Comparison of arsenate, chromate and molybdate binding on schwertmannite: surface adsorption vs anion-exchange

Exploring acid mine drainage treatment through adsorption: a bibliometric analysis

Discharge of acidic wastewater from mining activities (acid mine drainage (AMD)) is a major global environmental and public health issue. Although several approaches, including chemical precipitation and membrane technology, have been developed to treat AMD, adsorption has emerged as the most promising technology due to its cost-effectiveness and efficacy. Despite the wide adoption of adsorption in treating AMD, the evolution of research in this area remains poorly understood. To address this gap, a bibliometric analysis of the most recent literature involving the application of adsorption in AMD remediation was conducted by merging datasets of articles from Scopus (1127) and the Web of Science Core Collection (1422), over the past decade (2013-2022). This analysis revealed a yearly increase of 11% in research publications, primarily contributed by China, the United States, and South Africa. Keyword analysis revealed that natural schwertmannites and their transformations, activated carbon, zeolites, and clay minerals, are the most extensively employed adsorbents for the removal of common metals (arsenic, chromium, iron, manganese, among others). The findings underscore the need for future focuses on recovering rare earth elements, using nanoparticles and modified materials, pursuing low-cost, sustainable solutions, integrating hybrid technologies, pilot-scale studies, exploring circular economic applications of AMD sludges, and inter-continental collaborations. These insights hold significant future implications, serving as a valuable reference to stakeholders in the mining industry.

Phase transformation of schwertmannite in paddy soil under different water management regimes and its impact on the migration of arsenic in soil

Schwertmannite (Sch) holds a great promise as an iron material for remediating Arsenic (As)-contaminated paddy soils, due to its extremely high immobilization capacities for both arsenate [As(V)] and arsenite [As(III)]. However, there is still limited knowledge on the mineral phase transformation of this metastable iron-oxyhydroxysulfate mineral in paddy soils, particularly under different water management regimes including aerobic, intermittent flooding, and continuous flooding, and how its phase transformation impacts the migration of As in paddy soils. In this study, a membrane coated with schwertmannite was first developed to directly reflect the phase transformation of bulk schwertmannite applied to paddy soils. A soil incubation experiment was then conducted to investigate the mineral phase transformation of schwertmannite in paddy soils under different water management regimes and its impact on the migration of As in paddy soil. Our findings revealed that schwertmannite can persist in the paddy soil for 90 days in the aerobic group, whereas in the continuous flooding and intermittent flooding groups, schwertmannite transformed into goethite, with the degree or rate of mineral phase transformation being 5% Sch >1% Sch > control. These results indicated that water management practices and the amount of schwertmannite applied were the primary factors determining the occurrence and degree of mineral transformation of schwertmannite in paddy soil. Moreover, despite undergoing phase transformation, schwertmannite still significantly reduced the porewater As (As(III) and As(V)), and facilitated the transfer of non-specifically adsorbed As (F1) and specifically adsorbed As (F2) to amorphous iron oxide-bound As (F3), effectively reducing the bioavailability of soil As. These findings contribute to a better understanding of the mineralogical transformation of schwertmannite in paddy soils and the impact of mineral phase transformation on the retention of As in soil, which carry important implications for the application of schwertmannite in remediating As-contaminated paddy soils.

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 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.

A multi-scale assessment of the impact of salinity on the desorption of chromate from hematite: Sea level rise implications

The solubility and transport of Cr(VI) is primarily controlled by adsorption-desorption reactions at the surfaces of soil minerals such as iron oxides. Environmental properties such as pH, ionic strength, and ion competition are expected to affect the mobility and fate of Cr(VI). Sea level rise (SLR), and consequent seawater intrusion, is creating a new biogeochemical soil environment at coastal margins, potentially impacting Cr(VI) retention at contaminated sites. We employed in-situ ATR-FTIR spectroscopy and DFT calculations to investigate at the molecular level the adsorption of Cr(VI) on the hematite surface and its desorption by sulfate, as a function of pH and ionic strength. We further used a batch experiment to assess Cr(VI) desorption at varying artificial seawater (ASW) concentrations. IR results demonstrate the complexity of Cr(VI) adsorption, showing a combination of monodentate inner-sphere complexation at high pH and dichromate outer-sphere (∼75%) at low pH. The Cr(VI)-complexes exhibited desorption induced by increasing pH values (58% of desorption) and sulfate competition (∼40% desorption). ASW desorbed ∼20% more Cr(VI), even at just 1% concentration. Our findings provide insight into Cr(VI)-adsorption complexation that controls the retention and remobilization of Cr(VI) on Fe-oxide minerals. The results point to an elevated risk of Cr(VI) mobilization in contaminated soils affected by SLR.

A coupled electrochemical process for schwertmannite recovery from acid mine drainage: Important roles of anodic reactive oxygen species and cathodic alkaline

The increasing need for sustainable acid mine drainage (AMD) treatment has spurred much attention to strategic development of resource recovery. Along this line, we envisage that a coupled electrochemical system involving anodic Fe(II) oxidation and cathodic alkaline production will facilitate in situ synthesis of schwertmannite from AMD. Multiple physicochemical studies showed the successful formation of electrochemistry-induced schwertmannite, with its surface structure and chemical composition closely related to the applied current. A low current (e.g., 50 mA) led to the formation of schwertmannite having a small specific surface area (SSA) of 122.8 m² g^-1 and containing small amounts of -OH groups (formula Fe₈O₈(OH)_4.49(SO₄)_1.76), whereas a large current (e.g., 200 mA) led to schwertmannite high in SSA (169.5 m² g^-1) and amounts of -OH groups (formula Fe₈O₈(OH)_5.16(SO₄)_1.42). Mechanistic studies revealed that the reactive oxygen species (ROS)-mediated pathway, rather than the direct oxidation pathway, plays a dominant role in accelerating Fe(II) oxidation, especially at high currents. The abundance of •OH in the bulk solution, along with the cathodic production of OH^-, were the key to obtaining schwertmannite with desirable properties. It was also found to function as a powerful sorbent in removal of arsenic species from the aqueous phase.

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.

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.

Fe(II)-mediated transformation of schwertmannite associated with calcium from acid mine drainage treatment

Schwertmannite is an important Fe(III)-oxyhydroxysulfate in acid mine drainage (AMD) polluted areas and its stability depends on surrounding environmental factors and previously bound elements. The treatment and neutralization of AMD normally involve the use of lime, which leads to the discharge of abundant Ca in the mining area. Such an environmental disturbance brings up an important and less considered problem of how the reductive transformation of schwertmannite associated with coexisting Ca occurred. Here, the Fe(II)-mediated transformation of Ca-adsorbed schwertmannite and subsequent Ca repartitioning behaviors were investigated. Results showed that adsorbed Ca had a weak inhibitory effect on Fe(II)-mediated schwertmannite transformation. Release of SO₄^2- and SEM images both indicated that transformation rates of schwertmannite decreased under the influence of adsorbed Ca. XRD patterns indicated that adsorbed Ca altered schwertmannite transformation pathways and product compositions upon treatment with 0.4 mmol/L Fe(II). The end products of Sch notably contained both goethite and lepidocrocite; however, transformation products of SchCa only contained goethite all along. Approximately 33.5% of the surface adsorbed-Ca was released into solution within 6 hr after Fe(II) injection. Aqueous Ca behaved in a "first release and then im-mobilization" manner, which indicated dissolution and secondary mineralization drove Ca migration during the Fe(II)-mediated transformation of SchCa. Adsorbed Ca blocked the surface sites for subsequent Fe(II) adsorption, limited the Fe(II)-Fe(III) ETAE, and decreased the transformation rates. This work sheds light on the complex geochemical behavior of schwertmannite under the influences of environmental perturbations in AMD environments.

