Impact of Low-Molecular-Weight Organic Acids on the Transport of Polystyrene Nanoplastics

Plastic nanoparticles, widely used in various consumer products, have become a significant contributor to soil pollution, making it essential to understand their transport in soils, where organic substances are prevalent. This study aimed to investigate the influence of low-molecular-weight organic acids (LMWOAs) on the transport of polystyrene nanoparticles (PS-NPs) through saturated quartz sand. The focus was on seven specific organic acids: three dibasic acids—malonic acid (MA1), malic acid (MA2), and tartaric acid (TA)—and four monobasic acids—formic acid (FA), acetic acid (AA), propanoic acid (PA), and glycolic acid (GA). The effects were evaluated across a range of pH levels (4.0, 5.5, and 7.0) and in the presence of two cations, Na⁺ and Ca²⁺. The results showed that, in the presence of Na⁺, dibasic acids significantly enhanced the transport of PS-NPs, with TA being the most effective, followed by MA2 and MA1. This enhancement was attributed to the adsorption of LMWOAs onto the PS-NPs and quartz sand, leading to a more negative ζ-potential. This negative shift increased electrostatic repulsion between the particles, reducing their deposition and facilitating transport. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory further explained that higher pH levels increased the energy barrier, which reduced PS-NPs deposition by stabilizing them in the suspension. In contrast, the monobasic acids—apart from GA—exhibited minimal impact on PS-NP transport. These acids slightly diminished the hydrophilicity of the PS-NPs, as evidenced by a minor increase in the water contact angle, which in turn reduced their mobility. However, GA, with its additional hydroxyl group, acted similarly to dibasic acids, promoting both enhanced hydrophilicity and increased transport of PS-NPs. When Ca²⁺ was present, the transport enhancement was similar to that observed with Na⁺. The complexation and bridging effects of Ca²⁺ with the organic acids and PS-NPs contributed to this effect. Overall, these findings offer valuable insights into the factors influencing the mobility of PS-NPs in soils, which is crucial for understanding their environmental behavior and potential ecological impacts.