Increasing Solution pH Research Articles

Mechanisms of calcium-mediated As(V) immobilization by undissolved and dissolved biochar in saline-alkali environments

The environmental impact of arsenic (As) pollution has been a focal point within environmental science. In arsenic-polluted saline-alkali environment, the addition of exogenous biochar can affect the morphological transformation of As both through direct and indirect mechanisms, with calcium ions (Ca(II)) playing a crucial role. This study investigates the immobilization mechanisms of undissolved biochar (UOB) and dissolved biochar (DOB) on As(V) in the absence and presence of Ca(II) under alkaline conditions and aerobic atmosphere. While UOB and DOB alone are insufficient for As(V) immobilization, their combined action in the presence of Ca(II) achieves remarkable immobilization rates of 91.9% and 98.1%, respectively. Precipitation of calcium arsenate is identified as the primary immobilization pathway in both the UOB-Ca(II)-As(V) and DOB-Ca(II)-As(V) systems. Furthermore, Ca(II) acts as a mediator for As(V) immobilization through the formation of ternary UOB/DOB-Ca-As complexes, which are corroborated by Density Functional Theory (DFT) analysis from a microscopic perspective. Notably, the synergistic immobilization of As by DOB and newly generated CaCO3 in DOB-Ca(II)-As(V) system is highlighted. Additionally, the increase in Ca(II) concentration (0–100 mM) and solution pH (9.0–12.0) both significantly enhance the immobilization of As(V). An increase in the dosage of UOB (0.4–4 g/L) reduces the immobilization of As(V), while effect of the DOB concentration is insignificant. This study provides new insights into how the release of two biochar fractions into a typical Ca(II)-rich saline-alkali environment may alter the fate and transport of As species.

Transformation of sartan blood pressure regulators by ferrate(VI) and its activation systems: Kinetics and mechanisms

With the increasing number of patients diagnosed with hypertension, the annual consumption of sartan drugs has surged, resulting in their ubiquitous detection in aquatic environments. This increases concerns about their environmental persistence and unexpected toxicity. This study explores the applications of ferrate(VI) (Fe(VI)) in removing sartan blood pressure regulators (losartan (LOS), valsartan (VAL), and telmisartan (TEL)). Results demonstrate that sartans could be effectively removed by Fe(VI). Oxidation of LOS by Fe(VI) was pH-dependent at pH 7.0–9.0, with HFeO4– dominating the reaction. The second-order rate constants (kapp) of LOS with Fe(VI) decreased with increasing solution pH, and kapp was calculated to be 148.37 M−1 s−1 at pH 8.0. Furthermore, the Fe(VI)-Fe(III), Fe(VI)-S(IV), and Fe(VI)-S2O32– systems demonstrated the enhancement for the elimination of LOS, while the Fe(VI)–HCO3– system inhibited the reaction. Oxidizing product (OP) analysis suggested that the major transformation patterns of LOS, VAL, and TEL treated by Fe(VI) and its activation systems included oxidation, imidazole ring cleavage, cyclization, N-dealkylation, and bond cleavage. Multiple OPs were estimated to be more persistent and mobile than parent sartans, while their toxicity toward aquatic organisms might be several times lower than those of their parents. Overall, this study elucidates the oxidizing feasibility of Fe(VI) and related activation systems to eliminate sartans in water treatment.

Enhanced electrolytic production of hypochlorous acid using phosphorus-modified carbon felt electrodes: A study in disinfectant synthesis

