Mechanistic Investigation in Ultrasound-Assisted (Alkaline

Article pubs.acs.org/IECR

Mechanistic Investigation in Ultrasound-Assisted (Alkaline) Delignification of Parthenium hysterophorus Biomass Shuchi Singh,†,# S. T. P. Bharadwaja,‡,# Pawan Kumar Yadav,§ Vijayanand S. Moholkar,*,†,‡ and Arun Goyal*,†,∥ †

Center for Energy, ‡Department of Chemical Engineering, ∥Department of Biotechnology, Indian Institute of Technology Guwahati, Guwahati − 781 039, Assam, India § Department of Chemical Engineering, National Institute of Technology Tiruchirappalli, Tiruchirappalli − 620015, Tamil Nadu, India S Supporting Information *

ABSTRACT: Delignification of biomass is a primary step in biomass pretreatment in fermentation based synthesis of alcoholic biofuels. This paper attempts to give mechanistic insight into ultrasound-assisted delignification of biomass. Parthenium hysterophorus (carrot grass) has been used as the model biomass. The approach of study is to couple simulations of cavitation bubble dynamics to the experiments on delignification. Best values of delignification parameters with ultrasound have been identified as temperature = 303 K, NaOH concentration = 1.5% w/v, and biomass concentration = 2% w/v. Characterization of delignified biomass has been carried out using FTIR spectroscopy and XRD and FESEM techniques. Both physical and chemical effects of transient cavitation contribute to delignification. The physical effect of shock waves leads to depolymerization of lignin matrix through homolytic cleavage of phenyl ether α−O−4 and β−O−4 bonds. The chemical effect of radical generation causes hydroxylation/oxidation of the aromatic moieties and side chain elimination. Due to these peculiar mechanisms, ultrasound treatment gives effective delignification at ambient temperature and with lesser requirement of delignifying agents. Cavitation also causes decrystallization of cellulose due to partial depolymerization. Kinetic analysis of delignification at best values of parameters has revealed 2-fold enhancement with ultrasound as compared to mechanically agitated treatment.

1. INTRODUCTION Global crisis of fossil fuels in terms of their fast depletion and the detrimental effects on climate has encouraged research in renewable biofuels. Alcoholic fuels like bioethanol1 and biobutanol2 derived from biomass though have attracted the attention of researchers worldwide. However, commercial implementation of these biofuels requires an economic and cost-effective process for production. The major contribution to the production cost of alcoholic biofuels is from the substrate used for fermentation. Use of cheaper substrates like lignocellulosic biomass (in the form of agricultural or forest residues) is a viable solution to boost the economy of the production of biofuels. Agro residues from common crops like rice, wheat, corn, and sugar cane have been used as fermentation substrates. However, these biomasses also have alternate uses like animal fodder and domestic fuel, which raises their cost and reduces the availability. Use of waste biomasses is a solution to this problem. Parthenium hysterophorus is one such waste biomass, which is available abundantly in several parts of the world. P. hysterophorus, commonly known as carrot grass, is one of the world’s most invasive species. Although P. hysterophorus is included in the world’s seven most devastating and hazardous weeds, it has high potential to be the substrate for alcoholic biofuels production. It was demonstrated in a recent study3 from our group that total fermentable sugar yield of 397.7 mg/g raw biomass could be achieved after dilute acid pretreatment and enzymatic hydrolysis of P. hysterophorus biomass. The maximum ethanol yield from fermentation of these sugars is 202.8 mg/g raw biomass, which is at par with © 2014 American Chemical Society

conventional substrates such as molasses and corn. Thus, our study3 clearly demonstrates the potential of P. hysterophorus for bioethanol production. Another cost intensive component of alcoholic biofuels synthesis is the pretreatment and hydrolysis of the biomass prior to fermentation.4 For efficient and economic alcoholic biofuels production, it is necessary to extract the maximum amount of sugar monomers from raw biomass comprising of cellulose, hemicellulose, and lignin. Pentose sugars are usually obtained during hydrolysis of hemicellulose during acid pretreatment of biomass. The hexose sugars are obtained from enzymatic hydrolysis of the acid pretreated biomass (which mainly contains lignin and cellulose). For high hexose sugar yield in enzymatic hydrolysis, the cellulose moieties in the biomass need to be exposed to enzymatic action. However, the lignin content in the biomass acts as a barrier and hinders the accessibility of enzyme molecules to cellulose and the latter’s hydrolysis to hexose sugars.5,6 This necessitates lignin removal or delignification of pretreated biomass. The delignification process is generally carried out under acidic, alkaline, or oxidative conditions.7 For delignification under alkaline conditions, NaOH is a widely used reagent.8 The main chemical mechanism for degradation of lignin in alkaline and alkaline−oxidative environment is through the cleavage of the α- and β-aryl ether linkages to yield Received: Revised: Accepted: Published: 14241

June 10, 2014 August 12, 2014 August 24, 2014 August 24, 2014 dx.doi.org/10.1021/ie502339q | Ind. Eng. Chem. Res. 2014, 53, 14241−14252

Industrial & Engineering Chemistry Research

Article

Table 1. Summary of Literature on the Ultrasound-Enhanced Delignification of Biomass raw material

reaction parameters

wheat straw wheat straw

100 W, 20 kHz, 60 °C, 35 min Horn type reactor 0−35 min at 35 °C

newspaper pulp

100 W, 80% duty cycle, 0.5% (w/v) paper loading, 70 min, ambient temperature 570 W, 20 kHz, 25 °C, 30 min

poplar (Populus tomentosa) sawdust bamboo (Neosinocalamus af f inis) sugar cane bagasse

240 W, 22 kHz, room temperature

delignifying agents

results

ref

0.5 M NaOH (CH3OH:H2O 60:40) 67.4−78.5% lignin solubilized 0.5 M KOH pretreatment and 2% H2O2− 43.3−46.2% lignin and 27.1−28.1% 0.2% TAED post-treatment hemicelluloses solubilization 1 N NaOH 80% lignin removal

8 9

95% ethanol, methanol, dioxane, dimethyl sulfoxide aqueous soda solution

increased lignin extraction

11

improved delignification at room temperature and pressure 13.4−16.4% lignin isolation with β−O− 4 ether linked inter units

12

increased sugar yield after acid hydrolysis

14

100 W, 20 °C, 50 min

95% ethanol, 50 min NaOH (7%)/urea (12%)

24 kHz, 50 °C, 20 min

aqueous soda solution

10

13

resulted in removal of hemicellulose by hydrolysis to pentose sugars. Pretreated biomass was dried overnight at 60 ± 3 °C prior to the delignification process. Analysis of the chemical composition (i.e., determination of cellulose, hemicellulose, and lignin content) of pretreated and delignified biomass was done according to the standard TAPPI protocols.22 2.3. Parametric Investigation of the Delignification Process. Parametric investigation of the delignification for both ultrasound-assisted and mechanically agitated processes was done for the following parameters: (1) alkali concentration, (2) biomass loading or concentration, (3) temperature, and (4) reaction time. All experiments were conducted in a 100 mL glass beaker with reaction volume of 80 mL. Sequential investigation was done by assessing the effect of variation in one parameter at a time on the extent of delignification while keeping other parameters constant. The best value of the first parameter (corresponding to maximum lignin removal) was used while assessing the effect of the next parameter. To start with, the effect of NaOH concentration on delignification was assessed in the range of 0.5 to 3% w/v (corresponding to the concentration of 0.125−0.750 N) with values of other parameters as biomass concentration = 3% w/v, temperature = 30 °C, and time of treatment = 15 min. Next, biomass concentration was varied in the range of 0.5 to 8% w/v with values of other parameters as NaOH concentration = 1.5% w/v (in the case of ultrasound-assisted process) and 2.5% w/v (in the case of mechanically agitated process), temperature = 30 °C, and time of treatment = 15 min. Finally, the temperature of treatment was varied in the range 30−80 °C with values of other parameters as NaOH concentration = 1.5% w/v (in the case of ultrasound-assisted process) and 2.5% w/v (in the case of mechanically agitated process), biomass concentration = 2% w/v, and time of treatment = 15 min. All experiments were performed twice to check reproducibility of the results. The kinetics of the delignification process was measured at the best values of the three parameters obtained in the above investigation. 2.4. Ultrasound-Assisted Alkali Treatment of Biomass. A probe type microprocessor based programmable ultrasonic processor (Sonics and Materials, Model: VCX 500, max. power: 500 W, freq: 20 kHz) was used for sonication of the reaction mixture. The ultrasound probe was made of high grade titanium alloy and had a diameter of 13 mm. The probe was operated at 30% amplitude corresponding to peak theoretical power input of 150 W. The actual ultrasound power input to the system and the acoustic intensity (in terms of pressure amplitude of the ultrasound wave) was estimated using calorimetric method.23

