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Article

Effects of Low-Molecular-Weight Organic Acids on the Transport of Polystyrene Nanoplastics in Saturated Goethite-Coated Sand Columns

by
Feiyu Chen
1,
Xiaocheng Peng
2,
Xiaocheng Liu
2,
Biaodian Chen
3,
Lidong Chen
4,
Taotao Lu
1 and
Yi Gong
1,*
1
College of Hydraulic Science and Engineering, Yangzhou University, Yangzhou 225009, China
2
Hydrogeology and Engineering Geology Institute of Hubei Geological Bureau, Jinzhou 434000, China
3
Hubei Key Laboratory of Resources and Eco-environmental Geology, Geological Environmental Center of Hubei Province, Wuhan 430034, China
4
Jiangsu Surveying and Design Institute of Water Resources Co., Ltd., Yangzhou 225009, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(23), 3500; https://doi.org/10.3390/w16233500
Submission received: 6 November 2024 / Revised: 26 November 2024 / Accepted: 3 December 2024 / Published: 5 December 2024
(This article belongs to the Special Issue Environmental Fate and Transport of Organic Pollutants in Water)
Browse Figures
Versions Notes

Abstract

:
The widespread use of nanoplastics inevitably contributes to pollution in aquatic environments and soils. Therefore, it is crucial to understand how these particles migrate in soils with diverse organic matter. This study investigated the effects of low-molecular-weight organic acids (LMWOAs) on the migration of polystyrene nanoplastics (PS-NPs) in goethite-coated quartz sand. The experiments utilized two organic acids, propanoic acid (PA) and tartaric acid (TA), under varying aqueous conditions, including pH levels (4.0, 7.0), ionic strengths (1 mM, 10 mM), and cations (Na+, Ca2+, Ba2+). The experimental results indicated that with the presence of Na⁺, organic acids promoted the migration of PS-NPs through electrostatic forces and steric hindrance, with TA having a greater effect than PA. When pH < pHpzc, increased concentrations of positively charged goethite coating provided favorable deposition sites for the negatively charged PS-NPs, thereby increasing their deposition. Using the DLVO theory, low pH and high ionic strength (IS) decreased the energy barriers between PS-NPs and porous media, whereas high pH and low IS increased these barriers, thus enhancing PS-NPs transport. Divalent cations Ca2+ and Ba2+ enhanced the migration of PS-NPs through complex-forming and -bridging agents. These findings offered significant insights for predicting and analyzing the migration behavior of plastic nanoparticles.

