Removal of Hg2+ and methylmercury in waters by functionalized multi-walled carbon nanotubes: adsorption behavior and the impacts of some environmentally relevant factors

Abstract Adsorption of Hg2+ and methylmercury (MeHg) to multi-walled carbon nanotubes (MWCNTs) modified, respectively, with hydroxyl, amine and carboxyl groups was studied. The effect of various factors like the initial pH, natural organic matter (NOM), Cl- and adsorbent dose on the sorption efficiency were evaluated. It was found that amine-modified MWCNTs showed a strong adsorption capacity to Hg2+ and MeHg, and the removal efficiency could reach up to 92% (0.5 g/L MWCNTs, and 100 μg/L Hg2+ and MeHg) which is independent of pH. NOM had complex effects on the adsorption of Hg2+ and MeHg to MWCNTs. Cl- inhibited the adsorption of Hg2+ and MeHg to MWCNTs. The adsorption of Hg2+ and MeHg was found to be inhomogeneous and homogeneous chemisorption, respectively. Our results suggested that MWCNTs modified with different functional groups can efficiently adsorb both Hg2+ and MeHg in aqueous environment.


Introduction
Because of its long distance migration and bioaccumulation, mercury as a global pollutant has been attracting widespread attention [1,2]. Mercury in environment and biological body mainly exists in three forms including Hg 0 , Hg 2+ and methylmercury (MeHg). Hg 0 and Hg 2+ can be converted into MeHg by microorganisms [3][4][5][6]. MeHg is the most toxic organic mercury and is ubiquitous in the aquatic environment [7]. Mercury pollution comes mainly from industrial production, mercury mine and non-ferrous metals processing factory [8]. Various kinds of methods have been used to remove heavy metals from aqueous solutions including ion exchange, chemical precipitation, biological treatment, membrane filtration and adsorption [9]. Recently carbon nanotubes (CNTs), as adsorbents in the field of environment, have attracted more and more attention, because of their unique structure and chemical properties [10][11][12].
In this study, we studied the removal efficiency of Hg 2+ and MeHg from aqueous solution by pristine MWCNTs and MWCNTs modified with carboxylic functional group (MWCNTs-COOH), hydroxyl functional group (MWCNTs-OH), and amino functional group (MWCNTs-NH 2 ), respectively. The effects of environmentally relevant factors like pH, NOM, and Cl − on the transparent thin sheet was pressed and scanned from 400 to 4000 cm −1 , which the MWCNTs to KBr quality ratio was 1-100.

Adsorption experiments
The experiments were performed in 40 mL glass bottles with caps. The total volume of the solution was 20 mL, and Hg 2+ and MeHg working standard solutions were spiked into glass bottles to reach a final concentration of 100.0 μg/L Hg, respectively. After adding 10 mg MWCNTs, the solutions were placed on the oscillator to shake at 245 rmp and 20 °C for 90 min to reach adsorption equilibrium.
After achieving the equilibrium, 2 mL solution was taken out from each bottle and filtered through 0.22 μm PTFE membrane. The HPLC-ICP-MS hyphenated system was used to determine Hg 2+ and MeHg in the filtered solutions, and the detailed instrumentation and procedure could refer to our previous study [31].
To evaluate the influence of initial pH on adsorption, the pH of the solution was adjusted to 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0, respectively, by using 10 mmol/L phosphate buffer. The effects of other factors, including Cl − (0-800 mmol/L), NOM (0-20 mg/L, DOC) and adsorbent dose (0-3 g/L), on the adsorption were tested at pH 7.0. For each experiment, three parallel samples were prepared.
The removal efficiency of Hg 2+ or MeHg was calculated according to the equation [32]: where R is the removal efficiency of Hg 2+ or MeHg, C 0 is the initial concentration of Hg 2+ or MeHg (mg/L), and C e is the equilibrium concentration of Hg 2+ or MeHg (mg/L).
The adsorption capacity of adsorbent at equilibrium was calculated by the following equation [21]: where q is the adsorption capacity of adsorbent (mg/g), C 0 is the initial concentration of Hg 2+ or MeHg (mg/L), and C e is the equilibrium concentration of Hg 2+ or MeHg (mg/L). m is the adsorbent weight (g), V is the volume of solution (L).