fragmentation units (greater details are given in the Supporting Information). Recently, many researchers have employed ultrasound for intensification of delignification under alkaline treatment (summary of results of recent papers8−14 is given in Table 1). Despite these efforts, the exact physical mechanism of the ultrasound-assisted delignification is not established yet. Ultrasound and its secondary effect, cavitation, have physical and chemical effects on the reaction system. The physical effect is in terms of generation of strong microturbulence in the system through microconvection generated by transient bubble motion and microstreaming (i.e., small amplitude oscillatory motion of liquid elements induced by passage of ultrasound wave) generated by ultrasound wave.15−17 The chemical effect of cavitation is production of highly reactive radical species such as •OH, •O, and HO2• generated through thermal dissociation of vapor molecules entrapped in the bubble during transient collapse.18−20 These radicals may induce chemical reactions that lead to degradation of lignin through different mechanisms (greater details given subsequently).7,21 For effective use of ultrasound energy for delignification, relative contributions of the physical and chemical effects of ultrasound in enhancement of the delignification process needs to be identified. In this paper, we have treated this issue with approach of coupling experiments with simulations of cavitation bubble dynamics at the conditions of the experiments. Concurrent analysis of experimental and simulations results has revealed interesting mechanistic facets of the delignification process, as outlined in the subsequent sections.

2. MATERIALS AND METHODS 2.1. Materials. Biomass Collection and Processing. P. hysterophorus plant biomass was collected from the campus of our institute (IIT Guwahati). Ground biomass (particle size ∼1 mm) was pretreated with 1% (v/v) H2SO4 + 30 min autoclaving (121 °C, 15 psi pressure)3 and washed with tap water followed by distilled water until neutral pH was achieved. The solid residue (hereafter termed as pretreated biomass) was used for delignification. Chemicals and Enzyme. Sodium hydroxide pellets were procured from HiMedia Pvt. Ltd. (India) and were used as received. β-glucosidase from Aspergillus niger (Novozyme 188) was procured from Sigma-Aldrich, USA. 2.2. Compositional Analysis of P. hysterophorus Biomass. The biomass used in this study (termed as pretreated biomass) was the residue left after dilute acid pretreatment (with autoclaving) of raw biomass. The acid pretreatment 14242

dx.doi.org/10.1021/ie502339q | Ind. Eng. Chem. Res. 2014, 53, 14241−14252


Article

liberated total reducing sugar (TRS) was estimated by the method of Nelson27 and Somogyi.28

The actual power input to the system was determined as 1.045 W using an ultrasound probe with a tip diameter of 13 mm, which corresponded to acoustic intensity of 7873 W/cm2. The pressure amplitude of the ultrasound wave corresponding to this intensity was 1.5 bar. This processor also had facility of automatic frequency tuning and amplitude compensation, which ensures constant power delivery to the ultrasound probe and the liquid medium irrespective of the changes occurring in the liquid medium during sonication. 2.5. Characterization of Delignified P. hysterophorus Biomass. FESEM Analysis. The morphology of pretreated and delignified biomass was assessed using a field emission scanning electron microscope (Make: Zeiss, Model: Sigma). The samples for FESEM analysis were prepared by drying the biomass at 60 °C for 24 h. The dried samples were spread on the carbon tape placed over the surface of FESEM stub and were sputtered with 10 nm gold in a sputter coater (Leo, SCH 620). FTIR Characterization of Biomass Samples. Pretreated and delignified biomasses were characterized for the chemical composition using Fourier Transform Infra-Red spectroscopy. An FTIR spectrophotometer (Perkin−Elmer, Model: Spectrum Two) was used for characterization, and the samples were prepared by mixing biomass (10 mg) and KBr in a ratio (w/w) of 1:100. The mixtures were ground well, and the spectra were recorded in the range of 400−4000 cm−1 using 200 mg of biomass + KBr mixture in the form of pellets. XRD Analysis. The extent of delignification obtained in different experimental protocols was also assessed using an Xray diffractometer (Make: Bruker, Germany, Model: D8 Advance) with biomass crystallinity as the yardstick. The diffractometer was operated at 40 kV and 40 mA with Cu−Kα radiation (λ = 1.54 Å). Samples were scanned over the diffraction angle range of 2θ = 5°−30° with a step size of 0.05°. The crystallinity index (CrI) of all the samples was calculated as per the formula given by Segal et al.24 CrI(%) =

Icrystalline − Iamorphous Icrystalline

3. MATHEMATICAL MODEL FOR CAVITATION BUBBLE DYNAMICS We have used simulations of cavitation bubble dynamics to get a quantitative estimate of the physical and chemical effects induced by ultrasound and cavitation bubbles under different experimental conditions. For these simulations, we have used the diffusion limited ordinary differential equations (ODE) model (with boundary layer approximation).29 This model is based on the postulate proven by the partial differential equation (PDE) model of Storey and Szeri,30 which showed that vapor entrapment in the cavitation bubble during its radial motion is essentially a diffusion limited process. Storey and Szeri30 showed that not all of the solvent vapor which evaporates into the cavitation bubble during its expansion in the radial motion can escape in the ensuing compression phase. The entrapped vapor is subjected to the extreme conditions of temperature and pressure reached in the bubble during collapse. At these conditions, the vapor molecules can get dissociated into smaller fragments or chemical species. For the convenience of the reader, we have given the main components of the model and relevant thermodynamic data/boundary conditions in the Supporting Information. For greater details on this model, we refer the reader to our previous papers.31−33 The main components of the model are a set of 4 ordinary differential equations (ODEs) as follows: (1) Keller−Miksis equation for the radial motion of the bubble,34 (2) Equation for the diffusive flux of solvent vapor through bubble wall (or gas− liquid interface); (3) Equation for heat conduction through bubble wall; (4) Overall energy balance treating the cavitation bubble as an open system. The set of ODEs in the bubble dynamics model can be solved simultaneously using the Runge−Kutta adaptive step size method.35 The liquid medium in the present study is an alkaline solution with different NaOH concentrations and temperatures. The physical properties of the liquid medium were obtained either from the literature or by experimental measurement. The properties of density, vapor pressure, and viscosity of NaOH solutions were obtained from the literature.36,37 The surface tension of these solutions was measured using a tensiometer (Make: KRUSS GmbH, Germany and Model: K9-Mk1). These properties have been listed in Table 2. The biomass concentration in the delignification experiments was quite small, i.e. 2% w/v. Therefore, the amount of lignin from biomass getting dissolved in solution during the process is too small to cause any major change in the physical properties of the solution, which would in turn affect the radial motion of cavitation bubbles. Moreover, the biomass particles are soft (with low elasticity modulus) and of size = 1 mm. This size is much smaller than the wavelength of ultrasound, i.e. 7.5 cm for 20 kHz frequency. Due to these, the scattering and attenuation effect rendered by the biomass on ultrasound waves is also negligible. It is however possible some repolymerization and lignin condensation may occur in the bubble−bulk liquid interfacial region (due to reaction between the radicals generated by the bubble and the components of lignin dissolved in the liquid), which we have discussed in the result and discussion section. However, the reaction kinetics inside the cavitation bubble will stay unchanged as lignin is unlikely to evaporate into the bubble. Another important parameter in bubble dynamics model is the

