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Adsorption behavior and mechanism of Cr(VI) by modified biochar derived from Enteromorpha prolifera
Ecotoxicology and Environmental Safety 164 (2018) 440–447
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Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Adsorption behavior and mechanism of Cr(VI) by modified biochar derived from Enteromorpha prolifera
T
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Youyuan Chen, Baoying Wang, Jia Xin , Ping Sun, Dan Wu Key Laboratory of Marine Environment and Ecology, Ministry of Education; Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering (MEGE); College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Enteromorpha prolifera Magnetic biochar Adsorption Cr(VI)
Biochar that was derived from Enteromorpha prolifera and magnetically modified (BCF600) was evaluated for its physicochemical properties and Cr(VI) adsorption behavior and mechanism. The results showed that the modified biochar was less porous on surface and loaded with γ-Fe2O3 particles. BCF600 exhibited a maximum adsorption capacity for Cr(VI), which acquired from the Langmuir model of 88.17 mg g−1 and a removal efficiency of 97.71%, for 100 mg L−1 of Cr(VI). The adsorption of Cr(VI) by BCF600 decreased with increasing pH and background ion intensity. Based on the FTIR results, the change in the -OH groups on the surface after adsorption confirmed that electrostatic interaction was likely the preponderant mechanism. In addition, the BCF600 could be easily separated using a magnet and displayed high recyclability. Therefore, this magnetic biochar derived from Enteromorpha prolifera has the potential to serve as a highly efficient adsorbent for water pollution control.
1. Introduction Biochar is a carbon-rich product of biomass pyrolysis under hypoxic or anaerobic conditions (Dawood et al., 2017). It has a strong ability to adsorb various pollutants in water because of its stable porous structure, high specific surface area, and enriched oxygen-containing functional groups; thus, it has attracted much attention (N. Zhou et al., 2017). Presently, many raw materials exist for biochar preparation, such as sawdust (Y.Y. Zhou et al., 2017), rice straw (Qian et al., 2017), bamboo (Fan et al., 2010), rice husk (Ma et al., 2014), safflower seed (Angın, 2013), pinewood (Wang et al., 2015b), olive busk (Demirbas, 2004), Alternanthera philoxeroides (Yang et al., 2014), and other ligneous and cellulosic plant carbon, as well as fecal carbon and sludge carbon (Agrafioti et al., 2014a). However, finding cheap biomaterials with superior performance and easy steps for biochar preparation remains a major challenge in conventional wastewater treatment (Inyang et al., 2016). Enteromorpha prolifera is a marine macroalgal species that has high reproductive capacity owing to eutrophication and easily forms green tides, which negatively affect marine transportation, tourism, and water quality (Zhou et al., 2010). For instance, in the summer of 2008, an unprecedented outbreak of Enteromorpha prolifera occurred in China's Yellow Sea in the port city of Qingdao, with a total salvage of 100 million metric tons (Zhang et al., 2012). In the past ten years since that event, Enteromorpha prolifera has maintained its
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prosperity, thus threatening the economic development of many cities along the coast of the Yellow Sea. Determining how to use and dispose of Enteromorpha prolifera in an efficient and environmentally friendly manner is critical for marine environmental protection and efficient resource utilization. Therefore, the feasibility of using Enteromorpha prolifera to prepare biochar that can be applied in wastewater treatment has piqued interest. Until now, the literature on algal biochar and its application remains very limited (Shukla et al., 2017). Studies to date have indicated that biochars produced from algal biomass are fundamentally different from biochars produced from lignocellulosic feedstock (Maddi et al., 2011). Generally, biochars produced from various algal species tend to have low cation exchange capacity, carbon content, and specific surface area, but are high in pH, nitrogen, and extractable inorganic nutrients including P, K, Ca, and Mg (Yu et al., 2017). Given these characteristics of algal biochar, in most studies, it has been used as a soil improver in soil remediation (Li et al., 2011), but little attention has been paid to its adsorption potential. Later, some studies found that the cell wall of algae was composed of sugars, proteins, cellulose, lipids, etc. These constituents comprise hydroxyl, ketonic, phenolic, aldehydic, carboxylic, and other polar functional groups, which could serve as potential adsorption binding sites (Nautiyal et al., 2016), thus inspiring some researchers to hypothesize that algal biochar could be used as an adsorbent. In particular, for macroalgae, Roberts et al. (2015) observed
Corresponding author. E-mail address: [email protected] (J. Xin).
https://doi.org/10.1016/j.ecoenv.2018.08.024 Received 12 June 2018; Received in revised form 3 August 2018; Accepted 7 August 2018 0147-6513/ © 2018 Elsevier Inc. All rights reserved.
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2.2. Preparation of magnetic Enteromorpha prolifera derived biochar
that the specific surface area of biochar produced from macroalgae was relatively higher than that produced from other algal species. Subsequently, studies were conducted on the absorption capacity of algal biochar [Scenedesmus dimorphus (Bordoloi et al., 2017), Enteromorpha prolifera (Qiao et al., 2017), Spirulina platensis (Nautiyal et al., 2016), and Porphyra tenera (Park et al., 2016)] for Co(II), polycyclic aromatic hydrocarbons (PAHs), Congo red dye, and copper(II), respectively. However, the findings in these studies concerning algal biochar as an adsorbent for water pollutants were not as ideal as expected. Although these algal biomass sources are readily available and abundant, their adsorption capacities generally lie between 0.05 and 23.00 mg g−1 (Bordoloi et al., 2017; Nautiyal et al., 2016; Park et al., 2016; Qiao et al., 2017), which is significantly lower than that of biochar prepared from lignocellulosic feedstock. Therefore, the need to find technical methods to improve the adsorption capacity of algal biochars is urgent. Recently, many different biochar modification methods have been discussed: (1) chemical modification by increasing surface functional groups, aromaticity, and pore structures (Fan et al., 2010; Ma et al., 2014); (2) simple physical modification by improving biochar carbon structure, surface area, and pore volume (Lima et al., 2010); and (3) load modification by loading metals, oxides, or ions (Han et al., 2015; M. Zhang et al., 2013; Zhou et al., 2014). Among them, magnetic modification, an oxide load-modification method that can both increase adsorption capability and generate recyclability, is an efficient and promising alternative modification method. Wang et al. (2015b) synthesized a magnetic biochar using pinewood that had a As(V) adsorption capacity about 1.6 times higher than that of its corresponding original biochar. The adsorption capacity for As(V) of a magnetic biochar derived from chestnut shell (Z. Zhou et al., 2017) was 45.8 mg g−1, which was much greater than that of the pristine biochar (17.5 mg g−1). Han et al. (2015) used peanut hull to prepare magnetic biochar, yielding a product that had a Cr(VI) adsorption capacity about 1–2 orders of magnitude greater than that of its corresponding pristine biochar. However, according to our current knowledge, no attempt has ever been made to modify Enteromorpha prolifera with this method. Hence, whether the magnetic modification of Enteromorpha prolifera derived biochar could significantly increase its adsorption capability and magnetic properties is unknown and needs to be investigated. If the response is positive, deeper insights into the governing sorption mechanism of this modified biochar should be sought. In this study, a magnetic biochar was synthesized from macroalgae Enteromorpha prolifera biomass for the first time and its adsorption behavior and mechanism regarding the target heavy metal Cr(VI) were studied. The main purposes of this study were to: (1) synthesis and characterize the magnetic Enteromorpha prolifera derived biochar; (2) explore the adsorption properties and adsorption mechanism of Cr(VI) by magnetic Enteromorpha prolifera derived biochar; (3) evaluate the recyclability of the magnetic Enteromorpha prolifera derived biochar, and provide scientific guidance for the utilization of Enteromorpha prolifera.
