Optimizing Heavy Metal Removal with Iron‑Modified Magnetic Biochar Nanocomposites

Abstract

Magnetic biochar nanocomposites were fabricated by impregnating commercial biochar with zero‑valent iron. The study investigates how contact time, initial concentrations of Cd(II), Co(II), Zn(II) and Pb(II), sorbent dosage, solution pH, and temperature influence adsorption performance. Experiments revealed that equilibrium is reached after 360 min, with an optimal sorbent dose of 5 g dm^–3, pH 5, and 295 K. Kinetic data best fit the pseudo‑second‑order model, while Langmuir isotherms accurately describe equilibrium. Thermodynamic analysis (∆H⁰, ∆S⁰, ∆G⁰) indicates an exothermic, spontaneous, and physically driven adsorption process. Regeneration with 0.1 mol dm^–3 HNO₃ achieved a 97 % desorption yield. Characterization by FTIR, SEM, XRD, XPS, and TG confirmed successful iron loading and the presence of Fe(0). pH_PZC and pH_IEP of the biochar were determined.

Background

Agricultural residues—hazelnut husks, corn straw, rice husks, potato peel, sugar beet tailings, and others—are often landfilled or incinerated, posing risks to groundwater and air quality. These residues can be pyrolysed to produce biochar, a low‑cost sorbent with high porosity and surface area. Biochar not only enhances soil fertility but also removes heavy metals such as Cu(II), Cd(II), Cr(VI), Pb(II), and Ni(II) from water. However, separating biochar after use can be problematic. By integrating magnetic iron nanoparticles, the sorbent gains recoverability via an external magnetic field, as demonstrated by Fe, Fe₂O₃, and Fe₃O₄ modifications.

Previous work has shown that magnetic biochar derived from cottonwood, orange peels, and eucalyptus leaves can adsorb arsenate, phosphate, pentachlorophenol, and various heavy metals. Zero‑valent iron (Fe⁰) coatings provide high reactivity and affinity for organic pollutants and metal ions alike. This study builds on that foundation by preparing two magnetic biochar variants (MBC1 and MBC2) with differing FeSO₄ to NaBH₄ ratios (1:1 and 1:2) and evaluating their performance against Cd(II), Co(II), Zn(II), and Pb(II).

Methods

Preparation of Sorbents

The base biochar (Coaltec Energy, USA) was gasified biomass, yielding a carbon‑rich material. Zero‑valent iron coatings were applied by dissolving 0.18 mol dm^–3 FeSO₄·7H₂O in 100 cm³ water, adding 5 g of biochar, and reducing Fe²⁺ to Fe⁰ with NaBH₄ (1:1 or 1:2 molar ratios). The resulting nanocomposites, MBC1 and MBC2, were filtered, washed, and oven‑dried.

Chemicals

Analytical‑grade reagents (Avantor) were used. Stock solutions (1000 mg dm^–3) of Cd(NO₃)₂·4H₂O, CoCl₂·6H₂O, ZnCl₂, and Pb(NO₃)₂ were prepared. pH adjustments employed 1 mol dm^–3 HCl or NaOH.

Sorption and Kinetic Studies

Batch tests used 0.1 g sorbent in 20 cm³ of metal ion solutions (50–200 mg dm^–3), shaken at 180 rpm, pH 5, 295 K, and contact times up to 360 min. Residual concentrations were measured by atomic absorption spectroscopy (AAS). Equilibrium capacity was calculated via:
qe = ((C0 – Ce) V) / m

Sorption dose experiments varied sorbent amounts (5–10 g dm^–3), while pH studies spanned 2–6. Isotherm data were collected at 50–600 mg dm^–3 and temperatures 295, 315, and 335 K. Thermodynamic parameters were derived from:
ΔG0 = –RT ln Kd (2),
ΔG0 = ΔH0 – T ΔS0 (3),
Kd = Cs / Ce (4),
ln Kd = (ΔH0 / RT) + (ΔS0 / R) (5).

