Tartaric acid-induced photoreductive dissolution of schwertmannite loaded with As(III) and the release of adsorbed As(III)

Arsenic removal from coal by ferric chloride enhanced leaching under ultraviolet irradiation during flue gas desulphurization with coal slurry

During coal combustion, the harmful element arsenic can be released into environment and cause potential significant harm to human beings. Therefore, it is very important to study the removal of arsenic from coal before combustion. In this work, simulated SO2-containing flue gas was used to leach arsenic from coal in a 1 L UV photoreactor. The effects of FeCl3, ultraviolet (UV), pH and the Cl-/Fe3+ molar ratio on arsenic leaching and SO2 removal were experimentally investigated and the enhancing mechanism was analysed. Experimental results demonstrated that FeCl3 and UV could efficiently increase iron and arsenic leaching percentages and SO2 removal efficiency. UV irradiation could induce the oxidation of most trivalent arsenic. The arsenic leaching percentage was significantly larger than that of iron. Low pH was favourable for iron and arsenic leaching. The optimal Cl-/Fe3+ molar ratio was determined to be 3:1. The introduced ferric chloride could not only increase the concentrations of free radicals and ferric iron oxidants, the chloride ion might also impede the formation of passive coatings, thus increasing the arsenic leaching percentage, intensifying the oxidation of trivalent arsenic and enhancing the removal of SO2.

Effect of biochar-derived DOM on contrasting redistribution of chromate during Schwertmannite dissolution and recrystallization

Biochar-derived dissolved organic matter (BDOM), is extensively involved in the recrystallization of minerals and the speciation alteration of associated toxic metals. This study investigates how BDOM extracted from tobacco petiole (TP) or tobacco stalk (TS) biochar influences the speciation repartitioning of Cr(VI) in environments impacted by acid mine drainage (AMD), focusing on interactions with secondary minerals during Schwertmannite (Sch) dissolution and recrystallization. TP-BDOM, rich in lignin-like substances, slowed down the Cr-Sch dissolution and Cr release under acidic conditions compared to TS-BDOM. TP-BDOM's higher O/C component exerts a delayed impact on Cr-Sch stability and Cr(VI) reduction. In-situ ATR-FTIR and 2D-COS analysis showed that carboxylic and aromatic N-OH groups in BDOM could interact with Cr-Sch surfaces, affecting sulfate and Cr(VI) release. It was also observed that slight recrystallization occurred from Cr-Sch to goethite, along with increased Cr incorporation into secondary minerals within TS-BDOM. This enhances our understanding of BDOM's role in Cr(VI) speciation changes in AMD-contaminated sites.

Effects of pH on coprecipitation of As(III) with biogenic synthesized schwertmannite and jarosite

Secondary iron minerals play significant roles in the immobilization of As under acidic conditions, such as acid mine drainage. However, previous research works have not clarified the effect of pH on As(III) removal through coprecipitation with secondary minerals. Therefore, in this study, we aimed to investigate the discrepancy in As(III) coprecipitation with biogenic synthesized schwertmannite (Sch) and jarosite (Jar) at different pH values. For this, concentrations of Fe2+, TFe, S O 4 2 - , and As(III) in shake flasks were monitored during an overall incubation period of 83 h at initial pH of 1.5, 2.0, and 2.5. In addition, the physicochemical properties of collected minerals after incubation were identified using scanning electron microscopy, X-ray diffraction, pore size distribution, and Brunauer - Emmett - Teller surface area analyses. Our results showed that almost no mineral synthesis and no As(III) removal were detected in coprecipitated schwertmannite (Co-Sch) system and coprecipitated jarosite (Co-Jar) system at an initial pH of 1.5. The TFe precipitation efficiencies and As(III) removal efficiencies increased considerably and morphologies of Co-Sch and Co-Jar improved significantly when the initial pH value increased from 2.0-2.5. The maximum TFe precipitation efficiency and As(III) removal efficiency reached 30.8% and 89.6%, respectively, for the Co-Sch system, and were 47.5% and 37.4%, respectively, for the Co-Jar system. The overall results show that pH significantly affects the formation of Co-Sch and Co-Jar and the behaviour of As(III) coprecipitation.

