Article pubs.acs.org/EF
Influence of Carbonization Methods on the Aromaticity of Pyrogenic Dissolved Organic Carbon Minori Uchimiya,*,† Syuntaro Hiradate,‡ and Michael Jerry Antal, Jr.§ †
Southern Regional Research Center, Agricultural Research Service (ARS), United States Department of Agriculture (USDA), 1100 Robert E. Lee Boulevard, New Orleans, Louisiana 70124, United States ‡ National Institute for Agro-Environmental Sciences, 3-1-3 Kan-nondai, Tsukuba, Ibaraki 305-8604, Japan § Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States S Supporting Information *
ABSTRACT: Dissolved organic carbon (DOC) components of soil amendments, such as biochar, will influence the fundamental soil chemistry, including the metal speciation, nutrient availability, and microbial activity. Quantitative correlation is necessary between (i) pyrogenic DOC components of varying aromaticity and ionizable (carboxyl and hydroxyl) substituents and (ii) bulk and solution properties of biochars. This study employed fluorescence excitation−emission (EEM) spectrophotometry with parallel factor analysis (PARAFAC) to understand the influence of the pyrolysis platform (flash and high-yield carbonization, slow pyrolysis, and fast pyrolysis) and solution pH on the DOC structure of carbonaceous materials. The PARAFAC fingerprint representative of conjugated, polyaromatic DOC correlated (Pearson’s r ≥ 0.6; p < 0.005) with (i) volatile matter content and (ii) total organic carbon and nitrogen concentrations in water and base (50−100 mM NaOH) extracts. Electric conductivity of the extracts correlated with S (indicative of labile sulfate species) and Na + K concentrations (r > 0.9; p < 0.0005). The pH-dependent changes in fluorescence peak position and intensity suggested (i) protonation of carboxylate/phenolic functionalities and (ii) acid-induced aggregation of colloidal particles for ≤350 °C slow-pyrolysis biochars; DOC of high-yield/flash carbonization charcoals and ≥500 °C slow-pyrolysis biochars were less sensitive to pH. Solid-state 13C cross-polarization and magic angle spinning nuclear magnetic resonance analysis of bulk aromaticity (−CC− peak at 110−160 ppm) suggested that both recalcitrant and labile fluorescence DOC fingerprints are composed of polyaromatic structures that begin to form near 350 °C. These biochar-borne DOC components of varying aromaticity and carboxyl substituents will participate in hydrophobic and hydrogen-bonding interactions with soil components that will ultimately impact the biogeochemical cycles.
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INTRODUCTION Char is a form of pyrogenic carbonaceous material naturally comprising as much as 35% of total organic carbon (TOC) in fire-impacted soils1 and sediments.2 Highly carboxylated and hydrogen-deficient polycyclic aromatic structures were revealed by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and solid-state 13C nuclear magnetic resonance (NMR) analyses of natural organic matter (NOM) extracted from fire-impacted andosols3 and terra preta4 soils. Carboxyl and hydroxyl substituents on the aromatic rings result from the oxidative aging of charred biomass in soil,5 are responsible for the solubility of NOM in weak base, and are detectable by the benzenepoly(carboxylic acid) assay.3 The molecular weight (MW) of dissolved organic carbon (DOC) in fire-impacted soils ranged from 400 to 1200 Da3,5 and overlapped with the water extracts (25%, w/v) of slow- and fastpyrolysis biochars.6 Fluorescence excitation−emission (EEM) spectrophotometry is a sensitive, rapid, and non-destructive technique traditionally employed to comparatively characterize DOC in soils, sediments, and surface waters.7 Unlike ultraviolet−visible (UV−vis) spectrophotometry, fluorescence EEM offers distinguishable characteristic maxima for signature organic compounds present in a complex mixture of chromophores.8 Parallel factor analysis © XXXX American Chemical Society
(PARAFAC) is often employed on EEM to resolve overlapping spectra of fluorescence structures.9 The peak position and intensity of EEM depend upon (i) the molecular structure and corresponding extinction coefficient and quantum yield,10 (ii) concentration of fluorophores, and (iii) background media, especially pH and ionic strength. In river water samples, characteristic EEM peaks accounted for over half of all peaks detectable by FT-ICR MS, including non-fluorescence structures.11 Previous EEM/PARAFAC analysis on DOC of slowpyrolysis biochars showed confounding effects of pH and pyrolysis temperature.8 The DOC structure is expected to become (i) more aromatic (lower H/C ratio) and (ii) require higher pH to dissolve as a function of the pyrolysis temperature. However, the emission wavelengths of some, e.g., 500 °C broiler litter, biochars decreased relative to the feedstock, and the fluorescence intensity in 50 mM NaOH was lower than the water extracts.8 The objective of this study was to gain predictive correlations between the DOC structure and bulk property (especially the aromaticity) of pyrogenic carbonaceous materials. Two separate Received: January 20, 2015 Revised: March 5, 2015