1. Introduction

Plastic products are extensively utilized in various human activities due to their affordability, high ductility, and stability [1,2]. Plastics contribute to significant environmental pollution due to their low recycling rates and decomposition challenges [3]. Micro- and nano-plastics originate from various sources, including cosmetics, industrial processes, agriculture, sewage systems, packaging, and medical devices [4]. Microplastics are particles with sizes ranging from 100 nm to 5 mm, while those smaller than 100 nm are referred to as nanoplastics [5,6]. Micro- and nano-plastics enter the soil and groundwater via weathering, precipitation, runoff, and erosion, posing threats to human and aquatic health, crops, and drinking water sources [7,8]. According to research studies, annual plastic waste emissions on land significantly surpass those in marine settings [9]. Nanoplastics in agricultural soils originate from the degradation of residual mulch, sewage irrigation, sludge application, prolonged organic fertilizer use, and atmospheric deposition [10,11]. The accumulation of nanoplastics in agricultural soils can exert direct or indirect toxic effects on both soil flora and fauna [12]. Moreover, nanoplastics bind to pesticides and antibiotics in agricultural soils via electrostatic effects and van der Waals forces, creating complex pollutants and increasing risks to soil ecosystems [13]. Therefore, it is crucial to conduct experimental research to investigate the movement and ultimate fate of nanoplastics, particularly in agricultural soils.
Iron oxides, including goethite, hematite, ferrihydrite, and others, are naturally found in the soil environment [14]. In particular, goethite is common in soils and is characterized by a large specific surface area, high reactivity, and multiple adsorption sites, which together enhance the adsorption of nanoplastics and influence their mobility in soils [15,16]. Ma et al. demonstrated through column experiments that goethite influenced the migration of nanoplastics by forming complexes with both nanoplastics and dissolved organic matter and that the size of the nanoplastic particles affected their transport in goethite-coated sand columns [17]. Olena et al. found that the heterogeneous aggregation behavior of colloids with iron oxides and nanoplastics impacts nanoplastics migration [18].
Low-molecular-weight organic acids (LMWOAs) are commonly found in the environment, characterized by one or more carboxyl (-COOH) groups, and play a crucial role in processes such as contaminant transport in soil [19,20,21]. LMWOAs are produced by plant root exudation, organic soil material decomposition, and microbial processes [22,23]. Soils contain diverse LMWOAs such as oxalic, malic, citric, tartaric, and propanoic acids [24,25]. Studies have demonstrated that LMWOAs significantly influence the adsorption and mobility of pollutants in soil. Xu et al. reported that LMWOAs enhanced the migration of two neonicotinoid pesticides by competing with them for effective deposition sites on soil particle surfaces [26]. Qin et al. discovered that LMWOAs disrupt the equilibrium between metals adsorbed to solid phases and those in the soil solution by forming complexes, thereby facilitating metal mobility [27]. Chen et al. conducted column experiments and found that LMWOAs facilitated the migration of graphene oxide nanoparticles in saturated kaolinite- and goethite-coated sand columns through electrostatic interactions and steric hindrance [28]. However, current studies on polystyrene nanoplastics lack systematic rigor, and very few researchers have examined the influence of LMWOAs on nanoplastics migration in acicular ferrite-coated saturated porous media.
This study investigated the influence of various LMWOAs on the polystyrene nanoplastics migration in saturated porous media, examining how factors such as pH, ionic strength, and divalent cations influence these dynamics through column experiments with goethite-coated sand and additional characterization studies. The study results aimed to not only deepen our understanding of nanoparticles migration in complex environments but also provide a valuable reference for studying organic acids in the natural environment. Eventually, these studies will offer robust theoretical support for improving remediation technologies for soil and water contaminated with nanoplastics.

2. Materials and Methods

2.1. Materials

Polystyrene nanoplastics (PS-NPs), 99% pure, were obtained from Huge Biotechnology Tech Co., Ltd., Shanghai, China, for experimental use. The relevant properties and preparations of PS-NPs, quartz sand, propanoic acid (PA), and tartaric acid (TA) are described in detail in our previous study [29]. To remove surface impurities, the sand underwent purification using the method outlined by Mattison et al., involving the addition of an appropriate volume of 0.1 M HCl and stirring continuously for 2 h, followed by a similar treatment with 5% H2O2. Afterwards, the sand was rinsed several times with deionized water to reach a neutral pH, then dried at 90 °C and stored [30]. Additionally, PA, and TA with their physicochemical properties are detailed in Table 1.

2.2. Characterization of PS-NPs Suspensions

The hydrodynamic diameter (Dh) and ζ-potential of PS-NPs were measured using dynamic light scattering (DLS) and ZetaPALS (Zetasizer Nano Zen 3700, Malvern Panalytical, Ltd., Shanghai, China), respectively. The surface morphology of quartz sand and goethite-coated sand was observed and analyzed using energy dispersive spectroscopy (EDS) with an Apreo 2 SEM (Thermo Fisher Scientific Inc., Brno, The Czech Republic) field emission scanning electron microscope (SEM, Figure 1). The dispersion of PS-NPs in aqueous solutions containing TA was examined using SEM, which showed that the PS-NPs exhibited no significant aggregation (Figure 1).

2.3. Preparation of Goethite-Coated Sand

In brief, 87.5 mL of 0.17 M Fe(NO3)3 and 90.0 mL of 0.52 M NaOH were added to an evaporating dish to precipitate goethite onto 500 g of quartz sand. The mixture was placed in an oven at 105 °C for 72 h. To remove weakly adsorbed iron, the quartz sand was rinsed with 1.0 mM HCl and 1.0 mM NaOH [31].

