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In this study, the removal of monovalent and divalent cations, Na+, K+, Mg2+, and Ca2+, in a diluted solution from Chott-El Jerid Lake, Tunisia, was investigated with the electrodialysis technique. The process was tested using two cation-exchange membranes: sulfonated polyether sulfone cross-linked with 10% hexamethylenediamine (HEXCl) and sulfonated polyether sulfone grafted with octylamine (S-PESOS). The commercially available membrane Nafion® was used for comparison. The results showed that Nafion® and S-PESOS membranes had similar removal behaviors, and the investigated cations were ranked in the following descending order in terms of their demineralization rates: Na+ > Ca2+ > Mg2+ > K+. Divalent cations were more effectively removed by HEXCl than by monovalent cations. The plots based on the Weber–Morris model showed a strong linearity. This reveals that intra-particle diffusion was not the removal rate-determining step, and the removal process was controlled by two or more concurrent mechanisms. The Boyd plots did not pass through their origin, and the sole controlling step was determined by film-diffusion resistance, especially after a long period of electrodialysis. Additionally, a semi-empirical model was established to simulate the temporal variation of the treatment process, and the physical significance and values of model parameters were compared for the three membranes. The findings of this study indicate that HEXCl and S-PESOS membranes can be efficiently utilized for water softening, especially when effluents are highly loaded with calcium and magnesium ions.

Durian peel can be the main contributor to agricultural wastes and becomes an environmental problem when it is discarded into the landfill site or even burning it. The average of entire durian fruit weight is about 255,353 MT for all over the country. The loading rate of landfills is increasing, especially with the massive amount and size of the durian waste that requires a larger space to dispose of. Therefore, it is certainly important to utilize durian husk as an adsorbent to improve water quality, especially groundwater source. Groundwater commonly has a higher hardness level than surface water. The higher hardness in water becomes a major concern, especially in every cleaning task. This research aimed to investigate the potential of durian husk in reducing water hardness, electrical conductivity (EC), and total dissolved solids (TDS). Durian husk was treated with NaOH solution to improve adsorption efficiency. Laboratory analyses such as the ethylenediaminetetraacetic acid (EDTA) titration method were performed for the water hardness, total dissolved solids, and conductivity concentrations respectively to test the performance of the removal efficiency before and after the treatments. As the results, the removal of hardness concentrations by durian husk has significantly dropped with dosage and settling time. However, not for TDS and EC concentrations removal, which it went sudden increased for a higher dosage. The novelty of this study is that it is the first-ever experiment using the real on-site samples in the field as for initial concentrations, different from other previous studies by lab-scale the synthetic hard water and most of them using the highest concentration of hardness which up to 700 mg/L of CaCO3 as to find out the removal efficiency for water softening in water treatment using durian husk, compare to this study using a concentration that is too low within 300 mg/L and below.

An attempt has been carried out to remove hardness of borewell water by hydrodynamic cavitation. The performance of this approach is dependent on multiple parameters like pressure, orifice diameter, velocity, pipe material, etc. Under such circumstances it is difficult to decide most significant parameter governing efficiency of hydrodynamic cavitation. Observations were recorded at various pressures ranging from 1 bar to 10 bar. To achieve cavitation, venture meter and orifice with varying diameter were used. With varying pipe diameter and flow the efficiency of hardness removal was analyzed. In the present article assessment of most significant combination to remove hardness using hydrodynamic cavitation approach is carried out using Fuzzy Rule Base approach. Total 60 rules were formulated. A Mamdani type; Min-Max; Fuzzy method and triangular membership functions were adopted in fuzzification.

Monomer design and reconstruction is typically a preferred route to tune the inner structure and screen the performance of polyamide nanofilms for their efficiency and ease of scaling up in industrial production. Herein, two kinds of novel acyl chloride monomers, 1,2,3,4-cyclobutane tetracarboxylic acid chloride (BTC) and 1,2,4,5-cyclohexanetetracarboxylic acid chloride (HTeC), have been designed and synthesized. The BTC and HTeC monomers can rapidly react with amine molecule at interface to form an aliphatic polyamide nanofilm that is denser than that of TMC based polyamide membrane. The resulting mean effective pore size of the BTC and HTeC polyamide nanofilms is 0.184 nm and 0.197 nm, which is lower than that of the TMC nanofilm at 0.238 nm. Desalination experiments revealed that the aliphatic polyamide membrane shows more than a 98% rejection rate for CaCl2, MgCl2, and MgSO4 and a water flux of 84.4 kg m−2 h−1 MPa−1 (for 2000 ppm MgSO4). Moreover, the ion selectivity of Na+/Mg2+ and Na+/Ca2+ of the BTC membrane is as high as 126 and 31.5, respectively. These are much higher than those of the related TMC and commercial nanofiltration membranes. Fouling experiments indicate that the flux decline rate (FDR) of the aliphatic polyamide membrane is only 38%, whereas the FDR of the full-aromatic polyamide membrane is 60%. Further investigations confirmed that surface roughness is the main factor affecting the fouling behaviors of polyamide membranes. Our results demonstrate that BTC and HTeC monomers are unique potential materials in the fabrication of nanofiltration membranes used for water treatment such as water softening and ion sieving.

In this study, a new composite resin, based on the triple structure of bentonite clay, magnetite, and polyacrylonitrile, was synthesized, characterized, and applied for the removal of water hardness. The composite resins were characterized by Fourier-transform infrared, X-ray diffraction, and scanning electron microscope. The performance toward water hardness removal was evaluated using different approaches of batch and column experiments, using the ethylenediaminetetraacetic acid (EDTA) method, to determine different hardness levels of diverse water samples, which included synthetic and natural water samples. Different parameters were investigated that include contact time, resin dose, initial concentration, and the flow rate in the column process. The results showed an increase in the hardness removal with contact time. A batch process recorded 100% removal of water hardness by an equilibration time of about 60 min with a minimum of 3 g L−1. The adsorption results were found to be successfully fitted with the pseudo-second-order kinetic model and were better described with the Freundlich isotherm model. Furthermore, column studies revealed the effects of several operating parameters on the breakthrough profiles. Remarkably, the experimental results using the column approach revealed a high removal efficiency of 88–100% for the natural water samples, employing a 5 g packed column with a 2 mL min−1 flow rate for hard water that ranged from 237 to 680 mg CaCO3/L. The results illustrate the potential of this polymeric composite resin as an alternative to conventional softening treatment techniques in terms of low cost and high performance.

Semi-aromatic polyamide membranes have been widely commercialized and used in water treatment processes. Such membranes often have poor Ca2+ and Mg2+ rejection rates due to their intrinsic structural characteristics. Most approaches used to improve rejection are only focused on one kind of repulsion effect: lack of synergism. Thus, the enhancement of rejection is not remarkable. Herein, a novel monomer, trimesoyl-[4,4-dimethyl-5(4H)-azlactone] (TMDMA), was synthesized and used as a secondary reaction molecule in the interfacial reaction for simultaneously improving the steric hindrance and lowering the electrostatic attraction of semi-aromatic polyamide membranes. With the introduction of TMDMA, the mean effective pore size of the polyamide membrane is decreased from 0.23 nm to 0.18 nm. Moreover, the significantly decreased negative surface charge of the fabricated polyamide membrane is observed since the carboxyl from hydrolysis of the azlactone exhibits a higher pKa (acidity coefficient) than that of the benzoic acid. The desalination experiments reveal that the Ca2+ and Mg2+ rejection of the resulting membranes are increased by 26% (from 71.0% to 97%) and 14.0% (from 83% to 97%), respectively, while the water fluxes are maintained at 192 and 167 kg m−2 h−1 MPa−1, demonstrating a satisfactory water softening performance that exceeds commercial nanofiltration membranes. Additionally, the fabricated polyamide membranes can significantly resist the flux decline during fouling experiment that normally results from the deposition of Ca2+−foulant.

