New approach for the determination of stability constants of metal ions-soil organic matter

Abstract

The binding of metal ions to organic materials in soil and water is an area of considerable interest. A better understanding of the equilibria that determine the behaviour of the chemical elements in the superficial environment is clearly essential for effective geochemical research. Naturally occurring organic compounds are an integral part of soils and aquatic systems. These substances play a major role in the retention and deactivation of anthropogenic organic and mineral chemicals and are involved in the geotransport of metals. Humic substances make up the bulk of organic matter and they probably control the abundance and distribution of trace metals in the environment by complexation or chelation processes through functional groups including carboxylic, phenolic, alcoholic and enolic OH, carbonyl and NH 2 sites at which binding may occur [1]. The determination of stability constants of metal-humic and fulvic acid complexes permits the prediction of the chemical speciation of many metals and the elucidation of their dispersion cycles. The present work deals with the interactions of lead(II), cadmium(II) and copper(II) ions with water soluble soil organic matter. Cu(II) like Zn(II), Mn(II)…, is a metal which is essential to plants while Cd(II) and Pb(II) are without known function in plants but are of considerable interest in view of their toxicity and accumulation in the environment. A variety of methods have been applied for determining stability constants of metalhumate complexes, as reviewed by Stevenson [1]. Considerable progress has been made, but agreement has not yet been reached as to how the data can best be analyzed and interpreted. One of the most popular methods is base titration, which uses the competition of metal ions and protons for complexing sites on the ligand. It is such a modified approach that has been developed in this work. Humic substance samples (in this context, this term will be used to designate a system of organic water soluble compounds including polymers) were recovered by percolating rain-water through a A2 horizon of a podzolic soil and purified by filtration, ultracentrifugation, and treatment by passage through ion exchange resins [2]. Stock solutions of metallic perchlorates were prepared from commercial products and standardized against EDTA. Samples containing constant quantities of humic substances and perchloric acid were titrated by sodium hydroxide in the absence and in the presence of metal ion. Protometric measurements were performed at 25 °C in 0.1 NaClO 4 medium under nitrogen atmosphere. An automatic titrator ‘Tacussel TT Processor’ fitted with extension glass and calomel electrodes was used for recording direct and differential titration curves; pH readings were combined with determinations of free metal ion concentrations by means of ion selective electrodes (ISE). Titration and Protonation Constants of Humic Samples For humic substances which contain COOH and OH groups of various acid strengths we shall define the ligand concentration in terms of titratable acidity. The total acidity content was resolved into a strong, weak and weak acidity by applying linear titration plots derived by McCallum and Midgley [3] in combination with Gran's plots as previously used by Takamatsu and Yoshida [4]. In order to obtain pK a values for each of the two weak acidities, the so-called extended Henderson-Hasselbalch equation was used pH = pK a + n log α/(1 − α) During the alkali titration, the dissociation of humic samples was characterized by a coefficient that may be determined from titration curves of perchloric acid alone and in the presence of humic substances, according to a procedure derived from Irving-Rossotti's method [5]. Figure 1 shows typical Henderson-Hasselbalch plots leading for instance to results such as: • strong acidity: 0.32 meq 1 −1 • weak acidity groups (COOH): 0.33 meq 1 −1; pK a = 4.54; n = 2.15. • very weak acidity groups (OH): 0.34 meq 1 −1; pK a = 9.33; n = 2.59. ▪ Stability Constants of MetalHumate Complexes (HA) Three different methods have been investigated in the acidic range: • an approach closely related to that of Gregor . [6] but the experimental values of [HA] and [A −] are determined using an original derivation of equations resulting from the application of Irving and Rossotti's concepts [5]. • direct estimation of the free metal ion concentrations from ISE measurements. • application of Marinsky's method [7] accounting for complications arising from the electric field at the surface of the polyelectrolyte which determines the effective concentration [A −]. Reasonable agreement is observed. The existence of mono-complexes MA is expected for the three metal ions, while bis-complexes MA 2 are detected with Pb(II) and Cu(II) ions. Stability of the complexes followed the order Cu > Pb 》Cd. An increase in stability constant was observed with increase in pH and also with decrease in total metal ion concentration (at constant pH), as exemplified by Fig. 2.