Preparation of Co/Ti electrode by electro-deposition for aqueous nitrate reduction

Excess nitrate in water has caused a serious problem due to its threats to both human health and ecological safety. Nitrate can produce nitrite and nitrosamines that pose direct harm to proteins or cells, causing diseases such as gastrointestinal cancers (Powlson et al. 2008). Moreover, it is considered as one of the important factors leading to lake eutrophication and red tide (McIsaac et al. 2001). Therefore, it is vital to remove nitrate from both drinking water and wastewater. China has limited the maximum level of 15 mg L^–1 for total nitrogen for wastewater discharge which was issued in 2002 (NEPA 2002; Ye et al. 2019). The World Health Organization and many countries including the United States have set the limited maximum contaminant level of 10 mg L^–1 for nitrate-nitrogen or 50 mg L^–1 for nitrate in drinking water (NHCPRC 2006; USEPA 2010; WHO 2010).

Biological methods such as denitrification have been widely employed in wastewater plants, but the removal performance is negatively influenced by the limited carbon source and temperature (Molognoni et al. 2017; Zheng et al. 2021). Many physicochemical methods such as ion exchange, electrodialysis and chemical reduction have been developed. However, the concentrate waste produced from ion exchange, reverse osmosis, electrodialysis or the chemical release from chemical reduction led to secondary pollution and cost increases (Song et al. 2020; Hurtado-Martinez et al. 2021). Electrolysis for nitrate removal is recently causing attention due to its high removal efficiency, no chemical addition, simple operation and, in theory, no secondary pollution (Chen 2004). In an electrolysis cell, aqueous nitrate is reduced to nitrite (NO₂⁻) and subsequently to nitrous oxides (NO, N₂O), nitrogen gas (N₂) and/or ammonia (NH₃) on the cathode. The produced ammonia could be directly oxidized by anode or indirectly oxidized by free chlorine to N₂, which was generated by the anodic oxidation of the co-existing chloride. Up to now, many electrodes were developed for nitrate reduction by considering N₂ selectivity, as reviewed by Garcia-Segura et al. (2018) and Zhang et al. (2021). The composite cathode of metal/metal oxides coated plate substrate(s), such as Co₃O₄/Ti, Fe/Cu, Fe/C, Co₃O₄-TiO₂/Ti, Pd-Cu/NiAl-LMO, are generally better than a single-metal cathode for nitrate reduction (Su et al. 2017; Zhang et al. 2018; Gao et al. 2019; Bai et al. 2020). More specifically, Zhang et al. (2018) reported that the Fe/Cu electro-reduction system could remove 98.6% of NO₃⁻-N within 90 min. In contrast, the removal of NO₃⁻-N by a Cu or Fe electro-reduction system only reached up to 24.3 and 76.0%, respectively. Similarly, Gao et al. (2019) compared a TiO₂/Ti cathode with a Co₃O₄-TiO₂/Ti cathode for NO₃⁻-N removal efficiency and found that the Co₃O₄-TiO₂/Ti cathode had a higher NO₃⁻-N removal rate (80%) than that of the TiO₂/Ti cathode (45%).

Most of the previous studies did not focus on the life span of the electrode, which is an important factor for industrial application. In fact, the particles attached on the electrode surface are theoretically susceptible to fall off in the electrolyte solution during long-term work, which will reduce the performance for electrode reduction. It is reported that the electrode prepared by thermal decomposition showed a typical cracked mud-like structure, but its damage or life span has not been evaluated (Tian et al. 2007; Cheng et al. 2013). Electrodeposition possesses some advantages over the hydrothermal method, including low processing temperature, mildness of reaction, environmental friendliness, as well as low cost. Meanwhile, by simply changing the electrolyte composition and process adjustments, electrodeposition could obtain an optimized surface nanostructure and morphology, and better stability and reproducibility (Cai et al. 2019; Li et al. 2021). Thus, electrodeposition has received more attention. Moreover, natural water or wastewater contains complex inorganic/organic ingredients such as typical Ca, Mg, C, P, and organic matters, which could limit the nitrate reduction. The ions of Ca²⁺, Mg²⁺, HCO₃⁻, H₂PO₄⁻, HPO₄^2– and PO₄^3– could precipitate near/at the electrode surface during the electrochemical removal of Ca²⁺, Mg²⁺ or P removal and recovery (Zeppenfeld 2011; Lei et al. 2017). However, there are few studies about the effect of these factors on nitrate electro-reduction.

