Magnetic Porous Carbonaceous Material ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Magnetic Porous Carbonaceous Material Produced from Tea Waste for Efficient Removal of As(V), Cr(VI), Humic Acid, and Dyes Tao Wen,† Jian Wang,† Shujun Yu,† Zhongshan Chen,† Tasawar Hayat,‡,§ and Xiangke Wang*,†,‡,∥ †

School of Environment and Chemical Engineering, North China Electric Power University, No. 2 Beinong Road, Huilongguan Town, Changping District, Beijing 102206, People’s Republic of China ‡ NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia § Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan ∥ Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, School for Radiological and Interdisciplinary Sciences, Soochow University, Suzhou 215123, P.R. China S Supporting Information *

ABSTRACT: Magnetic porous carbonaceous (MPC) materials derived from tea waste were synthesized by an integrated biosorption−pyrolysis process and were applied as adsorbents for wastewater cleanup. On the basis of various characterizations, we demonstrated that the formation mechanism of γ-Fe2O3 anchored on the porous carbonaceous material surface consisted of the adsorption of iron ions and then the γ-Fe2O3 nucleation and growth through pyrolysis at alternative peak temperatures (300−500 °C). The sample pyrolyzed at 300 °C (MPC-300) showed good capacities for As(V) (38.03 mg g−1) and Cr(VI) (21.23 mg g−1) adsorption, outperforming that of commercial bulk Fe2O3 and many other materials. Moreover, the large available positive charge density can facilitate the effective adsorption of anionic dye (MO) and humic acid (HA) on the γ-Fe2O3 surface while the adsorption performance is sluggish for cationic dyes (MB and RhB). Relatively, the adsorption isotherms could significantly conform to the Langmuir model, and the pseudo-second-order dynamic equation was the optimal model to describe the kinetics for the adsorption of As(V), Cr(VI), humic acid, and dye pollutants on MPC-300. Kinetic studies show that MPC-300 can efficiently remove these pollutants in aqueous solution within 3 h. The presence of HA reduced Cr(VI) and As(V) adsorption on MPC-300 at pH < 6.0. The XPS and FTIR analysis further demonstrated that ion exchange between surface hydroxyl groups and Cr(VI)/ As(V) dominated the adsorption while the adsorption mechanism of MO and HA was attributed to electrostatic attraction on protonated−OH on the γ-Fe2O3 surface. The results suggested that the MPC material was a potential material to remove heavy metal ions, HA, and organic contaminants simultaneously with remarkable adsorption capacity, fast uptake rate, and easy magnetic separation. KEYWORDS: Magnetic porous carbonaceous material, As(V), Cr(VI), Humic acid, Dyes, Adsorption

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INTRODUCTION The global occurrence in water resources containing toxic organic or inorganic pollutants has raised concerns about potential effects on aquatic ecosystems and public health. Wastewaters released by mining, electroplating, paint, metallurgy, leather, and battery manufacturing industries have resulted in the accumulation of persistent organic pollutants and heavy metal ions in the environment. Among the heavy metal ions, both chromium and arsenic, which are known to be mutagenic, teratogenic, and carcinogenic to human health, pose adverse effects on human life and the environment in surface water as well as groundwater.1 At the same time, Cr(VI) and As(V) ions inevitably coexist with organic pollutants in aqueous solutions. For example, humic acid (HA), one of the principal humic fractions, which is mainly produced from the breakdown of animals and vegetables in the environment, is widely © 2017 American Chemical Society

distributed in soils, lakes, and rivers. HA contains many organic functional groups such as carboxylic, hydroxyl, amine, phenol, and quinine groups, which provide some different potential binding sites for metal ions. Although HA itself is not harmful to human health, it can react with halogen-based disinfecting agents during water treatment, thereby producing trihalomethanes, which are associated with an increasing risk of cancer. In addition, many dyes (MO, MB, and RhB) are toxic and have caused serious issues to aquatic living organisms even at low concentrations.2 The presence of these dyes in drinking water can give rise to taste, color, and odor problems, while ingestion through the mouth produces a burning sensation and Received: February 10, 2017 Revised: March 12, 2017 Published: March 22, 2017 4371

