Polarity reversal electrochemical process for water softening

Ni-P-PTFE cathode with low surface energy for enhancing electrochemical water softening performance

Efficient cathode regeneration is a significant challenge in the electrochemical water softening process. This work explores the use of an electroless plating Ni-P-PTFE electrode with low surface energy for this purpose. The Ni-P-PTFE electrode demonstrates improved self-cleaning performance at high current densities. By combining the low surface energy of the electrode with fluid flushing shear force, the precipitation rate on the Ni-P-PTFE electrode remains stable at approximately 18 g/m2·h over extended periods of operation. Additionally, the cleaning efficiency of the Ni-P-PTFE electrode surpasses that of stainless steel by 66.34%. The Ni-P-PTFE electrode can maintain a larger active area and a longer operational lifespan is attributed to its self-cleaning performance derived from low surface energy. Furthermore, the loose scale layers on the electrode surface are easily removed during electrochemical water softening processes, presenting a novel approach to cathode surface design.

A novel membrane-free electrochemical separation-filtering crystallization coupling process for treating circulating cooling water

The traditional electrochemical descaling process exhibits drawbacks, including low OH- utilization efficiency, constrained cathode deposition area, and protracted homogeneous precipitation time. Consequently, this study introduces a novel membrane-free electrochemical separation-filtering crystallization (MFES-FC) coupling process to treat circulating cooling water (CCW). In the membrane-free electrochemical separation (MFES) system, OH- is rapidly extracted by pump suction from the porous cathode boundary layer solution, preventing neutralization with H+, thereby enhancing the removal of Ca2+ and Mg2+. Experimental results indicate that the pH of the pump suction water can swiftly increase from 8.13 to 11.42 within 10 min. Owing to the high supersaturation of the pump suction water, this study couples the MFES with a filtration crystallization (FC) system that employs activated carbon as the medium. This approach captures scale particles to enhance water quality and expedites the homogeneous precipitation of hardness ions, shortening the treatment time while further augmenting the removal rate. After the MFES-FC treatment, the single-pass removal rates for total hardness, Ca2+ hardness, Mg2+ hardness, and alkalinity in the effluent reached 92 %, 97 %, 64 %, and 67 %, respectively, with turbidity of 3 NTU, current efficiency of 86.6 %, and energy consumption of 7.19 kWh·kg-1 CaCO3. This coupling process facilitates an effective removal of hardness and alkalinity at a comparatively low cost, offering a new reference and inspiration for advancements in electrochemical descaling technology.

Influence of Water Hardness and Complexing Agents on Electrochemical Hydrogen Peroxide Generation

Recently, many studies have been published regarding electrochemical oxygen reduction reaction for generating hydrogen peroxide (H2O2) using gas diffusion electrodes (GDEs) for various applications. Sodium salts solved in deionized water were usually used as supporting electrolytes. In technical applications, however, tap water-based electrolytes with hardeners are particularly relevant and have only been considered in a few studies to date. In this work, we investigated the influence of hardeners on H2O2-generation at 150 mA cm-2 and were able to show that scaling occurs predominantly on the GDE-surface and not in its pore structure. With the novel method in electrochemical synthesis by using complexing agents to bind hardeners, we were able to significantly reduce the scaling. Even after 10 h of operation, the reactor still achieves a faradaic efficiency (FE) of above 70 % (>67 mg h-1 cm-2), comparable to the experiments without hardeners and complexing agents in the electrolyte. Furthermore, we demonstrate that the complexing agents are not electrochemically converted at the carbon-based GDE and behave inertly. If the cell is operated with complexing agents and rinsed with acidic liquid (anolyte) between batches, scaling can be completely avoided.

Electrochemical water softening technology: From fundamental research to practical application

In recent decades, the environmentally benign electrochemical softening process has been gaining widespread interest as an emerging alternative for water softening. But, in spite of decades of research, the fundamental advances in laboratory involving electrolytic cell design and treatment system development have not led to urgently needed improvements in industrially practicable electrochemical softening technique. In this review, we firstly provide the critical insights into the mechanism of the currently widely used cathode precipitation process and its inherent limitations, which seriously impede its wide implementation in industry. To relieve the above limitations, some cutting-edge electrochemically homogeneous crystallization systems have been developed, the effectiveness of which are also comprehensively summarized. In addition, the pros and cons between cathode precipitation and electrochemically homogeneous crystallization systems are systematically outlined in terms of performance and economic evaluation, potential application area, and electrolytic cell and system complexity. Finally, we discourse upon practical challenges impeding the industrial-scale deployment of electrochemical water softening technique and highlight the integration of strong engineering sense with fundamental research to realize industry-scale deployment. This review will inspire the researchers and engineers to break the bottlenecks in electrochemical water softening technology and harness this technology with the broadened industrial application area.

