The widespread utilization of plastic products ineluctably leads to the ubiquity of nanoplastics (NPs), causing potential risks for aquatic environments. Interactions of NPs with mineral surfaces may affect NPs transport, fate and ecotoxicity. This study aims to investigate systematically the deposition and aggregation behaviors of carboxylated polystyrene nanoplastics (COOH-PSNPs) by four types of clay minerals (illite, kaolinite, Na-montmorillonite, and Ca-montmorillonite) under various solution chemistry conditions (pH, temperature, ionic strength and type). Results demonstrate that the deposition process was dominated by electrostatic interactions. Divalent cations (i.e., Ca²⁺, Mg²⁺, Cd²⁺, or Pb²⁺) were more efficient for screening surface negative charges and compressing the electrical double layer (EDL). Hence, there were significant increases in deposition rates of COOH-PSNPs with clay minerals in suspension containing divalent cations, whereas only slight increases in deposition rates of COOH-PSNPs were observed in monovalent cations (Na⁺, K⁺). Negligible deposition occurred in the presence of anions (F^-, Cl^-, NO^3-, CO₃^2-, SO₄^2-, or PO₄^3-). Divalent Ca²⁺ could incrementally facilitate the deposition of COOH-PSNPs through Ca²⁺-assisted bridging with increasing CaCl₂ concentrations (0-100 mM). The weakened deposition of COOH-PSNPs with increasing pH (2.0-10.0) was primarily attributed to the reduce in positive charge density at the edges of clay minerals. In suspensions containing 2 mM CaCl₂, increased Na⁺ ionic strength (0-100 mM) and temperature (15-55 ^◦C) also favored the deposition of COOH-PSNPs. The ability of COOH-PSNPs deposited by four types of clay minerals followed the sequence of kaolinite > Na-montmorillonite > Ca-montmorillonite > illite, which was related to their structural and surface charge properties. This study revealed the deposition behaviors and mechanisms between NPs and clay minerals under environmentally representative conditions, which provided novel insights into the transport and fate of NPs in natural aquatic environments.

Sorption of nanoparticles on mineral surfaces has been well recognized for its environmental implication. For certain gold ore deposits, sorption of gold nanoparticles (AuNPs) on pyrite may constitute a critical step in the metallogenesis process, yet it has not been adequately investigated. In this work, we synthesized negatively charged, positively charged AuNPs and pyrite respectively, and experimentally studied sorption of AuNPs on pyrite in an anaerobic environment. The effects of AuNPs charge characteristics, pH, hexadecyl trimethyl ammonium bromide (CTAB) concentration, and ionic strength were evaluated. For the negatively charged AuNPs, almost no sorption was observed owing to the electrostatic repulsion between the AuNPs and pyrite surface (with a measured isoelectric point of 2.0). In contrast, positively charged AuNPs could be significantly adsorbed on pyrite through electrostatic attraction, with the sorption extent decreasing with the increase of pH (2.2–9.1). However, the adsorption of CTAB from the solution was found to increase with the increase of pH. Furthermore, deliberately added CTAB seemed to inhibit the sorption of AuNPs. Our study not only demonstrates that the electrostatic interaction is the dominant mechanism in the sorption of AuNPs on pristine pyrite, but also reveals the important competitive relation between AuNPs and CTAB in the sorption process.

Nanocubic pyrite (FeS₂) crystals with exposed (100) crystal faces and sizes of 100–200 nm were successfully synthesized via a facile hydrothermal method using greigite (Fe₃S₄) as the iron precursor and thiourea (NH₂CSNH₂) as the sulfur source. When the concentration of thiourea was 40 mmol/L, both pyrite and hematite were observed in the as-prepared sample, indicating incomplete conversion of greigite into pyrite. With an increased thiourea concentration to 80 mmol/L, pyrite was found to be the only crystalline phase in the synthesized samples. All greigite could be transformed to pyrite within 24 h via the hydrothermal method, while further prolonging the hydrothermal time had insignificant effect on the crystal phase composition, crystallinity, and morphologies of the prepared nanocubic pyrite crystals. In contrast, when a mixture of Na₂S and S powder was used to replace the thiourea as the sulfur source, tetragonal, orthorhombic, cubic, and irregular pyrite crystal particles with sizes of 100 nm–1 μm were found to co-exist in the prepared samples. These results demonstrate the critical influence of sulfur source on pyrite morphology. Furthermore, our hydrothermal process, using a combination of greigite and thiourea, is proved to be effective in preparing nanocubic pyrite crystals. Our findings can also provide new insight into the formation environments and pathways of nanocubic pyrite under hydrothermal conditions.