Article pubs.acs.org/JAFC

Pyrolysis Temperature-Dependent Changes in Dissolved Phosphorus Speciation of Plant and Manure Biochars Minori Uchimiya*,† and Syuntaro Hiradate§ †

Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 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 S Supporting Information *

ABSTRACT: Pyrolysis of plant and animal wastes produces a complex mixture of phosphorus species in amorphous, semicrystalline, and crystalline inorganic phases, organic (char) components, and within organo-mineral complexes. To understand the solubility of different phosphorus species, plant (cottonseed hull) and manure (broiler litter) wastes were pyrolyzed at 350, 500, 650, and 800 °C and exposed to increasingly more rigorous extraction procedures: water (16 h), Mehlich 3 (1 mM EDTA at pH 2.5 for 5 min), oxalate (200 mM oxalate at pH 3.5 for 4 h), NaOH−EDTA (250 mM NaOH + 5 mM EDTA for 16 h), and total by microwave digestion (concentrated HNO3/HCl + 30% H2O2). Relative to the total (microwave digestible) P, the percentage of extractable P increased in the following order: M3 < oxalate ≈ water < NaOH−EDTA for plant biochars and water < M3 < NaOH−EDTA < oxalate for manure biochars. Solution phase 31P NMR analysis of NaOH−EDTA extracts showed the conversion of phytate to inorganic P by pyrolysis of manure and plant wastes at 350 °C. Inorganic orthophosphate (PO43−) became the sole species of ≥500 °C manure biochars, whereas pyrophosphate (P2O74−) persisted in plant biochars up to 650 °C. These observations suggested the predominance of (i) amorphous (rather than crystalline) calcium phosphate in manure biochars, especially at ≥650 °C, and (ii) strongly complexed pyrophosphate in plant biochars (especially at 350−500 °C). Correlation (Pearson’s) was observed (i) between electric conductivity and ash content of biochars with the amount of inorganic P species and (ii) between total organic carbon and volatile matter contents with the organic P species. KEYWORDS: organic fertilizer, remediation, charcoal, bioenergy, biomass



INTRODUCTION Solution phase 31P nuclear magnetic resonance (NMR) has been widely employed to understand the chemical speciation of dissolved phosphorus in soil and manure samples.1,2 Solid state 31 P NMR, X-ray diffraction (XRD),3 and X-ray absorption near edge structure (XANES)4 techniques are used to characterize P phases remaining on the solid phase after the sequential extraction. For example, poultry litter contained primarily inorganic P in the NaOH extract and organic P in the solid phase.4 Conversely, alum-amended poultry litter contained organic P in the solution phase, and aluminum oxidesassociated potassium and calcium phosphate in the solid phase.4 To our knowledge, 31P NMR analysis of slow pyrolysis biochar is currently limited to manure feedstock: swine bone5 and digested dairy fiber.6 Digested dairy fiber biochar (after exposure to dairy lagoons) contained orthophosphate in the soluble fraction and calcium phosphate in the solid phase.6 Acid−base chemistry has long been utilized to dissolve calcium-bound phosphate phases in acid and iron and aluminum (hydr)oxides-associated P phases in base.7 Oxalic acid, ethylenediaminetetraacetic acid (EDTA), and other chelating agents are often employed to dissolve strongly complexed phosphate in soil and manure samples.7 Similar extraction methods have been utilized to understand the phosphorus solubility and bioavailability of biochars produced from sewage sludge,8 manure,9 and wood10 slow and fast pyrolysis11 using various extraction procedures. However, these results from different literature sources are not directly © 2014 American Chemical Society

comparable. The objective of the present study was to understand how feedstock and pyrolysis temperature influence the P speciation of plant and manure biochars. This study employed biochars produced by slow pyrolysis of plant (cottonseed hull) and manure (broiler litter) feedstocks at 350, 500, 650, and 800 °C. This series of biochars has been previously investigated for the amount and structure of dissolved organic carbon (DOC)12 and interaction with heavy metals13,14 and agrochemicals.15,16 To understand the solubility of different phosphorus fractions, each biochar sample was exposed to increasingly more rigorous extraction procedures: water (16 h at room temperature), Mehlich 3 (1 mM EDTA at pH 2.5 for 5 min), basic EDTA (250 mM NaOH + 5 mM EDTA for 16 h), acidic oxalate (200 mM oxalate at pH 3.5 for 4 h), and total by microwave digestion (concentrated HNO3/ HCl + 30% H2O2). Basic EDTA extracts were then analyzed by solution phase 31P NMR to understand the chemical speciation of dissolved phosphorus fractions.



