Effect of some minerals and dissolved organic carbon on the behaviour of some heavy metals in soils

dissolved organic carbon on the behavior of heavy metals (Zn, Mn and Ni) in two In this study the effect of natural minerals such as zeolite and bentonite and the different soil samples (sand and clay soils) were carried out. The main results can be summarized as follows: 1- Adsorption and desorption experiment: The studied minerals zeolite and bentonite showed considerable variations in Zn, Mn and Ni sorption properties and DTP A extractability. Bentonite has the highest ability for Mn and Ni sorption.while zeolite has the highest ability for Zn sorption. Zeolite retains a relatively high percentage of the sorbed Zn agaInst the extractIon by DTP A. Zinc, manganese and nickel sorption data were described by a Freundlich and Langmuir adsorption model. The results showed that Langmuir constants for Zn, Mn and Ni sorption [maximum adsorption (b mmol kg-I)] have been in the following decreasing order: Bentonite Mn Bentonite Ni Zeolite Mn Bentonite Zn Zeolite Ni Zeolite Zn. While the [binding strength values (k in mmol l-1)] was in the order: Zeolite Mn Zeolite Zn Zeolite Ni Bentonite Mn Bentonite Zn Bentonite Ni. The results suggest that natural zeolite and bentonite minerals have a high potential for Zn, Mn and Ni retention. The availability of the retained Zn, Mn and Ni was higher for Zn compared with Mn and Ni, and zeolite seems to have the highest ability for Zn sorption and extractability by DTPA. Bentonite has intermediate characteristics for Zn. Mn and Ni sorption, 2- Incubation experiment: Data of the incubation showed that there are gradual increasing in the concentration of studied heavy metal in all fractions by increasing the rates of metals loading on zeolite and bentonite and as a results of incubation times. This increasing for the concentration of heavy metals (Zn, Mn and Ni) under study not only in the means of rates of doses of zeolite and bentonite but also in the means of incubating times. In the control clay soil, the background Zn, Mn and Ni in each fraction were generally in the order carbonate Fe-Mn oxides organic matter exchangeable water soluble fraction. While in the untreated sand soil, the background Zn, Mn and Ni in each fraction were generally in the order Fe-Mn oxides exchangeable carbonate organic matter water soluble. It was observed that after the period of incubation, metals in exchangeable fraction were predominant in all reatments of both soils with some exception. Moreover the increases in the other fractions were also observed. It may indicated that a fast process of metal distribution among the fractions occurred during the first 2-days incubation. The redistribution process from 2-weeks to 4-weeks is much slower comparing with that in 2-days. At the end of 4-weeks incubation, the percentages of Zn, Mn and Ni in exchangeable fraction were still very high. This observation was possibly due to the slow transformation of metals from loosely bound fractions such as exchangeable fraction to strongly bound. Column experiment: Addition of the natural mineral loaded by Zn, Mn and Ni to surface of the soil resulted in a marked increase in the amount of total Zn. Mn .and Ni in both soil. The increases corresponding, in general, to the concentration of metals loaded in the clay mineral. For the rate of natural clay mineral (0.4%) were chosen for application, total amounts were presented (mg/kg soil) as a function of soil depth. Maximum total concentrations of metals were found in the surface layer of soil (0-5 cm) for all metal ions. Data revealed that the amounts of Zn and Mn extracted by DTP A solution were generally decreased with increasing soil depth this in case of Zn and Mn. While Ni has a different pattern where it decreases by depth then increased in the last segment of column especially in case of sandy soil. Mobility of heavy metals were followed the order Mn Zn Ni.