Speciation Dynamics of Phosphorus during (Hydro)Thermal


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Speciation dynamics of phosphorus during (hydro)thermal treatments of sewage sludge Rixiang Huang, and Yuanzhi Tang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04140 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 15, 2015

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Speciation dynamics of phosphorus during (hydro)thermal treatments of

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sewage sludge

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Rixiang Huang, Yuanzhi Tang*

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School of Earth and Atmospheric Sciences, Georgia Institute of Technology

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311 Ferst Dr, Atlanta, Georgia 30324-0340, USA

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*Corresponding author. Email: [email protected]; Phone: 404-894-3814

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Submit to

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Abstract

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(Hydro)thermal treatments of sewage sludge from wastewater treatment process can

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significantly reduce waste volume and transform sludge into valuable products such as pyrochar

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and hydrochar. Given the global concern with phosphorus (P) resource depletion, P

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recycling/reclamation from or direct soil application of the derived chars can be potential P

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recycling practices. In order to evaluate P recyclability as well as the selection and optimization

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of treatment techniques, it is critical to understand the effects of different treatment techniques

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and conditions on P speciation and distribution. In the present study, we systematically

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characterized P speciation in chars derived from thermal (i.e. pyrolysis) and hydrothermal

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treatments of municipal sewage sludge using complementary chemical extraction and nuclear

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magnetic resonance (NMR) spectroscopy methods. P species in the raw activated sludge was

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dominated by orthophosphate and long-chain polyphosphates, whereas increased amounts of

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pyrophosphate and short-chain polyphosphates formed after pyrolysis at 250–600 °C. In contrast,

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hydrothermal treatments resulted in the production of only inorganic orthophosphate in the

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hydrochar. In addition to the change of molecular speciation, thermal treatments also altered the

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physical state and extractability of different P species in the pyrochars from pyrolysis, with both

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total P and polyphosphate being less extractable with increasing pyrolysis temperature. Results

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from this study suggest that P speciation and availability in sludge-derived chars are tunable by

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varying treatment techniques and conditions, and provide fundamental knowledge basis for the

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design and selection of waste management strategies for better nutrient (re)cycling and

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reclamation.

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Environmental Science & Technology

Introduction

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Tremendous amounts of sewage sludges are being produced as the byproducts of

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wastewater treatment processes, bringing a daunting task for the solid waste management

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industry. In addition to the intrinsic high water content and large volume, sludges often contain a

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wide range of organic and inorganic contaminants, such as heavy metals, pesticides, herbicides,

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microorganisms, and pharmaceuticals and personal care products (PPCPs).1, 2 On the other hand,

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sludge also consists of various nutrients and valuable metals at relatively high concentrations,

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and is increasingly recognized and treated as a resource for the recycling of critical nutrients

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such as phosphorus (P).3 In fact, a significant portion of P consumed by human activities is

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ultimately converged into wastewater treatment plants (WWTPs), making the sludges from

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WWTPs a great resource for P recycling and reclamation.4, 5 For example, enhanced biological

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phosphorus removal (EBPR) technique with the precipitation of inorganic P fertilizers (e.g.

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struvite) is a common P recycling practice by WWTPs.

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In recent years, thermal (e.g. pyrolysis) and hydrothermal treatments (e.g. hydrothermal

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carbonization; HTC) of sewage sludge have emerged as sustainable treatment techniques,

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because they can significantly decompose organic pollutants, reduce waste volume, and generate

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valuable by-products (e.g. pyrochar from thermal treatments or hydrochar from hydrothermal

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treatments).6-9 During the production of chars, nutrients (e.g. P) mostly remain in the solid

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phase,6, 10 making it a char-P composite with many potential applications, such as P reclamation

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from HTC produced hydrochars with acid extraction.11 (Bio)chars produced from thermal

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treatments have also been recognized as good soil amendments to adjust soil physical and

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chemical properties and improve soil qualities.12 With the significant decomposition of organic

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contaminants during thermal treatment processes, direct soil application of such char-P

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composite can also be an excellent alternative P recycling and fertilization practice with all the

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added benefits from chars.

