Microwave assisted hydrothermal preparation of rice straw hydrochars for adsorption of organics and heavy metals

Accepted Manuscript Microwave Assisted Hydrothermal Preparation of Rice Straw Hydrochars for Adsorption of Organics and Heavy Metals Yin Li, Nyamkhand Tsend, TiKai Li, Heyang Liu, Ruiqin Yang, Xikun Gai, Hongpeng Wang, Shengdao Shan PII: DOI: Reference:

S0960-8524(18)31488-3 https://doi.org/10.1016/j.biortech.2018.10.056 BITE 20620

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Bioresource Technology

Received Date: Revised Date: Accepted Date:

31 August 2018 20 October 2018 22 October 2018

Please cite this article as: Li, Y., Tsend, N., Li, T., Liu, H., Yang, R., Gai, X., Wang, H., Shan, S., Microwave Assisted Hydrothermal Preparation of Rice Straw Hydrochars for Adsorption of Organics and Heavy Metals, Bioresource Technology (2018), doi: https://doi.org/10.1016/j.biortech.2018.10.056

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Microwave Assisted Hydrothermal Preparation of Rice Straw Hydrochars for Adsorption of Organics and Heavy Metals Yin Li a, Nyamkhand Tsend a, TiKai Li a, Heyang Liu a, Ruiqin Yang a, Xikun Gai a, Hongpeng Wang a, Shengdao Shan b * a Zhejiang

Provincial Key Lab for Chemical and Biological Processing Technology of

Farm Product, School of Biological and Chemical Engineering, Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China b Key

Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province,

Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China * Corresponding author. * Shengdao Shan, Tel: 86-571-85070006. E-mail address: [email protected] (S. Shan) To be submitted to Bioresource Technology (Online) Declarations of interest: none Abstract A series of rice straw hydrochars were produced through a microwave-assisted hydrothermal treatment method, characterized and used for the adsorption of three organics and two heavy metals from aqueous solutions. The hydrochars have carbon contents from 37.44 % to 43.31 %, are rich in oxygen containing functional groups, and the equilibrium of hydrothermal carbonization reactions could be reached rapidly in microwave environment. The hydrochars can effectively adsorb the model pollutants, the maximum adsorption capacities of Congo red, berberine hydrochloride and 2-naphthol at 298 K and initial concentration of 0.5 mg/mL were 222.1, 174.0 and 48.7 mg/g, respectively, and

those of Zn2+ and Cu2+ were 112.8 and 144.9 mg/g, respectively. Adsorption thermodynamic parameters were calculated. These results suggest that microwave-assisted hydrothermal treatment is an effective method for the rapid production of hydrochars, and rice straw hydrochars are promising adsorbents for the removal of water pollutants such as organics and heavy metals. Keywords: hydrochar, microwave, pollutant, adsorption 1. Introduction The presence of toxic heavy metals and organic compounds in environment from waste disposal is currently an important environmental concern (Tong et al., 2011). Heavy metal ions, such as Cu2+ and Zn2+ are commonly detected in effluent from industrial and agriculture wastes. They become non-metabolized in living organisms when the concentration exceeds the tolerance limit, lead to the metal bioaccumulation in the soft tissues (Wang et al., 2017), and may further result in tissue and organ damages (Fu & Wang, 2011; Tong et al., 2011). Organic compounds such as dyes, antibiotics and aromatic compounds in waste water are toxic to microorganisms, hard to be degraded and could also lead to potential danger of bioaccumulation (Wang et al., 2016). Due to the recalcitrance and persistence of heavy metal ions and organic pollutants in environment, the treatment of wastewater containing these kinds of chemicals has been of special concern (Antonetti et al, 2016; Fu & Wang, 2011). Among different waste water treatment processes, adsorption is gaining wide acceptance because of their high efficiency and ability to separate a wide range of compounds from liquid (Rajapaksha et al., 2016). Commercial activated carbon provides good adsorption capability for water pollutants such as organics and heavy metals,