Adsorption isotherm models
To reveal the adsorption process and evaluate adsorption capacity, adsorption isotherms were studied. The Hg 2+ and MeHg adsorption isotherms for MWCNTs, MWCNTs-OH, MWCNTs-NH 2 , MWCNTs-COOH were modeled by various isotherms.
The Langmuir model is used to describe homogeneous monolayer adsorption on the surface generally. The linear form of the model is given as [33]: sorption efficiency were evaluated. In addition, the adsorption isotherms and kinetics were studied by fitting to various models to understand the sorption mechanisms.

Materials
Four kinds of multi-walled carbon nanotubes (MWCNTs > 95% in purity and special surface area, SSA > 140 m 2  was coupled to the ICP-MS instrument (Agilent 7700cs) by directly connecting the column outlet to the cross-flow nebulizer of ICP-MS through a commercial polytetrafluoroethylene (PTFE) connector. The mobile phase for the HPLC-ICP-MS system consisting of 1 g/L L-cysteine and 0.06 mol/L ammonium acetate was prepared daily.

Characterization of MWCNTs
The morphology and size of MWCNTs were characterized by transmission electron microscope (TEM, Hitachi, Japan). Ten mg pristine and functionalized MWCNTs were added into 20 mL deionized water to form aqueous solution respectively. Then 20 μL sample of pristine and functionalized MWCNTs were droped on carbon membrane copper net, drying in vacuum oven at room temperature for 12 h. The surface functional groups of pristine and functionalized MWCNTs were detected by Fourier transform infrared spectroscopy (FT/IR-6100, JASCO, Japan). A where q e is the equilibrium adsorption capacity of adsorbent (mg/g), C e is the equilibrium concentration of Hg 2+ or MeHg (mg/L). Q m and b are Langmuir constants indicating the capacity and energy of adsorption, respectively, and can be calculated from the intercept and slope value of the linear plot, 1/q e vs. 1/C e .
The character of the Langmuir isotherm can also be expressed by adopting a dimensionless equilibrium parameter, R L, which is defined as [33]: where b is the Langmuir constant (L/mg) and C 0 is the initial concentration of Hg 2+ or MeHg (mg/L). The R L value indicates the shape of isotherm. R L values between 0 and 1 indicate favorable adsorption, while R L > 1, R L = 1, and R L = 0 indicate unfavorable, linear, and irreversible adsorption isotherms, respectively.
The Freundlich model describes the heterogeneity adsorption system and can be expressed as the following equation [28,33,34]: where q e is the equilibrium adsorption capacity of adsorbent (mg/g), C e is the equilibrium concentration of Hg 2+ or MeHg (mg/L). K F (mg 1 − n L n /g) and n are Freundlich constants indicating the relative adsorption capacity and adsorption intensity. 1/n and lnK F are the slope and intercept value of the linear Freundlich equation, respectively.
The Dubinin-Redushkevich (D-R) model is related to adsorption energy and can be expressed as the following equation [33,35]: where q e is the equilibrium adsorption capacity of adsorbent (mg/g), q m is the maximum adsorption capacity, C e is the equilibrium concentration of Hg 2+ or MeHg (mol/L), β is related to the mean adsorption energy, ε is the Polanyi potential, and R (J/mol/K) and T (K) are gas constant and the temperature, respectively. E (kJ/mol) is the mean adsorption energy of adsorption per molecule of adsorbent, when it is transferred from infinity in the solution to the solid surface [33]: The Temkin isotherm model is based on a hypothesis that the adsorption energy decreases linearly with the surface coverage. The Temkin isotherm can be expressed as the following equation [33]: where RT/b T = B T , q e is the equilibrium adsorption capacity of adsorbent (mg/g), q m is the maximum adsorption capacity, C e is the equilibrium concentration of Hg 2+ or MeHg (mg/L), A T and B T are the constants of the linear plot q e vs. lnC e .

