盐度对镭同位素在海南红树林沉积物解吸行为的影响

天然放射性镭同位素在沉积物上的解吸行为是影响其在陆-海交换过程中的关键所在。采用沉积物室内解吸实验和现场采集间隙水测量224Ra含量两种方法,对水体盐度梯度控制镭在海南八门湾红树林沉积物的解吸行为进行了讨论。结果表明:沉积物上可交换态224Ra的最大量为0.44dpm/g,解吸比为35%。利用间隙水的224Ra含量确定镭的分配比Kd与水体盐度S呈反比例函数:Kd=8.4×102/S,与室内解吸实验的结果相比更能代表镭在沉积物上的真实解吸行为。在深度25—40cm内,湿地沉积物的224Ra处于与其母体228Th的平衡状态,但223Ra很可能处于相对其母体227Th亏损状态。

EFFECT OF SALINITY ON RADIUM DESORPTION FROM SEDIMENTS IN MANGROVE WETLAND, HAINAN

The desorption behavior of the four naturally occurring radium isotopes (224Ra, 223Ra, 228Ra, and 226Ra) from sediments is a key factor impacting their land-ocean interaction in coastal zones. By using laboratory desorption experiment and field observation of 224Ra activities in pore water of sediments, we discussed the effect of water salinity on radium desorption from bottom sediments in the mangrove wetland, Bamen Bay, Hainan. The results showed that the sediment contains 0.44 dpm g-1 of exchangeable 224Ra, which corresponds to approximately 35% of the total 228Th activity. On the basis of the field data of 224Ra activities in pore water, it was found that there is a inversely proportional relationship between the distribution coefficient (Kd) and the water salinity (S), Kd = 8.4×102 S-1, which is more representative of the real behavior of radium desorption from sediments, as compared with the results of the laboratory desorption experiments. It deduced that the 224Ra-228Th secular equilibrium was approached in the upper 25-40 cm sediment column; in contrast, the deficit of 223Ra relative to its parent 227Th possibly occurred in the mangrove wetland.