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XVI-th ARS SEPARATORIA – Borówno, Poland 2001 INSOLUBLE COMPOUNDS OF HEAVY METAL COMPLEXES Ona GYLIENĖ Institute of Chemistry, A. Goštauto 9, 2600 Vilnius, Lithuania, Tel.: (3702) 610042; Fax: (3702) 617018; e-mail: [email protected] Heavy metals such as Zn, Cu, Ni, Cr, Mo are essential nutrients for plants and animals. However, they become toxic at high concentrations. The toxicity of heavy metals depends mostly on a free ion, i.e. on a soluble metal compound concentration rather than on the total metal concentration. The insoluble metal compounds are less bioavailable and toxic. The industrial and agricultural emissions are the reason for enormous pollution of environment with heavy metals. Most of the heavy metals dissolved in wastewater effluents and surface runoff are complexed. Such complexing agents enhance metal solubility as well as their bioavailability. Moderately strong metal-complexing ligands which consist of a naturally occurring organic matter are responsible for the complexion of only about 5-20 % of such heavy metals as copper and nickel. The remaining part of heavy metals is complexed by synthetic chelating agents [1]. EDTA is one of the most widely used synthetic ligands in industry and household as a powerful complexing agent for heavy metal ions. EDTA is not degraded in sewage treatment plants and is present in the effluents with concentrations up to 18 µmol/l [2]. Beside to EDTA, the industries use other strong metal complexing agents viz. ethylendiamine, glycine (amino acetic acid), tartrate, citrate, malonate, oxalate. The oxidative destruction of ligands is used for metal removal from complexing agents containing solutions in main. Our investigations give the opportunity to recovery both metals and ligands by means of precipitation without ligand destroying and to reuse them in industry. Precipitation of citrate and tartrate The possibility to precipitate metals in the form of insoluble metal hydroxides in complexing agents containing solutions depends on the complex stability constant (β n) and hydroxide solubility product (S) which can be described by equation: MlL n = β n ⋅ S ⋅ Ln [ OH − ]m (1) XVI-th ARS SEPARATORIA – Borówno, Poland 2001 where MLn - is the metal residual concentration after hydroxide precipitation, Ln - ligand concentration, n - the number of ligand molecules in complex, m - the valenc of metal ions. Investigations were carried out with ligands EDTA, glycine (aminoacetic acid), citrate and tartrate which are used in the metal finishing industry. The excess of metals enables to precipitate citric and tartaric acids in alkaline solutions, where they form the particularly strong metal complexes. This property of carboxylic acids distinguishes then from other complexing agents such as EDTA and glycine. The results of precipitation of these complexing agents with metal ions are represented in Fig. 1 for Ni(II) complexes. 70 Residual Ni(II), mmol/l 60 1 50 2 3 4 40 30 20 10 0 0 50 100 150 200 Initial Ni(II), mmol/l Fig. 1: The dependence of residual Ni(II) concentration on total Ni(II) concentration at pH=12 in presence 20 mmol/l ligand: 1 - citrate, 2 - tartrate, 3 - EDTA, 4 - glycine. When Ni(II) concentration is twice higher than the concentration of Ni(II)-citrate complex in solution, the residual Ni(II) and citrate concentrations are very small. It means that 1 mole Ni(II)-citrate complex precipitates together with 1 mole of nickel hydroxide. The metal-citrate complexes coprecipitate with the metal hydroxides at a rather small excess of metal ions, i.e., only at 1-3 times higher concentration than the content of soluble metal complexes. In addition, complexes with citrate are more stable than those with EDTA or glycine. It can be assumed that chemical compounds of metal hydroxide and metal citrate are formed. However, the quantitative composition of these precipitates is not constant. The ratio of the metal, citrate and hydroxide in precipitates changes within a rather wide range. Tartrate and citrate easily can be precipitated with excess of Cu(II) ions in acidic solutions [3]. However, Cu(II) are not able to form insoluble compounds with citrate and tartrate in the case of participation of other metal ions, such as Ni(II). XVI-th ARS SEPARATORIA – Borówno, Poland 2001 Removal of glycine and EDTA. Experiments showed that glycine forms rather insoluble compound with copper ions of two forms of precipitates: cis-Cu(gly)2·H2O and transCu(gly)2·2H2O.. When NaOH or KOH are used as pH adjusters the formation of precipitates takes place during several hours or even several days. It depends on temperature, concentration of components, pH. EDTA with excess of Cu(II) forms insoluble Cu2EDTA⋅4H2O in mildly acidic solutions i.e at pH 2.5-4.5. The completeness of precipitation depends on initial concentrations of components. As a rule, the increase in initial concentration of Cu(II) decreases the solubility of the Cu2EDTA⋅4H2O. REFERENCES 1. Sedlak D.L, Phinney J.T.and Wudsworth W.W., 1997, Strongly Complexed Cu and Ni in Wastewater Effluents and Surface Runoff. Environmental Science and Technology, vol. 31, No 10, p. 3010-3016. 2. Nowack B., Xue H. and Sigg L., 1997, Influence of Natural and Anthropogenic Ligands on Metal Transport during Infiltration on River Water to Groundwater. Environmental Science. and Technology, vol. 31, No 3, p. 866-872. 3. Gylienė O., Šalkauskas M., Juškėnas R.. The Use of Organic Acids as Precipitants for Metal Recovery from Galvanic Solutions. J. Chemical Technology and Biotechnology, 1997, vol. 70, No 1, p. 111-115.