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Adsorption and Activated Carbon in Water Treatment

Explore activated carbon adsorption for water purification, its principles, applications, and factors influencing performance in modern water treatment.

Understanding Adsorption with Activated Carbon

Adsorption is a critical water treatment process where a solid material is utilized to effectively remove soluble substances from water. In this context, activated carbon serves as the primary solid adsorbent. Activated carbon is engineered to possess an exceptionally large internal surface area, typically ranging between 500 and 1,500 square meters per gram (m²/g). This extensive internal porosity makes activated carbon an ideal medium for adsorption.

Activated carbon is available in two main forms:

  • Powder Activated Carbon (PAC): Fine powder used for batch processes or intermittent dosing.
  • Granular Activated Carbon (GAC): Larger granules, predominantly used in continuous flow-through systems for water treatment applications due to its regenerability and suitability for fixed-bed filters.

Target Contaminants for Activated Carbon Adsorption

Granular Activated Carbon (GAC) is highly effective in removing a wide range of soluble substances from water, particularly organic, non-polar compounds. These include:

  • Organic, Non-polar Substances:
    • Mineral oil
    • BTEX (Benzene, Toluene, Ethylbenzene, Xylenes)
    • Polycyclic Aromatic Hydrocarbons (PAHs)
    • (Chloro)phenols
  • Halogenated Substances: Iodine, Bromine, Chlorine, Hydrogen, and Fluorine compounds
  • Aesthetic Impairments:
    • Odors
    • Tastes
  • Biological & Fermentation Byproducts:
    • Yeasts
    • Various fermentation products
  • General Non-polar Substances: Compounds that are poorly soluble in water.

Process Description: How Activated Carbon Filters Work

Water treatment with activated carbon typically involves pumping water through a column packed with activated carbon. The treated water then exits the column via a draining system. The efficiency and activity level of an activated carbon column are influenced by factors such as water temperature and the specific nature of the substances being treated.

As water continuously flows through the column, soluble substances accumulate on the carbon's surface. This accumulation gradually exhausts the filter, necessitating periodic replacement or regeneration of the activated carbon. Granular Activated Carbon (GAC) can often be regenerated by oxidizing the adsorbed organic matter. However, regeneration processes typically result in a 5-10% decrease in the activated carbon's efficiency and a small amount of carbon material is destroyed, requiring replenishment.

For enhanced treatment reliability and to prevent complete system exhaustion, it is common practice to operate multiple activated carbon columns in series.

The Mechanism of Adsorption

Adsorption involves the physical attachment of molecules from a gas or liquid phase onto the surface of a solid, in this case, the activated carbon. This process occurs in three main steps:

  1. Macro Transport: The movement of target organic material through the larger macro-pore system of the activated carbon. Macro-pores are defined as having diameters greater than 50 nanometers (nm), or 0.05 micrometers (µm).
  2. Micro Transport: The subsequent movement of organic material through the meso-pore and micro-pore systems within the activated carbon. Meso-pores range from 2-50 nm (0.002-0.05 µm) in diameter, while micro-pores are smaller than 2 nm (<0.002 µm).
  3. Sorption: The physical attachment (adsorption) of organic material onto the internal surface of the activated carbon, primarily within the meso-pores and micro-pores.

The overall activity level of adsorption is influenced by the concentration of the substance in the water, the water temperature, and the polarity of the substance. Non-polar substances (which are poorly soluble in water) are generally removed very effectively by activated carbon, whereas polar substances (highly soluble in water) are removed poorly or not at all.

