Bioresource Technology 101 (2010) 1477–1481

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Bioresource Technology

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Effect of modifying agents on the preparation and properties of the newadsorbents from wheat straw

Xing Xu, Baoyu Gao *, Wenyi Wang, Qinyan Yue, Yu Wang, Shouqing NiSchool of Environmental Science and Engineering, Shandong University, Jinan 250100, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 April 2009Received in revised form 18 June 2009Accepted 18 June 2009Available online 24 July 2009

Keywords:Modifying agentMWSPhosphateNitrogen contentKinetic

0960-8524/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.06.064

* Corresponding author. Tel.: +86 531 88364832; faE-mail address: bygao@sdu.edu.cn (B. Gao).

Three different types of new adsorbents modified from wheat straw were synthesized after the reactionbetween epichlorohydrin and triethylamine by using ethylenediamine (EDA), diethylenetriamine (DETA)and triethylenetetramine (TETA) as modifying agents. The performance of the modified wheat straws(MWS) was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron micro-scope (SEM) and elemental analysis. Results showed that the optimal dosages for the three modifyingagent (EDA, DETA and TETA) were 3, 4 and 3 ml. The optimum synthesis temperature for the threeMWS was 80, 85 and 95 �C, respectively. The IR spectra of the three MWS were analogical, and nitrogencontents of the MWS were found to be consistent with their adsorption capacity. The pseudo-second-order equation generated the best agreement with the experimental data for adsorption systems. Inaddition, the adsorption process of the three MWS reached equilibrium at 10–15 min. MWS (EDA)demonstrated the largest phosphate capacity than the other MWS.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Eutrophication is a serious problem of water pollution and iscaused by the nitrogen, phosphorus and other excessive nutrientsfor algae use. In surface freshwater systems, phosphorus is usuallythe limiting factor of algae growth, because in normal freshwatersystems, the content of phosphorus is usually limited in compari-son with that of nitrogen. As a result, the increase of phosphorusin surface freshwater systems will lead to excessive algae growth.So phosphorus removal is of great significance for the algae bloomcontrol.

Wheat straw (WS) is regarded as an abundant and biodegrad-able resource available for the preparation of adsorbents that canbe used for the removal of nitrate and phosphate. The idea of con-verting WS into an adsorbent is based on the predominant con-tents of cellulose (32.1%), hemicellulose (29.2%) and lignin(16.4%) in WS (Orlando et al., 2002a,b). Cellulose, hemicellulosesand lignin structures have a large amount of easily accessible hy-droxyl groups that can be used for preparation of various func-tional polymers (Kumar et al., 2009; Reddy and Yang, 2009).

The modification reactions for the preparation of an adsorbentfrom WS consist of polymerization (Zhu et al., 2005), chelating(Orlando et al., 2004) and crosslinking (Sarin and Pant, 2006; Con-rad and Hansen, 2007), which are commonly applied to enhancethe adsorption capacity by introducing functional groups to the

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WS. Some adsorbents prepared from other agricultural residuehave shown an excellent adsorption capacity for various ions byintroducing different functional groups, including sulphonyl, ami-do, carboxyl, amine and other chelating functional groups (Orlandoet al., 2004; Conrad and Hansen, 2007; Gong et al., 2005; Robinsonet al., 2002; Biswas et al., 2008).

Adsorbents prepared from agricultural residues are consideredas a low cost adsorbent and have shown significant adsorptioncapacity for organic pollutants and dye (Chi and Chen, 2009;Saratale et al., 2009; Memon et al., 2008; Huang et al., 2009).

The main objective of this paper is to examine the most effec-tive preparation method that produces an adsorbent for phosphateremoval. In the previous work, amine groups were introduced intoother agricultural residues after reaction with the epichlorohydrinand amine in the presence of catalyst and organic medium (Orlan-do et al., 2002a,b; Wang et al., 2007a,b). While it was difficult tochoose a suitable catalyst for the modification reaction; the effectof catalyst was to improve the synthesis of intermediate whichwas obtained after the reaction between cellulose and epichloro-hydrin, and to provide a weak-base (pH 8–11) reaction conditionto facilitate the reaction between intermediate and amine (Orlandoet al., 2002a,b; Navarro et al., 1996). Several weak-base catalystshave been considered for this reaction, and only pyridine hasshown an excellent catalytic effect. However, a serious secondarypollution with large mounts of odoriferous wastewater was pro-duced after using pyridine as catalyst in the preparation of theadsorbent, and thus, a further investigation will be carried out toaddress this problem.