In this study, we fabricated phosphorus-modified carbon felt electrode anodes for chloride oxidation in saline solutions to produce HClO via electrocatalysis, forming a compound fungicide saline applicable for debridement and disinfection. A low-cost phosphorus-modified carbon felt electrode (P@CF) with high chlorine evolution reaction activity was synthesized to address the reduced efficiency of CER and the solution's pH increase. Heteroatoms P and O were introduced into the carbon felt by phosphoric acid activation followed by heat treatment. The maximum active chlorine concentration on the P@CF electrode could reach 616.8 mg/L in 60 min under the optimal synthesis conditions of a phosphoric acid mass fraction of 30%, a phosphoric acid impregnation time of 3 h, and a heat treatment temperature of 300 °C. The active chlorine concentration was 1.8 times higher on the P@CF electrode compared to the original carbon felt electrode. The optimal reaction conditions for the generation of active chlorine were as follows: salt concentration of 9 g/L, voltage of 7 V, and electrode spacing of 2 cm as verified by response surfaces. This electrolysis reaction follows one-stage reaction kinetics. Subsequently, the disinfection efficacy of the produced disinfectants was examined. The prepared disinfectant was also compared to a commercially available hypochlorite disinfectant, showing similar disinfection effects on E. coli for both.

Enhanced removal of lead ions from wastewaters by electrochemical adsorption using nitrogen-doped Ti3C2Tx MXene

As a two-dimensional material containing transition metal carbides and nitrides, MXene is often used in capacitive deionization for its excellent hydrophilic and pseudocapacitive properties. Nitrogen doping is widely used to improve the adsorption properties of certain materials. However, the effect of nitrogen doping on the electrochemical adsorption properties of MXene remains unclear. In this work, nitrogen-doped Ti3C2Tx MXene was synthesized using a simple hydrothermal method to achieve highly selective and efficient removal of Pb(II) at a constant cell voltage. The concentration of Pb(II) decreased from 4.95 to 0.02 mg L−1 in a 100 mg L−1 NaCl supporting electrolyte after electrosorption. The removal efficiency of Pb(II) reached 99.5%, whereas it was only 15.2% for Na+. The introduction of nitrogen functional groups increased the electrical conductivity and layer spacing of MXene, thereby improving the pseudocapacitive adsorption performance in the presence of Pb(II). The complexation function of the Ti-OH group on nitrogen-doped Ti3C2Tx surface increased and contributed to the selective adsorption of Pb(II). The removal efficiency of Pb(II) increased with increasing solution pH and cell voltage, reaching the maximum value at pH 5.0 and 1.5 V, respectively. The effect of supporting electrolyte type on the removal of Pb(II) was negligible. In addition, the removal efficiency for mixed heavy metal ion solutions, including Cu(II), Ni(II), and Pb(II), exceeded 79.0%. It exhibited the highest removal efficiency for Pb(II) at 97.9%. After five cycles of electrochemical adsorption–desorption, the nitrogen-doped Ti3C2Tx maintained a stable Pb(II) removal efficiency of 97.0%. This work provides a promising material for the electrochemical removal of low-concentration heavy metal ions from wastewaters.

Efficient Phosphate Adsorption from Groundwater by Mn-FeOOHs

Manganese co-precipitated with goethite (Mn-FeOOH) is ubiquitous within (sub-)surface environments, which are considered one of the most important sinks for phosphorus pollution management. Accordingly, various mole ratios of Mn-FeOOHs are synthesized and characterized by XRD, FE-SEM, FTIR, BET, XPS, hysteresis loop, acid–base titration and zero potential. According to XRD and FESEM images, the substitution of Mn causes subtle alterations in the microstructure and crystal structure of goethite, and the morphology of Mn-FeOOHs is transformed from needle-shaped goethite to a short-rod-shaped rough surface with increasing Mn substitution. Based on the analysis of BET and acid–base titration, the substitution of Mn into goethite significantly improved the surface area, pore volume, surface properties and active sites of goethite, thereby establishing a theoretical basis for effective subsequent adsorption. Batch experiment results show that the removal rate of phosphate decreases with the increasing solution pH, indicating that acidic groundwater conditions are more conducive to the removal of phosphate. In addition, the adsorption of phosphate on Mn-FeOOHs is independent of ionic strength, indicating that the inner-sphere surface complexation predominated their adsorption behaviors. The isotherm experiment results showed that Mn-G15 exhibits the strongest adsorption capacity for phosphate at pH 5.5 and T = 318 K, with a maximum adsorption capacity of 87.18 mg/g. These findings highlighted the effect of Mn content on the fixation of phosphate onto Mn-FeOOHs from (sub-)surface environments in pollution management.

