Abstract
A highly selective cellulose-based adsorbent for mercury [(Hg)II] ion was prepared and characterized using Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectrometry, elemental analysis, and scanning electron microscopy. The results showed that functional thiosemicarbazide-grafted cellulose achieved equilibrium adsorption in 120 min, and the adsorbents had a Hg(II) ion removal rate of approximately 98.5% at a pH of 5.0. The adsorption kinetics fit the pseudo-second-order model, which indicated that the adsorption was a chemical process. Additionally, the adsorption isotherm data showed a best fit with the Langmuir isotherm model, with a maximum Hg(II) ion adsorption capacity of 331.1 mg/g. This adsorbent had a good selectivity for Hg(II) during competitive adsorption.
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Adsorption of Hg(II) Ions from Aqueous Solution by Thiosemicarbazide-modified Cellulose Adsorbent
Jiahao Jiang and Xiwen Wang *
A highly selective cellulose-based adsorbent for mercury [(Hg)II] ion was prepared and characterized using Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectrometry, elemental analysis, and scanning electron microscopy. The results showed that functional thiosemicarbazide-grafted cellulose achieved equilibrium adsorption in 120 min, and the adsorbents had a Hg(II) ion removal rate of approximately 98.5% at a pH of 5.0. The adsorption kinetics fit the pseudo-second-order model, which indicated that the adsorption was a chemical process. Additionally, the adsorption isotherm data showed a best fit with the Langmuir isotherm model, with a maximum Hg(II) ion adsorption capacity of 331.1 mg/g. This adsorbent had a good selectivity for Hg(II) during competitive adsorption.
Keywords: Cellulose; Thiosemicarbazide; Chemical modification; Adsorption; Hg(II)
Contact information: School of Light Industry and Engineering, State Key Lab of Pulp and Paper Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, Guangdong, People’s Republic of China; *Corresponding author: wangxw@scut.edu.cn
INTRODUCTION
Large amounts of wastewater are generated and discharged globally due to the continuous development of the textile, plastic, printing, and paper industries. This industrial wastewater contains many heavy metal ions, such as mercury, copper, cadmium, and palladium, which compose the principal metallic pollution caused by industrial production. Because they are not biodegradable, once these heavy metal ions are released into the environment they pollute the environment and destroy the ecology. They can also accumulate through entering the food chain, causing serious and sometimes fatal health disorders to humans, including various cancers (Fu and Wang 2011; Salman et al. 2015; Lofrano et al. 2016; Zhao et al. 2018).
Over the past several years, some common techniques have been utilized to remove heavy metal ions from industrial wastewater. These include chemical precipitation (Zhang et al. 2016; Gao et al. 2018), ion exchange (Khoiruddin et al. 2017; Andrzej et al. 2018), functionalized membrane (da Silva et al. 2016; Khoiruddin et al. 2017; Yang et al. 2018; Barros et al. 2018), ion flotation technique (Xu et al. 2017; Yenial and Bulut 2017; Micheau et al. 2018), and electrochemical treatment (Kabdaşlı et al. 2009; Ali et al. 2011). Mercury ion is one of the most hazardous heavy metal ions in wastewater, so these techniques have also been developed to remove mercury ions and have achieved great results (Hou et al. 2013; Oehmen et al. 2014; Xia et al. 2017). Although these techniques can yield positive achievement, they have disadvantages, including high cost, long duration, low sensitivity and stability, susceptibility to contamination, and secondary environmental pollution. Adsorption for heavy metal ion removal yields less environmental pollution and has a high removal efficiency, with additional advantages of lower cost and portable operation (Yu et al. 2014). Meanwhile, various adsorbents, a wide range of source materials, and reusability are also among the important factors that make adsorption one of the most common techniques in the treatment of industrial wastewater (Nayab et al. 2014; Yan et al. 2014).