2.4. Column Experiments

The column experimental setup primarily consists of a syringe pump with a syringe, a three-way valve, and a glass column (see Figure S1). The solution in the syringe is injected upward from the bottom through the syringe pump. Glass columns used in all experiments were 10 cm long and had an internal diameter of 1.35 cm. About 9 g of a blend of pure quartz sand and goethite-coated quartz sand (GQS) was added into each column. The average length of dry sand in the column was 5.1 cm. Before the transport experiment commenced, deionized water was slowly injected into the column using a syringe pump at the 0.018 mL/min rate to saturate the sand column. Noting that we measured its pH values with a pH meter and adjusted it using 0.1 M HCl and 0.1 M NaOH until it stabilized at the desired pH values before introducing the background electrolyte solution without PS-NPs. The background electrolyte solution was homogeneously mixed using a magnetic stirrer. Then, 150 mL of only the electrolyte solution was injected at 0.1 mL/min rate to reach saturation equilibrium. After 30 min of sonicating the PS-NPs solution to prevent aggregation, we applied the same treatment to the PS-NPs solution, measuring its pH before introduction and at the experiment’s conclusion. Afterwards, 60 mL of PS-NPs solution was pumped at 0.18 mL/min rate. The pH of the PS-NPs solutions introduced into all column experiments was consistently within the threshold range set by the experimental protocol, both before and after the experiments. To ensure all unbound PS-NPs were washed out, 20 mL of only background electrolyte solution was subsequently injected at the same rate. Effluent samples were collected in 5 mL brown glass vials from the outlet of the upper end of the sand column every 20 min (approximately 1 PV). Every column experiment collected twenty-one samples, and the amount of PS-NPs was quantified using a spectrophotometer. Absorbance was linearly related to the concentration of PS-NPs (see Figure S2 for the calibration curves). The effluent concentration C was normalized to the initial concentration C0 of the pumped PS-NPs solution. The experimental program is detailed in Table 2. Detailed experimental descriptions not only enabled readers to assess the soundness and scientific validity of the design but also facilitated the replication of experiments to verify and understand the findings.

2.5. DLVO Calculations

The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was utilized to estimate the total interaction energy between PS-NPs and sand grains [32,33].
V TOT = V VDW + V EDL
where VTOT represents the total interaction, VVDW is van der Waals, and VEDL is electrostatic double layer forces. For specific contents, please refer to the Supplementary Materials.

3. Results and Discussion

3.1. Impacts of the Content of Goethite-Coated Sand on PS-NPs Migration

Figure 2 illustrates the impact of varying contents of goethite coating (θ, θ is the mass fractions of goethite–coated sand) on PS-NPs migration in goethite-coated sand (GQS). It showed that the contents of goethite coating significantly influenced PS-NPs transport. PS-NPs exhibited the strongest migration ability in pure quartz sand (θ = 0), with the maximum C/C0 of 76% detected flowing out of the sand column at 12 pore volumes (PVs) [29]. In contrast, the C/C0 values reached the maximum of 42% and 10% at approximately 15 PV when θ was 0.05 and 0.10, respectively. At θ = 0.25, the effluent of PS-NPs was not significantly detected at any PV. The charge heterogeneity of the porous media at varying θ values was the primary reason for the differing degrees of PS-NPs transport [34,35]. The point of zero charge (pHpzc) of goethite ranges from 8.3 to 8.8 [36,37,38,39]. Consequently, under the experimental conditions (pH 4.0–7.0), the goethite surface was predominantly positively charged. As PS-NPs were electronegative (Table 2), the positively charged goethite on the surface attracted PS-NPs through electrostatic interactions, leading to the deposition on the GQS surface [40].
Scanning electron microscopy (SEM) revealed notable morphological differences between quartz sand and GQS (Figure 1). In comparison to pure quartz sand, GQS exhibited rougher surfaces and larger surface areas, offering PS-NPs more deposition sites [41]. Additionally, as θ increased, the amount of goethite coated on the quartz sand surface also increased, leading to a more uneven surface texture. At this stage, the pore volume of the quartz sand in the sand column became more complex, as shown by smaller pore throats and increased dead volumes within the pores [42]. This complexity facilitated the increased deposition of PS-NPs.

