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.

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.

Interaction of zero-valent iron and carbonaceous materials for reduction of DDT.

Dechlorination of dichlorodiphenyltrichloroethane (DDT) as a model compound was performed with zero-valent iron (micro-ZVI and nano-ZVI) as reductant and carbonaceous adsorbents as sink and catalyst in water. DDT is rapidly converted to dichlorodiphenyldichloroethane (DDD) in direct contact with ZVI. However, up to 90% of the DDD is transformed into non-identified, most likely oligomeric products. There is no indication of dechlorination at the aromatic rings. DDT is still rapidly dechlorinated when it is adsorbed on carbonaceous adsorbents, even though ZVI particles have no direct access to the adsorbed DDT. The carbonaceous materials function as adsorbent and catalyst for the dechlorination reaction at once. From electrochemical experiments, we deduced that direct physical contact between ZVI particles and the adsorbent is essential for enabling a chemical reaction. Electron conduction alone does not effect any dechlorination reaction. We hypothesize hydrogen species (H∗) which spill from the ZVI surface to the carbon surface and initiate reductive transformations there. The role of carbonaceous adsorbents is different for different degradation pathways: in contrast to hydrodechlorination (reduction), adsorption protects DDT from dehydrochlorination (hydrolysis).

In-situ treatment of herbicide-contaminated groundwater-Feasibility study for the cases atrazine and bromacil using two novel nanoremediation-type materials.

Two injectable reactive and sorption-active particle types were evaluated for their applicability in permeable reaction zones for in-situ removal of herbicides ("nanoremediation"). As model substances, atrazine and bromacil were used, two herbicides frequently occurring in groundwater. In order to provide recommendations for best use, particle performance was assessed regarding herbicide degradation and detoxification. For chemical reduction, Carbo-Iron® was studied, a composite material consisting of zerovalent iron and colloidal activated carbon. Carbo-Iron reduced bromacil with increased activity compared to nanoscale zerovalent iron (nZVI). The sole reaction product, 3-sec-butyl-6-methyluracil, showed 500-fold increase in half-maximal-effect concentration (EC50) towards the chlorophyte Scendesmus vacuolatus compared to the parent compound. The detoxification based on dehalogenation confirmed the dependency of the specific mode-of-action on the carbon-halide bond. For atrazine, neither nZVI nor Carbo-Iron showed significant degradation under the conditions applied. As novel subsurface treatment option, Trap-Ox® zeolite FeBEA35 was studied for generation of in-situ permeable oxidation barriers. Both adsorbed atrazine and bromacil underwent fast unselective oxidation. The transformation products of the Fenton-like reaction were identified, and oxidation pathways derived. For atrazine, a 300-fold increase in EC50 for S. vacuolatus was found over the duration of the reaction, and a loss of phytotoxicity to non-detectable levels for bromacil.

Hydrothermal treatment for regeneration of activated carbon loaded with organic micropollutants.

Hydrothermal treatment (HT) at 200 °C and 240 °C for 4 and 16 h was studied for the regeneration of granular activated carbon (AC) loaded with a range of organic micropollutants having a broad range of physico-chemical properties. Carbamazepine, diazinon, diclofenac, estrone, iohexol, metoprolol and sulfamethoxazole were fully converted. Limits were seen for the conversion of caffeine, ibuprofen and perfluorooctanesulfonate (PFOS). However, the degree of degradation was enhanced for the latter compounds in the adsorbed state as compared to experiments in aqueous solution. The methodology was tested in five loading and regeneration cycles for selected compounds with no change of the degradation potential and of the AC properties. In particular, the surface properties of the AC did not deteriorate upon HT as determined by the specific surface area (from BET isotherms), the point of zero charge, and the surface functional groups (from diffuse reflectance IR spectroscopy). As the total concentration of the loaded pollutants was minimized by HT, this method could be considered as a new low temperature regeneration technology for spent AC.