Study On The Effect Of Magnesium Modified Biochar On Nitrogen And Phosphorus Control In Bioretention

The collection,treatment and utilization of rainwater are important to solve the shortage of freshwater resources.As an important stormwater management measures,bioretention can not only effectively control pollutants in runoff rainwater,but also alleviate flood disasters to some extent.At present,the outstanding problem in the research and application of bioretention is that the removal effect of nitrogen and phosphorus is unstable.Biochar has a large specific surface area and strong stability,and has a good adsorption effect on a variety of pollutants.The magnesium modification of biochar can increase the specific surface area of the biochar and increase the types of surface active functional groups and the quantity,and then can increase the adsorption capacity of pollutants.Biochar is the secondary utilization of organic waste.The application of modified biochar in bioretention can effectively control runoff pollutants and provide a broader development direction for the application of biochar.In order to explore the feasibility of magnesium modified biochar used in bioretention,the main contents and results of this study are as follows:The biochar was modified by different concentrations of magnesium chloride hexahydrate solution.By measuring the adsorption effect of six kinds of biochar modified by Mg Cl₂·6H₂O solution on NH₄⁺-N and PO₄^3--P was measured,,it was found that the adsorption performance of rice husk biochar modified with 2mol/L magnesium chloride hexahydrate solution was the best.The modified biochar prepared under this condition was recorded as MgBC,and before modification,it was recorded as BC.The physicochemical properties of BC and MgBC showed that the properties of MgBC were stable,and the type and content of functional groups increased.Quasi second order kinetics and Langmuir model show that the adsorption process of NH₄⁺-N and PO₄^3--P by MgBC is dominated by chemical adsorption.Langmuir model fitting shows that the maximum adsorption capacity of MgBC for NH₄⁺-N and PO₄^3--P can reach 9.51mg/g and 51.37mg/g,respectively.Comprehensive analysis results of the properties of biochar before and after modification show that the modification can increase the number and types of functional groups inside the biochar to enhance the adsorption effect of pollutants.Therefore,the use of MgBC as a filler modifier for bioretention has great research potential.By adding different types of biochar to simulate the operation of the bioretention device,it was found that setting the submerged area could increase the comprehensive control effect of various pollutants.When the height of the submerged zone is 400mm,the control effect of NO₃^--N and NO₂^--N is the best.In general,the device No.1（with MgBC,soil and sand as filler）,device No.2（with BC,soil and sand）and device No.3（with soil and sand as filler）have the best comprehensive control effect on each pollutant in the 200 mm submerged zone.Regardless of the height of the submerged zone,the average control effect of the three devices on each pollutant is that the device No.1 with MgBC as the improved filler is the best,the device No.2 with BC as the improved filler is the second,and the device No.3 with sand and soil as the filler is the worst.The operation test results of the device under different dry period show that with the extension of the dry period,the control effects of devices No.4（with MgBC,soil and sand as fillers）,No.5（with BC,soil and sand as fillers）and No.6（with soil and sand as fillers）on various pollutants showed a downward trend.However,compared with device No.5 and device No.6,the device No.4 with MgBC as the filler modifier has a better and more stable control effect on pollutants under a longer drying period.Based on the comprehensive control effect of each pollutant,the device No.6 with sandy and soil as filler is not suitable for arid areas,while the device No.4 and device No.5 with BC and MgBC as improved fillers still have certain advantages in arid areas,and the device No.4 with MgBC as the modified filler has the best pollutant control effect.High-throughput sequencing can analyze the correlation between specific colonies in the microbial community structure and pollutant control effects from the perspective of microbial communities.The analysis results at the sample phylum and genus level show that using MgBC and BC as filler modifiers can not only repair the soil,but also improve the nitrogen and phosphorus removal bacteria in the bioretention system.Nitrifying bacteria are mainly distributed in 0-400mm depth packing,anaerobic ammonia oxidizing bacteria are mainly distributed in 200mm-800mm depth packing,and the content of device No.4（using MgBC as modified filler）is the highest,and denitrifying bacteria are mainly distributed in400mm-800mm deep filler,the content of phosphorus removal bacteria in the filler is relatively low,indicating that the removal of phosphorus is mainly controlled by the absorption of the filler itself,and the effect of microorganisms is small.There was a positive correlation between the content of functional bacteria and the control effect of pollutants.The analysis of functional bacteria shows that device No.4（with MgBC as the improved filler）has the best comprehensive removal effect on pollutants,and device No.4 is more suitable for longer dry period,because it can repair soil,increase Soil water holding capacity and beneficial to the survival of functional bacteria.All in all,MgBC used as a filler modifier for bioretention,can significantly improve the control effect of nitrogen,phosphorus and COD.Considering the simultaneous removal of nitrogen and phosphorus,it is recommended to use a bioretention with MgBC as a modifier,and set a 200mm submerged zone,the bioretention device can still have a good control effect on pollutants in arid and semi-arid areas.