Concurrent sorption of antimony and lead by iron phosphate and its possible application for multi-oxyanion contaminated soil

Concurrent stabilization of oxyanions such as antimony (Sb), arsenic (As), and heavy metals including lead (Pb) and manganese (Mn) in contaminated soils is difficult because of their diverse chemical properties. Antimony and As are stabilized by sorption with iron oxides while heavy metals are stabilized by phosphate. Hence, iron phosphate can be used to simultaneously stabilize Sb and Pb. Therefore, this study aimed to evaluate the possibility of simultaneous stabilization of Sb and Pb using iron phosphate. A single and a mixed solution of Sb and Pb were reacted with synthesized iron phosphate. Contaminated soil by Sb, As, Mo, Cr, and Mn was treated with iron phosphate, and bioavailable metal concentrations were evaluated by extracting the soil with 0.05 M ammonium sulfate. In a single solution, Sb(III) and Sb(V) sorption rate ranged up to 97% and 65%, respectively. In a mixed metal solution, Sb sorption increased compared to the single solution and Pb removal reached more than 95% in all cases. The sorption of Sb increased as the pH decreased, but the Sb(III) sorption was less affected by the pH than Sb(V). In various pH ranges, Sb(III) and Sb(V) sorption rates increased by 26 ~ 32% and 38 ~ 68%, respectively, compared to the single solution. Especially, Sb(V) sorption significantly increased in the presence of Pb at lower pH because of the lower solubility of iron phosphate. In soil, iron phosphate slightly decreased bioavailable As, Cr, Mo, Sb, and Mn concentrations. Therefore, metalloids and metals can be simultaneously stabilized by iron phosphate both in solution and soil.

Iron (oxyhydr)oxides are responsible for the stabilization of Cu and Zn in AMD after treatment with limestone

The formation and transformation of secondary iron (oxyhydr)oxides and their role in the stabilization of copper (Cu) and zinc (Zn) in acid mine drainage (AMD) after limestone treatment are worth studying to better understand the impacts of limestone AMD treatment. In this study, the wastewater from a copper mine ditch was sampled. Two different doses of limestone (S: 5.33 g L^-1 and SS: 8.00 g L^-1) were applied to adjust the pH range of the sampled AMD. The concentrations of Fe, Cu and Zn in the supernatant and the levels of iron (oxyhydr)oxides and heavy metals in AMD sediments were dynamically monitored for 300 days to analyze the transformation of the secondary iron mineral phase and the role iron (oxyhydr)oxides play in the removal and stabilization of Cu and Zn. The results showed that the pH rose rapidly to 6.82, decreased to 5.82 on the 150^th day, and finally decreased to approximately 4.63 by the 300^th day, when the dosage of limestone (S) was 5.33 g L^-1. Goethite was the main form of iron oxides in the sediments. As the incubation time increased, so did the content of crystalline Fe (oxyhydr)oxides. In addition, the Cu and Zn content in the fraction of crystalline Fe (oxyhydr)oxides increased as the corresponding iron (oxyhydr)oxide increased. When the high dosage of limestone (8.00 g L^-1 or SS) was applied, the pH remained at approximately at 7.46 during the whole period and goethite and lepidocrocite were present in the sediment. Amorphous/ poorly crystalline Fe-oxyhydroxide was the main product after SS limestone dosage, indicating that the risk of Cu and Zn reactivation in the sediment was higher with a higher limestone treatment dosage.

Mechanism of simultaneous lead and chromium removal from contaminated wastewater by a schwertmannite-like mineral

In this study, a schwertmannite-like mineral was synthesized for the removal of lead (Pb) and chromium (Cr) from contaminated wastewater. A shaking flask test was performed (150 r/min, 1 h) with FeSO₄·7H₂O, H₂O₂, Na₂SiO₃, and CaCl₂ added for the mineral synthesis reaction. Results show that optimal performance was achieved with the addition of 1.24 g/L FeSO₄·7H₂O, 0.75 g/L H₂O₂, 1.27 g/L Na₂SiO₃, and 0.44 g/L CaCl₂ at a water temperature of 28 °C, with coexisting ion (Na⁺, K⁺, Mg²⁺) concentrations of 1.50 mmol/L and 0.50 mmol/L EDTA as a complexing agent. Under these optimal conditions, maximum Pb and Cr removal rates of 95.08% and 97.99%, respectively, were achieved within the first 1 min of the mineral synthesis reaction, with the synthesis reaction completed by 6 min. The simultaneous removal of Pb and Cr during the schwertmannite-like material synthesis process occurred via electrostatic adsorption and coprecipitation. When the concentration of the complexing agent was increased from 0.75 to 6.03 mmol/L, the Pb removal rate decreased from 71.88 to 35.45%, and the Cr removal rate decreased from 95.13 to 75.07%, showing that Pb and Cr removal exhibited significant levels of inhibition. In contrast, varying reaction temperatures induced no significant differences. The Pb and Cr dissolution rates from Pb/Cr-containing schwertmannite-like minerals were 8.18% and 2.86% after 40 days, respectively. Therefore, the risk of secondary dissolution of heavy metals is small.

Sulfide-induced repartition of chromium associated with schwertmannite in acid mine drainage: Impacts and mechanisms

The cycling of Fe/S is often related to the formation and fate of schwertmannite which is particularly suitable as a scavenger for heavy metals and metalloids in acid mine drainage (AMD). However, the interactions between reactivity of S(-II) and schwertmannite with structurally incorporated Cr(VI) remain elusive. This work evaluated dissolution experiments in combination with XRD, SEM, FTIR, TEM, and XPS characterization to provide detailed information regarding the transformation of schwertmannite induced by S(-II) following changes in pH, Cr loading, and S(-II) concentration. Our results found that the presence of sulfide significantly decreased the stabilization of schwertmannite under acidic conditions. The reactivity of the three schwertmannite samples depended on the contents of Cr(VI) that were structurally incorporated and followed the order Sch > 0.13Cr-Sch > 0.17Cr-Sch. High S(-II) concentrations and low Cr doping favored the release of Fe and SO₄^2- from schwertmannite. Attenuation of Cr mobility occurred via elevating the S(-II) concentrations and pH values resulting in Cr concentrations spanning ∼1.39 to ∼0.10 mg L^-1 and ∼1.58 to ∼0.12 mg L^-1 for 0.13Cr-Sch and 0.17Cr-Sch, respectively. Combining the results of characterization, goethite was the dominant end product constituted secondary phase together with sulfide minerals (FeS, FeS₂), iron oxides (Fe₃O₄, Fe₂O₃), and CrFe minerals on the bulk mineral surface. The substituted Cr significantly inhibited the reductive transformation of schwertmannite by sulfide and led to the formation of lepidocrocite. Thus, we concluded that a three-stage transformation mechanistic pathway governed partial conversion of schwertmannite to goethite. This finding provides new understanding of the biogeochemical processes of iron minerals affected by reducing substances that control the transport and fate of immobilizing contaminants in an AMD-polluted area.

