Adsorptive removal of As(III) by biogenic schwertmannite from simulated As-contaminated groundwater

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.

Arsenic Removal via the Biomineralization of Iron-Oxidizing Bacteria Pseudarthrobacter sp. Fe7

Arsenic (As) is a highly toxic metalloid, and its widespread contamination of water is a serious threat to human health. This study explored As removal using Fe(II)-oxidizing bacteria. The strain Fe7 isolated from iron mine soil was classified as the genus Pseudarthrobacter based on 16S rRNA gene sequence similarities and phylogenetic analyses. The strain Fe7 was identified as a strain of Gram-positive, rod-shaped, aerobic bacteria that can oxidize Fe(II) and produce iron mineral precipitates. X-ray diffraction, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy patterns showed that the iron mineral precipitates with poor crystallinity consisted of Fe(III) and numerous biological impurities. In the co-cultivation of the strain Fe7 with arsenite (As(III)), 100% of the total Fe and 99.9% of the total As were removed after 72 h. During the co-cultivation of the strain Fe7 with arsenate (As(V)), 98.4% of the total Fe and 96.9% of the total As were removed after 72 h. Additionally, the iron precipitates produced by the strain Fe7 removed 100% of the total As after 3 h in both the As(III) and As(V) pollution systems. Furthermore, enzyme activity experiments revealed that the strain Fe7 oxidized Fe(II) by producing extracellular enzymes. When 2% (v/v) extracellular enzyme liquid of the strain Fe7 was added to the As(III) or As(V) pollution system, the total As removal rates were 98.6% and 99.4%, respectively, after 2 h, which increased to 100% when 5% (v/v) and 10% (v/v) extracellular enzyme liquid of the strain Fe7 were, respectively, added to the As(III) and As(V) pollution systems. Therefore, iron biomineralized using a co-culture of the strain Fe7 and As, iron precipitates produced by the strain Fe7, and the extracellular enzymes of the strain Fe7 could remove As(III) and As(V) efficiently. This study provides new insights and strategies for the efficient remediation of arsenic pollution in aquatic environments.

Effects of Fe(II) concentration on the biosynthesis of schwertmannite by Acidithiobacillus ferrooxidans and the As(III) removal capacity of schwertmannite

The effect of Fe(II) concentrations on schwertmannite bio-synthesis and the As(III) removal capacity of schwertmannite were investigated in this study. Acidithiobalillus ferrooxidans (A. ferrooxidans) were inoculated into five FeSO4 systems with initial concentrations of 50, 100, 200, 300, and 400 mmol/L, respectively, to bio-synthesize schwertmannite. The Fe(II) of the systems were almost completely oxidised at 48, 72, 120, 168, and 192 h, respectively, and the bio-schwertmannite yield was 1.99, 3.81, 9.36, 12.42, and 21.60 g/L. The results of this study indicate that all minerals harvested from the different systems are schwertmannite. As the initial Fe(II) concentration increases, the effect of the minerals removing As(III) decreases; moreover, the structure and extracellular polymeric substance (EPS) of schwertmannite may regulate the As(III) removal process. The EPS generated by the A. ferrooxidans can absorb As(III). The outcomes of this study provide fresh insights into the bio-synthetic regulation of schwertmannite and play a significant role in treating As-containing groundwater.

Isolation and characterisation of Fe(II)-oxidising bacteria and their application in the removal of arsenic in an aqueous solution

Arsenic (As) is a toxic metalloid disseminated in water, soil, and air. Arsenic contamination is currently a major public health concern. This study investigated arsenic removal by Fe(II)-oxidising bacteria in an aqueous solution. A bacterial strain, Z1, isolated from concentrated sludge, was identified as Sphaerotilus natans based on microscopic morphology, culture characteristics, and 16s rRNA gene sequences. After arsenic-resistant acclimation, Sphaerotilus natans Z1 successfully survived and propagated in high arsenic conditions (100 mg·L-1 As(V) or As(III)). To a certain extent, the isolated strain could decrease the concentration of As(III)/As(V) by biosorption under organic substance supply. Partial As(V) could be reduced to As(III) due to cytoplasmic arsenic reduction of bacteria. In addition, ferrihydrite, one of the iron oxides, was formed by the mediation of Sphaerotilus natans in the Winogradsky medium. Most of As(III)/As(V) could be effectively removed by sorbing onto the resultant ferrihydrite mineral. Thus, iron oxide minerals facilitated by Sphaerotilus natans may be an alternative remediation strategy for scavenging arsenic in the water environment.

Remediating flooding paddy soils with schwertmannite greatly reduced arsenic accumulation in rice (Oryza sativa L.) but did not decrease the utilization efficiency of P fertilizer

Planting rice (Oryza sativa L.) in As-contaminated paddy soils can lead to accumulation of arsenic (As) in rice grains, while the application of phosphorus (P) fertilizers during rice growth may aggravate the accumulation effect. However, remediating flooding As-contaminated paddy soils with conventional Fe(III) oxides/hydroxides can hardly achieve the goals of effectively reducing grain As and maintaining the utilization efficiency of phosphate (Pi) fertilizers simultaneously. In the present study, schwertmannite was proposed to remediate flooding As-contaminated paddy soil because of its strong sorption capacity for soil As, and its effect on the utilization efficiency of Pi fertilizer was investigated. Results of a pot experiment showed that Pi fertilization along with schwertmannite amendment was effective to reduce the mobility of As in the contaminated paddy soil and meanwhile increase soil P availability. The schwertmannite amendment along with Pi fertilization reduced the content of P in Fe plaque on rice roots, compared with the corresponding amount of Pi fertilizer alone, which can be attributed to the change in mineral composition of Fe plaque mainly induced by schwertmannite amendment. Such reduction in P retention on Fe plaque was beneficial for improving the utilization efficiency of Pi fertilizer. In particular, amending flooding As-contaminated paddy soil with schwertmannite and Pi fertilizer together has reduced the content of As in rice grains from 1.06 to 1.47 mg/kg to only 0.38-0.63 mg/kg and significantly increased the shoot biomass of rice plants. Therefore, using schwertmannite to remediate flooding As-contaminated paddy soils can achieve the dual goals of effectively reducing grain As and maintaining the utilization efficiency of P fertilizers.

