Adsorption - Activated Carbon

Adsorption removes dissolved organic pollutants, dyes, and micropollutants via surface binding to solid adsorbents achieving 80-95% color removal, 60-80% COD removal, and removal of trace contaminants (heavy metals, surfactants, phenols, pesticides) from textile effluent. Activated carbon dominant adsorbent due to high surface area (500-1,500 m²/g), porous structure (micro-pores <2 nm, meso-pores 2-50 nm, macro-pores >50 nm providing access), and versatile surface chemistry (non-polar and polar sites adsorbing diverse molecules). Adsorption mechanisms include physical adsorption (van der Waals forces, hydrophobic interactions binding organic molecules to carbon surface, reversible, multilayer possible), chemical adsorption (covalent or ionic bonding forming stronger irreversible attachment), pore filling (molecules entering pore structure, trapped by geometric constraints), and electrostatic interactions (charged carbon surface attracting oppositely charged ions, dyes). Carbon types include powdered activated carbon PAC (particle size 10-50 μm, rapid kinetics due to small size, dosed directly into wastewater 50-500 mg/L, contact time 15-60 min, requires separation via coagulation, sedimentation, or filtration, single-use non-regenerated, economical $800-2,000/tonne), and granular activated carbon GAC (particle size 0.5-5 mm, slower kinetics, used in fixed-bed columns, flow-through operation, regenerated via thermal or chemical methods, reusable 5-10 cycles, higher cost $1,500-3,500/tonne). Carbon production from carbonaceous materials (coconut shell—microporous, high hardness, premium $2,000-3,500/tonne; coal-based—balanced pore structure, economical $800-1,500/tonne; wood-based—mesoporous, moderate cost $1,000-2,000/tonne) via carbonization (pyrolysis at 400-600°C in inert atmosphere converting organic matter to char) and activation (physical activation at 800-1,100°C in steam or CO2 developing porosity, chemical activation at 400-700°C using H3PO4, ZnCl2, KOH creating pores and functionalizing surface). PAC process comprises dosing (50-500 mg/L depending on color, COD concentration, contact time required), mixing (30-60 min in contact tank or added to coagulation-flocculation providing dual benefit—adsorption + aiding flocculation), settling or filtration (removing carbon-laden solids, sand filtration 5-10 m/hr, membrane filtration 50-100 L/m²/hr), and disposal (spent carbon with adsorbed dyes non-regenerable at small scale, disposed via landfill $50-150/tonne or incineration recovering energy). GAC process uses fixed-bed columns (height 1-3 m, diameter 1-5 m, packed with GAC, flow upflow or downflow 5-20 m/hr, empty bed contact time EBCT 10-30 min), breakthrough monitoring (effluent color, COD measured continuously or periodically, breakthrough defined as reaching 5-10% of influent concentration signaling exhaustion), bed regeneration (thermal regeneration at 800-900°C in furnace oxidizing adsorbed organics, restoring 85-95% of capacity, carbon loss 5-10% per cycle, off-site regeneration $400-800/tonne or on-site $300-600/tonne if large scale >10 tonnes/cycle, alternatively chemical regeneration using acid, alkali, solvent extraction recovering 60-80% capacity, used for specific adsorbates). Adsorption capacity depends on carbon properties (surface area, pore size distribution, surface chemistry), adsorbate properties (molecular size, polarity, charge, concentration), and solution conditions (pH affecting charge, ionic strength affecting electrostatic interactions, temperature affecting kinetics and equilibrium, presence of competing species reducing capacity). Isotherms describe equilibrium relationship between adsorbate concentration in solution and on adsorbent: Langmuir isotherm (monolayer adsorption, q = qmax KL Ce / (1 + KL Ce) where q is uptake mg/g, qmax maximum capacity, KL affinity constant, Ce equilibrium concentration), Freundlich isotherm (multilayer, q = KF Ce^(1/n) where KF and n are constants), determining required carbon dose for target removal via isotherm experiments (batch equilibrium tests at varying doses 10-1000 mg/L, 24 hours contact, measuring residual concentration, fitting isotherm model). Performance includes color removal 80-95% (reactive, direct, acid dyes adsorbing well due to aromatic structure, affinity for carbon, disperse dyes less effective due to low solubility), COD removal 60-80% (organic molecules adsorbing, biodegradable organics may be partially removed improving overall COD reduction when combined with biological), removal of priority pollutants (heavy metals 50-90% depending on pH and speciation, surfactants 70-90%, phenols 80-95%, pesticides 85-98%, micropollutants at ng/L-μg/L levels reduced to below detection), and effluent polishing (post-biological, post-coagulation treatment achieving final color <20 Pt-Co, COD <80 mg/L suitable for reuse or discharge). Advantages include high removal efficiency (90%+ achievable for many dyes, organics), no sludge generation (unlike coagulation), simplicity (GAC columns automated, minimal supervision), flexibility (adjusting carbon dose, contact time, bed depth for varying influent quality), and producing high-quality effluent (suitable for reuse in non-critical applications—cooling, gardening, toilet flushing). Limitations include high carbon cost ($0.30-1.50/m³ depending on dose, regeneration frequency, influent strength), disposal or regeneration (spent PAC disposal $0.10-0.30/m³, GAC regeneration $0.20-0.60/m³ requiring off-site or on-site facilities), fouling by inorganics (calcium carbonate, iron, silica precipitating in pores reducing capacity 20-40%, requiring pretreatment softening, filtration), competition (biological organics, humic acids competing with dyes for sites reducing dye capacity 30-50%, requiring optimization of upstream treatment), limited for high-load effluent (PAC dose >500 mg/L or GAC exhausting in days uneconomical, suitable for polishing low-concentration effluent <500 mg/L COD, <200 Pt-Co color), and diffusion limitations (kinetics slow for large molecules, requiring long contact time 30-120 min). Applications include tertiary treatment (after biological and coagulation, final polishing for reuse or stringent discharge, 80-90% of adsorption use in textiles), standalone (for low-volume high-quality effluent like laboratory, sampling, color kitchen wastewater <50 m³/day), emergency treatment (PAC dosing during upset conditions, process failures providing temporary removal until normal operation restored), and advanced purification (reverse osmosis pretreatment removing organics preventing membrane fouling, potable reuse applications requiring >99% micropollutant removal). Alternatives to activated carbon include biosorbents (agricultural waste—rice husk, bagasse, sawdust, treated with acid/alkali providing capacity 10-100 mg/g vs. carbon 100-500 mg/g, low cost $100-500/tonne but lower performance, research/small-scale), zeolites (aluminosilicate minerals with ion-exchange capacity, effective for heavy metals, ammonia, less effective for dyes, cost $300-800/tonne), resins (synthetic polymeric adsorbents or ion-exchange resins, high capacity for specific dyes 200-500 mg/g, expensive $3,000-10,000/tonne, regenerable with acid/alkali/solvent, niche applications), and clays (bentonite, kaolinite providing low-cost adsorption 10-50 mg/g, effective for cationic dyes, less for anionic, cost $50-200/tonne, used in developing regions or low-budget projects). Regeneration economics determine viability: GAC regeneration economical if >10 tonnes/cycle (furnace capital $500,000-2,000,000 for 1-5 tonne/hour capacity, operating $300-600/tonne regenerated), smaller scales using off-site regeneration services ($400-800/tonne including transport), or switching to PAC if intermittent high-load treatment (PAC stockpiled, used as needed, avoiding continuous GAC replacement costs).