Sorption of Organic Compounds, Oxyanions, and Heavy Metal Ionson Surfactant Modified Titanate NanotubesHuan-Ping Chao,*,† Chung-Kung Lee,‡ Lain-Chuen Juang,‡ and Yin-Lung Han§
†Department of Bioenvironmental Engineering, Chung Yuan Christian University, Chung-Li, 32023, Taiwan, Republic of China‡Department of Environmental Engineering, Vanung University, Chung-Li 32061, Taiwan, Republic of China§Green Energy and Environment Research Laboratories, Natural Resources Technology Division, Industrial Technology ResearchInstitute, Hsinchu, 31040, Taiwan, Republic of China
ABSTRACT: The adsorption capacities and mechanisms of organic compounds with water solubilities (Sw) ranging from 55 to75 000 mg/L, oxyanions, and heavy metal ions onto surfactant (hexadecyltrimethylammonium, HDTMA) modified titanatenanotubes (TNT) were investigated. Effects of the HDTMA-modified process on the revolution of microstructures and surfacechemical characteristics of TNT were characterized with scanning electron microscopy (SEM), transmission electron microscopy(TEM), nitrogen adsorption−desorption isotherms, Fourier transform infrared spectroscopy (FTIR), and ζ potential. Theadsorption isotherms of selected adsorbates on the HDTMA-modified TNT (HMTNT) were measured to understand theeffects of the surfactant-modified process on the adsorption properties of TNT. It was found that HMTNT possessed bothhydrophilic and hydrophobic surfaces to uptake organic compounds with various Sw values. On the other hand, since thehydrophobic bonding by conglomeration of large C16 alkyl groups associated with HDTMA could render a positive chargedevelopment on the surface of TNT, HMTNT could simultaneously adsorb the oxyanions (on the surface covered by HDTMA)and heavy metal ions (on the surface not covered by HDTMA) through the anion and cation exchange mechanisms, respectively.It was experimentally concluded that HMTNT might be regarded as an amphiphilic and dual-electronic adsorbent, which couldeffectively remove many different kinds of contaminants. The regeneration of HMTNT was also briefly discussed.
1. INTRODUCTION
Industrial processes can generate a number of contaminantssuch as organic compounds, oxyanions, and heavy metal ions.They are all discharged into wastewater treatment plants. Thesecontaminants and their degradation products may becarcinogens and toxic to humans or to animals or plants livingin water. Some traditional chemical or biological processes havebeen developed for the treatment of contaminants inwastewater.1−3 Although traditional processes can removemost contaminants, advanced treatment can reduce the impactof contaminants on the environment. Among the methods ofadvanced treatment, the adsorption process is an efficient andeconomic process to remove contaminants from effluents.4
Most adsorbents can only adsorb one type of contaminant,such as metal ions or organic compounds, effectively based ontheir surface characteristics. For instance, the adsorption ofnonionic organic compounds is closely related to thehydrophilic or hydrophobic characteristics of adsorbentsurfaces, and it is difficult to adsorb organic compounds havinga diverse range of water solubility (Sw) values with only oneadsorbent. On the other hand, for ionic contaminants, oneadsorbent can only adsorb either cations or anions. Sincewastewater frequently contains various contaminants, it isnecessary to use different adsorbents to remove differentcontaminants. As a result, various chemicals are used to modifythe surfaces of adsorbents, which can enhance the adsorptioncapacities for the specific contaminants.5−11 These studies arefocused on altering the polarity or charge of an adsorbent’ssurface. Zeolites modified by octadecyltrichlorosilane have beenused as amphiphilic adsorbents that can adsorb both hydro-
philic and hydrophobic organic compounds simultaneously inthe wastewater.12,13 Moreover, hexadecyltrimethylammonium(HDTMA), which adsorbs on the surface of an adsorbent(such as zeolite or clay), may also produce an organic phasewhich acts as a partition medium into which nonionic organicsolutes with low Sw values can partition from aqueoussolution.14 In addition, the hydrophobic bonding by conglom-eration of large C16 alkyl groups associated with HDTMA toform the admicelle can render a positive charge developmenton the surface of the adsorbent, from which the anions can beremoved from aqueous solution with an anion exchangemechanism.15−19 Thus, HDTMA-modified adsorbent isregarded as an excellent adsorbent to uptake the low-Sworganic solutes and oxyanions.Recently, Kasuga et al.20 reported the preparation of TiO2-
