Batch and stirred flow reactor experiments on Zn sorption in acid soils: Cu competition

The adsorption and desorption of zinc in acid soils developed over granite and amphibolite were investigated with and without the simultaneous addition of copper. In stirred flow experiments, the maximum Zn sorption capacity (qmax) ranged from 1.29 to 4.36 mmol kg−1, and was significantly correlated (r = 0.924; p < 0.01) with KF Freundlich parameter obtained with batch experiments. Soil pH was the most influential variable on Zn adsorption by soils despite its low variability (4.08–5.06). In all cases more than 75% of Zn sorption was due to fast sorption reactions, an indication that in acid soils, Zn electrostatic bindings is predominant . Zn desorption ranged from 13 to 32% of sorbed Zn in batch experiments, and exceeded 62% in stirred flow experiments. Zn desorption was a slower process than its adsorption (46% on average). The presence of Cu reduced Zn sorption between 28 and 58% in batch experiments (with concomitant reduction of the Freundlich coefficient KF), and by 60–73% in stirred flow experiments, although during the first 2.5 min the influence of Cu was low. Besides a lesser sorption capacity in competitive sorption, also the rate of Zn binding fell faster than in noncompetitive sorption.