Zinc sorption by some Torrifluvents soil of Sub-Saharian region South of Libya

Zinc sorption by four Torrifluvents soils from Sub-Saharian region, south of Libya was evaluated by equlibirating soil samples with concentrations of Zn covers a range of 0 to 320 ug/ml. Effect of zinc carrier were also examined by using two carriers i.e. sulphate and EDTA. Results obtained show that all soils have high affinity for Zn sorption. Sorption isotherm of L (high affinity) type was noticed with zinc sulpate wheras S (low affinity) type was noticed with EDTA. Curviline relation between sorped zinc and zinc concentration in solution was proved by excellent fit of sorption to Frundlich equation. High mean values for sorption coefficient were noticed with sulphate (2.658) compared to EDTA carriers (0.350), While low mean value for regression coefficient (1/n) for mineral carrier (0.354) was found compared to organic carrier (1.376). Sorption coefficient were significantly correlated with clay, silt, carbonate and organic matter contents in case of sulphate carrier only. While negatively correlated with carbonate and organic matter contents when EDTA was used.


Intoduction
Zinc is known as essential micronutrient for plant and haevy metal contaminant.Its sorption by soil is important in understanding its soil chemistry.Which is in turn important for agricultural and environmental purpose, by determining their availability to plant and their movment through soil.Zinc supplying power of soil, extraction by leaching and uptake by plant roots depending essentially on the amount of zinc present at sorption sites, (Sidhu et al., 1977;Joshi et al., 1983).Jurinak and Bauer (1956) explained that there are two types of zinc sorption; non-specific sorption which occurred as a result of electrical attraction by active surface sites, and specific sorption which occurred as strong chemical bond at nonorganic soil colloidal surfaces.In the later type of sorption, metal ion replace O -2 and OH -1 ions in inter crystal structure of oxides and hydroxides (Shuman, 1977;Kalbaski et al., 1978).Zinc may be irreversibly fixed by clay through isomorphus substitutions or solid-state diffusion into the crystal structure of layer silicates (Adriano, 1986).
Zinc solubility also controlled by other reactions in soil such as mineral precipitation or adsorption onto Fe or Mn oxides (Catlett et al., 2002) and phosphate-Fe oxides interactions (Wang and Dustin, 2005).Continous application of large amount of zinc to soil may be caused possible accumulation of this element and potential harm to the environment, as zinc can be transported downward in soil and possibly deteriorate ground water quality.Downward movement of zinc in soil may be occurred under conditions of excess rainfall or irrigation water esspically in coarse-textured soils.All soils in Sub-Saharian regions, south of Libya have been formed from alluvium material.Most of them classified as Torrifluvents and characterized by medium or coarse texture, neutral to alkaline reaction and low content of organic matter.Our prelimenery investigations show that zinc defficiency is widely dispersed in crops produced in these regions.Studies dealing with chemistry of zinc in soils of these regions are very scarce.So this study was carried out to explore zinc sorption in four Torrifluvents soils from these regions using two zinc carriers.

Materials and Methods
Four soil samples from surface layer (0-30 cm) of some cultivated fields of Maknussa agricultuer project, in Sub-Saharian region, south of Libya, were obtained.All studied soils classified as Torrifluvents.Before use, the soil samples were air dried and ground to pass through 2-mm sieve.
Particle size analysis was carried out using pipet method.Organic matter by modified Walkley-Black method, electrical conductivity and pH, in soil paste extract, by conductometery and potontiometery respectively, total calcium carbonate equvielent by acid-neutralization method.All these analysis carried out as were described in Page et al. (1982).Available zinc was extracted by DTPA according to method proposed by Lindsay and Norvell (1978).Table 1 reported particle size distribution and some chemical characteristics of the studied soils.Sorption isotherm of Zn were determined by equalibrating 2 g soil in 70 ml plastic tube with 50 ml solution of Zn.Zinc equilibrating solution concentration were; 0, 10, 20, 40, 80, 160, and 320 ug ml -1 as ZnSO 4 .7H 2 O or Zn-EDTA.All solutions were adjusted to pH 7 before used.Tubes were stoppered and placed in reciprocating shaker and shaken for 1 hr at a speeding rate enough to keep the soil particles in suspenstion.Samples then incubated for additional 23 hrs at 25 o C .After the equilibration time, the tubes were centrifuged and the supernatant then filtered.The filtrate was reserved for determining Zn.Zinc was determined by using atomic absorption spectrophotometer.The amount of metal sorbed was calculated by subtracting the amount of metal remaining in the equilibrium solution from the initially added.Three physicochemical equations were used to characterize zinc sorption these are; Langmuir equation (X/C = Kb -KX), Freundlich equation (log X = log K f + 1/n log C) and Temkin equation (X = K t + B ln C), where C = concentration of zinc ion in equlibrating solution, X= quantity of sorbed zinc ion ; K, K f , K t constant (k constant releated to binding strength according to Langmuir equation) and b, B, n constant (b = maximum sorption according to Langmuir equation).Statistical parameters were used to choose most suitable equation characterize zinc sorption.

