Study On Effects Of Surfactants And Organic Matter On Heavy Metal Migration In Ferrihydrite

In recent years,industry has developed rapidly,but the development is accompanied by some environmental pollution problems such as heavy metal pollution,the control of heavy metal pollution is urgent.Heavy metals in the environment exist in various forms,such as hexavalent chromium（Cr（VI））in the form of oxygen-containing anions.Cr（VI）is a common heavy metal,which is harmful to human body and has strong toxicity and translocation in the environment.In addition,some heavy metals also exist in the form of complexes,such as heavy metal-EDTA complex formed by the complex of heavy metals and EDTA.These complexes were toxic and increased the migration capability and removal difficulty of heavy metals.The fate of these heavy metals in the environment is often related to ferrihydrite,which is due to the fact that the existence of ferrihydrite in the natural environment and engineering environment is extensive and has good adsorption capacity for various pollutants.In addition,ferrihydrite is easy to transform into other iron oxides with higher crystallinity,and the heavy metals on the ferrihydrite are likely to migrate during the transformation process.Therefore,the study on the influence of migration behavior of heavy metals during the transformation process of ferrihydrite can be used as a reference for the treatment of heavy metals with iron oxides.In this study,the migration behavior of Cr（VI）and metal-EDTA complexes（Ca（II）-EDTA and Ni（II）-EDTA）during the transformation of ferrihydrite was studied.Meanwhile,the influence of some environmental factors,such as p H value,Mn（II）,phytic acid and widely used surfactants on the migration behavior of Cr（VI）,and the migration behavior of Ca（II）-EDTA and Ni（II）-EDTA during the transformation process were studied.The study revealed the fate of Cr（VI）during the conversion of ferrihydrite,and the interaction of different metal-EDTA complexes with ferrihydrite and the migration behavior of complexes.At the same time,the solid phase in the transformation process was characterized by X-ray diffractometer,transmission electron microscope,scanning electron microscope,Fourier transform infrared spectroscopy,X-ray energy spectrometer and X-ray photoelectron spectroscopy.In this study,the migration behaviors of Cr（VI）and metal-EDTA complexes（Ca（II）-EDTA and Ni（II）-EDTA）under the influence of ferrihydrite transformation and some environmental factors were analyzed.The main conclusions drawn from this study were as follows:（1）Different surfactants have different effects on the transformation of ferrihydrite.The surfactant SDBS significantly hindered the conversion of ferrihydrite,while SDBS hindered the aggregation of ferrihydrite particles and thus hindered the formation of hematite.With the increase of SBDS concentration,the hindering effect decreased and more goethite was formed,which was attributed to SDBS promoting the dissolution of ferrihydrite and facilitating the formation of goethite.Besides,the CTAB did not affect the conversion rate of ferrihydrite,but the presence of CTAB favored the formation of hematite.（2）Different surfactants have different effects on the migration behavior of Cr（VI）during the conversion process.The anionic surfactant SDBS was not conducive to the fixation of Cr（VI）on ferrihydrite,because SDBS on ferrihydrite increased the negative charge on its surface and competed with the adsorption sites of Cr（VI）on ferrihydrite.The migration of Cr（VI）during transformation was slowed in the presence of SDBS,because the transformation of ferrihydrite was hindered by SDBS.On the contrary,the presence of cationic surfactant CTAB was beneficial to the fixation of Cr（VI）on ferrihydrite and facilitated the entry of Cr（VI）into the interior of ferrihydrite by promoting the aggregation of ferrihydrite particles.（3）Phytic acid affected the migration behavior of Cr（VI）.Phytic acid significantly hindered the fixation of Cr（VI）by ferrihydrite,and with the increase of phytic acid concentration,the effect of ferrihydrite to fix Cr（VI）became worse.This was because phytic acid itself has multiple phosphate groups that compete with Cr（VI）for adsorption sites on ferrihydrite.In addition,although phytic acid significantly hindered the transformation of ferrihydrite,it cannot hinder the migration of Cr（VI）,Cr（VI）still migrated from the solid phase to the liquid phase during the transformation process.（4）Mn（II）effectively offset the negative effect of phytic acid on the fixation of Cr（VI）in ferrihydrite.Ferrihydrite can promote the oxidation of Mn（II）.The oxidized Mn（II）was easy to precipitate and formed co-precipitation with Cr（VI）during the precipitation process,which facilitated the fixation of Cr（VI）on the ferrihydrite.Besides,no Cr（VI）in solid phase migrated to the liquid phase in the presence of Mn（II）during the ferrihydrite conversion process.（5）Different metal-EDTA complexes have different effects on the transformation of ferrihydrite.Ca（II）-EDTA promoted the dissolution of ferrihydrite and thus facilitated the formation of goethite.Ca（II）-EDTA has a small stability constant and it was easy to exchange with Fe（III）on ferrihydrite,resulting in the dissolution of ferrihydrite.However,Ni（II）-EDTA has an inhibitory effect on the conversion of ferrihydrite,Ni²⁺decomplexed from Ni（II）-EDTA inhibited the conversion of ferrihydrite.（6）The transformation of ferrihydrite affected the speciation and migration of metal ions in Ca（II）-EDTA and Ni（II）-EDTA complexes.Metals in Ca（II）-EDTA and Ni（II）-EDTA complexes exchanged with Fe（III）in ferrihydrite to become free metal ions.The existence of Fe（III）-EDTA proved that the process of metals in metal-EDTA complexes exchanged with Fe（III）in ferrihydrite.Besides,the transformation of ferrihydrite caused different forms of metal to migrate into solution.