The important role of the interaction between manganese minerals and metals in environmental remediation: a review

With illegal discharge of wastewater containing inorganic and organic pollutants, combined pollution is common and needs urgent attention. Understanding the migration and transformation laws of pollutants in the environment has important guiding significance for environmental remediation. Due to the characteristics of adsorption, oxidation, and catalysis, manganese minerals play important role in the environment fate of pollutants. This review summarizes the forms of interaction between manganese minerals and metals, the environmental importance of the interaction between manganese minerals and metals, and the contribution of this interaction in improving performance of Mn-based composite for environmental remediation. The literatures have indicated that the interactions between manganese minerals and metals involve in surface adsorption, lattice replacement, and formation of association minerals. The interaction between manganese minerals and metals plays an important role in environmental behavior of element and environmental significance of manganese minerals. The synergistic or antagonistic effect resulted from the interaction influence the purification of heavy metal and organism pollutant. The synergistic effect benefited from the coordination of adsorption and oxidation, convenient electron transfer, abundant oxygen vacancies, and fast migration of lattice oxygen. Based on the synergy, Mn-based composites have been widely used for environmental remediation. The synthesize methods of Mn-based composites mainly include homogeneous coprecipitation, chemical etching route, hydrothermal, homogeneous chelating sol–gel, and ethylene glycol reduction strategy. This review is helpful to fully understand the migration and transformation process of pollutants in the environment, expand the resource utilization of manganese minerals for environmental remediation.


Introduction
Manganese minerals are rich on Earth with an average content of about 0.1% in the crust (Barceloux 1999). They are particularly abundant in the crust, with an average abundance of 1000 mg⋅kg −1 . The world's total average manganese in the soil is 850 mg⋅kg −1 (Suttle et al. 2003). There are more than 30 kinds of manganese minerals including birnessite, vemadite, todorokite, romanechite, cryptomelane, and manganite, which are common in the soil (Robinson et al. 2013). Based on the structural type, manganese minerals can be divided into tunnel structure, layered structure and lowvalence unordered manganese mineral. Mutual transformation occurs between different types of manganese minerals under the certain conditions Wu et al. 2019a;Ruiz-Garcia et al. 2021). The combination of the [Mn(III)O 6 ] octahedron and [MnO 6 ] octahedron leads to Responsible Editor: Ioannis A. Katsoyiannis the transformation of the layered structure into the tunnel structure .
Because of high specific surface area and high chemical reaction activity, manganese minerals show strong interactions with metals (WegorzeWski et al. 2015;Wan et al. 2020;Yu et al. 2017a;Ibrahim et al. 2022;Jin et al. 2022). These interactions impact the environmental fate and biological toxicity of metals through adsorption and oxidation (Ma et al. 2015;Peacock and Moon 2012;Xu et al. 2011). The interaction between manganese minerals and metals also affects the structure, morphology, the adsorption performance, and oxidation capacity of manganese minerals Zhu et al. 2010a;Yin et al. 2020). Therefore, the interaction between manganese minerals and metals plays an important role in environmental behavior of element and environmental significance of manganese minerals (Atkins et al. 2014;Yin et al. 2020).
The interaction between manganese minerals and metals can also produce associated minerals (Chigrin and Kirichenko 2018;Najjar and Batis 2016). Fe-Mn minerals and ferromanganese crusts are the most representative associated minerals (Reykhard and Shulga 2019;Lu et al. 2019). Mnbased layered double hydroxides (Mn-LDHs) are important associated minerals in the environment (Chawla et al. 2017). These associated minerals further influence the migration and transformation of pollutants (Crowther 1982;Spinks et al. 2017). The oxidation degree of Tl(I) on ferromanganese crusts is determined by the proportion of birnessite, which controls the enrichment and isotopic fractionation of Tl(I) (Peacock and Moon 2012). Fe-Mn minerals affect the cycle of As in aquifer sediments by adsorption and fractions (Ma et al. 2015).
The synergistic or antagonistic effect resulted from the interaction influence the purification of heavy metal and organism pollutant. Base on the synergistic effect of the interactions between manganese minerals with metals, Mnbased composites have been widely applied in the field of environmental protection in recent years (Tarjomannejad et al. 2017;Ma et al. 2021;Ruiz-Garcia et al. 2021;Shaheen et al. 2022;Zhao et al. 2022;Xu et al. 2022a;Fang et al. 2022). The contribution of the synergistic effect between Mn and metal to environment purification has been confirmed Guo et al. 2018;Kuang et al. 2019;Peng et al. 2020;Sotiles et al. 2019;Yang et al. 2018a). Mn 0.52 Co 2.48 O 4 spinel has good oxidation activity because of its high specific surface area and the effective synergistic effect between Co and Mn during the cycles of different valence states of elements (Co 2+ + Mn 4+ ↔ Mn 3+ + Co 3+ ; Co 3+ + Mn 2+ ↔ Mn 3+ + Co 2+ ) (Gao et al. 2019;Wang et al. 2019a). Because of the synergistic effect of Fe and Mn, the catalytic activity of Fe-Mn composite exhibits a five-fold increase in oxygen reduction activity ). Application of the density functional theory calculation showed that the synergistic effect of bimetallic sites reduces the energy reduction of ·OH barrier, which is beneficial for oxidation and generation of contaminants (Sarkar et al. 2020).
This review firstly provides an overview the interaction mechanism between manganese minerals and metals, summarize the environmental importance of the interaction between manganese minerals and metals, and outline the role of synergistic effect between Mn and metal in improving performance of Mn-based composite for environmental remediation. This review not only helps to understand the migration and transformation processes of pollutants in the environment, but also expands the utilization of manganese minerals and Mn-based composite for environmental recovery.