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EDA (phosphate removal) DETA (phosphate removal) TETA (phosphate removal) EDA (Zeta potential) DETA(Zeta potential) TETA(Zeta potential)

zeta

pot

entia

l (

mv)

Phos

phat

e re

mov

al (

%)

Synthesis temperature (oC)

Fig. 1. Effect of synthesis temperature on the preparation of MWS.

1478 X. Xu et al. / Bioresource Technology 101 (2010) 1477–1481

In this work, a catalyst is replaced with a modifying agent.Three types of adsorbents were prepared from WS after reactionwith epichlorohydrin and triethylamine by using the three modify-ing agents as ethylenediamine (EDA), diethylenetriamine (DETA)and triethylenetetramine (TETA), respectively. Effects of synthesistemperatures and modifying agent dosages on the preparation ofMWS were considered, with phosphate removal and zeta potentialas measures of treatment efficiency. Fourier transform infraredspectroscopy (FTIR), scanning electron microscope (SEM) and ele-mental analysis were used for the characterization of the MWS.Three kinetic models were designed to describe MWS kineticbehavior.

2. Methods

2.1. Materials

WS, obtained from Liao Cheng, Shandong, China, was washedwith water, dried at 60 �C for 6 h and sieved into particles rangingfrom 100 to 250 lm.

2.2. Preparation of MWS

Four grams of WS were reacted with 20 ml of epichlorohydrinand 20 ml of N,N-dimethylformamide in a 250 ml three-neck roundbottom flask for 60 min (Orlando et al., 2002a,b, 2003, 2004); Batchvolume (1–5 ml) of different modifying agents was added and thesolution was reacted for 30 min, followed by adding 20 ml of 99%triethylamine (w/w) for graft reaction. Mixture was reacted for120 min. The synthesis temperature was controlled at 20–100 �C.

The primary product was washed with 250 ml of distilled waterto remove the residual chemicals first, then dried at 105 �C for 5 hand sieved to obtain particles of less than 250 lm. The final prod-uct was obtained after the second cycle of washing, drying andsieving and then used in all adsorption experiments.

The inter-reactive activities of triethylamine with cellulose arepoor. To improve this reactivity, cellulose is reacted with crosslink-ing agent (epichlorohydrin) first, and produces the cellulose ether(Liu et al., 2008). Cellulose ether can be efficiently reacted withEDA after the ring opening of epoxide group in cellulose ether,and the other amido group in EDA was induced to react with tri-ethylamine in an excess of epichlorohydrin.

2.3. Characterization of MWS

2.3.1. FTIR analysis and SEM analysisIR spectra were recorded on Perkin–Elmer ‘‘Spectrum BX” spec-

trometer in 4000–400 cm�1 region. SEM of the sample was ob-tained by JEOL JSM-6480LV scanning electron microscope. Thesample was coated with platinum before the SEM micrographwas obtained.

2.3.2. Nitrogen content and total exchange capacity (TEC mEq g�1)The nitrogen content of MWS was measured by element ana-

lyzer (Elementar vario EL III, Germany). TEC was estimated fromthe nitrogen content and calculated by following equation (Orlan-do et al., 2002a,b; Wang et al., 2007a,b):

TEC ðmEq g�1Þ ¼ N%

1:4ð1Þ

where TEC is the total exchange capacity (mEq g�1); N% is the totalnitrogen content; and 1.4 is the correction coefficient.

2.3.3. Zeta potential (mV)The three kinds of adsorbents prepared from WS were used for

the removal of anionic pollutant, so it was significant to determine

the change of surface charge of MWS in comparison with WS. Thezeta potential of MWS and WS were determined by electro-kineticanalyzer (JS94H Shanghai Zhongchen Digital Technical ApparatusCo., Ltd, China).

2.4. Batch adsorption

Phosphate solution with concentration of 50 mg (P) L�1 wasprepared by solving 2.198 g KH2PO4 into 1000 ml of distilled water,and the solution was stocked in a 1000 ml volumetric flask.

