Demineralisation of a crop soil by mild hydrofluoric acid treatment

J. Anal. Appl. Pyrolysis 71 (2004) 119–135

Demineralisation of a crop soil by mild hydrofluoric acid treatment Influence on organic matter composition and pyrolysis Y. Zegouagh a,b , S. Derenne a , M.F. Dignac b , E. Baruiso c , A. Mariotti b , C. Largeau a,∗ a

Laboratoire de Chimie Biorganique et Organique Physique, UMR CNRS 7573, ENSCP, 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b Laboratoire de Biogéochimie Isotopique, UMR CNRS-INRA 7618, UPMC, 4 place Jussieu, 75252 Paris Cedex 05, France c INRA Unité de Science du Sol, BP 01, 78850 Thiverval-Grignon, France Received 14 November 2002; accepted 1 April 2003

Abstract A recently developed demineralisation treatment, using 2% hydrofluoric acid, was applied to the <50 ␮m fraction of a silty loamy soil typical of crop soils from northern France. The material thus isolated was compared with an untreated control through elemental analysis and thermal degradation (Rock-Eval pyrolysis; analysis by combined gas chromatography–mass spectrometry of the pyrolysates obtained by off-line pyrolyses at 300 and 400 ◦ C). It appeared that: (i) efficient removal of the minerals was achieved by this treatment while only limited losses of organic carbon occurred; (ii) large retention of the pyrolysis effluents by the mineral matrix took place in the untreated sample; (iii) the extent of this retention, for the different types of pyrolysis products, is controlled by their molecular weight and polarity; (iv) through these off-line pyrolyses information can also be obtained on the origin of the components of Soil organic matter (SOM) pyrolysates (thermovaporized products vs. pyrolysis products formed via cracking reactions; lignin-derived phenolics vs. melanoidin-derived ones); and (v) pyrolytic studies limited to the untreated sample would have provided highly biased quantitative and qualitative information on SOM so that the presence of some important constituents would have passed unnoticed. © 2003 Elsevier B.V. All rights reserved.

∗

Corresponding author. Tel.: +33-1-4329-5102; fax: +33-1-4325-7975. E-mail address: [email protected] (C. Largeau).

0165-2370/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0165-2370(03)00059-7

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Keywords: Soil organic matter; Soil demineralisation; Hydrofluoric acid treatment; Rock-Eval pyrolysis; Off-line pyrolysis; Retention of pyrolysis effluents; Pyrolysis bias

1. Introduction Soil organic matter (SOM) represents one major pool of carbon in the global carbon cycle and plays a major role in ecosystems as: (i) an essential substrate for the development of the biological activity in soil; (ii) a stabilizer of soil structure; and (iii) a trap for organic and inorganic pollutants [1]. SOM comprises a complex mixture of plant, animal and microbial residues, at varying stages of decomposition. Chemical structure and preservation of SOM are controlled by humification and mineralisation processes that are responsible for extensive alteration of source materials ([2] and References therein). Elucidating the chemical structure of SOM is therefore an important but difficult task. Furthermore, the difficulties encountered in such studies are still heightened by the tight association of SOM with a dominant mineral matrix, especially for the finest granulometric fractions. As a result, a number of analytical tools can hardly be used for SOM examination in whole soils or whole soil fractions. Thus, the occurrence of high amounts of minerals makes difficult (if not impossible) direct studies via infra-red and nuclear magnetic resonance spectroscopy. Several types of methods can be considered to separate the organic fraction from the mineral matrix, mainly comprised of clay minerals and oxides. However, no efficient and universally accepted method has been so far established for SOM isolation [3]. Due to a very tight association between both constituents, physical fractionations (granulometric and/or densimetric) [4–6] are not highly efficient and chemical treatment has to be performed. Chemical fractionation of SOM is commonly carried out with aqueous base and acid solutions. Such a chemical separation was largely used to examine humification pathways and the effect of SOM chemical structure on carbon stability [7]. Two mineral-free fractions, the so-called humic and fulvic acids, are thus isolated. However, contrary to physical fractionation techniques which are non-destructive, such treatments may significantly modify the chemical composition of SOM. In fact, partial alteration can occur through SOM hydrolysis due to the alkaline or acid nature of the extraction solvents [8]. Alkaline or acid hydrolyses also can generate artifacts via the formation of humic-like macromolecules, termed melanoidins, by polycondensation of monomers and other alteration products of proteins and carbohydrates through Maillard reactions [9]. Furthermore, following this classical treatment, the insoluble fraction of SOM, i.e. the humin, remains associated with soil minerals and the origin and chemical structure of the latter fraction on a molecular level are still far from being elucidated [10]. The humin fraction is by far the main form of organic matter in soil [11] and a comprehensive knowledge of its structural features is necessary to understand the reactivity of SOM in chemical, biological and environmental processes [12]. Recently, a mild treatment with 2% hydrofluoric acid (HF), was shown to efficiently eliminate magnetic minerals from soils [13]. A treatment with HCl and a mixture of dilute HF/HCl was also recently developed to remove minerals and paramagnetic ions from sediment trap material and marine sediments for solid-state 13 C NMR studies [14]. For the five soils studied in [13], large improvements in the solid state 13 C NMR spectra of

