title: Performance enhancement of zero valent iron based systems using depassivators: Optimization and kinetic mechanisms.
abstract: The long-term ability of Zero-Valent Iron (ZVI) in contaminant removal relies on the effectiveness of iron to serve as electron donor, which makes it a versatile remediation material. However, the formation of oxide and hydroxide layers results in passive layer on ZVI surface during contaminant removal hinders its reactivity. The focus of this research was to evaluate the performance of corrosive agents such as acetic acid (HAc), aluminium sulphate (Alum) and potassium chloride (KCl) as depassivators to overcome passivation for sustainability and longevity. Batch experiments using seven combinations of the above chemicals were conducted to optimize the dosage of depassivators based on passive layer removal. The influence of depassivators in catalytic activity of ZVI in removing Cr(6+) was evaluated. The passive layer on ZVI particles was characterized using Scanning Electron Microscopy (SEM) and confirmed by Energy-Dispersive X-ray spectroscopy (EDAX) analysis. The major mechanisms in passive layer removal was found to be H(+) ion embrittlement followed by uniform depassivation when [HAc] was used and pitting corrosion when [Alum] and [KCl]were used. All the seven sets of chemicals enabled depassivation, but considering the criteria of maximum depassivation, catalytic activity and long term reactivity the depassivation treatments were effective in order as [HAc-Alum] > [HAc-Alum-KCl] >[HAc] > [Alum] > [HAc-KCl] > [KCl] > [Alum-KCl]. The kinetic rate of ZVI using [HAc-Alum] and [Alum] was relatively unchanged over the pH range of 4-10, made it suitable for ex-situ remediation. This insignificant influence of initial pH in catalytic activity of ZVI along with the improvement in longevity and sustainability makes it suitable for effective water treatment applications. The present work has successfully demonstrated that chemical depassivation can restore considerable reactivity of ZVI in the existing permeable reactive barriers.
authors.affiliation: Environmental and Water Resources Division, Department of Civil Engineering, Indian Institute of Technology Madras, Chennai, India.
authors.forename: Karim Vayalunkal Karottu
authors.initials: KVK
authors.lastname: Ansaf
authors.affiliation: Environmental and Water Resources Division, Department of Civil Engineering, Indian Institute of Technology Madras, Chennai, India.
authors.forename: Selvaraj
authors.initials: S
authors.lastname: Ambika
authors.affiliation: Environmental and Water Resources Division, Department of Civil Engineering, Indian Institute of Technology Madras, Chennai, India. Electronic address: indunambi@iitm.ac.in.
authors.forename: Indumathi Manivannan
authors.initials: IM
authors.lastname: Nambi
mesh_terms: D003343:Corrosion; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D014874:Water Pollutants, Chemical / Q000737:chemistry*; D018508:Water Purification
chemical_list: D014874:Water Pollutants, Chemical; D007501:Iron
keywords: Depassivation; Hexavalent chromium reduction; PRB’s life span; Pitting dissolution and hydrogen ion embrittlement; Zero valent iron