DOI: 10.1021/acssuschemeng.7b00418 ACS Sustainable Chem. Eng. 2017, 5, 4371−4380

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ACS Sustainable Chemistry & Engineering may cause nausea, gastritis, and vomiting problems.3,4 Therefore, it is critical to eliminate organic pollutants and heavy metal ions from wastewater. Among various treatment technologies, adsorption is regarded as one of the most significant methods in fundamental studies and industrial applications owing to its easy operation and wide availability of adsorption materials.5 For the sake of low-cost and easy-access advantages, biomaterials have great potential as a raw carbon material for synthesizing various functional materials. Up to now, a wide variety of biomaterials, including enteromorpha prolifera,6 watermelon,7 rice hull ash,8 corn stalk,9 and cotton,10 have served as adsorbents to eliminate environmental pollutants. In view of this point, bioderived carbon materials obtained from waste biomass exhibited enhanced adsorption performance compared to that of the commercial activated carbons.11 Generally, bioderived carbon materials have the advantages of superior thermal and chemical stability, high surface area, and abundant porosity, which ensure interactive sites with organic pollutants and heavy metal ions. Furthermore, magnetically functionalized carbon materials bring new prospects for the management of environmental pollutants because of their easy separation and fast uptake rate.12−14 Thus, several research efforts have been made to evaluate magnetic materials as an adsorbent in water decontamination for removing various contaminants, including heavy metal ions, nutrients, and organic compounds. Zhang et al.15 fabricated colloidal γ-Fe2O3 particles embedded in a porous biochar matrix from cottonwood for the removal of As(V) and methylene blue. Han et al.16 reported that magnetic biochar derived from peanut hull biomass showed an extreme capacity for Cr(VI) adsorption from aqueous solution. Chen et al.17 synthesized a novel magnetic biochar using orange peel powder for highly efficient removal of organic pollutants and phosphate from water. Unfortunately, most reported magnetic biochar usually exhibited low adsorption capacity and poor stability due to the surrounding chemical corrosion of their surface in polluted water. Hence, there is still a need to design and fabricate a high-efficiency adsorbent that presents large capacity, fast adsorption rate, easy separation, and long-term stability. Due to the great demand of tea products, large amounts of tea waste are generated every year in China, which could be reused as an ideal low-cost biomass source for wastewater treatment. Herein, we reported a facile, low-cost, and readily scalable approach for synthesizing a magnetic porous carbonaceous (MPC) material with nanosized γ-Fe2O3 nanoparticles (NPs) embedded in a porous carbonaceous matrix through thermal pyrolysis of FeCl3 treated tea waste. The presence of diverse chemical constituents of tea waste, including sugars, amino acids, and phenols, could be favorable to γ-Fe2O3 growth and prevent γ-Fe2O3 NPs from aggregating.18 In addition, the MPC material not only was an ideal candidate as a highefficiency adsorbent to remove contaminants from water but also could be easily separated from solutions. The prepared MPC materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), magnetic properties, and Brunauer−Emmett−Teller (BET) analysis. To achieve a better understanding of the pollutants’ adsorption on MPC material, Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses were carried out to explore the possible interaction mechanism. The recoverable MPC material derived from tea

waste is expected to have potential application in environmental pollution management.

