Study On The Oxidation Kinetics And Influencing Factors Of Sulfide By Manganese Oxide Minerals With Different Structures

As one of the most important component in soil, manganese oxide mineral has some excellent properties, such as adsorption, oxidation and catalysis, and has always been focused on its environmental attribute. High-concentration sulfides do harm to the health of human and animals directly for its toxicity, corrosivity and disagreeable odor. People draw more and more attention to the chemical behaviour and transformation of sulfide in contaminated environment. It will provide us basic data for illustrating the transport and transformation law of manganese oxide minerals and sulfide by studying their redox process, which also help us understand the theoretical knowledge of chemical behaviour of manganese oxide minerals and sulfide in environment.There are a variety of manganese oxide minerals for different crystal structures, and their redox characteristics are different for different chemical structures. In this work, different manganese oxide minerals with different structures, such as basic birnessite (Bir-OH), acidic birnessite (Bir-H), cryptomelane (CRY) and todorokite (TOD) were synthesized in the lab. X-ray diffractometer (XRD), scanning electron microscope (SEM) and ion chromatography (IC) and spectrophotometer were used to characterize the crystal structure of minerals and solid products, concentration of S2-, SO32-, S2O32- and SO42-during the process. The oxidation mechanism and kinetics of S2- by different manganese oxide minerals were investigated in simulated environment. The effect of reaction conditions, such as mineral structure, amount of manganese oxides, pH values and temperature, on the oxidation process and kinetics was studied systematacially. The main conclusions are as follows.1. Mn(OH)2 and MnOOH are the main solid products when S2- is oxidized by different manganese oxide mineral. It was observed that the solid reduction product of Bir-OH, Bir-H and cryptomelane was Mn(OH)2, and Mn(OH)2 could be oxidized to Mn3O4 in air. The reduction products of todorokite were Mn(OH)2 and MnOOH, and the latter was major product. It could be duduced that the reduction products of manganese oxide minerals were influenced by the chemical stability of mineral structure. Todorokite was rather stable, and the main solid product is MnOOH with higher valence state of manganese.2.S,SO32-,S2O32- and SO42- were the products of S2- oxidized by manganese oxide minerals,and elementary substance S was the main form of oxidation product. During early period of reaction,S2- was mainy transformed into S with a small amount of SO32-, S2O32- and SO42-. The maximum values of total percentage of S2- transformation into SO32-, S2O32- and SO42- were 14.53%,15.45%,13.36% and 25.25% when Bir-OH,Bir-H, CRY and TOD were applied as oxidants respectively. When the reaction was last for 7-14 d, elementary substance S was continuously oxidized into SO42- and S2O32-. For the oxidation dissolve of S,the characteristic peaks of corresponding manganese oxide mineral became stronger again in XRD patterns.3.The oxidation rate of S2- in the initial period followed a pseudo first-order kinetic law when S2- was oxidized by above manganese oxide minerals. Under different conditions,the oxidation capacity of above minerals were in the sequence of Bir-H> TOD>CRY>Bir-OH.The content of Mn(Ⅲ) in manganese oxide mineral and the amount of active Mn(Ⅲ)that could be bridged by dissolved sulfide were the main factors influencing the oxidation capacity of manganese oxide minerals. Manganese oxide mineral(0.1g) was adequately added in sodium sulfide solution with the concentration of sulfide about 200 mg/L(pH 12) at room temperature(20℃),and the apparent reaction rate constant Kobs was in order as Bir-H (0.4627 min-1)>TOD(0.3731 min-1)>CRY(0.2203 min-1)>Bir-OH(0.1888 min-1).4.Reaction solution temperature,pH value and the amount of minerals had some effect on the oxidation rate of S2-. The oxidation rate of sulfide could be enhanced by improving the amout of manganese oxide minerals. It had different influence on the oxidation rate by elevating reaction temperature for different minerals.Oxidation rate of S2- had no significant changes when it was oxidized by Bir-H and TOD with increasing solution temperature from 20℃to 30℃,however,it increased remarkably at 40℃.Oxidation rate of S2- by todorokite increased with the increase of reaction temperature.The amplitude of the increase of oxidation rate of S2- by Bir-OH was higher than that of oxidation rate of S2- by todorokite,and the oxidation rate of S2- was higher than that of by cryptomelane at 30℃.In the alkaline solution,pH values had no obvious influence on the oxidation rate of S2- when above four minerals were applied. Usually, the reaction rate increased with the decrease of pH. The apparent reaction rate constant Kobs was up to a maximum value when S2- was oxidized by todorokite with pH 10. On the whole, the oxidation capacity of Bir-H and TOD were higher than that of CRY and Bir-OH.