Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2018 Oct 15;3(10):13183–13194. doi: 10.1021/acsomega.8b01837

Chitosan–Thiobarbituric Acid: A Superadsorbent for Mercury

Rahul Bhatt †, Shilpi Kushwaha ‡, Sreedhar Bojja §, P Padmaja †,*
PMCID: PMC6644366  PMID: 31458039

Abstract

graphic file with name ao-2018-018373_0014.jpg

In the present investigation, chitosan (CH) was supramolecularly cross-linked with thiobarbituric acid to form CT. CT was well characterized by UV, scanning electron microscopy–energy-dispersive X-ray analysis, Fourier transform infrared, NMR, differential scanning calorimetry, thermogravimetric analysis, and X-ray diffraction analyses, and its adsorption potential for elemental mercury (Hg0), inorganic mercury (Hg2+), and methyl mercury (CH3Hg+) was investigated. Adsorption experiments were conducted to optimize the parameters for removal of the mercury species under study, and the data were analyzed using Langmuir, Freundlich, and Temkin adsorption isotherm models. CT was found to have high adsorption capacities of 1357.69, 2504.86, and 2475.38 mg/g for Hg0, Hg2+, and CH3Hg+, respectively. The adsorbent CT could be reused up to three cycles by eluting elemental mercury using 0.01 N thiourea, inorganic mercury using 0.01 N perchloric acid, and methyl mercury with 0.2 N NaCl.

1. Introduction

Water is one of the basic necessities required for sustenance of life. Deteriorating water quality has become a global concern due to extensive industrial, agricultural, and domestic activities.1 For instance, apart from natural processes such as weathering of rocks, volcanic eruptions, and geothermic activities, anthropogenic activities such as burning of fossil fuel, metal mining and refining, cement production, municipal waste incineration, and natural gas processing have also resulted in the release of mercury species into the atmosphere.2

Mercury is on the list of priority pollutants by many international regulatory agencies due to its toxicity, persistence, and bioaccumulation in the food chain.3 Mercury exists in three different forms, namely, elemental mercury (Hg0), inorganic mercury (Hg2+), and organic mercury compounds (CH3Hg+). The most severe toxic effects are caused by bioaccumulation of monomethyl mercury (CH3Hg+), formed from inorganic mercury, by the activity of sulfate and iron-reducing bacteria in sediments and soils primarily under reducing conditions.4 Elemental mercury is well known as a hazardous atmospheric pollutant for several decades.

A number of remedial techniques have been employed for the removal of mercury such as oxidation, electrofloatation, chemical precipitation, ion exchange, coprecipitation, adsorption, reduction, flocculation, and dry sorbent injection, among which adsorption has been proved to be the most practical and economical choice for the removal of mercury from wastewater and from flue gases.5,6 Due to high volatility of Hg0 and low solubility in water, conversion of Hg0 to Hg2+ has become a significant focus in the development of appropriate mercury control technologies because Hg2+ is more water-soluble and better adsorbed onto adsorbent materials.7

One of the most commonly used adsorbents is activated carbon due to its excellent adsorption capacity for organic and inorganic pollutants. However, the high cost of activated carbon has restricted its widespread use. This has led many researchers around the world to search for alternative low-cost adsorbents such as seaweed, wool wastes, agricultural wastes, chitin, chitosan, and clay materials.8 Chitosan (CH), in particular, has drawn attention of various workers as an effective biosorbent due to its low cost, chemical stability, high reactivity, and excellent chelation capability. Chitosan has been modified by several methods to enhance its adsorption capacity for different types of pollutants.

According to the hard and soft acid–base theory, soft bases would be favorable ligands (mainly containing nitrogen or sulfur groups) for binding to soft acids such as Hg(II). Ligands such as glycine, poly(dimethylsiloxane), thiourea, and maleic anhydride have been used to modify chitosan.2,9−12 Zhang et al. used chitosan modified with iodine salt and H2SO4, whereas Zhang et al. used chitosan modified with bentonite as adsorbents for elemental mercury.10,11 Chitosan and its derivatives were also reported to effectively remove Hg(II).12

Karthikeyan et al. used thiobarbituric acid (TBA) as the releasing agent during complexometric determination of mercury.13 Kala et al. developed a multimode ratiometric Hg2+-selective chemosensor system based on a carbazole–thiobarbituric acid conjugation.14 Shahzad et al. also reported the preparation of barbituric acid- and thiobarbituric acid-based chitosan derivatives and used them for cytotoxic studies.15

Kushwaha et al. have investigated the potential of barbituric acid-derivatized chitosan for the uptake of inorganic, methyl, and phenyl mercury species.16 However, till date, the potential of a single adsorbent for the uptake of inorganic, organic, and elemental mercury has not been studied. To study the mercury uptake using chitosan derivatives, chitosan (CT) supramolecularly cross-linked with 2-thiobarbituric acid (TBA) has been synthesized, characterized, and evaluated for the removal of Hg0, Hg2+, methyl mercury (CH3Hg+).

2. Results and Discussion

Chitosan was cross-linked with TBA by supramolecular interactions as shown in Scheme 1. The formation of a yellow solution on mixing chitosan with TBA under heating indicated that the supramolecular assembly was induced by heating, and further hydrogen bond formation and precipitation were facilitated by adding the solution mixture to 0.04 N NaOH solution.17−19

Scheme 1. Synthesis of CT Adsorbent.

Scheme 1

Open in a new tab

2.1. Characterization

2.1.1. UV Analysis

The electronic absorption spectra of CT exhibited an absorption maximum at 226 nm, which can be assigned to the π → π* transition, whereas the absorption bands at 282 and 363 nm can be assigned to n → π* intraligand transitions (Figure S1, Supporting Information).

2.1.2. Fourier Transform Infrared (FTIR) Analysis

Figure 1 shows the FTIR spectra of CH, CT, and mercury-loaded CT, and the attributions to their respective absorption frequencies are summarized in Table S1 (Supporting Information).

Figure 1.

Figure 1

Open in a new tab

FTIR analysis of CH, CT, and mercury-loaded CT.

The IR peak at 3424 cm–1 can be attributed to N–H and the O–H stretching vibrations in CH, whereas the bands at 1382 and 1462 cm–1 accounted for the presence of C–N bending vibrations.

The IR spectra of TBA exhibited bands in the 1436–1239 cm–1 region that can be attributed to C–H deformation as well as C–N and C–S stretching vibrations.

Intense broad bands in the 3300–3500 cm–1 region can be attributed to N–H and the O–H stretching vibrations in CT. The absence of peak at around 3600 cm–1 can be attributed to the keto-tautomer of TBA,20 evidencing the formation of CT. Furthermore, the absence of bands in the 2600–2500 cm–1 region for (SH) supports the presence of thione.21 The shoulder at ∼1700 cm–1 can be attributed to the C=O group of thiobarbituric acid. The 1640 cm–1 band could be due to the overlap of C=O and C=C group frequencies of CH and TBA. The IR bands in the ∼1640–1600 cm–1 region were broad, suggesting supramolecular interactions such as hydrogen bonds between CH and TBA in CT. Furthermore, the peak at 666 cm–1 indicated the presence of δ-ring in CT, whereas the peak at 1061 cm–1 indicated the presence of the N–C=S stretching.