Kinetic models
The adsorption kinetic of Hg 2+ and MeHg was studied according to pseudo-first-order and pseudo-second-order adsorption equations [35][36][37]. The pseudo-first-order equation is given as: where q t is the amount of adsorption at time t (mg/g), q e is the equilibrium adsorption capacity (mg/g), and k 1 (min −1 ) is the pseudo-first-order rate constant.
The pseudo-second-order equation is shown as: where k 2 (g mg −1 min −1 ) is the pseudo-second-order rate constant. Figure 1 shows the TEM images of four kinds of MWCNTs, in which no distinct changes in surface morphology was observed for the functionalized MWCNTs in comparison to the pristine MWCNTs.

FTIR spectroscopic characterization of MWCNTs
The FTIR spectrum were used to characterize the surface functional groups of MWCNTs, and the results were shown in Figure 2. While no major functional group was identified for the pristine MWCNTs, symmetric and asymmetric methylene stretching bands appeared at ~2962 and ~2884 cm −1 were detected in functionalized MWCNTs. Generally these functional groups located on the surface defect of the carbon nanotubes [38].
In the MWCNTs-COOH spectrum the characteristic peaks appeared at the ~3451 and ~1617 cm −1 , which are assigned to stretching vibrations of v(-OH) and v(C=O) of -COOH [34,39]. The peaks at ~3450 cm −1 correponded to -NH 2 stretching vibration in the MWCNTs-NH 2 spectrum. In addition, the presence of peaks at ~1415 and ~1262 cm −1 , origined from N-H in-plane and C-N bond stretching, respectively [39].
1 q e t buffers were adopted. The results shown in Figure 3 indicate that the pH had no significant effect on the removal of Hg 2+ and MeHg on MWCNTs-NH 2 . This is because the adsorption of Hg 2+ and MeHg onto MWCNTs-NH 2 is very strong that is independent of the initial pH. It is very important that the stability constant of the compounds of amino group and mercury can reach 10 18 [41]. It was found that the maximum removal efficiency is up to more than 92%, which is consist with the results in Ref. [41]. Figure 3 also shows that for pristine MWCNTs, MWCNTs-OH and MWCNTs-COOH, the removal efficiency of Hg 2+ and MeHg decreased gradually with the increase of pH. This is because with the increase of solution pH, the Hg(OH) + or MeHgOH compounds could be generated, which is stable in the water and reduced the Hg 2+ and MeHg adsorption [17,20]. It should be noted that a slight increase of removal efficiency of Hg 2+ and MeHg were observed for pH > 8.0. This might be attributed to the fact that when the solution pH is larger than the critial value pH pzc , mostly in the range of 4-6 [9,42], the negative charge surface, which can provide electrostatic interactions, enhances the absorption of Hg 2+ and MeHg [40].

Effect of NOM on adsorption
The impact of NOM on the adsorption of Hg 2+ and MeHg to MWCNTs seemed very complicated. There are at least The peaks at ~3439 and ~1265 cm −1 were attributed to -OH stretching vibration in the MWCNTs-OH spectrum [20]. Functional groups on the surface of the carbon nanotubes offered a lot of adsorption sites, which may be useful to increase the adsorption ability of carbon nanotubes.

Influence of initial pH on adsorption
The pH of solution had a great effect on the adsorption of metal ions [9,10,40]. To evaluate the effect of initial pH on the Hg 2+ and MeHg adsorption, a series of sample solutions with pHs ranging from 4.0 to 9.0 by phosphate