Softening ammonia sulfate wastewater is one of the vital issues in the rare earth industry. In this article, a novel softening process using [4,4′-isopropylidenebis (phenoxyacetate)] (H2IPOAA) for Ca2+ and Mg2+ (M2+) is developed. The effect factors, i.e., equilibrium time, pH value of aqueous phase and saponification degree are investigated. The extraction mechanism is proposed to be cation exchange. With a saponification degree of 85%, H2IPOAA is successfully used for the precipitation of Ca2+ and Mg2+ from ammonia sulfate wastewater within 10 min. The process can form precipitated particles with an average size of 35 μm, which are much larger than the precipitates formed by H2C2O4, Na2C2O4 and Na2CO3. The precipitated particles can be easily separated from the raffinate. In addition, H2IPOAA is repeatedly used for five times with a little loss. To a considerable extent, the reuse of H2IPOAA contributes to lowering the cost and reducing the pollution. The concentrations of Ca2+ and Mg2+ in the raffinate extracted by H2IPOAA are up to the MVR standard for wastewater treatment. The economical and efficient softening process meets the need of green chemistry, which is expected to replace traditional industrial processes for water softening based on H2C2O4, Na2C2O4 and Na2CO3.

An excellent positively charged nanofiltration (NF) membrane is of great significance for water softening. In this work, a novel positively charged composite PEI/TMA NF membrane was prepared by gradient cross-linking polyethyleneimine (PEI) with trimesic acid (TMA) on polyethersulfone ultrafiltration (PESU) membrane. The effects of concentration of PEI and TMA, the cross-linking reaction temperature and reaction time on membrane structure and performance were investigated in detail. The effects of PEI and TMA, reaction temperature and time on the membrane structure, composition and performance were investigated. The optimized preparation conditions of PEI/TMA NF membrane were fixed at 0.6% (m/v) PEI, 0.1% (m/v) TMA, 0.15% (m/v) SDS at 90 °C for 10 min. The as-prepared membrane exhibited a pure water flux of 19.1 L m−2 h−1 bar−1 and a rejection of MgCl2 93.0%, NaCl 38.2%, MgSO4 20.4%, and Na2SO4 8.8%. The functional layer is ultra-thin. Its high selectivity mainly results from Donnan effect. The long-term testing of our PEI/TMA NF membrane show a high stability. Compared with other positively charged membranes, the PEI/TMA NF membrane had unique advantages in the production process and a comprehensive performance.

In the current study, hard water softening for the removal of Ca2+ and Mg2+ ions was performed using hydrogel beads based on Gum Tragacance (GT) modified by using 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and poly(vinyl alcohol). The antibacterial spherical hydrogel beads were fabricated by instantaneous gelation of well dispersed mixture of poly(AMPS)-g-GT (1 g), poly(vinyl alcohol) (PVA, 1 g) flocculent, green-synthesized silver metal nanoparticles (AgNPs, 10 mg), and graphene oxide (GO, 10 mg) in the acetone solution of boric acid and then transferring into the different amounts (0.5–2.5 mL) of acidic solution of glutaraldehyde (GA) as cross-linker. The beads were fully characterized and their adsorption behavior matched well with the pseudo-second-order kinetic and the Langmuir isotherm models with the maximum adsorption of Ca2+(114.18 mg g−1) and Mg2+(162.46 mg g−1). The removal ability of the beads decreased by 6% after four adsorption/desorption cycles. The antibacterial performance of the hydrogel beads was also investigated against Gram-positive and Gram-negative bacteria.

The high electrode area requirement limited the applications of electrochemical water softening technology, despite of conspicuous advantages. This work introduces an electrochemical water softening system with multi-meshes coupled cathodes to address fouling in industrial circulating cooling water systems. Experimental data indicate that the coupled cathode increases the descaling rate of electrochemical water softening system due to the produced self-synergy effect based on its unique multilayer structure. The synergistic effect is constructed by separating and positioning the chemical reactions (alkalinity production and scale deposition) to different regions of the coupled cathode, thus a large amount of OH− ions from the internal layers and the preferentially deposited scale on external layers accelerate the subsequent scale deposition in later stage of the process. This system displays the reduced electrode area requirement and energy consumption depending on the increased descaling rate. This work provides a facile strategy for the development and applications of electrochemical water softening technology.

In this work, a new paired electrolysis procedure is successfully introduced. This is a combination of two different processes: electrochemical water softening (EWS) and electrochemical production of weak acid solutions (EPWA). This paired electrolysis method is tested for hardness removal in a simulated solution of concentrates, such as that produced by the electrodialysis of brackish water. The use of a divided electrochemical cell makes it possible to achieve removal efficiencies higher than 90% in optimum conditions. The good performance and stability of the cathode are also demonstrated by long-term experiments. Simultaneously, weak acids are obtained by protonation of their conjugate bases in the anodic compartment. Lactic acid is obtained from lactate with a purity of no less than 80% in this work without significant loss of efficiency in hardness removal. The main interest lies in the commercial value of weak acids such as lactic acid, citric acid or gluconic acid, among others. The methodology makes it possible to benefit from both cathodic and anodic processes, which may contribute to decreasing the final EWS treatment costs for a cost-effective desalination application. Keywords: Electrochemical softening, Weak acid, Lactic, Paired electrolysis, Concentrate

An integrally skinned dense ultrafiltration (UF) membrane was prepared using partially di-quaternized poly(sulfone-co-ethernitrile) random copolymer (PSF-co-DQAPEN) via classical nonsolvent induced phase inversion process (NIPS). Then the original UF membrane was molecularly restructured through a combined process of chemical quaternization-crosslinking reaction and thermal annealing for the preparation of a crosslinked nanofiltration (NF) membrane, in which p-xylylene dichloride (PXDC) and isopropanol were employed as crosslinking agent and nonsolvent treating medium, respectively. The influence of reaction time and crosslinker concentration on membrane performance were investigated. X-ray photoelectron spectroscopy (XPS) of membrane surface and solubility test of membrane were employed to study the chemical composition and crosslinking properties of membranes, and those results revealed that the original membrane had been successfully crosslinked after this post treatment process. The separation performance of membrane was ultimately changed from UF-level to NF-level after the quaternization-crosslinking reaction and thermal annealing treatment process. The optimum crosslinked NF membrane possessed MgCl2 rejection of 98.2% and water permeate flux of 4.12 L m−2 h−1 bar−1. Moreover, owning to the excellent chemical stability of membrane material, the crosslinked membrane had outstanding resistance to chlorine, acid and base, thus it can be used for water softening and removing of positive dyes. Given the convenience and feasibility of this modification method, integrally skinned NF membrane could be easily fabricated via the post chemical crosslinking and thermal annealing process of UF membrane using proper treating approaches.

