Adsorption studies of the removal of anions from aqueous solutions onto an adsorbent prepared from wheat straw

SCIENCE CHINA Chemistry

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*Corresponding author (email: [email protected])

• ARTICLES • June 2010 Vol.53 No.6: 1414–1419

doi: 10.1007/s11426-010-3100-6

Adsorption studies of the removal of anions from aqueous solutions onto an adsorbent prepared from wheat straw

XU Xing, GAO Yue, GAO BaoYu*, YUE QinYan & ZHONG QianQian

School of Environmental Science and Engineering, Shandong University, Jinan 250100, China

Received September 28, 2009; accepted November 2, 2009

Modified wheat straw (MWS) was prepared by the grafting of epichlorohydrin, triethylamine and ethylenediamine onto WS. The characteristics of MWS and its adsorption capacity for NO3

, PO43 and Cr2O7

2 were investigated. The results indicate that amine groups with positive charge have been introduced into the structure of MWS, and significantly increased its anion adsorption property. The functions of MWS dosage, the solution pH, the contact time and temperature have significant influence on the adsorption process, and the adsorption is well fitted with the Langmuir equation and pseudo second-order model. The maximum adsorption capacity of MWS for NO3

, PO43 (P) and Cr2O7

2 (Cr) is 53.5, 62.4 and 386.2 mg g1, respectively.

wheat straw, anion, adsorption, maximum adsorption capacity

1 Introduction

China is abundant in the biomass of raw materials from plantations, and approximately 600 million tons of agricultural by-products are produced every year [1]. The utilization of agricultural by-products is broadly divided into three cate- gories: (i) raw materials for pulp industry, (ii) animal feed- ing stuffs and (iii) biomass fuel and energy [1], which ac- count for 23.0%, 24.0% and 31.5% of the total annual pro- duction of agricultural by-products, respectively [2]. The remaining is deposed or burned without proper utilization, resulting in the nonpoint source pollution in the agroecologi- cal environment. Therefore, how to increase the utilization of agricultural by-products and reduce the pollution caused by agricultural by-products is of particular interest. Utiliza- tion of agricultural by-products has been investigated, and some studies reported the preparation of adsorbents from agricultural by-products [3–5]. An appropriate chemical composition in agricultural by-products with lignin, cellulose (as -cellulose), hemicelluloses, pectin and low mo-

lecular compounds, suggests a potential adsorbent in the application of wastewater disposal [5].

NO3, PO4

3 and Cr2O72 have become increasingly notice-

able for their adverse impacts on the sensitive water envi-ronment and organisms. In wastewater treatment technology, various techniques have been applied to remove these toxic anions [6, 7]. Among them, adsorption is recognized as one of the simplest and safest methods for the removal of pol-lutants from wastewater. In this paper, a new method for the preparation of an adsorbent based on wheat straw (WS) is reported. The adsorption studies indicate that modified wheat straw (MWS) is preferable in the removal of NO3

, PO4

3 and Cr2O72 from aqueous solutions.

2 Materials and methods

2.1 Chemical reagents and apparatus

Chemical reagents: the raw WS was obtained from Liaocheng, Shandong Province, China. Sulphuric acid, sodium hydroxide, potassium dichromate, diphenylcarbazide, acetone, iodine, epichlorohydrin, ethylenediamine, triethylamine and di-

XU Xing, et al. Sci China Chem June (2010) Vol.53 No.6 1415

methylformamide were analytically pure samples and ob-tained from Sinopharm, China.

Chemical apparatus: HH.S Water Bath (Jiangsu Jintan Medical Instrument Co., Ltd., China), HY-4 speed Govern- ing oscillator (Jiangsu Jintan Medical Instrument Co., Ltd. China), WFZ756 UV-visible spectrophotometer (Shanghai Optical Spectrum Instrument), JS94H microelectrophoresis apparatus (Shanghai Zhongchen Digital Technical Appara- tus Co., Ltd., China), Vario EL III element analyzer (Ger- many Elementar Co., Ltd., China); Perkin-Elmer “Spectrum BX” spectrometer (Co., Ltd., America).

2.2 Preparation of MWS

The synthetic method of MWS is simple with low cost and no secondary pollution. The details of the preparation are as follows: raw WS was washed with water, dried at 60 °C for 6 h and sieved into particles with diameters from 100 to 250 m. 10 g of WS was reacted with 5 mL of epichloro-hydrin, 5 mL of N,N-dimethylformamide, 2 mL of ethyl-enediamine and 4 mL of 99% triethylamine in a 250 mL three-neck round bottom flask at 80 °C for 12 h. The pro- duct was washed with 500 mL of distilled water to remove the residual chemicals, dried at 60 °C for 12 h and sieved to obtain particles smaller than 250 m in diameter and then used in all the adsorption experiments [3, 8, 9].

