Study On The Hydrogeochemistry Mechanism Of The Fly-ash Leachate And The Water-Rock Interaction Of Leachate

A large amount of studies of the fly ash leachate is pertaining to the effects on groundwater environment present evaluation and prediction;however,only relatively few studies have attempted to predicting the chemical properties of leachate and the reaction mechanism in leachate-mediator interaction.Experimental and hydrogeochemical simulation focused on analyzing the chemical characteristic of fly ash leachate derived from the fly ash in Baqiao coal-fired power plant and explains the reaction mechanism in water-rock interaction of the leachate.Under the guidance of reaction mechanism in water-rock interaction of leachate,from the characteristic of leachate,the single-phase mixing,polyphase mixing and column experiment,the author discusses the hydrogeochemistry about the leachate with different meidiator.Through those work,some important conclusions can be drawn,as follows:(1)From leaching experiment,the author analyzes the chemical properties of leachate evolving over different solid/liquid ratio and time.It is shown the leachate is highly alkaline solution for the CaO dissolution.The concentrations of Na⁺,Cl^-,Ca²⁺,SO₄^2- is decreasing and being in stable quickly with solid/liquid ratio increasing.The dominant solution species of Al³⁺,SiO₂ is Al(OH)₄^-,H₃SiO₄^- in high pH.In flyash-water system,as the liquid/solid ratio increased,the dissolution/precipitation of halite,gypsum,calcite and dolomite decrease.As the reaction time increased,the dissolution of aluminosilicate increase;and as the liquid/solid increased,the dissolution of aluminosilicate decrease.The combined effects of these reactions result in the chemical characteristics.(2)The experiment of leachate water-rock interaction indicates the chemical characteristic that the mixing solution of silty sand,sandy loam,loess is different.The major chemical styles of silty sand mixing solution show SO₄-Ca and HCO₃-Ca,the major chemical styles of sandy loam mixing solution show SO₄-Ca·Mg and SO₄-Ca,and the major chemical styles of the loess mixing solution show SO₄-Mg·Ca and HCO₃-Mg·Ca.The liquid/solid ratio have the maximun influence on concentration of Na⁺ in silty sand mixing solution,the concentration of Ca²⁺,Mg²⁺,SO₄^2- in sandy loam mixing solution,and the concentration of Cl^- in loess mixing solution.The liquid/solid ratio has the minimum influence on concentration of Na⁺,Cl^- in sandy loam mixing solution,and the concentration of Ca²⁺,Mg²⁺, SO₄^2- in mixing solution. (3)The geochemical modeling approach was used to predict the chemical reaction in water-rock interaction of leachate to the water composition by the PHREEQC software.The simulations lead to following conclusions:â‘ In single-phase mixing solution,the saturations of halite,gypsum,dolomite and calcite are decreasing with mixing ratio rising.The activities of CO₃^2- decreased as a function of pH, so the saturation of calcite and dolomite decrease.â‘¡In polyphase mixing solution,the evolution of chemical components,in mixing procedure of silty sand with solution,undergo the dissolution of halite,gypsum,CO₂, dolomite and k-feldspar,the precipitation of calcite,chalcedony,illite and cation exchange. The evolution of chemical component,in mixing procedure of loess with solution,undergo the dissolution of halite,gypsum,CO₂,dolomite and k-feldspar,the precipitation of calcite, chalcedony,illite,and cation exchange.â‘¢As the solutions mix with silty sand,sandy loam,loess individually,the dissolution amount order of halite is loess＞silty sand＞sandy loam,the halite dissolution influence the concentration of Cl^- in solution.The dissolution amount order of gypsum is sandy loam＞silty sandy＞loess,the gypsum dissolution influence the concentration of SO₄^2- in solution.The precipitation amount order of calcite is loess＞sandy loam＞silty sand.The dissolution amount order of dolomite is loess＞sand loam＞silty sand,the dissolution of dolomite influence the concentration of Mg²⁺ in solution.The cation exchanges make the Ca²⁺ into the solution from solid,make the Na⁺ into solid from solution,the exchange amount order is loess＞sandy loam＞silty sand.Under the combined effects of dissolution/precipitation of silicate and aluminosilicate,the concentration of Al³⁺ and SiO₂ is variation.(4) Finally,from the column experiment and geochemical simulation,the author puts up the leachate can dissolution more mineral than main-water,so permeability coefficient of the column filtration by leachate is bigger than by main-water,and the mineral dissolution capacity as so.