| In order to predict the environmental effect of heavy metals in calcareous purple soil r egion,alleviate the pollution of soil, improve soil quality, the calcareous purple soil from M ingshan River were selected as object of the study.In condition of adding different concentr ations of citric acid, using the method of equilibrium adsorption and desorption by citric a cid,HCL, investigate the effect of citric acid on the characteristics of Pb2+ adsorption-desorpt ion in bulk soil and different size fraction micro-aggregates(2-0.25mm,0.25-0.053mm,0.053-0.002mm,<0.002mm),affects, the influences of organic matter and calcium carbonate on the i mpact characteristics of citric acid.Ultimately determining the appropriate concentrations of citric acid to provide reference for improving the efficiency of phytoremediation in heavy metal contaminated soils, thus laid the foundation for the sustainable development of agricult ural production. The main conclusions are as follows:(1)In condition of different concentrations of citric acid, the adsorption quantities of P b2+ by the bulk soil and different size fraction of the micro-aggregates were increased wit h the increasing initial concentration of Pb2+, the adsorption capacities relation of bulk soil and the different size fraction micro-aggregates aggregates is (<0.002mm)>(2-0.25mm)>the bulk soil>(0.053-0.002mm)> (0.25-0.053mm). In different initial concentration of Pb2+, adso rption capacities show a single peak curve, the adsorption amount reached the highest valu e in citric acid(0.1mmol·L-1), after the peak, adsorption capacities decrease with citric acid concentration incerasing, and the adsorption capacities reach the lowest value in citric aci d (100mmol·L-1). After removal of organic matter or calcium carbonate,the adsorption charac teristics are similar to untreated soil,adsorption curve is a similar single peak curve;the adsor ption capacities decrease than the untreated untreated soil, the decreasing adsorption amount in creases with increasing concentration of citric acid.(2) Using Langmuir, Freundlich and Temkin equations to fit the isothermal adsorption pr ocess of untreated soil, organic matter removal soil and calcium carbonate removal soil. F reundlich equation was the most fitable to the adsorption, what shows that the adsorption o f Pb2+ in bulk soil and particle size microaggregates is Multilayer adsorption. Equation "n" indicates the adsorption isotherm linear degree, "n" of untreated soil is closer to the "l" tha n calcium carbonate removal soil and organic matter removal soil.After removal of organi c matter or calcium carbonate, the adsorption amount and the initial concentration of Pb2+ ar e not close to the linear positive correlation.The equilibrium constant "KF " reflects the com bination of strength and adsorption capacity, The relationship among"K.F " in bulk soil and p article size microaggregates of untreated soil, organic matter removal soil and calcium carbon ate removal soil are consistent with the organic matter, free iron oxide content and CEC . With the maximum KFincreasing rule,citricacid (100mmol L-1)<(10mmol L-1)< (Ommol L-1 )< (1mmol L-1)< (0.1mmol, L-1), and consistent to the maximum adsorption capacity ofXm, after the removal of organic matter or calcium carbonate"KF" decreased respectively. Afeter removal of organic matter or calcium carbonate, Pb2+ distribution coefficients" Kd"of bulk so il and microaggregates decrease which indicates the soil adsorption capacity decreases.(3) Using the same concentration citric acid as desorption agent which is in correspon ding with adsorption conditions, the citric acid desorption and adsorption quantities of bulk soil and particle size microaggregates in untreated soil, organic matter removal soil, calcium carbonate removal soil present the consistent relation:0.25-0.053mm<0.053-0.002 mm< bul k soil< 2-0.25 mm (<0.002 mm). The citric acid desorption of untreated soil increases with increasing Pb2+ initial concentration, and in initial Pb2+ concentration of 190 mg·L-1, citric a cid (100 mmol L-1) reached the maximum, when Organic matter was removed, in citric aci d (0-1 Ommol·L-1) interval, desorption with citri cacid concentration increased, after peaking, cit ric acid(100mmol·L-1) desorption began to decline, in the citric acid(10mmol·L-1), initial Pb2+190mg·L-1 reached the highest value.When calcium carbonate was removed, citric aci d desorption curve and the removal of organic matter of the desorption curve showed the sa me trend. The citric acid desorption rate of untreated soil increases with increasing initial Pb +and citric acid concentration, the relationship of desorption rate among bulk soil and par ticle size microaggregate is 0.25-0.053 mm>0.053-0.002 mm> bulk soil>2-0.25 mm> (<0.002m m),which is in contrast with the desorption characteristics. After removing organic matter or calcium carbonate respectively,desorption rates of bulk soil and microaggregates increase obviously, the increasing amount are significantly increased with the raising concentration of citric acid, in the citric acid (100mmol·L-1) peak, electrostatic and outer complex adsorptio n of Pb2+ to total adsorption capacity increases, non specific adsorption ratio is more large, inn er sphere complexation adsorption decreases, firmly adsorbed were decreased, and as the initi al Pb2+ concentration increased,the non specific adsorption rate increasing trend is more obvio us.(4)Using the HCl as desorption agent, desorption characteristics show differently that in bulk soil and microaggregates of untreated soil samples, organic matter removal soil sa mples and calcium carbonate removal soil samples. The desorption amount in bulk soil an d microaggregates of untreated soil are positively correlated to CEC, organic matter, calcium carbonate and free iron oxide content:(<0.002 mm)>2-0.25 mm>0.25-0.053 mm>0.053-0.00 2 mm. HCl desorption curve showed a peak type curve, the desorption amount reach the hig hest value in citric acid 0.1mmol· L-1,after the peak, the desorption amount decrease with the increasing concentration citric acid in adsorption test. After removal of organic matter , HCl desorption amount decreases with the increase of the concentration of citric acid, and n egatively correlated with increasing initial Pb2+concentration; HCl desorption rate decrease s obviously, the reduction amount histogram of desorption rate is valley type curve with the increasing citric acid desorption concentration, the HCl desorption rate in citric acid (0.1mm ol·L-1) reach the lowest; the inner chelate absorption of Pb2+percentage in total adsorption c apacity decreased significantly, and specific adsorption rate decrease. After removing calcium carbonate, HCl desorption shows trend that increased firstly and then decreased with increas ing citric acid concentration, and increasing trend is more obvious with increasing initial Pb2 +concentration; the HC1 desorption rate increase, in different initial concentration of Pb2+, H Cl desorption rate increase amounts decrease with the increase of citric acid desorption conce ntration; specific absorption rate increase compared with the untreated soil samples. non desor ption rates are different among untreated soil, organic matter removal soil samples, calcium carbonate removal soil samples, size relations of non desorption rate is untreated samples > organic matter removal samples> calcium carbonate removal samples.(5)After removal of organic matter or calcium carbonate, adsorption-desorption Pb2+ c haracteristics of citric acid are different. Compared with organic matter removal soil samp les, the adsorption amount of calcium carbonate removal soil samples increases more obvi ously with the increasing concentration of citric acid, citric acid desorption reduction amount is greater,adsorption and desorption curve is more obvious.lt shows that the effect of calci um carbonate on citric acid adsorption and desorption is of greater intensity. Citric acid des orption rate increasing amount and non desorption rate reduction of calcium carbonate remo val soil samples is more greater than the organic matter removal soil samples. At the same time,after removing the calcium carbonate, HCl desorption rate of bulk soil and microaggr egates is on the rise than that of the untreated soil samples, specific adsorption rate increase , while the organic matter removal, HCl desorption rate of soil and microaggregates are low er than that of the untreated soil samples, specific adsorption rate decrease. |
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