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

Acidic Cr(VI)-containing wastewater has received increasing attention in recent years. Schwertmannite is a suitable adsorbent for its acid resistance and good adsorption ability. However, it shows poor Cr(VI) adsorption performance under acidic conditions. Herein, inspired by the fast neutralization-mineralization process of acid mine drainage (AMD) triggered by alkaline rocks, a novel nano-scaled schwertmannite (Sch-2.7) with high Cr(VI) adsorption capacity was synthesized at constant pH of 2.7 via adding OH^-. Compared with common schwertmannite (Sch), appropriate OH^- effectively improved mineral yield (the precipitation efficiency of Fe: 96.75% vs. 29.93%), specific surface area (65.1 m²/g vs. 18.9 m²/g), surface group content, and further Cr(VI) adsorption ability of Sch-2.7. The maximum adsorption capacity was 54.17 (pH = 3), 61.59 (pH = 4), and 66.5 mg/g (pH = 5) for Sch-2.7, whereas only 20.35, 24.51, and 27.17 mg/g for Sch. On average, the former was 2.53 times higher than the latter. Temperature and coexisting ions had little influences on the sorption process of Sch-2.7. The mechanism analysis demonstrated that the Cr(VI) removal by Sch-2.7 was a more thermodynamic favorable process due to abundant reactive-active components on Sch-2.7 for adsorption reaction. This work provided new insight into performance optimization and application potential on Cr(VI) removal of schwertmannite.

Effects of aluminum incorporation on the schwertmannite structure and surface properties

Schwertmannite is a common nanomineral in acid sulfate environments such as Acid Mine Drainage (AMD) and Acid Sulfate Soils (ASS). Its high surface area and positively charged surface result in a strong affinity towards toxic oxyanions such as arsenate in solution. However, natural precipitation of schwertmannite also involves the accumulation of other impurities, in particular aluminum, an element that is often incorporated into the structure of Fe-oxide minerals, such as goethite and ferrihydrite, affecting their structural and surface properties. However, little is known about the effect of Al incorporation in schwertmannite on the removal capacity of toxic oxyanions found in AMD and ASS (e.g. arsenate). In this paper, schwertmannite samples with variable Al concentration were synthetized and employed in arsenate adsorption isotherm experiments at a constant pH of 3.5. Solid samples before and after arsenate adsorption were characterized using high energy X-ray diffraction and pair distribution function analyses in order to identify structural differences correlated with the Al content as well as variations in the coordination of arsenate adsorbed on the mineral surface. These analyses showed limited Al accumulation on schwertmannite (up to 5%) with a low effect on its structure. The maximum arsenate sorption capacity (258 mmolH2AsO4 molFe-1) was in the range of that with pure schwertmannite, but a higher proportion of inner-sphere coordination was observed. Finally, Al was found to desorb from schwertmannite, with adsorbed arsenate preventing this effect and increasing the stability of the mineral. These results are useful to interpret observations from the field, in particular from river water affected by AMD and ASS, where similar conditions are observed, and where aluminum incorporation is expected.

A novel approach for treating acid mine drainage by forming schwertmannite driven by a combination of biooxidation and electroreduction before lime neutralization

Acid mine drainage (AMD) contains abundant iron, sulfates, and various metal ions, and it causes environmental pollution. The traditional AMD lime neutralization forms a layer of iron hydroxide and gypsum on the surface of the lime particles, preventing continuous reaction and leading to excessive lime addition and neutralized sludge production. In this study, an approach for treating AMD using a cyclic process of biooxidation and electroreduction before lime neutralization was proposed, in which the Fe²⁺ in AMD was oxidized to Fe³⁺ and induced to form schwertmannite through Acidithiobacillus ferrooxidans. The remaining Fe³⁺ was reduced to Fe²⁺ using an electric field. After three biooxidation and two electroreduction cycles, 98.2% of Fe and 62.4% of SO₄^2- in AMD precipitated as schwertmannite (Fe₈O₈(OH)_5.16(SO₄)_1.37). The yield of schwertmannite reached 33.98 g/L_AMD, with a high specific surface area of 112.59 m²/g. The lime dosage and sludge yield of the treated AMD in the subsequent neutralization stage (pH = 7.00) decreased by 85.0% and 74.5%, respectively, than those of raw AMD. The pilot test results showed that the integrated treatment of biooxidation-electroreduction cyclic mineralization and lime neutralization has practical applications.

Modified chemical mineralization-alkali neutralization technology: Mineralization behavior at high iron concentrations and its application in sulfur acid spent pickling solution

Mineralization coupled with neutralization is a dual-function technology for disposing acidic iron-rich waters, which can recover the valuable iron in the form of secondary mineral and concurrently purify the wastewater. In this study, a modified technology for treating high Fe wastewater (sulfur acid spent pickling liquor, 62 g Fe/L) was proposed based on the specific investigation of the mineralization behaviors in Fe concentration range of 20-70 g/L. Results showed that high SO₄^2-/Fe²⁺ molar ratio (> 2.0) tended to trigger gelation phenomena at Fe concentrations above 30 g/L. Fe specie distribution suggested that the insufficient polymerization among Fe-OH complexes might be responsible for the gelation phenomena, since the strong Fe-SO₄ coordination almost completely suppressed the Fe_x(OH)_y^(3x-y)+ form (a general terms of Fe³⁺ hydrolysates and their polymers). Modified mineralization strategies were proposed, including pretreatment with dilution or BaCl₂/CaCl₂ precipitation, of which CaCl₂ pretreatment was a versatile and low-cost method. Following CaCl₂ pretreatment, chemical mineralization converted above 90% of iron into secondary mineral, which therefore drastically reduced the alkali consumption (from 164.2 g/L to 1.4 g/L) and sludge yield (from 328.1 g/L to 2.4 g/L) in subsequent neutralization treatment. The resultant mineral was identified as schwertmannite, and exhibited efficient adsorption capacity toward arsenite (364.2 mg/g). The modified chemical mineralization-alkaline neutralization is a cost-effective technology for the treatment of the acidic iron-rich waters. In practical applications, several regulating strategies should be further explored to improve the mineral purity, and the mineralization conditions must be optimized according to the Fe and SO₄^2- concentrations in wastewater.

Thermal Transformation of Natural Schwertmannite in the Presence of Chromium

Schwertmannite is a metastable mineral playing a crucial role in the immobilization of metal(oid)s in acid mine drainage (AMD) systems. High temperatures associated with wildfires could lead to a sudden schwertmannite transformation, changing the mobility of metal(oid)s. The objective of the present study was to examine the thermal transformation from schwertmannite to hematite, and the subsequent effect on the chromium partitioning. The immobilization of arsenate after thermal transformation and its implications on chromium mobility was also evaluated. Natural schwertmannite, with increasing contents of chromium, was thermally treated between 200 to 800 °C. Transformation products were characterized by solid-phase techniques and selective chemical extractions. Results indicated a transformation to hematite at temperatures above 400 °C. The presence of chromium barely affected the temperature at which the transformation occurred, although partitioning of chromium in the mineral changed with temperature. As the temperature increased from 25 °C to 400 °C, chromium was less mobile and less outcompeted by arsenic adsorption, suggesting a larger contribution of inner-sphere complexes with increasing temperature. At temperatures above 600 °C, non-mobile forms strongly associated with neo-formed hematite were found. Finally, neo-formation of hematite led to a decrease in arsenic adsorption, implying a potentially enhanced arsenic mobility in AMD systems upon wildfires.

