Environmental impact assessment of the electrokinetic adsorption barriers to remove different herbicides from agricultural soils.

In this study, the sustainability of the electrokinetic remediation soil flushing (EKSFs) process integrated without and with adsorption barriers (EKABs) have been evaluated for the treatment of four soils contaminated with Atrazine, Oxyfluorfen, Chlorosulfuron and 2,4-D. To this purpose, the environmental effects of both procedures (EKSFs and EKABs) have been determined through a life cycle assessment (LCA). SimaPro 9.3.0.3 was used as software tool and Ecoinvent 3.3 as data base to carry out the inventory of the equipment of each remediation setup based on experimental measurements. The environmental burden was quantified using the AWARE, USEtox, IPPC, and ReCiPe methods into 3 Endpoint impact categories (and damage to human health, ecosystem and resources) and 7 Midpoints impact categories (water footprint, global warming potential, ozone depletion, human toxicity (cancer and human non-cancer), freshwater ecotoxicity and terrestrial ecotoxicity). In general terms, the energy applied to treatment (using the Spanish energy mix) was the parameter with the greatest influence on the carbon footprint, ozone layer depletion and water footprint accounting for around 70 % of the overall impact contribution. On the other hand, from the point of view of human toxicity and freshwater ecotoxicity of soil treatments with 32 mg kg-1 of the different pesticides, the EKSF treatment is recommended for soils with Chlorosulfuron. In this case, the carbon footprint and water footprint reached values around 0.36 kg of CO2 and 114 L of water per kg of dry soil, respectively. Finally, a sensitivity analysis was performed assuming different scenarios.

Can the green energies improve the sustainability of electrochemically-assisted soil remediation processes?

The green powering of electrochemically-assisted soil remediation processes had been strongly discouraged. Low remediation efficiencies have been reported as a consequence of the reversibility of the transport processes when no power is applied to the electrodes, due to the intermittent powering of renewable sources. However, it has been missed a deeper evaluation from the environmental point of view. This work goes further and seeks to quantify, using life cycle assessment tools, the environmental impacts related to the electro-kinetic treatments powered by different sources: grid (Spanish energy mix), photovoltaic and wind sources. The global warming potential and the ozone depletion showed higher environmental impacts in case of using green energies, associated with the manufacturing of the energy production devices. In contrast to that, results pointed out the lowest water consumption for the treatment powered with solar panels. The huge water requirements to produce energy, considering a Spanish energy mix, drop the sustainability of this powering strategy in terms of water footprint. Regarding toxicities, the pollutant toxicity was highly got rid of after 15 days of treatment, regardless the powering source used. Nevertheless, the manufacturing of energy and green energy production devices has a huge impact into the toxicity of the remediation treatments, increasing massively the total toxicity of the process, being this effect less prominent by the electro-kinetic treatment solar powered. In view of the overall environmental impact assessed, according to mid and endpoint impact categories, it can be claimed that, despite the high energy requirements and affectation to the global warming potential, the use of solar power is a more sustainable alternative to remediate polluted soils by electrochemical techniques.

Toward more sustainable photovoltaic solar electrochemical oxidation treatments: Influence of hydraulic and electrical distribution.

