Study of Almond Shell Characteristics

materials

Article Study of Almond Shell Characteristics

Xuemin Li, Yinan Liu, Jianxiu Hao and Weihong Wang * Key Laboratory of Bio-based Material Science & Technology (Education Ministry), Northeast Forestry University, Harbin 150040, China; [email protected] (X.L.); [email protected] (Y.L.); [email protected] (J.H.) * Correspondence: [email protected]

Received: 17 August 2018; Accepted: 12 September 2018; Published: 19 September 2018

Abstract: A large amount of almond shells are disposed of every year. The anatomical and chemical characteristics of almond shells are investigated in this paper in order to contribute to better utilization of these shells. The micromorphology, surface elements, thermal stability, crystallization, chemical composition, and relative properties of almond shells are analyzed. Under observation by microscope and electron microscope, the diameter of almond shells is 300–500 µm for large holes, and 40–60 µm for small holes present in the shell. X-ray photoelectron spectroscopy shows the elements of almond shells include C (72.27%), O (22.88%), N (3.87%), and Si (0.87%). The main chemical constituents of cellulose, hemicellulose and lignin in almond shells account for 38.48%, 28.82% and 29.54%, respectively. The alkaline extract content of almond shells is 14.03%, and benzene alcohol extraction is 8.00%. The benzene alcohol extractives of almond shells mainly contain 17 types of organic compound, including benzene ring, ethylene, carbon three bond, and other mufti-functional groups. Thermal stability analysis shows almond shells mainly lose weight at 260 ◦C and 335 ◦C. These characteristics indicate that almond shells have the capacity to be used in composites and absorption materials.

Keywords: almond shells; anatomical structure; chemical composition; thermal stability

1. Introduction Biomass nutshell is a large yield crop residue which is generally discarded or incinerated. This not only pollutes the environment, but also wastes a large amount of resources. In order to avoid this, many scholars have undertaken research into its potential uses, including the use of biomass nut shells to produce activated carbon. Ahmad et al. used peanut shell adsorption of trichloroethylene for cleaning water [1], while Lan et al. prepared activated carbon from Hawaii nut shells [2]. Martínew et al. prepared activated carbon from walnut shells, and studied its properties [3], and Yang et al. formulated high speciﬁc surface area activated carbon from coconut shells by microwave heating [4]. Okutucu et al. produced fungicidal oil and activated carbon from pistachio shells [5]. In addition to the manufacturing of activated carbon, biomass nut shells have a number of other uses. Wongcharee et al. used macadamia nut shell residue as magnetic nanosorbents [6], and Pino et al. studied the biosorption of cadmium using coconut shells [7]. Notably, biomass nut shell has also been used in composites. Alsaadi et al. studied the effect of the content of particles on the mechanical properties of polymer composites [8], and Bae et al. have prepared composite by macadamia nut shell for capturing CO2 [9]. Bledzki et al. studied the effects of ﬁber physics and chemistry on the properties of composites prepared from barley shell and coconut shell reinforced polypropylene [10], and Ayrilmis et al. manufactured chestnut shell reinforced polypropylene composite [11]. In addition, Kasiraman et al. used cashew nut shells as fuel for camphor oil blending [12], and Bartocci et al. proved that biomass shells improve soil health and fertility when used as oil amendment [13]. Wang et al. demonstrated that biocarbon could possibly replace fossil fuel, and help reduce greenhouse gas emissions [14].

Materials 2018, 11, 1782; doi:10.3390/ma11091782 www.mdpi.com/journal/materials Materials 2018, 11, 1782 2 of 12

Almonds are a type of biomass nut shell and are widely grown in many regions of the world including India, Pakistan, Iran, and China. As of 2014, global output of almonds was three million tons annually (data). Almond shells account for around 35–75% of the total fruit weight, so about 10.5–22.5 million tons of shells were left [15]. This huge quantity of shells has great economic and practical potential, and investigation into the best way to utilize the shell is attracting increasing attention. Rodríguez-Reinoso et al. and Toles et al. prepared activated carbon with almond shells, studying its physical, chemical and adsorptive properties, as well as estimated cost of production [16,17]. Senturk et al. have used almond shells as adsorbents to remove dye rhodamine from aqueous solutions [18]. Mohan et al. prepared magnetic and activated carbon by almond shells to remove 2,4,6-trinitrophenol from water [19], and Essabir et al. reinforced polypropylene with almond shells by extrusion process [20]. Lashgari et al. studied the effect of the loading amount of nano-clay and content of almond shell powder on the strength and properties of polypropylene composites [21]. Due to such studies into almond shell utilization, there is limited knowledge of almond shell structure and chemical properties. Caballero et al. studied the kinetics of slowing the thermal decomposition of almond shells, and analyzed the pyrolysis of lignin and whole cellulose [22]. In the studies of Essabir et al. and Elleuch et al., it is shown that almond shells are absorbent and environmentally friendly [20,23]. This article focuses on the basic chemical composition and physical structure of almond shells in order to make better use of this large quantity bio-resource.

2. Materials and Methods

2.1. Materials The almond shells used in this study were purchased from Anhui Liang Zhong Liang Food (Huai’an, China). Poplar wood particles, as a control, were purchased from Jinan Yuanfang Wood Trading Company (Jinan, China). Both almond shells and poplar particles were screened into 40–60 mesh and 60–80 mesh.

2.2. Chemical Composition The main chemical composition of almond shell particles of 40–60 mesh were measured according to Chinese standard GBT 35816-2018 (Standard Method for Determination of Structural Carbohydrates and Lignin in Biomass Raw Material). The content sum of hemicellulose and cellulose was determined by using glacial acetic acid and sodium hypochlorite as a solvent. After repeated washing, the polysaccharide in the almond shells was dissolved, then dried and measured. The content of cellulose was determined by using nitric acid and ethanol as solvent. Other non-cellulosic materials including lignin, hemicellulose, and other materials in the sample are eluted to obtain cellulose. The lignin content was determined by sulfate method. After the sulfuric acid solvent was removed, the residue was measured to determine the quality of lignin. Particles of 40–60 mesh almond shell were used to measure extractives according to GBT 35818-2018 (Standard Method for Analysis of Forestry Biomass-Determination of Extractives Content). Extractive content of almond shell was measured after the shells were treated with cold water for 48 h, hot water for 6 h, 1% NaOH for 1 h, or benzene alcohol (V benzene/V ethanol = 2:1) for 6 h. In addition, the benzene alcohol extractive solution was diluted to 0.1 mg/mL (V benzene/V ethanol = 2:1), and its chemical composition was measured by GC/MS (7890A-7000B, American Agilent Corporation, Santa Clara, CA, USA).

