Bottom-Up Synthesis of De-Functionalized and Dispersible Carbon Spheres as Colloidal Adsorbent.

Recent innovative adsorption technologies for water purification rely on micrometer-sized activated carbon (AC) for ultrafast adsorption or in situ remediation. In this study, the bottom-up synthesis of tailored activated carbon spheres (aCS) from sucrose as renewable feedstock is demonstrated. The synthesis is based on a hydrothermal carbonization step followed by a targeted thermal activation of the raw material. This preserves its excellent colloid properties, i.e., narrow particle size distribution around 1 µm, ideal spherical shape and excellent aqueous dispersibility. We investigated the ageing of the freshly synthesized, highly de-functionalized AC surface in air and aqueous media under conditions relevant to the practice. A slow but significant ageing due to hydrolysis and oxidation reactions was observed for all carbon samples, leading to an increase of the oxygen contents with storage time. In this study, a tailored aCS product was generated within a single pyrolysis step with 3 vol.-% H2O in N2 in order to obtain the desired pore diameters and surface properties. Adsorption characteristics, including sorption isotherms and kinetics, were investigated with monochlorobenzene (MCB) and perfluorooctanoic acid (PFOA) as adsorbates. The product showed high sorption affinities up to log (KD/[L/kg]) of 7.3 ± 0.1 for MCB and 6.2 ± 0.1 for PFOA, respectively.

Photodegradation of Perfluorooctanesulfonic Acid on Fe-Zeolites in Water.

Perfluorooctanesulfonic acid (PFOS) decomposition in an aqueous environment remains a huge challenge because of its extreme chemical and biological inertness even when compared with other per- and polyfluoroalkyl substances (PFAS). In this work, we demonstrate for the first time a successful photochemical PFOS degradation by irradiation with 254 nm ultraviolet (UV) light after adsorption on µm-sized Fe(III)-loaded zeolites under ambient conditions with oxygen (O2) as the terminal oxidant. Twenty µM PFOS loaded on 0.5 g L-1 Fe-zeolites in aqueous suspension was degraded up to 99% within 96 h under acidic conditions (pH ≤ 5.5) in the presence of oxygen. Besides fluoride and sulfate, short-chain perfluorinated carboxylic acids (PFCAs) were identified and quantified as products. In addition, the effects of initial pH, catalyst dosages, and operation temperature on the degradation of PFOS were investigated. We also successfully applied the system to real groundwater samples where trace PFOS was present. Our results indicate that PFOS degradation is initiated by electron transfer from sulfonate to iron. The presented experimental study offers an option for a novel water remediation technology, comprising first a zeolite-based adsorption step followed by a step for photochemical regeneration of the adsorbent.

Hydrothermal Conversion of Triclosan-The Role of Activated Carbon as Sorbent and Reactant.

Triclosan (TCS) was treated under hydrothermal conditions at 240 °C for 4 h, either dissolved in aqueous solution or preadsorbed onto activated carbon (AC). Hydrothermal conversion of dissolved TCS led to formation of 2,8-dichlorodibenzo-p-dioxin (DCDD). Its yield was dependent on the pH of the aqueous solution increasing from 38% at pH 4 up to 67% at pH 12. Adsorption of TCS at neutral pH on three different kinds of ACs, powder, granular, and felt, changed the reactivity of the TCS molecule under hydrothermal conditions significantly. The conversion of TCS and, in particular, the formation of DCDD was inhibited in the presence of ACs. When TCS was adsorbed on powdered AC, the preferred reaction pathway was the reductive hydrodechlorination. The findings described herein may be valuable for a potential regeneration method for loaded AC based on hydrothermal treatment.

What Controls Selectivity of Hydroxyl Radicals in Aqueous Solution? Indications for a Cage Effect.

The oxidation of three isotopologues of methylcyclohexane (MCH: C7H14, C7D14, c-C6D11-CH3) by OH-radicals (•OH) in aqueous solution was investigated. Intermolecular and intramolecular H/D kinetic isotope effects (KIE = kH:kD) for the abstraction of H and D atoms by •OH were measured. These KIEs reflect inter- and intramolecular selectivities of hydrogen abstraction, i.e., the selection of •OH attack on carbon-hydrogen bonds in different molecules and in different positions of one molecule, respectively. The intermolecular selectivity of •OH attack in aqueous solution is largely discriminated against in comparison with the intramolecular selectivity. The observed extent of discrimination cannot be explained by partial diffusion control of the overall reaction rates. A cage model, where •OH and hydrocarbon molecules are entrapped in a solvent cage, is more appropriate. The much higher intramolecular KIEs compared to the intermolecular KIEs of the same chemical reaction, R-H + •OH → R• + H2O, indicate a high degree of mobility of the two reaction partners inside of the solvent cage. This mobility is sufficient to develop an intramolecular selectivity comparable to that of gas-phase reactions of •OH. Furthermore, literature data on KIEs of H-abstraction by •OH in aqueous and gas phases are discussed. There is a general tendency toward lower selectivities in the aqueous phase.

