Sodium Hydroxide Production Research Articles

Sustainable Desalination Waste Brine Management through Electrochemically Supported CO2 mineralization and Multiple Product Recovery

Seawater reverse osmosis desalination (SWRO) is a vital technology for low cost and energy requirement water production to ensure regional water security and continued economic development. However, the state-of-the-art technology typically exhibits a recovery rate of around 42% converting in 58% of the seawater input into desalination waste brine which are largely discharged into the sea1. Significant works on brine utilization have shown that SWRO desalination brine can be converted into usable feed for chlor-alkali process with simultaneous carbon capture capabilities2,3,4. In this work, the desalination effect of electrochemistry for simultaneous utilization and treatment of desalination brine (56.0 ± 2.1 g/L) into seawater level (32.0 ± 1.4 g/L) was explored. The results demonstrate chemical self-sufficiency for desalination brine treatment (removing Mg2+ and Ca2+ to below ppm level), electrochemically recovering chlorine (95% – 99% selectivity, and hydrogen gas (92% – 97% selectivity), and conversion of desalination brine into reclaimed seawater discharge (RSD). The RSD is further chemically dechlorinated and neutralised to pH 7.3 to be safe to discharge into the sea. The excess alkali brine is used to capture additional CO2 in the form of bicarbonates achieving net abatement in climate change impact (9.90 CO2 e/m3) after product carbon abatements are accounted. 1 Jones, E.; Qadir, M.; van Vliet, M. T. H.; Smakhtin, V.; Kang, S. The State of Desalination and Brine Production: A Global Outlook. Science of The Total Environment 2019, 657, 1343–1356. 2 Melián-Martel, N.; Sadhwani, J. J.; Ovidio Pérez Báez, S. Saline Waste Disposal Reuse for Desalination Plants for the Chlor-Alkali Industry: The Particular Case of Pozo Izquierdo SWRO Desalination Plant. Desalination 2011, 281, 35–41. 3 Du, F.; Warsinger, D. M.; Urmi, T. I.; Thiel, G. P.; Kumar, A.; Lienhard V, J. H. Sodium Hydroxide Production from Seawater Desalination Brine: Process Design and Energy Efficiency. Sci. Technol. 2018, 52 (10), 5949–5958. 4 Choi, W. Y.; Aravena, C.; Park, J.; Kang, D.; Yoo, Y. Performance Prediction and Evaluation of CO2 Utilization with Conjoined Electrolysis and Carbonation Using Desalinated Rejected Seawater Brine. Desalination 2021, 509, 115068. Figure 1

Seawater alkalization via an energy-efficient electrochemical process for CO2 capture

Electrochemical pH-swing strategies offer a promising avenue for cost-effective and energy-efficient carbon dioxide (CO2) capture, surpassing the traditional thermally activated processes and humidity-sensitive techniques. The concept of elevating seawater's alkalinity for scalable CO2 capture without introducing additional chemical as reactant is particularly intriguing due to its minimal environmental impact. However, current commercial plants like chlor-alkali process or water electrolysis demand high thermodynamic voltages of 2.2 V and 1.23 V, respectively, for the production of sodium hydroxide (NaOH) from seawater. These high voltages are attributed to the asymmetric electrochemical reactions, where two completely different reactions take place at the anode and cathode. Here, we developed a symmetric electrochemical system for seawater alkalization based on a highly reversible and identical reaction taking place at the anode and cathode. We utilize hydrogen evolution reaction at the cathode, where the generated hydrogen is looped to the anode for hydrogen oxidation reaction. Theoretical calculations indicate an impressively low energy requirement ranging from 0.07 to 0.53 kWh/kg NaOH for established pH differences of 1.7 to 13.4. Experimentally, we achieved the alkalization with an energy consumption of 0.63 kWh/kg NaOH, which is only 38% of the theoretical energy requirements of the chlor-alkali process (1.64 kWh/kg NaOH). Further tests demonstrated the system's potential of enduring high current densities (~20 mA/cm2) and operating stability over an extended period (>110 h), showing its potential for future applications. Notably, the CO2 adsorption tests performed with alkalized seawater exhibited remarkably improved CO2 capture dictated by the production of hydroxide compared to the pristine seawater.