An X-ray absorption spectroscopic study of the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite

The environmental chemistry of Cr is of widespread interest due to the hazardous nature of Cr(VI). Because of similar atomic size and charge, Cr^VIO₄^2- can substitute for SO₄^2- within schwertmannite - an Fe(III) oxyhydroxysulfate mineral that occurs widely in acidic and sulfate-rich systems. The presence of aqueous Fe(II) can induce transformation of schwertmannite to more stable Fe(III) phases (e.g. goethite) which may potentially impact the behaviour of co-associated Cr(VI). Here, we investigate the Fe(II)-induced transformation of Cr(VI)-substituted schwertmannite as a function of pH (4-8) and the degree of Cr(VI) substitution (0.16-13 mol% Cr^VIO₄^2--for-SO₄^2- substitution). Iron K-edge EXAFS spectroscopy revealed that higher levels of Cr(VI) substitution inhibited Fe(II)-induced schwertmannite transformation. Chromium K-edge XANES spectroscopy indicated that this outcome could be partly attributed to consumption of Fe(II) by reaction with Cr(VI), and the resulting formation of a passivating Cr(III)-Fe(III) hydroxide phase which stabilizes schwertmannite at greater levels of Cr(VI) substitution and at higher pH while also decreasing further reduction of structural Cr(VI). Overall, this study enriches our understanding of interactions between hazardous Cr(VI) and schwertmannite in environmental and engineered systems.

Unravelling biogeochemical drivers of methylmercury production in an Arctic fen soil and a bog soil

Arctic tundra soils store a globally significant amount of mercury (Hg), which could be transformed to the neurotoxic methylmercury (MeHg) upon warming and thus poses serious threats to the Arctic ecosystem. However, our knowledge of the biogeochemical drivers of MeHg production is limited in these soils. Using substrate addition (acetate and sulfate) and selective microbial inhibition approaches, we investigated the geochemical drivers and dominant microbial methylators in 60-day microcosm incubations with two tundra soils: a circumneutral fen soil and an acidic bog soil, collected near Nome, Alaska, United States. Results showed that increasing acetate concentration had negligible influences on MeHg production in both soils. However, inhibition of sulfate-reducing bacteria (SRB) completely stalled MeHg production in the fen soil in the first 15 days, whereas addition of sulfate in the low-sulfate bog soil increased MeHg production by 5-fold, suggesting prominent roles of SRB in Hg(II) methylation. Without the addition of sulfate in the bog soil or when sulfate was depleted in the fen soil (after 15 days), both SRB and methanogens contributed to MeHg production. Analysis of microbial community composition confirmed the presence of several phyla known to harbor microorganisms associated with Hg(II) methylation in the soils. The observations suggest that SRB and methanogens were mainly responsible for Hg(II) methylation in these tundra soils, although their relative contributions depended on the availability of sulfate and possibly syntrophic metabolisms between SRB and methanogens.

Sorption of arsenate(Ⅴ) to naturally occurring secondary iron minerals formed at different conditions: The relationship between sorption behavior and surface structure

Arsenic (As) is a problematic pollutant that can cause cancer and other chronic diseases due to its potential toxicity. Iron (oxyhydr)oxides can readily sorb As and play important roles in the geochemical cycle of As. Attention has mainly been given to the affinity and mechanism of As sorption by synthetic pure iron (oxyhydr)oxides, and little is known about the relationship between As behavior and multicomponent secondary iron minerals (SIMs) naturally formed in acid mine drainage (AMD). To investigate this relationship, we performed sorption kinetics, isotherm and competitive sorption experiments to investigate As(V) sorption behaviors on naturally formed SIMs harvested from different runoff zones of an abandoned coal mine. Several spectroscopic analyses were used to evaluate the structural and component changes and phase transformation. Three environmental SIMs formed at nascent (n-SIM), transient (t-SIM) and mature (m-SIM) stages were determined to be similar in the element components of Fe, S and O but different in structure. As(V) sorption behaviors on these environmental SIMs followed a pseudo-second-order kinetic model, and the sorption extent followed the sequence of n-SIM > t-SIM > m-SIM. As(V) sorption is not significantly influenced by Na⁺/Ca²⁺ concentration or ionic strength except for that of PO₄^3-, and it slightly decreases as the Cr(Ⅲ) concentration increases but increases with increasing Sb(Ⅲ)/(V) concentration. The results of spectral analyses indicate that As(V) immobilization mainly depends on exchange with SO₄^2- and surface complexation, along with the phase transformation of schwertmannite/jarosite to goethite and other phases. These findings are helpful for better understanding the geochemical behaviors of As(V) associated with environmental SIMs.

Microstructure of Al-substituted goethite and its adsorption performance for Pb(II) and As(V)

Naturally occurring goethite commonly undergoes Al-substitution, while how changes in microstructure induced by Al-substitution affect the interactive reaction of Pb(II) or As(V) at the goethite-water interface remains poorly understood. This study reveals the structural properties of Al-substituted goethite and its adsorption behavior for Pb(II) and As(V) by multiple characterization techniques and Charge Distribution-Multisite Surface Complexation (CD-MUSIC) modeling. Al-substitution caused an obvious decrease in the length-to-width ratio in goethite particles and a slight decrease in the proportion of (110) facets. The presence of Al-O sites and higher surface roughness induced by Al-substitution contributed to a higher inner Stern layer capacitance (C₁) and surface charge density of goethite. CD-MUSIC modeling results further revealed that the affinity constant of Pb(II) complex (log K_Pb) at the goethite-water interface and the adsorption capacity of goethite for Pb(II) decreased with increasing amount of Al-substitution, while an opposite tendency was observed for As(V) adsorption. The dominant species of both Pb(II) and As(V) on goethite were bidentate complexes, and Al-substitution had a minor impact on the abundance of Pb(II) and As(V) complexes on the surface of goethite. Overall, these experimental and modeling results provide new and important insights into the interfacial reactivity of Al-substituted goethite and facilitate the prediction of the environmental fate of heavy metals.

Insights into the improving mechanism of defect-mediated As(V) adsorption on hematite nanoplates

The fate of As(V) in subsurface environments is strongly affected by ubiquitous iron oxides. Defects are commonly present in natural hematite, while the impacts of defects on the active sites and complexation mechanism of hematite for As(V) remain poorly understood. In this study, the defect-rich hematite was employed to investigate the surface charge characteristics and As(V) adsorption behavior using potentiometric acid-base titration and CD-MUSIC model in comparison with corresponding defect-poor hematite. The total arsenate-active site density (5.7 sites/nm²) on defective hematite includes 1.2 sites/nm² of original sites and 4.5 sites/nm² of Fe vacancy-induced sites. The result revealed that the vacant Fe³⁺ sites in defective hematite was compensated by the protons in solution, thus resulting in a considerable increase in site density as well as positive charge. The CD-MUSIC modeling results demonstrated that the presence of Fe vacancies in hematite is beneficial to the improvement in affinity constants for both monodentate and bidentate arsenate complexes. The high adsorption capacity of defective hematite (2.60 μmol/m²) compared to defect-free hematite (1.33 μmol/m²) is attributed to its large affinity constants as well as its more active surface sites, thereby playing a vital role in reducing the threats of heavy metals in the environment.

