(340g) Effect of Microplastics on Nanoparticle Transport in Porous Beds.

Emerging contaminants, such as microplastics as well as nanomaterials, are continually increasing in natural environments, due to their widespread use and applications in the industry and manufacturing of new products with specific characteristics. Therefore, their presence in facility units to remove contaminants and waste using sand filters is a key concern that should be studied.

The objective of the present work is to determine if the presence of microplastics in porous beds could alter the fate and transport of nanomaterials that are present in aqueous matrices.

Materials and methods

The porous bed consisted of quartz sand in a tailor-made acrylic column. The selected nanomaterial was manufactured TiO₂ nanoparticles (P25, Evonik Degussa Corporation). The microplastic was commercial recycled micro polyethylene particles (Mundileno, Argentina). First, the column was filled with a mixture of sand and microplastic, the content of the latter was 0.40 or 2.00%. Then, the medium was homogenized by pumping a background solution (NaCl and CaCl₂ 0.5 and 10 mM ionic strength), trying to mimic natural environments. The addition of the suspension of nanoparticles followed and at last the background solution was injected to observe whether nanoparticles emerged from the column. The concentration of the nanoparticles was obtained by UV spectrophotometry.

Results

As it was determined in a previousl work, when no plastic was present and the ionic strength was low, the breakthrough curves showed a plateau that was reached after 1 pore volume (PV); while for suspensions in background conditions of high ionic strength, first peak was reached after 1 PV and then the retention of nanoparticles increased. The retention of this kind of nanomaterial is due to different principles, such as attachment to the clean sand, then straining and attachment to the ripe bed. This retention was dominated by electrostatic attraction, due to opposite surface charges, based on DLVO theory. Besides, larger aggregates were formed with time, clogging the pores, which was enhanced at 10 mM strength. Thus, retention of the new incoming TiO₂ was increased. At last, attraction of previously deposited TiO₂ contributed to this increased retention and could also be explained by DLVO theory.

In the presence of the recycled microplastic, there was no marked evidence of change in the pattern of the elution curves. For 0.5 mM ionic strength, the same plateau was obtained. In the case of NaCl a slightly lower retention was observed at 1.5 PV, indicating a moderate peak present. And, for CaCl₂, the plateau was reached at lower levels of elution, diminishing from C/C₀ (elution concentration in relation to inlet concentration of nanoparticles) = 0.35 to 0.25 and to 0.20 in the case of the lower and the higher plastic content in the bed, respectively. For NaCl 10 mM, a peak was obtained at 1.5 PV and C/C₀ = 0.3 in all the cases, and the elution diminished, indicating retention principally due to adhesion to the ripe bed, clogging of the pores, and attachment to previously deposited nanoparticles. A decrease in elution was evident when the plastic content was 2%. Conversely, for the case of CaCl₂ 10 mM ionic strength, the peak in elution was obtained at 1.75 PV in the absence of microplastic and at 1.6 PV when it was present, and the peak was considerably reduced as the amount of microplastics was increased to 0.4% and then to 2%, from 0.20 to 0.12 and 0.07, respectively. The presence of a divalent cation may suggest the increase in retention combined with the higher amount of plastic in the medium.

Conclusions

It is important to remark that no major changes were observed in the patterns of the elution curves during the transport of the nanoparticles in the porous medium modified with recycled microplastics. This can be explained by the characteristics of the plastic, inert, resistant, and stable, characteristics that are fundamental to enhance its use and durability. However, some increase in retention was found at three of the studied ionic strengths (NaCl 10 mM, CaCl₂ 0.5 and 10 mM), and a small decrease in the case of NaCl 0.5 mM.

Last, it is important to take into consideration that the presence of microplastics would have the potential to modify the filtration beds, by taking the place of the sand grains, with different affinities for the contaminants present that are sought to remove, therefore the efficiency could be modified according to the aqueous matrix that should be received.