Production Temperature Effects on the Structure of Hydrochar-Derived

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Sustainability Engineering and Green Chemistry

Production Temperature Effects on the Structure of Hydrocharderived Dissolved Organic Matter and Associated Toxicity Shilai Hao, Xiangdong Zhu, Yuchen Liu, Feng Qian, Zhi Fang, Quan Shi, Shicheng Zhang, Jianmin Chen, and Zhiyong Jason Ren Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04983 • Publication Date (Web): 27 May 2018 Downloaded from http://pubs.acs.org on May 27, 2018

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Environmental Science & Technology

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Production Temperature Effects on the Structure of Hydrochar-derived

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Dissolved Organic Matter and Associated Toxicity

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Shilai Hao,1 Xiangdong Zhu,1, 2,* Yuchen Liu,1 Feng Qian,1 Zhi Fang,3 Quan Shi,3 Shicheng

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Zhang,1,4* Jianmin Chen,1 Zhiyong Jason Ren2

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1 Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of

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Environmental Science and Engineering, Fudan University, Shanghai 200433, China

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2 Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder,

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Boulder, CO 80309, United States

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3 State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

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4 Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China

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* Corresponding author, Tel/fax: +86-21-65642297; E-mail: [email protected]

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(Xiangdong Zhu), [email protected] (Shicheng Zhang).

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Word Count:

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Words: 4664

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Figures: 6 × 300 = 1800 words

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Total: 6464

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ABSTRACT

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Hydrochar is a carbonaceous material derived from hydrothermal liquefaction, and it carries

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good potential as a new material for environmental applications. However, little is known about

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the dissolved organic matter (DOM) associated with hydrochar and the consequences of its

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release. The relationship between the production temperature and the characteristics of DOM

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released from hydrochar, as well as the associated biotoxicity were investigated using a suite of

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advanced molecular and spectroscopic tools. With the increase in production temperature, the

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resulted hydrochar-based DOM contained higher content of phenols and organic acids but less

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sugars and furans. Meanwhile, the molecular structure of DOM shifted to lower molecular

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weight with higher organic contents containing < 6 O atoms per compound, aromatics, and

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N-containing substances. While low-temperature hydrochar-derived DOM showed minimal

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biotoxicity, increase in production temperature to 330 °C led to a great rise in toxicity. This

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might be attributed to the increased contents of phenols, organic acids, and organics containing
240 °C).35, 36 In addition,

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the degree of decarboxylation, dehydration, and condensation reactions that produce large

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molecules increases greatly with increasing HTL temperature. As a result, high HTL

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temperatures yield highly aromatic and low molecular weight (MW) compounds.37 Due to the

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strong hydrophobicity of these compounds, significant amounts of these compounds accumulate

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on the hydrochar surface rather than in HTL aqueous solutions.20 Thus, the molecular

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composition of hydrochar-based DOM could be strongly affected by temperature. This change in

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molecular composition would ultimately result in significant differences in the biotoxicity of

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hydrochar-based DOM. If the relationship between HTL temperature and hydrochar-based DOM

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properties and the associated toxicities are clarified, upstream control of the hydrochar

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production process to decrease its potential environmental risk would be possible. Therefore,

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studying the effects of temperature on the molecular structure and potential biotoxicity of 4 ACS Paragon Plus Environment

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hydrochar-based DOM is important.

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Due to the compositional complexity of hydrochar-based DOM, single conventional analytical

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methods such as gas chromatography-mass spectrometry (GC-MS) cannot fully characterize the

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molecular and structural properties of DOM. Recently, electrospray ionization (ESI) Fourier

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transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has proven to be

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advantageous in analyzing DOM from different aquatic environments.38-42 Due to its ultrahigh

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resolution, a much broader scale of compounds in DOM and accurate molecular formulas can be

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determined unambiguously, including those containing hetero-atoms (e.g., N and S). This

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method has allowed researchers to determine the molecular fingerprints of complex DOM, such

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as aromaticity and double-bond equivalent (DBE), and track the molecular changes of DOM. By

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combining ESI FT-ICR MS with other conventional characterization methods, such as

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excitation-emission matrix fluorescence spectroscopy (EEM), the molecular structures of

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hydrochar-based DOM can be clearly identified, and the potential mechanism of toxicity of

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DOM can be better interpreted.