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One

critical

knowledge

gap

for

the

abovementioned

and

any

other

P

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recycling/reclamation approaches is the evolution of P speciation during the thermal treatments,

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as it is well known that the speciation of an element determines its mobility, transport, fate, and

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bioavailability. Speciation is defined as the chemical (e.g. oxidation state, molecular

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configuration, chemical phase) and physical states (e.g. distribution, association with other

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phases) of an element. In the case of phosphorus, it can exist in different molecular

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configurations (e.g., orthophosphate, phosphonate, organic phosphates, and polyphosphate) and

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these different entities can be present in different chemical states (e.g., cation complexes, surface

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adsorbed species on other minerals, incorporation into other mineral phases, or precipitation as

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P-containing minerals). Physically, these different P forms are likely to distribute

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heterogeneously within the solid feedstock and immigrate between solid and liquid phase during

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specific preconditioning or treatment processes. Thus, understanding the dynamic evolution of P

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speciation in the feedstock and the chars produced from (hydro)thermal treatments will provide

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fundamental knowledge basis for the evaluation, design, and development of suitable recycling

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and reclamation strategies. A variety of techniques can be used to characterize P speciation in

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environmental matrix such as soil, sediments, and wastes, such as sequential extraction, 31P solid

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state NMR, chemical extraction coupled with liquid

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spectroscopy, each with its own advantages and intrinsic limitations.13-15

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P NMR, P K-edge X-ray adsorption

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Although a few studies have tested pyrolysis16 and hydrothermal treatments6 for the

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transformation of sludge into char, little information exist on a systematic description of the

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change of P speciation during these treatments. For the few existing studies that characterized the

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P behavior during different treatment processes, P speciation was achieved indirectly through

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operationally defined fractionation and characterization methods.10, 17 However, some of these

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fractionation methods could potentially alter the speciation of P during the extraction process.18-

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20

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molecular structures.

Also, operationally defined P fractions are not equivalent to specific chemical species or

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In this study, we systematically characterized P speciation in raw sludges (activated and

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anaerobically digested sludge) as well as the pyro- and hydrochars derived from pyrolysis and

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hydrothermal carbonization (HTC) treatments of the sludges under varied conditions. Solid state

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speciation in the feedstock and the derived chars. Liquid 31P NMR was used to characterize the

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extractable P fraction from the solids to help validate the P speciation change in the solid and

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provide insights for the relatively labile P fractions for potential recycling and fertilization

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applications. The primary goal of this study is to understand the effect of treatment techniques

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and conditions on the transformation of different P entities, in order to provide critical

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knowledge basis for the potential application of (hydro)thermal treatments for sludge

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management and nutrient recycling/reclamation.

P nuclear magnetic resonance (NMR) spectroscopy was used to directly characterized P

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Experimental section

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Sludge collection and pretreatment. Sewage sludges were collected from F. Wayne Hill

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Water Resources Center (Gwinnett County, Atlanta, GA). The treatment plant has primary

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(physical), secondary (biological), and tertiary (membrane filtration) treatment units for treating

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municipal sewage from the County. Activated sludge was the waste sludge from the secondary

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treatment prior to dewatering treatment for anaerobic digestion, and anaerobically digested

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sludge was collected from the anaerobic digester receiving the activated sludge and primary

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sludge. The activated sludge collected still contain high amount of water, thus was centrifuged to

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further concentrate the sludge to ~26% dry mass content before storage at -20 °C. The anaerobic

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digested sludge (dry mass content ~21%) was stored at -20 °C without any pretreatment. For

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hydrothermal carbonization, a portion of both frozen samples was thawed at room temperature

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prior to treatments. For pyrolysis, a portion of both samples was freeze dried before treatments.

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The activated sludge sample represents sludge in its most unprocessed form, while the anaerobic

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sludge represent sludge experienced common processing (mixing with sludges from other units,

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dewatering, and anaerobic digestion). Details of the sampling and the chemical characteristics of

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these sludges are presented in Table S1.

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Pyrolysis and hydrothermal carbonization. Pyrolysis is a dry thermochemical process

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typically conducted on dry feedstock in the absence of oxygen and at elevated temperatures,

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while HTC is a wet thermochemical process that operates at relatively low temperature in

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combination with the autogenic pressure, and does not require specific atmosphere. Both of

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which can transform biomass into carbonaceous materials, while HTC has the advantage of

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accommodating wet feedstock. Pyrolysis of the activated sludge was conducted in a tube furnace

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(Thermo Scientific) under N2 flow (~1 mL/sec) at 250, 350, 450 and 600 °C, with a heating and

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cooling rate of 200 °C/h and a soaking duration of 4 h. The anaerobic sludge was treated at only

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at 450 °C. For each treatment condition, 1.0 g of dried activated sludge was added into a crucible

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and inserted into the glass tube. All samples were processed in duplicates.

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Hydrothermal carbonation (HTC) treatments used a solid:liquid ratio of 1:9 (w/w). To

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achieve this, 4 g centrifuged wet paste of activated sludge or anaerobic sludge (equivalent to ~1 g

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dried mass) and 6 g solution (DI water or 5% HNO3) were mixed in a 20 mL Teflon lined

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stainless steel hydrothermal reactor (Parr instrument). The reactor was sealed and heated in an

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oven at 225 °C for 24 h, then allowed to naturally cool down to 50 °C in an oven for overnight.

