Fate and distribution of nutrients and heavy metals during hydrothermal carbonization of sewage sludge with implication to land application

Accepted Manuscript Fate and distribution of nutrients and heavy metals during hydrothermal carbonization of sewage sludge with implication to land application Liping Wang, Yuzhi Chang, Qifeng Liu PII:

S0959-6526(19)31063-7

DOI:

https://doi.org/10.1016/j.jclepro.2019.03.347

Reference:

JCLP 16351

To appear in:

Journal of Cleaner Production

Received Date: 9 October 2018 Revised Date:

5 February 2019

Accepted Date: 31 March 2019

Please cite this article as: Wang L, Chang Y, Liu Q, Fate and distribution of nutrients and heavy metals during hydrothermal carbonization of sewage sludge with implication to land application, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2019.03.347. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrothermal carbonization of sewage sludge

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P-containing compounds

N-containing compounds

Resource potential

Heavy metals

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First increase and then decrease in solubility

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TN NH4+-N NO3--N

Fe

Reaction severity f (-)

Zn

As

Ni

Pb Mn

Cr

Direct effect fraction

F1

Except for As

F3

F2

 Soil conditioner with slow

Accumulation Immobilization

release of nutrients

Pyrrole-N Quaternary-N Pyridine-N

compound fertilizer

Soil application

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 Liquid nitrogen-potassium

Exponential increase in solubility

Linear increase in solubility

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（T, t）

Hg

Cd

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Liquid phase

Org-N

Cu

and heavy metals

F4

Protein-N Nitrile-N

F5

Potential effect Fraction

Stable fraction

Except for Mn and Cd

Except for As and Cd

ACCEPTED MANUSCRIPT Fate and distribution of nutrients and heavy metals during hydrothermal carbonization of sewage sludge with implication to land application Liping Wang1*, Yuzhi Chang2, Qifeng Liu1* School of Ecology and Environment, Ministry of Education Key Laboratory of Ecology and

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1

Resource Use of the Mongolian Plateau & Inner Mongolia Key Laboratory of Coal Chemical Industry Wastewater Treatment and Recycling, Inner Mongolia University, Hohhot, 010021,

Environmental Monitoring Center, Jining Environmental Protection Bureau, Ulanqab 012000,

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2

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Inner Mongolia, China

Inner Mongolia, China *

Corresponding author. Tel: +86 0471 4991469

Abstract

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E-mail address: [email protected] (L. Wang)

Hydrothermal carbonization has been considered effective to reduce sewage sludge volume and

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provide an opportunity to generate valuable byproducts for a wide range of potential applications. The fate and distribution of nutrients during hydrothermal carbonization of sewage sludge, along

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with that of heavy metals, is very important for realizing nutrient recovery and reuse by both hydrochar and liquid phase directly as soil amendments and/or organic fertilizer. In this study, we systematically investigated the migration and transformation of nutrients and heavy metals using chemical extractions and reaction severity (Ruyter model) methods at a temperature range of 120-300 °C for 30-180 min. With increasing reaction severity, carbon and nitrogen efficiency of hydrochar showed a liner and exponential decrease respectively, while phosphorus accumulated positively in hydrochar. Nitrogen species in hydrochar are mostly nitrogen-containing aromatic 1

ACCEPTED MANUSCRIPT heterocycles whereas in liquid phase are predominantly ammonia-N and organic-N. The organic phosphorus in sewage sludge was transformed to inorganic species but non-apatite phosphorus showed an exponential reduction in hydrochar with increasing reaction severity, together with

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the decrease of orthophosphate in liquid phase. As for heavy metals, hydrothermal carbonization promoted decrease in direct bioavailable fractions (except arsenic) and increase in stable fraction (except cadmium and arsenic) in hydrochar as reaction severity elevated. Since hydrothermal

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effects led to redistribution of nutrients speciation and immobilization of heavy metals, non-farm

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land application of both hydrochar and liquid phase seems to be possible and unrestricted. This study provides fundamental knowledges for the construction of sludge management strategies to better nutrients recycling and reclamation. Keywords

1. Introduction

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Hydrochar; Liquid phase; Reaction severity; Chemical speciation; Migration and transformation

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As a result of the increasing world population and the improving global living standards, it could soon require more fertilizer to guarantee food security than it is possible to supply from

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conventional means. It is estimated that by 2030 at least twenty percent as much global fertilizer gain as it is currently used will be required (Tenkorang and Lowenberg-DeBoer, 2009). Such enormous increase in fertilizer consumption must not be accompanied by an increase in environmental problems resulting from chemical fertilizer, such as widespread nutrient pollution and the eutrophication of lakes, rivers and coastal oceans (Foley et al., 2011). Consequently, natural and renewable forms of organic fertilizer must be considered over the traditional mineral fertilizer sources (e.g., phosphorus) which are severely diminishing. In this respect, the 2

ACCEPTED MANUSCRIPT production of organic fertilizer or organic soil amendments from municipal waste streams will be an alternative based on new technologies which are more efficient for the removal of harmful components and the uptake of nutrients by crops.

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Biological processes are widely used worldwide for municipal and industrial wastewater treatment, as the by-products of which a large amount of sewage sludge are being produced, bringing a considerable challenge for municipal solid waste management system. With more

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stringent regulations and increasing wastewater production, the amount of sewage sludge is

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continuously increasing not only in developing countries but also in developed countries (Libra et al., 2011). The annual sewage sludge production was estimated to be approximately 6 million tons on a dry weight basis in 2016 in China (Zhang et al., 2017). It is estimated currently that at least 13 million tons of sewage sludge will be produced in the EU up to 2020 (Kelessidis and

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Stasinakis, 2012). Sewage sludge as a renewable resource for the nutrients recovery has been believed to be an appealing solution to effectively solve the sludge-associated problems in order to meet stringent environmental quality standards (Tyagi and Lo, 2013). However, in addition to

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the high moisture content, the major disadvantage of sewage sludge is that various valuable

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nutrients (organic carbon, phosphorous, and nitrogenous compounds) and a wide range of hazardous substances (heavy metals, pathogens, and persist organic pollutants) are present in one mixture, which make the traditional application of sewage sludge as fertilizer in agricultural under pressure due to the potential risk to ecological system (Frišták et al., 2018). Even so, sewage sludge is still increasingly recognized and treated in recent years as a resource for recycling and reclamation of nutrients, especially phosphorus. Until recently, hydrothermal carbonization of sewage sludge have been developed as a 3

