Adsorbing Volatile Organic Compounds within Bitumen Improves Colloidal Stability and Air Quality

The mass loss of bitumen due to volatilization is one of the deterioration mechanisms through which the chemical composition of bitumen deviates from its original condition, leading to change in the physical and rheological properties of bitumen. The emissions from bitumen not only accelerate its aging but also negatively impact air quality and consequently human health. Here, we characterized the molecular compositions of the volatiles emitted from bitumen in an isothermal aging process (150 °C for 24 h). We also investigated the impact of losing these compounds on the association behavior of asphaltene stacks. Based on the GC-MS results, the main mass loss of SARA (saturates, aromatics, resins, asphaltenes) fractions is attributed to the emission of lightweight nonpolar aromatic and aliphatic compounds. The most dominant volatiles are single-ring aromatics carrying short side chains, followed by saturated and unsaturated aliphatic compounds including short-chain n-alkanes, iso-alkanes, cyclic alkenes, alkenes, and dienes. Using molecular modeling in the framework of density functional theory (DFT), we showed how the reduction in aromatics and saturates can impact the colloidal stability of asphaltenes in the matrix of bitumen. Based on the DFT results, the surface adsorption of aromatics and saturates on asphaltene planes provides the steric repulsion required to maintain the asphaltene planes at a distance sufficient to inhibit their agglomeration. In addition, aromatic molecules with the least hindrance repulsion can disturb the π-π interfacial interactions between asphaltene planes, facilitating their deagglomeration. Therefore, a direct consequence of the mass loss of bitumen due to volatilization is disturbing the asphaltene distribution within the bitumen and stimulating the aggregation of asphaltenes. We also studied the merits of incorporating biochar into the bitumen surface to reduce bitumen’s mass loss. Modification of bitumen separately with each of six biochars derived from biomass feedstocks (walnut shell, peanut shell, Douglas fir, pine bark, birch, and algae) showed the efficacy of each biochar to reduce the volatiles emitted from bitumen, thereby reducing the changes in the composition of maltene with consequent colloidal stability losses. The study outcomes highlight the importance of reducing emissions from bitumen to delay its aging and minimize negative impacts on air quality and human health.