The preparation of magnetic Enteromorpha prolifera derived biochar was based on the method reported by M. Zhang et al. (2013). Briefly, 50 g of Enteromorpha prolifera biomass was added to 500 mL FeCl3 solution at a concentration of 2 mol L−1 and stirred for 2 h with a magnetic stirrer at 80 °C. Then, the Enteromorpha prolifera biomass was separated from the solution and dried at 70 °C. The Enteromorpha prolifera biomass was then placed into a porcelain crucible and capped. The porcelain crucibles were subjected to heating in a muffle furnace for pyrolysis at desired temperature (400 or 600 °C) at the rate of 10 °C min−1 and then keep constant for 2 h. After naturally cooling to room temperature (25 °C), the biochar was sieved through a 0.6 mm sieve to produce particles less than 0.6 mm and washed with DI water a few times, then dried at 70 °C for 6 h, and finally put in a closed container before use. The pristine Enteromorpha prolifera derived biochar was prepared in the same way, except it was not pretreated with FeCl3. The original biochar samples were designated “BC” and the magnetic biochar samples were designated “BCF” with a suffix indicating the peak temperature of pyrolysis (i.e., BC400, BC600, BCF400, and BCF600). 2.3. Biochar characterization The surface morphology of the samples was characterized using scanning electron microscopy (SEM) (Hitachi S-4800, Japan). The surface functional groups of the samples were determined via Fourier transform infrared spectroscopy (FTIR) (Bruker TENSOR 27, Germany). The crystallographic structure of the samples was examined through Xray diffraction (XRD) analysis (Ultima IV, Japan). The specific surface area was quantified using the Brunauer, Emmett, Teller (BET) method (Autosorb-iQ3, USA). The elemental composition of biochars were determined via an elemental analyzer (Vario EL III, Elementar, Germany). 2.4. Sorption experiments 2.4.1. Optimization of preparation parameters Based on our previous study (Chen et al., 2017), two pyrolysis temperatures (400 and 600 °C) were chosen for biochar preparation. The pristine biochars (BC400 and BC600) and magnetic biochars (BCF400 and BCF600) were prepared at these two temperatures. BCF600, which had a greatest adsorption capacity, was selected based on preliminary experiments; hence, the physicochemical properties and the adsorption behavior and mechanism of BCF600 were further explored. 2.4.2. Adsorption and desorption characteristics To 50 mL conical vessels, 0.06 g of biochar and 30 mL of Cr(VI) solution were added. The initial pH of the Cr(VI) solutions with no biochar was tested as 5.03 ± 0.02 using a pH meter (HQ30D, Hach, USA). Then, the conical vessels were sealed and shaken in a 25 °C thermostat oscillator with 140 r min−1 shock oscillation for 120 h. After the adsorption equilibrated, the suspension was centrifuged at 3000 r min−1 for 5 min and the resultant supernatant was filtered at 0.45 µm. The concentration of Cr(VI) was tested with a UV–vis spectrophotometer (Pgeneral T6, China) using diphenylcarbazide spectrophotometry (C.Q. Chen et al., 2011). As a blank, different concentrations of Cr(VI) solution without biochar were separately measured via colorimetric analysis in experiments. No change in absorbance intensity of the solution illustrated that the Cr(VI) solution was stable. To determine the best dosage of each biochar, different dosages (1, 1.5, 2, 3, 4, 6, 8, 10, 12, 14, and 16 g L−1) of the four biochars (BC400, BC600, BCF400, and BCF600) were added to 30 mL of a 100 mg L−1 Cr (VI) solution and the adsorption capacities were compared. The kinetic experiments of Cr(VI) adsorption by BCF600 were carried out in 30 mL of 100, 300, and 500 mg L−1 Cr(VI) solution, and the concentration of
2. Materials and methods 2.1. Chemicals and materials Enteromorpha prolifera powder was purchased from Qingdao Seawin Biotech Group Co, Ltd. The Enteromorpha prolifera powder was thoroughly washed using deionized (DI) water and dried in an oven at 80 °C. Then, the biomass was stored in a container before pyrolysis. Ferric chloride hexahydrate (FeCl3·6H2O), potassium dichromate (K2Cr2O7), and other chemicals used were of analytical grade. A 1000 mg L−1 Cr(VI) stock solution was produced via dissolving 0.2829 g of K2CrO7 in 100 mL of DI water. The diverse concentrations of Cr(VI) solution used in this study were acquired via diluting the stock solution. 441
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interaction between the adsorbed molecules. The Freundlich model employs an empirical equation that is commonly utilized to depict the chemisorption of heterogeneous surfaces. The equations for these models are as follows: Langmuir model:
Cr(VI) was analyzed at disparate time intervals (1, 2, 4, 6, 8, 10, 12, 24, 36, 48, 60, 72, 84, 98, 108, and 120 h). The isotherm for Cr(VI) adsorption by BCF600 was determined for different initial Cr(VI) concentrations (50–500 mg L−1). After adsorption equilibrium, the Cr(VI) concentration was determined. The effects of pH on the adsorption of Cr(VI) by BC600 and BCF600 were researched by adjusting the pH value of the 200 mg L−1 Cr(VI) solution to 2–10 with HCl or NaOH solution, as appropriate. The effects of ion intensity on the adsorption of Cr(VI) by BC600 and BCF600 were studied by establishing different concentrations of NaCl (0–2 M) in the 100 mg L−1 Cr(VI) solution. Desorption experiments were promptly conducted following adsorption in order to study the material recyclability. The magnetic biochar was separated by a magnet, and the supernatant was decanted. Then, 30 mL of desorption reagent (H2O, HCl, NaCl, and NaOH) was added to the conical vessels. The experimental conditions and measurement methods were the same as those used in the adsorption experiments. The biochar after the adsorption experiment was denoted with the suffix “R”.
qe =
qe = KF Ce1/ n
(1)
η=
(C0 − Ce ) ×100% C0
(2)
where Ce is the equilibrium concentration(mg L ); qe and qm are the amounts of Cr(VI) adsorbed at equilibrium and the maximum adsorption capacity (mg g−1), respectively; KL and KF are the Langmuir (L mg−1) and Freundlich [mg(1-n) Ln g−1] model parameters, respectively; and n represents the adsorption intensity. The average values of three parallel samples were used as experimental data. The obvious differences between each group was evaluated by Turkey's multiple range tests, with P < 0.01 as obvious difference. All experimental data were plotted using OriginPro 9.0 (OriginLab, USA). 3. Results and discussion 3.1. Physicochemical properties of the biochars Compared with the high specific surface area of the pristine biochar, the specific surface areas of BCF400 and BCF600 after modification decreased by 55.64% and 94.89%, respectively (Supplementary Information, Table S1). This was possibly because the iron oxide particles loaded on the biochar surface after modification blocked the pores of the biochar. The richer and denser iron oxide particles of BCF600 would more easily trigger clogging, accounting for a lower specific surface area. The C, H, N, and S contents of the biochar decreased after modification, but the O content increased, probably due to the introduction of iron oxide. BCF600 had higher H/C and O/C than BCF400 did, indicating that BCF600 had lower aromaticity and higher polarity. The microstructure of the biochars as characterized by SEM was shown in Fig. S1 (Supplementary Information). For the four biochars, their surfaces all showed a highly porous network. In contrast with the pristine biochar, γ-Fe2O3 crystal particles appeared on the surface of the modified biochar. These particles were cubic on the biochar surfaces. Some splendent particles that appeared on the surface of BC400 were γFe2O3 (Zhang et al., 2015). The γ-Fe2O3 particles were distributed clustered and fine on the BCF400 surface, but were evenly dispersed on BCF600. In addition, it was observed that the particle size of the cubic γ-Fe2O3 on BCF600 was significantly larger than that on BCF400, and some γ-Fe2O3 particles were inserted in the biochar matrix, which indicated that the biochar matrix and iron oxide particles were bonded well mechanically. Compared with the pristine biochar (BC400, BC600), the functional groups on the modified biochar were dramatically different (Fig. 1). The characteristic peaks of water at 3741 cm−1 were enhanced for BCF400 and BCF600 (Tang et al., 2014), and the -OH stretching vibration of the carboxylic acid dimer occurred at 2358 cm−1, indicating that some -OH formed on modified biochar surface. By comparing BCF400 and BCF600, it was observed that the stretching vibration of -OH in BCF600 was significantly stronger, indicating that BCF600 has a higher surface polarity, which was consistent with its higher O/C molar ratio. In addition, the new peaks at about 462 and 533 cm−1 for BCF600 were the stretching vibration of Fe2O3, indicating magnetic Enteromorpha prolifera derived biochar was successfully prepared (Weng, 2016). The chemical structure of the magnetic biochar (BCF600) was
where qe is the equilibrium adsorption capacity for Cr(VI) of biochar (mg g−1). C0 and Ce are the initial and equilibrium Cr(VI) concentrations (mg L−1), respectively; V is the volume of the solution (L); and m is the biomass of the biochar (g). The Cr(VI) desorption efficiency r (%) was calculated via Eq. (3):
(qa − qb)
r=
qa
×100%
(3)
where qa is the adsorption capacity for Cr(VI) of biochar at adsorption equilibrium (mg g−1), and qb is the adsorption capacity for Cr(VI) of biochar at desorption equilibrium (mg g−1). Three kinetic models [pseudo-first-order (PF-order), pseudo-secondorder (PS-order), and Elovich] were utilized to depict the adsorption kinetics data (Lu et al., 2016; Taylor et al., 1995). The equations for such are as follows: PF-order kinetics equation:
qt = qe (1−e−k1 t )
(4)
PS-order kinetics equation:
qt =
k2 qe2 t 1+k2 qe t
(5)
Elovich equation:
qt =
1 1 ln (αβ ) + lnt β β
(8) −1
Adsorption capacity qe (mg g−1) and removal efficiency η (%) were calculated using Eqs. 1 and 2:
(C0 − Ce )⋅V m
(7)
Freundlich model:
2.5. Data processing
qe =
KL Ce qm 1+KL Ce
(6)
where qe and qt are the adsorption amounts of Cr(VI) by biochars at equilibrium and time t (mg g−1), respectively; and k1 and k2 are the corresponding adsorption rate constants of the PF-order (h−1) and PSorder (g mg−1 h−1) models, respectively. α and β are the initial adsorption rate (mg g−1 h−1) and desorption rate (g mg−1) constants, respectively. Two isotherm models (Langmuir and Freundlich) were utilized to describe the adsorption isotherm data (Nautiyal et al., 2016; N. Zhang et al., 2017; Y.Y. Zhang et al., 2017). The Langmuir model supposes that monolayer adsorption occurs on the homogeneous surface with no 442
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Fig. 1. Biochar characterization: (a) FTIR spectra of four Enteromorpha prolifera derived biochars; (b) FTIR spectra of BCF600 before and after absorption.
