Hexavalent Chromium Removal from Residual Ion-Exchange Brine Using Ferrous Sulfate
Creators: name: De Araujo Silva, Ana Luiza 
Publisher: name: University of Nevada, Las Vegas
Year: 2018

description: Removal of hexavalent chromium (Cr(VI)) from water and wastewater is required for health purposes and regulation compliance. Among the technologies available, ion exchange (IX) has been extensively applied and, in most cases, is followed by resin regeneration for the purpose of cost optimization. However, resin regeneration produces residual IX brine, also called waste regenerant brine, which is considered hazardous for containing the contaminant(s) treated in the column. This research focuses on the use of ferrous sulfate (FeSO4) to reduce and precipitate Cr(VI) contained in the regenerant brine from IX resins. The Cr(VI) reduction to trivalent chromium (Cr(III)) occurs due to the addition of FeSO4 to the solution, which reduces the electric potential of the water providing a suitable environment for Cr(VI) reduction. Once Cr(III) is formed, it will react with water forming chromium hydroxide (Cr(OH)3), which is a solid precipitate that can be removed out of the solution by settling and/or filtration. The brine solution tested in this research was collected from IX resin regeneration from a previous project, which used IX columns to treat a synthetic aqueous solution containing 10 mg-Cr(VI)/L. A total of 37 batch tests were performed to evaluate the impact of major parameters (i.e., initial Cr(VI) concentration, pH and dosage of reducing agent - FeSO4) on Cr(VI) removal from the ion-exchange brine solution. 
         Response Surface Methodology (RSM) was applied to statistically develop a model able to predict the removal of Cr(VI) from IX brines given the three identified major influencing variables (pH, initial Cr(VI) concentration, and ferrous sulfate dosage). Batch experiments were performed following experimental arrangements provided by the Central Composite Design (CCD) model, which is a type of RSM. A total of 34 batch tests were executed for the CCD model, 39 tests for evaluation of the impact of Fe(II):Cr molar ratio on Cr(VI) removal efficiency, and 32 tests to evaluate the impact of pH on Cr(VI) removal efficiency. 
         The findings of this research suggest that the major parameters and their interactions do have an impact on Cr(VI) removal from IX brine solution and complete Cr(VI) removal was observed in different scenarios. Experiments with the lowest initial Cr(VI) concentration tested (20 mg/L) showed 100% Cr(VI) removal efficiency when pH was 7.2, 6.4 and 3.5, for coagulant doses of 4.2, 1.9 and 3.08 mM, respectively. Experiments with the original brine collected from resin regeneration (40 mg-Cr(VI)/L) demonstrated successful results for a greater range of pH (2.8 to 7.2) but constant coagulant dosage (3.08 mM). Finally, for the most concentrated solution (130 mg-Cr(VI)/L) results as complete Cr(VI) removal was only observed for pH 7.2 and FeSO4 dose ³ 10 mM.  
         From the statistical analysis of the CCD quadratic model with interactions, it was observed that the interaction and some of the quadratic terms were not considered statistically significant at the 95% confidence level. The model could perhaps be improved by generating more experimental data. On the other hand, a second quadratic model was also generated by dropping all the non-significant variables (p-value > 0.05). The statistical analysis for the latter model showed that all terms included in the model were statistically significant at the 95% confidence level.
         The sludge generated in the experiments was observed to have a linear correlation with the amount of coagulant added, and therefore, tests containing the highest dose of FeSO4 produced the highest volume of sludge. Analysis other than volume measurement were not incorporated in this research, but it can be inferred that sludge composition consists of chromium and ferrous hydroxides.  
         Overall, the findings of the present study suggest that FeSO4 is an efficient Cr(VI) reducing agent and can be applied in the treatment of residual IX brine. Moreover, the regression equation referent to the quadratic model can be used to predict Cr(VI) removal from IX brine at the 95% confidence level.