Influence Of Humic Acid And Its Different Molecular Weight Fractions On Sedimentation Of Nanoscale Zero-Valent Iron

In recent years,water pollution problem has been growing increasingly serious along with the industrialization advancement,which threats the human health,economic and social sustainable development of the major obstacles.Nanoscale zero-valent iron?NZVI?particles have shown promise for in-situ remediation of groundwater contaminated with heavy metals,chlorinated organics and other reducible contaminants because of its high specific surf ace area and hence high reactivity.However,the colloid stability and reactivity of NZVI was influenced by the geochemical conditions,such as natural humic acid?HA?,ionic strength.HA can be regarded as highly heterogeneous nature compounds with complex structures and chemical properties,so the influence of HA is complex.The effects of HA and its different molecular weight?MW?fractions on the sedimentation of NZVI in the absence and presence of 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 10KDa,50KDa and 100KDa?.Differing sedimentation behavior was observed for NZVI with different MW fractions of HA.In the absence of HA,the degree of settling of NZVI particles became lower with increasing the concentration of HA.The degree of settling of NZVI particles in the presence of high MW fractions of HA?HA3?HA4?was lower than that with low MW fractions of HA?HA1?HA2?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.In the co-presence of Ca²⁺,distinct settling behavior of NZVI was observed in the presence of different MW fractions of HA.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 surface of NZVI,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.This study has far-reaching implications for the degradation of pollutants in groundwater by NZVI.