Abstract
The effects of humic acid (HA) and its different molecular weight (MW) fractions on the sedimentation of nanoscale zero-valent iron (NZVI) in the absence and presence of cations (i.e., Na+/Mg2+/Ca2+) were investigated. Ultrafiltration (UF) was used as the method of fractionation to obtain four different MW fractions (separated by ultrafiltration membranes of 10 kDa, 50 kDa, and 100 kDa). Differing sedimentation behavior was observed for NZVI with different MW fractions of HA. Generally, the degree of settling of NZVI particles in the presence of high MW fractions of HA was lower than that of low MW fractions of HA and that without HA. The results were mainly attributed to the steric stabilization provided by the high MW fractions of HA. The presence of Na+/Mg2+/Ca2+ alone had insignificant influence on the settling of NZVI, but both Mg2+ and Ca2+ exerted an obvious influence on the settling of NZVI in the co-presence of HA. The settling behavior of NZVI was further examined in the co-presence of different MW fractions of HA and Ca2+. The co-presence of low MW HA fractions and Ca2+ led to a lower settling of NZVI. This might be due to the formation of a layer of HA-Ca2+ complex on the particle surface, providing stronger steric stabilization. Nevertheless, in the co-presence of high MW HA fractions and Ca2+, the settling of NZVI was initially reduced but accelerated with time, which might be due to the gradual aggregation of NZVI with time resulted from the bridging effect of HA-Ca2+ complex.