Study On Aqueoeous Cr(Ⅵ) Removal By Highly Dispersed Graphene Supported Nanoscale Zero Valent Iron

Chromium(Cr)is one of the common heavy metal pollutants in China because of its wide spread use in industrial processes.The use of chromium in wood preservatives,leather tanning,paint formulation,steel fabrication,and metal finishing are the main sources of chromium based pollution.In nature,chromium exists primarily in two oxidation states(III and Ⅵ).Cr(Ⅵ)is a more toxic and soluble specie,posing a significant threat to the environment and human beings.Unlike many organic pollutants,chromium species are not removed and/or degraded through typical environmental and biological processes,thus it is critical to develop and identify an effective method for the removal of chromium from industrial wastewater.In recent years,the application of nanoscale zero valent iron(nZⅥ or nFe~0)for adsorption is attractive because of their large surface area,high reactivity,easy separation and recovery.Nevertheless,because of these characteristics,nZⅥ could rapid react with its surrounding media,resulted in formation of an oxidation layer.Also,agglomeration between nZⅥ particles hinders reaction of nZⅥ with contaminants and decreases their reactivity.Graphene(rGO)was used to carry out load stabilization treatment for nZⅥ,so as to prevent its agglomeration and provide an electron transfer path through the principle of ferrocarbon galvanic battery to improve its service efficiency.At the same time,in order to solve the problem of poor dispersion of rGO in water,and the surface negative charge is easy to reject Cr(Ⅵ)anions,this topic also introduced the water-soluble,highly adsorbent and positively charged layered magnesium aluminum bihydroxide(MgAl-LDH,or LDH),to further improve the use effect of rGO optimized nZⅥ system in the liquid phase system.RGO was modified by LDH,the water-soluble graphene system(LDH-rGO)was synthesized.Then,the stabilized nZⅥ@LDH-rGO system was prepared by liquid phase reduction method,which was applied to the removal of Cr(Ⅵ)in water.The main contents are as follows:(1)The stabilized nZⅥ@LDH-rGO system was characterized by dispersion,Zeta potential,SEM,XRD and XPS.LDH significantly improved the dispersibility of rGO in the aqueous solution.The Zeta potential of LDH-rGO was higher than that of rGO at the same pH.From SEM,XRD and XPS,it was also found that the LDH-rGO was capable of inhibiting the aggregation of nZⅥ particles.(2)In stabilized nZⅥ@LDH-rGO system,nZⅥ particles could loaded on the surface of LDH-rGO to avoid agglomeration.Compared to bare nZⅥ,Cr(Ⅵ)removal efficiency by nZⅥ@LDH-rGO within 120 min was increased from 25.08%to94.58%.The results also suggested that rGO:LDH=1:1 is appropriate to achieve high removal efficiency.(3)Other factors(nZⅥ and LDH-rGO addition,initial Cr(Ⅵ)concentration,initial pH value,reaction temperature,ionic strength and HA concentration)exhibited strong effects:efficiency decreased from 100%to 29.49%when pH value increased from 3.0to 11.0;increase of initial Cr(Ⅵ)concentration and ionic strength led to lower removal efficiency;most co-existing ions hinder Cr(Ⅵ)removal by competition for active surface;low concentration of HA facilitated the stabilization of nZⅥ,while high concentration HA resulted coagulation of particles and decreased their removal capacity.The used nZⅥ@LDH-rGO could be regenerated through several elution and reduction process,and maintained high efficiency in cyclic utilization.(4)Reaction kinetics fitted well with the pseudo first-order adsorption model.The reaction rate increased with the addition of nZⅥ,LDH-rGO,the initial pH value and the reaction temperature,but decreased with the increase of the initial concentration of Cr(Ⅵ).The stabilized nZⅥ@LDH-rGO system has good reactivity.(5)Finally,through the analysis of the reaction mechanism,it was found that the synergistic effect within the system was the main reason for the good reactivity of the stabilized nZⅥ@LDH-rGO system:first,rGO supported nZⅥ,which was conducive to the stability and dispersion of nanoparticles and increased the active sites that could be used for the reaction.Secondly,the existence of LDH improves the dispersion of the whole material in water and alleviates the repulsion of surface negative charge to cr-containing anions.In addition,the Cr(Ⅵ)removal achieved by stabilized nZⅥ@LDH-rGO system thanks to numerous Fe-C galvanic cells in aqueous solution,benefiting electrons transfer from nZⅥ to Cr(Ⅵ).