Application of biochar on mine tailings: Effects and perspectives for land reclamation
Researchers from the University of Udine have recently published the following paper: Fellet et. al. (2011) Application of biochar on mine tailings: Effects and perspectives for land reclamation. Chemosphere 83, 1262-1267. The paper is an important contribution to the biochar in reclamation space and describes the interaction of four different rates of a single biochar applied to mine tailings associated with historic lead and zinc extraction. Having reviewed and reduced the data presented in the paper, I do have a couple of general thoughts around the data set. My caveat to this interpretation is that I was looking for broader trends rather than statistical significance and used the mean values presented in the paper. Additionally, most of the data was presented in graphical rather than tabular form, so the data I used obviously has a degree of error associated with it. However, even with this in mind, the data provides some salient points:
· Water Holding Capacity: Linear regression with r2 = 1.00. This was observed in a clay texture class (3% sand, 14% silt, 83% clay) which is interesting as it challenges the opinion that biochar is most effective in coarse grained materials.
· pH: pH increased from 8.1-10.2 when applied to the soil. This needs to be considered when applying to alkaline soil. However, if the intent of biochar is for increased water holding capacity in semi-arid and arid conditions, then this may override any impact on plant growth associated with increased pH. The weathering effect of biochar in these soils over time may also play an important role in these systems, and the increased pH may only be a temporary consideration. However, pre-washing could be required prior to application if a negative and long-term effect on plant growth is observed.
· Electrical Conductivity: As for Water Holding Capacity, a linear increase was observed with increased biochar (r2 = 1.00). Again the effect on saline/sodic soils needs to be considered and pre-washing may be required. Long term leaching effects should also be considered.
· Cation Exchange Capacity: When all four values are used, the data again shows a linear correlation. The data presented in this paper indicates a reduced CEC when 1% biochar was incorporated into the tailings compared with no char addition to the mine tailings. Original data shows no statistical difference between 0% and 1%, so when I use data for the tailings with char addition the correlation coefficient becomes r2 = 1.00.
· Metals: Bar charts presented by Fellet et al. (2011) would suggest that for certain metals, biochar reduces leaching (when using TCLP extraction methods). When presented as scattergrams with log concentrations, the effect appears negligible, with the possible exception of Al. Even then I’m not getting too excited. These results may be a function of the pH effect described above. A similar experiment with metal loaded soils in the near neutral to acidic range will provide a useful comparison. Based on this data set, my conclusion is that in alkaline soils and mine tailings, biochar is ineffective in metal stabilization, despite the increase in CEC which thoeretically should increase metal adsorption.
I also have a PDF version of this review that includes the graphs I used for this interpretation. If you would like a copy, please email me at andrewharley@ascensionsoil.com.
