Adsorption of zinc onto peat from peatlands of Poland and Israel

Peat, defined as young Quaternary, mainly Holocene, organogenic sedimentary rock in the first stage of coalification, has received increasing attention due to its potential to act as an ubiquitous effective agent for the recovery of valuable metals, for the removal of metal ions from dumping site leachate and wastewater, or for retrospective monitoring of their migration and accumulation in the environment. For these purposes, adsorption capacity and binding strength of peat with respect to the metal ions under different conditions, as well as the mechanism of binding–release should be thoroughly known. To date, this knowledge is still limited. In this paper, results of studies on adsorption–desorption properties of Zn2+, which is one of the most mobile trace metals in the environment, in representative Polish and Israeli peat samples are presented. Batch studies were performed for Zn2+ in both single cation and a binary system (Zn–Cd) at pH 4.0. Column experiments under saturated flow conditions with Zn concentrations of 250 and 500 mg dm−3 in the input solution at pH 4.0 (monometallic system) were also conducted. Finally, batch and column zinc adsorption experiments on selected peat samples were carried out using real multi-component wastes from the electroplating process. To assess the binding strength of peat with respect to zinc, sequential extraction by Tessier et al. (1979)modified by Kersten and Förstner (1988) was applied. The adsorption capacity for zinc of low-moor Polish peat, which is typical for the Northern Hemisphere with pH 5.02–6.54, was 15,800 to 33,700 mg Zn kg−1. Lower values were found for the Israeli peat. The presence of mass equivalent amounts of Cd showed only a moderate suppressing influence on the uptake of Zn by different peat matter. This suggests a weak competition of metal ions for adsorption sites available, despite binding of these metals in similar mass ratios in the same most labile fractions. There was, however, a difference in the binding of Zn and Cd in the more stable fractions, where no competition between Zn and Cd occurred. A significant difference in fractionation of Zn sorbed in batch vs. column experiments, was indicated. In particular, higher Zn enrichment in the `insoluble organic' fraction under dynamic flow conditions was observed. Desorption experiments proved that zinc can be partially released with acid treatment from the labile fractions.