Single evaluation and selection of functional groups containing N or O atoms to heavy metal adsorption: Law of electric neutrality.

Adsorption capacities and characteristics of heavy metal adsorbents have been investigated thoroughly, but the essential functional groups to bind heavy metal pollutants in composites have not been identified and confirmed separately. Previous researches reported that –OH, –NH2, –CONH2 or their protonation had binding effects to heavy metals. However, these descriptions were all based on the complex composites. The composites were consisted of different functional groups. Thus, this article aims to evaluate and discuss (1) the adsorption properties of different functional groups containing N or O atoms, (2) the protonation of groups containing N atoms, (3) the basis properties of adsorbents related to adsorption, (4) the physical adsorption by network structure. By a series of single evaluations, the results showed that the hydrophobic ester group (R1-COO-R2), non-ionic groups, including –OH, –NH2, –CONH2, ether (C–O–C), CO, tertiary amine (R3N), and protonation of –NH2 and R3N, had no interaction with metals. Only negative groups, such as -COO-, –SO3-, could adsorb cationic heavy metals. And positive –N(CH3)3+ group could bind with Cr2O72−. Furthermore, these conclusions have been verified by the law of electric neutrality in the heavy metal solutions and solid adsorbents. This study determined that the combining process between negative functional groups and cationic metals, or between positive groups and anionic metals, can be applied to decrease the heavy metal concentration.