Arsenic (As) in Water Treatment

Quick Answer

Arsenic (As) is a metalloid with atomic number 33. PubChem lists its standard state as solid and its relative atomic mass as 74.92159.

Arsenic speciation is critical: As(III) is often oxidized to As(V) before adsorption or coagulation-filtration. Iron-based adsorbents, activated alumina, ion exchange and reverse osmosis are common options, but performance depends on pH and competing ions.

Interesting Facts

• At ordinary pressure, arsenic sublimes rather than passing through a normal liquid phase.
• In oxygen-poor groundwater, As(III) may dominate; after oxidation, As(V) is usually easier to capture on iron-based media.
• Arsenic treatment creates an arsenic-rich residual, so safe media or sludge management is part of the process—not an afterthought.

Water-treatment Design Implications

An element name alone is not enough to select treatment equipment. Confirm chemical form or speciation, total and dissolved concentration, pH, alkalinity, competing ions and the product-water target. Send the water-analysis results to Watermart to identify additional testing and shortlist processes before equipment is specified.

Summary Data Sources

• PubChem — Arsenic
• IUPAC Periodic Table of Elements
• WHO chemical hazards in drinking-water

Arsenic (As)

1. Basic Information

Atomic Number	33
Symbol	As
Atomic Weight	74.9216 g/mol
Category	Metalloid

2. Physical and Chemical Properties

Arsenic is a metalloid element that can be found in three allotropic forms: yellow, black, and gray. Its stable form is a fragile silvery gray solid crystal. Arsenic is easily oxidized in air and burns at high temperatures forming white clouds of arsenic trioxide. Arsenic can bind with many other elements and form various inorganic and organic compounds. In water, arsenic is usually found in two main oxidized forms:

• Arsenite, As(III): commonly uncharged over much of the drinking-water pH range and often harder to adsorb
• Arsenate, As(V): anionic over the drinking-water pH range and generally easier to capture on iron-based media

Arsenic mobility and treatment performance depend on pH, redox conditions, competing ions, and mineral surfaces.

3. Presence in Water and Health Effects

Arsenic can enter water sources through natural rock weathering, volcanic activity, and various human activities such as mining, metal smelting, and pesticide use. Arsenic concentrations in groundwater in some areas can reach dangerously high levels. Long-term exposure to arsenic through drinking water can cause a variety of serious health problems, including:

• Skin, lung, bladder and kidney cancer
• Cardiovascular disease
• Diabetes
• Neurological disorders
• Skin lesions
• Reproductive effects

The toxic effects of arsenic depend on its chemical form, with inorganic arsenic compounds generally being more toxic than organic compounds.

4. Water Treatment Applications and Removal Methods

Some of the main methods for removing arsenic from water include:

• Oxidation and Filtration: Arsenite, As(III), is oxidized to arsenate, As(V), which is generally easier to remove, followed by filtration.
• Coagulation-Flocculation: Addition of coagulants such as alum or iron salts to form flocs that bind arsenic, followed by settling or filtration.
• Adsorption: Use of adsorbent media such as activated alumina, granular iron hydroxide, or activated carbon to bind arsenic.
• Ion Exchange: Use of specialized ion exchange resins, especially strong base anion resins, to remove arsenate. However, the capacity of the resin may be limited due to the presence of other ions.
• Membrane Technology: Reverse osmosis (RO) and nanofiltration are effective in removing arsenic, especially in the form of arsenate.
• Electrocoagulation: Use of electrodes to produce an in-situ coagulant that binds arsenic.
• Phytoremediation: Use of certain aquatic plants to absorb arsenic from water. The choice of method depends on various factors such as arsenic concentration, water pH, presence of other contaminants, scale of operation, and cost considerations.

5. Process Safety and Residuals

Arsenic is a contaminant to remove, not a water-treatment chemical to recommend. Oxidation and capture move arsenic into spent media, sludge, regenerant, or membrane concentrate; those residuals require characterisation and a lawful disposal route.

8. Environmental Impact and Sustainability Considerations

Arsenic removal from drinking water poses several environmental and sustainability challenges:

• Waste Disposal: The arsenic removal process generates solid or liquid wastes containing high concentrations of arsenic. Disposal of these wastes must be done carefully to prevent contamination of the environment.
• Energy Consumption: Some arsenic removal technologies, such as reverse osmosis, require significant energy, which can increase the carbon footprint of the water treatment process.
• Chemical Use: Methods such as coagulation-flocculation require the use of chemicals, which can have environmental impacts in their production and disposal.
• Resource Sustainability: Adsorption technologies require periodic replacement of media, which can pose sustainability issues if the materials used cannot be recycled or produced sustainably.
• Impact on Aquatic Ecosystems: Discharge of arsenic-containing effluent, even in low concentrations, can affect aquatic ecosystems.
• Integrated Water Management: Arsenic removal should be integrated with broader water management strategies to ensure long-term sustainability of water resources.