Synthesis Of Modified Multi-walled Carbon Nanotubes And Their Adsorption Characteristic Of Tex And Heavy Metals

Groundwater contamination of organic contaminants and heavy metals has drawnwidespread attention. Adsorption is a simple, fast and efficient method to remove organiccontaminants and heavy metals, and the most critical technical problem is adsorbents.Carbon nanotubes （CNTs） have aroused widespread attention as a new type of adsorbentsdue to their outstanding ability for the removal of various inorganic and organiccontaminants. CNTs have been the focus of considerable research because of their highlyporous and hollow structure, large specific surface areas, and CNTs show stronginteractions with organic compounds.The pore structure and surface chemical properties of CNTs can be effectivelycontrolled by adjusting the synthesis parameters and modifying by liquid phase oxidation,solid phase activation and magnetic modification, and then some new types of modifiedmulti-walled carbon nanotubes （MWCNTs） were successfully synthetized in this study.The systematic study of the relationship between physicochemical properties andadsorption characteristics of TEX onto modified MWCNTs by qualitative and quantitativeanalysis has been investigated. Both heavy metals and organic contaminants may coexist atmany contaminated sites. Furthermore, the molecular science techniques were utilized toprobe and elucidate the relevant mechanisms from adsorption sites and coordinationenvironment of adsorbates by X-ray photoelectron spectroscopy （XPS）, X-ray absorptionnear edge structure （XANES） and extended X-ray absorption fine structure （EXAFS）spectroscopy. A facile one-pot method for synthesis of low-cost magnetic MWCNTs hasbeen reported, which will be of great significance for large-scale practical applications ofCNTs.The important conclusions of this work are summarized as follows: （1） Synthesis of MWCNTs with different oxygen contents and the influence of theirsurface oxygen content on TEX adsorptionMWCNTs with different oxygen contents were prepared by oxidization usingdifferent concentration NaClO solutions. With the increase of surface oxygen content,maximum adsorption capacities of TEX firstly increased, and then, began to decrease. Thesurface functionalization of the oxidized MWCNTs can improve their dispersibility inaqueous solutions, which consequently can be favorable for the aqueous phase adsorption.However, when oxygen content increases to a certain extent, oxygen-containing functionalgroups cause water clusters formation on the surface or tube end of MWCNTs, whichhinder the interaction between TEX and MWCNTs. The decrease indicates that theformation of water clusters plays a more important role than the better dispersion ofMWCNTs for TEX adsorption.（2） Synthesis of MWCNTs with different specific surface area and the influence oftheir physicochemical properties on adsorption characteristics of ethylbenzene and xyleneisomersMWCNTs with different specific surface area （SSA） by adjusting KOH/MWCNTsmass ratio were prepared using KOH activation. With the increase of KOH/MWCNTsmass ratio, adsorption capacities of ethylbenzene and xylene isomers （EX） greatlyincreased. A systematic quantitative analysis of the relationships between thephysicochemical properties of activated MWCNTs and adsorption characteristics of EXwas studied by correlation analysis. There are strong positive correlations between theadsorption affinity （or capacity） and SSA, mesopore volumes or hydroxyl groups, and theirregression equations have been obtained. Mesopore-filling and hydrogen-bond interactioncould be a predominant mechanism governing monoarmatics adsorption onto activatedMWCNTs. Results highlight the synergetic effects of porosity and surface chemistry incontrolling adsorption of monoaromatics onto MWCNTs in wastewater treatment. （3） A facile one-pot method for synthesis of low-cost magnetic MWCNTs and theirapplication for TEX adsorptionA facile one-pot method was devised to produce magnetic MWCNTs （APCNTs-KOH）by KOH etching method using as-prepared MWCNTs （APCNTs）. The adsorptioncapacities of TEX onto APCNTs-KOH are much higher than those onto APCNTs.Meanwhile, Fe nanoparticles of APCNTs were oxidized to Fe₃O₄/γ-Fe₂O₃, andAPCNTs-KOH could be effectively and immediately separated from aqueous solutions bya magnet, which reduces potential risks of CNTs as another source of environmentalpollution. Excellent magnetic materials APCNTs-KOH were prepared based on APCNTswith low cost, which is of great significance for large-scale practical applications of CNTs.Desorption of TEX showed a pronounced hysteresis for APCNTs-KOH, because someadsorbates may enter some micropores or rigid region of adsorbents.（4） Mutual effects of heavy metals and TEX adsorption on modified MWCNTs andtheir relevant mechanisms from molecular levelsPb²⁺suppressed the adsorption of TEX onto CNTs-3.2%O, and the degree ofsuppression decreased with the increase of initial concentration of TEX. Among fiveadsorbates, adsorption of toluene was diminished more than others. TEX diminish theadsorption of Pb²⁺or Cu²⁺onto CNTs-5.9%O. The differende in initial concentrations ofTEX had little influence on the suppression of Pb²⁺. The degree of suppression decreasedwith the increase of initial concentration of Pb²⁺. The molecular environmental sciencetechniques, such as XPS, XANES and EXAFS, were utilized to probe and elucidate therelevant mechanisms from molecular levels. Pb²⁺can form the O-Pb bond with surafaceoxygen-containing groups of CNTs-3.2%O, which may directly occupy part of the surfaceadsorption sites of CNTs-3.2%O. The large hydration sphere around surface complexes ofPb²⁺may also intrude or shield hydrophilic sites, leading to the “crowding out” of TEXaround the metal cations-complexed sites, and thus Pb²⁺can inhibit TEX adsorption onto CNTs-3.2%O. The results of XPS, NEXAFS and EXAFS indicate that the formation ofsurface or inner-sphere complexes of Cu²⁺through carboxylic groups （or hydroxyl groups）and hydration of CNTs-5.9%O. Pb²⁺can also form the O-Pb bond with oxygen-containinggroups of CNTs-5.9%O. There are some interactions between TEX and CNTs-5.9%Oindicated by the shift of bonding energy by XPS analysis, which could result in acompetition for adsorption sites between TEX and heavy metals ions. Thus, TEX directlyor indirectly inhibited Pb²⁺or Cu²⁺adsorption onto CNTs-5.9%O.