I am using biochar for heavy metal sorption. I did CHNS analysis and got wt% of C,H,N,S. I have calculated O/C, H/C, C/N and obtained different values for different samples at different pyrolysis temperatures. I would like to know what does these ratios indicate, in terms of-
- Sorption capacity of the biochar
- Abundance of surface functional groups
- Biochar yield, etc
Please also mention any other indications that could be obtained from these ratios that would help me find out the heavy metal sorption capacity of the biochar.
Also please mention if there is any range of values, that can be used for comparison.
Dear@ Uddipta Nath
O/C ratio indicates the polarity, and abundance of polar oxygen containing surface functional groups in biochar, higher the ratio more is the polar functional groups. Also, these groups actively takes part in adsorption of heavy metals. H/C ratio indicates the aromatically and stability of the biochar. Yes, all these ratios have a rage. You must visit the site of International Biochar Initiatives where you can find out more relevent information. Ample literature is also available for biochar characterization. You can refer the articles from J of analytical and applied pyrolysis, waste management and Bioresource technology,
As biomass undergoes thermolysis, its original polymeric molecules are broken up and transformed giving rise to three products: syngas (CO, H2, CH4), bio-oil (acetic acid, phenolic-like compounds, alcohols, H2O etc) and biochar+minerals. As temperature increases the biochar is more dominated by polyaromatic compounds. We can illustrate this transformation from original biomass comprising of hemicellulose, cellulose and lignin to biochar using the H/C and O/C ratio in a van Krevelen diagram (http://www.publish.csiro.au/?paper=SR10010; Article Influence of Pyrolysis Temperature on Physico-Chemical Prope...
). H/C and O/C ratios are used in the biochar community to estimate the stability of biochar in soil, which is important for calculating the carbon sequestration potential. In your case it may be of importance in determining the number of regeneration cycles. Not sure whether anyone looked at this. In addition, and as mentioned by Amita Shakya already, the O/C ratio can inform us about the general polarity of the material. To know the type and quantity of functional groups present you could try Boehm titration (Article Immobilization of heavy metal ions (CuII, CdII, NiII, and Pb...
; https://linkinghub.elsevier.com/retrieve/pii/S0008622313008981; http://linkinghub.elsevier.com/retrieve/pii/S0016706111001091). Be sure that your biochar has been washed to avoid interference.As O/C decreases with increasing temperature, the adsorption mechanism shifts from mainly ion exchange based to physisorption (van der Waals, London dipole). The sorption capacity should drop due to the weaker nature of physisorption, but typically the BET surface area increases exponentially (with increasing temperature) thus compensating for it. Something else you may wish to consider in certain adsorption systems is the presence of natural organic matter which competes with the heavy metals for the surface. An example can be found here (Article Adsorption of NPK fertilizer and humic acid on palm kernel s...
).As for the yield, since both H/C and biochar yield are correlated to temperature, these two (H/C vs biochar yield) should be too.
You could carry out a meta-analysis and develop a correlation between O/C ratio and heavy metal adsorption capacity. Since type of functional group, pH and BET surface area plays and important role too it may not be that straightforward. It may be advisable to contact an expert in statistics (e.g. Abbas F. M. Alkarkhi).
Hope this helps?
Thank you very much, I really appreciate you effort. Thank you!
Robert Thomas Bachmann
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