however, its widespread application is restricted to its relatively high cost. Biochar is a carbon rich product obtained from thermochemical conversion including pyrolysis and hydrothermal treatment of biomass (Belmonte et al., 2018; Jang et al., 2018; Licursi et al., 2015), and has been used as low cost adsorbent to remove organic contaminants and heavy metals for water cleaning purposes (Lee et al., 2018; Mohan et al., 2011; Sun et al., 2013). Biochar produced by hydrothermal carbonization is also called hydrochar or hydrothermal carbon (Ahmad et al., 2014; Licursi et al., 2017). Biochar from pyrolysis and hydrochar differ widely in physical-chemical properties (Bargmann et al., 2013), Usually, hydrochar has lower surface area and porosity than pyrolytic biochar, however, due to the rich oxygen-containing functional groups on its surface, hydrochar could observe considerably higher adsorption capability than pyrolytic biochar (Kambo & Dutta, 2015), which makes hydrochar a potential adsorbent in waste water treatment. Rice straw, a typical agricultural residue generated in large quantities every year, is a lignocellulosic material with 28.9846.01 % cellulose, 13.77-31.09 % hemicellulose, 3.51-10.53 % lignin (Huang et al., 2010) and loose structure, which makes it suitable for hydrochar production. The production of rice straw hydrochars with conventional heating method has been reported (Gao et al, 2018; Jin et al., 2017; Liu et al., 2017), their physical-chemical properties were compared with rice straw pyrochars, and the results indicated that hydrochars had more O-containing functional groups than pyrochars which might benefit their adsorption abilities for pollutants (Jin et al., 2017). Microwave heating offers many advantages compared with traditional heating methods such as homogeneous and rapid heating (Galletti et al., 2013; Galia et al., 2015), and chemical reactions in microwave environment could have higher reaction rates and

better selectiveness with lower energy consumption (Hui et al., 2016). On the other hand, hydrothermal carbonization of biomass undergoes a series of chemical reactions such as hydrolysis, dehydration and decarboxylation of hydrolyzed products, condensation, polymerization and aromatization of the intermediate compounds from dehydration and decarboxylation, and so forth (Reza et al., 2014), while microwave radiation has been reported to be capable of accelerating hydrolysis (Huang & Fu, 2013; Wu et al., 2012) and different types of polymerization (Kempe et al., 2011), which are important steps in hydrothermal carbonization to form solid product (Funke & Ziegler, 2010). Moreover, microwave-assisted hydrothermal carbonization has been conducted on cellulose, glucose, and some lignocellulosic materials such as P. africana shell and coconut shell (Elaigwu & Greenway, 2016a, 2016b, 2018; Guiotoku et al., 2012), and the results suggested that microwave assisted hydrothermal treatment could be a good option for the rapid production of hydrochar. However, the effects of microwave irradiation on the adsorption properties of the hydrochar products were not considered in these studies. In this work, a series of rice straw hydrochars were prepared in microwave environment, characterized and tested for adsorption properties of three model organics, Congo red (an anionic dye), berberine hydrochloride (an antibiotic) and 2-naphthol (an aromatic compound), and two model heavy metals, Zn2+ and Cu2+ from aqueous solutions. The primary objective of this study was to investigate the feasibility of using microwave assisted hydrothermal treatment method to produce rice straw hydrochars, explore the relationship between microwave treatment conditions and physical-chemical properties of the hydrochars, and evaluate the capabilities of the hydrochar adsorbents for the adsorptive removal of model pollutants from water.

2. Materials and methods 2.1 Materials Rice straw was obtained from Zhejiang province. Berberine hydrochloride was provided by Shanghai Darui Finechem Ltd. Congo red, 2- naphthol and copper sulfate were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Zinc sulfate was from Xilong Science Co., Ltd. All the chemicals are AR grade and were used as received. 2.2 Preparation of the rice straw hydrochars Rice straw hydrochars were prepared using microwave assisted hydrothermal treatment, the process temperatures were set to be from 160 °C to 200 °C and reaction time was from 40 min to 70 min. The ratio of solid to liquid was 1:10 (g of wet rice straw: mL of deionized water). Detailed process conditions for each hydrochar samples are shown in Table 1. Generally, 2 g of wet rice straw (with average moisture content of 10.5 %) was weighed in a microwave reaction tank, and 20 mL of deionized water was added. The reaction tank was then closed tightly, put into the microwave reactor, heated to the set temperature and held for a specific time. After this process, the reactor was cooled down to 60 °C. The solid phase of the mixture was collected by filtration, washed with 30 mL of ethanol for 1 time, 30 mL of deionized water for 3 times, and dried at 100 °C for 8 h. Yields of the rice straw hydrochars were calculated by dividing the dry mass of the hydrochar by the dry mass of the rice straw. 2.3 Characterization Ash contents of the rice straw hydrochars were determined using the method described in our previous work (Li et al., 2016; Li et al., 2018). Pore properties including BET specific surface area, pore volume and pore diameter of the rice straw hydrochars were