The interaction mechanisms between manganese minerals and metals
Because of the large specific surface area, abundant surface hydroxyl groups, rich vacancies and strong oxidation ability, manganese minerals have strong affinity towards metals (Machado-Infante et al. 2016;Qin et al. 2019a, b;Wu et al. 2019b). The interaction mechanisms between manganese minerals and metals include surface adsorption (Fig. 1a), lattice replacement (Fig. 1b) and formation of association minerals (Fig. 1c), which obviously affect the migration and transformation of metals Maeno et al. 2016;Nishi et al. 2017;Zhang et al. 2022;Liang et al. 2022a).

Surface adsorption
Previous studies have shown that a natural hexagonal birnessite with low crystallinity had a very high adsorption affinity to Pb(II) (Qin et al. 2019b;Wang et al. 2012). The adsorption capacity of birnessite for Pb(II) was 1.6-3.9 times higher than that of other metals. It was confirmed that Pb(II) formed two strong inter-sphere complex surfaces during adsorption: triple corner-sharing (TCS) on the Mn(IV) vacancy and double edge-sharing complexes (DES) on lateral edge surfaces . The maximum adsorption capacity of Cd(II) was 104.17 mg·g −1 , where the adsorption of Cd(II) on the manganese mineral was dominated by the formation of surface complexation in both outer and inner layers. According to the results of a surface complexation model, the adsorption of Cd(II) on manganese mineral can be well simulated by using the ion exchange at low pH and the inter-sphere surface complexation at high pH .
In a biofilm of hydrated bacteria, biological birnessite adsorbed Zn(II) by tetrahedral coordination on the crystal position (Toner et al. 2006). Zn(II) was adsorbed on the biological manganese mineral by generating two types of tetrahedral coordination complexes: a monodentate-mononuclear complex with a regular/distorted MnO 6 octahedron and a bidentate-binuclear complex with two MnO 6 octahedrons (Zhang et al. 2014). Zn(II) was anchored as a TCS surface complex above the vernadite vacancy, the complex was tetrahedral at a low Zn/Mn mole ratio and octahedral at a high Zn/Mn mole ratio (Grangeon et al. 2012). Triple-corner-sharing innersphere complexation on the vacant sites was the dominant Zn(II) sorption mechanism on birnessite (Tajima et al. 2022). As the result of the extended X-ray absorption fine structure spectrum (EXAFS) shown, there were two fixation modes of Ni on the biological manganese mineral: Ni was adsorbed by producing a triple-corner-sharing complex (Ni-TCS) at the vacancy when Ni concentration was low; Ni was embedded into the sheets of manganese mineral when Ni concentration was high (Peña et al. 2010). The maximum adsorption capacity of Ni(II) increases with the decrease of pH because the number of vacancies in biological manganese mineral increases with the increased acidity during the formation process (Zhu et al. 2010b).
The adsorption process showed high selectivity for Cu(II) on the surface of the manganese mineral with a maximum adsorption capacity of 11.926 mg⋅g −1 (Cano-Salazar et al. 2020). The result of the Cu-k-edge in EXAFS showed that most Cu(II) was adsorbed above the vacancy, Cu(II) was anchored at the edge of the birnessite in the form of polynuclear clusters when pH was 3.3-5.3 and the molar ratio of Cu/Mn increased from 0.08 to 0.23 (Qin et al. 2017). Cu(II) was adsorbed on birnessite by forming an inner-sphere complexation on the surface of the {001} crystal plane, and the bond length of Cu and Mn was 3.4 Å (Sherman and Peacock 2010).
