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.


Introduction
Schwertmannite (Sch) is a secondary iron (Fe)hydroxylsulfate mineral, which is found in acidic sulfaterich environments. It could potentially be used to treat multiple heavy metals in contaminated water or soils. It is probably one of the most frequently observed minerals that precipitate from ferriferous aqueous systems, such as mine drainages, soil solutions, and lake water, 1 similar to ferrihydrite.

Discovery
In natural environments, the Fe-sulfur (S) system automatically cycles through a reduction path based on Fe sulde and an oxidation path based on Fe sulfate. The Fe sulfates are commonly found in natural marine environments or lakes, etc., which oen also contain Sch. 2 Natural sources of Sch can be acid mine drainings (AMD), acid sulfate soils (ASS), 3 solid minerals, such as ochreous sediments, 4 natural Fe and S oxidation by-products in coal, 5 and some types of industrial liquid/solid waste produced during human activities, such as copper heap leachate solutions. 6 Early studies on Sch were undertaken by Brady et al. 7 and Murad 8 before it was recognized as a discrete mineral. There was no chemical analysis. Therefore, Sch was tentatively identied as a "well" crystallized ferrihydrite phase. In 1990, Bigham et al. 9 discovered a poorly crystallized oxyhydroxysulfate of Fe in sediment samples collected from mine effluents in the eastern Ohio coaleld and drainage exiting an abandoned copper-arsenic mine in Finland, which was identi-ed as the primary component of ochreous precipitates from sulfate-rich mine waters in the pH range of 2.5-4.0. This material was approved as an independent mineral species by the Commission on New Materials and Mineral Names of the International Mineralogical Association in 1992 (90-006). 10 Two years later, Bigham et al. 10 reported its occurrence at the Pyhäsalmi sulde mine, Province of Oulu, Finland. In the same year, this mineral was named in honour of Udo Schwertmann, Professor of Soil Science from the Technical University of Munich, who published the rst description of Sch. 9 In 1995, aer being frequently identied in precipitates from acid mine waters, a fully natural Sch was found in a small stream draining a pyritic Schist at PtScher Joch, Zillertaler Alps, Austria. 11 In Japan, Sch was discovered in 2003, based on a phenomenon of naturally attenuated arsenic (As) concentration form abandoned As mine in Nishinomaki, 12 and then the rst patent about Sch for absorbent was published. 13 In Chinese, Sch is called "shī weī tè màn shí", 14 and was rst found in AMD sediments from the Malan coal mine, Shanxi Province, China, in 2006, 15 and aerwards at the Dabaoshan mine in South China. [16][17][18][19] The research development of Sch and the primary contributions of researchers or groups are listed in Table 1. The table shows that natural Sch was discovered, identied, and structurally classied within the rst few years of its discovery. Aer this, related theoretical studies (over nearly 20 years) about its properties and further application, especially for environmental protection, began to attract more attention.

Microstructure
Characterization by X-ray diffraction (XRD) has suggested that Sch has poorer crystallinity characteristics than jarosite and goethite, but it is quite similar to ferrihydrite. 25 However, Loan et al. 26 demonstrated that most Sch whiskers have a structure that is consistent with maghemite, which is similar to some ferrihydrite components, whereas some sections also contain highly disordered ferrihydrite structural components and more amorphous regions. This description is mainly based on XRD data and some other direct images about the surface morphology of Sch. The Sch particles are transformed into crystal cubic and hexagonal phases, during which, sulfate and OH/H 2 O are ejected from the structure and sulfate is structurally incorporated. The microstructure of Sch is shown in Fig. 1A.
Schwertmannite is a novel mineral and possesses a 3D structure with ordered channels. Fukushi et al. 27 28 showed a deformed frame of an Fe octahedral that was similar to that of akaganéite. This was based on pair distribution functional data, XRD analysis, and density functional theory calculations. The simulations suggested that two sulfate molecules per unit cell were present in the structure, which formed one outer sphere and one inner sphere complex inside the channels created by the Fe octahedral. The material is described as "analogous in structure to basic Fe sulfate, but with a -Fe-Ocage, as shown in Fig. 1B. The Sch possessed a channel-like structure in the c axis orientation with a diameter of ca. 0.50 nm. 5 The SO 4 2À ions in the channel pores can be easily substituted by other ions that have a similar radius, such as PO 4 3À (r ¼ 0.238 nm), CrO 4 2À (Cr: chromium) (0.240 nm), or AsO 4 3À (0.248 nm). 29 Bigham et al. 30 reported that