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SOM were noted, but the 2% HF treatment resulted in substantial losses of organic carbon (10–17 wt.%). Nevertheless, the main feature of SOM, as reflected by such spectra which provide bulk information on the environment of carbon atoms, seemed not significantly affected. However, one could not exclude that the 2% HF treatment induces significant alteration in the chemical structure of SOM at a molecular level. The aim of the present study was to test the possible effects of the 2% HF treatment on the chemical composition of SOM and on its pyrolytic features. To this end, a silty loamy soil, typical of soils from North of France, was selected. The study was performed on the <50 ␮m fraction, which contains humified SOM tightly associated with minerals. This fraction was separated into two parts with only one treated by HF. The quantitative and qualitative features of SOM in the treated material were examined via pyrolytic methods and compared with those of the untreated control. Comparison was carried out by: (i) Rock-Eval pyrolysis; and (ii) off-line pyrolyses at 300 and 400 ◦ C followed by gas chromatography (GC) and combined gas chromatography–mass spectrometry (GC–MS) analyses of the trapped pyrolysis products. Pyrolysis, associated with GC–MS analysis of effluents, is well known as a suitable tool for the characterization of complex mixtures of geomacromolecules in various environments [2,3,12,15–19].

2. Experimental 2.1. Sample and treatment A sample of silty loamy soil (0–30 cm upper layer) was collected from a plot located at Grignon (25 km West of Paris, Yvelines, France). This plot has been cropped with wheat since 1973 and its main bulk features are reported in Table 1. Water soluble components were first eliminated by extraction of ca. 50 × g of soil (stirring at room temperature in H2 O 5 × 24 h) and centrifugation (2500 × g for 20 min) so as to avoid artifact formation via polycondensation of such compounds during the acid treatment. The extracted material was then submitted to granulometric fractionation according to [20] (disaggregation by stirring in water with glass beads and sieving) to separate the humified (<50 ␮m, accounting for ca. 85 wt.% of the whole soil) and the non-humified ( > 50 ␮m) SOM fractions. The former fraction, which contains the humified OM tightly associated with minerals (accounting for ca. 74% of the total organic carbon (TOC) of the whole soil), was separated into two parts. One part (18 g) was submitted to successive treatments with 2% HF and the second was used as control. The 2% HF solution was prepared from concentrated (48%) HF as previously described [13]. Each step of the treatment was performed by stirring at room temperature the soil sample with 250 ml of 2% HF, followed by centrifugation at 6800 × g Table 1 Main features of the Grignon soil Carbon (%)

pH (water)

Clay (%)

Silt (%)

Sand (%)

1.51

8.0

26.9

59.0

12.4

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for 20 min and elimination of the supernatant. This treatment was repeated nine times with stirring durations of 5 × 1, 3 × 16, and 1 × 64 h, respectively. The material thus obtained was finally washed four times until neutral with water and dried at 25 ◦ C. This treated material and the control were examined by elemental analysis, Rock-Eval and off-line pyrolyses and the off-line pyrolysates analysed by GC and combined GC–MS. 2.2. Analyses Elemental analyses were performed at the ‘Service Central de Microanalyse du CNRS’, Vernaison, France. Rock-Eval pyrolyses were carried out on 30–50 mg samples, as previously described [21]: heating under a helium flow at 300 ◦ C for 3 min, followed by programmed pyrolysis at 25 ◦ C min−1 up to 600 ◦ C and then oxidation at 600 ◦ C for 7 min under an oxygen flow. Off-line pyrolyses at 300 and 400 ◦ C were performed as previously described [22]. The samples (4 × 120 mg) into a quartz tube plugged with quartz wool were submitted to two successive heatings (20 min at 300 and 1 h at 400 ◦ C) under a 15 ml min−1 helium flow. The pyrolysis products swept away by the helium flow were bubbled into cold CHCl3 at −5 ◦ C. CHCl3 was eliminated under vacuum with a rotary evaporator before GC and GC–MS analyses. Heating at 300 ◦ C aimed at eliminating thermolabile and/or adsorbed components. Pyrolysis sensu stricto, i.e. extensive thermal cracking of macromolecular structures into smaller units was mostly achieved at 400 ◦ C. For GC analyses, a HP 5890 gas chromatograph was used equipped with a 25 m CPSil 5 CB capillary column (0.32 mm internal diameter, film thickness 0.25 ␮m) programmed from 100 to 280 ◦ C at 4 ◦ C min−1 , injector and detector held at 320 ◦ C, helium as carrier gas. GC–MS analyses were carried out using a HP 5890 gas chromatograph (same column and conditions as above) coupled with a HP 5989 mass spectrometer (electronic impact at 70 eV, mass range m/z = 40–600).

