Research On Performance And Mechanisms Of Phosphorus Adsorption By Modified Sewage Sludge-based Biochar

In recent years,the eutrophication of surface water caused by excessive nitrogen and phosphorus has attracted more and more attention.Due to the lack of economical and effective control strategies in rural areas,the uncontrolled discharge of phosphorus sewage is one of the main reasons for the eutrophication of the surface water.In order to achieve the recycling of sewage sludge and obtain high-efficiency phosphorus adsorbents for phosphorus removal in villages and towns,in this study,sewage sludge was used as the material,modified with four metals of Mg,Ca,Al,and Fe.The modified sludge-based biochar was pyrolyzed at temperature of 500-900℃,then analyzing the physical and chemical properties,the structure and functional groups of the biochar surface.Analyzing the effects of pyrolysis temperature and metal contents on the properties of sludge-based biochar,aims to select four modified sewage sludge-based biochars with the best phosphorus adsorption performance.Discussing its performance and influence factors to explore the mechanism of modified sewage sludge-based biochar adsorption of phosphate.Investigate the possibility of using modified sewage sludge-based biochar for phosphorus sewage,and find a feasible way for the efficient phosphorus removal of sewage in rural areas and the resource utilization of sewage sludge.The main findings are as follows:In order to explore its adsorption performance of phosphate,modified sewage sludge-based biochar was pyrolyzed at 500-900℃,and the properties of biochar were analyzed by characterization methods.The maximum adsorption capacities of SSBC900-Mg,SSBC900-Ca,SSBC500-Al,and SSBC500-Fe for phosphate are33.72mg/g,139.12mg/g,27.82mg/g and 6.06mg/g,respectively.The experimental results showed that,with the increase of the initial pH value,the adsorption capacity of SBC900-Mg and SBC900-Ca for phosphate increased first and then decreased;the adsorption capacity of SBC500-Al and SBC500-Fe for phosphate decreased gradually.The study found that the adsorption process of phosphate by SBC900-Mg and SBC900-Ca accorded with the pseudo-second-order kinetic model and Langmuir model,respectively,indicating that the adsorption of phosphate is mainly the chemical adsorption of the single layer of biochar.The SBC500-Al and SBC500-Fe conform to the pseudo-first-order kinetic model and Freundlich model,indicating that the adsorption process of phosphate is multi-layer adsorption under non-ideal conditions.The analysis of the adsorption thermodynamic model shows that the adsorption reaction is an endothermic process.Common ions in water,for example,Cl^-,SO₄^2-,HCO₃^-,NO₃^-,CA and HA can compete with phosphate for active adsorption sites on modified sewage sludge-based biochar,resulting in a decrease in the efficiency of biochar’s adsorption of phosphate.After five adsorption-desorption cycles,the four types of biochar still have adsorption capacity,indicating that the renewable modified sewage sludge-based biochar has a certain reuse value.The leaching results of biochar show that both SBC900-Mg and SBC900-Ca have great potential to be reused as farmland phosphate fertilizer after absorbing phosphorus.In order to further reduce the cost,limestone,basic magnesium carbonate,dolomite and scallop powder are used to modify sewage sludge-based biochar.The research results show that the modified material significantly enhances the adsorption capacity of biochar for phosphate.The order of phosphate adsorption capacity of biochar is DSBC<MSBC<SSBC<CSBC.In general,the higher the pH value of the system is beneficial to the adsorption of biochar on phosphate.Through the analysis of adsorption mechanism,it is shown that the adsorption process of modified sewage sludge-based biochar to phosphate mainly includes:surface precipitation of Ca/Mg and phosphate,electrostatic attraction between metal oxide and phosphate,Surface complexation and ion exchange with-OH.Therefore,a modified sewage sludge-based biochar production method established by this research can not only achieve the resource utilization of sewage sludge,but also obtain a high-efficiency phosphorus removal adsorbent for phosphorus sewage in villages and towns.