Previous studies have examined functionalized adsorbent preparation techniques, which offer flexibility in design and operation and in many cases facilitate the reacquisition of the precious metal ions, yielding additional economic value (Hoai et al. 2010). Activated carbon (AC) adsorbents (Hadi et al. 2015; Largitte and Pasquier 2016; Park and Lee 2018), carbon nanotubes (CNTs) (Wang et al. 2015; Oyetade et al. 2017; Zhang et al. 2017), and functionalized chitosan (Kheirandish et al. 2017; Dinu et al. 2018; Mousavi et al. 2018) are widely used in the removal of heavy metal contaminants. Bioadsorbents, such as fungi (Yusuf and Tekin 2017; Pourkarim et al. 2017), dead biomass (Cheng et al. 2017), and bacteria (Qian and Zhan 2016; Manasi et al. 2018), are also widely used.
The application of cellulose-based adsorbents for metal ion remediation and wastewater purification has received widespread attention in recent years, and it has the advantages of large surface area, strong mechanical properties, and being biodegradable. Most importantly, the chemically active hydroxyl enhances the adsorption capacity (Lindh et al. 2016; Huang et al.2017). Because of the distinctive properties of cellulose, many researchers have attempted to develop functionalized cellulose materials (Hokkanen et al. 2016).
There are many methods to functionalize cellulose materials, such as lipidization (Spinella et al. 2016; Gan et al. 2017), etherification (Abdel-Halim et al. 2015; Karatas and Arslan 2016; ), graft copolymerization (Deng et al. 2016; Sun et al. 2018a), and oxidization (Li et al. 2015, 2017; Beheshti Tabar et al. 2017). Oxidization is one of the most common techniques for cellulose-based material activation (Siller et al. 2015; Chen and van de Ven 2016; Cheng et al. 2016; Biliuta and Coseri 2016). In all oxidation reactions, the periodate can selectively oxidize the C2-C3 glucose molecule into two aldehyde groups by cleaving the glucopyranoside between them. This behavior allows for further cellulose modification while maintaining the main morphological structure and original properties of cellulose (Yang et al. 2015; Azzam et al. 2015; Tian and Jiang 2017).
In this study, thiosemicarbazide-modified cellulose (TSC) for Hg(II) selective removal was prepared, and Hg(II) adsorption using the TSC was conducted. The structure and morphology of TSC were characterized via Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), elemental analysis, and scanning electron microscopy (SEM). The thermodynamic properties were characterized using thermogravimetric analysis. The factors influencing adsorption behavior, including pH, reaction time, and aqueous solution concentration, were also investigated and optimized. The adsorption kinetic parameters of the Hg(II) removal were calculated, and the selectivity of TSC was also evaluated.
EXPERIMENTAL
Materials
Native cellulose extracted from palm wood was collected from the China National Tobacco Corporation (Beijing, China) and washed with deionized water after treatment in 5% (v/v) HCl. Then, the cellulose was oven-dried at 80 °C until reaching a constant weight. Sodium periodate, thiosemicarbazide, hydroxylamine hydrochloride, nitric acid, sodium hydroxide, and mercury chloride (HgCl2) were purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). Nitric acid was used after dilution, and the rest of the chemicals were used as received.
Preparation of thiosemicarbazide-modified cellulose
The Hg(II) ion adsorption cellulose was synthesized through a wet chemical process in an aqueous medium according to the following stepwise procedure. First, 1 g of the pretreated cellulose was dispersed into 100 mL of deionized water; 6 mmol sodium periodate per gram of cellulose was used to oxidize the cellulose. Then, the mixture was gently shaken for 6 h at 40 °C in the dark. Next, it was mixed with 10 mL of 10% aqueous ethylene glycol solution, and the mixture was shaken for 30 min to stop the oxidation reaction. After the reaction was over, oxidized cellulose was removed via filtration, dialysis, and centrifuging. Finally, the cellulose was washed with deionized water and dispersed. A special reaction between hydroxylamine hydrochloride and oxidized cellulose was conducted to determine the content of the aldehyde groups (Veelaert et al. 1997). The aldehyde content was calculated using Eq. 1,
Aldehyde Substitution Degree = 
(1)
where aldehyde substitution degree (%) is the amount of aldehyde per 100 glucoses, V1 (mL) is the amount of sodium hydroxide for oxidized cellulose, V2 (mL) is the amount of sodium hydroxide for raw material cellulose, c (mol/L) is the concentration of sodium hydroxide, and m (g) is the weight of the sample.
