Negatively-charged nanofiltration membrane and its hexavalent chromium removal performance

Electrically Controlled Adsorptive Membranes with Tunable Affinity for Selective Chromium (VI) Separation from Water

Ionic contaminants such as Cr(VI) pose a challenge for water purification using membrane-based processes. However, existing membranes have low permeability and selectivity for Cr(VI). Therefore, in this study, we prepared an electrically controlled adsorptive membrane (ECAM-L) by coating a loose Cl--doped polypyrrole layer on a carbon nanotube substrate, and we evaluated the performance of ECAM-L for Cr(VI) separation from water. We also used electrochemical quartz crystal microbalance measurements and molecular dynamics and density functional theory calculations to investigate the separation mechanisms. The adsorption and desorption of Cr(VI) could be modulated by varying the electrostatic interactions between ECAM-L and Cr(VI) via potential control, enabling the cyclic use of the ECAM-L without additional additives. Consequently, the oxidized ECAM-L showed high Cr(VI) removal performance (<50 μg/L) and treatment capacity (>3500 L/m2) at a high water flux (283 L/m2/h), as well as reusability after the application of a potential. Our study demonstrates an efficient membrane design for water decontamination that can selectively separate Cr(VI) through a short electric stimulus.

Chromium (III) and chromium (VI) removal and organic matter interaction with nanofiltration

Chromium (Cr) is a toxic inorganic contaminant for drinking water, in which the concentration has to be controlled for human health and safety. Cr retention was investigated with stirred cell experiments using sulphonated polyethersulfone nanofiltration (NF) membranes of different molecular weight cut-off (MWCO). Cr(III) and Cr(VI) retention follow the order of the MWCO of the studied NF membranes; HY70-720 Da > HY50-1000 Da > HY10-3000 Da with a pH dependency, especially for Cr(III). The importance of the charge exclusion was highlighted when Cr(OH)₄^- (for Cr(III)) and CrO₄^2- (for Cr(VI)) was the predominant species in the feed solution. In presence of organic matter, namely humic acid (HA), Cr(III) retention increased by 60 %, while no influence of HA was observed for Cr(VI). HA did not induce major modifications on the membrane surface charge for these membranes. Solute-solute interaction, in particular Cr(III)-HA complexation, was the responsible mechanism for the increase in Cr(III) retention. This was confirmed by asymmetric flow field-flow fractionation, coupled with inductively coupled plasma mass spectrometry (FFFF-ICP-MS) analysis. Cr(III)-HA complexation was significant at HA concentrations as low as 1 mgC/L. The chosen NF membranes were able to achieve the EU guideline (25 μg/L) for Cr in drinking water for a feed concentration of 250 μg/L.

Immobilization of Hexavalent Chromium Using Self-Compacting Soil Technology

A study of immobilization of hexavalent chromium in the form of Na₂CrO₄ salt by self-compacting soils (SCS) is presented. Carbofill E additive was used as SCS binder. The efficiency of immobilization of Cr (VI) was evaluated by washing out chromium compounds from SCS samples. The influence of the nature of the soil and the content of Carbofill E and Na₂CrO₄ in the SCS samples on the efficiency of Cr (VI) immobilization was studied. It was found that the nature of the soil and the content of Carbofill E in the SCS samples affect the immobilization of Cr (VI). Moreover, increasing the Carbofill E content in SCS samples further increases Cr (VI) immobilization. X-ray diffraction studies of the samples with immobilized hexavalent chromium showed that part of the sample transforms from a readily soluble form of salt into oxide forms of chromium and calcium-chromium, which are practically insoluble in water.

