Electrochemical Technologies for the Abatement of Endocrine Disrupting Compounds

Endocrine disrupting compounds (EDCs) are a class of emerging contaminants capable of interfering with the normal hormonal functioning of humans and other organisms. Even minute concentrations of EDCs in natural receiving water bodies can be minacious and are raising concern globally due to their connection with rising cases in reproductive, immune system, and developmental-related health disorders. These EDCs are released from anthropogenic sources and are routinely detected in the natural environment especially in wastewater. Incapacitated by poor infrastructure and design, conventional wastewater treatment facilities fail to satisfactorily remove the recalcitrant organic pollutants like, parabens, phenols, pesticides, etc., making them one of the biggest sources of EDCs discharged into the environment. Neoteric techniques such as, electrochemical oxidation, electro-Fenton process, electrocoagulation, and other electrochemical technologies (ETs) have emerged as a robust technology for eliminating biorefractory contaminants from different euents. The setups used in ETs are easy to fabricate, require low chemical additions, produce minimal sludge, and their faster/ecient degradation of recalcitrant compounds give them an edge over their conventional counterparts. However, expensive electrodes and substantial power consumption limit the scale-up application of ETs. Thus, this review attempts to present a holistic view of the applicability and feasibility of ETs with respect to EDCs attenuation from contaminated water and their advancement as a mainstream wastewater management technology. their advancement a wastewater remediation technology.