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.

Effect of pH regulation on the formation of biogenic schwertmannite driven by Acidithiobacillus ferrooxidans and its arsenic removal ability

The effect of pH regulation on schwertmannite bio-synthesis and its As removal ability were investigated in this study. The total Fe precipitation efficiency in a conventional schwertmannite bio-synthesis system (CK) reached 26.5%, with a mineral weight of 5.21 g/L and a mineral specific surface area of 3.18 m²/g. The total Fe precipitation efficiency increased to 88.4-95.8%, the mineral weight increased to 17.10-18.62 g/L, and the specific surface area increased to 3.61-90.67 m²/g of five different treatments in which the system pH was continually adjusted to 2.50, 2.70, 2.90, 3.10, and 3.30 every 3 h, respectively. The very small amounts of schwertmannite were transformed to goethite when the system pH was periodically adjusted to 2.90, 3.10 and 3.30. The increased specific surface area of bio-schwertmannite was due to the contribution of mesopores, with most pores having a diameter of 2-20 nm. For actual As-containing groundwater (27.4 μg/L), the As removal rate was 52.9% for bio-schwertmannite collected from the CK system. However, the removal rate of As increased to 92.7-97.8% for minerals which were collected after five adjusted pH treatments. The outcomes of this study provide a fresh insight into the bio-synthesis regulation of schwertmannite, and have great significance for the treatment of As-containing groundwater.

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.

Arsenic distribution in sediments of multi-tier sedimentary formation of coastal Pondicherry, India - Implications on groundwater quality

In this work, sediments collected from a 150-m deep litho-section of a coastal region and encompassing Quaternary, Tertiary and Cretaceous sedimentary formations were studied for arsenic distribution and other trace and redox sensitive elements. Arsenic concentration in the sediments is found to vary from <0.5 to 30 mg·kg^-1 and showed an increasing trend with the depth. The highest As content is observed at a depth of 129-131 m bgl belonging to Cretaceous formation. Though XRD studies do not indicate any arsenic mineral in sediments, positive correlations between As-Fe and S, as well as ESEM-EDS and TXRF studies confirm the presence of As sorbed to FeS mineral. The organic matter and As correlations suggest microbial mediated reduction process, which may pose future risk to water quality of this prolific coastal fresh water aquifer system. The elemental profiles infer that Cretaceous and Tertiary formations demonstrate inner shelf and marginal marine condition respectively.

A comprehensive study on amalgamation of sustainable solar powered distillation for arsenic and fluoride removal from groundwater

Due to the growing population and rapid urbanization, fresh water is becoming increasingly scarce. Apart from drinking, clean water is essential in a variety of applications such as batteries, pharmaceuticals, vehicles, and many research and development labs. Due to the unfeasible deficiency of the groundwater reserves, we must understand the importance of judicious usage and proper supervision of the groundwater resources. For the removal of various harmful substances in the groundwater such as arsenic, fluorides, and chlorides, we take the help of groundwater distillation. With the increase in environmental pollutions caused due to the use of injudicious usage of natural resources, we should look for an alternative for groundwater distillation techniques using renewable energy such as solar power. Groundwater is widely acknowledged as a major supply of water for drinking, agriculture, and a variety of other uses. Researchers have also employed solar distillation to transform groundwater into drinkable water. Hence, the main aim of the review paper is that we use solar power for groundwater distillation as solar energy is both a pressing necessity and a vital supply for all living things. Solar energy utilization is essential due to increased global warming and pollution, and it is being replaced by the traditional use of fuels in the globe today. We majorly focus on the removal of harmful compounds such as arsenic and fluoride from the groundwater with the help of solar-powered distillation techniques. The role of renewable energy technology in the sustainability of future water systems with a growing demanding share of groundwater distillation is presented in this paper.

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.

The coupling reaction of Fe2+ bio-oxidation and resulting Fe3+ hydrolysis drastically improve the formation of iron hydroxysulfate minerals in AMD

The oxidation of Fe²⁺ by Acidithiobacillus ferrooxidans (A. ferrooxidans) in acid mine drainage (AMD) is often accompanied by formation of iron hydroxysulfate minerals, such as schwertmannite and jarosite. This study reported that 80 mmol L^-1 of Fe²⁺ could be completely oxidized by A. ferrooxidans LX5 within 48 h, but only 27.7% of the resultant Fe³⁺ precipitated to form schwertmannite. However, the conversion efficiency to jarosite was much higher (54.5%). The formation of jarosite lasted 120 h, while only 24 h when conversed to schwertmannite. By constructing a cyclic process of 'Cu-reducing coupled with bio-oxidization', the total Fe in AMD could be fully converted into mineral precipitates. The resultant mineral specie could be regulated simply by control the K⁺ concentration. Thermodynamically, Fe³⁺ cannot hydrolyze spontaneously to form schwertmannite due to the positive Gibbs free energy (ΔrGm∘ = 6.63 kJ mol^-1) of the reaction. However, if Fe²⁺ were biologically oxidized by A. ferrooxidans, the resultant Fe³⁺ could spontaneously form schwertmannite because the aforementioned coupling reaction has a negative Gibbs free energy (ΔrGm∘ = -34.12 kJ mol^-1). Even though Fe³⁺ itself could hydrolyze to form jarosite spontaneously with ΔrGm∘ = -22.20 kJ mol^-1, the coupling reaction of Fe²⁺ bio-oxidation followed by Fe³⁺ hydrolysis in the presence of K⁺ could easily promote the formation of jarosite, which exhibited a great negative Gibbs energy (ΔrGm∘ = -67.45 kJ mol^-1).

