Preparation Of Magnetic Biochar And Its Phosphate Adsorption Properties

Caragana korshinskii biomass was pyrolyzed at 600?for 2 h to obtain the biochar,and a novel method of electric auxiliary modification was adopted to modify the biochar with metal Fe,Al,and Mg.?-Al₂O₃/Fe₃O₄ Caragana korshinskii biochar?AFBC?and magnetic hydrotalcite Caragana Korshinskii biochar?FCB/MAC?with both magnetic and high adsorption properties were synthesized.The adsorption of phosphate in water by AFBC and FCB/MAC was studied,the mechanism of phosphate removal was discussed,and the efficiency of phosphate recovery was investigated.The main findings are as follows:?1?AFBC analysis showed that the surface was rough and uniformly distributed with?-Al₂O₃ and Fe₃O₄particles.?-Al₂O₃ provided a large specific surface area.The saturation magnetization of Fe₃O₄ particles was 27.4 emu/g.The ability of phosphate adsorption and recovery of unmodified biochar were greatly improved.FCB/MAC analysis showed that Fe₃O₄particles and Mg-Al-LDH were successfully intercalated into the biocarbon matrix.The magnetization of Fe₃O₄ is 32.9 emu/g,which is beneficial to recovery from the solution.The maximum amount of phosphorus absorbed by FCB/MAC is 252.88mg/g,which is higher than that of most magnetic phosphate adsorbents.?2?The adsorption properties of AFBC and FCB/MAC were significantly affected by solution pH.With the increase of pH value,the adsorption amount of phosphate by AFBC first increased and then decreased.When the adsorption amount was at its maximum,the pH value was 5.0.The pH_pzc of AFBC was 6.8,and the pH_pzczc after phosphate adsorption was 5.1.The pH_pzc of FCB/MAC was about 8.21.When the solution pH<PH_pzc,the surface of the adsorbent was protonated and decreased with the decrease of pH value.The increase of positive charge in adsorption solution is beneficial to phosphate adsorption.When pH>pH_pzc,the electrostatic repulsive force between the negatively charged FCB/MAC surface and phosphate ions increases,resulting in a decrease in the adsorption of phosphate.The experiment of coexisting anions showed that there was no obvious effect of Cl^-,NO₃^-and SO₄^2-on phosphate adsorption in the process of phosphate adsorption by FCB/MAC.The inhibition of F^-on phosphate adsorption may be due to the fierce competition between P and F^-binding sites on the surface of adsorbents.However,the influence of coexisting anions on phosphate adsorption by FCB/MAC is very small.The results show that the adsorbent has high selectivity to phosphate and has important practical application value.?3?Isothermal and kinetic adsorption of phosphate by AFBC and FCB/MAC were studied under optimum experimental conditions.The adsorption kinetics experiment of AFBC shows that the pseudo-second-order kinetic model can describe the phosphate adsorption process well,and the Langmuir-Freundlich model fits the adsorption isotherm.It shows that the adsorption of phosphate on AFBC is mainly controlled by chemical adsorption.The adsorption experiment of phosphate by FCB/MAC is carried out.The adsorption kinetics of phosphate was well described by the pseudo-second-order and intraparticle diffusion models,and the Langmuir-Freundlich model fitted the isotherm well.The adsorption mechanism of FCB/MAC for phosphate included electrostatic attraction,ligand exchange,and inter-layer anion exchange of surface groups.It shows that the adsorption process of phosphate by FCB/MAC is multi-step.?4?Electrostatic adsorption and internal spherical complexation are the common mechanisms of phosphorus adsorption on AFBC.The phosphorous inner-sphere complexation may consist of a single dentate complex(congruent AlOPO₃^2-)and a double dentate complex?congruent Al₂O₂PO₂^-?.The adsorption mechanism of phosphate by FCB/MAC includes electrostatic attraction,ligand exchange of surface groups,and anion exchange between layers.?5?The optimal NaOH concentrations of AFBC and FCB/MAC for phosphate desorption were 1.0×10^-4 mol/L and 0.1 mol/L,respectively.The adsorption capacity and removal rate of the two adsorbents decreased with the increase of desorption times.After 5 recycling experiments,the removal efficiency of phosphate by AFBC remained at about 91.0%.The removal rate of FCB/MAC decreased significantly from 85.6%to 51.4%after five cycles.After 5 times of recycling,the two adsorbents still have high adsorption level and removal efficiency,which are promising new phosphate removal adsorbents.