Carbothermally Synthesized, Lignin-Based, Embedded And Surface Deposited Nano Zero- Valent Iron for Water Purification: Characteristics and Applications

Abstract

Synthesizing tailor-made materials for remediating wastewater polluted with contaminants of emerging concerns, has become a major concern of the scientific community. Nanoscale zero valent iron (nZVI) together with a biochar (BC) support provide advantageous materials for wastewater purification via adsorption, reduction, complexation and advanced oxidation mechanisms. Two approaches to nZVI-composite engineering have been reported: embedding in support matrix and surface depositing, where the latter being more common. Nonetheless, the behavior of embedded material towards contaminant remediation has not yet been sufficiently studied. Furthermore, the remediation capability of these two materials has not been comparatively evaluated. The present study focuses on preparing and extensively characterizing two materials; nZVI embedded in (Lig-e-nZVI) and surface deposited (Lig-s-nZVI) on lignin BC with subsequent comparative analysis of remedial action of the two materials. SEM, SEM/EDX, XRD, FTIR, proximate and ultimate analysis, point of zero charge, iron leaching and regeneration studies were carried out using pristine lignin BC as the control material. Porous and non-porous structures of the carbothermally prepared materials where crystalline iron was embedded or surface coated were compared along with their surface and bulk elemental compositions. It was evident that although the surface iron content is high in Lig-s-nZVI, the total iron content in both the materials were same. Loaded iron was confirmed to be in the zero valent state as per the observed XRD peak patterns. Surface functional groups and overall surface charge of the materials and iron leaching and depletion of remediation capacity were also analyzed. Pharmaceutical precursors p-nitroaniline (pNA) and p-nitrophenol (pNP) were used as sample molecules in this study due to their toxicity and health effects owing to persistence and bioaccumulation. Synergistic adsorptive and degradative behavior of the materials towards pNA and pNP showed an optimum pH of 3.0 and an optimum contact time of 60 minutes. Higher initial adsorption capacity was observed for Lig-s-nZVI and high sustainability and stability was portrayed by Lig-e-nZVI over the regeneration cycles. Both materials showed comparatively increased adsorption capacities in a simulated wastewater matrix. Therefore, it is conclusive that Lig-s-nZVI is of improved remedial capacity towards organic contaminants whereas, Lig-e-nZVI is a better candidate in action over time. Providing a thorough comparison of the properties and remediation performance of nZVI-BC composites synthesized using two commonly employed methods is expected to offer fresh insights to the scientific community.