Shift in Mass Transfer of Wastewater Contaminants from Microplastics in the Presence of Dissolved Substances

In aqueous environments, hydrophobic organic contaminants are often associated with particles. Besides natural particles, microplastics have raised public concern. The release of pollutants from such particles depends on mass transfer, either in an aqueous boundary layer or by intraparticle diffusion. Which of these mechanisms controls the mass-transfer kinetics depends on partition coefficients, particle size, boundary conditions, and time. We have developed a semianalytical model accounting for both processes and performed batch experiments on the desorption kinetics of typical wastewater pollutants (phenanthrene, tonalide, and benzophenone) at different dissolved-organic-matter concentrations, which change the overall partitioning between microplastics and water. Initially, mass transfer is externally dominated, while finally, intraparticle diffusion controls release kinetics. Under boundary conditions typical for batch experiments (finite bath), desorption accelerates with increasing partition coefficients for intraparticle diffusion, while it becomes independent of partition coefficients if film diffusion prevails. On the contrary, under field conditions (infinite bath), the pollutant release controlled by intraparticle diffusion is not affected by partitioning of the compound while external mass transfer slows down with increasing sorption. Our results clearly demonstrate that sorption/desorption time scales observed in batch experiments may not be transferred to field conditions without an appropriate model accounting for both the mass-transfer mechanisms and the specific boundary conditions at hand

Linear regressions of total element concentrations and TSS during different sampling campaigns in the lower Haraz catchment (March 2016 plus lab tests), 2012 data adapted from Nasrabadi et al. [18].

<p>Linear regressions of total element concentrations and TSS during different sampling campaigns in the lower Haraz catchment (March 2016 plus lab tests), 2012 data adapted from Nasrabadi et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0191314#pone.0191314.ref018" target="_blank">18</a>].</p

Map of the lower Haraz Basin with major land use as well as the sampling locations; numbers indicate sampling sites.

<p>Reprinted from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0191314#pone.0191314.ref018" target="_blank">18</a>] under a CC BY license, with permission from Elsevier, original copyright 2016.</p

Comparison of particulate <i>(C</i>_<i>SUS</i>) and dissolved <i>(C</i>_<i>W</i>) concentrations of metals in all four catchments [Ammer (_A, in black), Haraz (_H, in red), Steinlach (_S, in green) and Goldersbach (_G, in purple)].

<p>Comparison of particulate <i>(C</i>_<i>SUS</i>) and dissolved <i>(C</i>_<i>W</i>) concentrations of metals in all four catchments [Ammer (_A, in black), Haraz (_H, in red), Steinlach (_S, in green) and Goldersbach (_G, in purple)].</p

Linear regressions of total element concentrations and TSS during the 2013 and 2014 sampling campaigns in the Ammer and Goldersbach catchments, Germany.

<p>Linear regressions of total element concentrations and TSS during the 2013 and 2014 sampling campaigns in the Ammer and Goldersbach catchments, Germany.</p

Map of the Ammer, Goldersbach, and Steinlach catchments in Southwest Germany with major types of land-use as well as the sampling locations.

<p>Reprinted from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0191314#pone.0191314.ref011" target="_blank">11</a>] under a CC BY license, with permission from Elsevier, original copyright 2013.</p

A comparison among particle-bound concentration of metals (mg/kg) in Ammer, Steinlach, Goldersbach and Haraz River (calculated in this study) with similar studies/ background values.

<p>A comparison among particle-bound concentration of metals (mg/kg) in Ammer, Steinlach, Goldersbach and Haraz River (calculated in this study) with similar studies/ background values.</p

Linear regressions of total element concentrations and TSS during the 2014 sampling campaigns in the Steinlach catchment, Germany.

<p>Linear regressions of total element concentrations and TSS during the 2014 sampling campaigns in the Steinlach catchment, Germany.</p

Comparison of Sedimentary PAHs in the Rivers of Ammer (Germany) and Liangtan (China): Differences between Early- and Newly-Industrialized Countries

As a proxy to trace the impact of anthropogenic activity, sedimentary polycyclic aromatic hydrocarbons (PAHs) are compared between the early industrialized and newly industrialized countries of Germany and China, respectively. Surface sediment samples in the Ammer River of Germany and the Liangtan River of China were collected to compare concentration levels, distribution patterns, and diagnostic plots of sedimentary PAHs. Total concentrations of 16 PAHs in Ammer sediments were significantly higher by a factor of ∼4.5 than those in Liangtan. This contrast agrees with an extensive literature survey of PAH levels found in Chinese versus European sediments. Distribution patterns of PAHs were similar across sites in the Ammer River, whereas they were highly varied in the Liangtan River. Pyrogenic sources dominated in both cases. Strong correlations of the sum of 16 PAHs and PAH groups with TOC contents in the Liangtan River may indicate coemission of PAHs and TOC. Poor correlations of PAHs with TOC in the Ammer River indicate that other factors exert stronger influences. Sedimentary PAHs in the Ammer River are primarily attributed to input of diffuse sources or legacy pollution, while sediments in the Liangtan River are probably affected by ongoing point source emissions. Providing further evidence of a more prolonged anthropogenic influence are the elevated black carbon fractions in sedimentary TOC in the Ammer compared to the Liangtan. This implies that the Liangtan River, like others in newly industrialized regions, still has a chance to avoid legacy pollution of sediment which is widespread in the Ammer River and other European waterways