Cr(VI)-bioremediation mechanism of a novel strain Bacillus paramycoides Cr6 with the powerful ability to remove Cr(VI) from contaminated water

Extracellular bioreduction is the main Cr(VI) detoxification strategy of Bacillus sp. HL1

Bacterium with high Cr(VI) detoxification capability belonged to the genus Bacillus have been largely explored, yet their reduction strategies are still in debate. Cr(VI) removal performance and mechanism of Bacillus sp. HL1 isolated from tailings a site was comprehensively investigated in this study. Approximately 88.31% of 100 mg/L Cr(VI) was continuously removed within 72 h, while it could resist up to 300 mg/L Cr(VI). Metal ions Mn2+ and Cu2+ could effectively improve the Cr(VI) removal performance to 14.41% and 3.41% under the optimal conditions, respectively. Cr(VI) removal performances by subcellular extracts showed that nearly 45.28% of 100 mg/L extracellular Cr(VI) was efficaciously reduced to Cr(III), while only 14.27%, 6.40%, and 2.73% of the cell-free extract, resting cells, and cell debris were reduced, respectively. This suggested that extracellular bioreduction was the primary Cr(VI) detoxification strategy despite a small part of Cr(VI) reduction took place intracellularly. In particular, the reduction products of the intracellular and extracellular compounds significantly differed, with organo-Cr(III) complex outside the cell and crystalline Cr(III) precipitate inside. Such observation was also evidenced by the intracellular black precipitate observed in the TEM image. XRD, XPS, and EPR analysis showed different Cr(III) compositions of intracellular and extracellular products. This study deepens our insights into the different fates of microorganisms that reduce Cr(VI) intracellularly and extracellularly.

Growth-dependent cr(VI) reduction by Alteromonas sp. ORB2 under haloalkaline conditions: toxicity, removal mechanism and effect of heavy metals

Bacterial reduction of hexavalent chromium (VI) to chromium (III) is a sustainable bioremediation approach. However, the Cr(VI) containing wastewaters are often characterized with complex conditions such as high salt, alkaline pH and heavy metals which severely impact the growth and Cr(VI) reduction potential of microorganisms. This study investigated Cr(VI) reduction under complex haloalkaline conditions by an Alteromonas sp. ORB2 isolated from aerobic granular sludge cultivated from the seawater-microbiome. Optimum growth of Alteromonas sp. ORB2 was observed under haloalkaline conditions at 3.5-9.5% NaCl and pH 7-11. The bacterial growth in normal culture conditions (3.5% NaCl; pH 7.6) was not inhibited by 100 mg/l Cr(VI)/ As(V)/ Pb(II), 50 mg/l Cu(II) or 5 mg/l Cd(II). Near complete reduction of 100 mg/l Cr(VI) was achieved within 24 h at 3.5-7.5% NaCl and pH 8-11. Cr(VI) reduction by Alteromonas sp. ORB2 was not inhibited by 100 mg/L As(V), 100 mg/L Pb(II), 50 mg/L Cu(II) or 5 mg/L Cd(II). The bacterial cells grew in the medium with 100 mg/l Cr(VI) contained lower esterase activity and higher reactive oxygen species levels indicating toxicity and oxidative stress. In-spite of toxicity, the cells grew and reduced 100 mg/l Cr(VI) completely within 24 h. Cr(VI) removal from the medium was driven by bacterial reduction to Cr(III) which remained in the complex medium. Cr(VI) reduction was strongly linked to aerobic growth of Alteromonas sp. The Cr(VI) reductase activity of cytosolic protein fraction was pronounced by supplementing with NADPH in vitro assays. This study demonstrated a growth-dependent aerobic Cr(VI) reduction by Alteromonas sp. ORB2 under complex haloalkaline conditions akin to wastewaters.