3.2.3. Adsorption isotherms The adsorption isotherm data and the fitting results of the two models (Langmuir and Freundlich isotherm models) are shown in Fig. 3 and Table 2, respectively. The adsorption capacities for Cr(VI) of BCF600 increased with the initial concentration, finally reaching saturation. This was because more vacant adsorption sites were present under the initial concentration conditions, and these adsorption sites became fully utilized gradually as the concentration increased. As illustrated in Table 2, the Langmuir model offered a better fit for the experimental data (R2 > 0.999). This might be caused by the homogeneous distribution of active sites on BCF600 surfaces (Agrafioti et al., 2014b). If a site was occupied by a metal ion, this site would not occur further adsorption (Lu et al., 2016). The maximum adsorption capacity was about 91.5 mg g−1, which was higher than those of multiple other types of biochars and magnetic adsorbents (See Supplementary Table S2). For example, Wang et al. (2016) prepared pineapple-peel-derived biochar which had a large specific surface area and pore volume, and its maximum adsorption capacity for Cr(VI) was 7.44 mg g−1. Wang et al. (2017) prepared bamboo biochar through a facile one-step pyrolysis method that can preserved magnetization and assimilated the amine onto the surface of biochar, and the maximum adsorption capacity for Cr(VI) was 48.00 mg g−1. Different raw materials and modification methods had different ways to improve adsorption capacity. In this study, a large amount of γ-Fe2O3 particles which contributed to Cr(VI) adsorption were loaded on Enteromorpha prolifera derived biochar, and this modification method may be more effective for improving the Cr(VI) adsorption capacity. These results indicated that BCF600 could be utilized as a greatly efficient adsorbent for removing Cr(VI) from aqueous solutions.
investigated via XRD analysis, as shown in Fig. S2(a) (Supplementary Information). The main crystalline phase of the biochar was characterized by diffraction peaks at 30.2°, 35.5°, 43.2°, 57.3°, and 62.9° (M. Zhang et al., 2013). These peaks corresponded to the five index planes, i.e., (220), (311), (400), (511), and (440), of maghemite, manifesting that the iron oxide on the surface of the biochar was γ-Fe2O3 (Machala et al., 2011). This indicated that γ-Fe2O3 particles had been triumphantly loaded onto the surface of the Enteromorpha prolifera derived biochar, which further corroborated the FTIR and SEM results. 3.2. Cr(VI) adsorption behavior by biochars 3.2.1. Effect of adsorbent dosage The experimental results of Cr(VI) adsorption by biochars at different dosages are shown in Fig. S2(b). The modification significantly enhanced the Cr(VI) adsorption of the biochar. After modification, the Cr(VI) adsorption efficiencies of BCF400 and BCF600 increased by 3–5 times and 9–24 times, respectively, over that of their corresponding pristine biochars. With the increase of biochar dosage, the removal efficiencies of the four biochars all increased, yet to different extents. In comparison, BCF600 exhibited a much higher Cr(VI) adsorption capacity than BCF400 did. For instance, to achieve a removal efficiency of 97.7%, a dosage of 12 g L−1 was needed for BCF400, whereas the required dosage was only 2 g L−1 for BCF600. This was mainly because γFe2O3 particles dominated the Cr(VI) removal by the magnetic biochar. The larger and more uniform dispersion of γ-Fe2O3 particles on BCF600 increased the chances of interactions between adsorption sites and Cr (VI) (B.L. Chen et al., 2011). Considering its excellent Cr(VI) adsorption capacity, BCF600 at its optimal dosage of 2 g L−1 was selected as the model biochar for the following experiments.
3.2.4. Effect of pH To obtain deep research into the adsorption mechanism of BCF600, the effects of different pH levels on Cr(VI) adsorption by BC600 and BCF600 were compared. As illustrated in Fig. S3(a) (Supplementary Information), the adsorption capacity of Cr(VI) decreased with an increase of pH value. For BC600, the removal efficiency of Cr(VI) decreased sharply in the pH range 2–4, and then decreased slowly. This was because the adsorption of Cr(VI) by the biochar at different pH levels was largely dependent on the surface charge of the biochar and the existing form of Cr(VI). Cr(VI) is mainly present as chromate (HCrO4- and Cr2O72-) at pH 2.0–6.4 and as dichromate (CrO42-) at pH > 6.4 (Lu et al., 2016). The pHpzc (point of zero charge) of Enteromorpha prolifera derived biochar is 2.8–3.4 (Chen et al., 2017). When pH < pHpzc, the surface of biochar was positively charged, which was beneficial to the adsorption of Cr-containing anion (Wang et al., 2018; Zhang et al., 2018). When pH > pHpzc, the biochar surface was negatively charged, and electrostatic repulsion of the Cr-containing
3.2.2. Adsorption kinetics The adsorption kinetic curves of BCF600 at different initial concentrations of Cr(VI) are depicted in Fig. 2, and the corresponding fitting parameters are shown in Table 1. The adsorption capacity for Cr (VI) of BCF600 increased with adsorption time, and the adsorption process was divided into an initial rapid phase and a subsequent slow phase. The results showed that the model correlation coefficients (R2) followed the order of Elovich > PS-order > PF-order. The Elovich model fitted well with the experimental data. By comparing the initial adsorption rate constant obtained from the Elovich model, it was discovered that the Cr(VI) adsorption by BCF600 reached equilibrium much more quickly at a low Cr(VI) concentration of 100 mg L−1 with an initial adsorption rate constant of 222.55 mg g−1 h−1, whereas more time was needed to reach the adsorption equilibrium at higher Cr(VI) concentrations (300 and 500 mg L−1) with lower initial adsorption rate constants of 149.09 and 60.30 mg g−1 h−1, respectively. 443
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Fig. 2. Sorption kinetics data and fitted models of Cr (VI) by BCF600.
ions occurred, which was not conducive to Cr(VI) adsorption. The effect of pH on BCF600 followed a different rule compared to that for BC600. For BCF600, the removal efficiency of Cr(VI) decreased slowly in the pH rang 2–7, and the adsorption of Cr(VI) decreased rapidly as the pH increased. This was because BCF600 was loaded with many γ-Fe2O3 particles after the modification, and γ-Fe2O3 played a key role in Cr(VI) removal by the magnetic biochar. When pH < pHpzc (6.5) (Wang and Lo, 2009), Cr(VI) anions were adsorbed onto the positively charged maghemite surface by electrostatic interaction. In contrast, when pH > pHpzc (6.5), the surface of maghemite was deprotonated with a negative charge and the adsorption capacity was significantly reduced. With the increase in pH value, a large amount of OH- in the solution competed with Cr(VI) to occupy the sites on γ-Fe2O3, and thus the adsorption capacity decreased. This result was in accordance with previous research that indicated that electrostatic interaction was the adsorption mechanism of Cr(VI) on pure iron oxides (Shen et al., 2009) and the charge of iron oxide rely on pH through protonation and deprotonation (Han et al., 2015). Therefore, the acidic condition was more favorable for Cr(VI) adsorption by BCF600.
Fig. 3. Adsorption isotherms of Cr(VI) onto by BCF600.
