THE PROPERTIES OF NATURAL ORGANIC MATTER (NOM) AFFECT THE IMPACT OF NON-FUNCTIONALIZED AND FUNCTIONALIZED MULTI-WALLED CARBON NANOTUBES (MWCNTS) ON TOMATO PLANTS (SOLANUM LYCOPERSICUM)

Due to the high specific surface area and aspect ratio (length to diameter ratio, or L/D) of carbon nanotubes, they tend to bind strongly through the van der Waals interactions prevailing among tubes. Thus, they agglomerate and settle in water to form large conglomerates. The adsorption of natural organic matter (NOM) or surfactants onto raw multi-walled carbon nanotubes (MWCNTs) was shown to effectively enhance the dispersibility and stabilization of MWCNTs. The two kinds of dispersants used were humic acid and peptone. Also, two functionalized MWCNTs (-OH and -COOH functional groups) were also dispersed in humic acid (HA) to evaluate the effect of surface property on CNT biological interactions. Based on the dynamic light scattering (DLS) analyses, the use of surfactants increased the steric hindrance as well as the charge repulsion between adjacent CNT particles, thereby enhancing their suspension. HA and peptone sorption onto the surface of MWCNTs can cover their hydrophobic surfaces and help stabilize CNTs. Furthermore, hydroxyl modification of MWCNTs resulted in stable dispersions in water containing HA at 10 and 1000mg/L, while COOH-MWCNT suspensions displayed stable dispersion with lower negative surface charges solely at 100g/L. While, TEM images agreed with the DSL analysis that HA-stabilized MWCNTs were well-dispersed compared to pep-stabilized MWCNTs. Both f-MWCNT types showed a significant reduction in agglomerates as compared to the non-functionalized one. It was noted that the dispersion state as well as the surface properties of both MWCNTs and f-MWCNTs plays an effective role in the potential toxic effects of CNTs. Decreases in the growth rate, chlorophyll index, water uptake, dry weight, and root elongation rate along with a rise in mortality were detected as an indication of phyto-toxicity in both the pep-MWCNT suspensions at 1000mg/L and the peptone control seedlings in contrast to the seedlings treated with pep-MWCNTs at 10 and 100mg/L doses . This was an indicator for the presence of suspended MWCNTs as well as their unstable dispersion in the water column. However, the interaction between the HA-CNTs and the plants improved development in terms of water uptake, growth rate, chlorophyll index, dry weight and root elongation rate due to their well- dispersed stability in water. There were no differences among the f-MWCNT, MWCNT and HA plant groups in terms of their quantum yield and chlorophyll content. While the f-MWCNTs significantly enhanced the plants\u27 growth, water transpiration, and dry root and shoot weight as compared to the non-functionalized MWCNTs. It appeared that exposure to OH-MWCNTs improved the development of tomatoes in terms of water uptake, root elongation rate, and growth rate as well as light- and dark-adaptation, whereas COOH-MWCNTs and non-functionalized MWCNTs were apparently toxic in terms of root leakage and dark-adaptation. Overall, our results suggest that the surface properties of CNTs associated with their dispersion stability specify their influence on the growth of tomato plants. Moreover, the nature of the dispersant agent itself plays an active role in the toxicity of MWCNTs on tomatoes. Our investigation indicated that there is a significant correlation between the toxicity of unfunctionalized MWCNTs and f-MWCNTs and the toxicity of the dispersant agent