Simultaneous removal of lead(II) and cadmium(II) from acidic wastewater by Fe-modified sludge biochar

In natural environments, heavy metal water pollution often involves simultaneous presence of various heavy metal ions (HMs), posing significant challenges for their removal. In this study, Fe-modified sludge biochar was synthesized via the co-precipitation method, and the effects of various environmental factors on simultaneously removing lead(II) and cadmium(II) by Fe@SBC were investigated. Batch adsorption experiments showed that Fe@SBC has a higher removal capacities for Pb(II) than for Cd(II) in both single or binary solutions. Increasing solution pH promoted the HMs removal on Fe@SBC. Coexisting divalent cations (Mg2+ and Ca2+) exhibited stronger inhibitory effects on HMs removal by Fe@SBC than monovalent cations (K+ and Na+). The presence of humic acid (HA) did not affect Pb(II) removal on Fe@SBC in either single or binary systems. In single systems, increasing HA concentration enhanced Cd(II) removing on Fe@SBC, while in binary systems, increasing HA concentrations inhibited the Cd(II) removal on Fe@SBC. The sorption of Pb(II) and Cd(II) on Fe@SBC followed the pseudo-second-order kinetic model and the Langmuir model. The removal mechanisms involved electrostatic interactions, complexation with O-containing group, and mineral dissolution-precipitation. After three regenerations, the adsorption capacity of Fe@SBC for Pb(II) and Cd(II) decreased by approximately 5–10 %. In summary, this environmentally friendly composite material has great potential for simultaneously remediating of Pb(II) and Cd(II).

Effect of reagent addition and aging by microwave on the structure of layered double hydroxides

A series of MgAl-layered double hydroxides (MgAl-LDHs) were prepared by variable pH precipitation. The addition of the metal (M) containing solution to the base (B=NaOH) solution, either rapidly (r) or dropwise (d), led to the obtention of the MBr and MBd samples, respectively. In the other hand, by inverting the order of the added solution (pH increase methodology), BMr and BMd were obtained. The BMd sample was microwave aged at 80 °C, 150 °C and 220 °C for 45 min. The results of powder X-ray diffraction showed that rapid precipitation produces more crystalline LDHs than those obtained by dripping. The crystallinity of BMd was improved by microwave aging. All precipitation conditions produced nanoparticles with an average size of around 27.5 nm, which increased almost 5-fold with the microwave treatment. Precipitation by increasing pH gave LDHs with low pore volume, which increased during the heat treatment. Nevertheless, the specific surface area decreased significantly with aging temperature due to the increase in particle size.

Below ground chemical and microbial community responses of wood ash addition to a hardwood forest in central Ontario

The use of wood ash as a soil amendment remains restricted in many parts of Canada. To better understand belowground biogeochemical responses to wood ash, soil solution chemistry was measured over 3 years following the application of wood ash (0, 2.5, 5.0, and 7.5 Mg·ha−1) at a hardwood stand in Ontario, after which soil microbial response was assessed using 16S rRNA gene and internal transcribed spacer sequencing. Metal concentrations in the locally sourced wood ash were below provincial regulatory limits. Significant increases in soil solution pH were observed within the forest floor in the first year of the trial, and significant increases in calcium and magnesium were also observed in later years of the trial. Concentrations of most metals in soil water either decreased or exhibited no significant change in response to wood ash. There was an increase in diversity and richness of soil prokaryotic groups in the FH horizon at the highest wood ash treatment that is most likely linked to the large increase in pH. This study indicates that wood ash has a strong ameliorative effect on soil and soil water chemistry without major changes to soil microbial communities and is a viable amendment to forest soils at dosages below 5 Mg·ha−1.