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.

Modeling of microbial kinetics and mass transfer in bioreactors simulating the natural attenuation of arsenic and iron in acid mine drainage

Natural attenuation in acid mine drainage (AMD) due to biological iron and arsenic oxidation offers a promising strategy to treat As-rich AMD in passive bioreactors. A reactive transport model is developed in order to identify the main controlling factors. It simulates batch and flow-through experiments that reproduce natural attenuation in a high-As AMD. The 2-D model couples second-order microbial kinetics (Fe- and As- oxidation) and geochemical reactions to hydrodynamic transport. Oxidation only occurrs in the biofilm with an oxygen transfer from the air through the water column. The model correctly simulates the Fe(II)-Fe(III) and As(III)-As(V) concentrations in the outlet waters and the precipitates, over hydraulic retention times from 30 min to 800 min. It confirms that the natural attenuation at 20 °C is driven by the fast Fe(II) oxidation and slow As(III) oxidation that favors arsenite trapping by schwertmannite over amorphous ferric arsenate (AFA) formation. The localization of iron oxidation in the biofilm limits the attenuation of arsenic and iron as the water column height increases. The change in the composition of the bacterial iron-oxidizer community of the biofilm at the lowest pH boundary seems to control the Fe(II) oxidation kinetic rate besides the bacterial concentration.

Biosynthesized Schwertmannite@Biochar composite as a heterogeneous Fenton-like catalyst for the degradation of sulfanilamide antibiotics

Schwertmannite was successfully loaded onto biochar (Sch@BC) using a biosynthetic method. The physicochemical properties and structural morphology of Sch@BC were explored using XRD, SEM, BET, and XPS. The results showed that introducing biochar can effectively prevent the agglomeration of Sch. The catalytic activity of Sch@BC in the Fenton-like degradation of sulfamethoxazole (SMX) was also systematically investigated under different reaction conditions. Under optimum conditions ([SMX] = 10 mg L^-1, [H₂O₂] = 2.0 mM, Sch@BC = 1.0 g L^-1 and initial pH = 3.0), the removal efficiencies of the SMX and total organic carbon (TOC) were 100% and 45.9%, respectively, within 60 min of the reaction. The results of the radical scavenger effect and ESR studies suggested that the SMX degradation in the Sch@BC/H₂O₂ system was dominated by a heterogeneous Fenton-like reaction. The repeated use of Sch@BC for SMX degradation demonstrated its reusability and stability in Fenton-like reactions. There was also speculation about the degradation mechanism and pathways of SMX. Furthermore, under the same conditions, the removal efficiencies of sulfadiazine (SD) and sulfisoxazole (SIZ) under Fenton-like degradation in the Sch@BC system were 91% and 93%. The results provide a theoretical basis and practical guidance for the creation of a new catalyst using biochar as a support material for the degradation of sulfanilamide antibiotics.

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.

A novel approach for treating acid mine drainage through forming schwertmannite driven by a mixed culture of Acidiphilium multivorum and Acidithiobacillus ferrooxidans prior to lime neutralization

As the predominant treatment approach of acid mine drainage (AMD), lime neutralization often exhibits inefficiencies since the abundance of iron and sulfate in AMD usually form iron hydroxide and gypsum precipitate coatings on the surface of lime. In this study, a novel approach of biomineralization prior to lime neutralization for treating AMD was proposed, in which iron and sulfate were biologically precipitated as schwertmannite through iron biological reduction-oxidation driven by a culture mixed with Acidiphilium multivorum JZ-6 and Acidithiobacillus ferrooxidans LX5. It was found that only five cycles of iron reduction by A. multivorum JZ-6 followed by iron oxidation by A. ferrooxidans LX5 could remove completely iron and nearly 40% of sulfate in AMD, while non-ferrous metals (Al, Mn, Cu, Ni, and Zn) were hardly removed. Consequently, the amounts of lime required and sludge generated in the subsequent lime neutralization process were reduced by 56% and 68%, respectively. As a result, the content of non-ferrous metals in the sludge was increased by 3.2 folds. The level of Al was increased surprisingly to 19% (wt/wt), a level similar to the commercially valuable bauxite. The novel process of biomineralization prior to lime neutralization provides a sustainable way for AMD treatment.

Fluoride removal efficiencies and mechanism of schwertmannite from KMnO4/MnO2-Fe(II) processes

Fluoride has an adverse effect on both the environment and industrial production. In particular, wastewater from smelting systems containing high concentrations of fluoride is a major cause of fluoride pollution. Based on the characteristics of such wastewater, a targeted and integrated method for removal fluoride using schwertmannite is proposed. The schwertmannite, prepared from the oxidation of Fe(II) by KMnO₄ (Ksw) and MnO₂(Msw), effectively removed fluoride within 30 min under certain conditions. Under most experimental conditions, the removal efficiency of F ion by Ksw was always 13 % higher than that by Msw, which is attributed to the different concentrations of OH^- and SO₄^2- for ion exchange. The calculations showed that the chemical formulas of Ksw and Msw are Fe₈O₈(OH)_5.42±0.04(SO₄)_1.29±0.02 and Fe₈O₈(OH)_5.28±0.04(SO₄)_1.36±0.02, respectively. In the Ksw system, 0.70 mol of OH^- and 0.30 mol of SO₄^2- were released per mole of F ions sorbed; those released for the Msw system were 0.69 mol and 0.31, respectively. The results showed that OH^- played a primary role in the ion exchange and the schwertmannite showed good practicability for actual industrial wastewater.

