Separation of specific divalent cations and microplastics from dilute aqueous solutions
Date
2024
Authors
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Publisher
University of Delaware
Abstract
Historically, water quality has always been an important factor controlling human welfare. However, the presence of chemical contaminants in water bodies continues to raise public concerns worldwide. Existing technologies usually cannot achieve desired efficiency in the treatment of water containing specific contaminants and tend to be either chemical or energy intensive, especially when it comes to very dilute concentrations, e.g., heavy metal ions and hardness ions, or particles with nano-size, e.g., submicron microplastic (MPs) particles. This dissertation aimed to develop effective, low cost, and energy efficient techniques to separate and remove contaminants from aquatic systems. The key factors affecting the separation processes were explored. ☐ The specific chemical adsorption of selected divalent heavy metal ions was studied using hydrous γ-Fe2O3-biochar. The synthesis of γ-Fe2O3-biochar through Fe(VI) treatment of bamboo biochar increased surface acidity and magnetism strength, which enhanced metal ion adsorption and enabled easy magnetic separation of spent biochar after use. The adsorption characteristics of selected metal ions including Cu(II), Pb(II), Zn(II), Ni(II), Cd(II) and Co(II) onto hydrous γ-Fe2O3-biochar were studied. The adsorption isotherms were well fitted by the Langmuir adsorption model. The γ-Fe2O3-biochar exhibited significantly high monolayer metal coverage capacity. For example, the maximum Cu(II) adsorption density was 55.45×10-5 mol/g, which was 4.0 times that of plain biochar (13.75×10-5 mol/g) at pH 6. Metal ion adsorption reactions could be described by surface complex formation, involving all M(II) hydroxy species and surface hydroxy species, i.e., ≡ COH and ≡ FeOH. Both experimental and calculation results suggested the formation of inner- and outer-sphere complexes. Coulombic and specific chemical forces contributed to the total adsorption energy, with specific chemical energy being the dominating component for the adsorption of hydrolyzable metal ions onto γ-Fe2O3-biochar surface. Under the context of surface complex formation, it is visualized that metal ion hydrolysis reactions occurred on γ-Fe2O3-biochar surface, in parallel to that in the bulk phase. ☐ The electrosorption of divalent hardness ion, i.e., Ca2+, from aqueous solution was studied using a graphite supported activated carbon electrode (NSA@G). Zeta potential measurement revealed a low pHzpc of 3.0 for the NSA electrode, suggesting a negatively charged L-type carbon surface. The electrosorption behavior of Ca2+ ions followed the Langmuir adsorption isotherm and pseudo-first-order rate law. Electrosorption density and rate were influenced by the initial Ca2+ ion concentration, solution pH, and applied potential. Results showed that both reversible surface charge, regulated by the potential determining ions, i.e., H+ and OH- (or pH), and polarizable surface charge, controlled by the applied potential (or pE), contributed to the overall Ca2+ ion removal process. The contribution of reversible and polarizable surface charge varied with pH and applied potential. Specifically, the reversible surface charge played a more significant role at high pH value and low applied potential (60 ~ 83% of the total Ca2+ uptake), while the polarizable surface charge dominated at low pH and high applied potential (60 ~ 62% of the total Ca2+ uptake). The electrosorption capacity of NSA@G electrode for different divalent alkaline earth metals, i.e., Ca2+, Mg2+, Sr2+, and Ba2+, depended on ionic radius, hydration ratio, and hydration enthalpy. ☐ An innovative electro-assisted separation (EAS) system was developed to separate ultrafine MP particles, i.e., submicron polystyrene (PS) latex, from water solutions. Upon an application of an electric field, negatively charged MP particles migrated against the drag force exerted by water flow via electrophoretic movement. The critical electric field strength (Ec) required for effective separation was determined by the hydraulic condition, i.e., water velocity, and zeta potential of the MP particles. By applying an electric field exceeding Ec, complete removal of MPs from water could be accomplished, yielding an MP-depleted dilute stream at the cathode side and an MP-enriched concentrate stream at the anode side. A theoretical approach was developed to describe MP transport within the EAS system. The simulation results agreed well with the experimental observations across various operational conditions, including electric field strength, hydraulic condition, particle size, and particle concentration. ☐ The concentration and collection of MP particles from water was studied using an EAS system. By optimizing the interplay between electrophoretic and convection forces during the EAS process, MP particles in water solution can be efficiently enriched into a small volume. The concentration factor was influenced by the applied electric field, hydraulic conditions, and zeta potential of MP particles. Notably, at a specific hydraulic condition (i.e., qc = 0.1) and an electric field of 80 V/cm, the concentration factor of 500 nm MP particles reached 2.60 ± 0.15. However, smaller qc values, i.e., elevated filtration velocities, and higher electric fields may lead to higher mass loss of MPs. Results suggested that the specific energy consumption for MPs concentration increased with a decrease in qc and particle size. ☐ In summary, this dissertation offers solutions to address existing water quality challenges. It develops innovative, effective, and energy-efficient separation techniques for the removal of specific divalent cations and MPs from water. These findings hold great promise for advancing contaminant removal practices and enhancing overall water quality.
Description
Keywords
Hardness, Heavy metal ions, Microplastics, Separation