Improved nitrogen conservation capacity during composting of dairy manure amended with oil shale semi-coke as the porous bulking agent

Journal Pre-proof Improved nitrogen conservation capacity during composting of dairy manure amended with oil shale semi-coke as the porous bulking agent Xu Li (Conceptualization) (Methodology) (Software) (Investigation) (Formal analysis) (Data curation) (Writing - original draft) (Writing review and editing) (Visualization), Xiao-Shuang Shi (Conceptualization) (Methodology) (Resources) (Writing - review and editing) (Supervision), Ming-Yi Lu (Resources) (Data curation), Yu-Zhong Zhao (Resources) (Data curation), Rong-Bo Guo (Conceptualization) (Methodology) (Project administration) (Supervision), Hui Peng (Conceptualization) (Methodology) (Supervision)

PII:

S0304-3894(19)31696-6

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121742

Reference:

HAZMAT 121742

To appear in:

Journal of Hazardous Materials

Received Date:

27 August 2019

Revised Date:

21 November 2019

Accepted Date:

21 November 2019

Please cite this article as: Li X, Shi X-Shuang, Lu M-Yi, Zhao Y-Zhong, Guo R-Bo, Peng H, Improved nitrogen conservation capacity during composting of dairy manure amended with oil shale semi-coke as the porous bulking agent, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121742

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Improved nitrogen conservation capacity during composting of dairy manure amended with oil shale semi-coke as the porous bulking agent Xu Li

a,b,1

, Xiao-Shuang Shi

a,1

, Ming-Yi Lu

a,b

, Yu-Zhong Zhao a, Rong-Bo Guo

a,c*

,

Hui Peng b* a

Shandong Industrial Engineering Laboratory of Biogas Production & Utilization,

Key Laboratory of Biofuels, Shandong Provincial Key Laboratory of Synthetic Biology, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, Shandong Province 266101, PR China. University of Chinese Academy of Sciences, Beijing 100049, PR China.

c

Dalian National Laboratory for Clean Energy, Dalian 116023, PR China

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b

Corresponding author: Rong-Bo Guo ([email protected])

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*

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of

(Rong-Bo Guo).

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Sciences, No. 189 Songling Road, Qingdao, Shandong Province 266101, PR China

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Phone: +860532- 80662678 Fax: +860532-80662778 *

Hui Peng ([email protected])

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of

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Sciences, No. 189 Songling Road, Qingdao, Shandong Province 266101, PR China

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Phone: +860532-58782862 Fax: +860532-80662708

1

These authors contributed same to this work.

Graphical abstract:

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Highlights: Oil shale semi-coke was firstly utilized as the bulking agent during composting.

Utilization of semi-coke could reduce the nitrogen loss during composting.

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Addition of the semi-coke was beneficial for the growing of AOA and AOB.

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The PAHs in oil shale semi-coke could be removed effectively after

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composting.

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Abstract: Oil shale semi-coke is the solid waste produced from the retorting process of oil shale, which may cause pollution to the environment without reasonable disposing. In this study, semi-coke was used as the bulking agent during composting

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to accelerate biodegradation of the organics as well as decrease the nitrogen loss. Results showed that the addition of semi-coke could accelerate biodegradation of the

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organics, with a raise in the organic matter loss from 44.99% to 47.05% compared with the control. Furthermore, the nitrogen loss significantly decreased from 40.00%

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to 14.70% in the treatment added with semi-coke due to less emission of NH3 and much more transformation of NH4+-N to NO3--N by nitrification, which could be explained by the increasing abundance of ammonia-oxidizing bacteria and archaea at the late composting stage and drastic shift of the microbial community like Chloroflexi, Firmicutes and Actinobacteria. After the composting cycle, the maturity of the produced compost was elevated greatly in the treatments amended with semi-coke. The result of PAHs detection suggested that there were low PAHs content

in the raw oil shale semi-coke and they could be removed effectively to within the range for land application by composting especially when the surfactant was added.

Abbreviations PAHs

naphthalene

NAP

electrical conductivity

EC

acenaphtylene,

ANA

total solid

TS

fluorene,

FLU

organic matter

OM

phenanthrene,

PHE

total Kjeldahl nitrogen

TKN

anthracene

ANT

total nitrogen

TN

fluoranthene

FLT

water soluble carbon

WSC

pyrene

PYR

water soluble nitrogen

WSN

benzo[a]anthracene

BaA

ammonium nitrogen

NH4+-N

chrysene

CHR

nitrate nitrogen

NO3--N

benzo[b]fluoranthene

BbF

germination index

GI

benzo[a]pyrene

BaP

ammonia-oxidizing bacteria

AOB

ammonia-oxidizing archaea

AOA

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polycyclic aromatic hydrocarbons

Keywords: oil shale semi-coke, polycyclic aromatic hydrocarbons, composting,

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nitrogen conservation, AOA and AOB.

1. Introduction

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As a potential supplement and substitute for fossil fuels, oil shale has obtained an increasing attention of the world, especially with the gradual exhaustion of

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conventional resources like coal and oil. Semi-coke, the byproduct which is always abandoned or land filled as a kind of solid waste without reutilization, is produced from the retorting process of oil shale [1]. In China, huge amounts of oil shale resources have been detected and the reserves attain 720 billion tons, which make it the largest producer of unconventional oil from oil shale [2]. Generally, approximately 10-30 tons of oil shale semi-coke could be generated during the production of one ton

of shale oil [3]. It is a big problem to deal with such a large amount of oil shale semi-coke which may conversely hinder the development of oil shale industry if not managed well [2]. However, up to now, there are no effective methods which are economical and environmentally friendly for the disposal of the oil shale semi-coke [4]. Combustion is one of the common treatment which can realize the reutilization of semi-coke [2, 3], but semi-coke has a relative low calorific value and combustion requires a high level of technology and instruments which means high cost [4]. Also it

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is suggested that the semi-coke could be used as sorbent [5] or building materials [2], but it still cannot meet the demand of so large amounts of semi-coke disposal. So stacking or land filling is still the most common treatment which not only requires a

large area of land, but also may cause pollution to the soil and groundwater with the

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release of the leachate [1]. It will be a big contribution to the solid waste management and reutilization if an appropriate way could be found to effectively deal with the oil

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shale semi-coke with low cost and low environmental risk.

