Effect of Microbes on the Adsorption of Naphthalene by Graphene Oxide

The adsorption of naphthalene on graphene oxide (GO) nanosheets in presence ofPaecilomyces cateniannulatus (P. cateniannulatus) was conducted by the batch techniques. The morphology and nanostructure of GO were characterized by SEM, TEM, FTIR, XPS, and Raman. The adsorption kinetics indicated that the adsorption of naphthalene on GO and GO + P. catenlannulatus can be satisfactorily fitted pseudo-first-order and pseudo-second-order kinetic model, respectively. P. catenlannulatus inhibited the adsorption of naphthalene on GO at pH < 4.0, whereas the increased adsorption was observed at pH > 4.0. The adsorption of naphthalene on GO and GO + P. catenlannulatus can be better fitted by Langmuir and Freundlich model, respectively. The change in the conformation of GO was responsible to the increased adsorption of naphthalene by SEM and TEM images. According to FTIR analysis, naphthalene was absorbed by the oxygen-containing functional groups of GO, especially for –COOH. The finding in the study provides the implication for the preconcentration and removal of polycyclic aromatic hydrocarbons from environment cleanup applications.


Introduction
Graphene oxide (GO), as the precursor of graphene, exhibits the large number of hydroxyl and epoxy groups in the plane and a small amount of carboxyl and carbonyl group at the edge [1][2][3].Recently, it has been reported that GO displays the excellent adsorption performance for many organic contaminants, such as dyes [4][5][6][7], polychlorinated biphenyls (PCBs) [8][9][10], and polycyclic aromatic hydrocarbons (PAHs) [11][12][13].Wang et al. [13] investigated that the high affinities of the PAHs to GO were attributed to the carboxyl groups attaching to the edges of GO.Recent studies have revealed that the hydrophilic GO solution can transfer into the natural aquatic environments, which causes greater environmental risks [14].Therefore, understanding on the interaction between GO and microbes plays a vital role in evaluating the fate and transport of GO in the aquatic environments.
Paecilomyces cateniannulatus (P.cateniannulatus) is carnivorous fungi specialized in trapping and digesting nematodes, which has been extensively used to kill harmful nematodes by pathogenesis [15].It is demonstrated that P. catenlannulatus also can be used as a promising adsorbent to remove heavy metals [16][17][18][19][20][21].Li et al. [20] found that the maximum adsorption capacity of P. catenlannulatus calculated from Langmuir model was 140.85 mg/g for Hg(II).Such high affinity for heavy metals was attributed to a variety of oxygen-containing functional groups such as amine, phosphoryl, carboxyl, and hydroxyl groups [21].However, to the author's knowledge, little information on the effect of P. catenlannulatus on the adsorption of organic contaminants is available.
The objectives of this study are to (1) synthesize GO nanosheets and characterize the microstructure of GO by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transformed infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and Raman techniques; (2) investigate the effect of P. cateniannulatus on the adsorption of naphthalene onto GO under various environmental conditions; (3) determine the interaction mechanism between naphthalene and GO + P. cateniannulatus by TEM and XPS.The paper highlights the molecular mechanisms and conformations that influence the adsorption of microbes onto graphene-based nanomaterials.

Experimental
2.1.Materials.Flake graphite (225 mesh, 99.8%) was purchased from Qingdao Tianhe Graphite Co., Ltd.The strain 13 of P. catenlannulatus was cultured from the microbiology laboratory of University of Huangshan, China, which was isolated from infected pupae of the lepidopteran T. pityocampa in NE China.All chemicals as analytical reagent were purchased from Sinopharm Chemical Reagent Co., Ltd.

Synthesis of GO.
GO nanosheets were synthesized using a modified Hummers method [22].Typically, 1.5 g flake graphite and 100 mL concentrated H 2 SO 4 were poured into 500 mL round-bottom flask under vigorous stirring and icewater bath conditions; then 6.0 g of KMnO 4 was slowly added into the aforementioned suspensions.The suspensions were vigorously stirred for one week at room temperature.Then 12 mL H 2 O 2 (30 wt %) was added in the suspension, and the mixture was stirred for 2 hr at room temperature.After centrifugation at 23000 rpm for 60 min, the solid phase was dispersed using vigorous stirring and bath ultrasonication for 30 min at the power of 140 W.The centrifugation and ultrasonication were recycled for several times, and then the sample was rinsed with Milli-Q water until the solution was neutral.The few-layered GO was obtained by freeze-drying it in a vacuum tank overnight.

