Preparation Of Shell-based Materials And Their Adsorption Performance For Water Pollutants

Non-point source pollution is mainly related to the rapid development of modern industry and agriculture,which made China or even the world occur challenging problems water eutrophication and heavy metal pollution.Therefore,it is critical for them to effectively reduce the accumulation of nitrogen and phosphorus,and heavy metals in water,so that to alleviate the dilemma of water environment deterioration.Adsorption is one of the effective methods to reduce the load of nutrients and heavy metals in water.However the selection of adsorbents is a prerequisite for large-scale application of adsorption in pollutants removal.This paper is thus based on the fact that a large number of abandoned shells from aquaculture have adverse impact on water quality and land occupation.If we could reuse these materials characterized with good absorption and rich in Ca,great benefit could be achieved.As a consequence,a series of absorbents were prepared by modifying shells and the relationship between their structure,components and adsorption properties were studied.Firstly,phosphorus removal from water by a thermally modified shell-based absorbent was observed.It was found that low and high concentration of phosphorus can be efficiently removed.The shell-based materials COS900 was obtained by calcining the oyster shell powder at 900℃for 3 h.The structure and composition of original and calcined shells were investigated using SEM,XRD,FT-IR,BET.The results of isothermal adsorption showed that compared with the same calcined pure CaO powder（CaO900）(Q_m=1880 mg g^-1),COS900 has slightly lower adsorption capacity for phosphate(Q_m=1422 mg g^-1),respectively,which are consistent with Langmuir model adsorption.Kinetic adsorption showed that equilibrium can be achieved after 3 h adsorption,which was more in line with the adsorption process of pseudo-second-order kinetic model,belonging to chemical physical adsorption.The adsorption capacity of COS900 does not change with the increase of pH.FT-IR and XRD analysis found that-OH and Ca²⁺of COS900 played a crucial role in the adsorption of phosphate by internal complexation and electrostatic action.Secondly,hydroxyapatite（HAP）was prepared by chemical precipitation of COS900 and KH₂PO₄,where COS900 and KH₂PO₄were the calcium source and phosphorus source respectively.Isothermal adsorption experiment showed that the theoretical adsorption capacity of Pb（Ⅱ）by HAP reached 311.6 mg g^-1,which was consistent with the Langmuir adsorption model,belonging to single-layer adsorption.The absorption mechanisms was mainly the chemical change caused by the ion exchange of Ca（II）on the surface of the material by Pb（II）,and the entire adsorption belongs to chemical physical adsorption.With the increase of pH,electrostatic adsorption,physical adsorption,and ion exchange dominated the adsorption of HAP for Pb（II）.At pH≈5,the adsorption capacity of HAP for Pb（Ⅱ）reached the maximum.The result further verified the chemical and physical adsorption process of Pb（II）by HAP,which was consistent with the SEM and XRD analysis.Finally,to solve the seperation problem,the above HAP was furthur modified with activated carbon and Fe₃O₄to obtain HAP@Fe₃O₄@C by the impregnation-assisted chemical precipitation method.The isothermal adsorption result showed that the theoretical adsorption capacity of HAP@Fe₃O₄@C for Pb（Ⅱ）reached 723.6 mg g^-1,and the adsorption process was more consistent with the Langmuir model,which was a single-layer adsorption.The kinetic adsorption result found that the adsorption of HAP@Fe₃O₄@C composites on Pb（Ⅱ）was more in line with the pseudo-second-order kinetic model,which was a typical chemical physical adsorption process.The analysis of SEM and FT-IR showed that electrostatic attraction and ion exchange played major roles in the adsorption process.By comparison,the functional groups on the surface of activated carbon and the physical and chemical properties of Fe₃O₄played key roles in the adsorption of Pb（Ⅱ）by HAP@Fe₃O₄@C.Therefore,HAP@Fe₃O₄@C can not only efficiently adsorb lead ions in water,but also quickly be separated by an external magnetic field.