Synthesis, Characterization, and Environmental Applications of Hybrid Materials Based on Humic Acid Obtained by the Sol–Gel Route

Abstract

Humic acids (HAs) are ubiquitous macromolecules in the environment. Due to their high contents of oxygenated functional groups, they can interact with contaminants present in the natural environment and therefore influence the behavior of pollutants. However, a pH of 2 or lower is required to maintain HAs in the solid form. To increase the stability of HAs and their capacity to bind to contaminants, this work proposes the development of new hybrid materials based on alkoxysilanes and HAs for environmental applications such as dye adsorption. Three different materials with new functional groups were prepared by employing the following alkoxysilanes: tetraethyl orthosilicate, (3-aminopropyl)triethoxysilane, and N-[3-(trimethoxylsilyl)propyl]ethylenediamine. The final materials were denoted HWA, HOA, and HTA, respectively, and they were characterized by elemental analysis, diffuse reflectance Fourier-transform infrared spectroscopy (DRIFT), small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), and N2 gas-volumetric adsorption. The point of zero charge (pzc) and stability of these materials were also determined. Their selectivity was evaluated in adsorption experiments performed with two different charged dyes in aqueous medium, namely anionic rose bengal (RB) and cationic methylene blue (MB). The elemental, DRIFT, SAXS, SEM, and textural analyses confirmed the presence of a combination of the features of HAs and alkoxysilanes. The pzc results showed that the new materials displayed different characteristics and affinities. All the materials were stable in aqueous solution up to pH 10. For MB, the percentage removal values obtained by using HWA, HOA, and HTA were 98, 85, and 67%, respectively. As for RB, the percentage removal values were 19, 18, and 44% for HWA, HOA, and HTA, respectively. These hybrid materials have potential use as adsorbents for the removal of cationic or anionic species and could be viable alternatives to remove various substances present as contaminants in natural environments.

Value-Added Humic Acid Derived from Lignite Using Novel Solid-Phase Activation Process with Pd/CeO2 Nanocatalyst: A Physiochemical Study

“…Previous studies have suggested that the E 4 /E 6 ratio decreases with increasing content of condensed aromatic rings. 27 As shown in Table 5, the E 4 /E 6 ratio of RL, AL, ACL, and APL showed an increasing trend, suggesting that APL contained more low average molecular mass than the other treatments. 1 The humic acid was further fractionated into three sizes by a continuous flow analytical system: low (<10 000 Da), medium (10,000−50 000 Da), and high (>50,000 Da) average molecular mass.…”

“…Findings from previous studies have all pointed out that the Pd/CeO 2 nanocomposite is an excellent catalyst that can be used to oxidize and convert methane, formaldehyde, water gas, and other hydrocarbons with complicated carbon structures. ,, Because lignite has high volatile hydrocarbons and is rich in carboxyl, hydroxyl, and phenol functional groups, it is anticipated that Pd/CeO 2 nanocomposites can also oxidize and activate the humic substances from lignite to promote its applications. However, little research has been conducted on the use of a Pd/CeO 2 catalyst to active lignite via a solid-phase reaction at room temperature.…”

Structural characteristics of humic acids derived from Chinese weathered coal under different oxidizing conditions

“…Oxidation with H 2 O 2 can increase the oxygen–bearing group content of humic acids, possibly resulting from the oxidative reactions with hydrogen peroxide. It has been demonstrated by others that the E2/E3 and E4/E6 values are negatively correlated with the aromaticity, condensation and molecular weight of humic acids [33,44]. All the OHAs had a higher E4/E6 ratio than the HAs directly extracted from the Chinese weathered coal.…”

Targeted design of water-based humic substances-silsesquioxane soft materials for nature-inspired remedial applications

“…The obtained results are in good agreement with the reported data on enhanced sorption of dyes on humics-modied solid sorbents. 9,12 It should be specically stressed that as in case of the removal of the immobilized HS-SQ networks from the granular aquifer support, which we discussed above, the sorbed azodye can be also completely removed from the solid support by washing it off with 0.01 M NaOH. In our column experiments we have found that the azodye elutes together with the HS-SQ materials upon washing with 0.01 M NaOH (the results are not shown here).…”

“…8 Similar hybrid sorbents with immobilized humic layers were described by other authors as well. [9][10][11][12] Still, these lab-prepared solid sorbents do not meet the challenge of achieving in situ aquifer remediation with injectable reactive barriers (i.e., without excavating contaminated soils or sediments). 13,14 The same is true for restoring the humus content in soils which implies the in situ immobilization of large quantities of organic matter onto soil minerals.…”