Simultaneous removal of 2,4-DCP and Cr(VI) in water by gBC@nZVI: Cr(V) mediated ROS formation

In the actual polluted environment, the composite pollution system was usually composed of heavy metals and organic pollutants, especially Cr(VI) and chlorophenols often co-existed in industrial wastewater. In this study, a composite material consisting of multilayer graphene carbon shells coated with nZVI (gBC@nZVI) with various excellent properties was applied to deal with the composite system of Cr(VI) and 2,4-dichlorophenol (2,4-DCP) coexisting in the wastewater, and the intrinsic mechanism for the simultaneous removal of heavy metals and organic pollutants was systematically investigated. The results showed that in the composite pollution system (Cr(VI) = 20 mg/L, 2,4-DCP = 15 mg/L), complete removal of Cr(VI) could be achieved in 60 min, while 2,4-DCP achieved 95.2 % removal, with the contribution of oxidative degradation being 76.4 %. It was found that during the efficient adsorption and reduction of Cr(VI) by gBC@nZVI, the generation of the metastable intermediate Cr(V) was verified by probe compounds, electron spin resonance and UV–visible spectroscopy, and further experiments demonstrated that the generation of •OH was closely related to Cr(V) and promoted the oxidative degradation of the coexisting 2,4-DCP. Quenching experiments with the addition of specific scavengers indicated that •OH was the active species directly dominating the oxidative degradation of 2,4-DCP, whereas Cr(V) was indirectly involved in the degradation of 2,4-DCP by promoting the generation of •OH. In addition, the increase of solution pH negatively affected the degradation of 2,4-DCP in the composite polluted system, mainly due to the hindered generation of •OH in the alkaline environment. This work provided theoretical support for the promising application of gBC@nZVI in real environments contaminated with a combination of Cr(VI) and chlorophenols.

Mechanism investigation of food waste compost as a source of passivation agents for inhibiting pyrite oxidation

Pyrite oxidation causes the generation of acid mine drainage (AMD), posing significant challenges in the management of sulfide tailings. Passivation agents such as humic acid have been tested to prevent pyrite oxidation. The potential of using humate rich food waste compost as a source of passivation agents to suppress pyrite oxidation was explored in this study. Batch leaching tests were conducted at room temperature, involved oxidatively leaching a pure pyrite and a pyrite-containing coal refuse sample, respectively, with and without adding the compost at acidic pH. The involved passivation mechanisms were investigated by solution leaching tests and spectroscopic (XPS) and microscopic (SEM) analysis of the pure pyrite sample and its leaching residues. Compared to the control, adding the compost led to the increases in solution pH, removal of the Fe2+/Fe3+ ions from the solutions by adsorption, and remarkable decreases (≥58 %) in sulfur concentration in both samples. Both SEM and XPS analysis indicated that a coating layer was successfully established on pyrite surface after being leached in the presence of the compost. The coating layer comprised a mixture of organic species (e.g., O-CO and N-H/C groups) and phosphate, potentially improving the likelihood of providing antioxidant protection over acidic pH. As a low-cost material, food waste compost can benefit the source control of AMD not only in terms of its surface passivation, but also its intrinsic alkalinity, metal immobilization, and low environmental risk.

Synthesis and fabrication of magnetically separable phosphate-modified magnetic chitosan composites for lead(II) selective removal from wastewater

To address the urgent need for efficient removal of lead-containing wastewater and reduce the risk of toxicity associated with heavy-metal wastewater contamination, materials with high removal rates and easy separation must be developed. Herein, a novel organic-inorganic hybrid material based on phosphorylated magnetic chitosan (MSCP) was synthesized and applied for the selective removal of lead (II) from wastewater. From the characterization and the experimental results can be obtained that the magnetic saturation strength of MSCP reaches 14.65 emu/g, which can be separated quickly and regenerated readily, and maintains high adsorption performance even after 5 cycles, indicating that the adsorbent possesses good magnetic separation performance and durability. Also, MSCP showed high selective adsorption performance for lead in the multiple metal ions coexistence solutions at pH 6.0 and room temperature, with an adsorption coefficient SPb-MSCP of 78.85%, which was much higher than that of MSC (the SPb-MSC was 11.59%). Additionally, in the single lead system, the sorption characteristics of Pb(II) on MSCP and MCP had obvious pH-responsiveness, and their adsorption capacity increased with the increase of solution pH, reaching the maximal values of 80.19 and 72.68 mg/g, respectively. It is noteworthy that the acid resistance of MSCP with an inert layer coated on the core is significantly improved, with almost no iron leaching from MSCP over the entire acidity range, while MCP has 7.63 mg/g of iron leaching at pH 1.0. Significantly, MSCP exhibited a maximum adsorption capacity of 102.04 mg/g, which matches the Langmuir model at pH 6.0 and 298.15 K, and points to the pseudo-second-order kinetics of the chemisorption process of Pb(II) on MSCP. These findings highlight the great potential of MSCP for Pb(II) removal from aqueous solution, making it a promising solution for Pb(II) contamination in wastewater.