2.3. Infrared Spectrum Almond shells and poplar particles were scanned by Fourier infrared spectrometer (Nicolette 6700, American Water Corporation, Los Angeles, CA, USA). The KBr disk method was employed for the sample, and wavelength was 500–3500 cm−1 at a resolution of 5 cm−1. Materials 2018, 11, 1782 3 of 12

2.4. X-ray Diffraction Almond shells and poplar particles of 60–80 mesh were measured by X-ray diffraction (D/max-2200VPC, Japan science co., Ltd., Takatsuki City, Osaka, Japan) in the range of 2θ = 5–40◦ at scanning speed of 5 rad/cm. Two samples were repeated. The X-ray source was a Cu target (Cu Kα = 1.54056), and a nickel ﬁlter was used to excite alpha radiation. The crystallization index of the sample was calculated according to the protocol Formula (1) as follows [24]:

I − I CrI(%) = 002 am × 100% (1) I002 ◦ ◦ where I002 is the intensity at 2θ = 22 , and Iam is the intensity of background scatter at 2θ = 16 .

2.5. Surface Element Almond shells and poplar particles of 60–80 mesh were scanned by an X-ray photo-electron spectrometer (THERMO, American Thermoelectric Group, Waltham, MA, USA). Samples were excited with monochromatic Al Kα rays (1,486.6 eV). The sample binding energy (EB) charge correction was performed using a contaminated carbon C1s (284.8 eV). The X-ray beam was set at 6 mA, and the passing energy was 50 eV. The sputtering ion gun was 1000 eV, and the scanning step was 0.1 eV. Two samples were repeated.

2.6. Thermal Stability The thermal stability of almond shells and poplar particles was measured by high-precision thermo gravimetric analyzer (TG209F1, NETZSCH Scientiﬁc Instruments Trading, Bavaria, Germany). The temperature was increased from 0 ◦C to 800 ◦C at the rate of 10 ◦C/min. Two samples were repeated.

2.7. Microscopic Morphology The structure of almond shells was observed by a stereomicroscopic evaluation (JSZ6S, Jiangnan Yongxin Company, Guangzhou, China), and then by scanning electron microscopy (SEM) (QuanTa200, Dutch FEI company, Hillsboro, OR, USA). The cut cross-section of the almond shell was then sprayed with gold and observed at the acceleration voltage of 12.5 kV.

3. Results and Discussion

3.1. Microscopic Morphology Almond shells have well-developed pore structure, as illustrated in Figure1, where a row of large holes can be seen distributed in the cross section. Diameter of the big holes is 300–500 µm (Figure1b). The area around the holes is relatively dense; however, it is full of small hollow balls. Under SEM, almond shells appear to be composed of tightly compacted balls from the view of cross section shown in Figure1c. The diameter of these hollow balls is about 40–60 µm, and thickness of the ball wall is 20–40 µm. When enlarged 3000 times (Figure1d), tiny holes are found distributed on the wall of balls, and the wall is visibly layered. The almond shell wall is full of holes of varied diameter, making it light and potentially easily absorbed. Materials 2018, 11, 1782 4 of 12 Materials 2018, 11, x FOR PEER REVIEW 4 of 12

FigureFigure 1. 1.Almond Almond shell shell micro-topography. micro-topography. (a )(a Overall) Overall view; view; (b) ( Cross-sectionb) cross-section of shell;of shell; (c) Dense(c) dense part part of almondof almond shell; shell; (d) Hollow(d) hollow ball ball in thein the almond almond shell. shell. 3.2. Chemical Composition 3.2. Chemical Composition The content of cellulose in almond shells is 38.475%, which is lower than that of poplar (containing The content of cellulose in almond shells is 38.475%, which is lower than that of poplar 44.12% cellulose), but higher than other shells except pistachio shell (Table1). This indicates that the (containing 44.12% cellulose), but higher than other shells except pistachio shell (Table 1). This mechanical properties of composite prepared by almond shells might be higher than that of most indicates that the mechanical properties of composite prepared by almond shells might be higher biomass nut shells. than that of most biomass nut shells. Table 1. Proportion of cellulose, hemicellulose, lignin in six kinds of biomass (wt %). Table 1. Proportion of cellulose, hemicellulose, lignin in six kinds of biomass (wt %). Sample Cellulose Hemicellulose Lignin SampleAlmond shells 38.47Cellulose± 0.39 28.82Hemicellulose± 0.25 29.54 ± 0.11 Lignin Poplar 44.12 ± 0.23 30.21 ± 0.11 21.24 ± 0.31 AlmondCoconut shells shells 34.1238.47± ± 0.200.39 22.3628.82± 1.47 ± 0.25 28.04 ± 0.5729.54 ± 0.11 Walnut shells 36.38 ± 0.05 27.85 ± 0.31 43.70 ± 0.57 PoplarChestnut shells 21.4744.12± ± 0.270.23 16.2830.21± 0.35 ± 0.11 36.58 ± 0.2621.24 ± 0.31 Pistachio shells 43.08 ± 0.19 25.30 ± 0.46 16.33 ± 0.41 Coconut shells 34.12 ± 0.20 22.36 ± 1.47 28.04 ± 0.57 Note: The data of almond shells and poplar were measured by authors, and the rest of the measurements are derived from literature [25]. Walnut shells 36.38 ± 0.05 27.85 ± 0.31 43.70 ± 0.57