Synthetic strategies
The most common formation process for natural Sch (formula (1)) usually goes through two main steps, which are the oxidation of Fe 2+ and co-precipitation of Fe 3+ . However, aer nearly 20 years of development and improvement, Sch can be synthesized using various techniques that involve the chemical oxidation of ferrous species, abiotic precipitation of ferrous species (fast hydrolysis and slow dialysis), and/or biological ferrooxidation of ferrous species by Fe 2+ -oxidizing bacteria, such as Acidithiobacillus ferrooxidans, as Table 2 Chemical methods. Chemical methods for Sch preparation refer to the chemical oxidation of Fe 2+ species and the abiotic precipitation of Fe 3+ species, but these techniques are limited to basic theoretical research and small-scale synthesis in the laboratory. In these two methods Fe 3+ undergoes precipitation, but the former one needs another prior oxidation step for Fe 2+ species.
Chemical oxidation of Fe 2+ species. This method involves two main steps: pH adjustment of the Fe 2+ solution and the addition of H 2 O 2 . It was rst published by Regenspurg et al. 20 who followed a technique provided by Pentinghaus. In most associations, this method is carried out at room temperature. Previously, H 2 O 2 has been the most used oxidant for the Fe 2+oxidation process due to its clean reaction (formula (2) 35 The separation of Sch products are dependent on washing and fast centrifugation or ltration, the efficiency of which may be affected by the particle sizes.
Abiotic precipitation of Fe 3+ species. The Fe 3+ cations in water can be precipitated both over a short period of time and constantly over the long-term. During the hydrolysis of Fe 3+ species in innity dilutions, the constants of the hydroxo complexes of Fe 3+ were remarkably different when they were measured by Stefansson. 36 In the precipitation process, the Fe 3+ species rstly combines with water molecules to form complex intermediates, and then transforms into Fe-based oxysulfate (it becomes Sch when high concentrations of SO 4 2À exist). This process is shown by formula (3).
In comparison to the chemical oxidation method, the abiotic precipitation of Fe 3+ species is carried out at higher temperatures, which are usually higher than 60 C and lead to fast precipitation. Furthermore, the nucleation rate for abiotic precipitation is relatively higher than for chemical oxidation. Different abiotic precipitation techniques may lead to differences in crystal-growing time, but not the nucleation rate (fast completed in short time).
Fast hydrolysis. The Fe 3+ solutions were subjected to homogeneous hydrolysis at pH $ 1 by Reichelt et al. 37 6 ] + complexes transform into hydroxo complexes, and then polymerize into low molecular and positively-charged polynuclear complexes, such as [Fe 2 (OH) 3 ] 3+ or [Fe 3 (OH) 2 ] 7+ before precipitation. The creation of fast precipitation processes (involving a complex hydration-decomposition-polymerization-agingprecipitation process) causes the formation of kinetically favourable morphologies with low crystallization levels. When the precipitate remains in suspension, a thermodynamic optimum is approached due to crystallization and structural changes. This technique has been used on a small-scale, but scaled-up equipment has already been produced by a group led by Bertau. 37 They investigated the precipitation reaction of Fe 2 (SO 4 ) 3 solution with ammonia using a microjet mixer with gas discharge and a two impinging jet mixer to obtain nanostructured particles in the mm range. The fast hydrolysis method only requires a relatively fast wash and separation period for the nal product, which may make it easier to create a working system, but could also bring problems, such as a disordered structure and inaccurate composition due to fast nucleation and incomplete washing. Previous studies have suggested that this strategy provides Sch products with similar particle sizes and XRD peaks to the ones obtained through chemical oxidation. This is probably due to the similar separation process for Sch. Slow dialysis. In contrast to fast hydrolysis, slow dialysis has a remarkably long treatment time (usually longer than a month) for the crystal growing and product washing stages, which results in a longer reaction or crystallization time. The slow dialysis techniques for Sch synthesis involve long-term dialysis by cellulose membranes over about 30 days followed by a freeze drying process, which is expensive, complex, and timeconsuming. 38 However, the mineral particle diameters of the synthetic Sch produced by slow dialysis are larger than the synthetic Sch produced by the rapid methods. They also have better XRD signals and Fourier transform-infrared spectroscopy (FT-IR) characteristics. 35 It needs to clear that all the three methods we mentioned above involves chemical phase-transformation of Fe 3+ , without interferes of other composites. Such processes need precise control of condition factors to make sure the purity and structure of aimed product. Yet, the process droved by biological force may more effective and eco-environmental.
Biological ferrooxidation. Exactly speaking, biological ferrooxidation is also an oxidation path which uses bacteria or microorganisms as mediates, different from the pure chemical path. Biogenic fabrication takes place at room temperature and is dependent on bacteria or microorganisms that use a common Fe 2+ source as a feedstock. In this process, a high SO 4 2À concentration drastically inhibits akaganéite bioformation, but facilitates the occurrence of biogenic Sch. 39 Acidithiobacillus ferrooxidant cells have traditionally been used in the past, but novel bacteria, such as an acidophilic Fe-oxidizing strain C25, 40 can be used aer it has been cultured or isolated from its original environment. Furthermore, this method can also be combined with acidic and alkaline activation, 41 which may result in a higher SSA value, more hydroxyl groups, and innersphere sulfate complexes. Therefore, the biosynthesis pathway for Sch (bio-Sch) may be more effective when connected to other pathways. For example, bio-Sch combined with Brevibacterium sp. YZ-1 (used for microbial oxidation) has already been used to simultaneously treat As-contaminated soil, 42 and is considered a green remediation strategy for remediating As-contaminated soils.
In summary, the chemical oxidation methods were the earliest Sch production methods, whereas slow dialysis and the biogenic method can be more efficiently upscaled. However, chemical oxidation is more suitable for theoretical studies, especially when investigating crystal structure and surface morphology, because of its stricter requirement for heterogeneous synthesis and separation of the product.
In nature, the nal properties of Sch are not only dependent on the fabrication method, but are also closely tied with the postprocessing procedure, which includes the separation, drying, and smashing processes. Usually, drying methods in the laboratory involve natural drying, heat drying, freeze drying, and air drying. Freeze drying is more suitable for stable repeated preparation when the diffusion and transfer of water molecules in the samples is taken into consideration. Aer the drying step, physical steps, such as smashing and grinding, are used to obtain the powdered sample. These processes change the surface colour. Aer the post-processing steps and the separation process mentioned above, a whole preparation procedure that uses slow fabrication and freeze drying may result in Sch products with better crystallinity and larger particle sizes.

