A Sorbent Based on Liquor Distillers' Grains for the Removal of Pb(II ...

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ScienceDirect Procedia Environmental Sciences 31 (2016) 785 – 794

The Tenth International Conference on Waste Management and Technology (ICWMT)

A sorbent based on liquor distillers' grains for the removal of Pb(II) and Cr(III) from aqueous solution Yongde Zhanga,b,*, Xuegang Luo a,b,Xiaoyan Lin b , Suntao Huang a* a

General Research Institute for Nonferrous Metals, National Engineering Laboratory of Biohydrometallargy, BeiJin.P.R. China, 62014824 b Southwest University of Science and Technology, Mianyang, Sichuan 621010, China

Abstract A new sorbent based on liquor distillers' grain for removal of Pb(II) and Cr(III) from aqueous solution has been systematically studied. Comparison of the adsorption potential of the sodium hydroxide modified (MLDG) and raw (RLDG) liquor distillers' grain for the removal of Pb(II) and Cr(III) ions has been done by considering the effects of the following parameters: pH, dosage, contact time and initial concentration of metal ions. The samples were examined by particle size analyzer, specific surface area analyzer, SEM, FTIR, Zeta potential measurement and XPS. Results showed that MLDG has rougher surface and looser structure than RLDG and the -OH and -COOH are the main functional groups involved in the adsorption. The MLDG exhibited higher adsorption capacities on heavy metal ions than RLDG. The maximum adsorption capacities of MLDG were 44.13 mg g-1 for Pb(II) and 19.80 mg g-1 for Cr(III) while those of the RLDG were 11.69 mg g-1 for Pb(II) and 6.38 mg g-1 for Cr(III). Furthermore, the equilibrium sorption data are well demonstrated by Langmuir model and the kinetics adsorption data was well fitted to the pseudo-second-order equation. The results showed that sodium hydroxide modified liquor distillers' grains is a promising sorbent for the removal of heavy metal ions. © Published by Elsevier B.V This © 2016 2015The TheAuthors. Authors. Published by Elsevier B.V.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific. Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific Keywords: Distillers' grains; Modification; Adsorption; Pb(II); Cr(III);

1.

Introduction With the rapid development of industries such as electroplating, metal finishing, metallurgical work, tanning, chemical manufacturing, mining and battery manufacturing, etc, wastewater with heavy metals was yielded and discharged into the rivers, which have caused various diseases and disorders for their toxicity and tendency to accumulating in living organisms [1-3]. Among the various metal ions, lead, mercury, cadmium and chromium (VI) are regarded as the top four toxic metals [4]. At high exposure levels, lead causes encephalopathy, cognitive impairment, behavioural disturbances, kidney damage and many other problems for our health [4, 5]. Waters containing a high concentration of Cr can cause serious environmental problems as well as induce toxic and carcinogenic health effects on human beings [4, 6]. Therefore, the removal or recovery of lead and chromium from wastewater arouses great interests. Various techniques of wastewater treatment have been developed such as

* Corresponding author. Tel.: +86-816-6089009,; fax: +86-816-6089009. E-mail address: [email protected]

1878-0296 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific doi:10.1016/j.proenv.2016.02.074

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Yongde Zhang et al. / Procedia Environmental Sciences 31 (2016) 785 – 794

chemical precipitation, ion exchangers, electrolytic reduction, membrane processes, coagulation-flocculation, electro dialysis and ultra filtration etc [7-10]. However, most of these have disadvantages like high cost, secondary pollution and incomplete removal. Moreover, there exist concentration limits to those methods which are economical but become ineffective or too expensive to treat wastewater with metal ions with the concentrations of 100 mg/L [4]. Hence, whether its cost performance was acceptable was one of the most important prerequisites needs to be searched for an optimal technology. Recently, biomass, the byproducts or the wastes from agricultural operations was regarded as a potential alternative for the removal or recovery of metal ions from aqueous solutions due to its advantages such as low cost, high efficiency, minimization of chemical or biological sludge [9]. Liquor distillers' grains are abundantly available byproducts generated from white spirit industrial. There is a great deal of liquor distillers' grains from the white spirit industry in China every year. The liquor distillers' grains are usually used as animal feeding or randomly discarded [11]. Nevertheless, utilization of the liquor distillers' grains attracts extensive research interests. It was mainly composed of the cellulose, hemicellulose, lignin, polysaccharide and protein, thus might be a new inexpensive adsorbent with abundant functional groups such as hydroxyl, amino and carboxyl that are good for the binding of metal ions [12]. Studies on the comprehensive utilization of liquor distillers’ grains have been carried out recently, such as cultivating edible fungus, extracting products of high value-added, brewing vinegar, producing animal feed and agricultural fertilizers. Some investigations have been carried out for adsorbing metal ions with raw or modified brewer′s spent grains [12-15]. Few reports about using liquor distillers' grain as adsorbent to deal with heavy metal ions were found. In our study, the essential component of the liquor distillers' grains are rice husks, which are different from the barley spent grain. Acid or alkali modification process and the raw and acid modified liquor distillers’ grains used to adsorb Cu(II), Pb(II) and Cr(III) were reported in literature [2, 11]. Our previous studies indicate alkali modification of liquor distillers’ grains has potential for metal ions removal, thus we focused on the alkali modified liquor distillers' grains here, their ability for removal of Pb(II) and Cr(III) from aqueous solutions has been investigated. The equilibrium data was evaluated by Langmuir and Freundlich isotherm models, the kinetics of Pb(II) and Cr(III) adsorption onto the RLDG and MLDG were also studied to predict the nature of adsorption. 2. Materials and Methods 2.1. Materials and Instruments Wet liquor distillers' grains are obtained from Forgood Distillery Corporation located in MianYang, China. They are air-dried, crushed and screened over 80 mesh screens (177 microns). The major components of dried liquor distillers' grains (See Table 1) are protein, coarse fiber, crude fats, nitrogen-free extract and ash content. All chemical reagents (Sodium hydroxide, nitric acid, lead nitrate and chromium nitrate) were of analytical-reagent grade. Table 1 The components of dried liquor distillers' grains. Components percent crude protein

