The Study On Mechanism Of The Treatment Of Acid Mine Drainage Using PRB With Loess-steel Slag

Acid mine drainage(AMD)from abandoned coal mines is one of the primary pollution sources of the aquatic ecological environment in China.In recent years,with the closure of a large number of coal mines,the water level of AMD has been rising gradually and seeped out of the surface.The AMD not only severely affects the surface water body and their ecosystem,but also leaks into the underlying aquifer through the leakage section of the river,which cause the pollution of water resources and bring potential risk to the aquatic ecological environment.Therefore,it is urgent to research the treatment of AMD from abandoned coal mines.It provides scientific basis for the control of AMD,and has significance to protect water resources and ecological environment.Taking the AMD from abandoned coal mines in Shandi River basin of Niangziguan Spring catchment,Shanxi Province as the research object,and utilizing loess and various slags(iron slag,stainless steel slag and carbon steel slag)as the adsorption materials.By using batch adsorption experiments,the influences of solid-to-liquid ratio,adsorption time,initial concentration and temperature on the adsorption capacity of loess and various slags were investigated.The mineral characteristics of the adsorption materials before and after absorbing AMD was determined using scanning electron microscopy(SEM),X-ray diffraction(XRD),and infrared spectroscopy(FT-IR).Based on leaching experiments,the AMD was treated by PRB with different materials mixture ratio(loess and coarse sand;carbon steel slag powder and coarse sand;carbon steel slag powder and carbon steel slag particles).The dynamic adsorption law of PRB for the treatment of AMD and the material mixture ratio were studied.The fraction of sulfate and metal in adsorption materials was researched by sequential extraction procedure.Taking sulfate as a simulation factor,Hydrus-1D software was used to simulate the different width of PRB for treatment of AMD.The optimal width of PRB was studied.The results indicated that(1)The solid-liquid ratio,adsorption time,and initial concentration had significant effect on the adsorption property of the materials,while the temperature(25??45?)was not significant.The optimal solid-liquid ratio for loess and various steel slags adsorbing AMD was 100 g/L and 50 g/L,respectively.The removal of AMD for stainless steel slag and carbon steel slag is significantly higher than that of ferrous slag.The adsorption kinetic of AMD adsorbed onto adsorption materials was accurately described by the pseudo-second order model.(2)The negative values of the Gibbs free energy(?G is-0.12 k J.mol^-1?-27.18 k J.mol^-1)indicated that the adsorption of AMD on loess and various steel slags was spontaneous.The positive values of the enthalpy change(?H is 4.0k J.mol^-1?96.14 k J.mol^-1)and entropy change(?S is 15.49 J.mol^-1.K^-1?391.12J.mol^-1.K^-1)showed that the adsorption was endothermic,which implied that the increasing temperature was beneficial to the adsorption reaction.(3)The SEM proved that surface of loess,stainless steel slag,and carbon steel slag exhibited a rougher and more pore than ferrous slag,which could effectively increase the specific surface area and improve the adsorption properties.XRD,and FTIR showed that the alkaline substance of materials played a major role during the adsorption process.The iron ions formed hydroxides coated the surface of materials during the adsorption process,which led to the decrease of the effective adsorption sites and reduced the activity of materials.The mechanism of AMD removal by ferrous slag was mainly chemical precipitation,while the mechanism of AMD removal by loess,stainless steel slag,and carbon steel slag was mainly chemisorption,but also exhibited chemical precipitation.(4)The p H value increased with the increase of the adsorption materials and leaching time.The concentration of SO₄^2-,Fe,Mn,and Zn increased with the increase of leaching time under different materials mixture ratio,indicating that the removal is reduced.The removal of SO₄^2-,Fe,Mn,and Zn increased with the increase of adsorption material.The loess content of PRB with different material mixture ratio should be less than 20%,and the carbon steel slag powder content of PRB with different material mixture ratio should be less than 10%.(5)After the AMD was treated by PRB,the order of the sulfate fraction of adsorption material was less-soluble sulfate>exchange sulfate>and water-soluble sulfate.From the upper to the bottom in PRB,the water-soluble sulfate decreased,indicating that the water-soluble sulfate was converted into exchange sulfate and less-soluble sulfate.The adsorbed metal ions(Fe?Mn?Zn)mainly existed in the form of reducible state.The metal content(Fe?Mn?Zn)of adsorption materials gradually decreased from the upper to the bottom in PRB.(6)The available metal content of adsorption materials gradually decreased from the upper to bottom in PRB,indicating that the risk degree is reduced.When the PRB was filled with carbon steel slag and carbon steel slag particles,the available metal(Fe,Mn,Zn)content is less than 10%,implying that the environmental impact is in a low risk level.(7)The PRB incorporated with carbon steel slag could increase the distribution coefficient and the fraction of exchange sites,which enhanced the removal of pollutants from AMD.As the PRB width increased,the pollutants of AMD have been effectively removed using PRB.When the PRB width increased from 30 cm to 60 cm,the flow rate and the sulfate migration rate decreased rapidly.When the PRB width increased from 60 cm to 90 cm,the downward trend of the flow rate and the sulfate migration rate slowed down and it had a break point.When the PRB width increased from 90 cm to 150 cm,the flow rate and sulfate ion migration rate decreased slowly.Considering the the removal and the treatment quantity of AMD,the width of PRB is identified as 60 cm?90 cm.