Experimental Study On Soil Infiltration Technology For Treatment Of Pollutants In Road Pavement Runoff

In the rapid growth of traffic,the surface of the highway continues to accumulate pollutants,which enter nearby water bodies and soil after being washed by rainwater,forming water or land surface pollution.Compared with urban road runoff treatment,China’s highway land runoff treatment facilities are relatively missing,environmental pollution problems are becoming more and more prominent.Road runoff sources are scattered,uneven in volume,and have a variety of heavy metals,so it is not appropriate to directly adopt urban road runoff treatment model.And after substrate improvement,soil infiltration technology can be applied to road surface runoff treatment.Therefore,this study proposes to use quartz sand,biochar(BC),and carboxymethyl cellulose sodium(CMC)materials to improve the soil and strengthen the infiltration as well as the adsorption capacity of pollutants.By setting up soil infiltration systems with different substrates in the control group CK,the CM group with sodium carboxymethyl cellulose,the BC group with biochar,and the CBC group with sodium carboxymethyl cellulose and biochar(CMC-BC),we studied the changes of pollutant treatment performance under the long-term operation of soil infiltration systems and investigated the changes of physicochemical indexes and the accumulation of pollutants in soil substrates at different depths,with a view to providing information on soil improvement and soil infiltration technology in China for The main findings are as follows The main conclusions are as follows:(1)Through the experiments on substrate improvement,it can be concluded that rice husk biochar has the best effect on improving the pore structure of soil substrate.the addition of CMC can improve the soil pore structure and change the water holding capacity of soil.Static adsorption and infiltration experiments showed that compared with the control,the improved ratio of sand to soil ratio of 6:4,rice husk biochar addition of 5% and CMC addition of 0.1%,the infiltration coefficient was reduced by 60.69%,the saturated water content was significantly reduced,and the removal rates of ammonia nitrogen,phosphorus and copper were increased by38.97%,19.47% and 33.78%,respectively,and the removal rate of chromium was 30.14%.(2)Four soil percolation columns were constructed,and the start-up time of the soil percolation system was 30 d.In the soil percolation experiment,the removal effect of the original soil CK soil column on COD was relatively poor,with the average removal rate at59.44%.The removal efficiency of the modified system for COD,ammonia nitrogen,total nitrogen and total phosphorus was improved,and the COD of the effluent from the four soil columns could reach the standard for dryland crops in the Water Quality Standard for Agricultural Irrigation(GB5084-2021).The addition of biochar can make the system have higher permeability coefficient,stronger reoxygenation capacity and relatively higher COD removal efficiency.The removal rates of ammonia nitrogen in the effluent all reached over 51%,and the average removal rates of total nitrogen were 15.05%,18.48%,25.76% and 24.11%,respectively.And the removal rates of runoff total phosphorus in the four soil columns were67.11%,89.93%,40.27% and 75.17%,respectively,at the middle of operation,and the effluent ammonia nitrogen,total nitrogen and total phosphorus could reach the Class A standard in the Discharge Standards for Pollutants from Urban Wastewater Treatment Plants(GB18918-2002).However,the study on the removal of organic pollutants found that with the increase of operation time,the removal rate of phosphorus by soil percolation system is gradually decreasing trend.(3)The removal effect of the soil percolation system on heavy metals copper and chromium is more stable,and the effluent copper and chromium can reach the standard for dryland crops in the Water Quality Standard for Agricultural Irrigation(GB5084-2021).The change trend of copper concentration in the effluent from the four soil columns was similar,with lower effluent concentration in the early stage,and the removal rate of copper from the four groups of soil columns showed a decreasing trend as the operation time grew.The copper concentration in the effluent from the modified soil column CBC was lower than 0.5 mg/L.And due to the fluctuation of the influent hexavalent chromium,the effluent also showed the corresponding fluctuation characteristics.The adsorption capacity of CK soil column for hexavalent chromium decreased significantly from 70 to 96 d of operation,while the removal capacity of modified CBC soil column for hexavalent chromium remained stable.CMC could significantly improve the removal capacity of soil matrix for heavy metal hexavalent chromium,and the average removal rate of modified soil column CBC for hexavalent chromium was85.30%.(4)Before and after comparison of physicochemical indexes and elemental contents of soil substrates revealed that the p H of the improved substrates with rice husk biochar and CMC was more stable,with higher impact resistance,higher organic matter consumption and better buffering capacity.Compared with the CK soil column,the total nitrogen was not significantly elevated,while the total phosphorus content was significantly increased,while the addition of CMC and BC could effectively improve the effect of phosphorus adsorption.Compared with the CK soil column,the copper content of CM,BC and CBC soil columns increased by 31.92%,66.18% and 51.19%,respectively.The total amount of chromium in the substrate showed that CBC > BC > CM > CK.And the experiment showed that after runoff infiltration,copper accumulated in the soil surface layer,where the highest copper content was found in 0-15 cm.And there is no obvious pattern in the highest level of chromium accumulation of heavy metals.(5)Analysis of heavy metal Cu and Cr morphology of soil matrix at different depths showed that the addition of biochar and CMC synergistically improved the fixation of heavy metal Cu and Cr better.The unamended soil matrix fixed more active heavy metal Cu and had a greater rate and risk of migration.In the control soil column,the main form of heavy metal Cu in soil matrix was oxidizable state,while in the other soil columns,the residue state was dominant.