The ion exchange and adsorption properties of Sphagnum peat under acid conditions

Abstract

The effects of experimental conditions on ion exchange capacity of low- and medium-moor Sphagnum peat were tested in column-type experiments. The ion exchange mechanisms of the samples differed, and proton-cation exchange was more prominent with the low-moor sample. The typical adsorption curve consisted of a region with high adsorption level (> 99.5 %) and a descending region where the slope obeyed first order kinetics. The capacity of the low-moor sample was nearly twice that of the medium-moor sample, which was also indicated by the infrared spectra of the samples. pH had a strong influence on the adsorption in the range 2.0 - 4.9: increase of pH increased the adsorption. Concentration of the test solution was of smaller significance although increasing capacity was observed with increasing cation concentration. Column height and flow rate were of little or no importance in the studied range because of the rapidity of the adsorption. Adsorption experiments with representative amounts of ions on medium-moor Sphagnum peat at pH 3.0 - 3.3 revealed that with divalent transition metals the adsorption affinity obeys the complex stability order published in the literature. Trivalent cations were strongly adsorbed, as was expected. With alkalies and alkaline earths the adsorption capacity usually decreased with increasing ratio of molecular weight to charge. Sr2+ was an exception, proving the high selectivity of peat at pH 3.0 - 3.3. The adsorption of anions was extremely weak. The pure Sphagnum peat species differed in their adsorption properties. s. Fuscum was the strongest adsorber for most ions. Measurement of IR spectra proved to be a good method for rough estimation of capacity of a peat. The sampling place is of relevance to adsorption behaviour, as was shown with low-moor samples from three production areas. Pretreatment by freezing strengthened the adsorption by elongating the region of high adsorption level if the water content was 95 % at freezing. Extraction of the long-chain hydrocarbons may increase the capacity if the sample contains large amounts of those hydrocarbons. Like an ion exchange resin, peat can be used repeatedly as a cation adsorber, althoug h its capacity is lower. It behaves like selective, complexing resin.