Sustainable and green environmental remediations of water effluents and soils through electrochemical technologies
- Autores: María Millán Espinar
- Directores de la Tesis: Justo Lobato Bajo (dir. tes.), Carmen María Fernández Marchante (codir. tes.)
- Lectura: En la Universidad de Castilla-La Mancha ( España ) en 2021
- Idioma: inglés
- Tribunal Calificador de la Tesis: Cristina Saez Jiménez (presid.), Juan Manuel Ortiz Díaz-Guerra (secret.), Carlos Alberto Ponce de León Albarrán (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: RUIdeRA
- Resumen
The United Nations has exposed the need for moving our society toward a sustainable development because of the climate emergency that is currently taking place in the World as a consequence of the global warming. To fight for the fulfilment of the Sustainable Development Goals (SDGs), focused on promoting prosperity while taking care of the environment, it is essential to control and to stop the effects of the climate change. In this context, besides stopping the global warming, to recover environmental disasters, caused by the improper and uncontrolled production and use of the natural resources, must be a key issue to address. Nowadays, high concentrations of persistent pollutants can be found in soils and water bodies. The high chemical stability and low biodegradability of many of these pollutants make even possible their presence in the water supply, where they are known to affect the immune system of human beings. Consequently, their removal from the environment is necessary to reduce and to get rid of their negative impacts.
In the light of the aforementioned environmental emergency arises the main aim of this PhD, the development of sustainable and green remediation technologies to recover natural resources.
Within this context, the term green remediation was introduced by the Environmental Protection Agency (EPA) as “practice of considering all environmental effects of remedy implementation and incorporating options to minimize the environmental footprint of cleanup activities”. Among the wide variety of environmental remediation techniques, the electrochemical processes have been reported as clean, flexible and powerful technologies to recover polluted soils and liquid or gas effluents. It is important to note that despite those treatments are quite environmentally friendly, they require electrical energy to operate which may jeopardize their sustainability. According to this fact, a green powering of those treatments using renewable energies (REs) could turn these promising techniques into sustainable environmental remediation processes. Thus, their coupling could be the perfect tandem to get a total green remediation.
Keeping in mind the environmental premises previously exposed, Chapter 5 was focused on assessing the environmental impact of electrochemical technologies grid and green powered (solar and wind energies) using a Life Cycle Assessment (LCA) methodology. The carbon and water footprints of Electrochemical Advance Soil Remediation Processes (EASRPs) and Electrochemical Advance Oxidation Processes (EAOPs) were evaluated. Furthermore, to quantify the impact of those treatments on the human health, toxicity studies were also performed. Results reported higher environmental impacts related to the manufacturing of green energy production setups. Conversely, the use of a grid powering, considering a Spanish electricity grid, brings out higher environmental risks related to the production of energy because of its high dependence on fossil fuels. In short, the LCA analyses showed promising sustainability results in terms of carbon and water footprints by the use of renewable sources to power EAOPs. Regarding CO2 discharges, global warming potential (GWP) analyses reported emissions around 0.02 kg CO2 eq. L-1 under a green powering. Conversely, the grid powered EAOP showed higher environmental impact in terms of CO2 emissions, 0.14 kg CO2 eq. L-1. In contrast to that, the sustainability analyses of EASRPs claim against a green powering in terms of carbon footprint. In this case, solar and wind powering showed double and triple emission rates regarding a grid connection, 0.05 kg CO2 eq. kg-1. Notwithstanding, taking into account water footprint and toxicity factors, green energies are a suitable and sustainable powering strategy. Thereby, further studies at higher Technology Readiness Levels (TRL) are required to be able to make statements based on sustainability performance aimed at the real application of those technologies.