10% ~15%

crude fiber

17% ~27%

crude fat

2% ~7%

nitrogen free extract

26% ~ 49%

crude ash

12% ~ 18%

2.2. Modification of liquor distillers' grains Liquor distillers' grains particles (10 g), prepared as above procedure, were dispersed in 100 mL of sodium hydroxide aqueous solution with mass percent concentration of 10 %, and were oscillated for 12 h under 25 ć with the speed of 150 rpm. Then the distillers' grains particles were filtrated, washed with distilled water to remove the alkali and dried at 60 ć for 12 h. The modified liquor distillers' grains particles were collected to be used as absorbents. 2.3. Batch experiments. Stock solutions of lead nitrate and chromium nitrate (1000 mg L-1) were prepared in distilled water. All working solutions were prepared by diluting the stock solution with distilled water to the desired concentration. The adsorption oexperiments were carried out by adding 0.05 -0.25 g of RLDG and 0.05 -0.75 g MLDG to a series of 100 mL conical flasks containing 50 mL of single metal ion solution (20-800 mg·L-1). The pH value of the solution

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was adjusted to 2 - 6 by adding 0.1 mol L-1 HNO3 or 0.1 mol L-1 NaOH. The flasks were oscillated at 25 ć with the speed of 150 rpm for 3 h. For each determination of the metal ion concentrations three parallel samples were prepared. Atomic absorption spectrophotometer (TAS-990) was used to do the determination. Removal efficiency (E) and adsorption capacity (q) were calculated from the expressions (1) and (2): (1) C0 - Ce E (%)

u 100%

C0

q mg g -1

C0

- Ce V m

(2)

where C0 and Ce are the initial and final (equilibrium) concentrations of the metal ion in solution (mg L−1), V is the solution volume (L) and m is the mass of the sorbent (g). The particle size of samples was investigated by the laser diffraction granularity analyzer (MS2000). The specific surface area of RLDG and MLDG were detected by nitrogen adsorption/desorption test. The microstructures of RLDG and MLDG were investigated by the field emission scanning electron microscope (Inspect F50). The characterization of the functional groups of RLDG and MLDG was studied by Fourier transform infrared spectrometer (Nicolet 6700, FTIR). The surface zeta potential of the RLDG and MLDG under the different pH values were measured by Zeta potential analyzer (Zetaplus, Brookhaven, Co. Ltd). Surface analysis of the samples were studied by means of X-ray photoelectron spectroscopy (XPS, XSAM-800, KRATOS Co.) test with the exciting source of Al under Ultra HighVacuum (UHV). In order to compensate for the charging effects, all spectra were calibrated with the main C1s peak at 284.8 eV. Survey scans (0-1100 eV) were carried out with 0.65 eV step size. High-resolution scans with 150ms dwell time were carried out with 0.05 eV step size. The concentration of elements was calculated using the intensity of an appropriate line and XPS cross-sections. Experimental data were studied by pseudo-first-order and pseudo-second-order to elucidate the potential ratecontrolling steps and examine the kinetics of the adsorption process, respectively. Pseudo first-order kinetics The pseudo first-order equation (Lagergren’s equation) describes adsorption in solid-liquid systems based on the sorption capacity of solids. Its linear form can be expressed as [16, 17]: (3) ln q - q ln q - k t e