Characteristics
Previously reported studies on Sch materials have mainly focused on the structure-properties relationship. However, it is necessary to discuss the structure-related reports on synthetic methods because the different structures/morphologies are closely dependent on the synthetic strategies used and their tunable parameters.
Morphology. Schwertmannite has a multiple threedimensional structure that could be seen as a hierarchical model. Its particles range in size from the nanoscale to the microscale. The entire morphology can also vary. Previous reports suggested that there are mainly two morphologies. These are spheroids with a smooth surface, and hedge-hog-like particle and pin-cushion-like or sea urchin-like with a rough surface. Generally, a fast formation process results in a kinetically favoured structure containing spherical particles, whereas a slow fabrication process results in a thermally favoured structure with a hedge-hog-like morphology. Sometimes, secondary aggregates with large particle sizes are produced. Fig. 2 shows that most of the Sch samples obtained by chemical oxidation contain spherical particles that are larger than 0.5 mm and at the mm scale. Models ii and iii produce particles that have a hedge-hog-like or a sea urchin-like morphology, which is possibly caused by a higher synthetic temperature or a slow addition rate for H 2 O 2 .
The reaction temperature is important in the oxidation process. French et al. 43 stated that the synthetic Sch produced via chemical oxidation should not be described as a single phase with a repeating unit cell, but as a polyphasic nanomineral with crystalline areas spanning less than a few nano-meters in diameter within a characteristic "pin-cushion"-like amorphous matrix. The difference in synthetic temperature affects the density of the needles on the Sch surface. The needles on Sch produced at higher temperatures had a dendritic morphology, whereas the needles on Sch produced at room temperature were more closely packed.
The H 2 O 2 addition rate is intensively optimized for stable mineralization. Liu et al. 31 reported that a slow H 2 O 2 supply rate led to a low ferrous ions oxidation efficiency and low total Fe precipitation efficiency in Sch synthesis systems, which increased the SSA value by changing the morphology from a spherical to a "hedge-hog" structure. This improved the heavy metal ion removal efficiency.
There are too many examples of fast hydrolysis for an indepth discussion. Model ii in Fig. 2 shows that two kinds of remarkably different morphologies can be produced. The rst has a hedge-hog like structure with a diameter of ca. 100 nm, whereas the second type aggregates with each other and produces particles that cover a board range of sizes. Aer studying a large numbers of literatures, it is discovered that, the hedge-like morphology of Sch minerals are mainly formed at higher temperature (ca. 85 C) without control of pH value, whereas the aggregated one of Sch minerals are mainly formed at lower temperature (<60 C) and lower pH values (2.4-3.0). Considering the pH value are mainly decided by the detail strategies, such as lower pH values for oxidation reaction while relatively higher pH values for hydrolysis, it was rstly settled as an environmental factor. Different from that, the temperature factor may inuence the nucleation and aging process of crystallization, which in great degree decides the nal morphology of Sch products. Thus, once the pH condition has been set, the temperature factor becomes the main cause of morphology formation.
Another method involving slow dialysis is also based on the hydrolysis of Fe 3+ species and is quite similar to fast hydrolysis. However, the precipitation time for the dialysis is much longer than the fast hydrolysis pathway. This process mainly utilizes an extra cellulose membrane, which requires consistent treatment of the precipitate. Fig. 2 shows that the samples produced by model ii all possess a typical hedge-hog like structure, which may be due to the use of 60 C as the synthetic temperature. A higher temperature promotes dialysis, which thermally stabilizes the 3D structure.
In contrast, the biological ferrooxidation method can be easily classied and evaluated. This micro-organism mediated process is also an oxidation method. As model iii in Fig. 2 shows, this method produces many more dened particles than the other methods. However, these biogenic Sch particles have a limited SSA value of <50 m 2 g À1 , which may be due to the ultrane Sch particles. 44 Crystal structure. Schwertmannite is an amorphous mineral material, which means that its crystal structure is not as regular as other materials. This review has used classical characterizations by techniques such as XRD, IR, and extended X-rayabsorption ne structure (EXAFS) to provide a detailed description of the crystal structure.
X-ray diffraction spectroscopy. Previously, it was believed that Sch had a crystal structure that was similar to akaganéite (nominally b-FeOOH). 9 This structure was then used as a model for structure simulation studies. 27 However, some researchers believed that Sch particles (balls or whiskers) had a similar crystallinity to ferrihydrite or maghemite, although it also had some amorphous regions. 25,26 In reality, most researchers believe that Sch is a low-crystallinity mineral.