3.3. Desorption and recyclability To evaluate the recyclability of the magnetic biochar (BCF600), three consecutive desorption experiments and six cycles of adsorptiondesorption experiments were performed. The experimental results are demonstrated in Fig. 4. It can be found from Fig. 4(a)–(c) that the desorption efficiencies of Cr(VI) using different eluents after the first desorption were 0.96% (H2O), 1.34% (HCl), and 2.61% (NaCl), respectively. With the increase in the number of desorption, the desorption efficiencies of Cr(VI) by H2O, HCl, and NaCl decreased gradually. By the third desorption, Cr(VI) was scarcely desorbed by H2O (~0), HCl (~0), and NaCl (0.59%). These results indicated that Cr(VI) adsorbed on the magnetic Enteromorpha prolifera derived biochar was basically stable under acidic or neutral conditions. As shown in Fig. 4(a)–(c), Cr (VI) adsorbed on the magnetic Enteromorpha prolifera derived biochar could be desorbed by alkaline solutions and higher concentrations of alkaline solution were more favorable for Cr(VI) desorption, e.g., 20.49% removal of Cr(VI) with 0.1 M NaOH and 33.45% with 1 M NaOH. This coincided with the previous conclusion that the adsorption capacity was reduced under alkaline conditions (Fig. S3(a)), further confirming that electrostatic adsorption played an significant role in Cr (VI) absorption by BCF600. After threefold desorption, the desorption
3.2.5. Effect of background ion intensity To investigate the effect of background ion intensity on Cr(VI) removal, different concentrations of NaCl were used to adjust the ion intensities. As shown in Fig. S3(b), the results indicated that the existence of NaCl hardly influenced the removal of Cr(VI) by BCF600 at low concentrations (0.005 and 0.01 mol L−1). But, it significantly reduced the Cr(VI) removal capacity from 97.85% to 83.16% and to 76.15% at higher concentrations (1 and 2 mol L−1, respectively). The reasons for these phenomena are as follows: (1) Cl- anions may have competed with Cr(VI) for the adsorption sites on the adsorbent surface and the competition was more significant at high concentrations (Gan et al., 2015; Wang et al., 2015a) and (2) the solution ion intensity was increased at high concentration of NaCl, thus reducing the activity coefficient of Cr(VI) and greatly reducing the chances of collisions and contact between the sorbent (Z.B. Zhang et al., 2013). The effect of background ion intensity on Cr(VI) adsorption further showed that electrostatic interaction was a likely adsorption mechanism for Cr(VI) removal by BCF600.
Table 1 Kinetic parameters of Cr(VI) adsorption by BCF600. Cr(VI)
First-order
Second-order
Elovich
( mg L−1)
qe (mg g−1)
k1 (h−1)
R2
qe (mg g−1)
k2 (g mg−1 h−1)
R2
β (g mg−1)
α (mg g−1 h−1)
R2
100 300 500
45.99 73.74 81.83
0.63 0.76 0.66
0.913 0.841 0.843
48.46 77.97 86.90
0.02 0.01 0.01
0.982 0.939 0.945
0.22 0.14 0.12
222.55 149.09 60.30
0.985 0.998 0.997
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Table 2 Kinetic parameters of Cr(VI) adsorption by BCF600. Biochar
Langmuir model Qe (mg g
BCF600
91.56
−1
)
Freundlich model Qm (mg g 88.17
−1
)
KL (L mg
−1
R
KF (mg(1-n) Ln g−1)
n
R2
0.9996
38.66
4.26
0.6668
2
)
0.69
The specific surface area of BCF600 was significantly less than that of its corresponding pristine biochar (Table S1), but BCF600 still had a higher absorption capacity than the pristine biochar, indicating that the Cr(VI) adsorption by BCF600 did not mainly depend on its pore structure and that the γ-Fe2O3 loaded on the Enteromorpha prolifera derived biochar may play a key role in Cr(VI) removal (Fig. 1(a)). The SEM photos demonstrated that the biochar matrix was in charge of sustaining and dispersing maghemite (M. Zhang et al., 2013), which resolves the disadvantage of easy agglomeration of γ-Fe2O3. As can be observed from the FTIR spectra in Fig. 1(a), the water characteristic peaks at 3741 cm−1 were enhanced for BCF600 after modification, and BCF600 exhibited -OH stretching vibration of the carboxylic acid dimer at 2435 cm−1, in contrast to BC600, indicating that some -OH, which may come from γ-Fe2O3, was introduced after the modification. This was consistent with the fact that iron oxide had a hydroxylated surface with a large number of -OH functional groups (Ma, 2011). Therefore, we hypothesized that -OH played a key role in the removal of Cr(VI) by the magnetic biochar. To confirm this speculation, we provide the following supportive evidence. (1) Based on the results and discussion in Sections 3.2.4, 3.2.5, and 3.3, it was proposed that when the pH is < pHpzc (6.5) (Han et al., 2015; Wang and Lo, 2009), the -OH functional groups were protonated to form positively charged -OH2+ functional groups, so that the Cr-containing anions
efficiency of Cr(VI) by the alkaline solution was about 47.13%. Thus, Cr (VI) adsorbed biochar can be desorbed effectively using alkaline solution. Although Cr(VI) can be desorbed effectively by alkaline solution, the recyclability after desorption remained unclear. Therefore, six cycles of adsorption-desorption experimentation were carried out. As presented in Fig. 4(d), even though the adsorption capacity reduced gradually with the increase in number of cycles, in the sixth cycle, it was more than 23.12 mg g−1, which was still greater than that of the original biochar and many other types of biochar (Table S2). In addition, the magnetic Enteromorpha prolifera derived biochar itself was rich in magnetism and could be collected effectively after adsorption. Therefore, the magnetic Enteromorpha prolifera derived biochar could be regenerated efficiently via alkaline solution, indicating its high potential as a cheap adsorbent in Cr(VI) wastewater treatment. 3.4. Mechanism of Cr(VI) adsorption by magnetic Enteromorpha prolifera derived biochar To confirmed the possible adsorption mechanisms for Cr(VI) by the magnetic Enteromorpha prolifera derived biochar, both adsorbent structural information (specific surface area, SEM, FTIR, etc.) and sorption characteristics were analyzed.
Fig. 4. Three consecutive desorptions (a), (b), (c) and six consecutive adsorption-desorption cycles (d) of Cr(VI) on BCF600. 445
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could be adsorbed and migrated to the γ-Fe2O3 surface via electrostatic interaction (Shen et al., 2009). (2) By contrasting the FTIR spectra of BCF600 before and after the adsorption experiments, the reduction in the vibrational intensity of -OH at 3741 and 2358 cm−1 also proved that -OH was involved in the sorption (Fig. 1(b)). Therefore, electrostatic interaction might be the dominant mechanism adsorption for Cr (VI) by the magnetic biochar.
