Single and competitive adsorption of OMPs by carbon nanotubes – mechanism and fitting models

The adsorption of three organic micropollutants (diclofenac – DFN, pentachlorophenol – PCP and octylphenol – OP) on two kinds of carbon nanotubes (single walled carbon nanotubes – SWCNT and single walled carbon nanotubes with amine group – SWCNT-NH2) was investigated, in single and bicomponent solution at pH 5. SWCNT-NH2 had three times lower specific surface area than SWCNT. Significant differences were observed in sorption capacity of SWCNT and SWCNT-NH2 for given chemicals. The sorption uptake changes in the following order: OP > PCP > DFN for SWCNT and DFN > PCP > OP for SWCNT-NH2. A few times higher adsorption of OP on SWCNT came from low OP solubility in water in comparison to PCP and DFN. While, higher adsorption of DFN and PCP on SWCNT-NH2 was a result of electrostatic attraction between dissociated form of these chemicals and positively charged SWCNT-NH2 at pH 5. In adsorption from bicomponent solution, significant competition was observed between PCP and DFN due to similar adsorption mechanism on SWCNT-NH2. Opposite tendency was observed for SWCNT, DFN did not greatly affect adsorption of PCP and OP since they were very easily absorbable by sigma-sigma interaction.


Introduction
Organic micropollutants (OMPs) such as endocrine disrupting compounds, pharmacuticlas, pesticides, polycyclic aromatic hydrocarbons, surfactants usually enter into wastewater after their industrial and domestic application [1][2][3].Therefore, they occur in effluents, since the technologies used at conventional wastewater treatment plant does not allow their complete elimination [4,5].This is due to many of these contaminates are highly hydrophobic like PAHs, hardly soluble in water and toxic, like pesticides.A consequence of these properties is OMPs' bioaccumulation in environment, for example in animal's tissues and some environmental parts like sediments, soils [6,7].
Last years many efforts have been made to find the most favourable process for micropollutants removal from different aqueous media.Among many methods to remove these contaminants, membrane filtration, advanced oxidation processes and adsorption have been widely proposed [8][9][10][11].Although advanced oxidation and membrane filtration are very effective for the elimination of OMPs, these methods led to the generation of highly concentrated retentate and formation of undesirable for water oxidation by-products [12,13].
Therefore, adsorption is considered as superb process, since the high-quality effluent is produced.As an alternative for well-known activated carbon, carbon nanotubes are considered as highly effective adsorbent for OMPs removal.This is due to much faster adsorption of contaminants on carbon nanotubes than on activated carbon [14].Most of the adsorption studies were conducted for single adsorbate to describe adsorption mechanism or adsorbent potential.This approach does not bring useful information about real adsorption behaviour, for example in environmental water matrix containing a few different contaminates.In that conditions, we can observe cooperative or competitive behaviour, depending on affinity of coadsorbates to the adsorbent surface [15].
The aim of this study was to better understand the competition behaviour between OMPs with different properties (diclofenac, pentachlorophenol, octylphenol) during adsorption on carbon nanotubes.For that purpose, two kinds of experiments were conducted: (1) adsorption of OMPs from single solution, (2) adsorption of OMPs from bicomponent solution.

Materials
Selected OMPs i.e. 4-octylphenol (OP), diclofenac sodium salt (DFN) and pentachlorophenol (PCP) were purchased from Sigma-Aldrich (Poland) in analytical purity grade.Their physico-chemical properties are presented in Table 1.The stock solution of OMPs was prepared with methanol (1 g/L).The adsorbate feed solutions (single and bicomponent) were prepared by diluting the stock solution with pure water.pH of feed solution was adjusted to 5 using 0.5 M NaOH.

Carbon nanotubes and their characterization
Two different types of carbon nanotubes (CNTs): single walled carbon nanotubes (SWCNT) and single walled carbon nanotubes with amine group (SWCNT-NH2) were used.CNTs were purchased from Chengdu Organic Chemistry Ltd.Characteristic of the nanotubes as provided by the manufacturer is shown in Table 2. Nitrogen adsorption-desorption isotherms of CNTs were estimated at 77 K by means of a volumetric adsorption analyzer ASAP 2010 (Micrometrics USA) and on this basis the BET surface area and porous structure parameters were calculated.Streaming potential of SWCNT was studied using electrokinetic analyser SurPASS TM 3 and then zeta potential was calculated from the Smoluchowski equation.