ACCEPTED MANUSCRIPT sustainable technique due to the environmentally friendly and energy-efficient process, which can significantly reduce sewage sludge volume by improving dewaterability, decompose organic pollutants by hydrolysis and carbonization reactions, and generate valuable byproducts including

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the hydrochar and liquid phase (Danso-Boateng et al., 2015; Escala et al., 2012). It has been reported that the moisture in sewage sludge can be removed easily after hydrothermal carbonization at 180 °C above, resulting in the water removal as a liquid form as high as 92% by

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subsequent mechanical expression at increased temperature (Wang et al., 2014). Previous studies

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have shown that after hydrothermal carbonization of sewage sludge the aromatic network of hydrochar was dominated by polyfurans and N-heterocyclic aromatics, which could reduce the mobilization of nitrogen and avoid their fast release after fertilizer application (Paneque et al., 2017; Zhuang et al., 2017). Hydrothermal carbonization significantly promoted the accumulation

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of phosphorus in hydrochar with the form of only inorganic orthophosphate, the final forms of which are strongly correlated with the composition and speciation of metals in sewage sludge and the pH of system (Huang and Tang, 2015; Huang and Tang, 2016; Wang et al., 2017). The

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reported fraction of over 60% of carbon and 80% of phosphorus as well as less than 40% of

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nitrogen are generally remained in hydrochar after hydrothermal carbonization of sewage sludge (Escala et al., 2012; He et al., 2013; Huang et al., 2017). In addition, hydrothermal carbonization also resulted in the significant decomposition of organic contaminants and immobilization of heavy metals (Huang and Yuan, 2016; Weiner et al., 2013), making the hydrochar prone to direct soil application to improve soil qualities as soil amendments with improved water and nutrients holding capacity, cation exchange capacity, pH buffering capacity and microbial community (Libra et al., 2011; Mukherjee and Lal, 2013). On the other hand, the liquid phase separated with 4

ACCEPTED MANUSCRIPT hydrochar, which is commonly regarded as a wastewater and required to be appropriately treated further, has a high soluble concentration of both organics and nutrients (particularly ammonia and potassium), showing a potential application for liquid organic fertilizer production but is a

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less explored research area (Smith et al., 2016; Spitzer et al., 2018; Zhuang et al., 2017). Despite the apparent advantages of hydrothermal carbonization of sewage sludge and the extensive investigation of characteristics of hydrochar and liquid phase, little information is

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available on the systematic description of the nutrients and heavy metals speciation evolutions

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during the process for potential production of bio-fertilizer or nutrients recovery. The previous studies have suggested that the transformation and redistribution of nutrients and heavy metals are highly depended upon the reaction temperature and residence time during hydrothermal carbonization of sewage sludge (Shao et al., 2015; Shi et al., 2013; Sun et al., 2013), while most

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of which mainly focused on the effect of only reaction temperature or residence time under specific conditions. The severe hydrothermal conditions not only promotes the dissolution and decomposition of organic biopolymers but also leads to the bioavailable heavy metals being

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transferred into liquid phase and the stable fraction being accumulated in hydrochar (Huang and

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Yuan, 2016). The maximum nutrients recovery simultaneously by hydrochar as soil conditioner and liquid phase as organic fertilizer, however, is still the critical knowledge gap, as there is an absence of quantitative evaluation of the nutrients evolutions with the variation of employed conditions, together with that of heavy metals which are probably one of the limited factors for bio-fertilizer production from sewage sludge (Singh and Agrawal, 2008). Thus, systematic understanding the fate and distribution of nutrients and heavy metals during hydrothermal carbonization of sewage sludge is important for fundamental process design development of 5

ACCEPTED MANUSCRIPT bio-fertilizer production from sewage sludge using hydrothermal carbonization. In this study, based on the reaction severity (f (-)) of hydrothermal carbonization described by the Ruyter semi-empirical model (Eq. (1)), we systematically characterized the migration and

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transformation of main nutrients as well as various heavy metals under a wider range of hydrothermal conditions, specifically trying to fit the evolutions of total content and speciation of them with increasing reaction severity on the base of experimental data. The primary

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objectives of this study are to (1) understand the effect of reaction severity on the migration and

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transformation of both nutrients (carbon, nitrogen, phosphorus and alkali/alkaline earth metals) and heavy metals (Pb, Cd, Hg, Cr, Cu, Zn, Fe, Mn, and As) during hydrothermal carbonization of sewage sludge and to explore their evolution models as a function of the reaction severity; (2) evaluate the feasibility of soil conditioner and/or liquid organic fertilizer production from sewage

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sludge by hydrothermal carbonization. 2. Materials and methods 2.1. Sewage sludge sample

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Sewage sludge used in this study was no-stabilized but mechanically dewatered biological

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sludge collected from a municipal wastewater treatment plant in Hohhot, China. The wastewater treatment plant has a capacity of wastewater treatment of 200,000 m3 d-1 and is equipped with primary (preliminary sedimentation), secondary (activated sludge tank with anaerobic - anoxic aerobic process), and tertiary (membrane filtration) treatment units for treating municipal sewage from the city. The sewage sludge collected still contains a high moisture content of 86.14% and was kept in refrigerated plastic bags at -20 °C. Before hydrothermal carbonization, a portion of the frozen sewage sludge sample was thawed at ambient temperature. Details of the chemical 6

ACCEPTED MANUSCRIPT characteristics of sewage sludge sample are presented in Table S1. 2.2. Hydrothermal carbonization Hydrothermal carbonization of sewage sludge samples was performed in a laboratory-scale

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high pressure reactor where the vessel (500 mL) is constructed of type 316 stainless steel. In each experiment, about 250 g thawed sewage sludge was loaded in the reactor and heated up to the desired temperature in an electrically heated oven by a programmable controller with a

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heating rate of 3 °C min-1. The temperature of hydrothermal carbonization ranged from 120 to

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300 °C in interval of 30 °C with a duration of 30, 60, 120 and 180 min, respectively. These process conditions were chosen mainly in order to realize a wider range of reaction severity than that have been employed in previous literatures (Lin et al., 2015; Zhai et al., 2016). After processing, the reactor was cooled to quench the reaction by pumping circulating cooling water

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overnight. The produced hydrochar slurry was collected carefully and weighed, and then the liquid phase was separated by centrifugation at 12,000 rpm for 20 min. The samples of all liquid phase were kept in a refrigerated glass bottle at 4 °C and subsequently analyzed within 24 h after