A treatment chain including nanofiltration, crystallization and multi-effect distillation (MED) is for the first time proposed for the treatment of an effluent produced during the regeneration of Ion Exchange resins employed for water softening. The goal is to recover the minerals and to restore the regenerant solution to be reused in the next regeneration cycle. MED is the most crucial unit of the treatment chain from an economic point of view. A techno-economic analysis on the MED unit was performed and a novel performance indicator, named Levelized Brine Cost, was introduced as a measure of the economic feasibility of the process. Different scenarios were analysed, assuming different thermal energy sources and configurations (plane-MED or MED-TVC). It was found that the plane-MED fed by waste-heat at 1 bar is very competitive, leading to a reduction of 50% of the fresh regenerant current cost. Moreover, the thermal energy cost of 20US$/MWhth was identified as the threshold value below which producing regenerant solution in the MED is economically more advantageous than buying a fresh one. Overall, MED allows reducing the environmental impact of the industrial process and it results competitive with the state of the art for a wide range of operating conditions.

Electrochemical precipitation has attracted widespread attention in the treatment of recirculating cooling water in recent years. In order to guarantee continuous water softening efficiency, periodic scale detachment of cathode is indispensable. Nevertheless, periodic mechanical scrapping, prevalent scale detachment technique currently, increases the interelectrode gap, resulting in the decline of water softening efficiency and the rise of energy consumption. In this paper, polarity reversal was applied in the electrochemical precipitation process for removing scale deposits. The results showed that scale deposits were removed effectively and high water softening efficiency were accomplished steadily. The precipitation rate obtained was 53.9–73.4 g/h/m2. The reactor still could remove hardness ions efficiently after the scale detachment by polarity reversal. The energy consumption maintained in the range of 17.3–23.6 kWh/kg CaCO3. The total hardness removal efficiency was 16.4–21.4%. Repeated experimental results demonstrated that the electrochemical precipitation process adopting polarity reversal for scale detachment could operate steadily without any performance degradation detected. Cyclic voltammetric investigation showed good electrochemical stability of dimensionally stable anode (DSA) used. Accelerated life test indicated that it could keep active steadily in polarity reversal condition.

A polyvinyl chloride (PVC)-based positively-charged nanofiltration membrane was fabricated from polyvinyl chloride-graft-poly(N,N-dimethylaminoethyl methacrylate) by heating and crosslinking treatment. We characterized the surface chemical compositions, membranes structures, surface hydrophilicity and surface charges of the prepared membrane. The estimated membrane pore diameter is 1.98 nm and the pore diameters are in the range of 0.5–2.0 nm. The membrane exhibits excellent water permeability (9.3 L m−2 h−1 bar−1) and salt rejection (93.1% with MgCl2 solution). Both the heating treatment and crosslinking reaction can diminish the pore size of the precursor membrane. The membrane hydrophilicity is improved after quaternization, leading to higher water permeability compared with the membrane treated by heating only. Moreover, surface quaternization and crosslinking offer the membrane excellent pressure resistance and long-term stability. The prepared nanofiltration membrane has great promise in many applications including water softening and multivalent ion removal.

Nanofiltration membrane (NF) is one of the most important activities employed in wastewater treatment field. It is a relatively recent development in membrane technology and it can be aqueous or non-aqueous. Characteristics of NF fall between UF and RO, and functions by both pore-size flow (convective) and the solution-diffusion mechanisms. Membrane charges play an important role in membrane function and often NF membrane as have surface negative charges. NF technique is used in a variety of water and wastewater treatment (WWT) in different industrial applications. The main job of NF is the selective removal of ions and organic substances and it is used in some specified seawater desalination application. The main objective of this review is to illustrate the main applications of NF process in water reuse, WWT as tertiary treatment, water softening and desalination fields. Comparison of basic economic analyses with other alternative processes in profitability is also performed. Keywords: Membrane, Nanofiltration, NF applications, Wastewater treatment

Capacitive deionization (CDI) is an emerging membraneless water desalination technology based on storing ions in charged electrodes by electrosorption. Due to unique selectivity mechanisms, CDI has been investigated towards ion-selective separations such as water softening, nutrient recovery, and production of irrigation water. Especially promising is the use of activated microporous carbon electrodes due to their low cost and wide availability at commercial scales. We show here, both theoretically and experimentally, that sulfonated activated carbon electrodes enable the first demonstration of perfect divalent cation selectivity in CDI, where we define "perfect" as significant removal of the divalent cation with zero removal of the competing monovalent cation. For example, for a feedwater of 15 mM NaCl and 3 mM CaCl

The toxic heavy metals produced by the discharge of industrial wastewater pose a serious threat to the ecological environment and human health. Nanofiltration (NF) membrane separation technology is widely used in fields such as water softening, heavy metal removal and dye separation due to its environmental friendliness and low cost. Herein, a novel positively charged aliphatic polyamide NF membrane (PEI-BTC) has been developed by using 1,2,3,4-cyclobutane tetracarboxylic acid chloride (BTC) monomer bearing a stereoscopic structure which undergoes classic interfacial polymerization (IP) with polyethyleneimine (PEI) on the Polyether sulfone (PES) support membrane. The physicochemical properties revealed that the PEI-BTC membrane had a larger mean effective pore size (0.285 nm), a thinner separation layer (40 nm) and a stronger positively charged membrane surface (IEP = 7.25) than the traditional PEI-TMC membrane. Compared with previously reported PEI-based and commercial NF membranes, the optimized PEI-BTC membrane exhibits a higher MgCl2 (2000 ppm) rejection of 97.53% and pure water flux of 156.85 kg·m−2·h−1 at 1.0 MPa. Moreover, the prepared PEI-BTC NF membrane shows excellent toxic heavy metal (1000 ppm) removal efficiency in the order of Mn (98.78%) > Zn (98.32%) > Ni (97.74%) > Cu (95.67%) > Cd (90.49%). The results demonstrate that the prepared positively charged aliphatic polyamide NF membrane (PEI-BTC) has a unique industrial production potential for water softening and heavy metal removal.

Nanofiltration (NF) thin film composite membranes were prepared via interfacial polymerization using piperazine with isophthaloyl chloride on porous PES/PAN blend membrane. The prepared membranes were characterized through chemical structure, morphology, surface charge studies and separation performance. All the variables are taken into consideration including reactive monomers amount. The monomers ratio were 1: 0.3: 0.3: 0.3 that corresponds to piperazine: sodium hydroxide: dodecyl sodium sulfate: isophthaloyl dichloride respectively. Soaking time for amine solution was 30 min, while it was 10 min in organic chloride solution. After interfacial polymerization the membranes have been annealed at temperature 80 °C for curing time 10 min. These were the optimum conditions which greatly enhanced the flux and the salt rejection of NF thin film composite membranes. The membranes have rejection percent in the order of 99.9, 99, 82, and 72% for Na2SO4, MgSO4, CaCl2, NaCl respectively without reducing water productivity. This study offers separation of 7000 ppm salt solution with low operating pressure (5 bars) compared with previous studies. The fabricated membranes were applied for 30 days with steady-state flux keeping their polyamide layers. As well; the fabricated membrane with the optimum operating conditions has a better fouling resistance for humic acid and molecular weight cut off 600 Da.

