The effect of hydrolyzed humic acid on rare earth elements leaching and uptake by dicranopteris linearis

Rare earth elements (REEs) are essential for the manufacturing of smart electronics. Ion-adsorption clay (IAC) represents an alternative source of REEs, where these elements are weakly adsorbed on clay mineral surfaces and can be extracted through leaching processes. The effectiveness of leaching agents in enhancing REEs mobilization from IAC is crucial for the success of the In-situ extraction and subsequent bioavailability for plant uptake. However, the conventional use of ammonium sulfate (NH4)2SO4 in REEs extraction generates high concentrations of ammonium (NH4+) and sulfate (SO42-) ions, causing serious environmental risks. Humic acid (HA) may be an environmentally friendly alternative to (NH4)2SO4. However, its low solubility and complex structure limit its effectiveness in acidic IAC. A hydrolysis process on HA may produce smaller compounds with greater solubility for further improve REE extraction from IAC. Furthermore, native REEs hyperaccumulator such as Dicranopteris linearis (D. linearis) can accumulate REEs and radionuclides with greater efficiency, especially when IAC is treated with a leaching agent such as hydrolyzed humic acid (FA). This study aims to investigate the uptake capacity of D. linearis and the potential of using FA at various pH (0.3 % w/w FA pH 2 and 0.3% w/w FA pH 6) for extracting REEs and radionuclides from IAC samples. The physicochemical properties of IAC samples, including soil texture, moisture content, organic matter content, pH, electrical conductivity (EC), and X-ray diffraction (XRD), were first analyzed. The characteristics of FA were also examined using Fourier Transform Infrared (FTIR) Spectroscopy, CHNS elemental analysis, and Scanning Electron Microscopy with Energy Dispersive X-ray (SEM-EDX) Spectroscopy. Leaching experiments were performed using batch and column setups with different leaching agents (0.3% FA pH 2, 0.3% FA pH 6, 0.3% CaCl2, 0.3% (NH4)2SO4 pH 2, and 0.3% (NH4)2SO4 pH 6). In the plant uptake study, D. linearis was cultivated in IAC-amended soil for four months under different treatment conditions to assess its uptake of REEs and radionuclides. The REEs and radionuclide content were analyzed using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). The IAC soil was characterized as acidic sandy loam (pH 4.51) with low water-holding capacity and enriched in REEs (Ce, La, Nd, Pr, Sc, and Yb) and radionuclides (Th and U), while FA contained abundant oxygen-containing functional groups (−COOH, −OH, aromatic), indicating strong metal complexation potential. FA pH 6 performed better in batch leaching and was able to leach REEs, i.e., Ce (11.59%), La (354.11%), Nd (24.89%), Th (453.64%), and U (1.35%) more optimally. Meanwhile, (NH4)2SO4 pH 2 performed better in column leaching providing greater leaching efficiency for Ce (11.59%), La (95.56%), Nd (14.32%), and U (0.10%). The FA contains C, N, and Ca that support soil fertility, and the presence of oxygen-containing functional groups enhances REEs mobilization from IAC. The plant uptake study revealed roots preferentially accumulate radionuclides (Th and U) and Sc, while light rare earth elements (LREEs) including La, Nd, and Pr showed greater mobility and were more efficiently translocated to the shoots. These findings highlight the dual role of FA in improving REE extraction and nutrient supply, thereby supporting its application in phytoremediation.