Each type of activated carbon exhibits a unique adsorption isotherm, which graphically represents the relationship between the amount of substance adsorbed and its equilibrium concentration. In water treatment, the Freundlich isotherm is often used to describe this relationship:

$$ \frac{x}{m} = K_f \cdot C_e^{\frac{1}{n}} $$

Where:

  • $\frac{x}{m}$ = Amount of substance adsorbed per unit mass of active carbon (e.g., mg/g)
  • $C_e$ = Concentration difference (between before and after adsorption), representing the effective equilibrium concentration (e.g., mg/L)
  • $K_f$ and $n$ = Specific empirical constants for the adsorption system

Adsorption vs. Absorption: A Key Distinction

It is important to differentiate between adsorption and absorption:

  • Adsorption describes a surface phenomenon where a substance adheres to the surface of another material (e.g., molecules attaching to the internal surface of activated carbon).
  • Absorption refers to a bulk phenomenon where a substance is taken into the volume of another material, or when a gas is dissolved within a liquid solution.

Factors Influencing Activated Carbon Performance

The effectiveness of activated carbon is determined by several parameters, both in water and air purification applications.

In Water Treatment

  • Compound Characteristics: Compounds with higher molecular weights and lower solubility tend to be more effectively adsorbed.
  • Concentration: A higher concentration of the target compound generally leads to increased carbon consumption.
  • Competition: The presence of other organic compounds can compete for available adsorption sites on the carbon, potentially reducing the removal efficiency of the primary target contaminant.
  • pH: The pH of the water stream can significantly impact adsorption. For instance, acidic compounds are often better removed at lower (more acidic) pH levels.

In Air Purification

  • Compound Characteristics: Compounds with high molecular weight, lower vapor pressure (or higher boiling point), and high refractive index typically show better adsorption.
  • Concentration: Higher contaminant concentration leads to higher carbon consumption.
  • Temperature: Lower temperatures generally enhance adsorption capacity.
  • Pressure: Higher pressures tend to improve adsorption capacity.
  • Humidity: Lower humidity levels typically result in better adsorption capacity.

Chemical Adsorption Probabilities in Water

Based on chemical properties and empirical data, substances can be classified by their probability of being effectively adsorbed by activated carbon in water:

1. Chemicals with Very High Probability of Adsorption:

  • 2,4-D
  • Deisopropyltatrazine
  • Linuron
  • Alachlor
  • Desethylatrazine
  • Malathion
  • Aldrin
  • Demeton-O
  • MCPA
  • Anthracene
  • Di-n-butylphthalate
  • Mecoprop
  • Atrazine
  • 1,2-Dichlorobenzene
  • Metazachlor
  • Azinphos-ethyl
  • 1,3-Dichlorobenzene
  • 2-Methyl benzenamine
  • Bentazone
  • 1,4-Dichlorobenzene
  • Methyl naphthalene
  • Biphenyl
  • 2,4-Dichlorocresol
  • 2-Methylbutane
  • 2,2-Bipyridine
  • 2,5-Dichlorophenol
  • Monuron
  • Bis(2-Ethylhexyl)Phthalate
  • 3,6-Dichlorophenol
  • Naphthalene
  • Bromacil
  • 2,4-Dichlorophenoxy
  • Nitrobenzene
  • Bromodichloromethane
  • Dieldrin
  • m-Nitrophenol
  • p-Bromophenol
  • Diethylphthalate
  • o-Nitrophenol
  • Butylbenzene
  • 2,4-Dinitrocresol
  • p-Nitrophenol
  • Calcium Hypochlorite
  • 2,4-Dinitrotoluene
  • Ozone
  • Carbofuran
  • 2,6-Dinitrotoluene
  • Parathion
  • Chlorine
  • Diuron
  • Pentachlorophenol
  • Chlorine dioxide
  • Endosulfan
  • Propazine
  • Chlorobenzene
  • Endrin
  • Simazine
  • 4-Chloro-2-nitrotoluene
  • Ethylbenzene
  • Terbutryn
  • 2-Chlorophenol
  • Hexachlorobenzene
  • Tetrachloroethylene
  • Chlorotoluene
  • Hexachlorobutadiene
  • Triclopyr
  • Chrysene
  • Hexane
  • 1,3,5-Trimethylbenzene
  • m-Cresol
  • Isodrin
  • m-Xylene
  • Cyanazine
  • Isooctane
  • o-Xylene
  • Cyclohexane
  • Isoproturon
  • p-Xylene
  • DDT
  • Lindane
  • 2,4-Xylenol