analyzed by N2 sorption isotherms at 77K on a Sorptometer Quantachrome Autosorb iQ at the conditions described in our previous work (Li et al., 2016; Li et al., 2018). C, H, N and S contents of the rice straw hydrochars were performed on a Vario MICRO cube elemental analyzer, and O content was calculated by subtracting C, H, N, S and ash contents from the dry mass of the hydrochars. Bruker Vertex 70 fourier transform infrared spectrometer (FTIR) was used to record the FTIR spectra of the hydrochars. The surface structures of the hydrochar samples were determined by a Hitachi S3700 N scanning electron microscopy (SEM). 2.4 Adsoprtion experiments Rice straw hydrochars (0.06 g) were mixed with 30 mL of adsorbate aqueous solutions with known initial concentrations C0 (mg/mL) in conical flasks. The flasks were capped and shaken with a constant speed of 170 rpm at set temperatures for 8 h. After that, the mixtures were filtered, and the equilibrium concentrations Ce (mg/mL) of the adsorbates in the liquid phase were determined. The concentration of Congo red, 2-naphthol and berberine hydrochloride in aqueous solutions was analyzed by a UV-vis spectrometer at 499 nm, 273 nm and 345 nm, respectively, while the concentration of Zn2+ and Cu2+ was determined by an atomic adsorption spectrometry. Adsorption capacities Qe (mg/g) were calculated as follows: Qe = (C0 - Ce) × V/m (1) Where V (mL) is the volume of the adsorbate solution and m (g) is the amount of the hydrochar adsorbent. Langmuir and Freundlich equations were used to fit the adsorption isotherms and they were expressed as follows:

Qm K LCe 1+K LCe

(2)

Freundlich equation: Qe  K FCe1/n

(3)

Langmuir equation: Qe 

Where Qm (mg/g) is the maximum adsorption capacity to give a complete monolayer, KL (mL/mg) in Langmuir equation is a constant related to affinity. KF ((mg/g)(mL/mg)1/n) and 1/n in Freundlich equation are constants indicating adsorption capacity and intensity, respectively. 3. Results and discussion 3.1 Characterization Physical-chemical properties including yield, ash content, elemental composition, pore properties of the rice straw hydrochars are summarized in Table 1. The percentage yields of the rice straw hydrochars were determined to be between 37.00 % and 42.53 % which are lower than the average yields of hydrochars from some other waste biomass reported in literature (Kambo & Dutta, 2015) and the yields of rice straw hydrochars prepared with traditional heating method at similar temperatures (Liu et al., 2017), and increasing temperature and microwave treatment time did not reveal significant influences on the yields of the rice straw hydrochars. These results indicate the rapid decomposition of rice straw biomass under microwave irradiation at the temperatures of 160 °C to 200 °C. Hydrolysis is believed to be the first step of the hydrothermal carbonization of biomass, normally, hemicellulose hydrolysis starts above 180 °C, while cellulose starts hydrolyzing around 200-230 °C (Funke & Ziegler, 2010; Reza et al., 2014). Microwave irradiation has been reported to be able to activate cellulose molecules, heighten particle collisions and offer higher hydrolytic efficiency for lignocellulosic