Research on the descaling characteristics of a new electrochemical water treatment device

In recent years, chemical water treatment equipment has gained significant attention due to its environmental-friendly features, multifunctionality, and broad applicability. Recognizing the limitations of existing chemical treatment equipment, such as challenges in scale removal and the high water content in scale deposits, we propose a novel drum design for both anode and cathode, enabling simultaneous scale suction and dehydration. We constructed a small experimental platform to validate the equipment's performance based on our model. Notably, under the optimal operating parameters, the hardness removal rate for circulating water falls within the range of 19.6-24.46%. Moreover, the scale accumulation rate per unit area and unit time reaches 13.7 g h-1 m-2. Additionally, the energy consumption per unit weight of the scale remains impressively low at 0.16 kWh g-1. Furthermore, the chemical oxygen demand (COD) concentration decreased from an initial 106.0 mg L-1 to a mere 18.8 mg L-1, resulting in a remarkable total removal rate of 82.26%. In conclusion, our innovative electrochemical water treatment equipment demonstrates exceptional performance in scale removal, organic matter degradation, and water resource conservation, offering valuable insights for future research and development in chemical treatment equipment and electrochemical theory.

Subtle introduction of membrane polarization-catalyzed H2O dissociation actuates highly efficient electrocoagulation for hardness ion removal

Electrocoagulation represents a promising process for hardness removal from cooling water. Nevertheless, the slow hydrolysis reaction severely restricted the floc formation, inhibiting the hardness co-precipitation and simultaneously causing secondary pollution from dissolved Al3+. Inspired by the detrimental membrane fouling phenomenon in conventional electrodialysis, we reported a rational strategy to substantially enhance the hardness removal efficiency in electrocoagulation by introducing a special membrane polarization-catalyzed H2O dissociation herein. Leveraging the electron transfer between functional groups (-SO3- and -N(CH3)3+) of ion exchange membrane (IEM) and surface-adsorbed H2O under the electric field-induced ion depletion scenario, H2O dissociation could be effectively catalyzed, with this catalytic activity more intensive in -SO3- than in -N(CH3)3+. Such a special H2O dissociation beneficially created a widely distributed and well-simulated alkalinity zone around the anodic region of IEM, which promoted the conversion of dissolved Al3+ to floc Al, thereby enhancing floc formation and circumventing secondary pollution. All these features enabled the resulting membrane-enhanced electrocoagulation (MEEC) to achieve a super-prominent hardness removal rate of 318.9 g h-1 m-2 with an ultra-low specific energy consumption of 3.8 kWh kg-1 CaCO3, considerably outperforming those of other conventional hardness removal processes reported to date. Additionally, in conjunction with a facile air-scoured washing method, MEEC exhibited excellent stability and universal applicability in various reaction conditions.

Facilitated OH¯ diffusion via bubble motion and water flow in a novel electrochemical reactor for enhancing homogeneous nucleation of CaCO3

Electrochemistry is a potential method for water softening. An essential disadvantage is OH¯ ions from water electrolysis accumulate on cathode surface, inducing the generation of the insulating CaCO3 layer and then interrupting the electrochemical reaction. In order to propel OH¯ diffusion into the bulk solution instead of aggregation at cathode, we designed an electrochemical reactor, whose electrodes were placed horizontally in the middle of the reactor, and the bubbles created by water electrolysis move upward, while the water flows downward. The visual evidence displayed that the unique reactor structure allowed OH¯ to spread to almost all the solution rapidly. Average pH value of bulk solution reached 10.6 in only 3 min. Therefore, homogeneous nucleation of CaCO3 in bulk solution would take primary responsibility for water softening, and the softening efficiency is up to 212.9 g CaCO3/h/m2, higher than reported results. The reactor is easy to scale up, providing a new idea for the softening of circulating cooling water.

Spatial and temporal regulation of homogeneous nucleation and crystal growth for high-flux electrochemical water softening

Electrochemical softening is an effective technology for the treatment of circulating cooling water, but its hardness removal efficiency is limited because that nucleation and growth of scale crystals depended on cathode surface. In this study, a novel method was proposed to break through this limit via spatiotemporal management of nucleation and growth processes. A cube reactor was divided into cathodic chamber and anodic chamber via installing a sandwich structure module composed of mesh cathode, nylon nets, and mesh anode. Using this continuous-flowing electrochemical reactor, OH ̄ generated by water electrolysis was rapidly pushed away from cathode surface by water flow and hydrogen bubbles movement. As a result, a wide range of strongly alkaline regions was rapidly constructed in cathodic chamber to play a nucleation region, and homogeneous nucleation in liquid phase replaced heterogeneous nucleation on cathodic surface. Furthermore, the growth process of scale crystals in alkaline regions was monitored in situ. It took only 150 s of residence time to grow to 500 nm, which may be easily separated from water by a microfiltration membrane. With this new method, the precipitation rate was 290.8 g/(hˑm²) and corresponding energy consumption was 2.1 kW·h/kg CaCO₃, both were superior to those reported values. Therefore, this study developed an efficient electrochemical softening method by spatial and temporal regulation of homogeneous nucleation and crystal growth processes.

