Colloidal Organic Pollutants in Road Runoff: Sources, Emissions and Effective Treatment Technologies

Doktorsavhandling, 2020

Thousands of organic substances circulate in our society and are diffusely emitted through traffic, combustion and leaching from constructions and building materials into the urban environment. The research in this PhD thesis focuses on road runoff as the highest concentrations of pollutants are frequently found in runoff from areas with high traffic intensity. Many organic pollutants (OPs) emitted from vehicles and traffic-related activities exhibit environmental persistence and a tendency to bioaccumulate and may have detrimental long-term effects on aquatic life. Road runoff contains a cocktail of both particulate and non-particulate OPs. Hydrophobic OPs such as higher petroleum hydrocarbons, phthalates, and polycyclic organic hydrocarbons (PAHs) are not exclusively bound to particles, but also present in runoff in colloidal and truly dissolved forms. These hydrophobic compounds can also form nano- and microsized emulsions that may carry pollutants in stormwater. Hence, it is of great importance to develop treatment technologies that can remove non-particulate OPs from contaminated stormwater. The overall aim of this PhD research was to evaluate the best options to manage the colloidal fraction of OPs in road runoff, including road dust, water and sediments. The research also included to study the sources, emissions and the transport processes of OPs in road runoff.

In Paper I approximately 1100 compounds were chosen for further studies after comprehensive screening and assessment. The results of the developed iterative selection process used for identifying and selecting priority pollutants in urban road environments showed the following priority order: PAHs > aliphates C20–C40 > alkylphenols > phthalates > aldehydes > phenolic antioxidants > bisphenol A > oxygenated-PAHs > naphtha C5–C12 > amides > amines. Among these, PAH-16 were chosen for a substance flow analysis (SFA), which was performed for a highway case study area. The SFA showed that the main sources of PAHs emitted in the area were vehicle exhaust gases, followed by tyre wear, motor lubricant oils, road surface wear, and brake linings. Only 2–6% of the total 5.8–29 kg/ha annually emitted PAHs end up in the stormwater sewer system.

Particle size distribution and zeta potential measurements was performed on simulated road runoff, using laboratory prepared mixtures of ultrapure water and specific OPs with and without addition of humic acid and iron colloids (Paper II). The aim was to provide an understanding of the transport routes of OPs in the environment, and to determine whether OPs are transported with nano- and microparticles in the form of emulsions The following simulation mixtures were identified as potential emulsions: diesel (aliphates); alkylphenols (APs) and their ethoxylates (APEOs); diesel with APs and APEOs; phthalates, and a mixture of all OPs (including PAH-16) with and without colloids. Most of the particles in the samples were found in the nano-range of 30–660 nm, and a smaller portion of particles < 28% were found to be micro-sized.

In Paper III the potential of street sweeping to reduce the amounts of OPs and nano/microparticles reaching stormwater was investigated in a case study that included sampling road dust and washwater from a street sweeping machine, road dust before and after sweeping, and stormwater. The compound groups generally found in the highest concentrations in all matrices were aliphates C5–C35 > phthalates > aromates C8–C35 > PAH-16. The concentrations of aliphates C16–C35 and PAHs in washwater were extremely high at ≤ 53,000 µg/L and ≤ 120 µg/L, respectively, and the highest concentrations were found after a 3-month winter break in sweeping. The washwater contains a wide range of small particles, including nanoparticles in sizes from just below 1 nm up to 300 nm, with nanoparticles in the size range 25–300 nm present in the highest concentrations.

The design of an experimental car wash and subsequent laboratory analysis with a focus on OPs and particle size distributions was performed in Paper IV. The car wash experiment simulated high and a low intensity rain and carwash using conventional and eco-friendly detergents. Per driven km phthatales were emitted by 0.10­–0.40 µg, aliphates by 0.020–0.60 µg and PAH-16 by 2.5×10-4–2.5×10-3 µg, and were the OPs emitted in largest amounts from all cars. . The dominant phthalate was the high molecular weight di-iso-nonylphthalate (DINP) quantified up to 640 µg/L.  Nanoparticles in the size range 10–450 nm were also released in large amounts from the cars and the waters contained up to 3.3 ×106 of particles per liter.

In Paper V a pilot plant using column bed-filters of sand as a pre-filter, in combination with sorption filters of granulated activated carbon, Sphagnum peat or Pinus sylvestris bark, was used to investigate the removal of non-particulate OPs from urban stormwater. Samples from the filter effluents were collected weekly; during or after rain events; and during stress tests when incoming water was spiked with contaminated sediment and petrol or diesel. All sorption filters showed efficient reduction of aliphatic diesel hydrocarbons C16–C35, benzene, and the PAHs phenanthrene, fluoranthene, and pyrene during most of the operation time, which was 18 months. During the stress test events, all sorption filters showed 100% reduction of PAH-16, petrol and diesel aliphates C5–C35.

The following recommendations are suggested to prevent further spread of OPs and nanoparticles to the urban environment: (1) Frequent street sweeping of the most polluted streets in urban areas should be introduced as soon as possible; (2) Frequent washing of vehicles in urban areas should be mandatory, especially during winter when emissions of exhaust gases and vehicle wear are greatest; (3) The final step for treating highly polluted stormwater must contain sorption filters to effectively remove OPs and especially OPs in colloidal forms. Future research should perform multi-criteria decision analyses to compare the treatment options studied in this research with other options to find the most sustainable solutions to remove OPs and nanoparticles from stormwater.