The diverse roles of bromide in the degradation of tinidazole during UV-LED/chlorine process at different wavelengths: Kinetics, radical contribution, and degradation pathway

Bromide (Br–) is widely present in aquatic environment and plays important roles in pollutant degradation during advanced oxidation processes. This work systematically investigated the effect of Br– on the degradation of tinidazole (TNZ) during ultraviolet light-emitting diode (UV-LED)/chlorine process at different wavelengths (265 and 365 nm). The observed pseudo-first-order constant (k') for the TNZ degradation during UV-LED (365 nm)/chlorine process in the presence of 0.4 mM Br– (0.1125 min−1) was 1.56 times that during UV-LED (365 nm)/chlorine process in the absence of Br–. However, the k' value during UV-LED (265 nm)/chlorine process was reduced by 49.=7 % in the presence of 0.4 mM Br–. At 0–0.4 mM Br−, elevated levels of hydroxyl radicals (HO·) and active bromine species promoted TNZ degradation during UV-LED (365 nm)/chlorine/Br– process, whereas reduced HO· concentration was the main reason for the inhibition of TNZ degradation during UV-LED (265 nm)/chlorine/Br– process. With increasing solution pH from 5.8 to 7.8, the k' value of both processes decreased in the presence of Br–. Meanwhile, the k' value increased with increasing chlorine dosage and decreased with increasing initial TNZ concentration during both processes in the presence of Br–. The presence of Cl− and HCO3– presented negligible effects on the degradation of TNZ. According to the density functional theory calculation and detected intermediates, the degradation pathway of TNZ during both processes in the presence of Br– was proposed. The toxicity evaluation demonstrated that some of the degradation products of TNZ presented lower predicted toxicity values than TNZ. Overall, this work provides a thorough insight into the role of Br– in TNZ degradation during UV-LED/chlorine process at different wavelengths.

Unraveling the role of Mn(V)/Mn(III) in the enhanced permanganate oxidation under Vis-LED radiation

A novel approach of visible light-emitting diode (Vis-LED) radiation was employed to activate permanganate (Mn(VII)) for efficient organic micropollutant (OMP) removal. The degradation rates of OMPs by Vis-LED/Mn(VII) were 2–5.29 times higher than those by Mn(VII) except for benzoic acid and atrazine. Increasing wavelengths (445–525 nm) suppressed the degradation of diclofenac (DCF) and 4-chlorophenol (4-CP) owing to the decreased quantum yields of Mn(VII). Comparatively, light intensity and Mn(VII) dosage had a positive effect on the degradation of DCF and 4-CP. Experimental data revealed that Mn(V) dominated the DCF degradation whereas Mn(III) was the active oxidant in the 4-CP degradation. Mn(V) and Mn(III) formed from the photo-decomposition of Mn(VII), meanwhile, Mn(III) also formed from the Mn(V) photo-decomposition. The increase in solution pH inhibited DCF degradation but had a positive impact on 4-CP degradation, mainly due to the changing speciation of DCF and 4-CP. Inorganic anions (Cl− and HCO3−) had little impact on DCF and 4-CP degradation, while humic acid (HA) showed a positive impact because of the π-π interaction between HA and DCF/4-CP. The transformation products of DCF and 4-CP were identified and transformation pathways were proposed. Finally, the Vis-LED/Mn(VII) exhibited great degradation performance in various authentic waters. Overall, this study boosts the development of Mn(VII)-based oxidation processes.

