Physicochemical Characterization of Organic Matter in Bayer Liquor

Mar 20, 2014 - ABSTRACT: Organic matter in Bayer liquor from an alumina production facility in Australia was characterized in terms of its molecular w...
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Physicochemical Characterization of Organic Matter in Bayer Liquor Francesco Busetti,*,† Lyndon Berwick,‡ Suzanne McDonald,† Anna Heitz,§ Cynthia Ann Joll,† Joanne Loh,∥ and Greg Power∥ †

Curtin Water Quality Research Centre (CWQRC), Department of Chemistry, ‡WA Organic and Isotope Geochemistry Centre, Department of Chemistry, and §Department of Civil Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia ∥ CSIRO Light Metals Flagship (CSIRO Process Science and Engineering), Parker Centre, P.O. Box 7229, Karawara, Perth, Western Australia 6152, Australia S Supporting Information *

ABSTRACT: Organic matter in Bayer liquor from an alumina production facility in Australia was characterized in terms of its molecular weight distribution and molecular structure using a suite of complementary chromatographic, spectroscopic, and thermal and chemical degradation methods. The organic matter was characterized using high-performance size-exclusion chromatography with UV−vis detection (HPSEC−UV), Fourier transform infrared (FTIR) spectroscopy, and solid-state 13C nuclear magnetic resonance (13C NMR) spectroscopy. These techniques provided information on the apparent molecular weight distribution of the organic matter contained in the Bayer liquor, its alkyl/aromatic characteristics, and the presence of specific functional groups. The techniques of microscale sealed vessel (MSSV) pyrolysis−gas chromatography−mass spectrometry (GC− MS), flash pyrolysis−GC−MS, and online tetramethylammonium hydroxide (TMAH) thermochemolysis−GC−MS provided detailed information at a molecular level. Information on individual low-molecular-weight organic acids in the sample was also obtained using liquid chromatography−tandem mass spectrometry (LC−MS−MS). The novelty of this work is the molecular identification of nitrogen compounds, pyridines, pyrenes, quinolones, benzoquinolines, indoles, carbazoles, bipyridines, and phenylpyridines that derive from organic matter in the bauxite or its transformation products. The results from the other analysis techniques largely confirm the high aromatic content of the liquor, with varying degrees of alkyl (predominantly methyl), carboxylic, ketone, nitrile, and hydroxyl substitution. Aromatic acids were found to be abundant, although they were poorly detected using pyrolysis methods, highlighting the importance of using a suite of complementary techniques for the analysis of Bayer liquor samples.

1. INTRODUCTION The Bayer process is used to extract alumina from bauxite by digestion in a highly caustic solution under severe conditions, namely, high temperatures and pressures.1 Because humic-like substances are generally acidic in nature, under these conditions, more than 50% of the organic matter contained in the bauxite is extracted into the liquor.2,3 As the Bayer process is cyclic, the liquor is continually recycled, leading to the accumulation of these organic compounds. With each pass through the cycle, the higher-molecular-weight (-MW) compounds (e.g., humic and fulvic acids, lignin, cellulose, proteinaceous matter) undergo cleavage or oxidative breakdown to smaller compounds, usually anions because the highly basic conditions, such as oxalate and acetate. In alumina processing, the organic compounds cause detrimental effects on productivity (e.g., poisoning of gibbsite precipitation, decrease in liquor productivity, scaling) and product quality (e.g., purity, color, particle size, and strength).4,5 There is considerable knowledge on the nature of the most abundant organic constituents within Bayer liquor, which are primarily low-MW compounds with relatively simple structures, in their anionic forms (e.g., oxalate, acetate, and tartrate).1,2 Nevertheless, many important aspects of the organic composition of Bayer liquors are poorly understood because many hundreds of more complex, as yet mostly © 2014 American Chemical Society

unidentified, compounds are also present. The higher-MW compounds, comprising humic-like substances, are particularly difficult to characterize, and their impact on the Bayer process is not fully understood. It has been reported that organic compounds with MW > 50 kDa, even though present in Bayer liquor at relatively low concentrations, also adversely affect alumina precipitation and hinder removal of oxalate by precipitation from solution.6 Characterization of organic matter in Bayer liquors is a challenging task because of the high contents of dissolved aluminum, sodium carbonate, sodium chloride, sodium sulfate, and sodium oxalate, as well as unknown anions, in the liquors.1,7 Nevertheless, determination of the nature of the organic content in Bayer liquors is of significant industrial importance, because its presence is implicated in the formation of small particle-size hydrated alumina and other problems in alumina production.1,2,4,6 In this article, we present an integrated approach to the characterization of low- to high-MW organic species in Bayer liquors using a combination of complementary techniques for characterization of the “bulk” sample, as well as detailed Received: Revised: Accepted: Published: 6544

