Metal speciation in natural waters and metal complexing with humic matter

Abstract

The stability constants of metal and humic matter complexes extracted from different soils were calcu� lated. Speciation of metals in natural waters was deter� mined taking into account their chemical composition and content and properties of organic matter. We established metal speciation in water objects with account for competitive reactions resulting in forma� tion of hydroxide, hydrocarbonate, sulfate, and chlo� ride metal complexes and obtained a competitive series of metal activity in natural waters of the zones considered. An important specific feature of metals as contam� ination elements is the fact that when they occur in the environment, their potential toxicity and bioavailabil� ity depend significantly on their speciation. In recent years, lakes have been continuously enriched in haz� ardous elements such as Pb, Cd, Al, and Cr on a global (regional) basis [1]. Metals occur in natural waters as free ions, simple complexes with inorganic and organic ligands, and mineral and organic particles of molecules and ions sorbed on the surface. The occur� rence of soluble metal forms in natural waters depends on the presence of organic and inorganic anions. The most important organic ligands are humic matter (HM) washed out from soils; inorganic complexes formed include hydroxide groups, hydrocarbonates, sulfates, and chlorides. HM of natural waters com� posed of fulvic (FA) and humic (FA) acids contributes most significantly to metal inactivation. The concen� trations of these acids in surface waters attain 1– 100 mg/l [2, 3]. The concentrations of water biota exometabolites are much lower. Due to specific water formation in the whole humid territory of Russia, the colored waters are quite widespread. They are characterized by high concen� trations of humic matter, which is able to inactivate metals and to decrease their toxicity in natural waters. A characteristic feature of the water chemical compo� sition in these regions is elevated Fe, Al, and Mn con� centrations. Russian water quality standards concern� ing metals do not take into account regional and local specific features of the water chemical composition, which predefine speciation of metals and their ecotoxic properties. However, direct determinations are rather difficult. Our current task is to develop methods that would make it possible to forecast ele� ment speciation predefining toxic properties of water by the chemical composition of the water. The objective of this work was to investigate metal and humic matter complexing and to determine metal occurrence forms on the basis of experimentally obtained constants of metal and humic matter com� plexes extracted from typical soils of different natural climatic zones.