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EXPERIMENTAL SECTION

Materials. All reagents were of analytical grade and used without any further purification. The chemicals (anhydrous FeCl3, NaNO3, HNO3, NaOH, Na2HAsO4·7H2O, and K2Cr2O7) were purchased from Sinopharm Chemical Reagent Co., Ltd. Humic acid (HA) was extracted from Heilongjiang Province, China. The main HA constituents were approximately 60.44% C, 31.31% O, 4.22% N, 3.53 H, and 0.50% S. The starting material for the manufacture of biochar used in this work was Yellow Mountain Fuzz Tip, a naturally abundant biomass obtained from a local plant in Huangshan, China. Soluble and colored components were initially removed from tea by washing with boiling water and then dried in an oven at 85 °C overnight. The dried tea waste was subjected to ball milling, resulting in a particle size smaller than 120 mesh. Stock suspensions were prepared using Milli-Q water (18.2 M Ω cm−1) throughout the experiments. Synthesis of MPC Materials. The MPC material was synthesized by pyrolysis of Fe-loaded biomass, which was prepared in a biosorption process using the pristine tea leaves as an adsorbent. In a typical synthesis procedure, 6 g of biomass was immersed into 600 mL of FeCl3 solution with a concentration of 10 mM and stirred for 2 h at ambient temperature. Afterward, the mixture was washed with Milli-Q water and oven-dried at 80 °C overnight. The Fe-loaded biomass was transferred to a furnace for pyrolysis under a flowing high purity N2 atmosphere (99.999%) and heated at the appropriate temperature for 1 h at a heating rate of 2 °C/min. The final MPC materials obtained at different temperatures T (300, 400, and 500 °C) were denoted as MPC-300, MPC-400, and MPC-500, respectively. Porous carbonaceous (PC) material was made by the aforementioned procedure for comparison without FeCl3 added. Material Characterization. The morphology and microstructure of the samples were observed using SEM (JSM-6330F, JEOL, Japan). The TEM images were achieved on a Hitachi-7650 transmission electron microscope with at an accelerating voltage of 100 kV. The XRD patterns were obtained on a Philips X’Pert Pro Super X-ray diffractometer equipped with Cu Kα radiation (λ = 1.54178 Å) at a scan rate of 2θ = 0.05o/s. FTIR spectra were obtained on a Nicolet Magana-IR 750 spectrometer. The Raman spectra were recorded at room temperature in the backscattering configuration on a T64000 Jobin-Yvon (Horiba) spectrometer. Magnetic measurements were performed on powder samples using a MPMS-XL SQUID magnetometer. The N2−BET (Barrett−Emmett−Teller) surface area was measured with a Micromeritics ASAP 2010 system. XPS data were performed on a VG Scientific ESCALAB Mark II spectrometer with two ultrahigh vacuum (UHV) chambers. Thermogravimetric (TG) analysis was carried out using a TG-50 thermal analyzer (Shimadzu Corporation) under an air atmosphere at a heating rate of 10 °C min−1. Zeta-potential was measured by using a ZETASIEZER 3000 HSA system. Batch Adsorption Experiments. The adsorption of heavy metal ions and various organic pollutants on PC-300 and MPC-T material was carried out through a batch method. For Cr(VI) or As(V) adsorption, the MPC stock suspension, the NaNO3 background electrolyte solution, and the Cr(VI) or As(V) solutions were added in the polyethylene tubes to obtain the desired concentrations of different components. Negligible volumes of Milli-Q water with a given pH (adjusted with the dilute HNO3 or NaOH solutions) were added to the susupension to achieve the desired pH values. For the kinetic studies, MPC-300 (40 mg) was added to Cr(VI)/As(V) solutions (40 mL) with three different concentrations of 2, 5, and 10 mg L−1, respectively. The pH of the suspensions was adjusted to 5.0 and withdrawn at appropriate time intervals. In the adsorption isotherm studies, the resulting series of MPC materials were set as 1.0 g L−1 with different Cr(VI) or As(V) concentrations (from 2 mg L−1 to 120 mg L−1) at pH = 5.0. The above suspensions were oscillated for 24 h to achieve adsorption equilibration, and the MPC materials were 4372


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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration of the Synthesis of Magnetic Porous Carbonaceous (MPC) Materials

Figure 1. Low-magnification and high-magnification SEM images of biomass waste (a,b) and MPC-300 (d,e). The corresponding TEM images of biomass waste (c) and MPC-300 (f). separated by a permanent magnet. The concentrations of heavy metal ions in the supernatant were determined using inductively coupled plasma−atomic emission spectroscopy (ICP-AES). The adsorption percentage and adsorption capacity were calculated from eqs 1 and 2, respectively:19,20

adsorption (%) =

qe (mg/g) =

(C0 − Ce) × 100% C0

V × (C0 − Ce) m

For organic pollutants, methylene blue (MB), rhodamine B (RhB), methyl orange (MO), and humic acid (HA) were applied as typical organic pollutants. The stock suspensions (300 mg L−1) were prepared by dissolution of various organic pollutants into Milli-Q water. A certain amount of MPC-300 sample in the suspension was mixed with the aqueous soltutions of HA and other organic dyes. After gentle shaking for 2 h, the solid phases were separated from the suspension. The concentrations of residual MB, MO, RhB, or HA were determined on a UV−vis spectrophotometer (Hitachi, U-3900, Tokyo, Japan), and the calculation of the adsorption percentage and the adsorption capacity was the same as that for the heavy metal ions. All experimental data were the average of triplicate determination, and the relative errors were about 5%.

(1)

(2)

where C0 (mg L−1) and Ce (mg L−1) are the initial concentration and the equilibrium one after adsorption, respectively, and m (mg) and V (mL) are the dosage of MPC and the total volume of the suspension, respectively. The Langmuir isotherm model assumes a homogeneous adsorption surface with the equal adsorbate affinity, which is presented as21 Langmuir model: qe =

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RESULTS AND DISCUSSION Characterization of MPC Materials. Our strategy for the synthesis of the MPC sample was schematically described in Scheme 1. The Fe-loaded precursors were first converted into Fe-hydroxide (FeO(OH)) composites in the drying process.23 A subsequent pyrolysis step of Fe-preloaded biomass at 300− 500 °C leads to nucleation and growth of nanoparticles and the release of porosity, resulting in the metal oxide embedded in the amorphous carbon matrix. Details about the morphology of the pristine tea waste and MPC-300 were examined by SEM and TEM. The tea waste appeared as bulk stone-like features (Figure 1a), and no porous structure was observed in high magnification SEM (Figure 1b) and TEM (Figure 1c) images. After the pyrolysis of the as-loaded composite, numerous γFe2O3 NPs (Figure 1d) were decorated on the surface of biochar where nanometer range (50−80 nm) particles were individually formed without obvious aggregations (Figure 1e). The TEM image of MPC-300 (Figure 1f) identified the thin

bqmax Ce 1 + bCe

(3)