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separation. All experiments were processed in triplicate with the same processing condition to

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evaluate the reproducibility. 2.3. Analysis methods

2.3.1. Sewage sludge and hydrochar Sewage sludge and each of centrifuged hydrochar samples were dried at 105 °C for 12 h and homogenized in a mill at 25 Hz for 5 min. All following measurements were performed in duplicate and the mean values are reported. In the treated samples, ultimate analysis and ash content were determined by an elemental analyzer (Vario MACRO cube, Elementar, Germany) 7

ACCEPTED MANUSCRIPT and a combustion method at 650 °C for 2 h, respectively. Phosphorus speciation was measured by the Standards Measurements and Testing (SMT) sequential extraction protocol of the European Commission (Zhu et al., 2011), which allows the

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total phosphorus being fractionated into organic phosphorus (OP) and inorganic phosphorus (IP), and the latter includes non-apatite inorganic phosphorus (NAIP, the form associated with oxides and hydroxides of Al, Fe and Mn) and apatite phosphorus (AP, the form associated with Ca).

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The phosphorus concentration in the extract was analyzed as orthophosphate using the

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ammonium molybdate method by a HACH DR5000 spectrophotometer (Kumar et al., 2007). X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical nature of nitrogen on the surface of sewage sludge and hydrochar with an X-ray photoelectron spectrometer (ESCALAB 250, Thermo Fisher Scientific, U.K.) equipped with a monochromatized Al Kα

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radiation at 1486.6 eV. The measurements were carried out at identical pass energy of 20 eV and an energy step size of 0.1 eV. Semi-quantitative information of N-containing compounds was obtained by normalized areas of the fitted peaks using a Gaussian-Lorentzian shape (XPSPEAK

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4.1), with a Shirley type background.

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Tessier sequential extraction procedure was performed to analyze the speciation of heavy metals, which partitions heavy metals into five fractions including exchangeable (F1), bound to carbonates (F2), bound to iron and manganese oxides (F3), bound to organic matter (F4) and residual (F5) (Tessier et al., 1979). The concentration of each fraction of heavy metals was determined via Optima 8000 inductively coupled plasma-optical emission spectrometer (ICP-OES, PerkinElmer, USA). Alkali and alkaline earth metals were first dissolved in nitrohydrochloric acid by microwave pressure decomposition and then determined using flame 8

ACCEPTED MANUSCRIPT atomic absorption spectrometry (AA-7000, Shimadzu, Japan). 2.3.2. Liquid phase analysis The pH of the liquid samples was measured with a digital multi-parameter meter (HQ430d,

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HACH, USA). Total organic carbon (TOC) was measured by a total organic carbon analyzer (Vario TOC cube, Elementar, Germany). After a pre-digestion step with persulphate (5% K2S2O4 for 30 min at 125 °C), the total phosphorus (TP) and reactive phosphorus (PO43-) was determined

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by the above-mentioned spectrophotometry. The total dissolved nitrogen (TN) was also

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determined by the potassium persulfate digestion method (Ameel et al., 1993). The ammonia nitrogen (NH4+-N) was measured using a modified Nessler colorimetric method described by Jeong et al. (2013). The nitrate nitrogen (NO3−-N) was determined calorimetrically with nitration of salicylic acid according to Cataldo et al. (1975). The organic nitrogen was calculated by the

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difference of total dissolved nitrogen and the sum of nitrogen in ammonia and nitrate species. All the analyses were repeated in duplicate and the mean values were reported. 2.3.3. Statistical analysis

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The reaction severity (f (-)) of hydrothermal carbonization refers to the combined effects of

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both temperature (T) and residence time (t) employed, which was used in this study to evaluate evolutions of nutrients and heavy metals during hydrothermal carbonization of sewage sludge according to the model reported by Ruyter (Ruyter, 1982). − = 50

.

(1)

It has been observed that the reaction temperature and residence time are exchangeable parameters to achieve the same reaction severity during hydrothermal carbonization (Landais et al., 1994). Based on the experimental data, the statistical curves of nutrients and heavy metals 9

ACCEPTED MANUSCRIPT following reaction severity were fitted and empirical models were obtained, which enabled the predictions of their migration and transformation during hydrothermal carbonization of sewage sludge according to the time-temperature data. The correlations were considered statistically

3. Results and discussion 3.1. Fate of nutrients during hydrothermal carbonization

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significant at a 95% confidence interval (p < 0.05) and 95% prediction interval was presented.

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Hydrothermal carbonization is effective for stabilizing and minimizing of sewage sludge,

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however, a problem that needs to be considered carefully is the fate and redistribution of nutrients during hydrothermal carbonization if nutrients recovery from sewage sludge is still required in the near future. In this part, the evolutions of nutrients distribution and speciation with increasing reaction severity of hydrothermal carbonization will be investigated statistically

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to provide fundamental information for the potential application of hydrochar as soil amendment and liquid phase as organic fertilizer.

3.1.1. Mass balance and carbon efficiency

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The hydrochar yield is bound to decrease by elevating reaction severity during hydrothermal

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carbonization (Fig. 1). This is primarily due to the dissolution and chemical dehydration of organic compounds and secondarily due to the formation of gas as a result of the decarboxylation reactions. After hydrothermal carbonization of sewage sludge at the reaction severity of 0.51 (270 °C for 180 min), the hydrochar yield began to be below 60%. More importantly, a statistical model ( = 0.17

.

− 0.03) was obtained via fitting the experimental data to describe the

evolution of hydrochar yield as a function of the reaction severity, which provided a chance to predict the hydrochar production on the basis of the conditions applied. The model validity was 10

ACCEPTED MANUSCRIPT verified according to the hydrochar yield reported by Zhao et al. (2014). Fig. 1 Evolutions of hydrochar yield with increasing reaction severity during hydrothermal carbonization of sewage sludge. Hydrochar yield is defined as the dry mass ratio of hydrochar to original sludge.

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Carbon mass balance indicated that hydrothermal carbonization of sewage sludge leads to a significant fraction of carbon (higher than 60%) still retained in hydrochar (Fig. 2a), although the carbon efficiency of hydrochar shows a linear reduction (R2 = 0.97, p < 0.05) with increasing

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reaction severity. The loss of carbon were mainly transferred into the liquid phase (lower than .

