Study On Infiltration And Purification Of Road Rainwater Runoff By Strengthening Soil Substrate Of Dry Grass Swales

As a green landscape drainage measure,grass swales play an important role in controlling rainwater runoff and reducing runoff pollution.Due to the special soil matrix structure,dry grass swales has a strong capacity of stagnation storage and pollution interception on road rainwater runoff,so it is widely used in practical engineering as a LID measure.In this study,six kinds of inorganic fillers were added to optimize the infiltration and drainage performance of dry grass swales in view of the poor permeability of red soil substrate in east China.Then the best modified volcanic material was modified by LDHs film mulch,and the optimal modification conditions were determined by static adsorption experiment,and the effects of related factors such as dosage,solution p H and solution temperature on the adsorption effect of nitrogen and phosphorus of volcanic material before and after modification were determined.The kinetic adsorption and isothermal adsorption processes of volcanic rock materials before and after modification were analyzed by combining with related characterization methods,and the mechanism of nitrogen and phosphorus removal of volcanic rock materials before and after modification was discussed.On this basis,Mg/Fe-LDH modified volcanic rock material was added into the dry grass swales matrix layer,and the retardation and pollution interception capacity of different substrate types of straw gully was comprehensively analyzed through simulated rainfall and runoff test.The main experimental results are as follows:（1）By proportionally mixing inorganic fillers with pure red soil matrix（denoted as CHR）:20%perlite+80%red soil,20%green zeolite+80%red soil,20%medical stone+80%red soil,20%vermiculite+80%red soil,20%volcanic rock+80%red soil,20%ceramsite+80%red soil（HZZY,HLFS,HMFS,HZS,HHSY,HTL）,It can effectively improve the total porosity and permeability of red soil matrix.The total porosity of the improved matrix increased by 55.4%,51.38%,45.84%,42.9%,32.11%and 29.11%,respectively,and the corresponding permeability coefficients were HHSY>HTL>HLFS>HMFS>HZZY>HZS>CHR.In one-dimensional vertical infiltration simulation,the red soil matrix with volcanic material showed the best infiltration amount and infiltration rate.Under different rainfall intensities,the improved volcanic rock matrix shows strong outflow capacity,and from the aspect of drainage capacity improvement,the improved volcanic rock matrix is more adaptable to the change of rainfall intensity,and the permeability and drainage performance is more stable.（2）Static adsorption results show that the best preparation conditions of LDH modified volcanic rocks are solid liquid ratio of 10%and metal molar ratio of 2:1.The optimal dosage of volcanic material before and after modification is 10 g/L.The modified volcanic material shows a good adsorption effect of ammonia nitrogen in the range of p H 6～8.The modified volcanic material has a stable adsorption ability of phosphorus in the range of p H 3～7,while the adsorption effect of the original volcanic material to phosphate decreases gradually with the increase of p H value.With the increase of solution temperature,the adsorption capacity of nitrogen and phosphorus of volcanic rock material before and after modification is slightly improved.（3）The results of adsorption kinetics show that the adsorption rates of ammonia nitrogen and phosphate by LDHs film coated volcanic material are mainly controlled by chemical adsorption mechanism,and the whole adsorption process is not only controlled by the form of intra particle diffusion;The results of adsorption isotherm show that the adsorption process of ammonia nitrogen on volcanic rocks before and after modification fits the Langmuir isotherm model better,while the adsorption of phosphate fits the Freundlich isotherm model better.（4）According to SEM,EDS,FTIR and other characterization results,the surface of LDHs modified volcanic material becomes coarser due to the presence of metal particle attachment,and presents developed pore structure;In addition,the distribution of Mg,Fe,Al and other metal elements on the surface of the modified volcanic material is more uniform,and the corresponding metal content is also improved.The stretching vibration peaks of m-O,M-OH and M-O-M(Mainly Fe³⁺,Al³⁺and Mg²⁺)on the surface of LDHs coated volcanic rocks are strengthened.（5）The dry grass swales device improved by mixing volcanic rock material showed good infiltration and drainage ability in the whole experiment process,significantly reduced the surface outflow ratio,and the reduction rate of rainwater runoff was about two times that of the dry grass swales device with pure red soil substrate,which had strong runoff control ability.In addition,adding volcanic rock filler into the matrix layer can effectively improve the hydraulic load impact resistance of dry grass swales device,so that the improved grass swales can better adapt to the change of confluence water quantity.（6）Adding volcanic rock material to the soil substrate of dry grass swales can significantly improve its removal effect of pollutants in runoff,and the removal effect of COD,NH₄⁺-N,TN and TP in dry grass swale with Mg/Fe-LDH modified volcanic rock material is better than that of dry grass swale with original volcanic rock material.Adding Mg/Fe-LDH modified volcanic rock material as inorganic filler into the soil substrate of dry grass swales can significantly improve the runoff pollution control ability,and the maximum removal rates of COD,NH₄⁺-N,TN and TP in runoff can reach 68.24%,89.42%,69.75%and 91.44%.