3.2. Impacts of LMWOAs on PS-NPs Migration Under Varying pH Conditions

The soil in its natural environment is typically more acidic or neutral [43,44,45]. For example, Roelofsen et al. employed remote sensing techniques to estimate soil pH values in natural areas within the Netherlands, which primarily ranged from 2.1 to 7.9 [46]. Therefore, this subsection specified two pH conditions: pH 4.0 (acidic condition) and 7.0 (neutral condition). Figure 3 illustrates the effects of LMWOAs on PS-NPs migration in GQS under pH 4.0 and 7.0, showing enhanced transport with effectiveness ranked as follows: TA > PA. For example, as shown in Figure 3a, at pH 4.0 and without organic acids, the C/C0 values of PS-NPs were detected to reach the maximum of 10% from the sand column at 15 PV. In contrast, when PA and TA were present, the C/C0 values reached the maximum of 13% and 76%, respectively. We proposed that LMWOAs enhance the migration of PS-NPs primarily through two mechanisms. Initially, LMWOA adsorption on PS-NPs and GQS boosted their surface negative charges, leading to higher ζ-potentials (Table 2), which, in turn, strengthened the electrostatic forces, hindering PS-NPs adsorption onto GQS surfaces and thereby facilitating greater PS-NPs migration [47,48]. Furthermore, LMWOAs, adsorbing on PS-NPs and GQS, could increase the steric hindrance between the two and thereby enhance PS-NPs migration [49]. Notably, PA has one -COOH group, while TA possesses two carboxyl groups and two hydroxyl groups. More -COOH groups of LMWOAs carried additional negative charges, allowing for greater adsorption onto PS-NPs and GQS surfaces, thereby led to more electrostatic forces between PS-NPs and GQS [50,51]. Moreover, compared to PA, TA has a higher molecular weight and a larger surface area, resulting in greater surface coverage when adsorbed onto GQS. This larger coverage was more effective in competing to deposition sites with PS-NPs [52]. At pH 4.0, the electronegativity of both PS-NPs and GQS was diminished, independent of the presence of organic acids. Consequently, the DLVO theory could not well explain how LMWOAs affected PS-NPs migration in GQS (Table S1 and Figure S3).
As for pH 4.0, the migration of PS-NPs significantly increased at pH 7.0. At pH 4.0, the C/C0 values reached the maximum of 10%, 13%, and 76% at 15 PV both without organic acids and the presence of PA and TA, respectively. At pH 7.0, the values of C/C0 reached the maximum of 87%, 88%, and 94%, respectively. The pH-dependent deposition of PS-NPs could be attributed to the variable charge of the goethite coating. At pH 7.0, which is closer to the pHpzc of goethite (8.3 to 8.8), the surface of the goethite carried less positive charge compared to pH 4.0. At this point, fewer PS-NPs were attracted to goethite, which subsequently enhanced the transport within the GQS. Additionally, rising pH also increased the deprotonation of organic acids, converting -COOH to -COO⁻ and H⁺, which lowered the ζ-potentials of PS-NPs and GQS, facilitating PS-NPs movement. At pH 7.0, the maximum potential barrier (Φmax) was calculated to be 5.30, 9.83, and 32.63 KBT, while the secondary energy minimum (Φsec) was −0.43, −0.26, and −0.15 KBT without organic acids and the presence of PA and TA, respectively, based on the DLVO theory (Table S1 and Figure S3). An increased pH raised Φmax and reduced Φsec [53]. Consequently, at pH 7.0, it became more challenging for PS-NPs to overcome Φmax and then deposit. These factors collectively account for the significant differences observed in the penetration curves of PS-NPs at varying pH levels. Moreover, compared to pH 7.0, at pH 4.0, the zeta potentials of PS-NPs and GQS became less negative, leading to weaker electrostatic forces among PS-NPs particles. This increased likelihood of aggregation in aqueous solutions led to larger hydrodynamic diameters (Table 2) and reduced mobility, thereby enhancing the propensity for PS-NP deposition [54,55,56]. Additionally, due to weak electrostatic repulsion between PS-NPs and GQS, the PS-NPs were readily deposited onto GQS through heterogeneous aggregation [57].

