Sorption Behavior And Mechanisms Of Organic Contaminants By Natural Inorganic Nanorarticles

Nanotechnology has evolved as an interdisciplinary area, which has attracted great interest. Engineered nanoparticles (NPs) may enter the body and be toxic to humans, animals, and plants. Due to the perceived environmental risks of engineered NPs, recent researches has increasingly focused on natural NPs. An improved understanding of the capability of natural NPs as special sorbent for organic contaminants (OCs) removal and the mechanisms involved are required, in order to provide improved knowledge of useful applications of environmental-friendly natural NPs in polluted water treatment and environmental remediation. The main objectives of this study were to investigate the capability of nano-hematite, nano-montmorillonite, and nano-kaolin for the sorption of typical environmental OCs in aqueous solution. This was done by conducting sorption experiments under natural ambient conditions using the contrasting solution chemistry of electrolytic pH and ionic strength. Ionizable pentachlorophenol (PCP) and non-ionizable phenanthrene (PHE) were selected as representative OCs. The main results were as follows:The sorption capacities of PCP and PHE on the three different types of natural NPs were pH-dependent. A larger amount of PCP on nano-hematite was sorbed at pH values below its pKa (4.75). However, the PHE sorption capacity on nano-hematite was higher at relatively high or low pHs (e.g. below4and above12), possibly due to the larger available surface area of the nano-hematite, caused by the higher values of net charges and charge density. The affinity and the sorption capacity of nano-montmorillonite for PCP increased in the order of pH10<<pH6<<pH4. However, the affinity and the sorption capacity of nano-montmorillonite for PHE increased in the order of pH4<pH6<pH10. The affinity and the sorption capacity of nano-kaolin for PCP increased in the order of pH10<pH6<pH4. However, the affinity and the sorption capacity of nano-kaolin for PHE increased in the order of pH4<pH6<pH10. At pH6and10, hydrophobic interactions were employed to interpret the sorption of PCP and PHE by nano-kaolin, but hydrogen bonding interactions were stronger than hydrophobic interactions at pH4. It thus appears that the sorption of PCP and PHE on the three different types of natural NPs might be caused by a complicated interactive effect of speciation of surface functional groups on sorbent and specific electronic properties of sorbates at different solution pH.Base ions regulate the stability and aggregation behaviour of natural NPs. The influence of different ionic strengths indicated that the amounts of PCP and PHE sorbed by the three different types of natural NPs decreased as a concentration function of different types of ions (e.g. Na+, K+, Mg2+and Ca2+). PCP and PHE sorption were greater at low ionic strength. The increase of ionic strength in the solution greatly increased the aggregation of natural NPs, and such aggregation could contribute to the reduction of available sites for PCP and PHE sorption. Less adsorption sites provided by the smaller surface area of NPs may represent a dominant role in sorption of PCP and PHE, and so be responsible for the suppressed sorption of PCP and PHE, at relatively higher ionic strength. Moreover, competitive sorption by ions in the ambient solution, as well as the screening effects of the surface charge of natural NPs all played an important role in the suppressed sorption of PCP and PHE, as a concentration function of ions.Due to different surface properties and crystal structure, the sorption capacity and sorption mechanism of different natural NPs behaved differently. Overall, the affinity and the sorption capacity of nano-montmorillonite were superior to nano-hematite and nano-kaolin. The sorption capacity of PCP increased in the order of nano-kaolin<nano-hematite <<. nano-montmorillonite. The affinity and the sorption capacity of PHE increased in the order of nano-hematite<nano-kaolin<nano-montmorillonite. By comparison of both sorption capacity and ecologic advantages, our results suggested that natural NPs would be more competitive and efficient for PCP and PHE sorption than engineered NPs due to higher sorption capacity, shorter equilibrium time, lower cost and higher sorption affinity, without potential adverse consequences.