The XRD data shows that Sch consists of 8-line diffraction proles of between 0.486 and 0.146 nm (mainly at ca. 26.3, 35.2, 55.3, and 61.3 ) as Fig. 3 shows. 45 In all cases, the broad but consistent character of the diffraction maxima indicates a material that is poorly crystallized, but certainly not X-ray amorphous. However, the XRD diffraction patterns of the Sch samples are slightly different depending on the synthetic pathway used.  1120, 1070, 1033, and 990 cm À1 , which is similar to sulfate complexes adsorbed to hematite, goethite, and Sch in the presence of water; (II) bands at 1210, 1180, 1130, 1033, 972, and 966 cm À1 , which is comparable to the sulfate spectra for hematite and has the greatest degree of n 3 splitting; and (III) bands at 1108 and 972 cm À1 . Extended X-ray absorption ne structure. The EXAFS technique is mainly used for Sch phase identication in natural sediments. Collins et al. 47 combined analytical and eld measurements to probe the speciation and cycling of Fe in coastal lowland ASS. Fe K-edge EXAFS (Fig. 5) demonstrated that Sch dominated (43-77%) 2 nd Fe mineralization throughout the oxidized and acidied soil prole, which was the opposite to pyrite and illite. This result provides powerful evidence for the preferential existence, reactivity, and redox processes of Sch, including the transformation of Sch or 2-line ferrihydrite 48 to crystallin Fe oxyhydroxides.
Besides direct characterization, these technologies are now used to monitor As and Cr ions' ads/desorption on the Sch surface. In 1996, synthetic Sch, where the sulfate had been substituted by As and Se, was examined by X-ray absorption structure to characterize the location of anionic complexes. The results showed the direct substitution of SO 4 2À by Se within tunnels and the sorption of Se outside the tunnels. However, As was mainly found outside the structure, which destabilized it and reduced growth. 49 The X-ray absorption ne structure (XAFS) results for As adsorption on Sch 50 showed that As was bound to Fe octahedral, and individual Fe octahedral in the Sch became more regular aer adsorption. Pore structure. Generally speaking, Sch is a mesoporous material. The SSA values for Sch reported to date are in the range of 2-330 m 2 g À1 . This suggests that more extreme production methods lead to higher SSA values. Usually, the SSA value increases as the structure becomes more complex. However, because of its poor crystallin structure, the SSA value of Sch does not strictly follow the usual fabrication methods and condition rules. As Table 3 shows, only a few reports have directly described the pore structure of Sch as mainly having a "bimodal distribution" 51 with micropores (nearly microscale) and mesopores (mesoscale). 37,52 There is very little mention of macro-pores. The pore size distributions (PSDs) for the micro and mesopores are similar due to the interspaces among the nanoscale villi and the aggregated Sch particles, respectively.
The relationship between pore/channel structure and the SSA and pore size distribution parameters for the Sch samples is not quite consistent with the relationship between fabrication method and structure/morphology. Pore size distribution is mainly decided by the primary formation of mineral nanoparticles, whereas SSA is preferentially conrmed by the secondary stacking of primary nanoparticles during nucleation and aging. This may explain some of the structure/morphology aspects of Sch, but more research needs to be undertaken in this area. Zhang et al. 53 introduced fractal geometry (D f ), a novel mathematical tool, to describe the irregular and complex geometries of Sch that involves morphological characteristics and pore structures. The surface (shown by denoised SEM images) and inner core (shown by low-pressure N 2 adsorption analysis) D f values are considered to be the operative measurements of surface roughness and pore complexity, respectively. However, this structural evaluation model is based on microstructure and the entire morphology, which means that it needs further research and improvement.
Electro-kinetic (E-K) properties. Usually, the E-K properties of Sch are explored by measuring its zeta potential at various pH values. More specically, there should be a greater focus on the surface functional groups, surface charge density, zeta potential, and isoelectric point (pI) if more information about the interface between Sch and the environmental system is to be obtained.
Due to the structural similarities between Sch and ferrihydrite, it is easy to deduce that the most common functional group on the surface of Sch is Fe-OH. The Fe-OH bands exist widely in Fe-based clay or minerals. 54 The surface charge density of a Sch powder sample can be calculated via the experimental Other properties. The sample colour is the direct impression of the nal product. On most occasions, it's the easiest way to judge whether the production process has been effective or not. However, the colour is dependent on many factors, such as the drying temperature and the water percentage. Generally, the surface colour of Sch powders can be red, orange, yellow, ochre, brown, 53 or even black, which may be due to the Fe sulfate. Previous experiments have shown that colour is closely connected with the synthetic methods, drying methods, and crumbling methods used.
The estimated log K values for Sch are 2.01 AE 0.30, which seems to vary depending on the sulfate content. In comparison, the value for 2-line ferrihydrite is 8.46 AE 1.40 and 10.12 AE 0.74 for 6-line ferrihydrite. 1

