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ADSORPTION

Surface chemistry effects in activated<br>carbon adsorption of industrial<br>inorganic pollutants

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ADSORPTION

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  1. ADSORPTION SURFACE CHEMISTRY EFFECTS IN ACTIVATED CARBON ADSORPTION OF INDUSTRIAL INORGANIC POLLUTANTS BY: ALI DAHAN AL-MAQAS

  2. INTRODUCTION ADSORPTIONIS THE PROCESS THROUGH WHICH A SUBSTANCE, ORIGINALLY PRESENT IN ONE PHASEIS REMOVED FROM THAT PHASE BY ACCUMULATION AT THE INTERFACE BETWEEN THAT PHASE AND A SEPARATE (SOLID) PHASE. ADSORPTION IS THE ATTACHMENT OF PARTICALES TO A SURFACE.

  3. ADSORBENT, ADSORPTANDADSORBATE

  4. PHYSISORPTION→ABSENCE OF CHEMICAL BONDS THE MOLECULE RETAINS ITS GAS PHASE ELECTRONIC STRUCTURE, ALTHOUGH SOME DISTORTION IS STILL POSSIBLE. THE BINDING ENERGY DEPENDS ON THE POLARIZABILITY AND ON THE NUMBER OF ATOMS INVOLVED OF THE ATOMS AND VARIES BETWEEN FEW MEV (LIGHT GASES) AND SEVERAL EV (LARGE ORGANIC MOLECULES) CHEMISORPTION→STRONGER PERTURBATION OF THE MOLECULAR ELECTRONIC STRUCTURE WITH FORMATION OF CHEMICAL BONDS WITH THE SUBSTRATE. ENERGIES TYPICALLY OF SEVERAL EV

  5. FACTORS AFFECTING ADSORPTION THE MOST IMPORTANT FACTORS AFFECTING ADSORPTION ARE: • SURFACE AREA OF ADSORBENT. LARGER SIZES IMPLY A GREATER ADSORPTION CAPACITY. • PARTICLE SIZE OF ADSORBENT. SMALLER PARTICLE SIZES REDUCE INTERNAL DIFFUSIONAL AND MASS TRANSFER LIMITATION TO THE PENETRATION OF THE ADSORBATE INSIDE THE ADSORBENT (I.E., EQUILIBRIUM IS MORE EASILY ACHIEVED AND NEARLY FULL ADSORPTION CAPABILITY CAN BE ATTAINED). HOWEVER, WASTEWATER DROP ACROSS COLUMNS PACKED WITH POWDERED MATERIAL IS TOO HIGH FOR USE OF THIS MATERIAL IN PACKED BEDS. ADDITION OF POWDERED ADSORBENT MUST BE FOLLOWED BY THEIR REMOVAL.

  6. CONTACT TIME OR RESIDENCE TIME. THE LONGER THE TIME THE MORE COMPLETE THE ADSORPTION WILL BE. HOWEVER, THE EQUIPMENT WILL BE LARGER. • · SOLUBILITY OF SOLUTE (ADSORBATE) IN LIQUID (WASTEWATER). SUBSTANCES SLIGHTLY SOLUBLE IN WATER WILL BE MORE EASILY REMOVED FROM WATER (I.E., ADSORBED) THAN SUBSTANCES WITH HIGH SOLUBILITY. ALSO, NON-POLAR SUBSTANCES WILL BE MORE EASILY REMOVED THAN POLAR SUBSTANCES SINCE THE LATTER HAVE A GREATER AFFINITY FOR WATER.