A novel arsenic immobilization strategy via a two-step process: Arsenic concentration from dilute solution using schwertmannite and immobilization in Ca-Fe-AsO4 compounds

Acid mine drainage (AMD) with toxic arsenic (As) is commonly generated from the tailings storage facilities (TSFs) of sulfide mines due to the presence of As-bearing sulfide minerals (e.g., arsenopyrite, realgar, orpiment, etc.). To suppress As contamination to the nearby environments, As immobilization by Ca-Fe-AsO₄ compounds is considered one of the most promising techniques; however, this technique is only applicable when As concentration is high enough (>1 g/L). To immobilize As from wastewater with low As concentration (~10 mg/L), this study investigated a two-step process consisting of concentration of dilute As solution by sorption/desorption using schwertmannite (Fe₈O₈(OH)_8-2x(SO₄)_x; where (1 ≤ x ≤ 1.75)) and formation of Ca-Fe-AsO₄ compounds. Arsenic sorption tests indicated that As(V) was well adsorbed onto schwertmannite at pH 3 (Q_max = 116.3 mg/g), but its sorption was limited at pH 13 (Q_max = 16.1 mg/g). A dilute As solution (~11.2 mg/L As) could be concentrated by sorption with large volume of dilute As solution at pH 3 followed by desorption with small volume of eluent of which pH was 13. The formation of Ca-Fe-AsO₄ compounds from As concentrate solution (2 g/L As(V)) was strongly affected by temperature and pH. At low temperature (25-50 °C), amorphous ferric arsenate was formed, while at high temperature (95 °C), yukonite (Ca₂Fe_3-5(AsO₄)₃(OH)_4-10·xH₂O; where x = 2-11) and johnbaumite (Ca₅(AsO₄)₃OH) were formed at pH 8 and 12, respectively. Among the synthesized products, johnbaumite showed strongest As retention ability even under acidic (pH < 2) and alkaline (pH > 9) conditions.

Rare earth real wastewater treatment by pilot scale using new concept continuous treatment process

Rare earth (RE) containing radioactive species and a variety of toxic pollutants The treatment of real wastewater is very important for environmental protection. In this study, a new concept of continuous process consisting of precipitation, adsorption, and oxidation was developed without the use of chemicals. In the sedimentation step, waste oyster shell(WOS) and a PE tube diffuser(PE250) containing Na₂S (PE_Ca/S), PE_Ca/S were prepared, which were used to precipitate heavy metals with a removal efficiency of 97% or more. In the adsorption step, fluorine (F), arsenic (As), and thorium (Th) were precipitated and removed when heavy metals were removed using coal mine drainage sludge (CMDS) and an adsorbent (PU_CMDS) made of polyurethane (PU). Running a semi and pilot scale continuous process using PE_Ca/S, PU_CMDS and O₃/HC systems resulted in a semi and pilot scale operating period of 120 and 62 days, and 60.26, 797.84, 46.94, 78.62 g, and 7.120 kg and 266.35, 42556.8, 191.95, 3108.43 g and 629.84 kg for As, F, Th, Pb and CODcr has been removed respectively. In addition, the removal efficiencies of As, F, Th, Pb and CODcr were 99.75, 99.98, 93.60, 99.99, and 88.82%, respectively, when treating real RE wastewater using the pilot scale system. Without the use of agitated reactors and regulators, the new concept of continuous process can effectively treat RE real wastewater, and the quality of the process outlet has met the pollutant limits recommended by EPA and China for irrigation.

Mechanisms of Cr(VI) adsorption on schwertmannite under environmental disturbance: Changes in surface complex structures

Hexavalent chromium (Cr(VI)) mobility, reactivity and bioavailability in the acid mine drainage (AMD) are restricted by adsorption reactions on schwertmannite. However, the Cr(VI) adsorption mechanisms remain unclear. In this study, batch adsorption/desorption experiments, X-ray photoelectron spectroscopy (XPS), and in‒situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) in combination with a multivariate curve resolution- alternating least squares (MCR-ALS) analysis were employed to characterize Cr(VI) adsorption on schwertmannite. The results of batch experiments suggested that two kinds of anion exchange reactions occurred on Sch surface: the outer-sphere complexes and the inner-sphere complexes of sulfate were successively substituted by aqueous Cr(VI) to form inner-sphere complexes. XPS analysis showed that the adsorbed Cr (VI) tended to exchange with sulfate rather than with surface hydroxyl groups on schwertmannite. In-situ ATR-FTIR spectroscopic results confirmed that the Cr(VI) coordination species contained bidentate inner-sphere (C_2ν) and monodentate inner-sphere complexes (C_3ν). MCR-ALS analysis revealed that monodentate complexes were dominant at pH 5.0-8.0. The proportion of bidentate complexes decreased from 47% to 25% when pH increased from 5.0 to 8.0. Thus, we concluded that a transition occurred between bidentate to monodentate complexes. In addition, the Cr(VI) concentration exerted little influence on the change of surface complexes.

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.

Comparison of Cr(VI) Adsorption Using Synthetic Schwertmannite Obtained by Fe3+ Hydrolysis and Fe2+ Oxidation: Kinetics, Isotherms and Adsorption Mechanism

Good sorption properties and simple synthesis route make schwertmannite an increasingly popular adsorbent. In this work, the adsorption properties of synthetic schwertmannite towards Cr(VI) were investigated. This study aimed to compare the properties and sorption performance of adsorbents obtained by two methods: Fe3+ hydrolysis (SCHA) and Fe2+ oxidation (SCHB). To characterise the sorbents before and after Cr(VI) adsorption, specific surface area, particle size distribution, density, and zeta potential were determined. Additionally, optical micrographs, SEM, and FTIR analyses were performed. Adsorption experiments were performed in varying process conditions: pH, adsorbent dosage, contact time, and initial concentration. Adsorption isotherms were fitted by Freundlich, Langmuir, and Temkin models. Pseudo-first-order, pseudo-second-order, intraparticle diffusion, and liquid film diffusion models were used to fit the kinetics data. Linear regression was used to estimate the parameters of isotherm and kinetic models. The maximum adsorption capacity resulting from the fitted Langmuir isotherm is 42.97 and 17.54 mg·g-1 for SCHA and SCHB. Results show that the adsorption kinetics follows the pseudo-second-order kinetic model. Both iron-based adsorbents are suitable for removing Cr(VI) ions from aqueous solutions. Characterisation of the adsorbents after adsorption suggests that Cr(VI) adsorption can be mainly attributed to ion exchange with SO42- groups.

Molecular-Scale Understanding of Sulfate Exchange from Schwertmannite by Chromate Versus Arsenate

Schwertmannite effectively sorbs chromate (Cr(VI)), yet the sorption mechanisms remain elusive. We determined the Cr(VI) sorption mechanisms on schwertmannite at pH 3.2 and 5 using combined macroscopic sorption experiments with molecular-scale characterization and by comparing them to arsenate (As(V)) sorption. Cr(VI) adsorbs as bidentate-binuclear (BB) inner-sphere complexes through exchanging more sulfate and less >Fe-OH/OH₂, with 0.59-0.71 sulfate released per Cr(VI) sorbed. While As(V) also forms BB complexes, it exchanges sulfate and >Fe-OH/OH₂ equally with 0.49-0.52 sulfate released per As(V) sorbed. At high As(V) loadings, As(V) precipitates as amorphous FeAsO₄, particularly at low pH. The abovementioned differences between Cr(VI) and As(V) can be related to their different ionic radii and binding strength. Moreover, Cr(VI) and As(V) preferentially exchange sulfate inner-sphere complexes, increasing the proportion of sulfate outer-sphere complexes in schwertmannite. In turn, the concentration of sulfate outer-sphere complexes increases and then decreases with increasing Cr(VI) loading. Results suggest that an oxyanion, which would form inner-sphere complexes on a mineral surface, preferentially exchanges inner-spherically bound oxyanions than outer-spherically bound ones on the surface, even though both are exchanged. This study improves our understanding of the sorption of oxyanions on schwertmannite and their capabilities to template schwertmannite formation and stabilize its structure.

