Abstract:

Nanoplastics, particularly polystyrene nanoplastics (PSNPs), are widely detected in the environment owing to their extensive use in consumer and industrial products. Their high surface area and hydrophobicity enable them to strongly adsorb organic contaminants, such as triclosan (TCS), a persistent antibacterial agent commonly found in wastewater, soil, and sediments. While nanoplastics are known to enhance contaminant mobility, their co-transport behavior under dynamic hydrodynamic conditions is unknown. Flow rate variations and interruptions are common in natural and engineered systems, affecting nanoplastics-contaminant interactions by altering mobility, deposition, and desorption. While increased flow rates can enhance detachment, flow interruption may promote retention through prolonged contact times and irreversible adsorption. However, the mechanisms governing nanoplastic contaminant transport upon flow resumption, including stepwise release and delayed migration, remain poorly understood. This study systematically investigated the flow rate variations and flow interruption effects on PSNP-TCS co-transport in saturated quartz sand. Column transport experiments were conducted to examine breakthrough and retention behaviors, and a mathematical model was used to quantify attachment, detachment, and adsorption-desorption dynamics. These findings provide mechanistic insights into the hydrodynamic regulation of nanoplastics-contaminant transport, contributing to risk assessment and pollution mitigation strategies for aquatic environment. Quartz sand (0.6 mm mean grain size) was used as the porous medium and pretreated through acid washing, deionization, drying, and calcination at 600 ℃ to remove contaminants. The sand had a porosity of 0.44 and a saturated hydraulic conductivity of 1.44 cm·min⁻¹. PSNPs [(121.9±1.8) nm] were obtained commercially and dispersed in a 0.5 mmol∙L⁻¹ NaCl solution, followed by ultrasonication at 100 W for 5 min to ensure homogeneity. TCS stock solutions (2.5 g∙L⁻¹) were prepared in methanol and diluted to 5 mg∙L⁻¹, maintaining a methanol content below 0.2% (v/v) to minimize solvent effects. Transport experiments were performed in glass columns (10.0 cm length and 2.74 cm inner diameter) packed with quartz sand. The columns were pre-equilibrated with a 0.5 mmol∙L⁻¹ NaCl solution for 12 h. Two experimental conditions were investigated: (1) Flow rate variation: Transport behavior was evaluated at 0.3 mL·min⁻¹ and 1.0 mL·min⁻¹ to assess the shear force effects on breakthrough and retention. (2) Flow interruption: Following the introduction of the PSNP and/or TCS for 1.81 times the pore volume at 0.3 mL·min⁻¹, the flow was interrupted for 2, 12, or 24 h before resumption to examine the release dynamics. Effluent samples were analyzed using fluorescence spectrophotometry (PSNP, excitation/emission: 488/518 nm) and UV spectrophotometry (TCS, 210 nm). Zeta potential measurements were performed to evaluate the variations in the surface charge. A convective-dispersive transport model coupled with a dual-kinetic-site adsorption model was used to quantify the attachment, detachment, and sorption parameters. 1) PSNP-TCS co-transport Interactions. The results demonstrate a mutual promotion effect between PSNP and TCS, which significantly alters their transport behaviors in quartz sand. TCS reduced PSNP attachment at both reversible and irreversible sites, while increasing detachment at reversible sites, thereby enhancing PSNP mobility. Moreover, PSNP facilitated TCS transport, as evidenced by a 2.6-fold increase in TCS breakthrough recovery and a 76.2% reduction in TCS adsorption. These findings indicate that PSNP serve as mobile carriers for TCS, reducing its retention in porous media. 2) Flow Rate Effects on Transport Behavior. Increasing the flow rate from 0.3 mL·min⁻¹ to 1.0 mL·min⁻¹ significantly enhanced the transport of both PSNP and TCS. The detachment coefficient of PSNP at reversible sites increased, whereas TCS adsorption onto quartz sand decreased to 13.0-18.0% of its original value. These changes suggest that higher flow rates disrupt particle-collector interactions, reducing deposition and enhancing contaminant mobility. 3) Flow Interruption and Resuspension Mechanisms. Flow interruption induced significant PSNP and TCS retention, particularly at irreversible sites. The longer the interruption, the greater the retention, as reflected in a 3.9-5.1 fold increase in PSNP maximum attachment capacity and a 1.2-1.7 fold increase in TCS adsorption. Upon flow resumption, hydrodynamic shear and turbulence caused the stepwise release of previously deposited PSNP and TCS, leading to sharp concentration peaks before reaching equilibrium. This finding highlights the potential for delayed contaminant migration in transient flow environments. This study systematically examined the co-transport behavior of PSNP and TCS under variable hydrodynamic conditions, revealing critical mechanistic insights into their migration dynamics. These findings confirm that PSNP and TCS mutually enhance each other’s mobility, with PSNP acting as a contaminant carrier and TCS altering PSNP stability. AN increase in flow rate facilitated transport, whereas flow interruption promoted retention, followed by stepwise resuspension upon flow resumption. This study advances the fundamental understanding of nanoplastic contaminant transport, emphasizing the role of hydrodynamic forces in regulating their environmental fate. Future research should explore unsaturated flow conditions, heterogeneous porous media, and periodic hydrodynamic fluctuations to develop a more comprehensive framework for assessing nanoplastics-related environmental risks and informing pollution control strategies.

Key words: nanoplastics, triclosan, column experiment, transport, kinetic model

摘要：

聚苯乙烯纳米塑料（PSNP）和三氯生（TCS）作为新兴污染物，在环境介质中广泛存在，并可能通过相互作用影响彼此的迁移行为和环境归趋。然而，目前对于流体动力条件，特别是流速变化和水流中断对PSNP和TCS共迁移行为的影响机理尚不清晰。该研究系统分析了流速和水流中断对PSNP与TCS在饱和多孔介质（石英砂）中共迁移行为的影响特征，并基于数学模型揭示其动力学机制。研究发现，PSNP与TCS之间存在相互促进作用。TCS的加入降低了PSNP在可逆点位和不可逆点位上的附着系数，同时提高了可逆点位的分离系数。同时，PSNP的存在使TCS穿透曲线的回收率提升2.6倍，并显著降低其在石英砂表面的线性吸附分配系数（减少76.2%）。流速提升显著促进了两者的迁移，PSNP的可逆分离系数提高，而TCS的线性吸附分配系数下降至原来的13.0%-18.0%。PSNP和TCS在石英砂表面的沉积量，随水流中断时间的延长而增加。水流恢复后，流体动力和剪切力促使PSNP和TCS发生再悬浮，浓度逐渐恢复并趋于稳定。研究结果深化了对纳米塑料−污染物共迁移动力学的理解。

关键词: 纳米塑料, 三氯生, 柱实验, 迁移, 动力学模型