Coupling of Electric and Flow Fields to Enhance Ion Transport for Energy-Efficient Electrochemical Tap-Water Softening

Electrochemical-induced precipitation is a sustainable approach for tap-water softening, but the hardness removal performance and energy efficiency are vastly limited by the ultraslow ion transport and the superlow local HCO3-/Ca2+ ratio compared to the industrial scenarios. To tackle the challenges, we herein report an energy-efficient electrochemical tap-water softening strategy by utilizing an integrated cathode-anode-cathode (CAC) reactor in which the direction of the electric field is reversed to that of the flow field in the upstream cell, while the same in the downstream cell. As a result, the transport of ions, especially HCO3-, is significantly accelerated in the downstream cell under a flow field. The local HCO3-/Ca2+ ratio is increased by 1.5 times, as revealed by the finite element numerical simulation and in situ imaging. In addition, a continuous flow electrochemical system with an integrated CAC reactor is operated for 240 h to soften tap water. Experiments show that a much lower cell voltage (9.24 V decreased) and energy consumption (28% decreased) are obtained. The proposed ion-transport enhancement strategy by coupled electric and flow fields provides a new perspective on developing electrochemical technologies to meet the flexible and economic demand for tap-water softening.

Investigation of Calcium and Magnesium Removal by Donnan Dialysis According to the Doehlert Design for Softening Different Water Types

In this study, calcium and magnesium were removed from Tunisian dam, lake, and tap water using Donnan Dialysis (DD) according to the Doehlert design. Three cation-exchange membranes (CMV, CMX, and CMS) were used in a preliminary investigation to establish the upper and lower bounds of each parameter and to more precisely pinpoint the optimal value. The concentration of compensating sodium ions [Na⁺] in the receiver compartment, the concentration of calcium [Ca²⁺] and magnesium [Mg²⁺] in the feed compartment, and the membrane nature were the experimental parameters. The findings indicate that the CMV membrane offers the highest elimination rate of calcium and magnesium. The Full Factorial Design makes it possible to determine how the experimental factors affect the removal of calcium and magnesium by DD. All parameters used had a favorable impact on the response; however, the calcium and magnesium concentration were the most significant ones. The Doehlert design's Response Surface Methodology (RSM) was used to determine the optimum conditions ([Mg²⁺] = 90 mg·L^-1, [Ca²⁺] = 88 mg·L^-1, [Na⁺] = 0.68 mol·L^-1) allowing a 90.6% hardness removal rate with the CMV membrane. Finally, we used Donnan Dialysis to remove calcium and magnesium from the three different types of natural water: Dam, Lake, and Tap water. The results indicate that, when compared to lake water and tap water, the removal of calcium and magnesium from dam water is the best. This can be linked to the water matrix's complexity. Therefore, using Donnan Dialysis to decrease natural waters hardness was revealed to be suitable.

Softening with Ceramic Micro-Filtration for Application on Water Reclamation for Industrial Recirculating Cooling Systems

There is a global need for optimizing the use of water that has resulted from increased demand due to industrial development, population growth, climate change and the pollution of natural water resources. One of the solutions is to use reclaimed water in industrial applications that do not require water of potable quality, such as cooling water. However, for cooling water, (treated) wastewater's hardness is too high, apart from having a high load of suspended solids and organic matter. Therefore, a combination of softening with ceramic micro-filtration was proposed for treating wastewater treatment effluent containing fouling agents for potential use in industrial cooling systems. The effectiveness of the softening process on model-treated wastewater with calcium hydroxide in the presence of phosphate and sodium alginate was first evaluated using jar tests. Furthermore, membrane fouling was studied when filtering the softened water. The results showed that the inhibition of calcium carbonate precipitation occurred when inorganic substances, such as phosphate and organic compounds, were present in the water. The fouling of the membranes due to sodium alginate in water was only slightly negatively affected when combined with softening and phosphate. Therefore, this combination of treatments could be potentially helpful for the post-treatment of secondary effluent for cooling systems.