LigninChestnut is an amorphous shells substance21.47 without ± 0.27 fixed structure;16.28 however,± 0.35 it is three-dimensional36.58 ± 0.26 and highly branched. Lignin generally surrounds microfibers and large fibers. The covalent bond between Pistachio shells 43.08 ± 0.19 25.30 ± 0.46 16.33 ± 0.41 lignin and polysaccharide greatly enhances the bonding strength between the cellulose fiber and the ligninNote: matrix, The and data thus of playsalmond a role shells in bondingand poplar and we strengthening.re measured Theby authors, lignin content and the of rest almond of the shells is 29.54%,measurements higher than are derived that of from poplar, literature which [25]. contains 21.24%. Lignin may be beneficial to improve compatibility with thermoplastic, and it has a certain flame-retardant effect which is important for compositesLignin beingis an amorphous used as building substance materials. without The fixed special structure; ring however, structure it and is three-dimensional hydroxide structure and ofhighly hemicellulose branched. produces Lignin agenerally reactive activitysurrounds generally microfibers higher and than large that offibers. cellulose. The Thecovalent content bond of hemicellulosebetween lignin in and almond polysaccharide shells and greatly poplar enhances is similar, the and bonding higher strength than other between nuts the listed cellulose in Table fiber1. Thisand indicatesthe lignin that matrix, almond and shells thus may plays have a role more in prospectsbonding forand modification. strengthening. The lignin content of almondThe extractiveshells is 29.54%, content higher of almond than shellsthat of and poplar, poplar which is presented contains in 21.24%. Table2 .Lignin Extractives may be from beneficial alkali methodto improve were compatibility the highest, followedwith thermoplastic, by benzene and and it cold has water.a certain This flame-retardant result is related effect to solubility which is inimportant different for solvents. composites Cold being water used extractives as building of almond materials. shells The werespecial 3.14%, ring structure and poplar and was hydroxide 0.11%. Thisstructure indicates of hemicellulose that almond shells produces contain a reactive more terpenes, activity phenols, generally hydrocarbons, higher than andthat ester of cellulose. compounds The thancontent poplar of hemicellulose [26]. Terpenes in and almond phenols shells are the and main poplar component is similar, of and fragrance, higher thus than almond other nuts shells listed could in potentiallyTable 1. This be usedindicates as a rawthat materialalmond shells for creating may have essence. more The prospects content for of hotmodification. water extract was higher than thatThe ofextractive cold water content and this of isalmond due to shells the difference and popl inar temperature is presented from in Table solubility. 2. Extractives from alkali method were the highest, followed by benzene and cold water. This result is related to solubility in different solvents. Cold water extractives of almond shells were 3.14%, and poplar was 0.11%. This indicates that almond shells contain more terpenes, phenols, hydrocarbons, and ester compounds than poplar [26]. Terpenes and phenols are the main component of fragrance, thus almond shells could potentially be used as a raw material for creating essence. The content of hot water extract was higher than that of cold water and this is due to the difference in temperature from solubility.

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In In Table Table 3, 3, it it can can be be seen seen that that there there are are more more naphthenic naphthenic preparehydrocarbonshydrocarbonsprepareof monocyclic cosmetics. cosmetics. in inthe aromatic the almond almond hydrocarbons. shells, shells, so soit hasit has This the the indicatespote potentialntial to that tobe the beused used shell as asa contains raw a raw material material functional for for fuel, groupsfuel, or orto and to hydrocarbonshydrocarbons in in the the almond almond shells, shells, so so it it has has the the pote potentialntial to to be be used used as as a araw raw material material for for fuel, fuel, or or to to prepareprepareactive cosmetics. chemicalcosmetics. properties. TableTable 3. 3. Compound InCompound Table3 ,shell it shell can containing containing be seen thatex extractstracts there of of almond are almond more shells. shells. naphthenic hydrocarbons in prepareprepare cosmetics. cosmetics. TableTable 3. Compound3. Compound shell shell containing containing ex tractsextracts of ofalmond almond shells. shells. the almond shells,Table soTable it has 3. Compound 3. the Compound potential shell shell to containing be containing used as ex atracts ex rawtracts of material almondof almond forshells. shells. fuel, or to prepare cosmetics. ChemicalChemical MolecularMolecular StructuralStructural ChemicalChemical MolecularMolecular StructuralStructural ChemicalChemical MolecularTableMolecularTable 3. Compound3. Compound Structural shellStructural shell containing containing ex tractsexChemicaltractsChemical of almondof almond Molecularshells.Molecular shells. StructuralStructural CompoundCompound TableFormulaTableFormula 3. 3. Compound Compound Formulashell shellFormula containing containing ex CompoundextractsCompoundtracts of of almond almond shells.Formula shells.Formula FormulaFormula CompoundChemicalCompoundChemical MolecularFormulaMolecularFormulaTable 3. Compound StructuralFormulaStructuralFormula shell containing CompoundChemical extractsCompoundChemical of almond MolecularFormulaMolecularFormula shells. StructuralFormulaStructuralFormula CompoundChemicalCompoundChemical MolecularFormulaMolecularFormula StructuralFormulaStructuralFormula CompoundChemicalCompoundChemical MolecularFormulaMolecularFormula StructuralFormulaStructuralFormula ChemicalChemical MolecularMolecular StructuralStructural Cyclohexane,1,Cyclohexane,1,ChemicalChemical MolecularMolecular StructuralStructural Cyclopentane,CompoundCyclopentane,CompoundChemical FormulaMolecularFormula StructuralFormulaFormula Cyclohexane,1,CompoundCyclohexane,1,CompoundChemical FormulaFormulaMolecular FormulaStructuralFormula Cyclopentane,CompoundCyclopentane,Compound FormulaCFormula7CH7H14 14 FormulaFormula 2-dimethyl-,Compound2-dimethyl-,Compound FormulaCFormula8CH8H16 16 FormulaFormula ethyl-ethyl-Compound FormulaC7CH714H 14 Formula Cyclohexane,1,2-dimethyl-,Cyclohexane,1,2-dimethyl-,Compound C8FormulaCH816H 16 Formula Cyclopentane,Cyclopentane,ethyl-ethyl- ciscis- - C7HC147H 14 Cyclohexane,1,2-dimethyl-,Cyclohexane,1,2-dimethyl-,ciscis- - C8HC168H 16 Cyclopentane,Cyclopentane,ethyl-ethyl- Cyclohexane,1,Cyclohexane,1, Cyclopentane,Cyclopentane,Cyclopentane, C7HC147H 14 2-dimethyl-,Cyclohexane,1,2-dimethyl-,cis-cis - C8HC168H 16 ethyl-ethyl- CC7H77H14 14 2-dimethyl-,2-dimethyl-, CC8H8CH168 16H 16 ethyl-ethyl-ethyl- 2-dimethyl-,cis-cis - cis- Cyclohexane,Cyclohexane,ciscis- - TolueneToluene C7CH7H8 8 Cyclohexane,Cyclohexane, C8CH8H16 16 TolueneToluene C7CH78H 8 ethyl-ethyl- C8CH816H 16 Cyclohexane,Cyclohexane,ethyl-ethyl- TolueneToluene C7HC87 H8 C8HC168H 16 Cyclohexane,Cyclohexane,ethyl-ethyl- TolueneTolueneToluene C C7HC7H87 H88 Cyclohexane,Cyclohexane, C8HCC1688H H1616 TolueneToluene CC7H7H8 8 ethyl-ethyl- CC8H8H16 16 ethyl-ethyl- Cyclohexane,Cyclohexane, 1,3- 1,3- Cyclohexane,Cyclohexane, Cyclohexane,Cyclohexane, 1,3- 1,3- C8CH8H16 16 Cyclohexane,Cyclohexane, C9CH9H18 18 dimethyl-,dimethyl-,Cyclohexane, cis cis- - C8CH816H 16 1,1,3-trimethyl1,1,3-trimethyl C9CH918H 18 Cyclohexane,dimethyl-,Cyclohexane,dimethyl-, cis 1,3- cis- 1,3-- 1,1,3-trimethylCyclohexane,1,1,3-trimethylCyclohexane, 1,3-dimethyl-, CC8HCH168H 16 C9HCC189H H18 Cyclohexane,dimethyl-,Cyclohexane,dimethyl-, cis 1,3- -cis 1,3-- 8 16 1,1,3-trimethylCyclohexane,1,1,3-trimethyl1,1,3-trimethylCyclohexane, 9 18 8 16 9 18 Cyclohexane,Cyclohexane,cis 1,3- -1,3- C HC8H16 Cyclohexane,Cyclohexane, C HC9H18 dimethyl-,dimethyl-, cis -cis - CC8H8H16 16 1,1,3-trimethyl1,1,3-trimethyl CC9H9H18 18 dimethyl-,dimethyl-, cis cis- - 1,1,3-trimethyl1,1,3-trimethyl Cyclohexane,Cyclohexane, 1,1- 1,1- Cyclohexane,Cyclohexane,Cyclohexane, 1,1- 1,1- C8CH8H16 16 EthylbenzeneEthylbenzene C8CH8H10 10 dimethyl-dimethyl- CC8CHH816H 16 EthylbenzeneEthylbenzene C8CH C810H H10 Cyclohexane,Cyclohexane,dimethyl-dimethyl-1,1-dimethyl- 1,1- 1,1- 8 16 8 10 C8HC168H 16 EthylbenzeneEthylbenzene C8HC108H 10 Cyclohexane,Cyclohexane,dimethyl-dimethyl- 1,1- 1,1- Cyclohexane,Cyclohexane, 1,1- 1,1- C8HC168H 16 EthylbenzeneEthylbenzene C8HC108H 10 dimethyl-dimethyl- CC8H8H16 16 EthylbenzeneEthylbenzene CC8H8H10 10 dimethyl-dimethyl- CisCis- - Cyclopentane,1-Cyclopentane,1-Cyclopentane,1 Cis-bicyclo[4.2.0]CisCis- - Cyclopentane,1-Cyclopentane,1- CC8CH88H16 16 bicyclo[4.2.0]ocbicyclo[4.2.0]oc C8CH8CH148 14H 14 Materialsethyl-3-methyl-ethyl-3-methyl-Materials-ethyl-3-methyl- 2018 2018, 11,, 11x FOR, x FOR PEER PEERC REVIEW8CH8 16REVIEWH 16 bicyclo[4.2.0]ocbicyclo[4.2.0]ocCisoctaneCis- - C8CH814H 14 6 of6 12of 12 Cyclopentane,1-ethyl-3-methyl-ethyl-3-methyl-Cyclopentane,1- tanetane C8HC168H 16 bicyclo[4.2.0]ocbicyclo[4.2.0]octaneCistaneCis- - C8HC148H 14 Cyclopentane,1-ethyl-3-methyl-Cyclopentane,1-ethyl-3-methyl- CisCis- - Cyclopentane,1-Cyclopentane,1- C8HC168H 16 bicyclo[4.2.0]ocbicyclo[4.2.0]octanetane C8HC148H 14 ethyl-3-methyl-ethyl-3-methyl- CC8H8H16 16 bicyclo[4.2.0]ocbicyclo[4.2.0]oc CC8H8H14 14 (Z)-Hex-3-enylethyl-3-methyl-(Z)-Hex-3-enylethyl-3-methyl-(Z)-Hex-3-enyl(E) (E)- (E)- Hexane,3-taneHexane,3-tane Hexane,3-methyl-tanetane 2-methylbut-2- 2-methylbut-2--2-methylbut-2- CC11CHH1118HO182O O2 methyl-4-methyl-4- C8HCC168H H16 11 18 2 4-methylene- 8 16 enoate enoateenoate methylene-methylene-