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purchased from Sigma-Aldrich (Sydney, Australia), and formic acid (purity 99%) was purchased from Ajax FineChem (Sydney, Australia). The ultrapure water used for laboratory purposes and as the solvent of the HPLC mobile phase was purified using an IBIS Technology (Perth, Australia) Ion Exchange System followed by an Elga Purelab Ultra System (Sydney, Australia). Glass fiber filters (GF/F, 0.45 μm) were purchased from Whatman (Clifton, NJ). Stock solutions (nominal concentration of 10 μg/μL) were prepared by dissolving a known amount of an analytical standard or a surrogate standard in ultrapure water or MeOH. Two working solutions (nominal concentrations of 10 and 1 ng/μL), containing all of the analytical standard compounds, were prepared freshly for each analytical run by serial dilution of the single-compound stock solutions. All solutions were kept in a commercial refrigerator at 4 °C to avoid degradation. 2.4. Neutralization and Desalting of Bayer Liquor. An aliquot (50 mL) of Bayer liquor (ca. 21 g of organic carbon per liter) was diluted in ultrapure water (300 mL). The sample was neutralized with concentrated HCl solution under vigorous stirring. After neutralization, the pH was adjusted to 2.5 to eliminate carbonates as gaseous carbon dioxide. The solution was then reneutralized to pH 7 using a saturated solution of NaOH, prior to filtration (0.45-μm membrane). Excess water from the filtrate was removed by rotary evaporation to a final volume of 100 mL. Aliquots of the concentrated filtrate solution were placed in sealed 0.5-kDa dialysis bags. The dialysis bags were placed in ultrapure water in a 75-L stainless steel container and left to desalt for approximately 5−6 weeks or until the conductivity inside the dialysis bags had plateaued at 50 μS/cm. The water outside the dialysis bags was changed daily to expedite the dialysis process. The dialysis retentate was concentrated using a rotary evaporator system, and the sample was freeze-dried to recover the Bayer liquor organic matter (OM) in solid form. The solid sample of Bayer liquor OM with a nominal MW (NMW) of >500 Da was analyzed using FTIR spectroscopy, solid-state 13C NMR spectroscopy, MSSV pyrolysis−GC−MS, online TMAH thermochemolysis−GC− MS, and flash pyrolysis−GC−MS. 2.5. Liquid Chromatography−Tandem Mass Spectrometry. Liquid chromatography (LC) separations of organic acids were performed using an Agilent 1100 HPLC system (Palo Alto, CA) equipped with a solvent degasser unit, a quaternary pump, and a 100-well-plate autosampler. A Phenomenex Gemini C18 column (250 mm × 3.0 mm i.d.; 5 μm particle size) operated at 0.15 mL/min was used for the LC separation. The mobile phase for analysis of alkyl acids consisted of (A) 90:10 H2O/ACN containing 1% formic acid and 10 mM of ammonium formate and (B) ACN. The elution gradient was as follows: t = 0 min, 100% A; t = 15 min, 100% A; t = 20 min, 0% A; t = 25 min, 0% A; t = 26 min, 100% A; t = 46 min, 100% A. The mobile phase for analysis of aromatic acids consisted of (A) H2O containing 0.01% formic acid and (B) MeOH containing 0.1% formic acid. The elution gradient was as follows: t = 0 min, 70% A; t = 5 min, 70% A; t = 15 min, 0% A; t = 45 min, 0% A; t = 46 min, 70% A. The LC was coupled to a Micromass Quattro Ultima Triple Quadrupole system (Manchester, U.K.) fitted with an electrospray ionization (ESI) interface operated in negative-ion mode. For optimum signal intensity, the capillary and cone voltages were 2500 and 30 V. Hexapole1, aperture, and hexapole2 were set to 0.0, 0.1, and 0.1 V, respectively. Cryogenic liquid boil-off nitrogen gas (BOC Gases, North Ryde, NSW, Australia) was used as the

molecular characterization methods. Characterization of organic components was preceded by application of a variety of sample preparation techniques on the raw liquor, including neutralization through carbonation, ultrafiltration (UF), dialysis, and dilution. Various approaches were used for characterization. First, the bulk organic matter contained in Bayer liquor was characterized using a range of analytical techniques including high-performance size-exclusion chromatography with UV−vis detection (HPSEC−UV), Fourier transform infrared (FTIR) spectroscopy, and solid-state 13C nuclear magnetic resonance (13C NMR) spectroscopy. These techniques are relatively straightforward to apply and provide relevant information about the apparent molecular weight distribution of the organic matter contained in the Bayer liquor, the alkyl/aromatic characteristics, and the presence of distinct functionalities and chromophores in the sample. The second approach was to characterize, at a “molecular level”, the natural organic matter contained in Bayer liquor. A neutralized, desalted, freeze-dried sample of Bayer liquor with a nominal molecular weight (NMW) of >500 Da (in this article, we refer to this sample as a solid sample of Bayer liquor) was analyzed by online thermochemolysis using tetramethylammonium hydroxide (TMAH), followed by gas chromatography−mass spectrometry (GC−MS), as well as by microscale sealed vessel (MSSV) pyrolysis−GC−MS and conventional flash pyrolysis−GC−MS. Finally, specific lowmolecular-weight alkyl acids (i.e., C1−C4 species) and aromatic acids (i.e., C7−C9 species) were analyzed by liquid chromatography followed by tandem mass spectrometry (LC−MS−MS). This is the first study to apply a broad suite of complementary analytical methods of this nature to Bayer liquor organic matter (OM). Molecular characterization of complex mixtures of altered natural organic matter, such as Bayer liquor OM, is highly challenging, and the application of these complementary methods provided new insights into the structure and function of Bayer organic matter.

2. MATERIALS AND METHODS 2.1. Origin of Bayer Liquor Sample. The Bayer process liquor used in the present work originated from a lowtemperature alumina refinery processing Australian bauxite. 2.2. Carbonation of Bayer Liquor. The Bayer process liquor was carbonated prior to characterization. This involved bubbling CO2 gas through the solution at 80 °C for 3 h. The solid precipitate formed was removed by filtration. The resulting solution was essentially free of alumina and contained NaOH at a concentration of approximately 4 g/L and Na2CO3 at a concentration of approximately 47 g/L. 2.3. Analytical Standards and Chemicals. Alkyl acid analytical standards (oxalic acid, glyoxylic acid, tartaric acid, glyceric acid, glycolic acid, malic acid, malonic acid, acetic acid, lactic acid, succinic acid, and fumaric acid; purity > 95%) were supplied by Sigma-Aldrich (Sydney, Australia). Aromatic acids (salicylic acid, protocatechuic acid, phthalic acid, isophthalic acid, terephthalic acid, gallic acid, trimesic acid, and trimellitic acid; purity ≥ 95%) were supplied by Sigma-Aldrich (Sydney, Australia). The reagent used for online thermochemolysis− GC−MS was tetramethylammonium hydroxide (purity ≥ 98%) and was supplied by Sigma-Aldrich. The organic solvents were obtained as follows: Methanol (MeOH) and acetonitrile (ACN) were ChromAR HPLC (high-performance liquid chromatography) grade and were purchased from Mallinckrodt (Phillipsburg, NJ), ammonium formate (purity 99.995%) was 6545