The Freundlich isotherm model is applicable to a multilayer adsorption surface with a heterogeneous energetic distribution of active sites, which can be described by the following equation:22 Freundlich model: qe = kCe1/ n

(4)

−1

where qe (mg g ) is the amount of heavy metal ions adsorbed per unit weight of adsorbent. qmax (mg g−1) is the saturation adsorption capacity associated with complete monolayer coverage, and b (L mg−1) is a Langmuir constant that relates to the energy of the adsorbent. 1/n and k (mg g−1) are correlated to the relative adsorption intensity and adsorption capacity, respectively. 4373


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Figure 2. (a) XRD patterns and (b) FTIR spectra of MPC-T (300, 400, and 500 °C).

Figure 3. (a) TGA and (b) the corresponding DTA curves of MPC-T (300, 400, and 500 °C). (c) Hysteresis curves of MPC-T at 300 K, the corresponding close view of the hysteresis loops of MPC-T (300, 400, and 500 °C), and (d) MPC-300 material dispersed water solution and magnetic separation.

cm−1 (stretching vibration of −OH and −NH groups), 1643 cm−1 (CO stretching vibration conjugate with the NH2 (amide I band)), 1398 cm−1 (the symmetric stretch of C−N) and 1054 cm−1 (C−O stretching vibration).25,26 The band at 588 cm−1 was related to the stretching vibration of the Fe−O bond.27 Notably, the absorption bands at 2920−2850 cm−1 on MPC-300 were attributed to the stretching vibration of − CH and − CH2, while both functional groups were disappeared for MPC-400 and MPC-500. Figure S1a show Raman spectra of all samples has two strong peaks centered at 1330 and 1600 cm−1, which can be ascribed to the D band and G band of carbon material.28 While the peaks appeared around 222, 289, 392, and 501 cm−1 were characteristic Raman shifts of γ-Fe2O3, respectively.29,30 As the temperature was increased, thermal decomposition of original organic residues resulted in the release of the byproducts of H2O, CO2 and CO, which was accompanied by the formation of voids because of the volume

layer and the presence of magnetic NPs in the porous carbon matrix. To further confirm the existence of magnetic iron oxide, the obtained samples of MPC-T samples pyrolyzed at different temperatures were characterized by XRD. As observed in Figure 2a, typical peaks at 30.1°, 35.5°, 37.1°, 43.1°, 53.5°, 57.0 o , and 62.5° were assigned to the (220), (311), (222), (400), (422), (511), and (440) facets of γ-Fe2O3 (JCPDS No. 39− 1346), respectively.24 With increasing annealing temperature to 500 °C, the crystallinity of the powder increased because the intensities of diffraction peaks became stronger and sharper than those of the samples prepared at 300 and 400 °C. In addition, three patterns showed a broad and relatively weak diffraction peak at 23°, which corresponded to the amorphous carbon, composed of aromatic carbon sheets oriented in a relatively random manner. As shown in Figure 2b, the FTIR spectra of MPC-T samples showed characteristic bands at 3407 4374


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Figure 4. Adsorption isotherms of Cr(VI) (a) and As(V) (b) on PC-300 and MPC-T (300, 400, and 500 °C). m/V = 1.0 g L−1, pH = 5.0 ± 0.1, I = 0.01 M NaNO3.

Table 1. Langmuir and Freundlich Adsorption Isotherm Parameters for Cr(VI) and As(V) on PC-300 and MPC-T (300, 400, and 500 °C) Langmuir −1

Freundlich −1

2

species

sample name

qmax(mg g )

b (L mg )

R

Cr(VI)

PC-300 MPC-300 MPC-400 MPC-500 PC-300 MPC-300 MPC-400 MPC-500

8.53 21.23 17.14 12.33 9.25 38.03 31.67 27.81

0.042 0.117 0.071 0.051 0.049 0.029 0.012 0.008

0.973 0.964 0.979 0.998 0.908 0.925 0.958 0.965

As(V)