In the next step, the aqueous wet oxidized cellulose was mixed with thiosemicarbazide to graft the thiourea group onto cellulose. Before mixing with these chemicals, oxidized cellulose was pretreated via sonication, and 0.2 g thiosemicarbazide per gram of cellulose was dissolved at 65 °C for 30 min. The mixture was refluxed at 65 °C for 6 h to produce the TSC, and 5 mL of 0.1 M HCl solution was added after the reaction was over. Then, the TSC was removed via filtration and washed with deionized water. A special reaction was conducted to determine the amino group content. At 30 °C, 100 mg of sample was dispersed in deionized water, which changed the pH to 3.0. Sodium hydroxide solution was slowly titrated until the pH reached 12.0, and the changes in conductivity were recorded. The amino groups’ content was calculated using Eq. 2 (Da Silva Perez et al. 2003; Filpponen and Argyropoulos 2010),
Amino Group Content = 
(2)
where amino group content is in mmol/g, c is the concentration of sodium hydroxide solution, V1 (mL) is the minimum consumption of sodium hydroxide at the lowest conductivity, V2(mL) is the maximum consumption of sodium hydroxide at the lowest conductivity, and m (g) is the weight of the sample.
A schematic of the cellulose modification reactions is shown in Fig. 1.
Fig. 1. Synthesis of TSC
Methods
Characterization of samples
A Fourier-transform infrared spectrometer (Bruker VERTEX 70; Bruker Corporation, Billerica, MA, USA) was used to investigate the spectra of the native, oxidized, and modified cellulose. A quantitative analysis of the C, N, O, and S contents of the TSC was performed with an elemental analyzer (Vario EL cube; Elementar, Langenselbold, Germany). An X-ray powder diffractometer (D8 ADVANCE Plus; Bruker Corporation, Billerica, MA, USA) was utilized to investigate the crystallinity of the native, oxidized, and modified cellulose. A scan mode was applied to collect the data for 2θ = 5° to 40°. X-ray photoelectron spectrometry (XPS; ESCALAB 250Xi; Thermo Fisher Scientific Inc., Waltham, MA, USA) was used to analyze the binding energy of the adsorbents for analysis of its elements. A thermogravimetric analysis (TGA; TG 209 F1 Libra®; Netzsch, Selb, Germany) was conducted at a heating rate of 10 °C/min from 40 °C to 600 °C. The metal ion content was determined using inductively coupled plasma optical emission spectrometry (ICP-OES; Optima 5300 DV; PerkinElmer, Waltham, MA, USA). The surface morphology of the samples was examined via SEM (EVO 18; Zeiss, Oberkochen, Germany).
Adsorption of Hg(II) on TSC
Effect of initial pH on the adsorption of Hg(II)
The pH value of the solution was one of the key factors that affected the adsorption effectiveness of Hg(II) ions, it not only affected the presence of metal ions but also affected the adsorbent surface charge. In these experiments, 100 mg of TSC was placed in a 250-mL Erlenmeyer flask containing 200 mL of mercury aqueous solution with a main concentration of 100 mg/L. The adsorption of Hg(II) ions was performed with a pH ranging from 2.0 to 5.0 because metal precipitation interferes with and is indistinguishable from the adsorption phenomenon for pH values up to 7. All of the flasks were shaken using a constant temperature water bath oscillator at a constant shaking rate of 130 rpm for 24 h until adsorption equilibrium. Then, the cellulose was removed by a filter, and the residual Hg(II) ion content was estimated via ICP-OES.
The adsorption capacity of Hg(II) ions on TSC and the removal rate of Hg(II) ions were calculated using Eqs. 3 and 4, respectively,
qe = (ci – ce) 
(3)
Removal Rate (%) = (ci – ce) 
(4)
where qe (mg/g) is adsorption capacity, ci (mg/L) is initial mercury ion concentration, ce (mg/L) is equilibrium mercury ion concentrations, V (L) is the volume of aqueous solution, and m(g) is the weight of the sample.