There were either slight shifts in characteristic peaks of CT or change in intensities after adsorption of mercury species under study onto CT. The amide III band shifted from 1319 cm−1 in CT to ∼1304, 1302 and 1301 cm−1 in CT-Hg0, CT-Hg2+, and CT-CH3Hg+, respectively.15

2.1.3. Scanning Electron Microscopy–Energy-Dispersive X-ray (SEM–EDX) Analysis

The EDX spectra (Figure 2) showed the presence of sulfur in CT, indicating the incorporation of 2-thiobarbituric acid into chitosan. Furthermore, the presence of mercury peaks after adsorption of inorganic, methyl, and elemental mercury confirmed the adsorption of all of the three mercury species onto CT (Table S2, Supporting Information).

Figure 2.

Figure 2

Open in a new tab

EDX spectra of (a) CT, (b) CT-Hg0, (c) CT-Hg2+, and (d) CT-CH3Hg+.

2.1.4. NMR Analysis

The NMR spectrum of CT and proposed structure of CT are shown in Figure 3 and Scheme 1, respectively. The peaks at 2.05–2.07 ppm were attributed to the three methyl protons of N-acetyl glucosamine, and those at 3.18 ppm, to the 2 protons (H2) of glucosamine.22 The overlapping signals from 3.7 to 3.9 ppm were attributed to protons connected to the nonanomeric C3–C6 carbons in the glucopyranose ring of chitosan chain, and the signal at 5.029 ppm was attributed to the anomeric proton and to the proton connected to C5 of the pyrimidine ring of thiobarbituric acid.15

Figure 3.

Figure 3

Open in a new tab

NMR analysis.

2.1.5. Differential Scanning Calorimetry (DSC) Analysis

DSC thermal analysis curves are shown in Figure 4. The two exothermic peaks at 279 and 302 °C in CH and 285 and 301 °C in CT were attributed to the thermal decomposition of amino residues. Furthermore, no exothermic peak was observed at around 400 °C, thereby indicating that the N-acetyl groups were not present on the polymer matrix and only amino residues were observed, which are less thermally stable than the N-acetyl ones.23

Figure 4.

Figure 4

Open in a new tab

DSC analysis.

2.1.6. Thermogravimetric Analysis (TGA) Analysis

Both CH and CT exhibited similar thermal behavior (Figure S2, Supporting Information). The weight loss in the 40–100 °C region was due to the loss of adsorbed water or that absorbed in the inner polymeric network. The weight loss that occurred at ∼200–300 °C was slightly less in CT as compared to that in CH, which could be due to the supramolecular cross-linking of thiobarbituric acid in the polymer structure, leading to slower thermal degradation of the biopolymer. The weight loss observed at 300–400 °C can be ascribed to degradation of polysaccharide units and breakdown of the glucopyranose ring.24 The weight loss in the 400–500 °C range could be due to emission of volatile products, such as CO and CO2, and the formation of the carbon residue. The data has been summarized in Table S3 (Supporting Information).

2.1.7. X-ray Diffraction (XRD) Analysis

Figure 5 shows XRD analysis of CH, CT, and mercury-loaded CT samples. The XRD pattern of CT showed a broad peak at around 2θ = 20°, indexed as the 101 plane of crystal form I, which is present due to the amorphous state of chitosan. The higher peak intensity for CT at 19.93° could be attributed to the intermolecular hydrogen bonding patterns and crystalline structure.25 Furthermore, the position of the peak at 19.93° for CT shifted to 21.95, 19.98, and 20.27° on interaction with Hg0, Hg2+, and CH3Hg+, respectively. Furthermore, the binding of the sulfur species with Hg0, Hg2+, and CH3Hg+, respectively, was also supported with the generation of new peaks at 46.55, 41.21, and 42.73°, respectively26 (Table S4, Supporting Information)

2.1.7. 1

The crystalline index was calculated to be 50.07% using expression 1,27 where I110 denotes the maximum intensity at ∼20° and Iam is the intensity of amorphous diffraction at 16°, whereas the crystallite size was found to be 1.24 nm for the peak at 19.939° using Scherrer’s expression.28

Figure 5.

Figure 5

Open in a new tab

XRD analysis.

2.1.8. X-ray Photoelectron Spectroscopy (XPS) Analysis

In CT, the binding energy at 164.6 eV corresponded to organically bound sulfur.29 The typical S 2p3/2 binding energy for unbound thiols was between 163 and 164 eV, suggesting that the thiol group of thiobarbituric acid is free after layering with chitosan.30 After the adsorption of Hg(II), the appearance of peak at 161.7 eV suggested binding of thiol group to mercury, whereas the peak at 163.2 eV corresponded to the free thiol group. Similar signals were observed after adsorption of elemental mercury and methyl mercury (Figure S3, Supporting Information).

The carbon peak at 284.6 eV in CT was observed at lower binding energy than that for the mercury-bound species, which supported the adsorption of the mercury species onto the adsorbent. The nitrogen peak at ∼401 eV in CT-Hg2+ was observed at a higher binding energy due to donation of the lone pair of electrons on the nitrogen atom to a heavy metal ion and the resultant reduction of electron cloud density surrounding the N atom.31 Corresponding peaks were also observed in CT-Hg0 and CT-CH3Hg+. The binding energies of the Hg 4f7/2 level in the XPS spectra of CT-Hg2+ were observed at 100.4 and 101.6 eV, respectively. The two Hg(II) species were attributed to different coordination environments and the electronegativity of nitrogen and sulfur atoms.32 Similar signals were observed after adsorption of elemental mercury and methyl mercury. The mercury signals in CT-Hg0 at binding energies 100.8 and 101.9 eV suggested that mercury bonded to CT was present as Hg2+. Detailed assignments are tabulated in Table 1.

