The fractionation of fulvic acid and the optimal fraction as explanatory factors for binding characteristics of lead in aqueous solution

Impact of organic matter molecular weight on hexavalent chromium enrichment in green microalgae

In lightly polluted water containing heavy metals, organic matter, and green microalgae, the molecular weight of organic matter may influence both the growth of green microalgae and the concentration of heavy metals. This study elucidates the effects and mechanisms by which different molecular weight fractions of fulvic acid (FA), a model dissolved organic matter component, facilitate the bioaccumulation of hexavalent chromium (Cr(VI)) in a typical green alga, Chlorella vulgaris. Findings show that the addition of FA fractions with molecular weights greater than 10 kDa significantly enhances the enrichment of total chromium and Cr(VI) in algal cells, reaching 21.58%-31.09 % and 16.17 %-22.63 %, respectively. Conversely, the efficiency of chromium enrichment in algal cells was found to decrease with decreasing molecular weight of FA. FA molecular weight within the range of 0.22 µm-30 kDa facilitated chromium enrichment primarily through the algal organic matter (AOM) pathway, with minor contributions from the algal cell proliferation and extracellular polymeric substances (EPS) pathways. However, with decreasing FA molecular weight, the AOM and EPS pathways become less prominent, whereas the algal cell proliferation pathway becomes dominant. These findings provide new insights into the mechanism of chromium enrichment in green algae enhanced by medium molecular weight FA.

Structural analysis of lignite-derived humic acid and its microscopic interactions with heavy metal ions in aqueous solution

Understanding heavy metal environmental behavior with humic acid (HA) is critical. There is currently a lack of information on the control of its structure organization on its reactivity to metals. The difference in HA structures under non-homogeneous conditions is critical for revealing its micro-interaction with heavy metals. The heterogeneity of HA was reduced using the fractionation method in this study, the chemical properties of HA fractions were analyzed using py-GC/MS, and the structural units of HA were proposed. Pb2+ was used as a probe to investigate the difference in the adsorption capacity of HA fractions. The microscopic interaction of structures with heavy metal was investigated and validated by structural units. The results show that as molecular weight increased, the oxygen content and the number of aliphatic chains decreased, but the opposite was true for aromatic and heterocyclic rings. The adsorption capacity for Pb2+ was as follows: HA-1 > HA-2 > HA-3. According to the linear analysis of the influencing factors of maximum adsorption capacity and possibility factors, the adsorption capacity was positively correlated with the contents of acid groups, carboxyl groups, phenolic hydroxyl groups, and the number of aliphatic chains. The phenolic hydroxyl group and the aliphatic-chain structure have the greatest impact. Therefore, structural differences and the number of active sites play an important role in adsorption. The binding energy of HA structural units to Pb2+ was calculated. It was found that the chain structure is easier to bind to heavy metals than aromatic rings, and the affinity of-COOH to Pb2+ is greater than that of -OH. These findings can help improve the adsorbent design.

A Sandwich Structure of Fulvic Acid and PMIDA-Modified LDHs for the Simultaneous Removal of Cu2+ and Aniline in Multicomponent Solutions

The coexistence of organic and inorganic pollutants in industrial wastewater has emerged as a concerning environmental issue worldwide due to the critical levels of biological toxicity of these pollutants. In this context, the present study proposes a sandwich structure of fulvic acid and PMIDA-modified LDHs (FA/PMIDA-LDHs) for the simultaneous removal of Cu²⁺ and aniline from wastewater. The specific structure was synthesized using a combination of coprecipitation and impregnation methods. Abundant benzene rings and oxygen-containing functional groups greatly increased the number of sites for the adsorption of both Cu²⁺ and aniline. The maximum adsorption capacity of Cu²⁺ and aniline in solution with initial pH 5.0 at 25 °C could reach 221.24 and 132.28 mg/g, respectively. Cu²⁺ could be chelated by the functional groups in the FA/PMIDA-LDHs structure, and a coupled reduction-complexation mechanism was proposed for this process. The uptake of aniline on FA/PMIDA-LDHs was demonstrated to be a result of the combination of coordination forces, hydrophobic effects, π-π interactions, and hydrogen bonds. In a multicomponent solution, FA/PMIDA-LDHs exhibited excellent salt tolerance of up to 1000 mg/L of Na⁺ or Ca²⁺. The effects of Fe³⁺, Ni²⁺, Cl^-, Cr₂O₇^2-, SO₄^2-, and H₂PO₄^- on the uptakes of Cu²⁺ and aniline were also investigated.