Predictive Model Development for Adsorption of Organic Contaminants by Carbon Nanotubes

The main objective of the study was to investigate mechanisms and statistical modeling of synthetic organic contaminant (SOC) adsorption by carbon nanotubes (CNTs). First, predictive models were developed for adsorption of low molecular weight aromatic compounds by multi-walled carbon nanotubes (MWCNTs) using experimental data for 59 compounds. Quantitative structure-activity relationship (QSAR) and linear solvation energy relationship (LSER) approaches were employed and developed models were externally validated using an independent dataset obtained from the literature. Up to date, no QSAR model has been reported for predicting adsorption of organics by CNTs. No LSER model is available which comprehensively investigates the adsorption of organics on CNTs. Only recently, one study reported an LSER equation for the modeling of their experimental adsorption data on one MWCNT. Then, adsorption of ten environmentally relevant halogenated aliphatic SOCs by a single walled (SWCNT) and MWCNT was tested experimentally for the first time in the literature. Several LSER models were developed to further examine the adsorption mechanisms. The LSER equations constitute the first predictive models generated for adsorption of aliphatic SOCs by CNTs. In addition, the poly-parameter LSER model was compared to those previously generated for adsorption of aromatic SOCs by CNTs. The LSER model generated in this research is currently the most comprehensive models available in the literature. Finally, the role of carbon nanotube morphology (i.e. surface area, diameter, and length) on the adsorption of phenanthrene (PNT) was investigated by analyzing the adsorption isotherms obtained with several SWCNTs and MCWNTs in the laboratory and the literature. The QSAR (r2 = 0.88), and LSER (r2 = 0.83) equations and their external validation accuracies indicated the success of parameter selection, data fitting ability, and the prediction strength of the developed models. These models were developed for adsorption of low-molecular weight (/mol) aromatic SOCs by MWCNTs (with less than 5% oxygen content) in distilled and deionized water. For aromatic SOC adsorption models, the molecular volume term (V) of the LSER model was the most influential descriptor controlling adsorption at all concentrations. At higher equilibrium concentrations, hydrogen bond donating (A) and hydrogen bond accepting (B) terms became significant in the models. For halogenated aliphatic SOC adsorption models, at higher concentrations, the B parameter, capturing hydrogen bond accepting ability, was the most influential descriptor both for SWCNT and MWCNT. The negative dependence on B indicates that as the hydrogen bond accepting ability of an aliphatic compound increases, it becomes less likely to be adsorbed by CNTs. The other important LSER parameters were V (size) followed by P (polarizability), and they were positively correlated with adsorption, indicating that size and polarizability favors adsorption. The contribution of these parameters was 2 - 3 times less than the B parameter. However, there was no single parameter predominant in the aliphatic SOC models. The number of data points for aliphatic SOCs were much smaller than aromatic models. These results indicated that adsorption of aromatic SOCs by CNTs strongly depend on adsorbate hydrophobicity; while for aliphatic SOCs, in addition to hydrophobic driving force, other interactions (i.e., hydrogen bond accepting ability) also play a role. Additional investigation of CNT properties on adsorption of PNT showed that at low (e.g., 1 μg/L) equilibrium concentrations, MWCNTs with the larger outer diameters exhibit higher adsorption capacity on a specific surface area basis than those with smaller diameters. With increasing equilibrium concentration, adsorption on a specific surface area basis becomes independent of MWCNT diameter, and maximum adsorption capacity was controlled by the total surface area. A similar analysis for the adsorption of naphthalene (NPT), a planar molecule with one less benzene ring but twenty times higher solubility than PNT, showed no correlation with respect to MWCNT outer diameter at both low and high equilibrium concentrations. The results indicated that the surface curvature of MWCNT was more important on the adsorption of PNT than on the adsorption of NPT due to its smaller molecular size and lower adsorption capacity than PNT. Specific surface area normalized isotherms did not show a correlation between PNT adsorption and lengths of SWCNTs and MWCNTs. Carbon nanotube characterization results showed that the morphology of CNTs impacts their aggregation and plays an important role on the available surface area and pore volume for adsorption. Manufacturer\u27s data may not always represent the characteristics of CNTs in a particular batch. Therefore, accurate characterization of CNTs is essential to systematically examine the behavior of CNTs (e.g., adsorption, transport) in environmental systems. A fundamental understanding of CNT-SOC adsorption interactions is important to (i) assess the environmental implications of CNT releases and spills to natural waters, and their roles as the contaminant carriers in the environment and, (ii) evaluate the potentials of CNTs as adsorbents in water and wastewater treatment applications. Predictive LSER modeling can be used to gain insight to the adsorption mechanisms by examining the individual contribution of intermolecular interactions to overall adsorption. This study examined and showed adsorption mechanisms and CNT properties (such as surface area, pore volume, outer diameter, and surface oxygen content) on the adsorption behavior of different classes of SOCs by CNTs

S7E4: How can we eliminate PFAS?

