Biomass-Derived Carbon Sorbents for Cd(II) Removal: Activation and

Research Article pubs.acs.org/journal/ascecg

Biomass-Derived Carbon Sorbents for Cd(II) Removal: Activation and Adsorption Mechanism Zizhang Guo,†,‡ Xudong Zhang,† Yan Kang,† and Jian Zhang*,† †

School of Environmental Science and Engineering, Shandong Key Laboratory of Water Pollution Control and Resource Reuse, Shandong University, Jinan 250100, China ‡ School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164-2920, United States S Supporting Information *

ABSTRACT: Producing an ideal sorbent for removing heavy metals from aqueous solutions is still very challenging. A well-organized microporous structure and nitrogen-rich functional groups in a sorbent are proposed as an excellent platform for Cd(II) removal. Urea phosphate (UP) was used as an activating agent for preparing carbon sorbents derived from biomass. The activating procedure was examined by thermogravimetric analysis (TGA), which indicated that a well-developed microporous structure and nitrogen-rich functional groups were present in the sorbents due to the UP with urea and phosphoric acid molecules in its chemical structure and liberated in the activation process. The mechanisms of Cd(II) removal by sorbents were determined by X-ray photoelectron spectroscopy (XPS), which indicated that porous filtration and chemical reactions (ion exchange, electrostatic attraction and surface complexation) were involved in Cd(II) removal. KEYWORDS: Carbon sorbents, Urea phosphate, Preparation, Cd(II), Adsorption mechanisms

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INTRODUCTION Trace amounts of cadmium (Cd) show high toxicity in aquatic media, and the US National Toxicology Program has designated it as a carcinogen.1 Therefore, the development of an efficient technology to remove Cd from the environment is very necessary. Adsorption is a widely available method for removing various types of heavy metal contaminants, with the advantages of high efficiency, favorable costs, and technical flexibility.2−4 Because of the smaller hydrated ionic radius of Cd, the welldeveloped microporous volume of sorbents lends greater adaptation to the porous filtration for Cd removal.5 Furthermore, the surface nitrogen (N) functional groups of sorbent can share the electron pair for binding metal ions.6 Compared to oxygen functional groups, the lower electronegativity of nitrogen could more readily donate the lone pair of electrons for metal complex formation.6 The strong binding energy was produced between N groups and Cd(II) on the basis of hard−soft acid base theory.7 Thus, the well-developed microporous structure and rich N functional groups of sorbents are conducive to Cd(II) removal from aqueous solutions. Various methods for modifying N-containing functional groups onto adsorbents have been reported in a number of studies. N-functionalized silica was used to adsorb heavy metals from liquids by Heidari et al.8 Machida et al.9 used (3-mercaptopropyl) trimethoxysilane to modify mesoporous silica and activated carbon for removing heavy metals. The common disadvantage of these methods is the cumbersome steps required, first for preparation and then for modification. Urea phosphate (UP) is an effective and low-cost flame retardant and © 2017 American Chemical Society

its chemical formula is composed of urea and phosphoric acid molecules. Phosphoric acid is often used as an activating agent for activated carbon (AC) preparation owing to its advantages of low cost, high output, and little environmental pollution.10 Urea is usually used as a modifying agent for loading N functional groups on AC.11 However, there is powerful evidence for the advantages of preparing AC using UP as an activating agent. Moreover, the prepared AC was N-functionalized according to UP activation, sequentially improving the Cd(II) removal performance from wastewaters. The objectives of the present work were (1) to study the activation behavior of UP in the preparation of AC; (2) to investigate the physicochemical characteristics of carbon sorbents; and (3) to evaluate the adsorptive performance and explore the adsorptive mechanism of AC toward Cd(II).

