High Alkalinity Concentrations Research Articles

PH Level of Pore Solution in Alkali-Activated Fly-Ash Geopolymer Concrete and Its Effect on ASR of Aggregates with Different Silicate Contents

Durability has always been a major concern in concrete-based infrastructures. Alkali–silica reaction (ASR) is the result of a chemical reaction between hydroxyl ions in pore water and silica from aggregates and cement of the concrete matrix. Geopolymer is a type of alkaline reactivated binder which can be synthesized by a polycondensation reaction of geopolymeric precursors and alkali polysilicates. Due to the alkaline solution brought in by alkali polysilicates, it is intuitive that a higher alkaline concentration in pore solutions of geopolymer concrete will adversely affect long-term performance. This study researched the preceding hypothesis and reports the finding of an experimental investigation of alkali–silica reaction between reactive aggregates and the geopolymer matrix. Specimens were prepared using Class C fly ash with a high CaO content and three types of aggregates (granite, carbonate, and gravel), each with different alkaline activator ratios and Na2O doses. Each aggregate also had different percentages of silicate and aluminum content. Mechanical testing of potential resistivity of the aggregates was performed via length change. Pore solution of the hardened geopolymer concrete was extracted and the pH value of the pore solution was determined. Results suggest that the extent of ASR reaction based on the presence of all three types of aggregates in fly ash–based geopolymer concrete is substantially smaller than that of ordinary portland cement (OPC)-based concrete. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) tests also revealed that the amount ASR gel formed in geopolymer is less than that formed in ordinary portland cement concrete. Hence, utilizing ASR vulnerable aggregates in the production of geopolymer concrete might be permissible.

Biochars produced from coconut palm biomass residues can aid regenerative agriculture by improving soil properties and plant yield in humid tropics

Lignin-rich recalcitrant biomass residues of coconut palms viz. (i) mature coconut husk, (ii) tender (immature or green) coconut husk (iii) coconut leaf petiole and (iv) coir-pith were successfully pyrolysed using a simple charring kiln into carbon-rich, black, light weight and porous biochars. High alkalinity and good ash content made them fit for remediating acid soils. High potassium content in these biochars could help reduce the use of inorganic K. Thermogravimetric analysis showed the mass loss phases of husk and coconut leaf petiole biochars to be similar. However, all four biochars gave smooth curves indicating thermal stability of the product. Positive seed germination and earthworm avoidance tests proved their potential as soil amendment. Soil incubation studies with coconut biochars in graded doses, alone or in combination with coconut leaf vermicompost, increased the pH, organic carbon and potassium contents, and promoted plant-beneficial microbiota and enzyme activities. Pot studies with tender coconut husk biochar and coconut leaf vermicompost enhanced the dry weight of cowpea plants accompanied with increased arbuscular mycorrhizal sporulation and root colonization, and root nodule dry weight. A field trial resulted in higher chilli yields with tender coconut husk biochar + coconut leaf vermicompost addition. The results from our studies highlight the potential of pyrolysis as an innovative technology for quick recycling of highly recalcitrant coconut palm biomass residues to biochars as a local source of soil amendment to aid regenerative agriculture in humid tropics.

Pliocene–Holocene Alkali-Basaltoid Volcanism of the Tsushima Basin of the Sea of Japan: New Geochemical and Geodynamic Data

Abstract—The paper contains original data on the rock-forming and rare element compositions in the Pliocene–Holocene alkaline basaltoids of the Tsushima Basin Sea of Japan, obtained by the “wet chemistry” method, atomic emission spectrometry, and inductively coupled plasma mass spectrometry. Based on interpretation of these data, the geochemical and geodynamic features of this stage of volcanism were established; the source of magmogenesis was revealed, represented by garnet peridotites. The withinplate plume–continental nature of the alkaline basaltoids was established, indicated by the high alkalinity and potassium content of rocks, Ta–Nb maximum, high normalized La/Yb (22–28) and La/Sm (3–6) values, as well as high Ti/V, Nb/Y, Zr/Y, Zr/Hf, Th/Yb, and Nb/Ta indices close to chondrite. The withinplate nature is also to some degree confirmed by the closeness of the chemical features of Tsushima basaltoids to rocks of Zinbochu and Udalyanche volcanoes of the Central Asian withinplate province (CAWP) located to west from the Sea of Japan, and it was formed in the Late Cenozoic under the influence of the North Asian superplume.