Table 1. XPS Analysis.
sample name   binding energy (eV) FWHM (eV) area % inference
CT C 1s 284.6 1.960 10 320.7 52.2 C–C, C=C, C–H of aromatic bonds
C 1s 286.388 1.909 6803.6 34.4 C–S, C–N, C=N
C 1s 288.161 2.036 2652.6 13.4 C–OH
O 1s 532.393 2.489 9081.1 65.1 C–O–C
O 1s 533.338 2.105 4869.4 34.9 C–OH
N 1s 399.294 1.742 1367.8 70.1 nitrogen with unsaturated chemical bonds –N=
N 1s 400.493 1.640 583.9 29.9 aliphatic nitrogen of chitosan
S 2p 164.640 1.775 188.8 65.7  
S 2p 166.057 1.638 98.6 34.3 organically bound sulfur
CT-Hg0 C 1s 284.617 2.375 2602.1 60.9 C–C, C=C, C–H of aromatic bonds
C 1s 286.472 2.101 1162.3 27.2 C–S, C–N, C=N
C 1s 288.167 2.006 509.6 11.9 C–OH
O 1s 532.160 2.260 2138.9 72.9 C–O–C
O 1s 533.346 2.140 797.1 27.1 C–OH
N 1s 398.963 1.634 257.6 43.4 nitrogen with unsaturated chemical bonds –N=
N 1s 400.106 1.805 200.6 33.8 aliphatic nitrogen of chitosan
N 1s 401.534 1.905 135.0 22.8 N–Hg/protonated NH2 of chitosan
S 2p 162.228 1.952 230.6 63.3 binding of thiol group to mercury
S 2p 163.722 1.691 134.0 36.7 free thiol group/organically bound sulfur
Hg 4f 100.824 1.831 2338.2 41.5 mercuric ion bound to nitrogen
Hg 4f 104.826 1.872 1906.6 33.9  
Hg 4f 101.966 1.625 758.2 13.5 mercuric ion bound to sulfur
Hg 4f 105.934 1.638 625.1 11.1  
CT-Hg2+ C 1s 284.662 2.007 6869.9 63.5 C–C, C=C, C–H of aromatic bonds
C 1s 286.172 1.975 2705.9 25.0 C–S, C–N, C=N
C 1s 288.085 2.085 1249.0 11.5 C–OH
O 1s 532.376 2.189 3614.5 66.6 C–O–C
O 1s 533.802 2.076 1814.3 33.4 C–OH
N 1s 398.788 1.583 181.2 50.6 nitrogen with unsaturated chemical bonds –N=
N 1s 400.107 1.482 102.2 28.6 aliphatic nitrogen of chitosan
N 1s 401.223 1.591 74.5 20.8 N–Hg/protonated NH2 of chitosan
S 2p 161.738 1.668 168.0 66.2 S bound to mercury/organically bound sulfur
S 2p 163.218 1.542 85.6 33.8 free thiol group
Hg 4f7/2 100.404 1.955 759.6 39.0 mercuric ion bound to nitrogen
Hg 4f5/2 104.511 2.009 574.3 29.5  
Hg 4f7/2 101.675 1.789 396.1 20.3 mercuric ion bound to sulfur
Hg 4f5/2 105.580 1.811 217.5 11.2  
CT-CH3Hg+ C 1s 284.660 1.958 1525.3 54.5 C–C, C=C, C–H of aromatic bonds
C 1s 286.287 1.966 915.6 32.7 C–S, C–N, C=N
C 1s 288.192 2.171 355.5 12.7 C–OH
O 1s 532.370 2.188 4261.2 57.7 C–O–C–O–C
O 1s 533.651 2.068 3121.6 42.3 C–OH
N 1s 399.631 2.099 677.1 62.1 nitrogen with unsaturated chemical bonds –N=
N 1s 401.021 2.249 414.0 37.9 N–Hg/protonated NH2 of chitosan
S 2p 162.522 1.727 326.4 66.8 sulfur bound to mercury
S 2p 164.055 1.787 162.5 33.2 free thiol group/organically bound sulfur
Hg 4f7/2 100.883 1.932 2049.8 35.2 mercuric ion bound to nitrogen
Hg 4f5/2 104.841 1.618 1350.6 23.2  
Hg 4f7/2 102.218 1.883 1365.5 23.4 mercuric ion bound to sulfur
Hg 4f5/2 106.043 1.718 1061.3 18.2  
Open in a new tab

2.2. Batch Adsorption Studies

2.2.1. Effect of pH

It was noticed that the adsorption capacity did not change significantly with pH for all species of mercury under study (Figure 6). ζ-Potential analysis of CT (Figure S4, Supporting Information) revealed that the surface was negatively charged beyond pH 3. The results suggest that electrostatic attraction was not playing a role in the adsorption process. The adsorbent degraded in the medium with pH less than 2.

Figure 6.

Figure 6

Open in a new tab

Effect of pH variation.

2.2.2. Sorption Isotherm

It was observed that as the concentration of Hg0 was increased from 139 to 1395 mg/m3, almost all of the vapors were adsorbed onto the surface of the adsorbent (Figure S5, Supporting Information). The isotherm model analysis was carried out to determine the adsorption capacity of CT for Hg0, Hg2+, and CH3Hg+. The linear and nonlinear fitting curves of sorption isotherms are reported in Figures S6 and S7 (Supporting Information), respectively. The corresponding constants are presented in Table S5 (Supporting Information). The adsorption data for the adsorbent gave reasonably high correlation coefficient values for the models studied. The Freundlich model was observed to be the best fit with experimental Qe values close to calculated Qe values, suggesting heterogeneous nature of the adsorbent. The Freundlich constant n was observed to be more than unity, implying that the adsorption intensity was favorable over the entire range of concentrations studied. Furthermore, the high Kf value (indicator for adsorption capacity and intensity) calculated from the Freundlich model indicated that all of the three mercury species could be readily absorbed by CT.32

It can be further seen that the Langmuir isotherm model also fitted with high correlation coefficients 0.9938, 0.9901, and 0.9987, respectively. However, the calculated Qe values were observed to deviate significantly from experimental Qe values. This could be attributed to a multilayer adsorption process. The positive BT value calculated from the Temkin model indicated an exothermic adsorption process.

2.2.3. Effect of Time

It was noticed that the adsorption capacity changed with an increase in time (Figure 7). The results obtained revealed that CT exhibited the highest adsorption capacity at about 3 h for methyl mercury as well as inorganic mercury, whereas Hg0 attained equilibrium on the surface of the adsorbent after a period of 10 min.

Figure 7.

Figure 7

Open in a new tab

Effect of Time Variation on the adsorption of the mercury species under study onto CT.

2.2.4. Sorption Kinetics

To analyze the adsorption kinetics of the mercury species under study, the pseudo-first-order, pseudo-second-order, intraparticle, and Elovich models were applied to the data (Figure S8, Supporting Information) and the rate constants for the adsorption of Hg0, Hg2+, and CH3Hg+ by the adsorbents are presented in Table S6 (Supporting Information). Figure S9 (Supporting Information) shows the fitting of experimental data with the data calculated from the kinetic models for adsorption of the mercury species under study onto CT.

The correlation coefficients for the pseudo-second-order model were found to be greater than those for the other kinetic models for Hg0, Hg2+, and CH3Hg+, and the calculated equilibrium adsorption capacity values were closer to those obtained experimentally. This supported the assumption behind the model that the surface complexation may be the rate-limiting step involving valence forces through sharing or exchanging of electrons between adsorbent and adsorbate.33

The adsorption mechanism of an adsorbate onto the adsorbent can be described by three steps, viz., film diffusion, pore diffusion, and intraparticle diffusion.34 The slowest of the three steps controls the overall rate of the adsorption process. The adsorption step that controls the batch process for most of the contact time is intraparticle diffusion.35 In the present study, deviation of straight lines from the origin was observed, suggesting that intraparticle diffusion is not the sole rate-limiting step. A significant deviation between the estimated and experimental values of the Elovich model suggested that this model cannot be used to describe the kinetics of adsorption of mercury species onto CT.