Assessment of schwertmannite, jarosite and goethite as adsorbents for efficient adsorption of phenanthrene in water and the regeneration of spent adsorbents by heterogeneous fenton-like reaction

Schwertmannite, jarosite or goethite are commonly used to remove metals and/or metalloids from contaminated water via adsorption processes, but it is still unclear whether they can be used as adsorbents to remove hydrophobic organic pollutants (HOCs), such as polycyclic aromatic hydrocarbons (PAHs), from groundwater or wastewater. Here, the feasibility of using these iron (oxyhydr) oxide minerals as adsorbents for phenanthrene (a model PAH) adsorption and regenerating the spent adsorbents via heterogeneous Fenton-like reaction was investigated. Results showed that they exhibited rapid adsorption rates and considerable adsorption capacities for phenanthrene. The maximum Langmuir capacities (Qmax) for phenanthrene adsorption at 28 °C were in an ascending order of goethite (567 μg·g-1) < schwertmannite (727 μg·g-1) < jarosite (2088 μg·g-1). The adsorption process was a spontaneous and exothermic process along with the decrease of randomness at the solid/liquid interfaces, which was influenced by temperature, adsorbent dosage, and the coexistence of inorganic anions. Both schwertmannite and jarosite were superior to goethite in light of their easy separation from the bulk solution after the adsorption processes. A multi-cycle experiment demonstrated that the regeneration efficiency of schwertmannite (97.9-99.7%) was much higher than that of jarosite (80.1-87.2%), and the mineral structure, morphology and functional groups of schwertmannite were not changed during the successive adsorption-regeneration processes. Therefore, among the investigated three iron (oxyhydr) oxide minerals, schwertmannite was an attractive and regenerable adsorbent for the removal of phenanthrene from water owing to its high adsorption capacity, good separation ability, and excellent reusability.

Formation and transformation of schwertmannite in the classic Fenton process

The massive amount of sludge generated by the classic Fenton process, which has often been hypothesized to consist of ferric hydroxide, remains a major obstacle to its large-scale application. Therefore, reutilization of Fenton sludge has recently gained more attention. Understanding the formation, transformation, and properties of Fenton sludge combined with the stages of the Fenton reaction is pivotal, but not well illustrated yet. In this study, SEM-EDS, FT-IR, XRD, and XPS were applied to study the morphology, crystallinity, elemental composition, and valence state of Fenton sludge. The authors report that schwertmannite and 2-line ferrihydrite were generated and transformed in the oxidation phase and the neutralization phase of the Fenton process. SO₄^2- in the solution decreased by 8.7%-26.0% at different molar ratios of Fe(II) to H₂O₂; meanwhile, iron ion precipitated completely at pH 3.70 with the formation of schwertmannite containing sulfate groups in the Fenton sludge. The structural sulfate (Fe-SO₄) in schwertmannite was released from the precipitate with the addition of OH^-, and the production of Fenton sludge decreased with increasing pH when pH > 3.70. Goethite was found to form when the final pH was adjusted to 12 or at a reaction temperature of 80°C. Moreover, the possible thermal transformation to goethite and hematite indicated that Fenton sludge can be reused as a raw material for synthesizing more stable iron (hydro)oxides. The results provide useful insights into the formation and transformation of Fenton sludge, with implications for regulating the crystal type of Fenton sludge for further reuse.

Thiocyanate-induced labilization of schwertmannite: Impacts and mechanisms

Schwertmannite is an amorphous iron(III)-oxyhydroxysulfate that forms in acid mine drainage (AMD) environments. The characteristic of high heavy metal adsorption capability makes schwertmannite a potentially useful, environmentally friendly material in wastewater treatment. Unstable schwertmannite is prone to recrystallization. Understanding the mechanisms that induce schwertmannite labilization and affect its capacity to remove heavy metals are of great environmental and geochemical significance. Thiocyanate (SCN¯) is a hazardous pseudohalide that is also normally found in AMD. However, little is known about the impact of Fe(III)-binding ligand SCN¯ on schwertmannite stability and its subsequent capacity to bind trace elements. Here, we investigated the adsorption of SCN¯ on schwertmannite and subsequent mineral transformation to characterize this little-known process. The appearance of Fe²⁺ indicated that the interactions between schwertmannite and SCN¯ may involve complexation and reduction reactions. Results showed that the majority of the adsorbed-SCN¯ was immobilized on schwertmannite during the 60-days transformation. The transformation rates of schwertmannite increased with increasing concentrations of SCN¯. Goethite was detected as the dominant transformation product with or without SCN¯. The mechanisms of SCN¯-promoted dissolution of schwertmannite can be described as follows: (1) formation of Fe(III)-NCS complexes on the schwertmannite surface and in solution, a process which increases the reactivity of solid phase Fe(III); (2) the extraction of Fe(III) from schwertmannite by SCN¯ and subsequent schwertmannite dissolution; and (3) the formation of secondary minerals from extracted Fe(III). These findings may improve AMD treatment strategies and provide insight into the use and potential reuse of schwertmannite as a trace element sorbent.

