Analytical characterisation and PAH asessment of four biochars and a biochar amended cambisol from Southern Spain

Abstract

Producing biochar from a waste material can be classified as a waste recovery operation, but more intense research in the fields has only recently started. Therefore, biochar products are at a very early stage of development and their regulatory and testing requirements are liable to change. In April 2013, Switzerland has become the first country of Europe to officially approve the use of certified biochar in agriculture. Hungary also approved its use in 2012, however, the certification rules are still in progress in this case. Other nations are developing policies and strengthen the standing and potential funding for biochar projects (special mention in this particular case to New Zealand, Australia, USA and UK). Presently, the EU COST Action 'Biochar as option for sustainable resource management' is performing several experiments devoted to discern the properties and characteristics of a biochar before being officially approved as soil ameliorant. On the other hand, it is well known that polycyclic aromatic hydrocarbons (PAHs) are formed during combustion and pyrolysis processes, and as a consequence are likely to be components within biochar. PAHs form adducts with DNA and have, as a consequence, been prioritized by the US EPA and EU on account of their carcinogenetic, mutagenic and teratogenic properties. Consequently, their presence in biochars and biochar amended soils need to be assessed. In this study, we present an in-detail characterization of four biochars produced from different feedstock under different conditions. Biochar 1 to 3 (1-wood, 2-paper-sludge, 3-sewage sludge) were provided within a ring trail of the EU Biochar COST Action (http://www.cost.eu/domains_actions/fa/Actions/TD1107). Biochar 4 was produced by traditional char production method from grapevine wood by a Spanish winery. Characterization was performed by elemental analysis, microscopy and the determination of chemical and physical properties such as, pH, electrical conductivity (EC), ash content and water holding capacity (WHC). Fourier-Transform Infrared Spectroscopy (FT-IR) and 13C solid-state NMR techniques were also applied to elucidate the molecular structure and main chemical groups, whereas field emission scanning electron microscopy (FE-SEM) is a valuable research tool that was used to investigate the surface topography and chemical composition of biochars. Finally, the abundance of PAHs was determined in the biochar and in a Cambisol from SW Spain amended with 10, 20 and 40 t ha-1 biochar and subjected to plant growth pot experiments for 80 days.

Biochar 1, 2 and 4 revealed comparable elemental composition (C, H, N). In addition, values of pH, WHC and ash content were analogous (10.3-10.4, 178-266 and 7-25% respectively). Biochar 3, exhibited the lowest C (18%) and highest N (2%) contents, whereas physical properties were drastically different (pH= 6.7; WHC= 26.7; ash content~70%). The H/C and O/C atomic ratios suggested a generally high aromaticity for all the biochars, which was confirmed by the 13C solid-state NMR spectroscopy. However, Biochar 1 could not be elucidated by NMR most likely due to its graphenic properites, the latter were confirmed by IR spectroscopy. For wood-derived biochars, the IR spectra indicated the presence of lignin structures. The FE-SEM allowed distinguishing not only compositional but structural differences of the studied biochars. For instance, it showed the presence of metal rich crystals on the surface of wood biochar (biochar 1), collapsed structures for paper sludge biochar (biochar 2), mineral phases (rich in Al, Si, Ca and Fe) and organic residues for sewage sludge biochar (biochar 3) and etched surfaces for vineyard wood biochar (biochar 4) respectively. Biochars 1, 2 and 3, revealed PAH concentrations of approximately 2900, 1600 and 650 μg kg dw-1 respectively, whereas biochar 4 contained the highest amount of PAHs (Σ16 EPA PAH), reaching about 16000 μg kgdw-1, which is considerably above the maximum allowed threshold for PAHs (6000 μg kg dw-1 for the 16 EPA PAHs according to the International Biochar Initiative). Thus biochar 4 should not be used for agricultural purposes. Taking into account that biochar 4 was the only sample produced by the traditional kiln methods, the biochar production process seemed to affect significantly the PAHs levels. The Andalusian Cambisol used for the biochar amendment experiment showed values for the 16 EPA PAHs which are 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 indicated that the increase did not correlate with the biochar loads applied.