MATERIALS AND METHODS

Distilled, deionized water (DDW) with a resistivity of 18 MΩ cm (Millipore, Milford, MA, USA) was used for all procedures. All Received: Revised: Accepted: Published: 1802

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and broadband proton decoupling at 30 °C. The pulse delay time of 2 s was employed.23 Each spectrum was scanned 30000 times, and a broadening factor of 5.00 Hz was used in the Fourier transform procedure. Chemical shifts (ppm) were determined with respect to 85% H3PO4 solution (0 ppm). The total signal intensity and the fraction contributed by each P compound were calculated by integration of the spectral signals using Alice 2 for Windows version 5.1.1 (JEOL).

chemical reagents were obtained from Sigma-Aldrich (Milwaukee, WI, USA) with the highest purity available. Biochar Production. Feedstock property and pyrolysis conditions were described in detail previously.13,17 Briefly, cottonseed hull (CH25) was used as received from Planters Cotton Oil Mill (Pine Bluff, AK, USA).13 Broiler litter (BL25) was obtained from a USDAARS facility (Starkville, MS, USA) and was milled and pelletized.17 Feedstock was pyrolyzed at 350, 500, 650, and 800 °C under 1600 mL min−1 N2 flow rate for 1 h (for BL25) or 4 h (for CH25) using a laboratory-scale box furnace (22 L void volume) with a retort (Lindberg, type 51662-HR, Watertown, WI, USA). Biochar products were allowed to cool to room temperature overnight under N2 atmosphere. Biochars are hereby denoted by the feedstock abbreviation and pyrolysis temperature, for example, cottonseed hull feedstock (CH25) and biochars produced at 350 (CH350) and 500 °C (CH500). Extraction Procedures. To understand the solubility of different phosphorus species, each biochar sample was exposed to increasingly more rigorous extraction procedures (abbreviations in parentheses will be used throughout the text): DDW (water), Mehlich 3 (M3), basic EDTA (NaOH−EDTA), acidic ammonium oxalate (oxalate), and microwave digestion (total). Each extraction was performed independently, rather than sequentially. At the end of each extraction procedure, filtrate was acidified to 4 vol % nitric acid (trace metal grade, Sigma-Aldrich) for the determination of soluble P, K, Ca, Mg, Al, Fe, and Na concentrations using inductively coupled plasma atomic emission spectrometer (ICP-AES; Profile Plus, Teledyne/Leeman Laboratories, Hudson, NH, USA). Blanks, blank spikes, and matrix spikes were included for the quality assurance and control for the ICPAES analysis.18 Except for sizing, all biochars were extracted without modifications. For M3, oxalate, and water extractions, biochars were ground and sieved to 60 to 30% in the char fraction, whereas Fe/Al-bound fractions (NaOH extractable) did not change significantly.8 In a separate study, nearly an order of magnitude greater Colwell (8.5 sodium bicarbonate) P content was observed in 450 °C poultry litter biochar than in the steam-activated 550 °C biochar.33 Distinctive feedstock and temperature dependence was observable for oxalate and NaOH−EDTA extractions in Table 1. Especially at ≥650 °C, oxalate was nearly as effective as microwave digestion for extracting P from manure biochars, but not from plant biochars. On the other hand, NaOH− EDTA was particularly effective for dissolving P in 350−500 °C plant biochars (Table 1). Table S1 of the Supporting Information provides stability constants for the 1:1 complex of oxalate, EDTA, and phosphate with Ca2+, Fe3+, and other selected cations.34 EDTA has far greater stability constants for Ca2+, Fe3+, and other phosphate-binding metals than oxalate (Table S1). Greater P solubility of 350−500 °C plant biochars in NaOH−EDTA indicates the dominance of P species that are strongly interacting with the organic C fraction of biochar and/ or associated with iron/aluminum hydroxides and other mineral surfaces and are resistant to acid dissolution. For manure biochars, greater P dissolution in acidic media containing weaker ligand (oxalate) suggests the acid dissolution of calcium phosphate phases. Oxalate is able to release P from calcium phosphate phases by forming insoluble calcium oxalate35 having the solubility product constant (KSO) of 10−8.75.34 For both manure and plant biochars, Ca concentration in NaOH−EDTA was consistently higher than in oxalate extracts (Table S2, Supporting Information). Lower Ca concentration in the oxalate extract was set by the KSO of calcium oxalate. 31 P NMR Analysis of Plant and Manure Biochar Extracts. To understand the individual P species contributing to extractable P concentrations in Table 1, NaOH−EDTA extracts were analyzed using solution-phase 31P NMR. This extraction fluid (0.25 M NaOH + 0.05 M Na2EDTA) is widely utilized to enhance P extraction from soil while minimizing the hydrolysis of organic P to inorganic P.1 EDTA is used to release P from paramagnetic ions (see stability constant for Fe in Table S1), thereby decreasing line broadening.2 To our knowledge, no prior liquid-phase 31P NMR study systematically investigated the feedstock and pyrolysis temperature dependence. Figure 1 presents 31P NMR spectra for manure (left) and plant (right) feedstocks and biochars pyrolyzed at 350, 500, 650, and 800 °C. Spectra for manure biochars (left) indicate the dominance of inorganic orthophosphate (PO43‑ at 6.2 ppm) especially at higher temperatures. In contrast, pyrophosphate (P2O74− at −4.1 ppm) persisted in plant biochars (Figure 1, right). Pyrophosphate is produced by heating orthophosphate above 300 °C.36 Figure 2 presents Figure 1 with an expanded x-axis to visualize the monoester region (3−7 ppm). Both cottonseed hull and broiler litter feedstocks showed four peaks corresponding to phytate: 5.9 ppm (attributable to the P2

