Alumina Nanorods Synthesized from Chromium‑Containing Alumina Sludge: Influence of Cr, Fe, and Mg Doping on Crystal Transformation

Background

Low‑dimensional alumina—such as nanofibers and nanorods—offers exceptional strength, high elastic modulus, chemical stability, low thermal conductivity, and superior insulation, making it attractive for composites, catalysts, adsorbents, membranes, coatings, and battery anodes. However, the high cost of production limits widespread adoption. Numerous synthesis routes exist, including solid‑phase, vapor‑phase, and liquid‑phase methods. Among them, the liquid‑phase precipitation route is preferred for its mild conditions, homogeneous products, and low cost.

Chromium‑containing alumina sludge is a hazardous waste generated during non‑calcium roasting of chrome ore. Each ton of chrome product produces roughly 7 t of sludge, composed of 55–65 % Al₂O₃, 7–13 % chromium (as Cr(III) or the carcinogenic Cr(VI)), and trace silicon, iron, magnesium, and sodium. Conventional treatment involves detoxification—converting Cr(VI) to Cr(III)—or direct utilization of the sludge in refractory materials, both of which either occupy land or generate secondary pollution. Converting the sludge into high‑value alumina nanorods offers a dual benefit: pollution abatement and resource recovery.

In this study, alumina nanorods were prepared from the sludge via precipitation–calcination. Undoped, Cr‑doped, Fe‑doped, and Mg‑doped samples were also synthesized to isolate the effect of each ion. The resulting materials were characterized by XRD, FT‑IR, TG‑DSC, TEM, EDS, and XPS to elucidate phase evolution, microstructure, and chemical state changes.

Methods

Materials

Analytical‑grade reagents (aluminum sulfate octadecahydrate, chromium sulfate, ferric sulfate, magnesium sulfate, sodium hydroxide, sulfuric acid, sodium dodecyl benzene sulfonate) were used. Chromium‑containing alumina sludge was supplied by CITIC Jinzhou Metal Co., Ltd. All solutions were prepared with de‑ionized water.

Treatment of the Chromium‑Containing Alumina Sludge

The sludge was first washed and filtered (solid–liquid ratio 1:5 g/mL) to remove soluble Cr(VI). The residue was dissolved in sulfuric acid (ratio 1:3 g/mL), then oxidized with H₂O₂ to convert any remaining Cr(VI) to Cr(III). The resulting acidic solution was analyzed by titration and VIS spectroscopy (Varian 721N) and its composition is shown in Table 2.

Synthesis of Alumina Nanorods

A 0.25 M Al₂(SO₄)₃ solution was mixed with 2 M NaOH and dodecyl benzene sulfonate under stirring at 85 °C. The pH was adjusted to 9.0, the mixture was stirred for 5 h, then aged for 20 h. The precipitates were collected, washed with de‑ionized water and ethanol, and vacuum‑dried at 40 °C for 15 h. Calcination was performed in a programmable furnace: 250 °C / 1 h, 400 °C / 1 h, 770 °C / 1 h, 900 °C / 1 h, 1050 °C / 2 h. Undoped samples used pure Al₂(SO₄)₃; doped samples incorporated Cr, Fe, or Mg chlorates at concentrations matching those in the sludge (Table 2). Samples derived directly from the sludge were labeled “sludge‑derived.”

Characterization

Phase identification: XRD (Rigaku D/MAX‑RB, Cu Kα, 2θ = 10–70°, 2°/min). Functional groups: FT‑IR (Thermo Electron Scimitar 2000, 4000–400 cm⁻¹). Thermal stability: TG‑DSC (NETZSCH STA449F3, 15–1200 °C, 10 °C/min, air). Morphology and lattice: FETEM (JEOL Jem‑2100F). Surface chemistry: XPS (ESCAMABMKLL, Al Kα). Elemental distribution: EDS (JEOL).

Results and Discussion

XRD Characterization of the Alumina Nanorods

The XRD patterns (Fig. 1) reveal the presence of both α- and θ-Al₂O₃ phases. In undoped rods, θ-Al₂O₃ peaks are weak, indicating partial transformation to α-Al₂O₃ during calcination. Cr‑doped rods exhibit stronger θ-Al₂O₃ peaks and weaker α-Al₂O₃, confirming that Cr retards the phase transition. Conversely, Fe‑ and Mg‑doped rods show pronounced, sharper α-Al₂O₃ peaks, reflecting accelerated crystallization. Sludge‑derived rods display dominant θ-Al₂O₃ with low crystallinity, due to the high impurity load that impedes phase conversion.

XRD patterns of alumina nanorods doped with different ions: a undoped alumina, b Cr-doped alumina, c Fe-doped alumina, d Mg-doped alumina, and e sludge-derived alumina.

