Efficient adsorption of Cr(VI) in acidic environment by nano-scaled schwertmannite prepared through pH regulation: characteristics, performances, and mechanism

Effects of Fe(II) bio-oxidation rate and alkali control on schwertmannite microstructure and adsorption of oxyanions: Characteristics, performance and mechanism

Schwertmannite has attracted increasing interest for its excellent sorption of oxyanions such as AsO43-, CrO42-, and Sb(OH)6-. Controlling biomineralization by adjusting the Fe(II) oxidation rate and implementing alkali control can enhance the yield and adsorption performance of schwertmannite. However, the adsorption improvement mechanism is still unclear. The morphology, crystallinity, specific surface area (SSA) and oxyanion adsorption of schwertmannite synthesized with alkali control of solution pH and different Fe(II) oxidation rates were analyzed in this study. The differences in the adsorption mechanisms of As(V), Cr(VI) and Sb(V) on schwertmannite obtained under different synthesis conditions were also studied. Reducing the Fe(II) oxidation rate or maintaining the solution pH through alkali control significantly increased the SSA of schwertmannite and the proportion of outer-sphere sulfate. Alkali-controlled schwertmannite (Sch-C) exhibited superior As(V) and Sb(V) adsorption performance and slightly greater Cr(VI) adsorption than non-alkali-controlled schwertmannite. The As(V) and Sb(V) adsorption capacities of Sch-C greatly improved because the ultra-high SSA increased the surface hydroxyl content and reduced the passivation effect of amorphous precipitates on the mineral surface, allowing continuous sulfate exchange at inner mineral sites. An increased surface hydroxyl content had little effect on Cr(VI) adsorption, but an increased proportion of outer-sphere sulfate caused a slight increase in Cr(VI) adsorption. Sb(V) has a stronger hydroxyl exchange ability than As(V), but due to its octahedral structure, it exchanges only with outer-sphere sulfate on schwertmannite and hardly exchanges with inner-sphere sulfate.

The influence of Mn(II) on transformation of Cr-absorbed Schwertmannite: Mineral phase transition and elemental fate

Schwertmannite (Sch) is considered as an effective remover of Chromium (Cr) due to its strong affinity for toxic Cr species. Since the instability of Sch, the environmental fate of Cr deserves attention during the transformation of Sch into a more stable crystalline phase. The ubiquitous manganese(II) (Mn(II)) probably affects the transformation of Sch and thus the environmental fate of Cr. Therefore, this study investigated the impact of Mn(II) on the transformation of Cr-absorbed Sch (Cr-Sch) and the associated behavior of SO42- and Cr. We revealed that the transformation products of Cr-Sch at pH 3.0 and 7.0 were goethite and Sch, respectively. The presence of Mn(II) weakened the crystallinity of the transformation products, and the trend was positively correlated with the concentration of Mn(II). However, Mn(II) changed the transformation products of Cr-Sch from hematite to goethite at pH 10.0. Mn(II) replaced Fe(III) in the mineral structures or formed Mn-O complexes with surface hydroxyl groups (-OH), thereby affecting the transformation pathways of Sch. The presence of Mn(II) enhanced the immobilization of Cr on minerals at pH 3.0 and 7.0. Sch is likely to provide an channel for electron transfer between Mn(II) and Cr(VI), which promotes the reduction of Cr(VI). Meanwhile, Mn(Ⅱ) induced more -OH production on the surface of secondary minerals, which played an important role in increasing the Cr fixation. In addition, part of the Mn(Ⅱ) was oxidized to Mn(Ⅲ)/Mn(Ⅳ) at pH 3.0 and pH 7.0. This study helps to predict the role of Mn(II) in the transformations of Cr-Sch in environments and design remediation strategies for Cr contamination.

Multi-level Reactive Oxygen Species Amplifier to Enhance Photo-/Chemo-Dynamic/Ca2+ Overload Synergistic Therapy