Accelerated Catalytic Fenton Reaction with Traces of Iron: An Fe-Pd-Multicatalysis Approach.

An accelerated catalytic Fenton (ACF) reaction was developed based upon a multicatalysis approach, facilitating efficient contaminant oxidation at trace levels of dissolved iron. Beside the Fe(II)/H2O2 catalyst/oxidant pair for production of OH-radicals, the ACF system contains Pd/H2 as catalyst/reductant pair for fast reduction of Fe(III) back to Fe(II) which accelerates the Fenton cycle and leads to faster contaminant degradation. By this means, the concentration of the dissolved iron catalyst can be reduced to trace levels (1 mg L(-1)) below common discharge limits, thus eliminating the need for iron sludge removal, which is one of the major drawbacks of conventional Fenton processes. ACF provides fast degradation of the model contaminant methyl tert-butyl ether (MTBE, C0 = 0.17 mM) with a half-life of 11 min with 1 mg L(-1) dissolved iron, 500 mg L(-1) H2O2, 5 mg L(-1) Pd (as suspended Pd/Al2O3 catalyst) and 0.1 MPa H2, pH 3. The effects of pH, H2 partial pressure and H2O2 concentration on MTBE degradation rates were studied. Results on kinetic deuterium isotope effect and quenching studies are in conformity with OH-radicals as main oxidant. The heterogeneous Pd/Al2O3 catalyst was reused within six cycles without significant loss in activity.

Hydrophobic Fe-zeolites for removal of MTBE from water by combination of adsorption and oxidation.

Several zeolites were evaluated as adsorbents for the removal of MTBE from water in a screening process. It was observed that the SiO2/Al2O3 molar ratio is a decisive factor for the adsorption properties, at least in the case of ZSM5 zeolites. ZSM5 zeolites with SiO2/Al2O3 ratios >200 were found to provide the best sorption properties for MTBE. To design a combined sorption/reaction method, regeneration of the loaded zeolites by selected advanced oxidation processes (AOP) was studied: (1) Fenton treatment using H2O2 with dissolved iron salts and (2) heterogeneous Fenton-like oxidation with Fe immobilized on the zeolites. The first was ineffective in regenerating loaded zeolites. However, heterogeneous catalysis using Fe species immobilized on the zeolite by liquid ion exchange was markedly more effective. Although these hydrophobic zeolites have a low ion exchange capacity, resulting in iron loadings of ≤ 0.09 wt %, it was possible to obtain sufficiently active catalysts. Hydrophobic Fe-zeolites can therefore be regarded as promising materials for the removal of MTBE from water, since they allow the combination of efficient adsorption and oxidative degradation of MTBE by H2O2. In contrast to the homogeneous catalysis by dissolved iron ions, these heterogeneous catalysts work at near-neutral pH and can be easily reused. Fe-zeolites as adsorbents/catalysts showed a good stability in both batch and column experiments.

Modeling of a thermo-chromatographic pulse (TCP) as radio-frequency (RF)-induced selective heating effect.

By utilizing the specific influence of water adsorption on the dielectric loss factor and, consequently, selective heating of the zeolite NaY in a radio-frequency (RF) electromagnetic field, a hot zone moving through the packed bed can be realized initiated by water injection. While the effect of water adsorption on RF heating and the phenomenon of the thermo-chromatographic pulse (TCP) itself were described in a previous paper, the present study presents a detailed model for the description and explanation of this effect. It involves the complex dependence of dielectric loss on temperature and moisture content, a diffusion model considering both hopping and Knudsen mechanisms and a power balance for a representative segment of the packed bed. The developed model was successfully applied to adequately describe various experimental situations observed for selective RF heating in a NaY zeolite bed.

Influence of low- and high-frequency electrical heating on biodegrading microorganisms in soil: soil respiration.