Rejection Mechanism of Ionic Solute Removal by Nanofiltration Membranes: An Overview

The toxicity of heavy metals can cause water pollution and has harmful effects on human health and the environment. Various methods are used to overcome this pressing issue and each method has its own advantages and disadvantages. Membrane filtration technology such as nanofiltration (NF) produces high quality water and has a very small footprint, which results in lower energy usage. Nanofiltration is a membrane-based separation technique based on the reverse osmosis separation process developed in the 1980s. NF membranes have a pore size of 1 nm and molecular weight cut off (MWCO) of 300 to 500 Da. The properties of NF membranes are unique since the surface charge of the membranes is dependent on the functional groups of the membrane. The rejection mechanism of NF membrane is unique as it is a combination of various rejection mechanisms such as steric hindrance, electric exclusion, dielectric effect, and hydration mechanism. However, these mechanisms have not been studied in-depth due to their complexity. There are also many factors contributing to the rejection of NF membrane. Many junior researchers would face difficulty in studying NF membrane. Therefore, this paper is designed for researchers new to the field, and will briefly review the rejection mechanisms of NF membrane by both sieving and non-sieving separation processes. This mini-review aims to provide new researchers with a general understanding of the concept of the separation process of charged membranes.

Ionic Dendrimer Based Polyamide Membranes for Ion Separation

Separating low/high-valent ions with sub-nanometer sizes is a crucial yet challenging task in various areas (e.g., within environmental, healthcare, chemical, and energy engineering). Satisfying high separation precision requires membranes with exceptionally high selectivity. One way to realize this is constructing well-designed ion-selective nanochannels in pressure-driven membranes where the separation mechanism relies on combined steric, dielectric exclusion, and Donnan effects. To this aim, charged nanochannels in polyamide (PA) membranes are created by incorporating ionic polyamidoamine (PAMAM) dendrimers via interfacial polymerization. Both sub-10 nm sizes of the ionic PAMAM dendrimer molecules and their gradient distributions in the PA nanofilms contribute to the successful formation of defect-free PA nanofilms, containing both internal (intramolecular voids) and external (interfacial voids between the ionic PAMAM dendrimers and the PA matrix) nanochannels for fast transport of water molecules. The external nanochannels with tunable ionizable groups endow the PA membranes with both high low/high-valent co-ion selectivity and chemical cleaning tolerance, while the ion sieving/transport mechanism was analyzed by employing the Donnan steric pore model with dielectric exclusion.

Pressure-Driven Membrane Process: A Review of Advanced Technique for Heavy Metals Remediation

Pressure-driven processes have come a long way since they were introduced. These processes, namely Ultra-Filtration (UF), Nano-Filtration (NF), and Reverse-Osmosis (RO), aim to enhance the efficiency of wastewater treatment, thereby aiming at a cleaner production. Membranes may be polymeric, ceramic, metallic, or organo-mineral, and the filtration techniques differ in pore size from dense to porous membrane. The applied pressure varies according to the method used. These are being utilized in many exciting applications in, for example, the food industry, the pharmaceutical industry, and wastewater treatment. This paper attempts to comprehensively review the principle behind the different pressure-driven membrane technologies and their use in the removal of heavy metals from wastewater. The transport mechanism has been elaborated, which helps in the predictive modeling of the membrane system. Fouling of the membrane is perhaps the only barrier to the emergence of membrane technology and its full acceptance. However, with the use of innovative techniques of fabrication, this can be overcome. This review is concluded with perspective recommendations that can be incorporated by researchers worldwide as a new problem statement for their work.

Eco-Friendly Adsorbent from Waste of Mint: Application for the Removal of Hexavalent Chromium

