摘要
环境中的氧化锰矿物是可氧化Cr（Ⅲ）的唯一天然无机氧化剂,氧化锰矿物与Cr（Ⅲ）相互作用的反应速率与机制备受关注.本研究以水钠锰矿为对象,采用批量动力学方法研究了其结构中Mn（Ⅲ）在氧化Cr（Ⅲ）反应中的作用及动力学特点.结果表明,水钠锰矿氧化Cr（Ⅲ）符合准一级动力学方程,表观速率常数Kobs为0.0313 min-1,以Na4P2O7预处理水钠锰矿可络合出结构中的部分Mn（Ⅲ）,使其Mn的平均氧化度升高.当处理Na4P2O7浓度为10、20、50 mmol/L时,其Mn氧化度由3.50升高至3.63、3.73和3.78,处理后的水钠锰矿对Cr（Ⅲ）的平衡氧化率增加,但初始氧化速率变化并不明显,其相应表观速率常数分别为0.0351、0.0325和0.0309 min-1.水钠锰矿氧化Cr（Ⅲ）的反应历程中,Mn（Ⅳ）→Mn（Ⅲ）的电子转移过程生成的Mn（Ⅲ）显著影响水钠锰矿氧化Cr（Ⅲ）的速率.当新生成Mn（Ⅲ）被Na4P2O7络合后,反应速率降低45%-88%,且水钠锰矿的氧化度较低时,结构中Mn（Ⅲ）含量高,反应中被络合的新生态Mn（Ⅲ）多,反应速率降低幅度相对较大.因此,反应新生成的Mn（Ⅲ）具有较高的反应活性和较快的电子转移速率,而Mn（Ⅳ）→Mn（Ⅲ）的电子转移速率较慢,可能为水钠锰矿氧化Cr（Ⅲ）的反应速率控制步骤.
Cr（ Ⅲ ） could be oxidized only by manganese oxide minerals as natural inorganic oxidants in nature, and so the rate and mechanism of interaction between manganese oxide minerals and Cr（ Ⅲ ） were widely concerned. The effects of Mn（ Ⅲ ） in bimessites, the most common Mn oxide mineral in the environment, on the rate of Cr（ Ⅲ ） oxidation with birnessites and the kinetic characteristics were investigated through hatch kinetic technique. The results show that Cr（ Ⅲ ） oxidation rate follows a pseudo-first-order reaction, and the apparent rate constant Kobs is 0.031 3 min^-1 when the average oxidation state （AOS） of Mn is about 3.50 in birnessite. When the birnessite is pretreated with Na4 P2 07 solution, and the Mn（ Ⅲ ） can be eomplexed out from the solid oxides. Therefore the content of Mn（ Ⅲ ） in the hirnessites decreases and the AOS of manganese increases. The AOSs of Mn for the pretreated birnessites increase from 3.50 to 3.63, 3.73 and 3.78 when the concentrations of Na4P207 are about 10, 20 and 50 mmol/L respectively. The Mn（ Ⅲ ） content does not affect the initial oxidation rate of Cr（ Ⅲ ） markedly, although oxidation amount of Cr（ Ⅲ ） increases with the AOS of Mn. The apparent rate constants for the corresponding pretreated birnessites are 0.035 1, 0.032 5 and 0.0309 min^-1 respectively. The oxidation rate of Cr（ Ⅲ ） is markedly influenced by the amount of Mn（Ⅲ） produced in the transformation process of Mn（ Ⅳ ）→Mn（ Ⅲ ） . The newly formed Mn（ Ⅲ） is eomplexed by Na4P207 and the oxidation rate decreases to 45%-88%. The lower content of Mn（Ⅲ ） in hirnessites, the more Mn（Ⅲ） newly formed from the transformation of Mn（Ⅲ） is eomplexed out from the minerals, and the greater amplitude in the decrease of Cr（ Ⅲ ） oxidation rate. Thus the newly formed Mn（ Ⅲ） is highly active and possesses fast rate of electron transfer, however the rate of electron transfer in the transformation process of Mn（Ⅲ）→Mn（ Ⅲ ） is relatively slow. It could be deduced that the controlling step of initial oxidation rate of Cr（ Ⅲ ） with bimessites may he the electron transfer process of Mn（ Ⅲ ）→Mn（ Ⅲ ）.
出处
《环境科学》
EI
CAS
CSCD
北大核心
2009年第9期2779-2785,共7页
Environmental Science
基金
国家自然科学基金项目(40771102)
全国优秀博士论文作者专项基金项目(200767)
霍英东高校青年教师基金项目(111025)
湖北省自然科学基金计划青年杰出人才项目(2007ABB014)