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2.9. Fourier Transform Infrared Spectroscopy. Fourier transform infrared (FTIR) spectroscopy was carried out on a solid sample (∼1 mg) of Bayer liquor (NMW > 500 Da) using a Perkin-Elmer Spectrum 100 FTIR spectrometer fitted with a single-bounce diamond attenuated-total-internal-reflectance (ATR) accessory. Four background and four sample scans (4000−700 cm−1) were coadded and then normalized to produce an absorbance spectrum. A mathematical correction was applied to adjust the relative spectral intensities of the ATR spectrum to make it comparable to a transmission spectrum. 2.10. High-Performance Size-Exclusion Chromatography. High-performance size-exclusion chromatography (HPSEC) was performed using a Phenomenex BioSep-SEC-S 3000 column (300 mm × 7.80 mm i.d.; 5 μm particle size), fitted with a Phenomenex BioSep-SEC-S 3000 guard column (35 mm × 7.80 mm i.d.; 5 μm particle size). The highperformance liquid chromatography instrument was an Agilent 1100 Series apparatus equipped with a diode array detector set at 254 nm, the recommended optimal wavelength for the study of humic substances.10 Isocratic elution was conducted at a flow rate of 1.0 mL/min using phosphate buffer (10 mM, 1.36 g/L KH2PO4, and 10 mM, 3.58 g/L NaH2PO4; pH 7) as the mobile phase. Samples were diluted in phosphate buffer (pH 7) and filtered through a 0.45-μm nylon filter, and then an aliquot (100 μL) was injected onto the column. Polystyrene sulfonate (PSS) standards with molecular weights of 208, 1920, 3610, 4230, 6520, 15200, 35300, and 81800 Da and dextran blue and acetone were used to calibrate the SEC column, giving a linear equation of y = −0.3145x + 7.2006 (r2 = 0.985), where y represents log(MW) and x is the retention time (min). 2.11. Solid-State Cross-Polarization (CP) Magic-AngleSpinning (MAS) 13C Nuclear Magnetic Resonance (NMR) Spectroscopy. Solid-state CP-MAS 13C NMR spectroscopy was conducted using a Bruker AM300 instrument equipped with a Bruker 4-mm solid-state probe that was operated at 75.5 MHz. The spectra were collected using a magic-angle spinning speed of 8 kHz. A contact time of 2000 μs was used, with a 7.5μs pulse, a recycle delay of 1 s, and line broadening of 200 Hz. Chemical shifts were referenced to an external sample of glycine. Solid-state CP-MAS 13C NMR spectroscopy was preferentially used over solution 13C NMR spectroscopy because of the increased stability and the avoidance of concentration and solubility concerns with the solid-state technique.11,12 Peak areas were integrated to provide an approximate (semiquantitative) comparison of the relative proportions of the various functional groups. 2.12. Elemental Analysis. The analysis for total nitrogen, carbon, hydrogen, and sulfur on a solid sample of Bayer liquor (NMW > 500 Da) was performed at the Central Science Laboratory, University of Tasmania, using a Thermo Finnigan EA 1112 Series Flash Elemental Analyzer (Supporting Information, Table S4).

desolvation and nebulizer gas. Samples to be analyzed were diluted 1:100 in ultrapure water and neutralized to pH 7 with concentrated HCl solution, and 1−10 μL was injected in the LC−MS−MS system. Data processing was carried out using MassLynx NT 4.0 software. 2.6. MSSV Pyrolysis−GC−MS. A small amount (0.5 mg) of a solid sample of Bayer liquor (NMW > 500 Da) was loaded into the middle of a 5 cm × 5 mm i.d. glass tube. Glass beads were added to fill the void volume, and the tube was flamesealed. The sealed tube was then heated isothermally in an oven at 300 °C for 72 h, previously identified as appropriate thermal conditions to produce a wide range of natural organic matter (NOM) products in MSSV pyrolysis.8,9 The pyrolyzed sample tube was loaded into the MSSV GC injector (isothermal at 300 °C) and cracked open with a stainless steel plunger. The volatile products were transferred to the GC column with helium carrier gas at a constant head pressure of 19 psi. The products were initially trapped at the beginning of the column in liquid nitrogen for 1 min. GC−MS analysis was initiated upon removal of the liquid-nitrogen trap using a HewlettPackard (HP) 6890 GC coupled to an HP 5973 mass-selective detector (MSD). GC analysis was performed in split mode (20:1) with a 60 m × 0.25 mm i.d. × 0.25 μm film thickness DB5-MS capillary column (J&W Scientific). The GC oven was temperature programmed from an initial 40 °C (2 min isothermal) to 310 °C (20 min isothermal) at 4 °C/min. Full-scan (m/z 45−550) mass spectra were acquired at ∼3 scans/s. The mass spectrometer was operated in electronimpact mode at 70 eV with a source temperature of 230 °C and a transfer-line temperature of 310 °C. Product assignments were based on correlation of GC retention time and mass spectral data with library (Wiley 275 and NIST 05 databases) or other published data. 2.7. Flash Pyrolysis−GC−MS. Flash pyrolysis was conducted at 650 °C for 20 s on a solid sample (∼1 mg) of Bayer liquor (NMW > 500 Da) using a Chemical Data Systems 5250 pyroprobe with the pyrolysis chamber held at 300 °C.8 The same HP 6890/5973 GC−MS system and DB5-MS capillary column as used for MSSV pyrolysis was also used for flash pyrolysis. GC analysis was performed using helium carrier gas at a constant pressure of 19 psi and with a 40:1 inlet split. The GC oven was temperature programmed to increase from an initial −20 °C (1 min isothermal) to 40 °C at 8 °C/min and then to 310 °C (20 min isothermal) at 4 °C/min. Full-scan mass spectra were acquired in the range m/z 20−600 at ∼4 scans/s. The mass spectral parameters were the same as used for MSSV pyrolysis. 2.8. Online Thermochemolysis−GC−MS. Tetramethylammonium hydroxide was added as a methanolic solution (5 μL, 25% w/v) to a solid sample (1.2 mg) of Bayer liquor (NMW > 500 Da) just prior to pyrolysis at 550 °C for 20 s using a CDS 5250 pyroprobe, with the pyrolysis chamber held at 150 °C. GC−MS analysis was performed with the same HP 6890/5973 GC−MS system and DB5-MS capillary column as used for MSSV pyrolysis. Helium carrier gas was used at a constant flow of 1.1 mL/min and with a 30:1 inlet split. The GC oven was temperature programmed to increase from an initial −20 °C (1 min isothermal) to 40 °C at 8 °C/min and then to 320 °C (20 min isothermal) at 4 °C/min. Full-scan mass spectra were acquired in the range m/z 20−600 at ∼3 scans/s. The mass spectral parameters were the same as used for MSSV pyrolysis.

