A process for the preparation from a partially decomposed organic material like peat a granulated or pelletized sorption medium using low-temperature, thermal activation of the sorption medium to produce a high degree of granule or pellet hardness balanced against an efficacious level of ion-exchange and adsorption capacity, followed by chemical treatment of the thermally-activated sorption material via an acid solution and a salt solution to increase its ion-exchange and adsorption performance while minimizing the transfer of natural impurities found in the sorption medium to an aqueous solution is provided by this invention. The sorption medium of this invention can be used in a variety of aqueous solution treatment processes, such as wastewater treatment.