Aerobic composting is a complex biological process driven by a series of

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microorganisms [6]. It is widely used for the disposing of organic solid waste like animal manure, crop straw, food waste and sludge due to the low cost and

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environmentally friendly process [7]. But there are some existing problems of the composting process which may limit the further development of composting, such as long time maturation period, massive nitrogen loss and low maturation due to the

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incomplete composting [8]. To solve these problems, bulking agents with high porosity and large specific surface area like biochar [9], zeolite [10] and coal ash [11]

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are often added through which the aerobic microbial activities increased greatly with the improved pore structures in the composting pile, resulting in more rapid and complete aerobic composting [12]. In addition, the nitrogen retention capacity was enhanced due to the good adsorption ability to NH4+-N and NH3 [13] and favorable aerobic environment for nitrifying microbes like AOA and AOB during composting amended with porous bulking agents [14, 15]. Oil shale semi-coke could also be regarded as a kind of porous material [16]

which might be a potential good bulking agent for composting. During the retorting of the oil shale, large amounts of porous structures formed in the semi-coke with a surface area of 4.4-57 m2·g-1 and pore volume of 1-4 ml·g-1 [17], similar to some biochar [18]. After activation, the surface area could even attain 354.21 m2·g-1 with good adsorption capacity [19]. In the previous investigations, it has been used to as the absorbent and carrier for the microbial immobilization for the porous property [20]. But it had never been used as the bulking agent for composting, through which not only might the composting process be enhanced, but also the semi-coke could be

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disposed and reutilized with low cost and low harm to the environment. So in this study, we aimed to investigate the possibility of composting as the

disposing method for the reutilization of oil shale semi-coke. Firstly, effects of semi-coke on the physicochemical properties were evaluated when it was amended as

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the bulking agent. Secondly, influence of semi-coke on the relative abundance of nitrifying bacteria was analyzed to clarify the enhanced composting process. Thirdly,

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the biodegradation of PAHs in semi-coke was also examined after composting, because it was reported that there are PAHs left in the semi-coke during the retorting

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process of oil shale which might cause pollution [1] and research suggested the PAHs should be removed effectively if used as soil conditioner [21]. In addition, to

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accelerate the removal of PAHs, the surfactant of tween-80 was mixed into the composting pile [22].

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2. Methods

2.1 Materials collection and processing

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Fresh dairy manure was obtained from Qingdao Aote Highbred Dairy Farm,

Shandong province and stored at 4 ℃ before used. Air-dried corn straw was collected from Pingdu County, Shandong province and cut into pieces shorter than 0.5 cm before mixed with the dairy manure. Tween-80 (Lot.No.304A051, Beijing Solarbio Science & Technology Co., Ltd) was diluted into 20% solution (20 g/L) by deionized water. Oil shale semi-coke was purchased from Shandong Energy Longkou Mining Group Co., Ltd (Longkou, Yantai, Shandong, China), smashed and screened passing

through 100-mesh sieve before added into the composting pile as the porous bulking agent. The specific surface area was 30.11 m2·g-1 and the average pore size was 12.61 nm, measured by Brunauer-Emmett-Teller method [8]. Other physicochemical properties of the raw composting materials were shown in Table 1. 2.2 Experimental design and method A lab-scale continuous thermophilic composting method was used by placing each bioreactor (2.5 L) filled with raw composting resources in the incubator which was set at the constant high temperature of 55 ℃ during the whole composting

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experiment. Aeration rate was 0.2 L·min-1·kg-1-TS. The detailed structure and parameter setting of the composting experimental system were shown in our previous

study [8]. Raw composting feed stocks were prepared by mixing the fresh dairy manure and corn straw at the ratio of 3:1 (wet basis). Then deionized water was added

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to adjust the initial moisture content at about 65%. For the control group, the thoroughly mixed feedstock was put into the 2.5 L reactor, labeled as T1. For the

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treatments with bulking agent, extra semi-coke with a proportion of 15% on the wet basis of feed stock was added and mixed completely, labeled as T2 and T3

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respectively. In T2, distilled water was added to ensure the same moisture content with the control (65%). While in T3, the same volume of prepared 20% tween 80

conducted.

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solution was added. Replicate experiments for the control and each treatment were

The composting process for each group lasted for 40 days. Samples were

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collected from composting reactors at 0, 1, 4, 7, 12, 20, 30, 40 days after mixing them thoroughly. The obtained compost samples were divided into two parts. One part of

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the fresh samples was used to detect the pH, EC, moisture, TS, OM, TKN, TN, WSC, WSN, NH4+-N, NO3--N and GI immediately. The other part was freeze-dried and then stored at -80 ℃ before it was used to measure the PAHs content and AOA/AOB abundance. All results were calculated in triplicate samples and on the TS basis. 2.3 Physicochemical properties determination Solid compost samples were used to detect the content of moisture, TS, OM and TKN. Moisture and TS were measured by drying the fresh solid compost sample at

105 ℃ for about 24 h. Ash and OM content were obtained by putting the dried compost samples into the high temperature furnace (KSL-1200X-M, Heifei Kejing Materials Technology Co., Ltd) which was set at 550 °C for 6h. OM loss was calculated based on the initial and final ash content of the sample [23]. TKN was determined by Kjeldahl digestion method using the automatic Kjeldahl Apparatus (K9840, Jinan Hanon Instruments Co., Ltd). TN was the sum of TKN and NO3--N. TN loss was calculated according to the TS and TN content before and after composting [24].

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Water extract of the fresh solid compost samples were used to detect the pH, EC, WSC, WSN, NH4+-N, NO3--N and GI. First a aqueous suspension was prepared by mixing 30 mL deionized water with 3 g fresh compost sample and shaking for 2 h.

