Surface Area and Porosity
Activated carbon has an internal surface area of 800-1200 m² per gram — roughly the area of four tennis courts packed into a sugar cube. This enormous surface comes from a network of micropores (< 2 nm), mesopores (2-50 nm), and macropores (> 50 nm) created by thermal or chemical activation of raw materials like coconut shell, coal, or wood. Contaminant molecules diffuse into these pores and bind to the carbon surface through van der Waals forces.
The Freundlich Isotherm
The Freundlich equation q = K_F × C^(1/n) describes how much contaminant carbon can hold at a given water concentration. K_F quantifies adsorption capacity — higher values mean the carbon has greater affinity for the target compound. The exponent 1/n describes the isotherm shape: values below 0.5 indicate favorable adsorption where capacity changes little with concentration, ideal for treatment applications.
Breakthrough and Column Design
In a GAC column, adsorption occurs in a moving mass transfer zone (MTZ). Fresh carbon at the outlet provides polishing while saturated carbon at the inlet is exhausted. Breakthrough occurs when the MTZ reaches the outlet and effluent quality begins to degrade. The simulation models this process, showing how empty bed contact time (EBCT) and carbon properties affect the volume of water treatable before replacement.
Regeneration and Sustainability
Spent GAC can be thermally regenerated at 800-900°C in the absence of oxygen, driving off adsorbed contaminants and restoring 85-95% of original capacity. This dramatically reduces cost and waste compared to single-use disposal. Large treatment plants operate on-site reactivation furnaces. The carbon typically survives 5-10 regeneration cycles before requiring replacement.