Noble metal nanoparticles for water purification: a critical review

Water is one of the essential enablers of life on earth. Beginning with the origin of the earliest form of life in seawater, it has been central to the evolution of human civilizations. Noble metals have been similarly associated with the prosperity of human civilizations through their prominent use in jewellery and medical applications. The most important reason for the use of noble metals is the minimal reactivity at the bulk scale, which can be explained by a number of concepts such as electrochemical potential, relativisitic contraction, molecular orbital theory, etc. Recently, water quality has been associated with the development index of society. A number of chemical and biological contaminants have endangered the quality of drinking water. An overview of important events during last 200 years in the area of drinking water purification is presented. Realizing the molecular nature of contamination in drinking water, significant progress has been made to utilize the chemistry of nanomaterials for water purification. This article summarizes recent efforts in the area of noble metal nanoparticle synthesis and the origin of their reactivity at the nanoscale. The application of noble metal nanoparticle based chemistry for drinking water purification is summarized for three major types of contaminants: halogenated organics including pesticides, heavy metals and microorganisms. Recent efforts for the removal, as well as ultralow concentration detection of such species, using noble metal nanoparticles are summarized. Important challenges during the commercialization of nano-based products are highlighted through a case study of pesticide removal using noble metal nanoparticles. Recent efforts in drinking water purification using other forms of nanomaterials are also summarized. The article concludes with recent investigations on the issue of nanotoxicity and its implications for the future

Pressurized CO2 Electrochemical Conversion to Formic Acid: From Theoretical Model to Experimental Results

To curb the severely rising levels of carbon dioxide in the atmosphere, new approaches to capture and utilize this greenhouse gas are currently being investigated. In the last few years, many researches have focused on the electrochemical conversion of CO2 to added-value products in aqueous electrolyte solutions. In this backdrop, the pressurized electroreduction of CO2 can be assumed an up-and-coming alternative process for the production of valuable organic chemicals [1-3]. In this work, the process was studied in an undivided cell with tin cathode in order to produce formic acid and develop a theoretical model, predicting the effect of several operative parameters. The model is based on the cathodic conversion of pressurized CO2 to HCOOH and it also accounts for its anodic oxidation. In particular, the electrochemical reduction of CO2 to formic acid was performed in pressurized filter press cell with a continuous recirculation of electrolytic solution (0.9 L) at a tin cathode (9 cm2) for a long time (charge passed 67’000 C). It was shown that it is possible to scale-up the process by maintaining good results in terms of faradaic efficiency and generating significantly high concentrations of HCOOH (about 0.4 M) [4]. It was also demonstrated that, for pressurized systems, the process is under the mixed kinetic control of mass transfer of CO2 and the reduction of adsorbed CO2 (described by the Langmuir equation), following our proposed reaction mechanism [5]. Moreover, the theoretical model is in good agreement with the experimental results collected and well describes the effect of several operating parameters, including current density, pressure, and the type of reactor used. 1. Ma, S., & Kenis, P. J. (2013). Electrochemical conversion of CO2 to useful chemicals: current status, remaining challenges, and future opportunities. Current Opinion in Chemical Engineering, 2(2), 191-199. 2. Endrődi, B., Bencsik, G., Darvas, F., Jones, R., Rajeshwar, K., & Janáky, C. (2017). Continuous-flow electroreduction of carbon dioxide. Progress in Energy and Combustion Science, 62, 133-154. 3. Dufek, E. J., Lister, T. E., Stone, S. G., & McIlwain, M. E. (2012). Operation of a pressurized system for continuous reduction of CO2. Journal of The Electrochemical Society, 159(9), F514-F517. 4. Proietto, F., Schiavo, B., Galia, A., & Scialdone, O. (2018). Electrochemical conversion of CO2 to HCOOH at tin cathode in a pressurized undivided filter-press cell. Electrochimica Acta, 277, 30-40. 5. Proietto, F., Galia, A., & Scialdone, O. (2019) Electrochemical conversion of CO2 to HCOOH at tin cathode: development of a theoretical model and comparison with experimental results. ChemElectroChem, 6, 162-172