Arsenic removal and stabilization behavior of schwertmannite@BC (Sch@BC) in contaminated dual media (water/soil): Via sulfate exchange and chemical complexation

Arsenic (As) is extremely harmful to the ecological environment and human health owing to its high toxicity. The composite that biochar (BC) modified by Schwertmannite (Sch), marked as Sch@BC, were prepared to remediate As-contaminated water and soil with a high efficiency. The characterization results showed that the Sch particles were successfully loaded on the BC, providing more active sites for As(V) adsorption. Compared with the pristine BC, the adsorption capacity of Sch@BC-1 was significantly improved (50.00 mg/g), of which the adsorption capacity kept stable over a wide pH range (pH = 2-8). The adsorption process conformed to pseudo-second-order kinetics and Langmuir isotherm model, which indicated that chemical adsorption was the dominant mechanism and the adsorption rate was controlled by intraparticle diffusion. Sch@BC could adsorb As(V) through electrostatic interaction and ion exchange, forming a FeAsO₄ complex and removing As(V). The 5-week soil incubation experiment showed that 3% Sch@BC showed the optimal stabilization effect, while the proportion of stable crystalline Fe/Mn-bound fractionation (F4) increased. Moreover, the results of microbial community diversity showed that Sch@BC interacted with As-resistant dominant microorganisms such as Proteobacteria in soil, promoted their growth and reproduction, and improved the stability of As in soil. In summary, Sch@BC is an excellent agent with broad application prospects for remediating As-contaminated water and soil.

Effect of phosphate on ferrihydrite transformation and the associated arsenic behavior mediated by sulfate-reducing bacterium

Although PO₄^3- is commonly found in association with iron (oxyhydr)oxide, the effect of PO₄^3- on ferrihydrite reduction, mineralogical transformation, and associated As behavior in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this study, batch experiments, together with geochemical, mineralogical, and biological analyses, were conducted to elucidate these processes. The results showed that SRB can reduce ferrihydrite via direct and indirect processes, and PO₄^3- promoted ferrihydrite reduction by supporting SRB growth at low and medium PO₄^3- loadings. However, at high loadings, PO₄^3- stabilized the ferrihydrite. PO₄^3- shifted the transformation of ferrihydrite from magnetite and mackinawite to vivianite, which scavenges As effectively by incorporating As into its particle. In systems with 0.5 mM SO₄^2-, PO₄^3- exerted a weak effect on As mobilization. However, in systems with 10 mM SO₄^2-, substantial amounts of As were released into the solution, and PO₄^3- impacted As behavior strongly. Low PO₄^3- loadings increased the mobilization of As because of the competitive adsorption of PO₄^3- on mackinawite. Medium and high PO₄^3- loadings were beneficial for As immobilization because of the substitution of mackinawite by vivianite. These findings have important implications for understanding the biogeochemistry of iron (oxyhydr)oxide and As behavior in SRB-containing sediments.

Light-induced dissolution and concomitant crystallization of a Keggin-type polyoxometalate mimicking a naturally occurring phenomenon

A suspension of a yellow polycrystalline compound [PPh₄]₃[PMo^VI₁₂O₄₀] in N-methylformanilide (NMF) (in which it is not soluble), on irradiation with sunlight, initiates dissolution via its reduction followed by its crystallization leading to the isolation of single crystals of compound [PPh₄]₄[PMo^VMo^VI₁₁O₄₀]·3CH₃(C₆H₅)NCHO (1). Compounds [PPh₄]₃[PMo^VI₁₂O₄₀]·1.75 CH₃(C₆H₅)NCHO (2) and [PPh₄]₃[PMo^VI₁₂O₄₀]·2CH₃(C₆H₅)NCHO (3), each containing an oxidized Keggin anion, are obtained at two different temperatures when the corresponding mother liquor is kept in the dark.

Tetracycline-Induced Release and Oxidation of As(III) Coupled with Concomitant Ferrihydrite Transformation

Cocontamination with tetracycline (TC) and arsenic (As) is very common in paddy fields. However, the process and underlying mechanism of arsenite (As(III)) transformation on iron mineral surfaces in the presence of antibiotic contaminants remain unclear. In this study, the release and oxidation of As(III) on ferrihydrite (Fh) surfaces and Fh transformation in the presence of TC under both aerobic and anaerobic conditions were investigated. Our results indicated that the TC-induced reductive dissolution of Fh (Fe(II) release) and TC competitive adsorption significantly promote the release of As, especially under anaerobic conditions. The release of As was increased with increasing TC concentration, whereas it decreased with increasing pH. Interestingly, under both aerobic and anaerobic conditions, the addition of TC enhanced the oxidation of As(III) by Fh and induced the partial transformation of Fh to lepidocrocite. Under aerobic conditions, the adsorbed Fe(II) activated the production of reactive oxygen species (·OH and ¹O₂) from dissolved O₂, with Fe(IV) being responsible for As(III) oxidation. Under anaerobic conditions, the abundant oxygen vacancies of Fh affected the oxidation of As(III) during Fh recrystallization. Thus, this study provided new insights into the role of TC on the migration and transformation of As coupled with Fe in soils.

