Abstract: A process is disclosed for producing biosorbents by phosphorylation of cellulose-containing materials with phosphoric acid or ammonium phosphate in the presence of urea. Also disclosed are biosorbents with particular good use properties. Biosorbents of this type are used, in particular, for removing heavy metals from aqueous solutions. Proceeding from the disadvantages of the known prior art, purification costs of resultant raw products should be lowered and the formation of unwanted soluble organic polymer phosphates as by-products should be prevented. The biosorbents should have an improved mechanical strength and very good use properties. The disclosed biosorbents contain 5-10 % phosphorus and 1-4 % nitrogen in the form of carbamide groups. The synthesis mixture is mixed with elementary sulphur before starting the reaction in order to produce biosorbents with an increased mechanical strength. The proposed procedure allows reducing the otherwise customary reaction temperatures by about 40° C.