Cyclohexane,1,3-Cyclohexane,1,3- Cyclohexane,Cyclohexane, C8HC168H 16 C8HC148H 14 dimethyl-,dimethyl-, cis -cis - ethenyl-ethenyl-

Butanoicacid,2-Butanoicacid,2- Cyclopentane,Cyclopentane, methyl-,1,2-methyl-,1,2- C10CH1020HO202 O2 (1-(1- C8HC168H 16 dimethylpropyldimethylpropyl methylethyl)-methylethyl)- esterester

Cyclohexane,1,4-Cyclohexane,1,4- C8HC168H 16 dimethyl-dimethyl-

3.3.3.3. Infrared Infrared Spectrum Spectrum TheThe Fourier-transform Fourier-transform infrared infrared (FTIR) (FTIR) spectra spectra of poof poplarplar and and almond almond shells shells seen seen in Figurein Figure 2 and 2 and TableTable 4 are 4 are alike, alike, reflecting reflecting that that they they share share similar similar chemical chemical components, components, which which are are mainly mainly cellulose, cellulose, hemicellulose,hemicellulose, and and lignin. lignin. The The peak peak range range and and sh arpnesssharpness are are different different and and illustrate illustrate the the content content differencedifference of theirof their components. components. For For exam example,ple, the the characteristic characteristic peak peak of 890of 890 cm cm−1 for−1 for poplar poplar is sharperis sharper thanthan almond almond shells. shells. This This is theis the characteristic characteristic peak peak of ofcellulose, cellulose, indicating indicating that that poplar poplar contains contains more more cellulosecellulose than than almond almond shells shells (as (as Table Table 1). 1).The The characteristic characteristic peak peak at 1580at 1580 cm cm−1 and−1 and 820 820 cm cm−1 of−1 almondof almond shellsshells is sharperis sharper and and stronger stronger than than poplar, poplar, giving giving evidence evidence that that almond almond shells shells contain contain more more lignin lignin thanthan poplar. poplar. The The characteristic characteristic peak peak at 1730at 1730 cm cm−1 of−1 ofthe the two two samples samples was was similar, similar, indicating indicating that that the the hemicellulosehemicellulose content content is muchis much the the same same [27]. [27].

