MEV 013 Solved Guess Paper 2026: Environmental Chemistry for IGNOU Master of Science (Environmental Science)

Adsorption and its mechanism

Question: What is adsorption? Explain the mechanism of adsorption with suitable examples.

Meaning of adsorption

"Adsorption is a simplistic process in which adsorptive molecules (gas or liquid) bind to a solid surface."

Mechanism of adsorption

Adsorption is carried out in a column filled with porous sorbents, in batch or continuous mode, where mass transfer effects cannot be avoided. The process moves through three common steps:

1. Film diffusion (external diffusion): the adsorbate is carried to the outer surface of the adsorbent.
2. Pore diffusion: molecules travel through the pores of the solid, known as intraparticle diffusion (IPD).
3. Surface reaction: the adsorbate binds onto the adsorbent surface.

The slowest of these steps sets the overall speed and is called the rate-determining step.

Classification of adsorbents

1. Nonpolar solids: most molecular solids; adsorption is mainly physical, for example activated charcoal.
2. Polar solids: adsorb polar and nonpolar molecules but prefer polar ones; adsorption is chemical, for example silica and aluminium oxides.
3. Chemical adsorbing surfaces: hold molecules and release them after reaction, either catalytically or non-catalytically.

Factors and example

The key factors are the nature of the adsorbate and adsorbent, surface area, activation, air-flow rate, humidity, temperature and contact time. The Haber-Bosch process for ammonia uses a metal catalyst that adsorbs nitrogen and hydrogen gas, a classic example of adsorption at work in catalysis.

Conclusion: Adsorption proceeds through film diffusion, pore diffusion and surface reaction, and its efficiency depends on the adsorbent type and operating conditions. This makes it a cheap, effective tool for removing contaminants from water.

Soil profile and its horizons

Question: Describe a soil profile. Explain its various horizons and their importance.

Meaning of soil profile

"A vertical section displays of the soil components denoting its various zones from surface level to the bed rock is called soil profile."

As the parent material weathers over time, layers of soil form one above the other. They differ in thickness, structure, texture and composition, and are grouped into the organic O horizon and the mineral horizons A, E, B and C.

The soil horizons

1. O-horizon: the uppermost layer, also called the litter zone; found in forests, absent in deserts and cultivated fields.
2. A-horizon: the topsoil, several inches thick, holding most of the organic matter and humus, plant roots and the highest biological activity; essential for plant productivity.
3. E-horizon: the zone of eluviation, where clay and oxides of iron and aluminium are leached out, leaving sand and silt behind.
4. B-horizon: the subsoil or zone of accumulation, where material dissolved from the A-horizon is redeposited.
5. C-horizon: lacks organic matter and is made largely of weathered parent rock; the bedrock lies beneath it.

Conclusion: The soil profile shows distinct O, A, E, B and C horizons from surface to bedrock. Each differs in composition and together they reveal the soil's maturity, fertility and capacity to support plant life.

Global warming and the greenhouse effect

Question: What is global warming? How is it different from the greenhouse effect? What are its environmental effects?

Meaning of the greenhouse effect and global warming

"This phenomenon of trapping of heat inside earth's atmosphere is known as the greenhouse effect; the accelerated greenhouse effect is called global warming."

Greenhouse effect

Greenhouse gases let incoming solar heat reach the surface but reabsorb the outgoing infrared radiation, trapping heat in the lower atmosphere. Without them, the average global temperature would be about −18°C instead of 15°C. The principal greenhouse gases are CO₂, water vapour, methane, nitrous oxide and ozone, with CO₂ holding the largest share. Svante Arrhenius (1896) first linked rising CO₂ to warming.

Global warming and its effects

As the concentration of IR-absorbing gases rises, the greenhouse effect speeds up. This accelerated form is global warming, and its effects include:

1. Melting of glaciers and ice caps, raising sea levels by about 3.2 mm a year and threatening coastal areas.
2. Climate change: drought, reduced rainfall and irregular weather patterns.
3. Harm to wildlife and habitats, with a predicted warming of 3.0°C to 5.5°C.

Conclusion: The greenhouse effect is the natural trapping of heat by atmospheric gases, while global warming is its accelerated form, driving glacier melt, sea-level rise, drought and threats to wildlife.

Toxic effects of mercury and arsenic in water

Question: Explain the toxic effects of the following ions present in water: (a) Mercury (b) Arsenic.

(a) Mercury

"All forms of mercury are highly toxic but toxicities vary considerably."

1. Elemental mercury is itself non-toxic, but its vapours are highly toxic; they diffuse through the lungs into the blood and brain.
2. Alkyl mercurials (RHg⁺) are highly toxic, fat-soluble and cross the blood-brain and placental barriers easily.
3. Anaerobic bacteria methylate inorganic mercury (through methylcobalamin, vitamin B₁₂) into methyl mercury, which enters the food chain through phytoplankton and bio-accumulates.
4. The Minamata Bay tragedy in Japan (1953 to 1960) caused deaths, paralysis and congenital defects from fish contaminated with methyl mercury.

(b) Arsenic

"Arsenic exerts its toxic action by binding it strongly to the sulphydryl groups on protein."

1. It deactivates the enzyme pyruvate dehydrogenase, stopping ATP formation in the citric acid cycle.
2. Being chemically similar to phosphorus, arsenic interferes with the biochemical reactions of phosphorus and disrupts ATP synthesis.
3. Its three major biochemical effects are protein coagulation, complexation with co-enzymes and uncoupling of phosphorylation.
4. As(III) is more toxic than As(V); the antidote is the chelating agent British Anti-Lewisite (BAL).

Conclusion: Mercury, especially methyl mercury, attacks the nervous system after bio-accumulation, while arsenic blocks ATP-linked enzymes. Both are potent water toxins that can cause severe and often fatal poisoning.

Gas-Liquid Chromatography (GLC)

Question: What is the working principle of Gas-Liquid Chromatography (GLC)? Describe its instrumentation and applications in environmental monitoring.

Meaning of gas-liquid chromatography

"Gas-liquid chromatography (GLC) is a technique employed for the separation of two phases in a mixture where one is a stationary phase and other is a mobile phase."

Principle

GLC is a type of partition chromatography. Components partition between a non-volatile liquid stationary phase coated on a solid and an inert carrier-gas mobile phase such as neon, helium or argon. Separation follows the partition coefficient and the rule "like dissolves like", so more volatile, less polar compounds come out faster, with a shorter retention time.

Instrumentation

1. Injection port: the sample is injected by micro-syringe and vaporised in an oven.
2. Columns: packed or capillary (open-tubular) columns, held at 50 to 250°C.
3. Detectors: Thermal Conductivity (TCD), Flame Ionisation (FID) and Electron Capture (ECD).
4. Recorder: plots the detector signals as a chromatogram of peaks.

Applications

GLC is used for qualitative and quantitative analysis of organic compounds, estimating pesticide and insecticide residues in food, detecting pollutants in water and food, analysing breath, blood and saliva, and determining the components of air samples.

Conclusion: GLC separates volatile compounds by partition between a liquid stationary phase and an inert carrier gas, and its sensitive detectors (TCD, FID, ECD) make it a core tool for monitoring pesticides, pollutants and air quality.