100 g of poplar wood sawdust in a sieve fraction from 0 . 5 to 2 mm and with a water content of 11 % are mixed with 5 g of elemental sulfur in powder form and introduced into a solution of 446 g of urea in 848 ml of phosphoric acid ( 85 % strength ) and 100 ml of water and mixed . the resulting thin paste is distributed on porcelain dishes so that a layer about 1 cm thick is present . the porcelain dishes with the applied reaction mixture are dried in a ventilated drying oven at a temperature of 80 ° c . for 1 hour . the reaction mixture is then remixed , and the porcelain dishes are covered with stainless steel plates and treated further in the drying oven , feeding a gentle stream of nitrogen into the drying oven . over the course of one hour , the temperature in the drying oven is raised to the reaction temperature of 165 ° c . and is maintained for a period of 1 . 5 hours . after this period , the reaction is complete and the resulting biosorbent crude product is cooled . 100 g of chopped rye straw in a sieve fraction from 2 to 5 mm and with a water content of 12 % are introduced with a solution at 90 ° c . of 94 . 1 g of ammonium dihydrogen phosphate and 135 g of urea in 75 ml of water and mixed . the mixture is stored in a closed container for 2 hours and then put in a 2 l flask which is connected to a laboratory rotatory evaporator . the mixture is stirred at a temperature of 90 ° c . with an applied vacuum of 4 . 00 kpa until water no longer distills out . then 18 g of sulfur powder are added and vigorously mixed with the reaction mixture . subsequently , the temperature is raised to 145 ° c ., maintaining the vacuum and stirring further . the reaction temperature is maintained for a period of 2 . 5 hours . after the initial vigorous evolution of gas has subsided , after a reaction time of 2 . 5 hours , the reaction is complete . the resulting biosorbent crude product is cooled . 100 g of powdered spruce wood with a particle size of from 0 . 4 to 1 . 25 mm and a water content of 11 % is mixed with an amount of 1 g of elemental sulfur in powder form and , at room temperature in a closeable container , 50 ml of water are poured over and mixed . the container is then kept closed for two hours . after completion of the activation , the water content of the powdered spruce wood is 41 %. then 50 . 2 ml of 85 % strength phosphoric acid ( 8 . 26 mol / kg of wood ) are slowly added dropwise to the activated powdered spruce wood , and mixed with the powdered wood for a period of 30 min . after this mixing process , 148 . 2 g of prilled urea ( 3 . 35 mol / mol of phosphoric acid ) are added to the mixture and mixing is continued for 30 min . the moist , free - flowing mixture is then put into a laboratory rotatory evaporator ( 2 l flask ), a vacuum of 4 . 00 kpa is applied , and the temperature is raised to 90 ° c . by means of an oil bath while stirring continuously until water no longer distills out . while maintaining the applied vacuum and continuing stirring , the anhydrous mixture is heated to a reaction temperature of 135 ° c ., and this temperature is maintained during the mixing process . the initial evolution of gas subsided after 1½ hours , and the reaction process was terminated . the crude product was then cooled . the process was carried out in analogy to example 3 starting from various cellulose - containing materials but altering the starting components and the process parameters as follows . 1 ) mol of h 3 po 4 per kg of cellulose - containing material a biosorbent crude product is produced in analogy to example 1 but without addition of sulfur . a biosorbent crude product is produced in analogy to example 2 but without addition of sulfur . a biosorbent crude product is produced in analogy to example 3 but without addition of sulfur . the process is carried out in analogy to example 3 but without addition of sulfur and with an alteration in the added amounts of phosphoric acid and urea . compared with example 3 , 2 . 5 times the amount of phosphoric acid , 125 ml ( equivalent to 20 . 65 mol of h 3 po 4 per kg of spruce wood dry matter ) and 2 . 5 times the amount of urea , 370 . 5 g ( equivalent to 8 . 375 mol / mol of h 3 po 4 ) were added . the process was carried out in analogy to example 5 but with an alteration in the added amount of phosphoric acid . this was 19 . 6 ml ( equivalent to 4 . 0 mol of h 3 po 4 per kg of spruce wood dry matter ). the process was carried out in analogy to example 7 but with a change in the added amount of urea . this was 66 . 2 g , which is equivalent to a molar ratio of urea to phosphoric acid of 1 . 