2.2.5. Desorption Studies

Desorption studies (Figure 8) were carried out using different eluents for the recovery of elemental, inorganic, and methyl mercury after adsorption onto CT. The adsorbent (0.05 g) loaded with 692.16 mg/g Hg0, 1249.45 mg/g Hg2+, and 1248.27 mg/g CH3Hg+, respectively, was added to 25 mL of the desired eluents in a conical flask and agitated for 180 min. The supernatant was separated from the adsorbent by filtration, and the filtrate was analyzed for the presence of desorbed mercury.

Figure 8.

Figure 8

Open in a new tab

Desorption Studies of CT-Hg0, CT-Hg2+, and CT-CH3Hg+.

It was observed that about 95 and 78% of Hg2+ and Hg0, respectively, could be eluted using 0.01 N HClO4, whereas CH3Hg+ was not eluted using HClO4. On the other hand, CH3Hg+ was quantitatively desorbed using 0.2 N NaCl, whereas about 82–85% of Hg2+ and Hg0 were eluted using 0.01 N thiourea.

Figure S10 (Supporting Information) the adsorption cycles were studied by subjecting the dry adsorbent obtained from the previous cycle to 697 mg/m3 Hg0 vapors and 500 ppm Hg2+ and 500 ppm CH3Hg+ solutions, respectively. The desorption cycles were carried out for the adsorbents loaded with Hg0, Hg2+, and CH3Hg+ using 0.01 N thiourea, 0.01 N HClO4, and 0.2 N NaCl, respectively. The studies revealed that the adsorbent could be used for three cycles. The % uptake decreased from about 94 to 65, 99 to 75, and 99 to 78% for Hg0, Hg2+, and CH3Hg+, respectively, after the use of CT for three cycles.

2.2.6. Comparison with Other Adsorbents

The Qmax values that were obtained from the Freundlich isotherm were compared with those of other adsorbents, and it was observed that the adsorption capacity of CT superseded that of most of the adsorbents reported in the literature for all of the mercury species under study (Table 2). The adsorbent used in the present study, CT, was found to have high adsorption capacities of 1357.69, 2504.86, and 2475.38 mg/g for Hg0, Hg2+, and CH3Hg+, respectively. Yan et al. have reported a very high adsorption capacity of about 2000 mg/g Hg2+ using ZnS nanocrystals.36 Arshadi et al. reported that an immobilized dendritic adsorbent containing l-cysteine methyl ester onto the nanoparticles of mixed oxides has remarkable adsorption capacity and affinity toward mercury ions in water (3079 and 3232 mg/g).37,38 Furthermore, there are no reports where the potential of a single adsorbent has been studied for all three mercury species.

Table 2. Comparison with Other Adsorbents.
Sr. No. adsorbent metal pH Qmax (mg/g) references
1 CTS-I-S-410 Hg0   69 μg/g (10)
2 porous sulfur copolymer foams Hg0   151 μg/g (45)
3 biochar modified by nonthermal plasma Hg0   583.0 μg/g (46)
4 chitosan–poly(vinyl alcohol) Hg2+ 5.5 585 mg/g (47)
5 chitosan–thiourea Hg2+ 5 625.2 mg/g (48)
6 commercial chitosan Hg2+ 4 1127 mg/g (49)
7 propylthiol-functionalized mesoporous silica Hg2+ 3 560 mg/g (50)
8 chitosan–thioglyceraldehyde Schiff’s base cross-linked magnetic resin (CSTG) Hg2+ 5 98 ± 2 mg/g (51)
9 N-(2-hydroxy-3-mercapto propyl)-chitosan Hg2+ 4.5 588 mg/g (52)
10 chitosan cross-linked with glutaraldehyde and functionalized with magnetic nanoparticles-Fe3O4 (CSm) Hg2+ 5 152 mg/g (53)
11 poly(methacrylic acid)-grafted chitosan–bentonite nanocomposite (MACB) Hg2+ 6–7 125 mg/g (54)
12 chitosan-aluminum oxide composite Hg2+   900 mg/g (55)
13 cysteine–chitosan (CC) Hg2+ 5.5 1000 mg/g (56)
14 glutaraldehyde cross-linked chitosan Hg2+ 4.5 0.0077 mg/g (16)
MeHg+ 6–8 0.0089 mg/g
15 barbital–glutaraldehyde cross-linked chitosan Hg2+ 7 0.0065 mg/g
MeHg+ >8 0.0072 mg/g
16 Fe3O4@SiO2–RSH MeHg+ 8 14.40 μg/mg (57)
17 ZnS NCs on the surface of α-Al2O3 Hg2+ 4–6 2000 mg/g (36)
18 CT Hg0   1357.69 mg/g this paper calculated from Freundlich
Hg2+ 4 2504.86 mg/g
MeHg+ 4 2475.38 mg/g
Open in a new tab

3. Mechanism

The protonated form of TBA, i.e., HTBA, can be protonated or deprotonated depending upon the pH of the solution.39

3.
3.

Chitosan could be cross-linked with thiobarbituric acid through C=O···HN and C=S···HN hydrogen bonding.

Methyl mercury and inorganic mercury species could be covalently bound to sulfur and nitrogen group of thiobarbituric acid to form a stable chelate.40 The binding is also corroborated by XPS analysis.

CT further proved to be an efficient adsorbent for elemental mercury too. XPS studies indicated that mercury is in the +2 oxidation state after adsorption. The plausible mechanism could be that elemental mercury self-assembled onto CT, which further led to oxidation of mercury to Hg2+ species and complexation.41 Thiols are known to adsorb onto surfaces of soft metals such as Hg, Au, and Ag to form self-assembled monolayers of metal–thiol adducts, which further led to oxidation of Hg0(aq) and the formation of Hg+/2+–thiol complexes. Cohen-Atiya and Mandler suggested a two-step reaction mechanism involving (1) adsorption and transfer of negative charges from the thiol group to the metal surface and (2) transfer of the charge accumulated on the metal surface to electron acceptors.42 Gu et al. postulated that the reaction between Hg0(aq) and −SH in dissolved organic matter (DOM) may involve initial physicosorption of Hg0(aq) to −SH, followed by S–H bond cleavage and charge transfer, leading to oxidative complexation of Hg2+ by DOM. Possible electron acceptors could include H+ (from dissociated thiols), quinone moieties in DOM, and O2 under anoxic conditions.43 Zheng et al. have reported oxidation of elemental mercury by thiol compounds under anoxic conditions.44

4. Adsorption Study of Different Metals

Figure 9 shows the uptake of other heavy metals such as Cu2+, Cd2+, Pb2+, and Zn2+ onto CT. The metal ion solution (25 mL, 100 ppm) under study maintained at pH 4 was agitated with 0.01 g of CT for 3 h on an orbital shaker. It was observed that the adsorption of Hg2+ and CH3Hg+ was more on the adsorbent as compared to that on copper, cadmium, lead, and zinc.

Figure 9.

Figure 9

Open in a new tab

Uptake of Different Metals onto CT.

Furthermore, the interference of different metals on the mercury uptake was investigated (Figure 10), by taking 25 mL of solution containing 25 ppm Hg2+/CH3Hg+ and Cu2+, Cd2+, Pb2+, and Zn2+ at pH 4 and agitating with 0.01 g of CT for 3 h on an orbital shaker. It was observed that there was no significant change in the uptake of CH3Hg+ and Hg2+ in the presence of copper, cadmium, lead, and zinc.