t

e

1

where qe and qt are the equilibrium amounts of Pb(II) and Cr(III) adsorbed at and at time t in mg·g -1, respectively, and k1 is the pseudo-first-order rate constant (min-1). Pseudo-second order kinetics The pseudo second-order rate expression, which has been applied for analyzing chemisorption kinetics from liquid solutions, is linearly expressed as [18, 19]: (4) t 1 t qt

k 2 qe 2

h k2 qe2

+

qe

(5)

where k2 is the rate constant for pseudo second-order adsorption(g mg-1 min-1) and k2qe2 or h (mg g-1 min-1) is the initial adsorption rate. Adsorption isotherm models Adsorption isotherm is basically important to describe how adsorbate interacts with adsorbents and is critical in optimizing the use of adsorbents. Equilibrium modeling for the processes of removal of Pb(II) and Cr(III) were carried out by using the Langmuir and Freundlich adsorption isotherm models. The Langmuir model assumes that monolayer adsorption takes place at specific homogeneous sites on the surface of the adsorbent and also, when a site is occupied by an adsorbate molecule, no further adsorption can take place at this site. As such the surface will eventually reach a saturation point where the maximum adsorption of the

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surface will be achieved. The linear form of the Langmuir isotherm model is described as [16, 20]: Ce Ce 1 = qe qmax qmax b

(6)

where b (L mg-1) is Langmuir constant relating the free energy of adsorption, q max is the monolayer uptake capacity of the adsorbent, Ce (mg L-1) and qe (mg g-1) are the concentration and uptake capability of metal ions at equilibrium state, respectively. The Freundlich isotherm model is derived to model the multilayer adsorption and applicable to highly heterogeneous surface, and the well-known logarithmic form of Freundlich isotherm is given by the following form [21, 22]: lg qe

1 lg Ce lg K F n

(7)

where KF and n are Freundlich constants with n giving an indication of how favourable the adsorption process and KF ((mg g-1) (L mg-1)1/n) is the biosorption capacity of the adsorbent. 3. Results and discussion 3.1. Characterization of materials The particle size distributions of the RLDG (a) and MLDG (b) are shown in Fig. 1. It can be observed that the particle size distribution of the RLDG is broadened in a range of 0.6 to 700 µm inclusive of two maxima at about 10 and 250 µm, respectively. however, in case of MLDG, the particle size distribution mainly range from 2 to 10 µm inclusive of only one maxima at about 5 µm,which is probably conducive to the absorption for heavy metal ions.

Fig. 1. The particle size distributions of RLDG (a) and MLDG

(b) Fig. 2. The SEM photographs of RLDG and MLDG.

The specific surface area of RLDG and MLDG were 1.49 m2 g-1 and 0.30 m2 g-1, respectively. Despite a decrease in specific surface area, the adsorption capacities of MLDG for both Pb(II) and Cr(III) increased more than 3.0 times after base treatment. The adsorption capacities value was obtained from the batch test. Fig. 2 presents the surface morphologies of RLDG (a, b, c) and MLDG (c, d, e). It can be found that MLDG particles have rougher surface and looser structure, which is beneficial to adsorption for metal ions. Furthermore, the liquid of the modification of liquor distillers' grain by sodium hydroxide was measured using 3, 5-dinitrosaliculic acid method. The results show that there is reducing sugar in the degradation liquid. This may be resulted from degradation of carbohydrate during alkali treatment of liquor distillers' grains.

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Fig. 3. FT-IR spectra of RLDG and MLDG.

Fig. 4. Effect of pH on the removal of Pb(II) and Cr(III) on RLDG and MLDG (contact time: 180 min, and temperature: 25ć).

FT-IR spectroscopy was used to identify the chemical groups present in adsorbents. Fig. 3 shows FTIR spectra of RLDG and MLDG. The spectra display a number of absorption peaks, indicating the complex nature of the adsorbent material examined. Results showed that adsorption peaks are almost the same before and after modification. As seen in the spectrum of RLDG and MLDG, a broad and intense band at around 3200-3700 cm-1 is due to the overlapping of -OH, Si-OH and N-H stretching vibrations. The peaks at 2925 and 2853 cm-1 represent the asymmetrical and symmetrical stretching vibration of -CH2 group, respectively. The asymmetrical bending vibration peak of -CH2 and -CH3 could be ascribed to the band that observed at 1456 cm-1. A peak at 1384 cm-1 represents oxygen functional groups like a highly conjugated C=O stretching in carboxylic groups. The peak around 1657 cm-1 corresponds to the stretch vibration of the secondary amine group and the C=O group. The wave numbers at 1515 cm-1 and 1426 cm-1 are assigned to the stretching vibration of -COOH. The peak around 1074 cm-1 is mainly attributed to the bending vibration of O-H and the stretching vibration of C-O-C, and maybe antisymmetry stretching vibration of SiO2. the results of FTIR spectral peaks of the biosorbents provides a clear indication that the functional groups like acylamino, -OH, Si-OH and -COOH present on the RLDG and MLDG surface are beneficial to absorption for heavy metal ions [12, 23, 24]. XPS provides significant information on the chemical bonding of atoms [25]. The RLDG and MLDG adsorbents before and after adsorption were analyzed by XPS to determine the composition changes and the chemical states of the adsorbed species. The typical wide scan spectrums and high-resolution scans are illustrated in Fig. 4 and the relative percentage of surface composition and atomic ratios determined from the wide scan XPS spectra are summarized in Table 2. Table 2 Atomic concentrations on the adsorbent surfaces before and after adsorption obtained using XPS (data precision is ±5%). Sample C O N Si Cd Zn RLDG MLDG Pb(II)-loaded MLDG Cr(II)-loaded MLDG

