Highly efficient total nitrogen and simultaneous total organic carbon removal for urine based on the photoelectrochemical cycle reaction of chlorine and hydroxyl radicals

Critical review on on-site sanitation technologies: Typologies, treatment and transition towards circular economy

Around 4.2 billion people globally depend on on-site sanitation systems, with 43% relying on basic or unsafe facilities with key challenges of containment, emptying, transport, treatment and resource recovery from faecal sludge. This review paper critically examines faecal sludge characteristics and treatment technologies in terms of urine diversion capability, land requirements and capital as well as operational expenditure based on Indian and international practices. Apart from these, the review also provides reuse-recycle of processed manure, treated effluent and bio-energy from faecal sludge to create environmental and economic benefits. The review also highlights future research on the development of nature-based solutions for solid-liquid separation and treatment, low footprints and on-site treatment for fostering a circular economy. Ultimately this review emphasizes the importance of advancing faecal sludge management to address global sanitation challenges and to create sustainable solutions for environmental protection.

Nutrient Removal and Recovery from Urine Using Bio-Mineral Formation Processes

Harvesting nutrients from waste presents a promising initiative to advance and deliver the circular economy in the water sector while mitigating local shortages of mineral fertilizers worldwide. Urine, a small fraction of municipal wastewater, holds substantial amounts of nitrogen, orthophosphate (PO4-P), and chemical oxygen demand (COD). Separating urine aids targeted nutrient recovery, emissions reduction, and releasing capacity in wastewater treatment plants and taps into overlooked vital nutrients like magnesium (Mg2+) and potassium (K+), essential for plant growth. The ability of selected microorganisms (Brevibacterium antiquum, Bacillus pumilus, Halobacterium salinarum, Idiomarina loihiensis, and Myxococcus xanthus) to remove and recover nutrients from fresh urine through bio-mineral formation of struvite was investigated. The selected microorganisms outcompeted native microbes in open-culture fresh urine, and intact cell counts were 1.3 to 2.3 times larger than in noninoculated controls. PO4-P removal reached 50% after 4 days of incubation and 96% when urine was supplemented with Mg2+. Additionally, soluble COD was reduced by 60%; urea hydrolysis was only < 3% in controls, but it reached 35% in inoculated urine after 10 days. The dominant morphology of recovered precipitates was euhedral and prismatic, identified using energy dispersive spectroscopy and X-ray diffraction as struvite (i.e., bio-struvite), but K+ was also present at 5%. Up to 1 g bio-struvite/L urine was recovered. These results demonstrate the ability of bio-mineral producing microorganisms to successfully grow in urine and recover nutrients such as bio-struvite, that could potentially be used as sustainable fertilizers or chemicals.

A controlled non-radical chlorine activation pathway on hematite photoanodes for efficient oxidative chlorination reactions

Photo(electro)catalytic chlorine oxidation has emerged as a useful method for chemical transformation and environmental remediation. However, the reaction selectivity usually remains low due to the high activity and non-selectivity characteristics of free chlorine radicals. In this study, we report a photoelectrochemical (PEC) strategy for achieving controlled non-radical chlorine activation on hematite (α-Fe2O3) photoanodes. High selectivity (up to 99%) and faradaic efficiency (up to 90%) are achieved for the chlorination of a wide range of aromatic compounds and alkenes by using NaCl as the chlorine source, which is distinct from conventional TiO2 photoanodes. A comprehensive PEC study verifies a non-radical "Cl+" formation pathway, which is facilitated by the accumulation of surface-trapped holes on α-Fe2O3 surfaces. The new understanding of the non-radical Cl- activation by semiconductor photoelectrochemistry is expected to provide guidance for conducting selective chlorine atom transfer reactions.

Synergistic mechanisms between chlorine-mediated electrochemical advanced oxidation and ultraviolet light for ammonia removal