Figure 10.

Figure 10

Open in a new tab

Uptake of different metals in a synthetic mixture containing Hg2+, CH3Hg+, Cu2+, Cd2+, Pb2+ and Zn2+.

5. Conclusions

CT proved to be a superadsorbent for Hg0, Hg2+, and CH3Hg+. The binding of mercury species with the S moiety in CT was considered to be the main factor responsible for the high mercury uptake capacities. More than 99% of total mercury was removed within 180 min, suggesting instantaneous binding of mercury on surface sites followed by slower and nonspecific binding, resulting in high adsorption capacities of 1357.69, 2504.86, and 2475.38 mg/g for Hg0, Hg2+, and CH3Hg+, respectively. Hg0, Hg2+, and CH3Hg+ could be quantitatively eluted from CT using 0.01 N thiourea, 0.01 N HClO4, and 0.2 N NaCl, respectively.

6. Experimental Section

6.1. Materials

Chitosan (from crab shells) (75–85% deacetylated) (mean molecular weight ∼ 190 000–310 000 Da reported by the viscometric method) was purchased from Sigma-Aldrich. 2-Thiobarbituric acid (98%) (molecular weight = 144.15 g/mol) was purchased from Spectrochem. All chemicals were used as received without any further purification.

6.2. Preparation of Chitosan–Thiobarbituric Acid [CT] Adsorbent

For the preparation of the chitosan–thiobarbituric acid [CT] adsorbent (Scheme 1), a solution of 1% chitosan in 1% glacial acetic acid was prepared by dissolving 1 g of chitosan flakes in 1% glacial acetic acid. A 1% solution of 2-thiobarbituric acid in double-distilled water was prepared. Both the solutions were mixed in a ratio of 1:1 and then stirred for 4 h at 50–55 °C when a yellow solution was obtained. The solution obtained was then injected dropwise into 0.4 N NaOH solution to facilitate hydrogen bond formation and precipitation. The obtained material was then filtered, washed with double-distilled water until neutral, and then air-dried. The adsorbent was then used as such as adsorbent for the mercury uptake studies.

The adsorbent CT was characterized using UV, SEM–EDX, FTIR, NMR, DSC, TGA, and XRD techniques.

UV analysis of the adsorbent was performed using a JASCO V630 model UV-visible spectrophotometer within the 200–1100 nm range. FTIR spectra of the adsorbent (CT) and metal-loaded adsorbents (CT-Hg0, CT-Hg2+, and CT-CH3Hg+) were obtained using a PerkinElmer RX1 FTIR spectrometer in the wavenumber range of 500–4000 cm–1. SEM–EDX analysis of the adsorbent and metal-loaded adsorbent was done using a JSM 5610LV JEOL instrument. NMR analysis of CT was performed using a Bruker Avance-400 MHz instrument. DSC analysis of the adsorbent was done using a Mettler 822E DSC instrument from 30 to 550 °C under a nitrogen atmosphere. Thermogravimetric analysis of the adsorbent was done using an EXSTAR6000 TG/DTA 6300 instrument from 30 to 550 °C under a nitrogen atmosphere. XRD analysis of the adsorbent and metal-loaded adsorbent was done using a Bruker D2 Phaser model XRD instrument. The radiation source used was Cu Kα 1.5414 Å. XPS analysis of the adsorbent was done using a Kratos AXIS Ultra HSA X-ray photoelectron spectrometer. The ζ-potential of the adsorbent was analyzed using a Zetasizer ZetaPlus 90Plus/BI-MAS BrookHaven instrument at different pHs.

6.3. Preparation of Metal Solutions

Stock solutions of mercury (1000 ppm) were prepared by dissolving 1.379 g (98% pure) and 1.264 g (98% pure) of mercury(II) chloride and methylmercuric(II) chloride, respectively, in double-distilled water and made up to 1 L. Working standards were prepared by diluting different volumes of the respective stock solutions using double-distilled water to obtain the desired concentration.

6.4. Batch Adsorption Experiments

The adsorption potential of CT for the different mercury species was investigated by batch experiments. The effect of various parameters on the removal of inorganic and methyl mercury onto CT was investigated. For each experiment, 25 mL of Hg2+ and CH3Hg+ solutions, respectively, of known initial concentration and pH were taken in 100 mL conical flasks. A suitable adsorbent dose was added to each solution, and the mixture was shaken at a constant speed. The supernatant was separated from the adsorbent by the filtration method and analyzed for the presence of unadsorbed Hg2+ by the cold vapor atomic absorption spectroscopy technique (AAnalyst 200, PerkinElmer). The experimental procedure followed for the effect of pH (6.4.1) and effect of time (6.4.2) on the uptake of mercury species has been discussed in the Supporting Information.

The mercury uptake by CT was calculated as follows (eq 2)

6.4. 2

where Ci (mg/L) is the initial concentration of the metal ion, Ce (mg/L) is the final concentration of the metal ion, m (g/L) is the mass of the dried adsorbent, and qe (mg/g) is the adsorption capacity. The experiments performed without the adsorbent were treated as blank. Furthermore, to ensure the reproducibility of results, experiments were performed in triplicate and the data was presented as the mean value of these independent experiments.

6.5. Elemental Mercury Uptake Procedure

The experimental setup is as shown in Scheme 2. A 10 mL solution containing Hg2+ of desired concentration was taken in the sample tube, and 3% NaBH4 solution prepared in 1% NaOH was added to the sample tube to generate Hg0. The N2 gas pressure was set at 25 psig to obtain a flow rate of 0.350 L/min containing Hg0 (13.9–139 mg/m3). The N2 stream containing Hg0 was allowed to pass through an additional sample tube containing fixed weight (0.01 g) of the adsorbent for a fixed time interval at room temperature. The effect of dose was also studied where the amount of adsorbent was varied from 0.01 to 0.05 g. The unadsorbed mercury that reached the quartz tube atomizer (QTA) was measured.

Scheme 2. Elemental Mercury Uptake Procedure.

Scheme 2

Open in a new tab

The Hg0 removal efficiency was calculated as follows (eq 3)

6.5. 3

where C0 is the initial concentration of Hg0 and C is the final concentration of Hg0 reaching the QTA.

In each experiment, the mercury inlet gas was passed through the sorbent bed into the analytical system, until the inlet mercury concentration and the detected mercury were observed to be the same.

For calculation of the breakthrough capacity for Hg0 uptake (eq 4), the mercury mass balance equation was used to show that the mass of inflow mercury was equal to the sum of the mass of mercury adsorbed and the mass of mercury in off-gas.

6.5. 4

where Cin and Cout are the initial (mg/m3) and off-gas mercury concentrations, respectively, Q is the gas flow (mL/min = m3/min), tf is the time of adsorption experiment (min), mad is the mass of adsorbent (g), and Cad is mercury concentration in the adsorbent (mg Hg/gad).

The standard deviation was calculated as follows (eq 5)

6.5. 5

The experimental details of the effect of pH, time, adsorption isotherms, and kinetics models studied for the uptake of the mercury species have been discussed in the Supporting Information.