Effect of reaction time on the adsorption of Hg(II)
In these experiments, 100 mg TSC was placed in a 250-mL Erlenmeyer flask containing 200 mL of mercury aqueous solution with a main concentration of 100 mg/L, and the adsorption of Hg(II) ions was performed at a pH of 5.0. All of the flasks were shaken using a constant temperature water bath oscillator at a constant rate of 130 rpm with a reaction time from 0 min to 360 min until adsorption equilibrium. Then, the cellulose was removed using a filter, and the residual Hg(II) ion content was estimated via ICP-OES.
The adsorption ability of Hg(II) ions on TSC and the removal rate of Hg(II) ions were calculated using Eqs. 3 and 4, respectively.
Effect of solution concentration on the adsorption of Hg(II)
In these experiments, 100 mg TSC was placed in a 250-mL Erlenmeyer flask containing 200 mL of mercury aqueous solution with a main concentration ranging from 50 mg/L to 700 mg/L, and the adsorption of Hg(II) ions was performed at a pH of 5. All of the flasks were shaken using a constant temperature water bath oscillator at a constant rate of 130 rpm for 24 h until adsorption equilibrium. Then, the cellulose was removed by a filter, and the residual Hg(II) ion content was estimated via ICP-OES.
The adsorption capacity of the Hg(II) ions on TSC and the removal rate of Hg(II) ions were calculated using Eqs. 3 and 4, respectively.
Selectivity Experiments
The competitive metal ion adsorption studies were performed using a 100 mg sample dispersed in a 250-mL Erlenmeyer flask containing 200 mL of aqueous solution that was mixed together with mercury, lead, cadmium, and copper of an initial concentration of 100 mg/L The adsorption capacity of each metal ion was calculated, and the distribution coefficient (calculated using Eq. 5) was utilized for the selectivity evaluation (Liu et al. 2017a; Khan et al. 2017),
D = [(ci – ce) / ce] (5)
where ci (mg/L) is the initial mercury ion concentration, ce (mg/L) is the equilibrium mercury ion concentrations, V (L) is the volume of aqueous solution, and m (g) is the weight of the sample.
Desorption Experiments
Desorption experiments on TSC were performed using 0.1 M HNO3 and 0.1 M HCl solution as the desorption media. The amount of desorbed Hg(II) was measured using ICP-OES.
RESULTS AND DISCUSSION
Characterization of the Modified Cellulose
The results of the oxidation and modification reactions showed that the product yield was approximately 91.2%, and the degree of aldehyde substitution was approximately 46%. The weight loss of cellulose might have been caused by degradation during the chemicals’ interaction. The amino group content was approximately 1.15 mmol/g.
The results of the elemental analysis of native cellulose, oxidized cellulose, and modified cellulose are shown in Table 1. After the oxidation and modification of cellulose, there was an obvious increase in the nitrogen and sulfur content, which indicated that the thiosemicarbazide reactive groups were grafted onto the oxidized cellulose.
Table 1. Elemental Analysis of Native, Oxidized, and Modified Cellulose
The FTIR results are shown in Fig. 2. The spectra of the native cellulose exhibited diagnostic spectral peaks at approximately 1070 cm-1 to 1150 cm-1 due to C-O stretching vibrations, 1240 cm-1 to 1380 cm-1 because of O-H bending, and 3200 cm-1 to 3500 cm-1 due to O-H stretching. After the oxidation reaction, the adsorption band at 1710 cm-1 corresponded to C=O stretching vibration, indicating oxidization of the hydroxyl groups of the cellulose (Koprivica et al. 2016; Chen and Ven 2016; Kim et al. 2017). After thiosemicarbazide modification, the spectrum showed a new peak at approximately 1670 cm-1, which could have been due to the stretching vibration of C=N of the Schiff base formed between the aldehyde group of the oxidized cellulose and the amino group of the thiosemicarbazide (Monier and Abdel-Latif 2012). In Fig. 2, a peak appears to be present in the TSC spectrum at 1490 cm-1, and the peaks at approximately 767 cm-1 and 1230 cm-1 were related to the C=S bond. The FTIR spectra indicated that characteristic functional groups were present on the modified cellulose and that TSC was successfully created (Morcali et al. 2015; Essawy et al. 2016).
