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 .