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Therefore, this study comprehensively characterized the structural differences and biotoxicity

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of hydrochar-based DOM under different conditions with the aim to provide deeper

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understanding of this material to guide future production and application. A series of hydrochar

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products were prepared under different HTL temperatures (180 – 330 °C), and the characteristics

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of DOM released into an aquatic environment were investigated using various advanced

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molecular and spectroscopic tools. The biotoxicity of DOM was investigated based on

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cyanobacteria (Synechococcus sp), and possible growth inhibition mechanisms were discussed. 5 ACS Paragon Plus Environment


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Materials and Methods

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Hydrochar and DOM Samples

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Hydrochar was produced from 100 mesh bamboo (from Zhejiang Province) through

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hydrothermal liquefaction. In a typical experimental run, 15 g (dry weight) of bamboo and 150

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mL of water were loaded in the autoclave, heated up to the desired temperature for 60 min, and

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then cooled with tap water to room temperature. The resulting solid product was recovered by

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filtration and was then freeze-dried to obtain the hydrochar material. The bamboo-derived

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biochar was produced through pyrolysis under N2 flow of 500 mL min-1 at 500 °C for 60 min.

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The characterization methods of hydrochar were described in Supporting Information (SI) Text

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S1. The release of DOM from hydrochar was performed by soaking 0.1 g of hydrochar sample in

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50 mL of ultrahigh-quality Milli-Q water in a 60-mL amber glass vial. This mixture was shaken

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at 150 rpm for various durations (0.5, 1, 2, 4, 8, 12, and 24 h) to examine the release kinetics of

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DOM. To observe the effects of solution pH on the release of DOM, the solution pH was

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adjusted with a small volume of 0.1 M HCl or NaOH solution (See SI Table S1). The contact

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time was controlled at 24 h to evaluate the effects of hydrochar production temperature variation

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on its DOM release. The DOM solution was collected by filtering through a 0.45-µm

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polytetra-fluoroethylene membrane.

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Characterization of DOM Samples

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DOM samples were analyzed using a pH meter, a total organic C (TOC) analyzer, and

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ultraviolet-visible (UV-vis) spectrometry. Ion chromatography (IC) was used to analyze sugars 6 ACS Paragon Plus Environment

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and organic acids.

C nuclear magnetic resonance (NMR) spectra were used to analyze the

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molecular structure of the DOM, while GC-MS was used for phenol and furan compound

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analysis (See Text S2 for detailed methods).

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ESI FT-ICR MS analysis was performed on a Bruker Apex-ultra FT-ICR mass spectrometer

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equipped with a 9.4 T superconducting magnet and Apollo II ESI source. The DOM samples

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were diluted in methanol with a concentration of about 0.1 mg/mL and directly injected into the

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electrospray source at 180 µL/h using a syringe pump. The operating conditions for negative-ion

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formation consisted of 4.0 kV spray shield voltage, 4.5 kV capillary column introduced voltage,

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and -320 V capillary column end voltage. The 4M word size was selected for the time domain

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signal acquisition. A total of 128 scan FT-ICR data sets were accumulated to enhance the

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signal-to-noise ratio and dynamic range. The methodologies used for FT-ICR MS mass

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calibration, data acquisition, and processing are described elsewhere.43 Baseline scans of

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methanol were performed to ensure that the instrument was clean before analyzing the samples.

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EEM fluorescence spectroscopy spectra were recorded using a fluorometer (Aqualog;

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Horiba-Jobin Yvon, USA). The fluorometer was set up as follows: the excitation wavelength was

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incrementally increased from 240 to 550 nm in 3-nm intervals, with emission monitoring from

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220 to 600 nm at 2-nm intervals for each excitation wavelength. Quinine sulfate standards were

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used to calibrate the EEM spectra, and the fluorescence intensities were expressed in quinine

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sulfate equivalent units. After removing additional regions dominated by Rayleigh and Raman

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peaks, as well as regions without fluorescence, parallel factor (PARAFAC) modeling was

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conducted with non-negative constraints using MATLAB ver. 8.5.0197613 (R2015a) with PLS 7 ACS Paragon Plus Environment


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toolbox ver. 8.0.1.44

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The cyanobacteria (Synechococcus sp. (PCC 7942)) used in the assay were obtained from the

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Institute of Hydrobiology, Chinese Academy of Sciences (Wuhan, China). Bioassays of DOM

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using cyanobacteria were performed in 50-mL triangular flasks containing 15 mL of BG-11

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medium and 10 mL of different DOM samples to reach a final DOM concentration of ~ 16 mg

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C/L. The flasks were illuminated in an incubator for 7 days with a light: dark cycle of 12:12 h.