× 100

where Icrystalline = intensity of the crystalline peak at 2θ = 22° and Iamorphous = intensity of the amorphous peak at 2θ = 18°. 2.6. Enzymatic Hydrolysis of Delignified P. hysterophorus Biomass. The biomass residue after the delignification process was separated by filtration through a double layered nylon cloth. The biomass residue (comprising of mainly cellulose) was washed with hot water several times to remove traces of NaOH and until neutral pH was obtained. This residue was dried for 12 h in a hot air oven at 60 ± 3 °C and further used for enzymatic hydrolysis. The delignified P. hysterophorus biomass was subjected to enzymatic hydrolysis using the enzymes carboxymethylcellulase (CMCase, 1.0 U/ mg), produced by Bacillus amylolique-faciens SS3525,26 and βglucosidase from Aspergillus niger (Novozyme 188). The experiment was performed by using 5% w/v biomass loading, 200 U/g of biomass CMCase loading, and 25 U/g of biomass β-glucosidase loading in sodium acetate buffer (0.05 M, pH 5.0). The reactions were carried out in a 150 mL Erlenmeyer flask with a total reaction volume of 20 mL. 0.005% w/v sodium azide was added to the reaction mixture to avoid contamination. The flask was incubated at 30 °C and 150 rpm in an incubator shaker (Orbitek, Scigenics Biotech). The 14243



Article

Table 2. Effect of NaOH Concentration on the Delignification Processa,d lignin contentc in residue (mg/g)

NaOH loading (%, w/v) 0.5 1.0 1.5 2.0 2.5 3.0

US 107.3 104.0 69.5 75.2 74.4 74.6

± ± ± ± ± ±

MA 1.4 2.5 2.4 1.1 1.8 2.2

134.0 132.1 121.2 108.7 94.7 91.0

± ± ± ± ± ±

2.8 1.2 1.5 2.3 1.4 1.1

Vturb(r , t ) =

lignin removal (%)

R2 ⎛⎜ dR ⎞⎟ r 2 ⎝ dt ⎠

Pressure amplitude of shock waves (or acoustic waves):39−41

US

MA

enhancement in lignin removal with ultrasoundb (%)

66.0 67.1 78.0 76.2 76.5 76.4

57.6 58.2 61.7 65.6 70.0 71.2

14.6 15.3 26.4 16.2 9.3 7.3

PAW(r , t ) =

2 ρL d 2Vb R ⎡ ⎛⎜ dR ⎞⎟ d 2R ⎤ ⎥ ⎢ = ρ + 2 R L 4πr dt 2 r ⎣ ⎝ dt ⎠ dt 2 ⎦

where Vb is the volume of the bubble. A representative value of r is taken as 1 mm.

4. RESULTS AND ANALYSIS 4.1. Experimental Results. Compositional Analysis of P. hysterophorus. Raw P. hysterophorus biomass consisted of cellulose 45.2 ± 1.81% (w/w), hemicellulose 26.6 ± 1.23% (w/ w), and lignin 23.6 ± 0.83% (w/w). After dilute acid pretreatment in the autoclave, the biomass residue was found to contain cellulose and lignin as the main components. The composition of biomass residue after this pretreatment was as follows: cellulose 64.1 ± 2.5% (w/w), lignin 31.6 ± 1.04% (w/ w), and hemicellulose 1.02 ± 0.073% (w/w).3 Delignification of pretreated biomass resulted in cellulose content of 96.1 ± 0.94% (w/w) with traces of lignin 3.16 ± 0.07% (w/w) in delignified biomass, where ultrasound-assisted delignification was carried out under best values of parameters. These results indicate that most of the lignin content of the biomass was removed during delignification and maximum cellulose content was available for enzymatic hydrolysis. Parametric Investigation of Delignification. The results of the experiments for parametric investigation of delignification are presented in Tables 2−4. From these results the following definitive trends in delignification with experimental parameters could be identified: (1) The lignin removal in ultrasonic treatment at ambient temperature shows saturation with increasing NaOH concentration for ultrasonic treatment, as evident from the results depicted in Table 2. After NaOH concentration of 2% w/v, lignin removal stays practically constant. However, for mechanical agitation, a gradual rise in lignin removal is seen until NaOH concentration of 2.5% w/v. The percentage enhancement in lignin removal with ultrasound shows maxima at NaOH concentration of 1.5% w/v. (2) Table 3 shows the influence of biomass concentration on lignin removal at ambient temperature. In these experiments,

a

US = sonication of reaction mixture; MA = mechanical agitation of reaction mixture. bCalculated as (US − MA) × 100/MA. cLignin content values are mean ± std error (n = 2). dBiomass concentration = 3% w/v, temperature = 30 °C, time of treatment = 15 min).

initial or equilibrium radius of the cavitation bubbles. The bubble population in the liquid medium usually has a wide distribution. In the present simulations we have considered two representative values of initial bubble radii, viz. Ro = 5 and 10 μm. 3.1. Quantification of Physical and Chemical Effects of Ultrasound and Cavitation. Microstreaming Due to Ultrasound. The small amplitude of yet rapid oscillatory motion of fluid elements as ultrasound wave propagates through the medium is called microstreaming. This phenomenon gives rise to intense micromixing in the medium. The magnitude of the microstreaming velocity (u) is dependent on the pressure amplitude (PA) of the ultrasound wave as u= PA/ ρc. Substituting values of PA as 1.7 × 105 Pa, ρ = 995 kg/m3, and c = 1481 m/s, gives u = 0.115 m/s. Chemical Effect of Cavitation Bubbles (Sonochemical Effect). The diffusion limited mathematical model used in this study can predict the temperature and pressure reached in the cavitation bubble and also the number of gas and solvent molecules present inside the bubble at the moment of transient collapse. Due to very high temperature as well as concentration of species (due to extremely small volume of the bubble) inside the bubble, the kinetics of the reactions among these species is several orders of magnitude higher than the time scale of bubble dynamics (which is same as the period of the acoustic wave i.e. 50 μs for 20 kHz frequency).32,38 Therefore, thermodynamic equilibrium can be assumed to prevail inside the bubble at all times during radial motion. The equilibrium mole fraction of different chemical species in the bubble at the peak conditions reached at transient collapse can be calculated using Gibbs free-energy minimization technique.33 Physical Effect of Cavitation Bubbles. As noted earlier, radial motion of cavitation bubbles generates intense convection in the medium through two phenomena, viz. microconvection, shock or acoustic waves. Using time history of radial motion of a cavitation bubble (in terms series of radius, R, and bubble wall velocity, dR/dt versus time, t) obtained from the numerical solution of a bubble dynamics model, magnitudes of these entities can be determined as follows: Microconvection velocity:34

Table 3. Effect of Biomass Concentration on the Delignification Processb lignin contenta in residue (mg/g) biomass loading (%, w/v)

US

0.5 2.0 3.5 5.0 6.5 8.0

26.9 ± 1.0 31.6 ± 1.4 83.7 ± 0.7 96.4 ± 1.3 139.7 ± 1.6 149.8 ± 2.1

MA 29.4 43.0 84.0 86.49 95.25 94.7

± ± ± ± ± ±

0.7 0.5 1.3 0.8 1.4 1.0

lignin removal (%)

US

MA

enhancement in lignin removal with ultrasound (%)

91.5 90.0 73.5 69.5 55.8 52.6

90.7 86.4 73.4 72.6 69.9 70.0

0.9 4.2 0.1 −4.3 −20.2 −24.9

a Lignin content values are mean ± std error (n = 2). bNaOH concentration = 1.5% w/v with US and 2.5% w/v with MA, temperature = 30 °C, time of treatment = 15 min.