Green immobilization of toxic metals using alkaline enhanced rice husk biochar: Effects of pyrolysis temperature and KOH concentration

Biochar is a “green” material that has been widely used in environmental applications for its capability to remove or immobilize contaminants in different environmental media (i.e. soil, water and air) and mitigate climate change. In this study, the feasibility of using KOH enhanced biochar for soil Cd and Pb stabilization was investigated, and the effects of pyrolysis temperature and alkaline concentrations for modification were explored. Field-emission scanning electron microscopy (FESEM), N2 adsorption-desorption, and Fourier Transform Infrared Spectroscopy (FTIR) analyses were conducted to reveal the influence on biochar physiochemical properties. The immobilization performances were examined through Toxicity Characteristics Leaching Procedure (TCLP), and Response Surface Methodology (RSM) was adopted to visualize the results from leaching tests. The stabilization mechanisms of alkaline enhanced biochars were investigated using Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS), Tessier sequential extraction method and X-ray diffraction (XRD) analyses. The results indicated that rice husk biochar pyrolyzed at a relatively low temperature (i.e., 300 °C) and activated by moderate alkaline concentrations (i.e., 1 M or 3 M KOH) rendered optimum stabilization performance. KOH activation was a double-edged sword, with high alkaline concentrations destroying biochar's cell structures. Moreover, the integration of TOF-SIMS, XRD and sequential leaching method shed lights on the underlying mechanisms involved in metal stabilization. Surface complexation between toxic metals and oxygen-containing functional groups rather than liming or precipitation was proven to be the fundamental stabilization mechanism.

Electrodes modified with Ni electrodeposition decrease hexavalent chromium generation in an alkaline electrolysis process

Alkaline water electrolysis is one of the easiest methods used to produce hydrogen, offering the advantages of simplicity and low cost. The challenges for the widespread use of water electrolysis are to reduce energy consumption, cost and maintenance and to increase the reliability, durability and safety of the process. The alkaline electrolysis of water has been used for many years to obtain H2 and O2; however, less expensive, more active, endurable and efficient electrodes must be designed. Stainless steel (SS) is considered one of the least expensive electrode materials for alkaline electrolysers, since it is relatively chemically stable and has a low overpotential. Nevertheless, SS anodes do not withstand high concentration alkaline solutions because they undergo a corrosion process. If the electrolyser operates at a voltage of up to 1.6 V, it can generate Fe3O4 and hazardous hexavalent chromium (Cr6+) at the anode. Hexavalent chromium is generated when the chromium-containing stainless steel electrodes undergo an electro-oxidation process. In this work, a low power alkaline electrolyser was designed. In the first step, six electrodes were manufactured of stainless steel. In the second step, a nickel layer with matte or opaque finish was deposited on the SS surfaces to improve their resistance to corrosion and wear. Then, the performance curves and the production of hexavalent chromium were determined. The performance curve after 70 h of operation with nickel-plated electrodes showed an overpotential of 0.5 V at 10 A compared with stainless steel electrodes. Cr6+ was detected in the electrolyte and bubbler water of the system using SS electrodes at values that exceeded the standard (>0.5–1 mg L−1). If the electrolyser used Ni-electrodeposited electrodes, Cr6+ was observed in quantities within the normal range (<0.1 mg L−1). It is very important to prepare modified anodes with nickel electro-deposited, this process prevents the generation of hexavalent chromium, contamination of the electrolyte and reduces maintenance times. The above compensates the slight increase in electrodes cost and energy consumed by the electrolyser.

Ultrathin and porous Mo-doped Ni nanosheet arrays as high-efficient electrocatalysts for hydrogen evolution reaction

High-active and low-cost electrocatalysts is crucial in obtaining clean hydrogen energy via water splitting. Nickel (Ni) is an attractive alternative to the noble metal electrocatalysts for alkaline HER. However, in high concentration alkaline media, it suffers from severe corrosion and deactivation. To solve this problem an ultrathin and porous Ni nanosheet were fabricated with Molybdenum (Mo) doping by hydrogen reduction of NiMoO4-Ni(OH)2 nanosheets to fine-tune the morphology and HER activity. Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) demonstrate that the introduced Mo does not only help to control the morphology but also greatly boosts the HER activity of Ni due to the strong interaction between Mo and Ni. The enhanced HER performance is attested by the overpotential of 91 mV at 10 mA cm−2, low Tafel slope of 62 mV dec−1 and good stability, which is much superior to that of recently reported non-noble metal-based HER electrocatalysts. This study present Ni catalyst as a promising catalyst for the industrial production of H2.