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Solid hydrochar produced from HTC was collected by centrifugation, and was dried at 70 °C for

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at least 24 h till no further weight loss. All samples were processed in duplicates.

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Both raw and treated sludges were characterized for the content and speciation of total P

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in the raw sludge and their derived chars, as well as P in the liquid extracts and solid residuals of

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sludges and chars after P extraction (see section below).

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P extraction. P extraction was conducted on the raw sludges and chars to assess the

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speciation of the extractable P portion from these solids. A portion of the freeze-dried sludges

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and their (hydro)thermal-derived chars were extracted using a solution containing 0.25 M NaOH

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and 0.05 M EDTA at a solid:liquid ratio of 0.2:4 g/mL and was shaken at 150 rpm for 16 h at

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room temperature. At the end of extraction, liquid extracts and solid residuals were separated by

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centrifugation. All extractions were conducted in duplicates. The liquid extract and residual

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solids were analyzed for P content using combustion and for P speciation using liquid and solid

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state 31P nuclear magnetic resonance (NMR) spectroscopy. This NaOH/EDTA extraction method

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has been widely used for P extraction from various environmental matrix21 including sludge22, 23

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and has been demonstrated to be a relatively effective P extraction method with high P extraction

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efficiency and with the most diverse P peaks by liquid NMR analysis, suggesting more

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representativeness and less destruction than other extraction methods.24, 25

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P content quantification. Concentrations of P in all liquid or solid samples were

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measured using a combustion method. Solid samples were combusted at 600 °C for 2 h in a

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furnace, followed by an HCl extraction (1 M) for 16 h. The extracts were analyzed as

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orthophosphate using the ascorbic acid assay26 on an UV-vis spectrophotometer (Carey 60,

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Agilent). To quantify P in the liquid NaOH/EDTA extracts, due to the presence of organics, the

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solution was dried first, followed by the abovementioned combustion and acid extraction steps

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before quantification.

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P solid state NMR analysis. Solid-state

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P NMR spectra were acquired on the raw

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sludges and their derived chars with magic angle spinning (MAS) and proton decoupling on a

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Bruker Avance 400 spectrometer operated at a 31P frequency of 161.9 MHz. Solid samples (~ 20

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mg) were packed into the inserts for a 4 mm diameter zirconia rotor with Kel-F Caps (Wilmad,

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NJ) and spun at 12 kHz. Direct polarization (DP) data collection parameters were 2048 data

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points (TD) over an acquisition time (AQ) of 12.6 ms, a recycle delay (RD) of 180 s, and 128

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scans. Variable RD experiments were conducted and 180 s were verified to be sufficient to

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prevent signal saturation during data acquisition (see Supporting Information, Figure S3). The

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DP-MAS 31P-NMR spectra were acquired with a 31P 90° pulse of 5.0 µs and an attenuation level

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(PL1) of 12.1 dB. Chemical shifts were externally referenced to NH4H2PO4 at 0.72 ppm. 31P spin

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counting experiments were based on Dougherty et al,27 where NH4H2PO4 was used as an

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external intensity standard, and its

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equilibration for 1000 s (16.7 min). 31P{1H} cross polarization (CP)/MAS spectra were also

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collected on activated sludge and its hydrochar, as well as anaerobic sludge. The CP contact time

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was set at 1 ms, and 960 scans were collected for all samples.

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31

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P DP NMR spectrum was acquired in one scan after

P liquid NMR analysis. Because the P concentration in the extracted solution is high 31

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enough, the supernatant (600 µL) were directly mixed with D2O (100 µL) for liquid

P NMR

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measurements without further drying and re-suspension. NMR spectra of the liquid samples were

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obtained using a Bruker AMX 400 MHz spectrometer operated at 162 MHz at 297 K. Parameters

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of 90° pulse width, 6.5k data points (TD) over an acquisition time of 0.51 s, and relaxation delay

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of 15 s were applied. T1 value of all phosphorus signals in the activated sludge extracts (which

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has the most peaks) was determined in preliminary inversion-recovery experiments, which

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showed that all the signals have a T1 less than 3 s, thus a relaxation time of 15 s was used.

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Between 256 and 512 scans were acquired for each sample. An 85% H3PO4 was used as the

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external standard for chemical shift calibration.