14244



Article

Table 4. Effect of Temperature on the Delignification Processa lignin content§ in residue (mg/g) temperature (°C) 30 40 50 60 70 80

US 31.3 30.9 30.8 30.7 30.6 21.7

± ± ± ± ± ±

0.75 0.52 0.42 0.59 0.25 0.43

lignin removal (%)

MA

US

MA

enhancement in lignin removal with ultrasound (%)

± ± ± ± ± ±

90.1 90.2 90.3 90.3 90.3 93.1

73.5 77.3 83.9 84.9 86.7 87.1

22.6 16.7 7.6 6.4 4.2 6.9

83.7 71.7 50.9 47.5 42.0 40.7

0.52 0.37 1.09 0.46 0.35 0.62

§

Lignin content values are mean ± std error (n = 2). aBiomass concentration = 2% w/v, NaOH concentration = 1.5% w/v with US and 2.5% w/v with MA, time of treatment = 15 min.

Table 5. Kinetic Analysis of the Delignification Processc,e lignin contentd in residue (mg/g) time (min) 5 10 15 20 25 30

US 45.2 31.6 31.3 28.1 32.2 34.4

± ± ± ± ± ±

0.82 0.73 0.49 0.43 0.32 0.89

lignin removal with ultrasound

lignin removal with mechanical agitation

MA

% removal

kUSa (min−1)

% removal

kMAb (min−1)

± ± ± ± ± ±

85.7 90.0 90.1 91.1 89.8 89.1

0.262

73.3 81.0 86.4 87.1 87.5 89.1

0.126

84.4 60.0 42.8 40.7 39.6 34.4

0.76 0.68 0.46 0.37 0.30 0.86

kUS − pseudo-first-order kinetic constant for lignin removal with ultrasound. bkMA − pseudo-first-order kinetic constant for lignin removal with mechanical agitation. cEnhancement due to ultrasound: kUS/ kMA = 2.08. dLignin content values are mean ± std error (n = 2). eBiomass concentration = 2% w/v, NaOH concentration = 1.5% w/v with US and 2.5% w/v with MA and temperature = 30 °C with US and 80 °C w/v with MA. a

Kinetics of Delignification. The time profile of delignification with mechanical agitation and ultrasound treatment is given in Table 5. This data has been analyzed using pseudofirst-order kinetics (as an approximation of the complex delignification process). The graphical plots of pseudo-firstorder kinetic expression fitted to delignification data are given in the Supporting Information. The kinetic data for ultrasonic treatment shows that the extent of delignification remains constant after the first 10 min of treatment. In the case of mechanical agitation, the saturation in delignification is reached after 20 min of treatment. In view of these observations, the pseudo-first-order kinetic constant for delignification has been calculated as 0.262 min−1 using data for 10 min of ultrasoundassisted treatment. For a mechanically agitated system the kinetic constant has been calculated as 0.126 min−1 using data for 20 min. The ratio of two kinetic constants gives a quantitative estimate of the enhancement effect of ultrasound on the delignification process. Ultrasound enhances the kinetics of the process more than 2-fold. An interesting observation is that the lignin content of biomass for a longer time of ultrasound treatment (30 min) is higher as compared to the short duration of treatment of 20 min. This result is attributed to lignin condensation or repolymerization, which is induced by the radicals generated out of transient cavitation. These radicals are intercepted or scavenged by the soluble lignin moieties present in the interfacial region of the bubble, which gives rise to formation of phenoxy or similar radicals that induce repolymerization of lignin. The lignin droplets thus formed can get deposited on the biomass surface. This phenomenon is reflected in higher lignin content of the residual biomass for longer duration of ultrasonic treatment. Garcia et al.42 have also observed recondensation of lignin for longer duration of ultrasound treatment. The biomass concentration used by Garcia et al.42 (7% w/v) was much higher than used in our study (2% w/v).

the concentration of NaOH has been maintained at the best value of 1.5% w/v for ultrasonic treatment as per results of an earlier set of experiments. An interesting trend is seen in these experiments that the lignin removal with ultrasound reduces as the biomass concentration exceeds 2% w/v. Lignin removal for 8% w/v biomass concentration is nearly half of that at 2% w/v. A similar trend is seen for mechanical agitation; however, the effect of lignin reduction with biomass concentration is less marked. In the biomass concentration range of 5−8% w/v, lignin removal with mechanical agitation exceeds that with ultrasound. The best value of biomass concentration, for which maximum enhancement in delignification using ultrasonic treatment was seen (in comparison to mechanical agitation), was 2% w/v. (3) Table 4 depicts the effect of temperature on the delignification at the best values of NaOH concentration and biomass concentration determined earlier. As per these results, the extent of lignin removal with temperature remains practically the same for ultrasound in the range 30−80 °C, while for mechanical agitation a marginal rise of ∼16% is seen for the same temperature range. The percentage enhancement in lignin removal with ultrasound (in comparison to mechanical agitation) is highest for 30 °C, and thus this temperature is considered as the best value of temperature. Thus, on the basis of these experiments, the set of best values of experimental parameters for ultrasonic delignification are obtained as temperature = 30 °C, NaOH concentration = 1.5% w/v, and biomass concentration = 2% w/v. Similarly, the set of best values of the parameters for delignification with mechanical agitation are temperature = 80 °C, NaOH concentration = 2.5% w/v, and biomass concentration = 2% w/v. The composition of biomass after ultrasound-assisted delignification using the best values of the parameters (as mentioned above) was cellulose 96.1 ± 0.94% (w/w) and lignin 3.16 ± 0.07% (w/w). 14245



Article

Garcia et al.42 have noted that ultrasound treatment could yield effective delignification for treatments of long duration. Direct quantitative comparison between the results of Garcia et al.42 and our results is not possible due to differences in the characteristics of the ultrasound equipment used in the treatment such as frequency and pressure amplitude of ultrasound, power level, and geometry of equipment. 4.2. Results of Bubble Dynamics Simulations. Tables 6 and 7 depict representative results of simulations of cavitation

Table 7. Results of Cavitation Bubble Dynamics Simulations: Effect of Temperature parameters for simulations species

Table 6. Results of Cavitation Bubble Dynamics Simulations: Effect of NaOH Concentration parameters for simulations species

N2 O2 NO OH O H2O NO2 HO2 H N2O H2 HNO HNO2 N O3 H2O2

Ro = 5 μm Ro = 5 μm NaOH = 0.5% w/v NaOH = 1.5% w/v T = 303 K T = 303 K conditions at the first collapse of the bubble Tmax = 4441 K Tmax = 4444 K Pmax = 9023 atm Pmax = 9062 atm Vturb = 0.0071 m/s Vturb = 0.007 m/s PAW = 65.17 atm PAW = 66.4. atm NN2 = 1.27 × 10010 NN2 = 1.26 × 10010 NO2 = 3.39 × 1009 NO2 = 3.35 × 1009 09 NW = 4.32 × 10 NW = 4.22 × 1009 equilibrium composition of radical and other the bubble at collapse 7.09 × 10−01 7.09 × 10−01 −01 1.33 × 10 1.33 × 10−01 −01 1.19 × 10 1.20 × 10−01 1.23 × 10−02 1.23 × 10−02 1.23 × 10−02 1.23 × 10−02 −03 9.49 × 10 9.43 × 10−03 −03 2.20 × 10 2.21 × 10−03 7.12 × 10−04 7.12 × 10−04 6.11 × 10−04 6.11 × 10−04 −04 5.97 × 10 5.99 × 10−04 −04 3.36 × 10 3.35 × 10−04 1.06 × 10−04 1.06 × 10−04 2.06 × 10−04 1.67 × 10−04 −05 6.66 × 10 6.71 × 10−05 2.96 × 10−05 2.98 × 10−05 1.94 × 10−05 1.94 × 10−05