Adsorption isotherms experiments
The adsorption isotherms of PCP, DFN and OP were determined at 20°C in single and bicomponent solution system at pH 5. Two concentration levels of OMPs were used i.e. 0.1 and 0.5 mg/L.Experimentally, an adequate amount of a sorbent (from about 10 to 100 mg) was added into stoppered flasks containing 50 mL of single or bicomponent OMPs solution and shaked until equilibrium was reached.Then a sorbent was separated from the solution using PTFE syringe filter with a pore size of 0.45 µm and the resulting filtrates were analyzed for micropollutants concentration.The analytical procedure included solid phase extraction and chromatographic determination with HPLC.The amount of studied OMPs adsorbed on the nanotubes was determined by the following equation: where:   (mg/g) is adsorbed amount of the adsorbate, C0 and Ce (mg/L) are initial and equilibrium concentrations of the adsorbate, respectively, m (g) is weight of the sorbent, v (L) is a volume in which the adsorption was performed.

Data analysis
In this study, three common adsorption isotherm models including: where:   (mg/g) is the maximum adsorption capacity, and b (L/mg) is the Langmuir fitting parameter.
The Freundlich model is empirical and well describes adsorption on heterogeneous surface energy system.The model has significant importance for chemisorption and some cases of physisorption and can be written as: where: Kf ((mg/g) (L/mg) n ) is the Freundlich adsorption coefficient, n is the number describing surface heterogeneity and sorption intensity.
The Dubinin-Radushkevich model, which was originally proposed as an empirical adaptation of the Polanyi adsorption potential theory, has been the fundamental equation to quantitatively describe the adsorption of gases and vapours by microporous sorbents.The equation, based on the postulate that the mechanism for adsorption in micropores is that of pore-filling rather than layer-by-layer surface coverage, generally applies well to adsorption systems involving only Van der Waals forces and is especially useful to describe adsorption on microporous adsorbent.The Dubinin-Radushkevich equation is given in the following form: where:   (mg/g) is the amount of adsorbate that can be adsorbed in micropores, can be obtained by plotting   as a function of  2 ,  (kJ/mol) is the adsorption energy, can be read from the slope of the line,  is adsorption potential, is defined as: where: R (8.314 J•mol -1 •K -1 ) is the ideal gas constant, T (K) is temperature,   (mg/L) is the solubility in water.
The models were fitted to the experimental data using the Microsoft Excel spreadsheet with the Solver add-in.The least-squares method was used to determine the constants of the equations.
In order to compare competition strength between coadsorbates in bicomponent solutions following equation was used:  = where:  (%) is competition strength   and   (L/g) are adsorption distribution coefficient for studied OMPs without and with coadsorbate, respectively, calculated for selected equilibrium concentration, obtained from equation: 3 Results

Morphology and texture of CNT
The pore size distribution (PSD) of the SWCNT and SWCNT-NH2 are shown in Fig. 1 and their physical structure properties are listed in Table 2. SWCNT have three times larger surface area than SWCNT-NH2 that suggests better sorption potential of SWCNT.From PSD, SWCNT are dominated with micropores, while the SWCNT-NH2 structure is bidispersive, in large part consists of micropores and average fraction of mesopores.The zeta potential versus pH is presented in Fig. 2. The charge of both kinds of nanotubes was positive at pH around 4 and negative at pH around 7. What is more important, the SWCNT-NH2 have higher electric charge than SWCNT, which is attributed to dissociated form of amine group (NH3 + ) on their surface.The isoelectric point was found at pH 4.5 and 6.3 for SWCNT and SWCNT-NH2, respectively.