7.28

8.04

4.18

2.62

5.40

1.43

2.57

2.64

2.34

2.34

--

--

1.30

1.89

--

--

1.04

1.57

0.56

--

1.08

1.64

--

2.18

Fig. 5(a) and (b) show the results of the survey scan XPS spectra of the RLDG and MLDG surface, the characteristic signals for carbon (C 1s at 286.2 eV) and oxygen (O 1s at 532.9 eV) are clearly observed and a trace amount silicon (Si 2p at 103.5 eV and Si 2s at 156 eV) is also detected on the surfaces of RLDG and MLDG, indicating that carbon and oxygen are the main constituents. In comparison with RLDG, the relative atomic percentage of carbon, nitrogen and silicon on MLDG from Table 2 is lower and oxygen is higher, which maybe resulted from degradation of carbohydrate during alkali treatment of distillers' grains. In comparison with surface of MLDG, As shown in Fig. 5(c), (d), (e) and (f), the additional signals assigned to lead with the binding energies for Pb 4f at 143.7 eV and 138.9 eV and chromium with the binding energies for Cr 2p at 586.9 and 577.1 eV presented on MLDG after adsorption, which confirmed the presence of these cations on the MLDG.

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Fig. 5. XPS survey spectra of RLDG (a), MLDG (b), Pb(II)-loaded (c), Cr(III)-loaded(d) surfaces and high-resolution scans for Pb 4f (e) and Cr 2p (f).

3.2. Effect of pH on adsorption The metal speciation and the protonation of the chemical groups of the RLDG and MLDG surfaces are both influenced by the pH of the aqueous solution, which even lead to change the capacities of the RLDG and MLDG adsorption for metal ion. In order to determine those influences, sorption experiments were carried out varying the pH of the initial aqueous solution from 2 to 6, with dosage 10 g L-1 for RLDG and 2 g L-1 for MLDG, and the initial Pb(II) and Cr(III) concentration of 20±2 mg L-1 as shown in Fig. 5. Results indicated that the removal efficiencies of Pb(II) and Cr(III) on MLDG were both higher than those of RLDG when pH > 4.0. Adsorption by RLDG and MLDG were all highly pH-dependent and the removal efficiency increased sharply with an increase in pH from 2 to 4, then increased slightly. The results may mainly be attribute to the electrostatic interaction between the surface of the RLDG and MLDG and the heavy metal ions in the water solution. The FTIR analysis of the adsorbent indicated that the functional groups -OH and -COOH could be potential adsorption sites for interaction with the metal ions. At low pH (less than 4.0), an excess proton can compete with metal ions, and -OH and -COOH groups were more protonated on the absorbents (Eq (8) and Eq (9)), both the heavy metal ions and biosorbent surface carrying positive charges consequently, precluding the electrostatic attraction between them, decreasing the adsorbate uptake [26]. (8) ROH + H + o ROH 2 + RCOOH + H + o RCOOH 2 +

(9)

With an increase in the pH, the removal efficiency was greatly facilitated resulting from the weakened protonation of -OH and -COOH groups, This is owing to the presence of free lone pair of electrons on deprotonated oxygen atoms, which are suitable for coordination and complexation with the metal ions [27].