Then the water extract for detection was obtained after the aqueous suspension was

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centrifuged and filtered through 0.45-μm filter membranes. PH value was measured

by a digital pH meter (PB-10, Sartorius, Germany) and EC was measured by a

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conductivity meter (Five Easy Plus FE38, Mettler Toledo, Switzerland). WSC and WSN were determined by the TOC analyzer (Multi N/C 3100, Analytikjena,

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Germany). Nessler's Reagent Spectrophotometry and dual-wavelength ultraviolet spectrophotometry were used for the determination of NH4+-N and NO3--N

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respectively [25]. GI was measured to evaluate the compost maturity and phytotoxicity by seed germination experiment [26]. For the gas analysis, NH3 and CO2 evolution in each group were determined

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every day. The sampling, measuring and calculating methods were described in the previous study [8].

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2.4 PAHs extraction and determination A modified QuEChERS method was used for the extraction of PAHs [27].

Briefly, 2g of freeze-dried compost sample was extracted with 5 mL mixed solvents of acetone and dichloromethane (1:1, v/v) in the ultrasonic water bath for 30 min. Then centrifuged and collected the supernatant. The supernatant was evaporated and concentrated to dryness by termovap sample concentrator. The residue was dissolved in 2.5 mL acetonitrile and then the solvent was purified by adding 0.7g of MgSO4,

0.4g of primary and secondary amines and 0.4g of C18 phase. The purified solvent was centrifuged again and filtered through 0.45-μm filter membranes. Finally, the obtained extraction containing PAHs was analyzed by GC-MS (7890A-5975C, Agilent, USA). 2.5 DNA extraction, quantification of AOA and AOB by qPCR and high-throughput sequencing Total DNA sequences in the freeze-dried compost samples were extracted using the Fast DNA Spin Kit for Soil according to the manufacturer’s instructions. Then the

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abundance of AOA and AOB were determined by quantifying the related archaeal and bacterial amoA genes respectively, using a real time PCR instrument (ABI7500, Applied Biosystems, USA). The often used primers were applied for the amplification of

archaeal

amoA

(upstream

primer,

5’-STAATGGTCTGGCTTAGACG-3’,

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downstream primer, 5’- GCGGCCATCCATCTGTATGT-3’) and bacterial amoA (upstream primer, 5’- GGGGTTTCTACTGGTGGT-3’, downstream primer, 5’[28].

The

PCR

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GGGGTTTCTACTGGTGGT-3’)

program

began

with

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pre-denaturation at 95 °C for 5 min, then followed by 30 cycles of denaturation at

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94 °C for 30 s, annealing at 55 °C for 30 s, and extension at 72 °C for 30 s, finally ended with the last extension at 72 °C for 10 min. Every DNA samples were detected

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for triplicates and the absolute abundance of target genes was shown as mean gene copy numbers per gram of TS.

For the high-throughput sequencing, the DNA extraction, sequencing and

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analysis processes were the same as our previous study reported [8]. 2.6 Data analysis

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Data obtained in this study were the mean values of triplicates. Standard

deviations were presented in the tables and error bars were showed in the figures. Significant difference test at p=0.05 were analyzed by one-way analysis of variance using Excel 2010 (Microsoft Office Professional Plus 2010, USA). Correlations between the archaeal/bacterial amoA or microbial community and physicochemical properties were conducted using redundancy analysis (RDA) using Canoco 5.

3. Results and discussion 3.1 Variations of some basic properties PH and EC are important properties of the compost which can indicate variations of the composting material. As shown in Fig.1a, the pH value increased in the first few days, and then declined gradually until the end of the composting. The increase of pH was due to the degradation of organic acids and the formation of NH4+, while the decrease might be resulted by the dissolution of large amounts of CO2 into the compost and release of small acid substances [8]. Compared to T1, there were

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significant lower pH values (P<0.05) in T2 and T3 with the addition of semi-coke, especially at the later phase. The reason might be that large amounts of NH4+ was

transformed to NO3- by nitrification at the later composting stage (Fig.3a and Fig.3b), during which hydrogen ion was released causing the decline of pH [29]. Contrary to

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the variation of pH, the value of EC decreased in the initial stage and then increased

(Fig.1b). The declining EC implied the microbial immobilization of soluble salt and

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the rising value was resulted by the release of small organic acids and inorganic ions like NH4+ [18]. During the whole composting period of T2 and T3, the EC values

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were in the range of 1500-4000 μS·cm-1. However, significantly higher EC values were found (P<0.05) in T1 without semi-coke as the bulking agent which could be

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explained by the much more WSC and NH4+ existing in group of T1 (Fig.2a and Fig.3a). Even the value attained 5311.25 μS·cm-1 when the composting process finished at day 40, obvious higher than the mature compost standard of 4000μS·cm-1

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which has been suggested as the upper limit value of EC with no harm to plants [30]. Variation of the moisture content during composting was shown in Fig.1c. The

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initial moisture content in each group was adjusted consistently at about 65% for ensuring the optimal microbial activity [31]. It decreased gradually over the composting time due to evaporation, but a significantly higher value (P<0.05) was observed at day 40 in T2 and T3 indicating that water retention capacity of the compost was improved with the addition of semi-coke which was beneficial for the microbial activity. Similar phenomenon was found when biochar was utilized as the porous bulking agent during the composting of pig manure [32].

3.2 Effects of semi-coke on carbon dynamic and organic matter loss Carbon is one of the main elements during composting which can provide energy sources for the growth of microbes during aerobic fermentation resulting in the reduction of organic solid waste and the maturation of raw compost. So the metabolism of carbon has a tight relationship to the microbial activity during composting. In this study, variations of some important indexes related to carbon including WSC, CO2 evolution and OM content were investigated to analyze the carbon dynamic and organic matter biodegradation.