Electrodialysis (ED) is a promising and widely used technique for producing drinking water. Nevertheless, the presence of multivalent ions like Mg2+ and Ca2+ can be challenging in practical applications, because this leads to precipitation in the concentrated compartment, thus decreasing the performance of the ED process. Monovalent ion exchange membranes are capable of separating monovalent ions from a solution containing both mono- and multivalent ions, which can be applied for water softening. The monovalent ion selectivity of an ion exchange membrane in electrodialysis is greatly dominated by its surface structure and properties. In this study, a simple method is proposed by co-depositing mussel-inspired polydopamine (PDA) with polyethyleneimine (PEI) to modify a self-prepared sulfonated poly (ether sulfone) (SPES) ion exchange membrane. The effects of the PEI content on the monovalent selectivity were studied. When the optimized parameters were used in ED, the permselectivity of SPES-PDA/PEI-2 was 2.5 times higher than that of the SPES membrane. Especially the flux of H+ was enhanced. Furthermore, the PDA/PEI modified ion exchange membrane shows excellent operation stability for monovalent separation performance after immersion in acid and alkaline solution for 7 days. Comparing membranes prepared with different molecular weights of PEI, results revealed that modification with a lower molecular weight PEI yields a higher selectivity. This facile strategy may provide new opportunities not only to develop monovalent ion exchange membranes but also to engineer the surface of numerous materials in energy and environmental applications.

This study concerned aluminium exchanged strong acid cation resin for the removal of alkaline earth ions from simulated coal seam gas (coal bed methane) associated water samples which comprised of significant bicarbonate concentrations. The hypothesis was that use of aluminium exchanged resins for water softening would require only one regenerant stage, thus avoiding health and safety issues associated with strong acids and alkali on a coal seam gas water treatment site. Equilibrium exchange tests revealed that the selectivity of the aluminium exchanged resin was in the order Ba > Sr > Ca > Mg, albeit the exchange was unfavourable unless a stoichiometric excess of bicarbonate ions was present in solution. Equilibrium studies of multi-component solutions of alkaline earth ions indicated that at low loadings the alkaline earth ions co-sorbed on the resin surface, but the more preferred ion displaced ions of lesser affinity as monolayer exchange was approached. The presence of sodium ions not only reduced alkaline earth ion loading when in concentrations relevant to coal seam gas associated water application but also appeared to be incorporated into flocs in the form of salts. Column studies revealed significant loading of alkaline earth ions (0.787 eq/kg resin) but regeneration with AlCl3 (aq) did not recover all these species (0.67 eq/kg resin). Partial restriction of the flow was noted during column tests due to floc formation. A subsequent loading/regeneration cycle resulted in further diminishment in alkaline earth ion uptake and an inability to achieve low effluent concentrations of alkaline earth ions.

Treatment of recirculate cooling water using electrochemical method has obtained great attentions in recent years due to its environmental friendliness and easy operation. In order to ensure persistent hardness removal efficiency, periodical cleaning of cathode surface is essential. However, currently available electrode cleaning techniques are either inefficient or laborious. In this work, air-scoured washing was proposed to replace conventional scale detachment methods in the electrochemical water softening process. Mirror stainless steel was found to be the most suitable cathode material because it could enhance scale detachment significantly. Effective water softening and scale detachment were achieved. The precipitation rate was as high as 25.5–34.3 g/h/m 2 . After the air-scoured washing scale detachment, the cathode could resume their ability to soften. The energy consumption and the total removal efficiency were 17.6–22.3 kWh/kg CaCO 3 and 12.2–15.2%, respectively. Repetitive experimental results indicated the electrochemical water softening process using air-scoured washing could run steadily without any performance decay detected after repetitive operation.

The effect of post-treatment by acid-catalyzed hydrolysis of a commercial NE70 semi-aromatic polyamide (PA) membrane was systemically investigated to determine feasibility of use in water softening and antibiotic enrichment applications. The surface of a post-treated PA membrane was characterized using various analytical tools: SEM (Scanning Electron Microscopy) for surface morphology, ATR-FTIR (Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy) for chemical bonds, contact angle for hydrophilicity of membrane surface, and electrophoretic light scattering spectrophotometer for surface charge of membrane surface. Conversion of amide groups to amine and carboxyl groups by post-treatment increased hydrophilicity and absolute value of surface charge as well as pore size and molecular weight cut off (MWCO) value. Post-treated membrane under optimal condition showed enhancement of water flux (∼10%) as well as ∼20% ideal selectivity (Na + /Mg 2+ ) for water softening using a single electrolyte solution. In addition, mixture selectivity (Na + /Mg 2+ ) using a mixture solution at pH 3 was also improved ∼2.6 times. Post-treated membranes at pH 0.25 for 7 and 14 days as optimization points were also applied to enrichment of antibiotics which are erythromycin (ERY) and vancomycin (Van). Optimized post-treatment membranes showed higher water flux and lower NaCl rejection as well as competitive rejection of antibiotics when compared to virgin NE70 membrane or other commercial/fabricated membranes. The approach to post-treatment of semi-aromatic membrane by the acid-catalyzed hydrolysis method can be utilized as a multipurpose usage in the future depending on characteristics of the target compound (e.g. surface charge (positive/negative) or size diversity).

The instability and low permeability of graphene oxide (GO) membranes in water treatment limit their wide application. In this study, we described the fabrication and testing of GO/MXene composite membranes modified by polyethylenimine (PEI). The unique structure of the composite membranes exhibited a synergistic effect in terms of water permeability and metal cation rejection, which varied with the mass ratio of GO and MXene. The composite membranes with a GO/MXene mass ratio of 1: 4 exhibited a higher water permeability (9.5 ± 0.4 L m−2 h−1 bar−1). The PEI modified composite membrane had a rejection of over 70% for Ca2+ and Mg2+, which was about 7 times higher than that of the GO membranes. The removal mechanism of the PEI modified GO/MXene composite membrane on metal ions mainly involved steric effect and electrostatic effect. This kind of modified membrane has great potential in water softening.

The development of the positively charged nanofiltration (NF) membrane is of great importance for water softening. The relative low water permeance restricts its widespread application. Up to now, numerous efforts on the interlayer technologies have been implemented to manipulate the interfacial polymerization (IP) process of the negatively charged NF membrane. However, there are few reports on the interlayer-mediated positively charged NF membrane. Herein, an interlayer-mediated positively charged NF membrane is fabricated via an IP process between polyethyleneimine (PEI) and trimesoyl chloride. A facile interlayer is deposited based on the interaction of a macrocycle polyphenol of Noria and PEI. The fabricated NF membrane possesses a unique surface morphology, less intrusion of the polyamide layer, a low lateral mass transfer resistance and adequate separation layer thickness. Consequently, the interlayer-mediated NF membrane exhibits a high water permeance of 110.4 L·m−2·h−1·MPa−1, which is ~2.6 times over the control group of 41.6 L·m−2·h−1·MPa−1, while preserves a considerable rejection of 95.4% towards MgCl2 solution. More importantly, the water permeance is enhanced not at the expense of membrane thickness, which is conducive to the long-term running stability. This study paves a new way to fabricate a high performance positively charged NF membrane through an interlayer manipulation strategy.

Hard water and elevated ammonia are problems for many United States groundwater drinking water utilities, and some utilities, particularly those in the Midwest, face both challenges. Ion (cation) exchange (IX) is a common treatment technique for hardness reduction (i.e., softening) and may be used to remove ammonia as well, but these constituents may compete in IX and impact overall treatment performance. Few data have been reported on the impact on ammonia concentrations when using IX for softening in full-scale systems. This study investigated four full-scale groundwater treatment plants in Illinois that practice IX for softening (raw water hardness > 220 mg/L as CaCO3) and have elevated groundwater ammonia concentrations (> 2 mg N/L). Sampling throughout the year revealed consistent finished water hardness levels but variable ammonia concentrations. Ammonia removal varied and depended on how much water had been treated since the last regeneration. High ammonia removal (sometimes > 90%) occurred in the first half of the IX service cycle, while effluent ammonia concentrations increased compared to the influent (sometimes > 200%) towards the end of the IX cycle (total length 50,000-92,000 gallons [190-350 m3]). Ammonia removal efficiency varied among the plants, but the overall trends were similar. Because variable ammonia concentrations may make it difficult to produce a consistent total chlorine residual, they can negatively impact disinfection and water quality in the distribution system. Ammonia concentrations should be considered when designing softening systems to determine regeneration frequency, develop blending strategies, or include an alternative ammonia treatment process before IX softening to produce a more stable and consistent finished water.