Introduction
The advent of any novel material and technology is inherently followed by the formation of new types of waste products and contaminants. As these contaminants are newly identi ed, as these are from a new origin, and currently not or partially regulated by concerned regulatory bodies, hence these are often termed as 'emerging contaminants'(EmCs). The prevalence of EmCs in the aquatic environment is rampant and is well documented by many researchers (Ashton et al., 2004;Behera et al., 2011;Gros et al., 2012;Hughes et al., 2013) owing to the advancement in analytical technologies (Richardson, 2009). Arrays of anthropogenic activities, such as industrial operations, wastewater discharge, etc., are the major sources of release of these pollutants into the environment, though they can be of natural origin as well (Cabeza et al., 2012;Rout et al., 2021). There are innumerable EmCs present in the environment and can be broadly classi ed into three categories based on their origin, namely (a) personal care products, (b) pharmaceuticals, and (c) endocrine disrupting compounds (EDCs) (Gogoi et al., 2018).
Among these three types of EmCs, EDCs are considered the most potent ones due to their inherent ability to impede hormonal homeostasis, which subsequently develops into serious health related disorders. Any natural or synthetic chemical compound that is cable of interfering with the normal working of the endocrine system or hormonal functions of the body, such as the release of reproductive hormones, different metabolic activities, sustaining homeostasis, etc., is termed as EDC (Kabir et al., 2015;Kavlock et al., 1996). Many products and commodities of daily use, such as plastics products, food packaging, construction materials, personal care, and hygiene products, medications, pesticides, insecticides, and so on possesses some kind of EDCs in the form of paraben preservatives, bisphenol A (BPA), triclosan (TCS), phthalates, estrone (E1), estradiol, estriol (E3), etc., in their ingredients (Dodge et al., 2015;Kabir et al., 2015;Monneret, 2017).
Such widespread use of materials containing EDCs has led to their deposition and accumulation in the environment, especially in natural water bodies. The existence of EDCs is ubiquitous in the aquatic environment as evidenced by different investigations ascertaining their presence in surface water, groundwater, and wastewater, which is mainly due to industrial discharges and inadequate wastewater treatment systems (Vymazal et al., 2015;Zhang et al., 2015). Prolong residence of EDCs in the environment poses an imminent threat to both humans and natural biota due to their ecotoxicological effects. The EDCs have been known to cause serious health concerns in human beings including hormonal imbalance, hindering neurological functions, birth defects, and other developmental impairments in children (Monneret, 2017). Besides, exposure to EDCs can also lead to diseases like breast cancer in females, obesity, diabetes as well as reproductive ailments (Jambor et al., 2017;Legler et al., 2015;Nohynek et al., 2013). A similar adverse effect of EDCs has been observed in animals and aquatic biota like, interfering with the immune system, malfunctioning of hormone-secreting and reproductive organs, and formation of malignant tumours (Casanova-Nakayama et al., Fowler et al., 2012;Xie et al., 2018). Hence, the removal of EDCs from sewage and industrial e uents is imperative for safeguarding humans and natural ecosystems from any such deleterious effects.
Conventional wastewater treatment plants (WWTPs) are not designed to handle and remove EDCs from wastewater and consequently, they exhibit limited removal of EDCs from wastewater. Moreover, the recalcitrant and biorefractory nature of EDCs makes them resistant to degradation in biological treatment processes, such as activated sludge process, tricking lter, etc. Non-conventional and advanced treatment technologies like adsorption, membrane ltration, advanced oxidation processes (AOPs), etc., have been employed for the treatment of EDCs Vieira et al., 2020); however, all of these methods have some inherent drawbacks associated with their operation. In adsorption, the contaminant is not degraded rather only phase transformation occurs demanding separate attenuation for the safe disposal of the rejects. Moreover, regeneration of the adsorbent, once it has been spent during the adsorption, is a costly affair. On the other hand, membrane lters suffer from frequent fouling due to pore blockage and thus require regular replacement of lters, which becomes problematic while handling a large volume of wastewater. The AOPs are effective methods for eliminating ECs; nonetheless, expensive chemicals are required during their application for wastewater treatment that incurs a high cost for operation and maintenance of the these AOP-based treatment units .
The electrochemical technologies (ETs), such as electrochemical oxidation (EO), electro-Fenton process (EF), electrocoagulation (EC), electro otation process, etc., have emerged as cost-effective and eco-friendly technique for the e cacious abatement of EmCs from wastewater due to minimal chemical requirements, reduced sludge production, non-selective degradation of a vast variety of pollutants, lesser footprint and requirement of lesser detention time. The ETs are capable of effectively eliminating the target compounds by completely mineralizing them into carbon dioxide, water, and simple inorganic compounds via direct anodic oxidation (AO) and hydroxyl radical (•OH) mediated indirect oxidation (in case of EO and EF) or by separating out the contaminants from bulk solution either by skimming them from the surface (during EF) or by coagulation of contaminants followed by settling as in EC.
Many investigations have demonstrated the competency of EO, EF, and EC for the elimination of EmCs from the wastewater, such as caffeine, dyes, antibiotics, beta-blockers, parabens, phenolic compounds, and many more (Oturan, 2014;Raj et al., 2021;Seibert et al., 2020). Such promising result shown by ETs in the degradation of ECs exempli es their robustness in handling pollutants not amenable to biodegradation, rendering them as a potential solution for the treatment of wastewater containing EDCs and other persistent complex organic contaminants.
This review attempts to highlight the most recent developments made in the application of different ETs for the mitigation of EDCs and focuses on their removal e ciency, performance governing parameters, and salient features of each technology along with some key challenges in the upscaling of ETs for broader and eld-scale implementation.
Additionally, the bene ts and shortcomings of ETs have been critically reviewed, incorporating comparative economic analysis of ETs with conventional treatment methods and other advanced chemical treatments techniques. The attenuation capacity of ET-based hybrid systems and other advanced treatment techniques has also been brie y introduced to elucidate the integration abilities of ETs. To the best of our knowledge, no such review is available in the literature, which comprehensively presents the performance of ETs for EDCs abatement from wastewater. Besides, the review will also serve as a comprehensive guidebook for budding researchers interested in exploring ETs for the attenuation of EmCs from the aquatic environment, while the experts, presently working in the associated eld, will be bene ted from the updated literature, and would be enlightened with the recent insightful information pertaining to ETs that could assist their ongoing research. Moreover, the article also puts forward the research gap in the eld of ETs, which could be exploited by the researchers working in this domain for addressing the critical drawbacks for abetting ETs to realise their full potential.

Classi cation
The EDCs are a highly heterogeneous group of compounds, which have the potential to disrupt the endocrine (hormonal) system of the human body or any other organism. According to the de nition given by the United States Environmental Protection Agency, EDCs are "an agent that interferes with the synthesis, secretion, transport, binding, or elimination of natural hormones in the body that are responsible for the maintenance of homeostasis, reproduction, development and/or behaviour" (Crisp et al., 1998). In other words, any compound capable of triggering abnormal hormonal responses causing adverse health effects and hindering the natural growth and development of the body is categorized as an EDC. Classi cation of EDCs is a cumbersome task as there are innumerable substances that can affect the endocrine system of an organism directly or indirectly, and it requires a series of tests, and experiments for ascertaining the endocrine disrupting potential of a chemical compound and its metabolite. Moreover, most of these contaminants overlap with other subcategories of EmCs, making the categorisation of EDCs even more perplexing and complicated. Pojana et al. (2007) bifurcated EDCs into two categories, namely, synthetic chemicals, such as industrial solvents and their by-products, plasticizers, pesticides, etc., and natural chemicals like phytoestrogens. On the other hand, researchers proposed a product-based classi cation of EDCs, dividing them into three categories depending upon the commodities or material in which they are used or found, namely, pesticides, chemicals in products, and food contact materials (Forio and Goethals, 2020; Peña-Guzmán et al., 2019). Nonetheless, considering the heterogeneous nature of these chemicals and for the sake of simplicity in understanding, EDCs are broadly classi ed into six major groups in this article as (i) pesticides, (ii) phenolic compounds, (iii) phthalates, (iv) paraben compounds, (v) steroid estrogens, and (vi) estrogenic heavy metals, which are elaborated in the following section. Application and associated health hazards of the different classes of EDCs have been depicted in Table 1. pesticides are among the most widely applied pesticides, which are known to cause estrogenic imbalances in rats (Gellert, 1978). Adverse estrogenic activities of pesticides have been widely reported and DDT was among the rst few compounds identi ed as EDCs (Fisher et al., 1952).

