Adsorption and desorption of Zn in different soils and clay Minerals

Zinc adsorption was studied in three Ca-saturated soils that varied in their dominant clay minerals. organic matter and CaCO~ contents and also in three Ca- saturated pure clay minerals. kaolinite. montmorillonite. and palygroskite that represented the dominant clays in the selected soils. The pH of all systems was maintained at 7 except Qarn calcareous soil where it was 7.5. The adsorbed Zn was sequentially extracted with 0.5 M CaCI2. I M MgCI2. 0.005 M DTP A and 0.1 M HC!. The linear fit of the adsorption data of all systems to Langmuir equation was excellent when the curve was resolved into two linear portions. The adsorption maxima of clay mineral systems were in the order montmorillonite > palygros-kite» kaolinite at Iow and high Zn concentrations. Kaolinite adsorbed less Zn with Iow bonding energy with most Zn adsorbed in the exchangeable form. Palygroskite adsorbed slightly less Zn than montmoril-lonite but with much higher affinity as indicated by the higher calculated bonding energy and the much higher percentage of Zn adsorbed in the difficulty available form (0.1 M HCI- extractable). The adsorption of Zn by soils was largely affected by the high organic matter (in Baha soil) and the high CaCO3 content (Qarn soil) rather than by their dominant clay minerals. montmorillonite and palygroskite respectively. At low Zn concentrations. Zn adsorption maxima were in the order of their CEC; Baha > Qarn > Bakyria. At high Zn concentration the adsorbtion maxima were Qarn> > Baha> > Bakyria reflecting the role ofCaCO3 as an important adsorbent ofZn. Zinc adsorbed by Baha soil was mostly in the HCI-extractable fraction and such strong bonding was attributed to its high organic matter content. Zinc adsorbed by Qarn calcareous soil was largely in the DTPA extractable fraction and believed to be held on the CaCO