Facet-dependent surface charge and Pb2+ adsorption characteristics of hematite nanoparticles: CD-MUSIC-eSGC modeling

Accurate prediction of the environmental fate of Pb depends on the understanding of Pb coordination to mineral surfaces. Here, the proton and Pb adsorption and speciation on hematite nanocrystals with different exposed crystallographic facets were investigated. High-resolution transmission electron microscopy images revealed that hematite nanoplates (HNP) were of 75.3 ± 9.5% (001) facets and 24.6 ± 9.3% (012) facets, while hematite nanocubes (HNC) were of 76.0 ± 11.1% (012) facets and 24.0 ± 3.2% (110) facets. Our modeling results revealed that the proton affinity constant (log K_H) of ≡FeOH^-0.5 and ≡Fe₃O^-0.5 was 7.8 and 10.8 on hematite (012) facets, and changed to 7.7 and 11.7 on (110) facets, respectively. Owing to the different atomic arrangements, (012) facets not only have higher adsorption performance for Pb, but also present a greater dependence on pH than (110) facets. Additionally, our modeling further indicated that (012) facets bind Pb via both bidentate and tridentate complexes, while (110) facets bind Pb only through bidentate complexes at pH 3.0-6.5. These results facilitate a more detailed understanding of the complex species of Pb on hematite surface while also provide new insight into the reactivity mechanism of individual hematite facets.

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.

Characterization of Fe-based sediments received from chemical pre-treatment of hydrometallurgical waste leachate from the recycling of alkaline batteries

The waste leachate from the hydrometallurgical recycling of spent batteries contains a significant amount of undesirable iron that needs to be precipitated before the recovery of target metals. The produced Fe-sediments are usually disposed of or stored at the treatment site as waste and are often poorly managed. This work estimates the environmental stability and application potential of Fe-sediments produced from highly acidic hydrometallurgical leachate during the recycling of spent alkaline batteries. After pH neutralization of the leachate by Na₂CO₃, a primary Fe-sediment (PFS), mainly composed of highly unstable metal (i.e., Fe, Zn, and Mn) sulfates, was obtained. The subsequent rinsing of this unstable PFS sediment led to the production of a secondary Fe-sediment (SFS), which was composed of an amorphous-phased ferric iron sulfate hydrate - Fe₁₆O₁₆(SO₄)₃(OH)₁₀·10H₂O. The results of single extraction using chemical reagents and biological dissolution by iron-transforming bacteria confirmed that despite most of the ions in PFS were dissolvable, the processed SFS was environmentally safe. The sorption efficiency of SFS towards Pb(II) and As(V) (up to ~ 99% and 94%, respectively, with an initial concentration of 100 mg/L) was found to be promising, suggesting the high potential for economical reuse of SFS.

Arsenic(V) removal behavior of schwertmannite synthesized by KMnO4 rapid oxidation with high adsorption capacity and Fe utilization

Adsorption is a simple and efficient way for arsenic contamination purification in water, with a pressing challenge to find a cheap and efficient adsorbent. As a poorly crystalline Fe(III)-oxyhydroxy sulfate mineral, schwertmannite can be As(V) adsorbent because of its tunnel structure and low cost. However, the schwertmannite synthesized commonly by H₂O₂ rapid oxidation suffers from the low Fe utilization and limited As(V) adsorption capacity. In this research, the schwertmannite is synthesized by KMnO₄. The results show that the Fe utilization can be improved from 40% to 56%, with the As(V) adsorption capacities double times better than those synthesized by H₂O₂ at pH 7 and 2. The As(V) adsorption mechanisms at different pHs and the reason for the improvement of As(V) adsorption capacity are thoroughly investigated. The FTIR and EDS images confirm that As(V) adsorption exchange with SO₄^2- is the dominant mechanism at pH 7 and 2. At pH 11, the As(V) is mainly removed by surface complexation because the surface SO₄^2- is exchanged by OH^-. The intraparticle diffusion model fitting and XPS results further reveal that the tunnel structure built by Fe-SO₄ in the KMnO₄ oxidized schwertmannite is more stable, possibly resulting in the better As(V) adsorption performance.

Recent progress in understanding the mechanism of heavy metals retention by iron (oxyhydr)oxides

Heavy metals are widespread toxic environmental pollutants that can generate enormous health and public concern. Iron (oxyhydr)oxides are ubiquitous in both natural and engineered environments and have great retention capacity of heavy metals due to their high surface areas and reactivity. The sequestration of heavy metal by iron (oxyhydr)oxides is one of the most vital geochemical/chemical processes controlling their environmental fate, transport, and bioavailability. In this review, some of the common iron (oxyhydr)oxides are introduced in detail in terms of their formation, occurrence, structure characteristics and interaction with heavy metals. Moreover, the retention mechanisms of metal cations (e.g., Pb, Cu, Cd, Ni, Zn), metal oxyanions (e.g., As, Sb, Cr), and coexisting multiple metals on various iron (oxyhydr)oxides are fully reviewed. Principal mechanisms of surface complexation, surface precipitation and structural incorporation are responsible for heavy metal retention on iron (oxyhydr)oxides, and greatly dependent on mineral species, metal ion species, reacting conditions (i.e., pH, heavy metal concentration, ionic strength, etc.) and chemical process (i.e., adsorption, coprecipitaton and mineral phase transformation process). The retention mechanisms summarized in this review would be helpful for remediating heavy metal contamination and predicting the long-term behavior of heavy metal in natural and engineered environments.

Effects of aluminum substitution on the surface charge of colloidal goethite particles: experiments and MUSIC modeling

Proton adsorption behavior on the surface of Al-substituted goethites as a function of pH and ionic strength was investigated and simulated with the multisite surface complexation (MUSIC) model. In addition, X-ray diffraction, X-ray photoelectron spectroscopy, and field emission scanning electron microscope were used to characterize the crystal structure, chemical composition, micromorphology, and surface properties of the Al-substituted goethite. Al substitution was found to affect the crystal structure and micromorphology of goethite. The morphological differences did not result in significant differences in PZC value but largely affected the surface charge values. Goethite surface charge capacity increased progressively with increasing amount of Al substitution, which was attributed to increases in the density of surface coordinated sites due to the increase in (021)/(110) face ratio. The optimization calculations enabled a satisfactory fitting of the titration data of both pure goethite and Al-substituted goethite, and the MUSIC model facilitated a more specific understanding of the charging behavior of Al-substituted goethite. The singly (≡FeOH^-0.5 + ≡AlOH^-0.5) and triply coordinated (≡Fe₃O^-0.5 + ≡AlFe₂O^-0.5) surface groups were most likely responsible for the basic charging behavior of goethite in the pH range of 4-10. All results indicate that the MUSIC model has excellent performance in characterizing Al-substituted goethite, and the model has promising application prospect in other substituted metal (hydr)oxides.

