Copper and zinc adsorption onto poorly humified Sphagnum and Carex peat

Abstract

Peat generally has a high adsorption capacity and has been suggested as an adsorbent for metals in polluted waters. However, the adsorption potential of peat can be expected to be strongly dependent on the chemical properties of the water. In this study, the effect of pH, ionic strength (CaCl 2 and NaCl concentrations), and metal concentration on Cu and Zn adsorption onto poorly humified Sphagnum and Carex peat was investigated in batch experiments using a fractional factorial experimental design. The pH value was varied between 4 and 8, the CaCl 2 and NaCl concentrations between 1.2–6.2 and 0.4–43 mM, respectively, and the Cu and Zn concentrations between 0.05 and 0.5 mM. The amount of Zn adsorbed increased more with increasing pH than the amount of Cu adsorbed. The effect of NaCl/CaCl 2 concentration was minor. It was found that Zn adsorption in particular, but also Cu adsorption, increased more with pH onto Carex peat than onto Sphagnum peat. In the pH interval 4–8, the removal of Zn from the solution increased from 0% to 80% using Carex peat and from 10% to 65% using Sphagnum peat as the adsorbent. The Carex peat sample decreased the Cu concentration by 80% at pH 4 and by 95% at pH 8 and for Sphagnum peat a decrease of 85% was maintained in the pH range investigated. The differences between Sphagnum and Carex peat were attributed to the habitat conditions at the time of peat formation. Carex peat has higher ash, nitrogen and sulphur concentrations, while Sphagnum peat contains a higher amount of uronic acid. In treating polluted waters with peat, a higher degree of metal removal can be expected at high pH values than at low. The removal of Cu, which has a high affinity to the peat surface, was less dependent on pH than Zn removal. Poorly humified Carex peat should be chosen in treating wastewaters high in pH.