The sequestration of aqueous Cr(VI) by zero valent iron-based materials: From synthesis to practical application

In recent years, zero-valent iron (ZVI) has been extensively employed for the elimination of organic and inorganic contaminants. However, the performance of ZVI was restrained due to the inherent properties in the process of pollutants sequestration like agglomeration, surface passivation, and sensitivity to the pH and dissolved oxygen (DO) in the environment. To combat these issues, ZVI-based materials were utilized to attenuate the drawbacks of ZVI. Therefore, in this review, the representative hazardous hexavalent chromium (Cr(VI)) was chosen as the target pollutant to discuss the performance, limitations, and future of ZVI-based materials. The prevailing preparation methods of ZVI-based materials could be classified into aqueous reduction and mechanical procedures. Further, the conventional ZVI-based materials were mainly encompassed carbon-ZVI, sulfur-ZVI, bimetallic materials of ZVI, and magnetite-ZVI composites. A new insight into the co-effect of pH and DO on Cr(VI) removal by ZVI through five pathways was also proposed. The mechanism of Cr(VI) elimination by ZVI-based materials was dominant through the combination of reduction, adsorption, and co-precipitation, wherein the enhanced reduction capability of ZVI-based materials compared to their monometallic counterpart was critically scrutinized. Besides, some field applications of ZVI-based materials such as ZVI incorporation into the permeable reactive barrier (PRB) to remediate groundwater have also been examined. Finally, barriers in market penetration of ZVI-based materials in removing Cr(VI) have been highlighted which would open a new window for the researcher to accomplish the research gaps for shifting applications of ZVI-based materials from lab-scale to real or commercial implementations.