Removal of Hexavalent Chromium by Different ...

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Mar 13, 2014 - Removal of Hexavalent Chromium by Different. Modified Spruce Bark Adsorbents. SHA LIANG,1,2 XUEYI GUO,2 SILKE LAUTNER,3.
Journal of Wood Chemistry and Technology, 34:273–290, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 0277-3813 print / 1532-2319 online DOI: 10.1080/02773813.2013.869606

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Removal of Hexavalent Chromium by Different Modified Spruce Bark Adsorbents SHA LIANG,1,2 XUEYI GUO,2 SILKE LAUTNER,3 AND BODO SAAKE3 1

School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, P. R. China 2 School of Metallurgy and Environment, Central South University, Changsha, Hunan, P. R. China 3 Department of Wood Science, University of Hamburg, Hamburg, Germany Abstract: In this work, spruce bark was used as a raw material to remove Cr(VI) ions from aqueous solutions. Three kinds of chemically modified bark adsorbents were prepared by treatment with formaldehyde (FB), dilute sulfuric acid (AB), and concentrated sulfuric acid (CB), respectively. The chemical modifications mainly changed the relative lignin content in the bark. Lower pH facilitated the adsorption of Cr(VI) ions because reduction of Cr(VI) ions to Cr(III) ions occurred during the adsorption process which consumed a large amount of H + ions. Higher temperature accelerated the adsorption process, owing to the endothermic nature of the redox reaction. At initial solution pH around 1, the adsorption capacities of Cr(VI) ions on FB, AB, and CB were as high as 423, 503, and 759 mg/g, respectively, which were much higher than the reported adsorption capacities by other agricultural and forest biosorbents in the literatures. XPS analysis revealed the adsorption mechanism was adsorption-coupled reduction involving the electron-donor groups of lignin moieties. Keywords Spruce bark, biosorption, Cr(VI), reduction

Introduction Chromium is one of the most toxic heavy metal ions discharged into the environment through various industrial wastewaters, especially in electroplating, tanning, textile dyeing, wood preservation, etc.[1] The existing forms of chromium in effluents are Cr(III) ions and Cr(VI) ions. Cr(III) ions are generally only toxic to plants at very high concentrations and are less toxic or nontoxic to animals.[2] However, Cr(VI) ions are about 500 times more toxic than Cr(III) ions, and are well known to be carcinogenic and mutagenic to living organisms. Because of the differences, the discharge of Cr(VI) ions to surface water is regulated to below 0.05 mg/L by the US EPA, while total Cr ions, including Cr(III) ions, Cr(VI) ions, and their other forms, are regulated to below 2 mg/L.[3] Different technologies, including chemical precipitation, ion exchange, membrane technology, and adsorption, have been used for wastewater treatment. The most common Address correspondence to Xueyi Guo, School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, P.R. China. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lwct.