Stability
Although Sch has a tunable chemical composition and a unique pore/channel structure, its practical application is still somewhat limited. There are two possible reasons: unstable fabrication and storage, both of which originate from the low crystallinity of the Sch phase. For example, Sch can reductively transform into two 2 nd Fe 2+ minerals: siderite and mackinawite, and also relatively rapidly into goethite in ca. 25-65% anoxic reducing sediments, 63,64 depending on the SO 4 2À concentration. 37,65 However, many factors can inuence this transformation process, such as pH, Fe 3+ concentration, thermal treatment, existence of organic matter, and coexisting ions, etc. Environmental pH. Lake water samples almost all fall on a straight line for the relationship between Sch, pH value, and Fe 3+ concentration. This reects the solubility of Sch and shows that there is an equilibrium between them. Regenspurg et al. 20 discovered that the transformation rate of synthetic Sch into goethite rose with increasing pH between pH 2-7 aer a 1 year experiment. This suggested that Sch is the rst mineral formed aer oxidation and hydrolysis of a slightly acidic (pH 5-6) Fe 2+ -SO 4 solution, a process that directly affects the pH of the receiving water. The transformation of synthetic/natural Sch to goethite was studied in water with variable pH (2, 4, 6 and 8) for 1 year. 66 Only synthetic and one natural sample transformed to goethite with decreased oxalate solubility of Fe and trace elements and increased total Fe/S ratios and SSA. At pH 2, synthetic Sch fullytransformed to goethite, but at pH 4 and 6 only minor transformation occurred.
Concentrations of Fe species. Depending on the stability of pure Sch under Fe 2+ aq -rich anoxic conditions, 24 Fe 2+ sorption onto Sch is pH dependent, and surface coverage with Fe 2+ appears to be the key factor controlling transformation. Furthermore, a brown precipitate of ferrihydrite was always formed aer the hydrolysis of Fe 3+ at OH/Fe 3+ ratios >3.3. However, lower ratios induced the occurrence of ochre Sch. 52 The effect of dissolved Fe 2+ (0-1.0 mM) on the stability of Sch ($40 wt% Fe 3+ and 15 wt% SO 4 2À ) in anoxic alkaline conditions (pH ¼ 8) was investigated by Paikaray et al., 67 who found that product formation was accelerated when Fe 2+ levels increased. However, the sorbed AsO 3 3À markedly hindered lepidocrocite formation and almost 50-57% of the SO 4 2À was released in the absence of Fe 2+ . Therefore, the correct concentrations of Fe species are strictly required for the stable existence of Sch.
Temperature. Thermal transformation is another method that can be used to change Sch texture. Sch transforms into jarosite, hematite, and basic Fe sulfate at relatively low temperatures. 5 Basically, the crystal phase of Fe sulfate begins to form during thermal treatment, which triggers a series of domino effects. The thermal treatment discussed in this review includes two different paths, which are heating at low temperature during fabrication and calcination at high temperature aer fabrication.
French et al. 43 used high resolution transmission electron microscopy to show that the synthetic temperature in the chemical oxidation process used to produce Sch with "pincushion"-like morphology affects the density of needles on the surface. The needles on Sch produced at higher temperatures have a dendritic morphology, whereas those on Sch produced at room temperature are more closely packed. The low temperature treatment may cause different structural changes compared to the high temperature treatment. This factor does not change the inherent structure but does affect the acquired properties. Qiao et al. 44 reported that the temperatures used during the bio-Sch process do not greatly change Sch up to 250 C. Above this, there is a broad peak at 2q z 30-35 , which indicates a change from Sch to 2-line ferrihydrite (especially above 350 C). The SSA and As removal increased when the sample was heated at 250 C. However, removal then decreased at 550 C, but this was dependent on the loss of -OH from the Sch surface. Johnston et al. 68 used higher thermal temperatures (200-800 C) to transform As 5+ -Sch. Their results suggested that Sch was transformed into a nanocrystalline hematite with a greater SSA and a smaller particle size when heated to temperatures above 400 C.
Organic matter. Potential Fe 3+ coordination with chelating ligands, such as -COOH and -NH 3 , may facilitate the solubilization of Fe-bearing minerals, which would signicantly affect their bio-geochemical cycling. 69,70 Organic matter, such as amino acids, is capable of inducing redox reactions and involves various sorption mechanisms. 71,72 A typical 4-step mechanism was proposed by Xie et al. 73 for the interaction between L-tryptophan and CrO 4 2 -Sch, in which L-tryptophan acts as an inductor that causes the dissolution of CrO 4 2À -Sch.
This involves mass transfer (adsorption of L-tryptophan onto the Sch surface) and charge transfer (reduction of Cr 6+ to Cr 3+ ). The presence of various functional groups, such as -OH, -COOH, and -OH, means that organic matter is capable of carrying out different functions, such as redox reactions and binding processes. 74 Therefore, it strongly interacts with mineral oxides (e.g., Sch, ferrihydrite, goethite, and hematite) and contaminants, such as As. 75 There are generally two processes by which organic matter affects the transformation of Sch. These are reduction and adsorption, and, to date, adsorption has been more extensively reported than reduction. Fulvic acid (FA) and humic acid (HA) are also oen present when Sch is applied. 76,77 Fulvic acid is well-known to interact strongly with both Sch and As. Furthermore, As concentration has a strong inuence on the liberation of acidity from pure Sch and As-Sch. 78 However, knowledge about the fate of Fe and Cr aer the reduction and complexation of CrO 4 2À -substituted Sch by FA is poorly understood. The interaction between FA and CrO 4 2À -Sch was investigated by Xie et al., 79 who showed that the Fe and Cr concentrations in solution increased at rst, but then decreased when FA was at 10 mg L À1 and the pH was 6.5. This was possibly due to synergistic effects, such as interactions between ligand-promotion and reduction. The effect of FA on the liberation of acidity and the mobilization of As results in a greater liberation of acidity by As-Sch at pH 4.5 and 6.5 compared to pure-Sch. 78 The FA enhanced As mobilization at both pH values, but it was greater at pH 6.5. Humic acid has a skeleton of alkyl and aromatic units with carboxylic acid, phenolic hydroxyl, and quinine functional groups. 80 These large molecules can strongly adsorb to the outer particle surfaces (not the interior surfaces) of Sch powders. Therefore, these organic molecules can effectively modify species mobilization and acidity liberation. Furthermore, FA seems to affect Sch transformation more than HA. Coexisting ions. Pure solutions with only one solute usually do not exist in natural water or soil environments. This means that the inuence of other cations and anions needs to be considered. Acero et al. 81 used Sch to remove As from a solution with divalent trace metals (Zn, Cu, Pb, Cd, Ni, and Co) and found that the concentration of metal cations remained almost unchanged. When Dou et al. 82  . Research has shown that organic coexisting ions and P-based ions clearly inuence the removal of As by Sch due to the strong interaction between organic matter and As, and because organic coexisting ions and P-based ions have similar chemical properties. With regards to CrO 4 2À , the competition by the accompanying anions with . In conclusion, organic coexisting ions have a greater inuence on Sch adsorption ability than inorganic ones and, more specically, the metal cation and oxyanion effects on the adsorption process are strongly dependent on the pollutant species.
The factors mentioned above not only affect the formation of "fresh" Sch, but also inuence the long-term storage of Sch products. When Sch is formed in AMD, its transformation to goethite occurs within 2 years in meteoric waters. 85 Regenspurg et al. 20 suggested that Sch occurrence is transient and restricted to certain environments, such as acidic mining lakes, where Fe 3+ coordination is controlled by competition between SO 4 2À and OH. In contrast, synthetic Sch samples are more chemically stable and can remain unchanged aer years. This is due to their higher purity and better storage conditions compared to natural Sch forms, as previously mentioned. Furthermore, pH is an important factor that affects the long-term chemical stability of Sch. The effect of pH on the release of As from Sch-As is more obvious at higher pH values. This indicates that As release from Sch-As is mainly controlled by outside environmental factors, such as pH, rather than time. 86

