THE ROLE OF HUMUS ACIDS IN THE PROCESSES OF DISPERSION AND ACCUMULATION OF HEAVY METALS IN NATURAL WATERS, SOILS AND CARBONACEOUS ROCKS

Galina Varshal*, Tatiana Velyukhanova, Sabzbakhor Khushvakhtova, Irina

Koshcheeva, Oksana Tyutyunnik (Vernadsky Institute of Geochemistry and

Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 117975, Russia; Phone (095) 137-75-26; Fax (095) 938-20-54; E-mail varshal@geokhi.ru); Yuriy Kholin (Chemical Faculty, Kharkov State University, Svobody Sq.4, 310077 Kharkov, Ukraine).

ABSTRACT

The mechanism and quantitative characteristics of heavy metal and noble metal reactions with two main groups of humus acids, namely fulvic acids (FA) and humic acids (HA) were studied. It was found that the stable readily soluble high-molecular fulvate complexes of Me:FA=1:1 composition of anionic type are formed being a result of metal ion complex-formation with oxygen-bearing functional groups of FA. The formation of these complexes abruptly increases heavy and noble metal migration ability in the environment. The values of conventional stability constants of metal fulvate complexes were calculated. The data on sorption capacity of HA in relation to Hg(II), Pb(II), Cd(II), Cu(II), Sr(II), Au(III), Pt(IV), Pd(II), Rh(III), Ru(IV), Os(IV) and other metals were found from sorption isotherms.The values of conventional affinity constants of these metals by HA oxygen-bearing functional groups were calculated from sorption isotherms by the method of quantitative physical chemical analysis (QPCA). It was shown that high HA sorption capacity in relation to metal ions indicates that HA play their role as a strong geochemical barrier responsible for accumulation of pollutants and ore elements in soils, suspended matter of waters, fluvial and marine sediments.

INTRODUCTION

Among the important factors affecting the migration ability of metal ions in the environment are the reactions between metal ions and humus acids - the main organic complex-forming substances of natural waters, soils, fluvial and marine sediments (Gamble, Schnitzer 1973; Varshal et al. 1993; Varshal et al. 1998). It was proved that the interaction of metal ions with both main groups of humus acids proceeds by the same complex-formation mechanism with oxygen-bearing functional groups of humus acids. However, since FA and HA exhibit different properties, the interaction of metal ions with FA and HA leads to opposite geochemical results. Reactions with FA lead to high increase of metal dispersion in the environment. On the contrary, HA behave in natural processes as complexing sorbents favouring pollutants and ore element accumulation in soils, suspended matter of waters, fluvial and marine sediments.

For reliable evaluation of metal migration ability in the environment and prediction of the ratio of metal species coexisting in the solution phase of natural waters and metal distribution between solution phase of waters and bottom sediments the data on mechanism and quantitative characteristics of heavy metal and noble metal reactions with FA and HA are necessary. Main aim of this study was to obtain the complex compound stability data in modeling experiments. These complex compounds are a result of metal ion interactions with FA and HA. These data were applied for the prediction of heavy and noble metal behavior in the environment.

METHODS

Ashless preparations of FA were isolated from high-coloured waters of Moskva-river head; ashless preparations of HA were isolated from peat (Tver’ district) and from marine sediment samples of Peru oceanic shelf. The data on composition and stability of metal fulvate complexes were obtained by solubility, kinetic, potentiometric methods and by chromatography on ion-exchange paper. Gel chromatography was applied in the studies of molecular mass distribution of these complexes. The charge sign was determined by filtration through ion-exchange cellulose.

HA properties were investigated by use of potentiometric titration, IR-spectrometry, CHNS-analysis and X-ray diffraction analysis. The sorption of heavy and noble metal ions in a form of their chloride complexes on HA was studied by use of modeling experiments. Heavy and noble metal concentration in equilibrium solution was determined by AES-ICP methods by use of ICAP-61 polychromator. The method of quantitative physical chemical analysis (Kholin 1997) was applied for the treatment of potentiometric titration data and calculation of sorption isotherm parameters.

RUSULTS AND DISCUSSION

The results of modeling experiments on the study of solubility of metal fulvate and hydroxyfulvate complexes as a function of FA concentration in Me(OH)_n-FA system showed that the solubility of fulvate complexes of easily hydrolysed elements is one to three orders of magnitude above the corresponding values of hydroxides of the same elements. It was proved that the soluble fulvate complexes emerge only in reaction of metal compounds with FA highly associated forms. That is why high molecular forms predominate among the fulvate complexes of easily hydrolysed metal ions. The complexes of Me:FA = 1:1 composition of anionic type were found to be predominant for the majority of studied heavy and noble metals. The data on conventional stability constants for the metal fulvate complexes are presented in Table 1.

TABLE 1. Conventional stability constants of the metal fulvate complexes of Me:FA=1:1 composition

Cation	pH	Technique of determination	Average b11	lg b11
Ca (II)	-	pH - potentiometry	4.4×10^3	3.64
Sr (II)	5.0	Chromatography on ion exchange paper	3.7×10^3	3.57
Ce (III)	5.0	Idem	6.0×10^4	4.78
Y (III)	5.0	Idem	8.1×10^4	4.91
Fe (II)	5.0	Ion exchange on resins	4.7×10^4	4.67
Fe (III)	5.0	Solubility method	1.2×10^7	7.08
Hg (II)	6.4	Idem	1.7×10^11	11.23
Cu (II)	7.5	Idem	5.5×10^5	5.74
Sb (III)	5.8	Idem	8.7×10^7	7.94
Au (III)	5.8	Idem	5.6×10^8	8.75
Pt (IV)	5.3	Idem	6.8×10^7	7.83
Pd (II)	5.3	Idem	2.1×10^7	7.30
Ru (IV)	5.0	Idem	7.3×10^5	5.86

It is known that high stable complex compounds are formed in the fulvate systems. By use of these data it was shown that metal fulvate complexes should be considered as predominant heavy metal species in aqueous phase of surface fresh waters. At presence of FA solubility of heavy metal compounds, their migration ability and correspondingly the dispersion of heavy metals in the environment increases by one or two orders of magnitude.

