| Chromium pollution in soil will cause harm to the health of animals and plants,including human beings.Cr(VI)in soils is of wide concern due to its higher mobility and toxicity than Cr(Ⅲ).Nowadays,due to the emphasis on environmental protection,the anthropogenic sources of Cr(VI)have been significantly reduced and the oxidation of Cr(Ⅲ)has an increasingly heavy weight in the sources of soil Cr(VI).Manganese oxides,as natural inorganic oxidants widely found in nature,have strong oxidizing and adsorption abilities and play an important role in the oxidation of Cr(Ⅲ)by soils.Therefore,studying the oxidizing ability of manganese oxides on Cr(Ⅲ)can help to understand the oxidizing performance of different soil types on chromium.With the advantages of high tolerance,high biomass and ease of artificial culture,using Leersia hexandra Swartz to remediate chromium pollution is a cost-effective way.Therefore,in this paper,manganese oxides with different structures and oxidation capacities in soil,acidic birnessite,cryptomelane and black hausmannite,were selected for laboratory synthesis and investigated in hydroponic conditions and soil cultivation experiments for Cr(Ⅲ)oxidation ability in hydroponic conditions and soil culture experiments.Leersia hexandra Swartz was selected as a restoration plant to investigate the effect on its chromium restoration efficiency after chromium oxidation by different manganese oxides.The conclusions are as follows:(1)Qualitative and semi-quantitative analysis of birnessite,cryptomelane and hausmannite in acid water was carried out by means of XRD,SEM,TEM,FTIR,XPS and BET.It is determined that the synthesized birnessite is Mn(Ⅳ)oxide with layered structure and its micro-morphology is hydrangea.cryptomelane is a tunnel structure,rod-shaped crystal,Mn(Ⅳ)oxide.hausmannite is a massive cubic crystal and is Mn(Ⅱ,III)oxide.The BET specific surface area of acid birnessite and cryptomelane are 103.76 m~2/g and 95.92 m~2/g,respectively.The BET specific surface area of hausmannite is 28.59 m~2/g.(2)Hydroponic experiment:In 10mg/L Cr(Ⅲ)culture solution,the oxidation ability of manganese oxide to Cr(Ⅲ)is as follows:acidic birnessite>cryptomelane>hausmannite.The degree of biomass reduction of Leersia hexandra Swartz after the addition of the three manganese oxides was,in descending order:acidic birnessite>cryptomelane>hausmannite.The ratio of Cr concentration to material addition is positively correlated with the adsorption amount of Cr by manganese oxide.The oxidation of Cr(Ⅲ)by birnessite producesγ-Mn OOH and hausmannite,which improves the crystallinity and makes acidic birnessite more stable.After oxidation of Cr(Ⅲ)by cryptomelane,the crystals curl and the material stability rises.Hausmannite ore will dissolve in the oxidation experiment,and its stability will decrease.(3)Soil culture experiment:The addition of manganese oxides can improve soil Eh.The continuous oxidation ability of hausmannite is stronger than that of birnessite and cryptomelane.The addition of Cr(Ⅲ)will increase the concentration of soil Cr(Ⅵ).Under the experimental conditions,compared with the system with only Cr(Ⅲ),the addition of manganese oxide will increase the concentration of soil Cr(Ⅵ)in the early stage,but after enough flooding time,it will not increase the concentration of soil Cr(Ⅵ),and the appropriate amount of manganese oxide is beneficial to phytoremediation,but it will increase the content of oxidizable Cr in soil.To become a new source of pollution risk.In summary,although the oxidation efficiency of acidic birnessite and cryptomelane is high,the oxidation capacity decreases with time,and hausmannite has the ability of continuous oxidation,but the oxidation capacity is weak.When the oxidation capacity of manganese oxides in flooded soil is insufficient or the content of manganese oxides is low,the addition of a small amount of exogenous Cr(Ⅲ)will significantly increase the concentration of Cr(Ⅵ)in a short period of time,but it has no negative effect on the effect of soil Cr pollution remediation by Leersia hexandra Swartz after sufficient time of soil self-healing. |
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