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Results and Discussion

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Overall characteristics of the raw sludges and their derived chars

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The sources and general chemical characteristics of the untreated activated and

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anaerobically digested sludges were present in Table S1. The ash and metal content in the

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anaerobic sludge agree with previously reported range,16, 28 and are higher than that of activated

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sludge due to its mixing with primary sludge. We quantified the mass recovery of chars under

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different treatment conditions, total P content, and P recovery rates in the sludge derived pyro-

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and hydro-chars (Figure S1). Information regarding sample nature, thermal treatment conditions,

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sample labeling, and sample characterizations can be found in Table S2. For pyrolysis, P content

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increased with increasing temperature due to the loss of solid mass and the remaining of P in

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solid phase. For HTC treatment, mass recovery with DI and 0.5% HNO3 treatments was 37% and

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32%, respectively, and P content was 7.9% and 7.2%, respectively. The lower P recovery in

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HTC compared to pyrolysis is due to the dissolution of phosphate into the processed water

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(Figure 2B). FT-IR spectra confirmed the transformation of the carbohydrate (biomass)

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dominated starting feedstock to aromatics-intense chars after thermal treatment (Figure S2).29

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Both solid and liquid state 31P NMR were used to characterize the speciation of P in raw

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and treated sludges. Since the quantitative analysis of P by solid state NMR relies on the

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maximum observation of P in the sample,27, 30 variable recycle delays were tested to determine

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the minimal recycle delay. Spin counting experiments were also conducted to quantify the

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percentage of P being observed. As shown in the series of spectra collected for activated sludge,

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maximum intensity was achieved at 180 s recycle delays (Figure S3). Spin counting experiments

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showed that more than 85% of the P were observed for activated sludge and its chars pyrolyzed

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at 250 and 450 °C, with only 49%, 40%, and 30.9% for anaerobic sludge, 600 °C char and HTC

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hydrochars, respectively. The low P observation in anaerobic sludge and HTC hydrochars is

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most likely due to the homogenization of P species and their subsequent direct interactions with

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paramagnetic metals such as Fe and Mn. Indeed, studies on sewage sludges collected from

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different sources have shown the abundance of Fe (concentrations ranging from thousands of

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ppm to a few weight percent) as well as the increasing association between heavy metals and

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phosphate upon aging.2, 28

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Microorganisms in WWTPs remove phosphorous from wastewater and store it in a wide

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range of organic and inorganic forms at different cellular locations.31-34 As is shown in the NMR

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spectrum of activated sludge (Figure 1), there are broad signals ranging from 10 to -40 ppm and

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with two distinctive peaks. Signal from the low field (more positive chemical shift) is generally

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attributed to orthophosphate, while that from high field (more negative chemical shift) is from

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pyrophosphate and polyphosphate.13 However, the chemical shift of orthophosphate in solid state

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NMR can be shifted and even broader, a known phenomenon that is caused by a wide range of

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factors such as hydration, cation complexation, adsorption onto minerals, and different P salt

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types.30, 35, 36 For example, the chemical shift of different phosphates can range from 9 to -30

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ppm in solid state NMR.13 This phenomenon makes it difficult for the direct assignment of

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chemical shift regions to specific P species in these highly complex and heterogeneous matrix.

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Based on

P liquid NMR spectra of the NaOH/EDTA extracts (Figure 2A and 2C, see

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discussion later), the dominant P species in the extracts from activated sludge are

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orthophosphates and polyphosphate, qualitatively consistent with the solid state NMR results.

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Phosphate esters and pyrophosphate were also identified, but in a relatively small amount.

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Compared to that of activated sludge, solid state NMR spectrum of anaerobically

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digested sludge showed a predominant broad peak at the 10 to -30 ppm region (centering at ~-5

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ppm) (Figure 1). In agreement with that of solid state NMR,

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aqueous NaOH/EDTA extract from anaerobic sludge is dominated by orthophosphate, with small

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fractions of organic P and pyrophosphate (Figure 2A, see discussion later), suggesting that

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polyphosphate was possibly released from bacterial cells and hydrolyzed into orthophosphate

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during anaerobic digestion. This is also consistent with the low observation of P in these samples

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by solid state NMR, as orthophosphate is known to interact strongly with paramagnetic metals

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such as Fe and Mn as previously discussed.

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31

P liquid NMR spectrum of the

Many studies have been devoted to reveal P speciation in sewage sludge using solid state 31

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and liquid

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were found to be the dominant species in most activated sludges, with their relative abundance

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depending on WWTP techniques and operational conditions.23, 37, 38 Sludges that are ready for

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disposable usually have experienced mechanical and thermal dewatering as well as anaerobic

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digestion processes. In those sludges, orthophosphate was the primary P entity and can present in

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various metal complexed forms.39 These results were in agreement with our study. However,

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none of the previous studies systematically traced the P speciation dynamics during sludge

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treatment processes, and the percent of P observed by solid state NMR and the percent of P being

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extracted and characterized by liquid NMR were rarely constrained in these studies.

