Enhancement Principle Of Graphene Oxide Functional Modification For The Removal Of Heavy Metal Ions And Their Compound Pollutants

Heavy metal-containing wastewater can be treated by adsorption separation technology to obtain ultra-low concentration or high-quality treated effluent.However,due to the constraints of the specific surface area,surface electrical structure and spatial effect of conventional adsorbents,it is often difficult to obtain adsorbents within high adsorption capacity.Graphene,a representative of the emerging carbon nano materials,has the characteristics of large specific surface area,small size,high adsorption efficiency,and has been the research hotspot in recent years.However,traditional graphene-based adsorbents have some drawbacks,such as slow adsorption rate,low adsorption capacity,relatively high price,and difficulty in separation and reuse.On the basis of these considerations,this paper is to be devoted to:?1?increasing the type and quantities of functional groups that have chelating ability for heavy metal ions on the surface of graphene-based adsorbents to verify the effectiveness of adsorption enhancement;?2?introducing low-cost biomass such as cellulose,chitosan and starch into graphene-based adsorbents to improve the cost performance of adsorbent;?3?assembling three-dimensional graphene-based monoliths through the adjustment of experimental parameters for an ease to separate from wastewater.In addition,the regulatory mechanism of the metal'characteristic parameter and functional groups on the adsorption ability of graphene oxide towards metal were also investigated.The main contents of this paper were as follows:According to the hard-soft acid-base principle,metallic elements in the periodic table of elements were divided into soft,intermediate and hard ions.8 metal ions were selected as the representatives of soft ions(Cd²⁺and Pb²⁺),intermediate ions(Ni²⁺,Cu²⁺,Zn²⁺and Co²⁺)and hard ions(Na⁺and Mg²⁺).The adsorption performance of graphene oxide?GO?with respect to the 8 metal ions were investigated and the results showed that the adsorption capacity of GO towards hard ions was less than that for intermediate ions and soft ions.Mathematical relation was existed between the maximum adsorption capacity of GO towards metal ions and the characteristics parameters of metal ion including atomic number AN,change in ionization potential?IP,covalent index X_m²r,absolute value of the first hydrolysis constant decimal logarithm|lg K_OH|,atomic radius AR,polarization force or ionic index Z²/r and softness index?_p,The correlation model of these parameters on the maximum adsorption capacity of graphite oxide was established.Q_mvalues of GO towards different metals could be predicted by the following equation:Q_m=0.115AN+0.338?IP-0.110?lgK_OH?+0.114Z²/r+26.934?_p-1.479AR-1.162X_m²r-3.059?R²>0.885?.The binding energy data calculated by density functional theory?DFT?revealed that the reason why the Q_mvalue of GO with respect to hard ion was less than that of intermediate ion and soft ion.Meanwhile,it showed that nitrogen-containing functional groups such as amino group and amide group have strong chelating ability to Cu²⁺?intermediate ion?and Pb²⁺?soft ion?,which might be the reason why the functional groups containing nitrogen were mostly considered in the preparation of adsorbents.In order to improve the type and quantity of functional groups in adsorbent,three terminated hyperbranched amino polymersprepared by using diethylenetriamine?DETA?,triethylenetetramine?TETA?,and tetraethylenepentamine?TEPA?as amination reagents,respectively.HBP-NH₂was introduced into GO by graft copolymerization reaction.Then,three graphene oxide adsorbents modified by terminal amino hyperbranched polymers with different nitrogen-containing functional group densities were obtained.The N content of,,andwere 7.21%,10.20%and 12.43%,respectively.The adsorption amount of Cr?VI?by GO was 0.31 mmol/g.After introducing HBP-NH₂into GO,the adsorption ability of the obtainedadsorbent was greatly increased and the adsorption amount of,,andwith respect to Cr?VI?could be improved to4.71,4.95 and 5.79 mmol/g,respectively,indicating that the adsorption enhancement was effective.The adsorption mechanism of Cr?VI?adsorbed bywas analyzed by DFT theory.Carboxymethylcellulose?CMC?was compounded withto obtain ternary nanocomposite adsorbent.The combination of scanning electron microscopy?SEM,?Raman,Fourier infrared spectroscopic?FT-IR?and x ray photoelectron spectroscopy?XPS?confirmed that CMC had been successfully introduced to.The introduction of CMC could not only reduce the price of adsorbent,but also enrich the types of functional groups in adsorbent.Taking Pb²⁺and Cu²⁺as examples,the adsorption performance ofwith respect to heavy metals was investigated.Adsorption experiments showed that the the adsorption amount oftowards Pb²⁺and Cu²⁺could be reached 0.74 and 2.15 mmol/g,respectively.Fixed bed adsorption experimental data of Pb²⁺and Cu²⁺adsorbed bywere successfully correlated with the Adams-Bohart,Thomas and Yoon-Nelson models with the R²exceeding 0.79.Aiming at the heavy metal-organic dye compound polluted wastewater,a nitrogen and sulfur co-doped graphene macromer?GBM?was synthesized by hydrothermal method with2,5-dithiodiurea as modifier and reducing agent.Characterization analysis showed that GBM had network-like structures,exhibiting crumpled,loose porous structure and rough surface.After adsorption experiment completed,GBM could be easily separated from the water,solving reuse difficulties of traditional graphene-based adsorbent.Taking three common organic dyes,saffron O?SO?,crystal violet?CV?and methylene blue?MB?as examples,the adsorption performances of GBM towards binary solutions,Cd²⁺-SO,Cd²⁺-CV and Cd²⁺-MB were investigated.Adsorption experiments show that the maximum adsorption amount of GBM with respect to Cd²⁺,SO,CV and MB could be reached 1.76,0.95,0.54 and 0.39mmol/g in a single pollutant system,respectively.Adsorption synergism was observed with respect to Cd²⁺in presence of the dyes.The performance of GBM with respect to Cd²⁺removal from binary solutions,Cd²⁺-SO,Cd²⁺-CV,and Cd²⁺-MB,was enhanced by the presence of the dyes significantly,while the adsorption capacities towards the dyes were not affected by the presence of Cd²⁺.The above series of studies indicated that metal ion characteristic parameters would influence the adsorption performance,and the regulation of functional groups on graphene oxide adsorbent could significantly improve the adsorption capacity;The functional modification of graphene oxide by the combination of amino terminated hyperbranched polymer with high nitrogen content and low-cost biomass would greatly improve the cost performance of the adsorbent through the interaction of group increase and chelation enhancement;N/S co-doped graphene oxide three-dimensional macrovolume ocould not only showed the excellent adsorption performance towards heavy metal-organic dye composite system,but also realized the rapid separation,which had the prospect of engineering application.Therefore,the combination of metal ion characteristic parameters,adsorbent function and reaction condition control would constitute the future development of adsorption separation engineering.