José Maria De La Rosa: Assessment of Polycyclic Aromatic Hydrocarbons (PAHs) in biochar-amended Mediterranean agricultural soils

STSM at Agroscope Research Station in Zurich, Switzerland in August 2013

Under the guidance of Dr Thomas Bucheli, José conducted the first experiment dealing with total and bioavailable concentrations of the PAHs released from biochars incorporated into Mediterranean agricultural soils. José and Thomas found that grapevine wood Biochar can be problematic in terms of PAH relase. The STSM initiated strong collaborations between the host and the home institution and will result in a joint publication.

STSM Grantee

Jose María De La Rosa Arranz (fig.1)

Instituto de Recursos Naturales y Agrobiología de Sevilla IRNAS-CSIC (Seville, Spain)


Figure 1


Dr Thomas Bucheli

Agroscope Reckenholz-Tänikon Research Station ART (Zurich, Switzerland)


Highly condensed aromatic structures, including Polycyclic Aromatic Hydrocarbons (PAHs), are formed during biochar production due to incomplete combustion (pyrolysis step). Which compounds and to what extend such compounds are formed is greatly influenced by the process and its conditions. By now, there is no unique standard protocol to determine the total and bioavailable concentrations of the PAHs released from biochars. Additionally, up to now there is no publication concerning the effect of biochar amendment in Mediterranean agricultural soils.

For this study, four biochars produced from different feedstocks (including wood, pulp and paper sludge, sewage sludge and  grapevine wood) and a Luvisol chromic soil amended with the same biochars (previously incubated during 75 days) were be analysed. The total content of PAHs for each biochar and amended soil was analyzed and gave insight into the translocation potential and the distribution pattern of those compounds.


Figure 2: Incubation pots amended with Biochar

Materials and Methods

Four biochars produced from different feedstocks (including wood, pulp and paper sludge, sewage sludge and grapevine wood) were assessed. Luvisol chromic soils amended with 10, 20 and 40 T per ha of these biochars previously incubated during 79 days were used for this study. For comparative purposes, control soil without amendment was also incubated and analysed.


Figure 3: Milled soil and Biochar samples

The work carried out can basically be divided into five steps:

  1. Homogenisation of samples
  2. Sohxlet extraction.
  3. Concentration of the extracts.
  4. Clean-up of the biochar amended soils
  5. Separation of PAHs and determination by GC-MS


Figure 4: Soxhlet extraction apparatus

Main results

Solvent test with Hexane did not resulted in greater amounts of extracted PAHs. In fact the Toluene extractives resulted enriched in low weight PAHs such as Acenaphthylene and Fluorene. Thus, the use of Toluene was considered to the most suitable for both kind of samples (bulk biochars and biochar amended soils).

Biochar 4, produced from grapevine wood, resulted in the greatest amounts of PAHs (Σ16 EPA PAH). Biochars 1, 2 and 3, derived from wood, paper and sewage sludge presented concentrations of PAHs considerably lower. The maximum allowed threshold value for PAHs according to the International Biochar Initiative is 6,000 µg kgdw -1 for the 16 EPA PAHs. Thus biochar 4 is not valid for agricultural purposes.

The Andalusian Luvisol control soil used for the biochar amendment experiment presented values for the 16 EPA PAHs typically reported for non-polluted soils from rural areas.

The biochar addition resulted in a significant increase of the amounts of PAHs. However, preliminary results of samples corresponding to the amendment of 10 and 20 t ha-1 suggests that greater addition of biochar did not cause significant increases in the total amounts of PAHs. Analysis of soil samples amended with 40 t ha-1 is still in progress, these results will be essential to determine this finding.

The obtained concentrations were compared to those given as a threshold by European laws (CEN 15527: 2008 for biochar).  This approach is expected to reveal some insights into the behaviour of PAHs pollutants in biochar amended soils, which may help to improve predictions, to develop appropriate managing strategies as well as to avoid hazardous impacts.

Future collaboration with the host institution

The short stay at the laboratory of D. T. Bucheli at the Research Station Agroscope Reckenholz in Zurich was a great opportunity for José. With this stay he has initiated research collaboration between his group at IRNAS-CSIC (Spain) and Dr Thomas Bucheli’s group at the Research Station Agroscope Reckenholz (Switzerland). In fact, both groups intend to carry out the assessment of PAHs in biochar amended soils incubated with microorganisms in a controlled environment in the near future. The incubation experiment of biochars with microorganisms is actually in progress at the Respicond apparatus (IRNAS-CSIC). In November 2013 the colleagues participated in an exploratory workshop in Seville and discussed the experiment as well the analysis to be developed in greater detail.

Foreseen publications resulting from the STSM

It is planned to submit a joint manuscript devoted to the assessment of PAHs in biochar amended soils to a Q1 peer reviewed international journal before the end of 2013.

Many thanks to Dr Isabel Hilber and Franziska Blum for their support in the laboratory and to Dr Thomas Bucheli for his comments and support.


Bucheli, T., Blum, F., Desaules, E., Gustafsson, Ö., (2004). Polycyclic aromatic hydrocarbons, black carbon, and molecular markers in soils of Switzerland. Chemosphere 56, 1061–1076.

Hilber, I., Blum, F., Leifeld, J., Schmidt, H.P., Bucheli, T., (2012). Quantitative Determination of PAHs in Biochar: A Prerequisite To Ensure Its Quality and Safe Application. Journal of Agriculture and Food Chemistry 60, 3042–3050.

IUSS Working Group WRB, 2007. World Reference Base for Soil Resources, first update 2007. FAO, Rome.

Ledesma, E. B., Marsh, N. D., Sandrowitz, A. K., Wornat, M. J., (2002). Global kinetic rate parameters for the formation of polycyclic aromatic hydrocarbons from the pyrolysis of catechol, a model compound representative of solid fuel moieties. Energy Fuels 16, 1331–1336.