Effects of ionizable organic compounds in different species on the sorption of p-nitroaniline to sediment

In natural water systems, sorption of an organic pollutant to soil/sediment is often influenced by coexisting organic compounds. For ionizable organic compounds, molecular form may do differently competitive effect from ionic form. In order to examine competitive power of ionizable organic compounds in different forms, effects of some ionizable organic compounds including organic acids and organic bases in molecular form or ionic form on the sorption of p-nitroaniline to a sediment from water were investigated by batch equilibrium experiments. Results exhibit that molecular organic acids are much stronger in competitive power to p-nitroaniline than ionic ones, while ionic ones of organic bases compete with p-nitroaniline stronger than molecular ones. In addition, the competitive powers of coexisting organic acids in molecular form or ionic form in binary-solute systems are good correlated with the nonlinear sorption of cosolutes in single-solute systems, i.e., the stronger nonlinear sorption of the cosolute in single-solute systems, the stronger competitive effect of the cosolute on p-nitroaniline is showed in binary-solute systems. For organic amines, organic cations could be adsorbed through cationic exchange to the sediment more favorably than their molecular forms, and consistently, the former shows stronger competitive power than the latter. These results are of significance for one to describe accurately sorption of ionizable organic compounds in natural environment.