To describe the adsorption kinetic curves of the different typesof MWS, adsorption experiments were carried out by agitating 1 gof MWS with 500 ml of phosphate solutions (50 mg (P) L�1), and at20 ± 2 �C of temperature in a stirrer operating 120 rpm for 80 min.Samples (1 ml) were withdrawn at suitable time intervals and fil-tered to analyze for residual phosphate concentrations in solutionswith an UV–visible spectrophotometer (model UV754GD, Shang-hai) at an absorbance wavelength of 700 nm. The equilibrium con-centration in solid phase qe was given as:

qe ¼ðco � ceÞV

mð2Þ

where qe is the amount of phosphate sorption per gram MWS atequilibrium, co and ce are the concentrations of phosphate at origi-nal and equilibrium, respectively. V is the volume of solution, and mis the amount of MWS (g).

3. Results and discussion

3.1. Effect of synthesis conditions on the preparation of MWS

The preparation of MWS was effected by the synthesis condi-tions followed as synthesis temperature and dosages of differentmodifying agents. Phosphate removal and zeta potential were usedas the performance indicators to describe the effect of synthesistemperature and dosages of different modifying agents on thepreparation of MWS.

3.1.1. Effect of synthesis temperature on the preparation of MWSThe dosages of the three modifying agents for the MWS were all

designed as 2 ml in the preparation of the three MWS, and the ef-fect of synthesis temperatures (20–100 �C) on the preparation ofMWS is shown in Fig. 1.

The result shown in Fig. 1 indicates that less modification iscarried out when the temperature is lower than 40 �C, and the

X. Xu et al. / Bioresource Technology 101 (2010) 1477–1481 1479

phosphate removal of MWS with less modification are only about15%, slightly higher than that of WS (8.5%). The lowest synthesistemperatures which are required in the preparation of the threeMWS (MWS (EDA), MWS (DETA) and MWS (TETA)) are 40, 70and 75 �C, respectively. And the optimal synthesis temperaturesfor the three MWS are obtained at 85, 90 and 95 �C with thephosphate removal of 89.78%, 78.80% and 80.34%, respectively(Fig. 1). The highest phosphate removal achieved at lower synthe-sis temperature of MWS using EDA as modifying agent in com-parison with other modifying agents indicates that EDA is moreefficient in the synthesis of MWS, and the higher temperatureof the DETA and TETA required can be ascribed to the complexityof the structures in DETA and TETA with more amido groupswhich will participate in the reaction (Seko et al., 2007; Kolarzet al., 2001).

The zeta potential of MWS is also shown in Fig. 1. The zeta po-tential of MWS with less modification is close to 0 mV comparedwith that of WS detected as �30 mV. The trend of zeta potentialis similar to that of phosphate removal of the three MWS.

3.1.2. Effect of dosages of different modifying agents on thepreparation of MWS

On the base of the optimal synthesis temperatures of the threeMWS, the effect of dosages (1–5 ml) of the modifying agents on thepreparation of MWS was investigated. The results are shown inFig. 2.

The phosphate removal sharply increases with the modifyingagent dosage increasing from 1 to 2 ml (Fig. 2). The phosphate re-moval is stable with more modifying agents added. The optimaldosages of the modifying agents (EDA, DETA and TETA) for thepreparation of the three MWS are 3, 4 and 3 ml, respectively.EDA still demonstrates the excellent modifying effect on the prep-aration of MWS, followed by DETA and TETA, respectively. How-ever, a slight decrease of phosphate removal is observed whenthe dosages of modifying agents are more than 3–4 ml. The super-fluous modifying agents added in the reaction system will reactwith epichlorohydrin and triethylamine, causing a reduction of or-ganic chemicals for MWS synthesis; this will result in a decrease ofMWS in yield. Control synthesis experiments were performedwithout the addition of WS to produce the byproducts which aresynthesized with only organic chemicals mentioned above. Thebyproducts prepared from these organic chemicals are epichloro-hydrin–amine polymers. The adsorption experiment indicates thatthe phosphate removal efficiency of epichlorohydrin–amine poly-mers is only about 70–75%, which is relatively lower than that ofMWS; this indicates that the decrease of MWS caused by the

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zeta

pot

entia

l (

mv)

Pho

spha

te r

emov

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%)

Dosages of different modifying agents (ml)

EDA (phosphate removal) DETA (phosphate removal) TETA (phosphate removal) EDA (Zeta potential) DETA(Zeta potential) TETA(Zeta potential)

Fig. 2. Effect of dosages of different modifying agents on the preparation of MWS.

superfluous modifying agent dosage will result in a decrease inphosphate removal capacity.