Ro = 5 μm NaOH = 3.0% w/v T = 303 K

N2 O2 NO OH O H2O NO2 HO2 H N2O H2 HNO HNO2 N O3 H2O2

Tmax = 4443 K Pmax = 9093 atm Vturb = 0.0069 m/s PAW = 67.43 atm NN2 = 1.25 × 10010 NO2 = 3.32 × 1009 NW = 4.10 × 1009 oxidizing species in 7.08 1.33 1.20 1.23 1.23 9.40 2.22 7.12 6.07 6.01 3.32 1.06 2.07 6.67 2.99 1.94

× × × × × × × × × × × × × × × ×

10−01 10−01 10−01 10−02 10−02 10−03 10−03 10−04 10−04 10−04 10−04 10−04 10−04 10−05 10−05 10−05

Ro = 5 μm Ro = 5 μm NaOH = 1.5% w/v NaOH = 1.5% w/v T = 313 K T = 333 K conditions at the first collapse of the bubble Tmax = 4253 K Tmax = 3441 K Pmax = 8575 atm Pmax = 5310 atm Vturb = 0.0075 m/s Vturb = 0.0142 m/s PAW = 62.43 atm PAW = 21.15 atm NN2 = 1.21 × 10010 NN2 = 1.18 × 10010 NO2 = 3.22 × 1009 NO2 = 2.97 × 1009 NW = 9.10 × 1009 NW = 6.14 × 10010 equilibrium composition of radical and other the bubble at collapse 7.14 × 10−01 7.28 × 10−01 1.39 × 10−01 1.63 × 10−01 1.11 × 10−01 6.66 × 10−02 −02 1.13 × 10 7.41 × 10−03 −03 9.46 × 10 2.33 × 10−03 1.11 × 10−02 2.98 × 10−02 −03 2.18 × 10 1.64 × 10−03 −04 6.78 × 10 4.77 × 10−04 4.41 × 10−04 9.12 × 10−05 5.34 × 10−04 2.43 × 10−04 −04 2.90 × 10 1.68 × 10−04 −05 8.79 × 10 3.03 × 10−05 2.08 × 10−04 1.85 × 10−04 3.83 × 10−05 2.62 × 10−05 2.04 × 10−05 2.43 × 10−05

Ro = 5 μm NaOH = 1.5% w/v T = 353 K Tmax = 1672 K Pmax = 195.8 atm Vturb = 0.0442 m/s PAW = 1.01 atm NN2 = 9.95 × 1009 NO2 = 2.64 × 1009 NW = 4.12 × 10011 oxidizing species in 2.30 6.17 8.18 1.06

× × × ×

10−02 10−03 10−05 10−04

1.31 × 10−05

2.04 × 10−05

the cavitation bubble reach extreme values for ambient temperature of 303 K due to which the water vapor molecules entrapped in the bubble are dissociated into oxidative radicals like •OH and •O. Shock waves of amplitude >50 bar are also generated. It could be seen that physical and chemical effects of a cavitation bubble remain unaltered with NaOH concentration. The peak temperature and pressure reached in the bubble and the spectrum of chemical species generated in the bubble at transient collapse, in addition to the magnitude of the microturbulance velocity and acoustic waves generated by the bubble, are practically the same for all three concentrations of NaOH. The major oxidizing radical species generated in the bubble are •OH and •O. Effect of temperature on cavitation bubble dynamics shows interesting trends. The extent of water vapor entrapment in the bubble at the moment of transient collapse increases markedly with temperature. This is a consequence of large vapor pressure of water at higher temperature. Water vapor entrapped in the bubble not only reduces the peak temperature attained in the bubble at transient collapse but also cushions the collapse reducing the intensity of shock waves. The intensity of the bubble collapse (or the energy concentration at collapse) reduces concurrently, and the associated physical/chemical effects also diminish. At 353 K (80 °C), practically no oxidative radical formation is seen during bubble collapse. The magnitude of shock waves also shows reduction of an order magnitude.

bubble dynamics for a 5 μm air bubble. These tables list the peak temperature and pressure generated in the bubble at the moment of transient collapse, the amount of gas and vapor molecules present in the bubble at the instance of collapse, and the equilibrium composition of the species generated from the dissociation of these molecules in the bubble. The graphical simulations results, viz. radial dynamics of the cavitation bubble along with other effects (i.e., water vapor evaporation in the bubble, variation of temperature and pressure inside the bubble, and generation of microturbulence and shock waves), are shown in Figures 1 and 2 for a 5 μm air bubble at bulk temperature of 303 and 353 K. The complete simulations for 5 and 10 μm cavitation bubbles are given in the Supporting Information. The results given in Tables 6 and 7 essentially depict the influence of two parameters, viz. NaOH concentration and temperature, on caviation bubble dynamics for the representative values (0.5, 1.5, 3.0% w/v, for NaOH concentration and 313, 333, and 353 K for temperature). The simulation results reveal that temperature and pressure in 14246



Article

Figure 1. Representative simulation results (5 μm air bubble at 303 K, NaOH conc. 1.5% w/v). Time variation of (A) normalized bubble radius (R/Ro); (B) temperature in the bubble; (C) number of water molecules in the bubble; (D) pressure inside the bubble; (E) microturbulence generated by the cavitation bubble; (F) acoustic (or shock) waves emitted by the bubble.

Figure 2. Representative simulation results (5 μm air bubble at 353 K, NaOH conc. 1.5% w/v). Time variation of (A) normalized bubble radius (R/Ro); (B) temperature in the bubble; (C) number of water molecules in the bubble; (D) pressure inside the bubble; (E) microturbulence generated by the cavitation bubble; (F) acoustic (or shock) waves emitted by the bubble.

Explanation for the Trends in Delignification. The trends in the delignification process seen in the parametric investigation experiments can be explained as follows: (1) Leveling off of the extent of lignin removal with NaOH concentration could be attributed to a switch-over of the

delignification process from mass transfer limitation to kinetic limitation. Ultrasound and cavitation generates intense turbulence in the system due to which the accessibility of the biomass to ions/radicals is enhanced. Recent papers by Bussemaker and Zhang43,44 have also reported a beneficial 14247



