The method you mention is classic among the classics, but still what is inside is ill defined. The carbon content is not so high for a typical "carbon material", typically 70 - 80% as a mximum, and the rest is mainly O, H and P. And a few % of ashes of course. This is definitely not graphite or even amorphous carbon, this is a mixture of aromatic condensed carbonaceous residue with a lot of acidic moieties. Such heavy compounds appear because of the dehydrating effect of phosphoric acid, thereby forcing the condensation of cellulose and lignin into aromatic, highly disordered, macromolecules. As you can see, this kind of material is difficult to describe, but has an extensive micro or mesoporosity, depending on the precursor, and this is very good for adorption applications. After all, this is what is wanted after such a process.

Alain

Two methods are used for the production of activated carbon (AC): chemicalactivation (ChA) (activation with mineral salts) and physicalactivation (PhA) (activation with oxidizing agents such as CO2 or steam). In ChA,the precursoris impregnated with an activatingagent, such as ZnCl2, H3PO4, KOH, H2SO4, or NaOH, followed bycarbonization with conventional heating via an electrical furnacein an inert atmosphere at temperatures ranging from 400◦C to 800C, or carbonization with microwave heating.

Areview  production of activated carbon from agricultural byproducts via conventional and microwave heating

TamerM. Alslaibi, IsmailAbustan, Mohd Azmier Ahmad; and Ahmad AbuFoul

ChemTechnol Biotechnol 2013;88: 1183–1190

www.academia.edu/.../A_review_production_of_activated_carbon_from

Check the work of Foo KY and Baseem Hameed of Uni Sains Malaysia. 

I think there are many biomass starting materials with their own lignins and cellulosic combinations and the different methods of producing AC will also produce different chemical surfaces on the AC.