Fig. 2. FTIR spectra of (a) native cellulose, (b) oxidized cellulose, and (c) TSC
The XRD results of the investigation of the crystalline structure are shown in Fig. 3. The native cellulose presented the characteristic crystalline peaks at approximately 16° to 17°, an intense peak at 23°, and a peak at approximately 29°. Meanwhile, there was no noticeable change of characteristic peaks in the XRD patterns of oxidized and modified cellulose (Toba et al. 2013). One can make an initial conclusion that the inside aggregate crystalline structures of the oxidized and modified cellulose did not greatly change after the reaction. The crystallinity indices of the different samples are presented in Table 2. The crystallinities of the different cellulose samples were 72.9%, 70.6%, and 67.3% (Segal et al. 1959). This phenomenon might have been explained by most hydrogen bonds between the hydroxyls being broken during the oxidization reaction, with the amorphous region created from the subsequent modification reaction (Kim et al. 2000).
Fig. 3. XRD patterns of (a) native cellulose, (b) oxidized cellulose, and (c) TSC
Table 2. Crystallinity Indices of Different Cellulose Samples
The native cellulose and modified cellulose were also examined using XPS. Although it simply conducts an analysis of elements on the cellulose surface, XPS provides a valuable indication of changes in the chemical functional groups during the chemical reactions. The XPS survey scans of native cellulose and TSC are presented in Fig. 4. For native cellulose, only two peaks were observed: one at 285.7 eV and another at 533.1 eV, corresponding to C and O, respectively (Banuls-Ciscar et al. 2016). However, for the TSC, the XPS spectrum showed a nitrogen adsorption peak at 400.2 eV and the S2p specific peak at approximately 162.4 eV. These results were further evidence for TSC’s successful creation (Konshina et al. 2016; Ahmad et al. 2018).
Fig. 4. XPS patterns of (a) native cellulose and (b) TSC
Thermogravimetric analysis was employed to evaluate the thermal stability of the native cellulose, oxidized cellulose, and modified cellulose. The TGA curves are shown in Fig. 5. The thermal degradation temperature of native cellulose was approximately 260 °C. The mass of the native samples decreased while the temperature was below 120 °C due to water evaporation inside the cellulose. Although the samples were dried prior to TGA, some moisture could not be removed because of hydrogen bonding. The thermal degradation temperature of oxidized cellulose was approximately 200 °C; the thermal stability of oxidized cellulose was reduced compared to native cellulose. After cellulose oxidization by sodium periodate, the C2-C3 bonds on each glucose unit of the cellulose molecular chains were fractured, which destroyed the six-membered ring structure of the glucose units and led the oxidized cellulose to degrade at a lower temperature. The thermal degradation temperature of modified cellulose was greater than that of oxidized cellulose and was up to 220 °C. This might have been because the thiosemicarbazide functional group grafted onto the oxidized cellulose molecular chains, which lengthened the modified cellulose molecular chains.
Fig. 5. TGA of (a) native cellulose, (b) oxidized cellulose, and (c) TSC
The SEM results for the native cellulose and TSC are displayed in Fig. 6. As can be easily observed with approximately 500× magnification, the cellulose lengths were approximately several mm, and the widths were approximately 10 µm. Moreover, the native cellulose surface was smooth and had no swelling from dehydration. In contrast, the rough and porous surface of the modified cellulose indicated that after oxidation and thiosemicarbazide modification, functional and active sites were distributed on the cellulose surface, providing greater specific surface area and enhancing the ability to adsorb Hg(II). Meanwhile, the lengths and widths of the cellulose were distributed more evenly, and the widths were slightly reduced to approximately 7.0 µm. This result may have been the reason for chemicals touching the amorphous area of cellulose and causing degradation.