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Cyanobacterial growth was monitored daily by measuring the change in absorbance at 680 nm

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(algal chlorophyll absorbance peak) using a BioTek Synergy HT multimode microplate reader

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(BioTek Instruments; Winooski, VT). Microphotographs of selected cyanobacteria were taken

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under a microscope (Eclipse Ti-s; Nikon, Japan) with a CCD camera (DS-Ri1; Nikon, Japan).

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Results and Discussion

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Carbon Transformation in Hydrochar and Hydrochar-based DOM Release

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Figure 1 presents the basic elemental analyses of the hydrochar samples based on FTIR, NMR,

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and thermogravimetry. As the hydrochar production temperature increased, the graph describing

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hydrochar properties changed visibly. These changes included a decline of H/C and O/C atomic

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ratios based on a Van Krevelen (VK) diagram, as well as lower aliphatic and higher aromatic

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components as indicated by the FTIR (Text S3) and 13C NMR results. This was attributed to the

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increase in the degree of cellulose and lignin cracking at higher HTL temperatures. As seen in the

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thermograms (Figure 1c), high-temperature-derived hydrochar showed higher recalcitrance as

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suggested by a lower weight loss and higher retention weight.45 This higher recalcitrance 8 ACS Paragon Plus Environment

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indicated that hydrochar had higher aromaticity.45 Overall, hydrochar properties were strongly

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affected by the production temperature, which is expected to further affect hydrochar-based

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DOM release into the environment.

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The time-dependent DOM (mg C/g of char) release kinetics indicated that the amount of DOM

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released from hydrochar sharply increased within the initial 2 h and then plateaued after 12 h

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(Figure 2a). The calculated partitioning coefficients for DOM-like material that partitioned from

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hydrochar to water at varying pH indicated that only 3.5 – 6.5 % TOC in hydrochar was

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extractable (Table S2). Such variation of partitioning coefficients resulted from the synthetic

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effects of various surficial properties of hydrochars and aquatic environments. Generally, higher

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temperatures promoted DOM release from hydrochar, possibly caused by a gradual elevation in

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the amount of biomass cracking during the HTL process. With increasing hydrochar production

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temperature, more cracked hydrophobic organic compounds were enriched on the surface of

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hydrochar, resulting in the release of more DOM. Notably, when compared with biochar

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produced by pyrolysis at 500 °C, hydrochar released at least ten times more DOM than biochar

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(Figure 2a). This suggested that when used as an environmental amendment, hydrochar should

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be more carefully characterized and evaluated compared with biochar from the perspective of

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global C cycle and environmental impacts. In addition, alkaline conditions resulted in greater

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DOM release (Figure 2b), especially at pH 10, suggesting that alkalinity facilitated the release of

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DOM from hydrochar. This could be attributed to a larger amount of acidic organic compounds

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released from hydrochar in alkaline solutions due to the acidity of hydrochar.20



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Molecular Composition of DOM Released from Hydrochar Produced at Different

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Temperatures

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The chemical composition (i.e., compounds detectable with GC-MS and IC) of DOM changed

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significantly under different production temperatures (Figure 2c). With increasing temperature,

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the total amount of phenols and organic acids increased, the amount of sugar compounds

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decreased, while furans first increased followed by a decrease. Table S3 showed the results of

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quantitative analysis of each compound as a function of hydrochar production temperature. At

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lower temperatures between 180 – 240 °C, hemicellulose cracking dominated the process with

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more furans and sugars generation, while lignin was not readily hydrolyzed. Elevating the

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temperature above 240 °C enabled further degradation of furans and production of organic

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acids.46, 47 In addition, lignin was gradually cracked to produce more phenols with increasing

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hydrochar production temperature. These intermediates partially deposited on the surface of

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hydrochar and contributed to hydrochar-based DOM. This was confirmed by the strong linear

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correlation between hydrochar production temperature and the total amount of phenols in DOM

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(Figure 2d). Through GC-MS and IC analyses, the low molecular weight volatile organic

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compounds (i.e., furan and phenol) and the common organic acids (i.e., acetic acid) were well

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detected and quantified. However, these two techniques cannot work well for high molecular

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weight organic compounds with high boiling points. Furthermore, the C distribution of DOM

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indicated that up to 95.8% DOM was not identified and quantified with these conventional

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chromatography and mass spectral analyses (Figure S1). This was due to the fact that many polar

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organic compounds with high molecular weight cannot be analyzed by GC-MS and IC 10 ACS Paragon Plus Environment

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techniques.