3. Results and discussion 3.1. Elemental analysis and Rock-Eval pyrolysis A sharp decrease in total weight was observed following the 2% HF treatment and 1.53 × g of treated material were recovered from an initial sample of 18 g. Elemental analysis (Table 2) showed sharp increases in the concentration of total carbon (from ca. 2 to more than 10%) and of total nitrogen (from ca. 0.2 to ca. 1%). As confirmed by Rock-Eval analyses (see below) such enrichments reflect both limited elimination of SOM for the Grignon soil whereas extensive elimination of minerals is achieved. Nevertheless, due to low SOM concentration in the initial sample, the HF-treated material still contains a substantial level of mineral matter (ca. 72%). Rock-Eval pyrolysis has been extensively used for the examination of organic matter in sedimentary rocks [23,24]. In contrast, this method has been seldom applied in SOM studies [25,26]. In addition to the concentration of TOC, Rock-Eval analyses afford other indexes. S1 is the amount of effluents released per gram of sample at relatively low temperature

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Table 2 Carbon, nitrogen and ash content (wt.%) of the untreated sample (<50 ␮m fraction) and 2% HF-treated sample determined by elemental analysis

Untreated Treated

C

N

Ash

1.95 10.43

0.21 0.99

92 72

Together C+N+Ash account for ca. 94 and 83 wt.% for the untreated and treated samples, respectively. The difference to 100% corresponds to the elements of the organic matter (O, H and S) not taken into account and to the alteration of some minerals (such as dehydration and sulfate decomposition with sulphur dioxide release) upon the drastic thermal treatment used for ash isolation.

and reflects the abundance of thermolabile and/or adsorbed organic components. S2 is the amount of effluents released per gram of sample at higher temperature, during the main stage of formation of pyrolysis products. Tmax is the temperature corresponding to the maximum of the latter production during programmed heating. For the Grignon samples, a strong enrichment, of an order of magnitude, is observed for TOC in the treated material (Table 3). Large increases also occur for S1 and S2 values. Moreover, it can be noted that the latter increases (ca. × 37 and × 18 for S1 and S2, respectively) are markedly higher than simply accounted for by TOC enrichment (ca. × 10). Such differences reflect the extensive retention of pyrolysis effluents by the mineral matrix in the untreated material, as previously observed for organic-poor sedimentary rocks [27–29]. The especially high increase observed for S1 shows, as expected, that this retention is more pronounced at lower temperatures, i.e. during the first stages of heating. The decrease observed for Tmax also reflects the easier release of the effluents from the HF-treated material. Carbonates are readily eliminated under acid conditions and the total carbon determined by elemental analysis for the HF-treated material shall mostly correspond to organic carbon. This is in agreement with the very close values obtained for total carbon via elemental analysis and for TOC via Rock-Eval pyrolysis in this material (Tables 2 and 3). In contrast, for the untreated sample, a higher content is noted for total carbon when compared to TOC. This difference shall reflect the presence of low amounts of carbonates in the mineral matrix of the Grignon soil. Calculations of mass balances, based on total weight loss (1.53 × g of treated material obtained from 18 × g of the soil fraction) and on Rock-Eval data for organic carbon and elemental analysis data for minerals, showed that 17 of organic carbon and 93 wt.% of Table 3 Rock-Eval pyrolysis data for the untreated sample (<50 ␮m fraction) and 2% HF-treated sample

Untreated Treated a

TOC (%)

S1 (mg g−1 )a

S2 (mg g−1 )a

Tmax (◦ C)

1.1b 10.7

0.29 10.82

0.92 16.88

394 358

mg of effluents generated per g of sample. the difference between the TOC content of the whole soil (1.51%, Table 1) and this value reflects the elimination of both water soluble organic matter during the first extraction and of coarse non-humified organic debris during isolation of the <50 ␮m fraction. b