P NMR. Diverse P entities (mostly orthophosphate, polyphosphate, and organic P)

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P speciation in pyrochars derived from pyrolysis

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We used complementary solid state NMR, chemical extraction, and liquid NMR

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techniques to systematically and quantitatively characterize the change of P speciation during

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pyrolysis. As shown in Figure 1, the spectra of chars were significantly different from that of

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pristine activated sludge, with the signal became more homogenized and without clear peak

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separation.

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In order to ensure the representativeness of

31

P liquid NMR results for speciation

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comparison, we quantified the P extraction efficiency of the NaOH/EDTA method for different

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treatment conditions and also characterized their residual solids using solid state

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(Figure 3). Results showed that the extraction efficiency ranges from 40% to 70%, indicating that

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P in the extracts can account for a significant portion of the total P. As previously discussed, this

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extraction method pose little alteration of the P species, thus the speciation information from

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liquid NMR of the extracted solution can provide rich information on the speciation of

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extractable and more labile P species. Although the non-extractable P species were not directly

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characterized by liquid NMR, they can be identified and inferred by combining solid state NMR

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results of the solids before and after extraction, as well as liquid NMR of the extracts. As can be

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seen from Figure 3A, P extraction efficiency for the pyrochars decreases with increasing

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pyrolysis temperature, which can be caused by physical constraints as a result of carbon

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condensation during pyrolysis. We also note that the signals in the extracted residues shifted

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toward low field (more positive chemical shifts) (Figure 3B), which is likely due to the intrinsic

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heterogeneity of sludge and its derived chars. As previously discussed, the chemical shift of

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orthophosphate in solid state NMR can be broad and shifted as affected by a wide range of

31

P NMR

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factors such as hydration, cations, adsorbed minerals, and salt types.30, 35, 36 Different P species

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can present in very diverse physical and chemical forms, and it is possible that different P salts

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and/or cation-P complexes with NMR shielding effects were extracted from the solid, thus

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causing the observed spectra shifting.

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We now turned to discuss P speciation by liquid NMR observations before further

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comparison and interpretation of the solid state NMR spectra of the pristine and extracted

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samples. Although solid state NMR can directly observes the P species in solid samples,

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liquid NMR spectra of the liquid extracts possess better resolution and provide straightforward

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information for the identification and quantification of different P species.

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spectra of the extracts and their quantification were present in Figure 2 and Figure S4. After

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pyrolysis, the most distinct change is the appearance of pyrophosphate (chemical shift at ~ -5

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ppm) in the pyrochars of activated sludge, with relative abundance ranging from 10% to 24%, as

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compared to ~1% pyrophosphates in the raw activated sludge (Figure 2A and 2C). No significant

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alteration was found after the pyrolysis of anaerobic sludge, with orthophosphate still being the

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dominant P entity (with ~3% of pyrophosphate) in the extract of A450, except for the complete

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disappearance of the trace amount of mono- and diesters (Figure S4).

31

31

P

P liquid NMR

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Another noticeable change is that polyphosphate signals are affected by pyrolysis

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temperature. Polyphosphate is a polymer of phosphate ions joined by phosphoanhydride (P-O-P)

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bonds, with variable chain lengths ranging from a few to thousands of P atoms.40,

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Polyphosphate can occur in ring and branched structures (meta- and ultra-phosphates,

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respectively), although the linear structures are the most common form in nature. Polyphosphates

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can be organic (e.g. ATP and other nucleotide triphosphates), or inorganic. The NMR signal of

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linear inorganic polyphosphate typically consists of two regions: those from the end group P

41

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atoms (chemical shift ~ -4.5 ppm) and those from the middle group P atoms (chemical shift ~ -20

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ppm).42 The sum of these two peak areas represents the overall polyphosphate abundance,

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whereas the relative ratio of these two peak areas can be used to estimate the average chain

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length of polyphosphate (e.g. longer chained polyphosphate will result in higher middle group P

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signal than end group P signal). For the thermal treated solids, in general, polyphosphate

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abundance in their extracts decreases with increasing pyrolysis temperature. The relative

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abundance of polyphosphates was 31.5, 19.3, 27.8, and 5.6% for S250, S350, S450, and S600

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pyrochar, respectively (Figure 2A and 2C). Liquid NMR spectra also showed that the relative

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abundance of polyphosphate end groups increased after pyrolysis and was equal to or more than

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that of the middle groups, as compared to the predominance of middle group P signal in the raw

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sludge sample (Figure 2D).

