Sorption of chlorinated hydrocarbons from synthetic and natural groundwater by organo-hydrotalcites: Towards their applications as remediation nanoparticles

Functional materials contributing to the removal of chlorinated hydrocarbons from soil and groundwater: Classification and intrinsic chemical-biological removal mechanisms

Chlorinated hydrocarbons (CHs) are the main contaminants in soil and groundwater and have posed great challenge on the remediation of soil and ground water. Different remediation materials have been developed to deal with the environmental problems caused by CHs. Remediation materials can be classified into three main categories according to the corresponding technologies: adsorption materials, chemical reduction materials and bioaugmentation materials. In this paper, the classification and preparation of the three materials are briefly described in terms of synthesis and properties according to the different types. Then, a detailed review of the remediation mechanisms and applications of the different materials in soil and groundwater remediation is presented in relation to the various properties of the materials and the different challenges encountered in laboratory research or in the environmental application. The removal trends in different environments were found to be largely similar, which means that composite materials tend to be more effective in removing CHs in actual remediation. For instance, adsorbents were found to be effective when combined with other materials, due to the ability to take advantage of the respective strengths of both materials. The rapid removal of CHs while minimizing the impact of CHs on another material and the material itself on the environment. Finally, suggestions for the next research directions are given in conjunction with this paper.

Efficient degradation of trichloroethene with the existence of surfactants by peroxymonosulfate activated by nano-zero-valent iron: performance and mechanism investigation

In this study, the degradation of trichloroethylene (TCE) with the existence of tween-80 (TW-80) or sodium dodecyl sulfate (SDS) using peroxymonosulfate (PMS) activated by nano-zero-valent iron (nZVI) was investigated. Over 87.6% TCE (with 1.3 g L^-1 TW-80 presence) was degraded by 0.9 mM PMS and 0.12 g L^-1 nZVI, while 89.7% TCE (with 2.3 g L^-1 SDS presence) was degraded by 1.2 mM PMS and 0.20 g L^-1 nZVI, in which more than 71.9% TCE with TW-80 existence and 87.5% TCE with SDS existence were dechlorinated. Besides, the effects of some factors (i.e., PMS and nZVI dosages, initial solution pH, and inorganic anions) on TCE removal were evaluated. The degradation of TCE was restrained continuously with increasing surfactant concentration, and TW-80 was more easily decomposed than SDS in PMS/nZVI system. Furthermore, sulfate radical (SO₄^-•) and hydroxyl radical (HO•) were demonstrated the main reactive oxygen species (ROS) contributing to TCE degradation and SO₄^-• played a dominant role through EPR tests and ROS scavenging experiments. Finally, the results of TCE degradation in actual groundwater confirmed that PMS/nZVI process has great advantages and potential in remediation of actual TCE-contaminated groundwater with TW-80 or SDS existence.

30 Years of Vicente Rives’ Contribution to Hydrotalcites, Synthesis, Characterization, Applications, and Innovation

Hydrotalcite is the name of a mineral discovered in Sweden in 1842 whose formula is Mg6Al2(OH)16CO3·4H2O and presents a layered crystal structure that consists of positively charged hydroxide layers neutralized by interlayer anions as carbonate, also containing water molecules. The ease of their synthesis and the possibility of incorporating other layer cations and interlayer anions have made this type of layered double hydroxides (LDH) a group of very interesting materials for industry. In addition to LDH and due to the name of the most representative mineral, this group of compounds is commonly called hydrotalcite-like materials, or simply hydrotalcites. Another way of referring to them is as anionic clays because of their layered structure but, unlike classical clays, their layers are positive and their interlayers are anionic. The main fields of application of these solids comprise catalysis, catalyst support, anion scavengers, polymer stabilizers, drug carriers, or adsorbents. This paper briefly summarizes some of the work carried out by Professor Rives over more than thirty years, focused, among other topics, on the study of the synthesis, characterization, and applications of hydrotalcites. This research has led him to train many researchers, to collaborate with research groups around the world and to publish reference papers and books in this field. This contribution, written to be included in the Special Issue “A Themed Issue in Honor of Prof. Dr. Vicente Rives”, edited on the occasion of his retirement, only shows a small part of his scientific research and intends to value and recognize his cleverness and his enormous scientific and human quality.