Acknowledgments

The authors are thankful to University Grants Commission for financial assistance vide UGC-MRP (39-718/2010(SR) dated 13 Jan 2011), DST-FIST programme, Department of Chemistry, M S University of Baroda for NMR facility, SAIF Chandigarh for XRD studies, ERDA for SEM Analysis, Department of Metallurgy, M. S. University of Baroda for SEM−EDX analysis, SPARC, Vadodara, for DSC studies, and Department of Chemistry, M. S. University of Baroda, Vadodara, for laboratory facilities.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01837.

  • UV analysis, TGA analysis, XPS analysis, ζ-potential analysis, breakthrough capacity curve for Hg0, sorption isotherms, sorption kinetics, adsorption and desorption cycles, isotherm and kinetic model table, FTIR analysis table, EDX analysis table, TGA analysis table, XRD analysis table, sorption isotherm table, sorption kinetics table, and references (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao8b01837_si_001.pdf (1.9MB, pdf)

References

  1. Bhatnagar A.; Sillanpää M. Applications of Chitin- and Chitosan-Derivatives for the Detoxification of Water and Wastewater — A Short Review. Adv. Colloid Interface Sci. 2009, 152, 26–38. 10.1016/j.cis.2009.09.003. [DOI] [PubMed] [Google Scholar]
  2. Vieira R. S.; Oliveira M. L. M.; Guibal E.; Rodríguez-Castellón E.; Beppu M. M. Copper, Mercury and Chromium Adsorption on Natural and Crosslinked Chitosan Films: An XPS Investigation of Mechanism. Colloids Surf., A 2011, 374, 108–114. 10.1016/j.colsurfa.2010.11.022. [DOI] [Google Scholar]
  3. Lu X.; Huangfu X.; Zhang X.; Wang Y.; Ma J. Strong Enhancement of Trace Mercury Removal from Aqueous Solution with Sodium Thiosulfate by in Situ Formed Mn- (Hydr) Oxides. Water Res. 2014, 65, 22–31. 10.1016/j.watres.2014.07.016. [DOI] [PubMed] [Google Scholar]
  4. Jonsson S.; Skyllberg U.; Nilsson M. B.; Westlund P.; Shchukarev A.; Lundberg E.; Bjo E. Mercury Methylation Rates for Geochemically Relevant HgII Species in Sediments. Environ. Sci. Technol. 2012, 46, 11653–11659. 10.1021/es3015327. [DOI] [PubMed] [Google Scholar]
  5. Zhang J. R.; Huang W. T.; Zeng A. L.; Luo H. Q.; Li N. B. Ethynyl and π-Stacked Thymine–Hg2+–thymine Base Pairs Enhanced Fluorescence Quenching via Photoinduced Electron Transfer and Simple and Sensitive Mercury Ion Sensing. Biosens. Bioelectron. 2015, 64, 597–604. 10.1016/j.bios.2014.09.092. [DOI] [PubMed] [Google Scholar]
  6. Monterroso R.; Fan M.; Argyle M. Mercury Removal. Coal Gasif. Appl. 2011, 247–292. 10.1016/b978-0-8155-2049-8.10009-9. [DOI] [Google Scholar]
  7. Li X.; Zhang L.; Zhou D.; Liu W.; Zhu X.; Xu Y.; Zheng Y.; Zheng C. Elemental Mercury Capture from Flue Gas by a Supported Ionic Liquid Phase Adsorbent. Energy Fuels 2017, 31, 714–723. 10.1021/acs.energyfuels.6b01956. [DOI] [Google Scholar]
  8. Wu F. C.; Tseng R.; Juang R. A Review and Experimental Verification of Using Chitosan and Its Derivatives as Adsorbents for Selected Heavy Metals. J. Environ. Manage. 2010, 91, 798–806. 10.1016/j.jenvman.2009.10.018. [DOI] [PubMed] [Google Scholar]
  9. Vieira R. S.; Beppu M. M. Interaction of Natural and Crosslinked Chitosan Membranes with Hg(II) Ions. Colloids Surf., A 2006, 279, 196–207. 10.1016/j.colsurfa.2006.01.026. [DOI] [Google Scholar]
  10. Zhang A.; Xiang J.; Sun L.; Hu S.; Li P.; Shi J.; Fu P.; Su S. Preparation, Characterization, and Application of Modified Chitosan Sorbents for Elemental Mercury Removal. Ind. Eng. Chem. Res. 2009, 48, 4980–4989. 10.1021/ie9000629. [DOI] [Google Scholar]
  11. Zhang A.-c.; Sun L.; Xiang J.; Hu S.; Fu P.; Su S.; Zhou Y. Removal of Elemental Mercury from Coal Combustion Flue Gas by Bentonite-Chitosan and Their Modifier. J. Fuel Chem. Technol. 2009, 37, 489–495. 10.1016/S1872-5813(10)60005-9. [DOI] [Google Scholar]
  12. Shan C.; Ma Z.; Tong M.; Ni J. Removal of Hg (II) by Poly (1-Vinylimidazole)-Grafted Fe3O4@ SiO2 Magnetic Nanoparticles. Water Res. 2015, 69, 252–260. 10.1016/j.watres.2014.11.030. [DOI] [PubMed] [Google Scholar]
  13. Karthikeyan J.; Parameshwara P.; Shetty N. A.; Shetty P. Complexometric Determination of Mercury (II) Using 2-Thiobarbituric Acid as a Releasing Agent. Indian J. Chem. Technol. 2007, 14, 209–211. [Google Scholar]
  14. Kala K.; Vineetha P. K.; Manoj N. A Simple Cost Effective Carbazole-Thiobarbituric Acid Conjugate as a Ratiometric Fluorescent Probe for Detection of Mercury(Ii) Ions in Aqueous Medium. New J. Chem. 2017, 41, 5176–5181. 10.1039/C7NJ00805H. [DOI] [Google Scholar]
  15. Shahzad S.; Shahzadi L.; Mahmood N.; Siddiqi S. A.; Rauf A.; Manzoor F.; Chaudhry A. A.; Rehman I. u.; Yar M. A New Synthetic Methodology for the Preparation of Biocompatible and Organo-Soluble Barbituric- and Thiobarbituric Acid Based Chitosan Derivatives for Biomedical Applications. Mater. Sci. Eng., C 2016, 66, 156–163. 10.1016/j.msec.2016.04.056. [DOI] [PubMed] [Google Scholar]
  16. Kushwaha S.; Sudhakar P. P. Adsorption of Mercury(II), Methyl Mercury(II) and Phenyl Mercury(II) on Chitosan Cross-Linked with a Barbital Derivative. Carbohydr. Polym. 2011, 86, 1055–1062. 10.1016/j.carbpol.2011.06.028. [DOI] [Google Scholar]