3. RESULTS AND DISCUSSION 3.1. Molecular Weight Characterization of Bayer Liquor. To assess the molecular weight distribution of the organic carbon contained in the Bayer liquor, the neutral-pH sample was subjected to analysis by HPSEC. HPSEC has been extensively applied to the study of natural organic matter (NOM) in water samples13−15 and is a well-established separation technique used to determine the molecular weight distribution of NOM and its fractions such as fulvic and humic acids, conventionally with UV detection at 254 nm.16 6546

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to 1490 cm−1, and could be the result of the overlap of several infrared absorbance bands. It is also possible to discern the C O vibration between 1200−1280 cm−1, characteristic of carboxylic groups or possibly cyclic aryl ethers. Moreover, the small absorbance at 900−700 cm−1 is also indicative of aromatic carbon.23 Previous infrared analysis of organic extracts from a Bayer liquor sample identified carboxylic absorptions from hydroxyl (3400 cm−1), CO (1720 cm−1), aliphatic C H (2980) cm−1, and aromatic CC (1600 cm−1) peaks.21 In contrast to that previous work, CC stretching (1580 cm−1) characteristic of aromatic carbon was more prominent than the carboxylic acid signal (small shoulder at 1710 cm−1) in the present study. However, in both samples, there was a distinct band at 1100 cm−1 that has been ascribed to cyclic ethers in polycondensed aromatic compounds produced during pyrolysis of humic materials in the Bayer process.21 3.2.2. Solid-State 13C NMR Spectroscopic Analysis. The solid-state 13C NMR spectrum of a solid sample of Bayer liquor (NMW > 500 Da) is shown in Figure 2. The spectrum was

Calculation of polydispersivity (ρ) can provide an indication of the relative “uniformity” of polymeric organic matter in terms of MW. Mw is the weight of the molecule to which the “average” atom belongs, whereas Mn is the weight of the average molecule in the mixture. For a pure substance having a single molecular weight, Mn will be equal to Mw, whereas for a mixture of molecules, Mn < Mw and ρ > 1.10 The Bayer liquor in our study had a relatively low apparent molecular weight (Mw = 1883 Da, Mn = 1554 Da, ρ = 1.21), similar to those of aquatic humic acids (1500−5000 Da) or soil fulvic acids (1000−5000 Da).17 Size-exclusion chromatography (SEC) was used previously to obtain a continuous apparent size distribution of the organic matter present in a Bayer liquor.18,19 Previous separations were achieved on porous silica bead columns coupled in series to obtain a molecular weight distribution of Bayer liquor extracted with butanol. Because such different conditions were used, useful comparisons of SEC results with the current study are not possible. In other studies, Bayer humic substance fractions of >300000 Da were obtained using molecular-weight-specific bags.6,20,21 For a number of reasons previously highlighted,22 all separations based on molecular size should be treated with caution when applied to Bayer liquor extracts. 3.2. Spectroscopic Analysis of Solid Sample Obtained from Bayer Liquor. 3.2.1. Fourier Transform Infrared (FTIR) Spectroscopic Analysis. The FTIR spectrum of the solid sample of Bayer liquor (NMW > 500 Da) is presented in Figure 1. Specific infrared absorbance bands can be tentatively

Figure 2. Solid-state 13C NMR spectrum of a solid sample of Bayer liquor (NMW > 500 Da).

acquired using the cross-polarization (CP) technique that is usually used for acquiring 13C NMR spectra of organic matter.11,12 The NMR spectrum was integrated over four spectral regions, attributed to the following functionalities: 0− 45 ppm, alkyl carbon bonded to hydrogen; 45−110 ppm, Oalkyl or N-alkyl carbons (e.g., carbohydrates, amines, alcohols, ethers); 110−165 ppm, aromatic and unsaturated C; and 165− 185 ppm, carboxyl C in carboxylic acids, amides, and esters. The relative abundances of the aliphatic, aromatic, and carboxyl regions are listed in Table 1. These integrations enable qualitative comparison but do not reflect the absolute quantitative distribution of carbon types.

Figure 1. FTIR spectrum obtained from a solid sample of Bayer liquor (NMW > 500 Da).

assigned to various functional groups: The intense, broad absorption at 3600−3250 cm−1 is due to hydrogen-bonded hydroxyl groups of carboxylic acids, phenols, or alcohols. The unresolved band at 3000−2800 cm−1 and the bands at 1450 and 1370 cm −1 are characteristic of CH stretching and symmetric and asymmetric bending of methyl and methylene groups, respectively. Carboxylic acid functional groups of fulvic/humic acids exhibit a broad band at 2750−2500 cm−1.23 This is also confirmed by the presence of a shoulder at ca. 1710 cm−1, which is indicative of CO stretching due to carboxylic and possibly ketone, aldehyde, or ester carbonyl groups. The very prominent band centered at ca. 1580 cm−1 is indicative of CC stretching of aromatic carbons (1600−1580 cm−1). In our sample, this band is quite broad, going from 1720