The combination of chlorine-mediated electrochemical advanced oxidation (Cl-EAO) and ultraviolet (UV) radiation (UV-E/Cl) can efficiently remove ammonia from wastewater. However, the synergistic mechanisms between Cl-EAO and UV need to be explored in more detail. Thus, in this study, the ammonia oxidation performance of Cl-EAO and UV-E/Cl systems were compared, while the synergistic mechanisms were identified by the performance of UV/chlorine oxidation (UV-ClO) system and the results of electron paramagnetic resonance (EPR) analysis, free radical inhibition assays, and determination of steady-state concentration of free radicals. It was found that, compared with the Cl-EAO system, UV increased the ammonia removal rate by 42.85% and reduced the active chlorine concentration (56.64%) and nitrate yield (53.61%). In the Cl-EAO, and UV-E/Cl systems, Cl• were detected, and the free radical inhibition assays and determination of steady-state concentration of free radicals suggested that UV increased the concentration of Cl• by 51.47%, resulting in Cl• becoming the major contributor to ammonia oxidation in the UV-E/Cl system. Besides, UV also increase the concentrations of HO• and Cl2•-, which further promoted the organic matter removal in the real domestic wastewater. This study also discussed the ammonia oxidation performance of the UV-E/Cl system in real domestic wastewater, even with the presence of significant levels of organic and inorganic anions in the wastewater, UV increased the ammonia oxidation by 21.95%. The results of this study thus clarify the mechanisms and potential applications of UV-E/Cl technology.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

Enhanced •Cl generation by introducing electrophilic Cu(II) in Co3O4 anode for efficient total nitrogen removal with hydrogen recovery in urine treatment

Urine is a nitrogen-containing waste, but can be used as an attractive alternative substrate for H2 recovery. However, conventional urea oxidation reaction is subject to complex six-electron transfer kinetics and requires alkaline conditions. Herein, an efficient method of enhancing •Cl generation by introducing electrophilic Cu(II) into Co3O4 nanowires anode was proposed, which realized the highly efficient TN removal and H2 production in urine treatment under neutral conditions. The key mechanism is that the electrophilic effect of Cu(II) attracts electrons from the oxygen atom, which causes the oxygen atom to further attract electrons from Co(II), reducing the charge density of Co(II). Electrophilic Cu(II) accelerates the difficult conversion step of Co(II) to Co(III), which enhances the generation of •Cl. The generated •Cl efficiently converts urea to N2, while the electron transport promotes H2 production on the CuO@CF nanowires cathode. Results showed that the steady-state concentration of •Cl was increased to about 1.5 times by the Cu(II) introduction. TN removal and H2 production reached 94.7% and 642.1 μmol after 50 min, which was 1.6 times and 1.5 times that of Co3O4 system, respectively. It was also 2.3 times and 2.1 times of RuO2, and 3.3 times and 2.5 times of Pt, respectively. Moreover, TN removal was 11.0 times higher than that of without •Cl mediation, and H2 production was 4.3 times higher. More importantly, excellent TN removal and H2 production were also observed in the actual urine treatment. This work provides a practical possibility for efficient total nitrogen removal and hydrogen recovery in urine wastewater treatment.

Chlorine-mediated electrochemical advanced oxidation process for ammonia removal: Mechanisms, characteristics and expectation

Chlorine-Mediated Electrochemical Advanced Oxidation (Cl-EAO) technology is a promising approach for ammonia removal from wastewater due to its numerous advantages, including small infrastructure, short processing time, easy operation, high security, and high nitrogen selectivity. This paper provides a review of the ammonia oxidation mechanisms, characteristics, and anticipated applications of Cl-EAO technology. The mechanisms of ammonia oxidation encompass breakpoint chlorination and chlorine radical oxidation, although the contributions of active chlorine, Cl, and ClO remain uncertain. This study critically examines the limitations of existing research and suggests that a combination of determining free radical concentration and simulating a kinetic model would help elucidate the contributions of active chlorine, Cl, and ClO to ammonia oxidation. Furthermore, this review comprehensively summarizes the characteristics of ammonia oxidation, including kinetic properties, influencing factors, products, and electrodes. The amalgamation of Cl-EAO technology with photocatalytic and concentration technologies has the potential to enhance ammonia oxidation efficiency. Future research should concentrate on clarifying the contributions of active chlorine, Cl, and ClO to ammonia oxidation, the production of chloramines and other byproducts, and the development of more efficient anodes for the Cl-EAO process. The main objective of this review is to enhance the understanding of the Cl-EAO process. The findings presented herein contribute to the advancement of Cl-EAO technology and provide a foundation for future studies in this field.