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WSC is the easiest carbon source that can be directly utilized by the microorganisms, because the metabolism process proceeds in the water-soluble phase

[8]. Moreover, WSC is an important parameter during composting, for that it not only indicates the microbial activity but also assess the compost maturity [33]. As showed

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in Fig.2a, the initial WSC concentration in T1, T2 and T3 was 30.93, 22.24 and 25.05 g·kg-1-TS respectively. Much lower WSC content in T2 and T3 was due to the

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addition of semi-coke which almost had no water soluble organic matters, while higher value in T3 than in T2 was because of the addition of soluble organic matter of

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tween 80. After the composting began, overall the WSC decreased gradually due to the biodegradation except the period of day 1 to day 4 where there was a rising trend

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of the WSC for the release of small soluble organic compounds during the intensive biodegradation of macromolecular organic matters [6]. At day 40, the final value of WSC in T1, T2, T3 was 11.40, 5.76, 7.95 g·kg-1-TS respectively, and the

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corresponding biodegradation ratio was 63%, 74% and 68% (P<0.05) implying higher microbial activity during composting added with semi-coke. In addition, it is

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suggested a value of less than 10 g·kg-1-TS of the WSC concentration in mature compost [34], while only the group of T2 and T3 attained this standard, indicating that the addition of semi-coke could also promote the maturation of compost. CO2 is the end production during the aerobic fermentation of solid organic waste. Usually, there is a direct relationship between the CO2 evolution and microbial activity [8]. In this study, CO2 concentration in the exhausted gas and cumulative CO2 production were detected, shown in Fig.2d. The peak value of the CO2 concentration

in T2 and T3 occurred quickly at the first day, while it was observed much later in T1, implying an improved environment for the growth and reproduction of microbes when semi-coke was added. After the peak value, CO2 concentration in each group decreased gradually and became stable for the reduction of available carbon sources and poor microbial activity at the later phase. During the 40-day composting period, the cumulative CO2 production (P<0.05) in T1 was the lowest with a value of 374.19 g·kg-1-TS. Group of T2 amended with semi-coke placed the second (438.96 g·kg-1-TS) and T3 produced the most CO2 (495.88 g·kg-1-TS) for the extra addition of surfactant.

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The OM content and OM loss ratio can explain the overall biodegradation composition of carbon, which were shown in Fig.2b and Fig.2c. At the first 12 days, the OM content in each group sharply decreased due to the intensive biodegradation

of organic matters, consistent with the variation of WSC (Fig.2a). From day 12 to day

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20, the fluctuation became smaller. After day 30, the OM content in each group

remained almost unchanged, implying that the biodegradable organic matter was

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transformed into stable humus. The result of OM loss corresponded well with the cumulative CO2 evolution. Compared with T1 (44.99%), the final OM loss was higher

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in T2 (47.05%), which was probably caused by the improvement of pore structure in the composting pile with the addition of semi-coke, similar to the effect of other

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porous bulking agent [15]. In addition, even more biodegradation ratio of the organic solid waste was obtained in T3 (49.41%) added with surfactant, as it was revealed by the study that addition of surfactant during composting could promote the

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biodegradation of organics especially cellulose [35, 36]. 3.3 Influences of semi-coke on the nitrogen variation and total nitrogen loss

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Nitrogen is the other main element in composting because it plays an essential

part in the reproduction of microbes and it is the one of the most important nutrients beneficial for the growth of plants [37]. Nitrogen in the feedstock is present mainly in the form of organics. During composting, it goes through a series of complex biochemical reactions including ammonification, nitrification, denitrification, assimilation and volatilization, by which the nitrogen is transformed into several different forms such as NH4+-N, NO3--N and NH3. The variations of nitrogen were

showed in Fig.3. As shown in Fig.3a, similar changes of the NH4+-N content were found in T2 and T3, both of which declined dramatically from day 0 to day 12 for the massive volatilization of NH3 at high temperature and then became stable [38]. However, there was a slight increase in T1 at day 1 and day 40. The increase at day 1 might be due to the release of NH4+ by the biodegradation of nitrogen-containing compounds at the beginning of composting [38], while the increase at day 40 was probably caused by the incomplete composting process and poor nitrification as it was shown in Fig.3b

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[39]. During the whole composting period, there were significant lower NH4+-N contents in T2 and T3 (P<0.05) which might be explained by the great NH4+ adsorption capacity of semi-coke with porous structure and certain functional groups,

as it was revealed in studies using biochar and zeolite as the bulking agents [13, 40].

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The final NH4+-N concentration in T1, T2 and T3 was 0.99, 0.24, 0.28 g·kg-1-TS

respectively. Only the control group without semi-coke exceeded the maximum value

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of the mature compost (0.4 g·kg-1-TS) [41].

Profile change of NO3--N was exhibited in Fig.3b. The initial NO3--N content in

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T1 was 2.14 g·kg-1-TS, while the value declined to 1.36 and 1.45 g·kg-1-TS in T2 and T3 respectively due to the low content of NO3--N in semi-coke. After the composting

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process started, the NO3--N concentration decreased quickly and continuously until day 20, because the high temperature and NH3 toxicity would inhibit the growth of nitrifying bacteria which resulted in less NH4+-N shifted to NO3--N [39]. From day 20

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on, the composting came to the maturation stage where there was always an increase of the NO3--N concentration for the recovery of nitrifying bacteria [38]. During this

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stage, the content in T2 and T3 increased gradually, but it maintained very low value in T1 until the end of composting. At day 40, the control group contained the least NO3--N (0.05 g·kg-1-TS) for poor nitrification. The content increased to 0.98 g·kg-1-TS in T2 added with semi-coke which could enhance the nitrification by providing favorable aerobic environment for the nitrifying bacteria proliferation [42]. The highest NO3--N content was observed in T3 (1.72 g·kg-1-TS) where surfactant was mixed into the composting resources, probably resulting in further enhanced

nitrification. WSN is the important nitrogen nutrients that can be directly utilized by plant containing NH4+-N, NO3--N and some soluble organic nitrogen [43]. As shown in Fig.3c, overall the WSN exhibited a trend of sharp decrease in the initial stage for the massive NH3 emission (Fig.3d) and then became relative stable. At the late stage, there was a slight increase in T2 and T3 because of the enhanced nitrification which could transform more NH4+-N to NO3--N resulting in less NH3 emission as it was showed in Fig.3b and Fig.3d. After the 40-days composting period, there were more

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WSN in T2 and T3 amended with semi-coke which adversely had much less WSN before composting, implying that the addition of semi-coke could retain more easily utilized nitrogen nutrients during composting favorable for crop growth.

