Impact of particle size and surface charge density on redispersibility of spray-dried powders

Developing an equivalent toxicity area approach to comparing toxicity of urban road deposited sediments

Urban road stormwater has been seen as an alternative water resource for mitigating water scarcity. Road deposited sediments (RDS) is the primary source of road stormwater pollution since many toxic pollutants are attached to RDS and eventually enter stormwater runoff when rainfall events occur. This could threaten road stormwater reuse safety. This study developed an innovative equivalent toxicity area (ETA) approach to comparing overall toxicity of RDS among different road sites. In the ETA approach, the road surface area was used to represent how toxic the RDS generated from the area was. This ETA approach overcomes the difficulty to compare RDS toxicity among road sites due to variability of sampling volumes, which constrains the effective application of conventional toxicity comparison method of using pollutant amounts (loads/concentrations). Twelve road sites in Shenzhen, China were selected as a case study, where the ETA approach developed was used to compare their RDS toxicity using Chinese Hamster Ovary cells. Additionally, the key factors influencing RDS toxicity were identified by using a forward stepwise regression method. These outcomes provide a useful approach to comparing overall toxicity where various pollutant types with different amounts (loads/concentrations) and toxicity levels are present. Additionally, the results can also contribute to ensuring safety of RDS polluted stormwater reuse.

Comparative toxicity of organic mixture attached to road deposited sediments: Inadequacy of conventionally using individual pollutants to assess comprehensive hazard effects

Organic pollutants attached on road deposited sediments (RDS) during dry days can be washed-off into stormwater runoff during rainfall events, undermining stormwater reuse safety. Previous research studies commonly utilized individual pollutant groups and their quantity to evaluate the hazard effect of pollutants attached to RDS in terms of stormwater reuse. Since many types of organic pollutants are present together rather than individually, conventional approaches might not permit a comprehensive understanding of how appropriately the RDS polluted stormwater can be reused. This study undertook a toxicity test of organic pollutants attached to RDS using Chinese hamster ovary cells (CHO), testing a hypothesis that solely focusing on individual pollutant groups are not adequate to represent hazard effects of resulting stormwater and hence their adequacy for reuse. It is noted that comparative toxicity of RDS is not strongly related to total solids (commonly seen as the key carrier of pollutants) and chemical oxygen demand (COD, representing organic matters). Additionally, the comparison results of spatial distributions of toxicity (in this study) and individual pollutants in previous studies did not show a similar trend. These results imply that toxicity should be also used to indicate how stormwater can be safely reused while solely investigating individual pollutants can not adequately show a comprehensive hazard effect in terms of ensuring stormwater reuse safety. Based on study outcomes, a new assessment approach considering both pollutant and toxicity were proposed. This will assist on effective stormwater reuse and ensuring their reuse safety.

Core-Shell Morphology of Redispersible Powders in Polymer-Cement Waterproof Mortars

Redispersible powders based on soft core-hard shell polymer particles can be used as additives in polymer-cement mortars. The role of this morphology on the spray-drying production of these powders and on the crack-bridging properties of the corresponding cement-based membranes is investigated. Different polymer latexes at high solid content with varied core-shell ratio, shell thickness and chemical composition (hardness) were prepared from styrene and 2-ethylhexyl acrylate monomers via semi-batch emulsion polymerization. The latexes were characterized in terms of size, composition, and glass transition temperature (T g ), and spray-dried to obtain redispersible polymer powders (RPPs) using poly (vinyl alcohol) and limestone powder as anti-caking agents. The polymer powders were mixed with a mortar mixture and redispersed in water to produce cement-based membranes, which were tested for crack-bridging properties at different temperatures. The results showed that it was not possible to spray-dry a dispersion of homogeneous polymer particles with T g of -25 ∘ C, unless these particles are protected by much harder (high T g ) shell. In particular, it was observed that a thicker shell improved the spray-ability, but lowered the crack-bridging properties of the produced membrane. A trade-off between these two was revealed to be the key for the optimal design of the polymer nanoparticles, as proven by the systematic study of the core-shell morphology reported in this work. The best compromise was shown to consist of particles larger than 300 nm, shell thickness of about 5 nm, and core-shell ratio of 97%, with styrene content in the shell not larger than 80% to avoid excessive hydrophobicity.