Bentonite modification with hexadecylpyridinium and aluminum polyoxy cations and its effectiveness in Se(IV) removal

Preparation and Characterization of a Bentonite-Based Hybrid Gel for Coal Spontaneous Combustion Prevention

This paper presents an investigation of the feasibility of intercalating lignocellulose/xanthan gum (XG) and organic polymers into bentonite to obtain an efficient fire extinguishing gel material. The bentonite-based hybrid gel was prepared by adding polyacrylates, Al(OH)₃, lignocellulose, and XG into a bentonite suspension, and the resulting gel was characterized. The results showed that no cracking and powdering were found on the surface of the hybrid gel due to the formation of the cross-linked network in the bentonite, and a wide mesopore size distribution and good thermal stability were observed. The hybrid gel also exhibits a wide range of water adsorption ratios, excellent water retention, adjustable gelation times, shear thinning characteristics, and improved compressive strength (the yield stress reaches up to 13 MPa). Based on these characterizations, the mechanism of hybrid gel formation is proposed. The inhibition performance of the hybrid gel on coal spontaneous combustion indicates that the addition of the gel slows down the oxygen chemisorption and thus increases the ignition temperature. Due to the presence of the hybrid gel in the coal, the crossing point temperatures were increased and the lowest CO concentration was produced.

Materials interacting with inorganic selenium from the perspective of electrochemical sensing

Inorganic selenium, the most common form of harmful selenium in the environment, can be determined using electrochemical sensors, which are compact, fast, reliable and easy-to-operate devices. Despite progress in this area, there is still significant room for developing high-performance selenium electrochemical sensors. To achieve this, one should take into account (i) the electrochemical process that selenium undergoes on the electrode; (ii) the valence state of selenium species in the sample and (iii) modification of the sensor surface by a material with high affinity to selenium. The goal of this review is to provide a knowledge base for these issues. After the Introduction section, mechanisms and principles of the electrochemical reduction of selenium are introduced, followed by a section introducing the modification of electrodes with materials interacting with selenium and a section dedicated to speciation methods, including the reduction of non-detectable Se(VI) to detectable Se(IV). In the following sections, the main types of materials (metallic, polymers, hybrid (nano)materials…) interacting with inorganic selenium (mostly absorbents) are reviewed to show the diversity of properties that may be endowed to sensors if the materials were to be used for the modification of electrodes. These features for the main material categories are outlined in the conclusion section, where it is stated that the engineered polymers may be the most promising modifiers.

Investigation on the efficient separation and recovery of Se(IV) and Se(VI) from wastewater using Fe–OOH–bent

Decontamination of the toxic selenium compound, selenite (Se(IV)) and selenate (Se(VI)), from wastewater is imperative for environmental protection. Efficient approaches to remove Se(IV) and Se(VI) are in urgent needs. In this work, an accessible adsorbent Fe–OOH–bent was prepared and applied for the removal of Se(IV) and Se(VI) from wastewater. The batch experimental results demonstrate that Fe–OOH–bent exhibits high adsorption capacities of 5.01 × 10−4 and 2.28 × 10−4 mol/g for Se(IV) and Se(VI) respectively, which are higher than most of the reported bentonite based materials, especially in the case of Se(VI). Moreover, the Fe–OOH–bent displayed superior selectivity towards Se(IV) and Se(VI) even in the presence of excess competitive anions (Cl−, HCO3 −, NO3 −, SO4 2− and PO4 3−) and HA with concentrations of 1000 times higher than Se(IV) and Se(VI). By evaluating the adsorption ratio of Se(IV) and Se(VI), the reusability of Fe–OOH–bent was great through five adsorption-desorption cycles. For practical application, the column experiments were performed with simulated wastewater samples. The breakthrough and eluting curves of Se(IV) and Se(VI) were investigated through the columns packed with Fe–OOH–bent, and the results show that Se(IV) and Se(VI) can be successfully separated and recovered using 0.1 mol/L Na2SO4 (pH = 9.0) and 0.1 mol/L Na3PO4 (pH = 9.0), respectively. Our work provides a new approach for fractional separation as well as the recovery of Se(IV) and Se(VI) from wastewater.

An approach for the efficient immobilization of 79Se using Fe-OOH modified GMZ bentonite

Because of high mobility, the immobilization of long-lived fission product 79Se (often existed as 79Se(IV) and 79Se(VI) anions) is a critical consideration in the repository of high-level radioactive waste. In this work, a Fe-OOH modified bentonite (Fe-OOH-bent) was synthesized as a potential filling material in the repository site for effective adsorption and present the migration of different species of 79Se. The adsorbent was characterized using FT-IR, XRD, XFS, zeta potential and BET to clarify its physical properties, compositions and structures. A good thermal and radiation stabilities of Fe-OOH-bent was confirmed by its stable uptake ratio for Se(IV) and Se(VI) compared to original samples. The batch experimental results show that Se(IV) and Se(VI) can be efficiently removed from aqueous by Fe-OOH-bent within 60 min with maximum adsorption capacities of 68.45 mg/g for Se(IV) and 40.47 mg/g for Se(VI) in the optimal conditions, indicating its high potential application in consideration of its simple synthesis process, low cost and high adsorption capacity in view of immobilization of 79Se. The surface species and variation of oxide state of Fe as well as Se(IV) and Se(VI) onto Fe-OOH-bent were investigated by XPS analysis. The values of relative area of Se(IV)–O and Se(VI)–O in XPS spectra followed the same tendency as their adsorption ratio with the variation of system pH, suggesting that the formation of complexes between selenium species and Fe-OOH-bent surface.

