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3 pages, 171 KiB

Open AccessEditorial

Saline Wastewater: Characteristics and Treatment Technologies

by Jingtao Bi and Yingying Zhao

Molecules 2023, 28(4), 1622; https://doi.org/10.3390/molecules28041622 - 8 Feb 2023

Viewed by 1715

Abstract

The discharge of saline wastewater has significantly increased due to rapid urbanization and industrialization [...] Full article

(This article belongs to the Special Issue Saline Wastewater: Characteristics and Treatment Technologies)

Research

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14 pages, 2362 KiB

Open AccessArticle

Influence of Organic Impurities on Fractional Crystallization of NaCl and Na₂SO₄ from High-Salinity Coal Chemical Wastewater: Thermodynamics and Nucleation Kinetics Analysis

by Bo Shen, Bo Zhao, Hai Du, Yongsheng Ren, Jianwei Tang, Yong Liu, Quanxian Hua and Baoming Wang

Molecules 2024, 29(9), 1928; https://doi.org/10.3390/molecules29091928 - 23 Apr 2024

Cited by 1 | Viewed by 1269

Abstract

It is a valid path to realize the zero discharge of coal chemical wastewater by using the fractional crystallization method to recycle the miscellaneous salt in high-salinity wastewater. In this study, the thermodynamics and nucleation kinetics of sodium chloride (NaCl) and sodium sulfate [...] Read more.

It is a valid path to realize the zero discharge of coal chemical wastewater by using the fractional crystallization method to recycle the miscellaneous salt in high-salinity wastewater. In this study, the thermodynamics and nucleation kinetics of sodium chloride (NaCl) and sodium sulfate (Na₂SO₄) crystallization in coal chemical wastewater were systematically studied. Through analyses of solubility, metastable zone width, and induction period, it was found that the impurity dimethoxymethane would increase the solid–liquid interface energy and critical crystal size during the nucleation of Na₂SO₄. Ternary phase diagrams of the pseudo-ternary Na₂SO₄-NaCl-H₂O systems in simulated wastewater were plotted in the temperature range of 303.15 to 333.15 K, indicating that a co-ionization effect existed between NaCl and Na₂SO₄, and NaCl had a strong salting out effect on Na₂SO₄. Finally, the nucleation rate and growth rate of Na₂SO₄ crystals under simulated wastewater conditions were determined by the intermittent dynamic method, and the crystallization kinetic models of Na₂SO₄ were established. The crystallization nucleation of Na₂SO₄ crystals was found to be secondary nucleation controlled by surface reactions. The basic theoretical research of crystallization in this study is expected to fundamentally promote the application of fractional crystallization to realize the resource utilization of high-salinity wastewater in the coal chemical industry. Full article

(This article belongs to the Special Issue Saline Wastewater: Characteristics and Treatment Technologies)

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14 pages, 1052 KiB

Open AccessArticle

Heavy Metal Removal from Aqueous Solutions Using a Customized Bipolar Membrane Electrodialysis Process

by Samuel Bunani, Gudrun Abbt-Braun and Harald Horn

Molecules 2024, 29(8), 1754; https://doi.org/10.3390/molecules29081754 - 12 Apr 2024

Cited by 1 | Viewed by 933

Abstract

Lack of safe water availability and access to clean water cause a higher risk of infectious diseases and other diseases as well. Heavy metals (HMs) are inorganic pollutants that cause severe threats to humans, animals, and the environment. Therefore, an effective HM removal [...] Read more.

