Biochar: contaminant source or sink?

Biochar is a promising organic recycling material to be used as a soil conditioner and feed supplement. However, it may contain pollutants originating from feedstock, or formed during pyrolysis. The latter origin is eminent in the case of polycyclic aromatic hydrocarbons (PAHs). Inherent inorganic contaminants in biochars are unlikely and studies on this topic are rare. Concentrations of metals are probably similar to those in other recycling fertilizers, depending on the input material. Conversely, pyrolysis of organic matter produces condensed carbonaceous structures which strongly adsorb organic pollutants such as PAHs, pesticides, polychlorinated biphenyls, etc. Biochars are also used to immobilize heavy metal pollutions in soils. Carboxylized biochars are employed to enhance ionic interactions with the pollutants to avoid leaching into (ground) waters. The raise of pore water pH in biochar amended soil in contrast mobilizes anionic contaminants such as antimony or arsenic which exhibit mostly negative speciation in such alkaline environments. All in all, biochar is probably both, a source and sink of contaminants.

Organic pollutants, in particular polycyclic aromatic hydrocarbons (PAHs), may be produced during biochar production. These PAHs will enter the environment when the biochar is deployed, thereby potentially undermining its positive effects. The overall aim of biochar producers is to reduce PAH concentrations towards or below the threshold of 4 - 12 mg/kgdw for the sum of the 16 US EPA PAH for premium and basic grade biochar, respectively, as established by the European Biochar Certificate. To date there is only limited reliable data available on PAH residues in biochars, and influential (post-) pyrolysis process parameters have hardly been investigated systematically. Here, we present total PAH concentrations, as accounted for in legislation, produced from four different feedstocks. However, only a part of the PAHs in the biochar is available for organisms, and therefore, the biovavailable PAH concentrations were determined as well.

Besides organic contaminants, biochars may contain non-essential (toxic) elements such as cadmium (Cd), arsenic (As), lead (Pb), etc., or polluting levels of essential elements such as cupper (Cu), zinc, (Zn), etc. In contrast to organic contaminants formed during pyrolysis, heavy metals are inherent in the feedstock and remain in the organic material during pyrolysis. However, some metal can also be found in biochar as a result of friction during pyrolysis such as chromium (Cr) or nickel (Ni). Studies on metal contamination of biochars are rare. Along with the PAHs we show Ni, Cr, and Pb as a representative of heavy metal pollution in the biochars.

Copyright:	© HAWK Hochschule für angewandte Wissenschaft und Kunst - Fakultät Ressourcenmanagement
Quelle:	73. Symposium 2012 (Oktober 2012)
Seiten:	9
Preis:	€ 0,00
Autor:	Dr. Isabel Hilber Franziska Blum Sarah Hale Gerard Cornelissen Hans-Peter Schmidt

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