Fig. 6. The zeta potential with pH of on RLDG and MLDG. V

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Fig. 7. Effect of time on the removal of Pb(II) and Cr(III) on RLDG and MLDG (pH: 5.0, and temperature: 25 ć)

The effect of pH can be also explained by considering the surface charge on the adsorbent materials. The pHPZC of the RLDG and MLDG in aqueous phase was found a same value to be about 2.3 depicted in Fig. 6. Generally, At pH < pHPZC, the RLDG and MLDG surface may get positively charged due to the adsorption of H + (Eq (8) and Eq (9)), thus a force of repulsion occurs between the metal ions and the adsorbent surface resulting in low adsorption [26]. However, when the pH of solution is higher than pHpzc, the RLDG and MLDG adsorbent acts as a negative surface and the adsorption is greatly improved. Generally, the net positive charge decreases and negative charge increases with increasing pH value and leads to decrease in the repulsion between the adsorbent surface and metal ions and thus, enhances the adsorption capacity [28]. As shown in Fig. 6, for both of two adsorbents, the zeta-potential decreased with increasing the solution pH in the range from 1 to 6.14, while pH > 4.0, the zeta potential of MLDG is lower than that of RLDG, so the surface charge of MLDG is more negative than that of RLDG, which results in a lower electrostatic repulsion between the adsorbents surface and the metal ions [29] and the higher removal efficiency of Pb(II) and Cr(III) on MLDG than that on RLDG. However, precipitate could not be excluded at a higher pH. To make sure the maximum removal efficiency as well as to avoid precipitation of Pb(II) and Cr(III), all the following absorption experiments were conducted at pH 5.0 for both Pb(II) and Cr(III). 3.3 Effect of contact time and adsorption kinetics The adsorption experiments were carried out for different contact times with a fixed adsorbent quantity (0.5 g for RLDG and 0.1 g for MLDG) with 50mL of metal ion solution at pH 5.0, at a fixed initial concentration (20±2 mg L-1) of metal ions. Fig. 7 illustrates the effect of absorbed time on the removal of Pb(II) and Cr(III) by RLDG and MLDG. It can be seen that the adsorption capacities of Pb(II) and Cr(III) on MLDG are both higher than that on RLDG and the adsorption of Pb(II) and Cr(III) is very fast in the very early stage of absorption, for instance, the maximal adsorption capacities of Pb(II) and Cr(III), respectively, arrive at 10.075 and 9.510 mg g-1 for MLDG and 1.663 and 1.517 mg g-1 for RLDG within 5 minutes and equilibrium adsorption is gradually established within 180 min. For both the metal ions it was observed that 60 min was enough to reach adsorption equilibrium. As contact time increases, metal uptakes increase initially, and then become almost stable, denoting attainment of equilibrium. This rapid performance suggests the good diffusion ability of Pb(II) and Cr(III) in aqueous solutions and good binding capacity between the metal ions and the surface of RLDG and MLDG. These changes in metal uptake may be due to the fact that, initially, all adsorbent sites were vacant and the solute concentration was high. After that period, only a very low increase in the metal uptake was observed because there were few surface active sites on the RLDG and MLDG [17].

Fig. 8. Effect of sorbent mass on the removal of Pb(II) and Cr(III). Fig. 9. The adsorption isotherm for Pb(II) and Cr(III) on RLDG and MLDG(dose: 10 g L-1 for RLDG and 2 g L-1 for MLDG, contact time:

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180 min).

To evaluate the kinetics of the RLDG and MLDG adsorption process, the pseudo-first-order Eq. (3) and pseudo-second-order models Eq. (4) were tested to interpret the experimental data. The value of k1 and qe can be obtained from the slope of the plot of ln(qe-qt) versus t and qe and k2 can be evaluated from the plots of t/qt against t. The kinetic parameters are listed in Table 3. Table 3 Parameters of pseudo-first-order and pseudo-second-order models. pseudo-first-order Biosorbent Ion qe.exp qe.cal k1 R2 RLDG MLDG

Pb(II) Cr(III) Pb(II) Cr(III)