Fig. 6. SEM photos of (a) native cellulose and (b) modified cellulose
Adsorption Properties
Mercury(II) chloride was selected for the Hg(II) solute because it is ionized to (HgCl)– instead of Hg(II). Then, (HgCl)– was ionized to Hg(II) and Cl– through an ionization balance process. When Hg(II) ions were adsorbed by TSC, the decrease of Hg(II) ion concentration in the system promoted the further ionization of (HgCl)–. Therefore, the concentration of HgCl2could be raised to a higher level without fear of forming a precipitate. Inductively coupled plasma optical emission spectrometry detects the total amount of Hg(II) in the solution; its particular form would not affect the test results.
Effect of pH
Many previous studies have shown that the pH of a solution makes an essential impact in chemical heavy metal ion adsorption experiments (Monier and Abdel-Latif 2013; Guo et al. 2016). The pH value range of 2.0 to 5.0 was to determine the effect of the removal of Hg(II) by TSC. Higher pH values could not conduct, because with pH values of 6 and greater the Hg(II) ions would form a precipitate. The adsorption results are shown in Fig. 7. As shown in the adsorption curve, the removal rate of Hg(II) ions clearly increased when raising the pH from 2.0 to 5.0. This trend might have been because the Hg(II) ions adsorption mainly via the active sites on the surface of the modified cellulose could coordinate with the heavy metal cation. When pH was low, the quantity of hydrogen ions in the aqueous solution increased, and the majority of the active adsorption functional groups had been protonated. Consequently, the cellulose surface was full of positive charge, which subsequently weakened the coordination capability between the surface active sites and the Hg(II) ions. With increased pH, the quantity of hydroxide in the aqueous solution also increased, resulting in charge transfer in the carbon-sulfur double bond and the formation of a thioether structure, which subsequently enhanced the Hg(II) adsorption (Motahari et al. 2015; Sarkar et al. 2016; Zou et al. 2017).
Fig. 7. Effect of pH value on the removal of Hg(II) ions by TSC (initial concentration 100 mg/L, adsorbent 0.5 g/L, shaking rate 130 rpm, 30 C, and for 24 h)
Adsorption Kinetics
To investigate the time to reach saturation during the adsorption process, the adsorption rate throughout the experiment’s duration, and the interaction between the adsorbent and the heavy metal ions, Hg(II) ion adsorption kinetic experiments were conducted at 30 C and an initial concentration of 100 mg/L from 0 min to 360 min. The results of the adsorption rate throughout the experiment are exhibited in Fig. 8. As is clearly shown in the curve, the adsorption rate continuously decreased with the time elapsed until the adsorption capacity had reached saturation at 176 mg/g after approximately 100 min.
Fig. 8. Effect of contact time on the removal of Hg(II) ions by TSC (initial concentration 100 mg/L, adsorbent 0.5 g/L, pH 5.0, shaking rate 130 rpm, and at 30 C)
Because there are various functional groups in the TSC, such as N-H and C=S, it is essential to investigate the different possible interaction types. The Hg(II) ion adsorption data were subjected to kinetic adsorption model fitting and analysis for increased comprehension of the adsorption mechanism, and finally determined the limit for the rate that affected the adsorption process (Lee and Kim 2016; Zhu et al. 2017; Cruz-Tirado et al. 2017).
The most frequently used kinetic equations to describe the uptake kinetic mechanism are the pseudo-first-order equation (Eq. 6) and the pseudo-second-order equation (Eq. 7) (Ho and McKay 1999; Azizian 2004),
(6)
(7)
where qt (mg/g) is the adsorption capacity at time t (min), and k1 (min-1) and k2 (g/(mg·min)) are the adsorption rate constants of the pseudo-first-order kinetic model and pseudo-second-order kinetic model.
For both kinetic models, k and qe are usually calculated by calibrating the linear plots of ln(qe – qt) and t/qt versus t and then comparing the theoretical values with experimental values to evaluate the best kinetic models that fit with the work.