Evaluation of electrokinetics coupled with a reactive barrier of activated carbon loaded with a nanoscale zero-valent iron for selenite removal from contaminated soils

The range between dietary deficient and toxic levels for selenium is quite narrow. In this study, the synergistic effects of electrokinetics (EK) and a permeable reactive barrier (PRB) on the reductive sequestration of Se(IV) oxyanions from spiked soils were investigated in detail. Activated charcoal (AC)-supported Fe(II) and nanoscale zero-valent iron (nZVI) were prepared as the PRB media for use in an electrolyzer. In aqueous equilibrium adsorption tests, the AC-supported nZVI medium had a higher adsorption capacity than that of the other adsorbents. The Se(IV) removal isotherms were well-fitted using the Langmuir model. The Se(IV) removal rates were accurately predicted by both pseudo-first- and pseudo-second-order kinetic models. For the coupled systems, a moderate increase in the number of PRBs and decrease in the PRB thickness in the electrolyzer enhanced the removal and catalytic recovery of Se(IV) from the spiked soil samples. A Se(VI) removal efficiency of approximately 95% and Se(VI) reduction efficiency of 90% were achieved in the optimized electrochemical system. The Se(IV) species were reduced to Se° and FeSe by the AC-supported nZVI regardless of the pH distribution. The experimental results provide guidance for the multichannel recovery of Se from abandoned ore tailings or solid wastes.

Rapid Simultaneous Removal of Toxic Anions [HSeO3]-, [SeO3]2-, and [SeO4]2-, and Metals Hg2+, Cu2+, and Cd2+ by MoS42- Intercalated Layered Double Hydroxide

We demonstrate fast, highly efficient concurrent removal of toxic oxoanions of Se(VI) (SeO₄^2-) and Se(IV) (SeO₃^2-/HSeO₃^-) and heavy metal ions of Hg²⁺, Cu²⁺, and Cd²⁺ by the MoS₄^2- intercalated Mg/Al layered double hydroxide (MgAl-MoS₄-LDH, abbr. MoS₄-LDH). Using the MoS₄-LDH as a sorbent, we observe that the presence of Hg²⁺ ions greatly promotes the capture of SeO₄^2-, while the three metal ions (Hg²⁺, Cu²⁺, Cd²⁺) enable a remarkable improvement in the removal of SeO₃^2-/HSeO₃^-. For the pair Se(VI)+Hg²⁺, the MoS₄-LDH exhibits outstanding removal rates (>99.9%) for both Hg²⁺ and Se(VI), compared to 81% removal for SeO₄^2- alone. For individual SeO₃^2- (without metal ions), 99.1% Se(IV) removal is achieved, while ≥99.9% removals are reached in the presence of Hg²⁺, Cu²⁺, and Cd²⁺. Simultaneously, the removal rates for these metal ions are also >99.9%, and nearly all concentrations of the elements can be reduced to <10 ppb, a limit acceptable for drinking water. The maximum sorption capacities for individual Se(VI) and Se(IV) are 85 and 294 mg/g, respectively. The 294 mg/g capacity for Se(IV) reaches a record value, placing the MoS₄-LDH among the highest-capacity selenite adsorbing materials described to date. More interestingly, the presence of metal ions extremely accelerates the capture of the selenium oxoanions because of the reactions of the metal ions with the interlayer MoS₄^2- anions. The sorptions of Se(VI)+Hg and Se(IV)+M (M = Hg²⁺, Cu²⁺, Cd²⁺) are exceptionally rapid, showing >99.5% removals for Hg²⁺ within 1 min and ∼99.0% removal for Se(VI) within 30 min, as well as >99.5% removals for pairs Cu²⁺ and Se(IV) within 10 min, and Cd²⁺ and Se(IV) within 30 min. During the sorption of SeO₃^2-/HSeO₃^-, reduction of Se(IV) occurs to Se⁰ caused by the S^2- sites in MoS₄^2-. Sorption kinetics for the oxoanions follows a pseudo-second-order model consistent with chemisorption. The intercalated material of MoS₄-LDH is very promising as a highly effective filter for decontamination of water with toxic Se(IV)/Se(VI) oxoanions along with heavy metals such as Hg²⁺, Cd²⁺, and Cu²⁺.