Water treatment
Over recent decades, there have been many systematic studies on Sch and its potential use in environmental remediation. 87 They have mainly focused on the removal of contaminants in the water phase, such as AsO 4 3À , CrO 4 2À , SbO 4

3À
, and F À etc. These reports provide a solid foundation for theoretical research. The details are listed in Table 4.
Removal of pollutants. . The presence of As in water and soil systems (dominantly originating from parent rock and human activities) is a serious environmental problem for humans and other living organisms due to its high toxicity and wide distribution in natural environment. 88  within the Sch solid-phase, which can be interpreted as As Table 4 Composition, structure and potential applications of schwertmannite    sorption via incorporation into the Sch structure rather than merely surface complexation at the mineral-water interface. Similar results were also reported by Song et al., 89 who found that more AsO 4 3À was adsorbed on Sch at lower pH values, whereas AsO 3 3À sorption increased as the pH rose.
The As loading may also have different inuences on its removal. Sorption is governed by multilayer processes, as indicated by highly nonlinear Freundlich adsorption isotherms. 38  released, which was subsequently re-adsorbed into Sch during a 4 months stabilization period without mineralogical transformation. Schwertmannite can also be produced in other forms or shapes, such as irregular, cylindrical, and spherical granules. 82 The irregular and cylindrical granules have larger Sch loadings, higher porosities, a more abundant pore structure and larger V micro values than the spherical forms. The AsO 4 3À adsorption kinetics follows a pseudo second order rate equation and there is a two-stage of intraparticle diffusion process. The diffusion rate parameter order is irregular granules > cylindrical granules > spherical granules, which is in accordance with their V p and interparticle porosity. Furthermore, Q e values of 34, 21, and 5 mg g À1 , for irregular, cylindrical, and spherical granules, respectively, are achieved at an initial AsO 4 3À concentration of 20 mg L À1 and 0.5 g L À1 . Irregular and cylindrical granules perform much better over a wider pH range than spherical granules and can be used for four cycles without reducing the Q m value. Furthermore, the toxicity characteristic leaching procedure (TCLP) results show that the spent irregular and cylindrical granules are inert and could safely be disposed of in landlls. CrO 4

2À
. Chromium is a heavy metal contaminant that is frequently detected in the natural environment. It poses a threat to fauna when present as the CrO 4 2À species due to its higher solubility, mobility, and toxicity, when compared with Cr 3+ . 91 Previous studies have shown that Sch used for CrO 4 2À removal is usually prepared by biological ferrooxidation of the Fe 2+ species. Many batches are prepared using slow hydrolysis. Biogenic Sch has been used to absorb CrO 4 2À in a continuous ow column (1 mL min À1 , pH ¼ 6) with a Q m value of 35.30 mg g À1 . A possible adsorption mechanism for CrO 4 2À is the anion exchange process between various Cr 6+ species and SO 4 2À , a constituent of Sch. 83 Gan et al. 84  In general, the studies on Sb removal by Sch are quite similar to those about As, but are still limited to water phase removal. Previous research suggests that Sb is not easy to remove from water, even though it is in the same family as P and As, Dong et al. 34 combined Sch in its main active phase (conrmed by X-ray photoelectron spectroscopy) with graphite oxide (GO) as a composite adsorbent for SbO 4 3À removal so that there could be a synergistic effect between Sch and GO. The Q m value for SbO 4 3À was 158.6 mg g À1 (8.0 mg L À1 SbO 4

3À
, pH ¼ 7.0), which was superior to either GO or Sch alone. This was due to the effective incorporation of Sch among GO platelets and its high dispersion on the GO surface. They also used Sch granules to overcome the drawbacks of using small particle-sized adsorbents for SbO 3 3À and SbO 4 3À removal from water. 92 Sch granules were able to remove 32.9 mg g À1 SbO 3 3À and 23.2 mg g À1 SbO 4 3 , which was superior to most reported granular and powdered absorbents. The SbO 3 3À was effectively removed over a wide pH range, while the removal of SbO 4 3À was pH dependent and could be enhanced by lowering the solution pH. The SbO 3