One-step synthesis of ferrous disulfide and iron nitride modified hydrochar for enhanced adsorption and reduction of hexavalent chromium in Bacillus LD513 by promoting electron transfer and microbial metabolism

Microbial immobilization technology is effective in improving bioremediation efficiency and heavy metal pollution. Herein, Bacillus LD513 with hexavalent chromium (Cr(VI)) tolerance was isolated and immobilized on a novel ferrous disulfide (FeS2)/iron nitride (FeN) modified hydrochar (Fe3-SNHC) prepared from waste straws. The prepared Fe3-SNHC-based LD513 (FeLD) significantly improves Cr(VI) adsorption and reduction by 31.4 % and 15.7 %, respectively, compared to LD513 alone. Furthermore, the FeLD composite system demonstrates efficient Cr(VI) removal efficiency and good environmental adaptability under different culture conditions. Microbial metabolism and electrochemical analysis indicate that Fe3-SNHC is an ideal carrier for protecting LD513 activity, promoting extracellular polymer secretion, and reducing oxidative stress. Additionally, the carrier serves as an electron shuttle that accelerates electron transfer and promotes Cr(VI) reduction. Overall, FeLD is an environmentally friendly biocomposite that shows good promise for reducing Cr(VI) contamination in wastewater treatment.

Tetracoordinate Fe3+ Activated Li2ZnAO4 (A = Si, Ge) Near-Infrared Luminescent Phosphors

Fe3+-doped near-infrared (NIR) phosphors have received a lot of interest because they are nontoxic, inexpensive, and ecologically benign. In this work, Fe3+-activated Li2ZnAO4 (A = Si, Ge) phosphors were synthesized by solid-phase reactions, in which Fe3+ entered the Zn2+ tetrahedral site. When excited by 300 nm UV light, broad NIR emission bands at 750 nm (Li2ZnSiO4: Fe3+) and 777 nm (Li2ZnGeO4: Fe3+) were observed, with internal quantum efficiencies (IQE) of 62.70% (Li2ZnSiO4: Fe3+) and 30.57% (Li2ZnGeO4: Fe3+). The thermal stability was increased from 35.43 to 49.79% at 373 K via cationic regulation. The combination of activation energy, electron-phonon coupling, and Debye temperature explained the improved thermal stability of Li2ZnGeO4: Fe3+ phosphor. Besides, the as-synthesized phosphor demonstrated sensitive and selective Cu2+ ion detection.

Novel efficient capture of hexavalent chromium by polyethyleneimine/amyloid fibrils/polyvinyl alcohol aerogel beads: Functional design, applicability, and mechanisms

The harmful effects of hexavalent chromium (Cr(VI)) on the environment and human health have aroused wide public concern. In this study, bulk spherical aerogel beads (PAP) were synthesized from polyethyleneimine (PEI), protein amyloid fibrils (AFL), and polyvinyl alcohol (PVA) through green technology and its removal of Cr(VI) from wastewater was comprehensively studied. The results showed that although the bulk PAP beads (∼ 5 mm) only had an average pore size of 16.88 nm and a BET surface area of 12 m²/g, its maximum adsorption capacity for Cr(VI) reached 121.44 mg/g (at 298 K). Cr(VI) adsorption onto PAP conformed to pseudo-second-order adsorption kinetics and was endothermic. The adsorption of Cr(VI) decreased stepwise with the increase of solution alkalinity (pH = 2: 91.97%; pH = 10: 0.04%). Importantly, PAP showed high selectivity towards Cr(VI) in mixed heavy metal solutions (Cr(VI) > Pb(II) > Ni(II) > Cu(II) > Cd(II)) and good reusability (removal efficiency > 88% after 5 cycles). PAP had excellent anti-interference ability against FA and HCO₃^- with the overall removal rate exceeding 87% in the presence of 5 - 25 mg/L of these ions. Cations such as Na⁺, Mg²⁺, and other heavy metal ions at high concentrations could promote the removal efficiency of Cr(VI). The removal rates of Cr(VI) and Cr(III) by PAP in a tannery wastewater were 34.4% and 59.3%, respectively. Meanwhile, the removal rates of Cr(VI) in a electroplating wastewater and a contaminated soil leachate reached 84.4∼89.7%, showing high practicability. Mechanism studies revealed that electrostatic attraction, hydrogen bonding, reduction, and complexation were the main reactions for Cr(VI) removal by PAP. In general, the study of PAP provides a new insight into using bulk monolith materials for treating Cr(VI) contaminated wastewater.