Unraveling the mutual effect of different molecular weights natural organic matter fractions and heavy metals on ferrihydrite nanoparticles aggregation

Ferrihydrite is an iron mineral that widely exists in the environment as nanoparticles. The geochemical behavior of ferrihydrite nanoparticles (FNPs) can largely influence the fate of the correlated elements. Aggregation is the key factor to affects the geochemical fate of FNPs. Previous studies proved that the aggregation of FNPs is often influenced by natural organic matter (NOM), and the aggregation behavior was largely different with the different MW of NOM. However, little research considers the effect of the coexistence of heavy metal ions, which always interact with FNPs and NOM. Here, we studied the effects of different MWs of NOM on the aggregation and adsorption behaviors of FNPs in the presence of Cr(VI) and Cu(II), respectively. In Cr(VI) system, the presence of NOM with different MWs stabilized the FNPs, and the stabilization effect was higher with the increase of concentration and MW of NOM. However, in the presence of Cu(II), the addition of NOM first promoted the aggregation of FNPs and then inhibited, which was attributed to the change of surface charge from positive to negative. Furthermore, the adsorption and excitation-emission matrices parallel factor (EEM-PARAFAC) analysis results indicated that the adsorption properties of NOM were highly dependent on the humic substance components and MW distribution, and would affect the adsorption of NOM and Cr(VI)/Cu(II) on FNPs surface. These results can help us better understand the interaction mechanism of FNPs, NOM and heavy metals from the molecular level, and predict the fate of FNPs and heavy metals in the environment.