Use of Ethylamine, Diethylamine and Triethylamine in the Synthesis of Zn,Al Layered Double Hydroxides

Amines with two carbon atoms in the organic chain [ethylamine (EA), diethylamine (DEA), triethylamine (TEA)] have been used as precipitant agents to obtain a hydrotalcite-like compound with Zn (II) and Al (III) as layered cations and with nitrate anions in the interlayered region to balance the charge. This Layered Double Hydroxide was prepared following the coprecipitation method, and the effect on the crystal and particle sizes was studied. Also, the effect of submitting the obtained solids to hydrothermal post-synthesis treatment by conventional heating and microwave assisted heating were studied. The obtained solids were exhaustively characterized using several instrumental techniques, such as X-ray diffraction, Thermal Analysis (DTA and TG), Chemical Analysis, Infrared Spectroscopy (FT-IR), determination of Particle Size Distribution and BET-Surface area. Well crystallized solids were obtained showing two possible LDH phases, depending on the orientation of the interlayer anion with respect to the brucite-like layers. The results indicated that there is a certain influence of the amine, when used as a precipitating agent, and as a consequence of the degree of substitution, on the crystallinity and particle size of the final solid obtained. The LDHs obtained using TEA exhibited higher crystallinity, which was improved after a long hydrothermal treatment by conventional heating. Regarding the shape of the particles, the formation of aggregates in the former solid was detected, which could be easily disintegrated using ultrasound treatments, producing solid powder with high crystallinity and small particle size, with homogeneous size distribution.

Mechanism of trichloroethylene degradation in Fe(II)-activated peroxymonosulfate coupled with citric acid system in the presence of surfactants

This study demonstrated that peroxymonosulfate (PMS) activated by Fe(II)/citric acid (CA) could effectively degrade trichloroethylene (TCE) in the presence of Tween-80 (TW-80) or sodium dodecyl sulfate (SDS). Significant TCE removal of 91.6% (90.1%) with 1.3 g L^-1 TW-80 (2.3 g L^-1 SDS) were achieved at the PMS/Fe(II)/CA/TCE molar ratio of 4/4/4/1 (20/20/20/1). TCE degradation could be greatly elevated by Fe(II) and CA addition, while the existence of surfactants restrained TCE removal and the inhibitory effect increased with the higher surfactant concentration. The tests of the electron paramagnetic resonance (EPR) and reactive radicals scavenging experiments proved that sulfate radical (SO₄^-•), hydroxyl radical (HO•), and superoxide radical (O₂^-•) were responsible for TCE degradation and SO₄^-• acted as the major one. The influences of initial solution pH and inorganic anions k(Cl^- and HCO₃^-) on TCE removal were also investigated. Eventually, TCE removal in actual groundwater tests with surfactants confirmed that the PMS/Fe(II)/CA process has a huge potential of practical application in remediating the groundwater contaminated by TCE after the pretreatment by solubilization using surfactants.

Typical groundwater contamination in the vicinity of industrial brownfields and basic methods of their treatment

The article deals with simple methods of decontamination of groundwater from the vicinity of brownfields contaminated with organic and inorganic substances. In the literature, thousands of articles on this issue at various sophisticated levels of knowledge can be found. The articles are mostly suitable as an extension of scientific knowledge; however, regarding potential costs and respectively scale-up problems, the applications are limited. It turns out that the vast majority of contaminated water can be effectively decontaminated by simple methods, in a coagulation-sedimentation sequence → simple oxidation and reduction methods for separated water (Fenton reaction, photocatalysis, ozonation, reductive dehalogenation with zero metals) → adsorption of remaining pollutants on simple sorbents, eg on biochar → (possibly bioremediation or advanced physical methods such as membrane filtration) → final purification on activated carbon. Due to the usually limited volume loads of soils with pollutants in the vicinity of brownfields, it is not economically advantageous to build demanding decontamination units for water purification. Usually, the simplest solution is the system to pump-and-treat around the source of contamination, with the main emphasis on highly effective removal of pollutants from water that returns underground. Groundwater was taken from boreholes leading to the saturated zone in the vicinity of several selected industrial brownfields. The solutions are shown on individual typical cases.

Removal of Chlorine Ions from Desulfurization Wastewater by Modified Fly Ash Hydrotalcite

The effective removal of chlorine ion from the desulfurization slurry is of great significance to the stable operation of the desulfurization system. Modified fly ash hydrotalcites were prepared by alkali/acid-combined roasting and microwaving and used as an adsorbent for chlorine ion in desulfurized wastewater. The specific surface area and porosity of different adsorbents were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The impacts of pH, temperature, adsorbent dosage, and adsorption shaking time on adsorption performance were investigated. Results showed the alkali-combined roasting-modified fly ash hydrotalcite has the optimum removal effect on Cl^-. The optimal adsorption performance was achieved when the pH was 8, the adsorption temperature was 60 °C, the mass concentration of adsorbent was 10 g/L, the adsorption shaking time was 180 min, and the removal percentage of Cl^- was 68.1%. The adsorption isotherm was consistent with the Langmuir isotherm model, and the adsorption saturation was 694.4 mg/g, which belonged to monolayer adsorption.

