## 4 Discussion
Unlike FHA, the concentrations of BHA and FA were not correlated with grain size but were positively correlated with TOC and TN (\(r=0.76\)-\(0.92\)) (Table 1). Both BHA and FHA geochemical signatures indicated a mixed or dominantly terrestrial origin (Figure 10). BHA was the HS fraction with the maximum concentration of Cr, whereas FA were depleted in all minerals, relative to FHA and BHA (Figures 6-9). Like FHA, BHA also showed a positive correlation between V and Mn (Table 3), whereas FA did not show any significant correlation among metals (Table 4). In summary, the order of metal binding in HS observed in this study was as follows:  
* FHA: Cu (57.4-579.9 \(\mu\)g g\({}^{-1}\)) \(>\) As (6.0-205.3 \(\mu\)g g\({}^{-1}\)) \(>\) Zn (16.8-104.3 \(\mu\)g g\({}^{-1}\)) \(>\) Cr (19.1-81.2 \(\mu\)g g\({}^{-1}\)) \(>\) V (8.1-69.8 \(\mu\)g g\({}^{-1}\)) \(>\) Mn (7.0-50.0 \(\mu\)g g\({}^{-1}\)) \(>\) Ni (9.6-38.0 \(\mu\)g g\({}^{-1}\)) \(>\) Pb (1.3-18.7 \(\mu\)g g\({}^{-1}\)). Figure 10: Cross-plot of \(\delta^{13}\)C vs. TOC/TN values for SOM (squares), Free Humic Acids (circles), Bound Humic Acids (triangles), and FA (diamonds). Rectangles indicate the geochemical signatures of the different contributors to aquatic organic matter according to [18] (C3 plants, C4 plants, and marine and freshwater Dissolved Organic Carbon, Particulate Organic Carbon, and algae). * BHA: Cu (65.0-171.0 \(\upmu\)g g\({}^{-1}\)) \(>\) Cr (53.1-132.2 \(\upmu\)g g\({}^{-1}\)) \(>\) Mn (31.0-88.1 \(\upmu\)g g\({}^{-1}\)) \(>\) V (6.0-71.9 \(\upmu\)g g\({}^{-1}\)) \(>\) Zn (15.4-59.7 \(\upmu\)g g\({}^{-1}\)) \(>\) Ni (14.0-45.5 \(\upmu\)g g\({}^{-1}\)) \(>\) As (4.2-12.4 \(\upmu\)g g\({}^{-1}\)) \(>\) Pb (1.9-12.2- \(\upmu\)g g\({}^{-1}\)). * FA: Cr (20.5-104.0 \(\upmu\)g g\({}^{-1}\)) \(>\) Cu (10.6-80.1 \(\upmu\)g g\({}^{-1}\)) \(>\) Zn (11.2-63.7 \(\upmu\)g\({}^{-1}\)) \(>\) As (2.1-55.9 \(\upmu\)g\({}^{-1}\)) \(>\) Pb (0.7-28.3 \(\upmu\)g g\({}^{-1}\)) \(>\) Mn (3.3-18.4 \(\upmu\)g g\({}^{-1}\)) \(>\) Ni (2.3-10.6 \(\upmu\)g g\({}^{-1}\)) \(>\) V (1.2-10.2 \(\upmu\)g g\({}^{-1}\)). These data confirmed previous observations that Cu has a strong chemical affinity for HS, since Cu ions react with both the COOH and OH groups of humic ligands and tend to occupy most HS binding sites [33; 50; 51]. In this respect, results from this study added new information, indicating that FHA was the HS fraction with the highest affinity for Cu, BHA had the highest affinity for Cr, whereas As, Mn, Ni, V, Pb, and Zn showed the same affinity for HS, irrespective of the fraction considered. This study is the first dealing with the concentration of complexed metals in HS in the Cananeia-Iguape estuarine-lagoon complex. Previous studies have dealt with metal concentration in SOM or in sediments. Considering the concentrations of Cr and As in SOM, ref. [41] reported Cr concentrations four times lower than those observed in this study, whereas As concentration in SOM were 40 times lower. As regards metal concentration in sediments, ref. [37] reported Cu and Zn concentrations in the range of 15-61 and 31-101 \(\upmu\)g g\({}^{-1}\), respectively. Recent work by [43] showed that the maximum concentrations of environmentally available Pb, Cu, and Zn were 178, 18.7, and 123 \(\upmu\)g g\({}^{-1}\), respectively. These values were significantly higher than those measured in 2003 by [52] (56, 35, and 74 \(\upmu\)g g\({}^{-1}\) for Pb, Cu, and Zn, respectively). Although these results are not directly comparable with those presented in this study (because they show metal concentrations per gram of sediment rather than per gram of HS), we can use them to show that, unlike Cu and Zn, Pb is relatively abundant in the sediment but shows low concentrations in HS. This aspect deserves to be investigated further, because it may be interesting in terms of environmental mobility of this potentially toxic metal.