Effect of the air pressure on electro-Fenton process

Electro-Fenton process is considered a very promising tool for the treatment of waste waters contaminated by organic pollutants refractant or toxic for microorganisms used in biological processes [1-6]. In these processes H2O2 is continuously supplied to an acidic aqueous solution contained in an electrolytic cell from the two-electron reduction of oxygen gas, directly injected as pure gas or bubbled air. Due to the poor solubility of O2 in aqueous solutions, two dimensional cheap graphite or carbon felt electrodes give quite slow generation of H2O2, thus resulting in a slow abatement of organics. In this context, we report here a series of studies [7-9] on the effect of air pressure on the electro-generation of H2O2 and the abatement of organic pollutants in water by electro-Fenton process. The effect of air pressure, current density, mixing and nature of the organic pollutant was evaluated. [1] E. Brillas, I. Sirés, M.A. Oturan, Chem. Rev., 109 (2009) 6570-6631. [2] C.A. Martínez-Huitle, M.A. Rodrigo, I. Sirés, O. Scialdone, Chem. Rev. 115 (2015) 13362–13407. [3] M. Panizza, G. Cerisola, Chem. Rev. 109 (2009) 6541–6569. [4] I. Sirés, E. Brillas, M.A. Oturan, M.A. Rodrigo, M. Panizza, Environ. Sci. Pollut. Res. 21 (2014) 8336–8367. [5] C.A. Martínez-Huitle, S. Ferro, Chem. Soc. Rev. 35 (2006) 1324–1340. [6] B.P.P. Chaplin, Environ. Sci. Process. Impacts. 16 (2014) 1182–1203. [7] O. Scialdone, A. Galia, C. Gattuso, S. Sabatino, B. Schiavo, Electrochim. Acta, 182 (2015) 775-780. [8] J.F. Pérez, A. Galia, M.A. Rodrigo, J. Llanos, S. Sabatino, C. Sáez, B. Schiavo, O. Scialdone, Electrochim. Acta, 248 (2017) 169-177. [9] A.H. Ltaïef, S. Sabatino, F. Proietto, A. Galia, O. Scialdone, O. 2018, Chemosphere, 202, 111-118

Reductive dechlorination of TCE and cis-DCE by zero-valent iron and iron-based bimetallic reductants

CEs are the most frequently detected pollutants in groundwater. Several studies have been shown iron-based bimetallic reductants as a good method toward to chlorinated ethylenes degradation. However, many fundamental issues surrounding the chemistry of this phenomena remains elusive. In this study, kinetics and compound specific isotope analysis for reductive dechlorination of TCE and cis-DCE by unamended iron and iron-based bimetal reductants was evaluated. Generally, all the bimetals reductants tested revealed to increase the reactivity of the degradation, in which palladium and nickel were the additional metals more reactive. Ethene and ethane were the major products of TCE degradation. It is supported the simultaneous hydrogenolysis and β-elimination reaction hypothesis, however, the first step of TCE degradation by Au/Fe undergoes preferably by β-elimination, while by unamended iron, Pt/Fe and Co/Fe goes preferably by hydrogenolysis. No apparent elucidation was obtained to explain the high reactivity on bimetals systems; Degradação do TCE e cis-DCE por ferro de valência zero e redutores bimetálicos à base de ferro Resumo: Etilenos clorados são os poluentes mais frequentemente detetados na água subterrânea. Vários estudos têm mostrado que redutores bimetálicos à base de ferro são um bom método para a degradação dos etilenos clorados. Porém, muitas questões fundamentais acerca da química deste fenómeno permanecem elusivas. Neste estudo foi avaliada a cinética e a análise isotópica de compostos específicos para a degradação do TCE e cis-DCE por ferro e redutores bimetálicos à base de ferro. Genericamente, os redutores bimetálicos mostraram aumentar a reatividade da degradação, sendo paládio e níquel os metais adicionais mais reativos. Os produtos principais da degradação do TCE foram eteno e etano. É apoiada a hipótese da simultaneidade de hidrogenólise e β-eliminação, porém, o primeiro passo da degradação do TCE por Au/Fe é realizada preferencialmente por β-eliminação, enquanto por ferro, Pt/Fe e Co/Fe é realizada preferencialmente por hidrogenólise. Não houve uma elucidação aparente para explicar a reatividade nos sistemas bimetálicos