Todorokite has a high adsorption capacity for Cs(I), which influences the cycle of Cs(I) in the environment . Although the proportion of the central metal vacancy in biological birnessite was higher than synthetic birnessite, the adsorption density of Cs(I) on biological birnessite was lower (Sasaki et al. 2014). Since Cs(I) adsorbed on the two minerals was mainly in the form of outer spherical complexes, it cannot easily occupy the central metal vacancy in biogenic minerals. In summary, Pb(II), Cd(II), Zn(II), Ni(II), Cu(II) and Cs(I) were adsorbed by preferentially combining with the surface hydroxyl group and form surface complexes on manganese minerals.

Lattice replacement
The ability of metals to replace Mn(III)/Mn(IV) in layers has important influence on the removal efficiency of metals and the reactivity of manganese minerals (Holguera et al. 2018). Hexagonal birnessite can immobilize and oxidize Mn(II), and then transform it into Mn(III)-rich hexagonal birnessite, tripartite birnessite or tunnel manganese oxide, which significantly changes the environmental behavior of Mn oxide (Yang et al. 2019a, b). Under the conditions of low Mn(II)/Mn (5-24%) and pH ≥ 8, δ-MnO 2 and acid birnessite reacted with Mn(II) in solution and turned into orthogonal birnessite (Zhao et al. 2016). This transition was attributed to the co-distribution reaction between the Mn(II) fixed in the vacancy and the surrounding Mn(IV) in the layer. Therefore, the substitution Mn(IV) with generated Mn(III) impact importantly the structure of the birnessite.
The effect of Cu(II) on the characteristics of manganese minerals varies among different fixation patterns ). The replacement of Mn(II) with Cu(II) in the framework is considered to be a defect in the crystal structure but showed a stabilizing effect in the framework  (Nicolas-Tolentino et al. 1999). Previous study found that part of Cu(II) was embedded in the layer structure of MnO 2 by occupying vacancies at pH 8 (Sherman and Peacock 2010).
The substitution of Mn(III) with Co(III) in the MnO 6 layer plays a key role in the conversion of birnessite to todorokite (Xu et al. 2011). Co(II) and Ni(II) enter the vacancy of hexagonal birnessite and become part of the hexagonal sheet structure by replacing the missing Mn(IV) (Kwon et al. 2013). The compatibility of metals in the birnessite layer decreases in the order Co 2+ > Ni 2+ > Fe 2+ (Yin et al. 2013. Co(II) and Fe(II) are immobilized on birnessite by replacing Mn(III) and Mn(IV); however, only the replacement of Mn(III) has been found in the Ni(II) immobilization process. The grain boundary coordination radius (CR) determines the substitution of metals with Mn(III)/Mn(IV) in the birnessite layer: the smaller the difference between CR of metals and Mn(IV) or Mn(III), the more compatible they are with the Mn layer .
The particle size and vacancy in the structure of biological birnessite affect its fixing ability to Pb(II), Zn(II) and As(III, V), which determines the fate of metals in geochemical environment (Villalobos et al. 2014;Lee et al. 2013;Toner et al. 2006;Qin et al. 2018). The increase of internal vacancy causes negative electrostatic repulsion with the As(V) oxygen anion. XPS and XANES spectra indicated the enrichment mechanism of Pt(II) in marine ferromanganese crusts involved the oxidation reaction of Pt(II) and Mn(IV) on the Mn-O-Pt bond in [PtCl 4n (OH)n] 2− and Pt(IV) substitution on MnO 2 structure (Maeno et al. 2016).
There was a high correlation between thallium (Tl) and Mn, indicating a high geochemical affinity between Tl and manganese minerals (Cruz-Hernández et al. 2019). The adsorption of Tl(I) on the manganese minerals was through an irreversible oxidation mechanism. The produced Tl(III) was mainly retained in the vacancy. Manganese minerals show an extremely high capacity for Tl(I) oxidation, since Tl(I) can react with all internal Mn(IV) in the internal structure and form Tl 2 O 3 under a high concentration of Tl(III) (

