The interaction of humic acids with copper (II), cobalt (II), and nickel (II) ions at various pH values of the medium was investigated. It is established that the solubility of humic acids in water depends on the pH and increases with an increase in the pH of the medium from 3 to 8. At pH = 8, the solubility of acids reaches a maximum. Reducing the pH below 3 leads to a slight increase in dissolution, which is obviously due to the presence of amino groups in its structure. Investigation of the dependence of the amount of copper ions (II) associated with humic acids from the initial concentration of CuSO4 in the mixture showed that this value increases almost linearly with an increase in the initial concentration of copper salt. The linear dependence of the concentration of bound ions of copper (II) on their concentration in the solution indicates the equilibrium nature of the process. Increasing the pH of the mixture from 4.0 to 4.6 leads to an increase of concentration of copper salts with humic acids. At pH close to 7, and low concentrations of copper salts, water-soluble salts of humic acids are formed. These salts are difficult to undergo the deposition process. With an increase in the concentration of copper (II) salts and a pH close to 7, a precipitate of salts of humic acids and copper hydroxide (II) is formed. A higher concentration of bound copper ions (II) compared with the concentration of acidic groups in humic acids contributes to the formation of low-soluble copper salts. Low concentration of humic acids, the pH of the mixture is reduced. This is due to the interaction of metal ions with free acidic groups and the partial complexation of metal ions with amino groups of humic acids. Also, the displacement of anions of acids and hydrogen ions in the solution is carried out. The minimum pH of the mixture increases when the metal salts are converted into their hydroxides. The constant pH value is due to the formation of basic salts of metals with humic acids. Further increase in the concentration of sodium salts of humic acids in the mixture leads to an increase in the pH of the mixture. Investigation of the dependence of the amount of ion of cobalt (II) bound to sodium salts of humic acids from its concentration in aqueous solution at pH 4.6 showed that it is described as direct as for copper ions (II). Destruction of the main salts of cobalt with humic acids occurs faster than copper salts at the same values of pH of the medium. This is consistent with an increase in the pH of the aqueous solution during the conversion of cobalt (II) and copper (II) salts into their hydroxides and confirms the previously formulated opinion on the formation of basic metal salts with humic acids. Consequently, humic acids and their salts interact with ions of divalent transition metals in an aqueous solution with a pH value ranging from 3 to 5. The amount of bound metal ions increases with an increase in the pH of the medium from 4.0 to 4.6.
The article also gives a comparative description of the interaction of humic acids with ions of trivalent metals. It is shown that the addition of a salt solution of trivalent metal to dispersion of humic acids reduces its solubility. Humic acid is practically soluble at pH above 10, in the presence of aluminum salt.This, obviously, is due to the formation of sodium aluminate and sodium humate in this area of pH. The solubility of humic acids increases at pH above 7 in the presence of iron salts. At pH 11, the concentration of soluble humic acids is not more than 60%.In this case, insoluble particles of iron salts, which are sorbed on the macromolecules of humic acids, are formed. Investigation of the interaction of metal ions with humic acid at pH 3,2 showed that the concentration of metal ions bound to humic acid is proportional to the concentration of metal ions in the solution. At low initial concentration of iron ions in the mixture, its concentration in the solution is close to zero. The initial pH value of humic acid solution is in the range of 3-6.8 and practically does not depend on the concentration of iron ions bound by humic acid. The concentration of chromium ions bound to humic acids is lower than that of iron ions. The difference between the reaction of humic acids with chromium ions and iron ions may be due to the difference in the degree of hydrolysis of their salts. The study of the solubility of humic acids at pH 6.8 with the addition of aluminum salts, chromium, iron showed that at low salt concentrations, a dispersion is formed. An increase in salt concentration leads to an increase in the dispersion concentration. Most of humic acids precipitates with the addition of chromium or aluminum salts in relation to the concentration of metal salt in the mixture to the concentration of acid groups humic acids to about 3.At the same time, the pH of the solution decreases to about 4, while the pH of the mixture of humic acids with iron sulfate decreases to 3 at a lower concentration of trivalent iron salt in the mixture. Humic acids are practically insoluble at the same pH. Consequently, with the addition of low concentrations of aluminum, chromium, and iron salts to humic acids at pH 6.8, a dispersion is formed. An increase in salt concentration leads to an increase in the dispersion concentration. Since the solubility of humic acids depends on the pH of the solution, then with the addition of a solution of sodium hydroxide to the dispersion of humic acids, their solubility increases. At pH close to 8, they are practically dissolved. Adding a solution of the salt of trivalent metal to the dispersion of humic acids reduces its solubility.
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