Nitrogen-containing wastewater fuel cells for total nitrogen removal and energy recovery based on Cl•/ClO• oxidation of ammonia nitrogen

The excess nitrogen discharge into water bodies has resulted in extensive water pollution and human health risks, which has become a critical global issue. Moreover, nitrogenous wastewater contains considerable chemical energy contributed by organic pollutants and nitrogenous compounds. Therefore, the treatment of various kinds of nitrogen-containing wastewater for nitrogen removal and energy recovery is of significance. Biological methode and advanced oxidation processes (AOPs) are the main methods for nitrogen removal. However, biological treatment is easily inhibited by high-salinity, high ammonia nitrogen (NH₃-N/NH₄⁺-N), nitrite and toxic organics in wastewater, which limits its application. AOPs mainly induce in situ generation of highly reactive species, such as hydroxyl radical (HO•), sulfate radical (SO₄•^-) and chlorine radicals (Cl•, ClO•, Cl₂•^-), for nitrogen removal. Nevertheless, HO• shows low reactivity and N₂ selectivity towards NH₃-N/NH₄⁺-N oxidation, and SO₄•^- also demonstrates unsatisfactory NH₃-N/NH₄⁺-N removal. It has been shown that Cl•/ClO• can efficiently remove NH₃-N/NH₄⁺-N with high N₂ selectivity. The generation of Cl•/ClO• can be triggered by various techniques, among which the PEC technique shows great potential due to its higher efficiency for Cl•/ClO• generation and eco-friendly approach for pollutants degradation and energy recovery by utilizing solar energy. Cl•/ClO• oxidation of NH₃-N/NH₄⁺-N and nitrate nitrogen (NO₃^--N) reduction can be strengthened through the design of photoanode and cathode materials, respectively. Coupling with this two pathways, an exhaustive total nitrogen (TN) removal system is designed for complete TN removal. When introducing the mechanism into photocatalytic fuel cells (PFCs), the concept of nitrogen-containing wastewater fuel cells (NFCs) is proposed to treat several typical types of nitrogen-containing wastewater, achieving high-efficiency TN removal, organics degradation, toxic chlorate control, and energy recovery simultaneously. Recent research progress in this field is reviewed, summarized and discussed, and in-depth perspectives are proposed, providing new ideas for the resource treatment of nitrogen-containing wastewater.

Solar-Driven Catalytic Urea Oxidation for Environmental Remediation and Energy Recovery

The water-energy nexus is highly related to sustainable societal development. As one of the most abundant biowastes discharged into the environment, mild abatements and green conversions of urea wastewater have been widely investigated. Due to abundant sources, global distribution, and easy control, light-based catalytic strategies have become alternative on-site treatment approaches. After comprehensively surveying the recent progress, recent achievements of urea oxidation under light irradiation are reviewed herein. Several typical light-promoted systems employed in urea conversion, including photocatalysis, photo-electrocatalysis, photo-biocatalysis, and photocatalytic fuel cells, are meticulously introduced and discussed, from catalyst designs and medium conditions to established mechanisms. To realize the goal of sustainability, the chemical energy in urea-rich water could be utilized for the value-added production of hydrogen fuel and electricity. Finally, based on current developments, existing challenges are enumerated and developmental prospects in the future of light-driven urea conversion technologies are proposed.

Reactive species conversion into 1O2 promotes substantial inhibition of chlorinated byproduct formation during electrooxidation of phenols in Cl--laden wastewater

The generation of chlorinated byproducts during the electrochemical oxidation (EO) of Cl^--laden wastewater is a significant concern. We aim to propose a concept of converting reactive species (e.g., reactive chlorines and HO^• resulting from electrolysis) into ¹O₂ via the addition of H₂O₂, which substantially alleviates chlorinated organic formation. When phenol was used as a model organic compound, the results showed that the H₂O₂-involving EO system outperformed the H₂O₂-absent system in terms of higher rate constants (5.95 × 10^-2 min^-1vs. 2.97 × 10^-2 min^-1) and a much lower accumulation of total organic chlorinated products (1.42 mg L^-1vs. 8.18 mg L^-1) during a 60 min operation. The rate constants of disappearance of a variety of phenolic compounds were positively correlated with the Hammett constants (σ), suggesting that the reactive species preferred oxidizing phenols with electron-rich groups. After the identification of ¹O₂ that was abundant in the bulk solution with the use of electron paramagnetic resonance and computational kinetic simulation, the routes of ¹O₂ generation were revealed. Despite the consensus as to the contribution of reaction between H₂O₂ and ClO^- to ¹O₂ formation, we conclude that the predominant pathway is through H₂O₂ reaction with electrogenerated HO^• or chlorine radicals (Cl^• and Cl₂^•-) to produce O₂^•-, followed by self-combination. Density functional theory calculations theoretically showed the difficulty in forming chlorinated byproducts for the ¹O₂-initiated phenol oxidation in the presence of Cl^-, which, by contrast, easily occurred for the Cl^•-or HO^•-initiated phenol reaction. The experiments run with real coking wastewater containing high-concentration phenols further demonstrated the superiority of the H₂O₂-involving EO system. The findings imply that this unique method for treating Cl^--laden organic wastewater is expected to be widely adopted for generalizing EO technology for environmental applications.

