Mono-, Di-, and Tricarboxylic Acid Facilitated Lanthanum-Based Organic Frameworks: Insights into the Structural Stability and Mechanistic Approach for Superior Adsorption of Arsenate from Water

In this study, we tried to synthesize lanthanum organic frameworks (MOFs) with the linkers of benzoic acid (BA), 1,4-benzene dicarboxylic acid, and 1,3,5-benzenetricarboxylic acid (BTC), abbreviated as La-BA, La-BDC, and La-BTC, respectively. Interestingly, the BA linker approached La metal to form lanthanum methanoate (La(HCOO) ₃ ) instead of the La-BA MOF through an acid catalyst amide-hydrolysis mechanism, whereas La-BDC and La-BTC act as MOFs, confirmed by PXRD patterns. Various sophisticated instrumentation techniques such as FTIR, PXRD, XPS, BET, and TGA were utilized to understand the formation of MOF. This is the first report to investigate AsO ₄ ^3- adsorption and the dissolution behavior of La-BA, La-BDC, and La-BTC in detail using different spectroscopic methods. The maximum AsO ₄ ^3- adsorption densities obtained from the Langmuir isotherm model were found to be 2.623, 3.891, and 0.280 mmol/g for La-BA, La-BDC, and La-BTC, respectively, where the dose ratio was 1 g/L with the speed of 100 rpm at room temperature. The value for La-BDC was significantly superior to the previously reported adsorbents for AsO ₄ ^3- to date. The presence of AsO ₄ ^3- on both La-BA and La-BDC was confirmed by FTIR and XPS As 3d. After adsorption of 2.4 mM AsO ₄ ^3- , the precipitation mechanism controls the adsorption capacities on La-BA and the ligand exchange mechanism on La-BDC confirmed by solution as well as solid analyses. Sorption kinetic data of AsO ₄ ^3- followed a pseudo-second-order model, which is consistent with chemisorption involving the possible coordination of AsO ₄ ^3- on La-BA and La-BDC. These results suggested that the MOF materials can be developed to immobilize arsenic-rich wastewater.