5 : 1 . the biosorbent crude products produced in examples 1 to 3 and comparative examples 1 to 3 are treated to extract the by - products as follows . the cooled crude products are stirred with 1 l of water and put , free of bubbles , into a glass column usual for column chromatography or ion exchange processes , which is filled with water and has a diameter of 8 cm and a height of 20 cm . a flat perforated plate is attached above the bottom outlet . the level of liquid in the column is controlled by adjusting the height of the overflow connected to the bottom outlet . the sorbent packing in the column is 12 cm high . the crude product is purified by introducing a constant stream of 150 ml / h of water . at defined intervals , samples of the purifying water are taken at the overflow and investigated to detect phosphates . detection takes place by the method which is customary per se , using ammonium molybdate and nitric acid at the boiling point . the phosphate detection led to the following results for the various biosorbents : as proved unambiguously by the results for comparative example 3 , the biosorbent still has a considerable phosphate load after a purifying time of 50 hours , which is manifested by yellow color still being present in the phosphate detection in the purifying water . for comparative investigations , “ purified ” biosorbent particles according to comparative example 3 are taken out of the column and stirred with 300 ml of water and 100 ml of 1n naoh in a container . the biosorbent particles are then again packed into the glass column to continue the extraction process , and the purification is continued in the manner described at the outset by adding a further 5 l of water , and corresponding samples of the purifying water were taken and the phosphate detection was carried out . the following results were obtained thereby : these results show that complete removal of the soluble organic polymeric phosphates could not be achieved even by treating the biosorbents with sodium hydroxide solution . since no standardized test method for testing the mechanical stability of biosorbents yet exists , and what matters in practice is , in the final analysis , the mechanical strength of the sorbent packing , the following test method which is customary per se is carried out . after completion of the purification stages , the water is discharged from the glass columns packed with the biosorbents , and the sorbent packing is sucked free of drops by applying a vacuum of 33 . 33 kpa for a period of 30 min . the height of the sorbent packing is then measured and the sorbent packing is examined for fissuring . the results obtained in this way represent reliable information about the mechanical stability of the sorbent packing and the mechanical strength of the sorbent particles . the results obtained are as follows : as proved by the comparative examples , the addition of elemental sulfur suppresses the formation of soluble organic polymeric phosphates in the biosorbents , and the produced biosorbents show distinctly improved mechanical strength by comparison with the biosorbents obtained without added sulfur . the biosorbents produced in examples 3 to 13 and comparative examples 4 to 6 were obtained in the following yield and dry content with respective p and n contents : the biosorbents produced in examples 3 to 13 and comparative examples 4 to 6 were investigated for their sorption properties as follows . determination of the sorption equilibrium data took place by the following method : 250 ml graduated flasks were charged with the moist sorbent samples ( 0 . 1 to 0 . 025 g dry matter ) and in each case 1 to 5 ml of m / 10 solutions of salts of the metals cu , cd or pb , made up to the mark , provided with a magnetic stirrer bar and stirred at room temperature for 3 hours . after settling , the solutions were decanted off , and their ph was determined and their metal content was found by complexometry . the equilibrium concentrations of the sorbent were calculated from the equilibrium concentrations in the solution obtained in this way and from the initial concentrations fixed by the addition of metal salt solutions . addition of appropriate amounts of nitric acid before making up the graduated flask to the mark adjusted to ph = 3 for the sorption . several control measurements of the equilibrium concentrations in the solution via atomic absorption spectroscopy ( aas ) revealed deviations within the range of accuracy of the measurement and thus confirmed the reliability of complexometric analyses for the sorption investigations . the sorption capacities found for the individual sorbents are indicated in the following table . the strength of the sorption was determined on the basis of the equilibrium data at low equilibrium concentrations ( below 10 mg / l ) in the solution ( likewise at room temperature and ph 3 ). the metal - specific equilibrium coefficient k me , which is per se , was calculated by the formula in this , c s is the equilibrium metal concentration in the sorbent in mg / g and c 1 is the equilibrium concentration of metal in the in mg / l . the sorption results show that it was possible to achieve an ion exchange capacity of 4 . 5 to 5 . 6 meq / g on use of about 50 % less phosphoric acid and urea compared with the amounts customarily used to date . the lower values were found for sorbents based on beet pulp and barley straw , which show lower capacities based on their material structure . concerning the calculated equilibrium coefficients k me for the metals cu , cd or pb , very good results were achieved according to the examples according to the invention , which are considerably better than the strength values for known sorbents . the comparative examples prove that on use of amounts of the reactants phosphoric acid and urea which are outside those according to the invention , considerably poorer sorption properties are achieved , in particular in relation to the strength .