Table 1. Proportion of Carbon Types in the Solid-State 13C NMR Spectrum Obtained from a Solid Sample of Bayer Liquora 13

carbon type carboxyl aromatic O-alkyl/N-alkyl alkyl a

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C NMR signal (ppm)

proportionb (%)

185−165 165−110 110−45 45−0

38 59 − 3

NMW > 500 Da. bPercentage relative to total signal. dx.doi.org/10.1021/ie4028268 | Ind. Eng. Chem. Res. 2014, 53, 6544−6553

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Figure 3. Total ion chromatograms from (a) MSSV pyrolysis (300 °C, 72 h) and (b) TMAH thermochemolysis (550 °C, 20 s) GC−MS of a solid sample of Bayer liquor (NMW > 500 Da). Each product was allocated to one of 10 major groupings aligned with specific compound classes or other for miscellaneous or unidentified products as follows: For panel a (MSSV pyrolysis), A, low-MW aliphatic and cycloaliphatic compounds; F, benzofurans; C, cyclopentenones; B, benzenes; N, nitrogen-containing products; P, phenols; H, hydroaromatic compounds; PA, polycyclic aromatic compounds; OA, oxygenated aromatic compounds; O, other. For panel b (TMAH thermochemolysis), AlA, aliphatic carboxylic acids; ArA, aromatic carboxylic acids; NA, nitrogen-containing carboxylic acids; AH, aromatic hydrocarbons; N, nitrogen-containing products; PS, polysaccharide-derived products; OA, oxygenated aromatic compounds; T, TMAH byproducts; U, unknown. The pie charts reflect relative proportions of the major product groups from each analysis. Peak assignments correspond to products listed in Tables S1 and S3 (Supporting Information). Relative abundance (height of the most abundant peak) is indicated in italics in each panel.

Overall, the 13C NMR spectrum of the Bayer liquor OM was very similar to the 13C NMR spectrum published by Wilson et al.21 The signals from carboxyl C and aromatic C were dominant over those from alkyl C and O-alkyl carbon, indicating a large proportion of carboxylic and aromatic species. Very small (i.e., possibly within the background noise level of the spectrum) peaks at 240 and 260 ppm were also observed in the spectrum, possibly arising from spinning side bands (SSBs) from the aromatic C and carbonyl C peaks in the spectrum. The peak observed at 75 ppm could potentially arise from Oalkyl C or be a SSB from the carbonyl signal, and the signal at around 25 ppm could include a contribution from an aromatic SSB, as well as alkyl C. A low-intensity shoulder at 190 ppm, indicating the presence of ketones/aldehydes,24 was also present in the sample. Methyl groups attached to aromatic rings give rise to the prominent peak at 20 ppm in the aliphatic region,21 and methylene (30 ppm) carbons also make a minor contribution.25 The very low abundance of O-alkyl carbon is consistent with previous 13C NMR studies of solid samples of Bayer liquors20,26 and indicates very little contribution from carbohydrates, aliphatic alcohols, and ethers.27 3.3. Molecular Characterization of Solid Sample Obtained from Bayer Liquor. 3.3.1. MSSV Pyrolysis−GC− MS and Flash Pyrolysis−GC−MS. The total ion chromatogram (TIC) obtained from MSSV pyrolysis−GC−MS analysis of the

solid sample of Bayer liquor (NMW > 500 Da) is shown in Figure 3a. Overall, the product distribution showed many features similar to those previously reported from Bayer liquor pyrolysates.21,28,29 The 100 most abundant products listed in Table S1 (available in the Supporting Information) accounted for 79% of the total MSSV product yield. Each product was allocated to one of 10 major groupings aligned with specific compound classes or “other” for miscellaneous or unidentified products. The relative proportions of these 10 product groups are also indicated in Figure 3a. The product distribution from MSSV pyrolysis was polycyclic aromatic hydrocarbons (PA, 40%) > nitrogen-containing products (N, 18.5%) > alkylbenzenes (B, 14.5%) > other products (O, 7.3%) > alkyl phenols (P, 5.8%) > oxygen-containing aromatic compounds (OA, 5.3%) > hydroaromatic compounds (H, 3.6%) > benzofurans (F, 2.5%) > cyclic and acyclic aliphatic products (A, 1.6%) > cyclopentenones (C, 1.0%). Flash pyrolysis−GC−MS (major products, relative abundance, and classifications listed in Table S2 of the Supporting Information) was also conducted for comparison of the MSSV data with a more traditional pyrolysis data set. Although minor differences were evident in the presence and relative abundances of some products between MSSV and flash pyrolysis, the pyrolysate distributions from the two data sets were generally very similar and were therefore interpreted 6548

