Experimental Study Of The Reaction Process Between Acid Mine Water And Carbonate Rock In Closed Pit Coal Mine

Acid mine water（AMD）pollution to the environment is not only manifested during the mining process,but also poses a great threat to the environment after mine pit closure.High-sulfur coal resources are widely distributed in Guizhou Province,and the increase of coal mine pit closures will inevitably affect the water quality of karst groundwater systems.And when the closed pit mine void area is overlain by a carbonate formation,AMD will interact with it during the rebound process,but this reaction process is not yet clear.Therefore,this project uses indoor experiments to investigate the hydrogeochemical processes of AMD contacting and reacting with carbonate rock during the rebound of groundwater in closed pit mines by designing four influencing factors:different hydrodynamic conditions,different thickness of carbonate rock strata,different initial pH of AMD,and different initial Fe²⁺concentration of AMD.The characteristics of the changes of each water chemistry parameter in the reaction process under four different influencing factors were focused on,and the influence law of each factor on each parameter was elucidated.The spatially sampled test results reveal the mechanism of spatio-temporal variation of AMD in the response process.Various means such as mineral saturation index SI,Eh-pH diagram,XRD,SEM-EDS,Ca²⁺/Mg²⁺release were used to resolve the buffering mechanism of carbonate rock to AMD;and a mathematical model of AMD-carbonate-rock-groundwater interaction was constructed.The main research results obtained are as follows.（1）Characteristics of the changes in each water chemical parameter during the reaction.The pH values of all groups of experiments showed a trend of rapid increase,then slow increase and gradual stabilization.Fe showed a trend of decrease and then stabilization during the reaction process,and the decrease rate of Fe was accelerated when the hydrodynamic conditions were changed;while Mn in all groups of experiments showed an overall decreasing trend during the reaction process.（2）Effect of each factor on pH,Fe,and Mn.When the hydrodynamic conditions are stronger,the pH of AMD rises faster;the enhanced hydrodynamic conditions are favorable to the decrease of Fe and Mn concentration,and the decrease rate of Mn is smaller than that of Fe under the same hydrodynamic conditions.when the thickness of the carbonate formation is larger,the pH rises faster,and the decrease rate of Fe and Mn is also faster,and the change of the formation thickness has a greater effect on Fe than Mn.in the flooding stage,the lower the initial pH,the slower the pH rise rate of AMD.The lower the initial pH value,the slower the pH rise rate of AMD,and the lower the initial pH value,the faster the pH rise rate at the beginning of the drainage stage.During the reaction,the initial pH=3 was more favorable to the reduction of Fe and Mn concentration.When the initial Fe²⁺concentration of AMD was higher,the rate of decrease of Fe concentration was faster,and the initial Fe²⁺concentration had a certain influence on the decrease of Mn concentration,which showed that the increase of initial Fe²⁺would inhibit the decrease of Mn concentration.（3）Spatial diffusion mechanism of AMD.In the inundation stage,the recharge of karst groundwater makes the diffusion of H⁺,Ca²⁺,Fe and Mn in AMD in the horizontal direction appear as a process from far from the recharge end to the recharge end.In the vertical direction,the diffusion of H⁺,Fe and Mn in AMD is generally bottom-up;the H⁺concentration is stratified up and down throughout the reaction process,the Fe and Mn concentrations are stratified up and down in the early stage of the reaction,and gradually converge in the late stage of the reaction,while Ca²⁺is diffused in all directions.In the drainage stage,the water flow rate was the main control factor for the diffusion of AMD,and in general,the acceleration of the water flow rate made the concentration of each ion converge.（4）Buffering mechanisms of carbonate rocks against AMD.The buffering mechanism of carbonate rocks on the pH of AMD is that calcite and dolomite consume H⁺of AMD,thus raising the pH of AMD.The buffering effect of carbonate rock on AMD is related to the pH value of AMD itself,which shows that when the initial pH value of AMD is lower,the buffering effect played by carbonate rock is greater,and the stronger the hydrodynamic conditions,the greater the thickness of the stratum of carbonate rock,and the lower the initial Fe²⁺concentration of AMD,the faster the buffering effect of carbonate rock on the pH of AMD.For this reaction process of groundwater rebounding and contacting with carbonate rocks in closed pit mines,the following concept is proposed:the amount of Ca²⁺/Mg²⁺released during the reaction can be used as a buffering factor for the reaction of carbonate rocks with AMD.The buffering mechanism of carbonate rocks against AMD heavy metals is:increasing the pH of the solution and causing the oxidation of Fe（Ⅱ）,which releases·OH and generates deposits of fibrous iron ore（γ-FeOOH）and needle iron ore（α-FeOOH）during the oxidation of Fe（Ⅱ）.Mn in the reaction process is mainly reduced by adsorption or co-precipitation.（5）Mathematical model of AMD-groundwater-carbonate rock reaction.The mathematical expressions of pH in the flooding stage all follow a linear function of unity,while the mathematical expressions in the draining stage all exhibit an exponential function with base e.The mathematical expressions of Fe and Mn in both the flooding and draining stages exhibit an exponential function with base e.The mathematical expressions of Mn in the flooding and draining stages all exhibit an exponential function with base e.