Problems related to storage of acid substances in reinforced concrete tanks

In an industrial plant with continuous production, there were two breakdowns in the company’s sewage treatment plant for liquids with a high sulfuric acid content. Despite securing the underground walls of reinforced concrete tanks with protective coatings resistant to the impact of highly concentrated solutions with sulfuric acid as well as liquids with high alkaline concentration, there was sulphate corrosion destroying the insulation and structural reinforcement. The reasons for the failure were simultaneously improperly selected protective coatings and the technology of applying these coatings to the walls of tanks. In addition, the appearance of cracks in concrete and in polypropylene slabs is influenced by the rheology of construction materials, which is permanently ignored in the construction of multi-layer walls.

The combined effect of potassium, sodium and calcium on the formation of alkali-silica reaction products

Both alkalis and calcium play essential roles in the formation of alkali-silica reaction (ASR) products. Investigation of their combined effect helps to better understand the conditions of ASR. In this study, samples with a constant Ca/Si ratio of 0.3 but different K(or Na)/Si and K/Na ratios have been synthesized at 80 °C. Experimental studies and thermodynamic modelling show that a sufficient amount of K or Na is essential to initiate ASR; at low alkali concentrations C-S-H is stabilized instead. However, too high alkaline concentrations (≥900 mM at K(or Na)/Si ≥ 1) also favor C-S-H formation and suppress ASR product formation. The results reveal a strong effect of the alkalis (K and/or Na) on calcium concentrations and on the formation of ASR products; a maximum ASR product formation is observed at Na or K concentrations between 200 and 500 mM and at initial Ca/Si ratio between 0.1 and 0.4.

Carbon mass balance, isotopic tracers of biogenic methane, and the role of acetate in coal beds: Powder River Basin (USA)

A multi-isotope (C, H, O) investigation of biogenic gas (δ13C−CH4, δ13C−CO2, δD−CH4) and coal bed waters (δD, δ18O, δ13C-DIC, δ13C-acetate) was conducted along a ∼25-km basinward sampling transect in the Powder River Basin (PRB) in Wyoming and Montana (USA) to address inconsistencies among isotope fingerprinting techniques and to apply mass balance isotope modeling to biogenic coalbed methane (CBM). Results include the first published compound-specific δ13C-acetate values from a natural biogenic CBM system, providing new constraints on acetate’s role in biogenic CH4 generation. Coal bed water chemistry is anoxic and dominated by Na+ and HCO3−, with high alkalinity concentrations (14.8–33.0meq/L). The deep basin interior environment contains sulfate-free, Na+−HCO3−-dominated waters exhibiting signs of methanogenesis. CH4 and CO2 generally become 13C-enriched from the shallow basin edge to the deep basin interior (δ13C−CH4 −78.2 to −56.2‰ and δ13C−CO2 -24.7 to 4.7‰). Also, compound-specific δ13C-acetate is more positive in the basin interior compared to the basin edge setting (range −34.3 to −15.1‰). Using a steady-state mass balance isotope model, the observed variations in δ13C-CH4 and δ13C-CO2 can be explained in terms of the favorability of methanogenesis (fCH4) relative to heterotrophic bacterially-mediated non-methanogenic pathways, such as bacterial sulfate reduction, which compete with methanogenesis for substrates. At the basin interior, fCH4 exceeds 0.4, approaching its inferred limiting value hypothesized from the oxidation state of low-molecular weight compounds detected in PRB formation waters. Therefore, methanogenesis is likely a dominant biogeochemical pathway consuming substrates in the deeper coal beds. The basinward shift toward 13C-enriched acetate broadly indicates linkage between acetate, CH4, and CO2, and likely records acetate synthesis effects modified by methanogenic fractionation. These results suggest that acetate plays a role in methanogenesis in PRB coal beds (not necessarily a dominant role), while other C isotope and microbial evidence suggests that hydrogenotrophic methanogenesis is also a significant methane-producing pathway. The results of this study broadly suggest multiple viable methanogenic pathways and C cycling that affects acetate, a more complex view of coal bed geochemistry than implied by traditional C and H isotope fingerprinting of accumulated products (CH4, CO2) and coexisting water.

