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This article studies the effects of solubility, disperse features (average diameter dav and charge (ζ‑potential)) of the particles of heavy-metal slightly soluble compounds exemplified by aluminum(III), iron(III), zinc(II), and lead(II) hydrates related to the efficiency of their removal from aqueous solutions in the composition by electroflotation and filtration within the pH range 5–12. The efficiency of the electroflotation-based Pb(II) removal from aqueous solutions is determined by the sorption of ions and the dispersed Pb(II) phase on the Al(III) and Zn(II) dispersed phase. The size of the disperse phase of multicomponent systems is averaged compared to that of individual compounds. The maximum particle size of the dispersed phase of multicomponent systems comprising Al(III)–Zn(II)–Pb(II) at a pH of 8.0 reaches 36 μm, in the Fe(III)–Zn(II)–Pb(II) and this parameter reaches 46 μm at a pH of 9.0. The ζ-potential of the dispersed phase of the studied systems varies in the range from –2 to –9 mV for all the systems within the pH range 7–9. The research showed that the efficiency of the removal of lead(II) and accompanying metals compounds from wastewater through electroflotation reaches 95–97% within the pH range 8–9. Filtration of the solutions allows us to improve the removal to 99% and higher.

The effect of dielectric-barrier discharge plasma treatment on the valence state of metal ions in aqueous solutions of manganese, iron, cobalt, and nickel (Mn, Fe, Co, Ni) salts has been studied. The efficiency of recovery of sparingly soluble compounds of transition metals during the treatment of model solutions in a plasma reactor designed by the authors has been evaluated. The possibility of hydrogen peroxide formation in water by barrier discharge treatment has been shown, and the change in the redox potential of the system has been analyzed.

The process of removal of heavy metal ions as part of the multi-component system from electrolyte solutions by means of electroflotation and filtration with presence of the high-molecular compounds (HMC) of different nature has been studied. Studies were performed in solutions with ions of iron (III), nickel (II), copper (II), lead (II) and zinc(II), as well as NaCl and Na2SO4 (pH 9.5–10.5). It has been shown that addition of the sulphate and sodium chloride mixture in 50 g/L concentration to solutions containing metal ions with total 0.1 g/L concentration reduced the absolute value of dispersed phase ζ-potential from −2.5 mV to −6 mV. The dispersed phase size is getting smaller, two particle types appear in the solution − 54 µm (72% of the total particle quantity) and 5.0 µm (28%). Rise of the total metal ion concentration up to 0.2 g/L results in reduction of the dispersed phase size: 1.5 µm (63%) and 46 µm (37%) particles prevail, the ζ-potential is reduced slightly (-7.1 mV). In this case, the rate of removal of the studied metals by mean of electroflotation is not more than 32–46%. Addition of the anionic HMS (flocculants and surfactants) to the solutions with the total concentration of metal ions of 0.2 g/L and salts of 50 g/L results in rise of dispersed phase ζ-potential and tends its value to the isoelectric point (±0 mV). When the cationic surfactants are added, the particle surface is recharged, the ζ-potential acquires the + values (5.3–7.8 mV). It has been concluded, that the rate of electroflotation removal of heavy metals from concentrated electrolyte solutions with the anionic HMS runs to 98–99% for all components. Subsequent solution filtration with the anionic flocculant allows for reduction of the residual concentration to the following values: 0.5∙10-4 g/L for Fe (III), Cu (II) and Pb (II) ions and. 0.1∙10-4 g/L for for Ni (II), 0.14∙10-3 g/L for Zn (II). Addition of the cationic, non-ionic and amphoteric surfactants does not result in increase of electroflotation process efficiency.

Possibility of the electroflotation separation and extraction of cerium(II, IV), copper(II), and iron(II, III) from aqueous solutions is demonstrated. The optimal pH value and the concentration ratio of ions of the metals being separated, at which their electroflotation separation and extraction from aqueous solutions is the most efficient, was determined. It was shown that the electroflotation method is promising for selective separation and extraction of metal ions with various hydrate-formation pH values from aqueous solutions.