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4. Conclusions To efficiently utilize Enteromorpha prolifera resources, a magnetic biochar with γ-Fe2O3 particles uniformly distributed on its surface was first synthesized successfully by pyrolyzing FeCl3-pretreated Enteromorpha prolifera biomass. The maximum adsorption capacity of Cr(VI) by the magnetic biochar was far greater than that of pristine biochar. The pH and background ion intensity of the solution exhibited significant effects on Cr(VI) adsorption of. In addition, the magnetic Enteromorpha prolifera derived biochar could be effectively recovered magnetically and regenerated with alkaline solution. The results illustrated that the magnetic biochar produced from Enteromorpha prolifera has promising application in Cr(VI)-contaminated wastewater treatment. Acknowledgements The work was supported by Shandong Province key Research And Development Programs (2017GSF17119). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.ecoenv.2018.08.024. References Agrafioti, E., Kalderis, D., Diamadopoulos, E., 2014a. Arsenic and chromium removal from water using biochars derived from rice husk, organic solid wastes and sewage sludge. J. Environ. Manage. 133, 309–314. Agrafioti, E., Kalderis, D., Diamadopoulos, E., 2014b. Ca and Fe modified biochars as adsorbents of arsenic and chromium in aqueous solutions. J. Environ. Manage. 146, 444–450. Angın, D., 2013. Effect of pyrolysis temperature and heating rate on biochar obtained from pyrolysis of safflower seed press cake. Bioresour. Technol. 128, 593–597. Bordoloi, N., Goswami, R., Kumar, M., Kataki, R., 2017. Biosorption of Co (II) from aqueous solution using algal biochar: kinetics and isotherm studies. Bioresour. Technol. 244, 1465–1469. Chen, B.L., Chen, Z.M., Lv, S.F., 2011a. A novel magnetic biochar efficiently sorbs organic pollutants and phosphate. Bioresour. Technol. 102 (2), 716–723. Chen, G.Q., Zhang, W.J., Zeng, G.M., Huang, J.H., Wang, L., Shen, G.L., 2011b. Surfacemodified Phanerochaete chrysosporium as a biosorbent for Cr(VI)-contaminated wastewater. J. Hazard. Mater. 186 (2–3), 2138–2143. Chen, Y.Y., Hui, H.X., Lu, S., Wang, B.Y., Wang, Z.J., Wang, N., 2017. Characteristics of Enteromorpha prolifera biochars and their adsorption performance and mechanism for Cr(VI). Environ. Sci. 38 (9), 3953–3961 (in Chinese). Dawood, S., Sen, T.K., Phan, C., 2017. Synthesis and characterization of slow pyrolysis pine cone bio-char in the removal of organic and inorganic pollutants from aqueous solution by adsorption: kinetic, equilibrium, mechanism and thermodynamic. Bioresour. Technol. 246, 76–81. Demirbas, A., 2004. Effects of temperature and particle size on bio-char yield from pyrolysis of agricultural residues. J. Anal. Appl. Pyrolysis 72 (2), 243–248. Fan, Y., Wang, B., Yuan, S.H., Wu, X.H., Chen, J., Wang, L.L., 2010. Adsorptive removal of chloramphenicol from wastewater by NaOH modified bamboo charcoal. Bioresour. Technol. 101 (19), 7661–7664. Gan, C., Liu, Y.G., Tan, X.F., Wang, S.F., Zeng, G.M., Zheng, B.H., Li, T.T., Jiang, Z.J., Liu, W., 2015. Effect of porous zinc–biochar nanocomposites on Cr(VI) adsorption from aqueous solution. Rsc Adv. 5 (44), 35107–35115. Han, Y.H., Cao, X., Ouyang, X., Sohi, S.P., Chen, J.W., 2015. Adsorption kinetics of magnetic biochar derived from peanut hull on removal of Cr (VI) from aqueous solution: Effects of production conditions and particle size. Chemosphere 145, 336–341. Inyang, M.I., Gao, B., Yao, Y., Xue, Y.W., Zimmerman, A., Mosa, A., Pullammanappallil, P., Yong, S.O., Cao, X.D., 2016. A review of biochar as a low-cost adsorbent for aqueous heavy metal removal. Crit. Rev. Environ. Sci. Technol. 46 (4), 406–433. Li, Y.H., Du, Q.J., Liu, T.H., Yan, Q., Zhang, P., Wang, Z.H., Xia, Y.Z., 2011. Preparation of activated carbon from Enteromorpha prolifera and its use on cationic red X-GRL
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Adsorption behavior and mechanism of Cr(VI) by modified biochar derived from Enteromorpha prolifera
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Youyuan Chen, Baoying Wang, Jia Xin , Ping Sun, Dan Wu Key Laboratory of Marine Environment and Ecology, Ministry of Education; Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering (MEGE); College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Enteromorpha prolifera Magnetic biochar Adsorption Cr(VI)
Biochar that was derived from Enteromorpha prolifera and magnetically modified (BCF600) was evaluated for its physicochemical properties and Cr(VI) adsorption behavior and mechanism. The results showed that the modified biochar was less porous on surface and loaded with γ-Fe2O3 particles. BCF600 exhibited a maximum adsorption capacity for Cr(VI), which acquired from the Langmuir model of 88.17 mg g−1 and a removal efficiency of 97.71%, for 100 mg L−1 of Cr(VI). The adsorption of Cr(VI) by BCF600 decreased with increasing pH and background ion intensity. Based on the FTIR results, the change in the -OH groups on the surface after adsorption confirmed that electrostatic interaction was likely the preponderant mechanism. In addition, the BCF600 could be easily separated using a magnet and displayed high recyclability. Therefore, this magnetic biochar derived from Enteromorpha prolifera has the potential to serve as a highly efficient adsorbent for water pollution control.
1. Introduction Biochar is a carbon-rich product of biomass pyrolysis under hypoxic or anaerobic conditions (Dawood et al., 2017). It has a strong ability to adsorb various pollutants in water because of its stable porous structure, high specific surface area, and enriched oxygen-containing functional groups; thus, it has attracted much attention (N. Zhou et al., 2017). Presently, many raw materials exist for biochar preparation, such as sawdust (Y.Y. Zhou et al., 2017), rice straw (Qian et al., 2017), bamboo (Fan et al., 2010), rice husk (Ma et al., 2014), safflower seed (Angın, 2013), pinewood (Wang et al., 2015b), olive busk (Demirbas, 2004), Alternanthera philoxeroides (Yang et al., 2014), and other ligneous and cellulosic plant carbon, as well as fecal carbon and sludge carbon (Agrafioti et al., 2014a). However, finding cheap biomaterials with superior performance and easy steps for biochar preparation remains a major challenge in conventional wastewater treatment (Inyang et al., 2016). Enteromorpha prolifera is a marine macroalgal species that has high reproductive capacity owing to eutrophication and easily forms green tides, which negatively affect marine transportation, tourism, and water quality (Zhou et al., 2010). For instance, in the summer of 2008, an unprecedented outbreak of Enteromorpha prolifera occurred in China's Yellow Sea in the port city of Qingdao, with a total salvage of 100 million metric tons (Zhang et al., 2012). In the past ten years since that event, Enteromorpha prolifera has maintained its
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prosperity, thus threatening the economic development of many cities along the coast of the Yellow Sea. Determining how to use and dispose of Enteromorpha prolifera in an efficient and environmentally friendly manner is critical for marine environmental protection and efficient resource utilization. Therefore, the feasibility of using Enteromorpha prolifera to prepare biochar that can be applied in wastewater treatment has piqued interest. Until now, the literature on algal biochar and its application remains very limited (Shukla et al., 2017). Studies to date have indicated that biochars produced from algal biomass are fundamentally different from biochars produced from lignocellulosic feedstock (Maddi et al., 2011). Generally, biochars produced from various algal species tend to have low cation exchange capacity, carbon content, and specific surface area, but are high in pH, nitrogen, and extractable inorganic nutrients including P, K, Ca, and Mg (Yu et al., 2017). Given these characteristics of algal biochar, in most studies, it has been used as a soil improver in soil remediation (Li et al., 2011), but little attention has been paid to its adsorption potential. Later, some studies found that the cell wall of algae was composed of sugars, proteins, cellulose, lipids, etc. These constituents comprise hydroxyl, ketonic, phenolic, aldehydic, carboxylic, and other polar functional groups, which could serve as potential adsorption binding sites (Nautiyal et al., 2016), thus inspiring some researchers to hypothesize that algal biochar could be used as an adsorbent. In particular, for macroalgae, Roberts et al. (2015) observed
Corresponding author. E-mail address: [email protected] (J. Xin).
https://doi.org/10.1016/j.ecoenv.2018.08.024 Received 12 June 2018; Received in revised form 3 August 2018; Accepted 7 August 2018 0147-6513/ © 2018 Elsevier Inc. All rights reserved.
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2.2. Preparation of magnetic Enteromorpha prolifera derived biochar
that the specific surface area of biochar produced from macroalgae was relatively higher than that produced from other algal species. Subsequently, studies were conducted on the absorption capacity of algal biochar [Scenedesmus dimorphus (Bordoloi et al., 2017), Enteromorpha prolifera (Qiao et al., 2017), Spirulina platensis (Nautiyal et al., 2016), and Porphyra tenera (Park et al., 2016)] for Co(II), polycyclic aromatic hydrocarbons (PAHs), Congo red dye, and copper(II), respectively. However, the findings in these studies concerning algal biochar as an adsorbent for water pollutants were not as ideal as expected. Although these algal biomass sources are readily available and abundant, their adsorption capacities generally lie between 0.05 and 23.00 mg g−1 (Bordoloi et al., 2017; Nautiyal et al., 2016; Park et al., 2016; Qiao et al., 2017), which is significantly lower than that of biochar prepared from lignocellulosic feedstock. Therefore, the need to find technical methods to improve the adsorption capacity of algal biochars is urgent. Recently, many different biochar modification methods have been discussed: (1) chemical modification by increasing surface functional groups, aromaticity, and pore structures (Fan et al., 2010; Ma et al., 2014); (2) simple physical modification by improving biochar carbon structure, surface area, and pore volume (Lima et al., 2010); and (3) load modification by loading metals, oxides, or ions (Han et al., 2015; M. Zhang et al., 2013; Zhou et al., 2014). Among them, magnetic modification, an oxide load-modification method that can both increase adsorption capability and generate recyclability, is an efficient and promising alternative modification method. Wang et al. (2015b) synthesized a magnetic biochar using pinewood that had a As(V) adsorption capacity about 1.6 times higher than that of its corresponding original biochar. The adsorption capacity for As(V) of a magnetic biochar derived from chestnut shell (Z. Zhou et al., 2017) was 45.8 mg g−1, which was much greater than that of the pristine biochar (17.5 mg g−1). Han et al. (2015) used peanut hull to prepare magnetic biochar, yielding a product that had a Cr(VI) adsorption capacity about 1–2 orders of magnitude greater than that of its corresponding pristine biochar. However, according to our current knowledge, no attempt has ever been made to modify Enteromorpha prolifera with this method. Hence, whether the magnetic modification of Enteromorpha prolifera derived biochar could significantly increase its adsorption capability and magnetic properties is unknown and needs to be investigated. If the response is positive, deeper insights into the governing sorption mechanism of this modified biochar should be sought. In this study, a magnetic biochar was synthesized from macroalgae Enteromorpha prolifera biomass for the first time and its adsorption behavior and mechanism regarding the target heavy metal Cr(VI) were studied. The main purposes of this study were to: (1) synthesis and characterize the magnetic Enteromorpha prolifera derived biochar; (2) explore the adsorption properties and adsorption mechanism of Cr(VI) by magnetic Enteromorpha prolifera derived biochar; (3) evaluate the recyclability of the magnetic Enteromorpha prolifera derived biochar, and provide scientific guidance for the utilization of Enteromorpha prolifera.