Synthesis of a novel one-part alkali-activated material from solid wastes salt sludge and soda residue

In this paper, salt sludge (SS), a solid waste from sodium hydroxide production, was activated at high temperature for the first time. It was combined with soda residue (SR), a solid waste from sodium carbonate production, as a solid activator to activate the solid aluminosilicate precursor, blast furnace slag (BFS). This process developed a novel one-part alkali-activated material, termed salt sludge and soda residue co-activated blast furnace slag cementitious material (SRB), which effectively utilizes solid wastes. The effects of ratios of calcined SS (CSS) to SR, fine aggregate types, and curing methods on the physical and mechanical properties of SRB were investigated. The composition of the hydration products was characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and fourier transform infrared spectroscopy (FTIR), while the hydration process during the setting stage was examined with 1H low-field nuclear magnetic resonance (1H LF NMR). The influence of mix ratio, fine aggregate type, and curing method on the microstructure of SRB was analyzed using scanning electron microscopy (SEM). The results showed that the synergistic activation of BFS by CSS and SR was significantly more effective than individual activation. At the optimal mix ratio (CSS:SR:BFS = 15:15:70), the 3/28 days compressive strength of SRB was 14.3/53.7 MPa. The primary hydration products were C-(A)-S-H gels, along with Friedel's salt, ettringite, and hydrotalcite crystals. Using manufactured sand (MS) as a fine aggregate reduced mortar fluidity compared to river sand (RS) but enhanced the interfacial transition zone (ITZ) with the hardened paste, resulting in a denser microstructure. Heat curing promoted hydration product generation, increasing early (3 days) strength by a factor of 2.5. However, the rapid accumulation and insufficient diffusion of hydration products adversely affected the later strength.

An investigation of the mechanical strength, environmental impact and economic analysis of nano alkali-activated composite

The current study explores both the mechanical strength and life cycle assessments of fly ash (FA)-based alkali-activated concrete (AAC) with/without colloidal nano-silica (NS) addition at high and low molar concentrations of sodium hydroxide in the activator fluid (12 M and 3 M). The cradle-to-gate approach is used to establish the system boundaries for life cycle assessments (LCA). The environmental impact of AAC (with/without NS) and conventional cement concrete (CC) mixtures was determined using the ReCiPe midpoint and endpoint approaches across various impact categories. The AAC mixes with and without NS addition reduce greenhouse gas (GHG) emissions by 30 % and 25 % with respect to CC mix, respectively. Within the majority of impact categories, sodium silicate and sodium hydroxide production are the primary contributors, accounting for up to 91 %. However, in water depletion, global warming potential, and fuel depletion, the overall contribution is as high as 65 %. Also, the AAC mix with and without NS addition consumes 34 % and 14 % less electricity during its production with respect to the CC mixture. The CC mixes have almost 70 % more ecological damage and approximately 30 % more health risk than AAC with and without NS. However, the NS-modified AAC shows 20 % lower ecological and health risks than AAC without NS. The effect of resource scarcity on NS modified AAC is 50 % less than that of other mixes, including both AAC mix without NS and CC mix. In comparison to AAC without NS, the NS modified AAC demonstrates an 18 % reduced impact on resource scarcity. Finally, cost analysis demonstrates that AAC with NS is 16 % less expensive than CC, while AAC without NS is 26 % cheaper than CC. AAC containing 6 % NS inclusions exhibits environmental and economic benefits in addition to its enhanced mechanical performance, indicating its potential for widespread practical usage.