3.3. Effects of LMWOAs on PS-NPs Migration in the Presence of Cation Species

When divalent cations like Ca2+ and Ba2+ were present, LMWOAs could similarly enhance PS-NPs migration. For example, at pH 4.0 with the presence of Ca2+, the C/C0 value reached a maximum of approximately 33% at 15 PV without organic acids. In contrast, in the presence of PA and TA, the maximum was 36% and 74% at 15 and 14 PVs, respectively (Figure 4). The migration of PS-NPs was similar to those seen with Na+. The enhancement of PS-NPs transport by TA and PA might occur through several mechanisms. The primary mechanism was the bridging effect of divalent cations such as Ca2+. Without organic acids, this bridging occurred through the formation of PS-NPs-Ca2+-GQS complexes, facilitated by the -O functional groups present on both PS-NPs and GQS surfaces, leading to the deposition of PS-NPs [58,59,60]. When organic acids were present, their abundant -O functional groups (hydroxyl and carboxyl groups) could occupy active adsorption sites on GQS through the bridging effect of Ca2+ and GQS. The greater number of oxygen-containing functional groups in TA allowed it to occupy more active adsorption sites. Secondly, organic acids formed complexes with divalent cations through their oxygen functional groups. Consequently, PS-NPs in solution migrated as PS-NPs-Ca2+-LMWOAs complexes, which enhanced migration [61,62,63,64]. Additionally, LMWOAs adsorb onto PS-NPs and GQS via Ca2⁺ bridging, increasing the ζ-potentials of both, which intensifies electrostatic repulsion and steric hindrance (Table 2). Moreover, organic acids, particularly TA, facilitated PS-NPs transport more effectively than PA. This increased effectiveness was attributed to TA’s higher count of functional groups leading to increased negative charges and enhanced electrostatic repulsion. Additionally, TA, having a higher molecular weight, possessed a larger surface area and competed more effectively for deposition sites. DLVO calculations indicated that the maximum potential barrier height for TA (23.21 KBT) was greater than that for PA (4.82 KBT) (Table S1). Consequently, PS-NPs found it more challenging to overcome Φmax and deposit on the surface of GQS.
Differences in the maximum concentration of PS-NPs in the effluent and the breakthrough curves of PS-NPs migration within the column were observed when different divalent cations were present in the solution [65]. When Ba2+ was present, the C/C0 value reached the maximum of approximately 31% at 15 PV without organic acids. However, in the presence of PA and TA, C/C0 peaked at 33% and 68% at 15 PV, respectively (Figure 4). The order of PS-NP mobility inhibition induced by divalent cations was Ba2+ > Ca2+. This difference arises because Ba2+ was a more effective bridging agent than Ca2+ because it readily formed inner-sphere complexes with the -COOH groups on PS-NPs and the oxygen groups on GQS [66,67]. In contrast, Ca2+ typically formed outer-sphere complexes, resulting in weaker binding to the solid phase. DLVO theoretical calculations indicated that, in the presence of Ba2+, the θmax values were 3.65, 5.07, and 8.42 KBT without organic acids and in the presence of PA and TA, respectively. The corresponding θmin values were −0.05, −0.04, and −0.03 KBT. In contrast, in the presence of Ca2+, θmax values were 6.34, 4.82, and 23.21 KBT, with θmin values of −0.03, −0.02, and −0.02 KBT, respectively (Table S1). Compared to Ca2+, both the θmax and the θmin were lower with Ba2+, which made PS-NPs more easily overcome the θmax and deposit on GQS surfaces.