a

2053 (9%) 0 325 (1%) 0 0 800 (3.6%) 770 (2.5%) 343 (0.9%) 554 (1.4%) 400 (1.0%) 19261 (87%) 27309 (87%) 34810 (89%) 37537 (92%) 39273 (95%) 16566 (75%) 15187 (48%) 18859 (48%) 16910 (41%) 13832 (33%) 22022 31319 39240 40879 41371 20 3 3 5 7 58 37 19 12 10 12 28 31 34 32 29 35 50 54 58 9310 1724 126 32 5 0.3 9.9 9.4 10.0 9.1

509 614 623 655 694

424 (13%) 908 (27%) 1139 (22%) 1173 (7%) 577 (2%) 6766 521 72 52 0

cottonseed hull CH25 7.4 CH350 8.2 CH500 9.6 CH650 9.6 CH800 9.6 broiler litter BL25 5.0 BL350 7.6 BL500 9.3 BL650 9.7 BL800 10.0

0.0 2.6 2.5 5.1 5.8

104 306 423 293 540

3 6 8 9 10

23 56 72 77 77

75 37 20 14 13

12 7 7 8 10

3180 3405 5124 16816 37560

802 2383 2024 2717 2829

(25%) (70%) (40%) (16%) (8%)

2003 554 931 1735 2119

(63%) (16%) (18%) (10%) (6%)

201 27 36 119 80

(6.3%) (0.8%) (0.7%) (0.7%) (0.2%)

water M3f oxalatef NaOH−EDTA biochar

pHc

ECc (mS cm−1)

TOCc (ppm C)

densityd (g L−1)

ash

fixed C

VM

moisture

totale,f

extractable P in mg kg−1dry (% extracted) proximate analysis (wt %dry)b

Table 1. Bulk Parameters for Cottonseed Hull (CH) and Broiler Litter (BL) Feedstock and Biochars Pyrolyzed at 350, 500, 650, and 800 °C: pH, Electric Conductivity (EC), and Total Organic Carbon (TOC) of Hot Water Extracts (80 °C for 16 h); Density; Ash, Fixed Carbon, Volatile Matter (VM), and Moisturea