Evaluation of Strong Cation Ion-Exchange Resin Cost Efficiency in Manufacturing Applications-A Case Study

The effective ionic capacities of strong cation ion-exchange resins were investigated and compared using conditions similar to those found in white goods, in order to establish behavioral differences between commercial products and evaluate their capacity in a broader business context. Nine different products of equivalent TDS (Technical Data Sheet) capacity were observed to examine their differences in approximately real-life conditions. For a broader context of applicability analysis, besides the absolute ionic operating capacity, the following additional factors were included in the evaluation: the standard deviation in the resins’ performances and their relative prices. A complete method for material applicability evaluation was hereby proposed and shown to offer cost factor benefits of up to 21.1% within the range of products examined, in comparison to a cost-only evaluation for equivalent materials.

Intrinsic Pseudocapacitive Affinity in Manganese Spinel Ferrite Nanospheres for High-Performance Selective Capacitive Removal of Ca2+ and Mg2

Pseudocapacitor-type hybrid capacitive deionization (PHCDI) has been developed extensively for deionization, which enables to address the worldwide freshwater shortage. However, the exploitation of selective hardness ion removal in resourceful hard water via the intrinsic pseudocapacitive effect, rather than the ion-sieving or ion-swapping effect based on the electric double layer (EDL) of porous carbon, is basically blank and urgent. Herein, manganese spinel ferrite (MFO) nanospheres were successfully fabricated by one-step solvothermal synthesis and used as the cathode for PHCDI assembled with commercial activated carbon. The MFO electrode exhibited prominent capacities of 534.6 μmol g^-1 (CaCl₂) and 980.4 μmol g^-1 (MgCl₂), outperforming those of other materials ever reported in the literature. Fascinatingly, systematic investigation of binary and ternary ion solutions showed the high electro-affinity of hardness ions (Ca²⁺ and Mg²⁺) toward Na⁺, especially the leading affinity of Mg²⁺, in which the superhigh hardness selectivity of 34.76 was achieved in the ternary solution with a molar ratio of Na-Ca-Mg as 20:1:1. Unexpectedly, the ion-swapping trace in a multi-ion environment was also first detected in our pseudocapacitive-based electrode. The electrochemical response in unary and multiple electrolytes disclosed that the unique pseudocapacitive affinity based on the cation (de)intercalation-redox mechanism was from the synergistic effect of the relative redox potential, ionic radius, and valence, in which the redox potential was the dominant factor.

Theoretical Analysis of Constant Voltage Mode Membrane Capacitive Deionization for Water Softening

Water softening is desirable to reduce scaling in water infrastructure and to meet industrial water quality needs and consumer preferences. Membrane capacitive deionization (MCDI) can preferentially adsorb divalent ions including calcium and magnesium and thus may be an attractive water softening technology. In this work, a process model incorporating ion exclusion effects was applied to investigate water softening performance including ion selectivity, ion removal efficiency and energy consumption in a constant voltage (CV) mode MCDI. Trade-offs between the simulated Ca²⁺ selectivity and Ca²⁺ removal efficiency under varying applied voltage and varying initial concentration ratio of Na⁺ to Ca²⁺ were observed. A cut-off CV mode, which was operated to maximize Ca²⁺ removal efficiency per cycle, was found to lead to a specific energy consumption (SEC) of 0.061 kWh/mole removed Ca²⁺ for partially softening industrial water and 0.077 kWh/m³ removed Ca²⁺ for slightly softening tap water at a water recovery of 0.5. This is an order of magnitude less than reported values for other softening techniques. MCDI should be explored more fully as an energy efficient means of water softening.