According to the two renewable sources studied, the solar power seems to be the best alternative to recover natural resources from a greener point of view. Bearing in mind these promising insights, the use of solar panels to power electrochemical remediation techniques was assessed in detail to gain a deeper understanding of the kinetic of those treatments when they are electrically supplied by a fluctuating power source. In this way, Chapter 6 addresses an exhaustive study of EASRPs and EAOPs solar powered under realistic conditions. Keeping in mind the knowledge gained in the previous study and further studies of our group, the use of different powering and operational strategies were proposed in order to take advantage of this green energy to remediate environmental resources. Regarding EASRPs, traditionally these treatments have been working under constant electric potential gradients (V cm-1). Considering the limitations of those electrokinetic treatments and the promising data exposed by different research groups using solar energy to power EASRP, different powering strategies were evaluated. The process was performed under a solar powering and by means of constant electrical potential (V cm-1) and power (W cm-1) gradients. The random powering of solar energy turned into low removal efficiencies, dragging 10 g of clopyralid per kWh supplied to the treatment. Despite this operational strategy seems to be the most sustainable strategy, results claim against its use in electrochemical soil recovery processes in terms of efficiency. The lower transport of pesticide to the wells per unit of energy supplied (kWh) and the high evaporation flows observed, limit the potential advantage of coupling green energies to these electrochemical technologies. Furthermore, the reversion in the transport of pollutants overnight and the extreme electric fields applied at noon made the remediation process less efficient. Conversely, supplying constant electric potential gradients under mild operation conditions leads to a higher removal efficiency (111.40 g of clopyralid (kWh)-1) and a lower soil affectation in terms of extreme pHs, water contents and conductivities. On the other hand, the promising results exposed by solar powered EAOPs in terms of efficiency and sustainability working in batch mode, led to a thorough study under continuous operation conditions. The main limitation of this operational condition lies on the fluctuating powering in contrast to the continuous treatment flow. To strike a balance between both operational variables, an innovative operational strategy was tested. The EAOP worked at constant electric charge density (Ah dm-3). To do that, the treatment flow rate is adjusted according to the solar power fluctuations. Thus, optimum operational conditions are ensured throughout the remediation treatment regardless of the solar power supplied by the PV plant. Hence, issues associated with the null or low solar power production are getting rid of. Experimental tests confirmed that a random solar powering, without flow rate control, drops the efficiencies of the remediation treatment due to the fluctuating current densities supplied to the electrolyzer. In this case, an average removal of 0.42 g of TOC per kWh was quantified. Conversely, a targeted direct powering running at continuous electric charge density, allows the system to reach a removal equilibrium which makes the technology affordable to work under a semi-continuous mode. Furthermore, this operational strategy sheds light of a higher removal, reaching an average degradation of 0.46 mg of TOC (kWh)-1. Despite the positive results showed by this targeted powering method, smart control systems must be coupled to the treatment plant due to the complexity of this operational strategy. Furthermore, to solve the null remediation at night, energy storage systems should be used in order to power the EAOP at non-solar power production periods.
Considering the issues associated with the targeted solar powering of EAOP working in continuous mode, innovative solutions as smart control systems and energy storage devices were outlined in Chapters 7 and 8, respectively. Firstly, taking into account the simple, efficient and sustainable use of batch modes to treat wastewater effluents under a solar powering, the coupling of a set of two Conductive Diamond Electrochemical Oxidation (CDEO) reactors working under different hydraulic and electrical configurations was evaluated with the aim of shedding light on the operational strategy that allows to take advantage of the total solar power produced by a PV plant and to reach the highest and most efficient remediation. In view of the aforementioned premises, series and parallel hydraulic and electrical connections were tested between two CDEO reactors. The obtained results claim that to control and to manage the operational conditions (treatment flow and energy) of electrochemical remediation techniques is essential to develop efficient treatments. The treatment flow must be sufficient to harness the energy provided by the solar plant and to avoid its waste. Thus, to assess the hydraulic distribution of a wastewater effluent into an electrochemical reactor is key to ensure an optimum hydraulic retention time and hence a perfect contact between electrodes and waste. In this context, results showed higher removals while working under series hydraulic connections because of the higher hydraulic retention time of the effluent into the CDEO reactors. Regarding the solar power distribution, results showed an efficient remediation under parallel electrical connections. Taking these facts into account, the lowest remediation, 0.48 mg clopyralid (Wh)-1, was observed using a parallel-hydraulic and a series-electrical configuration. Conversely, the highest ratio of pesticide removal per Wh supplied to the treatment was observed under a series hydraulic distribution and a parallel electrical connection (2.52 mg clopyralid (Wh)-1). Consequently, those results confirm once again that higher current densities can lead to mass transfer limitations that reduce the efficiency of the electrooxidation treatment because of non-desired reactions. In short, to work under the best choice in terms of hydraulic and electrical connections among electrolyzers (series-parallel hydraulic-electrical strategy) can bring out an improvement four times higher in the specific removal of pollutants (g (Wh)-1).