Article

effect of enhanced turbulence generated in the system by ultrasound waves (with relatively higher frequencies of 376, 995, and 1179 kHz). After a certain concentration of NaOH, all biomass is accessible to the delignifying agent, and thereafter the process becomes kinetically controlled. Higher concentration of NaOH required in mechanical agitation for saturation of delignification is attributed to lower intensity of convection. (2) Reduction in extent of delignification with ultrasound at higher biomass concentration is attributed to large scattering and attenuation of ultrasound waves by the biomass particles as a result of which their pressure amplitude and hence the intensity of transient cavitation bubble collapse driven by these waves decreases. A similar effect in the case of a system with mechanical agitation can be explained in terms of nonexposure of all biomass to alkaline solution at higher concentrations of biomass. (3) As revealed by the simulations of cavitation bubble dynamics, the intensity of transient cavitation and its physical/ chemical effects reduces with temperature. However, the micromixing phenomenon in the delignification mixture is also contributed by the microstreaming (i.e., oscillatory motion of fluid elements) due to ultrasound, which remains unaffected by the temperature. Moreover, the intrinsic kinetics of various reactions induced by OH− ions in the delignification process increases with temperature. The rise in intrinsic kinetics compensates for the reduction of physical/chemical effect of transient cavitation due to which the net delignification obtained at different temperature remains constant. For the mechanically agitated system, the delignification shows a gradual rise with temperature due to higher diffusivity of ions at higher temperature and increased reactivity, as noted earlier. 4.3. Characterization of Delignified Biomass. FTIR Analysis. The change in composition of biomass after delignification with either mechanical agitation or ultrasound treatment was assessed using FTIR analysis. The FTIR spectra of pretreated biomass, delignified biomass with mechanical agitation and with ultrasound (under best values of experimental parameters as obtained in parametric investigation described earlier: NaOH concentration = 1.5% w/v, temperature = 30 °C, and biomass concentration = 2% w/v) are depicted in Figure S1 of the Supporting Information. The vibrational frequencies (or band positions) in the IR spectrum corresponding to different functional groups of biomass and the percentage change in the intensities of these bands for biomass treated with mechanical agitation and ultrasound are given in Table 8. The positive relative percentage change of intensity for a specific band corresponding to a component (either functional group or moiety) indicates reduction of that component.45 As seen from the results of Table 8, the percent relative change is positive for all bands corresponding to lignin removal (along with side chain), rupture of cellulose bonds and carbohydrate−lignin linkages for biomass treated under mechanical agitation, and ultrasound irradiation. Comparing between the FTIR spectra for biomass treated under mechanical agitation and ultrasound, we find that reduction in band intensities corresponding to aromatic ring stretch (1595 cm−1), O−H stretching (3348 cm−1), and cellulose band (1428 cm−1, indicating decrystallization of cellulose) are higher for biomass delignified with ultrasound treatment. This essentially indicates enhanced delignification with ultrasound treatment. These results are attributed to the physical and chemical effects of ultrasound and cavitation.

Table 8. Characterization of Delignified Biomass by FTIR Spectroscopyb,c band position (cm−1) 3348 2900 1745 1738 1720 1595 1508 1458 1428 1260 1245 1059 900

assignmenta

US

MA

O−H stretching (rupture of cellulose hydrogen bonds) C−H stretching (rupture of methyl/ methylene group of cellulose) carbonyl bonds (lignin side chain removal) CO stretching due to carbohydrate linked with lignin carboxylic acids/ester groups aromatic ring stretch (related to lignin removal) aromatic ring vibration (related to lignin removal) aromatic ring vibration (related to lignin removal) band of cellulose ester absorbance (related to removal of uronic acid) CO absorption (resulting from acetyl groups cleavage) CO stretching due to carbohydrate− lignin linkage band of cellulose

16.20

−4.85

8.71

−0.76

10.42

0.92

8.28

0.14

7.23 22.11

−3.15 7.48

9.94

7.18

8.79

6.14

12.88 9.79

5.82 5.58

9.65

5.62

5.95

0.00

10.91

3.87

Adopted from Kumar et al.45 bUS − under ultrasound irradiation, MA − under mechanical agitation. c% relative change = 100 × (Intensity of band for untreated solid − intensity of band for treated solid)/intensity of band for untreated solid, where positive value of % relative change indicates reduction. a

The general introduction to ultrasonic delignification and its possible mechanisms is given in the Supporting Information. The reactive sites in lignin are mainly the ether linkages and functional groups, since the carbon−carbon linkages are generally resistant to chemical attack. In alkaline and alkaline−oxidative environments, lignin degrades through the cleavage of the α- and β-aryl ether linkages through hydrolysis to yield fragmentation units. Ultrasound can cause depolymerization of the solubilized lignin moieties through homolytic cleavages of the phenyl ether β−O−4 and α−O−4 bonds.46 Hydroxyl radicals produced from transient collapse of a cavitation bubble can also induce degradation of lignin leading to hydroxylated, demethoxylated, and side chain eliminated products. A small extent of hydroxyl radical attack also occurs on the side chains leading to formation of dimers and oxidation of aromatic aldehydes to carboxylic acids. In addition to cleavage of interunitary bonds within the lignin matrix and between lignin−hemicellulose entities, condensation of lignin in the presence of ultrasound has also been observed. Lignin degradation results in release of low molecular weight phenolic species that are soluble in the liquid medium and, therefore, get extracted in the medium during sonication. These species (which have phenolic moieties) accumulate at the interface of cavitation bubble and can scavenge the radicals released during transient collapse leading to repolymerization. The repolymerized lignin, in the form of small globules or droplets, can get redeposited on the biomass surface. In essence, there are two parallel mechanisms leading to delignification of the biomass: first the hydrolysis reactions induced by OH− ions provided by the delignification agent, and second, the hydroxylation and oxidation reactions induced by • OH and •O radicals generated by transient cavitation bubbles. 14248



Article

Figure 3. FESEM micrographs of P. hysterophorus biomass (A) pretreated biomass, (B) delignified biomass with mechanical agitation, and (C) delignified biomass with ultrasound.

Greater rupture of cellulose−hydrogen bonds and decrystallization of cellulose (which essentially are indications of the depolymerization of cellulose matrix) could also be the effect of interparticle collisions driven by shock waves generated by cavitation bubbles at transient collapse. Greater reduction in aromatic ring stretch as well as aromatic ring vibration bands (1595, 1508, 1458 cm−1) for ultrasound-treated biomass could be a consequence of hydroxylation/oxidation of an aromatic ring by oxidative radicals like •OH and •O produced by cavitation bubbles. In a similar way, greater reduction of intensities for bands corresponding to lignin side chain removal (1745 cm−1) and carbohydrate−lignin linkages (1738 cm−1) for ultrasound-treated biomass can be attributed to physical as well as chemical effects of cavitation bubbles. Thus, the FTIR spectrum is a clear evidence for the beneficial effects of ultrasound and cavitation on the delignification process. XRD Analysis. The XRD spectra of the pretreated biomass, delignified biomass with mechanical agitation, and ultrasound treatment under best values of the parameters have been depicted in Figure S2 of the Supporting Information. The pretreated biomass comprises of lignin and cellulose and has a crystallinity index (CrI) = 57.36%. As the amorphous lignin is removed during treatment, one would expect a rise in the crystallinity index of biomass due to an increase in crystalline cellulose content of residual biomass. However, contrary to this expectation, the crystallinity index is found to decrease for both

The relative contributions of these mechanisms to the overall delignification depend on the reaction conditions. As revealed in simulations of cavitation bubble dynamics, the intensities of physical as well as chemical effects of cavitation bubbles are the highest for the best temperature of 303 K (obtained in parametric investigation of ultrasound treatment). The biomass particles get drifted in random directions at extremely high velocities in the shock waves generated during transient collapse of the bubbles. During such drifts they can collide with each other, and such collisions result in release of high energies.47 The energy released in such collisions can also lead to the rupture of bonds in the lignin matrix (such as lignin−carbohydrate linkages) leading to depolymerization of lignin. The interparticle collisions may also cause local heating that can induce reaction and/or accelerate the kinetics of a reaction (in the present context the kinetics of hydrolysis of ether linkages).48−50 In recent papers published by Bussemaker and co-workers21,43,44 similar conclusions have been drawn regarding the mechanism of delignification in that the lignin polymers degrade through mechanoacoustic and sonochemical effects of ultrasound from a cumulative effect of hydroxyl radicals, shear forces, and degradation of hydrophobic polymers in the interfacial region of transient cavitation bubbles, where the temperatures shoot up to a few hundreds of Kelvin at the instance of transient collapse (when the bubble core itself heats up to ∼5000 K, as noted earlier). 14249