1.828 1.666 10.430 10.563

0.228 0.147 0.055 1.633

0.018 0.006 0.022 0.029

0.837 0.904 0.731 0.823

pseudo-second-order qe.cal

k2

R2

1.828 1.646 10.428 10.427

0.247 0.312 0.369 0.056

0.999 0.999 1.000 1.000

According to Table 3, the calculated correlation coefficients are less than 0.904 for the first-order kinetic model, whereas the values of the correlation coefficient are greater than 0.999 for the pseudo-second-order kinetic model, suggesting that the adsorption kinetics could be well explained and approximated more favorably by the pseudosecond-order kinetic model for Pb(II) and Cr(III) absorption on the RLDG and MLDG. Furthermore, the estimated qe.cal value from the second-order model shows good agreement with experimental qe.exp value, indicating the pseudo-second-order mechanism is predominant and that chemisorption might be the rate-limiting step which controls the adsorption process [30]. 3.4. Effect of dosage The results for adsorptive removal of heavy metal ions with respect to adsorbent dose are shown in Fig. 8 over the range 1-15 g L-1 adsorbent amount in contact with 50 mL solution of 20±2 mg L-1, at temperature 25 ć and at pH 5.0. The removal efficiencies of Pb(II) and Cr(III) by MLDG are obviously higher than that on RLDG at the sorbent dose range of 1-15 g L-1. The removal efficiencies of Pb(II) on MLDG and RLDG reached saturation at 1 g L-1 and 10 g L-1, while that of Cr(III) on MLDG and RLDG were observed at 3 g L-1 and 10 g L-1, respectively. The percent removal of Pb (II) reached 99.77 % on MLDG at the adsorbent dosage of 1 g L-1 and increased from 51.25 to 97.46 % on RLDG with the increase of adsorbent dosage from 1 to 3 g L-1. In case of Cr(III), The percent removal on MLDG increased from 88.44 to 98.98 % for adsorbent dosage of 1 to 3 g L-1 while that on RLDG increased from 50.63 to 82.30 % for adsorbent dosage of 2 to 10 g L-1. In general, removal efficiency increases with increasing adsorbent dosage, which is due to the availability of more binding sites and larger surface area as the dosage of adsorbent increases [31]. On the other hand, the amount of Pb(II) and Cr(III) adsorbed (mg g-1) was found to decrease with increasing adsorbent dosage, which is attributed to more and more sorption sites on surface of the increasing RLDG and MLDG could not be saturated when the total Pb(II) and Cr(III) quantity in solution were fixed [32]. 3.5. Effect of equilibrium concentration and isotherm model The adsorption isotherm experiments were performed with the initial Pb(II) and Cr(III) concentrations varied from 5 to 800 mg L-1. The biosorption curves of Pb(II) and Cr(III) on RLDG and MLDG at the pH 5.0 and temperature 25 ć are shown in Fig. 9. It is apparent from the Fig. 9 the adsorption capacity increases with the increase in the Pb(II) and Cr(III) concentrations and the MLDG exhibited higher adsorption capacities than RLDG for both of the Pb(II) and Cr(III). the adsorption capacity of MLDG for Pb(II) and Cr(III) are 44.13 and 19.80 mg g-1 while that of the RLDG are 11.69 and 6.38 mg g-1.The reason for the enhancement in adsorption capacities may be the chemical modification promoting degradation of cellulose, protein and lignin of distillers' grains and formation of rough surface and loose structure. Moreover, the difference in the adsorption capacity of different metal ions is in agreement to some extent with the atomic weight and ionic radius of metal ions. Base on the fact that the affinity of ionic exchange increased with ionic valence and at the same charge value the cation with larger ionic radius was preferentially adsorbed [13], Pb(II) presented a better affinity to the MLDG surface than Cr(III), which is consistent with the fact that the Pauling ionic radius of Pb(II) (1.19Å) is higher than that of Cr(III) (0.62 Å).

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Fig. 10. The Langmuir curves for Pb(II) and Cr(III) adsorption

Fig. 11. The Freundlich curves for Pb(II) and Cr(III) adsorption on RLDG and MLDG in linear form.

The Langmuir and Freundlich isotherms (Eq (6) and Eq (7)) were used to model the adsorption isotherms with the quality of the fit assessed using the correlation coefficient. The Langmuir isotherm linear plots of Ce/qe versus Ce were shown in Fig. 10. Langmuir isotherm parameters from Table 4 indicate that the adsorption of Pb(II) and Cr(III) on RLDG and MLDG were in good agreement with Langmuir isotherm model. The Langmiur constant b can serve as an indicator for the isotherm rise in the region of lower metal concentration, which indicates the strength and affinity of the adsorbents for the solute [33]. The higher b for Pb(II) than Cr(III) suggested its higher affinity towards the adsorbent surface Ǆ The Freundlich isotherm constants KF and n are determined from the intercept and slope of a plot of lg q e versus lg Ce (Fig. 11). On average, a favorable adsorption tends to have Freundlich constant n between 1 and 10. Larger value of n (smaller value of 1/n) implies stronger interaction between biosorbent and heavy metal while 1/n equal to 1 indicates linear adsorption leading to identical adsorption energies for all sites [21]. In this study n values are greater than unity indicating a high affinity between adsorbate and adsorbent and is indicative of chemisorptions [18]. Comparing the correction coefficients (R2) of both models from table 4, The Langmuir isotherm model proved an excellent fit to the isotherm data of RLDG and MLDG, giving R2 values of 0.998 and 0.997 for Pb(II) and 0.993 and 0.980 for Cr(III), respectively, which indicates that the interaction between Pb(II) and Cr(III) and RLDG and MLDG is monolayer adsorption process rather than multiple-layer adsorption process. Table 4 The Langmuir and Freundlich adsorption parameters of RLDG and MLDG. Biosorbent RLDG MLDG

Ion

Langmuir isotherm constants

Freundlich isotherm constants

qmax(mg·g-1)

b(L·mg-1)

R2

KF

n

R2

Pb(II)

11.933

0.130

0.998

2.831

3.546

0.877

Cr(III)

6.639

0.060

0.993

0.708

2.294

0.940

Pb(II)

43.103

0.110

0.997

17.336

6.250

0.823

Cr(III)

19.084

0.030

0.980

10.186

12.195

0.827

According to Table 4, the maximum adsorption capacities of Pb(II) is higher than that of Cr(III) on both RLDG and MLDG, this may be attributed to the low tendency of lead ions to form strong complex although it can form hydrated ions [34]. 4. Conclusions The adsorption capacities of MLDG for Pb(II) and Cr(III) are improved obviously by a simple alkali modification. MLDG with rich functional groups -OH and -COOH, more rough surface and loose structure, exhibited higher absorption capacity for Pb(II) and Cr(III) ions than RLDG. The adsorption process depended significantly on the pH of solution. The kinetics data could be favorably described by pseudo-second-order model and the equilibrium data was satisfactorily represented by the Langmuir isotherm with a monolayer adsorption capacities of 44.13 mg g-1 for Pb(II) and 19.80 mg g-1 for Cr(III) on MLDG. Thus, all these indicated that MLDG can be a new potential adsorbent utilized for removal of heavy metal ions and it is worthy studying its dynamic adsorption for further industrial application.