2.3 Adsorption experiments

The NO3, PO4

3 and Cr2O72 removal experiments were oper-

ated as follows: a series of 250 mL flasks were filled with MWS at different mass loadings for NO3

, PO43 and Cr2O7

2 agitated in an orbital shaker at 120 rpm and liquid samples were taken out at a given time interval for residual concentration analyses with an UV-visible spectrophotometer. The equilibrium concentration in the solid phase (qe) was given as:

0 e( )C C Vq

m

(1)

where qe is the amount of anion adsorption per gram MWS at equilibrium, C0 and Ce are the concentrations of anions at the beginning and equilibrium, respectively. V is the volume of the solution, and m is the amount of MWS (g).

3 Results and discussion

3.1 Characteristics of MWS

3.1.1 Surface charge change in MWS and WS

To determine the zeta potential of MWS and WS at different pH values, the MWS and WS particles in the sediment phase were dispersed into distilled water with a pH range of 2.0–12.0. The results shown in Figure 1 indicate that the

Figure 1 Effect of pH on the zeta potential change of MWS and WS.

zeta potentials of WS remain negative in the range of pH values, and a larger negative zeta potential of WS is observed as the pH values increase. In contrast with the WS, the zeta potential of MWS is positive in the designed pH range, which indicates the existence of positive-charge functional groups on the MWS structure [4]. However, a gradual decrease in the zeta potential of MWS is observed with the increase in pH, which is attributable to such pH- dependent functional groups existing in MWS as hydroxyl and carboxyl groups. These groups will exhibit a greater negative charge when the pH is increased, resulting in the decrease of the positive charge of MWS.

3.1.2 Elemental change in MWS and WS

Table 1 shows the elemental changes of carbon, hydrogen and nitrogen in MWS in comparison with WS. As shown in Table 1, a slight increase was observed in the carbon con-tent and hydrogen content of MWS. However, the nitrogen content of MWS significantly increased from 0.35% to 3.463%, indicating the reactions proceed efficiently and a large number of amine groups from triethylamine had been introduced into the WS.

3.1.3 IR spectrum change in MWS and WS

The IR spectrum of WS (Figure 2) exhibits a sharp adsorp-tion band at 3385 cm1, which corresponds to the presence of free hydroxyl groups in WS. The bands at 2930 and 1625 cm1 indicate the presence of ketone and aromatic in the WS, whereas in the case of MWS, the special vibration in the bands at 1350 cm1 suggests the grafted amine groups in the structure of the MWS [4].

Table 1 Elemental changes in WS after chemical modification

N% C% H%

WS 0.35 41.11 6.102

MWS 3.463 44.84 6.823

1416 XU Xing, et al. Sci China Chem June (2010) Vol.53 No.6

Figure 2 FTIR analysis of MWS and WS.

3.2 Adsorption properties of MWS for anions

3.2.1 Effect of MWS dosage on the adsorption process

The effect of MWS dosage on the adsorption of NO3, PO4

3 and Cr2O7

2 is shown in Figure 3. An increase in the per- centage of adsorption with increasing MWS dosage is observed in all of the three anions. When the MWS dosage is 0.5 g L

1, the removal efficiencies of NO3, PO4

3 and Cr2O72

are 24.8%, 51.1% and 59.2%, respectively. As the dosage increases to 2 g L

1, the trend of removal efficiencies tends to be stabilized, and the adsorption reaches equilibrium at dosage of 4 g L

1, with the NO3, PO4

3 and Cr2O72 removal

of 84.4 %, 93.8% and 97.7%, respectively. Based on the consideration of practical applications, a dosage of 2 g L

1

seems to be optimal and is, therefore, maintained for the following experiments.

3.2.2 Effect of solution pH on the adsorption process

Table 2 shows the existing forms of NO3, PO4

3 and Cr2O72

at different pH values. As the solution pH increases from

Figure 3 Dosage of modified wheat straw (MWS) vs. removal of anions. (NO3

, 40 mg L1; PO4

3, 50 mg (P) L1; Cr2O7

2, 200 mg (Cr) L1; 20 °C; pH,

4.5).

2.0 to 6.0, the Cr2O72 gradually transforms into CrO4

2, and the dominant concentration of CrO4

2 is observed in the aqueous solution when the pH increases beyond 6.0. There-fore, the pH values for the Cr2O7

2 solution are regulated in the range of 2.0–6.0.