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conventional method for Cr(VI) ions removal is first to reduce Cr(VI) ions to Cr(III) ions by adding a reduction reagent like NaHSO3, Na2 SO3 , and FeCl2 under adjusted water pH around 2, then to precipitate Cr(III) ions as Cr(OH)3 by adding lime to increase pH to 9–10. The disadvantages of this method are the need to dispose of a large quantity of waste sludge and the fact that it may be ineffective when the metal concentrations are in the range of 1–100 mg/L. In recent years, adsorption technology is one of the effective methods to remove heavy metals from aqueous solutions. Agricultural wastes or by-products are considered to be the most cost-effective potential adsorbents for wastewater treatment.[4] Until now, different types of waste biomass have been utilized for adsorption of Cr(VI) ions. They include sugarcane bagasse,[5] plane leaves,[6] coir pith,[7] rice bran,[8] wheat bran,[9] and walnut hull,[10] etc. Bark is currently a huge residue stream for wood processing industries. In large mills, the bark is often burned in order to produce energy. Efficient alternative bark utilization for higher added-value products could improve the raw material utilization efficiency, the process economics of existing mills, and create new industries in this application field. Bark is mainly composed of cellulose, hemi-cellulose, lignin, and additionally contains a variety of extractives depending on the wood species. These components contain various functional groups, such as carboxyl and hydroxyl, which can combine metal ions from aqueous solutions. As early as in 1974, Randall investigated the potential of many different kinds of bark for the removal of heavy metal ions and proved bark was an effective material for wastewater treatment.[11] For the removal of Cr(VI) ions from aqueous solution, larch bark,[12] cedar bark,[13] and eucalyptus bark[14,15] were reported as adsorbents. Among the adsorption research works on Cr(VI) ions by biomass, the adsorption mechanism was discussed in most of the papers. Many studies reported that Cr(VI) ions were removed from aqueous solutions through an “anion adsorption” process; i.e., anionic Cr(VI) species were adsorbed by positive charged adsorption sites in the adsorbents.[15–20] However, Park et al.[21] suggested that these findings were misinterpreted due to errors in measuring total Cr ions in aqueous solution and insufficient contact time required for equilibrium, resulting from the lack of information about the valence state of Cr bound on the biomass. He pointed out a new, Cr(VI)-ions-removal mechanism which is now widely accepted as the true mechanism, named “adsorption-coupled reduction.” The objective of the present investigation was to produce adsorbents from spruce bark and clarify the mechanism that governs Cr(VI) ion removal. For the production of adsorbents in many previous studies, formaldehyde was used as a crosslinker in order to avoid the dissolution of low-molecular phenolic compounds from the modified bark. Since formaldehyde was not an environmentally benign reagent, one aim of our work was to find the necessity of the use of formaldehyde in the modification process. Therefore, three kinds of chemically modified bark adsorbents were prepared by treatment with formaldehyde (FB), dilute sulfuric acid (AB), and concentrated sulfuric acid (CB), respectively. For all three materials, the effects of various adsorption parameters, such as pH, contact time, temperature, and metal ion concentration, were evaluated extensively.

Experimental Preparation of Bark Adsorbents The spruce barks were kindly provided by a local wood company (Hamburg, Germany). After air drying, the barks were crushed by a cutting mill (Retsch SM2000) and then the

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obtained powder was sieved by a sieve shaker (Retsch AS 400) to get a particle size lower than 0.8 mm. The chemical modification of the bark powder was carried out using sulfuric acid and formaldehyde as modifiers. The modification conditions were optimized by DOE (Design of Experiment), choosing four factors, including the modification time, temperature, the mass percent of sulfuric acid (H2 SO4 %), and mass percent of formaldehyde (HCHO%), calculated based on the mass of bark. The ranges of these parameters were selected considering related literature on the crosslink of phenolic compounds in the bark.[12,15,22–25] The modification process was conducted by shaking 1 g of bark powder and 10 mL of solution mixed with formaldehyde and sulfuric acid in a water bath. After treating, the mixture was filtrated and washed several times with distilled water. The filter residue was air-dried in a climate room until water content was stable. Then, the prepared modified sample was used for Cr(VI) ion removal under the following adsorption conditions: initial concentration of Cr(VI) ions was 100 mg/L, solution pH was 2.0, solid to liquid ratio was 50 mg/20 mL, temperature was 30◦ C, and adsorption time was 24 h. Both the product yield after modification and the adsorption performance (percentage of adsorption) were taken into account for the model development. Two optima were calculated in order to obtain the best modification conditions for the bark with and without formaldehyde. When using formaldehyde, the optimal modification conditions were: 42.9◦ C, 2.2 h, 50% (w/w%) H2 SO4 , and 36% (w/w%) HCHO. Without formaldehyde, the optimal modification conditions were: 90◦ C, 2.5 h, and 47.6% (w/w%) H2 SO4 . Some papers also reported using concentrated sulfuric acid as a dehydrating reagent to immobilize polyphenol groups,[26–28] so this approach was also included in the study following the protocol of these references. Thus, three different modified bark adsorbents were prepared: 1. Ten grams of bark powder were mixed with 100 mL of solution containing 5 g of H2 SO4 and 3.6 g of HCHO under 43◦ C for 2.2 h. After modification, the mixture was filtrated and washed with distilled water until the filtrate reached a neutral pH value. The residue was air-dried in a climate room and the obtained adsorbent (9.3 g) was named FB (formaldehyde modified bark). 2. Ten grams of bark powder were mixed with 100 mL of solution containing 4.76 g of H2 SO4 under 90◦ C for 2.5 h. Through the same process, this obtained adsorbent (8.2 g) was named AB (acid modified bark). 3. Sixteen grams of bark powder were mixed with 100 mL of 72% H2 SO4 under 90◦ C for 2 h. Through the same process, this obtained adsorbent (11g) was named CB (concentrated acid modified bark). Characterization of Bark Adsorbents The chemical compositions of bark and prepared bark adsorbents were determined by sugar analysis. All samples were first extracted by accelerated solvent extraction (Dionex ASE200) through three-step extraction (petrol ether/acetone:water (9:1)/ethanol:water (8:2)). Then the extracted adsorbents were hydrolyzed by sulfuric acid through two-step hydrolysis, and the sugar content in the filtrate solution was determined by HPAEC-Borate analysis using an Ultimate 3000 system. The point of zero charge (pHpzc ) of bark adsorbents was determined by the solid addition method.[29] Dried adsorbents were coated with carbon on a sputter coater (Biorad, Germany) before being examined by a scanning electron microscope (SEM, S-520 Hitachi)