Addressing Freshwater Scarcity and Hydrogen Production: Offshore Wind and Reverse Osmosis Synergies

AbstractThe transition from fossil fuels to renewable energy sources is imperative to mitigate climate change and achieve sustainable development goals (SGDs). Hydrogen, as a clean energy carrier, holds great potential for decarbonizing various sectors, yet its production remains predominantly reliant on fossil fuels. This study explores a novel approach to sustainable hydrogen production by integrating offshore wind energy with reverse osmosis (RO) desalination technology. The proposed configuration harnesses offshore wind power to energize both a RO desalination system and water electrolysis unit. Initially, the wind energy powers the RO desalination process, purifying seawater, and then desalinated water is directed to water electrolysis system for generating green hydrogen directly from seawater. The resulting renewable hydrogen holds potential for diverse applications, including marine industries, and can be transported onshore as needed. The RO system is configured to treat 20 kg s−1 of seawater with a salinity of 35 000 ppm, aiming for a high recovery ratio and reduced freshwater salinity. A pressure exchanger (PX) is integrated to recover energy from high‐pressure brine stream and transfer it to the low‐pressure feed water, thus reducing the overall energy consumption of the RO process. The concentrated brine extracted from RO desalination is proposed to be utilized for the production of sodium hydroxide that can further pretreat incoming seawater and enhance the effectiveness of the filtration process by mitigating membrane fouling. This pressure exchanger increases the energy efficiency of the RO system from 63.1% to 64.0% and exergetic efficiency from 13.9% to 18.2% increasing the overall first and second law efficiencies to 37.9% and 33.5%. By leveraging offshore wind power to drive RO desalination systems, this research not only addresses freshwater scarcity but also facilitates green hydrogen generation, contributing to the advancement of renewable energy solutions and fostering environmental sustainability.

Optimizing operational parameters for highly-concentrated sodium hydroxide production via electrodialysis with bipolar membranes from concentrated brines

Sodium hydroxide necessitates the implementation of cleaner production methods that deviate from the conventional practices in the chlor-alkali industry. New sustainable sources such as concentrated brines, which are typically regarded as waste, are currently being investigated for the production of sodium hydroxide using Electrodialysis with Bipolar Membranes (EDBM). This process has the advantages of not producing gas by-products, reducing operational risks, and providing value to waste brines after converting them into acids or alkali solutions. However, the production of sodium hydroxide via EDBM using brines is limited by the low concentration of the final base (e.g. typically below 2 M), which increases the cost of transport. This study aims to overcome this limitation by optimizing parameters such as salt-to-base volume ratio, initial salt concentration, and current density values. Notably, this research is one of the few to employ Response Surface Methodology with a Central Composite Design to maximize the final sodium hydroxide concentration; two models using second-order polynomial equations were proposed. Results showed the possibility to achieve 5.3 M NaOH under a volume ratio range of 9.8:1–10:1, a current density range of 490 A/m−2 to 500 A/m−2, and an initial salt concentration in the range of 6 M NaCl. Moreover, the energetic analysis showed values of specific energy consumption in a range of 2.5 kWh kgNaOH-1-3.5 kWh kgNaOH-1 and specific productivity of the process up to 3.08 tonNaOH year−1 m−2, which are moderate in comparison with values previously presented in literature. Finally, an economic analysis showed that energy was the most significant cost component, making up 89 % of the total process cost, which was estimated to be 0.69 € kgNaOH-1 (concentration 21.2 % w/w). Overall, these high concentration values open a real chance to shift towards sustainable production of NaOH by means of EDBM from brines, highlighting the novelty of this study.

Life cycle assessment of textile fibre-to-fibre recycling by cellulose carbamate technology

The fashion industry faces major challenges in reducing its environmental impacts along the textile value chain, from fibre production, via various processing steps, use phase and to the end-of-life stage. A major challenge is how to shift from the current linear industry to a circular one, where textiles are both sustainably produced, and after the full life length, recycled into new fibres with high value applications. The aim of this study was to evaluate the environmental impacts of post-consumer textile fibre-to-fibre recycling by cellulose carbamate technology, in terms of climate impact, water scarcity impact, cumulative energy demand and land use impact. By performing life cycle assessment, it was shown that the chemically recycled cellulose carbamate fibre has a climate impact of about 2.2 kg CO2-eq per kg fibre, water scarcity impact of 1.6 m3 H2O-eq per kg fibre, cumulative energy demand of 90 MJ-eq per kg fibre and land use impact of about 92 Pt per kg fibre (when applying mass allocation of co-products). Hotspots identified during the fibre production technology were electricity use and production of sodium hydroxide. In a sensitivity analysis, it was shown that the choice of electricity has a major influence on the results, and by using a renewable electricity mix over an average Finnish electricity mix, the impact could be decreased for all impact categories, except when using bioenergy, which would increase the land use impact. Compared to primary fibres like viscose and conventional cotton, these impacts are in the lower to middle range, showing potential to lower environmental impacts when moving towards an increased amounts of recycled post-consumer textile fibre with high value applications, that can replace primary fibres.