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sources of terrigenous plants. However, their low abundance in the pyrolysate from the solid sample obtained from Bayer liquor contrasts the natural organic matter of aquatic systems and soils, in which alkyl phenols are generally dominant products.27,30,37 Wilson et al.21 suggested that phenolic compounds might be polymerized to cyclic aryl ethers (e.g., benzofurans) during the Bayer process, providing an alternative origin for the benzofuran products. A striking feature of the Bayer liquor OM analyzed here was the very high abundance of nitrogen-containing products detected by both pyrolysis methods. These included monoand polyaromatic nitriles and heterocyclic aromatic compounds, such as pyridines, pyrroles, quinolines, benzoquinolines, indoles, carbazoles, bipyridines, and phenylpyridines, again with varying degrees of alkyl and phenyl substitution. These N products have not been previously reported in such high concentrations in pyrolysates derived from Bayer liquors.21,28 Aromatic nitriles may derive from the dehydration of aromatic amides or from the reaction of aromatic carboxylic acids with ammonia during combustion,38 whereas low-MW heterocyclic N products (e.g., pyridines, pyrroles, indoles) are common pyrolysis products of proteins and amino acids.8,39 However, these products are not likely derived from intact proteins and peptides because these biochemicals are rapidly degraded by microbial activity during sedimentary diagenesis40 and would also not survive the severity of the Bayer treatment.21 The dominance of heterocyclic functionalities in the N product distribution from pyrolysis of the solid sample of Bayer OM indicates that the majority of these products originated from complex N-containing molecules resulting from the structural alteration of proteins. The thermal conversion of proteinaceous amide functionalities to N heterocyclic structures has been demonstrated by 15N NMR spectroscopic analysis of thermally treated biomass.40,41 Furthermore, organic nitrogen in mature sediments and petroleum is predominantly present as heterocyclic aromatic structures derived through diagenetic and catagenetic alteration of N-containing biochemicals.42 A further potential source of nitrogen-containing heterocyclic aromatic compounds is constituted by treatment chemicals that are added to the Bayer process. Although it is not possible to divulge the exact nature of chemicals that are used (i.e., commercial confidentiality), it is well-known that flocculants are used in the Bayer process in general and that some of these chemical may contain nitrogen moieties (e.g., polyamides, polyacrylamide). These organic nitrogen compounds may well accumulate in the process and be converted to the heterocyclic compounds detected in the pyrolysis process. The prevalence of N organic compounds indicates unusually high organic nitrogen content in the humic materials of the bauxite and the corresponding Bayer liquor. However, 13C NMR signals corresponding to many nitrogen functional groups overlap with those produced by alkyl, methoxyl, and carbonyl structures,43 so they are not easily identified. The impact of the high concentrations of N-containing organic compounds in Bayer liquor on the alumina precipitation process warrants further investigation. In contrast to the aromatic products, the low pyrolysis abundance of carboxylic acids was inconsistent with the significant carboxyl C signal in the 13C NMR spectrum. Only benzoic acid and methylbenzoic acid were detected, along with phthalic anhydride, benzaldehyde, acetophenone, and diacetylbenzenes, which are possibly derived from carboxylic acids.

together. However, the area integration of resolvable peaks from MSSV pyrolysis was 40 times greater than the area integration from the corresponding flash pyrolysis data, highlighting the sensitivity of the MSSV technique for the molecular characterization of complex, macromolecular organic material.30 The significance and potential origins of the various product groups are discussed below. The dominant products from both analyses were mono- and polycyclic aromatic hydrocarbons (PAHs) with varying degrees of alkyl substitution. These included benzenes, indenes, naphthalenes, biphenyls, fluorenes, phenanthrenes, pyrenes, and phenylnaphthalenes. PAHs are produced by incomplete combustion of organic matter.31,32 Their high abundance in the Bayer liquor reflects the elevated temperature and oxidizing conditions of the Bayer process and is consistent with the dominant aromatic carbon signals detected by FTIR and solidstate 13C NMR spectroscopies. Similar distributions of aromatic products were detected in previous pyrolysis studies on solid samples of Bayer liquors,21,29 whereas solid-state 13C NMR spectroscopy has shown that, compared to bauxite humic material, Bayer liquors are enriched in aromatic structures relative to aliphatic and carboxyl carbon.26 The low abundance of aliphatic pyrolysis products indicates that the majority of the aliphatic carbon signal detected in solidstate 13C NMR spectroscopy (0−45 ppm; Figure 2) and FTIR spectroscopy (aliphatic absorption bands at 2960, 1450, and 1370 cm−1; Figure 1) is likely to derive from alkyl substituents on aromatic rings. Indeed, the major peak in the alkyl C region in the NMR spectrum occurred at 20 ppm, a region attributed mainly to carbons of aromatic methyl groups.21 This correlates well with the high degree of methyl substitution evident in the pyrolysis products. In addition to the hydrocarbons, a variety of PAH products containing predominantly ketone and alcohol functional groups were detected. These included indanone, methylindanone, benzophenone, methylbenzophenone, naphthol, methylnaphthols, phenanthrol, hydroxyfluorenone, and anthrone. The strongly oxidizing environment of the refining process most likely gives rise to these oxygenated aromatics,33 although their precise origins remain uncertain. Their presence is nevertheless consistent with the detection of aldehyde/ketone species (small shoulder at 190 ppm) in the solid-state 13C NMR spectrum. The detection of several prominent phenyl-substituted aromatic compounds (e.g., phenylnaphthalenes, phenylpyridines, phenylaniline) provides additional evidence for the occurrence of oxidation reactions during the digestion process. The sedimentary occurrence of phenyl-substituted PAHs has been attributed to the formation of phenyl radicals in oxidizing environments.34 Other oxygen-containing pyrolysis products included C0−C2 alkylbenzofurans and dibenzofurans and alkyl phenols. Benzofurans are most likely derived from complex heterocyclic aromatic structures formed through diagenetic or thermal alteration of carbohydrates present in the bauxite humic material.35,36 The lack of detection of other characteristic products of carbohydrates is consistent with the very low abundance O-alkyl region in the solid-state 13C NMR spectrum and indicates little, if any, contribution from intact carbohydrates in the Bayer liquor. Alkylphenols (C0−C3) were relatively minor products from both pyrolysis analyses, consistent with the low phenolic contribution detected by solid-state 13C NMR spectroscopy. These compounds are typically attributed to lignin or tannin 6549

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of selected C2−C4 aliphatic and C7−C9 aromatic carboxylic acids. From a qualitative screening of the MS scan, SIM, and MRM LC−MS chromatograms, the Bayer liquor sample was found to contain most of the target analytes (Table 2).