Selected Beneficial Microbes Alleviate Salinity Stress in Hydroponic Lettuce and Pak Choi

Hydroponics is widely used in greenhouse and vertical farming production because these facilities can precisely control environmental conditions such as lighting, temperature, and vapor pressure deficit. However, the fertilizer solutions have a short life span, and they often do not have adequate microbial populations to enhance plant growth. Previous studies have shown the potential of beneficial microbes to promote plant production and alleviate abiotic and biotic stressors in the field, and studies on their use in controlled environments such as greenhouses and vertical farms are limited in the literature. In this study, we selected several plant growth promoting microbes (PGPMs) and tested their effects on alleviating salinity stress in ‘Rex’ lettuce (Lactuca sativa) and ‘Red Pac’ pak choi (Brassica chinensis) grown in deep water culture hydroponics. Our goal was to use one stressor, salinity, that induces profound symptoms in plant morphology. A three-cycle study was conducted using five PGPMs [Bacillus, Glomus, Lactobacillus, Trichoderma, and Bacillus/Pseudomonas/Trichoderma (B/P/T) mix] and two salinity levels [no salinity and salinity treatment, with 120 mM, 40 mM, and 80 mM sodium chloride (NaCl) solution used for the first, second, and third cycles, respectively]. We measured the effects of PGPMs and salinity on plant growth and quality and the solution pH and electrical conductivity (EC). Salinity stress decreased lettuce and pak choi leaf area and shoot fresh weight and increased plant leaf chlorophyll and anthocyanin contents with increased solution EC. Under high-salinity stress (120 mM NaCl), the addition of Trichoderma reduced pak choi leaf area and fresh weight but increased solution pH, whereas under low salinity stress (40 mM NaCl), Trichoderma increased pak choi leaf chlorophyll content. Under moderate-salinity stress (80 mM NaCl) condition, the addition of Glomus sp. increased lettuce fresh weight and leaf area, and B/P/T mix increased pak choi leaf area. In conclusion, using the selected PGPMs in low to moderate-salinity stress could increase lettuce and pak choi growth and quality parameters. These results have some practical applications in the future when more saline water is used for production.

Laboratory study on the transformation of oil-water interface properties under direct current electric field

Interface tension is one of the important mechanisms that affect crude oil recovery and plays an important role in the development of tight oil reservoirs. Electrical-enhanced oil recovery can be used as a technology to reduce interfacial tension and improve crude oil recovery. In the previous research process of Electrical-enhanced oil recovery, the focus was more on the influence of electric field on wettability, and the understanding of the changes in interfacial tension under electric field action is not yet comprehensive. This article designs an orthogonal experiment to study the effect of electric field on oil-water properties. The degree of influence of three factors, namely sodium chloride solution concentration (C), direct current voltage (V), and voltage action time (t), on interfacial tension is studied. Gas chromatography, four component analysis, and Fourier transform infrared spectroscopy are combined to discuss the mechanism of the influence of changes in solution pH, crude oil components, and oil phase functional groups on interfacial tension. The results show that an external electric field can effectively reduce the interfacial tension between oil and water, and the maximum change in interfacial tension after the experiment can be reduced from 23.21 mN/m to 0.37 mN/m; Through range analysis, it can be concluded that the concentration of sodium chloride solution has the greatest impact on interfacial tension, followed by DC voltage and voltage action time. The optimal experimental conditions are 0.5 mol/L sodium chloride solution, 10 V DC voltage, and a voltage action time of 18 h, and the change in interfacial tension is related to the increase in pH value. The change in crude oil composition is also an important reason for the decrease in interfacial tension under external electric fields. Finally, Fourier transform infrared spectroscopy was used to determine that under the action of an electric field, the increase in pH of the sodium chloride solution resulted in the formation of carboxylate salts through chemical reactions between alkaline substances of solution and acidic substances of the oil, which were the key factors in reducing interfacial tension. This study helps to understand and explore the principles and mechanisms of Electrical-enhanced oil recovery in improving oil recovery, with the aim of contributing to the development and promotion of Electrical-enhanced oil recovery technology.