biomass under mild conditions than conventional heating methods (Huang & Fu, 2013; Wu et al., 2012). The hydrolysis of cellulose could happen at a lower temperature in microwave environment, and hydrochar derived from cellulose could be formed at the temperature around 200 °C under microwave irradiation (Guiotoku et al., 2012). These could be favorable for the rapid establishment of hydrolysis equilibrium and lead to the similar yields and also the lower yields of the hydrochars compared with rice straw hydrochars prepared from conventional heating method at similar temperatures. The hydrochar samples prepared in this work have high ash contents of 14.27- 23.12 %, which should be attributed to the high ash content of raw rice straw (Fierro et al., 2010). Ash content of the hydrochars decreases with increasing hydrothermal treatment time at all the four reaction temperatures, which reflects the dissolution of inorganic minerals in water over time. As shown from the elemental analysis results (Table 1), the carbon contents of the rice straw hydrochars (37.44 % to 42.74 %) are lower than those of some hydrochars from other raw materials such as wood and grass reported in literatures (Reza et al., 2014) but higher than those of reported rice straw hydrochars prepared with conventional heating method (Liu et al., 2017). Hydrothermal carbonization has complex reaction network including hydrolysis, dehydration, decarboxylation, condensation polymerization and aromatization. The oligomers and monomers from the hydrolysis of biomass components are subject to subsequent degradation reactions. Reactive intermediates such as 5-(hydroxymethyl) furfural (HMF) from degradation then go through polymerization to form insoluble solids and finally partly form hydrochar product (Funke & Ziegler, 2010). Microwave radiation is able to accelerate both the hydrolysis (Huang & Fu, 2013; Wu et al., 2012) and polymerization (Kempe et al., 2011) steps in hydrothermal carbonization, and these could

result in a fast carbonization of biomass and higher carbon content of the final product. In addition, the hydrochars produced in this work exhibit much lower hydrogen contents than rice straw hydrochars prepared with conventional heating method (Liu et al., 2017), which further confirms the highly efficient carbonization of biomass under microwave radiation. On the other hand, the rice straw hydrochars have high oxygen contents from 30.73 % to 40.76 % indicating rich oxygen-containing groups on the surfaces of the hydrochar samples. As shown in Table 1, the rice straw hydrochars have BET specific surface areas of 8.21 m2/g to 25.47 m2/g, pore volumes from 0.03 cm3/g to 0.11 cm3/g and average pore diameters between 3.41 nm and 4.31 nm. The BET surface areas of the rice straw hydrochars are higher than those of some reported hydrochars produced from other raw materials such as Prosopis africana shell and urban food waste (Elaigwu et al., 2014; Parshetti et al., 2014). Rice straw has loose structure which could benefit the contact between the biomass and the water molecules, while microwave radiation is favorable for the hydrolysis of the biomass (Huang & Fu, 2013; Wu et al., 2012), and these might be the reasons for the higher BET surface areas of the rice straw hydrochars. Furthermore, these values are also higher than rice straw hydrochars prepared with conventional heating method at similar temperatures (180-240 °C) and much longer treatment time (1.5-4 h) (Jin et al., 2017; Liu et al., 2017), which further confirms the high efficiency of the hydrolysis reaction for the biomass under microwave irradiation. Increasing temperature exhibits slightly positive effect on the surface areas and pore volumes of the rice straw hydrochars. Ionic product of subcritical water (pressurized liquid water at the temperature range of 180350 °C) increases with increasing temperature while its dielectric constant decreases

(Kritzer & Dinjus, 2001), higher ionic product could make water act as a weak acid/base catalyst and accelerate the hydrolysis of biomass, on the other hand, lower dielectric constant might make water act more like a weak-polar solvent (Reza et al., 2014) to dissolve organic compounds from the hydrolysis and leave pore structures, and these could be responsible for the higher surface areas and pore volumes of the hydrochar products prepared at higher reaction temperatures. Process time did not show significant effects on the pore properties of the rice straw hydrochar samples, which might be because of the fast equilibrium of the hydrothermal carbonization reactions of biomass in microwave environment. Average pore diameters of the rice straw hydrochars are quite similar, which implies that the pore structure of hydorchar is also limited by the biological structure of the raw material. As can be found from the FTIR spectra of the raw rice straw and the prepared hydrochars, the hydrochar samples retained almost all the oxygen functional groups of the raw rice straw on their surfaces, such as O-H (near 3430 cm-1), C=O (near 1630 cm-1), and C-O (between 1049 cm-1 and 1200 cm-1). These results are consistent with the oxygen contents (Table 1) of the hydrochar samples, and the O-containing functional groups might lead to chemical or semichemical adsorption for organics on the hydrochars. On the other hand, aromatic C=C (between 1500 cm-1 and 1600 cm-1) and C–H vibrations in aromatic structures (between 650 cm-1 and 900 cm-1) were almost absent in the FTIR spectrums, indicating low aromatic contents of the rice straw hydrochars, and these results also agree with the low carbon contents (Table 1) of the hydrochars. Process conditions have no significantly effects on the surface functional groups of the rice straw hydrochars.