Water softening residuals disposal is a worldwide issue due to the lack of effective reuse alternatives. The current principal disposal methods of landfilling and land application are quite costly due to the dewatering and transportation costs involved, and these operations can also cause potential environmental harm from leaching of the additives used in the treatment process. This research is aimed at the use of water softening residuals in the production of biodegradable road deicers that would be beneficial in replacing the highly corrosive and environmentally harmful chloride salts that are currently used for road deicing. Experimental data developed show that calcium magnesium acetate (CMA) and calcium magnesium propionate (CMP) deicers produced using water plant sludges are effective in deicing applications. A mathematical model is developed for predicting freezing point depression of CMA and CMP deicers as a function of molal concentration. The model predictions are found to match well with the experimental data, providing confidence in the use of this model for the effective design water softening sludge based deicers. The information developed herein provides options for the sustainable management of softening residuals and the concommitant mitigation of environmental harm associated with road deicing operations.

Warm lime softening is commonly used to reduce hardness, silica, and a small fraction of organic matter from steam-assisted gravity drainage (SAGD) produced water through the addition of lime, soda ash, MgO, coagulant and flocculant. We report a systematic study on the impact of solution chemistry on the epichlorohydrin-dimethylamine coagulant demand for the treatment of synthetic SAGD produced water. Concentrations of magnesium, calcium, sodium bicarbonate, clay (mimicking suspended solids), sodium metasilicate (representing silica), and humic acid (mimicking dissolved organic matter) were varied to study their impact on coagulant demand. The impact of the concentration of lime, soda ash, and MgO on coagulant demand was also studied. Within the studied concentration range, the coagulant dose increased linearly with increasing concentration of humic acid (Ycoagulant = 29 + 0.703XHA) and silica (Ycoagulant = 52 + 0.537Xsilica), and increased slightly with increasing concentration of lime and soda ash, but remained almost unchanged with increasing concentration of dissolved hardness, clay, or MgO. The observations were correlated to the understanding of the electrokinetic properties of CaCO3 and Mg(OH)2 particles in lime softening. The findings provide insights for evaluating onsite coagulant dose and optimizing the process.

Chemical crystallization circulating pellet fluidized bed reactor (CPFBR) systems are widely used in water softening applications, and softening parameters and effects may be governed by certain rules for the optimum treatment of water at different levels of hardness. In this study, pilot tests were conducted on a CPFBR system with water at eight different levels of hardness. Specifically, the effects of the differences in hardness, alkalinity, and calcium and magnesium concentrations on the softening rules and regulation of the CPFBR were analyzed. The results showed that when Na2CO3 and NaOH were added simultaneously, the removal rate of Ca2+ was positively correlated with the dosage of Na2CO3, while pH was positively correlated with the dosage of NaOH. Under the experimental water quality conditions, when Ca2+

A novel three-chambered microbial desalination cell (MDC) was designed for evaluating desalination of synthetic ground water with simultaneous energy generation and resource recovery. The specific design enabled efficient interelectrode communication by reducing the distance of separation and also maintained an appropriate surface area to volume ratio. MDC were evaluated in different circuitry modes (open and closed) to assess the desalination efficiency, bioelectricity generation, resource recovery, substrate utilization and bioelectrokinetics. The closed circuit operation has showed efficient desalination efficiency (51.5%) and substrate utilization (70%). Owing to the effective electron transfer kinetics, closed circuit mode of operation showed effective desalination of the synthetic ground water with simultaneous power production (0.35 W/m 2 ). Circuitry specific biocatalyst activity was observed with higher peak currents (10.1 mA; −5.98 mA) in closed circuit mode. MDC can function as sustainable and alternative solution for ground and surface water treatment with power productivity and resource recovery.

In this study, kaolin-based geopolymer was synthesized using a two-step method that consists of a fusion step (fusion of kaolin and sodium hydroxide), and a hydration and dealkalization step. The fusion step was performed at a temperature range of 400–800 °C for 10 to 20 h, and the hydration and dealkalization step was carried out by washing the fusion product with demineralized water for a short time ( 2 + and Mg 2 + from both model media and groundwater. Results showed that sodium hydroxide to kaolin ratio, fusion time and temperature have profound effects on the performance of geopolymer in reduction of water hardness. Response Surface Methodology (RSM) was used to determine the optimal geopolymer synthesis conditions. RSM results indicated that the sodium hydroxide to kaolin ratio of > 2.2, the fusion time of > 14 h and the fusion temperature range of 500–700 °C provide the optimal synthesis conditions. Moreover, the synthesized geopolymer can efficiently adsorb Ca 2 + and Mg 2 + from both model media and groundwater. Based on the Langmuir isotherm model, the maximum Ca 2 + adsorption capacity was 76.34 mg/g at 25 °C, increasing to 94.34 mg/g at 45 °C. Similarly, the maximum Mg 2 + adsorption capacity increased from 39.68 mg/g at 25 °C to 51.55 mg/g at 45 °C. The adsorption experimental data at the above mentioned temperatures (i.e., 25 and 45 °C) fitted well with the pseudo-second-order model. Finally, efficient regeneration of the saturated geopolymer, using sodium chloride solution, suggested that the dominating mechanism of water softening by the synthesized geopolymer is ion exchange.

This review centers on the reverse osmosis (RO) and nanofiltration (NF) membranes for water treatment and desalination. Specifically the RO and NF membranes, whose rejection layer is being formed using semi-aromatic polyamide in various configuration and structure, have found prominent space in membrane arena. An interfacial polymerization (IP) is a distinctive technique for producing in - situ polyamide membrane in the form of films or composites using wide variety of amines and acid chloride monomers of various compositions. In this context, thin film composite membranes prepared by IP techniques are reviewed with their history in the present review. Emphasis is given on systematic development of membranes with its structural composition and separation characteristics. The functions of various additives and effect of IP reaction parameters on performance of membranes have been briefly discussed. Types of composite polyamide (PA) membranes prepared from comonomers, their derivatives and oligomers are reviewed. Commercial PA membranes are also discussed in details with their niches in the separation of monovalent and divalent ions and organics from water. Poly(piperazine amide) and polyethyleneimine based PA membranes show promising applications for water softening (hardness removal) in the form of flat sheet and hollow fiber configuration. This review may guide the researchers and separation scientists to develop novel approach for further development and modification of membranes for removal the toxic and harmful ions from water.