k

n

R2

1.06 5.47 3.18 1.81 1.34 2.56 0.87 0.79

2.38 3.35 2.82 2.55 2.56 1.83 1.41 1.27

0.927 0.867 0.893 0.953 0.815 0.855 0.922 0.907

g−1), suggesting that the Ms values were related to the heattreatment temperature and the load of γ-Fe2O3. Interestingly, the MPC-300 material can be efficiently separated from aqueous solution by using a permanent magnet (Figure 3d). Adsorption Isotherms of Cr(VI) and As(V) on the MPC Materials. To evaluate the adsorption performance, the resulting series of MPC-T samples pyrolyzed at different temperatures were expected to probe the uptake of Cr(VI) and As(V) ions from aqueous solutions. Two typical isotherm models (Langmuir and Freundlich) were applied to simulate the adsorption isotherms, and the results are shown in Figure 4. Langmuir and Freundlich isotherm constants for Cr(VI) and As(V) adsorption were summarized in Table 1. One can see from the R2 values that the Langmuir model fitted the experimental data better than the Freundlich model, indicating that the binding energy of MPC-T is uniform. In addition, the Freundlich constants n were found to be higher than 1, suggesting the good affinity of as-prepared samples for both Cr(VI) and As(V). According to the results of Langmuir model simulation (Figure 4a), the maximum adsorption capacity of Cr(VI) on MPC-300 was 21.23 mg g−1, which was higher than those of Cr(VI) on PC-300 (8.53 mg g−1), MPC-400 (17.14 mg g−1), and MPC-500 (12.33 mg g−1). Similar results (Figure 4b) were also observed for the adsorption of As(V) on MPC samples pyrolyzed at 300−500 °C, which possessed impressive capacities of 9.25, 38.03, 31.67, and 27.81 mg g−1 (still not saturated) for As(V) on PC-300, MPC-300, MPC-400, and MPC-500, respectively. Futhermore, the desorption isotherms were obviously higher than the adsorption isotherms (Figure S2), suggesting the irreversible adsorpiton and the adsorption of Cr(VI) and As(V) on MPC samples was mainly dominated by strong chemical interaction. As listed in Table S1, all of these

shrinking at high temperature. These results indicated that carboxyl and hydroxyl groups of biochar interacted with Fe−O bonds of γ-Fe2O3. From Figure S1b, the surface area of MPC300 calculated by the Brunauer−Emmett−Teller (BET) method was 63 m2 g−1, which was significantly higher than those of PC-300 (16 m2 g−1), MPC-400 (34 m2 g−1), and MPC-500 (31 m2 g−1). Therefore, the heat-treatment temperature played a crucial role in achieving various functional groups and the chemical bonds between biochar and hematite. The TG analysis was conducted to examine the weight percentage of γ-Fe2O3 in MPC-T samples (Figure 3a). The weight loss of the drying process below 125 °C was considered to be due to the evaporation of free water. The TGA curve of MPC-300 showed a slow weight loss of 5.3% from 125 to 260 °C, which could be attributed to the decomposition of residual organic moieties. However, the curves of MPC-400 and MPC500 maintained a stable tendency below 300 °C. Subsequently, a major weight loss (∼65%) occurred between 300 and 500 °C, indicating a large elimination of oxygen-containing functional groups.31 The DTA curves showed a corresponding intense endothermic peak around 400 °C (Figure 3b), which originated from the release of H2O, CO2, and CO molecules developed by the combustion of an organic carbon framework. The results showed that the mass percentages of γ-Fe2O3 were about 19.9% for MPC-300, 27.7% for MPC-400, and 28.8% for MPC-500. Such a difference of γ-Fe2O3 contents can cause diverse magnetization hysteresis curves. The magnetic properties of MPC-T samples measured at room temperature (Figure 3c) showed ferromagnetic behavior with small remnant magnetization and coercivity. The specific saturation magnetization (Ms) of MPC-300 was 12.15 emu g−1, which was lower than those of MPC-400 (34.81 emu g−1) and MPC-500 (45.25 emu 4375


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Figure 5. Effect of initial Cr(VI) (a) and As(V) (c) concentration (2, 5, and 10 ppm) on the Cr(VI) and As(V) removal efficiency of MPC-300. Pseudo-second-order linear plots for the removal of Cr(VI) (b) and As(V) (d) by MPC-300. m/V = 1.0 g L−1, pH = 5.0 ± 0.1, I = 0.01 M NaNO3.