Formation of association minerals
Associated minerals refer to a mixture of different kinds of minerals that appear in the same spatial range in nature (Marsh et al. 2019;Pirajno 2015). Because of the interaction of manganese minerals and metals, associated minerals are generated during the formation of manganese minerals in the environment. Figure 3 shows the effects of birnessite on the transformation of Fe 2+ to ferric oxides (Gao et al. 2015). The formation of ferric oxides during the interaction between birnessite and Fe. 2+ (Gao et al. 2015) Ferromanganese nodules on the seabed are the most representative associated minerals Marcus et al. 2004;Jiang et al. 2009). There are three types of ferromanganese nodules: (i) 1-1.5 cm smooth irregular nodules, Mn/ Fe = 2.50, trace metal abundance (Co + Ni + Cu = 2.10%); (ii) 2-4 cm smooth disc-shaped nodules, Mn/Fe = 3.65, trace metal abundance (Co + Ni + Cu = 2.35%); and (iii) 3-15 cm nodule, grape-like surface, Mn/Fe = 5.1, trace metal abundance (Co + Ni + Cu = 2.42%). The ferromanganese nodules fix metals through diagenesis during the growth of nodules under alternating oxygen and sulfite conditions (Reykhard and Shulga 2019). Ferromanganese nodules present alternating layers of iron-rich and manganese-rich nodules, reflecting the changes of the redox property in the water system (Gao et al. 2015).
In summary, the properties of manganese minerals, the concentration of metals and the reaction condition (pH, ion strength and temperature) affect the way of interaction between manganese minerals and metals, including surface adsorption (Table 1), lattice replacement, and formation of associated minerals.

Environmental purification of the interaction between manganese minerals and metals
Manganese minerals are critical components in the Earth system. Due to large specific surface area and high surface reactivities, they are recognized as ubiquitous geo-sorbents and geo-catalysts, regulating the immigration, transformation, crystallization, and bioavailability of metal ions in the environment through adsorption or redox reactions. In turn, metals can change the microstructural morphology, surface functional groups, electronic structure, adsorption, and oxidation catalysis of manganese minerals (Wang et al. 2019a, b, c;Li et al. 2019b). Thus, environmental purification of the interaction between manganese minerals and metals includes two aspects: (1) the immobilization of metals; (2) metals anchored on manganese minerals affect the environmental geochemical processes of other toxic pollutants (Timár et al. 2017;Yu et al. 2017a).