  7. AFFINITY OF THE SOLUTE FOR THE ADSORBENT (CARBON). THE SURFACE OF ACTIVATED CARBON IS ONLY SLIGHTLY POLAR. HENCE NON-POLAR SUBSTANCES WILL BE MORE EASILY PICKED UP BY THE CARBON THAN POLAR ONES. • NUMBER OF CARBON ATOMS. FOR SUBSTANCES IN THE SAME HOMOLOGOUS SERIES A LARGER NUMBER OF CARBON ATOMS IS GENERALLY ASSOCIATED WITH A LOWER POLARITY AND HENCE A GREATER POTENTIAL FOR BEING ADSORBED (E.G., THE DEGREE OF ADSORPTION INCREASES IN THE SEQUENCE FORMIC-ACETICPROPIONIC-BUTYRIC ACID).

  8. SIZE OF THE MOLECULE WITH RESPECT TO SIZE OF THE PORES. LARGE MOLECULES MAY BE TOO LARGE TO ENTER SMALL PORES. THIS MAY REDUCE ADSORPTION INDEPENDENTLY OF OTHER CAUSES. • DEGREE OF IONIZATION OF THE ADSORBATE MOLECULE. MORE HIGHLY IONIZED MOLECULES ARE ADSORBED TO A SMALLER DEGREE THAN NEUTRAL MOLECULES. • PH. THE DEGREE OF IONIZATION OF A SPECIES IS AFFECTED BY THE PH (E.G., A WEAK ACID OR A WEAK BASIS). THIS, IN TURN, AFFECTS ADSORPTION.

  9. Effects of functional groups Research on sorption of heavy metals in aqueous solutions typically examines adsorption kinetics and isotherms to evaluate the performance of carbon adsorbents.The pseudo second order (PSO) model is perhaps the most widely used to fit the adsorption kinetics data, signifying chemisorption dominated mechanisms for heavy metal adsorption by carbons. While both Langmuir and Freundlich models are often used to simulate the isotherm data. The Langmuir isotherm, suggesting monolayer adsorption onto carbon surface, is more popular than the Freundlich isotherm, probably because it predicts the maximum adsorption capacity (Qmax). In this section, we summarize heavy metal adsorption performance of various carbon adsorbents with oxygen, nitrogen, sulfur, and other functional groups in to Table 2, Table 3, Table 4, and Table 5, respectively.

  10. Oxygen containing functional groups Oxygen containing functional groups are so far the most important in influencing surface reactions, surface behavior, hydrophilicity, electrical and catalytic properties of carbons. Table 2 summarizes the effects of oxygen containing functional groups on the removal of heavy metals by carbon adsorbents in term of their maximum adsorption capacities, the best-fit kinetic and isotherm models, and the specific functional groups. The hydroxy and carboxyl groups, among the most common functional groups, have been found to contribute widely to the adsorption of heavy metals, such as Pb(II), Cr(VI), Hg(II), Cd(II) and Ni(II) ions onto various carbon adsorbents. In addition, -C=O and -C-O functional groups have been reported more likely to make effects on the surface of GO based adsorbents

  11. revealed the great adsorption capacity of Cu(II) ions onto GO through an aggregation process caused by oxygen containing functional groups binding to Cu(II) in aqueous solutions have also pointed out the important role of oxygen-containing groups on surfaces of GO played in the adsorption of Pb(II) and Cd(II), respectively. Adsorption of Cr (VI) on H3PO4 treated biochar was through forming inner sphere complex with the surface oxygenic functional groups on the biochar Different chemical and physical modification methods have been applied to oxidize biochars derived from various feedstocks and the modifications dramatically increased the contents of surface oxygen containing functional groups and thus biochars’ adsorption rates and capacities to various heavy metals including Pb(II), Cd(II), Cu(II), Ni(II), etc.