  17. Duan J.; Liang X.; Cao Y.; Wang S.; Zhang L. High Strength Chitosan Hydrogels with Biocompatibility via New Avenue Based on Constructing Nanofibrous Architecture. Macromolecules 2015, 48, 2706–2714. 10.1021/acs.macromol.5b00117. [DOI] [Google Scholar]
  18. Wu Y.; Wang Y.; Luo G.; Dai Y. In Situ Preparation of Magnetic Fe3O4-Chitosan Nanoparticles for Lipase Immobilization by Cross-Linking and Oxidation in Aqueous Solution. Bioresour. Technol. 2009, 100, 3459–3464. 10.1016/j.biortech.2009.02.018. [DOI] [PubMed] [Google Scholar]
  19. Takara E. A.; Marchese J.; Ochoa N. A. NaOH Treatment of Chitosan Films: Impact on Macromolecular Structure and Film Properties. Carbohydr. Polym. 2015, 132, 25–30. 10.1016/j.carbpol.2015.05.077. [DOI] [PubMed] [Google Scholar]
  20. Téllez Soto C. A.; Ramos J. M.; Costa Junior A. C.; Vieira L. S.; Rangel J. L.; Raniero L.; Fávero P. P.; Lemma T.; Ondar G. F.; Versiane O.; Martin A. A. Surface Enhancement Raman Scattering of Tautomeric Thiobarbituric Acid. Natural Bond Orbitals and B3LYP/6-311+G (d, p) Assignments of the Fourier Infrared and Fourier Raman Spectra. Spectrochim. Acta, Part A 2013, 114, 475–485. 10.1016/j.saa.2013.05.035. [DOI] [PubMed] [Google Scholar]
  21. Akbar Ali M.; Bose R. Metal Complexes of Schiff Bases Formed by Condensation of 2-Methoxybenzaldehyde and 2-Hydroxybenzaldehyde with S-Benzyldithiocarbazate. J. Inorg. Nucl. Chem. 1977, 39, 265–269. 10.1016/0022-1902(77)80011-0. [DOI] [Google Scholar]
  22. Kasaai M. R. Determination of the Degree of N-Acetylation for Chitin and Chitosan by Various NMR Spectroscopy Techniques: A Review. Carbohydr. Polym. 2010, 79, 801–810. 10.1016/j.carbpol.2009.10.051. [DOI] [Google Scholar]
  23. Varma A. J.; Deshpande S. V.; Kennedy J. F. Metal Complexation by Chitosan and Its Derivatives: A Review. Carbohydr. Polym. 2004, 55, 77–93. 10.1016/j.carbpol.2003.08.005. [DOI] [Google Scholar]
  24. Arora S.; Lal S.; Kumar S.; Kumar M.; Kumar M. Comparative Degradation Kinetic Studies of Three Biopolymers: Chitin, Chitosan and Cellulose. Appl. Sci. Res. 2011, 3, 188–201. [Google Scholar]
  25. Abdelgawad A. M.; El-Naggar M. E.; Hudson S. M.; Rojas O. J. Fabrication and Characterization of Bactericidal Thiol-Chitosan and Chitosan Iodoacetamide Nanofibres. Int. J. Biol. Macromol. 2017, 94, 96–105. 10.1016/j.ijbiomac.2016.07.061. [DOI] [PubMed] [Google Scholar]
  26. Srinivasulu B.; Dev P. B.; Murthy P. H. C. X-Ray Diffraction Study of a Panchavaktra Ras. Int. J. Pharm. Sci. Res. 2012, 3, 1853–1859. [Google Scholar]
  27. Kumirska J.; Czerwicka M.; Kaczyński Z.; Bychowska A.; Brzozowski K.; Thöming J.; Stepnowski P. Application of Spectroscopic Methods for Structural Analysis of Chitin and Chitosan. Mar. Drugs 2010, 8, 1567–1636. 10.3390/md8051567. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Guo L.; Liu G.; Hong R. Y.; Li H. Z. Preparation and Characterization of Chitosan Poly(Acrylic Acid) Magnetic Microspheres. Mar. Drugs 2010, 8, 2212–2222. 10.3390/md8072212. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Asasian N.; Kaghazchi T.; Faramarzi A.; Hakimi-Siboni A.; Asadi-Kesheh R.; Kavand M.; Mohtashami S.-A. Enhanced Mercury Adsorption Capacity by Sulfurization of Activated Carbon with SO2 in a Bubbling Fluidized Bed Reactor. J. Taiwan Inst. Chem. Eng. 2014, 45, 1588–1596. 10.1016/j.jtice.2013.10.012. [DOI] [Google Scholar]
  30. Bao S.; Li K.; Ning P.; Peng J.; Jin X.; Tang L. Highly Effective Removal of Mercury and Lead Ions from Wastewater by Mercaptoamine-Functionalised Silica-Coated Magnetic Nano-Adsorbents: Behaviours and Mechanisms. Appl. Surf. Sci. 2017, 393, 457–466. 10.1016/j.apsusc.2016.09.098. [DOI] [Google Scholar]
  31. Furukawa M.; Fujisawa H.; Katano S.; Ogasawara H.; Kim Y.; Komeda T.; Nilsson A.; Kawai M. Geometrical Characterization of Pyrimidine Base Molecules Adsorbed on Cu(110) Surfaces: XPS and NEXAFS Studies. Surf. Sci. 2003, 532–535, 261–266. 10.1016/S0039-6028(03)00464-3. [DOI] [Google Scholar]
  32. Li X.-Q.; Liang H.-Q.; Cao Z.; Xiao Q.; Xiao Z.-L.; Song L.-B.; Chen D.; Wang F.-L. Simple and Rapid Mercury Ion Selective Electrode Based on 1-Undecanethiol Assembled Au Substrate and Its Recognition Mechanism. Mater. Sci. Eng., C 2017, 72, 26–33. 10.1016/j.msec.2016.11.034. [DOI] [PubMed] [Google Scholar]
  33. Arshadi M.; Amiri M. J.; Mousavi S. Kinetic, Equilibrium and Thermodynamic Investigations of Ni(II), Cd(II), Cu(II) and Co(II) Adsorption on Barley Straw Ash. Water Resour. Ind. 2014, 6, 1–17. 10.1016/j.wri.2014.06.001. [DOI] [Google Scholar]
  34. Goswami S.; Ghosh U. C. Studies on Adsorption Behaviour of Cr (VI) onto Synthetic Hydrous Stannic Oxide. Water SA 2005, 31, 597–602. 10.4314/wsa.v31i4.5150. [DOI] [Google Scholar]
  35. Kannan N.; Sundaram M. M. Kinetics and Mechanism of Removal of Methylene Blue by Adsorption on Various Carbons—a Comparative Study. Dye. Pigment. 2001, 51, 25–40. 10.1016/S0143-7208(01)00056-0. [DOI] [Google Scholar]
  36. Qu Z.; Yan L.; Li L.; Xu J.; Liu M.; Li Z.; Yan N. Ultraeffective ZnS Nanocrystals Sorbent for Mercury(II) Removal Based on Size-Dependent Cation Exchange. ACS Appl. Mater. Interfaces 2014, 6, 18026–18032. 10.1021/am504896w. [DOI] [PubMed] [Google Scholar]