The trend of zeta potential of MWS prepared at different dos-ages of modifying agents in Fig. 2 is similar to that of phosphateremoval.

3.2. Characterization of MWS

3.2.1. FTIR analysis of MWSThe FTIR spectrum of WS and MWS reveals characteristic func-

tional groups. For the WS, the intensity of the band at 3380 cm�1 isattributed to O–H stretching vibration, and the band at 2920 cm�1

indicates C–H stretching vibration (Kumar et al., 2009). The IRanalysis of MWS displays a certain extent of change in the struc-ture between MWS and WS. The band at 1625 cm�1 is associatedwith N–H stretching of NH groups in MWS (Mishra and Jha,2009). C–X stretch of alkyl-halides is observed in the MWS bythe intense vibration of band at 623 cm�1. And an intense vibrationis observed in the band at 1410 cm�1, corresponding to character-istic for C–N stretching vibration of amine groups which have beengrafted into the structure of the MWS (Mishra and Jha, 2009; Gur-gel et al., 2009).

3.2.2. Elemental analysisTable 1 displays the elemental changes of carbon, hydrogen and

nitrogen of the three kinds of MWS in comparison with WS. Aslight increase is found in the carbon contents and hydrogen con-tents of MWS. However, nitrogen contents (N%) of the three MWS(EDA, DETA and TETA) increased significantly from 0.35% to14.50%, 14.01% and 11.46%, respectively, indicating that the reac-tions proceed efficiently and a large number of amine groups fromtriethylamine have been introduced into the MWS.

TEC is estimated from the nitrogen contents of the adsorbents,because there is a correlation between nitrogen content and ex-change capacity of adsorbents as determined by elemental analysis(Šimkovic and Laszlo, 1997; Orlando et al., 2002a,b). The result inTable 1 shows that TEC value estimated from the nitrogen contentof MWS with EDA as modifying agent is 10.36 mEq g�1, which ishigher than those of MWS with DETA and TETA as modifyingagents. In comparison with TEC value of WS, the markedly higherTEC values of all the three MWS once again validate the similar re-sult demonstrated in the previous FTIR analysis.

3.2.3. SEM analysis of MWSComparison results of the surface structure between MWS and

WS are obtained after the SEM measurement. Smoother surfacesare observed in the structures of the MWS in comparison withthe raw surface of WS, indicating that the order of cellulose is im-proved during the process of modification.

3.3. Adsorption kinetics of MWS

The adsorption kinetics of the three types of MWS is depicted inFig. 3a. For all the MWS, there is a steep ascending trend of qt at thebeginning of the adsorption process (0–10 min). A small increase ofphosphate adsorption is seen in the curves between 10 and 15 min,and then qt was steady and the adsorption reached equilibrium.

Table 1Change of element content of MWS and WS.

N (%) C (%) H (%) TEC (mEq g�1)

WS 0.35 41.12 6.13 0.25MWS (EDA) 14.50 46.32 8.37 10.36MWS (DETA) 14.01 41.78 8.74 10.01MWS (TETA) 11.46 44.25 8.25 8.19

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MWS (EDA) MWS (DETA) MWS (TETA) pseudo first-order

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(a) adsorption kinetics of MWS

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(d) pseudo second-order

Fig. 3. Adsorption kinetics and kinetic equation for adsorption of phosphate onto MWS.

1480 X. Xu et al. / Bioresource Technology 101 (2010) 1477–1481

The experimental data were discussed and analyzed by pseudo-first-order equation (3), modified pseudo-first-order equation (4)and pseudo-second-order equation (5), respectively. Kineticparameters for the three kinetic models and correlation coeffi-cients were analyzed and calculated in Fig. 3b–d and Table 2.

Pseudo-first-order equation (Özacar et al., 2005; Yang andBushra, 2005):

lnðqe � qtÞ ¼ ln qe � k1t ð3Þ

where qe (mg g�1) is the equilibrium concentration of phosphate inMWS; qt (mg g�1) is the average concentration of phosphate at timet (min) in MWS. k1 is the pseudo-first-order rate constant (min�1).

