Groundwater purification methods using elements of iron and manganese biogeochemical cycles
Authors:   Institute of Water Problems and Land Reclamation of National Academy of Agrarian Sciences of Ukraine, Kyiv, Ukraine, Charnyi Dmytro Volodimirovich 
Published: January 2023

For billions of years, natural biogeochemical processes are taking place on the Earth, one of the main driving forces of which is water. In the water constant changes take place in the concentrations of various elements, their compounds and substances in various physical and chemical states – solution, gas, suspension and the like. Natural processes provide both dissolution and conversion to insoluble forms of most ingredients of natural water. Based on this, we believe that on the basis of already existing water treatment facilities using natural processes, it is possible to prepare the clean water without using or minimally using traditional reagIn the case of the preparation of groundwater with excess Mn(II) content for demanganation, it is possible to attract the feature that in the underground waters prevalent in Ukraine, in the case of above normative manganese concentrations, iron ions are almost always present at much higher concentrations. Relying on the scientific research of geochemists studying the formation of iron-manganese nodules and the formation of manganese ores, we assumed that, by attracting abiotic and biotic factors, it is possible to remove surplus iron and manganese without the use of traditional pH-corrected approaches for demanganization and oxidizing potentials, higher atmospheric oxygen concentrations or artificial catalytic filter loadingsents. To test this hypothesis, a thermodynamic model of real underground water was created for a water intake in the town of Uzin, Kyiv Oblast with an excess permissible content of manganese, ammonium nitrogen and iron-silicon compounds in the form of inorganic ligands. The aim of the simulation was to determine during the aeration of this water the possibility of formation of compounds capable of catalyzing the oxidation of Mn2+ with soluble O2, primarily non-hydrolyzed Fe(OH)2+ and the already formed amorphous Fe(OH)3+ colloids, as possible Mn2+ adsorption sites. The simulation consisted in calculating the equilibrium in the system “natural underground water intake Uzin – atmospheric oxygen,” depending on the amount of atmospheric oxygen. Calculations, carried out by minimizing the Gibbs free energy, using the GEMS3 software. The gross composition of the system corresponds to the natural composition of the ground water of the Uzin water intake: 1000 g H2O + dissolved components. During the simulation, portions of oxygen were successively added to the feed water – at each step ~ 0,0031986 mg. A total of 2501 calculations were performed at 7,9997 mg of oxygen in the system. In the process of modelling, it was revealed that during the oxidation of Fe(II) to Fe(III) various iron-containing compounds are formed, including. Fe(OH)2+ and, in especially large amounts, amorphous Fe(OH)3. They contribute to the creation of conditions (oxidation, adsorption of Mn2+ and catalysis) to overcome the energy barrier and to obtain an efficient kinetical oxidation process Mn(II) → Mn(IV), with the aim of demanganating groundwater. Also, the simulation confirmed that during the aeration of water, Eh is increased, mainly due to oxidation of Mn2+ and Fe2+ by atmospheric oxygen and nitrogen compounds to nitrates. Correspondingly rH2 of aerated water increases to 14,43, and this already indicates favourable conditions for the development of chemolithotrophic microbiota, which further accelerates the oxidation of Mn2+. The oxygen concentrations necessary for the passage of these processes are also established. The simulated consumption of oxygen by abiotic oxidation of Mn2+ and Fe2+ and nitrogen compounds is rather insignificant and ranges from 2.5 to 3mg/dm3. Therefore, to saturate the water with oxygen, a simplified aeration system can be used, which is guaranteed to provide a dissolved oxygen concentration of 5 mg/dm3 or more. Based on the results of the simulation, a technology was developed for the reconstruction of the de-ironing station in Uzin. The main task was to create conditions that effectively remove excess manganese and iron – silicon ligands by introducing only one reagent – atmospheric oxygen. To do this, it is necessary to create conditions under which gradual changes in Fe2+→ Fe3+ concentrations occur continuously and where the partially hydrolyzed Fe(OH)2+ ion stably exists. Also, the constant presence of a significant amount of solid phases is necessary. Their role is played by polystyrene charge grains – the nucleus for the formation of ferromanganese concretions. After the formation of the ferromanganese concretions on their surface, the process Mn(II) → Mn(IV) will proceed as a heterogeneous catalysis. In addition, due to the mass distribution of chemilithotrophic microbiota, and at the given pH/Eh characteristics of the source water, – this should be Leptothrix ochracea and Galliumella ferruginea -, it is necessary to create carriers for their immobilization in a zone with a low concentration of Fe2+, to transfer their vital activity from Fe2+ to Mn2+ and further acceleration of the oxidation process Mn2+→ Mn4+. Technologically, this resulted in a new combined column-type structure. In this structure, a unit of simplified aeration, a sub-filtration layer of suspended sediment is combined, where in the primary water probes a gradual change in the concentration of dissolved oxygen in the Fe2+ → Fe3+ process occurs, that is, an oxygen concentration zone appears that corresponds to Fe(OH)2+ and in the final phase to amorphous Fe ferricite Fe(OH)3+. Above the Fe(OH)3+ layer is a light (lighter than water) filter load from the polystyrene pellets. These granules play the role of nuclei for the formation of ferromanganese concretions and carriers for the immobilization of chemolithotrophic microbiota. From the ascent of the granules, the filtering load is held by a false bottom, equipped with reverse filter elements. The water that has passed through the filtering charge is collected in the superfiltration space. As a result of purification, the concentration of manganese varied depending on the operation of these or other wells, within the range of 0,53 ÷ 0,64 mg/dm3, and after purification – within the range of 0,02 ÷ 0,08 mg/dm3, depending on the phase of the filter cycle. In the clean water reservoir the concentration did not exceed 0,05 mg/dm3. The iron content in the initial water was more stable 2 ± 0,4 mg/dm3. In purified water, according to the proposed technology, the concentration of iron in clean water reservoir did not exceed 0,2 mg/dm3. During the filtration process, the filtering granules became a substrate for the formation of the ferromanganese concretions crusts. The thicknesses of the existing MF film were: 0,518 ± 0,209 mm at P = 0,95 and  = 40,347%, the film mass on the granules was 0,0039 ± 0,0004 g at P = 0,95 and  = 10,256%. The concentration of manganese, was from Vilgouvani from one gram of filter load, was: 115,593 ± 4,332 mg / dm3 at P = 0,95 and  = 3,747%, the concentration of total iron, was Viluguvani from the film, was: 55,333 ± 30,853 mg /dm3 at P = 0,95 and 55,777%. The interpretation of the results of X-ray fluorescence spectrometry of the granules of the filtering charge of the combined structure showed the following elemental composition: Mn = 64,63%; Fe = 30,57%; Ca = 3,2%; Br = 1,6%. The crystal structure of the film was determined by X-ray diffractometry. This showed that the synthesized ferromanganese concretions of manganite are the most similar in its crystal structure to Todorokit – (Mn2+,Ca)Mn4++3O7*nH2O). Having a significant negative surface charge and open crystalline structures, manganites capture a number of low-valence cations, thereby neutralizing their charge. This effect explains some softening of water after its treatment and the content of Ca in the structure of CMM crusts. Obtained in practice, Todorokit consists of higher manganese oxides and accordingly has catalytic properties in the oxidation of Mn2+ and Fe2+. The results of the measurement – the potential of the obtained ferromanganese concretions t is -9,33 ± 0,341 mV, and the electrical conductivity is 0,168 mS/cm. The process of forming a cortex on the surface of polystyrene pellets is disclosed. It combines adsorption, electrostatic and electro-kinetic interactions and catalytically oxidative processes of both abiotic and biotic origin. In their formation a special role is played by a layer of suspended sediment from colloidal iron compounds. This is experimentally proved by comparing two technologies for the purification of groundwater of the same water intake. Both technologies are based on the aeration of the initial water and its subsequent filtration on the filtering charge from the polystyrene pellets, the only difference is that one unit, prior to filtration, ensures the passage of pro-aerated water in the sub-filtration space through a layer of suspended sediment from iron colloids of the general formula – nFe(X)+. Due to this, excess manganese is removed and a manganite film is formed on the polystyrene pellets. In the absence of a layer of suspended sediment from iron colloids, the deposition of manganese in technologically significant volumes does not occur, and a film mainly formed of amorphous Fe(OH)3 is formed on the surface of the granules. The electro-kinetic properties of colloidal particles forming a layer of suspended sediment in the sub-filtration space are established and their hydrodynamic diameters are determined. Empirical relationships are constructed that describe the regularities of the height distribution of a suspended layer of colloidal sediments in accordance with their hydrodynamic diameter, as well as changes in the concentrations of total iron. Developed on the proposed principles, the technology of deferrization and demanganation, they are implemented in the form of operating facilities in industrial and municipal water pipelines in the settlements of Kyiv, Zhytomyr, Poltava, Chernivtsi, Cherkassy, Sumy regions, and implemented as a technique for engineering the calculation of combined column structures. The monograph is intended for specialists in water treatment engaged in scientific and teaching activities, graduate and doctoral students in this area, as well as employees of water supply and sewerage facilities of various departments, primarily chief engineers of water supply and sewerage projects