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minerals were eliminated after the 2% HF treatment. The bulk of the SOM is therefore, retained after this acid treatment whereas extensive elimination of minerals is achieved. The organic carbon loss thus observed is similar to the maximal value previously obtained [13] through the study of a set of five soils of different chemical and mineralogical compositions. As stressed by the above authors, such a loss should mostly reflect the hydrolysis of some macromolecular constituents of SOM, like some labile proteinaceous and polysaccharidic components which are readily cleaved by acid hydrolysis. 3.2. Off-line pyrolyses 3.2.1. Bulk features The total weight loss observed, as wt.% of the organic carbon of the unheated material, following the two successive heatings at 300 and 400 ◦ C is of 11.9 and 29.1% for the untreated and treated samples, respectively (Table 4). Previous studies on organic-poor sedimentary rocks (TOC of a few %) showed that the abundant presence of minerals prevents an efficient release of pyrolysis effluents, due to the retentive capacity of the matrix [27,30]. Markedly higher losses were thus obtained from such samples after demineralisation [30]. However, this retention by the matrix appeared negligible as soon as the examined rocks exhibited moderate organic carbon contents (from TOC of ca. 5%) [27–29]. A similar situation is observed, with a soil sample, in the present study. It thus appears that the large removal of the mineral matrix, achieved by the 2% HF treatment, results in a conspicuous improvement in pyrolysis yield from the Grignon sample, as also reflected by Rock-Eval data. Molecular level studies were therefore performed so as to examine whether these quantitative changes are associated with changes in molecular composition of pyrolysates. The above mentioned total losses for the untreated and HF-treated samples were directly obtained by weighing and they correspond to the total amount of pyrolysis products swept away by the helium flow. Such products comprise high volatility organic compounds (not trapped in cold or lost during CHCl3 elimination under vacuum) and medium volatility organic compounds (corresponding to the trapped pyrolysate). As shown in a number of previous studies concerned with organic matter in sedimentary rocks and recent sediments,

Table 4 Amount of pyrolysis products generated upon 300 and 400 ◦ C heating of the untreated sample (<50 ␮m fraction) and 2% HF-treated sample Total lossa

Trapped productsb

Volatile productsc

Untreated

300 ◦ C 400 ◦ C

3.3 8.6

0.8 2.5

2.5 6.1

Treated

300 ◦ C 400 ◦ C

11.8 17.3

5.4 5.8

6.4 11.5

As wt.% of the organic carbon of the unheated material. a corresponding to volatile and trapped products. b products swept away by the He flow and trapped into a cold chloroform solution. c products too volatile to be trapped into the chloroform solution, determined by subtraction.

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Fig. 1. Total ionic current trace of the ‘untr/300’ pyrolysate showing a large hump of co-eluting peaks. The individual peaks chiefly correspond to n-alkanes and Cx refers to their total number of carbon atoms).

the molecular information that can be obtained through pyrolysis of complex mixtures of organic geomacromolecules and GC–MS analysis of pyrolysis effluents is largely contained in trapped pyrolysates [22]. In contrast the volatile compounds, even if they dominate the effluents, only afford more limited information. Indeed, such low molecular weight compounds, like CO2 , H2 S and short hydrocarbons, are ubiquitous pyrolysis products that cannot be related to specific moieties in the studied macromolecular structures. Accordingly, detailed molecular studies, via GC–MS analyses, were performed on the trapped pyrolysates obtained at 300 and 400 ◦ C from the untreated samples (termed ‘untr/300’ and ‘untr/400’, respectively) and from the HF-treated samples (termed ‘tr/300’ and ‘tr/400’, respectively). GC–MS analysis of the four pyrolysates showed complex compositions and the GC traces exhibit humps due to the occurrence of numerous co-eluting components, especially for the ‘untr/300’ pyrolysate as illustrated in Fig. 1. The main series in these complex mixtures were analysed through the detection of typical ions, so as to derive information on their carbon number range and distribution. Such features and the comparison between the four pyrolysates are discussed below for these series. 3.2.2. Molecular studies 3.2.2.1. Aliphatic hydrocarbons. The main hydrocarbons identified in the four pyrolysates correspond to an homologous series of n-alkanes. Such aliphatic hydrocarbons were previously identified in the pyrolysates of a number of samples, including SOM [2,18], humin [3] and residues from chemical degradation of humic acids and SOM [15,31–35]. The distribution of the n-alkanes released from the Grignon samples was determined by selective ion detection (m/z = 57). Two distinct origins, reflected by sharp differences in distribution, can be considered for such compounds.

