland using only sandy soil substrate,respectively.Under winter condition,the gap was 8.88%,11.45%and 15.39%,respectively.It indicates that the main reason of bagasse biochar improving the Cr(Ⅵ)removal rate is its ability to improve the substrate of the wetland.Bagasse biochar can markedly improve the amount of organic matter and number of microorganisms such as bacteria and fungi in the substrate,which improves the Cr(Ⅵ)absorption and reduction capacity of the substrate.3.After entering the Leersia hexandra Swartz constructed wetland adding bagasse biochar soil substrate,92.18%of Cr was absorbed by the substrate,in which 93.90%of Cr was Cr(Ⅲ)while only 9.10%was Cr(Ⅵ).This indicates that in the removal process,the substrate has efficiently reducted Cr(Ⅵ)to Cr(Ⅳ).FTIR analysis shows that there was coordination interaction between carboxyl on the substrate surface and Cr(Ⅳ).Hydroxyl,amino,C=O,C-O and C-H,etc.were involved in the Cr absoption process,indicating that organic matter acted as the electron doner during the reduction process.Only 6.67%of Cr entering the wetland system was absorbed by L.hexandra,in which 15%of Cr was Cr(Ⅵ)while 85%was Cr(Ⅳ).This indicates that in the removal process,L.hexandra has efficiently reducted Cr(Ⅵ)to Cr(Ⅲ).4.The Cr(Ⅵ)absorption process by litters was in accord with pseudo-first order kinetic equation.The isothermal adsorption process was in accord with Freundlich absorption model.The Cr(Ⅵ)absorption process was accompanied with redox reactions during which Cr(Ⅲ)was produced.Litters had a quick decomposition process in the initial phase,after which the decomposition rate decreased.Between 60 days an 70 days,the decomposition reached a balance and the weight loss ratio was 63.68%.Decomposition of the litters resulted in the realease of Cr.After 70 days of decomposition,only 11.95%of Cr was released into the solution,while most Cr was absorbed by litters.Therefore,the decomposition of litters does not have much influence on the effluent quality.5.The soil samples containing viable bacteria separated from the substrate had a Cr(Ⅵ)reduction rate of 76.52±2.34%,which was 2.36 times higher than that of soil samples containing bacteria killed by high temperature and was 3.18 times higher than that of soil samples containing bacteria killed by chloroform.The soil samples containing microorganisms had a much better Cr(Ⅵ)removal capacity than the control group without microorganisms,indicating that microorganisms in substrate played an important role in the Cr(Ⅵ)removal process.Compared with the microorganisms in the purified wetland,the biodiversity in Cr(Ⅵ)polluted wetland decreased.The dominant microorganism community also had an obvious change.Some of the dominant communities in Cr(Ⅵ)polluted soil were new species,and some were non-dominant microorganisms in the control group.6.A Cr(Ⅵ)reduction bacteria strain was sperated from the substrate near the root of L.hexandra,which was identified as Bacillus cereus.The optimal culture condition for Bacillus cereus to remove Cr(Ⅵ)is:Cr(Ⅵ)initial concentration=60 mg/L,pH=7,inoculum size=10%,culture temperature=35℃.Under this condition,the Cr(Ⅵ)reduction rate by Bacillus cereus reached 100%within 48 hours when the initial Cr(Ⅵ)concentration was 60 mg/L. |
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