Metal Binding and Sources of Humic Substances in Recent Sediments from the Cananéia-Iguape Estuarine-Lagoon Complex (South-Eastern Brazil)

Christian Millo,Maria De Nobili,Stefano Covelli,Camila Das Mercês Silva,Bruno Otero Sutti,Matteo Crosera,Carlo Bravo,Elisa Petranich,Marco Contin,Elisabete De Santis Braga,Elena Pavoni

doi:10.3390/app11188466

Christian Millo, Maria De Nobili + Show 9 more

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https://doi.org/10.3390/app11188466

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Abstract

The Cananéia-Iguape estuarine–lagoon complex (São Paulo state, Brazil) is a natural laboratory to study metal binding by humic substances (HS) in subtropical settings. This transitional environment is evolving into a freshwater environment due to water input from the Ribeira River, funneled through the Valo Grande Canal (Iguape). Past mining activities in the Ribeira River basin and maritime traffic are suspected to be potential sources of trace metals in the system. In this study, the trace metal contents of Free Humic Acids (FHA), Bound Humic Acids (BHA), and Fulvic Acids (FA) extracted from sedimentary organic matter were investigated. Moreover, the sources of HS were traced using their stable carbon isotope compositions and C/N ratios. The results suggested a mixed marine–terrestrial source of FHA, BHA, and FA. Copper and Cr were the most abundant trace metals bound to HS. On average, Cu showed concentrations of 176, 115, and 37.9 μg g−1 in FHA, BHA, and FA, respectively, whereas Cr showed average concentrations of 47.4, 86.3, and 43.9 μg g−1 in FHA, BHA, and FA, respectively. Marine FHA showed the highest binding capacity for trace metals, whereas terrestrial FA derived from the decay of mangrove organic matter showed the lowest binding capacity.

Highlights

Humic substances (HS) are important components of natural organic matter in water and sediments, where they represent the most refractory fraction [1,2]
Cu showed concentrations of 176, 115, and 37.9 μg g−1 in Free Humic Acids (FHA), Bound Humic Acids (BHA), and Fulvic Acids (FA), respectively, whereas Cr showed average concentrations of 47.4, 86.3, and 43.9 μg g−1 in FHA, BHA, and FA, respectively
The results presented in this study highlight the importance of humic substances (HS) as carriers and sinks for trace metals in tropical estuarine settings

Summary

Introduction

Humic substances (HS) are important components of natural organic matter in water and sediments, where they represent the most refractory fraction [1,2]. HS consist of heterogeneous mixtures of organic molecules formed during the decay of plant, animal, and microbial remains [3] Based on their solubility, HS are subdivided into three fractions: fulvic acids (FA, soluble in acidic and basic solutions), humic acids (HA, insoluble in acidic solutions), and humin (insoluble). The study of HS is relevant in coastal oceanography because HS can bind metals, either by complexation or by surface adsorption This binding ability is due to the fact that HS consist of a mixture of carboxylated and fused alicyclic structures, which can constitute ligands for metal binding [7]. The binding sites in HA and FA correspond mainly to the carboxylic and phenolic groups present, N- and S-containing groups may be relevant for metal ion binding [12]. Since HS are associated with fine sediments and particulate organic matter, they can contribute to the transfer of metals from the water column to the sediment [10,16]

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