Dang Duc Dung

A Co ₃ O ₄ –Fe ₃ O ₄ /ITO electrochemical sensor was fabricated via in situ formation of metal hydroxide precursors assisted by electrochemical nitrate reduction, enabling uniform film growth and strong interfacial adhesion. The composite electrode was applied for the simultaneous determination of Cd(II), Pb(II), Cu(II), and Hg(II) using differential pulse anodic stripping voltammetry (DP-ASV). Under optimized conditions (pH 5.25), the sensor exhibited well-defined stripping responses with detection limits of 1.40 nM for Cd(II), 1.33 nM for Pb(II), 0.44 nM for Cu(II), and 3.2 nM for Hg(II). Linear ranges were statistically determined using F -test and RMSE criteria, with correlation coefficients ( R ² ) exceeding 0.99 in most segments. To describe concentration-dependent behavior beyond linear regions, adsorption-based models were employed. Cd(II), Cu(II), and Hg(II) were well fitted by a Sips-offset model, while Pb(II) followed a Henry–Sips hybrid model. Speciation analysis using Visual MINTEQ indicated that Cd(II), Pb(II), and Cu(II) mainly exist as acetate complexes, whereas Hg(II) undergoes significant hydrolysis at higher pH. These findings highlight the effectiveness of the proposed electrode for sensitive multimetal detection and provide insight into adsorption-controlled accumulation in DP-ASV systems.

Magnetic spinel ferrites are attractive adsorbents for complex wastewaters because they couple high affinity with rapid magnetic recovery. Here, a series of Zn-substituted ferrites (Fe_1−xZn_xFe₂O₄, x = 0.0–1.0) was synthesized by co-precipitation and systematically characterized. The series samples display particles in the nanoscale range from 8–50 nm, single phase with spinel structure, tunable magnetization, and mesoporosity where the highest surface area of 228 m² g⁻¹ were estimated for Fe_0.4Zn_0.6Fe₂O₄ samples. The point of zero charge is obtained around 6.26, consistent with strong uptake of anionic species under mildly acidic conditions. Batch adsorption toward an organic dye (Direct Red 79, DR79) and an inorganic oxyanion (hexavalent chromium, Cr(VI)) shows optimal removal at pH = 3, with equilibrium contact times of ∼120 min for DR79 and ∼90 min for Cr(VI). Nonlinear kinetic fitting indicates Elovich behaviour for DR79 (heterogeneous chemisorption with site-energy distribution) and pseudo-second-order kinetics for Cr(VI). Intraparticle diffusion contributes to the rate of reaction but is not rate-limiting. Nonlinear isotherm analysis indicates that Freundlich is applicable to both solutes, while Langmuir capacities reach approximately 95.3 mg g⁻¹ and 62.1 mg g⁻¹ for DR79 and Cr(VI), respectively. Thermodynamic analysis reveals spontaneous adsorption in all cases. The uptake of DR79 is endothermic (ΔH° ≈ +79.1 kJ mol⁻¹; ΔG° ≈ −4.5 to −8.5 kJ mol⁻¹; ΔS° ≈ +0.271 kJ mol⁻¹ K⁻¹), whereas Cr(VI) is exothermic (ΔH° ≈ −40.0 kJ mol⁻¹; ΔG° ≈ −2.0 to −0.15 kJ mol⁻¹; ΔS° ≈ −0.126 kJ mol⁻¹ K⁻¹). The results highlight Fe_0.4Zn_0.6Fe₂O₄ composition displayed as a magnetically retrievable, dual-function adsorbent capable of treating mixed organic/inorganic contaminants with reliable nonlinear model parameterization for process design.