Question

Fluoride in water is removed using precipitation method by (a) lime and alum (b) lime, alum and bleaching powder (c) activated carbon (d) defluoron

Short answer

(a) lime and alum

Step-by-step solution

Understanding the Precipitation Method

Precipitation is a process where soluble substances are converted into an insoluble solid, called a precipitate, which falls out of a solution. This method can be used to remove certain ions, such as fluoride, from water by converting them into an insoluble compound.

Evaluating the Options

Each option describes a different method or combination of reagents that may act to precipitate fluoride from water: (a) **Lime and alum**: Lime (calcium hydroxide) and alum (aluminum sulfate) can cause precipitation. Lime increases the pH of water, allowing fluoride ions to precipitate as insoluble calcium fluoride (CaF2), while alum helps in coagulating and precipitating impurities. (b) **Lime, alum, and bleaching powder**: Similar to (a) but includes bleaching powder, which primarily acts as a disinfectant, not directly involved in fluoride removal by precipitation. (c) **Activated carbon**: This method is not a precipitation process; it involves adsorption, where fluoride ions stick to the surface of the carbon. (d) **Defluoron**: A specific chemical for fluoride removal, possibly proprietary, but usually not defined in terms of precipitation with traditional chemical names.

Selecting the Most Appropriate Option

Option (a), using lime and alum, is a well-known and widely used method to precipitate fluoride in the form of calcium fluoride. Lime increases solution pH to create conditions for precipitation, while alum serves as a coagulant, helping in aggregating and settling the particles.

Conclusion

The combination of lime and alum is effective for precipitating fluoride from water, due mainly to the creation of insoluble calcium fluoride and the coagulation properties of alum in water treatment processes.

Key concepts

Fluoride Removal

Fluoride removal from water is important because high fluoride levels can be harmful to health, leading to conditions like dental and skeletal fluorosis. The most effective method to remove fluoride is through precipitation. In this process, fluoride ions are transformed into insoluble compounds that settle out of the water, making them easier to remove. One of the popular methods is using lime and alum. Lime, which is composed of calcium hydroxide, increases the pH of the water. This change in pH enables the fluoride ions to bind with calcium, forming calcium fluoride ( CaF_2 ), an insoluble solid that precipitates out. The effectiveness of this method is grounded in its simplicity and ability to tackle high concentrations of fluoride, ensuring that the water becomes safe for consumption.

Coagulation with Alum

Coagulation is a chemical process that involves the addition of a coagulant to water, which causes contaminants to clump together and form larger particles. Alum, or aluminum sulfate, is a commonly used coagulant in water treatment. During the coagulation process, alum reacts with the alkalinity in water to form a gelatinous precipitate known as floc. This floc traps impurities and fine particulates such as dirt and microorganisms, allowing them to be removed easily in subsequent steps like sedimentation or filtration. In the context of fluoride removal, alum speeds up the process by aiding in the aggregation of precipitated calcium fluoride particles. It enhances the effectiveness by ensuring that these particles are substantial enough to settle out of the solution efficiently, thereby increasing the overall efficiency of the water treatment process.

Water Treatment Chemistry

Water treatment chemistry is an extensive field dedicated to ensuring safe, potable water by removing impurities and contaminants. It encompasses various chemical processes, one of the most critical being precipitation, which specifically targets ions or compounds like fluoride that otherwise remain dissolved. Through the addition of specific chemicals, such as lime and alum in the fluoride removal process, water treatment specialists can instigate chemical reactions that convert dissolved substances into insoluble particles. This transformation is crucial for removing harmful substances that could affect human health. Effective water treatment relies on a strong understanding of chemical reactions and pH levels. By carefully managing these variables, treatment facilities can optimize processes like coagulation and precipitation to maintain water quality and safety, thus safeguarding public health on a broad scale.