Powering electrochemical technologies with renewable energies is a promising way to get more sustainable environmental remediation techniques. However, the operational conditions of those processes must be optimized to undergo fast and efficient treatments. In this work, the influence of electrical and hydraulic connections in the performance of a set of two electrolyzers directly powered by photovoltaic panels was evaluated. Despite both electrolyzers were assembled using the same electrode material, they showed different performances. Results indicate that the electrolyzer with higher ohmic resistance and higher overpotential attained a greater production of oxidant species, being produced under the most efficient strategy around 4.8 and 15.1 mmol of oxidants per Ah by electrolyzer 1 and 2, respectively. Nevertheless, an excess of oxidant production because of an inefficient energy management, led to low removal efficiencies as a consequence of a waste of energy into undesirable reactions. Regarding the hydraulic distribution of wastewater between the cells, it was found to influence on the total remediation attained, being the serial connection 2.5 and 1.8 more efficient than a parallel wastewater distribution under series and parallel electrical strategies, respectively. Regarding electrical strategies, parallel connections maximize the use of power produced by the photovoltaic panels. Furthermore, this allows the system to work under lower current densities, reducing the mass transfer limitations. Considering both advantages, a hydraulic connection of the cells in series and an electrical connection in parallel was found to reach the highest specific removal of pollutant, 2.52 mg clopyralid (Wh)-1. Conversely, the opposite strategy (parallel hydraulic connection-series electrical connection) showed the lowest remediation ratio, 0.48 mg clopyralid (Wh)-1. These results are important to be considered in the design of electrolytic treatments of waste directly powered by photovoltaic panels, because they show the way to optimize the cells stack layout in full-scale applications, exhibiting significant impact on the sustainability of the electrochemical application.

Improving sustainability of electrolytic wastewater treatment processes by green powering.

This work focuses on the evaluation of the impact of powering electrolytic wastewater treatment processes with grid or renewable energy on the sustainability of this electrochemical remediation technology. To face this goal, it was performed an inventory of three bench-scale plants made up by the same treatment technology but powered from different supplies: connected to grid and directly coupled with solar photovoltaic panels or a wind turbine. Results show that the powering mode can significantly affect the environmental risks of the treatment, not only in terms of electricity demand but also on the formation of intermediates, which are more important in the cases in which the intensity profile varied. A life cycle assessment (LCA) is carried out in order to quantify the environmental impacts of green powering electrolytic wastewater treatment processes. Ecoinvent 3.3 data base, AWARE, USEtox, IPPC and ReCiPe methodologies are used to quantify the environmental burden into 5 midpoint (water footprint, global warming 100a, ozone layer depletion, human toxicity, freshwater ecotoxicity) and 17 endpoint impact categories. All impact categories are higher in the case in which the supplied power cames from a electricity grid mix. For the removal of 0.1 g 2,4-dichlorophenoxyacetic acid (2,4D) per liter (functional unit) of treated wastewater releases 0.14 kg CO2 eq. If the energy is provided by a wind turbine or a solar panel the processes emit 0.020 kg CO2 eq and 0.019 kg CO2 eq, respectively. A comparison of the impact based on the grid mix used in different countries is also made, which has pointed out the relevance of this input on the sustainability of the environmental electrochemical technologies.

Does intensification with UV light and US improve the sustainability of electrolytic waste treatment processes?

This work aims to assess the influence of ultrasounds (US) application or ultraviolet (UV) light irradiation on the efficiency and sustainability of the treatment of wastes by conductive diamond electrochemical oxidation (CDEO). To do this, a life cycle assessment (LCA) is carried out in order to quantify the environmental impacts of the intensified CDEO processes. Inventories of three bench scale remediation plants (CDEO, Sono-CDEO and Photo-CDEO) in which the different technologies are implemented are performed by means of Ecoinvent 3.3 data base. AWARE, USEtox, IPPC and ReCiPe methodologies are used to quantify the environmental burden into 5 midpoint (water footprint, global warming 100a, ozone layer depletion, human toxicity, freshwater ecotoxicity) and 17 endpoint impact categories. Photo-CDEO attains the faster and more efficient removal in terms of energy consumed. All impact categories are lower in the case in which UV light irradiation is coupled to the CDEO treatment, particularly if the electrolyte does not contain chloride anions. From the point of view of toxicity and ecotoxicity, it is essential to achieve a complete mineralization, because of the intermediates generated into wastes containing chloride anions can become more hazardous than the initial pesticide. The operation of these technologies at large current densities shows positive results from the sustainability point of view, despite the huge environmental impact related to the energy production. Data notice that almost a 99.0% of the total global warming potential is mainly due to the electricity required during the electrochemical treatment, being higher by the sono and photo CDEO treatments because of the use of additional devices. Nevertheless, this issue can be overcome by means of using renewable energies as power sources of these remediation treatments. According to results, it can be claimed that the electrochemical technologies may successfully compete with other AOPs in terms of sustainability.