Carboxylic acids are typically underestimated by analytical pyrolysis because of their structural polarity and tendency to decarboxylate.44 These compounds were therefore investigated by thermochemolysis (section 3.3.2) and LC−MS (section 3.3.3). 3.3.2. Online Thermochemolysis−GC−MS. Online thermochemolysis is an adjunct pyrolysis technique performed in the presence of tetramethylammonium hydroxide (TMAH).45,46 The approach overcomes some of the limitations of traditional analytical pyrolysis, allowing the separation and detection of many additional pyrolysis products of structural and source significance. It promotes the selective cleavage of ester and ether linkages in macromolecules through saponification/ transesterification reactions.47,48 The presence of the methylating agent yields methylated ester or ether derivatives of many polar constituents including fatty acids, aromatic and phenolic carboxylic acids, and alcohols. The TIC obtained from online TMAH thermochemolysis− GC−MS analysis of the solid sample of Bayer liquor (NMW >500 Da) is shown in Figure 3b. The 100 most abundant products, listed in Table S3 (available in the Supporting Information), accounted for 87% of the total product yield. The products were allocated to one of nine major groupings aligned with specific compound classes or “unknown” for unidentified products. The relative proportions of these nine product groups are also indicated in Figure 3b. The product distribution from thermochemolysis was aromatic carboxylic acids (ArA, 34.0%) > aliphatic carboxylic acids (AlA, 29.7%) > unknown products (U, 13.4%) > nitrogen-containing products (N, 8.9%) > TMAH byproducts (T, 4.3%) > aromatic hydrocarbons (AH, 4.1%) > nitrogen-containing carboxylic acids (NA, 2.6%) > oxygen-containing aromatic compounds (OA, 1.5%) = polysaccharide-derived products (PS, 1.5%). The detection of high abundances of aliphatic and aromatic mono-, di-, and tricarboxylic acids (as methyl esters), not detected by either flash or MSSV pyrolysis, is consistent with the high carboxyl group content observed in the solid-state 13C NMR spectrum. In addition, several methyl esters containing nitrogen functional groups were detected. Aliphatic and aromatic carboxylic acids are common components of Bayer liquors formed by the degradation and oxidation of humic material during repetitive caustic digestion.28,29 Of the aromatic products, only trace quantities of two lignin-derived compounds, 2-methoxyphenol and 4-hydroxy-3-methoxybenzoic acid (vanillic acid), were detected. This is consistent with the low abundance of phenolic products identified by MSSV and flash pyrolysis. The low concentrations and limited number of these products contrast the prolific distributions of guaiacyl/ syringyl-related methoxyaromatic compounds, typical of lignin rich NOM.37,44 Several of the major aromatic hydrocarbons (e.g., parent and alkyl-substituted benzenes, naphthalenes, and higher-MW PAHs) and N-containing pyrolysis products (e.g., pyridines) prominent by MSSV and flash pyrolysis were also detected by thermochemolysis. The low abundance of polysaccharidederived products was also consistent with the MSSV and flash pyrolysis data, indicating only minor contribution from carbohydrates to the Bayer liquor sample. 3.3.3. LC−MS−MS Analysis of C2−C4 Aliphatic and C7−C9 Aromatic Carboxylic Acids. A sample of carbonated and neutralized Bayer liquor was diluted and analyzed by LC−MS operated in MS scan, selected-ion-monitoring (SIM), and multiple-reaction-monitoring (MRM) modes for the presence

Table 2. Selected Aliphatic and Aromatic Organic Acids Analyzed in a Diluted (1:100 v/v) Sample of Bayer Liquora acquisition mode compound

formula (MW)

MS scan

C2−C4 Aliphatic Carboxylic Acids C2H2O3 (74.0) nob C2H4O3 (76.0) nob C3H6O3 (90.0) yes C3H4O4 (104.0) yes C4H4O4 (116.1) yes C4H6O4 (118.1) yes C4H6O5 (134.1) yes C4H6O6 (150.0) yes C7−C9 Aromatic Carboxylic Acids salicylic acid C7H6O3 (138.1) n.a.d protocatechuic acid C7H6O4 (154.1) n.a. phthalic acid C8H6O4 (166.1) n.a. isophthalic acid C8H6O4 (166.1) n.a. terephthalic acid C8H6O4 (166.1) n.a. gallic acid C7H6O5 (170.0) n.a. trimesic acid C9H6O6 (210.1) n.a. trimellitic acid C9H6O6 (210.1) n.a.

glyoxylic acid glycolic acid lactic acid malonic acid fumaric acid succinic acid malic acid tartaric acid

SIM

MRM

no yes yes yes yes yes yes yes

no noc yes yes yes yes yes yes

yes no yes yes yes no yes no

yes no yes yes yes no yes no

a

Sample was carbonated and neutralized to pH 7 before analysis. Outside MS scan range. cPoor sensitivity of the MRM transition prevented detection. dn.a.: data not available.

b

Figures S1 and S2 (available in the Supporting Information) show the detection of several C2−C4 aliphatic carboxylic acids and C7−C9 aromatic carboxylic acids in the sample of Bayer liquor analyzed. For convenience, LC−MS−MS chromatograms from analysis of single-compound standard solutions are also presented in Figures S1 and S2 (Supporting Information). For both aliphatic and aromatic acids, the LC−MS method, with the MS acquisition mode set to SRM or MRM, was generally found to be much more selective than when in SIM or MS scan mode. However, with the MRM mode, not all of the aliphatic acids could be detected for various reasons, including lack of fragmentation and/or production of small and unstable fragments and/or ion suppression. The main fragmentation pathways of the aliphatic acids include loss of H2O ([M − H]−, −18 Da), loss of CO ([M − H]−, −28 Da), and loss of COO− ([M − H]−, −44 Da). Some of the compounds analyzed, including glycolic acid and glyoxylic acid, have low MWs, so that, after fragmentation, ions of even lower MW were produced. These product ions may have been too unstable to be detected under MS-MS mode. Therefore, these two species were analyzed in SIM mode (Table 2). Oxalic acid is an important organic component of Bayer liquor.1 However, it could not be detected in our study because matrix effects completely suppressed the oxalic acid signal, to the extent that they prevented its detection; therefore, the present LC−MS method is not suitable for analysis of oxalic acid in Bayer liquor samples. Other methods employing, for example, ion chromatography with UV detection or liquid chromatography with UV detection could be used instead.1,22,49,50 6550