It can be seen from the SEM images of the raw rice straw and hydrochar sample M180/50 that the hydrochar sample M180/50 has groove-like orientations and serves slightly rougher surface than the raw rice straw which might be because of the removal of wax coated on the surface of raw rice straw after hydrothermal treatment. Uniformly distributed pores can be observed on the surface of the hydrochar sample, and the pore structures are clearly limited by the textile structure of the rice straw fiber. Rough surface and pore structure of the rice straw hydrochar might provide adsorption sites for different adsorbates. 3.2 Adsorption capacity Fig. 1 and 2 display adsorption capacities at initial concentration of 0.5 mg/mL and 298 K for the three organics (Congo red, berberine hydrochloride and 2- naphthol) and two heavy metals (Zn2+ and Cu2+) on the rice straw hydrochars, respectively. Generally, the rice straw hydrochars prepared in this study can adsorb all the three organics and two heavy metals, and process conditions do not present linear correlations with the adsorption capacities of the hydrochars for all the five model compounds. The rice straw hydrochars exhibit similar elemental compositions and surface functional groups, most of them display close BET surface areas and pore volumes, and the complex influences of these might be the reasons for their similar and seemingly random adsorption capabilities for the organics and heavy metals. The adsorption capacities of Congo red were 208.7 mg/g to 222.1 mg/g, which are higher than those on bamboo hydrochars (6.0-33.7 mg/g at the equilibrium concentration of 0.1 mg/mL) (Li et al., 2016) and some other reported biochars such vermicompost-derived biochars (with calculated maximum adsorption capacity of 31.28 mg/g) (Yang et al., 2016), Korean cabbage biochar (with maximum adsorption capacity of

95.81 mg/g) and wood chip biochar (with maximum adsorption value of 110.1 mg/g) (Sewu et al., 2017). The adsorption amounts of berberine hydrochloride on these hydrochars were 148.0 mg/g to 174.0 mg/g, which are higher than those on some commercial macroporous resins (such as about 120 mg/g on HPD100B and 118 mg/g on AB-8) (Li et al., 2013). The adsorption capabilities of the hydrochars for 2-naphthol were 18.0 mg/g to 48.7 mg/g, comparable to those on bamboo hydrochars (Li et al., 2016). In addition, the rice straw hydrochars served adsorption quantities of 64.6 mg/g to 112.8 mg/g for Zn2+ and 48.3 mg/g to 144.9 mg/g for Cu2+, which are much higher than reported maximum adsorption capacities of Zn2+ and Cu2+ (about 10.3 mg/g and 9.7 mg/g for Zn2+ and Cu2+ respectively in uniform unit) on raw rice straw (Rocha et al., 2009). Rough surface and pore structure (observed from SEM images) of the rice straw hydrochars could benefit the accessibility of adsorbates to the hydrochars’ surfaces and the adsorption process, and these could be responsible for the higher adsorption capabilities of rice straw hydrochars for Zn2+ and Cu2+ than raw rice straw. Moreover, these adsorption values are also much higher than the adsorption amounts of Zn2+ and Cu2+ on pyrolytic biochars produced from wood (with maximum Zn2+ and Cu2+ adsorption capacities of 4.54 mg/g and 6.79 mg/g) and corn straw (with maximum Zn2+ and Cu2+ adsorption amounts of 11.0 and 12.52 mg/g) (Chen et al., 2011; Jiang et al., 2016). Pyrolytic biochars are produced by heating biomass at the temperature of 300-650 °C in the absence of oxygen (Kambo & Dutta, 2015), and the biochar products, especially biochars obtained at high temperatures could have a high degree of carbonization along with the loss of oxygen-containing groups on the surfaces (Keiluweit et al., 2010). Compared with pyrolytic biochars, the rice straw hydrochars prepared in this work have oxygen-rich functional groups, which could assist chemical and

