| Summary: | Interactions of C.I. Basic Blue 3 with potassium humate in aqueous systems were investigated. Both the humic content and dye removal are of crucial significance in relation to water of a desired quality. Dye retention experiments demonstrated that potassium humate is an exceptionally efficient adsorbent. The effect of both the pH and temperature on the adsorption process was investigated. The temperature proved to only slightly influence the extent of dye sorption; contrarily, under mildly acidic conditions (pH = 4.0), the quantity of dye adsorbed was doubled on standing. At this pH value, interactions between the positively charged dye and the dissociated carboxyl groups of humic substances are encountered. This was also supported by the obedience of the experimental data to the pseudo-second-order kinetic model. Three adsorption models, i.e., Langmuir, Freundlich and BET, were fitted to the experimental data; the equilibrium adsorption conformed to the Langmuir and BET isotherm equations on the basis of electrostatic dye–humics interactions, while the fitting of the Freundlich model referred to the heterogeneities of humic substances attributed to their colloidal behavior. Thermodynamic quantities, i.e., enthalpy, entropy and free energy change in the adsorption, were calculated. The low <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>Δ</mo><msubsup><mi>H</mi><mrow><mi>a</mi><mi>d</mi><mi>s</mi></mrow><mi>θ</mi></msubsup></mrow></semantics></math></inline-formula> values verify the negligible effect of the temperature on the adsorption; <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>Δ</mo><msubsup><mi>S</mi><mrow><mi>a</mi><mi>d</mi><mi>s</mi></mrow><mi>θ</mi></msubsup></mrow></semantics></math></inline-formula> denotes a thermodynamically favorable reaction, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>Δ</mo><msubsup><mi>G</mi><mrow><mi>a</mi><mi>d</mi><mi>s</mi></mrow><mi>θ</mi></msubsup></mrow></semantics></math></inline-formula> denotes a spontaneous process.
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