Adsorption and migration of ammonia nitrogen in sediments in the presence of SiO2 and HA colloids

The release of internal pollutants in shallow lakes is a widely concerning environmental issue. Colloids widely existing in eutrophic shallow lakes will affect the release and migration of pollutants. In this context, the different effects of two colloids [humic acid (HA) and silica (SiO2)], either individual or in coexistence, on the adsorption and migration of ammonia nitrogen (NH4+-N) in sediments under different ionic strengths (IS) were analyzed, through a series of batch and column experiments. It was found that, under a relatively low IS condition (1 mM Na+), SiO2 colloids greatly enhanced the mobility of NH4+-N in sediments. With IS increased (20 mM Na+), the presence of colloids exhibited an impeding effect on the mobility of NH4+-N due to colloidal flocculation and precipitation. While HA colloids largely suppressed the mobility of NH4+-N in sediments under either low or high IS conditions. HA colloids are more likely to form aggregates and have more adsorption sites than SiO2 colloids. In the NH4+-N–SiO2–HA system, HA colloids also showed a retardant effect on the transport of both SiO2 colloids and NH4+-N, and the retardant effect increased with the increase of HA colloid concentration. Exchange coordination and hydrogen bond force, as well as the decreased stability of the colloid system, played an important role in such a retardant effect, which consequently resulted in more NH4+-N adsorption. The transport of colloids in the NH4+-N–colloids co-transport system can be successfully described by Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. In addition, with an increase of IS, NH4+-N transport was enhanced because of competitive adsorption of the cations and NH4+-N on colloids and the porous medium. These results implied that abundant HA colloids and low ion concentration can reduce the risk of NH4+-N release and migration in sediments and consequently its exchange flux across the sediment-water interface.