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– Firstly, they may occur as such in the samples and be released by thermovaporization upon heating. Indeed, the presence of long chain n-alkanes is well documented in higher plant waxes. These hydrocarbons occur in the C23 –C33 range, they are characterized by a pronounced odd-carbon-number predominance and usually exhibit a maximum at C27 , C29 or C31 [36,37]. Moreover, isotope and molecular evidence for direct input in crop soils of n-alkanes from leaf waxes was obtained via extract study [38]. Alteration of these hydrocarbons in soil should result in chain shortening and lowering in the predominance of the odd-carbon-numbered compounds. However, as observed in a number of sediments [39–41]] their typical distribution, i.e. a substantial contribution of long chain compounds and odd-carbon-number predominance in the C23 –C33 range, should be partly retained. – Secondly, in the case of the experiments at 400 ◦ C, n-alkanes should be produced, along with n-alkenes, via thermal cleavage of alkyl chains. Cracking of carbon–carbon bonds in alkyl chains generates primary radicals which undergo extensive rearrangements and chain shortening reactions before being stabilized by H• elimination or addition, resulting in alkene and alkane formation, respectively. The series of hydrocarbons thus generated are characterized by a regular decrease in intensity with increasing carbon number and a maximum carbon number equal to the length of the cracked chain. When off-line pyrolyses are performed, volatile hydrocarbons below C14 are lost during trapping and solvent removal. The series of n-alkanes and n-alkenes, originating from alkyl chain cracking, detected in this type of experiments therefore, exhibit a unimodal distribution with a maximum around C16 –C18 followed by a regular decrease in intensity. This type of distribution has been observed, via off-line experiments at 400 ◦ C, for the organic matter of a number of sedimentary rocks [22]. No series of n-alkenes was identified in the ‘tr/300’ pyrolysate and the series of n-alkanes ranges from C14 to C32 (Fig. 2a). There is no clear maximum for these thermovaporized hydrocarbons but a significant odd-carbon number predominance can be noted for the longest compounds (C28 –C32 range). The above features suggested that the bulk of these n-alkanes corresponds to partly altered hydrocarbons originating from higher plant waxes. No series of n-alkenes was also identified in the ‘untr/300’ pyrolysate. n-Alkane distribution in this pyrolysate exhibits a maximum at C17 , a rather regular decrease in intensity from C17 to C32 and low contribution of long chain compounds (Fig. 2b). The pronounced differences thus observed for these thermovaporized hydrocarbons, when compared to the treated sample (Fig. 2a), probably reflect retention by the mineral matrix during the thermal treatment. For apolar compounds, like n-alkanes, the extent of this retention should increase with molecular weight. Indeed, it was previously observed [30], for pyrolysis of sedimentary rocks, that long chain (C15 +) n-alkanes undergo preferential retention in sediment pores. Similarly, in the present study, long chain n-alkanes were preferentially retained by the mineral matrix during heating at 300 ◦ C of the untreated sample. As discussed below, such a selective retention was also supported by the composition of the pyrolysates at 400 ◦ C. The ‘untr/400’ pyrolysate exhibits a bimodal distribution of n-alkanes with a weak submaximum around C18 –C20 and a maximum at C31 . In addition, a substantial predominance of the odd-carbon-numbered n-alkanes is observed in the C28 –C32 range (Fig. 2c). A series of n-alkenes with a low relative intensity is detected. These features reflect the double origin of the n-alkanes in the ‘untr/400’ pyrolysate: (i) the thermovaporization of long chain,

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Fig. 2. Mass chromatograms (m/z = 57) showing n-alkane distribution in the 300 ◦ C pyrolysates of the 2% HF-treated (a) and untreated (b) samples and in the 400 ◦ C pyrolysates of the untreated (c) and 2% HF-treated (d) samples. Cx refers to the total number of carbon atoms. Other compounds exhibiting fragments at m/z = 57, such as isoprenoid and branched alkanes, are also observed on the traces.

predominantly odd-carbon-numbered, hydrocarbons (freed at 400 ◦ C but not released during previous heating at 300 ◦ C due, as discussed above, to selective retention by the mineral matrix at the latter temperature); and (ii) pyrolysis products originating from the cleavage of alkyl chains. In contrast, the ‘tr/400’ pyrolysate shows: (i) n-alkane/n-alkene doublets with similar intensities; and (ii) a unimodal distribution with a maximum around C17 –C19 , a low contribution of long chain compounds and no odd–even predominance (Fig. 2d). This latter distribution corresponds to the n-alkanes generated through the cracking of the alkyl chains. Large differences are therefore observed, regarding n-alkane distribution, between the pyrolysates at 300 and 400 ◦ C of the HF-treated and untreated samples. These differences