Lack of safe water availability and access to clean water cause a higher risk of infectious diseases and other diseases as well. Heavy metals (HMs) are inorganic pollutants that cause severe threats to humans, animals, and the environment. Therefore, an effective HM removal technology is urgently needed. In the present study, a customized bipolar membrane electrodialysis process was used to remove HMs from aqueous solutions. The impacts of the feed ionic strength, applied electrical potential, and the type and concentration of HMs (Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, and Ni²⁺) on the process performance were investigated. The results showed that feed solution pH changes occurred in four stages: it first decreased linearly before stabilizing in the acidic pH range, followed by an increase and stabilization in the basic range of the pH scale. HM speciation in the basic pH range revealed the presence of anionic HM species. The presence of HMs on anion exchange membranes confirmed the contribution of these membranes for HM removal in the base channels of the process. While no clear trend was seen in the ionic strength solution, the maximum HM removal was observed when 1.5 g/L NaCl was used. The initial HM concentration showed a linear increase in HMs removal of up to 30 mg/L. A similar trend was seen with an increase in the applied electrical potential of up to 15 V. In general, the amount of HMs removed increased in the following order: Cd²⁺ ˃ Ni²⁺ ˃ Co²⁺ ˃ Cu²⁺ ˃ Cr³⁺. Under some operational conditions, however, the removed amount of Cu²⁺, Co²⁺, and Ni²⁺ was similar. The mass balance and SEM-EDX results revealed that the removed HMs were sorbed onto the membranes. In conclusion, this process efficiently separates HMs from aqueous solutions. It showed the features of diluate pH adjustment, reduction in the overall stack electrical resistance, and contribution of anion exchange membranes in multivalent cation removal. The mechanisms involved in HMs removal were diffusion and migration from the bulk solution, followed by their sorption on both cation and anion exchange membranes. Full article

(This article belongs to the Special Issue Saline Wastewater: Characteristics and Treatment Technologies)

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13 pages, 3176 KiB

Open AccessArticle

In-Depth Study on the Effects of Impurity Ions in Saline Wastewater Electrolysis

by Qicheng Pan, Peixuan Zhao, Linxia Gao, Huimin Liu, Hongyun Hu and Lu Dong

Molecules 2023, 28(12), 4576; https://doi.org/10.3390/molecules28124576 - 6 Jun 2023

Cited by 4 | Viewed by 1778

Abstract

Concentration followed by electrolysis is one of the most promising ways for saline wastewater treatment, since it could produce H₂, Cl_2, and an alkaline solution with deacidification potential. However, due to the diversity and difference of wastewater, knowledge on the [...] Read more.

Concentration followed by electrolysis is one of the most promising ways for saline wastewater treatment, since it could produce H₂, Cl_2, and an alkaline solution with deacidification potential. However, due to the diversity and difference of wastewater, knowledge on the suitable salt concentration for wastewater electrolysis and the effects of mixed ions are still lacking. In this work, electrolysis experiments of mixed saline water were conducted. The salt concentration for stable dechlorination was explored, with in-depth discussions on the effects of typical ions such as K⁺, Ca²⁺, Mg²⁺, and SO₄²⁻. Results showed that K⁺ had a positive effect on the H₂/Cl₂ production of saline wastewater through accelerating the mass transfer efficiency in the electrolyte. However, the existence of Ca²⁺ and Mg²⁺ had negative effects on the electrolysis performance by forming precipitates, which would adhere to the membrane, reduce the membrane permeability, occupy the active sites on the cathode surface, and also increase the transport resistance of the electrons in the electrolyte. Compared to Mg²⁺, the damaging effect of Ca²⁺ on the membrane was even worse. Additionally, the existence of SO₄²⁻ reduced the current density of the salt solution by affecting the anodic reaction while having less of an effect on the membrane. Overall, Ca²⁺ ≤ 0.01 mol/L, Mg²⁺ ≤ 0.1 mol/L and SO₄²⁻ ≤ 0.01 mol/L were allowable to ensure the continuous and stable dechlorination electrolysis of saline wastewater. Full article

(This article belongs to the Special Issue Saline Wastewater: Characteristics and Treatment Technologies)

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16 pages, 1972 KiB

Open AccessArticle

Electrochemical Removal of Nitrogen Compounds from a Simulated Saline Wastewater

by Pasquale Iovino, Angelo Fenti, Simona Galoppo, Mohammad Saleh Najafinejad, Simeone Chianese and Dino Musmarra

Molecules 2023, 28(3), 1306; https://doi.org/10.3390/molecules28031306 - 30 Jan 2023

Cited by 19 | Viewed by 2800

Abstract

In the last few years, many industrial sectors have generated and discharged large volumes of saline wastewater into the environment. In the present work, the electrochemical removal of nitrogen compounds from synthetic saline wastewater was investigated through a lab-scale experimental reactor. Experiments were [...] Read more.

