Blog post submitted by Ana S. Fajardo 

Public awareness of the health effects of poor water quality is a driving force for the growing demand for new water technologies. One example is the increasing market for residential point-of-use drinking water purification systems (e.g., systems that fit under your home sink) [1]. To meet the demands of the public these systems need to be efficient, compact and user-friendly. Another example of a developing treatment need is industrial wastewater treatment. New technologies must reduce oxygen demand and organic/inorganic species to regulated values before discharge to a municipal sewer.

In this context, processes that use electricity to treat water have been attracting the attention of the scientists all over the world. These technologies are able to remove/convert in-situ difficult substances present in complex wastewaters or drinking water matrices, using autonomous, compact and chemical-free systems that can be deployed anywhere. So far, most of the studies related to electrochemically-driven technologies (e.g., electrochemical oxidation, electro-Fenton, photoelectron-Fenton, electrochemical reduction) are focused on treating synthetic matrices that contain a single target pollutant. This often delays the translation to higher technology readiness levels and commercialization of these systems for real water matrices. Why is this happening? This paper aims enlightening the gaps and needs in the transition from ideal lab made solutions to an actual commercial implementation.

According to the literature, most pollutant concentration ranges used in synthetic laboratory prepared solutions are several orders of magnitude higher than those that appear in real-world scenarios. These non-real conditions will mask the performance of the process, giving a wrong idea about energetic parameters such as faradaic efficiency and electric energy per order, which will mislead engineering decisions. In order to frame the readers, these two concepts will be defined. Faradaic efficiency determines the number of electrons consumed in an electrochemical reaction relative to the expected theoretical conversion governed by Faraday’s law and the electric energy per order quantifies the required electric energy to reduce pollutant concentration by one order of magnitude in a unit volume.
Fig. 1 – Box-and-whisker plot comparing concentration ranges of target pollutants found in true environmental concentrations (¢)  vs. synthetic solutions (¢) treated in the lab. Pollutants are classified by pollutant class: Per- and polyfluoroalkine substances (PFAS), pharmaceuticals, pesticides, and nitrates. Central mark (£) of box represents the median value, bottom and top edges are 25^th and 75^th percentiles, respectively. Whiskers extend to extremes not considered outliers. Outliers (u) are points greater than (Q1+ 1.5(Q3-Q1)) and less than (Q1-1.5(Q3-Q1)), where Q is quartile.

Beyond pollutant concentration, considering water matrix constituents in real water is also fundamental. Although conducting research in synthetic matrices has been essential to develop the mechanistic understanding of electrochemical principles for treating drinking waters and wastewaters, continuing in this path it will not allow identifying the competition between species commonly present in actual waters, or studying the effect of pH buffers or water hardness. Engineering challenges are being avoided; however they must be faced and solved prior to the technology being implemented. An example of this is the non-study of the electrode stability in a complex water matrix, that contains solutes and bacteria that may cause scaling and fouling on the surface of the electrode in long-term experiments, leading to the decrease of the performance of the process.

Fig. 2 – (a) Inorganic scaling on electrodes during electrocatalytic treatment of real groundwater (hardness: ~350 mg L^-1 as CaCO₃). (b) Biofouling on boron-doped diamond electrodes by Pseudomonas aeruginosa seen by optical coherence tomography microscopy [2].

NEWT researchers have recently tackled these challenges in a paper titled “Disparities between experimental and environmental conditions: Research steps toward making electrochemical water treatment a reality”. According to the research members from Thrust 1/NEWT (ASU, RICE, UTEP and YALE) that composes the team of this recent publication, it is time to stop avoiding pressing needs in the water treatment field and start redefining research questions to provide more mechanistically relevant process assessments and to overcome realistic challenges. In this specific case, identifying fit-for-purpose and niche opportunities for electrochemically-driven technologies is vital to continued research in the field.

Caption: Sergi Garcia-Segura (ASU), Alec Brockway Nienhauser (ASU), Ana S. Fajardo (ASU), Rishabh Bansal (ASU), Christian L. Coonrod (Rice), John D. Fortner (Yale) Mariana Marcos-Hernández (UTEP), Tanya Rogers (Rice), Dino Villagran (UTEP), Michael S. Wong (Rice), and Paul Westerhoff (ASU). 

“In our research field, learning and understanding the steps how to translate a process from bench to commercial scale is essential to our professional growth. Additionally, we are always trying to apply this concept within all our works,” said NEWT post-doc Ana Sofia Dos Santos Fajardo, one of the authors on this paper.

References

[1] R. Stirling, W.S. Walker, P. Westerhoff, S. Garcia-Segura, Techno-economic analysis to identify key innovations required for electrochemical oxidation as point-of-use treatment systems, Electrochim. Acta 338 (2020) 135874.

[2] D. Rice, P. Westerhoff, F. Perreault, S. Garcia-segura, Electrochemical self-cleaning anodic surfaces for biofouling control during water treatment, Electrochem. Commun. 96 (2018) 83–87.