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MATERIALS AND METHODS

Chemicals. Cd(II) stock solution (1000 mg L−1) was dissolved 1 g of CdCl2 by distilled water in a 1000 mL volumetric flasks, and it was diluted as required for further study. The properties of UP are given in Table S1. All chemical reagents were of analytical grade in this study. Preparation Methods. Phragmites australis (PA), which withers in wetlands in the winter, was sampled from Nansi Lake in Shandong Province (China). The specific pretreatment method is described in a previous study.12 The treated PA was fully soaked in a UP saturated Received: January 10, 2017 Revised: March 15, 2017 Published: March 24, 2017 4103

DOI: 10.1021/acssuschemeng.7b00061 ACS Sustainable Chem. Eng. 2017, 5, 4103−4109

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ACS Sustainable Chemistry & Engineering

next study. The final carbon sorbents are referred to as AC, and those impregnated without UP are referred to as BC. Characterization Methods. The thermal behaviors of original PA and impregnated PA with UP at different ratios (PA-UP-0.5, PA-UP-1.0, PA-UP-1.5, and PA-UP-2.0) were analyzed using TGA-50 analyzer for thermogravimetric analysis. The pore structure of sorbents was investigated by N2 adsorption/desorption using a Quantachrome porous analyzer at 77 K. Boehm’s titration method13 was quantized functional groups of sorbents. Elemental analyses (C, N, and H) of sorbents were measured by an Element Analyzer (Vario EI III, USA). The groups of ACs were determined by X-ray photoelectron spectrometry (XPS) (PerkinElmer PHI 550 ESCA/SAM) with a Mg Kα irradiation source. The comparison of before and after Cd(II) adsorption were discussed the removal mechanisms. Adsorption Experiments. The effects of contact time, initial concentration, pH, and ionic strength on the adsorption were studied by batch experiments. The temperature and the rotate speed in adsorption experiments were controlled at 25 ± 2 °C and 120 rpm, respectively. To ensure the adsorption reached equilibrium, the shaking was performed for 24 h. After equilibration, the sorbents were filtered through 0.45-μm membrane filters. The Cd(II) concentrations were measured by atomic absorption spectrometry (180−80, Hitachi, Japan). The following is the calculation formula of the Cd(II) sorption capacities of sorbents, qe (mg/g): qe = (C0 − Ce)V /M

(1)

where C0 is the initial concentration of Cd(II) (mg/L) and Ce is the equilibrium concentration of Cd(II) (mg/L). V represents the volume (L) of the adsorbed solution. M indicates the mass (g) of sorbents.

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RESULTS AND DISCUSSION Thermogravimetric Analysis. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) curves of the pristine PA and PA-UPs are shown in Figure 1. The activation was divided in three stages. Stage I was from 25 °C (room temperature) to 117.3 °C (melting point of UP), stage II was from the melting point to 600 °C, and stage III was the later stabilization section. At stage I, the samples show some peaks below the melting point of UP. The evaporated moisture may have reacted in the samples and the starting materials (PA) dissolved in the UP solution. The highly volatile matter was

Figure 1. TGA (a) and DTG (b) curves for the pyrolysis of original PA and PA-UP. solution at specific ratios (g UP/g PA: 0, 0.5, 1.0, 1.5 and 2.0) and then impregnated and dried at 105 °C overnight. The impregnant was heated in the muffle furnace under N2 atmosphere and maintained for 1 h at the desired temperatures (500, 600, or 700 °C). After carbonization and activation, the samples were washed with distilled water for removal of the impurities. At last, the carbons were dried and sieved for the

Figure 2. (a) SEM and (b) HRTEM micrographs of AC obtained PA at a temperature of 600 °C and impregnation ratio of 1.5 for UP. (c) XRD patterns of the carbons. (d) N2 adsorption/desorption isotherms of AC. (e) XPS spectra of BC and AC: survey scanning spectra of BC and AC. (f) FTIR spectra of BC and AC. 4104


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ACS Sustainable Chemistry & Engineering 2HPO3 → P2O5 + H 2O↑

Table 1. Textural Parameters and Yields of AC parameters

values

SBETa (m2 g−1) Smicb (m2 g−1) Smic/SBET % Sextc (m2 g−1) Vtotd (cm3 g−1) Vmice (cm3 g−1) Vmic/Vtot % Vext f (cm3 g−1) yield %

846.4 573.8 67.79 272.6 0.552 0.337 61.05 0.215 47.7

2P2O5 → 4P + 5O2 ↑

BET surface area (SBET) was determined by using the Brunauer− Emmett−Teller (BET) theory. bMicropore surface area. cExternal surface area. dTotal pore volume (Vtot) was determined from the amount of N2 adsorbed at a P/P0 around 0.95. eMicropore volume (Vmic) was evaluated by the t-plot method.

