离子类型、强度和铁氧化物影响下微塑料的迁移行为及模型计算

土壤中的微塑料（MPs）作为一种新型污染物而被广泛关注，其迁移行为受到自身理化性质、土壤溶液化学组成和土壤矿物等因素的影响.目前，铁氧化物存在时，离子类型和离子强度对MPs迁移行为的影响机制尚不明确.通过稳定性实验和迁移实验，重点考察了离子类型、离子强度和铁氧化物对带有不同官能团聚苯乙烯微塑料（PSMPs）迁移行为的影响，并用胶体迁移模型、CD-MUSIC模型及DLVO理论探讨其迁移机制.结果表明，PSMPs在NaH₂PO₄背景下的稳定性最强，标准化浓度（c/c₀）达到0.99，而在CaCl₂背景下的稳定性相对弱，c/c₀降低至0.94.不同离子种类对PSMPs的迁移行为产生不同影响，对于阳离子Na⁺和Ca²⁺，二价Ca离子有架桥作用和较强的电荷中和作用，更能阻滞PSMPs在石英砂中的迁移，出水溶液中PSMPs回收率低至（43.83±1.71）%且第二动力学位点上的一级保留系数k_2a为1.54 min^-1，铁氧化物会进一步加剧阻滞作用，回收率降低为（6.04±0.40）%且k_2a升高为5.33 min^-1；对于阴离子Cl^-和PO₄^3-，PSMPs在纯石英砂柱中的迁移主要受其自身表面电负性的控制，Cl^-离子下PSMPs电负性更低，回收率升高［（92.95±0.63）%］且k_2a降低（0.19 min^-1），而当铁氧化物存在时，PSMPs的迁移则受控于载铁石英砂表面的Zeta电位.CD-MUSIC模型计算结果显示PO₄^3-容易吸附在铁氧化物表面形成内圈络合物，降低载铁石英砂表面的电负性，因此PSMPs在PO₄^3-背景下的回收率升高［（76.22±1.39）%］且k_2a降低（0.66 min^-1）；同时，该内圈络合物的形态受PO₄^3-浓度的控制且不同形态的内圈络合物具有不同的表面负电荷数，因此高PO₄^3-浓度下载铁石英砂表面电负性反而高，不利于PSMPs迁移.此外，PSMPs的迁移均随着离子强度的增加而降低.最后，DLVO理论计算出不同条件下PSMPs和介质间的一级势垒变化趋势与PSMPs的迁移能力一致，即较高的一级势垒表明PSMPs和介质间较大的相互斥力有利于PSMPs的迁移，可以更好地阐明PSMPs的迁移行为.

Transport and Model Calculation of Microplastics Under the Influence of Ionic Type, Strength, and Iron Oxide

Microplastics (MPs) in soil have attracted extensive attention as an emerging pollutant, and the transport of MPs is affected by their own physical and chemical properties, the chemical composition of soil solutions, and soil minerals. However, in the presence of oxides, the underlying mechanism for the transport of MPs in different ionic types and ionic strengths is still not fully understood. In this study, the effects of ionic type, ionic strength, and iron oxide on the transport of polystyrene microplastics (PSMPs) with different functional groups were investigated through stability experiments and transport experiments. The colloid transport model, CD-MUSIC model, and DLVO theory were used to explore the transport mechanism. The results showed that normalized concentrations (c/c₀) of PSMPs were 0.99 in the NaH₂PO₄ background and 0.94 in the CaCl₂ background, respectively, which indicated that the strongest stability of PSMPs was observed in the former and the weakest in the latter. Different ionic types had different effects on the transport of PSMPs. For the cations Na⁺ and Ca²⁺, Ca²⁺ strongly inhibited PSMPs transport in pure quartz sand because of the bridging effect and strong charge neutralization effect; the recovery rate of the PSMPs in the effluent was (43.83±1.71)%, and a first-order retention coefficient on the second kinetic Site-2 (k_2a) was 1.54 min^-1. The presence of iron oxide enhanced the inhibition, the recovery rate of the PSMPs in the effluent decreased to (6.04±0.40)%, and k_2a increased to 5.33 min^-1. For the anions Cl^- and PO₄^3-, the transport of PSMPs in pure quartz sand was dominated by surface electronegativity of PSMPs, and PSMPs exhibited lower electronegativity under Cl^- background and thus showed higher recovery(92.95±0.63)%] and lower k_2a (0.19 min^-1). However, in the presence of iron oxides, the Zeta potential of the quartz sand surface was the controlling factor for PSMPs transport. According to results of the CD-MUSIC model, PO₄^3- could be easily adsorbed on the iron oxide surface to form innersphere complexes, which reduced the surface electronegativity of the iron-loaded quartz sand and enhanced the transport of PSMPs, higher recovery(76.22±1.39)%], and lower k_2a (0.66 min^-1). Moreover, the species of the formed innersphere complex was controlled by the PO₄^3- concentration, and different species of innersphere complexes had distinct negative surface charges. Higher surface electronegativity of the iron-loaded quartz sand was observed under higher PO₄^3- concentration, which was not conducive to the transport of PSMPs. Further, the transport ability of PSMPs decreased with the increase in ionic strength. Finally, the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to calculate the variation in the primary barrier between PSMPs and the collector under the conducted experimental conditions, which helped better elucidate the transport behavior of PSMPs. The variation in the primary barrier was consistent with the transport ability of PSMPs, and a higher primary barrier indicated a larger repulsion between PSMPs and the collector, which was in favor of PSMPs transport.