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P) and organic P species (phytate, lipids, DNA, and total organic P), based on Figures 1, 2, and S1. The monoester region was integrated as a whole because of well-documented signal overlap at 3−7 ppm.39 Absolute values in Table 2 are given in milligrams per kilogram biochar on a dry weight basis. These values were calculated by multiplying the fraction of integrated area (with respect to the total area) by total NaOH− EDTA extractable P in Table 1. Values in parentheses represent percent contribution of each P species to the total P extracted by NaOH−EDTA (Table 1). Broiler litter feedstock was dominated by phytate (58%) followed by inorganic orthophosphate (39%). Pyrolysis of broiler litter at 350 °C decreased phytate to 1%, and inorganic orthophosphate (63%) and pyrophosphate (36%) became dominant. Orthophosphate was the only species observable in 500, 650, and 800 °C broiler litter biochars (except for 3% pyrophosphate in BL500). In summary, pyrolysis of broiler litter transformed organic P dominance to inorganic P, and orthophosphate was the sole species at ≥500 °C. Cottonseed hull biochars showed a distinctively different temperature trend from the broiler litter biochars. Cottonseed hull feedstock was primarily (87%) phytate with lower orthophosphate fraction (12%) than BL25. At 350 °C, phytate decreased to zero, and pyrophosphate became the dominant species (71%); orthophosphate (27%) was much lower than BL350. The dominance of pyrophosphate continued until 650 °C: 68% pyrophosphate and 32% orthophosphate at 500 °C; 54% pyrophosphate and 46% orthophosphate at 650 °C (Table 2). Orthophosphate became the dominant species only at the highest temperature (800 °C): 81% orthophosphate and 19% pyrophosphate. In conclusion, EDTA was required to sufficiently dissolve P of plant biochar (Table 1) that was predominantly pyrophosphate (Table 2). The results suggest that pyrophosphate was stabilized by forming complexes with ash components (see stability constants in Table S1) or by interacting with organic C by electrostatic and hydrogen bonding interactions40 and other organomineral interactions.28 In contrast, orthophosphate was the dominant soluble P species of manure biochars (Table 2) and almost completely dissolved in acidic ammonium oxalate (Table 1). This observation suggests that soluble P of manure biochar consists of amorphous calcium phosphate, rather than the stable minerals such as apatite. Literature reviews on the XRD analyses of manure, biochar, and ash indicated the presence of apatite in ashed41 and gasified42 manures, but not in the slow-pyrolysis manure biochars29,30 containing as much as >40% C on a dry weight basis. Removal of organic matter by combustion is likely necessary to thermally form crystalline hydroxyapatite (Ca10(PO4)6(OH)2; OH may be replaced by F or Cl) and other stable calcium phosphate minerals. Calcination (heating in air) and other high-temperature processing are commonly employed to produce crystalline apatite, especially for the synthesis of biomaterials.43 Thermal stability of apatite is controlled by the Ca/P ratio, degree of substitution by carbonate and other ions, and heating atmosphere.43 In addition, pyrolysis produces reactive nanometer-sized precipitates on the biochar surface.42,44 These nanosized crystals can contain P and other nutrient elements that may be released into the aqueous phase in a more controlled fashion.45,46 Such nanomaterials are reactive sorbents for inorganic (e.g., heavy metals) and organic (e.g., agrochemicals) solutes44 and will add further complexity to the chemical interactions within rhizospheres that will ultimately influence crop growth.

Figure 1. 31P NMR spectra of broiler litter (BL25, left) and cottonseed hull (CH25, right) feedstock and biochar (pyrolyzed at 350, 500, 650, and 800 °C) extracts. Extraction method: 5 g of sample was shaken in 250 mM NaOH + 5 mM EDTA for 16 h. Chemical shifts (ppm) were determined with respect to 85% H3PO4 solution (0 ppm).