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To address this limitation, state-of-the-art negative ion ESI FT-ICR MS was used to further

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characterize the molecular composition of hydrochar-based DOM. The MW distribution of DOM

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ranged from 150 to 550 Da (Figure S2). Thousands of molecular formulas of DOM were

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identified by assigning molecular formulas (CaHbNcOd) to each peak (ESI FT-ICR MS molecular

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information file).48 It should be noted that ionization with negative ESI (-ESI) is suitable for

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polar compounds with acidic functionalities49 and less-volatile components that cannot be

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detected by GC-MS. Generally, high-temperature hydrochar-based DOM contained more types

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of organic compounds (Table S4). For example, 773 organic compounds were identified in

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hydrochar-based DOM (180 °C), while 1301 organic compounds were detected in

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hydrochar-based DOM (270 °C), indicating the progression of depolymerization reactions.

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However, when the hydrochar production temperature exceeded 270 °C, the number of organic

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compounds detected slightly decreased. This could be due to the re-polymerization of small

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organic compounds under the high temperature and high pressure HTL reactions. ESI FT-ICR

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MS greatly expands the scope of molecular information for DOM compared with conventional

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analyses. Furthermore, the peak of the m/z distribution representing the major compounds in

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DOM shifted from MWs of 350 – 400 to MWs of 250 – 300 with increasing HTL temperature

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(Figure S3a), which was consistent with the change of average MW of DOM (Table S4). The

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proportion of molecules with MWs of 350 – 400 showed a strong negative linear correlation with

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hydrochar production temperature (Figure S3b), supporting the initial observation that higher

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hydrochar production temperature promoted depolymerization reactions during HTL and resulted 11 ACS Paragon Plus Environment


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in a shift towards molecules with lower MWs. Figure S4 showed a scale-expanded view of the

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ESI FT-ICR MS mass spectra of DOM at m/z masses of 417 and 418. It was observed that DOM

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was composed of CHO and CHON compounds and higher production temperature led to higher

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molecular diversity. These results further confirmed the effects of hydrochar production

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temperature on the molecular composition changes in hydrochar-based DOM.

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Figure 3 and Figure S5 depicted the VK diagrams (from the ESI FT-ICR MS data) for the

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CHO and CHON molecules, respectively. The number of CHON compounds steadily increased

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in DOM as a function of hydrochar production temperature, which became evident at 330 °C

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(Figure S5). Table S4 also indicated that 330 °C hydrochar based-DOM had the highest

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percentage of CHON compounds. This indicated that more CHON compounds accumulated on

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the hydrochar surface under high temperature as a result of increased destruction of larger

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N-containing molecules.50 Similarly, DOM released from high-temperature hydrochar contained

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higher diversity of CHO compounds (Figure 3). It should be noted that the proportion of CHO

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compounds with low O/C and H/C ratios (i.e., aromaticity index) increased in DOM with

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increasing HTL temperature, indicating the progression of deoxygenation and dehydrogenation

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during HTL. Meanwhile, according to the aromaticity index AImod,51 most compounds (68~76%)

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in DOM were aliphatic compounds (Figure 4a). Moreover, increasing hydrochar production

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temperature resulted in less release of aliphatic compounds but more aromatic compounds. This

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was understandable, as hemicellulose and cellulose were degraded into aliphatic compounds at

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low HTL temperatures, while the cracking of lignin resulted in more aromatic compounds at high

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HTL temperature. The changes in aromatic compounds were in agreement with the changes in 12 ACS Paragon Plus Environment

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O/C values in the VK diagrams. Overall, DOM released from hydrochar produced at high

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temperatures contained more diverse molecules and aromatic compounds.

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The detailed variations of each class of species among the types of DOM were further

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analyzed. As HTL temperature increased, Ox class molecules (organic compound with x oxygen

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atoms) from hydrochar-based DOM shifted to organics that contained < 6 O atoms per

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compound (Figure 4b and Table S4). The dominant Ox classes were O6 to O9 at lower

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temperatures (180 – 240 °C) and O4 to O5 at high temperatures (270 – 330 °C). A similar shift of

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< 6 O atoms per compound in the N1Ox class (organic compound with 1 nitrogen atom and x

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oxygen atoms) was also observed (Figure 4c). These shifts in Ox and N1Ox classes indicated that

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hydrochar produced at higher HTL temperatures accumulated more compounds with < 6 O

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atoms as a result of increased depolymerization and deoxygenation. Notably, there was an

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increase in the proportions of N1O4 and N1O5 class compounds in DOM from hydrochar

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produced at 330 °C compared with other hydrochar-based DOM. Thus, hydrochar-based DOM

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(330 °C) exhibited distinct toxic effects compared to other DOM.

