Question

Describe methods by which arsenic can be removed from water.

Short answer

Arsenic can be removed from water by coagulation and filtration, adsorption, reverse osmosis, or oxidation and precipitation.

Step-by-step solution

Coagulation and Filtration

This method involves adding coagulants such as aluminum sulfate, ferric chloride, or iron salts to water. These chemicals cause arsenic particles to clump together into larger particles. Once these bigger particles are formed, they can be filtered out from the water using sand filters.

Adsorption

In this process, water passes through a material that attracts and holds onto arsenic. Materials such as activated alumina, granular ferric hydroxide, or ion exchange resins are commonly used. These adsorbents are effective in binding arsenic and removing it from the water.

Reverse Osmosis

Reverse osmosis employs a high-pressure pump that forces water through a semi-permeable membrane. This process removes not just arsenic, but also other dissolved substances, as the membrane only allows water molecules to pass through, leaving contaminants behind.

Oxidation and Precipitation

This method involves the chemical oxidation of arsenic from its trivalent form As(III) to the more easily removed pentavalent form As(V). Chemicals such as chlorine or potassium permanganate can be used for oxidation. Once oxidized, the arsenic can be precipitated out of the water using iron-based compounds.

Key concepts

Coagulation and Filtration

To remove arsenic from water through coagulation and filtration, specific chemicals known as coagulants are introduced into the water. Coagulants often used include aluminum sulfate and ferric chloride. These coagulants work by causing arsenic particles to aggregate into larger clumps called flocs. Once the particles clump together, they become easier to remove. This process is analogously like making crumbs into a big ball that's easier to pick out. After flocculation, the water is passed through sand filters that trap the larger particles, allowing cleaner water to pass through. This method is effective and widely used in treating water for safe consumption.

Adsorption Technology

Adsorption is another powerful technique for arsenic removal from water. This method involves passing water through materials that naturally attract and hold arsenic particles onto their surfaces, similar to a magnet. Some of these materials include activated alumina and granular ferric hydroxide. When water comes in contact with these adsorbents, arsenic binds to their surfaces and is thus removed from the flowing water. Even though the adsorbent material must be replaced after it reaches saturation, this method is efficient and quite effective in small-scale water treatment applications.

Reverse Osmosis

Reverse osmosis is a sophisticated filtration technique that utilizes pressure to force water through a semi-permeable membrane. The membrane in this process allows only water molecules to pass through, effectively trapping all other contaminants, including arsenic, on the other side of the membrane. Think of it like a sieve that only lets very small particles through. Not only does reverse osmosis remove arsenic, but it also filters out other dissolved substances like salts, making it a comprehensive purification strategy. It's highly effective, though it requires significant energy input and regular membrane maintenance.

Chemical Oxidation

Chemical oxidation and precipitation involve converting arsenic into a form that is easier to remove from water. Initially, arsenic typically exists in its trivalent form, As(III), which is difficult to manage and remove. By adding chemicals such as chlorine or potassium permanganate, As(III) is oxidized to As(V), a form that can more easily combine with other compounds. This chemical reaction facilitates the arsenic's transformation into a solid mass that can be precipitated out. During precipitation, often aided by iron compounds, the arsenic is effectively settled and can be filtered from the water. This method takes advantage of chemical reactions to ensure safer water.