Biochar already has a high pH due to carbonates and oxides formed during production. The more salts in the starting material the more carbonates and oxides left in the biochar. Grass,leaves, manures result in high carbonates where wood chips have less. Because calcium, sodium, potassium etc. end up as carbonates during char formation. Dissolving these with acid might clear them out creating more sites - but I believe your suggestion would just add to the coating of carbonates. 

The char will also contain variably concentrations of silica and silicates.  These may be soluble in alkali

Thanks Dr. Milham for your reply. What do you think would be an appropriate way to test this hypothesis?

Interesting idea. I might suggest a dilute acid (HCl) wash to remove carbonates. Then a soak in NaOH - perhaps 1N? Wash with dilute acid (0.5N?) again. Measure the activity before and after. I have a butane activity setup that would do this. It will differ with different chars so many will likely need be experimented with.  

The generation of the pores is as a result of subjecting the biochar to high temperature (activation) after impregnating the biochar with the chemicals. Like rightly stated by Frank Shields, the use of the alkali will increase the salts in the biochar, making it more alkaline in nature. You may want to use acidic solution. The impregnation reacts with the tars that blocked the pores of the biochar during pyrolysis. During the activation process, the pores are cleared off and then you can have a high porous material

I have tested many chars for activity starting at temperature of 450 C and increasing in 50 deg increments to 850 or 900 C. Found 700 to be about best as the voids collapse at higher temperatures. For this experiment I suggest a weak acid wash. Then measure activity. Then treat with NaOH (1N with heat?) and then wash again with dilute acid and re-test for activity. Something like that. Something like that.....

@Frank Shields, so you're suggesting that the best temp for activation without the pores collapsing should be around 700 C?

Don't you think that the precursor sometimes may also influence the pore development of the resulting activated carbon? 

When I test this using different biomass and plot the curve the optimum can change from 600 to 800. I am not sure the reason. Have not tried to determine the main reason for that. 

Mr. Frank and Mr. Oluwatosin, thank you for the answers and suggestions. Actually, in my case the problem is little different than just activation and creating more pores. The biochar I produced was saturated with CO2 and treated with strong alkali ( including calcium hydroxide and magnesium hydroxide). SEM images show closely spaced pores which is absent in fresh biochar and unsaturated biochar treated with alkali. Therefore, I was wondering if the alkali played a role in creating more pores in CO2 saturated biochar!

So after you produce the biochar you saturate it with CO2? and treat by soaking with alkali? then dry (105c)? and SEM?  OR the biomass soaked in Alkali before heating in a CO2 environment? Thanks and interesting. I wonder if the activity measurement would reflect what you are seeing in the SEM. 

Right. The biochar is soaked in alkali after being saturated with CO2 post production. Then SEM is conducted after drying it at 50-60 degrees for 24-36 hours. The biomass is not soaked in alkali at any stage.

Is there a wash to remove alkali? or is the alkali dried leaving residue of calcium and magnesium? carbonates? You then would not be removing anything but adding salts to the char. If you did an acid wash at the end are the additional voids still there?

No acid washing. Is there a possibility of interaction between alkali and the CO2 adsorbed in the pores of biochar?

Lots of questions that I think only research would answer. First question is the quality of the char. What the char is made from and the measured activity of the char. Second is if you were to wash out the alkali with dilute acid (0.5N HCl) and dry at 50 - 60 for 24 - 36h would you still see the new pores or are you seeing some crystal structure from the salts (I know nothing about SEM so this may be a stupid question). Third is how much CO2 can be absorbed in the char. The setup I use for butane could easily be switched to CO2 and the added weight determined. I don't think it would be much but have never tested it. Char does take up methane and that is interesting to me. Fourth is that alkali is good at reacting with organics esp tars so perhaps it is dissolving tars and you are seeing whats behind it.  - you have reached the extent of my knowledge so I just guess. I have no doubt that if our labs were side by side we could learn a lot and solve this mystery.

Hi,

I just came across your discussion and I am interested in the outcome of your research.

Mr.Gupta did you find an answer to your question?

Kind regards,

Christian

Hi Christian,

my research focus on application of biochar from different feedstock as admixture in cement-based construction materials ( for example, mortar, cement paste and concrete). Thus, I was curious about the effect of alkaline environment on additional pore generation or degradation in biochar. So far, my research outcome suggests that biochar prepared from certain feedstock (biomass) under controlled temperature and heating rate contribute positively to early and long term strength and durability of mortar and concrete. I haven't found any sign of degradation of biochar particles in highly alkaline cement mixes.

Would love to have your thoughts on my question and the topic.

Thank you!

I have a number of comments:

Alkali - with react with CO2 to produce carbonates. They are also used to remove carbon deposits from manufacturing plant, such as plastic injection moulding or extrusion , that use oil cooling. The alkali may strip away material from the pore spaces.

Carbonate formation is the slow process of cement cure. Initial cement cure is the hydration of tricalcium silicate which yields lime(3 moles per mole of Tricalcium silicate), this reacts with carbon dioxide, so the presence of CO2 in the char may accelerate this slower hardening phase.

I think you should speak with a cement chemist at a university (Leeds or imperial) or a cement manufacturer.

Andrew