Single adsorption of OMPs from waterisotherms and mechanism
The adsorption isotherms yield the most important information about the distribution of the adsorbate molecules between the liquid and the solid phase when the adsorption process reaches equilibrium.Fig. 3 shows the isotherms of DFN, PCP, and OP on SWCNT and SWCNT-NH2.The sorption of some OMPs on SWCNT occurs in different manners than on SWCNT-NH2.According to, the Giles classification, OP adsorption isotherms correspond to the L1 and L2 type for SWCNT and SWCNT-NH2 respectively.This type of isotherm is typical for microporous adsorbents [16], that is consistent with structural properties of SWCNT and SWCNT-NH2 obtained from nitrogen sorption desorption measurements.The initial curvature in the L type isotherm shows that as more sites in the adsorbent are filled it becomes increasingly difficult for a bombarding adsorbate molecule to find an empty site available.This implies that molecules are adsorbed non-vertically or there is lack of strong competition from the solvent.The shapes of the DFN and PCP isotherm curves resembles the L2 type and the H2 type for SWCNT and SWCNT-NH2.The H type isotherm is a special case of the L curve, in which the solute has high affinity for the adsorbent surface because of ion-ion attraction [17,18].Thus, we can notice that adsorption mechanism of given OMPs was similar for SWCNT (due to Lshape of isotherm), while for SWCNT-NH2 two different sorption behaviours occurred, first when OP is an adsorbate, the second for PCP and DFN..1051/e3sconf/20172200076ASEE17 Significant differences were observed in sorption capacity of SWCNT and SWCNT-NH2 for given OMPs.The sorption uptake changes in the following order: OP > PCP > DFN for SWCNT and DFN > PCP > OP for SWCNT-NH2, respectively.In order to elucidate the factors controlling adsorption mechanism and sorption affinity of OMPs we should consider their hydrophobicity, molecular weight, ionic form and solubility.A few times higher adsorption of OP on SWCNT in comparison to the two other adsorbates can be ascribed to the different solubility in water, which is 7, 10, 50 mg/L for OP, PCP and DFN, respectively.Affinity of the adsorbate to sorption is larger when solubility is low [19], therefore it explains very high sorption uptake of OP and very low uptake of DFN by SWCNT.Effect of molecular weight and log Kow of studied OMPs seems to be negligible because these properties were similar (Table 1).For SWCNT-NH2, PCP and DFN were found as easily absorbable compounds, while OP as hardly.It can be explained by chemical nature of these compounds at pH 5. When solution pH is around 4.5, both PCP and DFN are dissociated, in contrast to OP (Table 1).When we consider that surface charge of SWCNT-NH2 is positive at pH 5, we can assume that sorption of PCP and DFN was enhanced by ionic attraction.Mechanism does not occur for SWCNT due to neutral form at pH 5. The Langmuir, Freundlich and Dubinin-Radushkevich models were used to describe the experimental data, and the relevant parameters derived from these models are presented in Table 3.The fitting degree (based on correlation factors R 2 ) decreases in the following order: Langmuir, Dubinin-Raduskevich and Freundlich.Therefore, the subsequent discussion will be mostly concentrated on the parameters obtained from Langmuir and Dubinin-Radushkevich computations.The values of adsorption energy for both SWCNT and SWCNT-NH2 lie in the range of physical adsorption, apart from OP adsorption on SWCNT.Higher values of energy for SWCNT-NH2 can suggest that this sorbent mainly induces the adsorption of OMPs coming from strong electrostatic interactions [20].
From the value of maximum adsorption capacity derived from Langmuir and Dubinin-Radushkevich was found that OP and PCP were better adsorbed by SWCNT than SWCNT-NH2.Maximum adsorption capacity of PCP was 0.011 mg/g and 0.0068 mg/g on SWCNT and SWCNT-NH2, respectively, and for OP: 0.074 mg/g (SWCNT) and 0.0044 mg/g (SWCNT-NH2).On the one hand, it can be a consequence of the differences in structural properties of the adsorbents.As many authors suggest higher sorption capacity is found for the adsorbents with high specific surface area [14,21].Specific surface area (SSA) of SWCNT is around three times higher than SWCNT-NH2, thus both OP and PCP were better adsorbed by SWCNT than SWCNT-NH2.An interesting exception is noticed for DFN adsorption, because higher maximum sorption capacity was observed for SWCNT-NH2.This suggests that sorption behaviour, when DFN is adsorbate, is different for SWCNT and SWCNT-NH2.This fact was also revealed by isotherm type, discussed above.In the other words, normal dependence of SSA and sorption capacity does not apply for DFN adsorption 0 0,01 0,02 0 0,1 0,2 0,3 0,4 0,5 q e (mg/g) C e (mg/L) a 0 0,005 0,01 0 0,1 0,2 0,3 0,4 0,5