Effect of Cl − on adsorption
The effect of Cl − on adsorption of Hg 2+ and MeHg at pH 7.0 was shown in Figure 5, in which Cl − exhibited a strong inhibitory effect on the adsorption of Hg 2+ and MeHg. With the increase of Cl − concentration from 0 to 800 mmol/L, the removal efficiency of Hg 2+ and MeHg decreased sharply except MWCNTs-NH 2 , in which case the removal efficiency decreased gradually due to the strong adsorption of Hg 2+ and MeHg on MWCNTs-NH 2 .
It is well known that Hg 2+ and MeHg inclined to bind to Cl − to form complexes, which could reduce their sorption to MWCNTs. While HgCl 2 was the main existing form for Cl − concentration in the range of 5−115 mmol/L, HgCl 4 2− dominated when the concentration increased from 115 to 800 mmol/L ( Figure S1). High concentration of Cl − in solution could also promote the formation of very stable MeHgCl complex which showed lower sorption than MeHg to MWCNTs. These results agreed with those of previous studies [20,48]. de Diego et al. [48] found that a high concentration NaCl could led to declined adsorption rates of mercury species. Chen et al. [20] revealed that the adsorption of Hg 2+ dropped drastically from 94.3% to 1.5% as the increase of Cl − concentration from 0 to 1.0 mol/L and as the pH increasing from 4.3 to 10.5. Figure 6 shows the effect of adsorbent dose on the adsorption of Hg 2+ and MeHg at pH 7.0. The strong sorption of Hg 2+ to MWCNTs make it requires low dose of MWCNTs to remove Hg 2+ , and high (>90%) and almost equal removal efficiencies were obtained in the studied range of MWCNTs concentration (0.5−2.0 g/L) for all the four MWCNTs. Due to the relatively weak sorption of MeHg to MWCNTs, the removal efficiency four adsorbing processes in the solution: (i) NOM could combine with Hg 2+ and MeHg [25,43]; (ii) MWCNTs could adsorb Hg 2+ and MeHg [17,21,29,37,[44][45][46]; (iii) MWCNTs could adsorb NOM and form MWCNTs-NOM complexes [47]; and (iv) NOM could stabilize the dispersion of MWCNTs [26,27]. Depending on the characteristics of MWCNTs and NOM, as well as the aqueous chemistry parameters, these four processes showed different strength and therefore resulted in varied removal efficiency of Hg 2+ and MeHg.

Effect of adsorbent dose
The effect of NOM concentration on the adsorption of Hg 2+ and MeHg was studied at pH 7.0. As shown in Figure 4(a), the removal efficiency of Hg 2+ decreased gradually when the NOM concentration increased from 0 to 20 mg/L DOC. The influence of NOM on the adsorption of Hg 2+ can be understood from the following three aspects. Firstly, MWCNTs could adsorb NOM and Hg 2+ at the same time, which generated the competitive adsorption. Secondly, NOM occupied the binding sites on the MWCNTs, reducing the adsorption of Hg 2+ . Lastly, free NOM in the solution could increase the distribution of Hg 2+ in water, which could lead to reduced adsorption of Hg 2+ . Figure 4(b) shows that when the NOM concentration increased from 0 to 20 mg/L DOC, the removal efficiency of MeHg varied depending on the functional groups modified on MWCNTs. For MWCNTs and MWCNTs-OH, due to their relatively weak binding to MeHg, when low concentration of NOM was present, the NOM adsorbed on MWCNTs enhanced the adsorption and therefore removal of MeHg. For MWCNTs-NH 2 and MWCNTs-COOH that showed relatively strong combination with MeHg, when NOM was present, the NOM adsorbed on MWCNTs could not increase the adsorption of MeHg on MWCNTs; in contrast, the competitive binding of NOM to MeHg in the solution slightly reduced the removal efficiency of MeHg by MWCNTs. notes: in the solutions were spiked with 0.5 g/l different mWCnts, and 100 μg/l hg 2+ and mehg, respectively. the solution ph was adjusted by using 10 mmol/l phosphate buffer.

