The behavior of NOM-Cu(Ⅱ)colloids at the goethite interface.

Natural organic matter (NOM), due to its high reactivity, often facilitates the formation of NOM-heavy metal colloids. However, the impact of mineral components on the behavior of these colloids remains poorly understood. In present research, to investigate the interfacial reaction properties of NOM-Cu(II) colloids with goethite at varying C/Cu(II) ratios, nano-sized Cu(Ⅱ) colloids with different C/Cu(Ⅱ) ratios were synthesized under aerobic conditions by combining humic acid with Cu(Ⅱ). Adsorption experiments showed that NOM-Cu(Ⅱ) colloids enhanced Cu(Ⅱ) adsorption onto goethite at low C/Cu(Ⅱ) ratios (C/Cu(Ⅱ) ≤ 25). Conversely, the adsorption of Cu(Ⅱ) was hindered at high C/Cu(Ⅱ) ratios (C/Cu(Ⅱ) ≥ 50), while Fe release is promoted, facilitating further reactions with NOM-Cu(II) colloids to form Fe-NOM-Cu(II) colloids. HR-TEM and QCM-D experimental results indicated that NOM-Cu(II) colloids aggregated and formed a softer deposit layer on mineral surfaces at low C/Cu(II) ratios. Conversely, at high C/Cu(II) ratio, HA formed a dense adsorption layer on goethite, while Fe-NOM-Cu(II) colloids were observed in the liquid phase samples. In situ ATR-FTIR spectroscopy, ITC experiments, and theoretical calculations further demonstrated that the adsorption mechanism was not dominant at low C/Cu(II) ratios. Instead, the aggregation and deposition of colloids induced by goethite promote Cu(II) adsorption. At high C/Cu(II) ratio, the suppression of Cu(II) adsorption was attributed to the formation of a dense adsorption layer by free HA, which coordinated with goethite surfaces via carboxyl groups, occupying adsorption sites. Additionally, the high concentration of HA intensified the stability of colloids in solution. This research provides crucial insights into the interactions between NOM-Cu(II) colloid and environmental minerals, elucidating the molecular mechanisms influencing colloidal behavior on mineral surfaces, which is vital for understanding the geochemical cycling of heavy metals.