In this work, we report an experimental investigation on the synthesis of zeolites A and P, using kaolin as the main SiO 2 and Al 2 O 3 sources, by hydrothermal route, as well as on the structural, spectroscopic and magnetic properties of the produced zeolite composites with magnetite nanoparticles. The zeolites were successfully synthesized and the characterization of the obtained materials was carried out considering different techniques, including X-ray diffraction (XRD), infrared vibrational spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), as well as measurements of the magnetization as a function of the temperature and applied magnetic field. XRD analysis confirmed zeolites A and P as main crystalline phases, with a low intensity peak regarding the presence of small quantities of quartz remaining from the unreacted clay. SEM showed the well-defined morphologies of zeolites and, in the composites, magnetite nanoparticles dispersed over their surface. The average size of the magnetite nanoparticles was ca . 50 nm as determined by TEM analyses. The magnetic characterization confirmed the ferrimagnetic behavior of the magnetite nanoparticles and of the composites, as well as verified that the magnetic properties of the nanoparticles are not affected by the zeolites in the composite formation. Thus, the results evidenced that high quality zeolite composites with magnetite nanoparticles can be reached by considering the employed low cost method, placing this route as an attractive alternative for water softening reaching removal levels of about 97% in the first application times.

Good mechanical strength, high water flux and salt rejection, and highly chlorine-tolerant property are essential for nanofiltration (NF) membrane. To achieve these properties, polyethersulfone (PES) NF thin-film composite (TFC) membrane was prepared via interfacial polymerization using piperazine (PIP) and 2,2′-bis(1-hydroxyl-1-trifluoromethyl-2,2,2-triflutoethyl)-4,4′-methylenedianiline (BHTTM) as aqueous monomers, trimesoyl chloride (TMC) as organic monomers, and PES three-channel capillary ultrafiltration (UF) membrane as substrate. The prepared NF membrane displays pure water flux of 36.1 L m −2 h −1 and Na 2 SO 4 rejection of 96.8% at 4 bar. NaClO solution was used to treat NF membranes for the evaluation of NF membrane chlorine-tolerant property. The effects of NaClO concentration, contact time and operating pressure on the chlorine-tolerant property of NF membrane have been thoroughly investigated. Particularly, the durability of NF membrane versus Cl was tested though long-term test. The pure water flux and Na 2 SO 4 rejection of NF membrane after 5000 ppm-h NaClO immersion was 43.4 L m −2 h −1 and 99.8% at 4 bar, respectively. The fabricated NF membrane could tolerant 13000 ppm-h Cl under 4 bar, and the water flux and Na 2 SO 4 rejection are maintain at 51.6 L m −2 h −1 and 99.6% during NF running process, respectively. Moreover, NF membrane exhibits 3.84×10 5 ppm-h chlorine-tolerant at 0 bar. These results indicated that the fabricated PES three-channel capillary NF membrane could be applied to practical process of desalination or water softening.

Recently number of the micro power grid stations in India having capacity of 15 to 20 MW accepted the route of blending rice husk with coal to mitigate the rising cost of coal along with its depletion factor. India being the second largest producer o f rice paddy, rice husk has gained substantial consideration as a potential origin of vitality. The residual ash remains after combu stion refers as composite ash contain silica and alumina which promote the major respiratory diseases. Further, molecular sieve 4A (MS 4A) signify three dimensional network of silicon and aluminum atoms bridged by oxygen linkages. Pore opening of uniform dimensions with high sorption capacity proves the utility of this material in water softening, adsorption, membranes, petrochemical refining process etc.The current paper deals with the experimental attempts for the extraction of sodium silicate using composite ash. Further, it is enriched with sodium aluminate to design MS 4A in a rapid span without inclusion of templates. The investigation studies including XRD, SEM, PSA and IR reveals highly crystalline morphology in the sample. The research work carried out not only suggesting an eco-friendly way to generate energy but also provides a concomitant solution for disposal of waste ash to advanced material.

Recently, a novel class of low-pressure nanofiltration (NF) hollow fiber membranes, particularly suited for water softening and desalination pretreatment have been fabricated in-house using layer-by-layer (LbL) deposition with chemical crosslinking. These membranes can operate at exceedingly low pressures (2 bar), while maintaining relatively high rejections of multivalent ions. In spite of their great potential, our understanding as to what makes them superior has been limited, demanding further investigation before any large-scale implementation can be realized. In this study, the Donnan-Steric Pore Model with dielectric exclusion (DSPM-DE) is applied for the first time to these membranes to describe the membrane separation performance, and to explain the observed rejection trends, including negative rejection, and their underlying multi-ionic interactions. Experiments were conducted on a spectrum of feed chemistries, ranging from uncharged solutes to single salts, salt mixtures, and artificial seawater to characterize the membrane and accurately predict its performance. Modeling results were validated with experiments, and then used to elucidate the working principles that underlie the low-pressure softening process. An approach based on sensitivity analysis shows that the membrane pore dielectric constant, followed by the pore size, are primarily responsible for the selectively high rejections of the NF membranes to multivalent ions. Surprisingly, the softening process is found to be less sensitive to changes in membrane charge density. Our findings demonstrate that the unique ability of these membranes to exclusively separate multivalent ions from the solution, while allowing monovalent ions to permeate, is key to making this low-pressure softening process realizable., Massachusetts Institute of Technology (Pappalardo Fellowship), Singapore. National Research Foundation (its Campus for Research Excellence and Technological Enterprise (CREATE) programme), Singapore-MIT Alliance for Research and Technology (SMART) (015824-00194)

Fluidized bed and induced crystallization technology were combined to design a new type of induced crystallization fluidized bed reactor. The added particulate matter served as crystal nucleus to induce crystallization so that the insoluble material, which was in a saturated state, could precipitate on its surface. In this study, by filling the fluidized bed with quartz sand and by adjusting water pH, precipitation of calcium carbonate was induced on the surface of quartz sand, and the removal of water hardness was achieved. With a reactor influent flow of 60 L/hr, a fixed-bed height of 0.5 m, pH value of 9.5, quartz sand nuclear diameter of 0.2–0.4 mm, and a reflux ratio of 60%, the effluent concentration of calcium hardness was reduced to 60 mg/L and 86.6% removal efficiency was achieved. The resulting effluent reached the quality standard set for circulating cooling water. Majority of the material on the surface of quartz sand was calculated to be calcium carbonate based on energy spectrum analysis and moisture content was around 15.994%. With the low moisture content, dewatering treatment is no longer required and this results to cost savings on total water treatment process.

A new approach of highly fluxes thin film nanocomposite (TFN) nanofiltration (NF) membranes is reported. The fabricated module was made by incorporation of commercial-Al2O3 (CO.TFN) and camphor-Al2O3NPs (CA.TFN) into polyamide layers throughout the interfacial polymerization method. A simple biological reduction technique was adopted in preparation of camphor-Al2O3 NPs by using CinnamomumCamphora (CC) leaf extract. The crystallography of the commercial and camphor-Al2O3 NPs was examined by XRD and FTIR analyses. The CO.TFN and CA.TFN membranes were characterized by determining their surface roughness, pore size, porosity, zeta potential and contact angle parameters. The morphology and the cross-sectional of the NF membranes were studied by atomic force microscope (AFM) and scanning electron microscope (SEM). NF performance was investigated at various Al2O3 NPs loads, applied pressure, and time. The results, of the membranes fabricated at low cost, showed the high permeable flux and elimination of multivalent cations (Mg2+, Ca2+, and water softening). Incorporating 0.98 mM of camphor-Al2O3 NPs into the TFC membrane increased the water flux up to 4 times compared to only 1.5 times for commercial-Al2O3 NPs. Moreover, the salt rejection of CO.TFN and CA.TFN NF membranes increased to 95.1% and 96.5%, respectively for the feed solution (2 g/L Na2SO4 at 25 °C). The optimized NF membrane module of 0.98 mM camphor-Al2O3-NPs (CA.TFN) shows the maximum water flux 69.0,62.2, 60.5 and 55.4 L/m2.h for the feed solutions of following salts NaCl, Na2SO4, MgCl2 and MgSO4 with high salt rejections 92.4%, 96.5%, 91.7% and 95.3%, respectively. This proves that camphor-Al2O3 NPs have a significant role in increasing the membrane hydrophilicity. Hence, the CA.TFN membrane module proved to be a promising candidate for the real brackish water desalination as that collected from Marsa Alam, Egypt.