Emerging pollutants-Part II: Treatment

Emerging pollutants (EPs) refer to a class of pollutants, which are emerging in the environment or recently attracted attention. EPs mainly include pharmaceutical and personal care products (PPCPs), endocrine-disrupting chemicals (EDCs), and antibiotic resistance genes (ARGs). EPs have potential threats to human health and ecological environment. In recent years, the continuous detections of EPs in surface and ground water have brought huge challenges to water treatment and also made the treatment of EPs become an international research hotspot. This paper summarizes some research results on EPs treatment published in 2019. This paper may be helpful to understand the current situations and development trends of EP treatment technologies.

Fluorescing Layered Double Hydroxides as Tracer Materials for Particle Injection during Subsurface Water Remediation

Nowadays, the contamination of groundwater and soils by highly hazardous and toxic chlorinated solvents is a global issue. Over the past years, different remediation strategies have been developed, involving injection of reactive solutions and/or particles. However, a major difficulty is the monitoring of injected particles during the injection and after secondary mobilisation by groundwater flow. This study is focussed on the development of directly traceable particles by combining fluorescein with Layered Double Hydroxides (LDHs). We present here the facile and easily tuneable synthesis of fluorescing LDHs (Fluo-LDH) via co-precipitation under supersaturation conditions. Their ability to mimic particle sizes of previously studied reactive LDHs, which proved to be able to adsorb or degrade chlorinated organic solvents from aqueous solutions, was investigated as well. Tests using a novel Optical Image Profiler (OIP) confirmed that the fluorescent LDHs can be easily detected with this tool. Even LDHs with the lowest amount of fluorescent dye were detectable. Together with the use of an OIP, which is capable of exciting the fluorescent material and collecting real-time pictures, this can provide a new, efficient, and cost-effective method for in situ tracing of injected particles in the subsurface.

Hydrotalcite stability during long-term exposure to natural environmental conditions

Hydrotalcite-like compounds are a group of layered double hydroxides widely studied as sorbents to remove organic and inorganic contaminants under laboratory conditions. This study is a proof-of-concept of the long-term fate of hydrotalcite compounds under natural environmental conditions, to bridge the gap between laboratory studies and their field application as sorbents. Hydrotalcite (HT) with intercalated carbonate species (HT-CO₃) and dodecyl sulphate (HT-DS) were synthesised and placed in two groundwater monitoring wells in Denmark, one contaminated with chlorinated hydrocarbons and another with uncontaminated groundwater. To assess the structural and surface compositional changes of hydrotalcite compounds upon prolonged exposure to groundwater, the material was analysed with powder X-ray diffraction (PXRD), Fourier-transformed infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The results showed that the stability and dissolution behaviour of hydrotalcite compounds under groundwater conditions depended on the intercalated anion (CO₃^2- > DS) and groundwater dynamics (static flow > dynamic flow), while the hydrotalcite aggregate size only had a minor effect. Groundwater geochemistry influenced the precipitation of insoluble species (CaCO₃, and adsorbed sulphate) on the hydrotalcite surface. The instability of hydrotalcite compounds, especially in the case of HT-DS, may constitute a significant limiting factor on their future application as sorbents under dynamic flow conditions.

Layered Double Hydroxides with Intercalated Permanganate and Peroxydisulphate Anions for Oxidative Removal of Chlorinated Organic Solvents Contaminated Water

The contamination by chlorinated organic solvents is a worldwide problem as they can deeply penetrate aquifers, accumulating in the sub-surface as lenses of highly hazardous pollutants. In recent years, so called in situ oxidation processes have been developed to remediate chlorinated organic solvents from groundwater and soil by injecting solutions of oxidising agents such as permanganate or peroxydisulphate. We here present modified layered double hydroxides (LDHs) with intercalated oxidising agents that might serve as new reactants for these remediation strategies. LDHs might serve as support and stabiliser materials for selected oxidising agents during injection, as the uncontrolled reaction and consumption might be inhibited, and guarantee that the selected oxidants persist in the subsurface after injection. In this study, LDHs with hydrotalcite- and hydrocalumite-like structures intercalated with permanganate and peroxydisulphate anions were synthesised and their efficiency was tested in batch experiments using trichloroethene or 1,1,2-trichloroethane as the target contaminants. All samples were characterised using powder X-ray diffraction, thermal analysis coupled with mass spectrometry to directly analyse evolving gases, and Fourier-transform infrared spectroscopy. Additionally, particle size distribution measurements were carried out on the synthesised materials. Results of the batch experiments confirmed the hypothesis that oxidising agents keep their properties after intercalation. Permanganate intercalated LDHs proved to be most efficient at degrading trichloroethene while peroxydisulphate intercalated Ca,Al-LDHs were the most promising studied reactants degrading 1,1,2-trichloroethane. The detection of dichloroethene as well as the transformation of the studied reactants into new LDH phases confirmed the successful degradation of the target contaminant by oxidation processes generated from the intercalated oxidising agent.