Research On Leaching Of Phosphorus And Fluoride And Phosphorus Migration In Groundwater At Certain Phosphogypsum Stack In Sichuan Province

Sichuan Province is a highly developed area of the phosphorus chemical industry.A large amount of phosphogypsum is piled up in cities such as Deyang and Zigong.Phosphorus and fluorine in the phosphogypsum enter into the soil under the action of rainfall,and enter into the aquifer by leakage,which causes the groundwater resources to be polluted.When the contents of phosphorus and fluorine in groundwater are too high,on the one hand,high-fluorinated water bodies are formed;on the other hand,some groundwater contaminated by phosphorus pass through high hypertonic aquifers and flow into surface water,which exacerbated the eutrophication of water.As certain common inorganic pollutants,the contents of phosphorus and fluorine are higher in phosphogypsum and easily lost.This topic firstly investigated the phosphorus and fluorine in the surface water,groundwater and soil of the phosphogypsum yard.Secondly,through the indoor batch test,the phosphogypsum samples were leached,the influence of factors such as the temperature,the age of the stack,the pH of the extractant,and the size of phosphogypsum on the release of phosphorus and fluorine were investigated.Then the migration process of phosphorus in groundwater was simulated by percolation column,the influence of concentration of polluted liquid,pH value of groundwater and particle size of aquifer on phosphorus migration ability were found out,and the retardation effect and retention mechanism of sand column on leaching filtrate were investigated.Finally,this research established the migration model of phosphorus in simulated aquifer,explored its credibility,and predict the actual site in the end.Main conclusions are as follows:?1?The phosphorus over-standard rates of surface water,groundwater and soil around the phosphogypsum dumping area are:83.3%,57.1%,87.5%,respectively;the fluorine over-standard rates are:25.0%,21.4%,75.0%,respectively.It revealed that both water and soil were polluted by phosphorus and fluorine in different levels,and through the analysis of the source and distribution characteristics of characteristic pollutants,it proved that the phosphogypsum stack may be one of the important sources of groundwater phosphorus and fluorine pollution in this area.?2?Phosphogypsum leaching solution mainly contains anions such as SO₄^2–,Ca²⁺,PO₄^3–,F^-,and metal ions such as Mg²⁺,Al³⁺and Zn²⁺,which is a relatively complex strong acidic liquid?pH=3.5±0.5?;the existing state of phosphorus in the leaching solution is mainly inorganic phosphorus.The phosphorus release of new and old phosphogypsum ranged from 0.113 to 2.14 mg/g and 0.036 to 1.09 mg/g,respectively;the fluorine release ranged from 0.110 to 1.33 mg/g and 0.04 to 0.77mg/g,respectively.Both elevated temperature and strong acid environment are beneficial to the release of phosphorus and fluorine,and the larger the particle size of phosphogypsum,the shorter the stacking time and the greater the release of phosphorus and fluorine.?3?The adsorption of phosphorus by quartz sand is well fitted to the quasi-first-order and quasi-second-order kinetic equations.The fitting R² value with the Henry isotherm adsorption model is 0.939,and the partition coefficient k_d is62.846 ml/g,which indicated that it is a composite adsorption.When the contaminated liquid was inputted into the sand column,the relative concentration of phosphorus at each position increased from 0 to the peak value along with time and then gradually decayed to the original state.At the same time,the larger the particle size of the medium,the higher the concentration of the influent polluted liquid,the larger the pH value of the groundwater,the higher the relative concentration of phosphorus in different outlet holes,and the later the peak appears,indicating the retardation of the sand column to phosphorus became smaller and its migration ability wss strengthened.When the actual phosphogypsum leaching filtrate is input,the phosphorus could only migrate to 32cm in the sand column,and the peak concentration at each position is smaller than that of the simulated pollutant liquid.This indicated that the decay process is faster and the entrapment effect is greater,which mainly due to the presence of a large amount of Ca²⁺in the leachate,after contacted with quartz sand that containing hydroxyl functional groups on the surface,the hydroxyapatite precipitate was formed.?4?According to the adsorption of phosphorus by medium,a one-dimensional transport model of phosphorus in simulated aquifer was constructed,and the blocking coefficient of phosphorus in simulated aquifer was obtained by adsorption column method?R₂=1.85?.The variation trend of simulated and measured phosphorus concentration in different positions of sand column is basically consistent.The peak values of each hole in sand column appear at 1 h,1 h,3 h,5 h,and the concentration values corresponded with the distance at the same time were also merged well.The deviation ranges from 0 to 20.5%.The migration of phosphorus in groundwater of a certain phosphogypsum stack was predicted.It was found that within 200m,it needs about 2500 days until the phosphorus concentration decayed to 0 when the leakage accident occurred in the phosphogypsum yard.Within 800 days,the pollution range of phosphorus to the horizontal direction of groundwater is about 160 m.Therefore,the leakage of leachate will cause large and long-term phosphorus pollution to the surrounding groundwater,villagers water and Shitingjiang river system.