Reaction Behaviors of Nanoscale Fe3O4 and [Fe3O4]MgO Slurry Injection Coupled with the Electrokinetic Process for Remediation of NO3− and Cr6+ in Saturated Soil

The aim of this study was to investigate the reaction behaviors of nanoscale Fe3O4 and H1/10-[Fe3O4]MgO slurry injection coupled with the electrokinectic (EK) process for remediation of NO3− and Cr6+ in saturated soil. To assure the above-mentioned nanomaterials were capable of reductively adsorbing inorganic pollutants (e.g., NO3− and Cr6+) in the acidic environment in the anode reservoir of the ek remediation system, an investigation on transformation of the concerned nanomaterials in different aqueous solutions (de-ionized water and simulated groundwater ) of different initial pHs (2 and 3.5) was conducted. Due to a high dose of nanoscale Fe3O4 and a resulting serious agglomeration while adsorbing NO3− and Cr6+, the characteristic peaks of the X-ray diffraction (XRD) analysis for nanoscale Fe3O4 remained the same after adsorption experiments. But the situations were quite different in the case of nanoscale H1/10-[Fe3O4]MgO, the characteristic peaks of £\-Fe2O3 in the XRD pattern were detected, confirming that this nanomaterial could reductively adsorb NO3− and Cr6+ in the acidic environment.The effectiveness of using polyacrylic acid (PAA) and soluble starch (SS) to stabilize nanoscale Fe3O4 and H1/10-[Fe3O4]MgO in different aqueous solutions containing humic acid was compared. It was found the former yielded a better stability. Therefore, PAA was chosen to prepare the slurries of target nanomaterials. Then slurry injection coupled with the EK process was tested for remediation of NO3- and Cr6+ in saturated soil. The results showed that the removal efficiency of NO3− was more than 90%, and the NO3− concentration in the anode reservoir was below Taiwan¡¦s Pollution Control Standards of type¢¹Groundwater for NO3−-N. Under the same test conditions, however, the removal efficiency of Cr6+ was unsatisfactory. This might be ascribed to acidification of soil near the anode resulting in high adsorption of Cr2O72− by soil. Thus, a solution to solve this problem has to seeked.The solution lies in how to enhance the contact of the above-mentioned nanomaterials with Cr6+ in the anode reservoir. One possibility is to use the nature of SS would hydrolyze in the acidic environment. Therefore, SS-stabilized nanomaterials in the acidic environment would hydrolyze resulting in the exposure of the soil nanomaterials therein. To this end, SS was used to replace PAA for nanomaterial slurry preparation for remediation of Cr6+. In addition, polarity reversal was practiced in the EK system to maintain a neutral ph of soil and increase the mobility of Cr6+ in soil. Finally, the result showed that nanoscale Fe3O4 and H1/10-[Fe3O4]MgO slurry injection coupled with the polarity reversal electrokinetic system could really enhance the removal efficiency of Cr6+ in the saturated soil.In summary, nanoscale Fe3O4 and H1/10-[Fe3O4]MgO slurry injection coupled with the EK process has been proven to be capable of remedying NO3− and Cr6+ in saturated soil. Meanwhile, the concept of reductive adsorption was realized in this work as well.