Labile heavy metals in sediments controlled by terrestrial organic matter based on isotopic and molecular signatures in an anthropogenic estuary

Labile heavy metal (LHM) is greatly important as the most direct proxy of metal mobility, which is crucially controlled by organic matter (OM) in the estuaries. This work was performed in the Pearl River Estuary experienced eutrophication and heavy metal (HM) pollution, aiming to study the link between dissolved OM and LHMs in sediments, thereby understanding the metal risk mechanisms. LHMs exhibited pronounced accumulation in the west coast and river outlet areas. Co-occurrence of elevated LHMs and organic carbon versus nitrogen ratio, decreased stable carbon isotopic signature, suggested terrestrial OM largely facilitated the transport and accumulation of LHMs. Terrestrially derived sediment dissolved OM (SDOM) components with high humification may supply a stronger binding to most LHMs due to the powerful interactions. SDOM molecular analyses showed terrestrial, highly aromatic and stable substances enhanced HM mobility. Whereas, in situ bacteria or phytoplankton transformed DOM products containing nitrogen and sulfur made a greater contribution to labile Pb and Cr compared to others. These results implied SDOM with high aromaticity and low bio-reactivity promoted HM mobility and risk which was also considerably controlled by terrestrial SDOM. These observations offered new insights into the mechanisms involving the presence and risk of HMs in human-impacted estuaries.