Figure 2. Figure 1 with expanded x-axis to visualize the monoester region (3−7 ppm). Top spectra show authentic phytate having 5.9 ppm (attributable to the P2 position of phytate structure shown), 4.9 ppm (P4 and P6), 4.6 ppm (P1 and P3), and 4.4 ppm (P5) peaks.

position of phytate structure in Figure 2), 4.9 ppm (P4 and P6), 4.6 ppm (P1 and P3), and 4.4 ppm (P5).2 An authentic spectrum of phytate is shown in Figure 2 (top) for comparison. Above pH 9.5, phytate exists in a sterically hindered form having one equatorial (P2 in Figure 2) and five axial positions37 as shown in Figure 2. The majority of the phytate peaks disappeared upon pyrolysis at ≥350 °C (Figure 2). Additional minor peaks in manure feedstock (BL25) are attributable to diesters: alkali-stable phospholipids and teichoic acid at 0.3−2.1 ppm and DNA at −1.5 to −0.13 ppm in Figure 1.22,38 Polyphosphate (−20 ppm)2 was not observable in the spectra showing the entire −30 to 30 ppm region (Figure S1, Supporting Information). Table 2 presents 31P NMR peak integration results for inorganic (orthophosphate, pyrophosphate, and total inorganic 1805

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Table 2. Phosphorus Speciation of NaOH−EDTA (250 mM NaOH + 5 mM EDTA for 16 h) Extracts Determined by 31P NMRa inorganic P in mg kg−1dry (% total) biochar

orthophosphate

cottonseed hull CH25 93 (12%) CH350 654 (27%) CH500 646 (32%) CH650 1251 (46%) CH800 2286 (81%) broiler litter BL25 6445 (39%) BL350 9551 (63%) BL500 18241 (97%) BL650 100% BL800 100%

pyrophosphate 9 1692 1379 1466 542

(1%) (71%) (68%) (54%) (19%)

66 (0.4%) 5410 (36%) 618 (3%) 0 0

organic P in mg kg−1dry (% total) total

101 2346 2024 2717 2828

(13%) (98%) (100%) (100%) (100%)

6511 (39%) 14961 (99%) 18859 (100%) 16910 (100%) 13832 (100%)

phytate

lipids

701 (87%) 0 0 0 0

0 37 (2%) 0 0 0

9573 (58%) 226 (1%) 0 0 0

346 (2%) 0 0 0 0

DNA 0 0 0 0 0 136 (1%) 0 0 0 0

total 701 (87%) 37 (2%) 0 0 0 10055 (61%) 226 (1%) 0 0 0

a

Values in parentheses represent percent contribution of each P species to the total P in NaOH−EDTA extracts (Table 1 provides absolute values in mg kg−1dry.)

Table 3. Pearson’s Correlation (r > 0.878 for N = 5, p = 0.05) between Phosphorus Species in NaOH−EDTA Extracts (Table 2) and Bulk Property of Cottonseed Hull and Broiler Litter Biochars (Table 1)a [P] by 31P NMR (mg kg−1dry) orthophosphate

pyrophosphate

total inorganic

phytate

total organic

Cottonseed Hull Biochars proximate analysis (wt %dry)

extracted [P] (mg kg−1dry)

proximate analysis (wt %dry)

extracted [P] (mg kg−1dry) a

EC (mS cm−1) ash TOC VM moisture total NaOH−EDTA water EC (mS cm−1) ash TOC VM moisture total NaOH−EDTA water

X

X X X X

X X X Broiler Litter Biochars X X

X

X

X

X

X

X

Separate correlation was observed between the phosphorus species and total, NaOH−EDTA, and water-extractable P concentrations.

condensed P, for example, from pyrophosphate to orthophosphate.52 Thermal treatment of orthophosphate in turn forms less soluble polyphosphate species;51 pyrophosphate is an intermediate species in this process. The 350−650 °C plant biochars were primarily composed of pyrophosphate (Table 2), and polyphosphate was not observed (Figure S1). Subsequent discussions aim to elucidate the mechanisms favoring the formation of pyrophosphate from phytate or orthophosphate during pyrolysis of cottonseed hulls (Table 2). Polyphosphate is a food additive for meat and dairy products, partly to sequester Ca.53 When triphosphate-treated cheese spread was cooked, the product became enriched with pyrophosphate.54 In addition, pyrophosphate was more resistant to hydrolysis than triphosphate.53 The authors hypothesized that after heat-induced hydrolysis of triphosphate, pyrophosphate product became stabilized by forming complexes with Ca as well as protein in cheese.54 Similar enrichment of pyrophosphate product was observed when commercial polyphosphate was hydrolyzed by heating without