− 0.12) as

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30%), of which the carbon efficiency reveals an exponential increase ( = 0.15

the reaction severity increased (Fig. 2b). The concentration of total organic carbon (Fig. 3) in the liquid phase first increased and then decreased at a turning point of 0.14 (180 °C for 180 min) with increasing reaction severity from 0.03 to 0.71. This result suggested that organic substances

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in sewage sludge are first released since their solubilization and then suffer from a cleavage at the higher reaction severity, together with the increase in water production of the liquid phase

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(Ekpo et al., 2016b).

Fig. 2 Evolutions of carbon distribution in hydrochar (a) and liquid phase (b) with increasing reaction severity

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during hydrothermal carbonization of sewage sludge. Carbon efficiency is defined as the dry mass ratio of total carbon in hydrochar or liquid phase to that in original sludge.

Fig. 3 Evolution of TOC in liquid phase with increasing of reaction severity during hydrothermal carbonization of sewage sludge.

Hydrothermal carbonization significantly changes the elemental composition of sewage sludge. For resulting hydrochar, the atomic ratios of hydrogen/carbon (H/C) and oxygen/carbon 11

ACCEPTED MANUSCRIPT (O/C) decreased compared to the original sludge and the value of H/C is much lower than that of O/C (Fig. 4), showing the hydrothermal conversion process is predominantly governed by chemical dehydration, followed by decarboxylation reactions. The reduction of both atomic

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ratios was connected to increasing reaction severity, where a strong positive correlation (R2 = 0.98, p < 0.05) was observed for the evolution of H/C versus O/C during hydrothermal carbonization process of sewage sludge (Fig. 4). At the strongest reaction severity (f (-) =0.71

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(300 °C for 180 min)) in this study, the values of both ratios (H/C of 0.73 and O/C of 0.03) for

(2011) and Kim et al. (2014).

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the produced hydrochar are rather close to that of bituminous coals, as reported by Berge et al.

Fig. 4 Evolutions of atomic H/C and O/C ratios in hydrochar with increasing reaction severity during hydrothermal carbonization of sewage sludge.

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As previously observed, the organic matters in sewage sludge are reformed and redistributed during hydrothermal carbonization process, which provided a chance to make the hydrochar with

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some aromatic structure and labile carbon species (Peng et al., 2017; Wang, L. et al., 2017), implying that soil characteristics (aggregation, aeration and permeability) and organisms growth

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could be enhanced when the hydrochar is applied to degenerated soils (Kończak and Oleszczuk, 2018). Due to the high amounts of dissolved organic matters included in liquid phase, the land application of it could increase the soil organic matters to provide a steady release of nutrients to the plants when the organic matters are broke down (Martínez-Alcántara et al., 2016). Direct soil application of the hydrochar and liquid phase from hydrothermal carbonization of sewage sludge as soil amendments or fertilizers is hence meaningful to soil quality and productivity and wastes managements, and even carbon sequestration (Bargmann et al., 2014; Malghani et al., 2013). 12

ACCEPTED MANUSCRIPT 3.1.2 Nitrogen evolution The nitrogen efficiency of hydrochar against the reaction severity during hydrothermal carbonization of sewage sludge is illustrated in Fig. 5. It was observed that the increase of .

− 0.10) of nitrogen in

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reaction severity promoted an exponential decrease ( = 0.95

hydrochar, implying that the N-containing compounds (proteins) of sewage sludge are mainly decomposed into the liquid phase, with a smaller fraction being transferred into the gas phase as

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ammonia at higher reaction severity (Zhuang et al., 2017). As a result, a relatively lower nitrogen

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proportion of 29.02% was found in the hydrochar after hydrothermal carbonization of sewage sludge at the reaction severity of 0.71. Similar results under comparable conditions were observed for the effective nitrogen removal by hydrothermal carbonization (He et al., 2015b). Fig. 5 Evolutions of nitrogen efficiency for hydrochar with increasing reaction severity during hydrothermal

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carbonization of sewage sludge. Nitrogen efficiency is defined as the dry mass ratio of total nitrogen in hydrochar to that in original sludge.

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To gain insight into the evolution of N-containing functional groups in hydrochar, N 1s XPS (Fig. S1) was performed with various reaction severities during hydrothermal carbonization. The

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deconvolution results (Table 1) demonstrated that the nitrogen in sewage sludge was primarily composed of protein-N (36.46%), pyridine-N (31.38%), pyrrole-N (23.65%) and inorganic-N (8.51%). After hydrothermal carbonization at above 180 °C, the inorganic-N in hydrochar was completely hydrolyzed into the liquid phase with the form of NH4+-N and NO3--N (He et al., 2015b). The protein-N in hydrochar showed a continuous decrease with the increasing temperature, reaching a lower fraction of 22.03% at the reaction severity of 0.57 (300 °C for 60 min). This is attributed to the cleavage of peptide bonds in proteins, which results in the 13

ACCEPTED MANUSCRIPT formation of labile and stable amides and the NH4+-N is generated subsequently from the labile amides by deamination (Zhuang et al., 2017). Simultaneously, cyclization and dehydrogenation of amine intermediates derived from the degradation of stable proteins occurred, leading to the

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increase in pyrrole-N in hydrochar as the temperature increased, while the pyridine-N in hydrochar showed a decrease, which could lead to the increase in more stable quaternary-N compounds via the polymerization or condensation reactions (Wang et al., 2018; Xiao et al.,

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2017). It is noted that the transformations of these heterocyclic-N species (pyrrole-N, pyridine-N

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and quaternary-N) is closely related to the Maillard reactions between amino acids and the reducing sugar or their derivatives. Moreover, the elevated hydrothermal conditions (300 °C for 60 min) enhanced the dehydrogenation of stable amines and thus induced the formation of nitrile-N (Liu et al., 2017).

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Table 1 The normalized relative intensities of nitrogen functionalities (%) from N 1s XPS.

There is no doubt that the higher hydrothermal conditions could result in a dramatic cracking

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or dissolution of nitrogenous compounds in sewage sludge to the liquid phase. As the reaction severity increased, the concentration of TN for the liquid phase showed an exponential increase .

