Polymer–iron oxide nanofiber composites for lead removal: performance improvements through organic acid stabilization of nanoparticles to promote surface segregation during electrospinning†

Abstract

Herein we developed nanofiber composite membranes made of polyacrylonitrile (PAN) and iron oxide nanoparticles using a one-pot electrospinning synthesis method for application in point-of-use (POU) water treatment devices targeting both dissolved and particulate lead. With the goal of optimizing lead removal while minimizing raw material costs, we explored different commercially available iron oxides and incorporated simple organic acids (OAs) [e.g., ortho- and tera-phthalic acid (PTA and TPTA) and ethylenediaminetetraacetic acid (EDTA)] based on our previous observation that sodium dodecyl sulfate (SDS) promotes enrichment of iron oxide at the electrospun nanofiber surface (i.e., surface segregation). From sorption isotherm studies, we found that increasing iron oxide loading led to higher lead uptake (e.g., PAN with 5 wt% iron oxide exhibited a lead removal capacity of 10 mg g⁻¹ of mat versus 5 mg g⁻¹ for 1 wt% iron oxide). PAN with 5 wt% iron oxide (3.3 mg lead removal per $) also resulted in better cost-normalized lead removal than PAN with 1 wt% iron oxide (1.0 mg lead removal per $). The integration of OAs further improved performance; for example, PAN with 5 wt% iron oxide and 3 wt% PTA achieved approximately 40 mg g⁻¹. From nanofiber characterization via microscopic (SEM and TEM) and spectroscopic (XPS and FTIR) tools, OAs increase lead uptake through a combination of pathways: (1) stabilizing iron oxide particles and improving their dispersion in electrospinning sol gels; (2) promoting surface segregation that increases iron oxide concentration at the nanofiber surface; (3) functioning as a porogen that increases composite surface area; and (4) introducing some additional lead binding sites (e.g., carboxylates) within the nanofiber. Simulating point-of-use application in a dead-end filtration system (effective filter area of 12.6 cm², filter thickness of 120 μm, and flow rate of 20 mL min⁻¹), we observed lead-free permeate with just 0.24 g of our optimal formulation when challenged with 4 L of 150 μg L⁻¹ soluble lead solution and 90% removal when this filter was challenged with a feed solution containing both dissolved and particulate lead (160 μg L⁻¹ total lead with 30% of particulate lead; >0.1 μm). Our study highlights the potential for OAs to enhance the performance of polymer–metal oxide nanofiber composites via a one-pot synthesis that will help to minimize production costs for high-performing materials.