The aim of this work was to prepare a cathode by the electroplating method for improving the stability to remove aqueous nitrate. A Ti plate was used as substrate due to its excellent performance for nitrate reduction with Co₃O₄ attachment (Su et al. 2017; Yang et al. 2020). The prepared Co/Ti electrode was evaluated on its damage performance and nitrate removal behavior comparing with the Co₃O₄/Ti fabricated by thermal decomposition, and the possible nitrate reduction mechanism of the Co/Ti electrode was proposed. The effect of pH on NO₃⁻-N and total nitrogen (TN) removal was studied. Meanwhile, we explored the effect of Cl⁻ on TN and further clarified the mechanism. In addition, the effect of the coexistence of Ca and P on nitrate reduction were also investigated.

The Ru-IrO₂/Ti anode was purchased from Baoji Hengrui Titanium Anode Products Co., Ltd (Shanxi, China). The substrate for the cathode was titanium plate, which was purchased from Dongguan Huitai Alloy Material Co., Ltd (Guangdong, China). Cobalt chloride hexahydrate, sulfamic acid (≥99.5%), citric acid monohydrate (≥99.5%), and sodium dodecyl sulfate (SDS) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd (Shanghai, China). Saccharin sodium and basic potassium persulfate (≥99%) were obtained from Shanghai Yuanye Bio-technology (Shanghai, China) and Sigma-Aldrich (Shanghai, China), respectively. 1, 4-butynediol (≥98%), sodium sulfate, sodium chloride, phosphoric acid were purchased from Nanjing Chemical Reagent (Jiangsu, China). Hydrochloric acid, sulfuric acid, potassium nitrate, sodium hydroxide, acetone, ethanol, boric acid, and potassium sodium tartrate tetrahydrate were all obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). All reagents without purity noting were analytical grade without further purification during the experiment.

Titanium substrate was polished with sandpaper and then etched in boiled sulfuric acid (20%) for 2 h. After washing by acetone, ethanol and deionized water in sequence, the substrate was dried in a 45 °C oven for 5 h. The substrate was used as cathode for electroplating with an Ru-IrO₂/Ti anode in an electrolysis cell without chamber separating, which was filled with 100 mL of electrolyte solution containing cobalt chloride hexahydrate (2.75 g/L), boric acid (20.0 g/L), citric acid monohydrate (10.0 g/L), and one of sodium dodecyl sulfate (SDS, 0.15 g/L), saccharin sodium (0.20 g/L) or 1, 4-butynediol (0.50 g/L). The distance between anode and cathode was 2.0 cm, and the current density was 30 mA cm^–2 provided by an auto range DC power supply (IT6922A, ITECH Electronic (USA) Co., Ltd) for 2 h of electroplating. The obtained cathode was washed with 300 mL of deionized water eight times and then naturally dried for characterization or application. The nitrate removal and test of the prepared cathode were compared with the Co₃O₄/Ti cathode, which was prepared by thermal decomposition as reported (Su et al. 2017).

A scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) patterns and electrochemical workstation were all employed for one of the prepared cathodes. For linear sweep voltammetry (LSV) tests, the prepared electrode was used as the working electrode. The Ru-IrO₂/Ti electrode was used in electrolysis and a saturated calomel electrode (SCE) was used as a counter and reference electrode, respectively. The scan rate was 20 mV s^–1, and the scan range was –1.2–0 V. The instruments were the same and the operation parameter was similar to that in our previous work with the scan rate of 20 mV s^–1 between –1.2 and 0 V (Yang et al. 2020). For the mechanical stability test of different cathodes prepared under different conditions, ultrasonic interference was carried out in a CNC ultrasonic cleaner (KH-300DE, Kunshan-Hechuang Co. Ltd (JiangSu, China)) at 100 KHz. After 15 min the electrodes of ultrasonic interference were all dried at 45 °C for 5 h, and then the weight loss was calculated.

Nitrate reduction was conducted in an electrolysis cell without chamber separating at 25 °C, and the distance between anode (50 × 22 × 1 mm) and cathode (50 × 22 × 1 mm, l × w × t) was 2.0 cm. One hundred mL of solution with NO₃⁻-N (100 ppm) and Na₂SO₄ (0.05 M) were added into the electrolysis cell, and the electrodes were all submerged into the solution. The current density was controlled at 20 mA cm^–2 for the electrolysis process with magnetic stirring at 600 rpm. Different cathodes prepared from different conditions were all used in nitrate removal. After that, the effect of inorganic matters was investigated by using a 45 × 20 × 1 mm cathode. For the effect of Cl⁻, NaCl at different amounts was added into the electrolysis solution with different concentrations of NO₃⁻-N and Na₂SO₄ (0.05 M).