Competetive adsorption of OMPs
Fig. 4 presents adsorption isotherms of DFN, PCP and OP from bicomponent adsorbate solution.Shape of these isotherms, both for SWCNT and SWCNT-NH2 was similar to the single adsorption system.It means that second adsorbate in the solution did not cause significant change in adsorption mechanism of given OMPs.On the contrast, adsorption magnitude changed greatly as an effect of competition between some adsorbates.It was observed that the Langmuir and Dubinin-Radushkevich equations described the adsorption of OMPs with better correlation coefficient (Table 4), meaning that heterogeneous surface or porous structure played an important role in their adsorption and different binding sites with several adsorption energies were involved.Excluding adsorption of OP on SWCNT, adsorption energy of studied OMPs is higher than 8 kJ•mol -1 , indicating physical sorption.
Comparing the values of adsorption energy for single and bicomponent solutions we can notice that adsorption energy was slightly lower for competitive adsorption.Take, for an example, adsorption of DFN by SWCNT-NH2, energy is 18.3 kJ•mol -1 ; 11.2 kJ•mol -1 ; 14.1 kJ•mol -1 for single solution and for bicomponent solutions with PCP and OP, respectively.Similarly, from Langmuir and Freundlich models, the constants b and n, describing the affinity of binding sites and adsorption intensity, are lower for bicomponent system than for single, independently from adsorbent.It was found that between some of OMPs intense competition occurred, expressed as competition strength (Table 5).More specifically, for SWCNT stronger competition was observed between DFN and OP with competition strength of 61.7% than between the DFN and PCP with competition strength of 31%, while for SWCNT-NH2, opposite tendency was observed i.e. competition strength was 69% and 42% between DFN and PCP and between DFN and OP respectively (Fig. 4a, 4b).Similar results were obtained for PCP adsorption on SWCNT-NH2 in the presence of DFN and OP (Fig. 4d).In that case, competition strength was at the level of 70% and 23% for DFN and OP as coadsorbates, respectively.Interesting exception was observed for PCP adsorption on SWCNT, in the presence of DFN.Competition strength was 4.4% suggesting negligible impact of DFN on PCP adsorption.These results can be explained when we compare adsorption mechanism of given coadsorbates.Adsorption of DFN, in the presence of OP and PCP, on SWCNT was very low.Lower solubility of OP and its undissociated form (when pH<pKa) resulted in more preferential adsorption of OP than DFN on SWNCT.Similarly, DFN adsorption was inhibited by PCP as coadsorbate.Probably due to a role of ∏-∏ electron-donor-acceptor interaction and hydrogen bonds in PCP adsorption [22].Strong competition between DFN and PCP on SWCNT-NH2 was a result of the same mechanism of their adsorption, which was mainly electrostatic attraction.As was documented in the first part of this study, DFN was hardly adsorbable on SWCNT and easily on SWCNT-NH2, mainly due to its high solubility in water and dissociated form at pH 5. Therefore, dominant adsorption strength of DFN is ionic attraction, that occurs only for SWCNT-NH2 due to its positive charge at given 0 0,001 0,002 0,003 0,004 0,005 0 0,1 0,2 0,3 0,4 0,5 Separate discussion has to be done for OP adsorption with its coadsorbates (Fig. 4e and  4f).As was mentioned, OP was very easily adsorbable by SWCNT from single solution system.Moreover, competition between OP and DFN in bicomponent solution, was relatively low with competition strength at the level of 3%.This effect relates to negligible adsorption of DFN on SWCNT.Higher competition about sorption sites was observed between OP and PCP.Due to both sorbates can be adsorbed by hydrogen bonding and ∏-∏ interactions.Similar adsorption mechanism of PCP and DFN induces mutually competition.On the one hand, OP should be more preferentially adsorbed on SWCNT, on the other hand for two coadsorbates with different size, the smaller one can access the porosity in greater amount [23].Thus, PCP with smaller size than DFN (expressed as equivalent width and molar volume -Table 1) inhibits DFN uptake.Surprisingly, coadsorbate size did not affect adsorption of OP on SWCNT-NH2.A careful observation of Fig. 4f shows that competition was higher with coadsorbate molecules with higher size (DFN) than with smaller size (PCP).In other words, considering size of coadsorbates (PCP and DFN) we could have expected that higher competition will occur with PCP due to its lower steric hindrance.In fact, competition effect on SWCNT-NH2 was clearly controlled by ionic attraction, that was stronger for DFN.