+ 6560), from 1425 mg/L at 0.03 to 5767 mg/L at 0.71 (Fig. 6a). Among

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( = −5619

nitrogen species in TN, the evolution of Org-N concentrations exhibited a trinomial transformation throughout hydrothermal carbonization process (Fig. 6c) with a turning point at the reaction severity of 0.28 (240 °C for 60 min). It can be explained by the rapid degradation of protein in sewage sludge to liquid phase at above 150 ° C and the dissolved Org-N (mainly the labile organic N-containing compounds) is decomposed further at the temperature higher than 240 °C for 120 min. The similar observations were also reported in the studies of Yin et al. (2015) 14

ACCEPTED MANUSCRIPT and Zhuang et al. (2017). For the main inorganic-N species present in the liquid phase, the concentration of NH4+-N continuously increased as an exponential form ( = −6364

.

+

6487) with elevated reaction severity (Fig. 6b), whereas the NO3--N concentration decreased

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progressively to a low concentration of 14.45 mg/L (Fig. 6d). Therefore, the NH4+-N and Org-N are the predominant nitrogen species in liquid phase during hydrothermal carbonization of sewage sludge (Fig. S2).

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Fig. 6 The concentration evolutions of TN (a), NH4+-N (b), Org-N (c) and NO3--N (d) in liquid phase during

3.1.3 Phosphorus evolution

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hydrothermal carbonization of sewage sludge.

The TP content in hydrochar positively correlated with the reaction severity (R2 = 0.98, p < 0.05) during hydrothermal carbonization of sewage sludge (Fig. 7a), implying that hydrothermal

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effects enrich the phosphorus in the solid phase. It has been reported that phosphorus species in activated sewage sludge was dominated by orthophosphates, organic phosphates and long-chain

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polyphosphates (Huang and Tang, 2015; Huang et al., 2015). After hydrothermal carbonization, polyphosphate and organic phosphate were gradually degraded into orthophosphates (Huang et $.

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al., 2017), resulting in an exponential increase ( = −0.20

+ 0.93) of IP fraction in

hydrochar with increasing reaction severity (Fig. 7b). Nevertheless, the NAIP fraction (Fig. 7c) in hydrochar underwent an exponential decrease ( = 0.24

$.

+ 0.47), probably due to the

pH value of the carbonized sludge slurry (Fig. S3) decreased first from 7.75 to 5.28 (at reaction severity of 0.11 (180 °C for 60 min)) and then increased to 9.15 with the reaction severity ranged from 0.03 to 0.71 (Petzet et al., 2012; Shi et al., 2014; Xu et al., 2015). The changes in pH are closely associated with the presence of organic acids resulting from decomposition of the 15

ACCEPTED MANUSCRIPT biopolymers such as polysaccharides, lipid and proteins at low reaction severity, and a high production of alkaline groups like ammonia from the proteins degradation at higher reaction severity (Wang and Li, 2015; Wilson and Novak, 2009). As a result, the amount of both NAIP

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and OP, which are believed to play an important role in the potential mobility and bioavailability of phosphorus from hydrochar (He et al., 2016), were reduced gradually ( = 0.45

$.

+

0.54) as the reaction severity increased, while still over 57% of TP in hydrochar had a high

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potential mobility even at reaction severity of 0.71 (Fig. 7d).

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Fig. 7 Evolutions of phosphorus following hydrothermal carbonization of sewage sludge. (a) phosphorus concentration in hydrochar; (b) IP distribution in TP; (c) NAIP distribution in IP; (d) potentially bio-available phosphorus in hydrochar; (e) TP efficiency in liquid phase; (f) TP and PO43- concentration in liquid phase.

Significant amount of phosphorus were determined in the liquid phase during hydrothermal

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carbonization of sewage sludge with the highest solubility of 30% at reaction severity of 0.03 (Fig. 7e), which exponentially decreased ( = 0.42

.$

) with increasing reaction severity.

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The low phosphorus observation in liquid phase at higher reaction severity is most likely due to homogenization of phosphorus species during hydrothermal carbonization and the subsequent

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direct interactions of them with metals (Ca, Mg, Al, Fe and Mn) in sewage sludge (Huang and Tang, 2015). Along with the significantly increased liquid yields at higher reaction severity, the concentration of phosphorus in liquid phase also presented a decrease trends as reaction severity elevated (Fig. 7f). The similar observation was also reported for hydrothermal carbonization of swine manure in the study of Ekpo et al. (2016a). It is noted, however, that all the dissolved organic phosphorus in liquid phase was almost transformed into orthophosphates (PO43-) at the reaction severity over 0.18 (210 °C for 60 min), which can be calculate by the difference 16

ACCEPTED MANUSCRIPT between TP and PO43- in the liquid phase (Fig. 7f). Hence it is believed that hydrothermal carbonization has potential to resolve the nitrogen and phosphorus run-off problems, which are one of the main contributors to the eutrophication of

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water bodies due to the rapid phosphorus and nitrogen release rate of inorganic fertilizers or sewage sludge in agriculture use (Huang et al., 2017), by reducing the mobile phosphorus and nitrogen species (Table 1 and Fig. 7) to equilibrate the plant uptake and soil retention capacity.

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For the total content of effective nutrients (N, P and K) in hydrochar and liquid phase, it was

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closely correlated with the reaction severity during hydrothermal carbonization and reached up to 13.13% for hydrochar and 0.62% for liquid phase respectively at the reaction severity of 0.71 (Table 2), while the ultimate fertility of which depends actually upon the interaction among hydrochar, plants and soils.

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Table 2 The limits of heavy metals for land application of sewage sludge or compost.

3.1.4 Alkali/alkaline earth metals evolution

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There are a large amount of alkali (Na and K) and alkaline earth metals (Ca and Mg) in the original sewage sludge (Table S1), the information on the evolution and distribution of these

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metals during hydrothermal carbonization of sewage sludge is outstanding importance due to the potential fertilizer application of hydrochar and liquid phase. The concentration evolution of the alkali/alkaline earth metals in hydrochar and the distribution of them in the liquid phase and hydrochar are shown in Fig. S4. It was observed that there is a clear difference in the affinity of the divalent and monovalent metals with the solid and liquid phase. The stronger reaction severity resulted in an exponential increase in concentration of Ca ( = 7753

.

+ 27182) and Mg ( = 1060

.

+ 1303) in 17

ACCEPTED MANUSCRIPT hydrochar (Fig. S4a and c), whereas the distributions of them in the liquid phase were relatively lower (lower than 15% for Ca and 10% for Mg) and showed an upward inflection point with increasing reaction severity at a turning point of 0.32 (240 °C for 120 min) for Ca and 0.28

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(240 °C for 60 min) for Mg, respectively (Fig. S4b and d). This accumulation of Ca and Mg in hydrochar is closely associated with the immobilization of phosphorus in hydrochar (Ekpo et al., 2016a). Potassium and sodium, on the other hand, possessed a high affinity with the liquid phase,

.