Tailoring Charged Nanofiltration Membrane Based on Non-Aromatic Tris(3-aminopropyl)amine for Effective Water Softening

High-performance positively-charged nanofiltration (NF) membranes have a profound significance for water softening. In this work, a novel monomer, tris(3-aminopropyl)amine (TAEA), with one tertiary amine group and three primary amine groups, was blended with trace amounts of piperazine (PIP) in aqueous solution to fabricate a positively-charged NF membrane with tunable performance. As the molecular structures of TAEA and PIP are totally different, the chemical composition and structure of the polyamine selective layer could be tailored via varying the PIP content. The resulting optimal membrane exhibited an excellent water permeability of 10.2 LMH bar⁻¹ and a high rejection of MgCl₂ (92.4%), due to the incorporation of TAEA/PIP. In addition, this TAEA NF membrane has a superior long-term stability. Thus, this work provides a facile way to prepare a positively charged membrane with an efficient water softening ability.

Selective Pseudocapacitive Deionization of Calcium Ions in Copper Hexacyanoferrate

In recent years, the capacitive deionization (CDI) technology has gradually become a promising technology for hard water treatment. Up to now, most of the work for water softening in CDI was severely limited by the inferior selectivity and electrosorption performances of carbon-based electrodes in spite of combining Ca²⁺-selective ion-exchange resin or membranes. Pseudocapacitive electrode materials that selectively interact with specific ions by Faradic redox reactions or ion (de)intercalation offer an alternative strategy for highly selective electrosorption of Ca²⁺ from water because of brilliant ion adsorption capacity. Here, we first used copper hexacyanoferrate (CuHCF) as a pseudocapacitive electrode to methodically study the selective pseudocapacitive deionization of Ca²⁺ over Na⁺ and Mg²⁺. Using the hybrid CDI cell consisting of a CuHCF cathode and an activated carbon anode without any ion-exchange membrane, the outstanding Ca²⁺ electrosorption capacity of 42.8 mg·g^-1 and superior selectivity &(Ca²⁺/Na⁺) of 3.05 at a molar ratio of 10:1 were obtained at 1.4 V, surpassing those of the reported carbon-based electrodes. Finally, electrochemical measurements and molecular dynamics (MD) simulations provided an in-depth understanding of the selective pseudocapacitive deionization of Ca²⁺ ions in a CuHCF electrode. Our study would be helpful for developing high-efficiency selective electrosorption of target charged ions by intrinsic properties of pseudocapacitive materials.

Calcium and magnesium ion removal from water feeding a steam generator by chemical precipitation and flotation with micro and nanobubbles

This work summarises the results of calcium and magnesium ion removal from raw water feeding an industrial steam generation system. The cations were precipitated with sodium phosphate before separation of the solids by dissolved air flotation, with micro and nanobubbles. Studies were done at bench scale and validated at pilot scale (raw water feed = 1 m³h^-1; air-to-solids ratio = 0.046 mg of air mg^-1 of solids; residence time = 11 min). Results indicated that chemical precipitation followed by flotation significantly improved the quality of the boiler water. Best results were obtained after precipitating the cations with 50 mg L^-1 of sodium phosphate at pH 11.5 and flotation with a saturation pressure (P _sat) of 4 bar, a recycling ratio of 30% and a sodium oleate concentration of 20 mg L^-1 as an hydrophobizing reagent. The latter assisted the adhesion of the nanobubbles (100-500 nm) generated at 4 bar with a numeric concentration of about 2.5 × 10⁸NBs mL^-1. At pilot scale, the total hardness in the solution decreased by 80%; the residual calcium and phosphate ion concentrations were 12 and 2 mg L^-1 respectively. This cell was designed including lamellae and perforate plate to improve the superficial loading capacity (up to 9 m h^-1). The results were explained by chemical and interfacial phenomena and it is believed that this technique has great potential in water softening processes.