The effect of pH on the adsorption of NO3, PO4

3 and Cr2O7

2 is shown in Figure 4. As shown in Figure 4(a), a sharp increase in the removal of NO3

and PO43 occurs in a

narrow range of pH from 2.0 to 4.0. The removal efficiency is almost constant in neutral and alkaline conditions (pH range of 4.0–10.0). However, a gradual decrease in NO3

and PO4

3 removal is observed as the pH increases from 10.0 to 12.0, which is attributable to the significant increase of OH when the pH increases beyond 10.0. The presence of excess OH ions will compete with NO3

and PO43 for ad-

sorption sites and the effect of the adsorption decreases [10, 11].

Figure 4(b) shows the Cr2O72 removal with initial pH

change. The removal efficiency of Cr2O72 decreases as the

solution pH increases from 2.0 to 3.0. It is because that in a strong acidic condition, the Cr2O7

2 exists primarily in the form of HCrO4

, which will form a more effective interac-tion with amine groups than Cr2O7

2 [12]. As the pH further increases from 3.0 to 6.0, the OH increases significantly, and the Cr2O7

2 and CrO42 become the dominant forms of Cr

(VI) in the aqueous solutions. However, the available amine groups for Cr2O7

2 adsorption will decrease on the outer sur-face of MWS due to the more effective interactions between amine groups and OH, resulting in a further decrease in the Cr2O7

2 adsorption [13]. Based on the discussion above, it was found that a lower

pH value is favorable to the removal of CrO42, while acid

contamination may also occur when the pH is too low. Therefore, the optimal pH values regulated in the range of 4–5 are more practical in the potential applications.

3.2.3 Adsorption kinetics

The effect of agitation time on the adsorption of NO3, PO4

3 and Cr2O7

2 is shown in Figure 5. For all the tested anions there is a steep ascending trend of qe at the beginning of the adsorption process (0–15 min), with approximately 85% of the adsorbate removed. A small increase in the qe of NO3

, PO4

3 and Cr2O72 is observed in the curves as the agitation

time increases, and then qe is stabilized and the adsorption reached equilibrium at 25–30 min. Based on the results dis-cussed above, it is clear that the adsorption of NO3

, PO43

and Cr2O72 onto MWS is a rapid adsorption process.

To analyze the adsorption rate of NO3, PO4

3 and Cr2O72

onto MWS, the pseudo first-order equation and pseudo second-order equation were evaluated based on the experi-mental data. The results are shown in Table 3.


Table 2 Existing forms of NO3, PO4

3 and Cr2O72 at different pH values

pH range 2.0–3.0 3.0–6.0 6.0–10.0 10.0–12.0

NO3 NO3

PO43 H3PO4, H2PO3

, HPO32 H3PO4, H2PO3

, HPO32, PO4

3 H2PO3, HPO3

2, PO43

Cr2O72 HCrO4

, Cr2O72 Cr2O7

2, CrO42 CrO4

2

Figure 4 pH values vs. removal of anions (NO3, 40 mg L

1; PO43, 50 mg (P) L

1; Cr2O72, 200 mg (Cr) L

1; 20 °C; dosage of MWS, 2 g L1).

Figure 5 Adsorption time vs. adsorption amount of NO3, PO4

3 and

Cr2O72 (dosage of MWS, 2 g L

1; pH, 4.5; 20 °C).

The pseudo first-order model is [14]

e t e 1ln lnq q q k t (1)

And the pseudo second-order model is

2

t e2 e

1t t

q qk q (2)

where qe and qt are the amounts of dyes adsorbed per gram MWS at equilibrium and time t (mg g

1), respectively. k1 is the rate constant of pseudo first-order (min1). k2 is the equi-librium rate constant of pseudo second-order (g (mg

min)1). Intraparticle mass transfer resistance is considered as the

limiting factor in the model of pseudo first-order, while it is replaced by adsorption mechanism in the pseudo second- order model. Results shown in Table 3 indicate that the adsorption of NO3

, PO43 and Cr2O7

2 onto MWS could be well represented by the pseudo second-order kinetic model, which illustrates the fact that all the potential adsorption mechanism existing in the pseudo second-order model (external liquid film diffusion, surface adsorption, intra-particle diffusion, etc.) would be present in the adsorption process [15].

Table 3 Parameters of adsorption kinetics

Pseudo first-order Pseudo second-order Adsorbate

qe (mg g1) k1 (min1) R2 qe (mg g

1) k2 (g (mg min)1) R2

NO3 31.9 0.265 0.991 29.3 0.185 0.9998

PO43 (P) 43.9 0.277 0.983 39.2 0.152 0.9996

Cr2O72 (Cr) 81.9 0.254 0.972 73.1 0.095 0.9991

1418 XU Xing, et al. Sci China Chem June (2010) Vol.53 No.6

3.2.4 Adsorption isotherm

Temperature is an important parameter for the adsorption process. Figure 6 illustrates the effect of temperature (20, 30 and 40 °C) on the adsorption of NO3

, PO43 and Cr2O7

2 by MWS. The adsorption results were analyzed by the Lang-muir and Freundlich isotherm model equations [16], and the data is shown in Table 4.

