Why is soluble silicon in interstitial and lake water samples immobilized by freezing?

The reasons behind the lowering of the spectrophotometrically measurable silicon (Si) content in sediment interstitial and unfiltered lake water samples after storage in frozen form were explored. The storage container material had little effect on the immobilization, whereas lowering the pH of interstitial water samples before freezing decreased the “loss” of Si. When interstitial water samples were frozen, both native, naturally present Si and added standard solution Si were immobilized. The same reaction pattern was found for native Si in unfiltered lake water samples. On the contrary, neither the native Si in filtered lake water nor added standard solution Si was immobilized when lake water samples were frozen, indicating that high concentrations of particulate material, especially in the interstitial water, was the prime cause of the immobilization phenomenon. The results obtained in a sequential filtration (0.2 μm, 100,000 and 10,000 daltons) experiment with interstitial water samples supported the theory that Si is immobilized through polymerization or complex formation mediated by particulate material and molecules present in interstitial water. An incubation test revealed that the immobilization process was not entirely reversible. The amount, nature and behaviour of Si in interstitial water samples showed some seasonal variation.