A serious environmental disaster is looming on the horizon due to the indiscriminate release of heavy metals into the soil and wastewater from human industrial practices. In this study, waste mint (WM) was used to remove chromium(VI) from aqueous solution using batch experiments. The adsorbent material (WM) was characterized using scanning electron microscopy coupled with energy dispersive analysis of X-ray spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). The adsorption parameters optimized were as follows: pH solution (2–11), initial concentration of Cr(VI) (10–50 mg/L), adsorbent dose (0.1–10 g/L), and temperature conditions (298 K, 308 K, and 318 K). The experimental data fitted well to the fractional power kinetic model (0.97≤R2≤ 0.99) and Langmuir isotherm (R2 = 0.984) with a maximum adsorption capacity Qmax = 172.41 mg/g. The thermodynamic parameters for Cr(VI) sorption were also calculated, confirming that the adsorption process was spontaneous and accompanied by an exothermic adsorption (−4.83 ≤ ΔG ≤ −3.22 kJ/mol and ΔH = −28.93 kJ/mol). The Cr(VI) removal percentage was within the range of 41–98%, and the highest removal was noted at pH = 2. The results of the present study suggest that WM is a potential low-cost adsorbent for the removal of chromium(VI) from aqueous solutions.

Electrochemical Removal of Organic and Inorganic Pollutants Using Robust Laser-Induced Graphene Membranes

The removal of emerging environmental pollutants in water and wastewater is essential for high drinking water quality or for discharge to the environment. Electrochemical treatment is a promising technology shown to degrade undesirable organic compounds or metals via oxidation and reduction, and carbon-based electrodes have been reported. Here, we fabricated a robust, porous laser-induced graphene (LIG) electrode on a commercial water treatment membrane using the multilasing technique and demonstrated the electrochemical removal of iohexol, an iodine contrast compound, and chromium(VI), a highly toxic heavy metal ion. Multiple lasing resulted in a more ordered graphitic lattice, a more physically robust carbon layer, and a 3-4-fold higher electrical conductivity. These properties ultimately led to a more efficient electrochemical process, and the optimized LIG electrodes showed a higher hydrogen peroxide (H₂O₂) generation. At 3 V, 90% of Cr(VI) was removed after 6 h and reached >95% removal after 8 h at pH 2. Cr(VI) was mainly reduced to Cr(III), with small amounts of Cr(I) and Cr(0), which were partially deposited on the electrode membrane surface, confirmed with X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy analysis. Under the same conditions, 50% of iohexol was degraded after 6 h and the transformation products (TPs) were identified using ultra-performance liquid chromatography coupled with mass spectroscopy. A total of seven main intermediates were identified including deiodinated TPs (m/z = 695, 570, and 443), probably occurring via three transformation pathways including oxidative deiodination, amide hydrolysis, and deacetylation. The electrical energy costs calculated for the removal of 2 mg L^-1 Cr(VI) was ∼$0.08/m³ in this system. Taken together, the porous LIG electrodes might be utilized for electrochemical removal of emerging contaminants in multiple applications because they can be rapidly formed on flexible polymer substrates at low cost.

Magnetic magnesium ferrite-doped multi-walled carbon nanotubes: an advanced treatment of chromium-containing wastewater

Magnetic magnesium ferrite (MgFe₂O₄) nanoparticles (MMFNPs) were synthesized by employing the sol-gel method. These nanoparticles were ultrasonically decorated onto the multi-walled carbon nanotubes (MWCNTs) to produce magnetic magnesium ferrite nanocomposites (MMFNCs). The as-prepared materials were investigated for their capability to treat wastewater loaded with heavy metals. The synthesized nanocomposites were characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transmission infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and zeta analyzer. Besides, the effect of the environmental chemistry of the solution was determined by varying the critical parameters. The adsorption isotherm of Cr(VI) adsorption onto the as-synthesized MMFNC best fitted the Langmuir adsorption isotherm model. The high adsorption capacity of 175.43 mg/g was achieved at a temperature of 40 °C under optimized conditions. Due to the magnetic nature of MMFNC, they are easily recoverable from the aqueous solution making them cost-friendly. Even after seven consecutive adsorption-desorption cycles, the MMFNC presented an efficiency loss of less than 20% for the removal of Cr(VI) ions. The presented development method offers prospects in developing a highly effective magnetic adsorbent for heavy metal removal from wastewater.