The researchers first synthesized zirconium- and hafnium-based UiO-66 metal-organic frameworks through solvothermal reactions involving zirconium chloride or hafnium chloride, H2BDC, N,N-dimethylformamide and hydrochloric acid. Glacial acetic acid was then added to encourage gelation. Three precursor concentrations were examined for each metal, corresponding to 1, 2 and 3 g of ZrCl4 or HfCl4.

The resulting gels were washed with ethanol and DMF, centrifuged and evaluated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy and nitrogen-based Brunauer–Emmet–Teller analysis. All experiments were repeated over three trials. The materials had granular structures, broadly similar infrared profiles and no measurable mass change over five days of storage before the adsorption studies.

Material characterization indicated that the 3 g Hf-UiO-66 gel had a specific surface area of 365.54 m²/g, slightly above the 360.52 m²/g measured for 3 g Zr-UiO-66. Energy-dispersive X-ray mapping confirmed zirconium in the zirconium gels and hafnium in the hafnium gels. It also detected calcium, oxygen, aluminum, silicon and fluorine, which the authors attributed to impurities from starting materials, reaction vessels or equipment.

The zirconium-based gel remained visibly intact in the lead wastewater during adsorption, whereas the hafnium-based gel showed modest disintegration. Batch adsorption tests examined adsorbent doses of 1.0, 1.4, 1.8 and 2.2 g in 20 mL portions of lead-contaminated wastewater. Temperatures of 25, 50 and 75 °C and contact times of 30, 90 and 150 minutes were compared, with lead remaining in solution measured by atomic absorption spectroscopy.

In the optimized experiments using 3 g Zr-UiO-66 at 75 °C for 150 minutes, removal efficiency rose from 97.62% with 1.0 g of gel to 99.10% with 2.2 g. Under the same conditions, 3 g Hf-UiO-66 also reached 99.10% removal at a dosage of 2.2 g. The hafnium gel performed better at the two lower doses tested, while both individual gels produced their highest reported removal efficiencies under the longer, hotter treatment.

The hybrid produced a further, although modest, increase in performance. The researchers manually combined 3 g of the zirconium-based gel with 3 g of the hafnium-based gel, washed and centrifuged the blend, and repeated the adsorption experiments. At 75 °C and 150 minutes, the hybrid removed 98.870% of lead with a 1.0 g dose, 99.001% with 1.4 g, 99.172% with 1.8 g and 99.300% with 2.2 g.

In that final condition, the equilibrium lead concentration was 1.557 mg/L, compared with 2.00 mg/L for the individual gels at their best reported dosage. The authors attributed the improvement to a synergistic interaction between the two components, which they proposed created additional active sites and modified pore structures for Pb(II) capture. Model fitting suggested that lead capture was predominantly associated with chemisorption.

The pseudo-second-order kinetic model gave a reported coefficient of determination of 0.9965, while the Langmuir isotherm fitted the hybrid adsorption data better than the Freundlich isotherm. The reported Langmuir maximum capacities were 2.360, 4.160 and 13.680 mg/g at 25, 50 and 75 °C, respectively. The manuscript gives inconsistent descriptions of the thermodynamic values and their implications, including both favorable and unfavorable interpretations of the sign of the Gibbs free-energy change.

The clearest supported conclusion from those analyses is that the pseudo-second-order model provided the best reported fit and that the measured removal increased under the optimized adsorption conditions. Fourier-transform infrared spectroscopy provided a possible molecular explanation for the adsorption. A prominent band at 1,654.9 cm⁻¹, assigned to carbonyl stretching, decreased after adsorption, suggesting that Pb(II) ions occupied or interacted with carbonyl sites.

The researchers proposed coordination between the lone-pair electrons of Pb(II) and oxygen atoms in the C=O groups. The spectra before and after adsorption otherwise showed no major changes, which the authors interpreted as evidence that the gel framework remained stable during capture. The lead hydroxide produced later showed an O-H stretching feature at 3,324.8 cm⁻¹, supporting the reported formation of Pb(OH)2.

Recovering the lead was more demanding than removing it from the wastewater. To desorb Pb(II), the researchers repeatedly immersed the loaded individual and hybrid gels at 90 °C for 10-minute periods in a solution containing methanol and 0.1 M HCl, repeating the procedure ten times. The collected liquid was analyzed for Pb(II), after which the lead was precipitated by stirring the desorbed solution with 0.1 M NaOH.

A white solid formed, was allowed to settle for 90 minutes, filtered and washed with distilled water. Under repeated cycling, the hybrid retained better performance than the separate gels: after four cycles, reported desorption efficiencies ranged from 83.2% to 99.8%, depending on the initial loading. The zirconium-only gel reportedly approached the end of its useful cycle after about four rounds, whereas the hybrid sustained more cycles before losing approximately half of its lead-trapping capacity.

The hybrid also displayed different behavior in acidic and alkaline environments. A 2.4 g portion retained essentially its mass during a four-day exposure to 1 M hydrochloric acid, while exposure to 1 M potassium hydroxide produced measured mass losses ranging from 0.004 g after 48 hours to 0.125 g after 96 hours. These results support describing the hybrid as acid-resistant under the tested conditions, but not as universally stable: the researchers observed alkaline leaching, and the synthesis itself required acids and solvents.

Lead hydroxide also remains hazardous, so the conversion demonstrates chemical recovery rather than detoxification into a product suitable for general use. The study’s demonstrated cycle is therefore a laboratory sequence in which lead is captured from wastewater, released from the gel and precipitated as Pb(OH)2. The principal result is a materials strategy that combines lead capture with recovery of the captured metal.

Under the best reported hybrid condition, 2.2 g of gel removed 99.300% of lead from 20 mL of wastewater at 75 °C after 150 minutes, with a final equilibrium concentration of 1.557 mg/L. The work also shows that the hybrid could withstand the acidic test environment and retain lead-trapping performance over repeated desorption cycles. Its measured advantage over the separate zirconium and hafnium gels was small in absolute terms—0.2 percentage points in the best comparison—but consistent across the reported dosages. The evidence supports further investigation of hybrid metal-organic gels as a route for concentrating and recovering Pb(II), while the laboratory scale, single wastewater source and use of elevated temperatures limit what can be inferred about broader deployment.

Subject of Research: Laboratory adsorption and recovery of Pb(II) using a hybrid zirconium-hafnium metal-organic gel

Article Title: Lead adsorption, utilization and conversion via a novel hybrid acid-resistant Metal Organic Gel

Article References: Sanni, S. E., Shoroye, M., Okoro, E. E., & Patel, G. (2026). Lead adsorption, utilization and conversion via a novel hybrid acid-resistant Metal Organic Gel. Frontiers in Nanotechnology, 8, Article 1892777. https://doi.org/10.3389/fnano.2026.1892777

Image Credits: AI Generated

DOI: 10.3389/fnano.2026.1892777

Keywords: lead adsorption, metal-organic gel, UiO-66, zirconium, hafnium, wastewater treatment, lead hydroxide, chemisorption, mine wastewater, metal recovery, Lead, adsorption