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at 10 keV. XPS was employed to ascertain the oxidation state of the chromium bound on the adsorbents by X-ray photoelectron spectroscopy (K-Alpha 1063) using an Al Kα X-ray source, at 72 W and limiting vacuum of 10−9 mBar.

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Adsorption Experiments Aqueous solution of Cr(VI) ions was prepared by dissolving analytical grade K2 Cr2 O7 into 0.1 M HCl acid. The solution pH was adjusted with HCl and NaOH. Batch-wise adsorption experiments were carried out by shaking 20 mg of the modified bark adsorbent with 10 mL of Cr(VI) ions solution with a certain concentration in a water bath shaker at a certain temperature for a certain time. After adsorption and filtration, the concentration of Cr(VI) ions was determined by the diphenylcarbazide method using the Lambda 650 UV spectrophotometer (Perkin Elmer), and the concentration of total Cr ions was measured by an atomic absorption spectra-photometer (A300, Perkin Elmer). The concentration of Cr(III) ions was calculated from the difference between the total Cr ions and the Cr(VI) ions concentrations. The amount of metal adsorbed (qe ) was determined by using the following equation: qe =

(C0 − Ce )V m

(1)

where C0 and Ce represent the initial and equilibrium metal ions concentrations (mg/L), respectively; V is the volume of the solutions (mL) and m is the amount (mg) of adsorbent.

Results and Discussion Characterization of Bark Adsorbents The results of chemical analysis are depicted in Table 1. The extract content of the starting bark material adds to a total of 6%. This value is a bit low, which can be rationalized by the fact that the bark was obtained from a factory and had already experienced a longer storing time. However, this material is a realistic feedstock for an industrial implementation of the modification procedures under investigation. The hydrolysis residue reflects the acidinsoluble lignin equivalent to the Klason lignin. The contents of different sugars measured in the hydrolysis liquor give the ratio of cellulose, hemicellulose. From Table 1, it can be judged that the main components of bark are lignin and cellulose, around 41% and 27%, respectively. Comparing the data of modified bark adsorbents with raw bark, it can be seen that the sugar contents had little change after being treated with formaldehyde (FB), but the cellulose and hemicellulose were partly removed by modification with dilute sulfuric acid (AB). After treatment with concentrated sulfuric acid (CB), no sugar can be detected in the product. The lignin content based on the raw material increases to 63.8%, which means most likely as a result of condensation reactions with carbohydrate degradation products like furans. Lignin is the most important composition for Cr(VI) ion removal. Dupont and Guillon[30] conducted studies on Cr ions adsorption with pure cellulose and pure lignin at acidic pH and observed no significant adsorption onto cellulose, whereas Cr(VI) ion reduction to Cr(III) ions was carried out on lignin. From the lignin content in Table 1, the order is CB>AB>FB>Bark, which suggests the order of adsorption ability for Cr(VI) ions. Sample solutions for measurement of the total organic carbon (TOC) concentration were prepared by shaking 30 mg of bark or bark adsorbent with 15 mL of 50 mg/L