Zinc anode with artificial solid electrolyte interface for dendrite-free Ni-Zn secondary battery

Rechargeable nickel-zinc battery is regarded as a prospective choice for next-generation energy storage device due to its good safety, environmental friendliness and high energy density. However, zinc anode inevitably suffers from uncontrollable growth of zinc dendrites and the dissolution of zinc metal in high-concentration alkaline electrolytes, resulting in poor cycle stability and severely hampering the widespread applications of nickel-zinc batteries. Herein, a unique zinc anode with artificial solid electrolyte interface (ASEI) is facilely constructed via the rolling-tearing of tin and zinc foils and subsequent surface-based chemical reaction in a lead salt solution. The as-prepared ASEI composed of lead film has an efficient protective effect on preventing the dissolution of zinc anode. Meanwhile, the lead element and residual tin can not only enhance hydrogen evolution over-potential of zinc anode but affect the zinc growth mechanism. As a consequence, an excellent cyclic performances upto 100 cycles (capacity retention: 90%) with high reversible capacities are achieved for the zinc anode with AESI.

Highly Active Binder Free Plasma Sprayed Non-Noble Metal Electrodes for Anion Exchange Membrane Electrolysis at Different Reduced KOH Concentrations

Alkaline anion exchange membrane water electrolysis (AEMWE) is considered to be an alternative to proton exchange membrane water electrolysis (PEMWE) and conventional alkaline water electrolysis (AWE), owing to the use of non-precious metal and avoiding high alkaline concentration electrolyte, respectively. Here, we report a highly efficient AEMWE design using the fabrication of membrane-electrode assembly (MEA) by simply sandwiching anion exchange membrane between electrodes developed by plasma spraying of NiMoAl for cathode and NiAl for anode in various low concentrated KOH solutions (0.1 to 1.0M). The first impression from the electrochemical characterization is that a higher KOH concentration has positive effects on the overall cell performance. However, this effect is limited at higher KOH concentration. The cell operated in 1.0M KOH exhibits the highest current density of 0.44 A/cm² at 1.80 V, which is very close the one, 0.5 A/cm² at 1.80 V, in 6.0M KOH achieved in AWE.

Transition-Metal-Doped TiO2 Decorated Nife Layered Double Hydroxide Catalyst in Alkaline Oxygen Evolution Reaction

Efficient and earth abundant electrocatalysts for high-performance oxygen evolution reaction (OER) are essential for the development of sustainable energy conversion technologies. Here, the 3d transition-metal-doped TiO2 were synthesized and used the catalysts for alkaline oxygen evolution reaction. Commercial catalyst usually uses carbon as support such as Ir/C and Ru/C. But during oxygen evolution reaction, carbon will be corroded seriously. So, catalyst will detach from carbon support cause efficiency of catalyst to decay very quickly. In order to overcome these problems, we develop TiO2 as a support because TiO2 is a very durable material in high potential region and high concentration alkaline electrolyte. However, TiO2 is not a good catalyst for oxygen evolution reaction and suffer from its poor conductivity. To solve these drawbacks, we synthesized different 3d transition metal doping TiO2 in this work. After electrochemical tests, we choose (Ni, Fe) dual doped TiO2 was chosen because of its potential for oxygen evolution reaction. First of all, different different doping compositions of Ni and Fe were investigated, and it was found that Ni:Fe in molar ratio 3:1 showed the best oxygen evolution reaction performance. Next, different doping amounts of Ni to Fe, from 6:2, 3:1 to 1.5:0.5 were also evaluated, where Ni:Fe in 6:2 was the best composition for alkaline oxygen evoution. Furthermore, we modified catalyst by adding long chain anion, hydrogen treatment and loading NiFe LDH on (Ni, Fe) dual doped TiO2. Finally, NiFe/TiNiFe perform better oxygen evolution reaction than pure NiFe LDH. At the current density reach 10 mA/cm2, the required potential of NiFe/TiNiFe was 10 mV less than for pure NiFe LDH. After chronopotentiometric stability test for 48 hours, NiFe/TiNiFe showed almost no decay during reaction and is more superior than NiFe LDH. Our work successfully solves activity and stability issue at the same time by an excessive dual doping approach.

