The Effect Of Citric Acid And Humic Acid On Abiotic Interface Oxidation Of Mn (â…¡)

Manganese（oxyhydr）oxides are widely distributed in various surface environments,and more than 30 different types of Manganese（oxyhydr）oxides oxides have been found so far.Manganese（oxyhydr）oxides play an important role in heavy metal cycling and in redox reactions that occur in the environment.It is well known that varying pathways of Mn（Ⅱ）oxidation,including homogeneous oxidation,surface catalyzed oxidation,and microbial processes,are responsible for the diversity of Mn（oxyhydr）oxide formation.However,in the abiotic Mn（Ⅱ）oxidation process,the homogenous oxidation kinetics of Mn（Ⅱ）is extremely slow.The oxidation of Mn（Ⅱ）can be accelerated when there is catalytic interface or illuminated photosensitizer.Due to the similarity of crystal chemistry properties of Fe and Mn,they tend to coexist in soil,sediments and marine nodules in the natural environments.Therefore,the study of abiotic Mn（Ⅱ）oxidation by using iron oxides as the catalytic interface is likely to be the important pathway for the abiotic transformation of Mn（Ⅱ）in the environment.However,the environmental system is very complex.Natural organic matter,such as small molecular organic acids（citric acid）,large molecular natural organic matter（humic acid）,etc.,are widely distributed in various environments and play an important role in regulating the abiotic oxidation of Mn（Ⅱ）.In addition,humic acid,as colored dissolved organic matter（CDOM）,can activate molecular oxygen to generate oxygen active species（ROS）under the radiation of sunlight.These ROS can greatly promote the abiotic Mn（Ⅱ）oxidation process.This photochemical process might make the abiotic Mn（Ⅱ）oxidation pathway in the environment re-emphasized by researchers.Therefore,the subject uses ferrihydrite with weak crystallinity,large specific surface area and ubiquitous in environment as the catalytic interface,and uses citric acid,oxalic acid,ethylenediaminetetraacetic acid and humic acid as organic ligands to study the effect of organic ligands on the abiotic Mn（Ⅱ）oxidation.We use X-ray diffraction spectroscopy（XRD）,transmission electron microscopy（TEM）,field emission scanning electron microscopy（FE-SEM）,X-ray photoelectron spectroscopy（XPS）,Fourier transform infrared spectroscopy（FT-IR）,etc.and macroscopic experiments to explore:1)the effect of some common organic ligands in the environment on the kinetics and oxidation products of Mn（Ⅱ）oxidation on the surface of ferrihydrite;2)The effect of HA on the oxidation of Mn（Ⅱ）under light radiation,and the interaction between HA and the formed Mn oxides.The main conclusions are as follows:1.The effect of citric acid on the catalytic oxidation of Mn（Ⅱ）on the surface of ferrihydrite is studied.When the concentration of citric acid is less than 1 m M,citric acid inhibits the oxidative removal of Mn（Ⅱ）;When the concentration of citric acid is greater than 1 m M,citric acid promotes the oxidation of Mn（Ⅱ）,but still continues to inhibit the removal of Mn（Ⅱ）.The identification of oxidation products at the gradient concentration of citric acid can be obtained as follows:when the concentration of citric acid is greater than 1 m M,the oxidation products of Mn（Ⅱ）are mainly concentrated in the supernatant,and may exist in the form of Mn（III）CIA or Mn（III）CIA_ox complex.When the concentration of citric acid is less than 1 m M,the Mn（Ⅱ）oxidation products are mainly enriched on the surface of ferrihydrite.The oxidation products of Mn（Ⅱ）catalyzed by fresh ferrihydrite are mainly manganite,groutite and a small amount of hausmannite.Compared with the system without citric acid,the crystallinity of Mn（Ⅱ）oxidation product is weakened with the addition of citric acid,and only the characteristic peak of hausmannite appears on XRD pattern.However,the Mn AOSs results obtained by chemical titration and XPS fitting increase with the increase of the concentration of citric acid,indicating that the addition of citric acid might promote the oxidation of Mn（Ⅱ）to the higher valence Mn oxides.The effects of citric acid,ethylenediaminetetraacetic acid,oxalic acid and humic acid on the catalytic oxidation of Mn（Ⅱ）by ferrihydrite are compared.These organic acids have different inhibitory effects on the catalytic oxidation of Mn（Ⅱ）by ferrihydrite,and the order of inhibition is as follows:citrate>ethylenediaminetetraacetic acid>oxalic acid>humic acid.The order of inhibition may be related to the amount of organic acid adsorbed by ferrihydrite.2.We have investigated the pathway and mechanism of HA-mediated photochemical oxidation of Mn（Ⅱ）under ambient conditions.The results show that irradiant HA can promote oxidation of Mn（Ⅱ）and oxidation kinetics increases with increasing HA concentration.The major oxidants responsible for this reaction appear to be photoproduced superoxide radical anion（O₂^-）and singlet molecular oxygen（¹O₂）,among which O₂^-directly oxidize Mn（Ⅱ）,and ¹O₂ is the intermediate for the formation of O₂^-.The removal of Mn（Ⅱ）is inhibited at higher concentration of HA.The reason is that due to the electrostatic and steric effects of HA,the crystallization and growth of Mn（III,IV）oxides newly formed on the surface of HA colloid are inhibited,resulting in the formation of colloidal products.The morphology and structure characterization also indicate that the resulting products are highly disordered Mn（III,IV）oxides nanoparticles,possibly including bixbyite,hausmannite,manganite,feitknechite andδ-Mn O₂.Due to the high reactivity of Mn（III,IV）oxides generated in situ,part of HA is agglomerated and co-precipitated,and another part is degraded and mainly emitted as CO₂.These results suggest that photo-chemical Mn（Ⅱ）oxidation mediated by HA is an important pathway for abiotic formation of Mn（oxyhydr）oxides in surface environments.