Study On Preparation Of Hydroxyapatite Modified Sludge-based Biochar And Its Removal Of Cu²⁺ And Cd²⁺ From Water

Sludge,as a by-product in the process of sewage treatment at all levels,its output increases gradually with the increase of population and the diversification of industrial activities.The composition of sludge is complex and contains toxic and harmful substances.Considering the safety requirements and the urgency of recycling economy,the harmlessness and reduction of sludge is an environmental problem that must be faced.In recent years,the preparation of sludge biochar（SSBC）from sludge as an adsorbent for heavy metals in water has become a means of resource utilization to increase its value and reduce the cost of its treatment and disposal.Nevertheless,because SSBC itself has fewer functional groups and insufficient specific surface area,its adsorption performance is constrained.In this study,based on the resource utilization of sludge,SSBC was prepared by pyrolysis with the dehydrated sludge from municipal sewage treatment plants as raw material,aiming at the current situation that the removal ability of heavy metals in water by SSBC needs to be improved.Hydroxyapatite modified sludge biochar（HAP-SSBC）was prepared by loading HAP on SSBC through co-precipitation method to be a new and efficient adsorbent for the adsorption of Cu²⁺and Cd²⁺in water.The specific work and results are as follows:（1）SSBC was prepared by pyrolysis method,then,HAP and HAP-SSBC with different SSBC content were prepared by co-precipitation method.The results revealed that the adsorption capacities of Cu²⁺and Cd²⁺by the composites were much higher than that of SSBC and HAP.Besides,the adsorption capacity of HAP-SSBC was the highest when SSBC dosage in HAP-SSBC was 1.5 g.The results of SEM,EDS,XRD and BET showed that HAP-SSBC had been successfully prepared and had good surface characteristics.EDS and XRD results showed that HAP was successfully loaded on SSBC.SEM results showed that the morphology of HAP-SSBC was more losse and dispersed.BET results show that HAP-SSBC has larger specific surface area and total pore volume than SSBC,which provides a good site for adsorption of Cu²⁺and Cd²⁺.Zeta potential results show that HAP-SSBC has a low zero potential point,which is more conducive to the adsorption of Cu²⁺and Cd²⁺in water.（2）The adsorption capacity of HAP-SSBC on Cu²⁺and Cd²⁺was much higher than that of SSBC.At equilibrium,the adsorption capacity of 10 0 mg/L Cu²⁺and Cd²⁺reached 88.66 and 93.33 mg/g,which were nearly 15 and 8 times of SSBC,respectively.Kinetic fitting results reveal that the adsorption process is mainly chemisorption,and further study reveals that the speed-controlling step is liquid film diffusion.The initial p H of the solution has little effect on the adsorption capacity of Cu²⁺and Cd²⁺on HAP-SSBC,and its adsorption capacity of both gradually increases with the increase of p H,indicating that HAP-SSBC has a wide range of p H applicability.（3）The Langmiur isothermal model can best describe the adsorption process of Cu²⁺and Cd²⁺by HAP-SSBC at 15,25,35 and 45℃,indicating that the adsorption process of HAP-SSBC is monolayer uniform,and there is no interaction between heavy metal ions.At 25℃,the maximum adsorption capacities of Cu²⁺and Cd²⁺by HAP-SSBC were 89.93 and 114.68 mg/g,respectively,which were much higher than those of the same type of adsorbents.Thermodynamic analysis proved that the adsorption of Cu²⁺and Cd²⁺by HAP-SSBC was a spontaneous endothermic process.With the temperature increasing,the binding of Cu²⁺and Cd²⁺by HAP-SSBC was enhanced.The competitive adsorption experiments showed that Cu²⁺and Cd²⁺could inhibit each other’s adsorption,and Cu²⁺is more dominant in the system with the coexistence of Cu²⁺and Cd²⁺.（4）The XRD,FT-IR and XPS results before and after HAP-SSBC adsorption revealed the possible mechanism of the adsorption of Cu²⁺and Cd²⁺by HAP-SSBC.The results reveal that the adsorption mechanism of HAP-SSBC for Cu²⁺and Cd²⁺is mainly ion exchange,that is to say,Cu²⁺and Cd²⁺partially or completely replace Ca²⁺in HAP-SSBC cells.In addition,the surface complexation of Cu²⁺and Cd²⁺with-OH and-COOH on HAP-SSBC surface,or the combination with aromatic C=C to form Cu-πand Cd-πare also the removal mechanisms.The negatively charged functional groups on the surface of HAP-SSBC can also remove Cu²⁺and Cd²⁺from water by electrostatic attraction.