The influence of electrical heating on microbiological processes in soil has been studied to evaluate the potential for enhancing biodegradation of pollutants by controlling the temperature. A frequency of 50 Hz (power line frequency) was applied for resistive heating. Dielectric heating was realized using a frequency of 13.56 MHz (radio frequency). Both techniques were compared with conventional heating in a water bath. For experiments in laboratory and full scale, a model soil and a contaminated original soil were used. It was shown that under conditions capable for heating soil to 35 degrees C or even 60 degrees C, soil respiration as a measure for microbial activity was not hindered by electrical heating when temperature and moisture content were comparable with conventional heating. The variations of soil respiration were reversible upon temperature changes. Under certain conditions, periodical fluctuations of microbiological activity were observed. Several possible explanations including chronobiology are discussed without being able to provide an unambiguous interpretation for this effect.

Influence of low- and high-frequency heating on biodegrading microorganisms in soil: microbial degradation.

The influence of low-frequency (50 Hz) resistive and high-frequency (13.56 MHz, radio-frequency) dielectric heating in comparison to conventional heating on the microbial degradation of pollutants in soil was studied. The investigation of the biodegradation of model substances (benzoic acid, acetic acid, glucose, sodium acetate) added to a standard soil showed no significant influence of the electrical heating methods when compared with samples heated to the same temperature in a water bath. Therefore, a hindrance of the microbial degradation could be excluded as it was done for soil respiration in a previous study. This finding is especially relevant for the application of these electrical heating methods for thermally enhanced soil bioremediation as an option for making in situ or ex situ clean-up processes more efficient.

Fe-zeolites for the adsorption and oxidative degradation of nitroaromatic compounds in water.

Nitroaromatic compounds (NACs) are prominent explosives. In this context, these toxic substances were released into the environment and cause long-lasting groundwater contamination. In preparation of a possible in-situ remediation, colloidal Fe-zeolites were investigated for their capabilities as adsorbents and oxidation catalysts. It was shown that the Fe-zeolites FeBEA35 and FeFAU55 are potent inorganic adsorbents for NACs and simultaneously capable of activating H2O2 as Fenton-like oxidation catalysts. Adsorption isotherms of 15 NACs on both zeolites were measured to evaluate the option of coupling adsorptive contaminant enrichment with oxidative degradation. The faujasite-type zeolite FeFAU55 showed a distinct S-type adsorption behaviour and reached significantly higher NAC loadings of > 20 wt%. For FeBEA35, L-type adsorption isotherms and maximum loadings qmax of about 4 wt% were obtained. Degradation of all NACs, monitored by nitrate formation, was observed. Apparent rate constants of the NACs with hydroxyl radicals in a homogeneous, stoichiometric Fenton reaction were related to the heterogeneous system to examine the role of adsorption on the oxidative degradation. Beneficial influence of the adsorption on the oxidation rates was identified. The results of this work open up promising prospects for future application of Fe-zeolites for the in-situ remediation of NAC-contaminated groundwater.

Sulfur-36S stable isotope labeling of amino acids for quantification (SULAQ).

We introduce a universal metabolic labeling strategy using elemental heavy 36Sulfur (36S) called 36Sulfur stable isotope labeling of amino acids for quantification (SULAQ). In the proof of principle experiment, Pseudomonas putida KT2440 was grown in defined minimal medium with sodium benzoate or sodium succinate as the sole carbon and 32S- or 36S-sodium sulfate as the sole sulfur sources. Quantification using mass spectrometry resulted in 562 proteins with 1991 unique peptides. SULAQ technology can be a valuable alternative strategy for the quantitative comparisons in MS-based proteomics approaches characterizing bacterial and other biological samples in different growth conditions.

Sorption-induced effects of humic substances on mass transfer of organic pollutants through aqueous diffusion boundary layers: the example of water/air exchange.

