The Desorption Of Heavy Metals Adsorpted On Iron Oxides And Soil Colloids By EDTA And EGTA

Heavy metal contamination in soils in China is becoming serious in recent years.The geochemical behaviors of heavy metals in soils are controlled by their interaction with soil components through a series of physical-chemical reactions,incluing adsorption,complexation,precipitation,and substitution in various types.Finally,they exist in soils in different forms with different components,which determine their mobility,bioavailabity and thus toxicity.As soils in southern China contain a relative high amount of iron oxides that are highly reactive with respect to heavy metal pollutants,knowledges on how heavy metals associate with iron oxides provide a significant foundation for evaluating the metal environmental mobility,transportion and toxicity in environmental systems.Our previous studies showed that,more than 50% of the heavy metals is associated with crystalline iron oxides in polluted soils around the mining areas in Guangdong Province,but among these heavy metals the relative propertions in two different binding ways,i.e.adsorption v.s.substitution,are still unknown,and it is interesting and necessary for a further study to develop the method to distinguish and quantify them.This will greatly benefit in understanding the geochemical processes of heavy metals during the formation and transformation of soil iron oxides when environmental conditions change.In the present study,isothermal adsorption of several metals(Pb,Cu,Zn,Cd)on goethite and hematite at the environmental related pH conditions(5.0 and 6.5)were conducted,and then EDTA and EGTA were used to desorb heavy metals adsorbed on synthetic iron oxides and soil colloid samples.The results obtained were adopted to distinguish the adosorbed metals from that incorporated into mineral lattices,which will further enrich the contents in the heavy metal speciation method,and will provide deep insights into the potential environment risks of the metals associated with soil metal oxide minerals desorbent.The main results are listed as following:1.The maximum adsorption amounts of heavy metals on the synthesized goethite and hematite decrease in order: Pb2+ > Cu2+ > Zn2+ > Cd2+,and hematite adorbs more Pb2+,Cu2+ and Zn2+ than goethite,but less Cd2+.2.The solution pH value has an important impact on the adsorption kinetics and capacity.For example,the adsorption amounts of Pb2+,Cu2+,Cd2+ on goethite and hematite were greatly decreased and the adsorption reaction is hard to reach equilibrium at acidic conditions,with the exception of Zn2+.At a pH of 6.5,the maximum adsorption capacities of Pb,Cu,Zn,Cd on goethite were 22.2 mg/g,8.7 mg/g,3.2 mg/g and 2.5 mg/g,whereas those for hematite were 27 mg/g,17 mg/g,3.9 mg/g and 0.9 mg/g respectively.3.The removal efficiency of metal desorbing from goethite surfaces by EDTA decreased in the order: Pb?Cu>Zn>Cd.The remove rate of Pb2+ by EDTA firstly increased with increasing Pb2+ concentration and subsequently decreased,while that of slightly increased with increasing Zn2+ concentration.The removal efficiency of these metals from hematite surfaces by EDTA decreased in order: Pb>Cu>Zn>Cd,especially for almost no Cd2+ was desorbed.The desorbent concentration of had little effect on Pb2+ and Cd2+ desorption,while first increased and then decreased the removal of Cu2+ from hematite surfaces and slightly improved that of Zn2+.4.The efficiency of EGTA in desorbing these metals on goethite surfaces decreased in order: Cu>Pb>Zn>Cd.With increasing the desorbent concentration,the desorption efficiencies for Pb2+ and Cu2+ first increased and then decreased,while that for Zn2+ increased and that on Cd2+ almost constant.The removal efficiency of EGTA for four metals from hematite surface decreased in order: Cu>Pb>Zn>Cd,and were less effective than those on goethite surfaces.With increasing the concentration of desorbent,desorption efficiency for Pb2+ and Cd2+ did not change while for Zn2+ decreased and that for Cu2+ increased.Compared to EDTA,EGTA is more effective to desorb adsorbed metals on crystallized iron oxides,and it has the strongest desorption capacity on Cu2+ and the weakest on Cd2+ among these four metals.5.The desorption efficiency of heavy metals on the soil colloid samples by EDTA/EGTA was affected by the total amounts and species of the heavy metals,and desorption efficiencies by two chelants are very close in most conditions studied.Owing to the fact that part of heavy metals was in residual,the metals desorbed from soil colloids was much smaller than those from synthesized iron oxides.When the amount of Pb,Cu and Zn in residual was high in soil colloid samples,heavy metals desorbed were mainly from the residual part.Cd in the soil colloids was also hard to be desorbed.6.Based on the desorption amount of heavy metals from soil colloids and synthetic iron oxides,the proportion of heavy metal substitution in the lattice of soil iron oxidescan be evaluated.This is especially useful for Cd,which was mostly associated with crystalline iron oxides,However it should be kept in mind that this kind of evalution is subjected to the effect of the amount of heavy metals in the residual specie and their stability.