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The presence of significant amounts of pyrophosphate and polyphosphate end groups

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suggests that a portion of the longer-chained polyphosphates were broken down into shorter-

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chained polyphosphates or pyrophosphate (Figure 2A). Although the contribution from organic

303

phosphates such as adenosine triphosphate (ATP) and DNA can not be excluded, their

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contribution should be small since their abundance in the raw sludge is less than 6%, as

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calculated by the sum of integrated peak areas for monoester (chemical shift of ~2.5 ppm) and

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diester (chemical shift of ~0 ppm) P species (Figure 2C). The formation of pyrophosphates and

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increased abundance of polyphosphate end groups in liquid NMR results are also consistent with

308

the increased intensity at the middle range chemical shift (5–15ppm) in solid state NMR data

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(Figure 1). Previous studies on the pyrolysis of plants and manure also showed the formation of

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pyrophosphate after pyrolysis at a wide range of temperatures, although their starting P source

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mostly consisted of phytate.43 During pyrolysis, reactions such as dehydration, fragmentation,

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rearrangement, polymerization, and condensation are involved.44 These reactions possibly striped

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off functional groups associated with P species and fragmented the long chain polyphosphates

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into shorter chain polyphosphates.

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Comparison between the solid state NMR spectra of samples before and after extraction

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showed that P species at high field (-15 to -40 ppm) were preferentially extracted, especially for

317

activated sludge and S250 char (Figure 3B). This is in agreement with liquid NMR observations

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of more polyphosphate and pyrophosphate in extracts from raw sludge and S250 than those from

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higher temperature chars (e.g., S450 and S600) (Figure 2A). The causes of such non-equivalent

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extraction efficiency are most likely due to the heterogeneous cellular distribution of different P

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species in microbial cells (e.g. dissolved vs granular, intracellular vs extracellular) and the

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different degrees of alterations resulted from different treatment conditions. Although there are

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some polyphosphate in extracellular polymeric substances,38 most polyphosphates exist as solid

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granules within the cells.32, 34 It is possible that the charring and stabilization of carbon during

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pyrolysis physically constrained the intracellular polyphosphate granules and prevented them

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from being extracted by the NaOH/EDTA method.

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We note that a close comparison between the solid state NMR spectra of samples before

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and after extractions (Figure 3B) and of the relative abundance of different P species from liquid

329

NMR (Figure 2C) showed a discrepancy. If the signals in the high field (-15 to -40 ppm) of solid

330

state NMR were exclusively contributed from pyrophosphate and polyphosphate, preferential

331

extraction of them would result in more pyrophosphate and polyphosphate than orthophosphate

332

in the extracts. In contrast, the orthophosphate contents in the extracts were > 50% and increased

333

with increasing pyrolysis temperatures. Since polyphosphate is relatively stable during the

334

NaOH/EDTA extraction (as shown by Figure S5), the degradation of polyphosphate into

15 ACS Paragon Plus Environment

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335

orthophosphate during extraction is unlikely the cause of this discrepancy. Instead, this again

336

may be caused by intrinsic heterogeneity of P speciation in the sludge and its derived chars, as

337

well as the nature of

338

discussed. Signal in the high field (-15 to -40 ppm) may have partial contribution from

339

orthophosphates in different physical and chemical forms.

31

P solid state NMR being easily affected by many factors as previously

340 341

P speciation in hydrochars derived from hydrothermal treatment

342

Solid state 31P NMR spectrum of the HTC hydrochar from activated sludge is similar to

343

that of anaerobically digested sludge, with the presence of only one predominant peak centered

344

at -15 ppm chemical shift (Figure 1), which can be assigned to orthophosphate based on their

345

high P extraction efficiency (~53%, Figure 3A) and the simplicity of both the liquid and solid

346

NMR spectra (Figure 2). The percentage of observable P by solid state NMR is very low for the

347

hydrochar (30.9%; see Supporting Information Table S1), and this peak shifted toward high field

348

(more negative chemical shift) (Figure 1). We characterized both the process water from HTC

349

treatment and liquid extract by NaOH/EDTA, both showed only one peak at ~4 ppm chemical

350

shift (Figure 2B), and no pyrophosphates or polyphosphates were detected. Similarly, the extract

351

from hydrochar of anaerobic sludge consists of only orthophosphate, with the trace amount of

352

organic P and pyrophosphate disappearing after HTC treatment (Figure S4). Therefore, all P

353

species were hydrolyzed into orthophosphate in the HTC hydrochars, which can form various

354

types of phosphate salts or be associated with other mineral phases. This can also explain the low

355

P observation of HTC hydrochar by solid state NMR (30.9%). CP/NMR spectra showed that a

356

portion of P in the activated sludge was observable by CP, which suggests that they remain

357

protonated. However, there was no observable P for the activated sludge hydrochar and only

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358

slightly observable P for anaerobic sludge, which suggests the formation of more metal

359

complexes (Figure S6). This result at least substantiates that the P complexation environment

360

was significantly altered after anaerobic digestion and HTC. A previous study quantified P

361

transformation during supercritical water gasification of sewage sludge using chemical

362

fractionation method and showed that most organic P was transformed into inorganic P (apatite-

363

P and non-apatite inorganic P).17 Compared to pyrolysis, reactions in HTC mostly occur in the

364

liquid phase and hydrolysis is considered to be the first step.45, 46 Because sludge consists of

365

mostly biomass (e.g. microbial cells), small non-biological particles, and salts, they are likely to

366

be thoroughly mixed and altered during HTC process. Intracellular polyphosphates can also be

367

released, hydrolyzed, and subsequently interact with dissolved cations or other mineral phases.

