Synthesis and optimization of Fe₂O₃ nanofibers for chromate adsorption from contaminated water sources

Highlights

• We synthesized tunable Fe₂O₃ nanofibers via electrospinning.

• We examined property changes as a function of nanofiber diameter.

• Adsorption isotherm and kinetic studies toward chromate removal were conducted.

• The d = 23 nm Fe₂O₃ nanofibers showed optimal chromate adsorption performance.

• The enhanced performance was associated with increased specific surface area.

Abstract

In this work, α-Fe₂O₃ nanofibers were synthesized via electrospinning and characterized to observe optimal morphological and dimensional properties towards chromate removal. The Fe₂O₃ nanofiber samples were tested in aqueous solutions containing chromate $\left({{\text{CrO}}_4}^{2-}\right)$ to analyze their adsorption capabilities and compare them with commercially-available Fe₂O₃ nanoparticles. Synthesized Fe₂O₃ nanofibers were observed with a variety of different average diameters, ranging from 23 to 63 nm, while having a constant average grain size at 34 nm, point zero charge at pH 7.1, and band gap at 2.2 eV. BET analysis showed an increase in specific surface area with decreasing average diameter, from 7.2 to 59.2 m²/g, due to the increased surface area-to-volume ratio with decreasing nanofiber size. Based on ${{\text{CrO}}_4}^{-2}$ adsorption isotherms at pH 6, adsorption capacity of the Fe₂O₃ nanofibers increased with decreasing diameter, with the 23 nm sized nanofibers having an adsorption capacity of 90.9 mg/g, outperforming the commercially-available Fe₂O₃ nanoparticles by nearly 2-fold. Additionally, adsorption kinetics was also analyzed, increasing with decreasing nanofiber diameter. The enhanced performance of the nanofiber is suggested to be caused solely due to the increased surface area, in part by its size and morphology. Electrospun Fe₂O₃ nanofibers provide a promising solution for effective heavy metal removal through nanotechnology-integrated treatment systems.

Introduction

Hematite (α-Fe₂O₃) is one of the most abundant and most stable forms of oxidized iron minerals found on the Earth (Schwertmann and Cornell, 2000). α-Fe₂O₃, alongside other iron oxides and hydroxides, can be introduced naturally into the pedosphere and hydrosphere from the lithosphere during rock weathering (Schwertmann and Cornell, 2000). In these environments, iron oxides have shown to regulate the concentration of organic and inorganic constituents. In addition to its dominant usage in the iron and steel industry, α-Fe₂O₃ has also been used in a wide variety of applications, such as pigments, gas-sensors, field effect transistors, batteries, magnetic storages, and photoelectrolysis reactors (Zheng et al., 2009, Zhu et al., 2006). One prominent application of iron oxides, especially α-Fe₂O₃, is as an adsorbent for the removal of harmful heavy metals in water sources (Streat et al., 2008).

Heavy metals present a prevalent water supply dilemma through their discovery in groundwater sources, a significant supply of available drinking water. Heavy metals have been used for thousands of years, so their emergence and adverse effects have been known and understood for a long time (Jarup, 2003). However, exposure to these contaminants continues around the world and increasingly in developing countries (Jarup, 2003). Chromium (Cr), an example of a carcinogenic heavy metal, is very common in diverse metal industry processes, such as electroplating, leather tanning, wood preservations and chemical industries (Chowdhury and Yanful, 2010, Ren et al., 2013). It has been reported that occurrences of Cr entering the environment are due to leakage, poor storage or unsafe disposal practices from such metal industry processes (Chowdhury and Yanful, 2010).

Adsorption, a common treatment method for heavy metal removal, is the adhesion of atoms or molecules from a solution onto the surface of a highly porous material (Jeong et al., 2007). Fe₂O₃ is one of the most commonly used adsorbent materials for heavy metal removal. There has been extensive research on the performance of different iron minerals, such as iron oxides, iron hydroxides, iron oxide-hydroxides, and ferrihydrite, towards adsorption of heavy metals (Jeong et al., 2007). Of them all, α-Fe₂O₃ has attracted considerable attention as an adsorbent in water treatment due to its favorable properties, including its non-toxicity and being the most stable form of Fe₂O₃ under ambient conditions (Zhu et al., 2006). Additionally, studies comparing uptake of certain oxyanions, such as arsenate, across a suit of iron oxides and hydroxides (e.g., hematite, magnetite, and goethite) have found hematite to exhibit the greatest adsorption kinetics and capacity on a surface area basis (Giménez et al., 2007).

As with other metal oxide catalysts used in water treatment (e.g., TiO₂), Fe₂O₃ used for heavy metal removal is commonly in the form of nanoparticles. Fe₂O₃ nanoparticles are injected into the water supply, such as a groundwater basin, and slowly move with the flow of water while accumulating contaminants on its surface, and are subsequently captured by some filtration membrane at the endstream (Crane and Scott, 2012). The use of nanoparticle suspensions is discouraged due to the possible inadvertent release into the environment. Therefore, it is imperative to establish systems that utilize immobilized nanomaterials to encourage the future use of nanotechnology for water quality applications.

For this study, electrospun Fe₂O₃ nanofibers were investigated as an adsorbent for heavy metals. The synthesis of the nanofibers was conducted via electrospinning, a simple and cost-effective synthesis technique that produces solid nanoscale fibers from a polymeric solution with the use of an electrical charge (Dai et al., 2011; Ramakrisha et al., 2005; Thavasi et al., 2008). Fe₂O₃ nanofibers have been researched for gas sensors, magnetic devices, and information storage (Shao et al., 2011, Zheng et al., 2009, Zhu et al., 2006). Normally, iron(III) nitrate (Fe(NO₃)₃) is the popular iron precursor of choice during electrospinning, but precursors such as iron(II) acetate (FeAc₂) and iron(III) 2-ethylhexanoate diispropoxide (C₁₄H₂₉FeO₄) have also been used (Crane and Scott, 2012, Li et al., 2003, Zhu et al., 2006). Despite the large amount of research surrounding Fe₂O₃ nanofibers, the average diameter of these nanofibers is relatively large (100–300 nm) (Crane and Scott, 2012, Eid et al., 2010, Zheng et al., 2009) and there is very little work on Fe₂O₃ nanofibers used for heavy metal adsorption (Ren et al., 2013). The nanofiber dimensions will be heavily exploited to take advantage of the greater surface area-to-volume ratio compared to that of nanoparticles and larger sized nanofibers, where surface adsorption can be optimized.

The overall objective of this study is to synthesize Fe₂O₃ nanofibers and optimize their adsorption capacity for the adsorption of heavy metal pollutants in water. Fe₂O₃ nanofibers were synthesized through the electrospinning process and synthesis parameters were manipulated to control the dimensional and morphological properties. Synthesized nanofibers were characterized via various techniques to first relate observed changes in nanofiber properties to specific adjustments in our electrospinning procedure. Different adsorption experiments were conducted to analyze performance of the Fe₂O₃ nanofibers towards ${{\text{CrO}}_4}^{2-}$. Based on their adsorption capabilities, the structural modifications critical to optimizing treatment efficiency were identified.