Strategies for powering electrokinetic soil remediation: A way to optimize performance of the environmental technology.

The electro-kinetic remediation of soils using different powering strategies has been studied, in order to clarify which is the best strategy to couple solar powering with this remediation technology, in a context of developing more sustainable electrochemical remediation technologies. Direct powering from photovoltaic panels (Case a), application of constant electric fields with the same average value of Case a (Case b) and application of constant specific power with the same average value of Case a (Case c) have been compared. Results show an outstanding influence of the powering strategy on the removal efficiency of clopyralid (model of herbicide used in this work). The direct use of solar power profiles obtained the lowest removal efficiencies, which contrasts with the higher expected sustainability of this powering strategy. Reversion of pollutant transport overnight and extreme electric field values at noon help to explain the lower efficiency of this strategy. Evaporation mechanisms are promoted by operating at extreme large electric fields. In addition, harsher conditions lead to a higher negative soil affectation in terms of regions affected by extreme pHs, water contents and/or conductivities and to lower specific pollutant removals. Therefore, maximum efficiencies were found for Case b (constant electric potential gradient) with a total removal over 110 g kWh-1 and only a slight affectation into the final soil properties.

Influence of hydraulic retention time and carbon loading rate on the production of algae.

This paper is focused on the assessment of the production of algae in batch bioreactors. Hydraulic retention time, carbon loading rate and light color were the inputs of the study and algae production the main output. Bioreactors were operated in semi-continuous mode and tests lasted two months, more than two times the period required to meet a steady-state response. This steady-state was verified with plateau responses in both, soluble parameters and suspended solids. Results points out the great relevance of temperature. Likewise, they show that green light improves the production of algae, as well as long HRT and high CLR. Maximum production rates attained were in the range 4-14 mg d-1 L-1. The ratio COD /TSS for this biofuel was almost constant (3.13 mg COD mg-1 TSS) but the quality of the product obtained in terms of the Mean Oxidation State of Carbon is completely different. Longer HRT leads to lower MOSC and hence to potentially more valuable fuels.

Influence of the fuel and dosage on the performance of double-compartment microbial fuel cells.

This manuscript focuses on the evaluation of the use of different types and dosages of fuels in the performance of double-compartment microbial fuel cell equipped with carbon felt electrodes and cationic membrane. Five types of fuels (ethanol, glycerol, acetate, propionate and fructose) have been tested for the same organic load (5,000 mg L(-1) measured as COD) and for one of them (acetate), the range of dosages between 500 and 20,000 mg L(-1) of COD was also studied. Results demonstrate that production of electricity depends strongly on the fuel used. Carboxylic acids are much more efficient than alcohols or fructose for the same organic load and within the range 500-5,000 mg L(-1) of acetate the production of electricity increases linearly with the amount of acetate fed but over these concentrations a change in the population composition may explain a worse performance.

Bioelectro-Claus processes using MFC technology: Influence of co-substrate.

This work is focused on the removal of sulphide from wastewater using a two chamber microbial fuel cell, seeded with activated sludge and operated in semi-continuous mode. Two co-substrates were used in order to provide the system for carbon and nutrient source: actual urban wastewater and synthetic wastewater. Results show that sulphide is efficiency depleted (removals over 94%) and that electricity is efficiently produced (maximum power density is 150 mW m(-2)) meanwhile COD is also oxidised (removals higher than 60%). Sulphur and sulphate are obtained as the final products of the oxidation and final speciation depends on the type of co-substrate used. The start-up of the system is very rapid and production of electricity and polarisation curves do not depend on the co-substrate.