Overlooked effects of chlorides and bicarbonates on the intensity of peroxydisulfate activation in Fe(II)/citric acid-S2O82− process

The influence of chlorides (Cl−) and bicarbonates (HCO3−) on the Fe(II)/citric acid peroxydisulfate based (Fe(II)/CA-S2O82−) process has been commonly attributed to the formation of secondary reactive species, such as Cl• and CO3•, and their different reactivity with the target pollutant compared to primary reactive species (e.g., SO4•−). However, this conclusion has been entirely based only on analyzing target pollutants removal without the context of S2O82− consumption and dissolved Fe concentration throughout the process. To address this knowledge gap, we conducted a series of batch experiments to investigate the influence of Cl− (2 mmol∙L-1) and HCO3− (1 mmol∙L-1) on the intensity of S2O82− activation within the Fe(II)/CA-S2O82− process with tetrachloroethene (PCE) as the target pollutant. Throughout the experiments, PCE removal efficiency, S2O82− consumption, pH, redox potential, and dissolved Fe were analyzed. Equilibrium hydrochemical modelling (Visual MINTEQ 4) and high-resolution transmission electron microscopy (TEM) were employed to analyze and interpret the data obtained. We found that both Cl− and HCO3− significantly affected the intensity of S2O82− activation. While Cl− influenced the steady-state dissolved Fe(II) concentration, HCO3− primarily increased solution pH, which led to the formation of CA-stabilized ferrihydrite nanoparticles. This resulted in the inaccessibility of dissolved Fe(III) to the Fe(III)/Fe(II) regeneration cycle, consequently suppressing S2O82− activation intensity and PCE removal. Overall, the findings in this study have deepened the fundamental knowledge of how Cl− and HCO3− influence the Fe(II)/CA-S2O82− process, which may help improve the design and operation of S2O82−-based advanced oxidation processes using the Fe(II)/CA activation method.

Boron in wildfires: New insights into boron isotope fractionation during volatilisation, leaching and adsorption after combustion

Wildfires are hazards of increasing significance in recent decades. Our ability to forecast the evolution of fire regimes is inhibited by the lack of records of key fire parameters such as fire severity. Boron isotopes in the soil clay fraction have been shown to vary with fire severity, where increased δ11B coincides with higher fire severity. To elucidate the relationship between the B isotope composition of soil clays and fire events, we performed adsorption experiments by reacting rainwater with combusted leaves to analyse how B isotopes are fractionated during processes leading to the imparting of boron from plants to clay minerals in soil during and following combustion. We find that < 5% of B is volatilised during combustion of leaves and barks, where 11B is preferentially volatilised. No isotopic fractionation was detected during the leaching of leaves ash with rainwater, possibly due to the large water:clay ratio in our experiments. Adsorption of B from leaching solutions onto clay minerals shows isotopic fractionation, and hysteresis of the adsorbed B fraction. For experiments at pH between 7 and 9, the isotope fractionation between adsorbed and dissolved B (Δ11Badsorbed-dissolved) ranges from −8.8 to −14.5‰, indicating preferential adsorption of 10B onto clays compared to 11B. For experiments at pH > 10, the Δ11Badsorbed-dissolved values range from +11.2 to +19.4‰, indicating a preferential adsorption of 11B over 10B. Irregardless of pH, clay fractions in all experiments show increases in δ11B, as the leaching solutions have high δ11B relative to the soil clay minerals prior to their interaction. Ash of leaves combusted at 550 °C (highest temperature in our experiments) induce the greatest increase in solution pH and δ11B in clays. Our experiments suggests that the higher B isotope composition of clays following a high severity fire is likely imparted by solutions that leach isotopically heavy B from the combusted canopy.