3À
-loaded Sch was regenerated with 91.2% re-adsorption capacity, and this could be repeated ve times. However, the Sch adsorption capacity for Sb still needs further research because it is toxic even at low concentrations in water.
Fluorions. Fluorions have a small volume, which means that they easily enter into channel pores to form stable complex compounds. Related studies have shown that many Fe oxides or hydroxides have good adsorption capacities for uorions, especially, when the pH value is lower than the pH zpc value for Sch. In addition, Sch also possesses many -OH and SO 4 2À groups on the surface, which also exchange with uorions. Methods involving Sch in the water phase and within the soil system have great potential compared to traditional methods for uorion removal.
Research by Eskandarpour et al. 93 into the adsorption of uorions onto Sch in a batch system suggests that the adsorption of F by Sch is high, but it is insensitive to changes in temperature and equilibrium pH. The inner-sphere complexforming species had negative effects on uorion adsorption, whereas outer-sphere complex-forming species slightly improved uorion removal efficiency. A nano magnetic Sch prepared by introducing nano-magnetite into Sch 94 achieved the permissible limit dened by World Health Organization for removing F from water with a Q e of 17.24 mg g À1 according to the isotherm data. Interference by PO 4 3À , CO 3 2À , and HCO 3 À was more than by Cl À and SO 4 2À . The spent adsorbents were regenerated with basic solutions and they retained good adsorption efficiencies. Cd 2+ , Cu 2+ , etc. Schwertmannite can also remove cationic contaminants. For example, Cd is a typical toxic contaminant and needs to be removed from wastewater. Fan et al. 95 investigated the feasibility of using slow synthetic Sch to remove Cd 2+ from aqueous solutions. They used batch experiments that tested different Cd 2+ concentrations, temperatures, and pH values. The Langmuir isotherm successfully described the adsorption of Cd 2+ . A thermodynamic study suggested that absorption was spontaneous and endothermic. The Sch had a high Cd 2+ adsorption capacity of 110 mg g À1 at a dosage of 1 g L À1 and an initial pH of 8.0 at 25 C. At pH > 6.0, Cd adsorption dramatically increased and nearly 100% of the Cd was adsorbed at pH 8.0. In the natural pH range for Sch, the Sch that had adsorbed Cd 2+ had good regeneration ability. The Cd 2+ desorption proportion of the total sorbed quantity was 50-80% and the desorption rate increased as the pH decreased. Therefore, Sch can be employed as an efficient adsorbent for the removal of Cd from contaminated water. However, there are many other heavy metals, such as copper, 84 that could be removed by Sch or Sch-based materials, but these are not discussed in this review.
Organic. Schwertmannite also plays an important role as a catalyst in the degradation of organic pollutants. Organic pollutants that can be removed by Sch are mainly phenyl compounds, such as phenols. For the rst time, Wang et al. 62 applied Sch as a Fenton-like catalyst in the oxidation of phenols by H 2 O 2 and experimentally conrmed its good catalytic activity, especially in high salinity systems. Sch was more effective than goethite and could maintain phenol removal at 98%, even aer 12 cycles, although a phase transformation of Sch to goethite was observed. Duan et al. 33 used Sch in another Fenton-like process for nitrobenzene degradation. In this case, its catalytic activity was maintained at 92.5% (30 min, pH 3.0, 500 mg L À1 H 2 O 2 ) aer ve runs. Yang et al. 96 developed an in situ remediation technology that coupled the Sch/H 2 O 2 process with the E-K process for the removal of phthalates and acetaminophen in river sediments and found its remediation performance was superior to that of the batch degradation test. Clearly, Sch is a promising Fenton-like catalyst and its performance can be further enhanced by combining it with other technologies. Recently, Li et al. 51 established a Sch/H 2 O 2 system that also used ultrasonic technology for bisphenol A degradation. Removal was signicantly enhanced to 98% compared to the Sch/H 2 O 2 system at 69.6%. Furthermore, this remained at 95% aer ve cycles. Although there have only been a few application examples for organic remediation by Sch to date, it is clear that Sch can be combined with a number of traditional catalytic process, which means that it will play a more important role in the future.
As we concluded in Table 4, among all the related pollutants, Sch presents better adsorption properties for As and Cr-based pollutants than the other types. This is mainly caused by the unique channel-like structure whose pore diameter is quite close to the size of As or Cr-based ions like AsO 4 3À , AsO 3 3À , or CrO 4 2À , promoting the easy ion-exchange process. As for Asbased pollutants, the Fe element in Sch phase can be also used to rmly bond with As-based ions to form more stable phase. This point can also be conrmed by the amounts of literatures, as a proof. Regeneration in water systems. With regards to "green chemistry," Sch, especially the synthetic versions, are expected to be utilized in regeneration and recycling processes in the future. Typically, for As remediation, recovery by NaOH solution (0.1-1.0 M), 44,82,92,94 washing with deionized water (>5 times), 51 and dry processing (40-60 C) are common processes that ensure that the adsorbates can be nearly completely separated without the Sch structure being destroyed. It is worth noting that under the alkaline condition, Sch can transform into other phase possibly, which is not discussed too much before. As for Cr VI and Cu II remediation, a diluted H 2 SO 4 solution (pH 2.0) was used for regeneration of Sch considering that Cr VI can be desorbed from active sites through washing with low pH deionized water. 84 It turns out to be that there is no appreciable decrease in both metals owing to the well-kept morphology. Compared to the adsorption process, Sch used as catalysts for organic degradation are much easier to regenerate only by washing with deionized water several times and dry. 51 These processes are not time/energy-consuming, which means they may become common Sch regeneration techniques. Aer regeneration, the adsorption capacity of Sch decreases to a certain degree, but it can still reach the adsorption capacity of fresh batches if the adsorption conditions are adjusted. There are numbers of reasons why Sch can be easily regenerated. It is possible that the basic aqueous solution destroys the surface bonds on the outer surface of Sch or the inner surface of the Sch channels. Fast washing and quick separation without complex processes and special reagents do not inuence the Sch structure, which ensures that the adsorption capacity of Sch remains high in subsequent cycles. This easy separation of solid Sch and the regeneration process gives this material wide applications as adsorbent or catalyst in heterogeneous systems. Sch could also be reobtained by using some other functional composites, such as magnetic nanoparticles or hydrophilic/hydrophobic molecules. However, these possibilities require further research.

