Nanocrystalline iron hydroxide lignocellulose filters for arsenate remediation

Abstract

Harmful levels of environmental contaminants, such as arsenic (As), persist readily in the environment, threatening safe drinking water supplies in many parts of the world. In this paper, we present a straightforward and cost-effective filtration technology for the removal of arsenate from potable water. Biocomposite filters comprised of nanocrystalline iron oxides or oxyhydroxides mineralized within lignocellulose scaffolds constitute a promising low cost, low-tech avenue for the removal of these contaminants. Two types of iron oxide mineral phases, 2-line ferrihydrite (Fh) and magnetite (Mt), were synthesized within highly porous balsa wood using an environmentally benign modification process and studied in view of their effective removal of As from contaminated water. The mineral deposition pattern, minerology, as well as crystallinity, were assessed using scanning electron microscopy, transmission electron microscopy, micro-computed X-ray tomography, confocal Raman microscopy, infrared spectroscopy, and X-ray powder diffraction. Our results indicate a preferential distribution of the Fh mineral phase within the micro-porous cell wall and radial parenchyma cells of rays, while Mt is formed primarily at the cell wall/lumen interface of vessels and fibers. Water samples of known As concentrations were subjected to composite filters in batch incubation and gravity-driven flow-through adsorption tests. Eluents were analyzed using microwave plasma optical emission spectroscopy (MP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). By subjecting the filters to a flow of contaminated water, the time for As uptake was reduced to minutes rather than hours, while immobilizing the same amount of As. The retention of As within the composite filter was further confirmed through energy-dispersive X-ray mappings. Apart from addressing dangerously high levels of arsenate in potable water, these versatile iron oxide lignocellulosic filters harbor tremendous potential for addressing current and emerging environmental contaminants that are known to adsorb on iron oxide mineral phases, such as phosphate, polycyclic aromatic hydrocarbons or heavy metals.

Fig. 1. (A) Schematic of in situ synthesis of ferrihydrite (Fh) mineral within the cell wall of thin-walled balsa wood. (B) Schematic of in situ synthesis of magnetite at the cell wall/lumen interface. (C) Field-emission SEM image of ferrihydrite-balsa cross section. Scale bar equals 10 μm. (D) Schematic of batch incubation filtration and gravity-driven flow-through filtration of As-contaminated drinking water.

Fig. 2. X-ray microtomography of Fh mineralized balsa scaffolds. X-ray absorbent regions pertaining to the Fh mineral phase were false coloured in turquoise to visualize their three-dimensional distribution. (A) 3D volumetric rendering of the mineralized composite highlighting the extensive mineralization of vessels and rays throughout the sample. The positions of 2D transverse projections are marked. (B–D) Cross-sectional projections showing mineral deposition at the vessel/lumen interfaces, in rays and within fibre cell walls. Scale bars correspond to 1 mm.

Fig. 3. SEM/EDS mappings and microanalysis. Elemental mappings of Fh composite showing carbon (A) and iron (B) distribution. Scale bar corresponds to 50 μm. (C) TEM image shows formation of nanocrystalline Fh particles throughout compound middle lamella (CML), S1, and S2 cell wall layers and at S3 cell wall/lumen interface. Scale bar corresponds to 1 μm. Confocal Raman microscopy: chemical mapping illustrating the distribution of cellulose |1068–1190 cm⁻¹| (D) and ferrihydrite |650–750 cm⁻¹| (E) across Fh mineralized wood cells. Scale bars correspond to 10 μm. (F) Average Raman spectrum from region of interest marked in E (upper curve) and pure Fh powder (lower curve).

Fig. 4. Powder X-ray diffraction from respective mineral phases precipitated in reaction solution. (A) Magnetite powder XRD with labelled Bragg reflections. (B) Two broad XRD peaks indicate the formation of 2-line ferrihydrite.

Fig. 5. Adsorptive capacity of iron oxide-wood filters tested in (A–C) batch incubation tests and (D and E) gravity-driven flow-through tests. (A) MP-AES data were obtained from batch incubation tests in 1 mg L⁻¹ (1 ppm) As(v) solutions for 24 hours. (B) SEM-EDS mapping of Fh-modified wood material after exposure to 1 mg L⁻¹ (1 ppm) As(v) in batch incubation test. (C) ICP-MS data quantified As(v) concentrations after immersion of filters in 100 μg L⁻¹ (100 ppb) As(v) solutions for 24 hours. (D) As shown by MP-AES, As(v) concentration decreased as a function of filter length as 1 mg L⁻¹ (1 ppm) As(v) solutions of were passed through the samples. (E) ICP-MS data showed a substantial decrease in As(v) levels after gravity-driven flow-through test with 100 μg L⁻¹ (100 ppb) As(v) solutions.