Removal Of Heavy Metals From Water In The Presence Of Typical Nanomaterials And The Effects Of Humic Acid

Engineered nanomaterials (ENMs), including carbon nanotubes (CNTs) and nanosized oxides, offer the features of large specific surface areas and surface hydroxyl and carboxyl groups, and thus have attracted great interest for application as superior adsorbents for the removal and pre-concentration of organic pollutants and heavy metal ions from wastewater. What’s more, natural organic matter (NOM) widespread in water environmental can interact with ENMs through varieties of interactions and alter their migration, transport, fate in the environment, and consequently affect the adsorption of heavy metal ions with ENMs. Therefore, This research studies the removal of heavy metals(Cu2+、Zn2+、Pb2+、Cd2+) from water in the presence of typical nanomaterials(including carbon nanotubes and nanosized oxides) and the effects of humic acid. The research contents and conclusions are as follows:(1) Engineered nanomaterials including CNTs and nanosized oxides have been introduced widely as potential superior adsorbents for the removal and pre-concentration of heavy metal cations from water in previous studies. However, we observed that the removal of heavy metal ions should be largely attributed to the precipitation by forming the metal hydroxyl precipitates rather than the adsorption on CNTs and nanosized oxides. One evidence is that the pH-dependent percent removal curves of heavy metal ions in the presence of CNTs and nanosized oxidesare overlapped with the precipitation curves of the heavy metal ions. Another evidence is that the pH-dependent percent removal curves are independent on the added dosage of the CNTs and nanosized oxides as well as the initial concentration of the heavy metal ions. The unreasonable interpretation for the heavy metal ion removal with the adsorption mechanism on nanomaterials in previous studies could be mostly attributed to the ignoring of the solution pH changes after mixed with nanomaterials, which can act as a buffer, and consequently the ignoring or underestimating of the precipitation of metal hydroxyl precipitates. Another possible reason for the unreasonable interpretation with the adsorption mechanism is the impurity of nanomaterials that can dissolve into water and form precipitates with heavy metal ions. The awareness of the importance of precipitation of metal hydroxyl precipitates in the removal of heavy metal ions from water in the presence of nanomaterials makes us to conclude that CNTs and nanosized oxides cannot be the superior adsorbents for the removal and pre-concentration of heavy metal ions from water.(2) The adsorption capacity of Cu2+ by nano-TiO2 and nano-γ-Al2O3 coated HA correlated positively to the HA loading, while the adsorption affinity correlated positively to the percent of carboxylic functional groups ratio to total functional groups. Nanosized oxides exhibited remarkable adsorption of heavy metal ions by coating HA because of the complexation between metal ions and surface functional groups, especially the carboxylic functional groups. Compared with nano-γ-Al2O3, More carboxylic functional groups were adsorbed on nano-TiO2. HA phenolic OH groups were responsible for its ligand exchange with nano-TiO2, leading to the percentage of carboxylic functional groups ratio to total functional groups increasing. Therefore, nano-TiO2 coated HA not only possessed the larger adsorption capacity but also stronger adsorption affinity.