Chemical variability of groundwater samples collected from Needamangalam Region, Thiruvarur District, Tamil Nadu (India)

The aim of present study was to assess the quality of groundwater from Needamangalam region in Thiruvarur district. Eight different locations were selected to analyze the physicochemical parameters and compared with BIS drinking water quality guideline. From overall analysis, the collected groundwater samples of Needamangalam region are contaminated. These region groundwater samples not suitable for drinking purpose, because the water samples contain high concentration of hardness, alkalinity and total dissolved solids; if use the groundwater of Needamangalam region, shall be cause kidney problems due to excess of calcium hardness. Hence we need special treatment for improve the water quality.

Understanding groundwater salinization mechanisms to secure freshwater resources in the water-scarce city of Maputo, Mozambique

In this study hydrochemical, isotopic and multivariate statistical tools are combined with a recharge analysis and existing geophysical data to improve understanding of major factors controlling freshwater occurrence and the origins of high salinities in the multi-layered coastal aquifer system of the Great Maputo area in Mozambique. Access to freshwater in this semi-arid area is limited by an inefficient public supply network, scarce surface waters, long droughts and an increasing population growth. Groundwater has a large potential to enhance water security, but its exploitation is threatened by both coastal and inland salinization mechanisms that are poorly understood. A GIS approach is utilized to classify potential recharge zones based on hydrogeological properties and land use/cover, whereas potential recharge rates are estimated through a root zone water balance method. In combination with water stable isotope data results reveal that extreme rainfall events provide the most relevant contributions to recharge, and interception and evaporation play an important role in the low recharge areas. Hierarchical clustering of hydrochemical and isotopic data allows the classification of six water groups, varying from fresh to brackish/salt waters. Corresponding scatter plots and PHREEQC modelling show evaporation and mixing with seawater (up to 5%) as major processes affecting salinity in the area. The co-occurrence of high alkalinity and Cl concentrations, in combination with piezometric and geo-electrical data, suggests that: 1) inland brackish/salt groundwater is caused by mixing with seawater trapped within clay layers; and 2) brackish/salt surface waters result from seepage of brackish groundwater into rivers and wetlands, followed by evaporation, hence increasing salinity and δ18O values. Mixing with small fractions of trapped seawater as main salinity source, rather than halite dissolution, is further corroborated by Br/Cl ratios of brackish/salt water samples near the ocean ratio. Cation exchange upon salinization is mainly observed in the semi-confined aquifer, while freshening takes place in the phreatic aquifer, particularly in areas presenting high recharge rates.

Desilication of concentrated alkali solution by novel desilication reagent calcium hydroferrocarbonate: Part I. Synthesis of desilication reagent

For the deep removal of silica from sodium aluminate solution with a high alkaline concentration, we propose novel ferrite desilication reagents calcium hydroferrocarbonate (CHFC; 3CaO·Fe2O3·CaCO3·12H2O) because Fe3+ cations show extremely low solubilities of ~1–3 mg/L in highly concentrated caustic solutions. The main factors affecting the synthesis of CHFC were studied, including time, temperature, liquid-to-solid mass ratio, and particle size of CaO. The results showed that CHFC is a metastable material in the alkaline solution, and the synthesis factors, time and temperature, are related to each other. The optimum synthesis conditions of CHFC were determined. The mechanisms in the synthesis of CHFC, and the desilication mechanism by CHFC, are discussed, using the desilication reaction and X-ray Diffraction analysis. In the desilication process, silica combines with CHFC to form calcium hydroferrosilicate, calcium hydroferroaluminosilicate and.calcium ferroaluminosilicate.

A Study on the Treatment of Construction Wastewater Using Multi-Stage Settlement

Wastewater generated from tunnel excavation and construction site is constantly generating complaints and disputes due to environmental pollution problem caused by turbid water in a river near a construction site. Therefore, there is a lot of research on the processing method. In this study, a continuous multistage (3-stage) flocculation and precipitation reactor was developed to treat the construction wastewater with high concentration of suspended solids and strong alkalinity. This R & D technology can carry out mixing, coagulation, sedimentation and concentration in one reaction tank. And has a high response to the deterioration of the wastewater which is continuously changing. This technology is an economical and efficient technology for treating construction wastewater without the need for a separate sewer facility. In order to verify the performance of the equipment, we collected and analyzed the actual construction waste water. As a result, SS 88.31%, Turbidity 81.14% and CODcr 54.17% were removed. Therefore, it is expected that the use of this technology will reduce water treatment costs and increase treatment efficiency. This technology can reduce the facility cost and the required site area compared to the existing coagulation sedimentation technology. It is also possible to remove suspended matters with high efficiency without using coagulant and coagulant.

