Dissolved humic substances supplied as potential enhancers of Cu, Cd, and Pb adsorption by two different mangrove sediments

Purpose The external supply of humic substances has been recently suggested for the remediation of metal-polluted sediments; however, little is known about how to supply them and their effects on metal mobility. The study sought to investigate the sediment—metals—humic substance interaction in mangrove forest sediments. We aimed to evaluate the sediment adsorption potential in the case of large and rapid metal loads, as recently occurred in the Doce River (Brazil). Materials and methods In each mangrove forest sampling point of the Benevente River (RB) and Vitoria bay (MO), sediments were collected randomly along the river banks at a depth of 0–10 cm. Samples were characterized in terms of pH, CEC, organic carbon, texture, specific surface area, and elemental composition. The heavy metal content was measured by mass absorption spectrophotometry. Humic substances were extracted from the sediments according to the International Humic Substances Society (IHSS) method, avoiding separation of fulvic and humic acids. Original sediments were supplemented with humic substances and six Cu, Cd, or Pb concentrations. Freundlich and Langmuir equations were employed to create adsorption isotherms. Results and discussion The two sediments are significantly different, specifically with regard to organic carbon and Fe content, texture, and specific surface area. External humic substances increased the Cu adsorption capacity in both sediments but without an important change in Cu adsorption dynamics. Humic substances slightly increased the sediment adsorption capacity of Pb in RB sediment while they decreased in MO sediment, characterized by lower specific surface area, probably due to coverage of the active adsorption sites. Cd isotherms showed that the different characteristics of sediments alone do not affect Cd adsorption, but coupled with humic substances; Cd affinity for the soil surface increased five times in RB sediments confirming sediment-metal- humic substance interactions. Conclusions Humic substances affect soil metal retention mainly by altering the ion affinity for sediment surface, leading to contrasting results. The Fe concentration could be important depending on specific surface area and humic substance percentage, due to its capacity to form spheroids linked to molecules of humic substances on the clay surface. Several works have been carried out on this research area, but due to the many variables and different metal ions, we recommend further studies.