This study examines the effect of dissolved humic substances (DHS) on the rate of water-gas exchange of organic compounds under conditions where diffusion through the aqueous boundary layer is rate-determining. A synthetic surfactant was applied for comparison. Mass-transfer coefficients were determined from the rate of depletion of the model compounds by means of an apparatus containing a stirred aqueous solution with continuous purging of the headspace above the solution. In addition, experiments with continuous passive dosing of analytes into the water phase were conducted to simulate a system where thermodynamic activity of the chemical in the aqueous phase is identical in the presence and absence of DHS. The experimental results show that DHS and surfactants can affect water-gas exchange rates by the superposition of two mechanisms: (1) hydrodynamic effects due to surface film formation ("surface smoothing"), and (2) sorption-induced effects. Whether sorption accelerates or retards mass transfer depends on its effect on the thermodynamic activity of the pollutant in the aqueous phase. Mass transfer will be retarded if the activity (or freely dissolved concentration) of the pollutant is decreased due to sorption. If it remains unchanged (e.g., due to fast equilibration with a sediment acting as a large source phase), then DHS and surfactant micelles can act as an additional shuttle for the pollutants, enhancing the flux through the boundary layer.

Critical evaluation of the 2D-CSIA scheme for distinguishing fuel oxygenate degradation reaction mechanisms.

Although the uniform initial hydroxylation of methyl tert-butyl ether (MTBE) and other oxygenates during aerobic biodegradation has already been proven by molecular tools, variations in carbon and hydrogen enrichment factors (ε(C) and ε(H)) have still been associated with different reaction mechanisms (McKelvie et al. Environ. Sci. Technol. 2009, 43, 2793-2799). Here, we present new laboratory-derived ε(C) and ε(H) data on the initial degradation mechanisms of MTBE, ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) by chemical oxidation (permanganate, Fenton reagents), acid hydrolysis, and aerobic bacteria cultures (species of Aquincola, Methylibium, Gordonia, Mycobacterium, Pseudomonas, and Rhodococcus). Plotting of Δδ(2)H/ Δδ(13)C data from chemical oxidation and hydrolysis of ethers resulted in slopes (Λ values) of 22 ± 4 and between 6 and 12, respectively. With A. tertiaricarbonis L108, R. zopfii IFP 2005, and Gordonia sp. IFP 2009, ε(C) was low (<|-1|‰) and ε(H) was insignificant. Fractionation obtained with P. putida GPo1 was similar to acid hydrolysis and M. austroafricanum JOB5 and R. ruber DSM 7511 displayed Λ values previously only ascribed to anaerobic attack. The fractionation patterns rather correlate with the employment of different P450, AlkB, and other monooxygenases, likely catalyzing ether hydroxylation via different transition states. Our data questions the value of 2D-CSIA for a simple distinguishing of oxygenate biotransformation mechanisms, therefore caution and complementary tools are needed for proper interpretation of groundwater plumes at field sites.

Migrating temperature "thermo-chromatographic" pulses (TCP) initiated by radio-frequency (RF) heating.

In the present study, the astonishing influence of water dosage on a purged dry packed bed of NaY zeolite in the presence of an electric field with a frequency of 13.56 MHz was investigated. The injection of a small amount of water to the inlet of the bed led to pronounced selective heating of the inlet zone by more than 150 K. Thus, water represented a very effective coupling medium for dielectric heating. The selectively heated zone then slowly moved through the whole packed bed and a water pulse finally left the zeolite. This effect correlated with a coupled water and heat flux was called thermo-chromatographic pulse (TCP) emphasizing its analogy to chromatography. The phenomenon could not be performed by using conventional (convective) or microwave heating. It was demonstrated under various conditions and explained by a new model based on own experimental results as well as data from literature. The model will be the objective of a forthcoming publication.

Borohydride and metallic copper as a robust dehalogenation system: Selectivity assessment and system optimization.

Hydrodechlorination (HDC) using noble-metal catalysts in the presence of H-donors is a promising tool for the treatment of water contaminated by halogenated organic compounds (HOCs). Cu is an attractive alternative catalyst to noble metals since it is cheaper than Pd, Rh, or Pt and more stable against deactivation. Cu with borohydride (BH4-) as reductant (copper-borohydride reduction system; CBRS) was applied here for the treatment of saturated aliphatic HOCs. The HDC ability of CBRS was evaluated based upon product selectivities during reduction of CCl3-R compounds (R = H, F, Cl, Br, and CH3). For CHCl3, CH2Cl2, and CHCl2-CH3, the dechlorination reaction proceeds predominantly via α-elimination with initial product selectivities to CH4 and C2H6 of 84-85 mol-% and 70-72 mol-%. For CCl4, CBrCl3, CFCl3, and CCl3-CH3, stepwise hydrogenolysis dominates. CH2Cl-R compounds are formed as recalcitrant intermediates with initial selectivities of 50-72 mol-%, whereas CH4 and C2H6 are minor products with 16-35 mol-% and 30-35 mol-%. The effect of reaction conditions on product selectivities were investigated for CHCl3 as target. Solution composition, variation of reducing agents (BH4-, H* from H2) and increase of electron pressure (electric potential at Cu electrode and Fe0 as support) did not have marked influence on the selectivities (ratio of CH4 : CH2Cl2). Product selectivities for reduction of CCl3-R compounds were found to be substrate-specific rather than reductant-specific. Since the formation of halogenated by-products could not be avoided, transformation via a second reduction step was optimized by higher catalyst dose, addition of Ag, and vitamin B12 to the CBRS. Comparison between Pd and Cu based on costs, catalyst activities, selectivities, metal stability, and fate of halogenated by-products shows that the CBRS is a potent alternative to conventional HDC catalysts and can be recommended as 'agent of choice' for treatment of α-substituted haloalkanes in heavily contaminated waters.