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Table 1 Main components in spruce bark adsorbents Extract∗ % Adsorbent

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Bark

FB

AB

CB

1.52 3.50 0.94 1.48 2.57 0.77 1.67 3.40 0.70 0.93 2.43 0.57

Hydrolysis residue% (Lignin%)

Mannose % Xylose % 3.3

Glucose %

Others %

27.1

5.2

41.1

3.6

44.7 (41.6)∗∗

3.6 (3.4)∗∗

3.8 (3.5)∗∗ 29.2 (27.2)∗∗ 4.3 (4.1)∗∗

48.0 (39.3)∗∗

3.2 (2.6)∗∗

2.8 (2.3)∗∗ 30.6 (25.1)∗∗ 0.7 (2.0)∗∗

92.5 (63.8)∗∗

0.0

0.0

0.0

0.0



petrol ether/acetone:water (9:1)/ethanol:water (8:2). the data in the parentheses are the relative data after multiplying the yield of the adsorbents for better comparison with the starting bark material. ∗∗

Cr(VI) ions solution at pH around 1 at 30◦ C for 24 h. The TOC concentrations of the original bark, FB, AB, and CB measured by using a TOC Analyzer (DIN EN 1984) were 235, 108, 103, and 80 mg/L, respectively. This result showed that crosslinking helped to combine some water-soluble, low-molecular-weight compounds and decreased the leakage of low-molecular-weight compounds, thus improving the mechanical strength of the bark. The surface charge characteristics of adsorbents depend on the point of zero charge (pHpzc ). Their surface might be positively charged, negatively charged, or neutral when the solution pH is below, above, or equal to pHpzc , respectively. Adsorption of cations is favored at pH > pHpzc , while the adsorption of anions is favored at pH < pHpzc . The values of pHpzc of FB, AB, and CB were tested to be 3.0, 3.2, and 6.0, respectively, suggesting that CB has a positive surface charge over a wider pH range for binding Cr(VI) anions. It can be explained that the value of pHpzc is determined by the dissociation of the functional groups of the adsorbent, including hydroxyl of cellulose, phenolic hydroxyl, and carboxyl of lignin. Since the dissociation of phenolic hydroxyl and carboxyl of lignin occurs at higher pH, the adsorbent CB with the relatively higher content of lignin than FB and AB will exhibit positive surface charge over a wider pH range. Figure 1 shows the SEM micrographs of the bark, FB, AB, and CB. In the micrographs of bark, long-fiber-bundle and honeycomb-structure parenchyma cells can be clearly recognized. Little change can be detected in the structure of FB, but the structures of AB and CB appear clearly altered by sulfuric acid treatment. Especially from the micrographs of CB, irregular and porous surfaces could be observed, resulting from disintegrating cell structures associated with new cellular conglomeration due to the new compounds formed during concentrated sulfuric acid condensation. These structures are beneficial for the binding of metal ions.

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Figure 1. SEM graphs of the prepared spruce bark adsorbents. Along with the respective treatments (a-d) disintegration of bark cell wall structures can be observed. Bars represent 200 μm.

