Objective The aim of this study is to investigate the dealumination and micronized disintegration of zeolite 4A mediated by organic acids with varying carboxylic and hydroxyl functional groups. The physicochemical properties of the resulting liquid and solid phases were systematically characterized to enhance the understanding of the structure-activity relationship (SAR) between synthetic zeolites and the hydrothermal stability (T_s) of hide collagen. Furthermore, the correlation among solid-state dealumination, particle size distribution in solution and the subsequent tanning performance was elucidated.

Methods Four organic acids, namely formic acid (FA), oxalic acid (OXA), citric acid (CIA), and tartaric acid (TAA), were employed to dissolve excess zeolite 4A at concentrations of 0.25,0.5, and 1.0 mol/L, respectively, under controlled conditions. The solubility of the zeolite 4A was determined gravimetrically. The micromorphology and elemental composition of the undissolved residues were characterized via scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), while the average particle size (D_A) of the dispersed zeolite was measured using a nanoparticle size analyzer. Furthermore, the time-delay stirring effect of saturated zeolite solutions of four acids at a concentration of 0.5 mol/L was further investigated. Samples were collected at intervals of 30, 60, 90, 120, and 150 min to monitor the viscosity, D_A, and polydispersity index (P) using a rotational viscometer and size analyzer. Based on the observed D_A and P values, four modified powder zeolites (ADZ) were prepared via spray drying. The re-solubility and cloud point of the ADZ were evaluated, followed by tanning trials on pickled sheepskins. Finally, the hydrothermal stability (T_s) of the tanned leather and the exhaustion rate of the zeolite were quantified.

Results The solubility of zeolite in the four organic acid solutions exhibited a positive correlation with acid concentration. Specifically, the dissolution capacity followed the order of CIA＞TAA＞OXA＞FA (Fig.4), indicating that a higher number of hydroxyl or carboxyl groups enhances the dissolution efficacy toward zeolite 4A. EDS analysis revealed the Si/Al ratios in the following order: 1.239 (CIA)＞1.238 (OXA)＞1.009 (TAA)＞0.978 (untreated)＞0.940 (FA). These results suggest that CIA and OXA exert a potent dealumination effect, whereas FA and TAA exhibit negligible dealumination action (Tab.2). Particle size analysis showed that the D_A for FA, OXA, and TAA decreased as acid concentration increased; however, CIA was an exception. Given the low carbon (C) content on the solid surface in the CIA group according to EDS, it is hypothesized that high-concentration citric acid may facilitate the formation of soluble aluminum-citrate complexes in the solution (Fig.5). SEM observations confirmed that OXA, CIA, and FA caused significant structural degradation of the zeolite, while TAA-treated samples maintained their original tetragonal crystalline morphology (Fig.6). The D_A and polydispersity (P) analysis across the four acid systems indicated that the FA-treated particles were characterized by multiple components, a broad total P, and high sensitivity of D_A to stirring time. Conversely, the main particle components in the OXA system showed a relatively broad P with stable D_A, while those in CIA and TAA systems displayed a narrow P and minimal D_A variation (Fig.7). Finally, performance evaluations of the modified zeolites (ADZ) demonstrated high exhaustion rates and improved hydrothermal stability (T_s). Notably, OXA-modified zeolite achieved the highest exhaustion rate, while FA-modified zeolite yielded the superior T_s in the tanned leather.

Conclusions The dissolution capacity of organic acids toward zeolite 4A is positively correlated with the content of carboxyl and hydroxyl groups. While the D_A of the dissolved zeolite generally increased with higher acid concentrations, citric acid (CIA) exhibited an anomalous trend. Furthermore, the disintegration mechanisms, interaction modes, and the resulting particle size evolution significantly varied across different organic acid systems, which represents a promising strategy for the development of acid-modified zeolite tanning agents.