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should largely reflect the high retentive effects of the mineral matrix in the latter sample. Retention in the matrix therefore appears as an important process, even in the case of such apolar products, for long chain alkanes during heating at 300 ◦ C. The differences observed between the four pyrolysates can be accounted for by thermovaporization and chain cracking along with selective retention in the mineral matrix of the untreated sample. Accordingly, it seems that no substantial alteration, that would have been reflected by n-alkane distribution, took place, either for the free hydrocarbons or for the alkyl chain in macromolecular structures, during the 2% HF treatment. 3.2.2.2. Aromatic hydrocarbons. This type of compounds were previously observed in pyrolysates of humin and humic acids from soils [18]. The occurrence of aromatic hydrocarbons was also noted in the pyrolysates of the refractory organic fraction isolated from recent marine sediments [19] and in a number of pyrolytic studies concerned with organic matter in sedimentary rocks [22,42,43]. However, as stressed in the above studies, it is difficult to determine whether they are related to aromatic moieties that would pre-exist in the materials under examination and/or correspond to compounds secondarily formed upon heating via cyclization–aromatization of linear chains [22,44,45]. Moreover, contrary to n-alkanes, such aromatic hydrocarbons seem to be absent in living organisms. In fact, diagenetic cyclization–aromatization of fatty acids is generally considered as the source of the alkyl aromatic moieties in sedimentary organic matter [45,46]. The pyrolysates at 300 ◦ C of the untreated and treated samples do not contain significant amounts of alkylbenzenes. Therefore, as expected, contrary to n-alkanes, these aromatic hydrocarbons do not occur as such, as free compounds, in the Grignon samples. Alkylbenzenes were generated upon pyrolysis at 400 ◦ C but they only exhibit low relative abundances, especially in the case of the untreated sample. C4 –C23 n-alkylbenzenes (maximum at C8 ) and C5 –C17 n-alkylmethylbenzenes (maximum at C8 ) were identified in the ‘tr/400’ pyrolysate via selective ion detection at m/z = 91 and 105, respectively (Fig. 3). The ‘tr/400’ pyrolysate also comprises n-alkenylbenzenes (C6 –C23 , maximum at C8 ). For the ‘untr/400’ pyrolysate, only a series of C10 –C18 n-alkylbenzenes could be clearly identified. The lower abundance of the alkylbenzenes in the ‘untr/400’ pyrolysate, when compared to the ‘tr/400’ one, shall reflect some retention of these weakly polar compounds in the mineral matrix. 3.2.2.3. Alkanoic acids. A large part of the alkanoic acids in living organisms and natural samples is esterified in various structures. These ester links exhibit a relatively low thermal stability and the free radicals that result from their cracking (RCOO• ) are stabilized by H• addition to produce free acids. Alkanoic acids were thus previously observed in pyrolysates of humin and humic acids from various soils [2,3], insoluble and non-hydrolysable fractions isolated from recent sediments [19] and fossil organic matter [22,47]. Biological alkanoic acids are commonly dominated by even-carbon-numbered, n-saturated, fatty acids with a maximum at C16 or C18 and substantial amounts of C16 and C18 unsaturated fatty acids are also generally observed [48]. Alteration of fatty acids is reflected by: (i) some lowering in the predominance of the even-carbon-numbered homologues; and (ii) the selective degradation of the unsaturated compounds which are often entirely eliminated [49–51]. C15 and C17 branched acids (iso and anteiso) are also commonly detected in pyrolysates and considered

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Fig. 3. Mass chromatograms (m/z = 91+105) showing n-alkylbenzene distribution in the 400 ◦ C pyrolysates of the 2% HF-treated samples. n-Alkenylbenzenes were also detected as shown in the inset. Cx refers to the number of carbon atoms in the n-alkyl chain.