Uniform and dispersible carbonaceous microspheres as quasi-liquid sorbent.

Functional colloidal carbon materials find various applications, including the remediation of contaminated water and soil in so-called particle-based in-situ remediation processes. In this study, uniform and highly dispersible micro-sized carbonaceous spheres (CS) were generated by hydrothermal carbonization (HTC) of sucrose in the presence of carboxymethyl cellulose (CMC) as environmentally friendly polyelectrolyte stabilizer. In order to ensure their optimal subsurface delivery and formation of a self-contained treatment zone, a narrow size distribution and low agglomeration tendency of the particles is desired. Therefore, the obtained CS were thoroughly characterized and optimized with respect to their colloidal properties which are a crucial factor for their application as quasi-liquid sorbent. The as-prepared uniform CS are readily dispersible into single particles in water as confirmed by digital microscopy and form stable suspensions. Due to their perfectly spherical shape, particle sedimentation in aqueous suspensions is well predicted by Stokes' law. High sorption coefficients on the synthesized CS KD,CS were determined for phenanthrene (up to log (KD,CS/[L kg-1]) = 5) and other hydrophobic groundwater contaminants. This confirms the application potential of the CS, which were prepared by an economic low-temperature process using sucrose as bio-based precursor, for generating in-situ sorption barriers for groundwater and soil remediation.

Efficient removal of trifluoroacetic acid from water using surface-modified activated carbon and electro-assisted desorption.

Trifluoroacetic acid (TFA) is a very persistent, very mobile substance (vPvM) with potential toxicity, and causes increasing environmental concerns worldwide. Conventional wastewater treatment strategies are inefficient for selective TFA removal in the presence of inorganic anions. Here we show that surface defunctionalized activated carbon felt (DeACF) carrying anion exchange sites exhibits an outstanding adsorption efficiency towards TFA thanks to introduced electrostatic attraction and enhanced interactions between hydrophobic carbon surface and CF3 moieties (qmax = 30 mg/g, Kd = (840 ± 80) L/kg at cTFA = 3.4 mg/L in tap water). Flow-cell experiments demonstrated a strongly favored TFA uptake by DeACF from tap water over Cl- and SO42- but a remarkable co-adsorption of the inorganic water contaminant NO3-. Electro-assisted TFA desorption using 10 mM Na2SO4 as electrolyte and oxidized ACF as anode showed high recoveries of ≥ 87% at low cell voltages (< 1.1 V). Despite an initial decrease in TFA adsorption capacity (by 33%) caused by partial surface oxidation of DeACF after the 1st ad-/desorption cycle, the system stability was fully maintained over the next 4 cycles. Such electro-assisted 'trap&release' approach for TFA removal can be exploited for on-site regenerable adsorption units and as a pre-concentration step combined with degradation technologies.

Guest diffusion in interpenetrating networks of micro- and mesopores.

Pulsed field gradient NMR is applied for monitoring the diffusion properties of guest molecules in hierarchical pore systems after pressure variation in the external atmosphere. Following previous studies with purely mesoporous solids, also in the material containing both micro- and mesopores (activated carbon MA2), the diffusivity of the guest molecules (cyclohexane) is found to be most decisively determined by the sample "history": at a given external pressure, diffusivities are always found to be larger if they are measured after pressure decrease (i.e., on the "desorption" branch) rather than after pressure increase (adsorption branch). Simple model consideration reproduces the order of magnitude of the measured diffusivities as well as the tendencies in their relation to each other and their concentration dependence.