FigureFigure 2. Infrared 2. Infrared spectra. spectra.

MaterialsMaterials 2018 2018, 11, x, 11 FOR, x FOR PEER PEER REVIEW REVIEW 6 of 612 of 12 MaterialsMaterials 2018 2018, 11, ,11 x ,FOR x FOR PEER PEER REVIEW REVIEW 6 of6 of12 12 MaterialsMaterials 2018 2018, 11, 11, x, FORx FOR PEER PEER REVIEW REVIEW 6 of6 of12 12 Materials 2018, 11, 1782 6 of 12 (Z)-Hex-3-enyl(Z)-Hex-3-enyl (E)- (E)- Hexane,3-Hexane,3- (Z)-Hex-3-enyl (E)- Hexane,3- (Z)-Hex-3-enyl2-methylbut-2-2-methylbut-2- (E)- C11HC1811OH2 18O2 methyl-4-Hexane,3-methyl-4- C8HC16 8H16 (Z)-Hex-3-enyl (E)- Hexane,3- (Z)-Hex-3-enyl2-methylbut-2-2-methylbut-2- (E)- C11CH1118HO182O 2 methyl-4-Hexane,3-methyl-4- C8CH816H 16 enoateenoate Table 3. Cont. methylene-methylene- 2-methylbut-2-2-methylbut-2- C11CH1118HO182O 2 methyl-4-methyl-4- C8CH816H 16 enoateenoate methylene-methylene- enoateenoateChemical Molecular Structural methylene-methylene-Chemical Molecular Structural Cyclohexane,1,3-Cyclohexane,1,3-Compound Formula Formula Cyclohexane,CompoundCyclohexane, Formula Formula C8HC16 8H16 C8HC14 8H14 Cyclohexane,1,3-dimethyl-,Cyclohexane,1,3-dimethyl-, ciscis- Cyclohexane,ethenyl-Cyclohexane,ethenyl- C8CH816H 16 C8CH814H 14 Cyclohexane,1,3-dimethyl-,Cyclohexane,1,3-dimethyl-,Cyclohexane,1,3- cis cis- - Cyclohexane,Cyclohexane,Cyclohexane,ethenyl-ethenyl- CC8CH8H16H 16 C8CHC814H H14 dimethyl-,dimethyl-,dimethyl-, cis cis- - cis- 8 16 ethenyl-ethenyl-ethenyl- 8 14

Butanoicacid,2- Butanoicacid,2- Cyclopentane, Butanoicacid,2-Butanoicacid,2- Cyclopentane, methyl-,1,2-Butanoicacid,2-methyl-,1,2- Cyclopentane, Butanoicacid,2-Butanoicacid,2- C10HC2010OH2 20O2 Cyclopentane,(1- (1- C8HC16 8H16 dimethylpropylmethyl-,1,2-dimethylpropylmethyl-,1,2-methyl-,1,2- Cyclopentane,Cyclopentane,Cyclopentane, CC10CH10H20HO20O2O 2 methylethyl)-(1-(1- C8CHC816HH 16 methyl-,1,2-dimethylpropylmethyl-,1,2-dimethylpropyl 10 20 2 methylethyl)-(1-methylethyl)- 8 16 dimethylpropylesterester C10CH1020HO202O 2 (1-(1- C8CH816H 16 dimethylpropyldimethylpropyl methylethyl)-methylethyl)- esterester ester methylethyl)-methylethyl)- esterester Cyclohexane,1,4-Cyclohexane,1,4-Cyclohexane,1,4- CC8HHC16 8H16 Cyclohexane,1,4-Cyclohexane,1,4-dimethyl-dimethyl- 8 16 dimethyl- C8CH816H 16 Cyclohexane,1,4-Cyclohexane,1,4-dimethyl- dimethyl- C8CH816H 16 dimethyl-dimethyl- 3.3. 3.3.Infrared Infrared Spectrum Spectrum 3.3.3.3. Infrared Infrared Spectrum Spectrum 3.3.3.3. InfraredThe InfraredThe Fourier-transform Spectrum Fourier-transform Spectrum infrared infrared (FTIR) (FTIR) spectra spectra of po ofplar poplarpoplar and and almond almond shells shells seen seen in Figure in Figure 2 and2 2 and and TableTableTheThe4 Fourier-transform4 are areFourier-transform alike, alike, reﬂecting reflecting infrared thatinfrared that they they (FTIR) (FTIR) share share spectra similar spectrasimilar of chemical ofchemicalpo poplarplar and components, components,and almond almond shells whichshellswhich seen seen areare in mainly mainly inFigure Figure cellulose, cellulose,2 and2 and TableThe 4The are Fourier-transform Fourier-transformalike, reflecting thatinfrared infrared they (FTIR)share (FTIR) similar spectra spectra chemical of of po poplarplar components, and and almond almond which shells shells areseen seen mainly in in Figure Figure cellulose, 2 and2 and hemicellulose,TableTablehemicellulose, 4 are4 are alike, alike, and reflecting and reflectinglignin. lignin. Thethat that The Thetheypeak they peak peak share range share range range similar andsimilar and sh chemicalarpness chemicalsh sharpnessarpness components,are components, are different different whichand which and illustrate are are illustrate mainly mainly the cellulose, the contentcellulose, contentcontent TableTable 4 are4 are alike, alike, reflecting reflecting that that they they share share similar similar chemical chemical components, components, which which are− are1 mainly mainly cellulose, cellulose, differencehemicellulose,hemicellulose,difference of their ofof and their theirand components. lignin. components.components.lignin. The The Forpeak peak For Forexam range example,exam rangeple, ple, andthe and the thecharacteristicsh sharpness characteristiccharacteristicarpness are arepeak different peak peakdifferent of 890 of of 890 and890cm and cm− cm1illustrate for illustrate−1 poplarfor poplar the isthe sharpercontent iscontent sharper hemicellulose,hemicellulose, and and lignin. lignin. The The peak peak range range and and sh sharpnessarpness are are different different and and −illustrate1 −illustrate1 the the content content thandifferencedifferencethanthan almond almondalmond of oftheirshells. their shells.shells. components. Thiscomponents. This is the is thethecharacteristic For For characteristiccharacteristic exam example,ple, peakthe the peakcharacteristic of characteristic cellulose, ofof cellulose,cellulose, peakindicating peak indicatingindicating of of890 890 that cm cm thatthat poplar for for poplarpoplar poplar poplarcontains containscontains is issharper sharpermore moremore differencedifference of of their their components. components. For For exam example,ple, the the characteristic characteristic peak peak of of 890 890 cm cm−−11 −for1 for poplar poplar is− is1sharper sharper cellulosethanthancellulose almond almond than thanthan shells.almond shells. almondalmond This shellsThis shellsisshells isthe (as the (ascharacteristic Table(as characteristic Table Table 1).1 The). 1). The The characteristicpeak characteristicpeak characteristic of ofcellulose, cellulose, peak peak peak indicating at indicating at 1580 at 1580 1580 cm cm that− cm1 thatand −poplar1and and poplar820 820 820cm contains cm− containscm1 of− 1almond of more almond more thanthan almond almond shells. shells. This This is isthe the characteristic characteristic peak peak of of cellulose, cellulose, indicating indicating that −that1 − 1poplar poplar contains contains−1 −1 more more shellscellulosecelluloseshells is sharper than is than sharpersharper almond almondand and stronger shells shells strongerstronger (as than(as Table thanTablethan poplar, 1). poplar,poplar, 1). The Thegiving characteristic givinggivingcharacteristic evidence evidenceevidence peak that peak thatthat atalmond at1580 almondalmond1580 cm shellscm and shellsshells and contain 820 820 containcontain cm cm more of moremore ofalmond ligninalmond ligninlignin cellulosecellulose than than almond almond shells shells (as (as Table Table 1). 1). The The characteristic characteristic−1 peak peak at at 1580 1580 cm cm−1 −and1 and 820 820 cm cm−1 −of1 of almond almond shellsshellsthanthan is poplar.ispoplar.sharper sharper TheThe and and characteristiccharacteristic stronger stronger than than peakpeak poplar, poplar, atat 17301730 giving giving− cmcm1 −evidence1 ofofevidence the the two two that that samples samples almond almond was shells shells similar, contain contain indicating more more lignin thatlignin the shellsthan poplar. is sharper The andcharacteristic stronger thanpeak poplar, at 1730 givingcm of evidence the two samplesthat almond was similar,shells contain indicating more that lignin the thanshellsthanhemicellulose poplar. poplar.is sharper The The characteristic content and characteristic stronger is much peak than peak the atpoplar, same at1730 1730 [ 27cm giving ].cm−1 of−1 of theevidence the two two samples thatsamples almond was was similar, shellssimilar, containindicating indicating more that that lignin the the hemicellulosehemicellulose content content is much is much the thesame same [27]. [27]. −1 −1 thanhemicellulosethanhemicellulose poplar. poplar. The Thecontent contentcharacteristic characteristic is ismuch much thepeak peakthe same atsame at 1730 [27].1730 [27]. cm cm of of the the two two samples samples was was similar, similar, indicating indicating that that the the hemicellulosehemicellulose content content is ismuch much the the same same [27]. [27].