Water Softening Using a Light-Responsive, Spiropyran-Modified Nanofiltration Membrane

A novel technique for the covalent attachment of a light-responsive spiropyran onto polyamide thin film composite nanofiltration (NF) membranes in a one-step reaction using low-energy electron beam technology is described. The effect of illumination of the immobilized spiropyran was studied, as well as the resulting membrane properties with respect to MgSO₄ retention, water permeability rate, and chlorine resistance. Electron beam irradiation showed a direct effect on the transformation of the rough PA NF membrane surface into a ridge-and-valley structure. Upon UV light irradiation, the spiropyran transformed into zwitterionic merocyanine, which had shown MgSO₄ removal of >95% with water permeation rates of 6.5 L/(m²·h·bar). Alternatively, visible light was used to convert merocyanine to spiropyran, which achieved >95% of MgSO₄ retention with a water flux of around 5.25 L/(m²·h·bar). The modified NF membranes showed higher chlorine resistance as well as a higher normalized water flux as compared to the reference membrane, without a loss of ion retention. All the NF membranes were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. This study demonstrates a simple and inexpensive method for the immobilization of molecules onto polymeric membranes, which may be applied in water softening.

Ultrathin Polyamide Membrane with Decreased Porosity Designed for Outstanding Water-Softening Performance and Superior Antifouling Properties

Poly(piperazine-amide)-based nanofiltration membranes exhibit a smooth surface and superior antifouling properties but often have lower Ca²⁺ and Mg²⁺ rejection due to their larger inner micropore and thus cannot be extensively used in water-softening applications. To decrease the pore size of poly(piperazine-amide) membranes, we designed and synthesized a novel monomer, 1,2,3,4-cyclobutane tetracarboxylic acid chloride (BTC), which possesses a smaller molecular conformation than trimesoyl chloride (TMC). The thickness of the prepared BTC-piperazine (PIP) polyamide nanofilm via interfacial polymerization is as thin as 15 nm, significantly lower than the 50 nm thickness of the TMC-PIP nanofilm. The surface characterization reveals that the BTC-PIP polyamide membrane exhibits an enhanced hydrophilicity, a smooth surface, and a decreased surface-negative charge. The desalination performance (both rejection and water flux) of these membranes in terms of Ca²⁺ and Mg²⁺ exceeds that of the current commercial water-softening membranes. In addition, the BTC-PIP polyamide membrane also exhibits superior antifouling properties compared to the TMC-based polyamide membrane. More importantly, molecular simulations show that the BTC-PIP membrane has a lower average pore size than that of the TMC-PIP membrane, which demonstrates an enhanced steric hindrance effect, as confirmed by desalination performance. Our results demonstrate that in the household and industrial water-softening market, BTC-PIP membrane with decreased porosity, enhanced hydrophilicity, and smooth surface is preferred alternative to the conventional TMC-based polyamide membranes.

Poly(piperazine-amide)/PES Composite Multi-Channel Capillary Membranes for Low-Pressure Nanofiltration

The mechanical stability of conventional single-channel capillary fibres can be improved in a multi-channel geometry, which has previously found application in ultrafiltration. In this work, multi-channel polyethersulfone (PES) capillary membranes comprising seven feed channels were successfully fabricated in an enhanced steam–dry–wet spinning process and coated on the inner surface with a thin polyamide (PA) layer via interfacial polymerization (IP). The coating procedure consisted of impregnating the support multi-channel capillary membranes (MCM) with an aqueous piperazine solution, flushing with nitrogen gas to remove excess droplets, and pumping an organic trimesoylchloride solution through the channels. Insights into the interfacial polymerization process were gained through the investigation of various parameters, including monomer ratio, contact time, and drying time. Membranes were characterised via scanning electron microscopy (SEM), atomic force microscopy (AFM), and filtration experiments. The optimisation of both the PES support membrane and IP process parameters allowed for the fabrication of composite MCM with an MgSO₄ rejection of 91.4% and a solute flux of 68.8 L m⁻² h⁻¹ at an applied pressure of 3 bar. The fabricated composite MCM demonstrates that a favourable multi-channel arrangement can be upgraded with a PA layer for application in low-pressure nanofiltration.