Various aspects of the electroflotation technology for wastewater treatment and the extraction of valuable compounds from technological solutions have been reviewed. A brief description of the history of the appearance and improvement of the technologies based on the use of electroflotation process has been given. Their main operational parameters have been considered. Data on the mathematical simulation of the process have been provided. Achievements on the extraction of nonferrous and rare-earth metals from wastewaters, technological solutions, and technogenic waste have been discussed in detail. The results of investigations on the effect of various flocculants and surface-active compounds on the electroflotation process have been presented.

This paper reports on our study of the pH effect of solutions on the average hydrodynamic diameter (dav) of the particles of the disperse phase and the electrokinetic potential (ζ) of the particles of low-soluble iron subgroup metals compounds using Fe(II, III), Ni(II), and Co(II) compounds as an example. The pH effect of solutions on the efficiency of the electroflotation extraction of metal ions from aqueous solutions containing these ions in individual form or in mixture was studied. The efficiency of the electroflotation extraction of the low-soluble compounds of iron subgroup metals is directly related to the particle size and electrokinetic potential of the particles, which depend on рН. The maximum degree of particle extraction α reached 97–99% at рН values characterized by the maximum hydrodynamic diameter of particles (over 20 μm for Fe(II) and Co(II) compounds and over 50 μm for Fe(III) and Ni(II) compounds) at ζ potentials of up to–10 mV for systems approximated to real wastewater. In the case of the extraction of the disperse phase of the Fe(III)–Ni(II)–Co(II) multicomponent system, the synergic effect was observed: the coextraction of metals was more complete and effective, which may be due to suppressed negative charge. In the range of рН 10–11, the degree of extraction of the Fe(III) disperse phase did not exceed 74%; in the ternary system, it reached 94%.

The impact of concentrated NaCl, NaNO3, and Na2SO4 solutions on the average hydrodynamic diameter and charge of particles of slightly soluble nickel and copper compounds were found to be rather identical. In electrolytes solutions, the average size of the dispersed phases particles decreased from 50–54 µm to 30–45 µm, the electrokinetic potential became more negative (-(2–10) mV). In a result, the removal of slightly soluble nickel and copper compounds decreased from 98% to 94–97% and 30–94%, respectively. Minimal efficiency of electroflotation-based extraction of nickel was observed in the presence of NaNO3. Cationic flocculants promoted increased extraction up to 98–100% regardless of the electrolyte, and decreased electroflotation duration from10–15 min to 5–7 min.

The influence of pH of solutions on the size, surface characteristics, and electroflotation activity of particles of poorly soluble cerium(III, IV) compounds was examined. The recovery completeness and efficiency directly depend on the electrokinetic potential of poorly soluble cerium particles: the particles with the smallest absolute value of the surface charge are recovered most completely and efficiently. The effect of Superfloc series flocculants of different nature on the efficiency of the electroflotation process was demonstrated. Introduction of a cationic flocculant leads to the process intensification, to a decrease in the treatment time to 2 min, and to an increase in the degree of recovery of cerium compounds to 99%.

Method for purification of aqueous solutions to remove Fe(II), Ni(II), and Co(II) ions with ruthenium-titanium oxide electrodes (ORTA) was studied. The method is based on the process in which divalent metals are oxidized to the trivalent state at the phase boundary between the oxide layer on the electrode surface and the aqueous solution, followed by recovery of the metals from solution by filtration in the form of poorly soluble hydroxides M(OH)3. IT was found that technological parameters of the process affect the recovery efficiency of variable-valence metal ions from aqueous solutions. IT is shown that the method is promising for application in water purification and water treatment processes.

Applied aspects of the intensification of organic matter oxidation processes in water solutions have been studied. Data on chemical oxygen demand (COD) decrease and pH changes of the model solutions under the impact of barrier and spark pulsed high-voltage discharges have been presented.