derived nanotubes by a hydrothermal treatment of TiO2powder in 10 M NaOH aqueous solution. The obtainednanotubes possess cation exchange properties and are alsocharacterized by high surface area and pore volume.21,22 Theymay offer a special environment for adsorption of cations, suchas basic dyes or heavy metal ions, through the cation exchangemechanism.23−25 Moreover, when the sodium ions in thetitanate nanotubes (TNT) are replaced with HDTMA, thesurface properties of TNT are altered.26 As mentioned earlier,HDTMA-modified TNT (HMTNT) can be used to
Received: April 4, 2013Revised: June 25, 2013Accepted: June 25, 2013Published: June 25, 2013
adsorptively remove low-Sw organic solutes and anioniccontaminants. Moreover, since HDTMA only covers a partialsurface of the TNT, the remaining surface not covered byHDTMA can be used to adsorb the high-Sw organiccompounds and heavy metal ions, through the hydrophilicsurface and cation exchange property of TNT, respectively.In this study, the objective is to provide information on the
HDTMA exchange effects on the microstructures and surfacechemical characteristics of TNT and to understand theadsorption mechanisms of different contaminants, includingnonionic organic compounds with different Sw values rangingfrom 55 to 75 000 mg/L (propylbenzene, ethylbenzene, m-xylene, toluene, benzene, trichloromethane, 1-pentanol, m-cresol, and phenol), oxyanions (Cr2O7
2− and MnO4−), and
heavy metal ions (Cu2+ and Pb2+) onto the HMTNT. Therevolution of the microstructures of TNT is characterized withscanning electron microscopy (SEM), transmission electronmicroscopy (TEM), and nitrogen adsorption−desorptionisotherms. The change in the surface chemical properties ofTNT is identified with the Fourier transform infrared (FTIR)patterns and the zeta (ζ) potential. The adsorption isothermsof the selected adsorbates are then measured to identify theeffects of the surfactant-modified process on the adsorptionproperties of TNT. The relationships between the surfacecharacteristics of HMTNT and the adsorption selectivities fordifferent adsorbates on HMTNT are discussed.
2. MATERIALS AND METHODS
2.1. Adsorbents. TNT was prepared using a hydrothermalprocess that was presented in earlier reports.23,24 The TiO2source used for the TNT was commercial-grade TiO2 powderP25 (Degussa AG, Germany). It was well-known that thecharacteristics of TNT were highly dependent on thefabrication conditions, i.e., hydrothermal temperature, reactiontime, and postwashing procedures.26 To ensure the nanotubularstructure, high surface area, and high pore volume ofsynthesized TNT, 6 g of the TiO2 powder was mixed with120 mL of 10 M NaOH solution followed by hydrothermaltreatment of the mixture at 150 °C in a 200 mL Teflon-linedautoclave for 24 h. After hydrothermal reaction, the precipitatewas separated by filtration and washed with 0.01 N HClsolution and deionized water. First, the obtained TNT waswashed twice with 500 mL of deionized water. Then, thesamples were washed repeatedly with 200 mL of HCl aqueoussolution and twice with deionized water. Finally, the sampleswere immersed into 1000 mL HCl aqueous solution for 10 hand then washed with deionized water until the pH value of therinsing solution was nearly neutral. The obtained TNT sampleswere put in an oven at 105 °C for 8 h to remove moisture andthen stored in brown bottles until use.The organotitanate complexes were prepared by replacing
the Na+ in pure TNT with cationic surfactant HDTMA-Br(purchased from Sigma) through the cation exchange process.A 2 g sample of TNT was dispersed in 100 mL of distilledwater, and the solid suspension was then treated withHDTMA-Br solutions in a weight ratio of 0.5 HDTMA:1TNT. The concentration of added HDTMA was greater thanthe critical micelle concentration (about 0.9 mmol/L) ofHDTMA in pure water. After thorough mixing for 24 h, theorganotitanate complexes were washed with distilled water. TheHMTNT samples were then freeze-dried and stored at roomtemperature.
