REMOVAL OF HEAVY METALS FROM SOIL COMPONENTS AND SOIL BY NATURAL CHELATING AGENTS
Part I: Displacement from Clay Minerals and Peat by L-Cysteine and L-Penicillamine K. FISCHER University of Trier, FB VI – Geography/Geosciences, Department of Analytical and Ecological Chemistry, Trier, Germany (∗ e-mail:
[email protected], fax: 651 201 3617)
(Received 19 October 1999; accepted 28 September 2001)
Abstract. The β-thiol group containing amino acids L -cysteine and L -penicillamine were tested for their ability to release heavy metals (Cd, Cr, Cu, Hg, Ni, Pb, Zn) from bondings onto typical soil components (peat, bentonite, illite) at neutral pH formed by batch adsorption procedure. Following equilibration the metal loaded sorbents were extracted by aqueous amino acid solutions under various physico-chemical conditions. The extractability of metals from peat (metal contents between 62.0 and 6170 mg kg−1 ) increased in the presence of L -penicillamine following the order Cr < Hg < Pb < Ni ≈ Zn ≈ Cd, ranging from 9% (Cr) to 97% (Cd). The extractability by L -cysteine was slightly lower, following the sequence Cr < Pb < Cd < Ni < Zn. In both cases the sequences did not correlate with the order of the stability constants of the corresponding amino acid complexes. The recovery of metals from bentonite was higher generally. The extent of removal increased with increasing molar excess of the amino acids and, to a lower extent, with increasing extraction volume. Presumably initiated by oxidation of amino acids or by the formation of sparingly soluble polymeric metal complexes, the concentrations of dissolved metals dropped after an agitation period of 2 to 24 hr in most of the extracts. Aspects of this process were discussed in detail. Keywords: amino acids, bentonite, cysteine, heavy metals, metal extraction, peat, penicillamine
1. Introduction Organic chelating agents naturally occurring in soils and sediments influence the environmental distribution, cycling and effects of heavy metals in many ways. Often they increase metal solubility and enhance metal desorption from surfaces of minerals and organic substrates, leading to elevated metal mobilities and translocation within specific environmental compartments (McColl and Pohlman, 1987; Pohlman and McColl, 1988; Kuiters and Muilder, 1990). Depending on structure, functional groups, acid/base chemistry and charge of the ligands, modes of interaction are chelation including surface coordination, ion exchange, ligand exchange, and reductive dissolution. Besides aliphatic and aromatic bi- and polyfunctional carboxylic acids, amino acids constitute an important fraction of chelating compounds present in soil solutions (Stevenson and Ardakani, 1973; Stevenson, 1986; Monreal and McGill, 1985). Water, Air, and Soil Pollution 137: 267–286, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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Many biological, pedochemical and anthropogenic sources and processes, such as plant exudates (Marschner, 1986; Cakmak and Marschner, 1988), metabolic products of fungi and microorganisms (Gerloff et al., 1950; Watanabe, 1951), degradation of leaf litter and soil organic matter (Blaser et al., 1984; Nykvist, 1963), organic fertilization and discharge of silage effluents and sewages (Leidmann, 1995; Fischer, 1997) may contribute to the amino acid content of soils. Due to its β-thiol group, L-cysteine is the essential amino acid with the highest affinity to soft metal ions (Sillen and Martell, 1971; Berthon, 1995). Furthermore, L-cysteine is the principal metal binding unit of metallothioneins (Kägi and Nordberg, 1979). D -penicillamine [D -(–)-β,β-dimethyl cysteine] is a hydrolysis product of penicillin. It is used in the human therapy of metal intoxication and in the treatment of Wilson’s disease (Cu accumulation). The formation constants of most of the heavy metal complexes of L-penicillamine (the enantiomeric form used in this study) are greater than those of L-cysteine and L-cysteine is less stable against oxidation than L-penicillamine (Gergely and Sóvágó, 1979). The interaction of both thiol group containing amino acids with metal ions is investigated under biological and biochemical aspects predominantly. For instance, D -penicillamine is regarded as a model compound for studying the bonding sites of various metalloproteins. It seems interesting to consider geochemical viewpoints as well. These amino acids can be chosen as model compounds for the determination of possible effects of naturally occurring chelating agents on the speciation and phase distribution of heavy metals in soils. Since clay minerals and organic matter are important phases for the metal fixation in soils together with various metal (hydr)oxides, they have to be included in phase transfer studies for organically complexed metal ions. Furthermore, geochemical processes may offer innovative perspectives for geotechnical purposes. If a group of natural chelating agents is effective at mobilization of heavy metals in various environmental compartments, some of the inherent compounds may bear the potential to be utilized in soil remediation processes. It is important to mention, that the application of chelates for soil decontamination is not restricted to washing, flushing or extraction techniques. New developments utilize metal solubilization by chelates as a process element within phytoremediation (Blaylock et al., 1997; Huang et al., 1997), extraction by surfactants (Nivas et al., 1996; Doong et al., 1998) and electrokinetic remediation (Wong et al., 1997; Yeung et al., 1996). An interesting approach to bioremediate metal polluted soils is to adapt and to manipulate microorganisms so that the qualitative and quantitative exudation of chelating compounds is optimised (Wasay et al., 1998). Related to this topic, data on metal solubilization by cysteine are scarce and data for penicillamine are almost completely missing. Frimmel (1976) investigated the remobilization of mercury from representative sediment components by cysteine. Harris and Silberman (1983) considered cysteine as an important leaching agent but they did not consequently include the amino acid in their investigation on
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leaching of coal fly ash by chelating agents. Recently the stability of smectitecysteine-Cu2+ -systems was studied but possible phase transfer reactions were not mentioned (Brigatti et al., 1998). First attempts to determine the liberation of Cd, Cu and Ni from illite by L-penicillamine were made by Wagner et al. (1995). They found that between 80 and nearly 100% of adsorbed metals were removed by Lpenicillamine at pH 8.7. Therefore the purpose of this work was to examine the capability of aqueous solutions of L-cysteine and L-penicillamine to displace Cd, Cr, Cu, Hg, Ni, Pb, and Zn from typical soil components (bentonite, illite and peat) in batch experiments and to assess the influence of various physico-chemical parameters on metal removal rates and degree. Moreover experimental conditions which favor the elimination of already dissolved metals from the extracts were determined.
2. Materials and Methods 2.1. A DSORBENTS Two standard clay minerals and three almost identical peat samples were chosen for metal adsorption/extraction experiments. The characterization of the adsorbents is based on our own measurements and on company’s certifications (see Table I). Bentonite: (Schwaiba, Germany), a smectite rich mud stone, consists of 91% montmorillonite, 4% mica and 5% quartz. Exchangeable cations are calcium ions predominantly (Kraus, 1988). Illite; type ‘Lohrheimer Kaolin W’ (Erbslöh Geisenheim Industry-Minerals, Geisenheim/Rh., Germany); main components are illite (50.6%), kaolinite (31.4%) and quartz (17.9%). Minor components were iron (2.15%) and titanium (0.43%), each calculated as oxides. The grain size distribution is according to the company’s data:
