Cr^VI reductive transformation process by humic acid extracted from bog peat: Effect of variables and multi-response modeling

The present paper reports the efficiency of bog peat-derived humic acid (HA) in the reductive removal of hexavalent chromium (Cr^VI) from aqueous solution as affected by solution pH, the dose of Fe^III and reaction time (numeric variables) and light irradiation (categorical variable). A three-level Box-Behnken design (BBD) applied to design experimental matrix, model the effects and interactions of variables on four determined responses (residual concentration of dissolved Cr^VI, dissolved Cr^III, dissolved Fe^II and total Cr^VI) and optimize the experimental conditions for highest Cr^VI removal efficiency (Cr^VI RE). Reaction mechanisms are also well discussed. Regression models were developed and analyzed by the ANOVA test and models determination coefficient R². Obtained models were significant (F values > 13) and an excellent relationship between experimental and predicted responses (R²: 98.1–99.6%) was observed. The optimum conditions were established corresponding to the residual concentration of dissolved Cr^VI as an index for Cr^VI removal efficiency (RE). In the dark system, the highest Cr^VI RE (98.1%) was obtained under the following conditions: pH = 1, reaction time = 7 d and Fe^III dosage = 0.110 mM. In the light-irradiated system, the optimal Cr^VI RE of 98.3% was observed in pH = 1, reaction time = 5 d and Fe^III dosage = 0.075 mM. Almost all reduced Cr^III remained in the solution even at high pH value. No adsorption or precipitation of Cr^III on the HA surface at pH 5 was confirmed by surface analyses of HA using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).