| Heavy metals from anthropogenic activities, such as industrial wastes, mining activity, agricultural production and atmospheric disposition are commonly discharged into environment. Heavy metals, a kind of important pollutant, have prompted great interests due to their nonbiodegradable properties, bioaccumulation effects and toxicities even at low concentrations. Heavy metal contamination in soil is becoming serious. Pepole are paying more attention to adsorption and transportation of heavy metal in soil. Heavy metal pollution of soil and groundwater can be close associated. Once soil is polluted by heavy metal, heavy metal ion may be leached out gradually by the action of precipitation and moisture of soil and move down into groundwater, which leads to groundwater pollution. Many related studies have shown that adsorption/ desorption was one of the most important processes controlling the existence of trace metals on solid phases, including soils. Some studies found that heavy metal adsorption/desorption on soil was a complex process involving physical, chemical, and electrical interactions at soil surfaces. Heavy metal adsorption/desorption process in soils was affected by many factors such as soil characters, metal ion characters and pH value of the soil system. Environmental hazards derived from heavy metals are close linked to the mobilitiy and concentrations of heavy metals in soil. The mobility of heavy metals in terms of leachability depends not only on the total concentration in soil but also on soil properties and environmental factors. However, to assess the environmental impact of contaminated soils, the knowledge of the total concentration of a specific heavy metal without considering its speciation is not sufficient, because the fate and toxicity of heavy metals in contaminated soil is greatly controlled by chemical forms of heavy emtals.The presence of heavy metals in the agricultural lands has imposed a need for better understanding the processes of soil-heavy metal interactions, in particular, the mobility and retention mechanism of heavy metals. In this article, static adsorption/desorption characteristics of lead, cadmium, chromium(VI) and arsenic(V) by farmland soils from Northeast China were studied and the mobility of the four metals from aeration zone of soil to groundwater was investigated through soil column leaching experiment. Three dimensional simulative experiments were employed to investigate the transport law of heavy metals in deep aquifers. The method of selective sequential extraction was used to analyze the chemical fractions of lead, cadmium, chromium and arsenic in soils before and after leaching. It is well known that the mobility and toxicity of chromium and arsenic not only depend on their fractions, but also their oxidation and deoxidization state. Chromium and arsenic speciation were performed using the methods developed by Bank et al. and Chappell et al., respectively. A study about heavy metals in agricultural soils in Northeast area could provide valuable and insightful information for other regions in China.The content and chemical speciation of heavy metal element (lead, cadmium, chromium(VI) and arsenic(V)) in three farmland soils nearby contaminated area were analyzed in this study. The result indicated that, for three soils, the concentration of lead association with different fraction was in the sequence: Fe-Mn oxides fraction > residual fraction > organic matters fraction > carbonate fraction > exchangeable fraction. Cadmium was mainly associated with organic matters. There was no obvious difference between the percentage of cadmium associated with Fe-Mn oxides and organic matters. Exchangeable fraction and carbonate fraction of cadmium were all lower than others. Chromium and arsenic mainly existed in the form of residual fraction. Percentages of residual fraction for exotic lead and cadmium in soils were decreased, namely, latent mobility of lead and cadmium increased. The type of pollution source had great effect on the content of heavy metal in farmland soil and heavy metal mainly accumulated in the surface layer soil, which caused the content of cadmium in the soil in individual region to surpass the third level of"Soil Environment quality Standard"and posed threat to the safe production of agriculture.The adsorption/desorption of lead, cadmium, chromium(VI) and arsenic(V) were investigated. The adsorption/desorption of lead, cadmium, chromium(VI) and arsenic(V) obtained equilibrium in a few hours. Both Langmuir and Freundlich model gave good fits to the data of lead, cadmium and arsenic(V) adsorption, while chromium(VI) adsorption followed linear adsorption isotherm. Adsorption amounts of the four metals decreased in the order: lead > cadmium > arsenic(V) > chromium(VI). Anionic chromium(VI) and arsenic(V) could be only adsorbed by goethite, FeO(OH), aluminium oxides and other soil colloids with positively charged surface sites. However, soil surfaces were normally negatively charged, so, compared with lead and cadmium, the adsorption amounts of chromium(VI) and arsenic(V) were less. The difference between the adsorption amounts of lead and cadmium depended on ionic properties such as electronegativity, ionization potential, hydrolysis constants, ionic radius and redox potential. The amounts of cadmium desorption were all about 5 times of that of lead. lead and cadmium adsorption increased with the increase of pH.Column tests were used to investigate the transport of lead, cadmium, chromium(VI) and arsenic(V) in unsaturated zone. Selective sequential extraction (SSE) and solvent extraction were used to determine lead, cadmium, chromium and arsenic fraction and chromium and arsenic valent state [chromium(VI) or chromium(III), arsenic(V) or arsenic(III)] in soils before and after leaching experiment. Breakthrough curves (BTCs) fitted by Thomas model and Yoon–Nolson model showed that transport rates of the four metals in unsaturated zone followed the order: chromium(VI) > arsenic(V) > cadmium > lead. Effluent chromium and arsenic were almost entirely chromium(VI) and arsenic(V), respectively, and no chromium(III) and arsenic(III) were ever detected during the leaching experiment. The retention concentrations of lead, cadmium, chromium and arsenic in the soils decreased in the order of lead > cadmium > chromium > arsenic. After leaching experiment, the relative and absolute metal concentrations of exchangeable, carbonate, Fe-Mn oxide and organic fraction were all increased, which enhanced the potential mobility and risk of heavy metals to soil/groundwater system. Under long-term water saturated conditions, the concentrations of chromium(III) and arsenic(III) in different depth of soil profiles were increased, respectively.The effects of coexistent metallic ion, inorganic salt, organic matter, pH value and temperature on the adsorption of chromium(VI) and arsenic(V) by soil were studied. Among these coexistent components, pH value and KH2PO4 affected chromium(VI) and arsenic(V) adsorption more obviously. High pH value and the existence of KH2PO4 had reduced the adsorption of chromium(VI) and arsenic(V) by soil. The effects of pH value and KH2PO4 on the vertical transport of chromium(VI) and arsenic(V) in unsaturated zone were studied using soil column leaching tests. High pH value and the existence of KH2PO4 were advantageous to the vertical transport of chromium(VI) and arsenic(V) in unsaturated zone. Under lower pH, the relative and absolute concentrations of exchangeable chromium and arsenic in soil were increased after leaching tests. Therefore, the bioavailability of chromium and arsenic in soil were enhanced. Under higher pH or the existence of KH2PO4, the relative and absolute concentrations of exchangeable chromium and arsenic in soil were reduced after leaching tests. Averagely, 95% of Cr and 11% of As were deoxidized to chromium(III) and arsenic(III), which reduced the transport of chromium(VI) and arsenic(V). The transport velocity of chromium(VI) and arsenic(V) in different soils were different. With great content of sand and high pH value, the transport velocity of chromium(VI) and arsenic(V) were great. The contents of clay, iron oxide and CEC were negative correlation to the transport velocity chromium(VI) and arsenic(V).When chromium(VI) and arsenic(V) entered the aquifer, they transported in each direction. In the place which was near to the leaching point, the groundwater was easily polluted. After about a half year, the well water had been polluted. In the aquifer, the deoxidization of chromium (VI) and arsenic(V) was almost the same at different time. With the increase of time, the relative chromium and arsenic concentrations of residual fraction in solid phases were reduced gradually. The relative chromium concentration of Fe-Mn oxide fraction and the relative arsenic concentration of exchangeable fraction were all increased.
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