Enhancement of Paenibacillus sp. D9 Lipopeptide Biosurfactant Production Through the Optimization of Medium Composition and Its Application for Biodegradation of Hydrophobic Pollutants.

Interests in biosurfactant in industrial and environmental applications have increased considerably in recent years, owing to their potential benefits over synthetic counterparts. The present study aimed at analyzing the stability and oil removal efficiency of a new lipopeptide biosurfactant produced by Paenibacillus sp. D9 and its feasibility of its use in biotechnological applications. Paenibacillus sp. D9 was evaluated for optimal growth conditions and improved production yield of lipopeptide biosurfactant with variations in different substrate parameters such as carbon (C), nitrogen (N), C:N: ratio, metal supplements, pH, and temperature. Enhanced biosurfactant production was observed when using diesel fuel and ammonium sulfate as carbon and nitrogen source respectively. The maximum biosurfactant yield of 4.11g/L by Paenibacillus sp. D9 occurred at a C/N ratio of 3:1, at pH 7.0, 30°C, 4.0mM MgSO4, and 1.5% inoculum size. The D9 biosurfactant was found to retain surface-active properties under the extreme conditions such as high thermal, acidic, alkaline, and salt concentration. The ability to emulsify further emphasizes its potential usage in biotechnological application. Additionally, the lipopeptide biosurfactant exhibited good performance in the degradation of highly toxic substances when compared with chemical surfactant, which proposes its probable application in biodegradation, microbial-enhanced oil recovery or bioremediation. Furthermore, the biosurfactants were effective in a test to stimulate the solubilization of hydrophobic pollutants in both liquid environments removing 49.1 to 65.1% diesel fuel including hydrophobic pollutants. The study highlights the usefulness of optimization of culture parameters and their effects on biosurfactant production, high stability, improved desorption, and solubilization of hydrophobic pollutants.

Alkali Activated Slag Mortars Provide High Resistance to Chloride-Induced Corrosion of Steel

The pore solutions of alkali-activated slag cements and Portland-based cements are very different in terms of their chemical and redox characteristics, particularly due to the high alkalinity and high sulfide content of alkali-activated slag cement. Therefore, differences in corrosion mechanisms of steel elements embedded in these cements could be expected, with important implications for the durability of reinforced concrete elements. This study assesses the corrosion behaviour of steel embedded in alkali-activated blast furnace slag (BFS) mortars exposed to alkaline solution, alkaline chloride-rich solution, water, and standard laboratory conditions, using electrochemical techniques. White Portland cement (WPC) mortars and blended cement mortars (white Portland cement and blast furnace slag) were also tested for comparative purposes. The steel elements embedded in immersed alkali-activated slag mortars presented very negative redox potentials and high apparent corrosion current values; the presence of sulfide reduced the redox potential, and the oxidation of the reduced sulfur-containing species within the cement itself gave an electrochemical signal that classical electrochemical tests for reinforced concrete durability would interpret as being due to steel corrosion processes. However, the actual observed resistance to chloride-induced corrosion was very high, as measured by extraction and characterisation of the steel at the end of a 9-month exposure period, whereas the steel embedded in white Portland cement mortars was significantly damaged under the same conditions.

Investigation of the electrochemical dissolution behavior of tungsten during electrochemical machining

With the advantageous properties of a high melting point, high strength at high temperatures, low vapor pressure, and good thermal conductivity, tungsten is widely used in several advanced industrial applications. Micro structures such as micro holes and grooves are commonly prepared on the tungsten surface to enhance its heat transfer performance. Electrochemical machining (ECM) is a promising method of generating micro structures on the metal surface. Typically, an alkaline solution is employed as an electrolyte during tungsten ECM. Unfortunately, significant stray corrosion commonly occurs on the tungsten surface when high-concentration alkaline solutions (NaOH > 1 wt.%) are used. However, the machining efficiency decreases sharply when the concentration is less than 0.5 wt.%. In order to improve machining efficiency and accuracy, this paper proposes a novel mixed electrolyte consisting of NaClO3 mixed with a low concentration of NaOH for electrochemical machining of tungsten. The NaClO3 is used to increase the conductivity as well as to improve the formation of oxide films on the tungsten surface in order to avoid stray corrosion. NaOH is used to ensure continuous machining by removing the oxide film in the region being machined. The influence of the electrolyte composition and concentration on electrochemical dissolution of tungsten is analyzed by measuring the anode polarization curve and open circuit potential, and an optimized electrolyte with of 10 wt.% NaClO3 and 0.3 wt.% NaOH is identified. This optimized electrolyte enables generation of micro-through holes with inlet and outlet diameters of 500 and 410 μm, respectively, using a tube electrode with a diameter of 250 μm. The feed rate of the tool reaches 24 μm/min, which is 4 times faster than in previous studies.