Effect of pH Figure 2 shows the effect of initial solution pH on the adsorption% of Cr(VI) ions by the three modified bark adsorbents; the final solution pH values after adsorption are also given in the figure. It can be seen that the adsorption efficiencies towards Cr(VI) ions of all adsorbents were high (>99%) at low pH (0.5–3.0). At higher pH, the adsorption efficiencies decreased with small but significant differences between the various adsorbent materials. The general decline can be explained by the final pH values in Figure 2. The solution pH increased after adsorption, especially when the initial pH was higher than 2.0. At the same initial solution pH, the equilibrium pH after adsorption by CB was higher than that of FB and AB. Lower pH was beneficial for Cr(VI) ion removal because the active sites on the adsorbents were protonated under this condition and inclined to adsorb Cr(VI) ions, which exist as anion (mainly HCrO4 -) in solution. It suggests that high concentrations of H+ ions facilitate the adsorption whereas high concentrations of OH- ions suppress the adsorption reaction, thus accounting for the decrease in the percentage of adsorption of Cr(VI) ions ion at high pH. In order to understand the adsorption mechanism, the time dependency of solution pH as well as the concentration of Cr(VI) ions, Cr(III) ions, and total Cr ions were measured under different initial pH. The results depicted in Figure 3 indicated that all adsorbents exhibited similar adsorption trends. At initial pH 1.0 (Figure 3(a)), it can be seen that the

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100

7

90 FB AB CB

Adsorption%

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70

6 5

60 50

4

40

3

30 FB AB CB

20 10 0 0.5

1.0

1.5

2.0

2.5

3.0

final pH

80

2 1 0 3.5

inital pH Figure 2. Effect of initial pH on the adsorption of Cr(VI) ions by bark adsorbents and the final of pH after the adsorption. (Initial concentration of Cr(VI) ions: 50mg/L; solid/liquid: 20mg/10mL; 30◦ C; 24h.)

concentration of Cr(VI) ions decreased sharply and almost disappeared after 120 min for FB, 60 min for AB, and 40 min for CB. Instead of Cr(VI) ions, the Cr(III) ions, which were not initially present, appeared in the solution. Finally, the concentration of total Cr ions in the solution equaled the concentration of Cr(III) ions. This reveals that the reduction of Cr(VI) ions to Cr(III) ions occurred during the adsorption process according to the following reactions[1]: + − 3+ HCrO− 4 +7H +3e ↔ Cr + 4H2 O

(2)

The solution pH changed a little because there were enough H+ ions for reduction at pH 1.0. The reduced Cr(III) ions were present in the solution as cations, which could not be easily adsorbed at such low pH when the surface of the adsorbent was positively charged. Also, it can be calculated that removal efficiencies of total Cr ions by FB, AB, and CB were 68%, 64% and 80%, respectively. At initial pH 2.0 (Figure 3(b)), the time required to attain equilibrium increased up to 6 h for FB, 3 h for AB, and 2 h for CB. The solution pH increased during adsorption because of the consumption of H+ ions. Another phenomenon was that the final equilibrium concentration of Cr(III) ions was lower, which confirmed that adsorption of Cr(III) ions was preferred at higher pH. At initial pH 3.0 for FB and AB, and initial pH 2.5 for CB (Figure 3(c)), the solution pH increased dramatically during the incubation, and the required adsorption time to attain equilibrium was prolonged to a larger extent. The final pH was higher than 5.0, which was beneficial for Cr(III) ion removal. After adsorption, the concentration of total Cr ions equaled to the concentration of Cr(III) ions in the solution was almost zero. This means the

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1.2

residual Cr conc. /(mg/L)

1.0

FB pH0=1.0

40

0.8

Cr(VI) total Cr Cr pH

30

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0

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time/min 1.2

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Cr(VI) total Cr Cr pH

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pH

residual Cr conc. /(mg/L)

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time/min 1.2

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Cr(VI) total Cr Cr pH

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0.8 0.6

20

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0

0

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pH

50

residual Cr conc. /(mg/L)

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50

pH

(a)

0.0 250

time/min Figure 3. The variation of pH and ion concentrations during the adsorption of Cr(VI) ions at different initial pH (a) pH0 = 1.0;(b) pH0 = 2.0;(c) pH0 = 3.0 (2.5 for CB). (Initial concentration of Cr(VI) ions: 50mg/L; solid/liquid: 20mg/10mL; 30◦ C.) (Continued)

Removal of Cr(VI) Ions Using Spruce Bark 2.2

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residual Cr conc. /(mg/L)