In this study, calcium carbonate nanoparticles were proposed as a potential material for fluoride ion removal from aqueous solution. For this purpose, a new method based on the colloidal gas aphrons was applied for the preparation of the calcium carbonate nanoparticles. The synthesized nanoparticles were characterized by SEM, XRD, and FTIR analyses. Factors affecting the sedimentary reaction including the initial concentration of fluoride ion, pH, time, and temperature were individually investigated. The morphology of the successfully synthesized nanoparticles was quasi-spherical vaterite structure without important aggregation. The maximum fluoride ion removal capacity was obtained about 460 mg/g at 298 K. Also, the effect of temperature displayed that temperature change has a significant effect on the removal rate. The removal rate was highly dependent on the pH and by decreasing the pH, the removal rate decreased. Kinetic results indicated that the removal kinetics is in perfect agreement with the first-order model. The experiments showed that although the synthesized sample has a better performance than the industrial one, the purity and the low cost of calcium carbonate particles produced in the water softening process make them a great candidate for the removal of fluoride from aqueous solutions.

Electrochemical water softening (EWS) technology is considered to be a very promising descaling method due to its superiorities in clean, efficient and water-saving ability, although limited by the cleaning of cathode scale. Herein, an innovative EWS device coupled with an automatic descaling module was manufactured and applied to treat the industrial recirculating cooling water (RCW). Response surface methodology (RSM) analysis based on Box–Behnken design (BBD) was applied to optimize the combination effect of four important operation parameters, including initial hardness (1150 mg/L, as Ca2+), ratio of hardness and bicarbonate (0.5), current density (30 A/m2) and circulating water flow (20 m3/h). Next, a mathematical dynamic model was employed to reveal the relationship between the hardness removal percentage and the current density (ρ, A/m2) according to the RSM analysis results. Additionally, various configurations of the device were performed and demonstrated that the integration of automatic descaling system enabled a significant improvement on the descaling capacity (19.95 g/h/m2) and energy consumptions (27.1 kW h/kg CaCO3), which exhibited evident economic and environmental advantages compared to the chemical scale inhibitor method. This work provides a facile strategy for the development and industrial applications of EWS technology.

Highly positively charged nanofiltration (NF) membranes have been prepared via a layer-by-layer (LbL) self-assembly technique using graphene oxide (GO) and polyethyleneimine (PEI). The high aspect ratio and unique 2D structure of GO nanosheets enabled them to be easily assembled on the membrane surface, and the intrinsic low resistant channels within the GO nanosheets resulted in a high water flux of the membrane. By assembled a PEI layer on the membrane outer surface, the composite membrane exhibited high positive charge and resulted in the high rejections to multivalent ions. The effects of deposition time, PEI and GO concentrations on separation performance of the NF membranes were detailed studied. The best performance among all the membranes was achieved with salt rejections of 93.9% and 38.1% for Mg2+ and Na+, and a water flux of 4.2 L/m2 h bar at 30 °C and 0.5 MPa. The attractive performance of these NF membranes showed a great potential in the industrial application of water softening.

Thin film composite (TFC) hollow fiber nanofiltration (NF) membranes were fabricated via interfacial polymerization, with mixed diamines of piperazine (PIP) and 2,2′-bis(1-hydroxyl-1-trifluoromethyl-2,2,2-triflutoethyl)-4,4′-methylenedianiline (BHTTM) as water phase and trimesoyl chloride (TMC) as organic phase. Fabricated membranes were characterized for ATR-FTIR, SEM, AFM, and water contact angle. Formation conditions of the selective polyamide layer have been optimized for a good NF membrane performance. The pure water flux of NF membrane prepared under optimized conditions was 31.2 L·m− 2·h− 1 and the rejection for Na2SO4 was 99.7% at 6 bar. The rejections of NF membranes for different salts followed the order: Na2SO4 > MgSO4 > NaCl > MgCl2. The molecular weight cut-off of NF membrane was around 300 Da. The performance of NF membranes was stable during a 7-day filtration process for rejecting different species ions including heavy metal ferric ion (FeCl3 and Fe2(SO4)3) and oxyanions (SO42 − and HPO42 −), in addition to different inorganic salts. Also, the NF membranes exhibited chlorine tolerant property. This work demonstrates the potential of PES outer-selective TFC hollow fiber membranes for desalination or water softening process, provides database to fabricate suitable NF membranes for long-term salts removal and raise a simple technique to form a thin chlorine-tolerant polyamide layer.

One of the most critical issues encountered by thin film composite (TFC) nanofiltration (NF) membrane in water softening is the trade-off between water flux and salt rejection. A novel thin film nanocomposite (TFN) membrane was fabricated by incorporating graphene oxide (GO) of different quantity (ranging from zero to 0.5 wt.%) into polysulfone (PSf) microporous substrate. Polyamide layer was formed on top of the neat PSf and PSf-GO substrate to produce TFC and TFN membrane, respectively through the interfacial polymerization of piperazine and trimesoyl chloride monomers. With respect to pure water flux, TFN membrane made of 0.3 wt.% GO exhibited the highest water permeability with rejections for Na2SO4, MgSO4, MgCl2 and NaCl recorded at 95.2%, 91.1%, 62.1% and 59.5%, respectively. As compared to TFC control membrane, GO incorporated TFN membranes exhibited higher pure water flux, salt solution permeability and salt rejection. This proved that GO plays an important role in increasing both membrane hydrophilicity and surface negativity. Most importantly, TFN membrane made of nanocomposite substrate served as a promising solution to overcome the trade-off effect between water flux and salt rejection of TFC membrane owing to the improved properties of substrate upon incorporation of highly hydrophilic and negatively charged GO nanosheets.

Electrochemical water treatment is an attractive technology for water desalination and softening due to its low energy consumption. Especially, capacitive Deionization (CDI) is promising as a future technology for water treatment. Graphene (rGO) has been intensively studied for CDI electrode because of its advantages such as excellent electrical conductivity and high specific surface area. However, its 2D dimensional structure with small specific capacitance, high resistance between layers and hydrophobicity degrades ion adsorption efficiency. In this work, we successfully prepared uniformly dispersed Fe3O4/rGO nanocomposite by simple thermal reactions and applied it as effective electrodes for CDI. Iron oxides play a role in uniting graphene sheets, and specific capacitance and wettability of electrodes are improved significantly; hence CDI performances are enhanced. The hardness removal of Fe3O4/rGO nanocomposite electrodes can reach 4.3 mg/g at applied voltage of 1.5 V, which is 3 times higher than that of separate rGO electrodes. Thus this material is a promising candidate for water softening technology.