As shown in Fig.3d, NH3 was mainly emitted in the first 20 days during

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composting due to the intensive microbial activities and high NH4+-N concentration

[8]. The peak NH3 emission rate in T1 was 21.0 mg·day-1, much higher than T2 (14.0

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mg·day-1) and T3 (11.2 mg·day-1). The previous study revealed the porous bulking agent with rich micro-pores and large specific surface area had great affinity to NH4+

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and NH3, causing the reduction of NH3 emission during composting [14]. Also study showed porous bulking agent could enhance the nitrification resulting in less NH3

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emission [42]. Combined with the decreasing content of NO3--N (Fig.3b) and decreasing abundance of AOA/AOB (Fig.4) in the initial stage which means poor nitrification, what had more influence on the reduction of NH3 emission during the

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first 20 days was the good adsorption capacity of semi-coke. Then from day 20 to the end of the composting process, the NH3 emission became less and relative stable for

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the increasing stable compost [8]. Nevertheless, in the group of T1, the NH3 emission rate rose again from day 30 to day 40, which was predictable because the NH 4+-N concentration increased obviously at this period (Fig.3a). The reason might be the much weaker nitrification in the control group without semi-coke at the late composting phase, as it was indicated from the less NO3--N content and fewer AOA/AOB abundance (Fig.3b, Fig.4) in T1 without semi-coke. During the 40-day composting period, the cumulative NH3 emission was 233.8, 151.9 and 106.4 mg for

T1, T2 and T3 respectively (P<0.05). Compared with T1, the total NH3 emission was reduced by 35.03% and 54.49% in T2 and T3 respectively. While the previous study showed only a reduction of 21% was found during composting added with porous ceramsite and a slight increase value of 26% was found when added with vermiculite [8, 23]. Great reduction of NH3 emission could be realized when semi-coke was utilized as the bulking agent during composting. Furthermore, the effect might be furtherly improved with the addition of surfactant as the study showed the surfactant-modified zeolite had better capacity of nitrogen retention [44, 45].

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There was a similar trend of the variation of TKN (Fig.3e) and TN (Fig.3f). Both of them increased gradually over the composting time due to massive loss of organic

matter [39]. But in T1, there was a decline of the TKN and TN at the initial stage,

similar to the composting experiment of chicken manure added with bentonite [46].

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The reason might be the intensive NH3 emission at the first few days in T1 (Fig.3d). During composting, the content of TKN and TN were lower in T2 and T3 for the

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addition of semi-coke with lower nitrogen content, but their final increase ratios were significantly higher (P<0.05) when the composting experiment finished at day 40. For

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the TKN, the increase ratio was 55.41%, 64.35% and 65.40% for T1, T2 and T3 respectively. For the TN, it was 39.40%, 55.80% and 60.98% respectively.

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Nitrogen balance during composting was calculated in Table 2 to compare the overall nitrogen retention capacity in each group. Before composting, the weight of TN was 5.71, 5.84 and 5.90 g for T1, T2 and T3 respectively with no significant

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difference (P>0.05), while significant difference was found among groups after composting (P<0.05). The final weight of TN was 3.43, 4.98 and 5.42g respectively.

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Group of T1 without semi-coke (40.00%) had the most TN loss, similar to the value reported by the previous study [24]. Much less TN loss was found when semi-coke was used as the bulking agent in T2 (14.70%). Furthermore, the TN loss could be furtherly reduced when surfactant was added in T3 (8.16%). Compared with the control, reduction ratio of the TN loss in T2 and T3 was 64.45% and 78.89% respectively. The effect was even better than many good bulking agents often used in composting [25, 47]. As for the detailed forms of TN loss, studies revealed most of

the TN loss was due to the formation of NH3 under high temperature and production of N2O under anaerobic environment by denitrification [48]. N2O emission was not detected in this study, but it could be classified into other nitrogen loss. In this study, the NH3 emission loss in each group accounted for 10%-23 % of the TN loss, and other nitrogen (N2O) loss accounted for much more, similar with the previous study [49]. Comparing the loss weight of each kind of nitrogen among the treatments, there were significantly much more NH3 emission loss and other nitrogen loss in the control group than the groups added with semi-coke, indicating the addition of semi-coke

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could reduce the nitrogen gas emission such as NH3 and N2O. For the reduction of NH3, it was due to the enhanced nitrification and great affinity to NH4+ and NH3 of semi-coke as discussed above. For the reduction of N2O, it might be the improved aerobic composting when porous semi-coke was added as the bulking agent, which

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could inhibit the denitrification as it was revealed in the previous study [48]. 3.4 Dynamics of AOA/AOB abundance

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AOA and AOB are two important microorganisms for the ammonia oxidation, which were analyzed by quantifying the target amoA gene in AOA and AOB. As it

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was shown in Figure.4, both of AOA and AOB abundance exhibited significant change over the composting process. Before composting, the target amoA gene copy

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number of AOA and AOB was high, but it decreased drastically from day 0 to 7, implying a poor nitrification as it was showed from the high NH3 emission (Figure.3d) and decreasing NO3--N content (Figure.3b). The decreasing abundance of AOA and

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AOB might be caused by the thermophilic environment (≥ 55 ℃) as the previous study suggested that the optimum living temperatures for most kinds of

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ammonia-oxidizing microbes were in the range of 20 to 37 ℃ [50]. Also the high ammonia/ammonium concentration at the initial stage (Figure 3a, 3c) might inhibit the growth of AOA and AOB [48]. However, after day 7, AOA and AOB recovered gradually, causing large transformation of NH4+-N to NO3--N (Figure.3b). The recovery of AOA and AOB was mainly due to the reproduction of thermophilic AOA and AOB as the research revealed that the thermophilic ammonia-oxidizing microbes could live in the high temperature of 50-70 ℃ [50, 51], even above 80 ℃ [52].

