Design of Efficient Carbon-Based Adsorbents for the Removal of Organic and Inorganic Water Contaminants

Granular activated carbon (AC) and biochar (BC) are two carbon-based adsorbents commonly used for water and wastewater treatment. However, these adsorbents have drawbacks that suppress their aqueous contaminants removal efficiency. Their major disadvantages are that AC has low selectivity and reusability potential, and BC has a hydrophobic nature. The scope of this dissertation is to enhance the performance of commonly-used carbon-based adsorbents for the removal of organic and inorganic water contaminants and to understand the interactive mechanism of contaminants’ ions/molecules with adsorbents. Hexavalent chromium (Cr(VI)) and trichloroethylene (TCE) are two types of inorganic and organic water contaminants, respectively, which are introduced to receiving waters mainly by anthropogenic activities. Both Cr(VI) and TCE have been categorized as primary water contaminants by the United States Environmental Protection Agency (US EPA). In this work, AC and BC were modified, and their modification conditions were optimized. Activated carbon was modified through two different approaches. The first approach was coating granular activated carbon with polysulfide rubber, and the second approach was impregnating activated carbon with zero-valent iron nanoparticles. Modified and unmodified ACs were then compared for aqueous Cr(VI) removal efficiency. Biochar was also modified through two procedures. The first BC modification procedure is heat treatment aiming to make it hydrophilic, and the second approach is impregnating biochar with zero-valent iron nanoparticles. In the case of biochar modification, two modification procedures are finally combined to take advantage of the benefits of both. Modified and unmodified biochars were compared for the removal of p-nitrosodymethylanilin and TCE removal from aqueous phase. Adsorbents, before and after modification, were well characterized using various materials analysis techniques for surface morphology, surface chemistry, elemental composition, pore structure, surface charge, etc. Enhanced performances of the modified adsorbents were demonstrated by comparing contaminants’ adsorption capacity and removal rate. Kinetics studies were performed to investigate mechanisms and rates of the aqueous contaminant removal. Results of this study would benefit water treatment engineers and materials scientists to gain a better understanding of adsorbents’ drawbacks, correlate these characteristics to adsorbents’ physical and chemical properties, and purposefully modify them for adsorbents’ specific applications. This study helps to design highly-efficient materials by employing practical modification procedures on conventional adsorbents

Optimization of Photocatalytic Degradation of Acid Blue 113 and Acid Red 88 Textile Dyes in a UV-C/TiO₂ Suspension System: Application of Response Surface Methodology (RSM)

Textile industries produce copious amounts of colored wastewater some of which are toxic to humans and aquatic biota. This study investigates optimization of a bench-scale UV-C photocatalytic process using a TiO2 catalyst suspension for degradation of two textile dyes, Acid Blue 113 (AB 113) and Acid Red 88 (AR 88). From preliminary experiments, appropriate ranges for experimental factors including reaction time, solution pH, initial dye concentration and catalyst dose, were determined for each dye. Response surface methodology (RSM) using a cubic IV optimal design was then used to design the experiments and optimize the process. Analysis of variance (ANOVA) was employed to determine significance of experimental factors and their interactions. Results revealed that among the studied factors, solution pH and initial dye concentration had the strongest effects on degradation rates of AB 113 and AR 88, respectively. Least-squares cubic regression models were generated by step-wise elimination of non-significant (p-value > 0.05) terms from the proposed model. Under optimum treatment conditions, removal efficiencies reached 98.7% for AB 113 and 99.6% for AR 88. Kinetic studies showed that a first-order kinetic model could best describe degradation data for both dyes, with degradation rate constants of k1, AB 113 = 0.048 min−1 and k1, AR 88 = 0.059 min−1

Biochar Nanocomposite as an Inexpensive and Highly Efficient Carbonaceous Adsorbent for Hexavalent Chromium Removal

Biochar is commonly used for soil amendment, due to its excellent water-holding capacity. The Cr(VI) contamination of water is a current environmental issue in industrial regions. Here, we evaluated the effects of two-step modifications on boosting biochar’s performance in terms of the removal of aqueous hexavalent chromium (Cr(VI)), along with investigating the alterations to its surface properties. The first modification step was heat treatment under air at 300 °C, producing hydrophilic biochar (HBC). The resulting HBC was then impregnated with zero-valent iron nanoparticles (nZVI), creating an HBC/nZVI composite, adding a chemical reduction capability to the physical sorption mechanism. Unmodified biochar (BC), HBC, and HBC/nZVI were characterized for their physicochemical properties, including surface morphology and elemental composition, by SEM/EDS, while functional groups were ascertained by FTIR and surface charge by zeta potential. Cr(VI) removal kinetic studies revealed the four-time greater sorption capacity of HBC than BC. Although unmodified BC showed faster initial Cr(VI) uptake, it rapidly worsened and started desorption. After nZVI impregnation, the Cr(VI) removal rate of HBC increased by a factor of 10. FTIR analysis of biochars after Cr(VI) adsorption showed the presence of Cr(III) oxide only on the used HBC/nZVI and demonstrated that the carbonyl and carboxyl groups were the main groups involved in Cr(VI) sorption. Modified biochars could be considered an economical substitute for conventional methods

Assessment of p-Nitroso Dimethylaniline (pNDA) Suitability as a Hydroxyl Radical Probe: Investigating Bleaching Mechanism Using Immobilized Zero-Valent Iron Nanoparticles

Use of p-nitroso dimethylaniline (pNDA) as a selective hydroxyl radical (•OH) probe compound has been extensively reported in the literature for evaluating the performance of the catalysts used in Advanced Oxidation Processes (AOPs). Some recent studies speculated that pNDA might be oxidized by other oxidizing species, and might also be directly reduced. This study, for the first time, investigates pNDA bleaching mechanism by a Fe0-containing composite, capable of causing both reduction and Fenton-like oxidation. The composite consisted of Santa Barbara15 (SBA-15) mesoporous silica as the supporting medium for nZVI immobilization (nZVI/SBA15). The quantity of iron in the composite was optimized by synthesizing four various nZVI/SBA-15 samples with different iron to silica weight ratios and comparing their pNDA bleaching efficiency. Kinetic studies showed a pseudo-second-order model for pNDA bleaching using all nZVI/SBA-15 samples. The effects of •OH scavengers and dissolved oxygen concentration on pNDA bleaching rate were examined. FTIR analysis of pNDA solutions showed different molecular structures for pNDA bleached under different DO conditions, leading to the conclusion that pNDA is susceptible to reductive bleaching. Results demonstrated that when using pNDA as a probe for free radicals’ generation assessment, the reductive potential of the catalysts needs to be considered