A novel electrocatalytic system with high reactive chlorine species utilization capacity to degrade tetracycline in marine aquaculture wastewater

The problems of high salinity and coexistence of antibiotics in mariculture wastewater pose a great challenge to the traditional wastewater treatment technology. Herein, an electrocatalytic system based on cathodes to sustain reactive chlorine species (RCS) in a high chlorine environment was proposed. The results show that the content of RCS is affected by cathodes. The electrocatalytic system with FeNi/NF as cathode has the largest RCS retention capacity when compared with other cathode systems (carbon felt, nickel foam, copper foam, stainless steel, and nickel-iron foam). This is related to FeNi/NF's higher hydrogen production activity, which inhibits the reduction reaction of RCS. Furthermore, the degradation of tetracycline by the proposed FeNi/NF system maintained long-term effective performance across 20 cycles. Thus, the application of high chlorine resistance electrocatalysis system provides a possibility for practical electrocatalysis treatment of mariculture wastewater.

Powdered activated carbon-catalyzed chlorine oxidation of bisphenol-A and methylene blue: Identification of the free radical and effect of the carbon surface functional group

The effect of powdered activated carbon (PAC) on chlorine oxidation is not well understood, therefore this study was designed to further investigate the chlorine oxidation mechanism with the presence of PAC. The oxidation processes of two model organic pollutants (bisphenol-A and methylene blue) with chlorine were compared in the absence and presence of PAC. The results showed a significant increase in reaction rates with the addition of PAC. Electron spin resonance indicated that the PAC catalyzed the oxidation of chlorine to generate more Cl and O₂^-. Additionally, the analysis of the surface characteristics of thermally modified PACs under N₂ and their corresponding reaction rates revealed that there existed a significant correlation between the CO groups and the catalytic effect. PAC exhibited a much lower reaction rate under H₂ modification, which indicated that the π electrons of the basal plane might be involved in the catalysis. Density functional theory calculations confirmed that the various oxygen groups on PAC reduced the activation barrier for HOCl dissociation, particularly the carboxyl group. This investigation provides a better understanding of the interactions between chlorine and activated carbon materials, which could be useful for selecting suitable water treatment agents.

Electrocatalytic Methane Oxidation Greatly Promoted by Chlorine Intermediates

Renewable energy-powered methane (CH₄ ) conversion at ambient conditions is an attractive but highly challenging field. Due to the highly inert character of CH₄ , the selective cleavage of its first C-H bond without over-oxidation is essential for transforming CH₄ into value-added products. In this work, we developed an efficient and selective CH₄ conversion approach at room temperature using intermediate chlorine species (*Cl), which were electrochemically generated and stabilized on mixed cobalt-nickel spinels with different Co/Ni ratios. The lower overpotentials for *Cl formation enabled an effective activation and conversion of CH₄ to CH₃ Cl without over-oxidation to CO₂ , and Ni³⁺ at the octahedral sites in the mixed cobalt-nickel spinels allowed to stabilize surface-bound *Cl species. The CoNi₂ O_x electrocatalyst exhibited an outstanding yield of CH₃ Cl (364 mmol g^-1  h^-1 ) and a high CH₃ Cl/CO₂ selectivity of over 400 at room temperature, with demonstrated capability of direct CH₄ conversion under seawater working conditions.

Selective oxidation of pharmaceuticals and suppression of perchlorate formation during electrolysis of fresh human urine

Urine comprises only a small (~1%) volumetric fraction of municipal wastewater, but represents a dominant source of pharmaceuticals, many of which may pass through conventional wastewater treatment and pose risks to aquatic ecosystems. Point-source treatment of source-separated urine presents a unique opportunity to degrade pharmaceuticals before dilution with wastewater, and electrochemical advanced oxidation processes are one increasingly investigated option. However, they often lead to the formation of oxidation byproducts including chlorate, perchlorate at very high concentrations. Here, we show that the high urea content of fresh human urine suppresses the formation of oxychlorides by inhibiting formation of HOCl/OCl^‒ during electrolysis, while still enabling pharmaceutical degradation due to the slow rate of urea oxidation by ^•OH. This results in improved performance compared to equivalent treatment of hydrolyzed aged urine. This electrochemical oxidation scheme is shown to degrade the model contaminants cyclophosphamide and sulfamethoxazole with surface-area-to-volume-normalized pseudo-first-order rate constants greater than 0.08 cm/min in authentic fresh human urine. It results in ~100 × decrease in pharmaceutical concentrations in 2 h while generating ~1000 × lower oxychloride byproduct concentrations in synthetic fresh urine than synthetic hydrolyzed aged urine matrixes. Importantly, this proof-of-principle shows that simple and safe electrochemical methods can be used for point-source-remediation of pharmaceuticals in fresh human urine (before storage and hydrolysis), without formation of significant oxychloride byproducts.