SYNTHESIS AND IN VIVO BIOMEDICAL APPLICATIONS OF ULTRASMALL METAL NANOPARTICLES

Over the past decade, metal-based nanoparticles (MNPs) have gained much popularity in the field of nanomedicine owing to their exceptional physiochemical properties. Easy surface functionalization and conjugation with therapeutic moieties, stability, inertness, and inherent anticancer activities make MNPs promising diagnostic and therapeutic agents. Among different sizes of MNPs, which greatly affect their biodistribution and clearance, ultrasmall metal nanoparticles with the size less than 5 nm demonstrate unique pharmacokinetic properties, making them suitable for nanomedicinal applications. Therefore, many efforts have been made to synthesize various kinds of ultrasmall metal nanoparticles. In this study, a revolutionary synthesis method, termed as liquid diffusion synthesis (LDS) was developed to produce ultrasmall metal nanoparticles. In this new approach, simply immersing a dialysis bag containing an aqueous solution of a metal salt mixed with citric acid in a NaOH solution reservoir for tens of minutes, few-nm sized nanoparticles form inside the dialysis bag. Not only is this process exceptionally simple and cost effective, conducting at room temperature using aqueous solution of metal salt, citric acid and NaOH, but also it can produce a wide range of colloidal nanocrystals, covering all possible ultrasmall metal nanocrystals used as nanomedicine. Using this method, the synthesis of ultrasmall metal nanocrystals of Co, Ni, Cu, Au, Ag, Pd, Pt, and Lu have been demonstrated. Also, ultrasmall metal oxide nanoparticles can be produced using the same method. Ultrasmall nanoparticles of MnO, RuO2, Cu2O, FeO, ZnO2, and CeO2 have been synthesized. A mechanistic study was conducted to reveal the nanoparticle formation mechanism. It was found that the gradual change of the solution pH caused by the diffusion of OH- ions through the dialysis membrane played an essential role in the formation of these nanocrystals. Synthesized ultrasmall Cu nanoparticles have preliminarily been tested for its in vivo biomedical applications. It shows that Cu nanoparticles are stable in phosphate-buffered saline and fatal bovine serum. In vivo studies shows the renal clearability of Cu nanoparticles; about 67% of nanoparticles is excreted via urine after 48 hours of injection

Synthesis, characterization and aggregation behavior of carbon nanotube-metal oxide nanohybrids

Extracting multifunctional benefits by combining multiple nano-scale materials has driven materials science to develop nano-heterostructures, which are known as nanohybrids (NHs). Many such composite materials have been researched for applications in the energy sector and in biomedical devices and processes. Among these NHs, carbon nanotubes combined with metal oxides (MOs) are one of the most studied materials that provide unique advantages as electrocatalyst supports, and are currently being commercialized as embedded electrodes for fuels cells. NHs are not only a new class of complex materials but also brings in novel physicochemical properties that most likely cannot be captured by the sum of the properties of their components materials. Thus, understanding the environmental health and safety (EHS) of this new class of composite NHs is imperative. The first challenge that the nano-EHS community faces is to synthesize these materials with a range of MO loadings or composition under a controlled and comparable set of experimental conditions. In this dissertation, a set of carbonaceous-metal oxide NHs have been synthesized and characterized under comparable synthesis conditions. After synthesis, the underlying mechanisms of metal oxide formation on multiwalled carbon nanotubes (MWNT) surfaces has been enumerated, and finally, aggregation behavior of a select NH and its components has been assessed as a function of the metal oxide loading. A modified sol-gel technique has been developed to grow TiO₂, ZnO, Er₂O₃, and Pr₆O₁₁ nanocrystals on MWNT surfaces. The novelty of this technique is that, by varying reagent composition, metal oxide content on the MWNT surfaces can be controlled, keeping all other parameters unchanged. The modified synthesis protocol has been successfully developed to produce a relatively large amount of NHs (100s of mg per batch of synthesis), adequate for systematic nano EHS studies. Following detailed characterization of the materials, underlying hybridization and MO crystal formation mechanism(s) have been enumerated. Furthermore, standard electron potential of the metal species (while considering electron transfer between their oxidized state to zero valent form) has been found to be the controlling factor for the formation of metal or metal oxide crystals from the precursors on MWNT surfaces, using the sol-gel synthesis technique. Self-aggregation, one of the dominant environmental processes that particles undergo upon release into aquatic environment, has been assessed for one of the most used and commercialized NHs MWNT-TiO₂ and its components. This study investigated the role of TiO₂ loading on the aggregation behavior, MWNT-TiO₂ NH with three different TiO₂ loadings. Results suggested that TiO₂ loading on MWNT surfaces control aggregation behavior of the composite NHs. NHs with all TiO₂ loading demonstrated strong dependence on electrokinetics. Deoxygenation of the NHs with decreased TiO₂ loading due to the NH synthesis process appeared to be a key contributor on the electrokinetics of the NHs. The van der Waals interaction forces of the NHs decreased with decrease in TiO₂ loading. This study also concluded that classical DLVO theory may be inadequate to capture the aggregation behavior of the NHs. The controlled synthesis technique developed during this research, as well as the mechanisms of metal vs. metal oxide formation identified will be valuable to prepare a large set of NHs for nano-EHS studies. Aggregation behavior of the composites can be very complex in nature and cannot be predicted form the sum of the behavior of their component materials. The deviation of DLVO prediction from the experimental aggregation data calls for further investigation on direct measurement of other complex surface properties of the NHs upon hybridization such as surface roughness and surface charge heterogeneityCivil, Architectural, and Environmental Engineerin