as biomarkers of bacteria [52]. The presence of alkanoic acids in the four pyrolysates was examined by selective ion detection (m/z = 73). The acids of the ‘tr/300’ pyrolysate (Fig. 4a) are dominated by n-saturated C8 –C18 fatty acids whereas no unsaturated counterparts were detected. This series of saturated fatty acids exhibits a maximum at C16 and a pronounced even-carbon-number predominance. Substantial contributions of C15 –C17 , iso and anteiso, branched acids (abundance of ca. 1/4 compared to the predominant C16 n-saturated acid) were also observed. The above features therefore indicate the presence of partly altered fatty acids and of branched acids of bacterial origin. In agreement with the relatively low thermal stability of ester functions, these acids probably originate from the cracking of ester links in addition to the thermovaporization of free compounds. In contrast, the analysis of the ‘untr/300’ pyrolysate only showed very low relative amounts of C8 –C10 n-saturated fatty acids with a maximum at C9 . It therefore, appears that alkanoic acids, as expected for such high polarity compounds, are strongly retained by the mineral matrix. Accordingly, only minor relative amounts of short chain acids were released through heating at 300 ◦ C of the untreated sample. The presence of acids in the ‘tr/400’ pyrolysate (Fig. 4b) indicates that ester bonds were not entirely cleaved during the previous heating at 300 ◦ C of the HF-treated sample. These

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Fig. 4. Distribution of the saturated fatty acids in the 300 ◦ C pyrolysate of the 2% HF-treated (a) sample and in the 400 ◦ C pyrolysates of the 2% HF-treated (b) and untreated (c) samples. Cx refers to the total number of carbon atoms of the acids.

additional acids exhibit similar features to those mentioned above for the ‘tr/300’ experiment: major contribution of, predominantly even-carbon-numbered, C8 –C18 n-saturated fatty acids with a maximum at C16 , presence of C15 –C17 iso and anteiso acids. The ‘untr/400’ pyrolysate (Fig. 4c) contains alkanoic acids with similar distribution as noted above for the treated material. Accordingly, it appears that: (i) the extensive retention

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Table 5 Main methoxyphenols identified in the 300 ◦ C pyrolysate of the 2% HF-treated sample Main MS fragments

Structure

Type of basic unita

109, 124, 81 123, 138, 95 150, 135, 77, 107 154, 139 164, 77, 103, 149 137, 166 164, 77, 103, 149 168, 153, 125 151, 166 167, 182 137, 180 180, 165, 137 194, 119 167, 210 181, 196

2-Methoxyphenol 2-Methoxy-4-methylphenol 2-Methoxy-4-vinylphenol 2,6-Dimethoxyphenol 2-Methoxy-4-(2-propenyl)-phenol 2-Methoxy-4-propylphenol 2-Methoxy-4-(1-propenyl)-phenol 2,6-Dimethoxy-4-methylphenol 1-(4-Hydroxy-3-methoxyphenyl)-ethanone 4-Ethyl-2,6-dimethoxyphenol 1-(4-Hydroxy-3-methoxyphenyl)-2-propanone 2,6-Dimethoxy-4-vinylphenol 4-allyl-2,6-dimethoxyphenol 2,6-Dimethoxy-4-propenylphenol 1-(4-Hydroxy-3,5-dimethoxyphenyl)-ethanone

G G G S G G G S G S G S S S S

a

G = guaiacyl (2-methoxyphenol basic structure); S = syringyl (2,6-dimethoxyphenol basic structure).