Comparing the AOA and AOB amoA gene copies during composting, significant higher value (P<0.05) of the AOA was found, which was also observed in the previous study [28] that there was more AOA than AOB in compost due to the better thermo-tolerance of AOA [53]. Comparing the control and treatments, there were significantly more AOA and AOB in T2 and T3 amended with semi-coke (P<0.05) especially at the late phase of the composting, as it was reported that porous bulking agent could provide favorable environment for the aerobic microorganism like AOA and AOB [14, 15]. So a conclusion might be given that besides absorbing NH4+ and

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NH3, the addition of semi-coke during composting could favor the growing of AOA and AOB and promote the nitrification to improve the nitrogen conservation capacity. Better effect was found when surfactant was added.

3.5 Mechanism of the enhanced nitrogen conservation capacity during composting

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added with semi-coke

Redundancy analysis (RDA) of the AOA/AOB abundance and physicochemical

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properties during composting were shown in Fig.5. The first two canonical axes can totally explained 98.12 % of the change of AOA/AOB abundance (F=21.0, p=0.004).

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More detailed information about effects of each compost physicochemical property on AOA/AOB abundance from RDA was showed in Table 3. It could be easily observed

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from Fig.5 that compared with other properties except pH, parameters related to nitrogen such as NO3--N, NH4+-N, WSN, NH3 and TKN have much closer distance to AOA and AOB implying more tight correlations between them (Table 3). For the

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effects of pH, RDA results in Table 3 showed the pH exhibited the most explained variation (72.7%) on AOA/AOB, which meant the pH had the most tightly correlation

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with AOA/AOB. Similar phenomenon was found in the previous study [54], because extra H+ is produced when NH4+-N is transformed into NO3--N by AOA/AOB, resulting in the change of pH value. So in the final analysis, what correlated tightly with dynamics of AOA/AOB was the variation of nitrogen. The Pearson correlations were calculated for the further confirmation about the effects of semi-coke on the transformation of nitrogen during composting, shown in Table 4. There was not a significantly correlation between the NH3 emission and

NH4+-N (r2=-0.126, p>0.05), not as the study revealed, which might be caused by the continuous thermophilic environment and addition of semi-coke [14]. But in a whole, there was much more NH3 emission when more NH4+-N existed in the compost as it was showed in Fig.3a and Fig.3d, so semi-coke might reduce the free NH4+-N concentration in the composting pile due to its great adsorption capacity and enhanced nitrification, resulting in less NH3 emission. Accordingly the less NH3 emission resulted in much less TN loss during composting amended with semi-coke (Table 2) for the significantly positive correlation between cumulative NH3 emission and TN

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(r2=0.830, p<0.01). Also NH4+-N and NO3--N were found to be closely related with WSN. As shown in Fig.3c, the addition of semi-coke could retain more easily utilized nitrogen nutrients (WSN) favorable for the crop growth, because more NH4+-N was transformed into NO3--N (Fig.3b) which is very important nutrient, rather than

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volatile NH3. In addition, almost all the nitrogen compounds exhibited significant

positive or negative correlation with AOA and AOB, especially for NH4+-N and

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NO3--N. So it could be furtherly confirmed that the less NH4+-N content (Fig.3a) and much more NO3--N content (Fig.3b) in T2 and T3 especially at the late composting

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stage was resulted by the increasing number of AOA and AOB (Fig.4) when semi-coke added. As the previous study revealed that the growth of AOA and AOB

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was tightly related with the oxygen content [28], the aerobic environment was greatly improved with the addition of porous semi-coke, favorable for the growth of AOA and AOB in T2 and T3 [14, 15]. Furtherly the denitrification might be inhibited for the

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improved aerobic environment causing much less N2O emission as it was discussed above, but further investigation should be given about the N2O emission.

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For further investigation of the correlation between the nitrogen variation and

microbial community, 16S rRNA genes in the compost samples were sequenced by high-throughput sequencing method. As it was showed in Figure.6a, the dominant phyla in the three treatments were Actinobacteria (14.2%-61.2%), Firmicutes (7.4%-48.5%), Proteobacteria (12.2%-38.1%), Chloroflexi (0.4%-26.2%) and Bacteroidetes (0.9%-21.6%). All of these phyla exhibited drastic shift with the composting process and among the treatments as the PCA analysis showed

(Figure.6c), especially significantly difference was found between the control group (T1) and experimental groups (T2 and T3) added with semi-coke. Compared with T1, there were much less Actinobacteria from day 4 to day 40 in T2 and T3, but the phyla of Firmicutes in the initial stage from day 1 to day 7 and Chloroflexi in the late stage from day 20 to day 40 was significantly more. For the less Actinobacteria in T2 and T3, it was mainly due to the much less Thermopolyspora, Thermomonospora, Longispora and Iamia (Figure.6b). For the much more Firmicutes and Chloroflexi in T2 and T3, it was greatly due to the Bacillus and Roseiflexus respectively. In addition,

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correlations between the nitrogen variation and top 10 phyla or genera were conducted by RDA analysis, showed in Figure.6d and Figure.6e. Only the bacteria whose relative abundance attained 5% were chosen for the following correlation

discussion because of so many bacteria might showed potential correlation with the

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nitrogen variation. Almost all the nitrogen variations exhibited significantly positive

or negative relation with Rikenellaceae_RC9_gut_group or Roseiflexus. Besides, TKN

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showed positive correlation with the Iamia (r2=0.787, p<0.01) and Longispora (r2=0.732, p<0.01), both of which belong to Actinobacteria. And the NH3 emission

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rate was found to have significantly positive correlation with Bacillus (Firmicutes), Thermomonospora and Thermopolyspora (Actinobacteria). In a conclusion, the

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effects of oil shale semi-coke on the variation of nitrogenous compounds during composting were probably caused by some dominant phyla like Chloroflexi, Firmicutes and Actinobacteria, including Rikenellaceae_RC9_gut_group, Roseiflexus,

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Iamia, Longispora, Bacillus, Thermomonospora and Thermopolyspora. 3.6 Influences of semi-coke on the maturity and phytotoxicity

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Variations of the TC/TN ratio were showed in Fig.7a. The initial value was 21.19,