  37. Arshadi M.; Faraji A. R.; Amiri M. J. Modification of Aluminum–silicate Nanoparticles by Melamine-Based Dendrimer l-Cysteine Methyl Esters for Adsorptive Characteristic of Hg(II) Ions from the Synthetic and Persian Gulf Water. Chem. Eng. J. 2015, 266, 345–355. 10.1016/j.cej.2014.12.109. [DOI] [Google Scholar]
  38. Arshadi M.; Mousavinia F.; Khalafi-Nezhad A.; Firouzabadi H.; Abbaspourrad A. Adsorption of Mercury Ions from Wastewater by a Hyperbranched and Multi-Functionalized Dendrimer Modified Mixed-Oxides Nanoparticles. J. Colloid Interface Sci. 2017, 505, 293–306. 10.1016/j.jcis.2017.05.052. [DOI] [PubMed] [Google Scholar]
  39. Dave M. D.; Pande U. C. Photoinduced Electron Transfer Reaction of 2-Thiobarbituric Acid and Methylene Blue: Mechanism and Kinetics. Chem. Sci. Trans. 2013, 2, 749–760. 10.7598/cst2013.470. [DOI] [Google Scholar]
  40. Akbar Ali M.; Livingstone S. E. Metal Complexes of Sulphur-Nitrogen Chelating Agents. Coord. Chem. Rev. 1974, 13, 101–132. 10.1016/S0010-8545(00)80253-2. [DOI] [Google Scholar]
  41. Magnussen O. M.; Ocko B. M.; Deutsch M.; Regan M. J.; Pershan P. S.; Abernathy D.; Grubel G.; Legrand J.-F. Self-Assembly of Organic Films on a Liquid Metal. Nature 1996, 384, 250–252. 10.1038/384250a0. [DOI] [Google Scholar]
  42. Cohen-Atiya M.; Mandler D. Studying Thiol Adsorption on Au, Ag and Hg Surfaces by Potentiometric Measurements. J. Electroanal. Chem. 2003, 550–551, 267–276. 10.1016/S0022-0728(02)01145-2. [DOI] [Google Scholar]
  43. Gu B.; Bian Y.; Miller C. L.; Dong W.; Jiang X.; Liang L. Mercury Reduction and Complexation by Natural Organic Matter in Anoxic Environments. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 1479–1483. 10.1073/pnas.1008747108. [DOI] [PMC free article] [PubMed] [Google Scholar]
  44. Zheng W.; Lin H.; Mann B. F.; Liang L.; Gu B. Oxidation of Dissolved Elemental Mercury by Thiol Compounds under Anoxic Conditions. Environ. Sci. Technol. 2013, 47, 12827–12834. 10.1021/es402697u. [DOI] [PubMed] [Google Scholar]
  45. Abraham A. M.; Kumar S. V.; Alhassan S. M. Porous Sulphur Copolymer for Gas-Phase Mercury Removal and Thermal Insulation. Chem. Eng. J. 2018, 332, 1–7. 10.1016/j.cej.2017.09.069. [DOI] [Google Scholar]
  46. Wang T.; Liu J.; Zhang Y.; Zhang H.; Chen W.-Y.; Norris P.; Pan W.-P. Use of a Non-Thermal Plasma Technique to Increase the Number of Chlorine Active Sites on Biochar for Improved Mercury Removal. Chem. Eng. J. 2018, 331, 536–544. 10.1016/j.cej.2017.09.017. [DOI] [Google Scholar]
  47. Wang X.; Deng W.; Xie Y.; Wang C. Selective Removal of Mercury Ions Using a Chitosan–poly(Vinyl Alcohol) Hydrogel Adsorbent with Three-Dimensional Network Structure. Chem. Eng. J. 2013, 228, 232–242. 10.1016/j.cej.2013.04.104. [DOI] [Google Scholar]
  48. Zhou L.; Wang Y.; Liu Z.; Huang Q. Characteristics of Equilibrium, Kinetics Studies for Adsorption of Hg(II), Cu(II), and Ni(II) Ions by Thiourea-Modified Magnetic Chitosan Microspheres. J. Hazard. Mater. 2009, 161, 995–1002. 10.1016/j.jhazmat.2008.04.078. [DOI] [PubMed] [Google Scholar]
  49. Mohan D.; Gupta V. K.; Srivastava S. K.; Chander S. Kinetics of Mercury Adsorption from Wastewater Using Activated Carbon Derived from Fertilizer Waste. Colloids Surf., A 2001, 177, 169–181. 10.1016/S0927-7757(00)00669-5. [DOI] [Google Scholar]
  50. Aguado J.; Arsuaga J. M.; Arencibia A. Adsorption of Aqueous Mercury(II) on Propylthiol-Functionalized Mesoporous Silica Obtained by Cocondensation. Ind. Eng. Chem. Res. 2005, 44, 3665–3671. 10.1021/ie0487585. [DOI] [Google Scholar]
  51. Monier M. Adsorption of Hg2+, Cu2+ and Zn2+ Ions from Aqueous Solution Using Formaldehyde Cross-Linked Modified Chitosan–thioglyceraldehyde Schiff’s Base. Int. J. Biol. Macromol. 2012, 50, 773–781. 10.1016/j.ijbiomac.2011.11.026. [DOI] [PubMed] [Google Scholar]
  52. Cárdenas G.; Orlando P.; Edelio T. Synthesis and Applications of Chitosan Mercaptanes as Heavy Metal Retention Agent. Int. J. Biol. Macromol. 2001, 28, 167–174. 10.1016/S0141-8130(00)00156-2. [DOI] [PubMed] [Google Scholar]
  53. Kyzas G. Z.; Deliyanni E. A. Mercury(II) Removal with Modified Magnetic Chitosan Adsorbents. Molecules 2013, 18, 6193–6214. 10.3390/molecules18066193. [DOI] [PMC free article] [PubMed] [Google Scholar]
  54. Khalek M. D.; Mahmoud G. A.; El-kelesh N. A. Synthesis and Characterization of Poly-Methacrylic Acid Grafted Chitosan-Bentonite Composite and Its Application for Heavy Metals Recovery. Chem. Mater. Res 2012, 2, 1–8. [Google Scholar]
  55. Yang Y. L.; Zhao S. G.; Song C. G.; Gao M. The Adsorption of Chitosan-Aluminum Oxide Composite Material to Mercury Ions. Adv. Mater. Res. 2014, 1015, 647–650. 10.4028/www.scientific.net/AMR.1015.647. [DOI] [Google Scholar]
  56. Mangallo B.; Sjahrul M. Beneficial Effects of Cysteine in Modification of Chitosan. Eur. Chem. Bull. 2012, 1, 350–355. 10.17628/ecb.2012.1.350-355. [DOI] [Google Scholar]
  57. Li G.; Liu M.; Zhang Z.; Geng C.; Wu Z.; Zhao X. Extraction of Methylmercury and Ethylmercury from Aqueous Solution Using Surface Sulfhydryl-Functionalized Magnetic Mesoporous Silica Nanoparticles. J. Colloid Interface Sci. 2014, 424, 124–131. 10.1016/j.jcis.2014.03.026. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao8b01837_si_001.pdf (1.9MB, pdf)

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES