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Multipurpose use of a corncob biomass for the production of polysaccharides and the fabrication of a biosorbent Huawen Hu, Weixin Liang, Yuyuan Zhang, Siyong Wu, Quannu Yang, Yunbo Wang, Min Zhang, and Qianjun Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04179 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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ACS Sustainable Chemistry & Engineering

Multipurpose use of a corncob biomass for the production of polysaccharides and the fabrication of a biosorbent

Huawen Hu1, Weixin Liang1,2, Yuyuan Zhang1, Siyong Wu1, Quannu Yang2, Yunbo Wang2, Min Zhang1,* and Qianjun Liu3,*

1

College of Materials Science and Energy Engineering, Foshan University, Jiangwan 1st Road, Chancheng, Foshan 528000, Guangdong, China

2

College of Food Science and Engineering, Foshan University, Jiangwan 1st Road, Chancheng, Foshan 528000, Guangdong, China

3

College of Environmental Science and Engineering, Guangdong University of Technology, West Waihuan Street, Guangzhou 510006, Guangdong, China

* Correspondence: [email protected]; [email protected]

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Through fully exploiting waste biomasses as versatile matrices, value-added products and functional materials can be readily generated, which is of great significance to address many serious issues, especially environmental deterioration and waste of resource. Herein, a corncob biomass is selected as general platforms for achieving multipurpose applications, namely the extraction of polysaccharides that can be used as health care products and the fabrication of a functional biosorbent for environmental remediation. The polysaccharide extraction from the corncob particles is systematically investigated to optimize the extraction conditions and hence to obtain a good yield. After extraction, the corncob particles are further chemically modified with phosphoric acid to produce a biosorbent which is subsequently used to deal with malachite green (MG) as a typical aquatic pollutant. The adsorption mechanism underlying the efficient removal of the MG contaminant is also unraveled through a study of the various factors influencing the adsorption efficiency, in addition to investigations of the adsorption kinetics, thermodynamics and isothermal adsorption models. The modified corncob-based adsorption is found to be a spontaneous and endothermic process, and follows pseudo-second-order kinetics as well as a Freundlich adsorption isotherm. The modified corncob particles exhibit a favorable morphology, microstructure and surface properties that facilitate their uptake of MG in comparison to their unmodified counterparts. Phosphate functionalities and additional carboxyl groups are effectively incorporated onto the modified corncob bearing coarsened surface, pores and cracks, improving the adsorption performance by means of ionic bonding and electrostatic interactions between these newly introduced ionizable groups and the cationic MG molecules. Keywords: waste biomass, corncobs, polysaccharides, extraction, phosphate, carboxyl

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INTRODUCTION Waste biomass has been regarded as a renewable and inexpensive source of high-value biomaterials and has been widely employed as a platform to synthesize a diverse range of biomaterials for cleaning water, and to yield value-added products such as health foods and promising renewable energy sources.1-3 The effective utilization of waste biomasses can also help to prevent the environmental pollution as caused by these waste products themselves. For example, most waste biomasses emit hazardous methane, and their open combustion generates greenhouse gases such as CO2 and other local pollutants.4 From this viewpoint, improper management of waste biomasses poses a growing threat to the climate, as well as to water, soil and local air environments. In these regards, their effective utilization can not only minimize environmental pollution and strain on landfills, but also provide value-added products and functional materials for environmental remediation.

Scheme 1 Illustration of the main content of the present study, primarily including the crude polysaccharide extraction from corncobs, chemical modification of the 3



remaining corncobs by phosphoric acid, and the adsorption processing of MG using the modified corncob biomass as adsorbents.

Among waste biomasses, corncobs have been most extensively used as sources for yielding many value-added products, such as saccharides, cellulose, lignin, monophenols, xylose, nanocrystalline, and natural antioxidants.5-11 They have also been widely investigated as precursors for the generation of corncob derivatives and activated carbon for various applications such as water purification,1,12-13 hydrogen storage,14-16 methane storage,17 as well as the fabrication of supercapacitors18-19 and antibacterial materials.20 The widespread use of corncobs can be attributed to the large quantity of the corncobs produced annually (e.g., exceeding 20 million tons every year in China21). However, these abundant corncob-based bioresources have been poorly managed and often discarded, causing environmental contamination and waste of precious resource. Therefore, there is an urgent need to develop a feasible strategy for the scalable, affordable and effective utilization of corncobs as a renewable feedstock. Although significant efforts have been made to convert corncob biomasses into value-added products,22-33 little attention has been paid to both the extraction of value-added products and the generation of functional materials (e.g., adsorbents) from corncobs in a simultaneous way. For example, the ultrasound-assisted extraction of xylan,34 the enzyme-assisted microwave extraction of polysaccharides,35 the ultrasonication-enhanced NaOH solution-based extraction of xylan,36 and the Candida 4


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tropicalis-assisted, tetrabutylammonium hydroxide-induced extraction (combined with dilute acid hydrolysis) of xylitol.37 After extraction, large quantities of solid wastes are still left behind, thereby revealing that it is highly desirable to further harness the corncobs that remain after the extraction process. On the other hand, the direct use of the corncob for the fabrication of functional adsorbents without a process of value-added product extraction causes the waste of the precious resource. Only by the combination of these two aspects can the utilization efficiency be maximized to provide more valuable products and hence to achieve multipurpose applications. Despite that numerous strategies have been explored as potential corncob processing methodologies focusing on a single aspect of either the value-added product extraction or the direct fabrication of functional materials,6,23,32-33,38-40 most of them are tedious, time consuming, energy-inefficient, or environmentally unamiable, with demanding requirements for processing conditions, thus lowering the probability for plant-scale applications. For instance, energy-demanding processes (i.e., steam explosion and liquid hot pressured alcohol) have been employed to treat corncobs and obtain a value-added product.6 A lengthy and complex process consisting of soaking in a H2SO4 solution, filtrating, washing, steaming, mashing, and finally xylanase-assisted hydrolysis were reported to produce xylooligosaccharides from corncobs.41 Tedious procedures and hazardous chemicals were required for the preparation of a hyperbranched polyamide-modified corncob adsorbent.40 Meanwhile, the production of activated carbon from corncobs has called for high-temperature 5



calcination under inert gas conditions.23 Without a process of surface modification, corncobs have also been employed directly as adsorbents, but the resulting adsorption efficiency was unsatisfactory.21 This demonstrated that it was necessary to chemically modify corncobs to incorporate more active functionalities and hence to enhance their binding performance as adsorbents. For example, the introduction of carboxyl groups can facilitate stable complexation with heavy metal ions and the formation of stronger electrostatic, ionic and hydrogen bonding interactions between the adsorbent and organic wastes. In this study, the viability of corncobs as high-value materials for multipurpose applications was explored. The corncobs were firstly extracted to obtain value-added products (i.e., polysaccharides) by a facile temperature-assisted sonication treatment, and subsequently chemically modified with low-cost phosphoric acid under moderate reaction conditions to generate an adsorbent (see Scheme 1). An organic waste, in this case malachite green (MG), was consequently employed as a test pollutant to evaluate the performance of the modified corncob-based adsorbent. MG has a notorious reputation as a toxic environmental pollutant and can pose long-term carcinogenic and mutagenic threats to humans. Even worse, the enrichment of MG in humans can be readily achieved through the consumption of popular seafood such as fish.42 The study presented here provides a complete route for dealing with waste biomasses, in this case corncobs, for the generation of both value-added products and functional materials. Therefore, this study paves the way for the exploitation of other kinds of waste biomasses to address many issues related to health care and environmental 6


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protection. This work may also shed light on the optimization of the processing route, thus maximizing the efficiency of waste biomass usage for various applications.

EXPERIMENTAL SECTION Extraction of the polysaccharides from the corncobs. The main aspect of this study is illustrated in Scheme 1, and details of the raw materials employed, the sample characterizations and the adsorption tests are provided in the electronic supporting information (ESI). Firstly, to extract the polysaccharides from the corncobs, the following procedures were adopted. The dried and ground corncob powder was placed into a beaker which was then subjected to sonication treatment at a given temperature (30~80 oC), an extraction time (5~25 min), and a liquid-to-solid ratio (5~30). After the sonication extraction, the mixture was filtered, and then the filtrate was concentrated to a volume of 2~3 mL. The filtered solid was kept separate for subsequent modification with phosphoric acid. Ethanol (95%) was added to the concentrated solution at a volume ratio of 5:1 (ethanol:solution). After sufficient stirring, the mixture was allowed to stand for 1 h, centrifuged at 4000 r/min for 10 min, and subsequently filtered to yield the crude polysaccharide as a solid product. The impact of the different extraction temperatures, time, and liquid-to-solid ratios on the polysaccharide extraction yield was also investigated, on the basis of the L9(34) orthogonal test as designed. Further information regarding the evaluation of these impacts is provided in the ESI. Chemical modification of the extracted corncob with phosphoric acid. The 7



corncobs were subjected to the polysaccharide extraction process, and subsequently the extracted polysaccharides were placed into a 2 L beaker. At a liquid-to-solid ratio of 1:5 that allowed the solid corncob particles to be completely immersed in the liquid for achieving a sufficient extraction, the corncob particles were mixed with the chemical modifier, phosphoric acid with a concentration of 1 mol/L (refer to the Ref.43), and subsequently stirred at 50 °C for 2 h in a water bath to enable the reaction between phosphoric acid and the cellulose on the corncob surface to proceed. Subsequently, the mixture was filtered to obtain a filter cake that was initially dried in an oven at 50 °C for 2 h and then at 150 °C for 1.5 h in order to evaporate the water present on the surface. This dried residue was subsequently washed with an excess of deionized water to remove the excess phosphoric acid and was then dried at 50 °C, thus yielding the phosphoric acid-modified corncob.

RESULTS AND DISCUSSION Study of the extraction conditions and optimization of the crude polysaccharide extraction yield. A systematic study on the extraction of polysaccharides from the crushed and sieved corncob particles was performed, and the results are presented in Figure S1 of the ESI. Detailed analysis of the extracted polysaccharides by using HPLC and GPC is provided in Figures S2-S6 and Tables S1-S6 in the ESI. Among the studied parameters, the liquid-to-solid ratio exhibits the greatest influence on the extraction yield (Figure S1). More detailed description is presented in the ESI. L9(34) orthogonal test results revealed that the optimal design 8


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was A3B2C2 (i.e., a sonication temperature of 60 °C, a sonication time of 15 min and a liquid-to-solid ratio of 20:1, as seen in Tables S7 and S8 in the ESI). After three repeated verification tests, the optimized extraction yield of the crude polysaccharide was determined to be 6.96% on average. SEM observation. Figure 1 presents the SEM images of various corncob-based samples, including pristine corncob (Figure 1a and b), extracted corncob (Figure 1c and d), and chemically modified corncob (Figure 1e and f) samples. The pristine corncob particles exhibited sheet-like bulk morphology (Figure 1a and b), while the corncob particles became curved after the polysaccharide extraction, leading to the generation of large pores (Scheme 1, and Figure 1c and d). This morphological change can be attributed to the polysaccharide extraction, which altered the morphology of the remaining corncob and caused it to vary. The further chemical modification of the extracted corncob with phosphoric acid leads to a coarsened surface and more porous structure, together with the formation of cracks. This morphological transition resulting from the chemical modification can be attributed to the high temperature-induced local carbonization of the corncob containing abundant cellulose and lignin. It can be conjectured that these pores, cracks and coarsened surfaces could facilitate the adsorption of MG. The MG molecules are most likely to become immobilized in the pores, and if this is the case strong adsorption interactions may exist between the MG molecules and the adsorbent surface. In order to confirm the existence of these strong interactions, it is necessary to further explore the surface properties of the chemically modified corncob particles. 9



Figure 1 SEM images of various corncob-based samples at different magnification scales, including pristine corncob samples before the polysaccharide extraction (a,b), corncob samples after the polysaccharide extraction (c,d), and corncob samples observed after the extraction and subsequent chemical modification with phosphoric acid (e,f).

Analysis of the surface properties by FTIR, XPS and EDX mapping. Figure 2 shows the FTIR spectra of the extracted corncob particles before and after the chemical modification with phosphoric acid. The FTIR absorption peak centered at 10


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3427 cm-1 corresponds to the –OH stretching vibration of the associated water molecules,44-46 and becomes broadened after the modification with phosphoric acid and can be attributed to the high hydrophilicity of the phosphoric functionalities. Such a hydrophilicity facilitates the adsorption of a layer of water molecules. It is worth mentioning that the chemical modification results in the enhancement of FTIR absorption signals (~1716 cm-1) corresponding to the carboxyl groups.47-49 The absorption corresponding to C-O stretching vibrations (~1003 and 1049 cm-1 for the corncob sample before and after the modification, respectively) is enhanced as well after the modification with phosphoric acid. The enhanced C-O signals can be due to either the chemical modification-induced partial oxidation of carbon skeletons or the overlap of the absorption due to asymmetric stretching vibrations of the newly formed P-O-C with that resulting from C-O. A new absorption at 1119 cm-1 can also be detected, corresponding to the symmetrical stretching vibration of P-O-C.50-51 All of these spectral observations are consistent with the effective modification of the extracted corncob with phosphoric acid, which enables some of the cellulose to be converted to phosphocellulose (Figure S7 of the ESI).

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1635

1716

Wavenumber (cm-1) Figure 2 FTIR spectra of the extracted corncobs before and after modification with phosphoric acid.

The effective chemical modification can be further demonstrated by XPS analysis. The XPS survey spectra of pristine corncob, extracted corncob and modified corncob samples are presented in Figure 3. The survey XPS spectra of the corncob samples recorded before (Figure 3a) and after (Figure 3b) the polysaccharide extraction are similar. In contrast, the ratio of the C1s peak intensity to the O1s peak intensity is increased after chemical modification with phosphoric acid, as shown in Figure 3c. This confirms that the corncob particles underwent carbonization and became enriched with carbon during the chemical modification with phosphoric acid. The incorporation of phosphorous can also be validated by the small P2p peak. High-resolution C1s and P2p XPS spectra were recorded to further characterize the 12


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modified corncobs, as shown in Figure 4 and Figure 5, respectively. High-resolution O1s XPS spectra are also provided in Figure S8 of the ESI. The high-resolution C1s XPS spectra can provide an indication of the carbonization and introduction of more carboxyl groups,52 as a result of the temperature-assisted phosphorylation and oxidation by phosphoric acid (Figure 4c and Figure S9 in the ESI). The characteristic P2p XPS spectra with a 133.5 eV binding energy for [PO4]3- can be observed in Figure 5.53 The deconvoluted peaks at approximately 532.5 and 531.5 eV in the O1s XPS spectra can be assigned to the -O- and =O (in carbonyl, carboxyl and phosphate groups) moieties, respectively,54 as shown in Figure S8 in the ESI. A slight increase of the =O content can be noted for the modified corncob, as estimated by integration of the deconvoluted peaks, which provides further evidence of corncob oxidation as caused by the phosphoric acid-based modification. The EDX mapping images of the extracted corncob particles after the chemical modification with phosphoric acid are further provided in Figure 6. It can be visually found the effective incorporation of the element phosphorus onto the surface of the extracted corncob particles, with a uniform distribution, which indicates the homogeneous modification of the corncob particle surface by phosphoric acid (as also schematically displayed in Scheme 1).

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Figure 3 XPS survey spectra of the pristine corncob (a), the extracted corncob (b), and the corncob samples that had been extracted and subsequently modifies with phosphoric acid (c).

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Figure 4 High-resolution C1s core-level XPS spectra of the pristine corncob (a), the corncob remaining after the extraction process (b), and a sample of the corncob obtained after the extraction process and subsequent modification with phosphoric acid (c).

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Figure 5 High-resolution P2p core-level XPS spectrum of a corncob sample obtained after modification with phosphoric acid.

Figure 6. BSE and EDX mapping images of the extracted corncob particles after the chemical modification with phosphoric acid.

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Study of the adsorption performance of the chemically modified corncob toward MG. Figure 7 summarizes the results of the investigation regarding the adsorption capacities of the chemically modified corncob. Firstly, we compared the adsorption efficiency before and after the chemical modification, as shown in Figure 7a. After the modification with phosphoric acid, the corncobs exhibited a 24-40% average increase in their adsorption capacity. Such an enhancement can be attributed to four key considerations. First, an abundance of cellulose in the corncob after the polysaccharide extraction becomes carbonized during the modification process with phosphoric acid at an elevated temperature (Figure 3 and Figure 4), which could lead to an increase of the specific surface area and pore volume (see BET measurement results as shown in Figure S10 and Table S9 of the ESI). Second, the carbonization also results in the formation of complex cracks and pores on the surfaces of the corncob particles (Figure 2), thereby increasing the availability of adsorption sites (Scheme 1). Third, since the MG molecules are present in the cationic form, the additional carboxyl groups that are generated via the chemical modification (Figure 2 and Figure 4) can form strong ionic bonds and attractive electrostatic interactions with MG molecules, which is realized by the ionization of the carboxyl moieties to form anionic functional groups such as carboxylates. Fourth, the modification with phosphoric acid also contributes to the formation of the phosphate anions on the corncob surface (Scheme 1 and Figure 5, as well as Figure S7 and Figure S9 in the ESI), so that ionic bonding and electrostatic interactions also occur between these phosphate anions and the cationic MG molecules. 17



Figure 7 Investigation of the adsorption toward MG by using the chemically modified corncob. Comparison of the adsorption efficiency between the unmodified corncob and chemically modified corncob (a). Influence of the pH conditions on the adsorption efficiency of the modified corncob (b). Impact of the dosage of the modified corncob as the adsorbent on the adsorption efficiency (c). Comparison of the fitting results with respect to pseudo-first order and pseudo-second order kinetics (d). Comparison of the isothermal adsorption fitting results with regard to isothermal adsorption models including Langmuir and Freundlich isotherms (e). Plots of lnK0 as a function of 1/T (f). 18


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After it was demonstrated that modification with phosphoric acid enhanced the adsorption performance of the corncob particles, a systematic study was conducted to further evaluate their adsorption performance. Figure 7b depicts the impact of the pH on the adsorption performance. A large quantity of H+ ions can be adsorbed onto the surfaces of the modified corncob particles at lower pH values. Because MG molecules are present in the cationic form, electrostatic repulsion interactions occur between the cationic MG molecules and the H+-covered adsorbent, lowering the adsorption capacity at lower pH values. The adsorption capacity tends to become saturated in the pH range of 6.0-9.0. Considering that the adsorption percentage for MG reaches 92% in the pH range of 5.0-7.0, a pH of 7 was selected as the optimal pH value for the adsorption test. At the selected pH of 7, the dosage of the adsorbent (i.e., modified corncob), was further optimized, as shown in Figure 7c. The equilibrium adsorption capacity and adsorption percentage are estimated comprehensively, leading us to identify the optimal adsorbent dosage as 1 g/L. The adsorption kinetics were investigated further, with the results shown in Figure 7d and Table S10 in the ESI. The commonly-used adsorption kinetic models are based on pseudo-first order and pseudo-second order reactions,55 which can be respectively expressed by Eqs. (1) and (2):

qt = qe (1− e−k1t )

qt =

(1)

t 1 k2 qe

2

+

t qe

(2)

where k1, k2, qe, and qt denote the pseudo-first order adsorption rate constant (1/min), 19



the pseudo-second order adsorption rate constant (1/min), the equilibrium adsorption capacity (mg/g) and the adsorption capacity at time t (mg/g), respectively. It is noteworthy that the modified corncob-based adsorption reaches equilibrium relatively quickly, within 90 min. With the assistance of the standard curve, the adsorption capacity was calculated at each time point, and curve fitting was subsequently performed. The equilibrium adsorption capacities q1e and q2e were estimated to be 141.13 and 152.19 mg/g on the basis of the two kinetic models, with the correlation coefficients R2 of 0.811 and 0.977, respectively. Note that the degree of correlation not only can be exhibited by R2, but can also be reflected by the relative deviation D%. Although the equilibrium adsorption capacity (q1e = 141.13 mg/g), as calculated by the pseudo-first order kinetic model, is close to the experimental value (qe, exp = 148 mg/g), the correlation coefficient is low, indicating that the pseudo-first order kinetic model does not adequately describe the adsorption process. Concerning the pseudo-second order kinetic model, the correlation coefficient can reach as high as 0.977, with an equilibrium adsorption capacity (q2e = 152.19 mg/g) that is even closer to the experimental value (qe, exp = 148 mg/g). All of these findings demonstrate that the pseudo-second order kinetic model is more convincing and hence provides a more suitable description of the adsorption process. Since the pseudo-second order kinetic model is based on the assumption that the rate-limiting step in the adsorption process is chemical interactions,56 the adsorption interactions between the biosorbent (i.e., modified corncob) and MG molecules are strong. In comparison to an adsorption involving only physical interactions, the adsorption in this case can be significantly 20


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enhanced. The driving force under the present adsorption is a result of the effective modification of the extracted corncob with phosphoric acid, leading to the generation of numerous carboxyl and phosphoric groups. These functionalities readily form ionic bonds with the cationic MG species (Scheme 1). The isothermal adsorption measurements are shown in Figure 7e, with the specific parameter values summarized in Table S11 in the ESI. The isothermal adsorption reveals that the adsorption capacity of the adsorbent surface is determined by the concentration of the MG molecules at a constant temperature. When the temperature and MG concentration are fixed, the equilibrium adsorption capacity usually approaches a constant value. This behavior can thus be used to compare the adsorption performance of various adsorbents. Langmuir (Eq. 3) and Freundlich (Eq. 4) isotherm model equations are usually employed to investigate isothermal adsorption processes:57

qe =

bC e q m 1 + bC e

qe = k f Ce

(3)

n

(4) where qe, qt, Ce, b, k and n represent the equilibrium adsorption capacity (mg/g), the mono-layered and saturated adsorption capacity (mg/g), the equilibrium mass concentration (mg/L), the equilibrium adsorption constant (L/mg), and an arbitrary constant associated with the adsorption intensity, respectively. The Freundlich isotherm provides a better fit with the experimental data (with a correlation coefficient R2 of more than 0.995), than can be achieved with the 21



Langmuir isotherm (with an R2 value of below 0.93). Therefore, these findings reveal that the modified corncob-based isothermal adsorption better follows the Freundlich isotherm. Considering that the Langmuir model typically describes a homogeneous distribution of active sites on the adsorbent and a monolayer adsorption process,40 this suggests that the modified corncob-based adsorption does not occur via a monolayer adsorption phenomenon, but instead involves a multilayer adsorption and that the active sites are inhomogeneously distributed on the surface. We have demonstrated that the modified corncob-based adsorption process involves chemisorption which is widely believed to occur in the manner of monolayer adsorption,58 suggesting that both chemical and physical adsorption interactions are present in our case. As shown in the Freundlich isotherm model equation (Eq. 4), the n value reflects the inhomogeneity or the adsorption intensity; it is generally believed that an n value falling in the range of 2~10 reveals that the adsorption can readily proceed. For the modified corncob-based adsorption, the n values are in the approximate range of 2.8~2.9 (Table S11 in the ESI), confirming the favorable adsorption of the MG molecules onto the modified corncob. Also note that the n value is increased when the temperature is raised, which indicates that higher temperatures facilitate the adsorption.

Usually, the adsorption mechanism is also investigated by analyzing the thermodynamic functions including ∆G°, ∆H° and ∆S°, which can reflect the transfer and variation of thermal energy. The values of Gibbs free energy change ∆G° and the 22


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enthalpy change ∆H° can be an indication of spontaneous/non-spontaneous and exothermic/endothermic adsorption processes, respectively.59 These thermodynamic functions can be calculated according to Eqs. (5-8):

qe Ce

(5)

∆G = - RT lnK0

(6)

RT lnK0 = T∆S ° - ∆H °

(7)

∆H ° 1 ∆S ° + R T R

(8)