Article

5. CONCLUSIONS In this paper we have attempted to give mechanistic insight into the enhancement of the delignification process by ultrasound with NaOH as the delignifying agent. The approach has been to couple experiments with simulations of cavitation bubble dynamics. It has been revealed that both physical and chemical effects of ultrasound and cavitation contribute to delignification. The physical effects of transient cavitation, viz, microturbulence and shock waves, contribute to enhancement of depolymerization of lignin through hydrolysis. Microturbulence generates liquid flow through a biomass matrix that assists removal of monosaccharide units formed out of hydrolysis, while interparticle collisions driven by transient cavitation cause localized heating that boosts the kinetics of hydrolysis. The chemical effect of transient cavitation, i.e. radical generation, contributes to reduction in aromatic moieties in the lignin matrix by hydroxylation/oxidation. Transient cavitation is more intense at lower temperature, and the hydroxylation/oxidation induced by radicals generated from transient cavitation made significant contribution to overall delignification. The cavitation intensity reduces at higher temperature, but the intrinsic reactivity and kinetics of the ionic reactions is higher, due to which the contribution of hydrolysis induced by OH− ions (i.e., cleavage of α- and β-aryl ether linkages) to the depolymerization reaction is dominant. The intensity of transient cavitation and, hence, the magnitude of its physical and chemical effects is maximum at ambient or room temperature of 303 K. Thus, effective delignification is obtained under ultrasound treatment at room temperature, and with lower requirement (1.5% w/v) of the delignifying agent, as compared to the conventional technique of mechanical agitation. However, similar extent of delignification with ultrasonic treatment at elevated temperature of 353 K (where physical and chemical effects transient cavitation practically vanish) as compared to delignification at 303 K is a consequence of higher intrinsic reactivity of the delignifying agent that compensates for the physical and chemical effects of transient cavitation. A 2-fold enhancement in kinetics of delignification is seen with ultrasound treatment under the best operating conditions of temperature, biomass concentration, and alkali concentration.

ultrasonic treatment (CrI = 45.9%) and mechanical agitation (CrI = 46.9%). This essentially means that cellulose also undergoes partial depolymerization, with scission of β-1−4glycosidic bonds, giving rise to short chains of glucose monomer units. We attribute this effect to the interparticle collisions driven by shock waves generated by transient cavitation that cause localized heating at the point of impact inducing chemical reactions. FESEM Analysis. Figure 3 shows the FESEM micrographs of the pretreated biomass and delignified biomass with mechanical agitation and with ultrasound. The pretreated biomass (after acid + autoclaving treatment) clearly shows droplets of lignin that migrate to the surface of biomass (Figure 3A).51 The biomass surface after delignification with either mechanical agitation or ultrasound treatment shows removal of lignin droplets from the surface as revealed in Figure 3B and C. However, comparison of surface texture of the two biomasses reveals more surface roughness for ultrasound-treated biomass due to erosion or attrition. This erosion is attributed to physical effects of acoustic waves and the microturbulence generated by transient cavitation bubbles. 4.4. Enzymatic Hydrolysis of Delignified P. hysterophorus Biomass. Enzymatic hydrolysis of delignified P. hysterophorus biomass resulted in maximum reducing sugar concentration of 15.42 ± 0.33 g/L with sugar yield of 308.4 mg/g delignified biomass after 84 h of hydrolysis (Figure 4). This result indicates that the fermentation of the sugar obtained may lead to a theoretical ethanol titer of 7.86 g/L corresponding to 0.157 g ethanol/g delignified biomass.

■

ASSOCIATED CONTENT

S Supporting Information *

(1) Equations and thermodynamic data for the diffusion limited cavitation bubble dynamics model (including properties of NaOH solutions of different concentrations); (2) Graphical representation of the results of parametric investigations of delignification; (3) Graphical analysis of the kinetics of delignification; (4) Complete results of bubble dynamics simulations for 5 and 10 μm bubbles; (5) FTIR and XRD spectra of delignified biomass; (6) General introduction on sonochemical delignification. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 4. Time profile of enzymatic hydrolysis of delignified P. hysterophorus biomass.

The main aim of this study was to assess the effect of sonication on delignification from a mechanistic point of view. Thus, we have tried to find links between the physical and chemical effect of ultrasound and cavitation and the delignification process. We attempted to make biomass lignin free to the maximum possible extent so that the cellulose content in delignified biomass can be maximized, and thus the yield of sugars in enzymatic hydrolysis process is enhanced. However, the lignin dissolved in the solution during the delignification process can also be recovered by precipitation and can be utilized for various applications such as natural binder and adhesive, antioxidant, sub-bituminous coal, and sulfur-free solid fuel.52

■

AUTHOR INFORMATION

Corresponding Authors

*Phone: 91-361-258 2258. Fax: 91-361-258 2291. E-mail: [email protected]. *Phone: 91-361-258 2208. Fax: 91-361-258 2249. E-mail: [email protected]. Author Contributions #

These authors contributed equally.

14250



Article

Notes

(13) Li, Q.; Ji, G. S.; Tang, Y. B.; Gu, X. D.; Fei, J. J.; Jiang, H. Q. Ultrasound-assisted compatible in situ hydrolysis of sugarcane bagasse in cellulase−aqueous−N methylmorpholine−N−oxide system for improved saccharification. Bioresour. Technol. 2012, 107, 251−257. (14) Velmurugan, R.; Muthukumar, K. Ultrasound-assisted alkaline pretreatment of sugarcane bagasse for fermentable sugar production: Optimization through response surface methodology. Bioresour. Technol. 2012, 112, 293−299. (15) Young, F. R. Cavitation; McGraw Hill: London, 1989. (16) Leighton, T. G. The acoustic bubble; Academic Press: San Diego, 1994. (17) Shah, Y. T.; Pandit, A. B.; Moholkar, V. S. Cavitation reaction engineering; Plenum Press: New York, 1999. (18) Suslick, K. S. Sonochemistry. Science 1990, 247, 1439−1445. (19) Hart, E. J.; Henglein, A. Free radical and free atom reactions in the sonolysis of aqueous iodide and formate solutions. J. Phys. Chem. 1985, 89, 4342−4347. (20) Hart, E. J.; Henglein, A. Sonochemistry of aqueous solutions: H2-O2 combustion in cavitation bubbles. J. Phys. Chem. 1987, 91, 3654−3656. (21) Bussemaker, M. J.; Feng, Xu.; Zhang, D. Manipulation of ultrasonic effects on lignocellulose by varying the frequency, particle size, loading and stirring. Bioresour. Technol. 2013, 148, 15−23. (22) TAPPI. Technical Association of Pulp and Paper Industry; Atlanta, Georgia, USA, 1992. (23) Sivasankar, T.; Paunikar, A. W.; Moholkar, V. S. Mechanistic approach to enhancement of the yield of a sonochemical reaction. AIChE J. 2007, 53 (5), 1132−1143. (24) Segal, L.; Creely, J. J.; Martin, A. E., Jr.; Conrad, C. M. An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer. Text. Res. J. 1962, 29, 786− 794. (25) Singh, S.; Moholkar, V. S.; Goyal, A. Isolation, identification, and characterization of a cellulolytic Bacillus amyloliquefaciens strain SS35 from rhinoceros dung. ISRN Microbiol. 2013, 2013, No. 728134. (26) Singh, S.; Moholkar, V. S.; Goyal, A. Optimization of carboxymethylcellulase production from Bacillus amyloliquefaciens SS35. 3 Biotech 2014, 4 (4), 411−424. (27) Nelson, N. A photometric adaptation of the Somogyi method for the determination of glucose. J. Biol. Chem. 1944, 153, 375−380. (28) Somogyi, M. A new reagent for the determination of sugars. J. Biol. Chem. 1945, 160, 61−68. (29) Toegel, R.; Gompf, B.; Pecha, R.; Lohse, D. Does water vapor prevent upscaling sonoluminescence? Phys. Rev. Lett. 2000, 85, 3165− 3168. (30) Storey, B. D.; Szeri, A. J. Water vapor, sonoluminescence and sonochemistry. Proc. R. Soc. London, Ser. A 2000, 456, 1685−1709. (31) Kalva, A.; Sivasankar, T.; Moholkar, V. S. Physical mechanism of ultrasound-assisted synthesis of biodiesel. Ind. Eng. Chem. Res. 2009, 48, 534−544. (32) Krishnan, S. J.; Dwivedi, P.; Moholkar, V. S. Numerical investigation into the chemistry induced by hydrodynamic cavitation. Ind. Eng. Chem. Res. 2006, 45, 1493−1504. (33) Sivasankar, T.; Moholkar, V. S. Physical insights into the sonochemical degradation of recalcitrant organic pollutants with cavitation bubble dynamics. Ultrason. Sonochem. 2009, 16, 769−781. (34) Keller, J. B.; Miksis, M. J. Bubble oscillations of large amplitude. J. Acoust. Soc. Am. 1980, 68, 628−633. (35) Press, W. H.; Teukolsky, S. A.; Flannery, B. P.; Vetterling, W. T. Numerical Recipes; 2nd ed.; Cambridge University Press: New York, 1992. (36) Washburn, E. W. International critical tables of numerical data, physics, chemistry and technology; McGraw Hill: New York, 1928; Vol. 3. (37) Caustic Soda; SOLVAY Technical and Engineering Service; 1967; Bulletin 6. (38) Gong, C.; Hart, D. P. Ultrasound induced cavitation and sonochemical yields. J. Acoust. Soc. Am. 1998, 104, 2675−2682.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the Central Instrument Facility (CIF) at IIT Guwahati for providing the facility of the FESEM. Use of the XRD facility (procured through FIST grant No. SR/ FST/ETII-028/2010 from the Department of Science and Technology, Government of India) at the Department of Chemical Engineering, IIT Guwahati is also acknowledged. The authors acknowledge use of the FTIR spectrometer facility procured through the Indo−Finnish project grant from the Department of Biotechnology (DBT), Ministry of Science and Technology, Government of India. The authors express their gratitude towards the anonymous referees of this manuscript for their meticulous evaluation of the manuscript and constructive criticism.