values were much higher than previous reported flowerlike CeO2 (5.9 mg/g for Cr(VI) and 14.4 mg/g for As(V)),32 surfactant-modified zeolite Y (1.95 mg/g for Cr(VI) and 0.93 mg/g for As(V)),33 flowerlike α-Fe2O3 (5.4 mg/g for Cr(VI) and 7.6 mg/g for As(V)), and commercial bulk α-Fe2O3 (0.37 mg/g for Cr(VI) and 0.3 mg/g for As(V)).34 In addition, the poor adsorption performance of MPC-400 and MPC-500 may be ascribed to the relatively lower density of adsorptive sites on the γ-Fe2O3. This is because the magnetic nanoparticles obtained at higher temperatures have a relatively higher crystallization degree and larger particle diameter and are more closely in contact with the porous carbon layer.35 Nevertheless, it was expected that MPC-400 and MPC-500 hold a higher magnetic strength for easier separation, which was confirmed by the magnetic hysteresis curves (Figure 3c). Another interesting phenomenon was that MPC-300 showed a greater adsorption capacity for As(V) than Cr(VI), indicating that the surface of MPC-300 held a relatively greater affinity for As(V) than for Cr(VI). Such types of effects were reported in the formation of an inner-sphere complex between As(V) and iron (hydr-)oxide minerals, while the Cr(VI) anion was weakly adsorbed on the surface of iron oxides through an outer-sphere surface complex.36,37 Kinetic Studies of Cr(VI) and As(V) on MPC-300. On the basis of the above adsorption behavior of as-obtained samples, we investigated a series of kinetic experiments of Cr(VI) and As(V) adsorption on MPC-300. Figure 5a and c showed that the adsorption rates of Cr(VI) and As(V) on MPC-300 were considerably fast within the initial contact time of 1 h at three initial concentrations (2, 5, and 10 ppm). The time needed for complete removal of Cr(VI) and As(V) at low initial concentrations (2 and 5 ppm) was just 1.5 h, and the final removal efficiencies were up to ∼100%. In comparison, the adsorption efficiencies were found to be 81.8% and 90.7% for Cr(VI) and As(V) on MPC-300 at the initial concentration of

10 ppm. For the concentration of 10 ppm, the equilibrium time of Cr(VI) or As(V) on MPC-300 was longer than that of other concentrations (2 and 5 ppm), indicating that the active sites of γ-Fe2O3 on MPC-300 were gradually occupied by Cr(VI) or As(V) anions. The pseudo-first-order model and pseudosecond-order kinetic model were used to explore the kinetic adsorption processes (e.g., mass transfer or chemical reaction), which were described as follows: pseudo-first-order model: ln(qe − qt ) = ln qe − k1t

(5)

pseudo-second-order model: t 1 t = + 2 qt qe k 2qe

(6)

where qe (mg g−1) and qt (mg g−1) are the amounts of Cr(VI) or As(V) adsorbed after equilibrium and at time t (h), respectively. k1 (h−1) is the Lagergren adsorption rate constant, calculated from the plot of ln(qe − qt) versus t. k2 (g mg−1 h−1) represents the rate constant of the pseudo-second-order rate model. k2 and qe values can be determined experimentally by plotting t/qt versus t. From the adsorption kinetics shown in Figure 5b and d, the adsorption rate of Cr(VI)/As(V) on MPC-300 depended on its initial concentration, and the higher the concentration, the slower the adsorption rate. The corresponding detailed kinetic parameters were also tabulated in Table 2 and Table S2. The correlation coefficient values (R2) calculated from the pseudo-second-order kinetic model were above 0.999, which were higher than those of the pseudo-firstorder model. In addition, the calculated qe values were in good agreement with the experimental ones, showing quite good linearity. Therefore, the Cr(VI) and As(V) uptake onto MPC300 were favorable by the pseudo-second-order kinetic model, indicating a chemisorption process.38 4376


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the binding sites of solid particles. Thus, pH-dependent behavior of Cr(VI)/As(V) adsorption onto MPC-300 demonstrated that Cr(VI)/As(V) adsorption was believed to occur via electrostatic interactions.39 Actually, Cr(VI) and As(V) ions inevitably coexist with various electrolyte ions in wastewater, which would influence the migration of pollutants in environmental mediums. Thus, Na+, K+, Mg2+, and Ca2+ as well as Cl−, NO3−, and SO42− anions were the common coexisting ions. From Figure 6d, it was found that the presence of nitrate salts for cations and sodium salts for anions reduced the adsorption capacity of adsorbate on MPC-300. Most coexisting ions had no obvious influence on Cr(VI) and As(V) adsorption. Notably, the SO42− anion can significantly reduce both heavy metal ions’ adsorption on MPC-300, which was mainly ascribed to the strongest competition effect with target anions. In view of this point, the coexisting SO42− ions should be removed beforehand to obtain a high efficiency of MPC-300 for Cr(VI)/ As(V) adsorption. XPS Analysis. To study the interaction mechanism between the adsorbates and MPC-300, surface sensitive XPS spectra were measured with MPC-300 before and after Cr(VI) and As(V) adsorption. As shown in Figure 7a and b, there were four elements in the XPS survey spectrum of MPC-300, including 66.19 atom % of C, 23.44 atom % of O, 6.05 atom % of Fe, and 4.32 atom % of N. The fitted high-resolution spectrum of Fe 2p (Figure S3a) revealed that the peak positions located at ∼725.8 and ∼712.4 eV were assigned to Fe 2p1/2 and Fe 2p3/2 of γFe2O3 rather than Fe3O4,40 which were consistent with the XRD results. In addition, the peak centered at ∼719.1 eV was characteristic of γ-Fe2O3, which was not obvious in Fe3O4. Deconvolution of C 1s (Figure S3b) showed that there were three types of carbon atoms in MPC-300, i.e., the nonoxygenated carbon (284.6 eV), the carbon in C−O bond