+ 1.21 for K and =

+ 1.07 for Na) in liquid phase as the reaction severity elevated. The concentration

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−1.10

.%$

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which increased significantly as an exponential form ( = −1.26

of K and Na in hydrochar thus decreased with increasing reaction severity (Fig. S4e and f) and a strong negative correlation for K (R2 = 0.99, p < 0.05) and for Na (R2 = 0.98, p < 0.05) was observed. This distribution evolution of alkalis appears to be a little similar to the hydrothermal

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liquefaction of brown macro-alga (Anastasakis and Ross, 2011).

Due to the liquid phase with high percentages of potassium as well as nitrogen (especially NH4+-N) but less percentage of phosphorus (Fig. 7) and heavy metals (Table 2), it has potential

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to be as the liquid nitrogen-potassium compound fertilizer, including some other micronutrients

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such as boron (Fig. S8). The nutrients ratio is adjustable to adapt to the requirements of various plants and soils following the evolutions of nutrients with increasing reaction severity observed in this work.

3.2 Heavy metals evolution In addition to the transformation of nutrients, the evolution and distribution of heavy metals during hydrothermal carbonization of sewage sludge have also obtained increasing interests, because heavy metals account for a significant fraction and are adverse to the potential recovery 18

ACCEPTED MANUSCRIPT of nutrients from hydrochar and liquid phase. Understanding the migration and transformation of heavy metals during hydrothermal carbonization is helpful to optimize the process to make the minimization of heavy metals content and/or their bioavailability in the products with potential

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for fertilizer application. 3.2.1 Heavy metals distribution

For the sake of evaluation, it is reasonable to assume that there are no heavy metals being

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transferred into gases phase, because the adopted temperatures of hydrothermal carbonization are

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well below the boiling point of heavy metals. As expected, hydrothermal carbonization resulted in the dissolution of heavy metals into the liquid phase due to the degradation of sewage sludge particles and the extraction effect of subcritical water during the process (Fig. S7). Total content of the toxic heavy metals (Cd, Pb, Hg, As, and Cr) with major public concerns showed the

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exponential growth trends in the liquid phase with the increase of reaction severity, while almost a linear increase was observed for the other heavy metals including Fe, Mn, Cu and Zn, which are regarded as essential for plants growth and human health in small quantities, except for Ni

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with the highest release at reaction severity of 0.35 (240 °C for 180 min). However, the amount

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of heavy metals released in the liquid phase was still low (less than 30% for Cr, 25% for Cd, Pb, Hg, As, Cu and Zn, and 10% for Fe, Mn and Ni even at reaction severity of 0.71), showing the majority of heavy metals are remained in hydrochar after hydrothermal carbonization process of sewage sludge. The similar observations were also reported for the hydrothermal carbonization of sewage sludge in the studies of Huang and Yuan (2016) and Liu et al. (2018). Compared to the original sewage sludge, the concentration of heavy metals in hydrochar showed a positive linear increase (R2 > 0.96, p < 0.05) with increasing reaction severity, apart 19

ACCEPTED MANUSCRIPT from Ni having an exponential increase (Fig. S5). This could be explained by the first dissolution of heavy metals and the subsequent precipitation with other chemical species (like phosphorus) under the hydrothermal conditions, which provides a homogeneous environment to promote the

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occurrence of these reactions (McGowen et al., 2001; Shi et al., 2013). Since the different redistributions of heavy metals are associated with their physicochemical properties, the extent of their increase in concentration in hydrochar was ranked in the following order: Fe > Zn > Mn >

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Cu > Cr > Ni > Pb > As > Hg > Cd.

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There are obviously two partitions for the evolution of heavy metals in the liquid phase with increasing reaction severity, except for Mn (Fig. 8 and Fig. 8S). Below the reaction severity of 0.10 (180 °C for 30 min), the concentration of heavy metals in the liquid phase first increased and then decreased at a turning point of around 0.08 (150 °C for 120 min). This is mainly

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attributed to the significant growth of water yield in the liquid phase at above 150 °C but the degradation of sewage sludge matrix is still limited (Wang et al., 2014). While the reaction severity was over 0.10, the dissolution rate of heavy metals began to exceed the release rate of

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water, leading to an exponential increase in concentration for Cd, Pb, Hg, As, Cr and Cu in the

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liquid phase, and the linear increase for Zn, Fe and Mn. For Ni, the concentration in the liquid phase began to decrease at the reaction severity of 0.32 (240 °C for 120 min), which is consistent with the result of He et al. (2015a), showing good immobilization performance for Ni during hydrothermal carbonization process. Fig. 8 Evolutions of heavy metals concentrations in liquid phase with increasing reaction severity during hydrothermal carbonization of sewage sludge.

For the liquid phase as organic fertilizer, heavy metals could be not the limiting factor due to 20

ACCEPTED MANUSCRIPT their low solubility and high yield liquid production during hydrothermal carbonization (Appels et al., 2010). Nevertheless, some toxic intermediates, such as aldehydes, phenols, furfurals, heterocyclic aromatic hydrocarbons and their derivatives, are probably present in the liquid

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phase with higher concentrations as previously reported by Danso-Boateng et al. (2015), which could restrict the direct land application of liquid phase as organic fertilizer, together with offensive odour (Wang et al., 2016). Therefore, some post-treatments could be inevitable to

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3.2.2 Heavy metals speciation in hydrochar

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remove these undesirable substances, such as by washing and fermentation.

It has been believed that the chemical speciation of heavy metals is more important than the total concentration for the potential risk of hydrochar in land use, as well as for biogeochemical transformation and ultimate fate of them (Zhai et al., 2014). Based on Tessier’s sequential

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extraction method, the content evolution of bioavailable heavy metals in hydrochar were determined to evaluate the ecological risks of heavy metals in sewage sludge after hydrothermal carbonization (Fig. 9 and S6). In accordance with the activity of heavy metals speciation, the

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fractions of (F1+ F2 + F3) were identified as direct effect parts of eco-toxicity and bioavailability,

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while F4 and F5 were recognized as a potential effect fraction and a stable fraction, respectively. The statistical relationship between the evolution of chemical speciation and the reaction severity are shown in Fig. 9 and S6.

