The role of soil organic matter quality and mineralogy controlling the highest mercury concentration of the Brazilian mangroves

Authors: SILVA, GABRIEL H.M.C - Federal Rural University Of Pernambuco; ARAUJO, PAULA R. - Federal Rural University Of Pernambuco; VIEIRA, CLARISSA - Federal Rural University Of Pernambuco; ARAUJO, JANE K. - Federal Rural University Of Pernambuco; DE SOUZA JR., VALDOMIRO - Federal Rural University Of Pernambuco; DOS SANTOS, JEAN C. - Federal Rural University Of Pernambuco; Schmidt, Michael; YING, SAMANTHA - University Of California, Riverside; BIONDI, CAROLINE - Federal Rural University Of Pernambuco

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2025
Publication Date: 3/31/2025
Citation: Silva, G.H.M.C., Araujo, P.R.M., Vieira, C.B., Araujo, J.K.S., de Souza Junior, V.S., dos Santos, J.C.B., Schmidt, M.P., Ying, S.C., Biondi, C.M. 2025. The role of soil organic matter quality and mineralogy controlling the highest mercury concentration of the Brazilian mangroves. Science of the Total Environment. 975. Article 179274. https://doi.org/10.1016/j.scitotenv.2025.179274.
DOI: https://doi.org/10.1016/j.scitotenv.2025.179274

Interpretive Summary: Problem Statement Mercury (Hg) represents a threat to the health of polluted mangrove areas in coastal Northeastern Brazil. The role of soil organic matter (SOM) and minerals in the cycling and fate of Hg in these systems is not well understood currently. Accomplishment This research demonstrated the importance of SOM chemistry and mineralogy in Hg retention by mangrove soils in Northeastern Brazil. More specifically, lignin and tannin type functional groups within SOM played a role in retaining Hg in these soils. From a mineralogical perspective, reduced sulfur containing minerals like pyrite were also shown to retain Hg in these soils. These results collectively showed that these specific soil constituents may play a key role in regulating Hg fate and bioavailability in ecologically critical mangrove soils. Contribution This research has implications for broad environmental health as well as integrated aquaculture within mangrove systems worldwide. While integrated aquaculture systems within mangroves, including shrimp and fish ponds, may offer several ecological advantages, they are not without potential limitations. This includes the risk of Hg accumulation in sediments and bioaccumulation within products. Identifying important factors for controlling Hg bioavailability will, therefore, enhance understanding of how to more effectively manage Hg in mangrove systems. This will, in turn, help protect mangrove ecosystems and aquacultural products from Hg.

Technical Abstract: The ecological role of estuarine areas on organic matter storage is well known. However, the contributions of soil organic matter (SOM) quality and mineralogy as geochemical filters remain unclear. It requires further investigations in pursuit of contaminant retention understanding, such as mercury (Hg), one of the greatest threats to mangrove areas. We evaluated the highest Hg pollution case of the Brazilian mangroves to investigate the role of SOM and minerals composition in controlling this heavy metal fate. The selected area belongs to the Botafogo estuary, Northeast of Brazil, where about 30 tons of Hg residue were disposed by a chlor-alkali plant, between 1960 to 1980, near the estuary. Soils were sampled from different forests: Laguncularia racemosa, coexistence of species, and Rhizophora mangle distributed along the Botafogo estuary. Redox potential (Eh), pH, electrical conductivity (ECe), SOM content, and granulometry were performed. Total mercury (THg) and its distribution were also determined. SOM was analyzed by the evaluation of its thermal stability and molecular composition, while mineralogy was investigated due to XRD and microscopy procedures. We identified a severe contamination, in which THg concentration achieved values up to 14.3 mg kg-1, 161-fold higher than the local background. Besides the distance from the source, THg variation along the contaminated forests was controlled by the natural heterogeneity of C groups provided by different species domains. It clearly generates different scenarios for Hg retention in estuarine areas, especially where Rhizophora mangle develops, considering their characteristics to release more refractory C, such as tannin and lignin, responsible for blocking this contaminant. Simultaneously, but with a lower contribution, pyrite (Fe sulfide) acted as a retainer, also controlling Hg fate on the soil.