368 369

Environmental Implications

370

Growing global interests on sustainability has promoted the recycling and reclamation of

371

critical nutrients from waste streams47 and are pressing for the transition of waste water

372

management system and its integration with energy and agricultural management systems.

373

Significant amounts of phosphorus from human activities ultimately converge into WWTPs and

374

end up in sewage sludge, presenting an excellent opportunity for P recycling. P content in

375

sewage sludges can range from thousands of ppm to several weight percent, especially for

376

sludges from enhanced biological phosphorus removal (EBPR) process.4,

377

recently emerging (hydro)thermal treatment techniques can further enrich P in the produced

378

chars, which makes the sludge-derived char-P composite an excellent candidate for P recycling

379

and/or alternative fertilization.

5, 17

Furthermore,

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380

At present, little is known about the nutrient cycling processes following the application

381

of the residual P from various sources (e.g., sludge, manure, and crop residues) to soil, the effects

382

of pretreatments on the cycling processes, and the determining factors.48, 49 This information is

383

important for developing guidelines to incentivize P recovery from waste streams such as from

384

sewage sludge. A recent study showed that P in sludge and manure can be more recyclable than

385

in inorganic P fertilizer, depending on the pretreatment methods (e.g., anaerobic digestion,

386

composting, lime stabilization of biosolids, and P capture by metal precipitation),49 yet direct P

387

speciation information was missing.

388

Since speciation largely determines the mobility and availability of an element, P

389

speciation information is critical for the design and selection of waste treatments and P

390

recovery/recycling practices. First of all, depending on the feedstock and treatment conditions, P

391

entities potentially available from sludge and its derived products are different. This can possibly

392

affect the overall P recyclability by crops, since the mobility, transformation, and bioavailability

393

of different P entities are species-dependent. Activated sludge and its pyrochars possess more

394

diverse P configurations than its hydrochars and anaerobic sludge and derived chars, and the

395

pyrophosphate and polyphosphates that are not directly bioavailable may serve as P reservoir for

396

plants and/or organisms. Although the transport and transformation of polyphosphates in soil

397

environment is unclear and warrants future study. Secondly, results from chemical extraction and

398

NMR analysis indirectly suggest that treatment techniques and conditions may have significantly

399

altered the chemical and physical states of these identified P entities. The chemical and physical

400

states intrinsically determine whether particular P species can be dissolved into aqueous phase

401

and the associated kinetics, which is closely relevant to fertilization strategy with optimal P

402

release rate that enhance P recyclability while simultaneously mitigating P runoff. Due to the

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403

intrinsic complexity and heterogeneity of sludge and its chars, detailed information regarding the

404

phases (e.g., cation complexes, surface adsorption, structural incorporation, or mineral

405

precipitation) of the identified P entities are not addressed in the current study, but are under

406

investigation and will be published elsewhere. Regarding the physical states, it has been well-

407

established that biomass will be chemically stabilized after pyrolysis and HTC treatments, the

408

chars are more stable with increasing pyrolysis temperatures (correspondingly increasing

409

aromatic C condensation).50, 51Therefore, the pyrochars and hydrochars will be more stable than

410

their raw feedstocks and can potentially serve as slow released P reservoirs. Further speciation

411

characterization, P leaching test and bioavailability assessments based on simulated chemical

412

extraction or soil incubation test are needed to correlate intrinsic speciation and overall P

413

recyclability.

414

Moreover, soil application of sludges and their derived chars can alter the

415

biogeochemical processes in the soil environment to which they are applied, the alteration are

416

greatly affected by their overall physicochemical properties (e.g., elemental composition,

417

chemical structure and stability and dissolvable components). For example, depending on

418

pyrolysis or hydrothermal carbonization and their conditions, the leaching behaviors of dissolved

419

organic matters and inorganics are different, subsequently affects the receiving environments.16,

420

52

421

In summary, this study aims to provide the first systematic and direct characterization of

422

P entity dynamics during thermal treatments of sewage sludges. The combination of chemical

423

extraction and NMR spectroscopy provides unique information on the molecular entities of P

424

during the treatments. Information from this study lays the foundation for evaluating P

425

recyclability from solid biowastes.