Diamanti-Kandarakis et al. (2009) and
Despite worldwide agreement on curbing down the use of pesticides due to their devastating hormone-related disorders on wildlife and human health, the use of pesticides such as DDT is rampant, especially in developing countries like India, Ethiopia, Namibia, South Africa, etc., and it is responsible for causing acute toxicity and another health-related hazard in addition to endocrine disruption (Abhilash and Singh, 2009

Presence of endocrine disrupting compounds in the aquatic environment
The presence of EDCs in the natural water bodies can have dire health consequences on the aquatic ecosystem and human health. Human contact with EDCs is almost inevitable as they are found in a plethora of everyday products like, cosmetics, food packaging, plastic containers, medication, and so on. Unfortunately, most of these estrogenic compounds ultimately end up in the environment via different pathways ( Fig. 1) and pose serious maladies to the exposed organisms. The presence of these EDCs in different water bodies is frequently reported in the literature and it has been summarized in Table 1. contamination of rivers and other natural water bodies, as it was evident during the investigation carried out by Renganathan et al. (2021). In this investigation, they identi ed the presence of different pharmaceutically active compounds including 10.39 ng/L of TCS, 205.62 ng/L carbamazepine, and 28.51 ng/L diclofenac in the river Cauvery and its tributaries, which serves as the prime source of the freshwater supply for many states in southern India.
Though EDCs compounds may undergo degradation by means of sorption, biotransformation process, etc., under natural aerobic conditions, a substantial fraction of these xenobiotic compounds tend to settle at the bottom of the river bed, where they don't degrade further due to the absence of oxygen and thus accumulate in the sediments over a period of time (Koumaki et al., 2018). This may further aggravate the risk of toxic chronic exposure and bioaccumulation of EDCs in the ora and fauna inhabiting the contaminated water body. In the liver tissue of shes of the Pearl River system in south China, a maximum concentration of 40.920 µg/g of BPA and 5.978 µg/g of NP was discovered, which was suggestive of the bioaccumulation potential of these EDCs ( Moreover, NP was found to be more toxic than NP-9 with 24 h 50% lethal concertation (LC 50 ) for C. elegans being 26.88 mg/L and 1916.14 mg/L, respectively, for both compounds (De la Parra-Guerra and Olivero-Verbel, 2020). In a separate investigation, Sun and Liu (2017) observed embryotic malformations in zebra sh, when exposed to 0.6 mg/L of butyl benzyl phthalate. Furthermore, the presence of additional contaminants, such as microplastic can elevate the toxicity of NP. For example, the growth inhibition ratio (expressed as (increased density of control -increased density at different concentrations)/ increased density of control) of Chlorella pyrenoids was 69.2% for only NP, while it was 15.1% for NP combined with polyamide 1000 (

Electrochemical Technologies For The Abatement Of Endocrine Disrupting Compounds
Over the last few decades, the release of EDCs into natural water sources has become a global concern due to their substantial noxious effects on humans and wildlife. which makes this process a sustainable and effective panorama for wastewater remediation (Ifelebuegu and Ezenwa, 2011). Consequently, the requirement of applied potential and utilization of expensive catalysts and electrodes can render these treatment technologies uneconomical for large-scale applications. Therefore, further research is required to develop low-cost electrodes along with the reduction in the overall energy consumption of the system, which will take these technologies towards commercialisation.
A typical lab-scale electrochemical system comprises of a reactor tted with electrodes along with arrangement for power supply and mixing/recirculation of the wastewater. Sometimes to reduce electricity consumption, supplementary power source like solar power can also be used (Fig. 2).