Production of high purity MoO3 from spent catalyst of formaldehyde synthesis process via a novel two-step leaching-cementation method

This study presents a two-step methodology for producing high-purity MoO3 from spent FORMOX process catalysts. In the first step, a leaching process with a Na2CO3 solution is used to get molybdenum out of formaldehyde synthesis catalyst. The second step involves molybdenum-ion cementation in the presence of metallic zinc powder and plates. The leaching process is thoroughly explored using the design of experiments (DOE) to analyze critical parameters such as temperature, leaching time, Na2CO3/Mo stoichiometric ratio, and catalyst average particle size. A robust quadratic model with an R2 value exceeding 97 % is developed, serving as a predictive tool for molybdenum extraction. Through rigorous numerical optimization, the optimal conditions for 100 % extraction were found to be 74.5 °C, 92.3 min, a stoichiometric ratio of 2.52, and a particle size of 183 μm. However, the use of zinc powder in the cementation stage introduces impurities due to increased solution pH and reduced zinc dissolution. The strategic adoption of zinc plates mitigates these challenges, resulting in a notable enhancement of the final product's purity. Over 91 % of MoO4-2 ions with a purity surpassing 99.8 % are successfully obtained as hexagonal MoO3 powder at 85 °C with an initial solution pH of 1.0 after 15 min of the cementation process.

Comprehensive evaluation of the catalytic activity of alpha-ferric oxide in photocatalysis, Fenton-like and photo-Fenton systems for organics degradation: Performance and in-depth mechanism consideration

Although alpha-ferric oxide (α-Fe2O3) has been widely used in photocatalysis, Fenton-like and photo-Fenton systems for organics degradation, its catalytic activity is still necessary to be evaluated scrupulously in full consideration of organics adsorption and homogeneous Fenton oxidation. In this work, nanoscale α-Fe2O3 was prepared by the direct calcination of FeCl3·6 H2O, and its true catalytic activity in photocatalysis, Fenton-like and photo-Fenton systems was evaluated by the degradation of rhodamine B (RhB) and tetracycline (TC). It was found that the activity of α-Fe2O3 in Fenton-like and photo-Fenton systems at pH 3.0 was higher than that at pH 7.0 and pH 9.0. Compared with photocatalysis and Fenton-like systems, photo-Fenton system exhibited greater organics degradation performance. In the photo-Fenton system, photocatalytic reactions can not only degrade organics directly and accelerate the redox cycle of Fe(II)/Fe(III), but also supply more active sites to promote H2O2 activation. Photocatalytic reactions caused more Fe leached, whereas dissolved Fe could enhance degradation performance significantly even at a very low concentration (0.15 mg L–1). An interesting result was that the increase of solution pH enhanced H2O2 decomposition in the photo-Fenton system, but the degradation performance decreased obviously. Overall, α-Fe2O3 does not have high activity in photocatalysis and Fenton-like systems, and its heterogenous Fenton-like activity may be overestimated in many references. α-Fe2O3 is more suitable to be applied in photo-Fenton system to remove organic pollutants from acidic wastewaters considering H2O2 utilization efficiency.

Transition in solution dynamics of polyacrylic acid: An interplay between nonergodicity and triple mode relaxation

AbstractDynamic light scattering (DLS) measurements have been performed on the polyacrylic acid (PAA) to investigate the polyelectrolyte effect on its solution dynamics. Viscosity measurements confirm the solution to be in the nondilute regime. The polyelectrolyte impacts are contrasted by using two solution pHs, five electrolyte concentrations, and in each with four different polymer concentrations. An interesting ergodic nonergodic ergodic transitions are being observed with variation in solution pH and electrolyte concentrations, which is directly coupled to a similar transition between bi‐modal and triple mode relaxation. The partial heterodyne method is used to analyze the DLS data exhibiting nonergodicity; otherwise, the standard Siegert relation is being employed. For the new third mode of relaxation, in addition to the conventional fast and slow mode, a novel term of nonergodic mode being coined because of a direct association between its presence with the sample becoming nonergodic. The fast relaxation time being analyzed through Rouse model and both the nonergodic and slow relaxation time show a monotonous increase with rise in polymer concentration. The stretched exponent , a measure of inhomogeneity, decreases with increase in solution pH and polymer concentration while increases with the addition of salt to the solution.