FigureFigure 2. Infrared 2. Infrared spectra. spectra.spectra. FigureFigure 2. Infrared2. Infrared spectra. spectra. FigureFigure 2. 2.Infrared Infrared spectra. spectra.

Materials 2018, 11, x FOR PEER REVIEW 7 of 12

Table 4. Absorption band assignment in the infrared spectrum of almond shell and poplar.

Functional Wavenumber (cm−1) Vibration Type Cause Group

3300~3500 –OH stretching vibration cellulose, hemicellulose

2900~2935 –CH stretching vibration -

1640~1735 C=O stretching vibration lignin, hemicellulose

Materials 20181580~1605, 11, 1782 benzene ring stretching vibration lignin 7 of 12

1455~1465 –CH3O stretching vibration lignin Table 4. Absorption band assignment in the infrared spectrum of almond shell and poplar. 1320~1430 –CH bending vibration - Wavenumber (cm−1) Functional Group Vibration Type Cause 1221~1230 C–C C–O stretching vibration lignin 3300~3500 –OH stretching vibration cellulose, hemicellulose 2900~2935 –CH stretching vibrationcellulose, hemicellulose, - and 1640~17351025~1035 C=OC–O stretchingstretching vibration vibration lignin, hemicellulose 1580~1605 benzene ring stretching vibration ligninlignin 1455~1465 –CH3O stretching vibration lignin 1320~1430885~895 –CHR2C=CH2 bendingbending vibration vibration - - 1221~1230 C–C C–O stretching vibration lignin disubstituted 1025~1035810~833 C–Obenzene ring stretching vibration cellulose, hemicellulose,- and lignin 885~895 R2C=CH2 bending vibrationbenzene - 810~833 benzene ring disubstituted benzene -

3.4. X-ray Diffraction 3.4. X-ray Diffraction The X-ray diffraction curves in Figure 3 show that the diffraction peaks of almond shells The X-ray diffraction curves in Figure3 show that the diffraction peaks of almond shells appeared appeared near 16.6°, 21.48° and 34.67°, indicating its cellulose crystals belong to cellulose I type. In near 16.6◦, 21.48◦ and 34.67◦, indicating its cellulose crystals belong to cellulose I type. In addition, addition, using height method [28] to calculate the crystallization, the crystallization of almond shells using height method [28] to calculate the crystallization, the crystallization of almond shells is 32.88% is 32.88% lower than that of poplar (48.166%). Crystallization is mainly caused by cellulose, so low lower than that of poplar (48.166%). Crystallization is mainly caused by cellulose, so low crystallization crystallization may result in a decrease of mechanical properties. However, compared with other may result in a decrease of mechanical properties. However, compared with other commonly used commonly used nuts, crystallization of almond shells is higher, for example, crystallization is 23.25% nuts, crystallization of almond shells is higher, for example, crystallization is 23.25% for chestnut shells, for chestnut shells, 26.85% for peanut shells, 29.11% for melon shells, 30.61% for macadamia shells, 26.85% for peanut shells, 29.11% for melon shells, 30.61% for macadamia shells, and 23.69% for walnut and 23.69% for walnut shells [29]. Combined with cellulose content, almond shells may present shells [29]. Combined with cellulose content, almond shells may present higher mechanical properties higher mechanical properties than other nut shells, which is beneficial in the preparation of than other nut shells, which is beneﬁcial in the preparation of composite. composite.

Figure 3. X-ray diffraction spectra.