Electrochemical oxidation
Wastewater treatment through EO can be accomplished through different techniques, such as direct EO and indirect EO (Fig. 3). Generally, the direct EO process is also sometimes referred to as AO because

Electrocoagulation
The EC process is an advanced electrochemical technique employed for the removal of a broad spectrum of pollutants and emergent contaminants from wastewater as it is a combination of the functions of three processes, namely coagulation, oatation, and electrochemistry (Fig. 4). aluminium and iron are extensively utilized as the metallic anode material in the process of EC due to its high dissolution rate, favouring the production of hydroxides, polymeric hydroxides, and oxyhydroxides. As an example,

Electro-Fenton treatment for the disruption of EDCs
In the cathode-based electrochemical advanced oxidation systems, the most well-known technique is the EF process,   The oxidation of NaCl generates active chlorine species, such as Cl 2 , HClO, and OCl − , which is much more reactive than the S 2 O 8 2− that are formed during the oxidation of Na 2 SO 4 and therefore ameliorated the removal e ciency of phenol.
Concurring results were obtained during electrochemical degradation of phenol with an initial concentration of 100 mg/L using Ti mesh electrodes, wherein the addition of 10 mg/L of NaCl attained 98.21% removal e ciency for phenol, which was 9.5 times higher compared to the removal e ciency achieved using 10 g/L of Na 2 SO 4 as supporting The e cacy of such a BEF system was illustrated during the investigation by Sathe et al. (2021)  Therefore, biological treatment coupled with ETs represents a signi cant hybrid method for the removal of EDCs and other micropollutants from wastewater. However, further investigation is required to improve the holistic understanding of the biodegradation pathway of microbiota as well as the essential parameters that enhance the degradation e ciency of EDCs. Also, by-products and secondary metabolites, which are released during the biodegradation process need to be characterized and quanti ed because occasionally by-products can be more intoxicating and persistent compared to the parent compounds.

Bene ts And Drawbacks Of Electrochemical Technologies With Economical Perspective
The ETs have many advantages over other existing and advanced wastewater treatment systems (Fig. 6), though the most salient feature of ETs is the ability to mineralize a wide array of recalcitrant xenobiotic compounds at a relatively higher rate. Rosales    to be at least 0.39 Wh/g of TOC removed that again is unsustainable while dealing with a larger volume of wastewater.
In EO and EF process, the high current density is imposed on the electrodes to facilitate contaminant degradation that causes electrodes corrosion, while in EC sacri cial iron anodes are used for the formation of iron ocs in the solution, thus frequent replacement of electrodes is required in ETs for the consistent reactor performance.
Furthermore, a major concern during electrochemical degradation of biorefractory contaminants is the formation of toxic intermediates and metabolites due to partial mineralization. Degradation by-products like, 4-isopropyl phenol, phenol, divinyloxyethane, acetylacetone, and isobutyric acid were detected, when 20 mg/L of BPA was subjected to heterogeneous EF treatment

Future Scope
The non-selective destruction of contaminants by in situ electrochemically generated •OH radical in AO and EF makes them a very effective technology for managing xenobiotic pollution of hydric sources, while EC has displayed superior contaminant removal e ciency in comparison to the conventional coagulation process. Nonetheless, the application of ETs is still con ned to lab-scale investigations owing to their prohibitive operating cost and power demand.
Comprehensive research pertaining to the development of inexpensive durable electrodes and low-cost catalysts is required to reduce the initial investment and maintenance cost of ETs. Further investigations on holistic designing of continuous ow and energy-e cient reactors to suit more pragmatic applications will abet in the commercialisation of these neoteric technologies. More experiments on pilot-scale prototypes of ETs should be carried out to assess the behaviour of different cell parameters and associated operational di culties in the up-scaled reactors. Moreover, analysis of pilot-scale reactors will also aid in a better understanding of the hydrodynamic and electrochemical behaviour of the degradation process, which can be used for optimising the design of electrochemical reactors.
More recently, researchers are coupling ETs with different AOPs to boost the destruction of persistent contaminants such as, photo-EO process, photo-EF process, EF-peroxi coagulation, etc.

Conclusion
Page 25/42 The presence of EDCs in different hydric sources is an imminent threat to the wellbeing of the aquatic ecosystem and human health. Exposure to EDCs can have dire health consequences, such as hormonal imbalance, impairment of the reproductive system, cancer, obesity, developmental disorder in children, and so on. Conventional wastewater treatment plants are not designed to remove EDCs from wastewater and thus are the main culprit for EDCs discharge into natural receiving water bodies. The ETs have emerged as an effective process for the elimination of EDCs from contaminated water owing to their non-selective degradation of organic contaminants using highly potent •OH and other reactive species. Nevertheless, the broader application of ETs is awed by unreasonable operating costs and the high energy  Lab-scale electrochemical reactor setup operating with solar-generated electricity Advantages and limitations of electrochemical technologies