Metal ion adsorption by peat and solvent-extracted peat

The effect of the solvent extraction of peat on the sorption behaviour of three divalent metal ions [iron(II), lead and barium] was examined. Solvent extraction of peat resulted in particles of a smaller size with a significantly increased surface area. The extracted peat also demonstrated an increase in cation-exchange capacity from 127.2 to 149.5 mequiv/100 g, apparently due to an increase in available phenolic groups. Adsorption experiments carried out at pH 4.5 on peat and on the solvent-extracted peat using the three metal ions showed that the sorption behaviour of Fe(II) and Pb(II) ions correlated strongly with the Langmuir isotherm whereas the behaviour of Ba(II) ions was better described by the Freundlich expression. When the two peat substrates we re compared, the monolayer saturation capacity observed for the Fe(II) ion on peat was 0.357 mmol/g while a value of 0.420 mmol/g was recorded for the solvent-extracted peat, with solvent extraction of the peat leading to a more modest increase in Pb(II) ion adsorption. The increase in molar monolayer saturation capacity for the Fe(II) ion suggests that these smaller ions were bound preferentially to the strong phenolic sites, with Pb(II) ion adsorption occurring on the carboxylic sites. In the case of Ba(II) ions, interactions other than ion exchange contributed to the sorption behaviour.