The environmental problem the team targeted is both persistent and growing. Mining, processing, and industrial use of rare earth elements generate acidic leachate streams and tailings laden with dissolved lanthanide ions. When discharged untreated, these effluents contaminate groundwater, surface water, and marine ecosystems. Yet the same streams represent an opportunity: recovering rare earths from industrial process water, acid mine drainage, and electronic waste leachates offers a double dividend, cutting pollution while reclaiming strategically valuable materials. Conventional treatment routes such as chemical precipitation, ion exchange, solvent extraction, and membrane filtration all carry limitations in cost, selectivity, or performance in dilute solutions, which is why adsorption has long been viewed as the most promising alternative.

Metal-organic frameworks, or MOFs, have changed the calculus for adsorption chemistry. These materials consist of metal clusters connected by organic linkers into porous, crystalline lattices with surface areas that can reach thousands of square meters per gram, far exceeding conventional adsorbents like activated carbon, zeolites, and clays. Among the most studied families is UiO-66, a zirconium-based framework prized for exceptional chemical, thermal, and mechanical stability in water. The researchers in this work went a step further by incorporating amine groups into the framework, replacing the standard terephthalic acid linker with 2-aminoterephthalic acid. The lone electron pairs on nitrogen atoms can interact directly with lanthanum ions, adding a chemical binding dimension on top of simple pore-based capture.

Synthesis of the material followed a solvothermal route. Zirconium chloride and 2-aminoterephthalic acid were dissolved in dimethylformamide, acetic acid was added as a modulator, and the mixture was heated at 120 degrees Celsius for 16 hours. The resulting powder was washed and vacuum-dried. X-ray diffraction confirmed a highly crystalline structure with characteristic peaks matching the known UiO-66-NH2 pattern, while field-emission scanning electron microscopy revealed multimodal spherical particles averaging below 200 nanometers in size, though with some aggregation. Nitrogen adsorption measurements yielded a Brunauer-Emmett-Teller surface area of 682 square meters per gram, a dominant pore size of 1.79 nanometers, and a pore volume of 0.27 cubic centimeters per gram, values consistent with the 600 to 1,200 square meter per gram range reported in the literature for this material family.

To find the best operating conditions, the team turned to response surface methodology, specifically a Box-Behnken design. Thirteen experimental trials varied adsorbent dosage and contact time, and a quadratic polynomial model was fitted to the removal data. Analysis of variance confirmed the model was statistically significant, with an F-value of 6.44 and a p-value of 0.0149. The adsorbent dose emerged as the dominant factor, while the coefficient of determination reached 0.82, explaining most of the variability in lanthanum removal. The statistical exercise converged on an optimal recipe: 10 milligrams of adsorbent per 10 milliliters of solution, a pH of 9, and 150 minutes of contact time.

The influence of pH proved particularly revealing. As pH rose from 3 to 7, lanthanum removal actually fell from 98.5 percent to 75.21 percent. The explanation lies in acid-base chemistry at the adsorbent surface: in acidic conditions, the amine groups accept protons and become positively charged, while hydrogen ions compete directly with lanthanum for binding sites. Above pH 7, the picture reverses. The framework surface becomes negatively charged, electrostatic attraction between the surface and the positively charged trivalent lanthanum ions strengthens, and removal efficiency climbs, peaking at pH 9. Adsorbent dosage showed a similar decisive effect, with removal rising from 64 percent to 99.86 percent as the dose increased from 3 to 10 milligrams, reflecting the growing availability of active sites.

Modeling the equilibrium and kinetics of the process provided mechanistic insight. The Freundlich isotherm, which describes multilayer adsorption on heterogeneous surfaces, fit the data better than the Langmuir model, with an R-squared value of 0.9986 and a heterogeneity coefficient of 1.98 indicating favorable adsorption. The Langmuir fit still yielded a maximum capacity of 312.5 milligrams per gram, placing UiO-66-NH2 well above most conventional adsorbents reported for lanthanum, including magnetic clay composites at around 111 milligrams per gram and silica xerogels at under 7. Kinetic analysis showed the pseudo second-order model best described the uptake, with an R-squared of 0.9971, suggesting that chemisorption, the formation of chemical bonds through electron exchange, is the rate-limiting step rather than simple mass transfer.

The Weber-Morris intraparticle diffusion model added a final layer of understanding. The plot produced a single linear line across the entire time interval examined, with a diffusion rate constant of 7.14 milligrams per gram per minute to the one-half power and a positive intercept of 12.8 milligrams per gram. The non-zero intercept indicates that intraparticle diffusion is not the sole rate-limiting factor; boundary-layer resistance and external mass transfer also contribute. Crucially, the absence of multiple linear segments means no transition between diffusion regimes occurred during the experiment, pointing to a single, consistent mass-transfer mechanism governing the process from start to finish.

Perhaps the most compelling demonstration came in seawater, a brutally competitive environment for any adsorbent. Spiked with 100 parts per million of lanthanum, real seawater contains roughly 9,000 parts per million of sodium ions and 1,000 parts per million of magnesium ions, both of which could theoretically crowd out the target metal. Yet when 10 milligrams of UiO-66-NH2 were added and the mixture agitated for 150 minutes, inductively coupled plasma mass spectrometry showed the material captured 87.5 percent of the lanthanum while taking up only 1.1 percent of sodium and 0.5 percent of magnesium in the first cycle. Over five sequential adsorption and acid-elution cycles, lanthanum recovery declined only modestly to 85 percent, while competing ion uptake rose to just 3.2 percent for sodium and 2.2 percent for magnesium.

The authors attribute this selectivity and capacity to two factors working in concert: the microporous architecture with its substantial surface area and pore volume, and the electrostatic and coordinative interaction between electronegative nitrogen atoms in the linkers and the trivalent lanthanum ions. Because lanthanum concentrations in real contaminated waters are typically far lower than those of coexisting cations, the framework’s preference for lanthanum over sodium and magnesium is exactly the property a practical separation technology requires. The researchers suggest that this class of amine-functionalized zirconium MOFs could be extended to remove heavy metals and rare earth elements from seawater and industrial streams alike, turning a stubborn pollution problem into a source of recoverable strategic materials.

Subject of Research: Amine-functionalized zirconium metal-organic framework adsorbent for lanthanum removal from water

Article Title: Amine-terminated zirconium metal-organic framework as an efficient adsorbent for lanthanum removal

Article References: Saranya, J., Ramesh, S., Kumar, R. T. R., & Haldorai, Y. (2026). Amine-terminated zirconium metal-organic framework as an efficient adsorbent for lanthanum removal. Frontiers in Materials, 13, Article 1944659. https://doi.org/10.3389/fmats.2026.1944659

Image Credits: AI Generated

DOI: 10.3389/fmats.2026.1944659

Keywords: metal-organic frameworks, UiO-66-NH2, lanthanum, rare earth elements, adsorption, water treatment, response surface methodology, Freundlich isotherm, adsorption kinetics, seawater selectivity, zirconium MOF, heavy metal removal