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

Schwertmannite (SCH) has strong adsorption ability to As(III). However, there are few reports on the stability of SCH load with As(III) (SCH-As(III)). In this study, the effects of tartaric acid (TA), pH and coexisting ions including K⁺, Ca²⁺, Al³⁺ and CO₃^2- on the photoreductive dissolution of SCH- As(III) and the release of the adsorbed As (III) were investigated. The results showed that under UV irradiation TA could greatly enhance the release of total Fe and total As from SCH-As(III). Nevertheless, the total Fe and total As in the solution decreased when TA was consumed up. Compared to SCH, the reductive dissolution of SCH-As(III) was obviously suppressed. In the dark, TA could slowly enhance the dissolution of SCH-As(III), but its effect on the release of adsorbed As(III) was weak. Low pH was conducive to the release of iron and arsenic. Ca²⁺, K⁺, and CO₃^2- promoted the decrease of the dissolved total Fe in the later reaction. However, Al³⁺ inhibited the decrease of the dissolved total Fe and total As. The analyses of FTIR and XRD demonstrated that the mineralogical phase of SCH-As(III) after reaction changed. With light, the dissolved total Fe and total As existed mainly as Fe(II) and As(V), respectively. This is because Fe(II) was generated via ligand to metal charge transfer and As(III) was oxidized to As(V) by ·OH produced during the reaction. Thus, this study provides us with a comprehensive understanding of the stability of SCH-As(III) and the release of adsorbed As(III) in natural environments.

Transformation of cadmium-associated schwertmannite and subsequent element repartitioning behaviors

Schwertmannite is an important sink for cadmium (Cd) in acid mine drainage (AMD) environments and is unstable when environmental conditions change. However, the release and redistribution of Cd during schwertmannite transformation with respect to pre-bound Cd are poorly understood. In this work, the transformation of cadmium-associated schwertmannite and subsequent Cd repartitioning behaviors were investigated. The way of schwertmannite associated with Cd was predominant by absorption, and the diffuse layer model (DLM) showed that Cd²⁺ existed as monodentate complexes ≡Fe₍₁₎OCd⁺ and ≡Fe₍₂₎OCd⁺ on schwertmannite surfaces. Kinetics of SO₄^2- release and mineralogical characterization both showed that the mineral transformation rates decreased and more lepidocrocite aggregated with increasing adsorbed Cd levels. The shrinking core model revealed that Fe(II)-induced process would affect mineral dissolution by changing surface reaction-controlled step to internal diffusion-controlled step, and significantly promote the dissolution rate of Cd-adsorbed schwertmannite. Adsorbed Cd blocked the surface sites for later Fe(II) adsorption and the Fe(II)-Fe(III) electron transfer, then resulted in the decelerated transformation and the accumulation of intermediate phase lepidocrocite. The maximum release of aqueous Cd occurred after 1 mM Fe²⁺ addition, then over 69% of initial added Cd_(aq) re-bound to solid-phase accompanying with mineral transformation, and finally, Cd was mainly associated with the secondary minerals by complexation with surficial OH groups. These findings are useful for developing the strategies for treating Cd contamination in AMD affected areas.

A novel approach coupling ferrous iron bio-oxidation and ferric iron chemo-reduction to promote biomineralization in simulated acidic mine drainage

A novel Acidithiobacillus ferrooxidans-mediated approach coupling biological oxidation and chemical reduction for treating acid mine drainage (AMD) was investigated. The results showed that controlled addition of zero valent iron (ZVI) into the coupling system did not exhibit a significant adverse influence on the bacterial activity of Acidithiobacillus ferrooxidans but markedly increased the formation of secondary Fe-minerals. Nutrition did not affect the efficiency of coupling process, except for the bacteria density of A. ferrooxidans. 2 days cyclic treatment performed better than that of 4 and 8 days. After 14 cycles of the coupling process, 89.4% of total iron (2.23 g L⁻¹) was transferred into Fe-minerals finally. In addition, the combined system was highly effective in removing sulfate (63%) from a simulated AMD that contained soluble Cu, Zn, Al, and Mn. Valuable iron-sulfate material e.g. schwertmannite was formed with little co-precipitation of other metals. Therefore, the integration of A. ferrooxidans into the reduction by ZVI may have considerable potential in the enhancement of biomineralization efficiency, which may further decrease soluble TFe and sulfate loads in AMD before lime neutralization.

Controlled addition of ZVI into Acidithiobacillus ferrooxidans-mediated biomineralization can markedly improve the formation of secondary Fe-minerals in AMD.

Initial pH and K+ concentrations jointly determine the types of biogenic ferric hydroxysulfate minerals and their effect on adsorption removal of Cr(VI) in simulated acid mine drainage

It is of practical significance to promote the transformation of Fe in acid mine drainage (AMD) into ferric hydroxysulfate minerals with strong ability to remove heavy metals or metalloids. To investigate the types of biogenic ferric hydroxysulfate minerals generated in AMD by Acidithiobacillus ferrooxidans (A. ferrooxidans), different pH and K⁺ concentrations are tested for the formation of precipitates in media containing 160 mmol/L Fe²⁺. The Cr(VI) removal efficiencies of ferric hydroxysulfate minerals in AMD with different acidities are also compared. Results indicate that the mineralizing abilities of the initial pH levels (pH 3.0 > pH 2.5 > pH 2.0) and K⁺ concentrations (53.3 mmol/L > 3.2 mmol/L ≈ 0.8 mmol/L) differ, with cumulative Fe precipitation efficiencies of 58.7%, 58.0%, and 44.2% (K⁺ = 53.3 mmol/L), and 58.7%, 29.9%, and 29.6% (pH 3.0) after 96 h of A. ferrooxidans incubation, respectively. X-ray diffraction indicates that K-jarosites are formed in the treatments n(Fe)/n(K) = 0.1 and 3 at pH 2.0-3.0, while only schwertmannite is generated in a system of pH 3.0 and n(Fe)/n(K) = 200. X-ray photoelectron spectroscopy reveals that HCrO₄ ^- may be adsorbed as an inner-sphere complex on schwertmannite when the AMD pH is 3.0.