Hardness ions mainly including Ca2+ and Mg2+ can cause water quality issue. Softening of hard water is still challenging nowadays. Capacitive deionization (CDI) has attracted much attention in recent years because of its high efficiency, environmental-friendliness and low energy consumption. Mordenite is a porous ion exchange material. Herein, the porous mordenite modified activated carbon (MOR-AC) was used as cathode to selectively remove Ca2+ and Mg2+ by CDI technique. At the optimal MOR doping amount, the MOR-AC electrode shows good wettability and high specific capacitance of 162 F/g. The removal amount of Ca2+ ions on the MOR-AC cathode reached 248 μmol/g, much higher than that on the AC electrode (164 μmol/g). Distinct from AC electrode, the MOR-AC electrode delivers a much enhanced selectivity to hardness ions in multi-salt mixture solution. In the synthetic hard water, the MOR-AC electrode displays a highly selective adsorption to Ca2+ and Mg2+ with the selectivity coefficient αCa2+ Na+ and αMg2+ Na+ of 8.0 and 7.5, respectively. This CDI device is very stable and regenerable, and the adsorption capacity of Ca2+ retains 87% after 50 cycles in hard water. In addition, the scaling of electrode has been efficiently inhibited.

Significant effort has been made to measure and understand the surface charge of CaCO3 and Mg(OH)2 particles. Many laboratory experiments have been reported on zeta potential of natural and prepared CaCO3 and a few have also been published for Mg(OH)2, however, there are very few reported measurements of zeta potential of CaCO3 and Mg(OH)2 particles at conditions relevant to lime softening, despite lime softening being a common and established process for water treatment. The present study aims to understand the interactions and electrokinetic properties of these two particles in lime softening. Effects of various experimental parameters such as pH, temperature, aging, inorganic carbon (CO32−/HCO3−), and divalent cations (Ca2+/Mg2+) on the electrokinetic properties (i.e. zeta potential) of CaCO3 and Mg(OH)2 particles were individually studied. The interactions between humic acid (mimicking natural organic matter), silicate (representing silica), clay (mimicking suspended solids) and CaCO3/Mg(OH)2 particles were studied, as well as the interactions between CaCO3 and Mg(OH)2. Thermodynamic modeling was used to predict precipitates as a function of solution chemistry and assist with data interpretation. The results provide considerable insight into factors that are of importance to lime softening.

An electrochemical reactor based on a moving graphite–polymer composite (GPC) cathode and boron-doped diamond anode is developed and used for the electrochemical water softening and treatment of artificial vacuum toilet wastewater. The magnetically actuated cathode is designed for the in situ removal of insoluble compounds, which usually precipitate on the cathode during electrochemical water treatment processes. To obtain a suitable GPC cathode, the chemical stability and conductivity of various composites and their flexion and magnetic actuator characteristics are investigated. The most suitable GPC cathode is a 0.5 mm thick polypropylene-based composite, which presents chemical stability, the lowest resistivity of all analyzed samples (5.06 ± 1.80 mΩ cm), and enables flexions up to 2.4 mm. The water softening performance of the reactor featuring this electrode was evaluated using two configurations. Water hardness was decreased up to 72% and more than 90% in mixed and separated electrolyte modes, respectively. Further investigations demonstrate that this reactor can also be used for wastewater treatment. Artificial toilet wastewater was successfully discolored for reuse as toilet flushing water. Lastly, a treatment test over 120 h demonstrates that the magnetically actuated flexible GPC cathode removes in situ deposits on its surface and requires low maintenance. The performance of the new electrode configuration is similar to that of the state-of-the-art polarity reversal system and does not shorten the electrode service life.

High cathode area requirement limits industrial applications of electrochemical water softening technology. Based on previous works, the electrochemical water softening mechanism of high-efficient multi-layer mesh coupled cathode which consists of multilayers of stainless steel woven nets with different mesh sizes was studied systematically in this paper. Results reveal that the external and internal layers synergically enhance the performance of the coupled cathode. Calcium carbonate deposition reaction preferentially occurs on the external layers of the coupled cathode, due to the shielding effect caused large differences in current density and potential among the layers of the coupled cathode. Meanwhile, hydrogen bubbles can not only serve as soft templates, allowing nucleation sites to be transferred from the substrate surface to the formed calcium carbonate crystals, but also easily bring scale films with low coverage and adhesion force away from the substrate surface by mechanical stripping, prolonging the cathode deactivation time and making the cathode self-cleaning. Besides, the internal layers provide an alkaline environment through the stable hydrogen evolution process, further accelerating the deposition of hardness ions and reducing the cathode area required for water softening.

Here, a strategy to control surface negative charge, pore size and active layer thickness by manipulating the polyamide (PA) layer structure is proposed to fabricate an advanced water softening nanofiltration (NF) membrane. Based on this strategy, a dual-functional acyl dichloride monomer containing a protected amino group, 5-sulfinyl amino isophthaloyl dichloride (NSO), is designed as organic phase alternative. Because the protected amine group of NSO has higher hydrolytic activity and is prone to de-protect and form free amino group, it has important impact on the interfacial polymerization (IP) process and formed polyamide (PA) structure. To elucidate the role of the protected amino group in regulating PA structure, the desalination performances and structural characteristics of the three PA membranes prepared by NSO monomer and its structurally similar monomers (isophthalyl chloride (IPC) and trimesoyl chloride (TMC)) are systematically investigated. Compared with TMC-based NF membrane, the NSO-based membrane shows a smoother and lower carboxyl group density surface, decreased pore size and a thinner PA rejection layer (~30 nm). As a result, the novel NF membrane exhibits outstanding water softening performance in terms of ultrahigh divalent salts (Na2SO4, MgSO4, MgCl2 and CaCl2) rejections (above 98%) and preferable high water flux (138.25 ± 6.32 LMH), together with superior antifouling property. More importantly, its water softening capability remain stable even under the mixed salt solution including abundant SO42−, which far outperforms the typical poly(piperazine-amide)-based and state-of-the-art commercial NF membranes. This work further establishes the structure-property relationship and enables the rational design of high performance of NF membrane at the molecular level.

This review not only provided a summary of the state of the art with regards to commercially important zeolite LTA technology, but also identified strategies to further promote uptake of this material in industry. The hypothesis was that application of SWOT analysis principles would assimilate data regarding zeolite LTA in a manner which provided a concise and clear pathway for future development. Zeolite LTA synthesis innovation related to preparation of nano-zeolites, hierarchical materials, binderless zeolite granules, magnetized zeolites, loading of metals/metal oxides into the pore structure and introduction of heteroatoms. Waste resources included used glass, alum sludge from drinking water plants, fly ash from power stations, recycled aluminium, manufacturing waste, ash from organic material and various aluminosilicate rocks from the mining sector. In addition to traditional application of zeolite LTA for water softening opportunities; use of zeolite LTA for catalytic processes, membrane separation, alternate zeolite production, wastewater treatment, and anti-microbial purposes have been described. A key weakness identified was the lack of compelling economic models in relation to synthesis of zeolite LTA from waste resources or by new methods which produce hierarchical materials. Linked to the lack of economic studies was the limited information about process design of zeolite LTA manufacturing systems or industrial application of zeolite LTA. Key strategies developed included: (1) scaling-up of new synthesis methods to kg levels and beyond; (2) creation of process flow diagrams and piping and instrumentation diagrams; (3) detailed economic analysis of zeolite LTA synthesis from waste materials; (4) Utilization of methods to reduce the cost of zeolite manufacture such as ultrasonics, microwaves, “one pot” synthesis, continuous production and rapid synthesis; and (5) demonstration of zeolite LTA performance at pilot plant scale for the various applications identified. In conclusion, zeolite LTA has an expanded role to play in the area of zeolite science and this study has provided a roadmap to accelerate this development.