Table 2. Boehm’s Titration Results and Element Composition of the Carbons values −1

carboxylic groups (mmol g ) lactonic groups (mmol g−1) phenolic groups (mmol g−1) acidity (mmol g−1) basicity (mmol g−1) total groups (mmol g−1) C% N% H%

0.511 0.413 1.120 2.044 1.477 3.521 48.37 9.25 1.69

produced in the process of impregnation of PA by activating agents. Also, UP is volatilized and released urea and then decomposed to CO2 and NH3 at a temperature 50−60 °C below its melting point. Thus, the presence of these activities results in the emergence of peaks. As the temperature increased to 120−450 °C, PA-UPs displayed some distinct peaks due the decomposition of UP and the reaction of PA and UP. These are mainly divided into two parts: 120−190 °C (part I) and 220−450 °C (part II). The corresponding reactions are likely as follows: CO(NH 2)2 • H3PO4 → NH4PO3 + NH3↑ + CO↑ + H 2O↑

(part I)

NH4PO3 → HPO3 + NH3↑

(2)

(part II)

(part II)

(4) (5)

Therefore, more peaks occurred in the decomposition of PA-UPs between 120 and 600 °C in comparison with that of PA, which resulted from the acceleration of the carbonization of PA by the N-containing radicals and oxyacids of phosphorus. When temperature was up to 500 °C, the curves of weight loss of PAUPs were gentle due to the excellent flame-retardant property of UP. This characteristic also enhanced the productivity of carbon sorbents. Physicochemical Performance of Sorbents. From Figure S1, the highest Cd(II) removal performance of ACs shows the best preparation parameters (impregnation ratio 1.0, activation temperature 600 °C). This parameter was prepared for activated carbon for the next experiment. Figure 2a shows the SEM images of sorbents; these clearly show that abundant pore structure was present on the carbon surface, similar to the figures in refs 14 and 15. This also demonstrated that the good pore formation function of UP was expressed in AC preparation. However, the HRTEM images (Figure 2b) of sorbents show a regular microporous structure in the AC. The XRD patterns of sorbents are shown in Figure 2c. A broad peak appeared at 2θ = 22.5° in BC due to the presence of disordered cristobalite.16 For AC, a broad peak appeared at approximately 2θ = 25°, and the peak at 2θ = 22.5° disappeared after activation, indicating the formation of a turbostratic structure of disordered carbon and the removal of ash.17 The peaks at 2θ = 45° appeared in BC and AC simultaneously. Moreover, this peak was stronger for AC than for BC, indicating that carbon tends to crystallize with UP activation. The peaks at 2θ = 45° illustrated the formation of pores as a template of graphitic structures by the decomposition of carbon.18 Figure 2d shows the N2 adsorption/desorption isotherms of sorbents. The isotherms exhibited sharp increases in adsorbed volume at stages of low relative pressure (P/Po), pertaining to type I according to the classification of IUPAC. 19 The characteristics of a microporous structure were evident in the sorbents. The hysteresis loops appeared at 0.4, indicating that the adsorption isotherm belongs to type IV and presented the characteristics of a mesoporous structure. Therefore, microporous and mesoporous structures existed in the sorbents. The textural parameters are calculated in Table 1. A relatively higher BET surface area (846.4 m2/g) was seen in the sorbents, and the existence of microporosity generated the relatively low total pore volume (0.552 cm3/g). Furthermore, the higher Smic and Vmic contents of 68.0% and 61.0% evidenced the well-developed

a

parameters

(part II)

(3)

Figure 3. XPS spectra for AC before and after Cd(II) adsorption: (a) survey spectra and (b) Cd 3d. 4105


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Figure 4. XPS spectra for AC before and after Cd(II) adsorption: C 1s (a) and (b), O 1s (c) and (d), N 1s (e) and (f), and P 2p (g) and (h).