Effects of extreme pH conditions on the stability of As(V)-bearing schwertmannite

Schwertmannite (Sch) is known to be an effective scavenger of arsenic (As) due to its strong binding affinity for toxic As species. However, the evolution of As-bearing schwertmannite under extreme pH conditions is poorly understood. In this study, we investigated the effects of extremely acidic and alkaline conditions on the stability of schwertmannite with structurally incorporated As(V) (CO-Sch) and schwertmannite with adsorbed As(V) (AD-Sch). The results show that both extremely acidic and alkaline conditions have significant effects on the evolution of minerals and liberation of iron and sulfate. At extremely acidic pH, the maximal release of ferric iron (Fe(III)) and sulfate from CO-Sch were greater than that from AD-Sch, whereas 6.2% and 0.3% of total As released from AD-Sch and CO-Sch, respectively. At extremely alkaline pH, aqueous Fe(III) was not observed, and Fe(III) was retained in As-bearing schwertmannite due to the chemical equilibrium between the dissolution of schwertmannite and re-precipitation of goethite; structurally incorporated As(V) promoted the liberation of sulfate. In addition, the adsorbed As on schwertmannite is more stable, which led to a minor release of As (0.8%) over a 30-d period, however, the liberated As(V) from CO-Sch accounts for up to 3.2%. Under extremely acidic and alkaline conditions, portions of AD-Sch and CO-Sch transformed from schwertmannite to goethite after 30 d, while schwertmannite was still the dominant mineral. Adsorbed As(V) inhibited the transformation of As-bearing schwertmannite to goethite more significantly than structurally incorporated As(V).

Acidity and metallic elements release from AMD-affected river sediments: Effect of AMD standstill and dilution

In acid mine drainage (AMD) polluted rivers, considerable fraction of potential toxic elements are temporarily sequestered by sediments. There are two main potential environmental hazards associated with the sediments, acidity liberation and re-mobilization of metallic elements, during environmental conditions change. The effects of AMD standstill and water dilution on metallic elements migration were assessed in an AMD standstill test and a dialysis experiment. Maintaining AMD standstill, often occurring in AMD damming process, could induce the occurrence of iron secondary minerals precipitation along with attenuation of dissolved elements and a decrease in water pH value. Both field sediments and lab precipitates were confirmed as being dominant with schwertmannite which was the most important source and sink for acidity and metallic elements. The mechanism of cation heavy metals scavenging implied by FTIR results mostly depended on the exchanging of H⁺ from surface hydroxyl groups (-OH) in schwertmannite-rich sediments. For arsenic oxyanion, its adsorption included surface complexation with iron hydroxyl groups at the mineral surface, as well as anion exchange of SO₄^2- present in the structure. The quantities of acidity release differed significantly from 20 to 3714 mol H⁺/t depending on the iron hydroxyl minerals type and their contents in the corresponding sediments in 35 d dialysis, with the release rate well fitted by the second order model. Slight degree of phase transformation in schwertmannite dominant sediment had resulted in a high risk of metallic element release during the 35 d dilution duration. The significant risk of metallic elements release was ranked in the order of Cd > Mn > Zn > Pb, and with more than 89% of Cd released from FS6 and 82% from LPS1. Relatively, Cu and As in sediments were much more stable. Overall, damming was an effective and low cost pretreatment strategy for AMD pollution control. Knowledge of the characteristics of iron secondary minerals in river sediments is essential premise for both comprehensive assessment of site contamination status and effective remediation strategy decision.

Effects of hydrogen-peroxide supply rate on schwertmannite microstructure and chromium(VI) adsorption performance

Schwertmannite has attracted increasing interest for its excellent sorption for pollutants such as arsenite [As(III)] and arsenate [As(V)]. Limited studies were conducted with hexavalent chromium [Cr(VI)], especially for schwertmannite synthesized through Fe²⁺ oxidation. The effect of the hydrogen-peroxide (H₂O₂) supply rate on the structural characteristics and Cr(VI) adsorption capacity of schwertmannite is unclear. The morphology, crystallinity, specific surface area (SSA), pore volume and Cr(VI) adsorption of schwertmannite through Fe²⁺ oxidation at different H₂O₂ supply rates were analyzed. A slow H₂O₂ supply could improve the schwertmannite SSA and pore volume. Schwertmannite changed from nanoparticle aggregates (169.31 m²/g SSA, 0.20 cm³/g pore volume) to ball-with-whisker-shaped particles (228.75 m²/g SSA, 0.30 cm³/g pore volume) with a lower H₂O₂ supply rate. The Cr(VI) adsorption capacity increased by 6.25%-11.70% at different given Cr(VI) concentrations. The maximum Cr(VI) adsorption capacity of schwertmannite was 1.89 mmol/g at pH 6.0. Cr(VI) adsorption onto schwertmannite may be attributed mainly to ion exchange with the structural SO₄^2-. More than 91% of the original Cr(VI) adsorption capacity was maintained after four recycles. This study provides novel insights into the effects of H₂O₂ supply rate on the microstructure of schwertmannite and its adsorption capacity for Cr(VI) in aqueous medium.

Integrated electrokinetic processes for the remediation of phthalate esters in river sediments: A mini-review

Concerning the contamination of phthalate esters (PAEs) in river sediments, this mini-review introduces four recently reported novel "integrated electrokinetic (EK) processes" for the remediation purpose, namely two combined technologies of the EK process and advanced oxidation process (EK-AOP Processes) and two combined technologies of the EK process and biological process (EK-BIO Processes). The following is a comprehensive summary for these remediation processes: (1) the EK process coupled with nano-Fe₃O₄/S₂O₈^2- oxidation process - Test results have shown that nanoscale Fe₃O₄ played a significant role in activating persulfate oxidation. Even a recalcitrant compound like di(2‑ethylhexyl)phthalate (DEHP), its concentration in test sediment was reduced to 1.97 mg kg^-1, far below the regulatory levels set by Taiwan EPA; (2) the EK process integrated with a novel Fenton-like process catalyzed by nanoscale schwertmannite (nano-SHM) - Test results have revealed that simultaneous injection of nano-SHM slurry and H₂O₂ into the anode reservoir and sediment compartment is a good practice. 70-99% in removal efficiency was obtained for various target PAEs; (3) enhanced in situ bioremediation coupled with the EK process for promoting the growth of intrinsic microorganisms by adding H₂O₂ as an oxygen release compound (ORC) - Test results have demonstrated that an intermittent mode of injecting lab-prepared ORC directly into the contaminant zone would be beneficial to the growth of intrinsic microorganisms in test sediment for in situ bioremediation of target PAEs; and (4) coupling of a second-generation ORC (designated 2G-ORC) with the EK-biological process - Test results have proved that 2G-ORC is long-lasting and can be directly utilized as the carbon source and oxygen source for microbial growth resulting in an enhanced biodegradation of PAEs. Except DEHP having a residual concentration of 4 μg kg^-1, all other target PAEs in test sediment were totally removed by this novel combined remediation process.