Modified pseudo-first-order model (Yang and Bushra, 2005;Sulak et al., 2007):

qt

qeþ lnðqe � qtÞ ¼ ln qe � K1t ð4Þ

where K1 is the rate constant of modified pseudo-first-order(min�1).

Pseudo-second-order equation (Yang and Bushra, 2005; Sulaket al., 2007; Sud et al., 2008):

Table 2Kinetic parameters for adsorption rate expressions.

qea (mg g�1) Pseudo-first-order Modifie

qeb (mg g�1) k1 (min�1) R2 qeb (mg

MWS (EDA) 22.91 16.95 0.3703 0.9729 23.64MWS (DETA) 22.32 15.80 0.2473 0.9786 22.87MWS (TETA) 20.40 15.96 0.2222 0.9862 21.98

tqt¼ 1

k2q2eþ t

qeð5Þ

where k2 is the pseudo-second-order rate constant (g mg�1 min�1).Results shown in Fig. 3 indicate that pseudo-first-order equa-

tion and modified pseudo-first-order equation can only describethe adsorption process before equilibrium was reached (0–15 min). While it is found that pseudo-second-order model realizesthe fitting of the whole adsorption process shown in Fig. 3d. Thevalues of R2 (Table 2) of pseudo-second-order model for phosphateadsorption are satisfactory (>0.9996) and followed by those ofmodified pseudo-first-order equation and pseudo-first-order equa-tion, respectively. The experimental data of qea for the three typesof MWS (EDA, DETA and TETA) are 22.91, 22.32 and 20.40 mg g�1,respectively, which are very close to the data of qeb calculated frompseudo-second-order equation (Table 2). From the experimentalresults, it is suggested that the adsorption of phosphate ontoMWS follows the pseudo-second-order model.

The data of k1, K1 and k2 calculated from the pseudo-first-orderequation, modified pseudo-first-order equation and pseudo-sec-ond-order equation have shown an similar regulation as MWS(EDA) > MWS (DETA) > MWS (TETA), which indicates that the

d pseudo-first-order Pseudo-second-order

g�1) K1 (min�1) R2 qeb (mg g�1) k2 (g(mg min)-1) R2

0.2892 0.9962 22.88 0.1000 0.99980.1967 0.9975 22.52 0.0636 0.99980.1701 0.9932 20.53 0.0488 0.9996

X. Xu et al. / Bioresource Technology 101 (2010) 1477–1481 1481

phosphate adsorption process is faster with MWS (EDA) as adsor-bents (Yang and Bushra, 2005; Bakouri et al., 2009; Yang and Jiang,2009). The MWS (EDA) achieves the highest qea, followed by MWS(DETA) and MWS (TETA), respectively, which illuminates the moreexcellent phosphate adsorption capacity of MWS (EDA) than theother MWS types.

4. Conclusions

Different types of adsorbents were prepared from WS after thereaction with epichlorohydrin and triethylamine by using the threemodifying agents as EDA, DETA and TETA, respectively. Effect oftemperature and dosage of the modifying agents on the prepara-tion of MWS were determined. A relatively high temperature wasrequired in the modification, and the optimal dosage of the modi-fying agents was approximately 3–4 ml.

Characterization of the three MWS was evaluated. Aminegroups detected in the structure of MWS by the FTIR analysisand elemental analysis illuminated that MWS was very effectivein absorption of phosphate.

Pseudo-second-order provided the best correlation of theexperimental data for the adsorption kinetics of the MWS. Fastadsorption rates and high qea of MWS (EDA) illuminates the moreexcellent phosphate adsorption capacity than the other MWStypes.

Acknowledgements

The authors are thankful to the support of the Shandong Provin-cial Foundation of Natural Sciences, China. The project was sup-ported by the science and technology development key programby Shandong Province (No. 2006GG2206007), Key Projects in theNational Science & Technology Pillar Program in the EleventhFive-year Plan Period (2006BAJ08B05-2) the Postdoctoral Innova-tion Fund of Shandong Province (200802020) and the Key Scienceand Technology Research Project of Environmental Protection inShandong Province (No. [2006] 005).

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