Removal of cadmium and mercury ions from aqueous solution by sorption on treated Pinus pinaster bark: kinetics and isotherms

Abstract

Formaldehyde pretreated Pinus pinaster bark was used to sorb Cd²⁺ and Hg²⁺ from aqueous solutions. The sorption kinetics showed hyperbolic dependence of the proportion of cation adsorbed on time, and the sorption isotherms were satisfactorily fitted by Freundlich equations, with k and n values showing Hg²⁺ to be more efficiently sorbed than Cd²⁺. Except for low cation concentrations, for which sorption was practically total at all initial pH⩾6, sorption increased in this range, in keeping with a mechanism based on ion exchange with the hydroxyl protons of ring B of the procyanidin units of the tannins in the bark.

Introduction

Since industrial effluent containing heavy metals was recognized as a major threat to the environment and to health, research on the removal of these pollutants from solution has grown rapidly. The main offenders are cationic species of mercury, lead, nickel, chromium, cadmium, copper and zinc. At high concentrations these metals can successfully be recovered from solution by precipitation as hydroxides or carbonates, by membrane filtration, and by capture on synthetic ion exchangers. However, these methods are much less efficient for concentrations lower than about 100 ppm, for which they can be prohibitively expensive and can even fail to achieve legal limits (Matheichal et al., 1997). For these low concentrations it is preferable to use sorption techniques; and since activated carbon is too expensive for large scale use, numerous alternatives have been proposed, including peat (McKay and Porter, 1997), agricultural and forest waste (Deshkar and Dara, 1988; Marañón and Sastre, 1991) and other biosorbents (Kapoor et al., 1999; Peternele et al., 1999).

The forest wastes that have been investigated in this respect include the barks of certain timber trees. These barks are inexpensive, abundant, and contain polyphenolic compounds (generically termed tannins) that under appropriate conditions of pH and temperature are capable of sorbing significant quantities of metal cations from solution (Ruf et al., 1992; Vázquez et al., 1994). Furthermore, prior treatment to bring about crosslinking and/or functionalization can improve both the stability and sorption capacity of these compounds (Randall et al., 1976; Peternele et al., 1999).

Pinus pinaster bark is a by-product of the timber industry of Galicia (N.W. Spain) that is currently used only as fuel or for horticultural purposes. However, like the barks of other conifers it contains polyhydroxylated polyphenols based on procyanidin (Vázquez et al., 1987; Laks and McKaig, 1988). Since this kind of tannin can act as an ion-exchanger (Laks, 1991), the deprotonated hydroxyls of the procyanidin ring B chelating di-cations as shown in Fig. 1, the possibility arises in using this by-product to remove heavy metal cations from wastewater.

In this work, we studied the sorption of Cd²⁺ and Hg²⁺ ions by P. pinaster bark treated with formaldehyde. The kinetics and sorption isotherms of these processes were characterized, and the influence of pH was investigated.