Concentration distribution of molecules and particles in a chromium-containing model system: Fe-K₂Cr₂O₇-NaCl-H₂SO₄-H₂O at different temperatures of electrocoagulation process

Thermodynamic modeling of the electrocoagulation process in the system Fe-K₂Cr₂O₇-NaCl-H₂SO₄-H₂O at wide limits of temperature change (278-300 K) has been carried out for the purpose of water purification from chromium (Cr⁶⁺). Physicochemical (pH, I, Eh, Cp) and thermodynamic (H, S, U, G) parameters of the system at the established optimal ratios of the initial components have been calculated. Taking into account the calculated data the experimental research was carried out and the influence on the electrocoagulation processes was revealed: pH, current, type and concentration of electrolytes. The concentration distribution of individual molecules and particles (cations, anions) was established, including: Cr²⁺, Cr³⁺, CrO⁺ , CrOH²⁺, Fe⁺ , FeOH⁺, FeOH²⁺ in solution and thereby eliminating the use of ion chromatography for analytical purposes. An Eh-pH diagram showing the fields of presence of various forms of chromium has been drawn up, and a formula for calculating the value of the redox potential as a function of solution pH has been obtained. It is shown that Eh> 0, i.e., the medium is oxidizing, and the formation of Cr³⁺ in concentrated solution (I > 0.8) was observed. The electrocoagulation process achieved the binding of sulfur and iron in the form of FeS₂ and followed by the production of Fe(ОН)₃ and co-precipitation of Cr(OH)₃. The degree of water purification from chromium was more than 97% (reduction of Cr⁶⁺in water from 100 mg/L to 2.29-2.30 mg/L).