Characterization.
The nanostructures of GO were synthesized by SEM (JSM-6700F field emission scanning electron microscope, JEOL), TEM (JEOL-2010 transmission electron microscope, Japan), FTIR (JASCO FT-IR 410 spectrophotometer) with the KBr pellet technology, and XPS (ESCALAB 250Xi XPS analyzer, Thermo Scientific) with monochromatic Al  X-ray source (ℎ] = 1486.6eV) at a vacuum below 10 −7 Pa and Raman (LabRam HR Raman spectrometer, France) at 514.5 nm with Ar + laser.The samples used for the SEM and TEM observation were prepared by dropping the GO suspension on copper foil.

Batch Adsorption Experiments.
The adsorption kinetics and isotherms of naphthalene onto GO in the presence and absence of P. cateniannulatus were conducted in PTFE screw cap vials sealed with aluminum foil at 293 K.The sorption kinetic studies and pH-dependent experiments were performed with an initial naphthalene concentration of 5.0 mg/L, and the solid-to-liquid ratio was 0.25 g/L.To reach the maximum adsorption performance, the initial concentration of naphthalene solution was obtained over wide ranging from 30 g/L to 10.0 mg/L.After equilibrium (24 h), the naphthalene concentration in the supernatants was analyzed by an Agilent 1200 HPLC equipped with a fluorescence detector.The distribution adsorption coefficient (  , L/g) and sorption capacity (  , mg/g) can be calculated by ( 1) and ( 2), respectively: where  0 (mg/L) and  eq (mg/L) are initial concentration and equilibrated concentration after sorption, respectively. (g) and  (mL) are the mass of adsorbents and the volume of the suspension, respectively.All experimental data were the average of triplicate determinations and the error bars (within ±5%) were provided.

Results and Discussion
3.1.Characterization.The morphology and nanostructure of GO were characterized by SEM and TEM techniques.
As shown in the SEM image of GO in Figure 1(a), GO displayed the aggregated nanosheets, which were consistent with previous studies [23][24][25].The transparent and randomly accumulated with the wrinkles of GO was observed (Figure 1(c)).The BET specific surface area of GO was calculated to be 240 m 2 /g, which was significantly lower than the theoretical data (approximately 2630 m 2 /g) due to the incomplete exfoliation and aggregation [26].The deconvolution of the C 1s peak of GO was shown in Figure 1(e).As illustrated in Figure 1(e), the main peaks at 284.7, 286.6, 287.9, and 289.8 eV were ascribed to the C-C, C-O, C=O, and O-C=O, respectively [27][28][29].According to the XPS analysis, the contents of oxygen in GO and hydrogen were approximately 30.96 and 0.11 atomic %, respectively.In the Raman spectra (Figure 1(f)), two main peaks at 1350 ( band, the stretching vibration of sp 3 carbon atoms) and 1580 cm −1 ( band, the stretching vibration of sp 2 carbon atoms) were observed.The  and  band were related with the defects/disorders and first-order scattering of the  2 mode, respectively [13,30].The intensity ratio of the  band and  band (  /  ) of GO (0.83) was smaller than that reduced GO (1.13), which indicated the defects/disorders of GO nanosheets compared to reduced GO nanosheets [31,32].
Based on the characteristic results, it was demonstrated that GO nanosheets presented a variety of oxygen-containing functional groups such as epoxy, carboxyl, carbonyl, and hydroxyl groups.

