Study On Preparation And Phosphorus Removal Of A Novel Magnetic Calcium Silicate Hydrate Composite ?Fe₃O₄@CSH?

Phosphorus?P?is not only a non-renewable resource,but also plays a prominent role in eutrophication of water bodies.Exploring and researching efficient and stable materials for P removal has became the problem which demands prompt solution in the field of pollution prevention.Magnetic nanoparticles?MNPs?have received considerable attentions for their large surface area,strong P-binding capacity,as well as exceptive magnetism for rapid magnetic separation using an external magnetic field.However,MNPs with high surface free energy and fewer functional groups are prone to aggregation in aqueous solutions and susceptible to air oxidation,which negatively affects the P-removal capacity.The chemical stability and dispersity of MNPs could be further improved through the surface functionalization and modification,which results in the enhancement of P-removal performance.Previous studies have showed that calcium silicate hydrate?CSH?with a porous structure released calcium ions and hydroxide ions and could adsorbe phosphate in the solution,and then generated micro alkaline environment which was productive for Ca-P formation and P removal.Based on the above background,this work intends to synthesize a novel magnetic calcium silicate hydrate composite?Fe₃O₄@CSH?using CaO and white carbon black and Fe₃O₄nanoparticles as raw materials by the solvothermal method.The P removal and recovery properties of the synthesized Fe₃O₄@CSH was studied through batch experiments.The experimental results showed that:?1?The Fe₃O₄@CSH were synthesized from the as-synthesized Fe₃O₄ MNPs,CaO,and white carbon black using a dynamic hydrothermal synthesis method.The experiment results indicated that when the Ca/Si molar ratio was 2.2,reaction temperature was 170°C,reaction time was 7 h,and stirring rate was 90 r/min,the dissolved Ca²⁺and OH^-capacity were the best and the dissoluted Ca²⁺and OH^-concentration were 22.78 mg/L and 10^-2.952.95 mol/L separately.Under this condition,the synthesized Fe₃O₄@CSH materials had the best removal efficiency of P by 99.79%,and the residual P concentration in solution was only 0.04 mg P/L when the initial P concentration was 20 mg P/L.The four-factors and four-levels-orthogonal test was conducted and the optimum synthesis conditions were consistent with the single-factor test.The range analysis of test data showed that the order of factors are:Ca/Si molar ratio>reaction temperature>reaction time>stirring rate,and Ca/Si molar ratio is the most significant factors.?2?The Fe₃O₄@CSH synthesized under the optimum conditions was used for testing P-removal,reusability and magnetic separability of this material.The results showed that when the initial pH value was 5.35,initial P concentration was 20 mg P/L,adsorbent dosage was 0.6 g/L,reaction time was 1 h,and stirring rate was 150 r/min,the best P removal rate of 99.79%and the maximum P adsorption of 33.26 mg P/g were obtained.The Fe₃O₄@CSH could maintain relatively high capacity?over 20 mg P/g?over 8 cycles,indicating a good reusability for P removal.After P adsorption,the mixture could been rapidly separated from aqueous solution by a magnetic separation technique within 1 min.?3?Adsorption kinetic models,equilibrium isotherm models and thermodynamic analysis were performed to characterize the phosphate adsorption performance of the magnetic composite materials.Further TEM,SEM,FTIR analysis,XRD patterns,XPS analysis,zeta-potential,and BET analysis were used for exploring the P-removal mechanism of Fe₃O₄@CSH.The results indicated that the P adsorption on the surface of Fe₃O₄@CSH fitted well to the Freundllich equation and was a pseudo-second-order process.It was demonstrated that Fe₃O₄@CSH adsorbed P mainly via chemical adsorption,which is a spontaneous and endothermic process.Finally,most of P were removed in the form of CaHPO4.