semichemical interactions for organics, form complexes with heavy metal ions (Uchimiya et al., 2011), and so that lead to a more efficient adsorption of organics and heavy metals on the hydrochars. These results indicate that rice straw hydrochars could be promising adsorbents for removing organic pollutants and heavy metals from waste water. 3.3 Adsorption isotherms Five rice straw hydrochars providing higher adsorption amounts for each adsorbates and having higher yields among all the hydrochar samples were selected for the following test. Adsorption isotherms of the three organics and two heavy metals on selected hydrochar samples at 298, 308 and 318 K are displayed in Fig. 3 and 4, and the model fitting parameters along with the correlation coefficients from Langmuir and Freundlich equations were calculated. Correlation coefficients implied that both the Langmuir model (R2≥0.909) and the Freundlich model (R2≥0.918) can fit the adsorption isotherm data well for all the five model compounds. Normally, considering the heterogeneity of the hydrochars in physical-chemical properties (e.g., surface function groups and elemental compositions), it is reasonable to expect Freundlich model to give a better description for the isotherm curve. However, carbon materials are good adsorbents of microwave, and compared with traditional heating methods, microwave irradiation offers the advantages of heating from the interior of the material body, homogeneous heating and so forth (Menéndez et al., 2010). These may be favorable for the formation of a homogenous surface for the final hydrochar products and result in the good fit of Langmuir equation to the adsorption isotherms. The Qm values at 298 K from Langmuir model were 890.8, 565.0, 174.9, 216.9 and 226.8 mg/g for Congo red, berberine hydrochloride, 2- naphthol, Zn2+ and Cu2+ on selected hydrochars, respectively, the large values further confirm the effective

adsorption of the rice straw hydrochars for all the substances tested in this study. The 1/n values from Freundlich model were calculated to be 0.348-0.848 for Congo red, 0.2610.804 for berberine hydrochloride, 0.398-0.824 for 2- naphthol, 0.509-0.758 for Zn2+ and 0.244-0.703 for Cu2+, smaller than 1, indicating a favorable adsorption process for all these compounds on the rice straw hydrochars. Thermodynamic parameters of the adsorption for all the five model adsorbates on the selected rice straw hydrochars were calculated based on the Van't Hoff equation (Gupta et al., 2010), and the values were summarized in Table 2. The values of the Gibbs free energy (ΔG°, kJ/mol) were negative and within the range of physical adsorption (-20 to 0 kJ/mol) (Atkins, 1990) at all of the temperatures tested, indicating that the adsorption could occur spontaneously and is dominated by physical adsorption mechanism (Liu & Zhang, 2009) for all the three organics and two heavy metals on the selected rice straw hydrochars. The negative enthalpy (ΔH°, kJ/mol) values suggest an exothermic nature for the adsorption of all these adsorbates on the selected hydrochars. The entropy (ΔS°, KJ/mol.K) values of berberine hydrochloride, 2-naphthol and Cu2+ are positive, denoting an increased randomness at the solid liquid interface which reveals the irreversibility of the adsorption process for these adsorbates on the hydrochars, in addition, the absolute values of ΔH° for these substances are small (< 3.0 kJ/mol) implying weak interaction forces for the rice straw hydrochars. On the other hand, both the ΔS° and ΔH° values for the adsorption of Congo red and Zn2+ are negative, which indicates an enthalpically driven adsorption process of these substances on the selected hydrochars. 4. Conclusions

Rice straw hydrochars prepared from microwave-assisted hydrothermal carbonization were characterized and evaluated for the adsorption of five model substances from water. The hydrochars are rich in oxygen containing functional groups. The maximum adsorption amounts of Congo red, berberine hydrochloride, 2-naphthol, Zn2+ and Cu2+ on the hydrochars at 298 K and initial concentration of 0.5 mg/mL were 222.1, 174.0, 48.7, 112.8 and 144.9 mg/g, respectively, and the adsorption can happen spontaneously according to the thermodynamic analysis. These results suggest the potential application of rice straw hydrochars in wastewater treatment. E-supplementary data of this work can be found in online version of the paper. Acknowledgements This work was supported by the Foundation of Zhejiang Provincial Key Lab. for Chem. and Bio. Processing Technology of Farm Products, Zhejiang University of Science and Technology (grant numbers 2016KF0021), and the Interdisciplinary Pre-research Project of Zhejiang University of Science and Technology (grant numbers 2015JC05Y). References 1. Ahmad, M., Rajapaksha, A.U., Lim, J.E., Zhang, M., Bolan, N., Mohan, D., Vithanage, M., Lee, S.S., Ok, Y.S., 2014. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99, 19-33. 2. Antonetti, C., Licursi, D., Raspolli Galletti, A.M., Martinelli, M., Tellini, F., Valentini, G., Gambineri, F., 2016. Application of microwave irradiation for the removal of polychlorinated biphenyls from siloxane transformer and hydrocarbon engine oils. Chemosphere 159, 72-79. 3. Atkins, P.W., 1990. Physical chemistry, fourth ed. Oxford University Press, London.