3.4. Impacts of LMWOAs on the Migration of PS-NPs with Various Ionic Strength

Figure 5 illustrates that the C/C0 values reached the maximum of 71%, 72%, and 91% at the ionic strength (IS) of 1 mM and a pH of 4.0, corresponding to conditions without organic acids, with PA, and with TA, respectively. The C/C0 values reached the maximum of 10%, 13%, and 76% at the IS of 10 mM and a pH of 4.0. In the column, the migration of PS-NPs decreased as IS increased [68]. Table 2 shows that PS-NPs ζ-potentials with IS of 1 mM, without organic acids and PA and the presence of TA, were −10.33, −12.17, and −15.50 mV, respectively. The quartz sand zeta potentials were −19.20, −20.90, and −24.10 mV, respectively. PS-NPs migration decreased as ionic strength increased. At the IS of 10 mM, PS-NPs zeta potentials were −1.15, −2.62, and −4.65 mV, while those of quartz sand were −3.20, −3.96, and −5.71 mV. Increasing IS compressed the double electric layer of PS-NPs, raised the ζ-potentials of both PS-NPs and GQS [69], and reduced electrostatic repulsion, promoting PS-NPs deposition. Additionally, at the IS of 1 mM, the application of DLVO theory resulted in θmax values of 8.34, 11.35, and 19.09 KBT, corresponding to conditions without organic acids and with PA and TA, respectively. The θmin values were −0.02, −0.02, and −0.01 KBT, respectively (Table S1). At this point, the calculated θmax and θmin were higher than those at high IS (10 mM). This indicated that PS-NPs could easily overcome the maximum potential barrier at high IS and ultimately deposit on GQS surfaces.

4. Conclusions

This paper examined how various LMWOAs influenced the migration of PS-NPs through saturated GQS columns under various aqueous conditions. The charge heterogeneity of the goethite surface offered favorable deposition sites for PS-NPs, while the positive charge on the goethite surface under experimental conditions (pH 4.0–7.0) attracted PS-NPs to deposit on the GQS surface through electrostatic interactions. An increase in the content of goethite coating led to enhanced heterogeneous aggregation behavior of PS-NPs and reduced particle mobility. When the background solution was Na+, LMWOAs promoted the migration of PS-NPs under different pH values, with TA exhibiting stronger promotion due to their higher molecular weights and greater number of functional groups. Both pH and ionic strength also influenced PS-NPs transport. DLVO theoretical calculations indicated that high IS and low pH reduced θmax and θmin between particles. Additionally, higher IS compressed the double electric layer of PS-NPs and reduced the electrostatic forces between PS-NPs and GQS. The transport results for Ca2+ and Ba2+ as background cations were similar to those for Na+, with the bridging effect of the divalent cations and their ability to complex with various organic acids being the main mechanisms influencing PS-NPs migration. These experimental results not only deepened our understanding of nanoparticles migration in complex environments but also provided a valuable reference for studying various organic acids in environmental settings. The possibility of controlling contaminants transport by altering the water chemistry of groundwater and agricultural soils, or by employing specific organic acids, requires further investigation. These studies would offer robust theoretical support for improving remediation technologies for soil and water contaminated with nanoplastics.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16233500/s1, Figure S1: Experimental setup; Figure S2: Calibration curve as absorbance at the wavelength of 220 nm vs. concentration of PS-NPs in suspension; Figure S3: Calculated DLVO interaction energy profiles under LMWOAs (PA and TA) at different pH (at a fixed ionic strength, 10 mM NaCl): (a) pH 4.0 and (b) pH 7.0. The insets are plotted on a smaller y-axis scale to highlight the secondary energy minimum. Table S1: Calculated height of the energy barriers (Φmax), secondary energy minimum (Φsec), and the respective separation distances of particle–collector DLVO interaction energy profiles. References [70,71,72,73,74] are cited in the Supplementary Materials.

