Document Type : Research Paper
Authors
Department of Chemistry, College of Science, University of Thi-Qar, Iraq
Abstract
Keywords
INTRODUCTION
Contamination of water bodies by synthetic dyes remains one of the most pressing environmental challenges of our time. Textile, paper, and pharmaceutical industries routinely discharge dye-laden effluents into surface and groundwater, where their chemical stability and poor biodegradability allow them to persist [1,2]. Methylene blue (MB), a cationic thiazine dye, is of concern due to its carcinogenic potential and adverse effects on aquatic photosynthesis [3]. Among various treatment strategies, adsorption has gained traction owing to its simplicity, cost-effectiveness, and regeneration possibility [4,5].
Over the past decade, polymeric nanocomposites have attracted interest as adsorbents because their high surface area and tuneable functionality suit them for capturing pollutants [1,6]. Polyazomethines bearing imine (C=N) linkages offer coordination and electrostatic interaction sites, while poly(o-anisidine) brings amine and methoxy functional groups for dye binding [7,8]. Reinforcing with SiO₂ nanofillers boosts thermal stability and porosity [5,9].
The present study synthesizes a ternary SiO₂/PAM/P-o-Ans nanocomposite through sequential condensation–oxidative polymerization and evaluates its MB removal efficiency. The influence of pH, adsorbent dose, contact time, initial concentration, and temperature was investigated. Adsorption data were analysed using Langmuir and Freundlich models, and thermodynamic parameters were calculated.
MATERIALS AND METHODS
Raw Materials
1,3-Benzenedialdehyde and 1,4-diaminobenzene (Santa Cruz, USA), HCl (BDH), potassium persulfate, NaOH, DMF, o-anisidine, glacial acetic acid (CDH), methylene blue (C₁₆H₁₈ClN₃S, MW = 319.85), and deionized water were used as received.
Preparation of SiO₂ Nanoparticles
Nano-silica was synthesized from rice husks by alkaline extraction–acid precipitation [10–12]. Husks were washed, dried at 70 °C, and calcined at 900 °C for 7 h. The ash (5 g) was dissolved in 0.5 M NaOH (500 mL), heated 4 h with stirring. The filtrate was adjusted to pH 7 with HCl, aged 24 h, centrifuged, and dried at 70 °C for 6 h.
Synthesis of Polyazomethine (PAM)
PAM was synthesized by condensation of 1,3-benzenedialdehyde (0.1341 g in 10 mL DMF) with 1,4-diaminobenzene (0.1082 g in 10 mL DMF) at 90 °C with glacial acetic acid catalyst for 12–18 h [13–15]. The product was precipitated, washed, and dried under vacuum.
Synthesis of Poly(o-anisidine)
Poly(o-anisidine) was prepared by oxidative polymerization: o-anisidine (3 mL) in 1 M HCl (10 mL) was polymerized with K₂S₂O₈ (4 g in 10 mL HCl) in an ice bath for 2 h, then left 12 h [16].
Preparation of SiO₂/PAM and SiO₂/PAM/P-o-Ans
For the binary composite, 0.5 g nano-SiO₂ was sonicated in DMF (30 min) and used in the PAM condensation as described above [17,18]. For the ternary composite, SiO₂/PAM (0.5 g) was dispersed in water by ultrasonication, then o-anisidine was polymerized onto it using K₂S₂O₈ in ice-bath conditions (2 h stirring, 12 h aging) [19,20].
Characterization
Materials were characterized by FT-IR (Bruker ALPHA II; Shimadzu IRSpirit, 400–4000 cm⁻¹), XRD (PANalytical, Cu Kα), TEM, FESEM (FEI Inspect F50, 120,000×), AFM (MountainsSPIP Academic), TGA/DSC/DTG (20 °C/min to 900 °C), and GPC.
Batch Adsorption
Adsorption was studied in batch mode (10 mL dye solution + adsorbent, agitated). Residual MB was measured spectrophotometrically (calibration: Ce = (A − 0.0001)/0.0314). R(%) = (C₀ − Ce)/C₀ × 100 and qe = (C₀ − Ce) × V/m. Effects of pH (3–11), mass (0.01–0.05 g), time (15–150 min), concentration, and temperature (25–80 °C) were studied at C₀ = 100 ppm, V = 10 mL.