Study of the Production of Sodium Hydroxide by Bipolar Electrodialysis from Sodium Carbonate Solution

Study of the Production of Sodium Hydroxide by Bipolar Electrodialysis from Sodium Carbonate Solution

Energy, exergy, economic and environmental (4E) evaluation of a solar-integrated energy system at medium–high temperature using CO2 as the parabolic trough collector (PTC) working medium

This paper proposes a solar-integrated energy system at medium–high temperature (i.e., working temperature >300 °C) for power generation, desalination, and sodium hydroxide (NaOH) production. The system consists of four subsystems, including parabolic trough collector (PTC), organic Rankine cycle (ORC), multi-effect distillation (MED), and electrodialysis. The PTC system uses CO2 gas as the working medium to obtain medium–high temperature heat sources. The electrodialysis system electrolyzes the brine to obtain NaOH and HCl, thus avoiding environmental pollution. Energy, exergy, economic and environmental analysis were adopted to evaluate the system's performance, and six critical parameters were investigated. The results showed that the system production of desalinated water, NaOH, and HCl was 106.75 kg/h, 10.61 kg/h, and 48.46 kg/h, respectively, when the direct normal irradiance was 500 W/m2. The net output power of the ORC and the power consumption of the electroanalysis system reached 23.07 kW and 5.00 kW, respectively. The power consumption of the PTC compressor was 8.01 kW. The exergy destruction of solar collector accounted for the most significant proportion, reaching 76.98%. The exergy efficiency of the system reached 12.42%. Among the six different ORC working mediums (R134a, R152a, R600, R600a, R717, and Pentane), Pentane performed the best in the integrated system, while R134a performed the worst. The annual income of the system could reach 27,645 $, with a simple payback period and payback period of 8.62 years and 10.12 years, respectively. The net present value could reach 1,103,722 $. The system could reduce CO2 emissions by 5.64 tons per year. The research in this paper contributes to understanding the overall characteristics of solar-integrated energy systems when using gaseous working mediums to achieve medium and high-temperature thermal storage. It provides a reference for the development of more efficient solar-integrated energy systems.

Exploring the Production of Sodium Hydroxide via Bipolar Electrodialysis from Sodium Carbonate Solutions

This study explores the production of sodium hydroxide via bipolar electrodialysis using MB-3 membranes and a laboratory electrodialyzer-synthesizer with a three-chamber unit cell (five elementary cells, active area of each membrane 1 dm2). The research compares the use of sodium carbonate and sodium sulfate as initial solutions. The results show that sodium carbonate leads to a higher concentration of the resulting alkali under comparable process conditions. The alkali current efficiency is more than 70% when using sodium carbonate, while it drops sharply to 0.4–0.5 when using sodium sulfate. The energy consumption for transferring one kilogram of alkali ranges from 2.8–13.9 kWh/kg at operating current densities of 1–3 A/dm2.

