The Kinetic Mechanisms Of Arsenic Oxidation And Adsorption In Natural Ferruginous Manganese Ore/Water Interface

Arsenic?As?,a toxic metaloid,can be accumulated in organisms through the food chain and causes serious harm.In aquatic environment and soils,arsenite?As????is more mobile than arsenate?As?V??,and is not easily adsorbed and removed.For humans,the toxicity of arsenite rsenite is more toxic than arsenate..Therefore,the oxidation of As???could affect not only the arsenic migration and transformation in environment,but also the bioavailability and toxicity as well as the removal efficiency of arsenic.Natural ferruginous manganese ore?FMO?that is abundant in China is a natural oxidant of As???due to its low zero charge,strong reactivity and high porosity,and has a bright prospect in development and unitilization of new environmental mineral materials.This study mainly focus on the kinetic mechanisms of As???oxidation and adsorption by natural Fe-Mn ore as well as the effects of different impacting factors.The main results were summarised as follows.1.Analysis of chemical and physical properties of FMO showed that three natural FMOs with their Mn/Fe ratio of 31.94,4.88 and 2.2 had a great difference in PZC?3.5-6.4?,but a little difference in AOS?3.4-3.8?and pH?6.8-7.0?.Both MnO2% and AOS of H-birnessite were higher than those of todorokite,while their PZC and pH values were opposite.The specific surface area,pore volume and average pore size of the two synthetical Mn ores were larger than those of three natural FMOs,especially the pore volume being six times higher.2.A batch experiment was carried out to understand the kinetic characteristics and its influencing factors of As???oxidation.The oxidation capacitiy of As???for five Mn ores ranked as 4# H-birnessite?70.2%?> 1# low Fe/Mn FMO?49.2%?> 3# high Fe/Mn FMO?32.8%?> 2# moderate Fe/Mn FMO?26.8%?> 5# todorokite?24.1%?.And the adsorption capacity were 4# H-birnessite?58.18%?> 5# todorokite?50.42%?> 3# high Fe/Mn FMO?40.62%?> 2# moderate Fe/Mn FMO?36.45%?> 1# low Fe/Mn FMO?29.74%?.The oxidation rates of As???increased with the increase of pH.3.The mechanisms of arsenic oxidation and adsorption in FMO/water interface were studied by using XRD,FTIR and XPS.XRD spectra analysis showed that there was no new crystalline materials formed with oxidating and adsorpting.FTIR spectra analysis showed that the stretching vibration of M-O-As appeared at 856 cm-1,indicating arsenate being formed.XPS showed that the Mn could oxidize As???to As?V?,while the valence of Mn was reduced from Mn?IV?to Mn???/Mn?II?and the valence of Fe was not change in the process of oxidation and adsorption of As???.The results suggested that Mn played a dominant role in oxidation and Fe only played a role in adsorption.4.The regeneration of natural FMO after adsorption of arsenic was studied by means of adsorption and desorption experiment.The desorption rates were NaOH> NaHCO3> NaCl.Based on the desorption rate of 59% 94% by NaOH,it was suggested that FMO had a strong regeneration.5.Adding Na₄P₂O₇ in the solution promoted the oxidation of As???significantly in 2# natural FMO and 5# synthetical todorokite.Result showed that the oxidation rates of As???was increased by 1.31 times and 2.67 times respectively.This was due to the Na₄P₂O₇ complexed Mn???and formed Mn???-pyrophosphate which was back to the solution,and then provided more oxidation and adsorption sites to greatly contribute the continuation of oxidation reaction.