Soil remediation
Considering the huge amount of researches about Sch for water treatment, the potential of Sch for treatment on inorganic and organic pollutants could also be technically applied in soil remediation. The rst research on the use of Sch for soil remediation appeared in 2016. However, there have not been many studies to date. Biogenetic Sch pre-treated by acidic and alkaline activation (A-Sch) 41 achieved an immobilization efficiency of > 99.5% for water-soluble As with an A-Sch dosage of 5% and an Sch dosage of 10%, respectively, in As-contaminated soil. The immobilization percentages for NaHCO 3 -extractable As increased from 31.5% to 90.4% and from 40.2% to 93.8% when the dosage increase from 0.5 to 10 wt% for Sch and A-Sch, respectively. In general, both Sch and A-Sch effectively immobilize As in contaminated soil, and the immobilization performance of A-Sch is better than Sch, especially at lower dosages. This nding also has important implications for in situ immobilization of As in contaminated soils, especially in soils that have acidic Fe and sulfate-rich environments. This study was nished in 2016 and used an indirect application of Sch. The combination of microbial oxidation (AsO 3 3À -oxidizing bacterium strain YZ-1) (CGMCC No. 8329) and bio-Sch immobilization by Yang et al. 42 successfully treated highly As-contaminated soil with immobilization efficiencies of 99.3% and 82.6% for water-soluble and NaHCO 3 -extractable total As, respectively. This study suggested that the combination could potentially remediate highly As-contaminated soils. Studies about farmlands or other sites are needed so that the Sch remediation of heavy metal contaminated soils can be further evaluated. Sch is quite an effective remediator of Ascontaminated soil and perhaps Sb and Cr contaminated soil, although there have been no published reports on these latter two metals. Furthermore, potential problems about production technology, quality control, and cost control have still to be resolved, and the sustainability of contaminated soil remediation by Sch also needs to be examined.
As comparisons, Schs especially the synthetic ones own a comprehensive advantage for water and soil remediation. For example, Sch shows much higher SSA value than the reported organo-bentonites, 97 as well as wide application for more types of heavy metals. This mineral also needs facile fabrication, compared to the reported magnesium silicate-hydrothermal carbon composite 98 and graphene oxide (or its composites). 99,100 As for the adsorption for ionic typed pollutant like As III , Sch shows a signicant advantage with much higher adsorption than the reported CaMgFe-LDH. 101 Technically, the Sch mineral is an easily prepared, more effective, and more potential candidate for types of environmental process, even as a catalyst or carrier.

Inorganics
It is unrealistic to propose only one Sch method for the removal of heavy metals. Antelo et al. 102 proposed two mechanisms: complexation with Fe hydroxyl surface groups and anion exchange with sulfate complexes. Arsenic oxyanion adsorption mainly occurs through the 1 st mechanism, whereas CrO 4 2À oxyanions are absorbed by the 2 nd mechanism. The adsorption and post adsorption behaviour of Sch towards various oxyanions were investigated by Khamphila et al. 103

Organics
The treatment of organic contaminants in water or soil is best undertaken using the Fenton reaction. Although the reactants may differ, the reaction mechanism is quite similar. A possible mechanism for the oxidation of phenol with H 2 O 2 in the presence of Sch 62 is shown in Fig. 7. In the oxidation process, intermediates, such as quinines, catechol, oxalic acid, and acetic acid are produced. These organics play an important role in reducing Fe 3+ to Fe 2+ and lead to the dissolution of Fe oxides. Most of the intermediates are further oxidized and transformed into H 2 O and CO 2. In this mechanism, the identication of $OH is required. Methanol was chosen as an effective $OH scavenger to identify whether the $OH type reaction has degraded the nitrobenzene, 33 and Fe leaching and H 2 O 2 decomposition conrmed the success of the process. The effect of other kinds of $OH scavengers was also investigated during the degradation of bisphenol A. 51 In the degradation reaction, free $OH was generated in the bulk solution along with some intermediates, including p-isopropenyl phenol, p-benzoquinone, and methyl propionate, which proved the occurrence of a Fenton-like process on the Sch surface.

Conclusions and perspectives
Schwertmannite is a potential material that can be used for environmental remediation. This mineral has a unique channel structure and ion-exchange properties, which gives it an excellent adsorption capacity for inorganic contaminants in the water phase and catalytic degradation ability for organic contaminants, resulting in potential application in environmental remediation of water and soil. In the remediation process, the stability of Sch is an important factor that signicantly inuences its application, owing to the easy transformation of metastable Sch into other minerals. So far, Sch is not just a mineral or clay, but also an adsorbent or catalyst. Novel functional Sch materials in further investigation are always concerned. However, there are still lots of questions about the synthesis and application of schwertmannite as follows: (1) The schwertmannite samples fabricated via various methods and conditions present quite remarkable differences in material's characteristics and the adsorption properties of pollutants. The chemical oxidation, fast hydrolysis, slow dialysis, and biological ferrooxidation process are closely connected with distinct microstructures, pore-structure, and most importantly the adsorption properties. This phenomenon can be observed even at similar conditions. The relationship between the synthetic strategies or methods and materials' properties should be discussed from different views to provide more clarity about the controllable optimization aiming at the practical application.
(2) The environmental application of schwertmannite mainly focuses on water treatment while those about soil remediation are still in starting stage. Technically, the researches about the former aspect mainly revolve industrial waste water and acid mine drainage. In 2016, the study in schwertmannite for soil remediation was rstly reported with lots of attentions. Aerwards, more discussions are eagerly necessary for further application.
(3) Schwertmannite is a metastable amorphous Fe-based mineral, which is quite easy to transform into goethite phase. Such phase-transforming process has become an important and necessary issue. Schwertmannite can exist in long-term during the remediation process of soil and underground water, requiring necessity about the study for its phase-transformation and stability. Several researches about these factors of natural/ synthetic or As/Cr-doped Schwertmanites at different pH, temperature, co-existing ions, and organic matter have been reported in succession. Nevertheless, the issues about how the phase transformation occurs, how the pollutants release, and how long the release will last, are still lack of related simulation and tracking study, especially in the system of underground water and soil. Fig. 7 The Fenton reaction when catalysed by schwertmannite.
(4) As a potential catalytic material, schwertmannite can be used for the degradation of organic pollutants basically as efficient Fenton-like catalysts. Until now, kinds of aromatic compounds (such as phenolic and BPA) have already been examined as candidates, and excellent catalytic property in short time has been achieved. However, its recycling ability and chemical stability during the process of chemical reactions still need more deep study.
(5) Many paths can be used for the promotion of adsorption properties of Sch to various pollutants, such as thermal modi-cation, acidic modication, organic modication, and even shape granulations. Other strategies and technologies for structure optimization and functionalization are particularly concerned for more systematic researches.

Conflicts of interest
There are no conicts to declare.