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FB pH0=2.0

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40 Cr(VI) total Cr Cr pH

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Cr(VI) total Cr Cr pH

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residual Cr conc. /(mg/L)

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1.0 250

time/min 60

2.4 2.2

CB pH0=2.0

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Cr(VI) total Cr Cr pH

30 20

1.6 1.4

10 0

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0

50

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time/min Figure 3. (Continued)

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1.0 250

pH

50

residual Cr conc. /(mg/L)

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pH

(b)

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60 FB pH0=3.0

residual Cr conc. /(mg/L)

6 5

40 Cr(VI) total Cr Cr pH

30

4 3

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10 0

1

0

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0 30

time/h 7

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30 20

3 2

10 0

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pH

residual Cr conc. /(mg/L)

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0

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0 30

time/h 6

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40 Cr(VI) total Cr Cr pH

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time/h

Figure 3. (Continued)

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0 30

pH

residual Cr conc. /(mg/L)

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50

pH

(c)

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Table 2 Kinetic parameters of Cr(VI) adsorption by FB, AB, and CB Pseudo-first-order equation ◦

Adsorbent T/ C k1 (1/min) qe,cal (mg/g)

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FB

AB

CB

30 40 50 30 40 50 30 40 50

0.248 0.309 0.412 0.261 0.474 0.753 0.423 0.674 1.368

14.1 12.5 12.3 9.40 7.68 9.36 12.0 9.77 11.7

R2 0.994 0.958 0.979 0.985 0.944 0.954 0.994 0.956 0.979

Pseudo-second-order equation k2 (g/mg min) qe,cal (mg/g) 0.032 0.057 0.089 0.060 0.144 0.189 0.066 0.160 0.303

25.3 25.9 25.6 25.8 25.5 25.4 26.0 25.4 25.3

R2 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999

removal efficiencies of total Cr ions were larger than 99%, which is the desired result in actual wastewater treatment, although much longer adsorption time was needed. Effect of Temperature Batch experiments were performed at initial pH 3.0 for FB and AB, and at initial pH 2.4 for CB in order to examine the temperature dependency of Cr(VI) ion removal by bark adsorbents (Figure 4). Obviously, higher temperature accelerated the adsorption process, owing to the endothermic nature of the redox reaction. The pseudo-first- and second-order kinetic equations were used to examine the adsorption kinetics for better understanding of the dynamics of adsorption of Cr(VI) ions onto bark adsorbents. The pseudo-first-order kinetic model is known as the Lagergren equation[31]: log(qe − qt ) = log qe −

k1 t 2.303

(3)

where qt and qe are the amounts of ion adsorbed at time t and at equilibrium (mg/g), respectively, and k1 is the rate constant of pseudo-first-order adsorption process (1/min). The pseudo-second-order kinetic model can be expressed as the following equation[32]: t 1 1 = + t qt k2 qe2 qe

(4)

where k2 is the equilibrium rate constant of pseudo-second-order adsorption (g/mg min). The constants of kinetics models for the adsorption of Cr(VI) ions on FB, AB, and CB are listed in Table 2. It can be concluded from Table 2 that the pseudo-second-order equation provides better correlation coefficient and agreement between calculated qe values and the experimental data, whereas the pseudo-first-order does not give a good fit to the experimental data for the adsorption of Cr(VI) ions. This suggests that chemical adsorption is the rate-limiting step.[33] For the same adsorbent, the rate constants of pseudo-secondorder adsorption (k2 ) increase with the increase of temperature, which further proves the positive influence of temperature.

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FB pH0=3.0

residual Cr(VI) conc.(mg/L)

o

30 C o 40 C o 50 C

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30 C o 40 C o 50 C

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0 0

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time/h

Figure 4. The effect of temperature on the adsorption of Cr(VI) ions by FB, AB and CB. (Initial concentration of Cr(VI) ions: 50mg/L; solid/liquid: 20mg/10mL.)