For the effect of Ca, different amounts of CaSO₄·2H₂O were added in the electrolysis solution with 100 mg/L NO₃^–-N, 0.05 M Na₂SO₄ and 100 mg/L NaHCO₃ as the inorganic carbon. For the effect of co-existence of Ca, P and Na₂HPO₄·12H₂O were added in the electrolysis solution containing Ca²⁺ (50 mg/L), NO₃⁻-N (100 mg/L) and Na₂SO₄ (0.05 M). During the electrolysis, the solution was sampled for the measurement of nitrate, ammonia, nitrite, and total nitrogen (TN). For the stability evaluation, the optimized Co/Ti and Co₃O₄/Ti electrodes were used for ten batch tests. The 100 mL aqueous nitrate solution at the initial concentration of 100 mg/L was added in the electrolysis cell for a subsequent batch after 3 h of electrolysis. In addition, the nitrate solution containing Ca²⁺ (50 mg/L) and P (0.5 mg/L) was also used for these ten batch tests.

The analysis of nitrate, nitrite, ammonia and total nitrogen are described in the previous work (Yang et al. 2020). Briefly, the ion chromatography method was applied for nitrate and nitrite measurement. Nessler's reagent spectrophotometry method and alkaline potassium persulfate digestion UV spectrophotometric method were used respectively for the determination of the concentration of ammonia and TN. In this work, the concentration of Co²⁺ and Ca²⁺ was measured by using an inductively coupled plasma-atomic emission spectrometer (iCAP-7400, Thermo Fisher, USA).

In our electro-deposition process, the typical additives for surface improvement, such as SDS, saccharin sodium and 1,4-butynediol, respectively, at different concentrations, were employed to improve mechanical stability. To test the mechanical stability of the electrode, ultrasonic interference was applied on the electrode emerging in distilled water at 100 kHz. The result indicated that the mass loss of the electrode significantly increased as the concentration of saccharin sodium or 1,4-butynediol increased, while the SDS concentration increase improved the mass loss of the electrode with the mass loss from 0.21 mg decreasing to 0.055 mg. The effect of the three additives on Co dissolution was not obviously observed. Moreover, the addition of SDS or saccharin sodium had little effect on nitrate removal, while the nitrate removal decreased by nearly 10% as the 1,4-butynediol concentration increased to 0.5 g L^–1 in the electrode preparation process.

The Co/Ti electrode prepared by electro-deposition and the Co₃O₄/Ti electrode prepared by the typical thermal decomposition method were also used for ultrasonic interference treatment in order to compare their stability. As shown in Figure 4, the water immersed by the Co₃O₄/Ti electrode turned black, but the water immersed by the Co/Ti electrode showed little change in color, indicating that some materials on the electrode surface fell off. Specifically, the surface structure of the Co₃O₄/Ti electrode is easily affected by human operation and high temperature extreme environments, resulting in uneven surface distribution and different thickness. Thus, the Co₃O₄/Ti electrode could easily fall off. In contrast, the electrodeposition method used the electroplating solution to prepare the Co/Ti electrode, and the material on the Co/Ti electrode surface was tightly attached, making it more difficult to fall off. In addition, the mass loss and Co dissolution were further measured to clarify the phenomenon. Compared with the 0.294 mg of mass loss and 0.094 mg of dissolution for the Co₃O₄/Ti electrode, the Co/Ti electrode exhibited an excellent mechanical stability with 0.055 mg of mass loss and 0.024 mg of dissolution (Figure 4(a)). Meanwhile, the SEM images of the Co/Ti electrode before and after ultrasonic interference treatment were also measured (Supplementary Material, Figure S2). After the ultrasonic interference treatment, no significant effect on the electrode surface morphology was observed, and only a small part of the material attached to the Co/Ti electrode surface fell off, indicating excellent mechanical stability. Notably, although the mass loss of the electrode causes the secondary pollution of the treated water, both the electrodes after the mass loss showed no decrease in nitrate removal (Supplementary Material, Figure S3). This result is consistent with the observation from Figure 1(b), which suggests that the maximal removal obtained at a low Co electro-deposition amount (1.81 mg cm^–2). The Co electro-deposition amount we used was 4.41 mg cm^–2, which means that the loss of 0.055 and 0.024 mg had no effect on its removal performance.