■

LIST OF ABBREVIATIONS CrI = crystallinity index FESEM = field emission scanning electron microscope FTIR = Fourier transform infra-red (spectroscopy) MA = mechanical agitation US = ultrasound-assisted biomass treatment XRD = X-ray diffraction

■

REFERENCES

(1) Binod, P.; Sindhu, R.; Singhania, R. R.; Vikram, S.; Devi, L.; Nagalakshmi, S.; Kurien, N.; Sukumaran, R. K.; Pandey, A. Bioethanol production from rice straw: An overview. Bioresour. Technol. 2010, 101 (13), 4767−4774. (2) Ranjan, A.; Moholkar, V. S. Biobutanol: science, engineering, and economics. Int. J. Energy Res. 2012, 36, 277−323. (3) Singh, S.; Khanna, S.; Moholkar, V. S.; Goyal, A. Screening and optimization of pretreatments for Parthenium hysterophorus as feedstock for alcoholic biofuels. Appl. Energy 2014, 129, 195−206. (4) Barakat, A.; Chuetor, S.; Monlau, F.; Solhy, A.; Rouau, X. Ecofriendly dry chemo-mechanical pretreatments of lignocellulosic biomass: Impact on energy and yield of the enzymatic hydrolysis. Appl. Energy 2014, 113, 97−105. (5) Alvira, P.; Tomas-Pejo, E.; Ballesteros, M.; Negro, M. J. Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review. Bioresour. Technol. 2010, 101, 4851−4861. (6) Barakat, A.; de Vries, H.; Rouau, X. Dry fractionation process as an important step in current and future lignocellulosic biorefineries: A review. Bioresour. Technol. 2013, 134, 362−373. (7) Bussemaker, M. J.; Zhang, D. Effect of ultrasound on lignocellulosic biomass as a pretreatment for biorefinery and biofuels applications. Ind. Eng. Chem. Res. 2013, 52, 3563−3580. (8) Sun, R. C.; Sun, X. F.; Ma, X. H. Effect of ultrasound on the structural and physicochemical of organosolv soluble hemicelluloses from wheat straw. Ultrason. Sonochem. 2002, 9, 95−101. (9) Sun, R. C.; Tomkinson, J. Comparitive study of lignins isolated by alkali and ultrasound assisted alkali extractions from wheat straw. Ultrason. Sonochem. 2002, 9, 85−93. (10) Subhedar, P. B.; Gogate, P. R. Alkaline and ultrasound assisted alkaline pretreatment for intensification of delignification process from sustainable raw-material. Ultrason. Sonochem. 2014, 21, 216−225. (11) Yuan, T. Q.; Xu, F.; He, J.; Sun, R. C. Structural and physicochemical characterization of hemicelluloses from ultrasound-assisted extractions of partially delignified fast-growing poplar wood through organic solvent and alkaline solutions. Biotechnol. Adv. 2010, 28, 583− 593. (12) Baxi, P. B.; Pandit, A. B. Using cavitation for delignification of wood. Bioresour. Technol. 2012, 110, 697−700. 14251



Article

(39) Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; Wiley: New York, 1954. (40) Condon, E. U.; Odishaw, H. Handbook of Physics; McGraw Hill: New York, 1958. (41) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. Properties of Gases and Liquids; McGraw Hill: New York, 1987. (42) Garcia, A.; Alriols, M. G.; Llano-Ponte, R.; Labidi, J. Ultrasoundassisted fractionation of the lignocellulosic material. Bioresour. Technol. 2011, 102 (10), 6326−6330. (43) Bussemaker, M. J.; Zhang, D. A phenomenological investigation into the opposing effects of fluid flow on sonochemical activity at different frequency and power settings. 1. Overhead stirring. Ultrason. Sonochem. 2014, 21 (1), 436−445. (44) Bussemaker, M. J.; Zhang, D. A phenomenological investigation into the opposing effects of fluid flow on sonochemical activity at different frequency and power settings. 2. Fluid circulation at high frequencies. Ultrason. Sonochem. 2014, 21 (2), 485−492. (45) Kumar, R.; Mago, G.; Balan, V.; Wyman, C. E. Physical and chemical characterizations of corn stover and poplar solids resulting from leading pretreatment technologies. Bioresour. Technol. 2009, 100 (17), 3948−3962. (46) Yoshioka, A.; Seino, T.; Tabata, M.; Takai, M. Homolytic scission of interunitary bonds in lignin induced by ultrasonic irradiation of MWL dissolved in dimethylsulfoxide. Holzforschung 2000, 54 (4), 357−364. (47) Doktycz, S. J.; Suslick, K. S. Interparticle collisions driven by ultrasound. Science 1990, 247, 1067−1069. (48) Reddy, B. R.; Sivasankar, T.; Sivakumar, M.; Moholkar, V. S. Physical facets of ultrasonic cavitational synthesis of zinc ferrite particles. Ultrason. Sonochem. 2010, 17 (2), 416−426. (49) Choudhury, H. A.; Choudhary, A.; Sivakumar, M.; Moholkar, V. S. Mechanistic investigation of the sonochemical synthesis of zinc ferrite. Ultrason. Sonochem. 2013, 20 (1), 294−302. (50) Goswami, P. P.; Choudhury, H. A.; Chakma, S.; Moholkar, V. S. Sonochemical synthesis and characterization of manganese ferrite nanoparticles. Ind. Eng. Chem. Res. 2013, 52 (50), 17848−17855. (51) Donohoe, B. S.; Decker, S. R.; Tucker, M. P.; Himmel, M. E.; Vinzant, T. B. Visualizing lignin coalescence and migration through maize cell walls following thermochemical pretreatment. Biotechnol. Bioeng. 2008, 101 (5), 913−925. (52) Kamm, B.; Gruber, P. R.; Kamm, M. Biorefineries-Industrial processes and products: Status quo and future; Wiley-VCH Verlag GmbH and Co. KGa A: Weinheim, 2006.

14252


Mechanistic Investigation in Ultrasound-Assisted (Alkaline

Recommend Documents