Adsorption isotherms
Adsorption isotherm was conducted by varying the initial concentration of Hg 2+ and MeHg from 10-500 μg/L ( Figures S2-S5), and obtained isotherm parameters of MeHg increased with the MWCNTs dose. The maximum value of removal efficiency was observed at 1.0 g/L MWCNTs-NH 2 for the strong complex between the -NH 2 and MeHg, and at about 2.0 g/L of the other three MWCNTs.  notes: in the solutions were spiked with 100 μg/l hg 2+ and mehg, respectively. the solution ph was adjusted to ph 7.0 by using 10 mmol/l phosphate buffer. notes: in the solutions were spiked with 0.5 g/l different mWCnts, and 100 μg/l hg 2+ and mehg, respectively. the solution ph was adjusted to ph 7.0 by using 10 mmol/l phosphate buffer.
of Hg 2+ , which indicated that the adsorption of MeHg agreed more closely with Langmuir isotherms. The Q m of MWCNT-COOH for Hg 2+ was 133.5 mg/g. The result was higher than that in the literature [20].
The coefficient K F in the Freundlich model, representing the adsorption capacity, shows the same order as the Langmuir coefficient Q m . The n values ranged between 0 and 10 for all the absorbents, revealing the favorable adsorption process. The Freundlich isotherm also indicated that the adsorption presented in the surface of absorbents for Hg 2+ was heterogeneity with the regression coefficient 0.862 to 0.960.
were shown in Tables 1 and 2. The R 2 values in Tables 1  and 2 indicate that the Freundlich model and Dubinin-Radushkevich model can describe the Hg 2+ adsorption very well, while the Langmuir and Dubinin-Radushkevich models showed the best fit to the MeHg adsorption. From the Langmuir model, the maximum adsorption capacities Q m of Hg 2+ and MeHg are in the order of MWCNTs-NH 2 > MWCNTs-COOH > MWCNTs-OH > MW CNTs. For the initial concentration of 100 μg/L Hg 2+ and MeHg, the values of R L were between 0 and 1, indicating the favorable adsorption onto the four kinds of MWCNTs. The R 2 of MeHg was closer to one than that  In the Dubinin-Radushkevich model, the mean free energy value E DR is used to identify adsorption mechanism. The E DR value is between 8 kJ/mol and 16 kJ/mol for the chemical adsorption process, and below 8 kJ/mol for physical adsorption [37]. As the E DR values ranged from 8.15 to 12.37 kJ/mol, the adsorption of Hg 2+ and MeHg to the four MWCNTs is the chemical adsorption. From the Temkin model, the adsorption energy B T shows the biggest value in the adsorption of Hg 2+ and MeHg on MWCNTs-NH 2 , suggesting that Hg 2+ and MeHg could adsorb to MWCNTs-NH 2 .

Adsorption kinetics
The pseudo-first-order and pseudo-second-order kinetics models were adopted, to describe the adsorption kinetics of Hg 2+ and MeHg to the MWCNTs ( Figures S6 and S7), and the regression coefficients and the parameters of the kinetic models are shown in Tables 3 and 4. Since the R 2 values of the pseudo-second-order kinetic model are closer to one than that of the pseudo-first-order kinetic model, the pseudo-second-order kinetic model is more suitable to describe the adsorption processes of the Hg 2+ and MeHg.

Conclusions
This work demonstrated that MWCNTs-NH 2 , MWCNTs-COOH, MWCNTs-OH and MWCNTs are very efficient adsorbents for removal of Hg 2+ and MeHg from aqueous solution, with the order of adsorption capacity as MWCNTs-NH 2 > MWCNTs-COOH > MWCNTs-OH > MWCNTs. For all the four MWCNTs, the adsorption of Hg 2+ and MeHg is pH dependent, and NOM and Cl − concentration also plays an important role. Increase of NOM concentration caused a significant reduction of the removal efficiency of Hg 2+ , while Cl − produced a strong inhibiting effect on the adsorption of Hg 2+ and MeHg. The adsorption isotherm model of Hg 2+ agreed with the Freundlich and Dubinin-Radushkevich isotherms very well, suggesting it is the heterogeneity and chemical sorption. The MeHg adsorption followed the Langmuir and Dubinin-Radushkevich isotherms, which belongs to the homogeneity and chemical adsorption.

Disclosure statement
No potential conflict of interest was reported by the authors.