Author Contributions

Methodology, F.C.; software, T.L.; validation, X.P. and X.L.; investigation, F.C., X.P., X.L., B.C., L.C. and Y.G.; resources, Y.G.; data curation, T.L.; writing—original draft preparation, F.C.; writing—review and editing, F.C. and T.L.; supervision, B.C. and L.C.; funding acquisition, T.L. and Y.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Yangzhou Talent Program “LvYangJingFeng” (YZLYJFJH2022YXBS124).

Data Availability Statement

The dataset used in this study is not publicly available. Researchers with requests should contact the corresponding author. We will provide support and additional information within reasonable boundaries.

Acknowledgments

We acknowledge the assistance of the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Yangzhou University).

Conflicts of Interest

Author Lidong Chen was employed by the Jiangsu Surveying and Design Institute of Water Resources Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Water 16 03500 g001
Figure 1. The SEM and EDS analysis of (a) goethite-coated sand and (b) pure quartz sand and the SEM analysis of (c) PS-NPs in aqueous solution containing TA.
Figure 1. The SEM and EDS analysis of (a) goethite-coated sand and (b) pure quartz sand and the SEM analysis of (c) PS-NPs in aqueous solution containing TA.
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Water 16 03500 g002
Figure 2. Breakthrough curves (BTCs) of PS-NPs at varying θ ranging from 0 to 0.25 are depicted (the specific details θ = 0 are described in our previous study [29]). All BTCs were produced under the same conditions, with a pH of 4.0 and an ionic strength (IS) of 10 mM NaCl. Error bars stand for the standard deviation across two replicates.
Figure 2. Breakthrough curves (BTCs) of PS-NPs at varying θ ranging from 0 to 0.25 are depicted (the specific details θ = 0 are described in our previous study [29]). All BTCs were produced under the same conditions, with a pH of 4.0 and an ionic strength (IS) of 10 mM NaCl. Error bars stand for the standard deviation across two replicates.
Water 16 03500 g002
Water 16 03500 g003
Figure 3. Impacts of various LMWOAs on PS-NPs transport at 10mM NaCl (θ = 0.1): (a) pH 4.0 and (b) pH 7.0. Error bars stand for the standard deviation across two replicates.
Figure 3. Impacts of various LMWOAs on PS-NPs transport at 10mM NaCl (θ = 0.1): (a) pH 4.0 and (b) pH 7.0. Error bars stand for the standard deviation across two replicates.
Water 16 03500 g003
Water 16 03500 g004
Figure 4. Impacts of various LMWOAs on the PS-NPs migration of varying cation species (1 mM CaCl2 and BaCl2; θ = 0.1; pH 4.0): (a,c) breakthrough curves and (b,d) relative calculated DLVO theory. Error bars stand for the standard deviation across two replicates. Insets are plotted with a reduced y-axis scale to emphasize the secondary energy minimum.
Figure 4. Impacts of various LMWOAs on the PS-NPs migration of varying cation species (1 mM CaCl2 and BaCl2; θ = 0.1; pH 4.0): (a,c) breakthrough curves and (b,d) relative calculated DLVO theory. Error bars stand for the standard deviation across two replicates. Insets are plotted with a reduced y-axis scale to emphasize the secondary energy minimum.
Water 16 03500 g004
Water 16 03500 g005
