Acidic surface functional groups control chemisorption of ammonium onto carbon materials in aqueous media

Elucidation of mechanistic insight into the interaction of carbon materials' physicochemical surface properties and ammonium (NH₄⁺) adsorption in aqueous media was made by conducting a systematic study using a wide range of carbon materials. Three types of biochars (rice husk, poultry litter, and enhanced poultry litter) and activated carbons (fresh and aged coconut shell-based and charcoal-based) were used for investigating the NH₄⁺ adsorption mechanism. Poultry litter biochar, with lowest surface area (3 m² g^-1) and largest pore diameter (29 nm), showed the highest NH₄⁺ adsorption capacity (0.34 mg NH₄⁺g^-1), while charcoal-based activated carbon, with the highest surface area (1133 m² g^-1) and small pore diameter (6 nm), had the least NH₄⁺ adsorption capacity (0.09 mg NH₄⁺g^-1). The value of Freundlich isotherm constant 'n' was >1 for all tested carbon materials indicating chemisorption as the dominant sorption mechanism. Aging of the carbon surface resulted in 30% increase in NH₄⁺ retention. Surface chemical properties that most influenced NH₄⁺ chemisorption on to carbon materials were found to be acidic surface functional groups (ASFGs), elemental composition, ash content, and pH. The optimal conditions for NH₄⁺ adsorption, regardless of type and source of carbon materials, were solution pH of 8, a high amount of ash content, and carboxyl, carbonyl, and phenolic functional groups. Evaluation of CEC and ASFGs indicated that CEC and ASFGs are not equivalent terms. Through this study, conducted on carbon adsorbents derived from different sources, with different surface physical and chemical properties, we established that ASFGs, and not CEC, play a critical role in ammonium chemisorption on carbon materials. The study showed that low cost and eco-friendly biochars, with optimal surface chemistry, can replace expensive activated carbons for NH₄⁺ remediation in aqueous media.