The opposite geochemical conclusions were obtained in studies of metal ion interactions with humic acids isolated from peat and marine sediments. It was shown by use of potentiometric titration, IR-spectrometry, CHNS-analysis and X-ray diffraction analysis that properties of HA from peat are near to those from marine sediments. Energetic inhomogeneity of HA oxygen-bearing functional groups was established on the basis of the potentiometric titration data of HA, refined by method of QPCA. In both cases dissociation constants of HA carboxyl groups pK_a are varying in broad range: HA of peat samples are characterised by 5-7 ( mean value of pK_a =6.1), that of marine sediments are correspondingly 6-8 ( mean value of pK_a = 7.0 ). The distribution function of HA functional groups in relation to pK_a values is in accord with Gauss distribution low. According to X-ray diffraction data the contribution of aliphatic and aromatic components in HA of peat samples is evaluated as 77.2% and 22.8% correspondingly. Those values in relation to HA of marine sediments were found as 66.0% and 34% correspondingly.

Two main mechanisms, namely the ion exchange and complex formation were found to be exhibited in the process of heavy metal ion sorption on HA. The sorption capacity of heavy metals at certain pH values is considered as a quantitative characteristics of their interaction with HA. These data we obtained in respect of common pollutants, ore elements and isotopic carriers of longliving radionuclides. The values of sorption capacity of HA in relation to heavy and noble metals were found from sorption isotherms of Hg(II), Pb(II), Cd(II), Cu(II), Sr(II), Au(III), Pt(IV), Pd(II), Rh(III), Ru(IV), Os(IV) and other elements. It is shown that humic acids reveal high sorption capacity in relation to ions as following: of Hg 340 mg/g, Pb 120-150 mg/g, Cu 18 mg/g, Cd 9mg/g, Sr 17.5 mg/g, Au 320-350 mg/g, Pd 100-110 mg/g, Rh 11-12 mg/g, Ru 16-19 mg/g and Os 23 mg/g. The sorption of Hg(II), Cd(II), Pb(II), Cu(II), Au(III), Pd(II) and Rh(III) on HA from peat and marine sediment is convincingly ascribed by Langmuir isotherm equation. The isotherm parameters were calculated by non-linear least square method. As a result the values of HA sorption center conventional affinity constants in relation to heavy and noble metal ions were obtained ( Table 2).

TABLE 2. Conventional affinity constants of HA sorption centers in relation to heavy and noble metal ions

Metal ion	lgb	Metal ion	lgb
	HA from peat		HA from marine sediments
Hg(II)	6.30	Au(III)	6.0
Cd(II)	4.82	Pd(II)	5.0
Pb(II)	4.25	Rh(III)	3.2
Cu(II)	4.01	Pb(IV)	4.0
Ru(IV)	3.55

These constants are considered as quantitative parameters of metal ion chemical bond strength in relation to HA functional group. The distribution function of HA sorption centers in respect of affinity constant logarithms for every metal ion was carried out by use of CAS algorythm. It was found that for the most studied metal ions this distribution is in accord with Gauss law. But in the sorption of Hg(II) and Pb(II) on HA the energetic inhomogeneity of HA functional groups is exhibited very evidently. In particular, the distribution function of HA sorption centers in respect of affinity constant logarithms for mercury are of bimodal type. The calculations of sorption isotherms by QPCA metods and CAS algorithm in this case indicated that sorption centers with low and high affinity to metal ions do exist at HA surface. The metal ion sorption by low affinity centers is suggested to be performed by ion exchange mechanism, whereas high affinity centers could sorb metal ions by a mechanism of complex- formation. The conclusion on the prevailing contribution of complex-formation mechanism in the metal ion sorption on HA is supported by results of chemical phase analysis of soil, suspended matter of waters and bottom sediment samples by selective elution method.

Very interesting phenomenon was found in the study of Pt(IV) sorption: HA isolated from peat and marine sediments practically do not sorb Pt(IV) ions in contrast to other carbonbearing objects such as coal, carbonaceous particles of rocks and bitumen. The adjustment of this phenomenon causes is important for the understanding of black shale noble metal ore deposit genetical peculiarities. High sorption capacity of HA in relation to noble metals prove the dominant role of sorption on HA according to complex-formation mechanism in the processes of these elements primary accumulation in the formation of noble metal deposits in carbonaceous rocks

High HA sorption capacity in relation to heavy metal ions and high stability of produced complex-compounds determine HA role as a strong geochemical barrier responsible for accumulation of pollutants in soils, suspended matter of waters, fluvial and marine sediments. In general, the pollutant migration ability depends on the composition of soil and water humus acids and mostly due to the competition of metal ion complex formation with FA and HA.

Acknowledgements

This work was supported by the Russian Foundation for Basic Research (project no. 99-05-64195).

REFERENCES

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Kholin Yu.Y. (1997), A Quantitative Physical-Chemical Analysis of Equilibria on the Surface of Complexing Silicas. Functionalized Materials. V.2. Kharkov. Oko.

Varshal G M, Velyukhanova T K , Koshcheeva I Ya. (1993), In: Humic Substances in Biosphere. ( D.S.Orlov,Editor), Moscow, Nauka, pp. 97-117

Varshal G.M., Koshcheeva I.Ya., Khushvakhtova S.D. et al.(1998), Eurasian soil science, 31: 996-972