Electrochemical water softening as pretreatment for nitrate electro bioremediation

Electro bioremediation is gaining interest as a sustainable treatment for contaminated groundwater. Nevertheless, the investigation is still at the laboratory level, and before their implementation is necessary to overcome important drawbacks. A prevalent issue is the high groundwater hardness that generates scale deposition on electrodes that irreversibly affects the treatment effectiveness and their lifetime. For this reason, the present study evaluated a novel and sustainable approach combining electrochemical water softening as a preliminary step for electro bioremediation of nitrate-contaminated groundwater. Batch mode tests were performed at mL-scale to determine the optimum reactor configuration (single- or two-chambers) and the suitable applied cathode potential for electrochemical softening. A single-chamber reactor working at a cathode potential of -1.2 V vs. Ag/AgCl was chosen. Continuous groundwater softening under this configuration achieved a hardness removal efficiency of 64 ± 4% at a rate of 305 ± 17 mg CaCO₃ m^-2_cathode h^-1. The saturation index at the effluent of the main minerals susceptible to precipitate (aragonite, calcite, and brucite) was reduced up to 90%. Softening activity plummeted after 13 days of operation due to precipitate deposition (mostly calcite) on the cathode surface. Polarity reversal periods were considered to detach the precipitated throughout the continuous operation. Their implementation every 3-4 days increased the softening lifetime by 48%, keeping a stable hardness removal efficiency. The nitrate content of softened groundwater was removed in an electro bioremediation system at a rate of 1269 ± 30 g NO₃^- m^-3_NCC d^-1 (97% nitrate removal efficiency). The energy consumption of the integrated system (1.4 kWh m^-3_treated) confirmed the competitiveness of the combined treatment and paves the ground for scaling up the process.

Review of Techniques to Reduce and Prevent Carbonate Scale. Prospecting in Water Treatment by Magnetism and Electromagnetism

Carbonate scale is one of the main problems in hot water systems, and therefore, interest in this subject has grown since 2000s. Water treatments, based on magnetic and electromagnetic (EM) techniques to prevent scale, are being commercialized, but their effectiveness is not clearly demonstrated because it depends on temperature, pressure, dissolved CO2, pH, field intensity, water flow, etc. In this paper, a review of these techniques, together with other classical techniques, such as chemical softening, the use of inhibitors, ion exchange, electrochemical and membrane treatments is presented. The latter alter the composition of the water and generate hazardous waste for health and the environment, unlike magnetic and EM treatments, which are considered non-invasive techniques. Different hypotheses are used to explain the effect of these treatments, such as the formation of aragonite instead of calcite or crystal nuclei formation within the fluid. Analysis of salts formed with SEM, X-ray diffraction, or colorimetric tests seem to support the efficiency of these treatments since study in the fluid is not easy. Dissolution of the formed scale or its prevention endorse the commercialization of these techniques, but their effectiveness must be verified in each installation.

Optimal design of an electrochemical reactor for blackwater treatment

Electrolysis of blackwater for disinfection and nutrient removal is a portable and scalable technology that can lessen the need for cities to construct large-scale wastewater treatment infrastructure and enable the safe onsite reuse of blackwater. Several systems for treating wastewater from single toilets are described in the literature, but there are few examples of systems designed to use electrolysis to treat blackwater from nearby toilets, which is a situation more common in densely packed urban living environments. In order to scale a single toilet electrolysis system to one that could service multiple toilets, computational fluid dynamic analysis was used to optimize the electrochemical reactor design, and laboratory and field-testing were used to confirm results. Design efforts included optimization of the reactor shape and mixing to improve treatment efficiency, as well as automated cleaning and salt injection to reduce maintenance and service requirements. PRACTITIONER POINTS: Design of a reverse polarity mechanism to enable in situ electrode cleaning and improve long-term electrode performance. Optimization of a hopper design and drainpipe location to collect and remove flaking precipitates and mitigate maintenance issues. Design of an automated salt injection system to guarantee sufficient chloride levels for producing adequate chlorine residuals for consistent disinfection.

Novel S-scheme RP/NiCo-LDH composite with remarkably enhanced photocatalytic activity for H2 evolution under visible-light irradiation

A novel red phosphorus/nickel cobalt layered double hydroxide (RP/NiCo-LDH) heterojunction was successfully prepared and exhibited an excellent photocatalytic performance for hydrogen evolution.