Assessment of Key Environmental Factors Influencing the Sedimentation and Aggregation Behavior of Zinc Oxide Nanoparticles in Aquatic Environment
Zinc oxide nanoparticles (ZnO NPs) are among the most widely used engineered nanoparticles (ENPs) in various commercial sectors to achieve both social and economic benefits. The post-use release of these NPs to the environment is inevitable, and may pose threat to the human and eco-system. In the present study, we investigated the influence of single and multiple environmental factors on sedimentation behavior of ZnO NPs. The fractional-factorial method based on Taguchi orthogonal array (OA) L
... gonal array (OA) L 27 (3 13 ) design matrix was used for systematic investigation on the contribution and significance of multiple factors and their interactions. The result of single-factor showed that the ZnO NPs were unstable at or near pH zpc , with high electrolyte concentration; however, the adsorption of natural organic matter (NOM) i.e., humic acid, salicylic acid, and citric acid reverses the surface charge and enhanced NP stability. The Fourier transform infrared (FT-IR) analysis confirms the organic capping ligands on the NP surface. Moreover, the matrix result of analysis of variance (ANOVA) showed that electrolyte concentration and type, and NOM concentration were the most significant factors (p < 0.001) in promoting and influencing aggregation, while the interaction between the factors was also found insignificant. In addition, the result of aggregation kinetics and environmental water samples indicated that the mobility of ENPs may vary substantially in an environment with complex and heterogeneous matrices. This study may contribute to better understanding and prediction of the sedimentation behavior and fate of ZnO NPs in aqueous environments, to facilitate their sustainable use in products and process. Previous studies have shown that ENPs can enter into the human body directly or through bioaccumulation and exhibit some toxicity, such as inflammatory response, and cell membrane leakage [4, 5] . Similar hazardous effects on other aquatic biota including plants, mammalians, sea urchin and earthworms have also been reported [6, 7] . The tiny size of NPs may cause significant variation in the sedimentation and aggregation kinetics under various solution matrices  . Moreover, they are considered stable in suspension and high capability to convey toxic substance, while large size NPs have a tendency to aggregate which results in enhanced sedimentation  . Additionally, substantial inhibition effect of ZnO NPs was found on nitrogen and phosphorus removal activities in activated sludge or wastewater biofilms due to the release of zinc ion (Zn 2+ )  . In fact, the toxicity of these NPs is mostly attributed to surface properties, size, solubility, and various environmental factors of the aqueous solution [10, 11] . Once discharged in the aquatic environment, the NPs may transform into different forms i.e., either aggregate and dissolve or redistribute by means of sedimentation  . Consequently, such behavior will affect their fate and toxicity in the overall system. A number of studies      have shown that the sedimentation of NPs depends on various environmental factors mainly originated from water chemistry such as pH, ionic strength (IS), temperature and natural organic matter (NOM) type and content. It has been demonstrated that the surface potential and stability of ZnO NPs are highly reliant on the suspension pH, which influences through protonation/deprotonation of hydroxyl groups  . Similarly, the effects of various electrolytes i.e., monovalent (Na + , K + , etc.) and divalent (Ca 2+ , Mg 2+ etc.) cations, have been reported [14, 15] . The divalent cations efficiently compress the electrical double-layer (EDL) of NPs and consequently enhance the aggregation. In addition, the formation of zinc complexes with valence electrolytes also reduced the release of Zn 2+ [16, 17] . Some researchers have shown that changes in the water temperature may exert an effect on the solubility of NPs and may alter also their physicochemical interactions  . The stabilizing effect of NOM such as humic acid, citric acid etc. on NPs in the aquatic system has been reported in the literature [17, 19] . It has been shown that the high concentration of humic acid enhances the stability of ZnO NPs via electrostatic and/or steric repulsion mechanism, consequently decreases their aggregation rate  . It can also alter NPs dissolution behavior through chelation and/or complexation  . However, the low concentration of a humic substance in water bodies may increase the agglomeration and sedimentation process [16, 21] . While, low molecular weight NOM such as salicylic acid and citric acid, mostly found in lakes and rivers, may promote the dissolution of NPs and therefore elevate the toxicity of NPs in an aqueous environment [19, 22] . Some researchers [23, 24] have reported the destabilizing effect of NOM on NPs in the presence of divalent cations under varying pH conditions, which may be ascribed to intra-and/or intermolecular bridging effect between the NOM and NPs, resulting aggregation of NPs. The aggregation and sedimentation behavior are integrated phenomenon based upon complicated interaction among various physicochemical properties of NPs and solution chemistry. Understanding the environmental fate and mobility of ZnO NPs is essential for properly assessing their toxicity and risk. However, studies reported in the literature are limited to "one-factor-at-a-time" experiments to determine the sedimentation behavior of ZnO NPs. The literature also seems insufficient to study the interactive behavior of factors affecting this phenomenon. Therefore, it is significant to comprehensively understand the role of environmental factors in aquatic environment influencing sedimentation behavior of ZnO NPs in heterogeneous matrices. During last few decades, numerous statistical methods have been developed for analysis and optimization of experiment process parameters. Among them, the fractional factorial design based on Taguchi orthogonal array (OA)  utilizing the design of experiment approach (DOE), has been referred to [26, 27] as an efficient technique to study the effect of collective factors and their interaction. In this study, sedimentation behavior of ZnO NPs in various aqueous matrices was investigated using Taguchi standard OA L 27 (3 13 ) design matrix. The contribution and significance of individual and multiple environmental factors including, pH, temperature, electrolyte and NOM types, Water 2018, 10, 660 3 of 18 their concentration, as well as interaction among several parameters, were systematically studied. Additionally, the applicability of this method was verified against sedimentation experiments in various environmental water samples. In this study, we estimated ZnO NPs sedimentation behavior through concentration measurement rather than particle size. This approach may be another way to assess sedimentation behavior of ZnO NPs in an aqueous environment.