The pyrite ores are strategic industrial resources which generally serve as raw material for producing sulfuric acid. However, during the mining and industrial processing activities, associated toxic elements of cadmium (Cd) and lead (Pb) could be released into the surroundings, posing a significant threat to local environment and human health. In this study, the Institute for Reference Materials and Measurement (IRMM) sequential extraction scheme was used to investigate the geochemical fractionation of Cd and Pb in pyrite ores from a mining area located in Yunfu, western Guangdong, China. The results showed that most of Cd and Pb (>90%) were predominantly found in the geochemically mobile fractions, indicating that Cd and Pb were readily bioaccessible thus easily assimilated and accumulated by organisms. FESEM-EDS results showed that the studied pyrite ores were mainly composed of O, S, and Fe, while the XRD characterizations suggested that FeS 2 and SiO 2 were the major minerals. The high-resolution transmission electron microscope and element mapping characterization further confirmed that FeS 2 was the main mineral of pyrite ores which contained relatively enriched toxic heavy metals (e.g., Pb and Cd). The findings highlight that an extremely large amount of geochemically mobile heavy metals can be released into the environmental media during the mining and utilization processes of pyrite ores based on IRMM sequential extraction protocol. Therefore, proper countermeasures against environmental risks of utilizing pyrite ores should be taken to mitigate the impacts on local ecosystem and human health.

Tidal marshes are important recycling areas for biogenic silica (BSi) and macro- and microelements at the land–sea interface and are key locations for examining the decomposition process of wetland plant litter. In this study, in situ decomposition experiments were conducted with Phragmites australis , Cyperus malaccensis , and Spartina alterniflora in the Min River estuary wetland. Litterbags of 0.2-mm mesh size were used to evaluate the litter decomposition process and residual values of BSi and macro- and microelements, including C, N, Cr, Cu, Cd, Zn, Pb, Al, Mn, and Fe over 520 days. The litter decomposition rate significantly differed among species in the following order: C. malaccensis (0.005 d –1 ) > S. alterniflora (0.004 d –1 ) > P. australis (0.003 d –1 ) with BSi release rates of 98.64%, 96.75%, and 97.23%, respectively. Although there were net releases of BSi, C, and N from the three litter species, continuous decrease in the BSi/(C, N) ratio indicated that BSi was removed from the litter much faster than C and N. The accumulation index results showed that Cu, Pb, Al, and Fe were net-accumulated in the litter, whereas Cd, Mn, Cr, and Zn were predominantly released during litter decay. Pearson’s correlation analysis results showed that the amounts of N, Cu, Cd, Pb, Al, and Fe in the litter restrained BSi release with a significant negative correlation. These findings in the Min River estuary have important implications for geochemical cycles within wetland systems and the transport processes of potential nutrients out of the system.