The LSV test provides electroactivity of the electrodes for oxidation and reduction. In our previous work, Ti substrate and a Co₃O₄/Ti electrode were compared in the absence or presence of 100 mg/L nitrate with Na₂SO₄ as supporting electrolyte. Relevant literature demonstrated the current increase over the potential range of –0.7 to –1.0 V for a Ti based electrode were mainly attributed to NO₃⁻ reduction to NO₂⁻, and the current increase over the potential range of –0.9 to –1.2 V was mainly caused by the NO₃⁻ converted to NH₃ or the NO₂⁻ reduction (Wang et al. 2017; Yang et al. 2020). The process of nitrate reduction to nitrite was generally the rate-limiting step for nitrate removal (Montesinos et al. 2015; Yang et al. 2020). Figure 5 provides the LSV results of Ti substrate, and Co/Ti and Co₃O₄/Ti electrodes towards nitrate reduction. It can be seen that the current density was much larger for the Co/Ti and Co₃O₄/Ti electrode than that for Ti substrate over the potential increase from –0.7 to –1.2 V, indicating better electroactivity for both Co/Ti and Co₃O₄/Ti electrodes than Ti substrate. The difference could be ascribed to surface morphology improvement, thereby modifying the electronic structure and enhancing electron transfer (Wang et al. 2017). Obviously, compared with LSV of the Co₃O₄/Ti electrode, the smaller values of current density of the Co/Ti electrode over the range of –0.7 to –1.0 V, and the larger values of current density of the Co/Ti electrode over the range of –1.0 to 1.18 V were observed. The result suggests that the Co/Ti electrode had better electroactivity towards NO₃⁻ conversion to NH₃ and NO₂⁻ reduction and a weaker electroactivity towards NO₃⁻ reduction to NO₂⁻. This result on the improved electroactivity on nitrite reduction could decrease the nitrite generation, as confirmed in the results in Figure 6(c). It is also noted that the potential of hydrogen evolution overlapped the potential range (–1.0 to –1.2 V), which implied a possibility that there is a higher electroactivity of hydrogen evolution for the Co/Ti electrode than the Co₃O₄/Ti electrode.

To evaluate the nitrate reduction performance of the prepared electrode the Co₃O₄/Ti electrode, which has excellent treatment ability, was used for comparison. The concentration of NO₃⁻-N, NH₄⁺-N, NO₂⁻-N and TN during the 3 h electrolysis is shown in Figure 6. The Co/Ti electrode exhibited a comparable kinetic performance on NO₃⁻-N reduction with 97% of nitrate removal, 62% of NH₄⁺-N generation and 37% of TN removal. NO₂⁻-N generation was not observed for the Co/Ti electrodes, while 4% of nitrogen was reduced to NO₂⁻-N for the Co₃O₄/Ti electrode, which is consistent with the LSV result of the better electroactivity on nitrite reduction. A small decrease of NH₄⁺-N generation after 2 h could be explained by anode oxidation and NH₃ volatilization, as was discussed in previous work (Yang et al. 2020). Thirty-seven per cent of TN removal means that 36% of NO₃⁻-N may be eventually removed from solution as N₂ or NH₃ gas. Considering the maximum generation of NH₄⁺-N (66.5%), it can be seen that the main products of Co/Ti cathode reduction were NH₄⁺-N (66.5% NO₃^–-N) and nitrogen-containing gas (N₂ or NH₃, 33.5% NO₃^–-N), of which the NO₃⁻-N transfer portion was coincidentally consistent with the reaction as described in previous work (Yang et al. 2020). It means that the Co/Ti cathode had no selectivity towards N₂ or NH₄⁺-N. In addition, these results on NO₃⁻-N, NH₄⁺-N and TN change for the Co/Ti cathode are all comparable to that of the Co₃O₄/Ti electrode. However, there was no NO₂⁻ detected in the system of the Co/Ti cathode, which could be explained by the better electroactivity towards nitrite reduction of the Co/Ti cathode than the Co₃O₄/Ti electrode.

In the electrochemical treatment process, pH also plays an important role in the removal of contaminants (Zou et al. 2019; Zhu et al. 2020). The effect of pH on NO₃⁻-N and TN removal was explored in our experiment, and the result indicated that a slight decrease as the pH increased from 3 to 10 (Figure 7). The result was different from that reported by Yao et al. (2021), who found that the NO₃⁻-N removal was the lowest at pH 7, which was nearly 50% lower than that of pH 3 by using the Ti electrode as cathode. However, Tang et al. (2019) found that pH 9 was best for nitrate reduction as the function of Fe⁰, Cu⁰ and the chelating resin by the Fe-Cu/D407 cathode. Therefore, the different effect of pH value may be attributed to the different electrode materials used.