Fig. 9 Evolutions of heavy metals speciation in hydrochar following hydrothermal carbonization of sewage sludge.

For the original sewage sludge, there was a high direct bioavailable fraction for Cd, Mn, Pb, Cr, Fe, Ni and As, with a value of 64.5%, 52.4%, 35.0%, 23.7%, 25.1%, 24.9% and 19.3%, 21

ACCEPTED MANUSCRIPT respectively, whereas Hg (0.0%), Cu (6.4%) and Zn (12.2%) were in a relative low level. With the increase of reaction severity, the direct bioavailable fraction of most heavy metals decreased regularly (Fig. 9 and S6), implying the positive effect of hydrothermal carbonization on the

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immobilization of heavy metals, with the exception of As (Fig. 9). It is noted that there was an upward peak observed at the reaction severity of 0.07 (150 °C for 60 min) and 0.18 (210 °C for 60 min) for Cd and Fe, respectively, which were mostly contributed by the conversion of F3

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fraction from F5 fraction for Cd and from F4 fraction for Fe. With regard to As, hydrothermal

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carbonization lead to an increase in the direct bioavailable fraction and the evolution began to level off after the reaction severity of 0.24 (240 °C for 30 min), whereas the fractions of both F4 and F5 presented an significant decrease. This evolution of As speciation in hydrochar was probably influenced by the pH and redox potential of subcritical water, along with the solubility

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in liquid phase (Carbonell-Barrachina et al., 2000).

After hydrothermal carbonization, the content of Cr and Pb in F4 fraction increased first due to the transformation of exchangeable and carbonate fraction. The subsequent decrease in F4 at

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above reaction severity of 0.18 for Cr and 0.10 for Pb was probably attributed to the degradation

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of organic matters under hydrothermal conditions, leading to the reduction of potential toxicity. Accordingly, the F5 fraction of Cr and Pb sharply increased with increasing reaction severity from 0.03 to 0.71 and became the main chemical speciation (over 50%) when the reaction severity of hydrothermal carbonization elevated to 0.28 for Cr and 0.21 (210 °C for 120 min) for Pb (Obrador et al., 2001). Cd and Mn were tendentiously associated with organic and sulfide substances during hydrothermal carbonization process, resulting in the increase of F4 fraction with the rise of reaction severity, while the F5 fraction increased for Mn but decreased for Cd, in 22

ACCEPTED MANUSCRIPT accordance with the results of Shi et al. (2013). The enhancement of residual fraction (F5) for various heavy metals is generally desired if hydrochar is applied to the degenerated soils, which are formed through complexation, precipitation, adsorption or other procedures and finally are

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fixed within the crystal structure of primary and secondary minerals during hydrothermal conditions (Liu et al., 2018; Tessier et al., 1979). Similar observation was found for Cu, Zn, Fe,

increase in F5 fraction as the reaction severity increased.

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Ni and Hg, of which the amount showed a continuous decrease in F4 fraction and a persistent

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Heavy metals levels are generally the restrictions, although in most countries there are no regulations that are specific to land application of the products from hydrothermal carbonization of sewage sludge and usually the regulations governing bio-solids and fertilizer are considered as a reference (Libra et al., 2011). From the point of the bioavailable concentrations of heavy metals

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in hydrochar, all of them except As at reaction severity of 0.71 were far below the control levels of Class B of sludge agricultural use from China sewage sludge ordinance (GB 4284-2018), as well as that from German sewage sludge ordinance (1992) and the United States 40 CRF Part

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503 rule (1993), while they (especially As, Cd, Hg and Cu) are far beyond the limits of the China

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and German fertilizer ordinances, as shown in Table 2. This suggested that unrestricted land application of hydrochar in non-farm lands may be possible if the appropriate hydrothermal conditions are selected according to the evolutions of heavy metals demonstrated in Section 3.2. Furthermore, hydrochar with abundant oxygen-containing groups on surface has additional advantage for the adsorption of heavy metals and organic pollutants from soils (Alatalo et al., 2013; Higashikawa et al., 2016; Shen et al., 2018). In the future, the loading rate, background values and plant uptake as micronutrients (apart from As, Hg, Cd and Pb) for heavy metals in 23

ACCEPTED MANUSCRIPT soils should also be considered for the land application of hydrochar, and the impact of long-term application of hydrochar on soil biological activity and plant yield is also required (Siebielec et al., 2018).

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4. Conclusions This study provides the systematic understanding of nutrients and heavy metals evolutions

conclusion of this study can be summarized as follows:

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with increasing reaction severity during hydrothermal carbonization of sewage sludge. The major

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(1) With increasing reaction severity, carbon, nitrogen, phosphorus and potassium in sewage sludge are reformed and redistributed in hydrochar and liquid phase, leading to a liner reduction in carbon efficiency, an exponential decrease in nitrogen and potassium efficiency and an exponential increase in phosphorus efficiency for hydrochar. The nutrients speciation and

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availability in hydrochar and liquid phase are thus tunable by varying the reaction severity of hydrothermal carbonization.

(2) Heavy metals solubility and species are influenced significantly by reaction severity and

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presented different evolutions for individuals, which resulted in a regular decrease in direct

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bioavailable fractions (except As) in hydrochar and an increase in their stable fraction (except Cd and As), implying immobilization of heavy metals during hydrothermal carbonization. (3) Land application of the products from hydrothermal carbonization of sewage sludge to non-farm soils seems to be possible from the points of nutrients and heavy metals speciation evolutions, whereas it is doomed not to be a straight forward road. Further researches are indispensable to investigate the effects of the hydrochar and liquid phase as fertilizers and/or amendments on the improvement of soils characteristics, the uptake of macro and micronutrients, 24

ACCEPTED MANUSCRIPT and the plants growth and their nutritional quality. Acknowledgments This study is supported by the programs of Natural Science Foundation of Inner Mongolia

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33

ACCEPTED MANUSCRIPT List of tables and figures captions Table 1 The normalized relative intensities of nitrogen functionalities (%) from N 1s XPS. Table 2 The limits of heavy metals for land application of sewage sludge or compost.

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Fig. 1 Evolutions of hydrochar yield with increasing reaction severity during hydrothermal carbonization of sewage sludge. Hydrochar yield is defined as the dry mass ratio of hydrochar to

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original sludge.