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Table 3 Cr(VI) adsorption capacity of some agricultural by-products reported in the literature

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Adsorbent Formaldehyde modified spruce bark Acid modified spruce bark Concentrated sulfuric acid modified bark Sugarcane bagasse London plane leaves Coir pith Rice bran Wheat bran Walnut hull Japanese cedar bark Grape waste

pH0 1 1 1 3 3 2 2 2 1 3 4

Temperature/ ◦ C q/ mg/g References 30 30 30 25 40 60 20 40 30 50 30

423 503 759 103 83 318 286 311 98 91 99

This study This study This study [5] [6] [7] [8] [9] [10] [13] [26]

Adsorption Isotherms The equilibrium adsorption isotherms of FB, AB, and CB at different initial pH were evaluated by plotting the adsorbed chromium ions (qe ) against the equilibrium concentration of chromium ions (Ce ) in solution (Figure 5). The isotherm profiles of different adsorbents showed the same trend: with the increase of metal ion concentration, the adsorption capacity increased until adsorption saturation was reached. Some conclusions can be revealed from Figure 5. As for Cr(VI) ion adsorption, the adsorption capacity of Cr(VI) ions by three bark adsorbents increased with the decrease of initial solution pH. This is because the reduction reaction of Cr(VI) ions consumed a large amount of H+, and higher concentration of Cr(VI) ions needed more H+. At initial solution pH around 1, the highest adsorption capacities of Cr(VI) ions on FB, AB, and CB were nearly 423, 503, and 759 mg/g, which were much higher than the selected high adsorption capacities for Cr(VI) ions removal by other agricultural and forest biosorbents reported in the literature (Table 3). However, high adsorption of Cr(VI) ions does not mean high adsorption of total Cr ions in the solution. A comparison between the adsorption capacity of Cr(VI) ions and total Cr ions has rarely been reported in the previous literature. Comparing the adsorption isotherms of Cr(VI) ions and the total Cr ions in Figure 5, it can be seen that when the initial solution pH was around 3 for FB and AB, and 2 for CB, there was little difference in the adsorption capacity between Cr(VI) ions and total Cr ions. This is because, along with the reduction reaction at this initial pH, the equilibrium pH increased significantly to a higher pH, which was good for Cr(III) ion adsorption; then the final concentration of total Cr ions in the solution almost equaled the concentration of unabsorbed Cr(VI) ions. When the initial solution pH was lower, the solution pH increased to a lesser extent, and the reduced Cr(III) ions were less absorbed. Especially at initial solution pH around 1, there was a big difference between the adsorption capacity of Cr(VI) ions and total Cr ions. It can be understood that there were enough H+ ions for the reduction reaction of Cr(VI) ions and solution pH to remain at 1.1–1.2, which resulted in many Cr(III) cations left in the solution. Therefore, the calculated adsorption capacity of total Cr ions was less than that of Cr(VI) ions. In the practical wastewater treatment, both the removal of Cr(VI) ions and total Cr ions should be considered.

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FB

350

Cr(VI), pH0=3

300

Cr(VI), pH0=2 Cr(VI), pH0=1

250

total Cr, pH0=3

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total Cr, pH0=1

100 50 0

0

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CB

600

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qe/(mg/g)

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qe/(mg/g)

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Cr(VI), pH0=1 total Cr, pH0=2

400

total Cr, pH0=1

300 200 100 0

0

100

200

300

400

500

600

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Ce/(mg/L)

Figure 5. Adsorption isotherms of Cr(VI) ions and total Cr ions adsorption by FB, AB and CB at different initial pH. (Initial concentration of Cr(VI) ions: 50-2000 mg/L; solid/liquid: 20mg/10mL; 30◦ C; 24h.)

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CB-Cr

Intensity

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AB-Cr

FB-Cr

594 592 590 588 586 584 582 580 578 576 574 572 570

Binding energy/ev Figure 6. XPS spectra collected from the Cr 2p core regions of the Cr-laden spruce bark adsorbents (FB, AB and CB).

