I have been testing , making, engineering charcoal since i am in Thailand… What i learned is in contradiction with this Phd researcher you mentioned.
Yes, retort charcoal is more pure and more conductive and No, the adsorption level decreases with higher temperatures and no oxygen. ( i have a lot of lab results for that )
Activated charcoal is charcoal made in a retort and subsequently “activated” by adding O2 from air, Co2 , H2o … removing the soft carbon from the hard carbon pores.
( tried to put all the knowhow short version it in a nutshell )
Depending the purpose and the quality of the water to treat, you construct a open barrel type, kiln, or similar i have seen already here on DOW, where as the glowing char falls down the bottom hole and is subsequently smothered with sand or water. ( kind of a gasifier )
If using open top barrel, the fire must be kept untill its smokeless, or after the initial covering from the drum, air must be added untill full glowing, hot as hell can be, but must be total smokeless, then cover and cool down.
If you test that conductivity, the best pieces are less then 3 Ohm
summerising: burning, glowing untill smokeless, cool down
The pure carbon will be alkaline if not washed, but sometimes that is preferable for drinking water with low PH
a simple test to check adsorption rate: how much alcohol can a piece of charcoal hold by weight, to test with methanol, alcohol, water and so on… where as i use methanol as “field test”
Observation, after washing the carbon with methanol, it becomes more hydrophilic hence adsorbs more water.
Hope this helps a bit