More thoughts on biochar and water by William Stevenson.
From the research I have been doing, I have found that although it is true that hydrophobic char has a high cation exchange capacity (high electrostatic surface charge), this is not the cause of it being water-repulsive. The main element to this is that its surface is composed of aliphatic functional groups.
So, these aliphatic functional groups are the cause of char hydrophobicity and it is through soaking or flooding the char that we can remove them.
The wettability of biochar seems to be governed mainly by pyrolysis temperature, although other factors such as feedstock also play a role. Pyrolysis temperature influences: surface area, pore radius and aliphatic groups (all relevant).
A low pyrolysis temperature also creates hydrophobic tars, clogging surface pores and rendering the char hydrophobic.
Interestingly, when char is not hydrophibic, it is not hydrophilic (as per definition) either. It does not form hydrogen bonds. It can however be said to have a high affinity towards water due to a high absortion capacity.
Based on these findings, it would be interesting to get an idea of the charcoal dust formation temperature.
I have not yet found anything relating to hydrophobicity caused by dry cooling rather than water-quenching. If anyone reading this has any information about this please contact us.
I have also been reading about the complex hydraulic biochar-soil relationships and about obtaining plant-available water, which is what we are ultimately interested in.
Observations by William Stevenson - 13.02.16
Notes from dfg: I have included Williams observations and the notes below to give the general reader some idea of the complexity and science behind the work we are doing in Namibia.
Cations and Cation Exchange Capacity
The clay mineral and organic matter components of soil have negatively charged sites on their surfaces which adsorb and hold positively charged ions (cations) by electrostatic force. This electrical charge is critical to the supply of nutrients to plants because many nutrients exist as cations (e.g. magnesium, potassium and calcium). In general terms, soils with large quantities of negative charge are more fertile because they retain more cations (McKenzie et al. 2004) however, productive crops and pastures can be grown on low CEC soils.
Organic matter has a very high CEC. Sandy soils rely heavily on the high CEC of organic matter for the retention of nutrients in the topsoil.
In organic chemistry, hydrocarbons (compounds composed of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds. Aliphatics can be cyclic, but only aromatic compounds contain an especially stable ring of atoms, such as benzene. Aliphatic compounds can be saturated, like hexane, or unsaturated, like hexene. Open-chain compounds (whether straight or branched) contain no rings of any type, and are thus aliphatic.
Aromatic hydrocarbons are unusually stable compared to other geometric or connective arrangements of the same set of atoms. It is difficult to to break the structure apart and to have them react with other substances.