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To further investigate the DOM structure, relative abundance maps of DBEs as a function of C

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number were plotted for each chemical class for different DOM (Figure S6). Generally, as

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hydrochar production temperature increased, the diversity of each class compound increased and

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showed a shift to higher DBE areas. This indicated that these species became more scattered and

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unsaturated, further illustrating the enhancement of diversity and aromaticity in DOM molecules.

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With exact C number and DBE value, the changes in organic acid compounds in different

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hydrochar-based DOM could be further elucidated. More long-chain organic acids (e.g., 13 ACS Paragon Plus Environment


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C14H28O2, tetradecanoic acid, C15H30O2, pentadecanoic acid, and C17H34O2, heptadecanoic acid)

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were released from the surface of high-temperature hydrochar (Figure 4d). In general, with

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increased HTL temperature, the MWs of the molecules in DOM, the amount of sugar compounds,

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the ≥ 6 O class compounds, and aliphatic compounds decreased, while the proportion of CHON

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compounds, < 6 O class compounds, acidic compounds, and aromatic and unsaturated

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compounds increased.

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Structural Properties of DOM from Hydrochar Produced at Different Temperatures

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The UV-vis spectrum of DOM had a characteristic absorbance at 232 nm corresponding to

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π-π* transitions of aromatic C=C bonds (Figure S7). DOM derived from hydrochar produced at

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higher HTL temperatures showed higher absorbance values, indicating that it contained

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substances with more aromatic structures, which was in agreement with the FT-ICR MS

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molecular composition analysis.

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The EEM spectra showed a marked increase in fluorescence intensity in DOM from hydrochar

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produced at high temperatures (Figure S8). Moreover, the range of fluorescence increased with

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increasing hydrochar production temperature, indicating that more types of optical substances

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were released from the hydrochar. The EEM-PARAFAC analysis of the fluorescence spectra

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supported a model containing four components (I, II, III, and IV) (Figure 5). Components I and

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II presented excitation/emission maxima at 250/325 and 240/325 nm, respectively. These two

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components could be substances with phenolic, less aromatic, or protein-like structures which

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are located in the low excitation/emission region of the EEM.27 Component III featured two 14 ACS Paragon Plus Environment

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peaks at 240/450 nm and 340/450 nm, which are classified as UVC and UVA humic-like

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substances (hydrophobic fraction with large molecular size).52 Component IV showed a peak in

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270/300 nm, which had lower emission wavelengths than components I and II. This red shift in

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the excitation/emission maximum in component IV may be associated with an increased

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aromaticity and higher molecular weight relative to components I and II.27, 53, 54 The standard

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EEM spectra of the compounds in DOM from the GC-MS and IC analyses were compared to the

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four PARAFAC components to clarify the connection between the optical properties and

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molecular composition of DOM (Figure S9 and Table S5). Most phenols showed fluorescence

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absorptions similar to the components I, II, and IV, while furans, sugars, and organic acids

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exhibited weak fluorescence absorption. This suggested that the main optical substances in DOM

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were phenol-like aromatic compounds. The relative abundances of the model components (based

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on released concentrations) showed that component I accounted for > 40% of the optical

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substances in DOM (Figure S10a). Notably, the relative abundance of component IV showed a

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marked increase in DOM from hydrochar produced at 330 °C compared to other DOM (Figure

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S10a). Furthermore, component IV may have been derived from < 6 O atoms compounds in the

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N1Ox class, which was strongly supported by the positive correlations between the percentage of

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N1O5 and N1O4 classes and relative abundance of component IV (Figure S10b, c). Component IV

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may also have been derived from increased aromatic Ox and N1Ox class compounds, since ESI

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FT-ICR MS results showed that each class compound showed a shift to higher DBE areas (more

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aromatic) with increasing production temperature. Overall, aromatic substances, especially

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phenol-like compounds, were the most abundant optical substances in DOM. With increasing 15 ACS Paragon Plus Environment


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hydrochar production temperature, the changes in the molecular compositions of DOM (e.g.,

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2,6-dimethoxyphenol) were in accordance with the changes in the relative abundance of their

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attributed modeled components.