Anode Materials for Sulfide Oxidation in Alkaline Wastewater: An Activity and Stability Performance Comparison

Hydrogen sulfide (H2S) is a toxic and corrosive gas. It occurs naturally in aquatic environments and is a common contaminant of sewage, industrial wastewater and gas streams. Various methods have been applied for its removal but generally issues exist such as high cost, chemicals consumption and sludge production. Electrochemical treatment, whereby sulfide is oxidized and NaOH can be recovered via cathodic water splitting, provides an interesting alternative, as it is chemical-free and may allow for the recovery of elemental sulfur (So) and caustic (NaOH) as by-products (Vaiopoulou et al., 2016). Whereas the operational costs, mainly power consumption, could be limited, particularly the anodes (dimensionally stable anodes, DSA) come at high cost and limited lifetime due to passivation or poisoning of the active sites. DSA are in many cases titanium based electrodes coated with mixed metal oxides (MMO). They are widely used in electrochemical processes such as chlorine and oxygen generation, due to their long life and reduced energy consumption, and have been recently investigated for electrochemical treatment of sulfide streams. In the particular case of spent caustic streams (SCS) where a combination of high alkalinity, sulfide concentration and current densities takes place, DSA have to our knowledge not been evaluated. Therefore, we tested 6 electrode materials (Ir MMO, Ru MMO, Pt/IrOx, Pt, PbOx and TiO2/IrTaO2) to determine how the different catalyst coatings affected the sulfide oxidation efficiency and the stability and lifetime of the electrode when applied for the treatment of SCS. A first series of tests was conducted in a multielectrode reactor, to assess the activity and stability of the different electrode materials. Activity tests were performed in a batch mode and controlled potentiostatically at 50 A m-2 current density, while stability tests were performed in a continuous mode for 305 h and controlled at a current density range from 50 to 200 A m-2. The counter electrode used was stainless steel mesh and the anolyte consisted of 50 mM Na2S and 50 mM NaOH. At regular intervals cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed. The change in the electrode surface texture and composition upon sulfide oxidation was examined with scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX). Sulfide removal in an alkaline electrochemical cell is proceeding through a number of different oxidation reactions, that happen in a wide range of anode potentials and result in the production of So or different sulfur oxyanions (Mao et al., 1991). This distribution was also observed in this study, with the sulfur species produced from sulfide oxidation differing with the anode materials, yet with So as the first product of sulfide oxidation, followed by S2O3 2-, SO3 2- and SO4 2-.This work demonstrated that Ru MMO was the most active material towards So recovery with a Coulombic efficiency (CE%) of 65 %. Moreover, it performed sulfide oxidation at an onset anode potential of 0.78 V vs. SHE and completed the batch cycle with an average of 1.1 ± 0.3 V vs. SHE. The Ru MMO was also found to be the most stable electrode with only a decrease of 17% of the initial Ru/Ti ratio after 305 hours. This reduced catalyst over base metal ratio can be explained by the sulfur deposition observed on the surface of the electrode. More specifically, the Ru MMO electrode preserved a stable potential of 1.57 V vs. SHE at a high current density of 200 A m-2, which is in good agreement with the SEM/EDX results. In contrary, the harsh conditions of high alkalinity and sulfide concentration affected the stability of the Pt/IrOx electrode which showed an increase of the anode potential from 2.79 to 6.61 V vs. SHE at 200 A m-2. The decline of stability was confirmed with SEM/EDX, where no significant amount of the Pt and Ir catalysts were found on the surface of the electrode at the end of the test. This study confirms the importance of the anode material selection as a crucial aspect of the electrochemical cells for sulfide oxidation. The choice of the anode materials can directly affect the capital costs of the application and it also has a direct impact on the energetic efficiency and the operational maintenance of the system. The electrochemical sulfide oxidation on Ru MMO demonstrated that a combination of activity and stability is a desired property for the anode material, although it still needs to be determined whether these results will compensate for the high costs of most of the precious metals used in DSA electrodes. Figure 1