Comparison of the Biological and Chemical Synthesis of Schwertmannite at a Consistent Fe2+ Oxidation Efficiency and the Effect of Extracellular Polymeric Substances of Acidithiobacillus ferrooxidans on Biomineralization

Schwertmannite is an environmental mineral material that can promote the natural passivation of heavy metal elements, thereby reducing environmental pollution from toxic elements. However, the fundamental reason for the difference between the chemically (H₂O₂-FeSO₄) and biologically (Acidithiobacillus ferrooxidans-FeSO₄) synthesized schwertmannite is still unclear. In this study, X-ray diffraction, scanning electron microscopy, the Brunauer⁻Emmett⁻Teller method, and X-ray fluorescence spectrometry were used to compare the structure, specific surface area, and elemental composition of schwertmannite synthesized by biological and chemical methods. The removal capacity of As(III) by the two kinds of schwertmannite and the effects of extracellular polymeric substances (EPS) on biogenetic schwertmannite were also investigated. At a consistent Fe2+ oxidation efficiency, the chemical method synthesized more schwertmannite than the biological method over a 60-h period. The biosynthesized schwertmannite had a "chestnut shell" shape, with a larger particle size and specific surface than the chemically synthesized schwertmannite, which was relatively smooth. The saturated adsorption capacities of the biologically and chemically synthesized schwertmannite were 117.0 and 87.0 mg·g-1, respectively. After exfoliation of the EPS from A. ferrooxidans, the biosynthesized schwertmannite displayed a "wool ball" shape, with rough particle surfaces, many microporous structures, and a larger specific surface area. The schwertmannite yield also increased by about 45% compared with that before exfoliation, suggesting that the secretion of EPS by A. ferrooxidans can inhibit the formation of schwertmannite.

Synthetic Iron Oxides for Adsorptive Removal of Arsenic

Removal of arsenic from water reservoirs is the issue of great concern in many places around the globe. As adsorption is one of the most efficient techniques for treatment of As-containing media, thus the present study concerns application of iron oxides-hydroxides (akaganeite) as adsorbents for removal of this harmful metal from aqueous solution. Two types of akaganeite were tested: synthetic one (A) and the same modified using hexadecyltrimethylammonium bromide (A_M). Removal of As was tested in batch studies in function of pH, adsorbent dosage, contact time, and initial arsenic concentration. The adsorption isotherms obey Langmuir mathematical model. Adsorption kinetics complies with pseudo-second-order kinetic model, and the constant rates were defined as 2.07 × 10^-3and 0.92 × 10^-3 g mg^-1 min^-1 for the samples (A) and (A_M), respectively. The difference was caused by significant decrease in adsorption rate in initial state of the process carried out for the sample A_M. The maximum adsorption capacity achieved for (A) and (A_M) akaganeite taken from Langmuir isotherm was 148.7 and 170.9 mg g^-1, respectively. The results suggest that iron oxides-hydroxides can be used for As removal from aqueous solutions.

Ultrasonic-enhanced Fenton-like degradation of bisphenol A using a bio-synthesized schwertmannite catalyst

Schwertmannite (Sch) was synthesized by Acidithiobacillus ferrooxidans and used as Fenton-like catalyst for bisphenol A (BPA) degradation combining with ultrasonic technology (US). Physicochemical characterizations showed that the bio-synthesized Sch particles had a pompon-like morphology with high BET surface area of 92.92m²/g. The degradation reaction showed a two-stage pseudo-first-order kinetic process consisting of an induction period and a followed rapid degradation period. A synergistic effect existed between US and Sch on activating H₂O₂ and the synergy factor was calculated to be 2.32. The catalytic efficiency of the system was mainly affected by pH, Sch dosage and temperature, but less relevant to H₂O₂ concentration. Free OH radicals in the bulk solution were identified to be the dominant oxidant, which were produced by both heterogeneous and homogeneous processes. The promotional effect of US on Fenton-like degradation of BPA can be ascribed to the reasons of (1) increasing the radical generation by ultrasonic cavitation; (2) reducing the apparent activation energies of degradation reaction; (3) accelerating the dissolution of iron and (4) keeping the high surface area of catalyst by continuous surface cleaning. Ecotoxicity tests indicated lower toxicities of intermediates than BPA. In addition, Sch exhibited high reusability in the recycle study.

Evaluation and optimization of a new microbial enhancement plug-flow ditch system for the pretreatment of acid mine drainage: semi-pilot test

Acid mine drainage (AMD) is typically characterized by low pH, a high concentration of sulfate and dissolved heavy metals. Therefore, it is of practical significance to promote the transformation of soluble Fe and SO₄²⁻ into iron hydroxysulfate minerals by biomineralization of Acidithiobacillus ferrooxidans. This enhances the lime neutralization efficiency of AMD by reducing the production of ferric hydroxide and waste gypsum. In this study, a new microbial enhanced plug-flow ditch reaction system was developed for the pretreatment of AMD on a semi-pilot scale. System stability under different hydraulic retention times (HRTs) was examined and the effects of microbe enhancement-lime neutralization technology and direct lime neutralization technology were compared. The bio-oxidation efficiency of Fe²⁺ (5 g L⁻¹) reached 100% in some parts of the system when HRT was 3 and 2 days, and the time taken to reach steady state was 6 and 4 days, respectively. When the HRT was 1 day, the reaction system had operated for 4 days before the equilibrium was lost. At the optimum HRT (2 days) and after the system was stable, the average precipitation rate of total Fe was 53.62% and the average removal rate of As(iii) was 17.27%. Following microbial enhanced pretreatment, the amount of lime required and waste residues generated for AMD neutralization decreased by 75.00% and 85.25%, respectively. This result supports the application of microbial enhancement-lime neutralization passive treatment technology for AMD.

Acid mine drainage (AMD) is typically characterized by low pH, a high concentration of sulfate and dissolved heavy metals.

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.

