Catalytic Oxidation And Adsorption Mechanism Of Cr/As In The Presence Of Soil Iron-manganese Nodules

Iron-manganese nodules,as one of the main deposition forms of manganese oxide,have both the adsorption and oxidation characteristics for the presence of iron and manganese oxides,therefore they significantly affect the formation,migration,transformation and bioavailability of various micronutrients and pollutants in soils.Mn???exists in the environment where iron-manganese nodules are formed.Mn???can be adsorbed on the surface of iron-manganese nodules and undergo redox reaction to deposite on the mineral surface,which in turn affects the migration and transformation of pollutants such as Cr???and As???.There have been more studies on manganese oxides and Mn???,the oxidation of Mn???on the surface of manganese oxides will affect the surface reactivity of minerals,and even affect the crystal structure of manganese oxides.In this work,the catalytic oxidation process of Mn???on the surface of iron-manganese nodules and the possible reaction pathway of manganese?hydrogen?oxide formation were studied.Then,the interaction mechanism between Cr???and natural iron-manganese nodules was studied under oxic conditions.Finally,the interaction process between iron-manganese nodules and As???under ultraviolet light was investigated.The main results are as follows:1. During the interaction between iron-manganese nodules and Mn???,the effects of different Mn???concentrations,p H values and dissolved oxygen concentration were further investigated.The results showed that Mn???was mainly adsorbed on the surface of iron-manganese nodules at p H 4.0 and 6.0.At p H 8.0,iron-manganese nodules could adsorb and oxidize Mn???.The oxidation rate of Mn???increased with increasing p H.The single Mn???solution had no obvious oxidation at p H 8.0 oxygen atmosphere.The consumed Mn???increased with the increase of dissolved oxygen concentration during the reaction between iron-manganese nodules and Mn???,so it was shown that iron-manganese nodules had a catalytical effect on Mn???oxidation.The adsorbed Mn???on the surface of iron-manganese nodules was oxidized and transfered electrons to O₂on the mineral surface,likely forming Mn???oxides,however,it needs to be further characterized due to their complicated crystal structure and chemical composition of nodules.No new mineral phase was formed after the reaction of iron-manganese nodules for 30 days,but the crystal structure changed to some extent during the reaction.With increasing dissolved oxygen concentration,it promoted the oxidation of Mn???,and helped to strengthen the crystal structure of minerals.2. During the interaction between iron-manganese nodules and Cr???under oxic condition,the effects of chemical composition,dissolved oxygen concentration,p H and ionic strength were further investigated.The results showed that iron-manganese nodules could effectively oxidize dissolved Cr???,and most of the newly formed Cr???was adsorbed on the surface of iron-manganese nodules.In iron-manganese nodules,manganese oxides mainly contributed to Cr???oxidation,and iron oxides facilitated the adsorption and immobilization of Cr???.In addition,Cr???could be catalytically oxidized to Cr???on the surface of manganese oxides through the generation of Mn???intermediate or Mn?IV?oxides from released Mn???under oxic conditions.At constant p H 5.0 in air,the adsorption capacity of Cr???by iron-manganese nodules reached 30.7mg g^?1;the highest oxidation capacity was 1.3 mg g^?1;and the adsorption capacity of the generated Cr???reached 0.8 mg g^?1.Oxygen accelerated the oxidation of Cr???by iron-manganese nodules under oxic conditions.The oxidation rate rose from 2.2 to 2.8 mg L^?1with increasing dissolved oxygen concentration from 0.3 to 40 mg L^?1.The oxidation rate of Cr???by the nodules decreased with increasing p H from 2.0 to 8.0,and increased with increasing ionic strength.3. During the interaction between iron-manganese nodules and As???under UV light, the effects of different bands of light,dissolved oxygen concentration,p H values??on the oxidation of As???by iron-manganese nodules were further studied.The results showed that iron-manganese nodules can adsorb and oxidize As???,and the light of far-UV promote the As???oxidation of and the release of Mn???from iron-manganese nodules.The oxidation capacity of As???by iron-manganese nodules increased with increasing p H.At p H 5.0,7.0,and 9.0,20%,50%,and 75%of As???are oxidized,respectively.With increasing dissolved oxygen concentration,the amount of oxidized As???increased,while the amount of released Mn???almost remained stable.During the photoreaction,As???on the surface of minerals generated a variety of reactive oxygen species(OH^·,O₂^·-,etc.),which further promotes the oxidation of As???.The presence of radical scavenger affected the oxidation of As???,but the release of Mn???was not affected too much.The oxidation of As???and the release of Mn???under UV condition should be two relatively independent processes.The oxidation of As???is mainly due to the generation of free radicals under far-UV light,and the release of Mn???is mainly due to the photoreduction of iron-manganese nodules under far-UV conditions.Soil iron-manganese nodules taken from the natural environment were used as adsorbents and oxidants to study their adsorption,oxidation and catalytic oxidation behaviors of heavy metals.It improves to understand the reaction process of Mn???during the formation of natural iron-manganese nodules,and reveals the migration and transformation mechanism of Cr???and As???by iron-manganese nodules,providing basic data for their environmental behaviors in soils.