19 ACS Paragon Plus Environment

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426

Page 20 of 32

ASSOCIATED CONTENT

427

Supporting Information: Chemical characteristics of the feedstock sludges; Summary of

428

sample and treatment conditions and NMR results; Comparison of the solid and P recovery, and

429

P content after different (hydro)thermal treatment conditions; FT-IR spectra of the sludge and

430

derived chars; solid-state

431

NMR spectra of sludge extracts after 1 and 4 d and of the extracts of anaerobic sludge chars.

432

This material is available free of charge via the Internet at http://pubs.acs.org.

31

P NMR spectra of sludge with variable recycle delays;

31

P liquid

433 434

ACKNOWLEDGMENT

435

This work is supported by American Chemical Society Petroleum Research Fund #

436

54143-DNI5. We thank Brandon Brown (F. Wayne Hill Water Resources Center) for help with

437

sewage sludge collection, Dr. Johannes E. Leisen and Dr. Leslie Gelbaum (Georgia Tech NMR

438

Center) for assistance with NMR setup, and Dr. Hailong Chen (Georgia Tech) for help with the

439

hydrothermal treatment setup.

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Sludge *

*

Ana *

*

S250 *

*

S450 *

*

S600 *

*

HTC * 90

60

*

30

0

-30

-60

-90

-120 -150

Chemical shift (ppm)

Figure 1. 31P solid-state NMR spectra of raw activated sludge (sludge), raw anaerobically digested sludge (ana), and the chars produced from HTC and pyrolysis (250, 450 and 600 °C) treatments of the activated sludge (HTC, S250, S450, and S600, respectively). Asterisks denote spinning side bands.

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Environmental Science & Technology

A Sludge-ex

20

15 10 Ana-ex

OrthoP Monoester

PolyP-middle

B

Diester 5

0

-5

-10 -15 -20 -25 -30

PolyP-end

S250-ex

Sludge-ex

PyroP

20

15

10

5

0

-5

-10 -15 -20 -25 -30

5

0

-5

-10 -15 -20 -25 -30

0

-5

-10 -15 -20 -25 -30

HTC-ex

S350-ex

20

S450-ex

15

10

HTC-pro

S600-ex

20

15

10

5

0

-5

-10 -15 -20 -25 -30

20

15

10

5

Chemical shift (ppm)

OrthoP Ester-M Ester-D PyroP PolyP-middle PolyP-end

C 100

D

60

40

20

2.0 1.5 1.0 0.5 0.0

x -e

Cex

00

x

x

-e

S6

00

-e

-e x

50

50

S4

-e x

x -e

50

50

x

S3

-e

HT

x -e

50

S6

50

aex

S4

x

x

S3

-e ge

-e

S2

ud

ge

An

Sl

Sl ud

S2

0

3.0 2.5

End/Middle ratio

80

Relative Abundance

Chemical shift (ppm)

Treatments

Treatments

Figure 2. (A) 31P liquid NMR spectra of the extracts from raw sludge (sludge-ex), anaerobically digested sludge (ana-ex) and pyrolyzed biochars produced at 250–600 C (S250-ex to S600-ex). (B) Spectra of the extracts from raw sludge, HTC hydrochar extract (HTC-ex), and the process water from the HTC treatment (HTC-pro). Chemical shifts for main P species are labeled, including orthophosphate (orthoP), monoester P species (monoester), diester P species (diester), polyphosphate middle P groups (polyP-middle), polyphosphate end P groups (polyP-end), and pyrophosphate (pyrophosphate). (C) Relative abundance of different P species in the liquid extracts of raw sludges (sludge-ex, ana-ex) and sludge-derived chars produced by pyrolysis at 250–600 C (S250-ex to S600-ex) and by HTC (HTC-ex). Data were calculated from the area

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Page 30 of 32

integration of their 31P liquid NMR spectra. (D) ratio of polyphosphate end group to middle group abundance in sludge and its pyrolyzed biochars.

100 A

Extracted fraction (%)

80

60

40

20

0 Sludge

Ana

S250

S350

S450

S600

HTC

Treatments

B Sludge Sludge-ex *

100

*

75

50

25

0

-25

-50

-75 -100 -125 -150

S250 *

S250-ex

*

S450 *

*

S450-ex

S600 S600-ex *

*

100

75

50

25

0

-25

-50

-75 -100 -125 -150

Chemical shift (ppm)

Figure 3. (A) Extraction efficiency of the NaOH/EDTA method for raw sludges (sludge, ana) and their derived chars, as calculated by P content in extracted solution divided by total P in the pristine samples; (B) 31P solid-state NMR spectra of activated sludge and its pyrolyzed biochars produced at 250–600 C (S250 to S600) (black lines), as well as their corresponding solid residuals after extraction (red lines). Asterisks denote spinning side bands.

3 ACS Paragon Plus Environment

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Environmental Science & Technology

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