K0 =

lnK 0 = −

where qe, Ce, R, ∆H° and ∆S° denote the equilibrium adsorption capacity (mg/g), equilibrium mass concentration (mg/L), gas constant (R = 8.314J/mol·K), the enthalpy change (kJ/mol) and the entropy change (J/molK), respectively. As shown in Figure 7f, the plots of lnK0 versus 1/T are highly linear. The enthalpy change can be estimated as the intercept of the linear plot, ∆S°/R, while the entropy change can be obtained from the slope of the plot, -∆H°/R. For a solid-liquid system, the adsorption of the MG molecules onto the adsorbent surface can lower the chaos of the system and hence reduce the entropy, so that ∆S° should be below 0.60 However, the ∆S° value observed during the modified corncob-based adsorption of MG is larger than 0 (Figure 7f and Table S12 in the ESI), which is inconsistent with the thermodynamic theory. This can be attributed to a monolayer of adsorbed water molecules residing on the modified corncob surface before the adsorption of the MG molecules takes place. As a consequence, the water molecules are desorbed and the MG molecules are adsorbed simultaneously. Because the number of desorbed water 23



molecules exceeds that of the adsorbed MG molecules, the chaos of the system is increased in the macroscopic view and hence the entropy is increased. Under the given adsorption test conditions, the Gibbs free energy change ∆G° is always negative and becomes further lowered at elevated temperatures, and thus the adsorption enthalpy change ∆H° is found to be a positive value (Table S12 in the ESI). These findings demonstrate that the modified corncob-based adsorption is a spontaneous and endothermic process, revealing that elevating the temperature facilitates the adsorption, in consistence with the reported cellulose/activated carbon composite monolith-based61 and oxidized corncob-based62 adsorption processes. According to the laws of thermodynamics, a spontaneous adsorption process should also be exothermic,63-64 which is inconsistent with our experimental data. Two explanations may account for this discrepancy. First, the quantity of the desorbed water molecules is much higher than that of the MG molecules adsorbed onto the modified corncob, and the desorption of water molecules is endothermic, thus rendering the overall adsorption as an endothermic process from a macroscopic viewpoint. Second, the endothermic chemical adsorption plays a more important role in the present modified corncob-based adsorption process as compared to the exothermic physical adsorption from the thermodynamic viewpoint.

CONCLUSIONS This study demonstrated that corncob waste can serve multiple roles as a feedstock for the production value-added materials such as polysaccharides and for 24


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the fabrication of functional adsorbents. In addition to conducting a systematic investigation to maximize the yield of extracted polysaccharides, the corncobs remaining after the extraction were modified with phosphoric acid and investigated as adsorbents for the removal of MG, a toxic compound. The adsorption mechanism underlying the efficient removal of the MG molecules was also disclosed through an investigation of various impacts on the adsorption efficiency, in addition to a study of the adsorption kinetics, thermodynamics and isothermal adsorption models. The modified corncob-based adsorption is a spontaneous and endothermic process, and follows pseudo-second-order kinetics and the Freundlich adsorption isotherm. As evidenced by the SEM observation as well as FTIR and XPS analyses, agricultural waste biomass (i.e., corncobs), can be developed into an excellent adsorbent with a favorable morphology, microstructure and surface properties for this role, as a result of the polysaccharide extraction and effective chemical modification with phosphoric acid. The morphologies and surface properties of the corncob are significantly altered after the polysaccharide extraction and chemical modification, endowing the modified corncob with enhanced adsorption capacities in comparison with the unmodified counterparts. Coarsened surface, porous structure and cracks are generated on the surfaces of the modified corncob particles. Meanwhile, an abundance of phosphate and carboxyl groups are also formed. This thereby reveals that attractive ionic bonding and electrostatic interactions can occur between these newly introduced anionic groups and the cationic MG molecules, significantly enhancing the adsorption capacity of the modified corncobs. 25



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ASSOCIATED CONTENT Supporting Information. Information as mentioned in text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors. *(M.Z.) e-mail: [email protected]; *(Q.L.) e-mail: [email protected]

ACKNOWLEDGMENTS The authors acknowledge the National Natural Science Foundation of China (51702050), the Science and Technology Project of Guangdong Province (2015A030401106), the Innovation Project of Department of Education of Guangdong Province (2015KTSCX150), and the Science and Technology Project of Foshan City (2015AG10011, and 2016GA10162).

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Simultaneous process of the extraction of polysaccharides and the fabrication of biosorbents from renewable, sustainable, inexpensive, and biocompatible corncob biomasses.


Multipurpose Use of a Corncob Biomass for the ... - ACS Publications

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