In the last few years, many industrial sectors have generated and discharged large volumes of saline wastewater into the environment. In the present work, the electrochemical removal of nitrogen compounds from synthetic saline wastewater was investigated through a lab-scale experimental reactor. Experiments were carried out to examine the impacts of the operational parameters, such as electrolyte composition and concentration, applied current intensity, and initial ammoniacal nitrogen concentration, on the total nitrogen removal efficiency. Using NaCl as an electrolyte, the N_TOT removal was higher than Na₂SO₄ and NaClO₄; however, increasing the initial NaCl concentration over 250 mg·L⁻¹ resulted in no benefits for the N_TOT removal efficiency. A rise in the current intensity from 0.05 A to 0.15 A resulted in an improvement in N_TOT removal. Nevertheless, a further increase to 0.25 A led to basically no enhancement of the efficiency. A lower initial ammoniacal nitrogen concentration resulted in higher removal efficiency. The highest N_TOT removal (about 75%) was achieved after 90 min of treatment operating with a NaCl concentration of 250 mg·L⁻¹ at an applied current intensity of 0.15 A and with an initial ammoniacal nitrogen concentration of 13 mg·L⁻¹. The nitrogen degradation mechanism proposed assumes a series–parallel reaction system, with a first step in which NH₄⁺ is in equilibrium with NH₃. Moreover, the nitrogen molar balance showed that the main product of nitrogen oxidation was N₂, but NO₃⁻ was also detected. Collectively, electrochemical treatment is a promising approach for the removal of nitrogen compounds from impacted saline wastewater. Full article

(This article belongs to the Special Issue Saline Wastewater: Characteristics and Treatment Technologies)

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8 pages, 1494 KiB

Open AccessCommunication

Investigation on the Removal Performances of Heavy Metal Copper (II) Ions from Aqueous Solutions Using Hydrate-Based Method

by Xiaobing Lan, Jun Chen, Yang Xie, Fenglong Hu, Changzhong Chen, Dongdong Li, Jianhong Jiang and Bin Deng

Molecules 2023, 28(2), 469; https://doi.org/10.3390/molecules28020469 - 4 Jan 2023

Cited by 3 | Viewed by 1770

Abstract

Since heavy metal ion-contaminated water pollutionis becoming a serious threat to human and aquatic lives, new methods for highly efficient removal of heavy metal ions from wastewater are important to tackle environmental problems and sustainable development. In this work, we investigate the removal [...] Read more.

Since heavy metal ion-contaminated water pollutionis becoming a serious threat to human and aquatic lives, new methods for highly efficient removal of heavy metal ions from wastewater are important to tackle environmental problems and sustainable development. In this work, we investigate the removal performances of heavy metal copper (II) ions from aqueous solutions using a gas hydrate-based method. Efficient removal of heavy metal copper (II) ions from wastewater via a methane hydrate process was demonstrated. The influence of the temperature, hydration time, copper (II) ions concentration, and stirring rate on the removal of heavy metal copper (II) ions were evaluated. The results suggested that a maximum of 75.8% copper (II) ions were removed from aqueous solution and obtained melted water with 70.6% yield with a temperature of −2 °C, stirring speed 800 r/min, and hydration time of 4 h with aninitial copper concentration of 100 mg/L. The initial concentration of copper (II) ions in the aqueous solution could be increased to between 100 and 500 mg/L. Meanwhile, our study also indicated that 65.6% copper (II) ions were removed from aqueous solution and the yield of melted water with 56.7%, even with the initial copper concentration of 500 mg/L. This research work demonstrates great potential for general applicability to heavy metal ion-contaminated wastewater treatment and provides a reference for the application of the gas hydrate method in separation. Full article

(This article belongs to the Special Issue Saline Wastewater: Characteristics and Treatment Technologies)

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13 pages, 5810 KiB

Open AccessArticle

Study on the Structure of a Mixed KCl and K₂SO₄ Aqueous Solution Using a Modified X-ray Scattering Device, Raman Spectroscopy, and Molecular Dynamics Simulation

by Mengdan Qiao, Fei Li, Xianze Meng, Meiling Wang, Hanyu Zhu, Zhiyong Ji, Yingying Zhao, Jie Liu, Shizhao Wang, Xiaofu Guo, Jingtao Bi and Junsheng Yuan

Molecules 2022, 27(17), 5575; https://doi.org/10.3390/molecules27175575 - 30 Aug 2022

Cited by 3 | Viewed by 1905

Abstract

The microstructure of a mixed KCl and K₂SO₄ aqueous solution was studied using X-ray scattering (XRS), Raman spectroscopy, and molecular dynamics simulation (MD). Reduced structure functions [F(Q)], reduced pair distribution functions [G(r)], [...] Read more.

