Cotransport and deposition of colloidal polystyrene microplastic particles and tetracycline in porous media: The impact of ionic strength and cationic types

The cotransport behaviors of colloidal polystyrene microplastic particles (PSMPs) and tetracycline (TC) (20 mg/L) were investigated in saturated porous media in KCl and CaCl2 solutions of various ionic strengths (1, 10, 50, 100 mM). Furthermore, the effects of TC concentration (0, 1, 5, 10, 20 mg/L) on the cotransport behaviors of PSMPs and TC in 100 mM KCl solution were assessed. The cotransport behaviors were analyzed by comparing the individual transport behaviors of PSMPs or TC. When cotransported, the presence of TC (20 mg/L) slightly inhibited PSMPs mobility in K+ solutions (the C/C-0 decreased in the range of 0-5.9%), but facilitated it in Ca2+ solutions (the C/C-0 increased in the range of 6.7-42.6%). In KCl solutions, although the presence of TC (PSMPs) did not significantly affect the transport behaviors of PSMPs (TC), the attachment efficiencies of both PSMPs and TC showed a non-linear and non-monotonic change with increase in ionic strength. However, in CaCl2 solutions, the effects of TC (PSMPs) on the transport behaviors of PSMPs (TC) were remarkable and a non-linear non-monotonic change was observed. The adsorption of TC on PSMPs might play a critical role during the cotransport. Thus, the balance between the transport-inhibiting (e.g., the reduction in electrostatic repulsive force) and transport-facilitating effects (e.g., the effects on hydrophilicity/hydrophobicity of PSMPs due to TC adsorption) may be responsible for the observed changes. Overall, the results demonstrated that the cotransport behaviors of PSMPs and TC were more complicated than their individual transport behaviors in porous media, which might vary considerably with environmental conditions. This work could greatly improve our understanding of complex cotransport behaviors and environmental risk of PSMPs. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The co-transport behaviors of colloidal polystyrene microplastic particles (PSMPs) and tetracycline (TC) were found to be complex and influenced by ionic strengths in saturated porous media. The presence of TC affected the mobility of PSMPs differently in K+ and Ca2+ solutions, while the attachment efficiencies of both PSMPs and TC showed non-linear and non-monotonic changes with increases in ionic strength, particularly in CaCl2 solutions. The balance between transport-inhibiting and transport-facilitating effects, such as TC adsorption on PSMPs, played a critical role in the observed changes.