3.5. X-ray Photoelectron Spectroscopy Characterization

As shown in Figure4, both almond shells and poplar have strong peaks near 532.39 eV/285 eV in X-ray photoelectron spectroscopy, which is the peak of O 1s and C 1s, respectively. In addition, the peak position of N was detected at 398 eV in almond shells, while the peak position of Si at 101.8 eV was detected in poplar. This indicates that there are more N elements in almond shells and Si elements in poplar, and the N may be contained in the protein. Materials 2018, 11, x FOR PEER REVIEW 8 of 12

3.5. X-ray Photoelectron Spectroscopy Characterization As shown in Figure 4, both almond shells and poplar have strong peaks near 532.39 eV/285 eV in X-ray photoelectron spectroscopy, which is the peak of O 1s and C 1s, respectively. In addition, the peak position of N was detected at 398 eV in almond shells, while the peak position of Si at 101.8 eV Materialswas detected2018, 11, 1782in poplar. This indicates that there are more N elements in almond shells and8 of 12Si elements in poplar, and the N may be contained in the protein.

FigureFigure 4.4. X-rayX-ray photoelectronphotoelectron spectroscopy.spectroscopy.

TableTable5 5shows shows that that thethe mainmain chemical elements elements co containedntained in in the the almond almond shells shells are are essentially essentially the thesame same as those as those in other in other nuts. nuts. Compared Compared with the with sh theell of shell Chinese of Chinese chestnut, chestnut, peanut, peanut, Sunflower, Sunflower, Hawaii Hawaiinut, and nut, walnut, and walnut, almond almond shells present shells presentthe highes thet highestcontentcontent of oxygen, of oxygen, and relatively and relatively low carbon low carboncontent. content. Compared Compared to poplar, to nut poplar, shells nut generally shells generally contain less contain silicon less and silicon more and nitrogen more elements. nitrogen elements.Almond shells Almond might shells therefore might therefore have advantages have advantages in fire resistance in fire resistance when preparing when preparing composite, composite, due to duethe toflame the flameinhabitant inhabitant capabilities capabilities of N. of N.

TableTable 5. 5.Surface Surface chemicalchemical composition composition and and relative relative content content of of nut nut and and poplar poplar (wt (wt %). %).

Sample Type n n n s n Sample Type n C C n O O in si N n N Chestnut shells 80.26 17.28 0.85 1.61 Chestnut shellsPeanut shells80.26 74.1617.28 21.72 0.340.85 3.78 1.61 Sunﬂower shells 78.86 18.13 0.42 2.59 Peanut shellsHawaii nut shells74.16 79.1621.72 18.92 0.260.34 1.67 3.78 Walnut shell 78.84 19.32 0.16 1.67 Sunflower shellsPoplar 78.86 74.5618.13 20.93 4.510.42 - 2.59 Almond shells 72.27 22.88 0.87 3.87 Hawaii nut shells 79.16 18.92 0.26 1.67 Note: The data of almond shells are measured by authors, and the rest are derived from reference [29]. Walnut shell 78.84 19.32 0.16 1.67 The software XPS Pesk was used to ﬁt the peak of the curve for Figure5, and C 1,C2, and C3 was used to representPoplar the different74.56 states of carbon.20.93 The main form4.51 of carbon in almond- shells is C1,C2, andAlmond C3, as listed shells in Table6.72.27 The characteristic22.88 peak of binding0.87 energy 284.383.87 eV was mainly produced by C–H bond in Figure5. Its electronegativity was small, so the binding energy was small. The characteristicNote: The data peak of almond of binding shells energy are measured 286.08 eV by wasauthors, mainly andproduced the rest ar bye derived C–OR, from in which reference binding [29]. energy was higher. The binding energy of C3 (287.79 eV) was mainly produced by C=O. In Table6,C 2 content of almond shells was higher than that of other biomass nut shells, The software XPS Pesk was used to fit the peak of the curve for Figure 5, and C1, C2, and C3 was indicating that there are more –OR groups in the almond shells. This result is consistent with Table1, used to represent the different states of carbon. The main form of carbon in almond shells is C1, C2, showing that almond shells were likely to contain more cellulose. The content of C3 is lower than that and C3, as listed in Table 6. The characteristic peak of binding energy 284.38 eV was mainly produced of other biomass nut shells, which illustrates that the C=O in the almond shell was less than that of by C–H bond in Figure 5. Its electronegativity was small, so the binding energy was small. The other biomass nut shells.

Materials 2018, 11, x FOR PEER REVIEW 9 of 12

characteristic peak of binding energy 286.08 eV was mainly produced by C–OR, in which binding energy was higher. The binding energy of C3 (287.79 eV) was mainly produced by C=O. In Table 6, C2 content of almond shells was higher than that of other biomass nut shells, indicating that there are more –OR groups in the almond shells. This result is consistent with Table 1, showing that almond shells were likely to contain more cellulose. The content of C3 is lower than Materials 2018, 11, 1782 9 of 12 that of other biomass nut shells, which illustrates that the C=O in the almond shell was less than that of other biomass nut shells.

FigureFigure 5. High-resolution 5. High-resolution C C 1s 1s maps maps of almond almond shells. shells.

