Copper nanoparticles/graphene modified green rusts for debromination of tetrabromobisphenol A: Enhanced galvanic effect, electron transfer and adsorption

Enhanced removal of Pb(II) from acid mine drainage using green reduced graphene oxide/silver nanoparticles

Mining activities can potentially release high levels of Pb(II) in acid mine drainage (AMD), which thereafter poses a significant threat to ecological security. In this study, green reduced graphene oxide/silver nanoparticles (rGO/Ag NPs) were successfully synthesized via a one-step approach using a green tea extract and subsequently used as a cost-effective absorbent to remove Pb(II) from AMD. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated that organic functional groups in the green tea extracts, such as C=O-C, CO, and CC, acted both as reductants and stabilizers in the synthesis of rGO/Ag NPs. In addition, the removal efficiency of Pb(II) by rGO/Ag NPs (84.2 %) was much better than either rGO (75.4 %) or Ag NPs (12.3 %) alone. Also, in real AMD, the distribution coefficient (Kd) of Pb(II) (4528 mL/g), was much higher than other heavy metal indicating the adsorbent had a high selective affinity for Pb(II). Interestingly, after five cycles of use, the removal efficiency of Pb(II) by rGO/Ag NPs from AMD actually increased from 46.4 to 65.2 % due to iron oxides (i.e., Fe2O3 and Fe3O4) being generated when rGO/Ag NPs was exposed to AMD. The removal of Pb(II) via adsorption on the rGO/Ag NPs surface involved formation of hexagonal rod-like precipitates. This work demonstrated the potential of rGO/Ag NPs to be continuously used for the removal of Pb(II) from AMD.

Adsorption and Removal of Tetrabromobisphenol A by Adsorption on Functionalized Mesoporous Silica Nanotubes: Isotherms, Kinetics, Thermodynamics, and Optimization via Box-Behnken Design

In pursuit of environmental safety, a novel and efficient method-dispersive solid-phase extraction based on functionalized mesoporous silica nanotubes (FMSNT nanoadsorbent)-was developed to remove tetrabromobisphenol A (TBBPA) from water samples. Characterization and comprehensive analysis of the FMSNT nanoadsorbent, including maximum adsorption capacity of 815.85 mg g^-1 for TBBPA and its water stability, confirmed its potential. Subsequent analysis revealed the impact of multiple factors, for instance pH, concentration, dose, ionic strength, time, and temperature, on the adsorption process. The findings revealed that the adsorption of TBBPA followed the Langmuir and pseudo-second-order kinetics models while primarily driven by hydrogen bond interactions between bromine ions or hydroxyl groups of TBBPA and amino protons around the cavity. The novel FMSNT nanoadsorbent showed high stability and efficiency even after five times of recycling. Moreover, the overall process was identified as chemisorption, endothermic, and spontaneous. Finally, the Box-Behnken design was applied to optimize the results, confirming good reusability even after five cycles.

Catalytic degradation of brominated flame retardants in the environment: New techniques and research highlights

Due to the extensive commercial use of brominated flame retardants (BFRs), human beings are chronically exposed to BFRs, causing great harms to human health, which imposes urgent demands to degrade them in the environment. Among various degradation techniques, catalytic degradation has been proven to be outstanding because of its rapidness and effectiveness. Therefore, much attention has been given to catalytic degradation, especially the extensively studied photocatalytic degradation and nanocatalytic reduction techniques. Recently, some novel advanced catalytic techniques have been developed and show excellent catalytic degradation efficiency for BFRs, including natural substances catalytic degradation, new Fenton catalytic degradation, new chemical reagent catalytic degradation, new material catalytic degradation, electrocatalytic degradation, plasma catalytic degradation, and composite catalytic degradation systems. In addition to the common features of traditional catalytic techniques, these novel techniques possess their own specific advantages in various aspects. Therefore, this review summarized the degradation mechanism of BFRs by the above new catalytic degradation methods under the laboratory conditions, simulated real environment, and real environment conditions, and further evaluated their advantages and disadvantages, aiming to provide some research ideas for the catalytic degradation of BFRs in the environment in the future. We suggested that more attention should focus on features of novel catalytic techniques, including eco-friendliness, cost-effectiveness, and pragmatic usefulness.