Efficient urine removal, simultaneous elimination of emerging contaminants, and control of toxic chlorate in a photoelectrocatalytic-chlorine system

Urine, which is an important waste biomass resource, is the main source of nitrogen in sewage and contains large quantities of emerging contaminants (ECs). In this study, we propose a new method to efficiently remove urine, simultaneously eliminate ECs, and control the generation of toxic chlorate during urine treatment using a photoelectrocatalytic-chlorine (PEC-Cl) system. A type-II heterojunction of WO₃/BiVO₄ was used as a photoanode to generate chlorine radicals (Cl•) by decreasing the oxidation potential of WO₃ valence band for the highly selective conversion of urine to N₂ and the simultaneous degradation of ECs in an efficient manner. The method presented surprising results. It was observed that the amount of toxic chlorate was significantly inhibited by circumventing the over-oxidation of Cl^- by holes or hydroxyl radicals (•OH). Moreover, the removal of urea nitrogen reached 97% within 90 min, while the degradation rate of trimethoprim in urine was above 98.6% within 60 min, which was eight times more than that in the PEC system (12.1%). Compared to the bare WO₃ photoanode, the toxic chlorate and nitrate generated by the WO₃/BiVO₄ heterojunction photoanode decreased by 61% and 44%, respectively. Thus, this study provides a safe, efficient, and environmentally-friendly approach for the disposal of urine.

Semiconductor Electrode Materials Applied in Photoelectrocatalytic Wastewater Treatment—an Overview

Industrial sources of environmental pollution generate huge amounts of industrial wastewater containing various recalcitrant organic and inorganic pollutants that are hazardous to the environment. On the other hand, industrial wastewater can be regarded as a prospective source of fresh water, energy, and valuable raw materials. Conventional sewage treatment systems are often not efficient enough for the complete degradation of pollutants and they are characterized by high energy consumption. Moreover, the chemical energy that is stored in the wastewater is wasted. A solution to these problems is an application of photoelectrocatalytic treatment methods, especially when they are coupled with energy generation. The paper presents a general overview of the semiconductor materials applied as photoelectrodes in the treatment of various pollutants. The fundamentals of photoelectrocatalytic reactions and the mechanism of pollutants treatment as well as parameters affecting the treatment process are presented. Examples of different semiconductor photoelectrodes that are applied in treatment processes are described in order to present the strengths and weaknesses of the photoelectrocatalytic treatment of industrial wastewater. This overview is an addition to the existing knowledge with a particular focus on the main experimental conditions employed in the photoelectrocatalytic degradation of various pollutants with the application of semiconductor photoelectrodes.

Exhaustive denitrification via chlorine oxide radical reactions for urea based on a novel photoelectrochemical cell

Urea is a major source of nitrogen pollution in domestic sewage and its denitrification is difficult since it is very likely to be converted into ammonia or nitrate instead of expected N₂. Herein, we propose an exhaustive denitrification method for urea via the oxidation of amine/ammonia-N with chlorine oxide radical, which induced from a bi-functional RuO₂//WO₃ anode, and the highly selective reduction of nitrate-N on cathode in photoelectrochemical cell (PEC). Under illumination, the WO₃ photoanode side promotes the quantities hydroxyl and reactive chlorine radical, and these radicals are immediately combined to stronger chlorine oxide radical by RuO₂ side, which obviously enhances the efficiency and speed of the urea oxidation. Synchronously, the over-oxidized nitrate can be selectively reduced by Pd and Au nanoparticles on the surface of cathode. Eventually, exhaustive denitrification is realized by the circulative reaction. Experimental observations and theoretical calculation revealed that chlorine oxide radical promoted significant denitrification of urea with an efficiency of 99.74% in 60 min under the optimum condition. The removal rate constant of the RuO₂//WO₃ anode was 3.08 times than that of single WO₃ anode and 2.64 times than that of single RuO₂ anode, confirming the chlorine oxide radical had stronger ability on denitrification than reactive chlorine radical. Also, the bi-functional anode contributed to best current efficiencies, utilizing the energy availably. This work proposes a promising method of exhaustive denitrification for urea.