Humic acids from coals of the North-Bohemian coal field: II. Metal-binding capacity under static conditions

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Abstract

The interactions of metal cations (Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Zn) with solid humic acids (HAs) were investigated in a batch arrangement. The sorption of metals on the surface of HAs depends strongly on the pH — sorption decreases with decreasing pH. The metal-binding capacities estimated from the metal–humate binding model range from ca. 0.4 mmol/g (Fe, Ni, Zn) to 1.7 mmol/g (Pb). Of the examined metal cations, Pb2+ binds most strongly to solid HAs. The order of the metal–humate binding strength is consistent with series found in the literature and is based on the stability constants of metal–humate complexes. In comparison with coal-derived HAs, soil HAs from chernozem (and also low molecular weight HAs) exhibited markedly higher metal-binding capacities, whereas the capacity of oxyhumolite (the raw material for HA production) was lower.

Introduction

Humic acids (HAs) are highly functionalized carbon-rich biopolymers with a great capacity to bind metal cations. Their elemental composition as well as the functional group content vary depending on their origin and preparation procedure. For soil HAs, a helical secondary structure has been proposed with building blocks composed of C36H30N2O15·xH2O (x=0–15) [1]. The main functional groups are carboxylic and phenolic, but minor amounts of other groups, such as amino acids, are always present. The interactions of humic substances with metals play an important role in metal mobility and bioavailability in the environment. Various methods have been used to study metal–HA interactions, starting with the ‘classical’ Schubert’s method [2], [3], [4]. Potentiometric methods in various arrangements are the most frequently used [5], [6], [7], [8], whereas voltametric methods are used less often [9], [10]. An assessment of the stability constants of metal–humate complexes was reviewed recently with the emphasis on the application of the ultrafiltration method [11]. Useful information on the properties of complex compounds of humic substances with heavy metals was obtained using physico-chemical (‘direct’) methods such as X-ray spectrometry, NMR, FTIR, laser fluorimetry, gel chromatography, radioactive isotope labelling, etc. [1], [12].

The present work focused mainly on the potential industrial applications (e.g., for heavy metal removal from waste waters) of HAs manufactured on a large scale from North-Bohemian brown coal. HAs from other sources were also included for comparison. The basic properties of HAs were described in the first part of this study [13]. Metal-binding capacities were investigated in closed stirred systems. In heterogeneous systems (solid HA–metal solution), the equilibrium metal concentrations were measured at various pH values. The application of HAs for metal trapping in a column arrangement (dynamic system) is described in the third part of this study [14].

Section snippets

Metal–humate binding model

The modelling of metal–humate interactions has been a field of active research for several decades, but, despite all efforts, we are still far from a consensus concerning binding models. Existing models were reviewed and classified by Logan et al. [15]. It is usually assumed that oxygen-containing functional groups act as the metal-binding sites. However, the involvement of other groups cannot be discounted. In some models, HAs are considered as mixtures of mono- and polyprotic acids [7].

Preparation of HAs

Most of the measurements were performed with HAs prepared from young coal (oxyhumolite) from the North-Bohemian Vaclav mine near Bilina.

Results and discussion

Using the above procedure, the dependence of X (the amount of metal bound to 1 g of HAs) on pH was measured for various initial concentrations of metals. Measurements were performed in an acidic pH range, where HAs as well as metal–humate complexes are insoluble. A typical set of experimental dependencies is shown in Fig. 2 for lead and purified HAs. Additional experiments (not shown in Fig. 2) were performed at other metal concentrations and pH values to cover the examined range more

Acknowledgements

Financial support from the Ministry of Industry and Trade of the Czech Republic (project ‘The Evaluation of Low-Rank Coal to Chemically and Biologically Active Matters’, grant No. MP-3321/51) is acknowledged.

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