FT‑IR Spectra of the Alumina Nanorods

All samples exhibit O–H stretching (3500–3300 cm⁻¹) and H₂O bending (1635 cm⁻¹), indicating adsorbed water. The 2360 cm⁻¹ band corresponds to CO₂. In the fingerprint region, undoped rods show characteristic α-Al₂O₃ vibrations at 829, 589, and 449 cm⁻¹ and θ-Al₂O₃ at 762, 663, and 488 cm⁻¹. Cr doping reduces the α-Al₂O₃ signals, while Fe and Mg doping weaken the θ-Al₂O₃ bands and shift peaks slightly, reflecting changes in bonding. Sludge-derived rods lack α-Al₂O₃ features, confirming the absence of the stable phase.

FT‑IR spectra of nano alumina rods doped with different ions: a undoped alumina, b Cr-doped alumina, c Fe-doped alumina, d Mg-doped alumina, and e sludge-derived alumina.

TG‑DSC of Alumina Nanorods

The TG–DSC curves (Fig. 3) show three mass‑loss stages for undoped precursors: 40 % loss below 250 °C (moisture), 35 % between 250–730 °C (dehydroxylation and transformation to θ-Al₂O₃), and a minor loss above 730 °C (conversion to α-Al₂O₃). Doping shifts the endothermic peaks to higher temperatures and broadens them, indicating that Cr, Fe, and Mg alter the kinetic pathways of phase transformation. These thermal trends align with XRD and FT‑IR observations.

TG and DSC of the nano alumina rod precursors doped with different ions: a undoped alumina, b Cr-doped alumina, c Fe-doped alumina, d Mg-doped alumina, and e sludge-derived alumina.

TEM, SAED, and HRTEM Images of Alumina Nanorods

Undoped rods are uniformly dispersed with diameters of 4–6 nm and lengths of 20–60 nm. SAED patterns confirm mixed θ- and α-Al₂O₃ phases, and HRTEM shows interplanar spacings of 0.273 and 0.284 nm (θ) and 0.255 and 0.348 nm (α). Cr-doped rods are longer (50–120 nm) but maintain similar lattice spacings, indicating that Cr does not drastically alter the crystal lattice. Fe- and Mg-doped rods exhibit improved crystallinity with sharper diffraction spots and slightly altered interplanar distances, consistent with their accelerated phase transition. Sludge-derived rods are dispersed, 4–6 nm in diameter and 50–100 nm in length, but lack any α-Al₂O₃ spots, reflecting the suppression of stable phase formation by co‑dopants.

TEM, SAED, and HRTEM of alumina nanorods doped with different ions: a undoped alumina, b Cr-doped alumina, c Fe-doped alumina, d Mg-doped alumina, and e sludge-derived alumina. (1) TEM; (2) SAED; (3) HRTEM.

EDS Characterization of Alumina Nanorods Precursor Doped with Different Ions

EDS quantification (Table 3) shows dopant levels of 2.06 % Cr, 0.99 % Fe, and 0.58 % Mg in single‑doped precursors—close to the target amounts. In sludge‑derived precursors, Cr is 2.11 %, Fe 0.14 %, and Mg 0.96 %, confirming that Cr and Mg dominate the incorporation, while Fe is largely suppressed.

XPS Characterization of Nanometer Alumina Fibers Doped with Different Ions

O 1s spectra reveal binding energies of 531.90 eV (undoped), 531.85 eV (Cr-doped), 531.15 eV (Fe-doped), 531.20 eV (Mg-doped), and 532.00 eV (sludge-derived). These values indicate a slight shift toward higher energy for samples with more θ-Al₂O₃. Al 2p peaks at 74.00, 74.25, 74.75, 74.38, and 73.90 eV correspond to the respective samples, confirming the Al³⁺ oxidation state. Fe 2p spectra show both Fe(II) and Fe(III) signals in Fe-doped rods, whereas Cr 2p spectra display Cr(VI) and Cr(III) components, with Cr(VI) largely absent in sludge-derived rods. Mg 2p indicates MgO formation in Mg-doped rods, but is weak in sludge-derived samples, consistent with low Mg content.

XPS spectra of a O 1s and b Al 2p for alumina nanorods doped with different ions.

XPS spectra of a Cr 3+ 2p, b Fe 3+ 2p, and c Mg 2+ 2p.

Conclusions

Chromium, iron, and magnesium are successfully incorporated into alumina nanorods. Cr doping impedes the θ→α transformation, while Fe and Mg accelerate it. Co-doping from the sludge severely restricts phase conversion, yielding predominantly θ-Al₂O₃ with limited crystallinity. The dopants modify the lattice, alter O–Al bonding, and influence thermal behavior, as confirmed by XRD, FT‑IR, TG‑DSC, TEM, EDS, and XPS. These insights demonstrate that chromium-containing alumina sludge can be valorized into high‑performance alumina nanorods, offering a sustainable solution to waste management and resource recovery.

Abbreviations

BE

Binding energy

DSC

Differential scanning calorimetry analysis

EDS

Energy‑dispersive spectrometer

FETEM

Field‑emission transmission electron microscopy

FT‑IR

Fourier transform infrared spectra

HRTEM

High‑resolution transmission electron microscopy

SAED

Selected area electron diffraction

TEM

Transmission electron microscopy

TG

Thermogravimetric analyzer

XPS

X‑ray photoelectron spectroscopy

XRD

X‑ray powder diffraction