Figure 5. Impacts of diverse LMWOAs on PS-NPs migration at 1 mM NaCl (θ = 0.1, pH 4.0): (a) BTCs and (b) relative calculated DLVO interaction energy profiles. Error bars stand for the standard deviation across two replicates. Insets are plotted with a reduced y-axis scale to emphasize θmin.
Figure 5. Impacts of diverse LMWOAs on PS-NPs migration at 1 mM NaCl (θ = 0.1, pH 4.0): (a) BTCs and (b) relative calculated DLVO interaction energy profiles. Error bars stand for the standard deviation across two replicates. Insets are plotted with a reduced y-axis scale to emphasize θmin.
Water 16 03500 g005
Table 1. Structures and acidity coefficient (pKa) of various organic acids.
Table 1. Structures and acidity coefficient (pKa) of various organic acids.
Organic AcidStructureMolecular Weightpka1pka2
Propanoic acid (PA)Water 16 03500 i00174.084.87
Tartaric acid (TA)Water 16 03500 i002150.093.044.37
Table 2. Selected properties of PS-NPs suspensions and sand under different solution chemistry conditions.
Table 2. Selected properties of PS-NPs suspensions and sand under different solution chemistry conditions.
Column
No.		θPorous
Media		Background SolutionpHζ Potential of PS-NPs a (mV)ζ Potential of Sand b (mV)Zave-PS-NPs c (nm)
10.05GQS10 mM NaCl4.0---
20.25GQS10 mM NaCl4.0---
30.10GQS10 mM NaCl4.0−1.15−3.20190.8
40.10GQS10 mM NaCl + 0.1 mM propanoic acid4.0−2.62−3.96150.2
50.10GQS10 mM NaCl + 0.1 mM tartaric acid4.0−4.65−5.71120.0
60.10GQS10 mM NaCl7.0−10.61−21.93182.4
70.10GQS10 mM NaCl + 0.1 mM propanoic acid7.0−13.68−26.00133.9
80.10GQS10 mM NaCl + 0.1 mM tartaric acid7.0−21.53−44.03104.2
90.10GQS1 mM NaCl4.0−10.33−19.20124.6
100.10GQS1 mM NaCl + 0.1 mM propanoic acid4.0−12.17−20.90118.9
110.10GQS1 mM NaCl + 0.1 mM tartaric acid4.0−15.50−24.10119.3
120.10GQS1 mM CaCl24.0−7.98−14.17203.7
130.10GQS1 mM CaCl2 + 0.1 mM propanoic acid4.0−8.15−15.27137.2
140.10GQS1 mM CaCl2 + 0.1 mM tartaric acid4.0−19.20−16.87160.9
150.10GQS1 mM BaCl24.0−6.20−11.07262.6
160.10GQS1 mM BaCl2 + 0.1 mM propanoic acid4.0−7.26−13.40207.4
170.10GQS1 mM BaCl2 + 0.1 mM tartaric acid4.0−9.48−14.33196.8
Notes: a ζ-potential of PS-NPs. b ζ-potential of quartz sand. c Hydrodynamic diameter of PS-NPs based on DLS analysis.
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Chen, F.; Peng, X.; Liu, X.; Chen, B.; Chen, L.; Lu, T.; Gong, Y. Effects of Low-Molecular-Weight Organic Acids on the Transport of Polystyrene Nanoplastics in Saturated Goethite-Coated Sand Columns. Water 2024, 16, 3500. https://doi.org/10.3390/w16233500

AMA Style

Chen F, Peng X, Liu X, Chen B, Chen L, Lu T, Gong Y. Effects of Low-Molecular-Weight Organic Acids on the Transport of Polystyrene Nanoplastics in Saturated Goethite-Coated Sand Columns. Water. 2024; 16(23):3500. https://doi.org/10.3390/w16233500

Chicago/Turabian Style

Chen, Feiyu, Xiaocheng Peng, Xiaocheng Liu, Biaodian Chen, Lidong Chen, Taotao Lu, and Yi Gong. 2024. "Effects of Low-Molecular-Weight Organic Acids on the Transport of Polystyrene Nanoplastics in Saturated Goethite-Coated Sand Columns" Water 16, no. 23: 3500. https://doi.org/10.3390/w16233500

APA Style

Chen, F., Peng, X., Liu, X., Chen, B., Chen, L., Lu, T., & Gong, Y. (2024). Effects of Low-Molecular-Weight Organic Acids on the Transport of Polystyrene Nanoplastics in Saturated Goethite-Coated Sand Columns. Water, 16(23), 3500. https://doi.org/10.3390/w16233500

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