Application and optimization of bipolar membrane process for drinking water production from Black Sea

This study presents an alternative process that produces drinking water from seawater, which can solve the increasing drinking water shortage. The aim of this study is to produce drinking water from original Black Sea water with three-compartments unit bipolar membrane electrodialysis. In the experiment's operation parameters, the effects of flow rate, potential, and temperature of drinking water, hydrochloric acid, and sodium hydroxide production were investigated. Response surface method Box-Behnkem design was applied for modeling and optimization. The optimum operating conditions were determined as a temperature of 35 °C, a flow rate of 0.5 l/min, and a potential of 12.5877 V. Under these conditions, the highest removal efficiency was found at 98% in Na+, 97% in Cl−, Ca +2 77%, Mg+2 85%, and K+ 59% ions. The microbiological content was cleaned in 15 min without any additional processing. As a result, drinking water production in compliance with the ‘Regulation on Water for Human Consumption/Turkey’ was achieved. The results obtained are promising for the production of drinking water from cold sea waters with low salinity such as the Black Sea and the production of drinking water from low salinity, cold sea waters with bipolar membrane processes. It can reduce energy consumption by incorporating alternative energy sources such as seawater wave energy into the process.

Wastewater Treatment for Mercury and Arresting Mercury Contaminants Movement Within Industrial Soil

We have proposed optimal methods of treating both the waste- and stormwater for metallic and soluble mercury within the soil of mercury-based chlorine- and sodium-hydroxide production sites. The problem has been solved by combining physical, chemical and physicochemical treatment methods. Through the use of various filtering materials, coagulants and flocculants followed by sorption through AB-17-8 and Lewatit TR-214 ion exchangers, the level of mercury within wastewater has been decreased by about 99.9%. Demercurization of mercury-contaminated areas to the complete extraction of metallic mercury has been carried out by draining and the subsequent transfer of water-based compounds into a stable form through chemical immobilization without any prior soil removal. To achieve the aforesaid result we have treated the soil with stearic-acid salts solutions, which resulted in almost complete arrest of mercury within the soil.

Factors Influencing the Formation of Salicylic Acid by Bipolar Membranes Electrodialysis.

Salicylic acid is an intermediate product in the synthesis of dyes, medications and aspirin. An electrodialysis module has been constructed with commercial cationic, anionic and bipolar membranes for the conversion of sodium salicylate into salicylic acid. The effect of operating conditions such as applied electric potential, salt concentration, initial acid concentration and volumetric flow on bipolar membrane electrodialysis (BMED) yields were investigated using Taguchi analysis. The results obtained in 210 min of work show an average concentration of salicylic acid of 0.0185 M, an average electric current efficiency of 85.3%, and a specific energy consumption of 2.24 kWh/kg of salicylic acid. It was concluded that the proposed bipolar membrane electrodialysis process is an efficient alternative to produce salicylic acid (SAH) from sodium salicylate (SANa) in an environmentally friendly manner. Furthermore, the production of sodium hydroxide was obtained as a by-product of the process carried out.

Enhanced Optimization of Bioethanol Production from Palm Waste Using the Taguchi Method

In the present study, palm fiber (PF) and palm fronds (PFN) were selected as local agricultural wastes for the extraction of different biopolymers (cellulose, hemicelluloses, and lignin) by alkaline sodium hydroxide (PF, 2.37% NaOH at 86.5 °C for 1.6 h; PFN, 6% NaOH at 90 °C for 1 h) and bioethanol production. The processes of extraction were optimized by the experimental design method of Taguchi. The total carbohydrates of PF and PFN obtained were 24.4% and 31.0%, respectively. In addition, the untreated palm fiber (UPF), untreated palm frond (UPFN), cellulose palm fibers (CPF), and cellulose palm fronds (CPFN) were subjected to enzymatic hydrolysis processes using crude enzymes and commercial enzymes at 48 °C and pH 5.5. The results indicate that the maximum reducing sugars used were CPF 229.90, CPFN 243.69, UPF 120.19, and UPFN 100.00 (mg/g), which were obtained at a crude enzyme loading. CPF and CPFN hydrolysates were then successfully converted into bioethanol by a separate enzymatic hydrolysis and fermentation by Saccharomyces cerevisiae. Anaerobic cultivation of the hydrolysates with S.cerevisiae resulted in 0.222 g/g and 0.213 g/g bioethanol in the case of CPF and CPFN, respectively. Optimization processes could be an innovative approach to the sustainable development of bioethanol production.