Phosphate loading alters schwertmannite transformation rates and pathways during microbial reduction

Acid sulfate systems commonly contain the metastable ferric oxyhydroxysulfate mineral schwertmannite, as well as phosphate (PO₄^3-) - a nutrient that causes eutrophication when present in excess. However, acid sulfate systems often experience reducing conditions that destabilize schwertmannite. Under such conditions, the long-term fate of both schwertmannite and PO₄^3- may be influenced by interactions during microbially-mediated Fe(III) and SO₄^2- reduction. This study investigates the influence of PO₄^3- on Fe(III) and SO₄^2- reduction and the subsequent mineralogical transformation(s) in schwertmannite-rich systems exposed to reducing conditions. To accomplish this, varied PO₄^3- loadings were established in microbially-inoculated schwertmannite suspensions that were incubated under anoxic conditions for 82 days. Increased PO₄^3- attenuated the onset of microbial Fe(III) reduction. This delayed consequent pH increases, which in turn had cascading effects on the initiation of SO₄^2- reduction and subsequent mineral species formed. Under zero PO₄^3- loading, goethite (αFeOOH) formed first, followed by mackinawite (FeS) and siderite (FeCO₃). In contrast, in higher PO₄^3- treatments, vivianite (Fe₃(PO₄)₂) and/or sulfate green rust (Fe^II₄Fe^III₂(OH)₁₂SO₄) became increasingly important over time at the expense of goethite and mackinawite compared to PO₄^3--free conditions. The findings imply that PO₄^3- loading alters the rates and onset of microbial Fe(III)- and SO₄^2-- reduction and the subsequent formation of secondary Fe-bearing phases. In addition, schwertmannite reduction and the associated mineralogical evolution under anoxic conditions appears to sequester large quantities of PO₄^3- in the form of green rusts and vivianite.

Crystallization Behaviour of Iron-Hydroxide Sulphates by Aging under Ambient Temperature Conditions

The crystallization behaviour of jarosite and schwertmannite has been studied by precipitation-aging experiments performed using different parent-solution concentrations at acidic conditions and ambient temperature. Schwertmannite exhibits low crystallinity and is the only mineral identified during low-concentration (LC) experiments. However, in high-concentration (HC) experiments, a relatively rapid Ostwald ripening process leads to the transformation of schwertmannite into natrojarosite. The presence of sodium modifies the morphology and stability of the obtained phases. TEM observations reveal that schwertmannite particles consist of disoriented nanodomains (~6 nm) spread in an amorphous mass. In contrast, natrojarosite particles exhibit a single-domain, highly crystalline core, with the crystallinity decreasing from core to rim. The thermal behaviour of these phases depends on both their composition and their degree of crystallinity. TG and DTG analyses show that, below 500 °C, the amount of structural water is clearly higher in schwertmannite than in natrojarosite. The present results highlight the role of the ripening processes in epigenetic conditions and could be important in interpreting the formation of jarosite in Earth and Martian surface environments.

Chromium(III) substitution inhibits the Fe(II)-accelerated transformation of schwertmannite

Schwertmannite is an Fe(III)-oxyhydroxysulfate which is common in acid mine drainage (AMD) and acid sulfate soil (ASS) environments. Natural schwertmannite is often enriched in Cr(III), yet the effects of Cr(III) substitution on schwertmannite transformation to more stable Fe(III) minerals has not been addressed. Here we examine, for the first time, the effects of Cr(III) substitution on the Fe(II)-accelerated transformation of schwertmannite. X-ray diffraction (XRD) and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy shows that Cr(III) substitution inhibits schwertmannite transformation. Substitution at a Cr(III):Fe(III) ratio of 0.025 decreased schwertmannite transformation (at pH 6.5) by 18–49% (depending on Fe(II) concentrations) relative to that of Cr(III)-free schwertmannite. Formation of crystalline secondary phases (predominantly goethite) caused associated decreases in solid-phase Fe and Cr extractability by 1 M HCl. The extractability of Cr was consistently greater than that of Fe, suggesting some accumulation of Cr(III) at the residual schwertmannite surface–a phenomenon which passivates the surface against Fe(II)/Fe(III) electron transfer and atom exchange required for the Fe(II)-accelerated transformation process. The finding that Cr(III)-substitution inhibits schwertmannite transformation implies that it may also significantly impact associated Fe, S and trace metal(loid) behaviour.

Migration and fate of metallic elements in a waste mud impoundment and affected river downstream: A case study in Dabaoshan Mine, South China

Fate of metallic elements and their migration mechanisms in a waste mud impoundment and affected downstream were assessed. Physicochemical and mineralogical methods combined with PHREEQC calculation, statistical analysis and review of relevant literatures were employed. Results showed that the waste in mud impoundment had been severely weathered and acidized. Metallic elements exhibited high mobility and activity, with a mobility ranking order of Cd > Zn > Mn > Cu ≈ Cr > As ≈ Pb. Hydraulic transportation originating from elevation variation was the most important driving force for metallic elements migration. Although damming standstill was considered as an effective strategy for controlling coarse suspended particulate pollutants, metallic elements were still transported to the Hengshi River in both dissolved phase and fine suspended particle phase accompanied by the overflow of acid mine drainage. The concentrations of dissolved metallic elements were attenuated significantly along the Hengshi River within 41 km stretch. Precipitation/ co-precipitation of iron oxyhydroxides, especially schwertmannite, ferrihydrite and goethite minerals, were established as the most critical processes for metallic elements attenuation in river water. Accompanied by metals migration in the river, two pollution sensitive sites with notably high content of metals in the stretch of S6-S8 and S10, were identified in gently sloping river stretch.

Schwertmannite: occurrence, properties, synthesis and application in environmental remediation

Schwertmannite is a typical iron-derived mineral, which was originally discovered in acid mine drainings and subsequently synthesized in the laboratory. Increasingly, it is seen as having considerable potential as an adsorbent material, which could be used for environmental remediation (such as the treatment/remediation of arsenic, chromium, antimony, fluoride, and organic contaminants). This study reviews current developments, mainly in the preparation, structure, and water treatment of Schwertmannite. Several key issues are discussed in detail, such as synthetic strategy, the structure-property relationships, potential environmental applications, and related mechanisms. Soil remediation by schwertmannite is compared to water treatment, and its application is further evaluated. Finally, the methodologies for water treatment and soil remediation using schwertmannite are also taken into consideration from an environmental point of view.

Effect of Cu(II) on the stability of oxyanion-substituted schwertmannite

Schwertmannite, a Fe(III)-oxyhydroxysulfate mineral formed in acidic (pH 3~4), iron- and sulfate-rich acid mine drainage (AMD) environments, tends to undergo phase transformations with changes in pH and redox condition, which may depend on the presence of various trace anions and cations. In the present study, the effects of Cu(II) on the stability of arsenate-, chromate-, and molybdate-substituted schwertmannite were investigated. The release of Fe(III) and sulfate from schwertmannite seems to be accelerated in the presence of Cu(II) at pH ~ 3, while Cu(II) retarded the dissolution of schwertmannite at pH ~ 5. XRD and SEM results showed that pure schwertmannite and chromate-substituted schwertmannite underwent transformation to goethite over a 2-month period, the presence of Cu(II) enhanced the stability of the mineral's structure at both pH 3 and 5. However, the structures of arsenate- and molybdate-substituted schwertmannites showed no significant changes in the presence or absence of Cu(II) at both pH 3 and 5. During the phase transformation process, the amount of released oxyanions followed the sequence of chromate > molybdate > arsenate. Moreover, the release of arsenate and chromate from schwertmannite was retarded in the presence of Cu(II) at pH 5, whereas the release of molybdate was promoted. These results have important environmental implications for the stability of schwertmannite and its potential to immobilize contaminant trace elements under AMD conditions.