Fig. 2 Evolutions of carbon distribution in hydrochar (a) and liquid phase (b) with increasing

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reaction severity during hydrothermal carbonization of sewage sludge. Carbon efficiency is defined as the dry mass ratio of total carbon in hydrochar or liquid phase to that in original sludge.

Fig. 3 Evolution of TOC in liquid phase with increasing of reaction severity during hydrothermal

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carbonization of sewage sludge.

Fig. 4 Evolutions of atomic H/C and O/C ratios in hydrochar with increasing reaction severity

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during hydrothermal carbonization of sewage sludge. Fig. 5 Evolutions of nitrogen efficiency for hydrochar with increasing reaction severity during

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hydrothermal carbonization of sewage sludge. Nitrogen efficiency is defined as the dry mass ratio of total nitrogen in hydrochar to that in original sludge. Fig. 6 The concentration evolutions of TN (a), NH4+-N (b), Org-N (c) and NO3--N (d) in liquid phase during hydrothermal carbonization of sewage sludge. Fig. 7 Evolutions of phosphorus following hydrothermal carbonization of sewage sludge. (a) phosphorus concentration in hydrochar; (b) IP distribution in TP; (c) NAIP distribution in IP; (d) potentially bio-available phosphorus in hydrochar; (e) TP efficiency in liquid phase; (f) TP and 34

ACCEPTED MANUSCRIPT PO43- concentration in liquid phase. Fig. 8 Evolutions of heavy metals concentrations in liquid phase with increasing reaction severity during hydrothermal carbonization of sewage sludge.

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Fig. 9 Evolutions of heavy metals speciation in hydrochar following hydrothermal carbonization

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of sewage sludge.

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ACCEPTED MANUSCRIPT Table 1 The normalized relative intensities of nitrogen functionalities (%) from N 1s XPS. Sample

Inorganic-N

Quaternary-N

Pyrrole-N

Protein-N

Pyridine-N

Nitrile-N

402.9+0.3

401.2±0.3

400.2±0.3

399.8±0.3

398.8±0.3

398.7±0.1

Original

8.51

0.00

23.65

36.46

31.38

0.00

S180

0.00

16.63

26.18

32.47

25.69

0.00

S240

0.00

23.65

27.48

25.37

S300

0.00

24.82

28.62

22.03

23.24

0.00

12.66

11.84

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ACCEPTED MANUSCRIPT Table 2 The limits of heavy metals for land application of sewage sludge or compost. Sample

Heavy metals (mg/kg)

Total nutrients (%)

As

Cd

Cr

Cu

Pb

Hg

Ni

Zn

N+P2O5+K2O

Original sludge

40

5

78

141

57

4

27

602

10.15

Hydrochar f (-) = 0.18 (210 °C-60 min)

50

6

75

145

43

2.2

26

598

11.14

Hydrochar f (-) = 0.71 (300 °C-180 min)

78

10

61

179

29

1.45

Liquids f (-) = 0.18 (210 °C-60 min)

0.53

0.06

2.34

2.08

1.00

0.18

Liquids f (-) = 0.71 (300 °C-180 min)

1.62

0.23

5.95

6.67

2.81

0.59

A (farmland, garden plot, pastureland )

30

3

500

500

300

3

100

1200

B (Other land)

75

15

1000

1500

1000

15

200

3000

China organic fertilizer standard (2012)b

15

3

German sewage sludge ordinance (1992)

-

10/5c

German fertilizer ordinance (2015)

40

1.5

USA 40 CFR Part 503

75

85

21

622

13.13

0.45

8.22

0.53

0.05

22.78

0.62

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China sewage sludge ordinance (2018)a

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1

150

-

50

2

-

-

900

800

900

8

200

2000/2500c

300/2d

70

150

1

80

1000

-

4300

840

57

420

7500

2

a

3

sludge into two categories (A and B) according to the land types.

4

b

Industry standards of organic fertilizer in China (NY 525-2012).

5

c

The limit for soils with a clay content below 5% or pH below 5.

6

d

The limit of chromium (VI) in fertilizer.

7

e

The lower limit for total nutrients in organic fertilizer.

8

- No limit was regulated.

5.00e

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Control standards of pollutants in sludge for agricultural use in China (GB 4284-2018), which classifies the

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Fig. 1 Evolutions of hydrochar yield with increasing reaction severity during hydrothermal carbonization of

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sewage sludge. Hydrochar yield is defined as the dry mass ratio of hydrochar to original sludge.

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Fig. 2 Evolutions of carbon distribution in hydrochar (a) and liquid phase (b) with increasing reaction severity

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during hydrothermal carbonization of sewage sludge. Carbon efficiency is defined as the dry mass ratio of total

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carbon in hydrochar or liquid phase to that in original sludge.

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2

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Fig. 3 Evolution of TOC in liquid phase with increasing of reaction severity during hydrothermal carbonization

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of sewage sludge.

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40

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Fig. 4 Evolutions of atomic H/C and O/C ratios in hydrochar with increasing reaction severity during

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hydrothermal carbonization of sewage sludge.

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Fig. 5 Evolutions of nitrogen efficiency for hydrochar with increasing reaction severity during hydrothermal

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carbonization of sewage sludge. Nitrogen efficiency is defined as the dry mass ratio of total nitrogen in

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hydrochar to that in original sludge.

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Fig. 6 The concentration evolutions of TN (a), NH4+-N (b), Org-N (c) and NO3--N (d) in liquid phase during

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hydrothermal carbonization of sewage sludge.

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43

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3

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Fig. 7 Evolutions of phosphorus following hydrothermal carbonization of sewage sludge. (a) phosphorus

5

concentration in hydrochar; (b) IP distribution in TP; (c) NAIP distribution in IP; (d) potentially bio-available

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phosphorus in hydrochar; (e) TP efficiency in liquid phase; (f) TP and PO43- concentration in liquid phase.

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Fig. 8 Evolutions of heavy metals concentrations in liquid phase with increasing reaction severity during

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hydrothermal carbonization of sewage sludge.

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Fig. 9 Evolutions of heavy metals speciation in hydrochar following hydrothermal carbonization of sewage

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sludge.

4

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ACCEPTED MANUSCRIPT Highlights Fate and distribution of nutrients with increasing reaction severity are identified. Evolution of heavy metals speciation with increasing reaction severity is detected. Hydrothermal effects promote the immobilization of heavy metals except for As.

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Non-farm land application of orthophosphate rich hydrochar is possible.

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Liquid phase is rich in NH4+-N with implication to organic fertilizer production.