Preparation Of Magnesium Modified Biochar For Nitrogen And Phosphorus Removal From Waste Water

As an important source of life,water was the material basis for survival of all living things on the earth.However,water pollution became increasingly serious due to the development of agricultural intensification,industrialization and urbanization.Among them,the problem of eutrophication caused by nitrogen and phosphorus was particularly serious.Biochar was a new type of functional material,which had been widely used in recent years.Due to its large specific surface area and abundant functional groups,it had attracted more attention in the field of water pollutant purification.However,the original biochar had a poor removal for nitrogen and phosphorus.In this study,the peanut shell modified biochar prepared by different magnesium impregnation methods was screened to study its adsorption performance of nitrogen and phosphorus in water,and combined with characterization methods such as BET,FTIR and XRD,to explore the adsorption mechanism of Mg O-BC on nitrogen and phosphorus.In addition,the effect of Mg O-BC on the dynamic removal of nitrogen and phosphorus under different factors was explored through sand column simulation experiments.The goal of this research was to prepare cost-effective,green and environmentally friendly adsorption materials,and to provide a basis for the recycling of nitrogen and phosphorus.The main conclusions were as follows:（1）The adsorption capacity of nitrogen and phosphorus by magnesium-modified biochars was as follows:Mg O-BC>Mg Cl₂+Na OH-BC>Mg（OH）₂-BC>Mg Cl₂-BC,1mol L^-1 Mg O was selected to Modify biochar in this study.The specific surface area of Mg O-BC increased by more than two times compared with BC,and the average pore diameter increased slightly.The EDS energy spectrum showed that the Mg content of Mg O-BC significantly increased than BC,which proved the successful preparation of Mg O-BC.The Mg content of Mg O-BC decreased,the diffraction peaks of Mg O and Mg（OH）₂ disappeared after the adsorption of nitrogen and phosphorus,the stretching vibration peaks of O-H,Mg-O and O-Mg-O also disappeared.stretching vibration peaks of N-H,P-O and the diffraction peak of Mg HPO₄appeared,indicating that the main mechanisms of Mg O-BC adsorbing nitrogen and phosphorus were ion exchange and electrostatic effects,respectively.（2）The pseudo-second-order kinetic model could better fit the adsorption data,showed that the adsorption of nitrogen and phosphorus by Mg O-BC was physical-chemical complex process,The correlation coefficients of Elovich and the two-constant model also reached about 0.8,indicating that there was chemical adsorption and complexation during the adsorption of nitrogen and phosphorus by Mg O-BC.The Langmuir model was more consistent with isotherm adsorption data,indicating that the adsorption was dominated by monolayer adsorption,and the maximum theoretical adsorption capacity of nitrogen and phosphorus was 126.58 mg g^-1 and263.15 mg g^-1,respectively.The thermodynamic adsorption reaction showed that the adsorption of nitrogen and phosphorus by Mg O-BC proceed spontaneously,and it was exothermic reaction and entropy value reduced.The increase of ionic strength hindered the adsorption of nitrogen and phosphorus,and the effect on phosphorus was more significant.The adsorption of nitrogen and phosphorus by Mg O-BC first increased and then decreased with the increase of p H.The adsorption capacity of nitrogen and phosphorus was the maximum p H of 7 and 6,respectively.When there were K⁺,Na⁺,NO₃^-,Cl^-and SO₄^2-,they basically had no effect on the adsorption of nitrogen and phosphorus by Mg O-BC;Ca²⁺and Al³⁺inhibited the adsorption of nitrogen and promoted the adsorption of phosphorus,and the effect of Al³⁺was more significant.Increasing ionic strength promoted desorption of nitrogen and phosphorus by Mg O-BC,and the desorption effect on phosphorus was stronger.Increasing p H promoted desorption of phosphorus by Mg O-BC,acidic and alkaline conditions promoted the desorption of phosphorus.（3）In the quartz sand column,increasing flow rate and ionic strength had almost no effect on the retention of nitrogen and phosphorus;increasing p H decreased the retention of nitrogen,but had almost no effect on the retention of phosphorus.In the Mg O-BC packed sand column,the more Mg O-BC was packed,the more nitrogen and phosphorus were retained,the outflow time of phosphorus was delayed,and its tailing phenomenon was more obvious;increasing flow rate promoted the outflow of nitrogen and phosphorus,and the higher the flow rate,the faster the penetration time of phosphorus,and the less obvious the tailing phenomenon;The removal rate of nitrogen and phosphorus was inversely proportional to ionic strength,and the recovery rate of phosphorus was higher than that of nitrogen.The increase of p H reduced the retention of nitrogen and phosphorus,and the retention of phosphorus was less than that of nitrogen.Desorption of nitrogen and phosphorus by the Mg O-BC packed sand column showed that under the same flow rate,ionic strength and p H,the amount of nitrogen desorption gradually increased,while the amount of phosphorus desorption showed the opposite trend,and eventually tended to desorb equilibrium state;the desorption amount of nitrogen and phosphorus gradually increased when the flow rate and ionic strength increased.An increase in p H would promote nitrogen desorption,while acidic and alkaline conditions were more conducive to phosphorus desorption.