Multi-length-scale heterogeneous structured ion exchange membranes for cost-effective electrolysis and hydrogen production

The design of an ultrabubble-repellent ion exchange membrane (IEM) with high ion transport efficiency for the electrolysis process remains an ongoing challenge. Herein, a multi-length-scaled heterogeneous ZrO2 coating was successfully introduced onto a sodium perfluorinated ion exchange membrane (SPIEM) surface by a facile coating method. Surface morphology optimization technology is developed to control the mesopore size, density and surface coverage of particles, thus forming a “popcorn-like” hierarchical micro/nanostructured ZrO2 coating (M-Ⅱ), which is conducive to bubble repellency in time and efficient ion transport. By using this membrane in chlor-alkali electrolysis, the power consumption can reach 2037.8 kWh t−1, representing reductions of 166.2 kWh and 113.2 kWh, respectively, for producing one ton of 100% NaOH compared with pristine SPIEM (2204 kWh t−1) and commercial Nafion TM N2030 (2151 kWh t−1) at 6 kA cm−2. As a result, M-Ⅱ can realize the production of sodium hydroxide and hydrogen at low cost. The M-Ⅱ also showed remarkable stability with negligible fluctuation for 360 h. Furthermore, after simple purification, the hydrogen from chlor-alkali electrolysis shows excellent performance in fuel cell tests. This study demonstrates the effectiveness of micro/nanoscale engineering on chlor-alkali electrolysis performance, providing a simple and efficient method for solving the gas adhesion problem in the electrolysis process.

A Review on Extraction Processes of Salts from Different Salt Lakes and their Environmental Impact in Industry

Because of its direct and indirect penetration into numerous chemical industries, salt, also known as sodium chloride or halite, is one of the most frequent forms of salts in industrial applications. Evaporation is a typical method of extracting this chemical all around the world. Halite is also a low-cost material because it is found in concave rocks along the coast or in lakes, where saltwater is confined and subjected to evaporation, which concentrates the components in the water and deposits salt, generally by sun evaporation. Several functions of human consumption, salt manufacturing is incredibly significant. Salt is also regarded as one of the essential elements in the extraction of riches throughout history, such as oil extraction in contemporary times, as salt began to be utilized as a food additive, and thus its economic worth emerged. This is because it is common in the all-terrain and has vast origins. It is one of the five essential chemicals that make up the backbone of chemical manufacturing, alongside petroleum. It contributes significantly to the production of chlorine and sodium hydroxide, as well as being consumed by animals and humans. This study aims to describe the method of extraction of salt and its relationship to the environmental aspects so that it gives the reader a comprehensive analysis of all the problems that are related to the extraction process and what are the appropriate methods to deal with the problems associated with the extraction processes and give glimpses of the direct impact on the environment.

Obtaining Caustic Soda and Burkeite by Caustification of Mixture of Corbonate and Sodium Sulphate

The optimal technological parameters for the production of caustic soda from sodium carbonate and sulfate and calcium hydroxide have been established. The influence of the concentration of lime milk on the rate of filtration by sediment and filtrate has been studied.
 Purpose of the Work: The purpose of this work is to determine the physicochemical basis for the production of sodium hydroxide and burkeite by causticization of sodium carbonate and sulfate.
 Scientific Novelty: In comparison with the known works, a theoretical profoanalytical analysis of the one-cation of four-component system and its constituents was carried out for the first time, and also determined the optimal technological parameters of causticization of solutions for local carbonates and sodium sulfates.
 Features of the Work:
 - the influence of the main technological parameters on the processes of causticization, filtration, evaporation;
 - research of intermediate and finished products by modern physicochemical methods;
 - study of the rheological properties of the solution depending on the temperature and concentration of the resulting solutions.