  12. Nitrogen containing functional groups Because incorporation of nitrogen groups onto carbon surface can increase its basic properties to enhance the adsorption of heavy metals, the nitrogen containing functional groups have received increasing attentions in the research of carbon adsorbents. On the other hand, the introduction of nitrogen functional groups also had negligible effects on porous properties of carbons . Table 3 displays the related studies on heavy metal adsorption of carbon materials with nitrogen containing functional groups, summarizing the relevant adsorption capacity and the best-fit kinetic and isotherm models. The nitrogen containing functional groups, such as -NH2 and -NH, contribute mostly to selective adsorption of heavy metals onto carbon nanomaterials,

  13. which are commonly synergistic with the oxygen containing functional groups. The nitrogen-containing ACs derived from ammonium humates show high adsorption ability to aqueous Cu(II) and Pb(II), even better than many commercial ACs .Nitrogen-fictionalized carbon (biochar/AC) can be obtained directly from diammonium hydrogen phosphate treatment biomass through pyrolysis and have strong sorption ability to aqueous Cr(VI) . Several other carbon materials including microporous carbon, mesoporous carbon, magnetic carbon nanoparticles, and carbon film have been doped with nitrogen to introduce nitrogen containing functional groups on carbon surfaces to increase their adsorption ability to various heavy metals in aqueous solutions

  14. Sulfur containing functional groups In general, sulfur containing functional groups on carbon adsorbents are beneficial to their adsorption of heavy metals. Depending on the experimental conditions, however introducing sulfur containing functional groups can either increase or decrease specific surface area and pore volume of the carbon adsorbents , which may also affect their adsorption performances. In addition, elemental sulfur has been loaded into carbon adsorbents through sulfuration treatment such as solution infiltration and chemical reaction-deposition , which can expand their adsorption ability to various aqueous heavy metals.Table 4 lists selected studies on the effects of sulfuric groups on carbon adsorption of heavy metals, summarizing the corresponding adsorption capacity as well as the best-fit kinetic and isotherm models.

  15. As can be seen, the sulfuric functional groups significantly enhance the adsorption performance of heavy metals onto carbons, especially with respect to the adsorption of Cu(II) and Cd(II) ions onto GO and CNT based adsorbents. Feng et al. revealed that the elemental sulfur species deposited on carbonaceous surfaces after continuous exposure to hydrogen sulfide are the most effective mercury uptake sorbents comparing thiophene and sulfate. The preparation of sulfur-functionalized ordered mesoporous carbons with sulfur bearing salts activating can simultaneously increase its specific surface area (837-2,865 m2/g) and pore volume (0.71-2.3 cm3/g), the affinity of the sulfurized carbons towards heavy metals is in order of Hg(II)>Pb(II)>Cd(II)>Ni(II) at pH of 7 or greater

  16. Sulfonated graphene nanosheets not only hold a relatively complete sp2-hybridized plane with a high affinity to aromatic pollutants in water, but also have sulfonic acid groups and partial original oxygen-containing groups that exhibit fast adsorption kinetic rate and superior adsorption capacity towards aqueous Cd(II) . It has also been reported that dithiocarbamate groups functionalized MWCNTs prepared by ethylenediamine and carbon disulfide have very high adsorption capacities for Cd(II), Cu(II), and Zn(II) in aqueous solutions

  17. Other functional groups Selected studies about the effects of other functional groups on carbon adsorption for heavy metals are summarized in Table 5 along with the corresponding adsorption capacity and the best-fit kinetic and isotherm models. While oxygen, nitrogen and sulfur containing functional groups are predominant for the adsorption of heavy metals, other functional groups (e.g., -C=C,-CH2, -O-CH3 and -P-O-C) can also be introduced to carbon surface to promote heavy metal adsorption (Table 5). Xu et al. confirmed precipitation of Pb(II) with the CO32- group released from waste-art-paper biochar to form PbCO3 and Pb2(OH)2CO3 crystals on its surface and indicated that high temperature incineration of the spent biochar can turn it into value-added (high purity >96%) PbO nanoparticles

  18. revealed that ACs prepared from Glebionis coronaria L. biomass by phosphoric acid activation are rich in phosphorus containing surface functional groups, which are beneficial to the removal of Cd(II) from aqueous solution.Amidoxime-grafted MWCNTs synthesized by plasma techniques can selectively adsorb U(VI) ions from nuclear industrial effluents, which was due to the binding contribution of different functional groups especially -C=N-OH

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