Adsorption Mechanism To characterize the main mechanism of Cr(VI) ion removal, it is very important to ascertain the oxidation state of the chromium bound on the adsorbents.[34] Thus, XPS spectra were collected from the Cr 2p core regions of the Cr-laden bark adsorbents (Figure 6). According to the literature,[21,35] Cr(III) compounds showed bands at binding energies of 577–579 and 586–588 ev, which corresponded to 2p3/2 and 2p1/2 orbital, respectively, while those of Cr(VI) compounds appeared at binding energies of 579–581 and 588–590 eV, respectively. From Figure 6, it can be concluded that the spectra of Cr-laden bark adsorbents were well matched with those of Cr(III) compounds. These results reveal that the removal of Cr(VI) ions was through an adsorption-coupled reduction mechanism. Enough electrondonor groups on the adsorbents are important to reduce Cr(VI) ions to Cr(III) ions. In our research, the reduction of Cr(VI) ions mainly involved lignin moieties, including carboxyl groups[36] or methoxy and carbonyl groups, as electron-donors.[37,38] Thus, in Figure 5, at initial solution pH around 1, CB showed higher adsorption capacity than FB and AB. This was because at low pH there was enough H+ that could participate in the reduction of Cr(VI) ions and solution pH changed a little during the reduction reaction. Also, CB had enough functional groups, especially the carboxyl, carbonyl, and methoxy in lignin content, to act as electron-donors to bind chromium. However, FB and AB, with lower content of lignin, could not provide enough adsorption sites for binding Cr(VI) ions when the concentration of Cr(VI) ions increased, so the adsorption capacities of them were much lower. At initial pH around 2, with the reduction reaction quickly proceeding, the solution pH greatly increased. There were not enough H+ ions in the solution, even though CB had enough electron-donors. Thus, there was less difference among the adsorption capacities of FB, AB, and CB. Enough H+ ions and enough electron-donors were the two important factors contributing to the high adsorption capacity for the removal of Cr(VI) ions.

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1. binding Cr(VI) ions to positively charged groups on the surface of bark adsorbent; 2. reducing of Cr(VI) ions to Cr(III) ions by adjacent electron-donors; 3. releasing reduced Cr(III) ions into solution by electronic repulsion between the positively charge groups and Cr(III) ions (under high pH condition, Cr(III) ions can be adsorbed onto the surface of the adsorbent by complexation). Desorption experiments were carried out to remove the chromium from Cr-laden bark adsorbents by soaking them in the 1 mol/L HNO3 solution for 24 h. In such strong acidic medium, only 31%, 33%, and 16% of adsorbed Cr could be recovered from Cr-loaded FB, AB, and CB, respectively. This result further confirmed the irreversible nature of Cr(VI) ion adsorption due to the strong bond between the chromium ions and the functional groups of the bark adsorbents.

Conclusions Three kinds of chemically modified bark adsorbents were prepared by treatment with formaldehyde (FB), dilute sulfuric acid (AB), and concentrated sulfuric acid (CB). All products were used to remove Cr(VI) ions from aqueous solutions. Lower pH and higher temperature facilitated the adsorption of Cr(VI) ions because the reduction reaction of Cr(VI) ions to Cr(III) ions occurred during adsorption process. The results showed that the adsorption of Cr(VI) ions by bark adsorbents was through an adsorption-coupled reduction mechanism, which consumed a large amount of H+ ions and involved the electron-donor groups of lignin moieties. At initial solution pH around 1, the adsorption capacities of Cr(VI) ions on FB, AB, and CB were as high as 423, 503, and 759 mg/g. However, it should be noted that high adsorption of Cr(VI) ions does not mean high adsorption of total Cr ions in the solution. Another important conclusion is that it is unnecessary to use formaldehyde for immobilizing phenolic compounds because a treatment with diluted sulfuric acid already gives a better adsorbent material. The treatment with concentrated sulfuric acid is the best choice regarding the performances of the product, although the product yield is relatively low and the production process with concentrated acid might be more complicated compared to the modification with diluted acid.

Funding This work was financially supported by the China Scholarship Council (2011) and by the German Federal Ministry of Education and Research (funding agency PTJ; FKZ: 03SF0345).

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