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50. Yang, G., Wu, L., Xian, Q., Shen, F., Wu, J., Zhang, Y., 2016. Removal of Congo Red and Methylene Blue from Aqueous Solutions by Vermicompost-Derived Biochars. Plos One 11(5), e0154562. Figure Captions Fig. 1. Adsorption capacity of (a) Congo red, (b) berberine hydrochloride and (c) 2naphthol on the rice straw hydrochars at 298 K Fig. 2. Adsorption capacity of (a) Zn2+ and (b) Cu2+ on the rice straw hydrochars at 298 K Fig. 3. Adsorption isotherms of (a) Congo red onto sample M180/50, (b) berberine hydrochloride onto sample M180/60 and (c) 2- naphthol onto sample M200/60 Fig. 4. Adsorption isotherms of (a) Zn2+ onto sample M200/50 and (b) Cu2+ onto sample M160/60 Tables and Figures Table 1. The main properties of the rice straw hydrochars

Samples

Temperature (oC)

Reaction time (min)

Yield (%)

Ash content (%)

N (%)

C (%)

H (%)

S (%)

O (%)

BET surf area (m2/

M160/40

160

40

37.00

22.49

0.66

42.74

3.18

0.21

30.73

16.8

M160/50

160

50

38.23

20.38

0.41

38.99

5.53

0.04

34.66

11.0

M160/60

160

60

42.53

17.56

1.15

40.34

5.11

0.09

35.76

8.21

M160/70

160

70

39.91

14.36

0.93

43.31

5.62

0.06

35.72

10.2

M180/40

180

40

32.93

23.12

0.50

38.98

5.43

0.04

31.93

19.4

M180/50

180

50

42.31

22.13

0.48

40.06

5.52

0.03

31.79

15.9

M180/60

180

60

38.06

18.00

0.45

39.87

5.55

0.02

36.12

19.9

M180/70

180

70

37.28

14.87

0.40

41.66

5.72

0.02

37.34

16.7

M200/40

200

40

31.81

20.13

0.75

38.94

5.06

0.05

35.08

16.6

M200/50

200

50

36.22

19.53

0.75

37.44

4.85

0.04

37.4

25.4

M200/60

200

60

37.84

14.27

0.81

40.69

5.12

0.05

39.07

18.9

M200/70

200

70

37.73

15.00

0.54

38.31

5.05

0.35

40.76

16.0

Table 2. Adsorption thermodynamic parameters for the three organics and two heavy metals on the selected hydrochars Adsorbate

Hydrochar sample

Congo red

ΔG°(kJ/mol)

ΔH°

ΔS°

298 K

308 K

318 K

(kJ/mol)

(KJ/mol.K)

M180/50

-5.336

-4.961

-4.916

-11.54

-0.021

Berberine hydrochloride

M180/60

-4.961

-5.064

-5.212

-1.202

0.013

2-naphthol

M200/60

-4.002

-4.743

-4.278

-0.101

0.014

Zn2+

M200/50

-4.960

-4.412

-4.357

-13.861

-0.030

Cu2+

M160/60

-4.614

-4.415

-4.781

-2.032

0.008

250

200

Congo red

200

Berberine hydrochloride

150

Qe (mg/g)

100

100

50

50

0

0 M 16 0 M /40 16 0/ M 50 16 0 M /60 16 0/ M 60 16 0 M /70 18 0/ M 40 18 0 M /50 18 0 M /60 18 0/ M 70 20 0 M /40 20 0/ M 50 20 0/ 60

M 16 0 M /40 16 0/ M 50 16 0 M /60 16 0 M /60 16 0 M /70 18 0 M /40 18 0 M /50 18 0 M /60 18 0 M /70 20 0 M /40 20 0 M /50 20 0/ 60

Qe (mg/g)

150

Rice straw hydrochars

Rice straw hydrochars

(a) (b)

50

2-naphthol

Qe (mg/g)