The preparation of magnetic Enteromorpha prolifera derived biochar was based on the method reported by M. Zhang et al. (2013). Briefly, 50 g of Enteromorpha prolifera biomass was added to 500 mL FeCl3 solution at a concentration of 2 mol L−1 and stirred for 2 h with a magnetic stirrer at 80 °C. Then, the Enteromorpha prolifera biomass was separated from the solution and dried at 70 °C. The Enteromorpha prolifera biomass was then placed into a porcelain crucible and capped. The porcelain crucibles were subjected to heating in a muffle furnace for pyrolysis at desired temperature (400 or 600 °C) at the rate of 10 °C min−1 and then keep constant for 2 h. After naturally cooling to room temperature (25 °C), the biochar was sieved through a 0.6 mm sieve to produce particles less than 0.6 mm and washed with DI water a few times, then dried at 70 °C for 6 h, and finally put in a closed container before use. The pristine Enteromorpha prolifera derived biochar was prepared in the same way, except it was not pretreated with FeCl3. The original biochar samples were designated “BC” and the magnetic biochar samples were designated “BCF” with a suffix indicating the peak temperature of pyrolysis (i.e., BC400, BC600, BCF400, and BCF600). 2.3. Biochar characterization The surface morphology of the samples was characterized using scanning electron microscopy (SEM) (Hitachi S-4800, Japan). The surface functional groups of the samples were determined via Fourier transform infrared spectroscopy (FTIR) (Bruker TENSOR 27, Germany). The crystallographic structure of the samples was examined through Xray diffraction (XRD) analysis (Ultima IV, Japan). The specific surface area was quantified using the Brunauer, Emmett, Teller (BET) method (Autosorb-iQ3, USA). The elemental composition of biochars were determined via an elemental analyzer (Vario EL III, Elementar, Germany). 2.4. Sorption experiments 2.4.1. Optimization of preparation parameters Based on our previous study (Chen et al., 2017), two pyrolysis temperatures (400 and 600 °C) were chosen for biochar preparation. The pristine biochars (BC400 and BC600) and magnetic biochars (BCF400 and BCF600) were prepared at these two temperatures. BCF600, which had a greatest adsorption capacity, was selected based on preliminary experiments; hence, the physicochemical properties and the adsorption behavior and mechanism of BCF600 were further explored. 2.4.2. Adsorption and desorption characteristics To 50 mL conical vessels, 0.06 g of biochar and 30 mL of Cr(VI) solution were added. The initial pH of the Cr(VI) solutions with no biochar was tested as 5.03 ± 0.02 using a pH meter (HQ30D, Hach, USA). Then, the conical vessels were sealed and shaken in a 25 °C thermostat oscillator with 140 r min−1 shock oscillation for 120 h. After the adsorption equilibrated, the suspension was centrifuged at 3000 r min−1 for 5 min and the resultant supernatant was filtered at 0.45 µm. The concentration of Cr(VI) was tested with a UV–vis spectrophotometer (Pgeneral T6, China) using diphenylcarbazide spectrophotometry (C.Q. Chen et al., 2011). As a blank, different concentrations of Cr(VI) solution without biochar were separately measured via colorimetric analysis in experiments. No change in absorbance intensity of the solution illustrated that the Cr(VI) solution was stable. To determine the best dosage of each biochar, different dosages (1, 1.5, 2, 3, 4, 6, 8, 10, 12, 14, and 16 g L−1) of the four biochars (BC400, BC600, BCF400, and BCF600) were added to 30 mL of a 100 mg L−1 Cr (VI) solution and the adsorption capacities were compared. The kinetic experiments of Cr(VI) adsorption by BCF600 were carried out in 30 mL of 100, 300, and 500 mg L−1 Cr(VI) solution, and the concentration of
2. Materials and methods 2.1. Chemicals and materials Enteromorpha prolifera powder was purchased from Qingdao Seawin Biotech Group Co, Ltd. The Enteromorpha prolifera powder was thoroughly washed using deionized (DI) water and dried in an oven at 80 °C. Then, the biomass was stored in a container before pyrolysis. Ferric chloride hexahydrate (FeCl3·6H2O), potassium dichromate (K2Cr2O7), and other chemicals used were of analytical grade. A 1000 mg L−1 Cr(VI) stock solution was produced via dissolving 0.2829 g of K2CrO7 in 100 mL of DI water. The diverse concentrations of Cr(VI) solution used in this study were acquired via diluting the stock solution. 441
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interaction between the adsorbed molecules. The Freundlich model employs an empirical equation that is commonly utilized to depict the chemisorption of heterogeneous surfaces. The equations for these models are as follows: Langmuir model:
Cr(VI) was analyzed at disparate time intervals (1, 2, 4, 6, 8, 10, 12, 24, 36, 48, 60, 72, 84, 98, 108, and 120 h). The isotherm for Cr(VI) adsorption by BCF600 was determined for different initial Cr(VI) concentrations (50–500 mg L−1). After adsorption equilibrium, the Cr(VI) concentration was determined. The effects of pH on the adsorption of Cr(VI) by BC600 and BCF600 were researched by adjusting the pH value of the 200 mg L−1 Cr(VI) solution to 2–10 with HCl or NaOH solution, as appropriate. The effects of ion intensity on the adsorption of Cr(VI) by BC600 and BCF600 were studied by establishing different concentrations of NaCl (0–2 M) in the 100 mg L−1 Cr(VI) solution. Desorption experiments were promptly conducted following adsorption in order to study the material recyclability. The magnetic biochar was separated by a magnet, and the supernatant was decanted. Then, 30 mL of desorption reagent (H2O, HCl, NaCl, and NaOH) was added to the conical vessels. The experimental conditions and measurement methods were the same as those used in the adsorption experiments. The biochar after the adsorption experiment was denoted with the suffix “R”.
