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Abstract
The main objective of this study was to provide better understanding of the behavior of heavy metals and its relation to the mineralogical composition of some Egyptian desert soil. This objective was achieved through the realization of the several specific objectives including:
1- study of the mineralogical composition of selected samples.
2- study depth wise distribution of heavy metals ( Fe, Mn, Zn, Ni, Pb, and Co).
3- study factors effecting behavior of heavy metals in soil environment.
4- Estimation of the different chemical forms of each element in the various profile layers
To achieve these objectives. Thirty-seven soil samples were collected from (11) soil profiles. Representing Four locations named El-Hammam, Burg El-Arab, Wadi Sudr and Wadi El-Nutrun.There are four uncultivated (virgin) profiles (1, 2, 9, 10 ) and seven profiles ( 3, 4, 5, 6, 7, 8, 11 ) were cultivated. Four profiles representing the Eastern desert at Wadi Sudr and two profiles representing the Western desert at wadi El-Natrun, two profiles representing Northwestern coast at Burg El-Arab, three profiles representing El-Hammam. Four soil profiles ( 1, 6, 7, 11 ) were selected from these soils to investigate the distribution of chemical fractions of heavy metals and soil mineralogy. The selection was based on soil salinity, texture, and total content of CaCO3.
Main soil physical and chemical characteristics of the selected soil profiles were obtained using standard methods, the soil samples were analyzed for total, available content of (Fe, Mn, Zn, Ni, Co, and Pb) and fractionated by sequential extraction to estimate the quantities of the different forms of these elements. Water soluble (F1), exchangeable (F2), carbonate bound (F3), Fe& Mn oxidies (F4), organically (F5), residual (F6). Mineralogical analysis for sand and clay fraction were also estimated.
The present study indicated the following points:
i) Heavy minerals analysis showed the abundance of amphiboles and pyroxenes which are considered the less stable minerals and can be taken as an indicator of the negligible effect of the past-depositional changes and considered these soil as weakly developed and heterogenetic.
In conclusion the obtained results indicated the importance of heavy minerals in the sand fraction as reservoir of Fe, Mn, Zn, Ni, Co, and P b in the studied soils.
ii) The semi-quantitative clay mineralogical analysis based on the relative intensity of specific x-ray diffraction peaks, showed the presence of different contents of montmorillonite, kaolinite, illite, smectite, interstratified and palygorskite (attapolgite).
iii) measuring the total amounts of heavy metals in soils did not provide reliable data upon their bioavailability. The iron content in the studied soils, It is clear that the average values of total Fe content were 2580.3, 2761.7, 3346.7 and 3482.7 mg kg -1 in soil samples of Wadi Sudr, El-Hammam, Wadi El-Natrun and Burg El-Arab, respectively. It is clear that the average values of total amounts of Mn were 220.5, 189.2, 135.7 and 128.3 mg kg-1 in soil samples of Wadi Sedr, Wadi El-Natrun, El-Hammam and Burg El-Arab, respectively. It is clear that the average values of total Zinc contents were 13.8, 12.49, 11.3 and 8.68 mg kg-1 for soil samples of Wadi Sudr, Burg El-Arab, Wadi El-Natrun and El-Hammam respectively. The average values of total content of Ni were 5.04, 4.75 3.49 and 2.59 mg kg-1 for soil samples of Wadi Sedr, Burg El-Arab, Wadi El-Natrun and El-Hammam, respectively. It is clear that the average values of total amounts of Co were 2.27 2.49, 2.64 and 3.57 mg kg-1 in soil samples of El-Hammam, Wadi El-Natrun, Wadi Sudr and Burg El-Arab, respectively. The average values of total amount of Pb were 1.23, 1.93, 2.8 and 3.6 mg/kg for wadi sudr, El-Hammam, wadi El-Natrun and Burg El-Arab, respectively.
As a result, the sequential extractions scheme revealed reliable identification of the chemical fractions of Fe, Mn, Zn, Ni, Co, and Pb in soils. These fractions gave reliable information on distribution of metals in the different soils. Most of the immobile fractions of Fe, Mn, Zn, Ni, Co, and Pb are associated with (Fe&Mn oxides (F4), strongly complexed with organic matter (F5) and residual (F6) ). Higher portions of Pb occurred in mobile fractions; (F1), (F2) and (F3) than those of Fe, Mn, Zn, Ni, and Co.The order of fractions (Fe, Mn, Zn, Ni, Co, and Pb) content of studied soils are as follows:
1) In general, Fe fractions can be arranged in the following sequence:
Res. > Oxid. > Org. > Carb. > Exch.> Water soluble. for Wadi Sudr
Res. > Oxid. > Exch > Carb. > Water soluble > Org. for Wadi El-Natrun
Res. > Oxid. > Org. > Carb. > Exch.> Water soluble. for Burg El-Arab
Res. > Oxid. > Org. > Carb. > Exch.> Water soluble. for El-Hammam soils
2) In general, Mn fractions are presented in the following sequence:
Oxid. > Carb. > Rcs. > Org. > Exch. > Water soluble. for Wadi Sudr
Res. > Oxid. > Carb. > Org. > Exch. > Water soluble. for Wadi El-Natrun
Oxid. > Carb. > Res. > Exch. > Org. > Water soluble. for Burg El-Arab
Oxid. > Res.> Carb. > Exch. > Org. > Water soluble. for El-Hammam soils
3) In general, the association of Zinc in the soils was in the following decending order:
Res.> Fe – Mn Oxid. > Org. > Carb. > Exch. > Water soluble.
4) Generally, the distribution Ni among fractions was in the order:
Oxid. > Res. > Org. > Carb> Exch= Water soluble., for Wadi Sudr soil.
Res. > Org. > Oxid > Carb>Water soluble = Exch., for Wadi El-Natrun
Oxid. > Res. > Org. > Carb. > Water soluble = Exch. , for Burg El-Arab soil and
Oxid. > Org. > Res. > Carb. >Water soluble = Exch., for El-Hammam soils
5) In general, the associat of Co in the soils was in descending order:
Res. > Fe-Mn Oxid > Org. > Carb > Water soluble > Exch.
Exception Wadi Sedr:
Res. > Oxid. > Carb > Org. > Water soluble > Exch.
6) In general, the partitioning of Pb among the different fraction in Wadi Sudr.
Oxid > Res. > Org. > water soluble > Carb. = Exch.
Oxid > Exch > Res. > Carb> Org. > water soluble. ,for Wadi El- Natrun
Oxid > Res. > Carb. > Org. > Exch. > water soluble., for Burg El-Arab
Oxid > Res. > Org. > Water soluble. >Carb.>Exch. ,for El-Hammam
The data showed that the studied elements (Co, Zn, Pb) decreased with soil depth, while (Ni, Mn, Fe) increased with depth. With some exception, where these element accumulated in the subsurface and deepest layers.
iv) The factors affecting the forms of studied elements were determined by correlation coefficient between forms of studied elements and different soil properties. The behaviour of heavy metal in soils was influnced by as pH, O.M and the contents of CaCO3 and clay plus EC, SAR and the values of the active carbonate.
v) Statistical analysis of the elemental composition of the investigated fine sand fractions indicated the presence of Fe, Mn, Zn, Ni, Co, and Pb in the elemental composition of most of the identified heavy minerals in the sand fraction.