Table 2. Kinetic Parameters Calculated from PseudoSecond-Order Model species Cr(VI)

As(V)

C0 (mg L−1)

qe,exp (mg g−1)

qe,cal (mg g−1)

k2 (g mg−1 h−1)

R2

2 5 10 2 5 10

1.88 4.96 8.18 1.91 4.86 9.07

1.90 4.98 8.21 1.91 4.88 9.12

10.05 3.20 1.25 15.39 5.77 0.96

0.999 0.999 1 0.999 0.999 0.999

Effect of pH, Humic Acid, and Coexisted Ions. The effect of solution pH on Cr(VI) and As(V) adsorption on MPC-300 was also investigated. As shown in Figure 6a and b, the tendency of both heavy metal anions’ adsorption on MPC300 decreased slowly at pH < 4.0 and quickly at pH 4.0−10.0. Similar effects of pH on Cr(VI) and As(V) adsorption in the presence of HA were also observed. However, the presence of HA slightly decreased Cr(VI)/As(V) adsorption at pH < 6.0. No obvious difference was found for As(V) adsorption in the HA+MPC-300 system at pH > 6.0. On the basis of the investigation of zeta-potential (Figure 6c), one can see that the presence of HA slightly influenced the pHzpc values, which decreased from 6.8 (absence of HA) to 6.0 (presence of 10 mg/ L HA). At pH < pHzpc, HA adsorbed on MPC-300 became more negative, and HA occupied the active sites, resulting in the decrease of binding negative charged Cr(VI)/As(V) target ions, whereas at pH > pHzpc, the negative charged Cr(VI)/ As(V) and soluble HA did not easily adsorb on the negative charged surface of MPC-300 due to the electrostatic repulsion. We speculated that in the case of the HA+MPC-300 system, Cr(VI)/As(V) anions might compete against HA molecules for

Figure 6. Effect of humic acid on the adsorption of Cr(VI) (a) and As(V) (b) on MPC-300. (c) Zeta potential of MPC-300 and HA+MPC-300 as a function of pH. (d) Effect of coexisting ions on the adsorption of Cr(VI) and As(V) by MPC-300. m/V = 1.0 g L−1, I = 0.01 M. 4377


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Figure 7. (a) XPS spectra and (b) elemental compositions of MPC-300 before and after As(V) and Cr(VI) adsorption. And the corresponding O 1s spectrum of MPC-300 (c) before adsorption, (d) after As(V) adsorption, and (e) after Cr(VI) adsorption.

MPC-300 can be perfectly fitted by a pseudo-second-order model. The total organic carbon (TOC) concentration as a function of contact time (Figure S7) showed clearly that the content of TOC in residual solution was reduced quickly with an increase of contact time, demonstrating the strong interaction of organic pollutants with MPC-300. The experimental data of MB, RhB, MO, and HA adsorption were regressively simulated with the Langmuir and Freundlich models (Figure S8), and the relative parameters were tabulated in Table S4. From Table S4, one can see that the isotherms could be described well by the Langmuir model rather than the Freundlich model. This was due to the assumption of an exponentially increasing adsorption amount in the Freundlich model. Moreover, MPC-300 possessed impressive capacities of 95.92 mg g−1 toward HA and 73.12 mg g−1 toward MO, which were higher than those toward MB (45.74 mg g−1) and RhB (31.94 mg g−1; as shown in Table S4). Obviously (Figure S6d), the adsorption performances of PC-300 toward HA, MO, MB, and RhB were 34.14, 25.52, 22.69, and 21.36 mg g−1, respectively. Generally, MO and HA are organic molecules with a negative charge in aqueous solution, whereas MB and RhB have a positive charge.9 The surface of γ-Fe2O3 was positively charged, which was favorable for the binding of the negatively charged HA and MO targets due to the strong electrostatic. Furthermore, the mutual effects of HA and MO adsorption on MPC-300 were also investigated. As shown in Figure S9a, the presence of HA led to slight inhibition of MO adsorption on MPC-300, suggesting the stronger coordination of γ-Fe2O3 for HA. The abundance of various organic pollutants adsorbed on MPC-300 can be confirmed by means of FTIR spectroscopy, as shown in Figure S9b. After adsorption of various dyes and HA by MPC-300, the characteristic spectra of MB, RhB, MO, and HA were almost recorded in the