Mineralization of tribromophenol under anoxic/oxic conditions in the presence of copper(II) doped green rust: Importance of sequential reduction-oxidation process

The groundwater environment often undergoes the transition from anoxic to oxic due to natural processes or human activities, but the influence of this transition on the fate of groundwater contaminates are not entirely understood. In this work, the degradation of tribromophenol (TBP) in the presence of environmentally relevant iron (oxyhydr)oxides (green rust, GR) and trace metal ions Cu(II) under anoxic/oxic-alternating conditions was investigated. Under anoxic conditions, GR-Cu(II) reduced TBP to 4-BP completely within 7 h while GR only had an adsorption effect on TBP. Under oxic conditions, GR-Cu(II) could generate •OH via dioxygen activation, which resulted in the oxidative transformation of TBP. Sixty-five percentage of TBP mineralization was achieved via a sequential reduction-oxidation process, which was not achieved through single reduction or oxidation process. The produced Cu(I) in GR-Cu(II) enhanced not only the reductive dehalogenation under anoxic conditions, but also the O₂ activation under oxic conditions. Thus, the fate of TBP in anoxic/oxic-alternating groundwater environment is greatly influenced by the presence of GR-Cu(II). The sequential reduction-oxidation degradation of TBP by GR-Cu(II) is promising for future remediation of TBP-contaminated groundwater.

Pyridinic nitrogen enables dechlorination of trichloroethylene to acetylene by green rust: Performance, mechanism and applications

Carbonous materials were found to catalyze the dechlorination of trichloroethylene (TCE) by green rust (GR), but the catalytic mechanism was not fully understood. We have developed a facile ball milling method to synthesize N-doped graphene (NG) with various N species, catalyzing fast dechlorination of TCE to acetylene by GR with the highest acetylene production rate of ~0.1 d^-1. The adsorption of TCE onto NG is mainly derived from the graphene region of NG, and high pyridinic N is essential for the enhanced TCE reduction by GR. Oxygen species did not enhance the TCE reduction in GR/NG system. High dechlorination rates are correlated to a high amount of defect in NG and a high electron conductivity of NG. Pyridinic N has the highest adsorption energy for TCE among all the N species, which leads to the highest catalytic performance. High electrochemically active surface area resulted from the high content of pyridinic N facilitate the NG-catalyzed dechlorination. The acetylene production rate in real groundwater is still around one-third of that in ultrapure water. This work not only reveals the catalytic mechanism of NG-catalyzed dechlorination by GR, but also provide a feasible approach for practical remediations of TCE-contaminated groundwater using GR-NG mixture.

Generation of atomic hydrogen by Ni-Fe hydroxides: Mechanism and activity for hydrodechlorination of trichloroethylene

Atomic hydrogen (H•) is highly reactive for the hydrodechlorination of trichloroethylene (TCE). In this work, we found that the coprecipitation of Ni²⁺ and Fe²⁺ at neutral pH led to an unprecedented catalytic generation of H•. The generated H• effectively dechlorinate TCE to nontoxic ethylene and ethane, and Fe²⁺ is the only electron donor. The abundant adsorbed H• produced with a Ni/Fe ratio of 0.4 enhances hydrogen evolution reaction causing a low efficiency for hydrodechlorination. In contrast, the active absorbed H• is generated in the crystal lattice of Ni-Fe hydroxides with a Ni/Fe ratio of 3.0 causing highly efficient hydrodechlorination of TCE. This work not only reveals the mechanism of catalytic hydrodechlorination by Ni-Fe hydroxides at neutral pH, but also provides a novel approach to detoxify TCE in contaminated water using facile precipitated Ni-Fe hydroxides.

A green rust-coated expanded perlite particle electrode-based adsorption coupling with the three-dimensional electrokinetics that enhances hexavalent chromium removal

A green rust-coated expanded perlite (GR-coated Exp-p) microelectrode was synthesized and incorporated into a column-mode three-dimensional electrokinetic (3D-EK) platform to effectively pursue a continuous Cr(VI) removal from the aqueous solution. Brucite-like layers of GR were decorated onto the Exp-p material. The molar ratio of Fe(II) to Fe(III) played a most vital role among the three synthesis factors in influencing the performance of the particle electrode. For the equilibrium adsorption experiments, the target maximum adsorption capacity of 122 mg/g was predicted by a target optimizer and desirability function at the conditions following the pH of 4.7, the initial concentration of 172.4 mg/L, the dosage of 0.28 g/L, and the temperature of 28.96 °C, respectively. SO₄^2-, Cl^-, and NO₃^- fiercely competed with Cr(VI) anions in the acidic conditions for the locally positive sites. A low concentration and a slow flow were favored in the column-mode 3D-EK platform. The pseudo-first-order and Langmuir models were suitable for describing the kinetics and isotherms of the adsorption process, respectively. Cr(VI) anions were electrostatically attracted to the silanol groups and GR surface of the adsorbent, subsequently reduced in both heterogeneity and homogeneity, and finally immobilized by coordinating with silanediol groups and silanetriol groups.