In recent years, communities across Maine and the U.S. have discovered the presence of toxic chemicals called per- and polyfluoroalkyl substances, or PFAS, in their land and water. Also known as forever chemicals because they are difficult to destroy, PFAS have been incorporated in various products, including food containers, clothing, rugs, teflon pans, fabrics and dental floss, for decades. Emerging research, however, has linked PFAS to several health issues, including weakened immune systems, increased risk of obesity and multiple cancers, developmental problems in children and harm to negative effects on reproduction. Onur Apul, assistant professor of environmental engineering at the University of Maine, is researching how to eliminate PFAS. He is one of many UMaine faculty members studying these forever chemicals and ways to mitigate them, and providing technical assistance to Maine farmers and other stakeholders. In Episode 4 of Season 7 of “The Maine Question,” Apul elaborates on the origins of PFAS, the threats they pose and efforts to stop their widespread contamination

Estimated Greenhouse Gas Emissions from PFAS Treatment of Maine Drinking Water

State law requires the adoption of a maximum contaminant level (MCL) for PFAS contamination in drinking water by June 1, 2024. While discussion of mitigation options has included the degree of risk reduction and the cost of treatment, indirect environmental effects are missing from the conversation. Until other technologies are developed, water treatment in Maine will likely rely on the established, energy-intensive method of PFAS adsorption (binding) to granular activated carbon. We conducted an inventory of greenhouse gas emissions associated with water treatment using this treatment media to fill gaps in the discussion. We found that greenhouse gas emissions for water treatment to bring PFAS down to the current interim standard are substantial, raising the footprint of an average user by 6.7–18 percent. We use this information to discuss implications for policymaking

Repeatable use assessment of silicon carbide as permanent susceptor bed in ex situ microwave remediation of petroleum-impacted soils

Efficiency of microwave-enabled ex situ soil remediation can be improved by dielectric susceptors. Cost, and environmental burden of these susceptors can be minimized if they are used repeatedly in a permanent bed set-up. In this study, carbon nanofibers, activated carbon, magnetite, and silicon carbide were tested at the lab scale for repeated use in permanent microwave-induced thermal soil remediation. Despite their superior ability to convert microwaves into heat, carbon nanofibers experienced electrokinesis and activated carbon partially combusted in the microwave cavity, which complicates their pragmatic use in remediation applications. Magnetite was also able to convert microwaves into heat effectively and it was relatively more stable; however, repeated heating/and cooling cycles changed its physicochemical properties, which was attributed to oxidation of iron oxides at the air-soil interface. Silicon carbide, on the other hand, was an efficient heating agent and was stable during repeatable heating and cooling cycles. Through 25 heating/cooling cycles, an average peak temperature of 329 ± 55°C was achieved for a 29 cm3 sample and analysis of dielectric properties after every 10th and 25th cycle indicated that there were no significant losses in thermal conductivity or permittivity of the material. Subsequent remediation experiments with silicon carbide demonstrated that between 89 and 97% of the total petroleum hydrocarbons were removed from soil including a marked fraction of heavy hydrocarbons when 20.2 kJ g−1 of microwave energy was introduced. In addition, post-treatment experiments demonstrated that soil conditions were capable of supporting seed germination indicating that some conditions of soil were recovered after microwave remediation

Arıtma çamur minimizasyonu :

Sludge management is one of the most difficult and expensive problems in wastewater treatment plant operation. Consequently, ‘sludge minimization’ concept arose to solve the excess sludge production by sludge pretreatment. Sludge pretreatment converts the waste sludge into a more bioavailable substrate for anaerobic digestion and leads to an enhanced degradation. The enhanced degradation results in more organic reduction and more biogas production. Therefore, sludge pretreatment is a means of improving sludge management in a treatment plant. Among pretreatment methods, acidic pretreatment has been subject of limited successful studies reported in the literature. On the contrary; ultrasonic pretreatment was reported as an effective pretreatment method. Main objective of this study was to investigate the effects of these two pretreatment methods and their combination in order to achieve a synergistic effect and improve the success of both pretreatment methods. Experimental investigation of pretreatment methods consists of preliminary studies for deciding the most appropriate pretreatment method. Anaerobic batch tests were conducted for optimization of the parameters of selected method. Finally, operation of semi-continuous anaerobic reactors was to investigate the effect of pretreatment on anaerobic digestion in details. Preliminary studies indicated that, more effective pretreatment method in terms of solubilization of organics is ultrasonic pretreatment. Fifteen minutes of sonication enhanced 50 mg/L initial soluble COD concentration up to a value of 2500 mg/L. Biochemical methane potential tests indicated that the increased soluble substrate improved anaerobic biodegradability concurrently. Finally, semi-continuous anaerobic reactors were used to investigate the efficiency of pretreatment under different operating conditions. Results indicate that at SRT 15 days and OLR 0.5 kg/m3d ultrasonic pretreatment improved the daily biogas production of anaerobic digester by 49% and methane percentage by 16% and 24% more volatile solids were removed after pretreatment. Moreover, even after pushing reactors into worse operating conditions such as shorter solids retention time (7.5 days) and low strength influent, pretreatment worked efficiently and improved the anaerobic digestion. Finally cost calculations were performed. Considering the gatherings from enhancement of biogas amount, higher methane percentage and smaller amounts of volatile solid disposal from a treatment plant; installation and operation costs of ultrasound were calculated. The payback period of the installation was found to be 4.7 years.M.S. - Master of Scienc

