Valorization of Refinery Sulfur in Asphalt and Its Environmental and Health Implications

With the spread of hydrodesulfurisation in petroleum refining and increasingly strict regulations on the sulphur content of transport fuel, especially marine fuel, a global surplus of elemental sulphur has gradually appeared. There is now a surplus that will be used to improve high-volume infrastructure materials, such as asphalt binders. Sulfur-modified asphalt has the following advantages: it is less dependent on petroleum-based binders; sulfur polymerisation and inverse vulcanisation can be used to improve mechanical properties; and it is economically viable because sulfur is significantly cheaper than bitumen. Partial replacement of bitumen with sulfur can reduce binder costs and utilise the excess sulfur from the refinery productively at the same time. Meanwhile, the asphalt materials will also release various hazardous substances and other harmful factors over time. Emissions occur in binder production, mixture hauling, pavement application, and over the life of the pavement, and potential exposure routes for construction workers and nearby communities are created. Sulfur in the asphalt system can change the emission characteristics by promoting the production of sulfur-containing volatile compounds, such as sulfides and thiols, which are often highly odorous and may also be more toxic. Research has been conducted on the emissions of traditional asphalt; however, the impact of adding sulphur on the chemical structure of emissions and pathways for human health exposure remain unexplored. The three main knowledge gaps are as follows: (1) The effect of sulfur chemistry on the emission characteristics of asphalt and the generation of sulfur-containing volatile organic compounds have not been systematically studied; (2) Data on prolonged human exposure and the resulting health changes, especially neurological and respiratory problems, are scarce; and (3) How emissions from sulfur-containing asphalt are altered in the presence of environmental stress, such as heat ageing, UV degradation and oxidation, is poorly understood. At the same time, the following three new opportunities have appeared: (1) Stabilising sulphur through inverse vulcanisation and sulphur-based polymers; (2) Adding bio-derived additives and functional carbons to reduce volatile emissions; (3) Combining materials science, exposure assessment and toxicological modelling to better evaluate the health risks of sulphur-containing asphalt systems. The above deficiencies need to be rectified in order to promote the all-weather construction of roads with sulphur valorisation and safeguard the environment and public health.