40

30

20

10

0

0 60 70 40 50 60 70 40 50 60 70 40 /5 / / / / / / / / / / 0/ 60 60 60 80 80 80 80 00 00 00 00 16 M1 M1 M1 M1 M1 M1 M1 M2 M2 M2 M2 M Rice straw hydrochars

(c) Fig. 1. Adsorption capacity of (a) Congo red, (b) berberine hydrochloride and (c) 2naphthol on the rice straw hydrochars at 298 K 120

160

2+

Zn

2+

Cu

140

100

120 100

Qe (mg/g)

Qe (mg/g)

80 60 40

80 60 40

20

20

0

0

M 16 0/ 40 M 16 0/ 50 M 16 0/ 60 M 16 0/ 70 M 18 0/ 40 M 18 0/ 50 M 18 0/ 60 M 18 0/ 70 M 20 0/ 40 M 20 0/ 50 M 20 0/ 60 20 0 70

0 0 0 40 50 60 70 40 50 60 70 /40 /5 /6 /7 0/ 0/ 0/ 0/ 0/ 0/ 0/ 0/ 0 00 200 200 16 16 16 16 18 18 18 18 20 M2 M M M M M M M M M M M

Rice straw hydrochars

Rice straw hydrochars

(a) (b) Fig. 2. Adsorption capacity of (a) Zn2+ and (b) Cu2+ on the rice straw hydrochars at 298 K

250

200 298K 308K 318K Langmuir Freundlich

200

298K 308K 318K Langmuir Freundlich

150

Qe(mg/g)

Qe(mg/g)

150

100

100

50

50

0

0 0.0

0.1

0.2

0.3

0.4

0.0

0.1

Ce(mg/mL)

0.2

0.3

Ce(mg/mL)

(a) (b)

80

298K 308K 318K Langmuir Freundlich

Qe(mg/g)

60

40

20

0 0.0

0.1

0.2

0.3

0.4

Ce (mg/mL)

(c) Fig. 3. Adsorption isotherms of (a) Congo red onto sample M180/50, (b) berberine hydrochloride onto sample M180/60 and (c) 2- naphthol onto sample M200/60

0.4

160

140

298K 308K 318K Langmuir Freundlich

140 120

100

Qe(mg/g)

Qe(mg/g)

100

298K 308K 318K Langmuir Freundlich

120

80 60

80 60 40

40

20

20 0

0

0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

Ce(mg/mL)

0.2

0.3

0.4

0.5

Ce(mg/mL)

(a) (b) Fig. 4. Adsorption isotherms of (a) Zn2+ onto sample M200/50 and (b) Cu2+ onto sample M160/60

Microwave Assisted Hydrothermal Preparation of Rice Straw Hydrochars for Adsorption of Organics and Heavy Metals Yin Li a, Nyamkhand Tsend a, TiKai Li a, Heyang Liu a, Ruiqin Yang a, Xikun Gai a, Hongpeng Wang a, Shengdao Shan b * a Zhejiang

Provincial Key Lab for Chemical and Biological Processing Technology of

Farm Product, School of Biological and Chemical Engineering, Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China b Key

Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province,

Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China Graphical abstract * Corresponding Author.

* Shengdao Shan, Tel: 86-571-85070006. E-mail address: [email protected] (S. Shan)

Microwave-assisted hydrothermal carbonization

Rice straw

Hydrochar

O N+

O

Cu2+ Zn2+

Cl-

OH

OCH3

OCH3 NH2

NH2 N

O

Before adsorption

S ONa

O

N

N

N

O

S

O

ONa

After adsorption

Graphical abstract

Microwave Assisted Hydrothermal Preparation of Rice Straw Hydrochars for Adsorption of Organics and Heavy Metals Yin Li a, Nyamkhand Tsend a, TiKai Li a, Heyang Liu a, Ruiqin Yang a, Xikun Gai a, Hongpeng Wang a, Shengdao Shan b * a Zhejiang

Provincial Key Lab for Chemical and Biological Processing Technology of

Farm Product, School of Biological and Chemical Engineering, Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China b Key

Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province,

Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China Highlights * Corresponding Author.

* Shengdao Shan, Tel: 86-571-85070006. E-mail address: [email protected] (S. Shan) Highlights 

Microwave-assisted hydrothermal carbonization of rice straw



Relation between microwave conditions and hydrochars’ physical-chemical properties



Adsorption of three organics and two heavy metals on the hydrochars