Electrochemical synthesis of chemicals and treatment of wastewater promoted by salinity gradients using reverse electrodialysis and assisted reverse electrodialysis

In the last years, an increasing attention has been devoted to the utilization of waters with different salt content to drive valuable redox processes at the electrodes by reverse electrodialysis processes (RED) or assisted RED (A-RED), thus allowing to significantly reduce the energetic costs associated to conventional electrolyses. In this review, the most relevant findings were presented and were critically discussed. The use of RED and A-RED for the synthesis of chemicals, the conversion of CO2, and the treatment of wastewater contaminated by organic and inorganic pollutants resistant to traditional biological processes was analyzed. The main advantages and disadvantages of these routes were commented, as well as the key points that should be addressed to favor the utilization on an applicative scale

Transesterification of rapeseed oil over acid resins promoted by supercritical carbon dioxide

The methanolysis of rapeseed oil catalyzed by commercial styrene-divinylbenzene macroporous acid resins was performed in a batch reactor at 100-140 \ub0C and 10-46 MPa to study the effect of supercritical carbon dioxide (scCO2) on the performances of the process. Reaction temperatures of 120-140 \ub0C were necessary to obtain high enough yields of fatty acid methyl esters. Upon addition of scCO2 faster transesterification kinetics was obtained also at the lowest investigated operating pressure (10-11 MPa), working in two fluid phase systems. Experiments performed changing the reaction time indicated that most of the esters were formed during the first 3 h. When the pressure was increased at 38-46 MPa, the fluid phases merged in a single one without significant modification of the performances of the process. The enhancement effect of scCO2 on the transesterification kinetics is tentatively discussed in terms of modification of the phase behaviour of the reaction system and swelling of the polymeric acid resin. \ua9 2010 Elsevier B.V. All rights reserved

ELECTROCHEMICAL MICROREACTORS FOR THE ABATEMENT OF ORGANIC POLLUTANTS IN WATER SOLUTION

Electrochemical methods can offer new sustainable routes for the abatement of organic pollutants resistant to biological processes. These methods use a clean reagent, the electron, and very mild operative conditions (ambient temperature and atmospheric pressure) with limited operative costs. However, electrochemical processes present some important disadvantages when performed in conventional reactors. In particular, to achieve reasonable cell voltages when the medium has not an adequate conductivity, one needs adding to the system a supporting electrolyte. This is certainly a main obstacle for a wide application of electrochemical tools. Indeed, adding chemicals is often a problematic issue, since this may lead to the formation of secondary products, makes more difficult the separation procedures and increases the operative costs. Recently it has been shown that the electrochemical processes can strongly benefit from the utilization of microfluidic electrochemical reactors (i.e. cells with a distance between the cathode and the anode of tens or hundreds of micrometers) allowing to minimize or even remove some of the above mentioned disadvantages. Thus, very small distances between electrodes lead from one side to a drastic reduction of the ohmic resistances, (allowing to operate with lower cell voltages and without supporting electrolyte), and on the other side to intensify the mass transport of the reagents towards electrodes surfaces. The utilization of micro devices may present the drawback of a more easy fouling but also other potential advantages such as an easier scale-up procedure through simple parallelization of many small units. In this work, the possible utilization of various electrochemical oxidation methods for the treatment of aqueous solutions of Acid Orange 7 (AO7) chosen as a model compound (namely, direct electrochemical oxidation, indirect oxidation with active chlorine and electro-Fenton) used alone or in a combined way was studied for the sake of comparison of various electrochemical approaches. The abatement of AO7 was performed successfully in the micro reactors under a single-pass mode without supporting electrolyte at low cell voltages. A very high conversion for passage can be achieved, allowing to operate the process under a continuous mode and to achieve a fast screening of the effect of operative parameters due to very short times of treatment. The utilization of three micro reactors in series open interesting new perspectives, including the opportunity to modulate the current density among the reactors, in order to optimize the figures of merit of the process. The effect of various operating parameters such as the initial concentration of the AO7, the electrode surface, the flow rate and the current density was also investigated in detail

Electrical power production from low-grade waste heat using a thermally regenerative ethylenediamine battery

Thermally regenerative ammonia-based batteries (TRABs) have been developed to harvest low-grade waste heat as electricity. To improve the power production and anodic coulombic efficiency, the use of ethylenediamine as an alternative ligand to ammonia was explored here. The power density of the ethylenediamine-based battery (TRENB) was 85 \ub1 3 W m2-electrode area with 2 M ethylenediamine, and 119 \ub14Wm2 with 3 M ethylenediamine. This power density was 68% higher than that of TRAB. The energy density was 478 Wh m3-anolyte, which was ~50% higher than that produced by TRAB. The anodic coulombic efficiency of the TRENB was 77 \ub1 2%, which was more than twice that obtained using ammonia in a TRAB (35%). The higher anodic efficiency reduced the difference between the anode dissolution and cathode deposition rates, resulting in a process more suitable for closed loop operation. The thermal-electric efficiency based on ethylenediamine separation using waste heat was estimated to be 0.52%, which was lower than that of TRAB (0.86%), mainly due to the more complex separation process. However, this energy recovery could likely be improved through optimization of the ethylenediamine separation process

A critical review on latest innovations and future challenges of electrochemical technology for the abatement of organics in water

Updated water directives and ambitious targets like the United Nations’ Sustainable Development Goals (SDGs) have emerged in the last decade to tackle water scarcity and contamination. Although numerous strategies have been developed to remove water pollutants, it is still necessary to enhance their effectiveness against toxic and biorefractory organic molecules. Comprehensive reviews have highlighted the appealing features of the electrochemical technologies, but much progress has been made in recent years. In this timely review, a critical discussion on latest innovations and perspectives of the most promising electrochemical tools for wastewater treatment is presented. The work describes the performance of electrocatalytic anodes for direct electrochemical oxidation, the oxidation mediated by electrogenerated active chlorine, the electrocatalytic reduction as well as coupled approaches for synchronous anodic and cathodic processes combined with homogeneous and heterogeneous catalysis. The last section is devoted to the assessment of scale-up issues and the increase in the technology readiness level

Hydrothermal liquefaction of sewage sludge: use of HCOOH and KOH to improve the slurry pumpability in a continuously operated plant

We studied the hydrothermal liquefaction (HTL) of digested sewage sludge (DSS) as model of waste biomass in batch and continuous reactors. HCOOH and KOH were used to improve the slurry pumpability. HTL experiments were conducted at the same kinetic severity factor in a batch reactor of 25 mL of volume and in a continuously operated tubular reactor with 350 mL of volume. The observed outcomes suggested that it was not possible to achieve the pumpability of native DSS when a high concentrated stream of suspended solid particles has been fed to the HTL continuous plant. Using acidic or basic homogeneous additives, as potassium hydroxide or formic acid, it was possible to enhance the pumpability of a concentrated slurry of DSS in the continuous plant achieving yields of heavy oil (fraction of biocrude) similar to those obtained in the batch reactor and with higher H/C ratios. Hence, we found that HCOOH and KOH are promising additives for the practical implementation of a continuous HTL process