自然界水钠锰矿日光催化作用模拟实验研究

Experimental study on the natural photocatalytic reduction of birnessite

以水钠锰矿为代表的锰氧化物在地表环境中广泛存在,其日光催化作用是驱动地表锰元素价态循环的重要途径之一.在合成并系统研究不同结构锰氧化物半导体矿物学性质的基础上,重点开展了其光催化还原反应及其影响因素的对比实验研究,探讨了矿物晶体结构、电子结构与介质条件对光催化还原效果的影响机制,提出了光催化反应过程中光电子的转移途径.实验结果表明:不同结构锰氧化物在可见光照射下均可发生自光催化自还原从而释放锰离子,其中层状的单斜水钠锰矿自还原效率最高,光照5h可释放11.65%锰离子,孔道状的锰钾矿自还原效率最低(3.58%);锰氧化物的光催化反应速率随pH的降低而升高,其中单斜水钠锰矿在pH为6.5条件下的锰离子溶出率是pH为9.5中的2.66倍;锰氧化物的光催化反应速率受有机电子供体的影响,其中腐殖酸做空穴捕获剂时锰的还原量是甲醇时的1.78~4.46倍;焦磷酸根可迅速络合光催化反应中产生的三价锰而显色,其中δ型二氧化锰与六方水钠锰矿光照5h可持续产生0.058mg·L^(-1)三价锰,表明锰氧化物的光还原是单电子转移方式,三价锰是主要的中间产物.

Manganese oxides are widely found in the environment,and their photocatalytic activity is one of the important ways to drive the manganese cycle.An investigation into the photocatalytic reduction reaction and its influencing factors（e.g.mineral crystal structure,electronic structure,etc）was conducted based on the systematic study of the properties of manganese oxides with different structures.Our results demonstrate that all the manganese oxides used in this experiments can occur the photocatalytic reduction under visible light irradiation and release manganese ions.The monoclinic birnessite with layered structure has the highest efficiency of photocatalytic reduction,and it can release 11.65% manganese ions in 5 hours.As the pH value decrease,the photocatalytic reaction rate of manganese oxide increase.The release rate of manganese ion of monoclinic birnessite at pH 6.5 is 2.66 times higher than that at pH 9.5.The photocatalytic reaction rate of manganese oxide is also affected by the electron donor.The results show that the amount of released manganese ions using humic acid as the hole capture agent is 1.78~4.46 times higher than using methanol.Furthermore,δ-MnO_2 and hexagonal birnessite under light in 5 hours can produce 0.058mg·L^（-1） trivalent manganese ions,indicating that the reduction of manganese oxide isasingle-electron-transfer mode and trivalent manganese is the main intermediate product.