Solubility and dissolution kinetics of particle-bound metals in a surrogate lung fluid

Environmental context Metals in airborne particles can interact with lung fluids once particles are inhaled. Water solubility of particle-bound metals is often used to define their bioavailability. We show that particle-bound metals exhibit a complex dissolution kinetics in a surrogate lung fluid, their solubility and dissolution kinetics are driven by the fluid composition, and only to a minor extent by atmospheric aqueous phase processing of aerosols. Rationale Metals in airborne particles can interact with lung fluids once particles are inhaled and cause oxidative stress. Their oxidative potential is dependent on their solubility and dissolution kinetics in the lung fluids. Methodology In this study, we collected PM2.5 samples from the city of Padova (Italy), in the northern Italian Po Valley. We investigated the solubility and dissolution kinetics of particle-bound metals in a surrogate epithelial lining fluid (SELF) and compared the results to those obtained for a fog water. Results We found that most elements present a complex dissolution kinetics characterised by three phases: (1) immediate dissolution, (2) first-order dissolution, and (3) first-order precipitation. We observed that the speciation of metal ions in the SELF influences the dissolution kinetics of each element. We also found that atmospheric aqueous phase processing in fog can influence the dissolution kinetics of the elements in SELF. Discussion The speciation study showed that the SELF composition is the main driver of metal solubility and dissolution kinetics with only a minor influence of atmospheric fog processing. Therefore, metal bioavailability is mainly determined by the biological fluid composition.