At the same time, the presence of Cl⁻ will inhibit the reduction of nitrate, thereby decreasing the efficiency of reducing NO₃⁻ to nitrogen. In the process of ammonia oxidation under the effect of ClO⁻, ammonia nitrogen will also produce NO₃⁻-N and NO₂⁻-N, resulting in a decrease in the denitrification efficiency. Therefore, the addition of Cl⁻ has no synergistic effect on the removal of nitrate. Noteably, compared with Figure 6(a), (100%), the addition of Cl⁻ reduced the removal of nitrate (83%) by 18.2%.

The effect of co-existing Ca on nitrate removal was investigated in this work. Figure 9 shows the effect of Ca²⁺ in the presence of NaHCO₃ as inorganic carbon of 100 mg/L. It was found that the increased weight of electrode was 9.8 mg at the initial Ca²⁺ concentration of 150 mg/L, which is equal to the formation of CaCO₃ precipitation as calculated by the decreased Ca²⁺ concentration in solution (Figure 9(a)). The white crystal-like matter on the cathode surface center could be observed, and the result of XRD pattern confirmed the white crystal-like matter is CaCO₃ (Supplementary Material, Figure S4). This phenomenon indicated that the HCO₃⁻ converted to CO₃^2– as the result of pH increase and generated CaCO₃ on the surface of the cathode. All the precipitation formed on the cathode surface rather than solution suggested that this precipitation process in solution was limited by the OH⁻ diffusion. However, this precipitation effect on the performance of nitrate reduction was not observed. Figure 10 shows the effect of P on nitrate removal in the presence of Ca²⁺ at 50 mg/L. As the P concentration increased from 0 to 50 mg/L, the nitrate removal greatly decreased from 83.1 to 45.5%. It was found that the weight of electrode increased by 4.8 mg when the initial P concentration was increased to 20 mg/L and the concentration of Ca²⁺ and P was decreased. The ratio of the decreased amount of Ca²⁺ and P was nearly 2:1, suggesting the precipitation was Ca₃(PO₄)₂. This result indicated that the coexistence of Ca and P have a greater adverse effect on nitrate removal.

The stability or electrode working life is an important index for the industrial application. We replaced nitrate solution with fresh solution after 3 h of electrolysis nine times, and the removal performance is shown in Figure 11. It was found that the Co/Ti electrode exhibited excellent stability on nitrate removal of 90.0 ± 2.2% without a significant decrease after ten reuse cycles. In the presence of Ca (50 mg/L) and P (0.5 mg/L), the nitrate removal of the Co/Ti electrode significantly decreased from 85.4 ± 1.5 to 57.7 ± 3.5%, indicating the presence of Ca (50 mg/L) and P (0.5 mg/L) has a significant adverse effect on Co/Ti electrode. A possible explanation could be that the generation of hydroxide ions around the surface of the cathode promotes the formation of the Ca, P precipitation on the electrode surface.

A Co/Ti electrode was prepared by electrodeposition and characterized by XRD and SEM. The removal of nitrate and the effect of coexisting Cl^–, Ca²⁺ and HPO₄^2– ions on nitrate removal by using the Co/Ti electrode as cathode were all investigated. Comparing to a Co₃O₄/Ti electrode prepared by the typical thermal decomposition method, the Co/Ti electrode exhibited an excellent mechanical stability with 18.7% of mass loss and 25.5% of Co dissolution of Co₃O₄/Ti electrode after ultrasonic interference. Aqueous nitrate was reduced to ammonium nitrogen (66.5%) and nitrogen gas (33.5%) with nearly 100% of nitrate removal. Nitrite was not detected during the electrolysis process because of its high electroactivity towards nitrite reduction. The presence of chlorine ion (1,000 mg/L) could convert all the nitrate (100 mg/L) to nitrogen gas after 3 h of electrolysis. The presence of calcium ion (150 mg/L) has little effect on nitrate removal regardless of CaCO₃ precipitation on the surface of the electrode in our experiment short-term test, while the coexistence of calcium ion and hydrogen phosphate ion have a greater adverse effect on nitrate removal, with a 49% reduction. Although the Co/Ti electrode exhibited excellent stability on nitrate removal without a significant decrease after ten reuse cycles, the presence of Ca (50 mg/L) and P (0.5 mg/L) decreased the removal from 85.4 ± 1.5 to 57.7 ± 3.5%. This result suggests that Ca and P should be pre-removed before the electrolysis process.

We gratefully acknowledge the generous support provided by the High-level Talent Team Project of Quanzhou City (2018CT006) and the Xuzhou Science and Technology Project (KC19049).

All relevant data are included in the paper or its Supplementary Information.