and MnO4−), and heavy metal ions (Cu2+ and Pb2+) were
selected as adsorbates. All of the selected organic compoundswith purities greater than 95% were obtained from Merck. Theywere divided into two groups according to their Sw values.Benzene, toluene, m-xylene, ethylbenzene, and propylbenzenewere regarded as the low-Sw organic compounds. Trichloro-methane, 1-pentanol, m-cresol, and phenol were regarded as thehigh-Sw organic compounds. The Sw values of the organiccompounds are listed in Table 1. Both potassium dichromate(Cr2O7
2−) and potassium permanganate (MnO4−) were
obtained from Merck. Both Cu2+ and Pb2+ were obtainedfrom their nitrate salts (Merck).
2.3. Adsorption Measurements. Each experiment wasrun only for a single compound. Each experiment wasduplicated and the data were averaged. If the bias of dataexceeded 15%, triplicate repetitions were made. A 0.2 g sampleof adsorbent was added to a Teflon centrifuge tube containing50 mL of distilled water. For organic adsorbates, the organiccompound was added to the tube to allow a concentration ofapproximately 20−80% Sw in the solution. When the Sw valuesof organic compounds were greater than 1000 mg/L, theconcentrations were limited to less than 1000 mg/L. Thecentrifuge tubes were put into a reciprocating shaker with 180rpm equilibrated for 48 h at 25 °C. Then, the centrifuge tubeswere centrifuged for 30 min at 9000 rpm (9020g) to separatethe solid from the solution. The concentrations of 1-pentanol,trichloromethane, benzene, toluene, m-xylene, ethylbenzene,and propylbenzene were analyzed using a gas chromatograph(GC; Perkin-Elmer Clarus 500). Aliquots of 2 mL of solutionwere transferred into glass vials containing 2 mL of carbondisulfide. These vials were sealed with Teflon foil lined screwcaps and shaken for 3 h on a reciprocating shaker to extract theorganic compound. The collected samples were injected into aGC equipped with a flame ionization detector to determine theconcentrations of the test organic compounds. A J&W DB-5capillary column (30 × 0.53 mm i.d.; film thickness 3 μm) wasused to separate the organic compounds. Phenol and m-cresolwere analyzed directly using a GENESYS 10 ultraviolet (UV)spectrometer. For both phenol and m-cresol, the pH value ofsolution in the adsorption process was controlled at 5.0 toavoid their dissociation. The solution was centrifuged for 30min at 9000 rpm (9020g), and the supernatant was filteredthrough a 0.2 μm filter. The concentrations of phenol and m-cresol were analyzed using UV absorbances at wavelengths of270 and 265 nm, respectively. The detection limits for allinstruments were less than 0.1 mg/L.
Table 1. Water Solubilities (at 25 °C) of the SelectedOrganic Compounds27
For heavy metal ions, the initial pH value for the adsorptionexperiments was controlled at 5.0 to avoid precipitation. Tomake a comparison, the initial pH value for the oxyanionadsorption experiments was also controlled at 5.0. The liquidand solid phases were separated by centrifugation and filtration.Then, a 15-mL aliquot of the supernatant was collected andanalyzed for both oxyanions and heavy metal ions by atomicabsorption spectrometry (Avanta/AAS, GBC).The adsorption capacity of adsorbates was calculated using
the relation Q = VΔC/m, where V was the volume of liquidphase, m was the mass of adsorbent, and ΔC was the differencebetween the initial and final concentrations of adsorbates inaqueous solution, which could be computed simply from theinitial and final instrument readings.2.4. Instruments. Field emission scanning electron
microscopy (SEM; JEOL JSM-6700F) was used to observethe particle size and morphology of TNT. Transmissionelectron microscopy (TEM) with an H-7500 electron micro-scope (Hitachi, Japan) using a 120 kV accelerating voltage wasapplied to examine the inside structure of TNT. Energydispersive X-ray (EDX) spectra were obtained with an S-4000scanning electron microscope (Hitachi, Japan), using a 25 kVaccelerating voltage. The BET surface areas, pore sizes, andpore volumes of the obtained TNT and HMTNT weredetermined by nitrogen adsorption−desorption isotherms at 77K with a Micromeritics TriStar 3000 apparatus. FTIR spectrafrom a Perkin-Elmer Model 1600 FTIR spectrophotometerwere recorded in the range 4000−800 cm−1 with a scan rate of0.2 cm/s. A ζ potential analyzer (Zetasizer, 3000HS, MalvernCo.) was used to measure the ζ potentials of the TNT andHMTNT at pH 5.0.2.5. Data Handling. In this study, both the Freundlich28
and Langmuir29 adsorption equilibrium equations were used toexplain the adsorption characteristics between adsorbate andadsorbent. The Freundlich equation can be expressed as
= =Q x m K C/ nF
1/(1)
where x was the amount of the adsorbate adsorbing onto theadsorbent (mg), m was the adsorbent weight (kg), KF was theequilibrium constant (mg/kg (L/mg)1/n), C was the equili-brium concentration (mg/L) of adsorbate in the solution, and nwas an empirical constant. When n was equal to 1, the isothermwas linear and partitioning process might be the primarymechanism between the adsorbate and adsorbent.The Langmuir equation can be expressed as
= =+
Qxm
K bCK C1
L
L (2)
where x, m, and C were defined as above, KL was the Langmuiradsorption constant (L/mg), and b was the maximumadsorption capacity (mg/kg). The organic compounds withdifferent Sw values might adsorb on the TNT surface notcovered by HDTMA or partition into the HDTMA on thesurface of TNT. Moreover, the HMTNT might uptake theoxyanions and heavy metal ions through anion and cationexchange processes, respectively. The profiles of isotherms canbe used to determine the potential adsorption mechanisms.The adsorption characteristics were discussed herein accordingto the obtained isotherms and adsorption capacities.
3. RESULTS AND DISCUSSION3.1. Microstructures and Surface Chemical Character-
istics of TNT and HMTNT. Figure 1 shows the SEM image of
TNT obtained by the hydrothermal reaction and subsequentlywashed with HCl aqueous solution. The synthesized TNT has acylinder-like shape with a diameter of 10−30 nm and a lengthof several hundred nanometers. The result indicates that theraw TiO2 particles are converted into a nanotubular structureafter the hydrothermal treatment. Further observation with theTEM image in Figure 2 demonstrates that the synthesizedTNT possesses uniform inner and outer diameters along itslength.
Figure 3 shows the FTIR spectra of TNT and HMTNT. Theabsorbance near 3400 cm−1 demonstrates the existence of theOH groups on the surfaces of TNT and HMTNT. This resultalso implies that HDTMA only occupies the partial sites of theTNT surface. The weak absorbance of the signal near 1600cm−1 indicates the presence of the bending vibration of the H−O−H group on the surface. For HMTNT, the signal from 2850to 2960 cm−1 is the absorbance for the C−H bond, indicatingthat HDTMA has adsorbed on the TNT surface.
Figure 1. SEM image of TNT.
Figure 2. TEM image of TNT.
Industrial & Engineering Chemistry Research Article
To confirm HDTMA on the surface of TNT, the elementalcompositions of both TNT and HMTNT are analyzed. Table 2
lists the percentage of main elements of TNT before and afterthe HDTMA modification. As shown in Table 2, after theHDTMA modification process, the bromine content increasesfrom 0 to 2.23% and the carbon content increases from 1.91 to8.90%. The changes in elemental composition suggest thatHDTMA is definitely present on the TNT surface. Especially,the content of sodium atoms only decreases from 5.73 to2.46%. The presence of Na on the HMTNT surface indicatesthat HDTMA cannot replace all the Na ions. In other words,the HMTNT surface still possesses exchangeable cation sites.This can be further verified with the result that the cationexchange capacity (CEC) of TNT still has 125 mequiv/100 g(see Table 3) after the HDTMA modification process isadopted.
The porous structure characteristics, including the BETsurface area, pore size, and pore volume, obtained from theconventional analysis of nitrogen isotherms are also collected inTable 3. As listed in Table 3, owing to the insertion effects ofHDTMA cations, both the surface area and pore volume ofHMTNT are slightly smaller than those of the primary TNTsample. Since HDTMA may adsorb on the inside and outsideof nanotubes, which may decrease the internal space or coversome small pores on the external surface, however, the averagepore size increases.As mentioned earlier, when the HDTMA is adsorbed on the
surface of TNT, the hydrophobic tails interacting with each
other may produce an organic phase into which nonionicorganic solutes with low Sw values can partition. Moreover,since the surface of TNT that is not covered by HDTMA ishydrophilic, it still can adsorb the organic compounds with highSw values. The above results reveal that HMTNT may beregarded as an amphiphilic adsorbent, i.e., possessing hydro-phobic and hydrophilic surface simultaneously. Accordingly,HMTNT can exhibit complicated sorption mechanisms fororganic compounds with a wide range of Sw values.
3.2. Sorption of Low-Sw Organic Compounds. Figure 4presents the isotherms of the low-Sw organic compounds on
TNT and HMTNT. As shown in Figure 4a, the adsorptioncapacities for the relatively low Sw organic compounds on TNTare very low and the isotherms are irregular. Moreover, asdemonstrated in Figure 4b, an obvious increase in theadsorption capacity is observed for the adsorption onHMTNT and the isotherms are close to linearity. SinceHDTMA on the TNT surface can be regarded as an additionalorganic matter, the partition of organic compounds with low Swvalues into HDTMA may occur.30,31 According to thepartitioning concept, the slope of a linear isotherm representsthe distribution (or partitioning) coefficient (or the KF value inthe Freundlich equation with n = 1) and it should be inverselyproportional to the Sw values of the examined organiccompounds.31 As demonstrated in Figure 4b and Table 4, theslopes of the isotherms decrease in the order propylbenzene >ethylbenzene > m-xylene > toluene > benzene. The result
Figure 3. FTIR spectra of TNT and HMTNT.
Table 2. Elemental Compositions (%) of TNT and HMTNT
demonstrates that the low-Sw organic compounds partition intothe HDTMA on the TNT surface.3.3. Sorption of High-Sw Organic Compounds. Figure 5
displays the adsorption isotherms for organic compounds with
relatively higher Sw values on TNT and HMTNT. For TNT,trichloromethane has the smallest adsorption capacity amongthe examined organic compounds. For HMTNT, the isothermsof 1-pentanol, m-cresol, and phenol are L-type, while theisotherm of trichloromethane is linear. The results may beascribed to the fact that 1-pentanol, m-cresol, and phenol eachpossess a hydroxyl group that is able to form a hydrogen bondwith the oxygen atom on TNT and HMTNT surfaces.
Trichloromethane cannot form the hydrogen bond with thesurface of TNT and thus has low affinity toward the surface ofTNT. In this case, the formation of a hydrogen bond becomes acritical factor to determine the adsorption capacity of anorganic compound with high Sw value onto the TNT. Asobserved in the FTIR spectra, the HMTNT surface stillpossesses OH groups that can form hydrogen bonds with theorganic compounds. Since the amount of OH groups on theHMTNT surface is lower than that on TNT, the adsorptioncapacity of HMTNT is smaller than that of TNT. Moreover,the isotherms are concave downward curves (i.e., L-type),revealing that these organic compounds have high affinity forthe TNT and HMTNT surfaces. On the other hand, theisotherms of trichloromethane on TNT and HMTNT are S-type (concave upward) and linear type, respectively. Since thehydrophilic characteristics of the TNT surface may lead to aweak affinity toward trichloromethane, the S-type isotherm is areasonable result. Moreover, the linear isotherm implies thattrichloromethane can partition into HMTNT. It should benoted that although the high-Sw organic compounds may alsopartition into HDTMA on the HMTNT surface, their relativelyhigher Sw values imply that such partitioning is quite weak andthe degree of partitioning does not make a significantcontribution to the adsorption capacity. As a result, theisotherms exhibit concave downward curves. As listed in Tables4 and 5, except for trichloromethane, the adsorption data canbe well described with the Langmuir equation and R2 values aregreater than 0.97. Moreover, the maximum adsorptioncapacities (i.e., b values) are proportional to the Sw values ofthe organic compounds.
3.4. Adsorption of Oxyanions. Zhang et al.32 haveproposed that the adsorption of quaternary amines on the clayinvolves at least three types of reactions, viz., a cation exchangereaction, adsorption of ion pair, and tail−tail interactions.Moreover, it was found that the adsorption mechanisms wereclosely related to the size of the quaternary amines. Xu andBoyd33 pointed out that HDTMA was initially adsorbed bycation exchange in the interlayer, which caused extensive clayaggregation. As the loading increased, HDTMA adsorbed to theexternal surfaces of aggregates via cation exchange andhydrophobic bonding, with the latter causing a positive chargedevelopment on surfaces and ultimately clay dispersion. Thepositive charges on the surface can uptake anionic contami-nants through an anion exchange process. As listed in Table 3,the TNT surface possesses a negative charge, while theHMTNT surface has a positive charge. Since the surface of
Table 4. Langmuir and Freundlich Isotherm Constants for Adsorption of Selected Adsorbates onto HMTNT
adsorbate ads eq KF or KL n b (mg/kg)/(mmol/kg) R2
HMTNT has positive charges, it can be used to adsorboxyanions.Figure 6 displays the adsorption isotherms of Cr2O7
2− andMnO4
− on TNT and HMTNT. As expected, the oxyanions
exhibit low (<5000 mg/kg) and high (>25 000 mg/kg)adsorption capacities on TNT and HMTNT, respectively.For HMTNT, the profiles of isotherms exhibit a sharp riseunder low equilibrium concentrations and reach a plateau uponincreasing the equilibrium concentration continuously. Suchconcave downward isotherms (L-type) indicate that anionexchange process could be the primary mechanism for theadsorption of oxyanions on the HMTNT. As listed in Table 4,the adsorption of oxyanions on HMTNT can fit the Langmuirequation well. The adsorption capacity of Cr ion (Cr2O7
2−) isslightly higher than that of Mn ion (MnO4
−). This result maybe closely related to the fact that a divalent anion (dichromate)generates higher affinity for the HMTNT than a monovalentanion (permanganate). In addition to the anion exchange, thecoexistence of other mechanisms for the adsorption of
oxyanions onto HDTMA on TNT surface is also possible.For instance, Haggerty and Bowman34 proposed the concept ofsurface complexation or surface precipitation, i.e., the oxyanionsassociating with the cationic head groups of the HDTMA onthe TNT surface to form organic salts, to explain the increase inadsorption capacity.
3.5. Adsorption of Heavy Metal Ions. It is well-knownthat TNT possesses a negative surface charge and caneffectively adsorb the cations through the cation exchangeprocess.25 Moreover, the cationic metal ions cannot replace theHDTMA on the surface of adsorbent.35 Accordingly, ifHDTMA only partially occupies the cation exchangeable siteson the TNT surface, HMTNT still can take up the cations.Figure 7 shows the adsorption isotherms of Cu2+ and Pb2+ onTNT and HMTNT.As demonstrated in Figure 7, the isotherm profile of TNT is
similar to that of HMTNT. The L-type isotherms reveal thehigh affinity between the heavy metal ions and the TNT orHMTNT, and the cation exchange process is a potential
Table 5. Langmuir and Freundlich Isotherm Constants for Adsorption of Selected Adsorbates onto TNT
adsorbate ads eq KF or KL b (mg/kg)/(mmol/kg) n R2
adsorption mechanism. The adsorption capacity of Pb2+ onTNT is greater than that on HMTNT. This may be ascribed tothe fact that HDTMA occupies some exchangeable cation sitesand thus resulting in a decrease in the adsorption amount.However, the adsorption capacity of Cu2+ on HMTNT isgreater than that on TNT. In general, the presence of HDTMAshould decrease the adsorption of the heavy metal ions.However, the presence of nitrogen atoms in the structure ofHDTMA may lead to a greater adsorption capacity of Cu2+
through the complexation process. The functional groupscontaining nitrogen atoms can provide nonbonding electronsto coordinate with divalent metal ions. Cu2+ possesses arelatively higher potential to form such complexes and, then, ahigher adsorption capacity on HMTNT than on TNT. As listedin Tables 4 and 5, the adsorption data can be well describedwith the Langmuir equation. Moreover, the adsorption capacityof Pb2+ based on millimoles per kilogram is higher than that ofCu2+.Since TNT derived from hydrothermal method possesses a
cation exchange property and is also characterized by highsurface area and pore volume, it may offer a specialenvironment for the adsorption of cations, such as heavymetal ions and basic dyes, through the cation exchangemechanism. Moreover, when the sodium cations in the TNTare replaced with HDTMA, the hydrophobic tails of HDTMAinteracting with each other may produce an organic phasewhich acts as a partition medium into which nonionic organicmolecules partition from aqueous solution. On the other hand,the hydrophobic bonding by conglomeration of large C16 alkylgroups associated with HDTMA can render a positive chargedevelopment on the surface of TNT, from which the anionscan be removed from aqueous solution with an anion exchangemechanism. According to the above results, it may beexperimentally concluded that HMTNT can be regarded asan amphiphilic adsorbent wherein it can effectively uptake thehydrophilic and hydrophobic organic contaminants through theadsorption and partitioning process, respectively. On the otherhand, it is also a dual-electronic adsorbent that cansimultaneously adsorb cationic and anionic contaminantsthrough the cation and anion exchange processes, respectively.As listed in Table 4, HMTNT possesses high adsorptioncapacities toward the various contaminants. For Cu2+, theadsorption capacity of HMTNT is higher than that of bothactivated carbon36,37 and Na−montmorillonite,36 and iscomparable to that of carbon nanotubes.37 For Cr2O7
2−, theadsorption capacity of HMTNT is comparable to that of somenew materials, such as layered double hydroxides38 andmalachite nanoparticles.39 Those results indicate thatHMTNT may be an attractive adsorbent for the removal ofheavy metal ions and oxyanions from wastewater.The reuse of an adsorbent is an important factor because it
allows for a reduction in both treatment costs and wastegeneration. Currently, two major regeneration techniques areoften used, namely, high temperature calcination for removal oforganic and carbon deposits on adsorbents and ion exchangeregeneration for restoring the exchange capacity.40 Recently, asimple washing process with alkaline or acid solution has beenproposed to recover the surfactant-modified adsorbents.41−43
For the regeneration of HMTNT, high temperature calcinationis inappropriate, because the heat-treating process has asignificant effect on both the pore structure and surfacechemical characteristics of HMTNT,44 which in turn results inan obvious decrease in the adsorption capacity for adsorbates.
However, HMTNT may be heated at a lower temperature (e.g.,100 °C) to volatilize the adsorbed organic compounds.45 Onthe other hand, the simple washing process with alkaline or acidsolution may be taken into consideration due to the fact thatthe adsorption of heavy metal ions and oxyanions ontoHMTNT is closely related to the solution pH. Finally, it is well-known that the as-prepared TNT may be used as an effectivephotocatalyst46 and the photocatalytic degradation of theadsorbed organic compounds may also be used for theregeneration of HMTNT.
4. CONCLUSIONS
TNT possessed high surface area, pore volume, and cationexchange capacity. It had great potential as an adsorbent for theadsorptive removal of organic compounds with high Sw valuesand heavy metal ions. When the partial surface of TNT wascovered by HDTMA, it became an amphiphilic and dual-electronic adsorbent. The low-Sw organic compounds couldpartition into the HDTMA on the HMTNT surface to enhancetheir sorption capacities. Both the high-Sw organic compoundsand heavy metal ions might also adsorb on the HMTNTsurface that was not covered by HDTMA. Moreover, oxyanionscan be removed by HMTNT from an aqueous solution with ananion exchange mechanism. It may be tentatively concludedthat HMTNT might be a good adsorbent for the uptake ofheavy metal ions, oxyanions, and organic compounds with awide range of Sw values. For the regeneration of HMTNT,simple washing with alkaline or acid solution can be used todesorb both the heavy metal ions and oxyanions. Thermaltreatment to volatilize the adsorbed organic compounds can beadopted. Moreover, the photocatalytic degradation of theadsorbed organic compounds by using the HMTNT asphotocatalysts may also be taken into consideration.
NotesThe authors declare no competing financial interest.
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