Combination of microbial oxidation and biogenic schwertmannite immobilization: A potential remediation for highly arsenic-contaminated soil

Here, a novel strategy that combines microbial oxidation by As(III)-oxidizing bacterium and biogenic schwertmannite (Bio-SCH) immobilization was first proposed and applied for treating the highly arsenic-contaminated soil. Brevibacterium sp. YZ-1 isolated from a highly As-contaminated soil was used to oxidize As(III) in contaminated soils. Under optimum culture condition for microbial oxidation, 92.3% of water-soluble As(III) and 84.4% of NaHCO₃-extractable As(III) in soils were removed. Bio-SCH synthesized through the oxidation of ferrous sulfate by Acidithiobacillus ferrooxidans immobilize As(V) in the contaminated soil effectively. Consequently, the combination of microbial oxidation and Bio-SCH immobilization performed better in treating the highly As-contaminated soil with immobilization efficiencies of 99.3% and 82.6% for water-soluble and NaHCO₃-extractable total As, respectively. Thus, the combination can be considered as a green remediation strategy for developing a novel and valuable solution for As-contaminated soils.

The influence of fractal nature on schwertmannite adsorption properties

Schwertmannite (SCH), a ferric oxyhydroxy sulfate mineral, had attracted extensive interests due to its excellent adsorption performance in decontamination processes.

Immobilization of As(V) in Rhizopus oryzae Investigated by Batch and XAFS Techniques

Arsenic (As) contamination in aqueous solutions has become an increasing public concern due to the immense harm to human health. Herein, bioaccumulation of arsenate (As(V)) by Rhizopus oryzae in aqueous systems was investigated under different environmental conditions, such as different pH's, ionic strengths, mycelia dosages, mycelia growths, and temperatures. The results showed that As(V) could be bioaccumulated efficiently by R. oryzae, and the maximum bioaccumulation capacity of As(V) in R. oryzae was 52.4 mg/g at T = 299 K, which was much higher than that for other biomaterials under similar conditions. R. oryzae generated a higher content of thiol compounds under As(V) stress to immobilize As(V) from aqueous solutions. X-ray absorption near-edge spectroscopy analysis indicated that As(V) was partly reduced to As(III) with increasing contact time, which increased As(V) bioaccumulation in mycelia. In addition, extended X-ray absorption fine structure analysis showed that the As-S complex played an important role in As(V) immobilization by mycelia. This study provided an in-depth investigation of intracellular As speciation and coordination in R. oryzae on the molecular scale, which was crucial to understand the interaction mechanisms of As(V) with fungi during environmental cleanup.

Adsorption and degradation of Congo red on a jarosite-type compound

Natrojarosite particles were prepared by forced hydrolysis. X-ray diffraction and field-emission scanning electron microscopy were used to characterize the resulting products.

Schwertmannite Synthesis through Ferrous Ion Chemical Oxidation under Different H2O2 Supply Rates and Its Removal Efficiency for Arsenic from Contaminated Groundwater

Schwertmannite-mediated removal of arsenic from contaminated water has attracted increasing attention. However, schwertmannite chemical synthesis behavior under different H2O2 supply rates for ferrous ions oxidation is unclear. This study investigated pH, ferrous ions oxidation efficiency, and total iron precipitation efficiency during schwertmannite synthesis by adding H2O2 into FeSO4 · 7H2O solution at different supply rates. Specific surface area and arsenic (III) removal capacity of schwertmannite have also been studied. Results showed that pH decreased from ~3.48 to ~1.96, ~2.06, ~2.12, ~2.14, or ~2.17 after 60 h reaction when the ferrous ions solution received the following corresponding amounts of H2O2: 1.80 mL at 2 h (treatment 1); 0.90 mL at 2 h and 14 h (treatment 2); 0.60 mL at 2, 14, and 26 h (treatment 3); 0.45 mL at 2, 14, 26, and 38 h (treatment 4), or 0.36 mL at 2, 14, 26, 38, and 50 h (treatment 5). Slow H2O2 supply significantly inhibited the total iron precipitation efficiency but improved the specific surface area or arsenic (III) removal capacity of schwertmannite. For the initial 50.0 μg/L arsenic (III)-contaminated water under pH ~7.0 and using 0.25 g/L schwertmannite as an adsorbent, the total iron precipitation efficiency, specific surface area of the harvested schwertmannite, and schwertmannite arsenic(III) removal efficiency were 29.3%, 2.06 m2/g, and 81.1%, respectively, in treatment 1. However, the above parameters correspondingly changed to 17.3%, 16.30 m2/g, and 96.5%, respectively, in treatment 5.

Arsenic immobilization in the contaminated soil using poorly crystalline Fe-oxyhydroxy sulfate

A low crystalline Fe-oxyhydroxy sulfate (FeOS) was used to immobilize arsenic (As) in soils in this study. The effects of FeOS amount, treatment time and soil moisture on As immobilization were investigated. The results showed that water-soluble and NaHCO3-extractable As were immobilized by 53.4-99.8 and 13.8-73.3% respectively, with 1-10% of FeOS addition. The highest immobilization of water-soluble (98.5%) and NaHCO3-extractable arsenic (47.2%) was achieved under condition of 4% of FeOS and 80% of soil moisture. Further, more amounts of FeOS addition resulted in less time requirement for As immobilization. Sequential chemical extraction experiment revealed that easily mobile arsenic phase was transferred to less mobile phase. The FeOS-bonded As may play a significant role in arsenic immobilization. Under leaching with simulated acid rain at 60 times pore volumes, accumulation amount of As release from untreated soil and soil amended with FeOS were 98.4 and 1.2 mg, respectively, which correspond to 7.69 and 0.09% of total As amounts in soil. The result showed that the low crystalline FeOS can be used as a suitable additive for arsenic immobilization in soils.