Table 3 lists the constants qe, k, and R2, which describe the adsorption mechanism and models. Clearly, the pseudo-second-order equation showed a better match with the experimental data between the experimental and calculated qe and the high value of R2 (> 0.999). Furthermore, the better fit of the pseudo-second-order equation signified that the adsorption (Yao et al. 2016; Dogan et al. 2018; Song et al. 2018). Additionally, TSC exhibited a high initial adsorption rate of 22.35 mg/(g·min) for Hg(II), which was greater than those in previous reports (Yao et al. 2016). Noticeably, the pseudo-first-order model was not an appropriate model of the adsorption kinetic behavior because of the poor value of R12 (< 0.86).
Table 3. Kinetic Parameters for Hg(II) Adsorption by TSC
Adsorption isotherms
Two extensively applied adsorption isotherm models, the Langmuir and Freundlich models, are important in explaining the relation between the Hg(II) ions on the adsorbent samples and the initial concentration of the metal ions. These two common isotherm models are formulated as the Langmuir equation (Eq. 8) and the Freundlich equation (Eq. 9) (Yao 2000; Senniappan et al. 2016; Taoufik et al. 2017),
(8)
(9)
where qe (mg/g) is the adsorption capacity of metal ions at equilibrium, ce (mg/L) is the metal ion concentration of solution at equilibrium, kL (L/mg) is the Langmuir constant, qm (mg/g) is the maximum adsorption capacity of the adsorbent, kF is the Freundlich constant, and 1/n is the heterogeneity factor.
Comparing these two adsorption isotherm models in principle, the Langmuir model assumes that the heat of adsorption was constant throughout the adsorption process and that the adsorption process was an isothermal monolayer interaction that was established on adsorbent molecules and metal ions. Meanwhile, the Freundlich model is an isothermal adsorption equation utilized as a semi-empirical formula (Afonso et al. 2016; Terdputtakun et al. 2017).
The adsorption experiments for isotherm models were conducted at 30 C, and the results are exhibited in Fig. 9. Clearly, the adsorption capacity continuously increased, but the adsorption rate continuously slowed until reaching equilibrium.
The experimental data were fitted with both the Langmuir and Freundlich isotherms. The results and parameters are listed in Table 4. Clearly, compared with the Freundlich isotherm, the Langmuir isotherm had a better fitness for the equilibrium with R2 = 0.992. This result indicates that adsorption of Hg(II) ions by TSC mainly followed the monolayer adsorption pattern, and the theoretical maximum adsorption capacity was 331.1 mg/g.
Fig. 9. Adsorption isotherms of Hg(II) ions by TSC (initial concentration 0 mg/L to 700 mg/L, adsorbent 0.5 g/L, pH 5, shaking rate 130 rpm, and at 30 C)
Table 4. Parameters for Hg(II) Ion Adsorption by TSC According to Different Equilibrium Isotherms
The essential characteristic of the Langmuir model can be reflected by the constant RL, which evaluated the suitability of the adsorption process. It can be calculated as follows in Eq. 10 (Monier et al. 2014; Wang et al. 2017),
(10)
where c0 is the initial concentration of metal ions in solution. The value of RL presents the tendency of the isotherm to be unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). Further, a smaller RL value predicts greater affinity between the adsorbent and metal ions. The specific values for Hg(II) adsorption by TSC were between 0.034 and 0.308, indicating that TSC is a suitable adsorbent for Hg(II) ions (Tian et al. 2011b; Liu et al. 2017b).
The results of Hg(II) ion adsorption on different adsorbents under similar experimental conditions are exhibited in Table 5. The maximum adsorption capacity for Hg(II) of TSC is the greatest among several other adsorbents. The factors affecting the disparity in Hg(II) ion removal were the properties of the different adsorbents.
