Black carbon in soils: The use of benzene polycarboxylic acids as specific indicators

Bruno Glaser, Ludwig Haumaier, Georg Guggenberger and Wolfgang Zech

Institute of Soil Science, University of Bayreuth, 95440 Germany. Fax: ++49-921-552246. Email: bruno.glaser@uni-bayreuth.de

Mitteilungen der Deutschen Bodenkundlichen Gesellschaft (1997) 85: 237 - 240

 

  1. Introduction

    2. Black carbon (BC) is formed during incomplete combustion of biomass. Being highly resistant to microbial and chemical oxidation, it is considered to be a significant sink in the global carbon cycle. Most likely, it is also a source of stable aromatic carbon in soils.

    A simple method is presented for the estimation of black carbon (BC) in soil samples. Our method uses benzene polycarboxylic acids (BPCA) as specific indicators for BC, since these compounds long have been known to be formed upon oxidation of coal or carbonized organic materials. The analytical procedure includes hydrolysis, oxidation, sample cleanup, derivatization, and gas liquid chromatography. To test the usefulness of BPCA as marker for BC, we compared the yield of BPCA from charred wood with that from "humic substances" produced in the laboratory by enzymatic browning, non-enzymatic browning and aerobic incubation of barley straw.

     

  2. Materials

    3. We studied the oxidation behavior of perylen, a condensed aromatic PAH produced by incomplete burning processes. Pinus silvestris wood was charred at 300 °C at increasing time. The Maillard reaction was simulated by incubation of an aqueous mixture of fructose and glycine at 105 °C. The enzymatic browning was modeled by aerobic incubation of Malus domestica with a high polyphenol content. Bacterial and fungal degradation reactions took place due to aerobic incubation of Hordeum vulgare with pure bacterial and fungal colonies, respectively. For method optimization we used soil samples enriched in charcoal and their native counterparts. We quantified BC with the aid of activated charcoal as a reference substance.

     

  3. Method

    4. Samples were hydrolyzed very rigorously (conc HCl 170 °C, high pressure digestion apparatus) prior to oxidation in order to remove polyvalent cations such as Fe3+ and Al3+. Then the hydrolyzed soil was digested with 65 % HNO3 at 170 °C in a high pressure digestion apparatus, where SOM was oxidized and soil aromatic carbon (SAC) was converted to BPCA. A subsequently cation exchange resin cleanup was necessary to remove polyvalent cations again. BPCA could be separated with GLC after derivatization with BSTFA. All steps have been optimized, especially the critical hydrolysis and oxidation steps.

     

  4. Results and discussion

    5. Oxidative degradation of perylen as a model substance for coalified material gave only mellitic acid and hemimellitic acid in a mass ratio of 9 to 1 (Fig. 1). Chemical structures of the analyzed BPCA can be seen from Fig. 2 and a FID chromatogram of these compounds in Fig. 3. Table 1 informs about the concentrations of individual BPCA, the sum of all BPCA with 3 to 6 carboxylic groups and the BC concentration refered to activated charcaol.

    Fig. 1. Oxidative degradation of perylen as a model substance for coalified material gave only mellitic acid and hemimellitic acid in a mass ratio of 9 to 1.

     

    Fig. 2. Structures of the analyzed BPCA.

     

    Fig. 3. GLC-FID chromatogram of a BPCA mixture, citric acid and 2,2´-biphenyl dicarboxylic acid. 1 benzoic acid, 2 phthalic acid, 3 isophthalic acid, 4 citric acid, 5 terephthalic acid, 6 hemimellitic acid, 7 2,2´-biphenyl dicarboxlyic acid, 8 trimellitic acid, 9 trimesic acid, 10 pyromellitic acid, 13 benzene pentacarboxylic acid, 14 mellitic acid.

    Fig. 4. GLC-FID chromatogram of the Aric Anthrosol after nitric acid digestion. 4 citric acid (internal standard), 6 hemimellitic acid, 7 2,2´-biphenyl dicarboxylic acid (recovery standard), 8 trimellitic acid, 9 pyromellitic acid, 10 mellophanic acid, 11 prehnitic acid, 13 benzene pentacarboxylic acid, 14 mellitic acid.

    Fig. 5. GLC-FID chromatogram of the enyzmatic browning. 1 benzoic acid, 4 citric acid, 7 2,2´-biphenyl dicarboxylic acid.

     

    Table 1. BPCA yield of the humification reactions, charred wood, and the soil samples

    soil

    hemimellitic

    trimellitic

    trimesic

    pyromellitic

    mellophanic

    prehnitic

    b5ca

    mellitic

    sum b3ca

    -b6ca

    BC

    ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ [ mg C / g C ]¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾ ¾

    bacterial incub

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    fungal incub

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    enzymatic browning

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    Maillard

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

    n.d.

     

    n.d.

    n.d.

    n.d.

    charred wood

    (240 min/300°C)

    6.2

    20.2

    0.6

    22.1

    25.6

    35.2

    74.4

    73.2

    257.4

    585

    Aric Anthrosol

    2.2

    3.1

    0.5

    8.0

    7.2

    13.8

    34.5

    38.1

    107.4

    244

    Ferralsol

    n.d.

    n.d.

    n.d.

    2.3

    2.8

    3.1

    9.6

    6.5

    24.3

    55

    Fimic Anthrosol

    0.9

    1.8

    n.d.

    4.2

    5.3

    7.1

    25.0

    24.5

    68.8

    156

    Cambisol

    n.d.

    n.d.

    n.d.

    traces

    traces

    traces

    1.3

    2.3

    3.6

    8

     

    n.d. not detectable

  5. Conclusions

Nitric acid oxidation of charred material released BPCA. These acids have been found exclusively after charring of biomass but not in the "humic substances" produced in the laboratory. Therefore we conclude that BPCA are indicators specific for charring or incomplete burning processes.