Synthesis method of tetravalent manganese feroxyhyte (δ-Fe (1-x) Mn x OOH, x=0.05-0.25), characterized by the fact that it is conducted in two continuous flow stirred-tank reactors arranged in serial configuration, where the first reactor has weakly acidic conditions and the second weakly alkaline conditions and high redox potential for the production of a material having a high negative charge surface density, which is applied by the following steps: i) FeSO 4 or FeCl 2 is added to the reactor as an iron source at a concentration of 1-50 g/L and KMnO 4 is added as a source of manganese at a concentration of 1-20 g/L to perform the reaction for nuclei production of the δ-Fe (1-x )Mn x OOH, with the pH value being adjusted to the weakly acidic region of 5-6 by adding one or a combination of more than one of the reagents NaOH, NaHCO 3 , Na 2 CO 3 , KOH, KHCO 3 , K 2 CO 3 , while at the same time the value of the redox potential is adjusted to the range of 450-550 mV with the appropriate adjustment of the FeSO 4 or FeCl 2 and KMnO 4 flowrates, then ii) the reaction of the negatively charged surface nanocrystals growth proceeds to the reactor by further addition of FeSO 4 or FeCl 2 and KMnO 4 reagents with a pH value in the reactor adjusted to the alkaline region 9-10 by adding one or a combination of more than one of the reagents NaOH, KOH and at the same time the value of the redox potential in the 600-700 mV range, with the appropriate adjustment of the FeSO 4 or FeCl 2 and KMnO 4 flowrates, at iii) a residence-reaction time of maximum 20 minutes in the reactor and at least 2 hours in the reactor ; and iv) the product exiting the reactor enters the thickening tank , where with mild stirring for a period of 6-36 hours, grains are formed in an irregular shape, having a specific surface area of 100 m 2 /g to 300 m 2 /g, high negative surface charge (>1.5 mmol H + /g) and isoelectric point less of than 5.5. i) FeSO 4 or FeCl 2 is added to the reactor as an iron source at a concentration of 1-50 g/L and KMnO 4 is added as a source of manganese at a concentration of 1-20 g/L to perform the reaction for nuclei production of the δ-Fe (1-x )Mn x OOH, with the pH value being adjusted to the weakly acidic region of 5-6 by adding one or a combination of more than one of the reagents NaOH, NaHCO 3 , Na 2 CO 3 , KOH, KHCO 3 , K 2 CO 3 , while at the same time the value of the redox potential is adjusted to the range of 450-550 mV with the appropriate adjustment of the FeSO 4 or FeCl 2 and KMnO 4 flowrates, then ii) the reaction of the negatively charged surface nanocrystals growth proceeds to the reactor by further addition of FeSO 4 or FeCl 2 and KMnO 4 reagents with a pH value in the reactor adjusted to the alkaline region 9-10 by adding one or a combination of more than one of the reagents NaOH, KOH and at the same time the value of the redox potential in the 600-700 mV range, with the appropriate adjustment of the FeSO 4 or FeCl 2 and KMnO 4 flowrates, at iii) a residence-reaction time of maximum 20 minutes in the reactor and at least 2 hours in the reactor ; and iv) the product exiting the reactor enters the thickening tank , where with mild stirring for a period of 6-36 hours, grains are formed in an irregular shape, having a specific surface area of 100 m 2 /g to 300 m 2 /g, high negative surface charge (>1.5 mmol H + /g) and isoelectric point less of than 5.5. <EOS>