Do drinking water plants retain microplastics? An exploratory study using Raman micro-spectroscopy.

The retainment of microplastics (MPs) down to 1 µm by a Danish drinking water plant fed with groundwater was quantified using Raman micro-spectroscopy (µRaman). The inlet and outlet were sampled in parallel triplicates over five consecutive days of normal activity. For each triplicate, approximately 1 m3 of drinking water was filtered with a custom-made device employing 1 µm steel filters. The MP abundance was expressed as MP counts per liter (N/L) and MP mass per liter (pg/L), the latter being estimated from the morphological parameters provided by the µRaman analysis. Hence the treated water held on average 1.4 MP counts/L, corresponding to 4 pg/L. The raw water entering the sand filters held a higher MP abundance, and the overall efficiency of the treatment was 43.2% in terms of MP counts and 75.1% in terms of MP mass. The reason for the difference between count-based and mass-based efficiencies was that 1-5 µm MP were retained to a significantly lower degree than larger ones. Above 10 µm, 79.6% of all MPs were retained by the filters, while the efficiency was only 41.1% below 5 µm. The MP retainment was highly variable between measurements, showing an overall decreasing tendency over the investigated period. Therefore, the plastic elements of the plant (valves, sealing components, etc.) likely released small-sized MPs due to the mechanical stress experienced during the treatment. The sub-micron fraction (0.45-1 µm) of the samples was also qualitatively explored, showing that nanoplastics (NPs) were present and that at least part hereof could be detected by µRaman.

Tuning Porosity of Reduced Graphene Oxide Membrane Materials by Alkali Activation.

The alkali-activation method allows for obtaining highly porous carbon materials. In this study, we explored the effect of activation temperature and potassium hydroxide concentration on the pore structure of reduced graphene oxide (rGO), as potential membrane material. Above 700 °C, potassium species react with the carbon plane of rGO to form nanopores. This activation process is deeply studied through DSC measurements and isothermal gravimetric analysis. The porosity of the formed materials consists of both micro- and mesopores, with most of the pores having a size smaller than 10 nm. The specific surface area and pore volume increase with increasing the potassium hydroxide/graphene oxide weight ratio (KOH/GO) up to 7 (897 m2∙g-1 and 0.97 cm3∙g-1, respectively). However, for a synthesis mixture with KOH/GO of 10, the specific surface area of the produced material drops to 255 m2∙g-1. The film-forming ability of the porous reduced graphene oxide (PRGO) was tested by drop-casting on porous silicon carbide substrates. In this case, continuous PRGO films were obtained only from dispersions with 5 g∙L-1 GO loading and KOH/GO ≤3. Such films can still have high specific surface area and pore volume (up to 528 m2∙g-1 and 0.53 cm3∙g-1) and main pore volume <10 nm. Hence, they can potentially be applied as membrane devices, but their scalability and their adhesion on the substrate under realistic filtration conditions still remain challenges.