Conclusions
The adsorption behavior of OMPs in single and bicomponent solution was evaluated.Adsorption of three OMPs by SWCNT and SWCNT-NH2 were compared with each other.
From the shape of isotherms, similar adsorption mechanism of OMPs was observed for SWCNT and two different behaviors for SWCNT-NH2first for OP, the second for PCP and DFN.In addition, the loadings of OMPs on adsorbents decreased in the following sequence: OP > PCP > DFN for SWCNT and DFN > PCP > OP for SWCNT-NH2.Favorable adsorption of OP and PCP by SWCNT was an effect of their low solubility in water and sigma-sigma interactions.Easy adsorption of DFN and PCP by SWCNT-NH2 was assigned to electrostatic attraction between dissociated form of PCP and DFN and positively charged sorbent surface at pH 5. It was found that the values of adsorption energy for both sorbents indicated physical adsorption, apart from OP adsorption on SWCNT.Higher values of energy for SWCNT-NH2 can suggest that this sorbent mainly induces the adsorption of OMPs coming from strong electrostatic interactions.
From results obtained in bicomponent solution, different competitive effects were observed.Take, for an example adsorption of DFN with its coadsorbates PCP and DFN by SWCNT, OP inhibited DFN adsorption much stronger than PCP.For comparison, when DFN was adsorbed by SWCNT-NH2, opposite tendency was observed, stronger inhibition of DFN adsorption was caused by PCP.This is due to control of PCP and DFN adsorption on SWCNT-NH2 by electrostatic attraction.on the other hand, between some coadsorbate, a lack or negligible competition was observed, like for OP and PCP adsorption on SWCNT with DFN as coadsorbate.Comparing the values of adsorption energy for single and bicomponent solutions we can notice that adsorption energy was slightly lower for competitive adsorption.
The paper has been prepared within the frame of the National Science Centre project based on decision no DEC-2013/11/B/ST8/0439.

( 1 )
Langmuir model, (2) Freundlich model, and (3) Dubinin-Radushkevich model were used to fit experimental data.Each of the model is briefly described below.The Langmuir equation assumes monolayer adsorption, where molecules interact only with the surface of sorbent.The form of the Langmuir isotherm is represented by the following equation:

Fig. 4 .
Fig. 4. Adsorption isotherms of DFN (a, b), PCP (c, d) and OP (e, f) by SWCNT and SWCNT-NH2 from single solution and two bicomponent solutions.The solid lines represent Langmuir model fit.
solution OP & DFN solution E3S Web of Conferences 22, 00076 (2017) DOI: 10.1051/e3sconf/20172200076 ASEE17 solution pH.The same adsorption mechanism was documented for PCP and therefore strong competition between these adsorbates was observed.

Table 2 .
Textural characteristic of studied carbon nanotubes.

Table 3 .
Parameters of Langmuir, Freundlich and Dubinin -Radushkevich equations and correlation coefficients for the adsorption of OMPs on carbon nanotubes from single solution.

Table 4 .
Parameters of Langmuir, Freundlich and Dubinin -Radushkevich equations and correlation coefficients for the adsorption of DFN, PCP and OP on carbon nanotubes from single and bicomponent solutions.

Table 5 .
Competition strength (calculated for selected equilibrium concentration) between coadsorbates in bicomponent solution.