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DOI: 10.1021/acs.energyfuels.5b00146 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Fast-pyrolysis biochar (FP) was employed to represent a pyrolysis platform where the solid co-product was not given sufficient heattransfer time for carbonization. The FP was produced at 500 °C from mixed sawdust by Dynamotive Energy Systems (Vancouver, British Columbia, Canada) using an industrial-scale bubbling fluidized-bed reactor designed to yield 60−75 wt % bio-oil, 15−20 wt % biochar, and 10−20 wt % syngas.16 The FP was a fine powder and was used asreceived in a sealed drum. Proximate Analysis. Moisture, ash, volatile matter (VM), and fixed carbon contents of flash/high-yield carbonization charcoals were determined in triplicate by following ASTM method D758224 using a LECO thermogravimetric analyzer (TGA701, LECO, St. Joseph, MI). Moisture was determined as the weight loss after heating the sample under a N2 atmosphere in an open crucible to 107 °C and holding at this temperature until sample weight stabilized. The VM was determined as the weight loss after heating the sample under a N2 atmosphere in a covered crucible to 950 °C and held for 7 min. Ash was defined as the remaining mass after subsequently heating the sample under an O2 atmosphere in an open crucible to 750 °C and holding at this temperature until sample weight stabilized. After the determination of moisture, ash, and VM, fixed carbon was calculated by difference. Proximate analysis results for oat and pecan high-yield carbonization charcoals were in agreement with the literature values.12 Proximate analysis results for slow- and fast-pyrolysis biochars were obtained from the literature.20,25−27 Sequential Extraction. To investigate the influence of extraction fluid, 13 flash/high-yield carbonization charcoals, 15 slow-pyrolysis manure feedstocks and biochars, and 1 fast-pyrolysis biochar were first sequentially extracted: 16 h in cold (room temperature) water, 16 h in hot (80 °C) water, and then 16 h in cold (room temperature) 50 mM NaOH. Each extraction step employed the biochar/extraction fluid ratio of 2 g/20 mL and is hereby denoted E1 (cold water), E2 (hot water), and E3 (50 mM NaOH). Suspension was centrifuged (900g for 30 min), and the supernatant was filtered (0.45 μm Millipore Millex-GS, Millipore, Billerica, MA). Filtered extracts (87 total for E1−E3) were analyzed without dilution or pH adjustment. TOC [in parts per million (ppm) of C] and total nitrogen (TN, in ppm of N) were determined using a torch combustion TOC/TN analyzer (Teledyne Tekmar, Mason, OH). The pH was determined using Sartorius Professional PP15 m (Sartorius, Bohemia, NY). To estimate the ionic strength, electric conductivity (EC) was determined using a YSI 3200 conductivity meter (YSI, Yellow Springs, OH). Sequential extracts of flash/high-yield carbonization charcoals were analyzed for dissolved Ca, K, Na, P, Al, Fe, Mn, S, and Si concentrations after acidifying the filtered extract to 4 vol % nitric acid (trace metal grade) using inductively coupled plasma−atomic emission spectroscopy (ICP−AES, Profile Plus, Teledyne/Leeman Laboratories, Hudson, NH). Blanks, blank spikes, and matrix spikes were included for the quality assurance and control for the ICP−AES analysis.28 To understand the pH effects, selected flash carbonization charcoal, slow-pyrolysis manure and plant biochars, fast-pyrolysis biochar, and model humic substances were separately extracted using 0.1 M NaOH at room temperature. A portion (1 mL) of filtered (0.45 μm) base extract was then added to 9 mL of 100 mM acetate buffer (pH 5) to compare the same extract at basic and acidic pH. Fluorescence EEM and PARAFAC. Fluorescence EEM of each extract was obtained using a F-7000 spectrofluorometer (Hitachi, San Jose, CA) set to 220−400 nm excitation and 280−600 nm emission wavelengths in 3 nm intervals, 5 nm excitation and emission slits, 0.5 s response time, and 2400 nm min−1 scan speed. The blank EEM for background solution (100 mM pH 5 acetate for buffered samples and DDW for all other samples) was obtained daily and was subtracted from each sample to remove the lower intensity Raman scattering.29 After the removal of additional regions dominated by Rayleigh and Raman peaks and the region without fluorescence,30 PARAFAC modeling was conducted with a non-negativity constraint using MATLAB, version 8.2.0.701 (R2013b, Mathworks, Natick, MA) with a DOMFluor toolbox.30 Solid-State 13C NMR. To investigate the aromaticity and other bulk structural components of biochar as a function of the pyrolysis
extraction procedures were used to (i) fractionate DOC into cold-water (room temperature), hot-water (80 °C), and 50 mM NaOH (16 h each at 100 g L−1) sequential extracts and (ii) systematically investigate the pH effects using 100 mM NaOH extracts with and without 1:10 dilution in pH 5 acetate buffer. Although slow-pyrolysis biochars produced in a well-controlled, externally heated laboratory box furnace allow for a systematic investigation of the pyrolysis temperature, low capacity and yield make such a pyrolysis platform impractical for purported fieldscale soil amendment. This study will focus on the high-yield12,13 and flash carbonization14 charcoals produced at pilot scale under elevated pressure and low gas flow rate to approach the theoretical fixed C yield and high throughput (8−9 kg m−2 min−1). To understand how the bulk aromaticity of pyrogenic C translates into the DOC structure, solid-state 13C crosspolarization and magic angle spinning (CPMAS) NMR analyses of model slow-pyrolysis biochars were compared to EEM/ PARAFAC of DOC extracted from high-yield12,13/flash14 carbonization, slow-pyrolysis (≈10 °C min−1),15 and fastpyrolysis (>1000 °C s−1)16 biochars produced from diverse (plant, manure, and sewage sludge) feedstocks.
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MATERIALS AND METHODS
Distilled, deionized water (DDW) with a resistivity of 18 MΩ cm (APS Water Services, Van Nuys, CA) was used for all procedures. Elliott soil humic acid (ESHA; 1S102H), reference Suwannee River natural organic matter (NOM; 1R101N), and standard Suwannee River II humic acid (HA II; 2S101H) were obtained from the International Humic Substance Society.17 All other chemical reagents were obtained from Sigma-Aldrich (Milwaukee, WI) with the highest purity available. Slow Pyrolysis. Pecan shells (PS25) were obtained from a sheller and were ground (SM 2000 cutting mill, Retsch Gmbh, Haan, Germany) and sieved to