qe =
qe = KF Ce1/ n
(1)
η=
(C0 − Ce ) ×100% C0
(2)
where Ce is the equilibrium concentration(mg L ); qe and qm are the amounts of Cr(VI) adsorbed at equilibrium and the maximum adsorption capacity (mg g−1), respectively; KL and KF are the Langmuir (L mg−1) and Freundlich [mg(1-n) Ln g−1] model parameters, respectively; and n represents the adsorption intensity. The average values of three parallel samples were used as experimental data. The obvious differences between each group was evaluated by Turkey's multiple range tests, with P < 0.01 as obvious difference. All experimental data were plotted using OriginPro 9.0 (OriginLab, USA). 3. Results and discussion 3.1. Physicochemical properties of the biochars Compared with the high specific surface area of the pristine biochar, the specific surface areas of BCF400 and BCF600 after modification decreased by 55.64% and 94.89%, respectively (Supplementary Information, Table S1). This was possibly because the iron oxide particles loaded on the biochar surface after modification blocked the pores of the biochar. The richer and denser iron oxide particles of BCF600 would more easily trigger clogging, accounting for a lower specific surface area. The C, H, N, and S contents of the biochar decreased after modification, but the O content increased, probably due to the introduction of iron oxide. BCF600 had higher H/C and O/C than BCF400 did, indicating that BCF600 had lower aromaticity and higher polarity. The microstructure of the biochars as characterized by SEM was shown in Fig. S1 (Supplementary Information). For the four biochars, their surfaces all showed a highly porous network. In contrast with the pristine biochar, γ-Fe2O3 crystal particles appeared on the surface of the modified biochar. These particles were cubic on the biochar surfaces. Some splendent particles that appeared on the surface of BC400 were γFe2O3 (Zhang et al., 2015). The γ-Fe2O3 particles were distributed clustered and fine on the BCF400 surface, but were evenly dispersed on BCF600. In addition, it was observed that the particle size of the cubic γ-Fe2O3 on BCF600 was significantly larger than that on BCF400, and some γ-Fe2O3 particles were inserted in the biochar matrix, which indicated that the biochar matrix and iron oxide particles were bonded well mechanically. Compared with the pristine biochar (BC400, BC600), the functional groups on the modified biochar were dramatically different (Fig. 1). The characteristic peaks of water at 3741 cm−1 were enhanced for BCF400 and BCF600 (Tang et al., 2014), and the -OH stretching vibration of the carboxylic acid dimer occurred at 2358 cm−1, indicating that some -OH formed on modified biochar surface. By comparing BCF400 and BCF600, it was observed that the stretching vibration of -OH in BCF600 was significantly stronger, indicating that BCF600 has a higher surface polarity, which was consistent with its higher O/C molar ratio. In addition, the new peaks at about 462 and 533 cm−1 for BCF600 were the stretching vibration of Fe2O3, indicating magnetic Enteromorpha prolifera derived biochar was successfully prepared (Weng, 2016). The chemical structure of the magnetic biochar (BCF600) was
where qe is the equilibrium adsorption capacity for Cr(VI) of biochar (mg g−1). C0 and Ce are the initial and equilibrium Cr(VI) concentrations (mg L−1), respectively; V is the volume of the solution (L); and m is the biomass of the biochar (g). The Cr(VI) desorption efficiency r (%) was calculated via Eq. (3):
(qa − qb)
r=
qa
×100%
(3)
where qa is the adsorption capacity for Cr(VI) of biochar at adsorption equilibrium (mg g−1), and qb is the adsorption capacity for Cr(VI) of biochar at desorption equilibrium (mg g−1). Three kinetic models [pseudo-first-order (PF-order), pseudo-secondorder (PS-order), and Elovich] were utilized to depict the adsorption kinetics data (Lu et al., 2016; Taylor et al., 1995). The equations for such are as follows: PF-order kinetics equation:
qt = qe (1−e−k1 t )
(4)
PS-order kinetics equation:
qt =
k2 qe2 t 1+k2 qe t
(5)
Elovich equation:
qt =
1 1 ln (αβ ) + lnt β β
(8) −1
Adsorption capacity qe (mg g−1) and removal efficiency η (%) were calculated using Eqs. 1 and 2:
(C0 − Ce )⋅V m
(7)
Freundlich model:
2.5. Data processing
qe =
KL Ce qm 1+KL Ce
(6)
where qe and qt are the adsorption amounts of Cr(VI) by biochars at equilibrium and time t (mg g−1), respectively; and k1 and k2 are the corresponding adsorption rate constants of the PF-order (h−1) and PSorder (g mg−1 h−1) models, respectively. α and β are the initial adsorption rate (mg g−1 h−1) and desorption rate (g mg−1) constants, respectively. Two isotherm models (Langmuir and Freundlich) were utilized to describe the adsorption isotherm data (Nautiyal et al., 2016; N. Zhang et al., 2017; Y.Y. Zhang et al., 2017). The Langmuir model supposes that monolayer adsorption occurs on the homogeneous surface with no 442
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Fig. 1. Biochar characterization: (a) FTIR spectra of four Enteromorpha prolifera derived biochars; (b) FTIR spectra of BCF600 before and after absorption.
3.2.3. Adsorption isotherms The adsorption isotherm data and the fitting results of the two models (Langmuir and Freundlich isotherm models) are shown in Fig. 3 and Table 2, respectively. The adsorption capacities for Cr(VI) of BCF600 increased with the initial concentration, finally reaching saturation. This was because more vacant adsorption sites were present under the initial concentration conditions, and these adsorption sites became fully utilized gradually as the concentration increased. As illustrated in Table 2, the Langmuir model offered a better fit for the experimental data (R2 > 0.999). This might be caused by the homogeneous distribution of active sites on BCF600 surfaces (Agrafioti et al., 2014b). If a site was occupied by a metal ion, this site would not occur further adsorption (Lu et al., 2016). The maximum adsorption capacity was about 91.5 mg g−1, which was higher than those of multiple other types of biochars and magnetic adsorbents (See Supplementary Table S2). For example, Wang et al. (2016) prepared pineapple-peel-derived biochar which had a large specific surface area and pore volume, and its maximum adsorption capacity for Cr(VI) was 7.44 mg g−1. Wang et al. (2017) prepared bamboo biochar through a facile one-step pyrolysis method that can preserved magnetization and assimilated the amine onto the surface of biochar, and the maximum adsorption capacity for Cr(VI) was 48.00 mg g−1. Different raw materials and modification methods had different ways to improve adsorption capacity. In this study, a large amount of γ-Fe2O3 particles which contributed to Cr(VI) adsorption were loaded on Enteromorpha prolifera derived biochar, and this modification method may be more effective for improving the Cr(VI) adsorption capacity. These results indicated that BCF600 could be utilized as a greatly efficient adsorbent for removing Cr(VI) from aqueous solutions.
investigated via XRD analysis, as shown in Fig. S2(a) (Supplementary Information). The main crystalline phase of the biochar was characterized by diffraction peaks at 30.2°, 35.5°, 43.2°, 57.3°, and 62.9° (M. Zhang et al., 2013). These peaks corresponded to the five index planes, i.e., (220), (311), (400), (511), and (440), of maghemite, manifesting that the iron oxide on the surface of the biochar was γ-Fe2O3 (Machala et al., 2011). This indicated that γ-Fe2O3 particles had been triumphantly loaded onto the surface of the Enteromorpha prolifera derived biochar, which further corroborated the FTIR and SEM results. 3.2. Cr(VI) adsorption behavior by biochars 3.2.1. Effect of adsorbent dosage The experimental results of Cr(VI) adsorption by biochars at different dosages are shown in Fig. S2(b). The modification significantly enhanced the Cr(VI) adsorption of the biochar. After modification, the Cr(VI) adsorption efficiencies of BCF400 and BCF600 increased by 3–5 times and 9–24 times, respectively, over that of their corresponding pristine biochars. With the increase of biochar dosage, the removal efficiencies of the four biochars all increased, yet to different extents. In comparison, BCF600 exhibited a much higher Cr(VI) adsorption capacity than BCF400 did. For instance, to achieve a removal efficiency of 97.7%, a dosage of 12 g L−1 was needed for BCF400, whereas the required dosage was only 2 g L−1 for BCF600. This was mainly because γFe2O3 particles dominated the Cr(VI) removal by the magnetic biochar. The larger and more uniform dispersion of γ-Fe2O3 particles on BCF600 increased the chances of interactions between adsorption sites and Cr (VI) (B.L. Chen et al., 2011). Considering its excellent Cr(VI) adsorption capacity, BCF600 at its optimal dosage of 2 g L−1 was selected as the model biochar for the following experiments.
3.2.4. Effect of pH To obtain deep research into the adsorption mechanism of BCF600, the effects of different pH levels on Cr(VI) adsorption by BC600 and BCF600 were compared. As illustrated in Fig. S3(a) (Supplementary Information), the adsorption capacity of Cr(VI) decreased with an increase of pH value. For BC600, the removal efficiency of Cr(VI) decreased sharply in the pH range 2–4, and then decreased slowly. This was because the adsorption of Cr(VI) by the biochar at different pH levels was largely dependent on the surface charge of the biochar and the existing form of Cr(VI). Cr(VI) is mainly present as chromate (HCrO4- and Cr2O72-) at pH 2.0–6.4 and as dichromate (CrO42-) at pH > 6.4 (Lu et al., 2016). The pHpzc (point of zero charge) of Enteromorpha prolifera derived biochar is 2.8–3.4 (Chen et al., 2017). When pH < pHpzc, the surface of biochar was positively charged, which was beneficial to the adsorption of Cr-containing anion (Wang et al., 2018; Zhang et al., 2018). When pH > pHpzc, the biochar surface was negatively charged, and electrostatic repulsion of the Cr-containing
3.2.2. Adsorption kinetics The adsorption kinetic curves of BCF600 at different initial concentrations of Cr(VI) are depicted in Fig. 2, and the corresponding fitting parameters are shown in Table 1. The adsorption capacity for Cr (VI) of BCF600 increased with adsorption time, and the adsorption process was divided into an initial rapid phase and a subsequent slow phase. The results showed that the model correlation coefficients (R2) followed the order of Elovich > PS-order > PF-order. The Elovich model fitted well with the experimental data. By comparing the initial adsorption rate constant obtained from the Elovich model, it was discovered that the Cr(VI) adsorption by BCF600 reached equilibrium much more quickly at a low Cr(VI) concentration of 100 mg L−1 with an initial adsorption rate constant of 222.55 mg g−1 h−1, whereas more time was needed to reach the adsorption equilibrium at higher Cr(VI) concentrations (300 and 500 mg L−1) with lower initial adsorption rate constants of 149.09 and 60.30 mg g−1 h−1, respectively. 443
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Fig. 2. Sorption kinetics data and fitted models of Cr (VI) by BCF600.