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The solution-state

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C NMR spectra of DOM were used to further investigate the molecular

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structure of DOM (Figure S11). The compositions of functional moieties were compiled in Table

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S6. Aromatic C (90 – 146 ppm) and alkyl C (0 – 45 ppm) were the main C-containing functional

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groups in all DOM samples (Table S6). Typically, with increasing hydrochar production

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temperature, hydrochar-based DOM contained more aromatic C and alkyl C but less O-alkyl

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derivatives. This is because the feedstock (bamboo) underwent more intensive dehydration and

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condensation at higher temperature that resulted in the corresponding change in DOM structure.

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In addition, the proportion of ester functional groups (COOR, 165 – 185 ppm) increased slightly,

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in accordance with the increase of carboxylic acids (e.g., tetradecanoic acid) in DOM as a

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function of hydrochar production temperature. Overall, the NMR-derived characteristics of the

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DOM samples were in agreement with the results of the UV-vis and EEM analyses.

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Bioassay Shows Biotoxicity of DOM from Hydrochar Produced at Different Temperatures

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The bioassay results showed that most hydrochar-based DOM (210 ~ 300 °C) caused slight

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inhibition of cyanobacterial growth (Figure 6a). Interestingly, hydrochar-based DOM (180 °C)

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and biochar-based DOM (500 °C) showed no biotoxic effects on cyanobacteria, rather it

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exhibited a stimulatory effect. However, when production temperature was increased from

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210 °C to 300 °C, the biotoxicity of hydrochar-based DOM increased with an inhibition rate 16 ACS Paragon Plus Environment

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from 6-15%. At the highest production temperature (330 °C), hydrochar-based DOM completely

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inhibited cyanobacterial growth. The microphotographs of the growth of cyanobacteria

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confirmed the trend of DOM biotoxicity (Figure 6b). From the perspective of biotoxicity, more

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attention should be placed on the environmental risk of high temperature-derived hydrochar (>

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330 °C). Furthermore, hydrochar production needs to be optimized to ensure the environmental

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safety of hydrochar.

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The difference in biotoxicity among different DOM samples could be related to several distinct

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changes of the chemical components. In a previous study, the toxicity of DOM from biochar was

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believed to be caused by phenolic compounds or low-MW organic acids.24, 55 Based on the

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molecular composition changes in the hydrochar-based DOM under different temperatures, the

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increased amounts of phenol-like compounds and organic acids in DOM partially contributed to

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the increased toxic effects on cyanobacteria. In addition, hydrochar-based DOM (330 °C) had the

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highest abundance of N1Ox compounds with < 6 O atoms and component IV. This suggested that

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< 6 O class compounds and CHON compounds in DOM may contributed to the increased

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toxicity of DOM from high-temperature hydrochar. Furthermore, the proportions of N1O5, N1O4,

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and N1O3 classes positively correlated with the inhibition rate of cyanobacteria (Figure S12 and

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Figure 6c), suggesting that N1Ox class compounds with < 6 O atoms strongly inhibited

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cyanobacterial growth. This might be explained by the N1Ox class compounds’ ability to easily

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penetrate into the cell membranes of cyanobacteria, inducing strong biotoxicity. However, each

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compound’s exact contribution to toxicity remained to be further analyzed due to the complex

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mixture of different compounds in DOM; the toxic effects could be due to a combination of 17 ACS Paragon Plus Environment


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multiple mechanisms. Overall, these findings indicated that phenols, organic acids, and

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N-containing compounds with < 6 O atoms contribute to the inhibition of cyanobacterial growth.

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Environmental Implications

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This study showed for the first time that hydrochar generated from HTL can release high

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concentrations of DOM in aquatic environments. This will significantly impact hydrochar

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production and its environmental applications. The hydrochar produced at high production

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temperatures released more phenols, organic acids, aromatic substances, as well as species with

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< 6 O atoms. This was likely due to the progression of cracking reactions during the HTL process,

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such as dehydration and deoxygenation. Markedly, hydrochar-derived DOM (330 °C) exhibited

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the greatest inhibition of cyanobacterial growth, which was probably related to the increase in

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N1Ox class compounds or higher aromaticity compounds. The comprehensive molecular and

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spectroscopic characterizations used in this study offered a critical dataset of hydrochar-based

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DOM for future research at a molecular level. Such understanding of the DOM composition,

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structure, and toxicity provided insights on the potentials and challenges of the eco-friendly use

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of hydrochar materials. Therefore, material optimization and functionalization should be

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conducted in a more controlled manner with high functionality and low toxicity in the future of

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hydrochar commercialization. While this study revealed the characteristics of DOM from

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hydrochar, future studies are needed to investigate HTL operation methods in order to generate

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hydrochar with less toxicity without affecting bio-oil quality. It is also necessary to develop

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post-treatment methods to detoxify hydrochar and improve beneficial use of this material. 18 ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS

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This research was supported by the National Key Research and Development Program of

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China (Grant No. 2017YFC0212205), the National Key Technology Support Program (Grant No.

