几种吸附剂吸附亚砷酸根离子过程中pH值的动态变化

观察了三种吸附剂吸附亚砷酸根离子过程中溶液pH值随时间的变化。氢氧化铁在吸附过程中的pH值变化大致可分为三个阶段，0～6分钟为第一阶段，体系的pH值快速上升，指示亚砷酸根置换了固相中的氢氧根；7～40分钟为第二阶段，pH值变化不大，指示Fe(OH)3凝胶对亚砷酸根离子的吸附达到动态平衡；40分钟后为第三阶段，溶液的pH值呈单边下降趋势，这一阶段吸附剂与被吸附的阴离子之间发生了缩合反应，并释放出H＋。Mg-Al-LDO在吸附亚砷酸根的反应过程中pH值持续上升，它具有从水溶液中获取阴离子以恢复其前驱体结构的能力，这一反应中有氢氧根生成；Mg-Fe-LDO兼有前两者的吸附机理，吸附反应过程pH值的变化趋势此介于二者之间。氢氧化铁在加热前后吸附容量变化不大，25?C和90?C时分别为69.7mg/g和73.7mg/g，而Mg-Al-LDO和Mg-Fe-LDO的吸附容量在25?C时分别为62.4mg/g和82.5mg/g，在90?C时分别增加到114.9 mg/g和199.0 mg/g。Mg-Al-LDO和Mg-Fe-LDO在90?C条件下吸附容量的大幅增加，可能和溶解CO2的干扰被抑制有关。

pH Variation as Indicator for the Reaction between Arsenite and Absorbents

Three types of absorbents, ferric hydroxide, Mg-Al layered double oxide (Mg-Al-LDO) and Mg-Fe layered double oxide (Mg-Fe-LDO) were studied for their absorption on arsenite. Initial pH value of NaAsO2 solution is 9.71. After ferric hydroxide being added, the pH variation with time can be divided into 3 stages: in the first 6 minutes, pH increased from 9.71 to 10.36, which indicated that OH- from the solid phase was released into the solution; during the 7 to 40 minutes, pH changed only a little between 10.36-10.34, which indicated that the absorption of ferric hydroxide on AsO2- reached dynamic equilibrium; after 40 minutes, pH value decreased continuously to 9.72 at the 360th minute, which was caused by the condensation reaction between arsenite and the absorbent, H+ was released as a result. During the absorption of Mg-Al-LDO on arsenite, pH values increased with time continuously, this is because that Mg-Al-LDO takes anions from the solution to recover the crystal structure of its precursor, in this reaction hydroxyl is formed. It is found that Mg-Fe-LDO has double absorption effects on arsenite as shown by ferric hydroxide and Mg-Al-LDO, for that its pH-time curve is between the two absorbents. The absorption capacity of ferric hydroxide on arsenite changed a little with temperature, from 69.7mg/g at 25℃ to 73.7mg/g at 90℃. Absorption capacity of Mg-Al-LDO and Mg-Fe-LDO increased apparently when the medium was heated, from 62.4mg/g and 82.5mg/g at 25℃, to 114.9mg/g and 199.0mg/g at 90℃ respectively. Ascending absorption capacity of Mg-Al-LDO and Mg-Fe-LDO on arsenite at 90℃ is quite possibly because that the competition from carbonic anion on arsenite is suppressed by heating, which expelled dissolved CO2 from the solution.