Study On Pollution Control Effect Of Coal Gangue Based Permeable Pavement Systems

With the large-scale construction and rapid development of the city,the impervious pavement area of the city is constantly increasing,leading to rainwater cannot be intercepted by plants and groundwater conservation.Urban rainwater contains suspended matter（TSS）,nitrogen and phosphorus nutrients,pathogenic bacteria heavy metals and other substances,which lead to runoff pollution increasing.These pollutants discharged into rivers,lakes and other water bodies will cause the deterioration of urban water quality.Green stormwater infrastructure,such as permeable pavement system,has been widely used in urban stormwater management and urban road construction.Permeable pavement shows excellent effect in reducing total runoff,reducing peak runoff,removing runoff pollutants and so on.With the development and utilization of mineral resources,a large number of industrial solid wastes such as coal gangue will be produced in the process of coal mining,washing and processing.The output of coal gangue in our country is huge.A large amount of coal gangue can not be properly disposed,occupying a large amount of land resources and causing pollution to the surrounding environment.However,coal gangue itself has good adsorption properties after treatment and can be recycled.In this study,coal gangue was crushed and screened after pretreatment such as calcination.Then it is used as filler to replace traditional filler in permeable pavement facilities.Relying on the good adsorption performance of calcined coal gangue to a variety of pollutants,the infiltration of runoff and rainwater pollutants in permeable pavement facilities can be removed.So as to improve the water quality of the permeable pavement facilities,and then improve the water quality of groundwater and surface water.Research shows that:（1）After coal gangue is calcined at 900℃for 6 h,the removal effect of TP is the best.After calcination,the pollutants such as sulfide,chloride and organic matter contained in the coal gangue can be removed.The content of Al₂O₃and SiO₂in this kind of calcined coal gangue is 50.26%and 33.50%by mass,respectively.And it has a large specific surface area（16.66 m²/g）,which provides adsorption site for the removal of pollutants.This kind of calcined coal gangue shows different degree of adsorption removal effect on TP,Cu and Zn.The saturated adsorption capacity can reach 2.63 mg/g,0.39 mg/g and 0.60 mg/g,respectively.（2）Under dynamic conditions,large amounts of Zn and Cu will be leached from gravel.Coal gangue will leach NH₄⁺-N and TN.And the leaching amount is greatly related to the particle size of coal gangue,the smaller the particle size is,the more the leaching amount is.River sand also leaches small amounts of NH₄⁺-N and TN.The removal effect of calcined coal gangue on TP,Zn and Cu is obviously higher than that of other fillers.However,the removal efficiency of COD,NH₄⁺-N,TN and Cr by the four fillers should be improved.For coal gangue,the removal effect of small particle size coal gangue on most pollutants is better than that of large particle size coal gangue.However,a large amount of Cr can be leached from coal gangue with small particle size just like river sand.In summary,the removal effect of the four fillers on pollutants under dynamic conditions is ranked from high to low as follows:small particle size coal gangue>river sand>large particle size coal gangue>gravel.（3）The leaching quantities of COD,TN,TP and NH₄⁺-N in coal gangue base permeable pavement facilities and traditional pavement facilities are very small.This indicates that coal gangue as the filling material of permeable paving facilities will not cause environmental risk.With the increase of rainfall duration,the removal rates of COD,TN,NH₄⁺-N and Cr in the traditional permeable pavement and coal gangue based permeable pavement decrease.The removal efficiency of TP by coal gangue based permeable pavement is very good,and the removal rate is above 93.02%.The removal rates of Cu and Zn in the four paving facilities are high.The removal rates of Cu and Zn were all above 83%and 75%respectively.The removal effect of Cu and Zn in the facilities filled with coal gangue is better than that in the traditional facilities.The Cr removal efficiency of the four facilities is basically the same,and the removal rate is all low,and the removal efficiency needs to be improved.The mechanism of pollutant removal by coal gangue includes physical interception,chemical adsorption and microbial degradation.The influences of rainfall return period and rainfall fall-dry period on the pollutant removal rate of the four permeable pavement facilities are not obvious.No matter for the rainfall return period of 0.5–5 years or the rainfall dry period of 1–12 days,in general,the facilities with the sand layer and gravel layer replaced by coal gangue with corresponding particle size（CG-SG）in the permeable paving facilities have the best removal effect on runoff pollutants.（4）Compared with traditional permeable bricks,coal gangue based permeable bricks do not leach more dissolved heavy metals Cu,Zn and Cr,so the use of coal gangue aggregate is safe.The removal effect of coal gangue based permeable brick on Cu and Zn is higher than that of traditional permeable brick,and the more coal gangue aggregate is added,the better the effect is.The removal rate of Zn can reach about 96%,and the removal rate of Cu can reach about 82%.This is mainly due to the adsorption of coal gangue to these two heavy metals.There is no significant difference between the Cr removal efficiency of coal gangue based permeable bricks and that of traditional permeable bricks,because the adsorption capacity of coal gangue to Cr is limited,and the removal rate of permeable bricks to Cr is very low,so the removal capacity needs to be strengthened.This study not only solves the problem of solid waste reuse from the perspective of resource utilization,but also provides a new idea for low-impact development facilities from the perspective of sponge city construction,which effectively reduces the construction and operation cost of sponge facilities,and has significant environmental,economic and social benefits.