2. Chemicals with High Probability of Adsorption:

  • Aniline
  • Dibromo-3-chloropropane
  • 1-Pentanol
  • Benzene
  • Dibromochloromethane
  • Phenol
  • Benzyl alcohol
  • 1,1-Dichloroethylene
  • Phenylalanine
  • Benzoic acid
  • cis-1,2-Dichloroethylene
  • o-Phthalic acid
  • Bis(2-chloroethyl) ether
  • trans-1,2-Dichloroethylene
  • Styrene
  • Bromodichloromethane
  • 1,2-Dichloropropane
  • 1,1,2,2-Tetrachloroethane
  • Bromoform
  • Ethylene
  • Toluene
  • Carbon tetrachloride
  • Hydroquinone
  • 1,1,1-Trichloroethane
  • 1-Chloropropane
  • Methyl Isobutyl Ketone
  • Trichloroethylene
  • Chlorotoluron
  • 4-Methylbenzenamine
  • Vinyl acetate

3. Chemicals with Moderate Probability of Adsorption*:

  • Acetic acid
  • Dimethoate
  • Methionine
  • Acrylamide
  • Ethyl acetate
  • Methyl-tert-butyl ether
  • Chloroethane
  • Ethyl ether
  • Methyl ethyl ketone
  • Chloroform
  • Freon 11
  • Pyridine
  • 1,1-Dichloroethane
  • Freon 113
  • 1,1,2-Trichloroethane
  • 1,2-Dichloroethane
  • Freon 12
  • Vinyl chloride
  • 1,3-Dichloropropene
  • Glyphosate
  • Dikegulac
  • Imazypur

* For these chemicals, activated carbon is only effective in specific circumstances.

4. Chemicals for which Adsorption with Activated Carbon is Unlikely to Be Effective:

  • Acetone
  • Methylene chloride
  • Acetonitrile
  • 1-Propanol
  • Acrylonitrile
  • Propionitrile
  • Dimethylformaldehyde
  • Propylene
  • 1,4-Dioxane
  • Tetrahydrofuran
  • Isopropyl alcohol
  • Urea
  • Methyl chloride

AquaChain Engineering Tip

When operating multiple activated carbon columns in series, continuously monitor the effluent of the lead column. A slight breakthrough in this column indicates it's approaching exhaustion, allowing for timely rotation or regeneration before overall treatment efficiency is compromised in the subsequent columns.

Frequently Asked Questions

Q: What is the primary difference between Powdered Activated Carbon (PAC) and Granular Activated Carbon (GAC)? A: PAC is a fine powder typically used for batch treatment or periodic dosing, offering high surface area for rapid adsorption. GAC consists of larger granules, making it suitable for continuous flow-through systems in fixed filter beds, and is often preferred for regenerability in large-scale water treatment.

Q: Why is activated carbon more effective at removing non-polar substances than polar ones? A: Adsorption is primarily a physical process where non-polar substances, which have low solubility in water, are preferentially drawn to and adhere to the non-polar surface of activated carbon. Polar substances, being highly soluble in water, have a stronger affinity for water molecules and are less readily adsorbed onto the carbon's surface.

Q: How is spent activated carbon typically regenerated? A: Granular Activated Carbon (GAC) is commonly regenerated by thermal oxidation, where organic contaminants are burned off in a controlled furnace. While effective, this process typically results in a 5-10% loss of carbon efficiency and some material destruction, requiring periodic replenishment of the lost carbon.

For more information on filtration methods, see our guide on [Filtration]( {{ '/en/tech-library/filtration-2075b0022' | url }} ).