368

2015BAD15B06), the National Natural Science Foundation of China (Grant No. 21577025), the

369

International Postdoctoral Exchange Fellowship Program of China Supported by Fudan

370

University and the State Key Laboratory of Heavy Oil Processing.

371

Supporting Information

372

The hydrochar characterization procedures are provided in Text S1. The analytical procedures

373

to determine the basic characteristics of DOM are presented in Text S2. FTIR analysis of

374

hydrochar is shown in Text S3. The volume of 0.1 M HCl or NaOH solution added to adjust

375

DOM solution pH is shown in Table S1. The partitioning coefficients for DOM-like material that

376

partitioned from hydrochar to water at varying pH are in Table S2. The main chemical

377

compounds detected by GC-MS and IC are in Tables S3. The characteristic information of DOM

378

by ESI FT-ICR MS is listed in Table S4. The fluorescence peak positions of the main identified

379

compounds in DOM are shown in Table S5. Relative proportions of the chemical functional

380

groups are listed in Table S6.

381

The C distribution, broadband ESI FT-ICR MS spectra, MW distribution, VK plots for the

382

CHON molecular formulas, and expanded ESI FT-ICR mass spectra of DOM are listed in

383

Figures S1 – S5. The DBE versus C number distribution of the different DOM are shown in

384

Figure S6. The UV-vis spectra, EEM, and

385

S11. The correlations between cyanobacterial inhibition rate and proportions of N1Ox class

13

C NMR spectra of DOM are listed in Figures S7 –



386 387 388

compounds are shown in Figure S12. The molecular formulas and intensity for the compounds in DOM by ESI FT-ICR MS was attached as an Excel file (FT ICR MS Information).

389


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Page 21 of 32


390

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521 522



3.0

a

2.5 Dehydration

H/C

2.0

Demethanation Bamboo 180°C

1.5

210°C 240°C

1.0

270°C 300°C 330°C

0.5 0.0 0.00

0.25

0.50

0.75 1.00 O/C

1.25

-OH

Transmittance (arbitrary units)

Decarboxylation

Page 26 of 32

aliphatic CH2

330 °C

C=O

300 °C 270 °C 240 °C

1200

210 °C 550

180 °C 2915

2840

3415

1703 1603 1113 1030 1504, 1460

1.50 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1)

Aromatic C 100

b

C=C C=C C-O-C C-O

d 330 °C

c

300 °C

Mass (%)

80 60 40

270 °C

180 °C 210 °C 240 °C 270 °C 300 °C 330 °C

240 °C

210 °C

180 °C

20 100 200 300 400 500 600 700 800 200 Temperature (°C)

160

120 80 40 Chemical shift (ppm)

0

Figure 1. (a) Effects of production temperature on H/C and O/C atomic ratios for hydrochar samples in Van Krevelen diagram, (b) FTIR spectra, (c) thermograms plots, and (d) Solid-state 13

C CP/MAS NMR spectra of hydrochar samples derived from varied production temperature.



330 °C

28

240 °C 270 °C

24

300 °C 210 °C

20

180 °C

16 2 0

500 °C (biochar)

0

Mass release (mg /g Char)

1.6 1.2

4

8

12 16 Time (h)

Phenols Organic acids Furans Sugars

20

DOM Released (mg C/g Char)

a

32

36 30 24

24

c

0.8 0.4 0.0

b

180 °C 210 °C 240 °C 270 °C 300 °C 330 °C

42

18 4

Total released phenols (mg/g char)

DOM Released (mg C/g Char)

Page 27 of 32

180 210 240 270 300 330 Hydrochar production temperature (°C)

5

6

7 pH

8

9

10

d

1.6

1.2

y=0.007x-1.06 R2=0.97

0.8

0.4 180 210 240 270 300 330 Hydrochar production temperature (°C)

Figure 2. (a) Release kinetics of DOM from biochar and hydrochar produced at different temperatures, (b) the effect of solution pH on the amount of DOM released from hydrochar (24 h release), (c) the effect of hydrochar production temperature on the amount of detailed organic compounds in DOM, and (d) the correlation between hydrochar production temperature and total phenols in DOM.