Photoreductive dissolution of schwertmannite loaded with Cr(VI) induced by tartaric acid

Schwertmannite (SCH) as an adsorbent for Cr(VI) removal has been widely investigated. However, there are limited reports on photoreduction driven dissolution of SCH loaded with Cr(VI) (SCH-Cr(VI)) and the fate of Cr(VI) in the presence of dissolved organic matter (DOM). In this study, the effect of tartaric acid (TA) on the stability of SCH-Cr(VI) exposed to simulated solar radiation was examined. The results demonstrated that TA could greatly enhance the release of the dissolved total Fe (TFe) from SCH-Cr(VI). Conversely, the dissolved total Cr (TCr) obviously declined. Low pH promoted the liberation of TFe and TCr. The presence of ions including Al³⁺, Ca²⁺, K⁺ and CO₃^2- exerted different impact on the photoreductive dissolution of SCH-Cr(VI) induced by TA. On the basis of the species distribution of iron and chromium and the characterization of the solid samples, the underlying mechanism is proposed for the transformation and the fate of Cr(VI). Cr(VI) was reduced to Cr(III) by Fe(II) generated from Fe(III)-TA_n via ligand to metal charge transfer. The produced Cr(III) was adsorbed by SCH or co-precipitates with Fe(III). Thus, this study helps us to gain an insight into the mobility and fate of Cr(VI) in acid mining drainage containing DOM, and will help design remediation strategies for Cr contamination.

Co-adsorption of As(III) and phenanthrene by schwertmannite and Fenton-like regeneration of spent schwertmannite to realize phenanthrene degradation and As(III) oxidation

Co-contamination of arsenic and polycyclic aromatic hydrocarbons (PAHs) in groundwater is frequently reported, and it is thus necessary to develop efficient techniques to tackle this problem. Here, we evaluated the feasibility of utilizing schwertmannite to co-adsorb As(III) and phenanthrene from water solution and regenerating spent schwertmannite via a heterogeneous Fenton-like reaction to degrade adsorbed phenanthrene and meanwhile oxidize adsorbed As(III). The results suggested that schwertmannite with a hedgehog-like morphology was superior to that with a smooth surface for the adsorption removal of As(III) or phenanthrene because of the much higher BET surface area and hydroxyl proportion of the former one, and schwertmannite formed at 72 h incubation effectively co-adsorbed As(III) and phenanthrene from water solution. The adsorption of As(III) and phenanthrene on schwertmannite did not interfere with each other, while the acidic initial solution pH delayed the adsorption of As(III) on schwertmannite but enhanced the adsorption capacity for phenanthrene. The adsorption of As(III) on schwertmannite mainly involved its exchange with SO₄^2- (outer-sphere or inner-sphere) and its complexation with iron hydroxyl surface groups, and phenanthrene adsorption mainly occurred through cation-π bonding and OH-π interaction. During the adsorption-regeneration processes, schwertmannite adsorbed As(III) and phenanthrene firstly, and then it can be successfully regenerated via Fenton-like reaction catalyzed by itself to effectively degrade the adsorbed phenanthrene and meanwhile oxidize the adsorbed As(III) to As(V). Therefore, schwertmanite is an outstanding environmental adsorbent to decontaminate As(III) and phenanthrene co-existing in groundwater.

Mobilization of arsenic during reductive dissolution of As(V)-bearing jarosite by a sulfate reducing bacterium

Microbial sulfidization of arsenic (As)-bearing jarosite involves complex processes and is yet to be fully elucidated. Here, we investigated the behavior of As during reductive dissolution of As(V)-bearing jarosite by a pure sulfate reducing bacterium with or without dissolved SO₄^2- amendment. Changes of aqueous chemistry, mineralogical characteristics, and As speciation were examined in batch experiments. The results indicated that jarosite was mostly replaced by mackinawite in the system with added SO₄^2-. In the medium without additional SO₄^2-, mackinawite, vivianite, pyrite, and magnetite formed as secondary Fe minerals, though 24.55 % of total Fe was in form of an aqueous Fe²⁺ phase. The produced Fe²⁺ in turn catalyzed the transformation of jarosite. At the end of the incubation, 41.99 % and 48.10 % of As in the solid phase got released into the aqueous phase in the systems with and without added SO₄^2-, respectively. The addition of dissolved SO₄^2- mitigated the mobilization of As into the aqueous phase. In addition, all As⁵⁺ on the solid surface was reduced to As³⁺ during the microbial sulfidization of As-bearing jarosite. These findings are important for a better understanding of geochemical cycling of elements As, S, and Fe in acid mine drainage and acid sulfate soil environments.

Thermodynamic and Spectroscopic Studies of the Complexes Formed in Tartaric Acid and Lanthanide(III) Ions Binary Systems

Binary complexes of tartaric acid with lanthanide(III) ions were investigated. The studies have been performed in aqueous solution using the potentiometric method with computer analysis of the data for detection of the complexes set, determination of the stability constants of these compounds. The mode of the coordination of complexes found was determined using spectroscopy, which shows: Infrared, circular dichroism, ultraviolet, visible as well as luminescence spectroscopy. The overall stability constants of the complexes as well as the equilibrium constants of the reaction were determined. Analysis of the equilibrium constants of the reactions and spectroscopic data allowed the effectiveness of the carboxyl groups in the process of complex formation.