Download insoluble compounds of heavy metal complexes

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.