Hollow Polyaniline Microparticles Deliver 127.9 mg/g Cr(VI) Removal for Sustainable Wastewater Treatment

Abstract

We report a scalable synthesis of hollow polyaniline (PANI) micro/nanospheres via soft‑templated monomer polymerization in an alkaline medium using Triton X‑100 micelles. The resulting nanostructure exhibits rapid, high‑capacity adsorption of hexavalent chromium (Cr(VI)) across a broad pH spectrum, achieving a maximum removal of 127.88 mg g⁻¹ at pH 3. Acid regeneration restores the material’s performance, underscoring its practical reusability.

Background

Cr(VI) is a carcinogenic pollutant released by chromium plating, leather tanning, metal finishing and textile manufacturing. Its high mobility and toxicity pose severe health risks, including kidney failure and liver cancer. Conventional reduction methods using sulfites or ferrous salts generate non‑recyclable waste, driving the need for novel, eco‑friendly adsorbents. Conductive polymers, particularly polyaniline (PANI), have attracted attention for Cr(VI) remediation due to their facile synthesis, low cost and rich amine functionality that can donate electrons for reduction. However, bulk PANI suffers from limited porosity, restricting its surface area and adsorption efficiency. Hollow PANI microspheres, with their internal cavities, dramatically increase the available active sites, enhancing Cr(VI) uptake. Existing hard‑templated approaches are laborious and compromise structural integrity. Our soft‑templated method overcomes these drawbacks, delivering high‑yield, reproducible hollow spheres.

Methods

Aim

To fabricate a scalable, recyclable hollow PANI nanomaterial capable of removing Cr(VI) from industrial effluents.

Materials

Aniline, NaOH, Triton X‑100, and ammonium persulfate (APS) were used as received.

Synthesis

In a typical run, 32 mmol aniline, 32 mmol NaOH, and 0.82 mmol Triton X‑100 were dispersed in 20 mL deionized water under magnetic stirring for 20 min, cooled in an ice bath, then mixed with 32 mmol APS (pre‑cooled). The reaction proceeded for 12 h at 0 °C. The product was repeatedly washed with water and ethanol until the filtrate was clear, then dried at 60 °C for 24 h.

Characterization

Morphology: FESEM (Sirion 200) and TEM (JEOL‑2010). Structure: XRD (Philips X’Pert) and FTIR (JASCO FT‑IR 410). Composition: ICP and UV‑Vis for Cr quantification; XPS (ESCALAB 250) for oxidation states. Size and zeta potential: Zetasizer 3000HS.

Cr(VI) Removal Test

Potassium dichromate solutions (initial 62.4 mg L⁻¹) were prepared and diluted to desired concentrations. 10 mg PANI was dispersed in 20 mL of the Cr(VI) solution at specified pH (adjusted with NaOH/HCl), stirred for 3 h, centrifuged, and the supernatant’s pH was readjusted to 7.5–8.5. The equilibrium removal capacity (q_e) was calculated by q_e=((c₀−c_e)V)/m.

Kinetics

Removal was monitored at pH 3, 4, 5. At each time point, the solution was centrifuged and c_e measured by UV‑Vis at 350 nm. Data were fitted to the pseudo‑second‑order equation t/q_t=1/(k₂q_e²)+t/q_e.

Isotherm

Equilibrium adsorption at various initial Cr(VI) concentrations was analyzed with the Langmuir model: c_e/q_e=1/(q_mk_L)+c_e/q_m.

Results and Discussion

Morphology

FESEM images reveal uniform hollow spheres (0.5–2 µm diameter) with visible central cavities (Figure 1a,b). TEM confirms the hollow architecture.

Structure

FTIR spectra show characteristic bands at 1569 cm⁻¹ (quinoid C‑N), 1496 cm⁻¹ (benzenoid C‑N) and 1142 cm⁻¹ (N‑Q‑N), indicating predominant emeraldine form. XRD displays sharp peaks at 20.2° and 25.4°, confirming high crystallinity.

Cr(VI) Removal Kinetics

Rapid decoloration occurs within 90 min at pH 3. The pseudo‑second‑order model fits the data excellently (R²>0.998), yielding q_e values of 80.65, 37.48 and 21.56 mg g⁻¹ for pH 3, 4, 5 respectively.

Effect of pH

Removal capacity increases sharply as pH decreases from 12 to 2, reaching 127.88 mg g⁻¹ at pH 3. Below pH 2, total Cr rises due to Cr(III) desorption, likely driven by electrostatic repulsion. Thus, optimal removal occurs between pH 2–4.

Isotherm Analysis

Langmuir fitting (R²>0.998) yields maximum capacities q_m of 127.88 mg g⁻¹ (pH 3), 43.20 mg g⁻¹ (pH 4) and 25.61 mg g⁻¹ (pH 5), confirming monolayer adsorption on a homogeneous surface.

Mechanism

Cr(VI) is first electrostatically adsorbed as HCrO₄⁻ onto protonated emeraldine PANI, then reduced to Cr(III) while PANI oxidizes to pernigraniline. Cr(III) remains ionically bound to the PANI surface. XPS shows a Cr 2p₃/₂ binding energy of 577.4 eV, characteristic of Cr(III). The hollow morphology provides extensive surface area (BET 32.8 m² g⁻¹) and internal pores for rapid mass transfer.

Regeneration

Immersion of spent spheres in 1 M HCl for 30 min restores the emeraldine state and recovers ~95 % of the original capacity, demonstrating robust reusability.

Comparison with Literature

Table 1 summarizes that our hollow PANI spheres surpass many reported adsorbents in capacity and recyclability.

Conclusions

We have demonstrated a facile, soft‑templated synthesis of hollow PANI micro/nanospheres that achieve 127.9 mg g⁻¹ Cr(VI) removal at pH 3, with rapid kinetics, monolayer adsorption behavior, and simple acid‑based regeneration. These attributes position the material as a promising, sustainable solution for industrial Cr(VI) remediation.