microporous structure. The carbon sorbents prepared from PA by UP activation possessed high BET surface area and microporous volume. The yield values (47.7%) were higher than those in refs 20−22. These outcomes occurred because of the excellent flame-retardant property of UP as an activating agent in the AC preparation procedure, as mentioned in the Thermogravimetric Analysis section. Figure 2e shows the typical survey XPS spectra of sorbents. The elemental surface compositions were calculated according to the areas of element. The C, O, and N relative content of the carbon sorbents was evaluated by XPS. The differences of BC

and AC are that the 7.56% of N existed in AC but BC, indicating that AC included the N functional groups. Boehm’s titration method results were calculated in Table 2, acidic and basic groups of BC and AC existed significant differences. AC contains a large amount of basic groups than BC under the influence of UP activation. The different types of functional groups of BC and AC were investigated by FTIR and are shown in Figure 2f. For the original BC, the peaks at 1384 and 1714 cm−1 correspond to the C−H and CO stretching vibration.23,24 For AC, the absorptions at 1200, 2920, and 3419 cm−1 correspond to C−N, C−H, and N−H or O−H, respectively.25,26 Consequently, the 4106


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ACS Sustainable Chemistry & Engineering carbon adsorbent treated by UP increases the number of pore structure and functional groups, and the types of functional groups were also changed. Cd(II) Adsorption Mechanism. In the Supporting Information, the Cd(II) adsorption data and fitting of kinetics and isotherms are shown in Figures S2 and S3 and Tables S2 and S3. The adsorption behavior was well fitted pseudo-second-order model of kinetics and Langmuir model of isotherm, suggesting that the adsorption processes of the carbons were controlled by chemisorption.15 Figure S4 shows the pH and ion effects for Cd(II) adsorption. It is noted that the ion exchange mechanism was obtained by the proton release during Cd(II) adsorption. Also, noncovalent electron donor−acceptor interactions give rise to a pair of free electrons of C, O, and N atoms in surface functional groups in adsorption process. XPS study was used to explain the adsorption mechanisms, which determined the changes of surface binding groups of AC before and Cd(II) adsorption. The survey and Cd 3d spectra are presented in Figure 3. From XPS survey spectrum of AC-Cd (Figure 3a), the presence of Cd(II) complexes were observed. The peaks at 398.42, 399.87, 400.81, 405.30, 405.63, and 412.07 eV (Cd 3d, Figure 3b) indicated bonding between surface groups and Cd(II). The C 1s, O 1s, and N 1s spectra for AC before and after Cd(II) adsorption are shown in Figure 4. The C 1s deconvolution of AC (Figure 4a) was at 284.67, 285.89, 287.27, 288.69, 290.74, and 294.34 eV corresponding to graphitized carbon, C−N, carbonyl, carboxylic or ester groups, alkyl or ester groups, and π−π* transitions,27−31 respectively. From Figure 4b, the changes of C 1s spectra after Cd(II) adsorption were mainly that the peak at 288.69 eV (CC) was reduced and the peak at 294.34 eV (π−π* transitions) disappeared. It is found that Cd(II) and the π electrons of AC were coordinated during the adsorption. The O 1s spectrum of AC showed peaks at 530.32 eV (oxide ion O), 531.05 eV (CO), 532.71 eV (hydroxyl or ether), 535.36 eV (−CO), and 538.84 eV (C−OH).32,33 From Figure 4c and d, it can be observed that the differentiated peaks disappeared or shifted, obviously to other binding energy sites, after Cd(II) adsorption, reflecting the fact that the lone pair of electrons from O atom in the groups to form complexes with Cd(II) ions. The role of the O-containing groups in Cd(II) adsorption may be described by the following reaction.6 R−COOH + OH− ↔ R−COO− + H 2O −

−

R−OH + OH ↔ R−O + H 2O −

R−O + Cd(II)X ↔ R−O Cd(II)X −

−

R−COO + Cd(II)X ↔ R−COO Cd(II)X

R−NH3+ + Cd(II)X → R−NH 2Cd(II)X + H+

(12)

Table 3. Comparison of the Maximum Adsorption Capacities of Cd(II) onto Various Adsorbents adsorbents precursors

activating agents

PA

(6)

PA

UP

olive stone bagasse pitch apricot stones briquettes coconut shell coffee residue

ZnCl2 H2SO4 H2SO4 steam steam H3PO4 ZnCl2 fluorophore

mesoporousor ganosilica attapulgite/carbon nanocomposites sulfonated styrofoam nitrated styrofoam commercial GAC (F 400)

