Study On The Mechanisms Of Sorption-Desorption Of Heavy Metals And Its Application In The Irrigated Desert Soils From The Oasis Regions, Northwest China

Sorption -desorption of heavy metals is one of the primary processes that affects the fate and mobility of heavy metals in soils.Sorption-desorption play a dominant role in the plant nutrition and pollution control.For heavy metals such as Cu,Zn,Pb,Cd,Ni etc,these reactions have a large effect on their availability to plants and on the extent to which these heavy metals pass into the foood chain.Laboratory batch experiments,single extraction procedure,sequential extraction procedure,and simple correlation analysis were carried to study the sorption and desorption in the irrigated desert soil from the oasis in the arid regions,northwest China.These studies will provid the basic data for chemical behavior in the irrigated desert soils from the oasis,and will provide a scientific basis for the assessment of ecological risk and remediation of soils contaminated by heavy metals.The main results show as follows:(1)Total Cu content of soils used in the sorption-desorption tests for single heavy metal and Combined heavy metals ranged from 31.73 to 59.12 mg/kg;Total Zn content of soils ranged from 33.53 to 61.22 mg/kg;Total Pb content of soils ranged from 12.16 to 34.13 mg/kg;Total Cd content of soils ranged from 0.26 to 0.52 mg/kg;Total Ni content of soils ranged from 22.64 to 41.58 mg/kg.The concentration of Cu,Zn,Pb,Cd,and Ni were much less than Cu,Zn,Pb, Cd,and Ni limit ofâ…¡grade soil environmental quality in the the Environmental Quality Standard for Soils in China,indicating all the soils studied for sorption-desorption tests could be considered uncontaminated.(2)Under the conditions of a single heavy metal,the amount of heavy sorbed varied with soil properties,the concentration and type of heavy metal.The sequence of heavy metal adsorption capacity has been found for the irrigated desert soils in the order Pb＞Cu＞Zn. Sorpfion of Cu,Zn,Pb in the irrigated desert soils was well described by Freundlich sorption model;Sorption of Pb in the irrigated desert soils was also well described by Henry sorption model.Heavy metals sorption capacity of the irrigated desert soils is likely to be controlled by a combination of soil characteristics such as texture,pH,organic matter,CaCO₃,and CEC rather than a single factor such as pH Or organic matter.At low concentration,the percent of desorption of heavy metals in soils is smaller;At higher heavy metal additions,the percent of desorption of heavy metals in soils is larger.The sequence of heavy metal desorption percent was in the order Cu＞Zn＞Pb,indicating the sequence of heavy metal bioavailability is also in the order Cu＞Zn＞Pb.Secondary power function can be used to simulate the relationship between desorption amount of heavy metals and sorption amount before desorption in the irrigated desert soils.(3)Copper was distributed mainly in the residual fraction in the natural irrigated desert soils.The high adsorption capacity of copper and unreversible adsorption of copper(low exchangeable fraction)in the irrigated desert soils,indicating that copper can be very strong maintained by the irrigated desert soils,thus reducing the risk of groundwater and food chain contamination under the general conditions.At low copper concentration,the percent of Cu in organic fraction is larger than that of at higher copper additions,which indicates that copper gives priority to the organic matter in the irrigated desert soils.Copper gradually occupied the sorbing sites in other constituents of soil when the concentration of Cu increased.But, organically-bound Cu was not the largest Cu fraction in the irrigated desert soils,probably because the concentration of added copper is larger,and organic matter contents of the irrigated desert soils is low compared to temperate soils.In general,the percent of Cu in inorganic fraction is larger than the percent of Cu in organic fraction at the studied concentration of copper, probably because the CaCO₃ content and pH is high in the soils of add regions.The percent of Cu in residual fraction decreased with the increase of added copper concentration in the irrigated desert soil samples,indicating that it is difficult for added Cu to enter copper residual fraction. The anaount of copper transformed into residual fraction increased with the increase of added copper concentration.At higher metal additions most of the metals were extracted in their more mobile forms,which contrasted with their original partitioning in the soils,indicating the presence of larger numbers of surface active sites responsible for the retention of more mobile fractions than the number of high energy sites for retaining the more resistant fraction.