Table 5. Adsorption Capacities of Hg(II) Ions onto Other Similar Adsorbents
Metal ion competitive adsorption
In this study, it was necessary to consider the selectivity of adsorption of different heavy metal ions by modified adsorbent samples. Table 6 shows the selective separation and distribution coefficient of Hg(II) adsorption on TSC compared with Pb(II), Cd(II), and Cu(II) at a pH of 5.0, with an initial concentrations of 100 mg/L, and at 30 C. The adsorption capacity of Hg(II) ions was 186.2 mg/g in the case of interference by other heavy metal ions, which indicated an excellent selectivity of Hg(II) by TSC.
The distribution coefficient was calculated by Eq. 5, and the selectivity coefficient that indicated the modified adsorbent’s ability to adsorb Hg(II) ions compared to other metal ions is also shown in Table 6. As shown, TSC presented a distribution coefficient for Hg(II) over 50 times greater than for the other heavy metal ions. These results indicated that the functional sites grafted on TSC after the modification reaction can better form complex structures with Hg(II) ions (Chen et al. 2018; Wu et al. 2018).
Table 6. Selective Adsorption of Hg(II) from Multi-component Mixtures by TSC
Note: Initial concentration 100 mg/L, adsorbent 0.5 g/L, shaking rate 130 rpm, solution pH 5.0, and at 30 C
Desorption and reusability of TSC
After measuring the Hg(II) concentration in the desorption medium, the desorption ratios for Hg(II) metal ions using 0.1 M HNO3 and 0.1 M HCl solutions were 91.3% and 92.6%, respectively. The adsorption capacity of TSC could still reach 85% after five cycles, so there was no particularly notable decrease in the adsorption efficiency of TSC.
Adsorption mechanism
Figure 10 shows the sulfur element (S2p) XPS spectra of TSC before and after Hg(II) adsorption. As shown, the binding energy of the narrow XPS spectrum for S2p was 162.5 eV, which may have been due to the C=S bonds. After adsorption of Hg(II), the S2p binding energy moved to a slightly greater value of 165.4 eV, corresponding to the transfer of electrons from S atoms of -C=S- to Hg(II) (Bai et al. 2011; Ding et al. 2016). Figure 11 shows the mercury element (Hg4f) XPS spectrum of TSC after adsorption. As shown in the curve, 101.1 eV and 104.9 eV Hg4f binding energies could suggest coordination via -C=S- groups, as illustrated in the molecularly modeled schematic in Fig. 12 (Yao et al. 2016; Jie et al. 2018).
Fig. 10. S2p narrow XPS scan for TSC before (a) and after (b) adsorption of Hg(II) (initial concentration 100 mg/L, adsorbent 0.5 g/L, pH 5.0, shaking rate 130 rpm, and at 30 ℃)
Fig. 11. Hg4f narrow XPS scan for TSC after adsorption of Hg(II) (initial concentration 100 mg/L, adsorbent 0.5 g/L, pH 5.0, shaking rate 130 rpm, and at 30 C)
Fig. 12. Proposed model of complexation of Hg(II) with TSC
CONCLUSIONS
- A novel and functional adsorbent for Hg(II) removal in aqueous solutions was prepared by employing thiosemicarbazide to modify dialdehyde cellulose. The mercapto group on the surface of this derived chelating cellulose could efficiently and effectively adsorb Hg(II) ions with high selectivity.
- The performance of TSC was investigated by various techniques, and the adsorption properties of Hg(II) ion removal were also studied under different conditions. The adsorption behaviors of modified cellulose were superior to native cellulose both in equilibrium adsorption ability and in adsorption rate.
- The adsorption kinetics of Hg(II) onto TSC was fast and followed the pseudo-second-order model, which indicated that chemical adsorption occurred during the adsorption process.
- The Langmuir isotherm model had the best fit with the Hg(II) ion adsorption data through a monolayer adsorption capacity of 331.1 mg/g.
- The results of the multi-component system adsorption experiments supported the TSC’s selectivity toward Hg(II) ions in the presence of interference from other ions.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (31470608).
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Article submitted: January 10, 2019; Peer review completed: March 29, 2019; Revised version received: April 20, 2019; Accepted: April 22, 2019; Published: April 24, 2019.
DOI: 10.15376/biores.14.2.4670-4695