Adsorption Kinetics.
Figure 2 showed the adsorption kinetics of naphthalene on GO and GO + P. catenlannulatus.As shown in Figure 2(a), the adsorption of naphthalene on GO significantly increased with increasing reaction times from 0 to 3 h; then high-level adsorption was observed.However, the increased adsorption of naphthalene on GO + P. catenlannulatus was observed with increasing reaction times.It should be noted that P. catenlannulatus facilitated the adsorption of naphthalene on GO.The adsorption kinetics of naphthalene on GO and GO + P. catenlannulatus were fitted by pseudo-first-order and pseudo-second-order kinetic models.Their linear forms of pseudo-first-order [33] and pseudo-second-order kinetic models [34] were given in (3) and ( 4), respectively:   where   and   (mg/g) are the amount of naphthalene adsorbed at equilibrium and at time , respectively.  and   are the pseudo-first-order and pseudo-second-order kinetic rate constant, respectively.As shown in inset in Figures 2(a) and 2(b), it was observed that the adsorption of naphthalene on GO and GO + P. catenlannulatus can be satisfactorily fitted pseudo-second-order and pseudo-first-order kinetic model (correlation coefficient  2 > 0.999), respectively (Table 1).The results from adsorption kinetics indicated that P. catenlannulatus significantly influenced the adsorption mechanism of naphthalene on GO nanosheets.

pH Effect.
The effect of pH on naphthalene adsorption by GO and GO + P. catenlannulatus was shown in Figure 3.As shown in Figure 3(a), the adsorption of naphthalene on GO was independent of ionic strength, whereas the naphthalene adsorption on GO + P. catenlannulatus significantly increased with increasing pH from 2.0 to 5.0 (Figure 3(b)).Sun et al. [12] also found that the adsorption of naphthalene on GO was independent of pH; therefore the authors demonstrated that pore filling and flat surface adsorption dominated the adsorption of naphthalene on GO.The high-level adsorption of naphthalene on GO + P. catenlannulatus was also observed at pH 5.0-6.0;then adsorption of naphthalene on GO + P. catenlannulatus slightly decreased at pH > 6.5.
It should be noted that P. catenlannulatus inhibited the adsorption of naphthalene on GO at pH < 4.0, whereas the P. catenlannulatus facilitated the adsorption of naphthalene on GO at pH > 4.0.The inhibited adsorption of naphthalene on GO in the presence of P. catenlannulatus at pH < 4.0 could be attributed to the attachment of P. catenlannulatus on GO surface, resulting in the block of reactive sites of GO.The facilitated adsorption at pH > 4.0 was due to the electrostatic attraction of positively charged GO and negatively charged naphthalene at pH > pKa (∼9.3).The pH-dependent adsorption further demonstrated that the P. catenlannulatus significantly influenced the adsorption of naphthalene on GO.

Adsorption Isotherms.
Figure 4 showed the adsorption isotherms of naphthalene on GO and GO + P. catenlannulatus.As illustrated in Figure 4(a), the adsorption of naphthalene on GO obviously increased with increasing naphthalene concentration and then maintained high-level adsorption.However, the adsorption of naphthalene on GO + P. catenlannulatus significantly increased with increasing initial naphthalene concentration.The Langmuir and Freundlich models were utilized to fit the adsorption isotherms of naphthalene on GO.The linear forms of Langmuir [35] and Freundlich model [36] can be defined as ( 5) and ( 6), respectively: ln  where   (mg/L) and   (mg/g) are the concentration of naphthalene after equilibrium at solution and solid phase, respectively;   (L/mg) and   ((mg/g)/(mg/L) 1/n ) are the Langmuir and Freundlich adsorption coefficient, respectively. max (mg/g) is maximum adsorption capacity, and 1/ is the heterogeneity of the adsorption sites and an indicator of isotherm nonlinearity.The fitted results of naphthalene adsorption on GO and GO + P. catenlannulatus were shown in Figure 3 and Table 2.As listed in Table 2, the adsorption of naphthalene on GO can be better fitted by Langmuir model ( 2 > 0.999), whereas the adsorption of naphthalene on GO + P. catenlannulatus can be satisfactorily simulated by Freundlich model ( 2 > 0.996).The maximum adsorption capacities of GO and GO + P. catenlannulatus calculated from Langmuir model were 0.167 and 112.3 g/g for naphthalene at pH 5.0 and 293 K, respectively.The fitted results indicated that the adsorption of naphthalene on GO was attributed to the monolayer adsorption, whereas the high affinity of GO + P. catenlannulatus for naphthalene was due to a variety of reactive sites with different adsorption energy.These reactive sites with the different energies were derived from the sieving effect of the powerful groove regions of GO [37], which was further demonstrated by the below characteristic results.

Adsorption Mechanism.
Based on the aforementioned batch adsorption, it was observed that P. catenlannulatus significantly influenced the adsorption of naphthalene on GO under various environmental conditions.Therefore, the adsorption mechanism between naphthalene and GO + P. catenlannulatus was demonstrated by SEM, TEM, and FTIR techniques.As shown in Figure 1(b), the GO surface became fairly flat and rigid after adsorption.It was also demonstrated from TEM images (Figure 1(d)) that the scrolls on the edge of the GO were mainly unfolded, which was consistent with previous study [13].The slight accumulation of the GO nanosheets was observed, which indicated the layers tended to aggregate tightly.The surface functional groups of GO nanosheets could form intrasheet bridging; therefore interaction of naphthalene and GO retained a fairly planar geometry because the steric hindrance inhibited the GO nanosheets from wrapping [38].Therefore, the change in the conformation of GO was responsible to the increased adsorption of naphthalene.Figure 5 showed the FTIR spectra of GO and GO + P. catenlannulatus after naphthalene adsorption.For GO, the peaks at approximately 1725, 1620, 1390, and 1085 cm −1 corresponded to the stretching vibration of the C=O, skeletal C=C, carboxyl O=C-O, and alkoxy C-O bond, respectively [2,25,39,40].The widen peaks at 3450 cm −1 (data not shown) were assigned to the -OH stretching vibration of adsorbed water [41].For GO + P. catenlannulatus, the peaks of C=C bonds shifted from 1625 to 1633 cm −1 after naphthalene adsorption, which indicated that - interactions dominated the adsorption process between GO + P. catenlannulatus and naphthalene [4].The shift of carboxyl O=C-O bending vibration (from 1390 to 1381 cm −1 ) was greater than that of alkoxy C-O bending vibration (from 1085 to 1079 cm −1 ), which suggested that high adsorption of naphthalene on GO + P. catenlannulatus was attributed to oxygen-containing functional groups of P. catenlannulatus, especially -COOH groups.These observations were consistent with previous studies [13,39].

Conclusions
The effect of P. catenlannulatus on the adsorption of naphthalene on GO was investigated by batch techniques.Based on the characteristic results, it was demonstrated that GO nanosheets presented a variety of oxygen-containing functional groups such as epoxy, carboxyl, carbonyl, and hydroxyl groups.The adsorption kinetics indicated that the adsorption of naphthalene on GO and GO + P. catenlannulatus can be satisfactorily fitted pseudo-first-order and pseudo-secondorder kinetic model, respectively.The pH-dependent adsorption showed that the high-level adsorption of naphthalene on GO + P. catenlannulatus was observed at pH 5.0-6.0;then adsorption of naphthalene on GO + P. catenlannulatus slightly decreased at pH > 6.5.It was observed that P. catenlannulatus inhibited the adsorption of naphthalene on GO at pH < 4.0, whereas the increased adsorption was observed at pH > 4.0.The adsorption of naphthalene on GO and GO + P. catenlannulatus can be better fitted by Langmuir and Freundlich model, respectively.According to the SEM and TEM analysis after adsorption, the change in the conformation of GO was responsible to the increased adsorption of naphthalene.The GO surface became fairly flat and rigid after adsorption.It was also demonstrated from TEM images that the scrolls on the edge of the GO were mainly unfolded.According to FTIR analysis, naphthalene was absorbed by oxygen-containing functional groups of GO, especially -COOH groups.The finding in the study provides the implication for the preconcentration and removal of polycyclic aromatic hydrocarbons from environment cleanup applications.

Figure 1 :Figure 2 :
Figure 1: The characterization of GO (a) and (b) SEM images of GO before and after adsorption; (c) and (d) TEM images of GO before and after adsorption; (e) XPS (C 1s) spectra; (f) Raman spectra.

Figure 5 :
Figure 5: FTIR spectra of GO (a) and GO + P. cateniannulatus (b) after adsorption of naphthalene.

Table 1 :
Kinetic parameters of naphthalene adsorption on GO and GO + P. cateniannulatus.

Table 2 :
Parameters for Langmuir and Freundlich models of naphthalene adsorption on GO and GO + P. cateniannulatus.