Observed predominance of phytate in both plant and manure feedstocks and decomposition at ≥350 °C (Table 2) are in agreement with the reported properties of phytate. Phytic acid forms during ripening of seeds and cereal grains and composes 60−80% of total P in cereals, oilseeds, and legumes.47 Poorly digestible phytate composes >60% P in swine and poultry (nonruminant) feed.47 Phytate can hydrolyze in water1 but is stabilized by forming complexes with metal cations. 48 Compared to phosphate monoesters, diesters are more mobile because of lower charge.1 Mononucleotides and other monophosphate esters degrade within hours of release, whereas inositol phosphate is stabilized by interacting with iron/ aluminum oxides and becomes the major organic P in soil.49 However, phytate nonenzymatically decomposes at ≥150 °C within an hour.50 Phytic acid visually darkened at 150 °C and lost C and H at 380 °C.50 Orthophosphate can be formed by hydrolysis and thermal degradation of phytate.24,51 In general, more condensed P in a polyphosphate mixture reacts with water to form less 1806

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Table 4. Pearson’s Correlation (r > 0.878 for N = 5, p = 0.05) between P and Fe, Ca, Mg, Al, K, or Na in Total, NaOH−EDTA, Oxalate, and Water Extractsa total Fe

Ca

Mg

NaOH−EDTA Al

K

Na

Al

oxalate Ca

Mg

Al

K

Na

X

X

X X

X X X

Cottonseed Hull Biochars mg kg−1dry extracted [P]

[P] by 31P NMR

total P NaOH−EDTA P oxalate P orthophosphate total inorganic

X X X X Broiler Litter Biochars

mg kg−1dry extracted [P]

[P] by 31P NMR

total P NaOH−EDTA P oxalate P orthophosphate total inorganic

X

X

X

X

X

X X X

X X

X

a

Separate correlation was observed between (i) orthophosphate or total inorganic P in NaOH−EDTA extracts by 31P NMR and (ii) Fe, Ca, Mg, Al, K, or Na in different extracts.

a food product.53 In addition to pyrophosphate, the formation of trimetaphosphate ring product was catalyzed by Ca2+ when commercial polyphosphate was heated at >120 °C.53 Pyrophosphate (P2) can form nonenzymatically at neutral and alkaline pH and room temperature via coupled exergonic hydrolysis of polyphosphate (P3) and endergonic condensation of orthophosphate (P): P3 + P = 2P2;55 this reaction can be catalyzed by Ca2+ and other cations. Surface catalysis lowers the temperature at which high-energy phosphoanyhydride bonds can form nonenzymatically between orthophosphate to produce pyrophosphate.56 Orthophosphate was converted to pyrophosphate on dry SiO2 and CaPO4 surfaces at 70 °C.56 Similarly, the surface of cottonseed hull biochars may catalyze the formation of pyrophosphate from orthophosphate (Table 2). Because water is the product of condensation reaction to form the phosphoric anhydride (P−O−P) bond,57 drying alone can alter the P speciation.38,56 Air-drying decreased waterextractable (labile) P in feed by 13−61%, primarily by decreasing orthophosphate.38 In contrast, drying increased NaOH−EDTA extractable P by as much as 48%38 In summary, thermal degradation of phytate produces orthophosphate, which thermally converts to pyrophosphate. Pyrophosphate was likely stabilized by forming complexes on the biochar surface and did not convert to polyphosphate. It must be noted that pyrophosphate has an order of magnitude higher water solubility (0.5 M) than phytate (0.07 M; both Na salts) (SciFinder Scholar). Pearson’s Correlation between P Species and Bulk Properties of Biochar. To understand the feedstock- and temperature-dependent origins of different P species, Pearson’s correlation was conducted between P species (Table 2) and bulk parameters of biochar samples (Table 1). Table 3 presents parameters that showed positive correlation (r > 0.878 for N = 5, p = 0.05) with orthophosphate, pyrophosphate, total inorganic P, phytate, or total organic P determined by the 31P NMR method: EC, ash, TOC, VM, and moisture. For the plant biochars, (i) orthophosphate correlated with EC; (ii) total inorganic P correlated with EC and ash; and (iii) total organic P correlated with TOC and VM. For the manure biochars, (i) total inorganic P correlated with EC, similarly to the plant