Results and Discussion
All soils used showed a high sorption capacity at low concentration of zinc added ( 0-80 ppm) as ZnSO 4 .7H 2 O.A sharp increase in the amount sorped was found as the concentration of Zn initialy added increased from 0 to 80 ppm.Continuous increase in the amount of sorped Zn with increasing concentration of the added was noticed by all soils, but with decreasing percent (Table 2).This clearley indicates the high ability of these soils to retain Zn.While, the amount and percent of sorbed zinc added as Zn-EDTA were low at low levels of addition and increased with increasing concentration of addition (Table 2).Sorption isotherms of Zn added as zinc sulfate for soils used are presented in Figure 1.The L type (high affinity type) of sorption isotherms for all soils indicates the high affinity of these soils for Zn.Zinc sorption show curvilinear relation with its concentration in equlibrium solution within the range 5-30 ppm.This explained that reddy sorption sites were decreased.High increment in the sorped amount with incresing the concentration of the added Zn from 160 to 320 ppm indicate that this concentration was not large enough to saturation stage for all soils.Amount of Zn sorped by soils, as indicated by the position of sorption isotherms, was in the order: soil 12 > soil 7 > soil 31 > soil 60.This order of sorption was significantly correlated with clay, silt, calcium carbonate and organic matter contents (Table 3).This reslut was in agreement with the result of Choudhari and Dhir (1983); Razaq et al. (1993) and Falatah and Sheta (1999).Where; Y is the mean of adsorped quantity (ug g -1 ) and X is the percent of clay, CaCO 3 or OM ; ns, not significant ,** significant at 0.01 level.

B A
Sorption isotherm of Zn added as Zn-EDTA presented in Figure 2. The S type of sorption isotherm for all soils with Zn-EDTA indicates low sorption affinity with low concentration.Continuous increase in the amount of Zn sorped also noticed with increasing added zinc.Sorption isotherm of Zn sorped from Zn-EDTA for the studied soils was in the order : soil 12 > soil 60 > soil 31 > soil 7.This indicate that reduction in the amount of sorped znic as a result of using chelate carier was different between soils used.This is may be attributed to differences in their chemical characteristics such as active carbonate, iron and manganes oxides and mineralogy.Sorption of zinc from Zn-EDTA was not correlated with carbonate and organic matter contents, but significantly correlated with clay and silt contents (Table 3).This result indicated that carbonate and organic matter effect on zinc sorption were sufficieny inhibited by EDTA carrier .
Statistical parameters ( r 2 , t, SE e ) were used to find the best fit of Zn sorption to the three physicochemical equations used.Results obtained indicate that Frundlich equation was excellent to describe the relation between Zn concentration in equlibrium solution and its sorption by soils.Frundlich equation has highest value of r 2 and t in comparision to langmuir and temkin equations (Table 4).The excellent fit of Zn sorption to Frundlich equation was in agreement with Saeed andFox (1979), El-Gundy et al. (1994), Falatah and Sheta (1999) and Al-Ameri (2001).The confirmation of Zn sorption to Frundlich equation suggest that sorption of this metal at high concentration in all soils used is curviline process (Shukla andMittal, 1979, Elrashidi andO'Conner 1982) .
Frundlich equation's constant (Table 5) explain that soils vary in zinc sorption.Mean sorption coefficient (log k f ) for soils used were in the order: soil 12 > soil 7 > soil 60 > soil 31.This result are in good agrement with the clay, clay + silt and calcium carbonate contents.High values for log k f was noticed with zinc sulphate in comparison with chelated zinc.Over all mean of log k f with sulphate and EDTA carriers were 2.658 and 0.350 respectively.This clearly explain a reduction of 7.594 times in log k f with Zn-EDTA compared to Znsulphate carrier.Differences in log k f are related to differences in properties and behaviour of the two carriers used, such as stability in soil system and ability to form precipitated minerals and components in soil solution.McBride (1989) indicated that some metal complexing ligands formed with EDTA suprpress metal adsorption.Thus sorped amount from each fertilizer and release constant of them will effectively affected (Lahav and Hochberg, 1975;Norvell, 1977 ).
Sgnificant correlation were found between sorption coefficient (log k f ) and content of clay (0.663*), silt (0.655*), calcium carbonate equvilent (0.761*) and organic matter (0.516*) when mineral carrier was used.Using organic carrier show low correlation between log k f and the content of clay (0.420) and silt (0.394) and negative correlation with calcium carbonate equivelent (-0.692 *) and organic matter (-0.797**) (Table 6).This clearly explain the differences in sorption reduction value occured between the studied soils and confirmed the previous finding that organic carrier inhibit sufficiently zinc sorption by carbonate and organic matter in the studied soils.
Mean regression coefficient (1/n) value for mineral and organic zinc carriers were 0.354 and 1.376 respectively, which indicate that this factor was increased 3.887 times with using organic carrier compared to sulfate carrier.This confirmed the previous results found with log k f .

Conclusion
In conclusions, zinc sorption in the studied soils highly correlated with clay, silt, calcium carbonate and organic matter contents.Effect of calcium carbonate and organic matter were highly inhibted by using EDTA as a carrier.This must be considerd when planning for fertlization program with zinc in these soils.

Figure 2
Figure 2. (A) Relationship between Zn sorbed (X) and equilibrium concentration (C) when added as Zn-EDTA, and (B) Sorption isotherm of Zn according to freundlich equation.