Sensitivity of the Transport of Plastic Nanoparticles to Typical Phosphates Associated with Ionic Strength and Solution pH

Abstract

The influence of phosphates on the transport of plastic particles in porous media is environmentally relevant due to their ubiquitous coexistence in the subsurface environment. This study investigated the transport of plastic nanoparticles (PNPs) via column experiments, paired with Derjaguin-Landau-Verwey-Overbeek calculations and numerical simulations. The trends of PNP transport vary with increasing concentrations of NaH₂PO₄ and Na₂HPO₄ due to the coupled effects of increased electrostatic repulsion, the competition for retention sites, and the compression of the double layer. Higher pH tends to increase PNP transport due to the enhanced deprotonation of surfaces. The release of retained PNPs under reduced IS and increased pH is limited because most of the PNPs were irreversibly captured in deep primary minima. The presence of physicochemical heterogeneities on solid surfaces can reduce PNP transport and increase the sensitivity of the transport to IS. Furthermore, variations in the hydrogen bonding when the two phosphates act as proton donors will result in different influences on PNP transport at the same IS. This study highlights the sensitivity of PNP transport to phosphates associated with the solution chemistries (e.g., IS and pH) and is helpful for better understanding the fate of PNPs and other colloidal contaminants in the subsurface environment.

Keywords: phosphates; plastic nanoparticles; release; retention; solution chemistry.

Figure 1

Breakthrough curves of PNPs at various NaH₂PO₄ (0–1 mM) (a) or Na₂HPO₄ (b) concentrations in the absence of NaCl under pH 7. The release of PNPs was initiated by eluting with ultrapure water under pH 7 (phase 2) and pH 10 (phase 3). Replicate experiments were performed under all experimental conditions.

Figure 2

Breakthrough curves of PNPs with NaH₂PO₄ (0–1 mM) under pH = 7 (a) or pH = 10 (b); breakthrough curves of PNPs with Na₂HPO₄ (0–1 mM) at pH = 7 (c) or pH = 10 (d). All the experiments were carried out in the presence of 1 mM NaCl. The release of PNPs was initiated by eluting with ultrapure water under pH 7 (phase 2) and pH 10 (phase 3), respectively. Only the elution of water at pH 10 (phase 2) was performed when the PNPs were retained under pH 10 in phase 1.

Figure 3

Breakthrough curves of PNPs at NaH₂PO₄ (0.25 mM) under pH = 5–10 (a); breakthrough curves of PNPs at Na₂HPO₄ (0.25 mM) under pH = 5–10 (b). All the experiments were carried out in the presence of 1 mM NaCl. The release of PNPs was initiated by eluting with ultrapure water under pH 7 (phase 2) and pH 10 (phase 3), respectively.

Figure 4

Breakthrough curves of PNPs under IS = 1 with different combinations of phosphate and NaCl at pH = 7. The release of PNPs was initiated by eluting with ultrapure water under pH 7 (phase 2) and pH 10 (phase 3), respectively.