23.39 and 22.76 for T1, T2 and T3 respectively. Then it decreased gradually in T2 and T3 until the end of composting for the more rapid mineralization of carbon than nitrogen. While in T1, there was a slightly rise of the TC/TN ratio at the beginning of composting corresponding well with the decline of TN (Fig.3f) due to the excessive volatilization of NH3 (Fig.3d). At day 40, the TC/TN ratio in compost was 12.90, 11.23 and 10.36 in T1, T2 and T3 respectively, all of which are in the range of

maturation standard [32]. The relative lower value of TC/TN ratio in T2 indicated more stable compost with the addition of semi-coke and the effect was better in T3 when surfactant was mixed into the reactor. The conclusion was also confirmed by the result of seed germination test which is considered as the most effective method to evaluate the maturity and phytotoxicity of the compost [40]. Usually, the maturity of compost is divided into three degrees according to the value of GI, including immature (< 80%), mature (80-90%) and very mature (> 90%) [8]. As it was showed in Fig.7b, the GI value increased gradually

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over the composting time because of the decline of toxicity substance like NH4+ and WSC [8]. There was significant difference (P<0.05) between the control group (T1) and treatments (T2 and T3), but no significant difference was found between T2 and

T3 (P>0.05) during composting. At day 7, the compost in T2 and T3 became mature

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with a GI value of 90.62% and 90.35 respectively, while the GI value in T1 was only

57.09% which could not attain the mature standard until day 12. When the 40-day

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composting period finished, the GI value in T1 was 94.88% much lower than T2 (120.00%) and T3 (120.94%), implying that semi-coke could shorten the composting

3.7 PAHs biodegradation

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cycle and elevate the compost maturity.

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There were 11 kinds of PAHs observed in the raw semi-coke of the 16 USEPA-listed PAHs, including NAP, ANA, FLU, PHE, ANT, FLT, PYR, BaA, CHR, BbF and BaP. After mixing with the feedstock in T2 and T3, the initial concentration

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of each PAH was in the range of 0.16-0.43 mg·kg-1-TS (Fig.8a). The total PAHs content was very low with a value of 3.44 mg·kg-1-TS, while PAHs with short

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benzene rings accounted for the most with a value of 2.85 mg·kg-1-TS including 0.26 mg·kg-1-TS of the 2 rings (NAP), 1.10 mg·kg-1-TS of the 3 rings (ANA, FLU, PHE, ANT) and 1.49 mg·kg-1-TS of the 4 rings (FLT, PYR, BaA, CHR) (Fig.8b). When the composting finished at day 40, each type of PAH was degraded in various degrees (Fig.8c). The total content was significantly (P<0.05) decreased to 2.74 mg·kg-1-TS and 2.45 mg·kg-1-TS in T2 and T3 respectively (Fig.8b), all of which were much lower than the maximum PAHs content (6.00 mg·kg-1-TS) for land application [55]

after the continuous thermophilic composting. In the group of T2, the total PAHs biodegradation ratio was 41.94%. The removal percentage of PAHs containing 2, 3 and 4 benzene rings (52.65%, 41.73% and 42.20%) was higher than those with 6 benzene rings (39.38%) (Figure.8d), as it was observed that the small and medium PAHs were easier to be degraded during composting [56]. In the group of T3 added with surfactant, the amount of each PAH was furtherly decreased to the range of 0.12-0.30 mg·kg-1-TS (Fig.8a). The total PAHs biodegradation ratio (Figure.8d) was 47.23%, higher than the value of 41.94% in T2, indicating the addition of tween 80

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could enhance the removal of PAHs during composting [57]. Similar PAHs loss (15.8-48.6%) was observed during composting of sewage sludge, but it need a much longer time of 76 days [58] implying more rapid biodegradation of the PAHs in this

study utilized with surfactant. Significant difference (P<0.05) was found among the

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loss of PAHs with different benzene rings. The removal percentage of PAHs in T3 was 59.22%, 46.40%, 47.14% and 41.48% respectively for the 2, 3, 4 and 5 rings, and

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the corresponding increase ratio was 6.57%, 4.67%, 4.94% and 2.10% compared to T2, which exhibited the better effect of surfactant on the removal of PAHs with short

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benzene rings.

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4. Conclusion

During the 40-day composting experiment, it was revealed that not only could the oil shale semi-coke be utilized as a good porous bulking agent beneficial for the

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composting process, but also its contaminant decreased greatly after composting, meeting the requirement of the maximum PAHs content for land application.

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Compared with the control, there were lower WSC content, more CO2 emission and OM loss in the treatment added with semi-coke, implying much higher microbial activity caused by the more favorable environment with the addition of semi-coke. In addition, less NH3 emission was observed in the treatment resulting in much less nitrogen loss, which was due to the more transformation of NH4+-N to NO3--N by the great adsorption capacity of semi-coke to NH4+ and NH3 and promoted nitrification of AOA/AOB. The effects of semi-coke on the nitrogen variation were also probably

caused by the drastic shift of some dominant phyla like Chloroflexi, Firmicutes and Actinobacteria. Moreover, elevated compost maturity was obtained when semi-coke was used as the bulking agent. After composting, the PAHs in semi-coke were removed greatly especially for those with short rings and the total degradation ratio was furtherly increased to 47.23% when tween 80 was added. Besides the increasing removal of PAHs, other effects discussed above were also improved at different degrees with the addition of tween 80. So as the study suggested, composting might

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be an effective method for the treatment of oil shale semi-coke.

Author Contributions CRediT author statement:

Li Xu: Conceptualization, Methodology, Software, Investigation, Formal analysis,

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Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization,

Editing, Supervision Lu Mingyi: Resources, Data Curation

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Shi Xiaoshaung: Conceptualization, Methodology, Resources, Writing - Review &

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Zhao Yuzhong: Resources, Data Curation

Guo Rongbo: Conceptualization, Methodology, Project administration, Supervision

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Peng Hui: Conceptualization, Methodology, Supervision

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Declarations of interest: none

Acknowledgements The project was supported by the "Transformational

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Technologies for Clean Energy and Demonstration", Strategic Priority Research Program of the Chinese Academy of Sciences (XDA 21060400) and Qingdao Science and Technology Project for People's Livelihood (17-3-3-45-nsh).