(4)Under the conditions of coexistence of five heavy metals,Cu,Zn,Pb,Cd and Ni, adsorption and desorption of five heavy metals and total heavy metals in irrigated desert soils was well described by both Langmuir and Freundlich equations.According to the amount of heavy sorbed and sorption percentage at the largest concentration of added heavy metals in irrigated desert soils,the sequence on sorption capacity of Cu,Zn,Pb,Cd and Ni in irrigated desert soils was Pb＞Cu＞Cd＞Zn＞Ni;and total heavy metals sorption capability of irrigated desert soil profile samples were ranked as S_80-100cm＞S_40-60cm＞S_0-20cm＞S_60-80cm＞S_20-40cm. According to desorption amounts of heavy metals at the same sorption amounts,the sequenceof five heavy metals desorption capability in irrigated desert soils is,on the whole,in the order Ni＞Zn＞Cd＞Cu＞Pb.Heavy metals desorption capability of S_0-20cmand S_20-40cmis larger than other soil samples.Heavy metals sorption behaviors and desorption processes are closely related; Secondary power function can be used to simulate the relationship between desorption amount of five heavy metals and sorption amount before desorption in the irrigated desert soils.At the same equilibrium concentration,sorption amounts of five single heavy metals and total heavy metals based on sorption experiments were smaller than sorption amounts of five single heavy metals and total heavy metals based on desorption experiments.This suggests that sorption of Cu,Zn,Pb,Cd and Ni in the irrigated desert soils is not reversible.(5)Under the conditions of coexistence of five heavy metals,the salient features of the selectivity sequences for sorption on the basis of Kd100 were as follows:the selectivity sequences of heavy metals sorption in the irrigated soils were Pb＞Cu＞Cd＞Zn＞Ni;all soil samples sorbed a greater proportion of total Pb than of any other metal;the Kd100 value of Pb was more than four times that of Cu in each soil sample;the Kd100 value of Pb more than ten times that of Cu in S_60-80cmsoil sample.The sequences of affinity on the basis of Kd100 of heavy metals desorption in the irrigated soils were also Pb＞Cu＞Cd＞Zn＞Ni.The Kd100 value of Pb was more than four times that of Cu on the basis of heavy metals desorption in each soil sample;the affinity of Pb and Cu is far larger than other heavy metal.(6)Under the conditions of coexistence of five heavy metals,Copper was distributed mainly in the organic fraction in the irrigated desert soils at low added heavy metals concentration(25 mg/l),except for S_20-40cmsoil sample.At added high heavy metals concentration(100 and 200mg/l),most of Cu sorbed were extracted in their carbonate bound fraction in each soil sample.For each soil samples at different added heavy metals concentrations,zinc and lead sorbed were basically distributed mainly in the carbonate bound fraction and Fe-Mn oxides bound fractionin the irrigated desert soils;the amount of the carbonate bound fraction was greater than that of Fe-Mn oxides bound fractionin.Cd sorbed was distributed mainly in the carbonate bound fraction and exchangeable fraction in the irrigated desert soils;in other words, Cd added had hiher trend to be transformed into the carbonate bound fraction and exchangeable fraction in the irrigated desert soils,therefore,it is difficult to eliminate Cd pollution.Ni sorbed was distributed mainly in the carbonate bound fraction,but a considerable portion of Ni added had transformed into the residual fraction and exchangeable fraction.(7)In nine extraction procedures,generally,extraction capacity of Cu was as follow:EDTA＞Citric acid＞NH₄Ac＞TCLP＞NH₄NO₃＞MgCl₂≈Tartaric acid＞Acetic acid＞Oxalic acid;extraction capacity of Ni was as follow:TCLP＞Citric acid＞EDTA＞NH₄Ac＞Tartaric acid＞NH₄NO₃≈Acetic acid≈Oxalic acid＞MgCl₂.The correlatuin is significant between soil Cu concentration obtained by Citric acid and NH₄Ac extraction procedures and Cu concentration in the roots of the wheat plant,indicating Citric acid and NH₄Ac extraction procedures provided a good indication of bioavailability for Cu under evaluation.None of the studied nine extraction procedures may predict the bioavailability for Ni,which suggest the bioavailability for Ni may depend not only on soil heavy metal concentration,but also on soil characteristic,interface between soil and plant or other environmental factors affecting growth.