Effects Of Phosphate And Humic Acid On The Transport Mechanism Of Modified Nano Iron Composite And Hexavalent Chromium Remediation In Soil

"Environmental protection and governance should focus on solving outstanding environmental problems that damage the health of the people."Nowadays,our society and country have focused on the construction of ecological civilization.However,due to the excessive discharge of industrial waste water,heavy metal chromium was released into the natural environment,and its toxicity and high fluidity have caused serious environmental and health problems.Therefore,the issues of remediation on the chromium-contaminated soil needs to be solved urgently.Iron nanoparticles(nFe)have been considered as a promising nanomaterial due to its excellent reactivity and cost-effectiveness,and are suitable for in-situ remediation of Cr(?)contaminated soil.This article takes carboxymethyl cellulose sodium(CMC),kaolin and molybdate(Mo O₄^2-)modified green synthesized nFe composite materials as the research object,exploring the physical and chemical properties of soil(environmental factors such as phosphate,humic acid,cationic species,p H value,etc.)on their reactivity and mobility,hence further explaining the actual efficiency of in-situ Cr(?)remediation in soil by green synthesized nFe composite materials.The research conclusions have been showed as below:(1)The effect of phosphate and humic acid on Cr(?)remediation by CMC modified nFe(CMC-nFe):This chapter researched the influence mechanism of phosphate and humic acid on the transport of CMC-nFe and Cr(?)remediation by CMC-nFe in different electrolytes(NaCl,Ca Cl₂).Through the characterization by XRD,TEM and FTIR,it could be proposed that CMC is connected to the surface of nFe particles through COO^-,hence forming a black spherical chain structure catched in CMC.In NaCl and Ca Cl₂ solutions,phosphate and humic acid both enhance the electrostatic repulsion between particles through adsorption and promote the transport of particles in porous media.In particular,due to the complexation of humic acid and Ca²⁺,CMC-nFe exhibits the lower transport in Ca Cl₂.In the presence of phosphate and humic acid,the transport of CMC-nFe is significantly enhanced.In the batch test,the removal on Cr(?)by CMC-nFe(including adsorption and reduction)was significantly inhibited by phosphate or humic acid due to the competing for adsorption sites.However,the inhibitory effect on the removal of Cr(?)by CMC-nFe was weakened in the coexistence of phosphate and humic acid because humic acid could hinder the adsorption of phosphate on the surface of CMC-nFe.In the column experiment,in the presence of phosphate or/and humic acid,the reduction efficiency on Cr(?)by CMC-nFe was slightly weakened in the NaCl solution,and the inhibitory effect of phosphate and humic acid on CMC-nFe was not significant;In Ca Cl₂,phosphate or/and humic acid obviously inhibited the Cr(?)remediation efficiency by CMC-nFe.(2)The effect of phosphate and humic acid on Cr(?)remediation by kaolin supported nFe(K-nFe):This chapter researched the effects of phosphate and humic acid on the transport of K-nFe and Cr(?)remediation efficiency in different electrolyte solutionsat different p H.The surface morphology of K-nFe was characterized via SEM,EDS etc.,the lamellar structure of kaolin enhanced the dispersity of nFe particles,hence improving the stability and reactivity of K-nFe.In an acidic environment(p H 2.5),the removal capacity of Cr(?)by K-nFe in water reached 100 mg g^-1,and in the presence of phosphate and humic acid,the Cr(?)reduction ability of K-nFe significantly reduced;in a neutral environment(p H 6.8),phosphate and humic acid have little effect on the Cr(?)removal capacities of K-nFe.During the K-nFe and Cr(?)co-transport in saturated quartz sand,phosphate and humic acid significantly inhibited the Cr(?)reduction efficiency of K-nFe in an acid environment,thereby weakening the immobilization of Cr(?).In a neutral environment(p H 6.8),compared with acidic environment the Cr(?)reduction efficiency of K-nFe greatly decreased due to the low H⁺environment,while phosphate and humic acid exhibited the unobvious impaction on the Cr(?)remediation by K-nFe.(3)The effect of phosphate on Cr(?)remediation by molybdate modified nFe(Mo-nFe):In this chapter,Mo-nFe with different Mo/Fe molar ratios were synthesized,and the difference of suspension stability,transport,Cr(?)removal efficiencies of Mo-nFe with different Mo/Fe ratios was investigated,meanwhile,the influence mechanism of phosphate on the Cr(?)remediation by Mo-nFe in soil.FTIR and EDS results showed that Mo O₄^2-modified nFe via the adsorption on the binding site of nFe.The increase of Mo/Fe ratio could make the surface zeta potential of Mo-nFe become more negative,and enhanced the electrostatic repulsion between particles,thereby improving the suspension stability and transport of Mo-nFe.Mo-nFe(0.06)exhibited the best removal capacity on Cr(?).Phosphate has a strong promoting effect on the transport of Mo-nFe in the porous medium.However,as Mo-nFe and Cr(?)co-transport in soil,phosphate significantly inhibited the Cr(?)remediation efficiency of Mo-nFe due to the competitive adsorption.From the perspective of electron selectivity,the research in this chapter leads to a more efficient selective reaction with Cr(?)by changing the surface structure of nFe,providing the potential application materials and theoretical guidance on improving the reactivity of nFe composites with Cr(?)at the molecular level and reaction mechanism.