Mechanism Study On Passivation Stability Of Different Adsorbents To Heavy Metals

With the rapid development of industrialization,the problem of soil heavy metal pollution in China is becoming more and more serious.What’s more,the promulgation of "the 10-Chapter Soil Pollution Action Plan" actuated “how to solve the aforementioned mire” into paramount importance.Compared with the traditional methods,the in-situ passivation technology is simple and economical,and more suitable for solving the heavy metal pollution in china.The crucial of the in situ passivation technology is to choose a kind of good performance,high efficiency,economy and environment friendly passivation material.In situ remediation technology is through the change of heavy metal rich deposit form,and then fixed on the passivation agent.Nevertheless,changes of soil environment will lead to the canned eavy metals release in the soil again.Hence maintaining the reciprocal balance between passivation materials’ stability and removal efficiency is sagacious.Herein,black carbon,bentonite,titanium dioxide for raw materials,were modified by three methods i.e.,oxidation,grafting and coating.Subsequently,the modified carbon black,organic bentonite and Fe₃O₄@TiO₂-GO composite materials were obtained.Meanwhile,the passivation effect of different passivation materials for heavy metal ions were compared in the light of adsorption isotherms and adsorption kinetics experiment.In addition,the adsorption thermodynamics and adsorption desorption experiment were conducted to explore the stability mechanism of different passivation passivation materials for heavy metal ions.Ultimately,the removal efficiency and passication stability of the modified black carbon were verified in the soi-plant system through the soil culture test.The motif of this work is to provide theoretical basis for the practical application of the passivating agent.The main results of this paper are as follows:（1）Through the ways of oxidation,grafting,and coating,three materals（i.e.,modified black carbon,organic bentonite,Fe₃O₄@TiO₂-GO organic inorganic composite materials）were acquired.The characterization of SET,TEM,EDS,and FTIR shows that the modified carbon black surface is oughness,forming a porous structure,and coupling-NO₂ organic oxygen-containing functional groups on oxidated surface.Furthermore,the bentonite surface is roughness,uneven,and grafted onto organic functional groups.Regarding to the core-shell structure of Fe₃O₄@TiO₂-GO organic inorganic composite materials,the core is Fe₃O₄ combined with TiO₂ as the shell,and GO as the outer coating layer.The thickness of TiO₂ shellcan be effectively controlled by the molar ratio of Fe₃O₄/TiO₂ precursor.（2）Modificated black carbon was choose as the best passivation agent ascribed to the superiority on saturated adsorption capacity.The adsorption rate of heavy metals in organic bentonite increased rapidly,suitable for the adsorption rate of heavy metal containing wastewater treatment;FTG composite materials with magnetism and light catalysis,more suitable for the wastewater containing refractory organic compounds the treatment.The maximum adsorption capacity of heavy metal ions Cu2+ and Cd2+ from modified carbon black was 23.42 mg·g-1 and 35.59 mg·g-1respectively,and the absorption kinetic curve accords with the pseudo-second-order kinetics model;maximum adsorption of heavy metal ions Cu2+ and Cd2+ from organic bentonite was 11.21 mg·g-1and 10.30 mg·g-1 respectively,and the absorption kinetic curve accords with the pseudo-second-order kinetic model;The adsorption of As（III）via FTG composite material conformed to the Langmuir adsorption isotherm model,the maximum adsorption capacity was 8.57 mg·g-1,the maximum adsorption capacity of FTG on enrofloxacin was 5.3 mg·g-1,and after 90 min visible light irridation it can be completely photodegraded.（3）The adsorption of heavy metal ions on the organic inorganic composite materials mainly ascribed to chemical adsorption,and adsorption of heavy metal ions on inorganic materials owning to physical adsorption,and the organic-inorganic composited material is more stable in heavy metals adsorption.The adsorption morphology of Cu2+ on the surface of modified black carbon is presented following: the ion exchange>the physical action>chelation.While the adsorption of Cd2+ on the surface of modified black carbon is presented following: Chelation>ion exchange>physical action.The mechanism of the modified black carbon to heavy metal is mainly divided into three kinds: electrostatic adsorption,chelation and micropore capture.The binding forms of Cu2+ in organic bentonite are shown as follows: Chelation > physical action > ion exchange;the binding forms of Cd2+ in organic bentonite,the chelation,ion exchange,and physical function.The passivation mechanism of heavy metals by organic bentonite is mainly divided into three types: ion exchange and chelation.（4）After the application of MBC,the content of available heavy metals in soil decreased,and reached a significant level.It was found that MBC had the best passivation effect in Cu（or Cd）-MBC-Indian mustard soil plant system.The adsorption of MBC on heavy metals Cu2+ and Cd2+ in soil occurred after 21 days.With the exposure time extended,the cumulative adsorption capacity increased but counld not achive saturation even after 56 days.In different plant soil systems,CaCl2 analytical Cu2+ account for ⁵0% in total desorption concentration,CaCl2 state Cd2+ percent of the total desorption are below 20%,the passivation stability is better MBC in soil Cd2+.（5）Through the plant root tip SEM and ultrathin sections TEM heavy metal Cu2+ and Cd2+ polluted the plant roots leading plant root surface is roughness,shrinkage and fracture,shrinkage,irregular shape,root tip cells arranged disorderly,cell wall thickening,organelle vacuolization,plasmolysis.The degree of damage about heavy metals in India mustard roots is much greater than that of ryegrass,showing the type of detoxification mechanism hinge on different plants after applying MBC carbon package,the root surface is smooth and no significant structural damage,cell wall thinning,protoplast and cell is basically normal,showed that MBC can effectively improve the toxic effects of heavy metals on plants.