The microstructure of a mixed KCl and K₂SO₄ aqueous solution was studied using X-ray scattering (XRS), Raman spectroscopy, and molecular dynamics simulation (MD). Reduced structure functions [F(Q)], reduced pair distribution functions [G(r)], Raman spectrum, and pair distribution functions (PDF) were obtained. The XRS results show that the main peak (r = 2.81 Å) of G(r) shifted to the right of the axis (r = 3.15 Å) with increased KCl and decreased K₂SO₄. The main peak was at r = 3.15 Å when the KCl concentration was 26.00% and the K₂SO₄ concentration was 0.00%. It is speculated that this phenomenon was caused by the main interaction changing, from K-O_W (r = 2.80 Å) and O_W-O_W (r = 2.80 Å), to Cl⁻-O_W (r = 3.14 Å) and K⁺-Cl⁻ (r = 3.15 Å). According to the trend of the hydrogen bond structure in the Raman spectrum, when the concentration of KCl was high and K₂SO₄ was low, the destruction of the tetrahedral hydrogen bond network in the solution was more serious. This shows that the destruction strength of the anion to the hydrogen bond network structure in solution was Cl⁻ > SO₄²⁻. In the MD simulations, the coordination number of O_W-O_W decreased with increasing KCl concentration, indicating that the tetrahedral hydrogen bond network was severely disrupted, which confirmed the results of the Raman spectroscopy. The hydration radius and coordination number of SO₄²⁻ in the mixed solution were larger than Cl⁻, thus revealing the reason why the solubility of KCl in water was greater than that of K₂SO₄ at room temperature. Full article

(This article belongs to the Special Issue Saline Wastewater: Characteristics and Treatment Technologies)

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Review

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18 pages, 1858 KiB

Open AccessReview

Research Updates on the Mechanism and Influencing Factors of the Photocatalytic Degradation of Perfluorooctanoic Acid (PFOA) in Water Environments

by Jie Liang, Lingling Guo, Biao Xiang, Xueyi Wang, Jiaxi Tang and Yue Liu

Molecules 2023, 28(11), 4489; https://doi.org/10.3390/molecules28114489 - 1 Jun 2023

Cited by 4 | Viewed by 3045

Abstract

Perfluorooctanoic acid is ubiquitous in water bodies and is detrimental to the health of organisms. Effectively removing perfluorooctanoic acid (PFOA), a persistent organic pollutant, has been a hot topic around the world. With traditional physical, chemical, and biological methods, it is difficult to [...] Read more.

Perfluorooctanoic acid is ubiquitous in water bodies and is detrimental to the health of organisms. Effectively removing perfluorooctanoic acid (PFOA), a persistent organic pollutant, has been a hot topic around the world. With traditional physical, chemical, and biological methods, it is difficult to effectively and completely remove PFOA, the costs are high, and it is easy to cause secondary pollution. There are difficulties in applying some technologies. Therefore, more efficient and green degradation technologies have been sought. Photochemical degradation has been shown to be a low-cost, efficient, and sustainable technique for PFOA removal from water. Photocatalytic degradation technology offers great potential and prospects for the efficient degradation of PFOA. Most studies on PFOA have been conducted under ideal laboratory conditions at concentrations that are higher than those detected in real wastewater. This paper summarizes the research status of the photo-oxidative degradation of PFOA, and it summarizes the mechanism and kinetics of PFOA degradation in different systems, as well as the influence of key factors on the photo-oxidative degradation and defluoridation process, such as system pH, photocatalyst concentration, etc. PFOA photodegradation technology’s existing problems and future work directions are also presented. This review provides a useful reference for future research on PFOA pollution control technology. Full article

(This article belongs to the Special Issue Saline Wastewater: Characteristics and Treatment Technologies)

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