Preparation Of Nano Phosphorus Biochar Composite Materials And Study On Migration Law In Soil

The pollution of lead(Pb)and cadmium(Cd)in the soil and groundwater is very serious.It is urgent and important to prepare environmental remediation material with a good affinity for Pb and Cd.Nano-hydroxyapatite has good adsorption properties for heavy metals,has the advantages of environmental self-purification and seldom causes secondary pollution,which is treated as one of the most effective materials for remediation of heavy metal polluted environment.Preparation of composite materials by biochar and nano-hydroxyapatite can achieve efficient and economical remediation of various heavy metal-contaminated soils.Aiming at Pb and Cd pollution,this study used rice husks and straw crops as raw materials to prepare biochar which was then modified by nano-hydroxyapatite,and selected a composite material with lower cost and good performance.The migration characteristics of composite material in saturated and unsaturated soil have certain guiding significance for the application of such material for Pb and Cd contaminated site remediation.In this study,the composite material of nano-phosphorus and biochar(n Hap/BC)was synthesized by using nano-hydroxyapatite to modify the selected straw biochar.The research shows that the way of adding biochar in first stage under ultrasonic is the best one.The characterization results of SEM,XRD and FTIR all show that the nano-hydroxyapatite was successfully loaded on biochar.When the carbon-to-phosphorus ratio is 1,the specific surface area of the material is 149.46 m²·g^-1,which is close to the specific surface area of nano-hydroxyapatite 151.39 m²·g^-1;When the material dosage is 1 g/L,the removal rate and adsorption amount of Pb²⁺at a concentration of 400 mg/L are twice higher than those of straw biochar,and the removal rate and adsorption amount of Cd²⁺at a concentration of 200 mg/L are both 9 times higher than those of straw biochar.Considering the economic cost and treatment effect,the composite material with the carbon-to-phosphorus ratio of 1 is the optimum.The best performance of n Hap/BC was used for the migration experiment in saturated natural sand.Studies have shown that increasing humic acid concentration,p H and decreasing ionic strength can increase the negative charge on the surface of n Hap/BC and natural sand,of which the ionic strength has the most significant effect,thus increasing the absolute value of Zeta potential as well as the electrostatic repulsion and the dispersion stability of n Hap/BC,then promoting the migration of n Hap/BC;Brown diffusion dominated the deposition mechanism of particles in n Hap/BC suspension while the gravity settling and interception accounted few.The interception efficiency of natural sand to n Hap/BC manifests that the interception efficiency gradually decreased with increasing humic acid concentration,increasing p H and decreasing ionic strength,leading to the more migration of n Hap/BC and an increase of penetrating platform.From the DLVO theoretical analysis,the larger ionic strength will compress the electric double layer and reduce the electrostatic repulsion between n Hap/BC and natural sand,thus decreasing the maximum energy barrier value in the system from 2219.37k T to 836.74 k T.Besides,n Hap/BC is prone to permanent polymerize and the absolute value of the second energy minimum in the system becomes larger,which also facilitates the easier adsorption of n Hap/BC particles on the surface of natural sand;The maximum migration distance of n Hap/BC in natural sand can reach 39.28 cm without humic acid and interferential ion.The maximum migration distance is positively correlated with humic acid concentration and negatively correlated with ionic strength.The farthest migration distance is smaller in acid soils than that in medium alkaline soils.The migration of the wetting front of n Hap/BC material suspension in unsaturated clean natural sand shows that:the factors affecting the distance of the wetting front movement follow the orders of:pore water flow rate>intermittent time>colloidal suspension concentration;The wetting front movement rate is positively correlated with the pore water flow rate,while negatively correlated with the intermittent time and the colloidal suspension concentration.The Kostiakov model is used to obtain the correlation between wetting front movement distance,and the pore water flow rate,intermittent time as well as colloidal suspension concentration during intermittent and continuous dosing.When Continuous dosing,the migration rate of the composite material wetting front in clean soil is not different from that in polluted soil;When intermittently added,the migration of composite material wetting front in clean soil is significantly faster than that in contaminated soil.The study of the farthest migration distance of n Hap/BC in saturated and unsaturated columns and the content of heavy metals in the effluent indicates that the prediction formula for the farthest migration distance of composite materials in saturated medium can be approximated applied to the prediction of in unsaturated medium;When continuous and intermittent dosing,the content of heavy metals in the effluent has a linear negative correlation with the length of soil column.The content of heavy metals in the effluent with continuous dosing is higher than that with intermittent dosing.Intermittent dosing can slow the migration of n Hap/BC materials and reduce the ability of heavy metals migration.The effect of n Hap/BC on polluted natural sand shows that when n Hap/BC material is injected into polluted natural sand,it can immobilize Pb and Cd to some extent with a better effect on Pb than Cd.The leaching toxicity of Pb²⁺and Cd²⁺was positively correlated with the depth of soil column.Intermittent injection facilitate n Hap/BC has a better heavy metal immobilization effect than continuous injection.The research results of this study play a certain guiding role in the theoretical research and engineering practice of the remediation of Pb and Cd polluted soil or groundwater.