Environmental impact assessment of alkali-activated mortar with waste precursors and activators

Synthetic activators used in alkali-activated binder are the primary contributors to its adverse environmental effects. Therefore, this study aims to perform a life cycle assessment of alkali-activated mortar (AAM) prepared from industrial waste as precursors and activators. The impact analysis is performed through ReCiPe methodology for six different AAM mixes. Environmental impacts of replacing conventional precursors (fly ash and slag) with waste ceramic powder (WCP) and red mud (RM) and conventional activators (sodium silicate (SS) and sodium hydroxide (SH)) with RM, desulfurization dust, and silica fume are assessed. The efficiency of ReCiPe is compared with recommendations from the International Reference Life Cycle Data System. Sensitivity analysis for SH produced using three different techniques and simplified cost analysis for the mortar mixes is conducted. SS is observed to contribute 50–59% of climate change for mixes with conventional activator combination. WCP as a precursor warrants higher dosage of SS and SH, thereby increasing the environmental impact. Replacing SS with silica fume has the least negative effects on ecosystem quality and human health. AAM prepared with conventional activators shows lower resource depletion due to a lower dosage of activators and the absence of pretreatment of fly ash. ReCiPe can be used to evaluate all the impact categories except freshwater ecotoxicity and human toxicity, for which USEtox is suggested. Sensitivity analysis shows that the membrane cell process of SH production is eco-efficient. Savings of up to 14% are observed by replacing both precursors and activators with industrial residues.

Electrochemical Characteristics of Modified Heterogeneous Bipolar Membrane and Electromembrane Process of Nitric Acid and Sodium Hydroxide Recuperation from Sodium Nitrate and Boric Acid Solution

Characteristics of an аMB-2m bipolar membrane containing ion-polymer with phosphoric-acid groups, catalytically active in the water dissociation reaction, were studied by the electrochemical-impedance spectroscopy. The study is carried out in 0.1 М nitric acid–0.1 М sodium hydroxide system. The аMB-2m membrane bipolar region resistance is shown to be an order of magnitude less than that of the MB-1 and MB‑2 commercial membranes. The process of nitric acid and sodium hydroxide production from 0.5 М sodium nitrate solution, as well as from 0.5 М sodium nitrate solution containing 0.75 М boric acid is studied in an electrodialysis apparatus with three-chamber unit cells comprised of Ralex CMH cation-exchange membrane, аMB-2m bipolar membrane, and Ralex AMH anion-exchange membrane. The electrochemical characteristics (the current efficiency, the specific energy consumption, and the specific productivity) are shown to remain unchanged in the presence of boric acid, its transfer into the acid and alkali chambers of the electrodialyzer does not exceed 7%. A probable mechanism of the boric acid and borate ion transfer through the ion-exchange membranes into the acid and alkali chambers is suggested.

Compressive strength of geopolymer concrete with fly-ash from Paiton Steam Power Plant and variations of substitution sodium silicate (Na2SiO3) with natural zeolite

Geopolymer concrete is one of the technological innovations of concrete without cement. It was prepared using fine aggregates, coarse aggregates, fly-ash, sodium hydroxide (NaOH), and sodium silicate (Na2SiO3). In the production of geopolymer concrete, sodium silicate and sodium hydroxide serve as an alkaline activator. One of the natural materials that can be used as an alternative material is natural zeolite. Zeolit contains high silica and alumina, that expected to increase the compressive strength of geopolymer concrete. This research aims to develop geopolymer concrete technology by understanding the setting time behaviour and compressive strength by substitution sodium silicate (Na2SiO3) with natural zeolite. The percentage of zeolite that substitute for sodium silicate is 0%, 2.5%, 5%, and 7.5 %. The molarity of NaOH is 12 M. Meanwhile, the ratio of sodium hydroxide and sodium silicate is 1:2.5. Compressive strength testing is carried out on cylindrical specimens 10 cm × 20 cm. The results show that natural zeolite can accelerate the setting time of geopolymer paste. The fastest initial setting time is 48 minutes. Meanwhile, the final setting time is 60 minutes in geopolymer paste with substitution sodium silicate using 7.5 % zeolite. In addition, for compressive strength results, zeolite can increase compressive strength. The highest average of compressive strength results is 46.07 Mpa in substitution of Na2SiO3 with 7.5 % zeolite.