CrVI 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 (CrVI) from aqueous solution as affected by solution pH, the dose of FeIII 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 CrVI, dissolved CrIII, dissolved FeII and total CrVI) and optimize the experimental conditions for highest CrVI removal efficiency (CrVI 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 CrVI as an index for CrVI removal efficiency (RE). In the dark system, the highest CrVI RE (98.1%) was obtained under the following conditions: pH = 1, reaction time = 7 d and FeIII dosage = 0.110 mM. In the light-irradiated system, the optimal CrVI RE of 98.3% was observed in pH = 1, reaction time = 5 d and FeIII dosage = 0.075 mM. Almost all reduced CrIII remained in the solution even at high pH value. No adsorption or precipitation of CrIII 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). [Display omitted] •The CrVI removal process in a CrVI/HA/FeIII/dark-light-irradiated system was modeled.•RSM and BBD were applied to predict responses and optimize influential factors.•Influence mechanism of each factor on the responses was discussed.•HA was analyzed before and after reaction with CrVI in selected run by XPS and AFM.