Wolfram Buss: organic contaminants in biochar and hydrochar

STSM at the Karlsruher Institut für Technology (KIT) and the University of Hohenheim in June 2013

The PhD topic of Wolfram Buss deals with contaminant issues in biochar and involves investigation of heavy metals, PAHs and other organic contaminants in biochar. He started with investigating mobile organic compounds in biochar that are under normal circumstances only produced in small quantities in advanced pyrolysis facilities. This work is important to understand to what degree unnoticed problems during production (re-condensation) can contaminate biochar.

After having performed different phytotoxicity studies, he focused on the causes of toxicity and how they can be mitigated (make use of contaminated char). To gain further insights into the nature of these compounds he needed to analyze their composition. Due to lack of analysis facilities like GC-MS, GC-FID and similar equipment in our research group he went on this STSM to Germany. The aim of this trip to the “Karlsruher Institut für Technology” (KIT) and the University of Hohenheim was to extract biochar samples and analyze the composition of organics contaminants.

Host: Prof. Andrea Kruse, University of Hohenheim

Description of the work and main results

Overhead-shaker: water extractable organic compounds

Biochar samples were crushed and extracted in overhead shakers for 20h at a constant turning rate.

Phenol index

Using a tool kit (LCK 346 phenol test) and a spectrophotometer the phenol index was determined.

Table 1: Determination of phenol index after a water extraction of biochar with an overhead shaker for 20h. A tool kit for preparing  samples was used followed by measurement in a spectrophotometer

water extractable phenol-index µg/g    
clean char < 45    
gas contaminated char 1797    
liquid char 2734    

HPLC-UV/RI: organic acids, aldehydes, alcohols

Additionally, common organics acids, alcohols and aldehydes were analyzed using a HPLC in combination with an UV and a RI detector.

  • Compounds: acidic acid, formic acid, propionic acid, tartaric acid, malaric acid, citric acid, levulinic acid, glycolaldehyde dimer, acetaldehyde, formaldehyde, methanol, ethanol
  • Results not received yet

 

Soxhlet extraction: solvent extractable organic compounds

We decided to use soxhlet extraction with an extraction solvent that has a relatively low vaporization temperature and that can be analyzed by the GC-FID directly, so that we don’t have to transfer it into another solvent which always leads to reductions in concentrations. We used 100 mL ethyl acetate (vaporization temperature 77-78°C), 20h extraction time and 1 g of biochar sample.

GC-FID: phenolic compounds

A quantitative measurement of a selection of phenolic compounds was performed using a GC-FID

  • Compounds: o-cresol, p-cresol, phenol, 4-ethyl-phenol, catechol, 4-methyl catechol, 4-ethyl catechol
  • Detection limit for all: 100 mg/L
  • Results: all compounds were under the detection limit but for all compounds peaks were visible in the contaminated chars but not in the clean chars (apart from 4-ethyl phenol which was not present in gas contaminated treatment)

GC-MS: phenolic compounds

A qualitative screening of phenolic compounds and similar compounds was done using a GC-MS

  • Results not received yet

Conclusions

The contaminated chars possess high concentrations of phenols (phenol index) for example compared with 30-40 µg/g water soluble phenols that have been found in wood charcoal from pyrolysis at around 350°C (Gundale 2007). It is also obvious that all kinds of phenol derivatives are present as well, but only in the contaminated chars. All these compounds or only a few could be the cause for the toxicity to germination of cress seeds. This should be further investigated.

Future collaboration and planned publications

Due to the fact that most compounds were under the detection limit of the GC-FID people from the KIT will perform another soxhlet extraction with more biochar sample this time. Additionally, other extraction solvents apart from ethyl acetate are planned to be tested to optimize the biochar extraction method.

Furthermore, it is planned to test a new extraction facility which can separate between water extractable and water vapour extractable compounds to extract HTC- char as well as my biochars.

The composition and concentration of water and solvent extractable compounds in the contaminated biochar samples will be used for an article publication. The publication will be about mobile (volatile and easily leachable) organic compounds in (contaminated) biochars.