Interaction at the solid-water interface of 0d and 2d carbon nanoparticles : environmental impacts and applications for contaminant removal

Abstract

Carbon dots (CDs) and graphene oxide (GO) are rapidly emerging carbon-based nanomaterials that, due to their growing applications, will inevitably find their way to natural waters; however, their environmental fate is mostly unknown. Carbon dots with different surface functionality and graphene oxide were fabricated and characterized. Their surface charge, given by the zeta potential, and their hydrodynamic diameter in suspension were investigated under a variety of environmentally relevant conditions. The effect of ionic strength was studied in the presence of monovalent (NaCl) and divalent (CaCl2) cations, for pH levels from 3 to 11; humic acid was used as a model for dissolved natural organic matter. Total potential energies of interactions were modeled by classical DLVO theory. The experimental results for CDs showed that water chemistry altered the surface charge of the nanomaterials, but their hydrodynamic size could not be correlated to those changes. Nanoparticles remained largely stable in suspension, with some exception at the highest ionic strength considered. DLVO theory did not adequately capture the aggregation behavior of the system. Moreover, cation and/or humic acid adsorption negatively affected the emission intensity of the particles, suggesting limitations to their use in natural water sensing applications. The results for GO showed that the GO is negatively charged over a wide range of pH and the pH did significantly affect GO stability at a level of 4 or higher, but the particles became unstable below pH 3 due to protonation of −COOH at the edge. Ionic strength (IS) and salt type had observable effects on stability as a result of electrical double layer compression and specific interactions. CaCl2 affects GO more noticeably than NaCl because of the binding ability of Ca2+ ions with carboxyl and hydroxyl functional groups. The applicability of DLVO theory as a predictive tool was investigated by modeling GO sheets in two different geometries; three-dimensional sphere like particles and two-dimensional particles at different values of pH and IS. Overall, the specific interactions and chemical structure of adsorbed organics had a dominant role in GO stability. Laser induced graphene (LIG) can be fabricated in one-step, scalable, reagent-free process, by irradiation of a commercial polyimide (PI film) by a CO2 infrared laser under ambient conditions. This approach is environmentally and economically promising fabrication method. Laser induced graphene (LIG) was successfully fabricated on microporous ceramic membranes. The surface area, morphology, and chemical characterizations were performed on the LIG layer. Water contact angle measurements showed the hydrophobicity of LIG. Pure water and solvents with different polarities were used to understand the solvent flux behavior of LIG membrane. The LIG membrane showed very high non-polar solvent fluxes and remarkably low water permeability, and thus, the transport through the LIG membrane is related to dipole moment and dielectric constant, represented by solvent polarity. The LIG membrane achieved 90 [percent] rejection for 255 nm diameter silica particles, suggesting the presence of submicron size connecting pore channels that dominate the transport mechanism. Furthermore, LIG was used as adsorbent for the removal of atrazine (ATZ) from water. The effect of water chemistry, presence of humic acid (HA), adsorption time, and initial ATZ concentration on the adsorption process was explored. The prepared LIG exhibited significant removal of ATZ from aqueous solutions; hydrophobicity and [pi]-[pi] interactions played important roles in the process. Adsorption of ATZ on LIG followed a pseudo-second order kinetic model and Langmuir model for the isotherm with maximum adsorption capacity of 15.0 mg ATZ/g LIG. Adsorption was more favorable at higher pH and not affected by ionic strength. LIG exhibited enhanced performance over many previously reported adsorbents. The introduction stage of HA with respect to ATZ influenced the results: pre-introduction of HA reduced ATZ adsorption by 32 [percent], whereas post-introduction of HA resulted in a slight release of ATZ from the LIG.