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Acknowledgements This research was supported by The National Decommissioning of Nuclear Facilities and Radioactive Waste Management Research Projects Focus, SASTIND China (2014-806); Science and Technology Project of Qinghai Province ˄2014-GX-Q04˅. References [1] S.O. Lesmanaa, N. Febriana, F.E. Soetaredjo, J. Sunarso, S. Ismadji, Studies on potential applications of biomass for the separation of heavy metals from water and wastewater, S.O. Biochem. Eng. J. 44 (2009) 19-41. [2] W.S. Wan Ngah, M.A.K.M. Hanafiah, Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: A review, Bioresour. Technol. 99 (2008) 3935-3948. [3] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review, J. Environ. Manage. 92 (2011) 407-418. [4] U. Farooq, J.A. Kozinski, M.A. Khan, M. Athar, Biosorption of heavy metal ions using wheat based biosorbents-A review of the recent literature, Bioresour. Technol. 101 (2010) 5043-5053. [5] D.W. O’Connell, C. Birkinshaw, T.F. O’Dwyer, Heavy metal adsorbents prepared from the modification of cellulose: A review, Bioresour. Technol. 99 (2008) 6709-6724. [6] Y. Wu, S. Zhang, X. Guo, H. Huang, Adsorption of chromium(III) on lignin, Bioresour. Technol. 99 (2008) 7709-7715. [7] M. Ahmaruzzaman, Industrial wastes as low-cost potential adsorbents for the treatment of wastewater laden with heavy metals, Adv. Colloid. Interface. Sci. 166 (2011) 36-59. [8] E. Katsou, S. Malamis, K.J. Haralambous, Industrial wastewater pre-treatment for heavy metal reduction by employing a sorbent-assisted ultrafiltration system, Chemosphere. 82 (2011) 557-564 [9] D. Sud, G. Mahajan, M.P. Kaur, Agricultural waste material as potential adsorbent for sequestering heavy metal ions from aqueous solutionsA review, Bioresour. Technol. 99 (2008) 6017-6027. [10] L.V.A. Gurgel, O.K. Júnior, R.P.F. Gil, L.F. Gil, Adsorption of Cu(II), Cd(II), and Pb(II) from aqueous single metal solutions by cellulose and mercerized cellulose chemically modified with succinic anhydride, Bioresour. Technol. 99 (2008) 3077-3083. [11] S.I. Mussatto, G. Dragone, I.C. Roberto, Ferulic and p-coumaric acids extraction by alkaline hydrolysis of brewer’s spent grain. Ind. Crops. Prod. 25 (2007) 231-237. [12] Q. Li, L. Chai, Z. Yang, Q. Wang, Kinetics and thermodynamics of Pb(II) adsorption onto modified spent grain from aqueous solutions, Appl. Surface Sci. 255 (2009) 4298-4303. [13] Q. Li, L. Chai, Q. Wang, Z. Yang, H. Yan, Y. Wang, Fast esterification of spent grain for enhanced heavy metal ions adsorption. Bioresour. Technol. 101 (2010) 3796-3799. [14] K.S. Low, C.K. Lee, S.C. Liew, Sorption of cadmium and lead from aqueous solutions by spent grain. Process. Biochem. 36 (2000) 59-64. [15] S.G. Lu, S.W. Gibb, Copper removal from wastewater using spent-grain as biosorbent. Bioresour. Technol. 99 (2008) 1509-1517. [16] A.M. Awwad, N.M. Salem, Kinetics and thermodynamics of Cd(II) biosorption onto loquat (Eriobotrya japonica) leaves. J. Saudi Chem. Soc. (2011), doi:10.1016/j.jscs.2011.10.007. [17] D.H.K. Reddy, K. Seshaiah, A.V.R. Reddy, S.M. Lee, Optimization of Cd(II), Cu(II) and Ni(II) biosorption by chemically modified Moringa oleifera leaves powder. Carbohydr. Polym. 88 (2012) 1077-1086. [18] H.K. Boparai, M. Joseph, D.M. O’Carroll, Kinetics and thermodynamics of cadmium ion removal by adsorption onto nano zerovalent iron particles, J. Hazard. Mater. 186 (2011) 458-465. [19] A. Hawari, Z. Rawajfih, N. Nsour, Equilibrium and thermodynamic analysis of zinc ions adsorption by olive oil mill solid residues, J. Hazard. Mater. 168 (2009) 1284-1289. [20] N. Feng, X. Guo, S. Liang, Y. Zhu, J. Liu, Biosorption of heavy metals from aqueous solutions by chemically modified orange peel, J. Hazard. Mater. 185 (2011) 49-54. [21] W.S. Wan Ngah, M.A.K.M. Hanafiah, Adsorption of copper on rubber (Hevea brasiliensis) leaf powder: Kinetic, equilibrium and thermodynamic studies, Biochem. Eng. J. 39 (2008) 521-530. [22] J. Febrianto, A.N. Kosasih, J. Sunarso, Y.H. Ju, N. Indraswati, S. Ismadji, Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent: A summary of recent studies, J. Hazard. Mater. 162 (2009) 616645. [23] X. Luo, Z. Deng, X. Lin, C. Zhang, Fixed-bed column study for Cu2+ removal from solution using expanding rice husk. J. Hazard. Mater. 187 (2011) 182-189. [24] J. Umeda, K. Kondoh, High-purification of amorphous silica originated from rice husks by combination of polysaccharide hydrolysis and metallic impurities removal. Ind. Crops. Prod. 32 (2010) 539-544. [25] V. M. Nurchi, G. Crisponi, I. Villaescusa, Chemical equilibria in wastewaters during toxic metal ion removal by agricultural biomass, Coord. Chem. Rev. 254 (2010) 2181-2192. [26] M.T. Uddin, M. Rukanuzzaman, M.M.R. Khan, M.A. Islam, Adsorption of methylene blue from aqueous solution by jackfruit (Artocarpus heteropyllus) leaf powder: A fixed-bed column study. J. Environ. Manage. 90 (2009) 3443-3450. [27] Q.F. Lü, Z.K. Huang, B. Liu, X. Cheng, Preparation and heavy metal ions biosorption of graft copolymers from enzymatic hydrolysis lignin and amino acids. Bioresour. Technol. 104 (2012) 111-118. [28] E. Demirbas, N. Dizge, M.T. Sulak, M. Kobya, Adsorption kinetics and equilibrium of copper from aqueous solutions using hazelnut shell activated carbon. Chem. Eng. J. 148 (2009) 480-487. [29] G. Vázquez, O. Mosquera, M.S. Freire, G. Antorrena, J. González-Álvarez, Alkaline pre-treatment of waste chestnut shell from a food industry to enhance cadmium, copper, lead and zinc ions removal, Chem, Eng, J. 184 (2012) 147-155. [30] A.V. Maldhure, J.D. Ekhe, Preparation and characterizations of microwave assisted activated carbons from industrial waste lignin for Cu(II) sorption. Chem. Eng. J. 168 (2011) 1103-1111. [31] J. Zhang, X. Lin, X. Luo, C. Zhang, H. Zhu, A modified lignin adsorbent for the removal of 2,4,6-trinitrotoluene. Chem. Eng. J. 168 (2011) 1055-1063. [32] Y. Ding, , D. Jing, H. Gong, L. Zhou, X. Yang, Biosorption of aquatic cadmium(II) by unmodified rice straw. Bioresour. Technol. 114 (2012) 20-25. [33] T. Shi, S. Jia, Y. Chen, Y. Wen, C. Du, H. Guo, Z. Wang,. Adsorption of Pb(II), Cr(III), Cu(II), Cd(II) and Ni(II) onto a vanadium mine tailing from aqueous solution. J. Hazard. Mater. 169 (2009) 838-846. [34] M.R. Lasheen, N.S. Ammar, H.S. Ibrahim, Adsorption/desorption of Cd(II), Cu(II) and Pb(II) using chemically modified orange peel: Equilibrium and kinetic studies, Solid. State. Sci. 14 (2012) 202-210.