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and potential ammonia oxidation rate during agricultural waste composting, Bioresource Technology, 270 (2018) 278-285.

na

[54] R. Tao, S.A. Wakelin, Y. Liang, G. Chu, Response of ammonia-oxidizing archaea and bacteria in calcareous soil to mineral and organic fertilizer application and their

20-30.

ur

relative contribution to nitrification, Soil Biology and Biochemistry, 114 (2017)

[55] J. Poluszyńska, E. Jarosz-Krzemińska, E. Helios-Rybicka, Studying the Effects

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of Two Various Methods of Composting on the Degradation Levels of Polycyclic Aromatic Hydrocarbons (PAHs) in Sewage Sludge, Water Air & Soil Pollution, 228 (2017) 305.

[56] Y. Zhang, Y.G. Zhu, S. Houot, M. Qiao, N. Nunan, P. Garnier, Remediation of polycyclic aromatic hydrocarbon (PAH) contaminated soil through composting with fresh organic wastes, Environmental Science & Pollution Research, 18 (2011) 1574-1584.

[57] K.Y. Cheng, K.M. Lai, J.W.C. Wong, Effects of pig manure compost and nonionic-surfactant Tween 80 on phenanthrene and pyrene removal from soil vegetated with Agropyron elongatum, Chemosphere, 73 (2008) 791-797. [58] P. Oleszczuk, Changes of polycyclic aromatic hydrocarbons during composting of sewage sludges with chosen physico-chemical properties and PAHs content,

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Chemosphere, 67 (2007) 582-591.

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Table 1

Physicochemical properties of the raw composting materials Corn straw

Semi-coke

7.69±0.02

7.60±0.02

7.83±0.03

1.43±0.02

2.57±0.02

2.59±0.03

84.23±0.12

9.17±0.15

10.84±0.17

OM (%)

87.18±0.35

90.41±0.13

16.86±0.46

TKN (g·kg-1-TS)

36.87±0.00

21.08±0.13

7.39±0.36

WSC (g·kg-1-TS)

46.03±0.29

11.84±0.17

0.16±0.03

WSN (g·kg-1-TS)

6.23±0.11

1.45±0.01

0.41±0.00

NH4+-N (g·kg-1-TS)

5.05±0.00

1.08±0.03

0.11±0.01

NO3--N (g·kg-1-TS)

2.79±0.13

0.86±0.02

0.09±0.02

pH EC (mS·cm-1)

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Moisture (%)

Dairy manure

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Properties

TS: total solid; EC: electrical conductivity; OM: organic matter; TKN: total Kjeldahl

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nitrogen; WSC: water soluble carbon; WSN: water soluble nitrogen

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Table 2

Nitrogen balance during composting Initial TN (g)

T2

TN

loss (g)

TN loss

NH3-N

NH3-N

Other

Other

ratio

emission

loss

loss

loss

(g)

ratio

(g)

ratio

5.71±0.01

3.42±0.04

2.29

40.00%

0.23±0.02

10.23%

2.05

89.77%

5.84±0.02

4.98±0.25

0.86

14.70%

0.15±0.00

17.70%

0.71

82.30%

5.90±0.09

5.42±0.04

0.48

8.16%

0.11±0.01

22.05%

0.38

77.95%

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T3

(g)

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T1

Final TN

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Table 3

Summarize effects of each compost physicochemical property on AOA/AOB from

Properties

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redundancy analysis

Variation explains (%)

F value

P value

71.9

33.3

0.004

64.8

23.9

0.004

52.3

14.3

0.006

42.1

9.5

0.014

TC/TN

38.3

8.1

0.012

NH4+-N

37.2

7.7

0.014

WSC

29.1

5.3

0.042

NH3

24.0

4.1

0.052

MC

8.1

1.1

0.29

pH

TKN

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WSN

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NO3--N

6.2

0.9

0.376

Table 4

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EC

Pearson correlations between the nitrogen variations and nitrifying microbes

1

NO3--N

0.620*

WSN

0.929** -0.126

NH3(1)

NH3(2)

TKN

TN

AOA

0.665**

1

-0.489

0.011

1

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NH3(1)

WSN

AOB

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NH4+-N

NO3--N

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NH4+-N

NH3(2)

-0.585*

-0.521*

-0.755**

-0.704

1

TKN

-0.360

-0.363

-0.462

-0.471

0.874**

1

TN

-0.275

-0.216

-0.375

-0.405

0.830**

0.988**

1

AOA

0.675**

0.785**

0.590*

0.437

-0.528*

-0.508

-0.403

1

AOB

0.733**

0.744**

0.713**

0.626**

-0.645**

-0.633**

-0.541*

0.917**

1

WSN: water soluble nitrogen; TKN: total Kjeldahl nitrogen; TN: total nitrogen; AOA:

ammonia-oxidizing archaea; AOB: ammonia-oxidizing bacteria; NH3(1): NH3 emission rate; NH3(2): cumulative NH3 emission. : p < 0.05; **: p < 0.01.

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Fig.2 Biodegradation of the organics during composting: (a) WSC, (b) CO2 evolution, (c) OM, (d) OM loss Note: T1, dairy manure; T2, dairy manure + semi-coke; T3,

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dairy manure + semi-coke + tween 80

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Fig.3 Dynamic changes of the major nitrogen during composting: (a) NH4+-N, (b) NO3--N, (c) WSN, (d) NH3 emission, (e) TKN, (f) TN

Note: T1, dairy manure; T2,

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dairy manure + semi-coke; T3, dairy manure + semi-coke + tween 80

Fig.4 Dynamics of AOA and AOB abundance during composting Note: T1, dairy

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Fig.5 Redundancy analysis of the AOA/AOB abundance and physicochemical

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properties during composting

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Figure.6 Relative abundance of (a) the top 10 bacterial phyla and (b) the top 10

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bacterial genera during composting; (c) principal component analysis of the bacterial community; Redundancy analysis of the compost physiochemical properties and

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bacterial community: (d) phyla, (e) genera

Fig.7 Maturity of the compost during composting: (a) TC/TN, (b) GI Note: T1, dairy

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Fig.8 Removal of the PAHs during composting Note: T1, dairy manure; T2, dairy

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manure + semi-coke; T3, dairy manure + semi-coke + tween 80