Ultrasonic pretreatment and subsequent anaerobic digestion under different operational conditions

In this study ultrasonic pretreatment was investigated in order to improve anaerobic digestion. First, the most effective sonication time was selected during the preliminary studies conducted on waste activated sludge samples. Then the optimal time selected was confirmed running batch anaerobic reactors. In the last part of the experiments, the effect of sonication was investigated for different operational conditions of semi-continuous digesters. Preliminary studies showed 15 min of sonication increased 50 mg/L initial soluble COD concentration up to a value of 2500 mg/L. Batch anaerobic digester results indicated that the increased soluble substrate improved anaerobic biodegradability concurrently, again with the maximal improvement observed for 15 min of sonication. Results from semi-continuous reactors indicated that at SRT of 15 days and OLR of 0.5 kg/m(3) d, ultrasonic pretreatment improved the daily biogas production, methane production and volatile solids reduction significantly when compared to control system. During the operation of reactors at 7.5 days of SRT, pretreatment helped to keep the reactors working. A simple economical analysis of the system was performed using the data obtained during the laboratory study

Can Capillary Suction Time be an Indicator for Sludge Disintegration?

Sludge disintegration is a tool for sludge minimization goal. Sludge disintegration (or sludge pretreatment) principally converts the waste activated sludge into a more bioavailable form prior to anaerobic digestion and contributes to the enhancement of anaerobic digestion. The indicator of success for sludge disintegration is generally taken as the concentration of released organics to the liquid phase. There are different examples of expressing the success of disintegration; such as degree of disintegration, soluble COD release, ratio of soluble to total COD, the released sCOD per unit amount of solids present or constituents of released sCOD. Capillary suction time (CST) is a simple method used to measure dewaterability of sludge samples. Relationship between dewaterability measured by CST and a number of sludge properties such particle size, bound water content, etc. has been shown in the past. Therefore, this study investigates whether CST can be used to express the effectiveness of sludge disintegration. For this purpose a number of sludge pretreatment techniques were tested and correlations between CST and sCOD were analyzed. Results show that CST can act as a good indicator of disintegration for some pretreatment methods but is not useful for indicating the disintegration levels if any of the pretreatment method involves some sort of conditioning

Adsorption of organic contaminants by graphene nanosheets, carbon nanotubes and granular activated carbons under natural organic matter preloading conditions

The effect of NOM preloading on the adsorption of phenanthrene (PNT) and trichloroethylene (TCE) by pristine graphene nanosheets (GNS) and graphene oxide nanosheet (GO) was investigated and comparedwith those of a single-walled carbon nanotube (SWCNT), a multi-walled carbon nanotube (MWCNT), and two coal based granular activated carbons (GACs). PNT uptake was higher than TCE by all adsorbents on both mass and surface area bases. This was attributed to the hydrophobicity of PNT. The adsorption capacities of PNT and TCE depend on the accessibility of the organic molecules to the inner regions of the adsorbent which was influenced from the molecular size of OCs. The adsorption capacities of all adsorbents decreased as a result of NOM preloading due to site competition and/or pore/interstice blockage. However, among all adsorbents, GO was generally effected least from the NOM preloading for PNT, whereas there was not observed any trend of NOM competition with a specific adsorbent for TCE. In addition, SWCNT was generally affected most from the NOM preloading for TCE and there was not any trend for PNT. The overall results indicated that the fate and transport of organic contaminants by GNSs and CNTs type of nanoadsorbents and GACs in different natural systems will be affected by water quality parameters, characteristics of adsorbent, and properties of adsorbate. (C) 2016 Elsevier B.V. All rights reserved

Towards Selective Removal of Bromide from Drinking Water Resources using Electrochemical Desalination

Disinfection of drinking water is crucial in water treatment as it suppresses waterborne pathogenic diseases. However, as an unintended consequence, disinfection generates disinfection byproducts (DBP). DBPs are cytotoxic, carcinogenic, and nephrotoxic especially when they are brominated. Brominated DBP formation is governed by the interaction of reactive precursors such as natural organic matter (NOM), and Brˉ with oxidants that are added as disinfectants (e.g., chlorine, chloramines, ozone). Historically, the main strategy to control the formation of DBP was to remove NOM from water by coagulation, adsorption, bio-filtration, pre-oxidation, or membrane separation; however, processes that remove NOM do not necessarily remove Brˉ. Herein, we investigated the utilization of combined capacitive and faradaic ion removal in a flow cell to remove Brˉ as well as Clˉ concurrently with more selectivity towards the former. The effectiveness of the proposed technique was evaluated by determining the maximum salt adsorption capacity and measuring the specific ion concentration with ion chromatography. In a binary equimolar mixture of Brˉ and Clˉ, Brˉ was more selectively adsorbed over Clˉ at 1.2 V applied potential due to the contribution of bromine gas evolution to the capacitive deionization