Reactive oxygen species (ROS)-involved photodynamic therapy (PDT) and chemodynamic therapy (CDT) hold great promise for tumor treatment. However, hypoxia, insufficient H2O2, and overexpressed glutathione (GSH) in the tumor microenvironment (TME) hinder ROS generation significantly. Herein, we reported CaO2@Cu-TCPP/CUR with O2/H2O2/Ca2+ self-supply and GSH depletion for enhanced PDT/CDT and Ca2+ overload synergistic therapy. CaO2 nanospheres were first prepared and used as templates for guiding the coordination between the carboxyl of tetra-(4-carboxyphenyl)porphine (TCPP) and Cu2+ ions as hollow CaO2@Cu-TCPP, which facilitated GSH-activated TCPP-based PDT and Cu+-mediated CDT. The hollow structure was then loaded with curcumin (CUR) to form CaO2@Cu-TCPP/CUR composites. Cu-TCPP prevented degradation of CaO2, while Cu2+ ions reacted with GSH to deplete GSH, produce Cu+ ions, and release TCPP, CaO2, and CUR. CaO2 reacted with H2O to generate O2, H2O2, and Ca2+ to achieve O2/H2O2/Ca2+ self-supply for TCPP-based PDT, Cu+-mediated CDT, and CUR-enhanced Ca2+ overload therapy. Thus, this multilevel ROS amplifier enhances synergistic therapy with fewer side effects and drug resistance.

The purification of acid mine drainage through the formation of schwertmannite with Fe(0) reduction and alkali-regulated biomineralization prior to lime neutralization

Acid mine drainage (AMD) contains abundant Fe (II), Fe(III), and SO42-, as well as a large amount of dissolved toxic metals and metalloids, posing a serious threat to the environment. In this study, an integrated technique for the treatment of AMD was proposed. The technique started with pre-oxidation followed by Fe(0) reduction and alkali-regulated biomineralization and then ended with lime neutralization. The technique removed toxic metal oxyanions in the pre-oxidation stage and recovered pure schwertmannite during the subsequent alkali-regulated biomineralization. Fe(III), which could not be directly biomineralized, was reduced to Fe(II) by Fe(0). A small amount of alkali was added to regulate the hydrolytic mineralization reaction after Fe(II) oxidation in AMD, which in a single biomineralization could remove in the form of schwertmannite >95 % of soluble Fe in the AMD. In the subsequent lime neutralization process, the amount of lime required and the sludge produced were reduced by 75.4 % and 84.9 %, respectively, compared to the raw AMD. Additionally, the content of non-ferrous metals in the sludge increased 5.6-fold. Compared with non-alkali-regulated biomineralization, the schwertmannite obtained by the alkali-regulated biomineralization had a higher adsorption capacity for oxyanions (e.g., arsenic, chromium, and antimony). The new approach should significantly reduce the treatment cost of AMD and recover Fe and S elements in the form of valuable secondary minerals, such that it is reasonable to expect that it will be widely adopted in practical applications.

Schwertmannite and akaganéite for adsorption removals of Cr(VI) from aqueous solutions

Iron hydroxides have received high attention in the treatment of chromium (Cr) polluted wastewater. In this study, the obtained chemical (or biological) pincushion-schwertmannite spheres had a diameter of 2 - 5 μm (0.5-1 μm), and akaganéite rods had a length of 300-500 nm (100-150 nm) at an axial ratio of about 3. The average diameters (μm) of their agglomerated particles in solutions were 20.6-32.5 (only 0.480 for Aka-Chem). Schwertmannites and akaganéites were used to investigate Cr(VI) adsorption behaviors in aqueous solutions by batch experiments, under various reaction times, initial Cr(VI) and adsorbent levels, pH values, temperature and anions of NO₃^-, Cl^-, CO₃^2-, SO₄^2-, and H₂PO₄^-. Adsorption data well fitted to pseudo-second-order rate model (R² = 0.999), and Langmuir (R² = 0.954-0.988) and Freundlich (R² = 0.984-0.996) isothermal models at pH 7.0. Maximum Cr(VI) adsorption capacities were 119/133 for Sch-Chem/Sch-Bio, and 14.6/83.6 for Aka-Chem/Aka-Bio. The H₂PO₄^- than SO₄^2-/CO₃^2- had a stronger effect on Cr(VI) adsorption. Adsorbents with pH_ZPC of near to 4.0 still had a good Cr(VI) removal ability at pH 3.0-8.0. The possible Cr(VI) adsorption mechanisms by FTIR and XPS results for schwertmannite and akaganéite were electrostatic attractions and ion exchanges between hydroxyl (or sulfate) and chromate ions. The Cr(VI) adsorption of optimal schwertmannite and bioakaganéite was a spontaneous, endothermic and random process at the temperatures of 288-318 K. They had a good regeneration ability for Cr(VI) adsorption, and removal ratios could reach to about 80% of original values (60-70% in aqueous solution with 60 mg/L Cr(VI) and pH7.0, and 35-50% in wastewater with 120 mg/L Cr(VI) and about pH4.0), after three cycles. Herein, schwertmannite/bioakaganéite have a promising application in treatment of acidic/neutral wastewater with chromate.