in the mineral matrix observed at 300 was overcome at 400 ◦ C; and (ii) the 2% HF treatment did not largely affect the fatty acid moieties. 3.2.2.4. Phenolic compounds. Numerous phenolic compounds were previously observed in pyrolysates of SOM [15] and of humin and humic acids from various soils [2,3,53]. Phenol, C1 –C3 alkylphenols and methoxyphenols were identified in the ‘tr/300’ pyrolysate (Table 5). The latter comprise mono- and dimethoxy compounds and some are substituted by short alkyl chains up to C3 . The production upon pyrolysis of such alkylphenols and methoxyphenols with short alkyl substituents (
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therefore indicated that lignin derivatives are unlikely to be a significant source for these phenolics. A similar distribution of alkylphenols was previously observed in the pyrolysates of SOM [15], dissolved and particulate marine organic matter [57–59] and refractory organic matter from recent marine sediments [19]. Such pyrolysis products were considered as derived from melanoidins [57]. Accordingly, the phenolic products released via heating at 400 ◦ C of the HF-treated sample should mostly originate from the cracking of such components. It therefore, appears that lignin was extensively cracked during the first heating at 300 ◦ C whereas the thermal cleavage of melanoidin units chiefly took place at 400 ◦ C. This relatively low thermal stability of lignin moieties is consistent with the abundance of monoaromatic rings in lignin structure, lignin monomeric units being based on a C3 -alkylphenyl skeleton. Lignin components therefore comprise a number of thermally weak bonds, like benzylic bonds, since the generated radicals are conjugated with the aromatic ring. In the case of melanoidins, the level of aromatic rings is lower and a higher resistance to thermal stress is also promoted by a high level of cross-linking. No aliphatic alcohols were observed in the four pyrolysates. For the untreated soil, this may reflect alkanol dehydration catalysed by clay minerals, as previously observed [60] via flash pyrolysis at 610 ◦ C of alkanol/clay mixtures. The lack of alkanols in the pyrolysates of the treated sample may reflect dehydration during the acid treatment. 3.2.2.5. Organic sulphur compounds (OSCs). Several minor series of OSCs were identified in the ‘untr/300’ pyrolysate. These OSCs are mainly comprised of C8 –C17 n-alkylthiophenes, C1 –C3 alkylbenzothiophenes and C2 –C12 n-alkylthiolane. In recent sediments and sedimentary rocks, such compounds are typical pyrolysis products of OM fractions formed via the so-called ‘natural sulphurization’ pathway [61]. This pathway is based on intra- and intermolecular incorporation, in functionalized lipids, of reduced sulphur species produced by anaerobic sulphate-reducing bacteria [62–64]. Such bacteria may occur in anaerobic micro-environments in soils, thus accounting for the low amount of OSCs found in the ‘untr/300’ pyrolysis products. In contrast, no OSCs were detected in the ‘untr/400’, ‘tr/300’ and ‘tr/400’ pyrolysates. The pronounced difference thus observed between the ‘untr/300’ and the ‘untr/400’ pyrolysates, with respect to OSC occurrence, indicates that the low amount of sulphur-containing units occurring in the untreated sample corresponds to thermolabile material. The latter feature is in agreement with the low thermal stability well documented for sulphur-containing groups [62,65–67]. In addition, this difference is consistent with the low polarity and the relatively low molecular weight of the generated OSCs and hence their negligible retention by the mineral matrix upon heating at 300 ◦ C. The lack of OSC in the ‘tr/300’ pyrolysate indicates that the 2% HF treatment resulted in the alteration of the minor sulphur-containing moieties in the SOM of the Grignon sample.

4. Conclusion The main results obtained via the comparative study of the untreated and 2% HF-treated samples from the humified fraction of a crop soil are as summarized below. This comparison, based on pyrolytic methods, provided information on: (i) the retention of the main families of pyrolysis products in the mineral matrix of the untreated sample; (ii) possible biases

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related to such retention in the interpretation of the pyrolysis results; and (iii) the influence of the 2% HF treatment on the humified organic matter of the Grignon soil. – Efficient removal of mineral components (ca. 93 wt.%) was achieved by this relatively mild acid treatment whereas a limited loss (ca. 17 wt.%) was observed for TOC. – Extensive retention of pyrolysis effluents by the mineral matrix in the untreated sample was reflected by indexes derived from Rock-Eval pyrolysis and by the yields of pyrolysis products observed upon off-line pyrolyses. Information at a molecular level on such a retention was obtained via GC–MS analysis of off-line pyrolysates at 300 and 400 ◦ C. – For the apolar compounds (n-alkanes) this retention increases with the molecular weight. The release of long chain n-alkanes is thus affected during heating at 300 ◦ C but this retentive effect is overcome at 400 ◦ C. – For the polar compounds (OSC, alkylbenzenes, alkanoic acids and phenolics) retention by the mineral matrix increases, as expected, with polarity. Thus: (i) only a negligible to limited retention was noted for the two former families at 300 ◦ C; (ii) alkanoic acids are strongly retained by the matrix at 300 and released at 400 ◦ C; and (iii) phenolic compounds are totally retained at both temperatures. – The compounds which occurred as such in the samples before heating and those generated by cracking could be distinguished by comparison of the four pyrolysates and, also, the phenolics derived from lignin and melanoidin moieties. – Direct pyrolysis of the untreated sample would have afforded highly biased quantitative and qualitative information, especially on phenolic components. Some false conclusions would have been derived from such a pyrolysis and the presence of lignin and melanoidin components would have passed unnoticed. – The organic matter loss due to the 2% HF treatment should mostly reflect the hydrolysis of labile proteinaceous and polysaccharidic components into water soluble products. The minor sulphur-containing moieties that occur in the humified organic matter of the Grignon soil are also affected by the treatment. In contrast, alkane and fatty acid distributions suggest that the corresponding moieties in this organic matter probably remained largely unaffected.

Acknowledgements This study was supported by the ‘Programme National Sol et Erosion’ (INSU-CNRS). We thank Yves Pouet for technical assistance in GC–MS analyses.

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