UPSC Prelims 2025 Q50: Activated Carbon and Adsorption — Explained
Consider the following statements:
Statement I: Activated carbon is a good and an attractive tool to remove pollutants from effluent streams and to remediate contaminants from various industries.
Statement II: Activated carbon exhibits a large surface area and a strong potential for adsorbing heavy metals.
Statement III: Activated carbon can be easily synthesized from environmental wastes with high carbon content.
Which one of the following is correct in respect of the above statements?
- Both Statement II and Statement III are correct and both of them explain Statement I
- Both Statement II and Statement III are correct but only one of them explains Statement I
- Only one of the Statements II and III is correct and that explains Statement I
- Neither Statement II nor Statement III is correct
Answer: (a) Both Statement II and Statement III are correct and both of them explain Statement I
Statement-wise explanation
- Statement-I: correct — Activated carbon is one of the most widely used adsorbents for treating industrial and municipal wastewater. It takes out dyes, phenols, pesticides, pharmaceutical residues, chlorine, odour-causing compounds and dissolved metals. It is also used in remediation, for example in groundwater pump-and-treat systems and in air filters and gas masks. Its low cost, the fact that it can be regenerated, and its ability to capture many kinds of pollutant make it an 'attractive tool'.
- Statement-II: correct — Activation (steam, CO2, or chemicals such as KOH, ZnCl2 or H3PO4) opens a dense network of micropores and mesopores. This usually gives a surface area of more than 500 m²/g, and some grades exceed 2,000 m²/g. That huge internal surface, along with surface oxygen groups that bind metal ions, lets it adsorb heavy metals such as lead, cadmium, chromium and mercury. It explains Statement-I, because adsorption on a large surface is exactly how activated carbon pulls pollutants out of effluents.
- Statement-III: correct — Any material rich in carbon can be carbonised and then activated. Common feedstocks are coconut shells, nutshells, rice husk, sawdust, fruit peels, bagasse and even waste PET bottles and sludge. Because the raw material is cheap waste, activated carbon is low-cost and fits the idea of a circular economy. That is part of why it is an 'attractive' tool for pollution control, so this statement also explains Statement-I.
How to eliminate
Statement-I is a general, positive claim with no absolute words such as 'only' or 'always'. Anyone who has seen a water-purifier carbon filter knows it is true. Statement-II rests on the textbook fact that adsorption depends on surface area, and activated carbon is the classic high-surface-area adsorbent. That rules out the option saying neither is correct. Statement-III uses the word 'easily', which may look like a trap. But carbonising and activating farm waste is a simple, well-known process, and coconut-shell carbon is sold everywhere. Once you accept that all three are true, ask whether each one gives a reason for Statement-I. Statement-II explains why it works (it adsorbs well). Statement-III explains why it is 'attractive' (cheap and made from waste). The word 'attractive' in Statement-I is the clue that the cost/source argument also counts as an explanation. The one fact to remember: activated carbon works by adsorption on a very large internal surface, and it can be made from almost any carbon-rich waste.
Know more
Activated carbon (activated charcoal) is carbon processed to be extremely porous. It removes pollutants by adsorption: molecules or ions stick to its surface. This is different from absorption, where a substance is taken into the bulk of a material.
How it is made
- Carbonisation: a carbon-rich material is heated without oxygen, which drives off volatile matter.
- Activation: the pores are opened up, either physically (steam or CO2 at about 800–1,000 °C) or chemically (KOH, ZnCl2 or H3PO4 at lower temperatures).
The result has a surface area of roughly 500 to more than 2,000 m² per gram.
| Feature | Detail |
|---|---|
| Mechanism | Physical adsorption (van der Waals forces) plus chemical binding at surface functional groups |
| Common feedstocks | Coconut shell, wood, coal, rice husk, nutshells, bagasse, fruit peels, waste plastics |
| Forms | Powdered (PAC), granular (GAC), extruded/pellet |
| Removes | Organics, dyes, pesticides, phenols, chlorine, odour, heavy metals (Pb, Cd, Cr, Hg), VOCs |
| Uses | Water purifiers, effluent treatment, gas masks, gold recovery, poison treatment in medicine, mercury control at power plants |
| Regeneration | Heating or steam drives off the adsorbed material so the carbon can be reused |
Related terms UPSC mixes up
- Biochar: charcoal made by pyrolysis for use in soil. It stores carbon and improves fertility, and is usually not activated.
- Black carbon: soot from incomplete burning. It is an air pollutant and a short-lived climate forcer.
- Phytoremediation: using plants, not adsorbents, to clean soil or water.
- Bioremediation: using microbes to break down pollutants.
India is a large producer of coconut-shell activated carbon, which ties waste valorisation to the coconut economy.
NCERT Concepts to Revise
- Water pollution: Statement I is about removing inorganic pollutants such as heavy metals from effluents, which is a core type of water pollution.
- Black carbon: Black carbon is soot, a pollutant. Don't confuse it with activated carbon, the engineered adsorbent this question is about.