Clay minerals can adsorb heavy metal cations via both permanent negatively charged and pH-dependent surface functional groups such as aluminol and silanol. Organic contents of clays (mostly derived from humus substance) enhance heavy metal cation adsorption. Fourty percent of organic matter in sediments consists of humic acids which may mobilize metals in sediments. Because of having higher solubility than humic acids, fulvic acids induce metal mobilization to deeper zones. Despite its lower solubility, humic acid gives stable complexes with metal cations. Thus, it provides controlled mobilization of metal ions and keeps them in shallow zones. In order to enable a better understanding of humic acid-metal interaction in environment, we determined the stability constants of Cu(II)-humate complexes, and modeled Cu(II), Cd(II), Pb(II) adsorption on clay minerals in the presence of humic acid. The acidity constants of -COOH and Ar-OH groups and the stability constants of Cu(II)-humate complexes were calculated by surface complexation modeling. As a result, the acidity constants of humic acid samples were restricted to three different pH ranges of 4 &lt; pH &lt; 7, 8 &lt; pH &lt; 11 and 11 &lt; pH &lt; 13. These pH ranges may correspond to carboxylic groups, phenolic-OH groups and polyol side chains of phenolic polymers, respectively. Stability constants (Log K1 and Log K2) of Cu(II)-humate complexes were found to be 7-40 and 16-27, respectively. The interactions among clays, metal cations and humic acid were investigated as regards the species distribution of binary surface-metal, surface-ligand (humate) and ternary surface-metal-ligand complexes. It was observed that metal adsorption is decreased in the presence of humic acid, and clay minerals behave as a chelating ion exchanger for metal cations. Thus, the mobilization of unhydrolyzed metal ion and hydrolyzed metal species in marine, surface water, sediment and groundwater was modeled with the aid of metal ion and humic acid sorption on clay minerals.

Formation of metal-humate complexes in sediments and environmental impact of heavy metal adsorption in the presence of humic acids

253rd National Meeting (2017)

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