Regeneration Tests

Cd‑loaded MBC2 was shaken with 20 cm³ of distilled water or HNO₃ (0.1–5 mol dm^–3) for 360 min. Desorption yield was calculated as:
%Desorption = (Cdes / (C0 – Ce)) × 100 (6).

Apparatus and Analysis

Experiments used a 358A shaker (Elpin Plus). pH was measured with a pHM82 meter. AAS (Spectr AA 240 FS) quantified metal ions. FTIR (Cary 630) spanned 650–4000 cm^–1. SEM (Quanta 3D FEG) visualized morphology. XRD (PANalytical Empyrean) identified crystalline phases. XPS (Prevac) examined surface chemistry. TG/DTG (TA Q50 TGA) assessed thermal stability. Zeta potential was measured with a Zetasizer Nano‑ZS90, and surface charge density calculated via:
σ0 = ΔV C F / (Sw m) (7).

Results and Discussion

Adsorption Kinetics

Rapid uptake occurred within the first 60 min, reaching equilibrium after ~240 min at the highest concentration (200 mg dm^–3). The pseudo‑second‑order model (R² > 0.97) best described the kinetics, confirming chemisorption as the rate‑determining step. Rate constants decreased with increasing initial concentration, indicating site saturation.

Effect of Sorbent Dose

Increasing the sorbent dose from 5 to 10 g dm^–3 lowered the per‑gram capacity, confirming an optimal dose of 5 g dm^–3 for maximum efficiency.

Effect of Initial pH

Adsorption sharply increased from pH 2 to 5, then plateaued, reflecting the protonation state of surface functional groups and the dominant Cd²⁺ species. pH 5 was chosen as the optimal condition.

Adsorption Isotherms

Langmuir fits (R² > 0.95) indicated monolayer coverage, with maximum capacities (q_m) of 20.65 mg g^–1 (MBC1) and 23.55 mg g^–1 (MBC2) for Cd(II). Temperature dependence revealed lower capacities at higher temperatures, consistent with exothermic adsorption.

Thermodynamics

Negative ΔH⁰ values (< 40 kJ mol^–1) confirm physisorption. ΔG⁰ values between –20 and 0 kJ mol^–1 indicate spontaneous uptake, with greater spontaneity at lower temperatures.

Regeneration of Spent Sorbent

0.1 mol dm^–3 HNO₃ achieved a 97 % desorption yield. Desorption plateaued after ~180 min, demonstrating efficient regeneration and potential for repeated use.

Characterization

FTIR spectra showed shifts in hydroxyl, carboxyl, and C=O bands after metal uptake, indicating surface complexation. SEM images revealed well‑dispersed Fe(0) nanoparticles, with smaller particles in MBC2 correlating with higher capacity. XRD confirmed the presence of quartz, calcite, dolomite, and Fe(0) peaks. XPS identified C, O, Fe, Mg, Si, Al, P, Ca, Cd, and K, verifying successful iron impregnation. TG/DTG curves indicated ~35 % weight loss up to 1273 K, reflecting thermal stability.

Surface Charge

Potentiometric titration yielded a pH_PZC of 10.5 and an isoelectric point pH_IEP < 3. Zeta potential measurements showed negative values across the pH range, supporting colloidal stability.

Conclusions

Two iron‑modified magnetic biochar nanocomposites (MBC1 and MBC2) effectively remove Cd(II), Co(II), Zn(II), and Pb(II) from aqueous solutions. Key operating parameters—contact time, sorbent dose, pH, and temperature—significantly influence performance. MBC2, with a higher reducing agent content, exhibits superior affinity and magnetic recoverability. Regeneration with dilute nitric acid achieves > 97 % desorption, enabling repeated use. Characterization confirms Fe(0) incorporation, thermal resilience, and favorable surface charge, underscoring the material’s potential for water remediation applications.