Implementation of zero-valent iron (ZVI) into drinking water supply - role of the ZVI and biological processes

Arsenic in drinking water is concerning millions of people around the world, even though many solutions to the problem have come up in recent years. One of the promising solutions for removing arsenic from water is by implementation of a zero-valent iron (ZVI) in the drinking water production. The purpose of this work was to study a treatment of As pollution based on the ZVI, aeration and sand filtration that was monitored for period of 45 months. In applied configuration and conditions ZVI was not able to remove arsenic alone, but it worked as a source of ferrous ions that during its oxidation enabled to co-precipitate arsenic compounds in the sand filter. The results show that after a lag phase of about 6 months, it was possible to achieve water production with an As content from 20 μg L(-1) to below 5 μg L(-1). The treatment also enabled to remove phosphates that were present in groundwater and affected As uptake by hindering its co-precipitation with Fe compounds. Determination of colony forming units on As amended agar helped to find arsenic resistant bacteria at each stage of treatment and also in the sand filter backwash sludge. Bacterial communities found in groundwater, containing low concentration of As, were found to have high As resistance. The results also indicate that the lag phase might have been also needed to initiate Fe ions release by corrosion from elemental Fe by help of microbial activity.

Arsenic(V) Removal in Wetland Filters Treating Drinking Water with Different Substrates and Plants

Constructed wetlands are an attractive choice for removing arsenic (As) within water resources used for drinking water production. The role of substrate and vegetation in As removal processes is still poorly understood. In this study, gravel, zeolite (microporous aluminosilicate mineral), ceramsite (lightweight expanded clay aggregate) and manganese sand were tested as prospective substrates while aquatic Juncus effuses (Soft Rush or Common Rush) and terrestrial Pteris vittata L. (Chinese Ladder Brake; known as As hyperaccumulator) were tested as potential wetland plants. Indoor batch adsorption experiments combined with outdoor column experiments were conducted to assess the As removal performances and process mechanisms. Batch adsorption results indicated that manganese sand had the maximum As(V) adsorption rate of 4.55 h^-1 and an adsorption capacity of 42.37 μg/g compared to the other three aggregates. The adsorption process followed the pseudo-first-order kinetic model and Freundlich isotherm equations better than other kinetic and isotherm models. Film-diffusion was the rate-limiting step. Mean adsorption energy calculation results indicated that chemical forces, particle diffusion and physical processes dominated As adsorption to manganese sand, zeolite and gravel, respectively. During the whole running period, manganese sand-packed wetland filters were associated with constantly 90% higher As(V) reduction of approximate 500 μg/L influent loads regardless if planted or not. The presence of P. vittata contributed to no more than 13.5% of the total As removal. In contrast, J. effuses was associated with a 24% As removal efficiency.

Redox stability of As(III) on schwertmannite surfaces

As(III)-enriched mine discharge often drains through Fe(III)-mineral abundant land covers which makes the understanding of its fate and redox behaviour extremely important. We therefore conducted batch kinetic and equilibrium studies at pH 3.0±0.05 in anoxic media coupled with spectroscopic and microscopic examinations at variable conditions to understand possible As(III) binding mechanisms and the redox stability of As(III) on schwertmannite, a prominent ferric mineral in acid mine drainage environments. Schwertmannite acted as an efficient scavenger for As(III) compared to goethite at identical sorbent:solute ratios. As K-edge X-ray absorption near-edge structure (XANES) demonstrated partial oxidation of sorbed As(III) to As(V) on both the minerals depending on the Fe(III)/As(III) ratios (goethite acted as a better oxidant than schwertmannite). Sorbed As(III) and As(V) coordinated in a bidentate binuclear binding mechanism with As(III)/As(V)-O and As(III)/As(V)-Fe interatomic distances as 1.78/1.69 and 3.37/3.31Å, respectively. Scanning (SEM-EDX) and transmission (TEM) electron microscopic, and IR spectroscopic measurements revealed the formation of As-containing surface coatings by sorbed As on schwertmannite.

Arsenate adsorption on three types of granular schwertmannite

Schwertmannite was synthesized on a 2 m(3)-scale and fabricated to irregular, cylindrical and spherical shape granules using drum granulation, extrusion and spray coating, respectively. The granules were systematically evaluated for As(V) removal from drinking water in terms of both performance and safety. The irregular and cylindrical shape granules (IS and CS) had larger schwertmannite loadings, higher porosity, more abundant pore structure and larger micropore volumes than those with a spherical shape (SS). As(V) adsorption kinetics on IS, CS and SS schwertmannite granules followed a pseudo-second order rate equation and two-stages of intraparticle diffusion. The rate parameters were in an order of IS > CS > SS granules. The faster uptake kinetics of the IS granules was due to their largest pore volume and interparticle porosity. Furthermore, adsorption capacities of 34, 21 and 5 mg/g, for IS, CS and SS granular schwertmannite samples were achieved at an initial As(V) concentration of 20 mg/L and adsorbent dose of 0.5 g/L. IS and CS samples performed much better over a wide pH range versus SS samples. Except for humic acid, PO4(3-) and SiO4(4-) did not inhibit As(V) adsorption on IS and CS granular specimens. SS samples worked poorly even in the absence or presence of co-existing anions. Regeneration was achieved using 0.1 M NaOH. The recycled IS and CS granular specimens can be used for 4 different cycles with no or nominal loss of adsorption capacity. Column experiments were also conducted. The IS, CS and SS granular specimens treated 8100, 4200 and 120 bed volumes (BVs) of contaminated water. No heavy metals leached from the packed granular adsorbent and appeared in the column effluent. Furthermore, the toxicity characteristic leaching procedure (TCLP) showed that the spent IS and CS granules were inert and could safely be disposed of in landfills. In short, irregular-shaped granules (IS) fabricated by drum granulation is a good candidate for arsenic removal from drinking water with a high future application potential.