the fractionation of some heavy metals in calcareous soils affected by land uses of central area of zanjan provine (northwest of iran)

introduction: heavy metals are found to be one of the major environmental hazardous contaminants, for human health, animal life, air quality and other components of environment. they can affect geochemical cycles and accumulate in animal tissues since physical processes are not able to remove them, so they are consistent in long term. the analysis of the total concentration of heavy metals in soil may provide information about soils enrichment but in general, it is widely used to determine the potential mobility of heavy metals in environmental behavior under chemical forms of metals in soils. heavy metals existat several phases including water-soluble, exchangeable, bounded to organic matter, bounded to carbonates, bounded to fe-mn oxides, secondary clay minerals and residual fraction within primary minerals network. there is a dynamic equilibrium between different fractions of elements in soil. the main objectives of the present study were a) the analysis of the total concentration of heavy metals such as fe, mn, ni, cr, co, pb, zn, cd and cu and b) the fractionations of heavy metals and identification of controlling factors to distribution and behavior of heavy metals in soils at different land uses. materials and methods: the study was performed at central area of zanjan province (iran). the study area was over 2000 km2 in coordinates 20´ 36° to 41´ 36° e and 19´ 48° to 53´ 48° n. the average altitudes were over 1500 meters above sea level. the major land uses of the study area included agriculture (ag), rangeland (ra) and urban (ur). sample collection was done based on the random grid method in august 2011. surface soil samples (0-10 cm depth) were taken from grid centers included 137, 77 and 27 samples from ag, ra and ur land uses, respectively. the samples were digested in nitric acid 5 normal (sposito et al., 1982) and total concentration of pb, zn, ni, mn, cu, cr, fe and co were measured by perkin-elmer: aa 200 atomic absorption instrument and cadmium was measured by atomic absorption equipped with rayleigh: wf-1e graphite furnace. 75 soil samples were selected, dtpa-extraction and sequential extraction were performed and physiochemical characteristics of these samples analyzed. to extract the metals by dtpa, the method developed by lindsay and norvell, (1978) was used and sequential extraction was done by tessier et al., (1979) method. all statistical parameters were calculated using spss 16.0 software, and mean comparison (mean separation) was carried out using duncan test at probability level of 5%. results and discussion: the results indicated that heavy metals concentrations and patterns were evidently affected by different land uses. co concentration was between 17.0 – 35.7 mg/kg and had the lowest total coefficient of variation (14%). the maximum total cr and ni values were measured in ag land use (26.1 and 52.6 mg/kg, respectively) and lowest was in ur land use (17.0 and 37.2 mg/kg, respectively). the highest total average value of mn was found in ra (698.9 mg/kg) and the lowest in ur (629.1mg/kg) land use. the highest fe concentrations were measured in ag and ra land uses (17.2 and 17.0 g/kg, respectively) and the lowest in ur land use (14.0 g/kg). the maximum concentration of total cd was observed in ur land use (2.47 mg/kg) and its minimum values were found in ra and ag (0.83 and 0.75 mg/kg, respectively) in the study area. in ur land use, cu and zn were more significant than ag and ra land uses. pb variation was the same as zn so that its increased concentration was found in urban land use (90.2 to 1357.5 on average 220.1 mg/kg). the highest pb values were measured in ur land use (220 mg/kg) while the lowest concentrations were found in ra and ag land uses (80.6 and 69.0 mg/kg, respectively). different elements showed various fractional distribution in different land uses. the highest co percentage was related to residual fraction at all land uses, with values up to 48.4%, 54.0% and 48.1% in ag, ra and ur land uses, respectively. ni fractionation had approximately the same pattern with co in all factions and land uses, except exchangeable fraction of ni that showed the lowest percentage in all land uses. the dominant fraction of cu was residual fraction with the amounts of 73.3% 76.0% and 61.9% in ag, ra and ur land uses, respectively. the second dominant fraction in ur and ag land uses was related to that was bounded to om, with 16.5% and 10.1%, respectively. zn distribution in the ag and ra land uses had the same trend: residual>bounded to fe-mn oxides>bounded to om>bounded to carbonate>exchangeable fraction. whereas, zn distribution showed different trend in ur land use as bounded to fe-mn oxides>residual>bounded to carbonate>bounded to om>exchangeable fraction. pb distribution was different in each land use. pb showed similar distribution to zn in ur. in ag and ra land uses residual fraction of pb was measured as highest value while other fractions of pb had these distributions: pb bounded to carbonate>bounded to fe-mn oxides>exchangeable >bounded to om fraction in ag land use and pb bounded to fe-mn oxides>bounded to om>bounded to carbonate>exchangeable fraction in ra land use. conclusion: based on the results of this study, cr, co, ni, mn and fe magnitudes are uneven in soils. the total heavy metal concentrations fractionation can provide information on the contaminant metals sources. high levels of exchangeable fractions, acid soluble and easily reducible perhaps indicates anthropogenic activities. naturally, chemicals are associated to resistant soil fractions such as oxy hydroxides, organic matter and sulfides. in soil fractionation, the contribution of each fraction in soil was a function of metal type and land uses. those metals were affected by anthropogenic activities such as lead, zinc, and partly copper, showed the highest percentage in the fraction that influenced by external input sources. those were characterized by lithogenic origin (cobalt and nickel) was mainly found to be highest in residual fraction. however, in all metals, those fractions affected by anthropogenic activities (non-resistant fractions) were much more in urban land use than agriculture and rangeland ones.