Inactivation of Escherichia coli planktonic cells by multi-walled carbon nanotubes in suspensions: Effect of surface functionalization coupled with medium nutrition level.

While earlier studies have identified the antibacterial activity of carbon nanotubes (CNTs) and proposed that cell membrane damage by direct contact with CNTs is likely the main toxicity mechanism, the relative importance of chemical versus physical properties of CNTs in controlling their bacterial cytotoxicity is understudied. Given that CNT is commonly modified via acid treatment to enhance its dispersivity and surface chemistry, in this study commercially available multi-walled carbon nanotubes (MWCNTs) with high purity were processed carefully by acid reflux, resulting in differences in surface charge of MWCNTs without altering their physical properties. The surface condition of MWCNTs was also modified by adsorption of organic matter to compare bacterial toxicity of functionalized and non-functionalized MWCNTs in suspensions. Results show that although overall electrostatic repulsion and steric obstruction resulted from surface modifications led to elevated dispersivity of MWCNTs and mitigated toxicity on planktonic Escherichia coli cultures, no correlation between the dispersivity and bacterial toxicity of MWCNTs was observed, suggesting that dispersity alone may not be a proper index to estimate the CNT antibacterial effect on planktonic cells in the aqueous phase. In addition, viability recovery of MWCNT-treated cells was observed to be nutrition level-dependent, implying that availability of proper nutrients may be another important factor to be considered when assessing the ecotoxicity of CNTs in the aquatic system.

The origin of Al(OH)(3)-rich and Al(13)-aggregate flocs composition in PACl coagulation.

The composition of hydrolyzed Al species is essential for the understanding of coagulation with Al-based coagulants. Surface characteristics of flocs formed by coagulation with two distinct polyaluminum chloride (PACl) coagulants were identified. One commercial coagulant (PACl-C) with voluminous monomeric Al and colloidal Al(OH)(3) and a custom-made PACl (PACl-Al(13)) containing high Al(13) content were applied to destabilize kaolin particles. The flocs formed by PACl-C and PACl-Al(13) at neutral and alkaline pH ranges, respectively, were observed by FE-SEM and HR-TEM. In addition, the Al composition of these flocs was characterized by XPS and HR-XRD, and the imaging of Al(OH)(3) precipitates and Al(13) aggregates were conducted by SEM as well as tapping mode AFM in liquid system. The observations of flocs indicate that the morphology of Al(OH)(3)-rich flocs are fluffy and porous around the edge of flocs, while the Al(13)-aggregate flocs have a glossy contour and irregular structure. Both Al(OH)(3)-rich and Al(13)-aggregate flocs do not possess well-formed crystalline structure except for the Al(13)-like crystal exists in the Al(13)-aggregate flocs. Among Al(OH)(3) precipitates, colloidal Al(OH)(3) is micro-scale in size, while amorphous Al(OH)(3) is nano-scale. During the formation of Al(13) aggregates, some coiled and clustered Al(13) aggregates with smoother surface were observed. The XPS study on floc surface showed that tetrahedral (Al(IV)) /octahedral (Al(VI)) Al ratio on the surfaces of PACl-C and PACl-Al(13) flocs is 1:1.6 and 1:9.9, respectively. Of the in situ formed Al(13), almost half of Al-hydroxide precipitates on the surface of Al(OH)(3)-rich flocs possess the Al(IV) center. It also found that the irregularly aggregated Al(13) with a similar Al(13) crystalline structure subsists on the surface of Al(13)-aggregate flocs.

Coagulation dynamics of fractal flocs induced by enmeshment and electrostatic patch mechanisms.

The size and structure of flocs during floc formation were monitored for various coagulation mechanisms. Two distinctive mechanisms, namely, enmeshment and electrostatic patch, govern the dynamics of kaolin particles coagulation by polyaluminum chloride (PACl). They were investigated by small angle static light scattering (SASLS) and solid-state (27)Al NMR. In addition, a novel wet SEM (WSEM) was used in-situ to image the morphology of the aggregate in aqueous solution. Synthetic suspended particles were coagulated by two PACl products, a commercial product (PACl) and one laboratory product (PACl-E). The PACl-E contained more than 60% Al(13) while the PACl contained only 7% Al(13), with large percentage of colloidal Al. For coagulation by PACl at neutral pH and high dosage where the strong repulsion between particles occurs, the enmeshment ruled by reaction-limited aggregation (RLA) results in larger sweep flocs as well as higher fractal dimensional structure. For coagulation by PACl-E at alkaline pH and low dosage, the flocs were coagulated predominately by electrostatic patch with Al(13) aggregates. At such condition, it is likely that diffusion-limited aggregation (DLA) predominately rule PACl-E coagulation. The fractal dimension (D(s)) values of PACl and PACl-E flocs formed at enmeshment and electrostatic patch increased with dosage, respectively. When breakage of flocs occurs, the breakage rate of PACl-E flocs is slower than that of sweep flocs. By WSEM imaging, the adsorption of spherical Al precipitates onto the particles was observed to form sweep flocs with a rough and ragged contour, while the PACl-E flocs were formed with a smooth and glossy structure.

Coagulation behavior of Al(13) aggregates.

The coagulation behavior of Al(13) aggregates formed in coagulation of kaolin was investigated by small angle static light scattering (SASLS), solid-state (27)Al NMR and tapping mode atomic force microscope (TM-AFM). A kaolin suspension was coagulated by PACl containing high content of Al(13) polycation (PACl-Al(13)). The results indicated that Al(13) was predominant in destabilizing kaolin particles for PACl-Al(13) coagulation even though at alkaline pH (pH 10). At such high pH, Al(13) aggregates were observed when the dosage of PACl-Al(13) was increased. In addition, the mechanism of coagulation by PACl-Al(13) at alkaline pH was affected by dosage. When the dosage was insufficient, coagulation was caused by electrostatic patch, which led to compact flocs with high fractal dimension (D(f)). Interparticle bridging dominated the coagulation when the coagulant dosage approached the plateau of adsorption, which caused the looser flocs with low D(f). The in-situ AFM scanning in liquid system proved that the existence of linear Al(13) aggregates composed of a chain of coiled Al(13) in coagulation by PACl-Al(13) at a high dosage and alkaline pH. Meanwhile, several coiled Al(13) aggregates with various dimensions were observed at such condition.