biochars; and (ii) both phytate and total organic P correlated with TOC as well as the moisture content. In summary, bulk property for the inorganic component (EC and ash) correlated with inorganic P species. Bulk property for organic carbon (TOC and VM) correlated with organic P species. Hydrophilicity (moisture content) of manure biochars may contribute to the release of organic P species in NaOH−EDTA. Table 3 shows a separate correlation between (i) the same series of P species in NaOH−EDTA extracts and (ii) total, NaOH−EDTA, and water-extractable P concentrations of plant and manure biochars. Interestingly, most labile (waterextractable) P fraction correlated with the signature P species in each feedstock category: pyrophosphate for plant biochars; phytate and total organic P for manure biochars (Table 3; note that water-extractable P was zero for BL350, 650, and 800 in Table 1). Total P (determined by the most rigorous microwave digestion method) correlated with orthophosphate for both plant and manure biochars. Plant biochars also showed a correlation between the total inorganic P and NaOH−EDTA extractable P (Table 3). Pearson’s Correlation between P and Other Extractable Elements. Phosphorus species are stabilized in aqueous systems by binding Al, Ca, Mg, Fe, and K (Table S1); these elements often dissolve together during extraction procedures.7 For example, Mg2+ may dissolve together with orthophosphate from struvite (MgNH4PO4·6H2O) in poultry litter.3 Alum (aluminum sulfate), lime, and iron sulfate are often intentionally added to poultry litter to decrease P runoff.4 The Ca stability constant for a 1:1 complex increases from 2.66 for orthophosphate, to 8.1 for triphosphate, to approximately 22 for phytate (25 °C and zero ionic strength).34 In contrast, the stability constant for Ca is similar for different phosphorus species containing one phosphate group: orthophosphate (2.66), CH3−O−PO32− (1.47), and phenyl−O−PO32− (1.45) (Table S1). Table 4 presents observed correlation (r > 0.878 for N = 5, p = 0.05) between P and Fe, Ca, Mg, Al, K, or Na in total, NaOH−EDTA, and oxalate extracts. Overall, total P of broiler litter biochars correlated with total Fe, Ca, Mg, Al, K, and Na. Similarly, oxalate-extractable P correlated with oxalate-extractable Mg, Al, K, and Na of broiler litter biochars. In contrast, no 1807

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correlation was observed between EDTA-extractable P and these elements. Table 4 also shows observed correlations between (i) Fe, Ca, Mg, Al, K, and Na in different extracts and (ii) orthophosphate or total inorganic P in NaOH−EDTA extracts determined by 31P NMR. Orthophosphate correlated with oxalate-extractable Mg, K, and Na for broiler litter biochars and Ca, K and Na for cottonseed hull biochars. Of different elements composing biochar ash, alkali metals (Na, K) dissolve in water most readily.10 Alkaline earth metals (Ca, Mg) exist as sulfate, phosphate, and carbonate phases and dissolved in acidic oxalate solution (Table 4). At respective pyrolysis temperature, manure biochar contained >2-fold greater total Ca (by microwave digestion) than the plant biochar (Table S2). The persistence of pyrophosphate in cottonseed hull (but not broiler litter, Table 2) biochar having lower Ca (compared to manure biochars, Table S2) suggests the interaction of pyrophosphate with the organic C fraction of plant biochars. Strong chelating agent EDTA at elevated pH (Table 2) was necessary to compete with this interaction to release pyrophosphate from the biochar surface.



ASSOCIATED CONTENT

S Supporting Information *

Stability constants for phosphate, carboxylate, and aminocarboxylate ligands, entire 31P NMR spectra for plant and manure biochars, and total, NaOH−EDTA, and oxalateextractable elements for Table 4. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*(M.U.) Fax: (504) 286-4367. Phone: (504) 286-4356. E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We thank Dr. Isabel Lima for providing the broiler litter biochar samples and Lynda Wartelle for laboratory assistance. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.



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dx.doi.org/10.1021/jf4053385 | J. Agric. Food Chem. 2014, 62, 1802−1809

Pyrolysis temperature-dependent changes in dissolved phosphorus speciation of plant and manure biochars.

Pyrolysis of plant and animal wastes produces a complex mixture of phosphorus species in amorphous, semicrystalline, and crystalline inorganic phases,...
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