tailieunhanh - PHYSICAL - CHEMICAL TREATMENT OF WATER AND WASTEWATER - CHAPTER 11

Như đã đề cập trong Chương 10 làm mềm nước, miễn là nồng độ của CaCO3 và Mg (OH) 2 vượt quá solubilities của họ, các chất rắn có thể tiếp tục kết tủa. Tình trạng này có thể gây ra quy mô hình thức, chất rắn do kết tủa của các ion trong dung dịch tiền gửi. Để ngăn chặn hình thành quy mô, nước phải được ổn định. Nước là ổn định khi nó không bị giải thể cũng không phải tiền gửi kết tủa. Nếu độ pH cao, ổn định có thể được thực hiện bằng cách. | 11 Water Stabilization As mentioned in Chapter 10 on water softening as long as the concentrations of CaCO3 and Mg OH 2 exceed their solubilities the solids may continue to precipitate. This condition can cause scale to form a solid that deposits due to precipitation of ions in solution. To prevent scale formation the water must be stabilized. A water is said to be stable when it neither dissolves nor deposits precipitates. If the pH is high stabilization may be accomplished using one of several acids or using CO2 a process called recarbonation. If the pH is low stabilization may be accomplished using lime or some other bases. Because of the universal presence of carbon dioxide any water body is affected by the reaction products of carbon dioxide and water. The species produced from this reaction form the carbonate system equilibria. As discussed later the stability or instability of water can be gaged using these equilibria. Thus this chapter discusses this concept. It also discusses criteria for stability and the recarbonation process after water softening. CARBONATE EQUILIBRIA The carbonate equilibria is a function of the ionic strength of water activity coefficient and the effective concentrations of the ionic species. The equilibrium coefficients that are calculated from the species concentrations are a function of the temperature. This functionality of the coefficients can in turn be calculated using the Van t Hoff equation to be addressed later. One of the major cations that can form scales as a result of the instability of water is calcium. Calcium plays an important role in the carbonate equilibria. We will therefore express the carbonate equilibria in terms of the interaction of the calcium ion and the carbonate species which are the reaction products of carbon dioxide and water. In addition since the equilibria occur in water the dissociation of the water molecule must also be involved. Using calcium as the cation the equilibrium equations of the .