ions occurred, which was not conducive to Cr(VI) adsorption. The effect of pH on BCF600 followed a different rule compared to that for BC600. For BCF600, the removal efficiency of Cr(VI) decreased slowly in the pH rang 2–7, and the adsorption of Cr(VI) decreased rapidly as the pH increased. This was because BCF600 was loaded with many γ-Fe2O3 particles after the modification, and γ-Fe2O3 played a key role in Cr(VI) removal by the magnetic biochar. When pH < pHpzc (6.5) (Wang and Lo, 2009), Cr(VI) anions were adsorbed onto the positively charged maghemite surface by electrostatic interaction. In contrast, when pH > pHpzc (6.5), the surface of maghemite was deprotonated with a negative charge and the adsorption capacity was significantly reduced. With the increase in pH value, a large amount of OH- in the solution competed with Cr(VI) to occupy the sites on γ-Fe2O3, and thus the adsorption capacity decreased. This result was in accordance with previous research that indicated that electrostatic interaction was the adsorption mechanism of Cr(VI) on pure iron oxides (Shen et al., 2009) and the charge of iron oxide rely on pH through protonation and deprotonation (Han et al., 2015). Therefore, the acidic condition was more favorable for Cr(VI) adsorption by BCF600.
Fig. 3. Adsorption isotherms of Cr(VI) onto by BCF600.
3.3. Desorption and recyclability To evaluate the recyclability of the magnetic biochar (BCF600), three consecutive desorption experiments and six cycles of adsorptiondesorption experiments were performed. The experimental results are demonstrated in Fig. 4. It can be found from Fig. 4(a)–(c) that the desorption efficiencies of Cr(VI) using different eluents after the first desorption were 0.96% (H2O), 1.34% (HCl), and 2.61% (NaCl), respectively. With the increase in the number of desorption, the desorption efficiencies of Cr(VI) by H2O, HCl, and NaCl decreased gradually. By the third desorption, Cr(VI) was scarcely desorbed by H2O (~0), HCl (~0), and NaCl (0.59%). These results indicated that Cr(VI) adsorbed on the magnetic Enteromorpha prolifera derived biochar was basically stable under acidic or neutral conditions. As shown in Fig. 4(a)–(c), Cr (VI) adsorbed on the magnetic Enteromorpha prolifera derived biochar could be desorbed by alkaline solutions and higher concentrations of alkaline solution were more favorable for Cr(VI) desorption, e.g., 20.49% removal of Cr(VI) with 0.1 M NaOH and 33.45% with 1 M NaOH. This coincided with the previous conclusion that the adsorption capacity was reduced under alkaline conditions (Fig. S3(a)), further confirming that electrostatic adsorption played an significant role in Cr (VI) absorption by BCF600. After threefold desorption, the desorption
3.2.5. Effect of background ion intensity To investigate the effect of background ion intensity on Cr(VI) removal, different concentrations of NaCl were used to adjust the ion intensities. As shown in Fig. S3(b), the results indicated that the existence of NaCl hardly influenced the removal of Cr(VI) by BCF600 at low concentrations (0.005 and 0.01 mol L−1). But, it significantly reduced the Cr(VI) removal capacity from 97.85% to 83.16% and to 76.15% at higher concentrations (1 and 2 mol L−1, respectively). The reasons for these phenomena are as follows: (1) Cl- anions may have competed with Cr(VI) for the adsorption sites on the adsorbent surface and the competition was more significant at high concentrations (Gan et al., 2015; Wang et al., 2015a) and (2) the solution ion intensity was increased at high concentration of NaCl, thus reducing the activity coefficient of Cr(VI) and greatly reducing the chances of collisions and contact between the sorbent (Z.B. Zhang et al., 2013). The effect of background ion intensity on Cr(VI) adsorption further showed that electrostatic interaction was a likely adsorption mechanism for Cr(VI) removal by BCF600.
Table 1 Kinetic parameters of Cr(VI) adsorption by BCF600. Cr(VI)
First-order
Second-order
Elovich
( mg L−1)
qe (mg g−1)
k1 (h−1)
R2
qe (mg g−1)
k2 (g mg−1 h−1)
R2
β (g mg−1)
α (mg g−1 h−1)
R2
100 300 500
45.99 73.74 81.83
0.63 0.76 0.66
0.913 0.841 0.843
48.46 77.97 86.90
0.02 0.01 0.01
0.982 0.939 0.945
0.22 0.14 0.12
222.55 149.09 60.30
0.985 0.998 0.997
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Table 2 Kinetic parameters of Cr(VI) adsorption by BCF600. Biochar
Langmuir model Qe (mg g
BCF600
91.56
−1
)
Freundlich model Qm (mg g 88.17
−1
)
KL (L mg
−1
R
KF (mg(1-n) Ln g−1)
n
R2
0.9996
38.66
4.26
0.6668
2
)
0.69
The specific surface area of BCF600 was significantly less than that of its corresponding pristine biochar (Table S1), but BCF600 still had a higher absorption capacity than the pristine biochar, indicating that the Cr(VI) adsorption by BCF600 did not mainly depend on its pore structure and that the γ-Fe2O3 loaded on the Enteromorpha prolifera derived biochar may play a key role in Cr(VI) removal (Fig. 1(a)). The SEM photos demonstrated that the biochar matrix was in charge of sustaining and dispersing maghemite (M. Zhang et al., 2013), which resolves the disadvantage of easy agglomeration of γ-Fe2O3. As can be observed from the FTIR spectra in Fig. 1(a), the water characteristic peaks at 3741 cm−1 were enhanced for BCF600 after modification, and BCF600 exhibited -OH stretching vibration of the carboxylic acid dimer at 2435 cm−1, in contrast to BC600, indicating that some -OH, which may come from γ-Fe2O3, was introduced after the modification. This was consistent with the fact that iron oxide had a hydroxylated surface with a large number of -OH functional groups (Ma, 2011). Therefore, we hypothesized that -OH played a key role in the removal of Cr(VI) by the magnetic biochar. To confirm this speculation, we provide the following supportive evidence. (1) Based on the results and discussion in Sections 3.2.4, 3.2.5, and 3.3, it was proposed that when the pH is < pHpzc (6.5) (Han et al., 2015; Wang and Lo, 2009), the -OH functional groups were protonated to form positively charged -OH2+ functional groups, so that the Cr-containing anions
efficiency of Cr(VI) by the alkaline solution was about 47.13%. Thus, Cr (VI) adsorbed biochar can be desorbed effectively using alkaline solution. Although Cr(VI) can be desorbed effectively by alkaline solution, the recyclability after desorption remained unclear. Therefore, six cycles of adsorption-desorption experimentation were carried out. As presented in Fig. 4(d), even though the adsorption capacity reduced gradually with the increase in number of cycles, in the sixth cycle, it was more than 23.12 mg g−1, which was still greater than that of the original biochar and many other types of biochar (Table S2). In addition, the magnetic Enteromorpha prolifera derived biochar itself was rich in magnetism and could be collected effectively after adsorption. Therefore, the magnetic Enteromorpha prolifera derived biochar could be regenerated efficiently via alkaline solution, indicating its high potential as a cheap adsorbent in Cr(VI) wastewater treatment. 3.4. Mechanism of Cr(VI) adsorption by magnetic Enteromorpha prolifera derived biochar To confirmed the possible adsorption mechanisms for Cr(VI) by the magnetic Enteromorpha prolifera derived biochar, both adsorbent structural information (specific surface area, SEM, FTIR, etc.) and sorption characteristics were analyzed.
Fig. 4. Three consecutive desorptions (a), (b), (c) and six consecutive adsorption-desorption cycles (d) of Cr(VI) on BCF600. 445
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could be adsorbed and migrated to the γ-Fe2O3 surface via electrostatic interaction (Shen et al., 2009). (2) By contrasting the FTIR spectra of BCF600 before and after the adsorption experiments, the reduction in the vibrational intensity of -OH at 3741 and 2358 cm−1 also proved that -OH was involved in the sorption (Fig. 1(b)). Therefore, electrostatic interaction might be the dominant mechanism adsorption for Cr (VI) by the magnetic biochar.
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