The Langmuir equation is

e max max e

1 1 1 1

q Q bQ C (3)

and the Freundlich equation is

e F e

1ln ln lnq K C

n (4)

where Ce is the equilibrium dye concentration in solution (mg L

1), Qmax is the monolayer capacity of the sorbent (mg g

1), b is the Langmuir constant (L mol1) and related to the free energy of adsorption, KF is the Freundlich constant (L g

1), n (dimensionless) is the heterogeneity factor. Results shown in Table 4 indicate that Langmuir iso-

therm generates the better agreement with experimental data for adsorption systems in comparison with Freundlich iso-therm. The Qmax of MWS for NO3

and PO43 adsorption de-

creases as the temperature increases, while it is observed that the Qmax for Cr2O7

2 increases with the increase in tem-perature, indicating that the adsorption of Cr2O7

2 onto MWS is an endothermic process in contrast with the exothermic nature in the NO3

and PO43 adsorption process. Table 4

shows that the qe of MWS for Cr2O72 (386.2 mg (Cr) g

1) is much higher than those of NO3

(53.5 mg g1) and PO4

3 (62.4 mg (P) g

1), suggesting the more effective interaction between amine groups and Cr2O7

2.

3.2.5 Comparison of the Qmax of various adsorbents

Table 5 shows the comparison of the Qmax of various adsorbents. Experimental data shown in Table 5 indicates that MWS is excellent in the adsorption of NO3

, PO43 and

Cr2O72, and could compete with some commercially avail-

able anion exchange resins. This provides strong evidence for the potential of MWS in technological applications of toxic anions’ removal from aqueous solutions.

Figure 6 Adsorption isotherms of NO3, PO4

3 and Cr2O72 (pH, 4.5; contact time, 120 min; dosage of MWS, 2 g L

1).

Table 4 Adsorption isotherm parameters

Langmuir equation Freundlich equation Adsorbate T (K)

Qmax (mg g1) KL (L mg1) R2 Kf 1/n R2

293 53.5 0.0527 0.989 5.22 2.42 0.979

303 52.1 0.0386 0.991 4.98 2.30 0.981 NO3

313 50.9 0.0314 0.993 4.13 1.99 0.963

293 62.4 0.0715 0.998 7.23 2.72 0.985

303 59.6 0.0472 0.997 7.50 2.90 0.971 PO43

313 58.1 0.0371 0.995 6.38 2.25 0.954

293 386.2 0.104 0.999 24.5 7.73 0.973

303 367.6 0.171 0.999 26.3 6.75 0.984 Cr2O72

313 332.6 0.266 0.998 28.7 4.14 0.967


Table 5 Qmax of NO3, PO4

3 and Cr2O72in different adsorbents

Qmax (mg g1)

Adsorbent NO3

PO43 (P) Cr2O7

2 (Cr)

MWS 53.5 62.4 386.2

Modified hull rice [3, 16] 62.4 37.2 204

Activate carbon [17] 6.8 5.8 -

Amberlite IRA-900 [18] 71.6 42.1 153.3

4 Conclusions

(1) A mass of amine groups were observed in the struc-ture of MWS, and the amine groups with positive charges significantly enhanced the adsorption capacity of MWS.

(2) The adsorption of NO3, PO4

3 and Cr2O72 onto MWS

was influenced by the dosages, agitation time, temperature and pH. The adsorption was a rapid process, and reached equilibrium at 25–30 min. The best removal results were achieved at the dosage of 2.0 g L

1 and initial pH of 4.0–5.0. (3) The fit of the data suggested that the adsorption iso-

therm could be well represented by the Langmuir model and the pseudo-second-order equation generated the best agreement with experimental data for adsorption systems.

(4) The Qmax of MWS for NO3, PO4

3 (P) and Cr2O72 (Cr)

were 53.5, 62.4 and 386.2 mg g1, respectively.

The research was supported by the Key Projects in the National Science & Technology Pillar Program in the Eleventh Five-year Plan Period (2006BAJ08B05-2), National Major Special Technological Programmes Concerning Water Pollution Control and Management in the Eleventh Five-year Plan Period (2008ZX07010-008-002) and the National Natural ScienceFoundation of China (50878121).

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Adsorption studies of the removal of anions from aqueous solutions onto an adsorbent prepared from wheat straw