(8) (9)

where R represent other components of the molecules and Cd(II)X represent Cd(II) species. Like O-containing groups, N-containing groups also contribute to Cd(II) adsorption. From Figure S4, Cd(II) adsorption onto the surface of sorbents was accompanied by the release of H+. From Figure 4e and f, after Cd(II) adsorption, the peak area ratios of 398.3 eV (pyridine) and 399.9 eV (amide or amino) decreased from 21.23% to 12.19% and from 41.95% to 28.87%, respectively. Moreover, the peaks of 401.06, 401.89, 403.77, and 406.13 eV disappeared,34 indicating that nitrogen atoms in the adsorption process as an electron donor. R−NH 2 + H+ ↔ R−NH3+

(11)

Accordingly, Cd(II) were binding on amide/amino group with covalent bonds according to replace H in the amide/amino group or H+ protonated amide/amino groups.4 The coordination compound of [Cd(NH3)6]2+ or [Cd(CN)4]2− occurred between Cd(II) and amide/amino group. The role of phosphate in Cd(II) removal was also determined by XPS and is shown in Figure 4g and h. After Cd(II) adsorption, the two peaks (133.9 and 137.8 eV) of the original AC were shifted. In particular, the peak of the P2O74− group was reduced by 11%. A possible reaction is that the P2O74− mixed with Cd2+ generated Cd2P2O7. In conclusion, Cd(II) adsorption experiments and XPS study coindicated the Cd(II) adsorption mechanism. The porous filtration of sorbents with a well-developed microporous structure also played a role due to the smaller size of Cd species. The ion exchange was occurred between Cd(II) species with the protons in the groups of sorbents. The electrostatic attraction of the deprotonated phenolic and carboxylic groups enhanced adsorption of Cd(II). The O atoms in deprotonated carboxylic, ether phenol, ketone, and ester groups, and N atoms in OC−NH and C−NH2 groups, as well as P atoms in pyrophosphate, could donate their electrons to form coordination complexes or covalent binding with Cd(II). Cd(II) Removal Performance of Different Sorbents. Table 3 shows the maximum Cd(II) adsorption capacities of

(7)

−

R−NH 2 + Cd(II)X → R−NH 2Cd(II)X

adsorption temperature (°C)

qmax (mg/g)

refs

30

8.55

30

40.65

30 25 25 room 25 25 25 room

1.65 38.3 33.57 9.1 8.9 39.61 32.77 186.6

30

46.72

42

room

57.2 67.4 8.21

43

room

this work this work 35 36 37 38 39 40 41

44

various carbon sorbents. The carbon sorbents in this work show the highest adsorptive performance for Cd(II) from aqueous media compared with the references. These results demonstrate that the use of the new activating agent UP in carbon sorbent preparation is feasible and efficient.

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CONCLUSION The present work revealed that carbon sorbent preparation derived from PA by UP activation is feasible. The produced carbon sorbents showed a well-developed microporous structure

(10) 4107


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and rich nitrogen functional groups. This excellent performance promoted the Cd(II) adsorption from aqueous solutions. Based on the Cd(II) adsorption studies, AC presented a higher adsorptive capacity than other adsorbents. The pseudo-secondorder model of kinetics and Langmuir model of isotherm best represented the adsorption data. The adsorption mechanism is mainly attributed to microporous filtration, ion exchange, electrostatic attraction, and surface complexation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00061. Table S1 shows characteristics of urea phosphate. Figure S1 shows effects of impregnation ratios and activation temperatures on the Cd(II) adsorption capacities. Adsorption kinetics were described by Table S2 and Figure S2. Adsorption isothermals were described by Table S3 and Figure S3. Figure S4 shows effects of pH and ionic strengths on adsorption (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: + 86 531 88363015. Fax: + 86 531 88364513. E-mail address: [email protected]; [email protected] (J.Z.). ORCID

Jian Zhang: 0000-0002-9934-8888 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Natural Science Foundation of China (51578321) supported this work. Z.G. (201606220157) would like to acknowledge the fellowship from the China Scholarship Council (CSC).

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REFERENCES

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Biomass-Derived Carbon Sorbents for Cd(II) Removal: Activation and

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