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Data for the C7−C9 aromatic carboxylic acids was acquired in both SIM and SRM modes, with SRM showing better sensitivity and selectivity. Decarboxylation of the main parent ion (i.e., [M − COOH]−) was the primary transition observed for all of the aromatic carboxylic acids analyzed. Several of the aromatic acids [i.e., isophthalic, terephthalic, and phthalic acid (M1 = C8H6O4); trimellitic and trimesic acid (M2 = C9H6O6)] are isomers and share the same MW for both the precursor ions ([M1 − H]− = 165 m/z and [M2 − H]− = 219 m/z, respectively) and the same product ions ([M1 − COOH]− = 121.1 m/z and ([M2 − COOH]− = 165 m/z, respectively). For these isomers, compound identification was ensured through comparison of chromatographic retention times with those obtained from analysis of standard solutions. All of the C2−C4 aliphatic acids identified in this study have been previously reported in Bayer liquor samples, namely, succinic acid,18,51 tartaric acid,18,52 malic acid,18 malonic acid,50,52−54 fumaric acid,55 and lactic acid.2,18,19,49,50,53,54,56−58 The presence of five of the C7−C9 aromatic carboxylic acids is also in agreement with several other studies that also reported their presence in Bayer liquor samples, namely, salicylic acid,1,2,18,19,49−54,59 trimellitic acid,18,53,54 phthalic acid and terephthalic acid,18,54 and isophthalic acid.51−54,60 In contrast, trimesic acid, which has been reported before in Bayer liquor samples,18,19,53 could not be detected in the current study. Gallic acid and protocatechuic acid were analyzed in the current study, as representatives of poly(hydroxybenzoic) acids, but were not detected; these compounds have not been reported previously in Bayer liquor samples.22 The absence of these compounds is in agreement with the low abundance of phenolic and polyphenolic pyrolysis products, indicating very little phenolic character in the OM in this Bayer liquor sample. Quantitative assessment of the detected acids by LC−MS was not attempted in this study but the presence of many of the acids detected by LC−MS was consistent with observations from the thermochemolysis analysis of the solid sample of NMW > 500 Da obtained from the Bayer liquor. Methyl esters of acids were abundant in the TMAH thermochemolysis product mixture and these might have been present either as free acids in the original Bayer liquor or they may have been released through the thermochemolysis process. For example, lactic acid, detected in the carbonated, neutralized, and diluted Bayer liquor using LC−MS, would have been converted to the methyl ester of 2-methoxypropanoic acid [compound 20 in Table S3 (Supporting Information), 7% abundance] in the thermochemolysis product mixture; succinic acid would have been converted to the dimethyl ester of butanedioic acid [compound 34, Table S3 (Supporting Information), abundance = 8.21%], similarly to fumaric acid [compound 32, Table S3 (Supporting Information), abundance = 0.39%] and phthalic acid [compound 67 in Table S3 (Supporting Information), abundance = 0.96%]. The methyl esters of isophthalic acid and terephthalic acid were major components in the thermochemolysis analysis [compounds 70 and 71, Table S3 (Supporting Information), abundances = 11% and 7.4%, respectively), suggesting that macromolecules containing these functional groups were abundant in the Bayer liquor; their presence as free acids was confirmed using LC−MS. Similarly, the presence of the methyl ester of trimesic acid [1,3,5-tricarboxylic acid, compound 96, Table S3 (Supporting Information), 1.77%] in the thermochemolysis product mixture was also consistent with the detection of the nonmethylated version of this compound

using LC−MS. However, trimellitic acid was not detected using LC−MS, but the methyl ester of this compound [1,2,4-benzene tricarboxylic acid, compound 93, Table S3 (Supporting Information), 0.87%] was detected in the thermochemolysis product mixture, suggesting that this compound might have been bound within a larger molecular structure in the original Bayer liquor and was released only upon thermochemolysis.

4. CONCLUSIONS The structural information provided by the complementary spectroscopic and molecular characterization techniques employed in this study showed that the Bayer liquor OM exhibited a highly aromatic structure with varying degrees of alkyl (predominantly methyl), carboxylic, ketone, nitrile, and hydroxyl substitution. The pyrolysis-based analytical methods of MSSV pyrolysis−, online thermochemolysis−, and flash pyrolysis−GC−MS consistently showed that the major groups in the Bayer liquor OM of NMW > 500 Da were aromatic in nature, and these indications were supported by data from 13C NMR and FTIR spectroscopies. NMR analysis showed contributions from alkyl groups, but it was concluded that these comprised primarily short-chain (methyl and ethyl) substituents on aromatic rings. Heterocyclic O- and Ncontaining structures also contributed significantly to the Bayer liquor OM of NMW > 500 Da, whereas phenolic carbon was present in low abundance. A relatively high abundance of nitrogenous organic matter was detected using MSSV pyrolysis; such a relative abundance has not been previously reported for Bayer liquor OM. The high temperatures, pressures, and caustic concentrations induce structural alteration of the humic material in the bauxite, resulting in a high proportion of oxidized moieties, such as aromatic acids, as determined using LC−MS and TMAH thermochemolysis. Organic acids are poorly represented in conventional flash pyrolysis and MSSV pyrolysis, highlighting the importance of the alternative techniques of LC−MS and TMAH thermochemolysis that are more suited to the detection of this compound class.

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ASSOCIATED CONTENT

S Supporting Information *

Major products from MSSV pyrolysis−GC−MS, flash pyrolysis−GC−MS, online TMAH thermochemolysis−GC− MS along with some examples of LC−MS−MS chromatograms of C2−C4 aliphatic and C7−C9 aromatic carboxylic acids, and elemental analysis data (total nitrogen, carbon, hydrogen, and sulfur). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +61-0-892663273. Fax: +61-0-892662300. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the CSIRO Flagship Collaboration Fund is gratefully acknowledged. We would like to thank Dr. J. Pringle from Monash University for acquiring the 13C NMR spectra and for the insightful data interpretation provided. We also gratefully acknowledge the contribution of two anonymous reviewers whose expertise helped to improve the manuscript. 6551

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dx.doi.org/10.1021/ie4028268 | Ind. Eng. Chem. Res. 2014, 53, 6544−6553