Study On The Mode Occurrence And Environmental Stability Of Fe And Mn In The Sediments Of AMD-wetland Treatment System

Artificial wetland is a new type of ecological wastewater treatment technology,which has been gradually used in treatment of Acid Mine Drainage（AMD）.A large number of heavy metals will be gathered by sediments after AMD entering the wetland treatment system and will be released back after the external pollution under control,which directly affect the quality of overlying water and threaten water security and human health.Therefore,it has important practical significance to study the content and distribution characteristics of heavy metals in sediments and the environmental behaviors of heavy metals in different exogenous conditions.In this paper,the physical and chemical properties and the distribution characteristics of Fe and Mn of sediments in the AMD-wetland treatment system were studied.The particle size composition characteristics of sediments and the distribution characteristics of Fe and Mn in different particle size distribution of sediments were discussed by screening experiments.The effects of environmental factors（pH,litters,low molecular weight organic acids）on the mode of occurrence and environmental stability of Fe and Mn in sediments were studied by laboratory simulation experiments.The purpose of carrying out these experiments was to provide the reference for the heavy metal prevention and wetland sediments treatment.The main conclusions of this paper were as follows:（1）The physical and chemical indicators of sediments in the AMD-wetland treatment system were that,the pH value was 7.73,the EC value was 1098.5μs/cm,the content of organic matter was 40.56 g/kg.The main mineral components of sediments were mainly quartz,pyrite and spinel.The contents of Fe and Mn in the sediments were3.71×10⁵ mg/kg,8.01×10³ mg/kg,respectively,and both of which have significantly exceeded the background value of the epibiotic sediments in Guizhou Province.In the sediments,the main way of existence of Fe was residue state,but there were much potential harm to the environment;Mn element was mainly in the form of weak acid extract state,which has direct harm to the ecological environment.（2）The sediments in AMD-wetland treatment system were mainly concentrated in the particle size range of<75μm and>450μm.pH value increased with the decrease of particle size,while the organic matter decreased with decreasing particle size.The maximum conductivity value of sediments was 1253μs/cm,which appeared in150²50μm particle size range.Within each grain size,the mineral composition of sediment was mainly quartz,pyrite and spinel.The higher content of Fe and Mn concentrated on the smaller size.In the sediments of different particle sizes,Fe was mainly in the stable residue state form（95.7%）,so the harm to the ecological environment was very small;Mn mainly existed in the form of weak acid extract state,and the proportion increased with increasing particle size.In the sediments with particle size>450μm,the proportion of weak acid extract state Mn was 65.4%,which was harmful to the ecological environment.（3）Under different pH conditions（1.5⁷.5）,the pH value of the overlying water of sediments gradually increased and the EC value gradually decreased with the increase of the initial pH value of extractant.Acidic conditions would stimulate the release of Fe and Mn and dissolution of the sulphate in the sediments,increase the concentration of Fe,Mn and SO₄^2- in overlying water,and decrease the content of the organic matter of sediments.After treatment with different pH conditions,there was no significant change in the main mineral composition of the sediments.At the same time,the ratio of unstable states of Fe and Mn increased in the sediments proceeded by low pH conditions,leading to enhance dangers caused by Fe and Mn elements in the sediments to environment and increase the potential ecological risk to environment.（4）Decomposition of litters could decrease the pH value and increase EC value of overlying water.Meanwhile,it would also promote the release of Fe and Mn in the sediments and affect the concentration of SO₄^2- in the overlying water.After litters decomposition,the content of organic matter in the sediments increased obviously,and the main mineral components of the samples were quartz,pyrite and gypsum.The weak acid extraction state and reducible state of Fe and Mn in the sediments changed after litters decomposition,but the overall proportion of unsteady state increased with the increase of the amount of adding litters,so as to increase the environmental risk.（5）In the AMD-wetland treatment system,the organic acid secreted by Commelina diffusa had a strong effect on Fe and Mn in the sediments.Through the laboratory simulation,it was found that adding organic acid would decrease the p H value of overlying water,and promote the release of Fe and Mn in the sediments,but the ability of releasing was affected by the nature and the concentration of organic acid.At the same time,adding organic acids could inhibit the dissolution and release of sulphate to a certain extent,and significantly increase the content of organic matter in the sediments.After organic acid treatment,the main mineral components of sediments were quartz,needle iron ore and diamond manganese ore.Low molecular organic acid treatment would increase to the content of weak acid extraction state and decrease the content of reducible state,oxidizable state and residue state of Fe and Mn in samples,and thus increased risk to the environment significantly.