Table 6. X-rayTable photoelectron6. X-ray photoelectron spectroscopy spectroscopy (XPS) (XPS) testtest results results of ofalmond almond shells shells C 1s. C 1s. Binding Energy (eV) A (%) Sample Binding Energy (eV) A (%) Sample C1 1s C2 1s C3 1s C1 1s C2 1s C3 1s C1 1s C2 1s C3 1s C1 1s C2 1s C3 1s AlmondAlmond shells shells 284.38284.38 286.08286.08 287.79287.79 55.88 32.35 32.35 11.76 11.76 ChestnutChestnut shells shells 284.8284.8 286.28286.28 286.93286.93 63.18 17.70 17.70 19.12 19.12 Peanut shells 284.8 286.24 286.79 48.93 16.34 34.73 SunﬂowerPeanut shells shells 284.8284.8 286.25286.24 288.07286.79 48.93 61.99 16.34 25.94 34.73 12.07 Hawaii nutSunflower shells shells 284.8284.8 286.25286.25 287.70288.07 61.99 54.58 25.94 30.85 12.07 14.57 Walnut shells 284.8 286.25 287.10 55.22 23.17 21.61 Hawaii nut shells 284.8 286.25 287.70 54.58 30.85 14.57 Note: The data of almond shells are measured by authors, the rest of the data is derived from reference [29]. Walnut shells 284.8 286.25 287.10 55.22 23.17 21.61 3.6. Thermal StabilityNote: The data of almond shells are measured by authors, the rest of the data is derived from reference [29]. The pyrolysis process of almond shells is presented in Figure6. From room temperature to 200 ◦C, 3.6. Thermal Stability almond shells and poplar have a small weight loss peak due to the water loss stage. In the range of 200–350 ◦C, bothThe poplar pyrolysis and process almond of almond shells shells display is presen ated weight in Figure loss 6. peakFrom room which temperature is mainly to 200 formed by °C, almond shells and poplar have a small weight loss peak due to the water loss stage. In the range hemicelluloseof 200–350 and lignin. °C, both Weight poplar lossand almond of almond shells shellsdisplay occurreda weight loss earlier peak which than poplar,is mainly however,formed by and the Materials 2018, 11, x FOR PEER REVIEW 10 of 12 peak of the almondhemicellulose shells and was lignin. sharper Weight than loss of poplar. almond The shells highest occurred loss earlier peaks than of poplar, poplar however, and almond and shells ◦ ◦ appeared atthe 300–500The peak carbonization of theC. almond The poplarphase shells occurs was had sharper after the highest500 than °C poplar.and peak pyrolysis The at highest 350 is basicallyC, loss while peaks completedthe of poplar almond at thisand stage.almond shell peak was at 338 ◦C.Thus, Poplar’sshells the appeared curve peak of isnutrientat wider300–500 biomass and °C. The sharper after poplar 500 °C than had tends the almond to highest be gentle. shells. peak In atconclusion, Thermal 350 °C, while the interpretation thermal the almond stability shell of cellulose peak was at 338 °C. Poplar’s peak is wider and sharper than almond shells. Thermal interpretation and ligninof is poplar formed is superior by volatiles, to that of mainly almond shells, relying but on compared the pyrolysis with other of nut cellulose shells, the [30 thermal]. The stability higher peaks of of ofalmond cellulose shells and is ligninbetter, isfor formed example, by thevolatiles, highest mainly pyrolysis relying temperature on the pyrolysis of pistachio of cellulose shells is 337[30]. °C, The poplar indicateandhigher cashew that peaks nut it containsof shells poplar is 318 indicate more °C, which cellulose.that it is contains lower than more the cellulose. almond shells (338 °C).

FigureFigure 6. Thermal 6. Thermal gravity gravityand and derivative thermogravimetric. thermogravimetric.

4. Conclusions In this study, the physical structure and chemical properties of almond shells were determined. The following conclusions were obtained. 1. Observed by microscope and electron microscope, almond shells have the diameter of 300–500 μm for large holes, and 40–60 μm for small holes. 2. The elements of almond shells include C (72.27%), O (22.88%), N (3.87%), and Si (0.87%). The main chemical constituents of cellulose, hemicellulose, and lignin account for 38.48%, 28.82% and 29.54%. 3. The alkaline extract content of almond shells was 14.03%, and benzene alcohol extraction was 8.00%. The benzene alcohol extractives of almond shells mainly contain 17 types of organic compound including benzene ring, methylene, carbon three bond, and other multi-functional groups. 4. Thermogravimetric analysis shows almond shells mainly lose weight at 260 °C and 335 °C. This shows that its pyrolysis temperature is lower than poplar; however, it is higher than pistachio shells and cashew nut shells. 5. X-ray diffraction spectra showed that diffraction peaks of almond shells appeared near 16.6°, 21.48° and 34.67°. Cellulose crystals belong to cellulose I type. The crystallization of almond shells is 32.88%, which is lower than poplar; however, it is higher than chestnut shells, peanut shells, melon shells, macadamia shells, and walnut shells. 6. X-ray photoelectron spectroscopy showed that almond shells contained more N than poplar and five other kinds of nut shell (chestnut shells, peanut shells, sunflower shells, Hawaii nut shells, and walnut shells). There was less Si in the almond shells compared to poplar.

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The carbonization phase occurs after 500 ◦C and pyrolysis is basically completed at this stage. Thus, the curve of nutrient biomass after 500 ◦C tends to be gentle. In conclusion, the thermal stability of poplar is superior to that of almond shells, but compared with other nut shells, the thermal stability of almond shells is better, for example, the highest pyrolysis temperature of pistachio shells is 337 ◦C, and cashew nut shells is 318 ◦C, which is lower than the almond shells (338 ◦C).

4. Conclusions In this study, the physical structure and chemical properties of almond shells were determined. The following conclusions were obtained.

1. Observed by microscope and electron microscope, almond shells have the diameter of 300–500 µm for large holes, and 40–60 µm for small holes. 2. The elements of almond shells include C (72.27%), O (22.88%), N (3.87%), and Si (0.87%). The main chemical constituents of cellulose, hemicellulose, and lignin account for 38.48%, 28.82% and 29.54%. 3. The alkaline extract content of almond shells was 14.03%, and benzene alcohol extraction was 8.00%. The benzene alcohol extractives of almond shells mainly contain 17 types of organic compound including benzene ring, methylene, carbon three bond, and other multi-functional groups. 4. Thermogravimetric analysis shows almond shells mainly lose weight at 260 ◦C and 335 ◦C. This shows that its pyrolysis temperature is lower than poplar; however, it is higher than pistachio shells and cashew nut shells. 5. X-ray diffraction spectra showed that diffraction peaks of almond shells appeared near 16.6◦, 21.48◦ and 34.67◦. Cellulose crystals belong to cellulose I type. The crystallization of almond shells is 32.88%, which is lower than poplar; however, it is higher than chestnut shells, peanut shells, melon shells, macadamia shells, and walnut shells. 6. X-ray photoelectron spectroscopy showed that almond shells contained more N than poplar and ﬁve other kinds of nut shell (chestnut shells, peanut shells, sunﬂower shells, Hawaii nut shells, and walnut shells). There was less Si in the almond shells compared to poplar.

Author Contributions: Conceptualization, W.W. and X.L.; Methodology, X.L.; Software, J.H.; Validation, X.L.; J.H. and Y.L.; Formal Analysis, X.L. and Y.L.; Investigation, X.L.; Resources, X.L.; Data Curation, X.L. and Y.L.; Writing-Original Draft Preparation, X.L. and Y.L.; Writing-Review & Editing, X.L. and W.W.; Visualization, X.L. and W.W.; Supervision, X.L. and W.W.; Project Administration, W.W.; Funding Acquisition, W.W. Funding: This research was funded by [Nature Science Foundation of China] grant number [31670573] and the 2017 Special Project of Sustainable Development for Central University Innovation Team of the Ministry Education grant number [2572017ET05]. Acknowledgments: The authors would like to acknowledge the financial support by the Nature Science Foundation of China (31670573) and the 2017 Special Project of Sustainable Development for Central University Innovation Team of the Ministry Education (2572017ET05). Conflicts of Interest: The authors declare no conflict of interest.

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