2.0 Lipids

210 °C

240 °C

CHO: 777

CHO: 847

270 °C

300 °C

330 °C

CHO: 929

CHO: 866

CHO: 651

Proteins/amino sugars 180 °C

1.0

Unsaturated hydrocarbons

H/C

CHO: 772

1.5

Lignins

Condensed aromatics

0.5 2.0

H/C

Page 28 of 32

1.5

1.0

0.5 0.0

0.2

0.4 O/C

0.6

0.8

0.2

0.4 O/C

0.6

0.8

0.2

0.4 O/C

0.6

0.8

Figure 3.Van Krevelen plots for CHO molecular formulas assigned to the ESI FT-ICR MS spectral peaks in different HTL temperature derived hydrochar-based DOM. The size of sphere represents the relative abundance of one type of molecular formula. The CHO number represents the numbers of CHO molecules in DOM. Boxes overlain on the plots indicate biomolecular compound classes.


Page 29 of 32


a

28

Aliphatic Aromatic Condensed Aromatic

60

40

b 330 °C

300 °C

270 °C

240 °C

210 °C

180 °C

24

Percentage (%)

Relative abundance (%)

80

20

20 16 12 8 4

0 180

210

240

270

300

0

330

O2 O3 O4 O5 O6 O7 O8 O9 O10 Ox Class Species

Hydrochar production temperature (°C)

O

d 7

HO O

c

330 °C

300 °C

270 °C

240 °C

210 °C

C15H30O2

HO

180 °C

O

6

C17H34O2

HO

330 °C

5

DBE = 1

Percentage (%)

C14H28O2

4

300 °C 270 °C

3

240 °C

2

210 °C

1 180 °C

0 16

N1O3 N1O4 N1O5 N O N1O7 N1O8 1 6 N1Ox Class Species

20 24 Carbon Number

28

Figure 4. (a) Aliphatic and aromatic compounds distribution in DOM from hydrochar produced at different temperatures. Formulas are classified as aliphatic (AImod < 0.5), aromatic (0.5 < AImod < 0.67), and condensed aromatic (AImod > 0.67). AImod (aromaticity index) was calculated based on reference 45. (b) Ox and (c) N1Ox species distribution in different hydrochar-based DOM. (d) DBE (double-bond equivalent) versus carbon number distribution of possible organic acids in different hydrochar-based DOM detected by ESI FT -ICR MS. The sphere size represents the relative abundance of one type of molecular formula.



I

Page 30 of 32

II

III

IV

Figure 5. Fingerprint EEM from 4 component PARAFAC (parallel factor) modeling of hyrochar-based DOM.


Page 31 of 32


1.2

a Biochar

100

Inhibition Rate (%)

Absorbance

Hydrochar

120

1.0 0.8

80 60 40 20

0.6

0

180 210 240 270 300 330 500 Temperature (°C)

0.4 180 °C 210 °C 240 °C 270 °C

0.2

300 °C 330 °C blank 500 °C

0.0

Cyanobacteria inhibition rate (%)

0

1

2

3 4 5 Time （ Day）

c

100

6

7

330 °C

80 60 y=24.6x-23.1

40

R2=0.95 240 °C

20

210 °C

300 °C

270 °C

0 180 °C

-20 0

1 2 3 4 Percentage of N1O4 class (%)

5

Figure 6. (a) Cyanobacterial growth treated with DOM from hydrochar produced at different temperatures (the blank represents the cyanobacteria treated with 25 mL of pure water), (b) the microphotos of cyanobacterial growth with different hydrochar-based DOM, the cyanobacterial growth can be estimated by the depth of green and the size of cyanobacterial cluster and (c) the correlation between cyanobacterial inhibition rate and percentage of N1O4 class chemicals of different hydrochar-based DOM.



Table of Contents

Hydrochar-DOM Influence Algae Growth 180 oC 240 oC 270 oC 330 oC +4.0% -13.1% -15.3% 180 oC

330 oC

Molecules Changes -98.7%

ACS Paragon Plus Environment

Page 32 of 32

Production Temperature Effects on the Structure of Hydrochar-Derived

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