On the other hand, to overcome the issue related to the null removal at night, the use of energy storage systems (ESSs) as booster of photovoltaic solar electrochemical oxidation (PSEO) treatments was assessed in Chapter 8. Those devices may store the excess of renewable energy and electrically assist electrochemical remediation processes at null green electricity production hours. In this way, those storage systems may also be able to smooth the fluctuating powering of solar energy and to keep soft operational conditions which may turn into efficient and sustainable remediations. Focused on this issue, it was assessed the remediation performance of an EAOP directly powered by an ESS (Pb - battery). Experimental data point out that the resistance offered by the electrochemical technology determines the current and voltage supplied by the ESS. Furthermore, the remediation treatment time is limited by the capacity of the ESS. According to those facts, the operational conditions of the treatment can be altered. For comparative purposes, the electrooxidation performances of different CDEO reactors fitted with different electrodes were assessed. Electrochemical reactors equipped with electrodes which provide higher ohmic resistance were powered by lower current densities, keeping the treatment powered for a longer time. Conversely, low resistant electrodes bring out shorter and higher powered electrooxidation treatments. These results prove that it is essential to fit the electrical features of the electrochemical remediation technique and the ESS with the aim of working under optimum operational conditions and, consequently, performing efficient remediations. In short, these results exposed an important and relevant breakthrough in terms of energy dosage by means of energy storage systems. This knowledge is essential to reach an efficient coupling of renewable energy systems and electrochemical technologies in order to get an environmental green remediation. Within this context, the design and development of a vanadium redox flow battery (VRFB) stack was performed to electrically assess those wastewater treatments. It is worth mentioning that an RFB was selected as the suitable ESS to address this aim because its strengths, independent sizing of power and energy, allow a straightforward and an exhaustive design and scaling up according to the electrical features required by the electrochemical treatment. Considering this fact, results pointed out that a stack made up of at least four single cells is required to power the chosen CDEO reactor. To evaluate the performance of the RFB stack to electrically assist the EAOP, experimental tests under real operational conditions were performed. Data show promising results when the RFB stack was charged under different weather conditions (rainy, cloudy and sunny days). Soft weather conditions, high current densities, reported lower capacities due to the faster kinetic of the system, 4.17 Ah. In contrast to that, harsher weather conditions, low current densities, noticed slow charge steps and more energy was stored, 5.81 Ah. Despite the fluctuating current densities supplied by the PV plant to the RFB stack, large capacities and efficiencies were reached. Hence, those promising results claim in favour of the use of VRFBs as booster of EAOPs directly coupled by solar panels with the aim of reaching the most suitable green remediation.
Finally, keeping in mind the unpredictable remediation of solar powered electrochemical treatments because of their dependence on weather conditions, Chapter 9 addresses this issue by the development of a highly versatile software tool. This predictive simulator allows to know in advance the kinetic performance of a PSEO treatment directly coupled to a PV plant and assisted by an RFB stack. This mathematical tool aims at controlling and managing the solar power coming from a photovoltaic (PV) plant to carry out a PSEO under the most optimum and efficient operational conditions. To meet this aim, different predictive modules were developed. Firstly, a Solar Radiation Prediction Module (SRPM) consisting of a pragmatic model able to forecast solar radiation in a specific location and time was carried out, attaining correlation coefficients of 0.89. Additional modules faced up the modelling of a CDEO reactor and an RFB directly connected to a PV plant. Those modules are interconnected in an Energy Management Module (EMM) capable of distributing the energy coming from the PV plant towards its direct use in a PSEO treatment or its storage into an RFB stack. Thus, this innovative software tool allows to determine the best solar powering strategy in order to reach the highest and most efficient remediation. To quantify the level of remediation, the Environmental Remediation Module (ERM) predicts the oxidation of a persistent organic pollutant model (clopyralid) as a function of the current density supplied to the treatment, considering the most relevant operational regimens of an electrochemical oxidation process. Once the individual modules were validated and interconnected, deeper theoretical studies were carried out in order to achieve an exhaustive understanding of those technologies when they are working under realistic conditions. Those results demonstrated that the management of the energy coming from a PV plant is key to ensure the most suitable powering of an EAOP coupled with a green energy source. The regulation and storage of solar energy allow to smooth the powering conditions and to extend the treatment time of the process, increasing consequently its overall efficiency. Despite the positive results reported by the proposed models to predict the electrical and kinetic performance of those devices, some issues arise according to the interconnection of the modules because of the different electrical connections that could be performed between the electrochemical systems (CDEO reactor and RFB stack) when they are directly coupled to the PV plant. At this point, series and parallel electrical powering strategies were tested. Straightforward series or parallel connections, without energy regulation, showed different performance in terms of remediation and energy efficiency, because each electrical connection turns into a quite different energy distribution. Predictive results reported higher remediation values when both electrochemical devices were directly coupled in parallel due to the lower current densities supplied by the PV plant to both systems. This powering strategy gets rid of operational drawbacks related to high current densities as mass transfer limitations or waste of energy, which may shed light on removal efficiency drops on the EAOP and low capacities on the RFB.
According to the promising approaches deducted thanks to the predictive modelling, a deeper analysis using a targeted powering strategy was carried out. This study met an exhaustive power control by means of the use of variable resistances. Theoretical results suggested that the use of a parallel connection under an RFB control allows the systems to reach higher remediations. Charging the RFB under lower current densities, points out higher capacities which extends the remediation treatment and the total remediation reached.
Considering the results obtained in this Doctoral Thesis, it can be concluded that renewable energies can work as suitable power supplies of electrochemical remediation techniques. Nevertheless, to perform the most sustainable and efficient treatment, the management, control and prediction of green energy must be carried out.