(286.2 eV), and the carboxylate carbon in O−CO bond (289.0 eV).41 There were two main types of carbon in MPC300, i.e., the carbon in C−O and the nonoxygenated carbon. After adsorption, the signals of Cr(VI) and As(V) were found on the MPC-300 sample. The As 3d spectrum of the As(V)adsorbed sample (Figure S3c) showed a characteristic peak located at 45.4 eV, which was attributed to the As−O bonding, while the high-resolution of the Cr 2p XPS spectrum (Figure S 3d) showed the characteristic Cr 2p peaks at 577.2 and 587.1 eV, which were ascribed to Cr 2p3/2-O and Cr 2p1/2-O bonding, respectively. The complex O 1s spectrum (Figure 7c−e) of MPC-300 can be deconvoluted into four different oxygen containing functional groups: (a) O−H at 533.5 eV, (b) C−O at 532.3 eV, (c) CO at 531.0 eV, and (d) Fe−O 530.0 eV. It was found that the intensities of O−H peaks after As(V) and Cr(VI) adsorption decreased slightly, whereas Fe−O species became significantly higher, suggesting that the partial replacement of hydroxyl groups was exchanged by H2AsO4− or HCrO4−/Cr2O72− (the predominate species in aqueous solution at pH 5, presented in Figure S4). The results were consistent with a recent study, in which Cao et al. found that the O−H groups on flowerlike α-Fe2O3 were replaced by As(V) or Cr(VI) anions, resulting in the decreased ratio of t2g/ eg in the synchrotron-based XANES spectra.34 Adsorption of Humic Acid and Organic Pollutants. The adsorption performance of MPC-300 with various dyes (MB, RhB, and MO) and HA were also studied, followed by UV−vis adsorption examination of the residual solutions (Figure S5). From a kinetic aspect (Figure S6a), MPC-300 can remove ∼80 wt % of pollutant (10 ppm) from aqueous solution within the contact time of 30 min, exhibiting a fast adsorption rate. The high correlation coefficients (Table S3) suggested that the adsorption kinetics of organic pollutants on 4378


Research Article


Education Institutions are acknowledged. X.W. acknowledges the CAS Interdisciplinary Innovation Team of Chinese Academy of Sciences.

corresponding spectrum of the adsorptive adduct. From the ring stretching band at 1643 cm−1 and C−O stretching vibration at 1054 cm−1, the organic pollutants that adsorbed MPC-300 showed blue shifts to 1617 and 1034 cm−1,42 respectively. And the symmetric stretch of C−N at 1384 cm−1 appeared after their adsorption onto MPC-300, implying the interaction between various pollutants and γ-Fe2O3.43

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CONCLUSIONS To get rid of heavy metal ions and organic pollutants from wastewater, the MPC materials were synthesized via a simple biosorption method followed by heat treatment process. The results showed that the sample prepared at 300 °C (MPC-300) was the best among the series with the maximum adsorption capacities of 38.03 mg g−1 for As(V) and 21.23 mg g−1 for Cr(VI) at pH 5.0. Ion exchange between the hydroxyl on the γFe2O3 surface and As(V) or Cr(VI) was the main mechanism of As(VI)/Cr(VI) interaction with MPC-300. Further investigations revealed that the adsorption capacities were in the order of RhB < MB < MO < HA, largely because the MPC-300 can selectively adsorb negatively charged anionic dyes and HA through electrostatic interaction. Therefore, the novel MPC material is practically capable for the efficient separation of heavy metal ions, humic acid, and dyes from wastewater in environmental pollution cleanup.

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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.7b00418. N2 adsorption−desorption isotherms; adsorption−desorption isotherms of Cr(VI) (a) and As(V) on MPC300; X-ray photoelectron spectroscopy (XPS) spectrum of Fe 2p and C 1s for MPC-300; XPS spectrum of As 3d and Cr 2p of MPC-300 after As(V) and Cr(VI) adsorption; relative distribution of As(V) and Cr(VI) species in aqueous solutions; UV−vis spectra of MB, RhB, MO, and HA adsorption on MPC-300 as a function of reaction time; kinetic parameters of As(V), Cr(VI), MB, RhB, MO, and HA adsorption on MPC-300; dyes and humic acid adsorption on MPC-300; Langmuir and Freundlich adsorption isotherm fitting and parameters for MB, RhB, MO, and HA sorption on MPC-300; TOC concentrations as a function of reaction time (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-10-61772890. Fax: +86-10-61772890. E-mail: [email protected]. ORCID

Xiangke Wang: 0000-0002-3352-1617 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from NSFC (21225730, 91326202, 21377132, 21577032), the Fundamental Research Funds for the Central Universities (JB2015001), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection and the Priority Academic Program Development of Jiangsu Higher 4379


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