RESULTS AND DISCUSSION
Characterization
The FT-IR spectrum of nano-SiO₂ (Fig. 1a) showed bands at 3630 cm⁻¹ (O–H of Si–OH), 1221 cm⁻¹ (symmetric Si–O–Si), 1043 cm⁻¹ (asymmetric Si–O–Si), 949 cm⁻¹ (Si–OH), 808 cm⁻¹ (Si–O–Si bending), and 471 cm⁻¹ (Si–O rocking) [21,22]. The nanocomposite spectrum (Fig. 1b) confirmed azomethine formation at 1567 cm⁻¹ (C=N), with bands at 3374 cm⁻¹ (N–H), 1477 cm⁻¹ (C=C aromatic), 1279 cm⁻¹ (C–N), 1003 cm⁻¹ (C–O–C methoxy), and 742 cm⁻¹ (C–H aromatic) [23–25]. The XRD pattern (Fig. 1c) showed a broad hump at 2θ ≈ 22°, confirming amorphous structure [21]. TEM of SiO₂ (Fig. 1d) revealed spherical particles (20–100 nm) with agglomeration, while the nanocomposite TEM (Fig. 1e) showed particles embedded in a polymeric matrix [26,27]. FESEM (Fig. 1f) showed granular surface with particles of 44–77 nm within the polymer matrix [28]. AFM analysis (Fig. 1g,h) gave Sa = 54.08 nm, Sq = 67.98 nm, Sdr = 10.36%, mean diameter = 81.20 nm, and 369 detected features [29]. TGA (Fig. 1i) showed multi-stage decomposition with 26.2% residual mass at 897 °C, corresponding to the SiO₂ fraction [30,31]. GPC yielded Mn = 21,200, Mw = 38,700, Mz = 66,300 g/mol, and Đ = 1.82 [32,33].
Adsorption Studies
The effect of pH (Fig. 2a) showed R% increasing from 88.28% (pH 3) to 92.20% (pH 9) then declining to 90.23% (pH 11). At low pH, protonated surface amines repel cationic MB; deprotonation at higher pH creates negative surface charge enhancing attraction [34,35]. The effect of adsorbent mass (Fig. 2b) showed R% rising from 82.49% (0.01 g) to 92.61% (0.05 g), while qe decreased from 82.49 to 18.52 mg/g due to unsaturated sites at higher doses [36]. An optimum of 0.03 g was selected (R% = 91.79%, qe = 30.60 mg/g). Contact time (Fig. 2c) showed rapid uptake to 91.02% at 30 min, with equilibrium at 90 min (91.34%) [37,38]. The concentration study (Fig. 2d) showed R% increasing from 15.00% at 10 ppm to 88.67% at 100 ppm, with qe rising from 0.50 to 29.56 mg/g [39].
Adsorption Isotherms
The Langmuir model (Ce/qe = 1/(Qm·b) + Ce/Qm) and Freundlich model (log qe = log Kf + (1/n) log Ce) were applied to the equilibrium data (Fig. 3). The isotherm parameters derived from the linear plots are summarised in Table 1.
Temperature and Thermodynamics
R% decreased from 92.26% at 25 °C to 88.06% at 80 °C (Fig. 4a), confirming exothermic character. Thermodynamic parameters (Table 2) were obtained via ΔG° = −RT ln K and the Van’t Hoff equation: ln K = −ΔH°/RT + ΔS°/R (Fig. 4b, R² = 0.9603) [40,41]. All ΔG° values are negative (−3.47 to −2.64 kJ/mol), confirming spontaneous adsorption. The magnitudes (< 20 kJ/mol) indicate physisorption [38]. Negative ΔH° (−7.943 kJ/mol) confirms exothermic process. Negative ΔS° (−14.804 J/mol·K) indicates ordered MB arrangement on the surface [42,43].
CONCLUSION
A ternary SiO₂/PAM/P-o-Ans nanocomposite was successfully prepared via sequential condensation and oxidative polymerization. FT-IR confirmed azomethine (C=N) linkage formation at 1567 cm⁻¹ alongside poly(o-anisidine) incorporation. XRD revealed a predominantly amorphous structure (2θ ≈ 22°). TEM and FESEM showed spherical nanoparticles (44–77 nm), while AFM gave a mean particle diameter of 81.20 nm (Sa = 54.08 nm). TGA indicated 26.2% residual mass at 897 °C, and GPC yielded Mw = 38,700 g/mol with Đ = 1.82. Batch adsorption experiments demonstrated a maximum methylene blue removal of 92.20% at pH 9, adsorbent dose 0.03 g, contact time 90 min, and 25 °C. Removal decreased with rising temperature (88.06% at 80 °C), confirming an exothermic process. Thermodynamic analysis yielded ΔG° = −3.42 to −2.64 kJ/mol, ΔH° = −7.943 kJ/mol, and ΔS° = −14.804 J/mol·K, indicating spontaneous physisorption. These results highlight SiO₂/PAM/P-o-Ans as a promising adsorbent for cationic dye remediation.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.
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