研究水钠锰矿对重金属离子的吸附,其结构中八面体空穴特点与重金属离子吸附的关系是人们关注的热点.采用酸性和碱性2种介质条件合成了具有不同锰氧化度的2个系列水钠锰矿,研究了它们的锰氧化度(average oxidation state of Mn,AOS)与其d110面网间距和Pb2+的最大吸附量的关系,以及Pb2+吸附量与Mn2+、H+和K+释放量的关系.结果表明,同系列的水钠锰矿具有相似的晶体形貌,它们的锰氧化度与其d110面网间距呈极显著的负相关(r=-0.9035<-0.6614,n=14,α=0.01),而与Pb2+的最大吸附量呈极显著的正相关(r=0.9779>0.6614,n=14,α=0.01),供试水钠锰矿锰氧化度的高低表观上反映了结构中八面体空穴数量的多少,水钠锰矿随着锰氧化度的提高,导致其结构中的八面体空穴数增多,对Pb2+的吸附容量增大,八面体空穴数量对Pb2+的吸附量的大小起着非常重要的作用.Pb2+的吸附量与吸附过程中Mn2+、H+和K+的释放量之和呈极显著的正相关(r=0.9962>0.6614,n=14,α=0.01),Pb2+吸附过程中伴随的Mn2+、H+和K+的释放主要来自于水钠锰矿结构中八面体空穴处吸附的相应阳离子.吸附前Mn2+,Mn3+与H+、K+占据水钠锰矿结构中八面体空穴上下方位点的相对多少受水钠锰矿的锰氧化度高低的影响,锰氧化度低时,八面体空穴上下方位点吸附Mn2+、Mn3+较多,吸附的H+、K+则较少,反之亦然.
The characteristics of Pb2+ adsorption on the surface of birnessites with different average oxidation states (AOS) of Mn, synthesized under acidic and alkali conditions, were investigated. The results indicated that the amount of adsorbed Pb2+ increased with the increase of Mn AOS in birnessites. The amount of Pb2+ adsorbed positively correlated with the amount of released Mn2+, H+, and K+ (r = 0.9962 > 0.6614, n = 14, α = 0.01). The released Mn2+, H+, and K+ were derived mostly from the corresponding cations adsorbed on the vacant sites. The maximum amount of adsorbed Pb2+ increased with the increasing vacant cation sites, leading to an increase of the total amount of released Mn2+, H+, and K+, and the increased likelihood for two Pb2+ adsorbed in the region of one side of a vacant site.
ZHAO Wei, FENG Xionghan, TAN Wenfeng, LIU Fan, DING Shuwen Key Laboratory of Subtropical Agriculture Resource & Environment, Ministry of Agriculture of China, Huazhong Agricultural University, Wuhan 430070, China.
Oxidation of As(Ⅲ) by three types of manganese oxides and the e ffects of pH, ion strength and tartaric acid on the oxi- dation were investiga ted by means of chemical analysis, equilibrium redox, X-ray diffraction(XRD) and transmission electron microscopy(TEM). Three synthesized Mn oxide minerals, bir nessite, cryptomelane, and hausmannite, which widely occur in soil and sediments , could actively oxidize As(Ⅲ) to As(V). However, their ability in As(Ⅲ) -oxidation varied greatly depending on their structure, composition and surface properties. Tunnel structured cryptomelane exhibited the highest ability of As( Ⅲ) oxidation, fol- lowed by the layer structured birnessite and the lower oxid e hausmannite. The maximum amount of As(V) produced by the oxidation was in the order(mmol/kg) of cryptomelane(824.2) > birnessite(480.4) > hausmannite(117.9). As pH increased from the very low value(pH 2.5), the amount of As(Ⅲ) oxidized by the tested Mn oxides was firstly decreased, then negatively peaked in pH 3.0 —6.5, and eventually increased remarkably. Oxidation of As(Ⅲ) by the Mn oxides had a buffering effects on the pH variation in the solution. It is proposed th at the oxidative reaction processes between As(Ⅲ) and birnessite(or cryptomel ane) are as follows: (1) at lower pH condition: (MnO2)x + H3AsO3 + 0.5H+ 0 .5H2AsO4- + 0.5HAsO42- +Mn2++ (MnO2)x-1 + H2O; (2) at higher pH condition: (MnO2 )x + H3AsO3 0.5H2AsO4- + 0.5HAsO42- + 1.5H+ + (MnO2)x-1·MnO. With increa se of ion strength, the As(Ⅲ) oxidized by birnessite and cryptomelane decreased and was negatively correlated with ion strength. However, ion strength had litt le influence on As (Ⅲ) oxid- ation by the hausmannite. The presence of tart aric acid promoted oxidation of As(Ⅲ) by birnessite. As for cryptomelane and haus- mannite, the same effect was observed when the concentration of tartar ic acid was below 4 mmol/L, otherwise the oxidized As(Ⅲ) decreased. These findings are of great significance in improving our understanding of As geochem
