The Adsorption Properties And Mechanism Of Cd(?) Removal Onto Calcium-containing Biosorbents And Its Modifided Materials

With the rapid development of industry and agriculture,cadmium pollution has become increasingly serious,and is far exceeding the environmental self-purification capacity.The technology of adsorption is highly advantageous for the treatment of heavy metal contamination due its low-cost,high-efficiency,easily recyclable adsorbents Two potential calcium-containing bio-materials,shell powder and bone meal,are inexpensive and widely available,and can be used as the best substitute for artificial adsorbents of calcium carbonate and calcium hydroxyphosphate.The purpose of this study is developed to investigate the effects and mechanisms of calcium-containing bio-materials?shell powder,bone meal and its modified materials?,and then it provide technological support for remediation of Cd contamination.The research results were as follows:?1?Shell powder?SP?was used as raw material for Cd removal.A series of characterization methods included FT-IR,XRD,SEM-EDS,specific surface area were combined to investigate the morphology,pore structure and functional groups of the material.The results shown that the main component of SP was CaCO₃,which contained a large number of mesopores and macropores.The surface was rich in CO₃^2-functional groups,and the removal of trace element can be achieved by the precipitation of CO₃^2-and Cd?II?.At lower pH,H⁺inhibited the removal of Cd?II?onto SP by the competitive adsorption with Cd?II?.The adsorption kinetics of Cd?II?accords with the pseudo-first-order kinetic model and the isothermal adsorption process could be well fitted with Temkin and Langmuir isotherms.The adsorption process was favorable and the maximum adsorption capacity was161.75 mg·g^-1.?2?Fish bone meal?FBM?was modified by the NaOH?FBM_T?.A series of above-mentioned methods were conducted to investigate the physicochemical characterization of samples.The main component of the materials was hydroxyapatite?HAP?.Compared with FBM,FBM_T obtained a certain number of mesopores and macropores,with higher pore volume and average pore size,which increased by 68.57%and 24.69%,respectively,compared with FBM.After modification,the content of functional groups such as-OH,C=O and PO₄^3-on the surface of FBM_T increased,showing a certain buffering performance.The mechanism of Cd?II?removal onto functional materials mainly included cation specific adsorption,electrostatic adsorption,ion exchange,surface complexation and precipitation.The adsorption kinetics of Cd?II?onto FBM and FBM_T fitted the pseudo-second-kinetic model well and the main rate-limiting step was chemisorption.The isothermal adsorption datas fitted well with the Langmuir model.A,and the amount of saturated adsorption amount increased from 9.28 mg·g^-11 in FBM to 22.47 mg·g^-11 in FBM_T.After modification,the corresponding adsorption strength K_F value also increased and the separation factor 0<R_L<1 indicated that the removal process of Cd?II?was favorable.?3?After treatment of fish bone meal?FBM?at different temperatures of 200,400,600°C and800°C,a series of reactions such as the adsorbed of water in the modified materials evaporated,and the decomposition of residual organic matter,collagen and its intermediates occurred.The microporous and mesoporous was mainly in FBM and B₂₀₀,and with increasing the temperature,lots of mesopores and macropores was obtained in the B₄₀₀,B₆₀₀00 and B₈₀₀,the number of PO₄^3-functional groups also increased.However,the main component of the modified material was still HAP after treatment at different temperatures.The adsorption kinetics of Cd?II?onto B₂₀₀,B₄₀₀,B₆₀₀00 and B₈₀₀00 fitted well with pseudo-second-order model and the main rate-limiting step was chemisorption.The isothermal adsorption process conformed to the Langmuir model,mainly for monolayer adsorption,which was favourable.The saturated adsorption capacity of Cd?II?increased from 9.284 mg·g^-11 to 37.799 mg·g^-1,and the maximum value of saturated adsorption was obtained for B₆₀₀.?4?B₆₀₀00 was modified by Chitosan?Cs?and Fe₃O₄,and obtained a new material(Cs-Fe₃O₄-B₆₀₀)with magnetic responsiveness.After modification,the macropores in the materials disappeared,and while the amount of pore volume and specific surface area were increased,and an amino?II?functional group appeared.The main mechanism of Cd?II?onto Cs-Fe₃O₄-B₆₀₀00 were mainly the complexation of amino groups,and a small part of ion exchange between Cd?II?and Fe₃O₄.The state of the amino group was affected with the change of pH.At lower pH,the competitive adsorption of H⁺and the electrostatic repulsion generated by the protonation of the amino group inhibited the adsorption of Cd?II?.The adsorption process was extremely quick and the more suitable adsorption kinetics of Cd?II?removal onto Cs-Fe₃O₄-B₆₀₀00 was pseudo-second-order,the main rate-limiting step was chemical adsorption.The isothermal adsorption process conformed to the Langmuir model,indicateing a monolayer and favorable adsorption.The saturated adsorption capacity of Cd?II?by modified materials increasedby70.1%.