Interaction between manganese mineral and Mn
A positive correlation has been observed between the adsorbed Mn(II) and the structural Mn(III) in the whole formation process of bio-manganese oxide (BMO) (Watanabe et al. 2012). The structural Mn(III) played a leading role in the reactivity of BMO, since Mn(III) is not only main adsorption site for Mn(II), Mn(III) is also a highly active intermediates in manganese minerals (Sun et al. 2015). The adsorption performance of coexisting As(V) for manganese minerals was closely related to the structural Mn(III) and adsorption of Mn(II) (Liu et al. 2019a, b). This shows that the interaction of manganese minerals and Mn(II)/Mn(III) affect the circulation of As(V) in the environment.
In the absence of Mn(II), Zn(II) was adsorbed as a tetrahedral and TCS complex at the layer vacancy of hexagonal birnessite (Lefkowitz and Elzinga 2015). However, the Zn(II) fixed on hexagonal birnessite was transformed to spinel (Zn(II) 1−x Mn (II) x Mn(III) 2 O 4 ) after the addition of Mn(II). This conversion was driven by the electron transfer from the adsorbed Mn(II) to the structural Mn(IV), then generated Mn(III) surface species for co-precipitation with Zn(II) and Mn(II). For the absence of Mn(II) in aqueous solution, the adsorbed Ni(II) located at the vacancy of hexagonal birnessite in the form of a TCS complex (Lefkowitz and Elzinga 2017). The introduction of Mn(II) to solution led to desorption of Ni(II) and formation of the edge-sharing Ni(II) complex, which was attribute to the competitive displacement between Ni(II) and Mn(II). The interaction of birnessite and Mn(II) at pH 7.5 caused the transformation of birnessite into secondary feitknechtite, which enhanced Ni(II) removal from the solution by incorporating Ni(II).
The stability of Cd(II) fixed on birnessite was slightly improved because of the addition of Mn(II) (Bochatay and Persson 2000;Elzinga 2011). Under alkaline conditions, the addition of low-concentration Mn(II) to the vacancy-rich birnessite caused the migration of Cd(II) from the vacancy to the edge site; In contrast, the high concentration of Mn(II) reacted with birnessite and generated abundant Mn(III), which co-precipitated with Cd(II) to form an amorphous Cd(II)-Mn(III) complex ). This co-precipitation process reduces the mobility of Cd(II) in the environment. In summary, the interaction of manganese mineral and Mn(II) impacts the geochemical circulation of Mn(II) and other coexisted heavy metals in the environment.

Interaction between manganese minerals and Fe
Interactions between Fe and Mn/manganese minerals commonly occur in nature and produce ferromanganese nodules, which impact the migration and transformation of pollutants (Wu et al. 2019c;Sun et al. 2018;Liu et al. 2017). Because of the similar octahedral Sb(V)(OH 6 ) structure to MnO 6 of Mn oxides, Sb(V) preferentially adsorbed on Mn oxides in ferromanganese nodules (Qin et al. 2019a); in contrast, tetrahedral As(V)O 4 3− was adsorbed on ferrite and manganese oxides, mainly through the formation of bidentate-binuclear complexes. The Mn/Fe ratio and mineral composition controls their distribution on the ferromanganese nodule. Platinum group elements (platinum, iridium and ruthenium) were significantly related to some redox sensitive trace metals (Co, Ce and Tl) in ferromanganese nodules (Berezhnaya et al. 2018;Marcus et al. 2018); similar to the behavior of Co and Ce, platinum group elements were also removed from seawater by suspended ferromanganese nodules. The enrichment mechanism of platinum group elements was due to adsorption and oxidation on the surface of δ-MnO 2 .
Fe-Mn binary oxides (BMFO) as one of production of interactions between Fe and Mn/manganese minerals, are also widely found in the natural environment (Foroutan et al. 2019;Pranudta et al. 2020). Because of their strong Table 1 The forms of interaction between manganese minerals and metals  Kwon et al. 2009) affinity to As(III), Fe-Mn binary oxides significantly impact the environmental behavior of As. The synergistic effect between hematite and manganese oxide in Fe-Mn binary oxide exhibited a synergistic effect on the removal of As(III) (Zheng et al. 2020). The coordination of adsorption-oxidation plays an important role in As(III) removal (You et al. 2020). The in-situ formation of BFMO can eliminate the species of Fe(II), Mn(II) and As(III & V) simultaneously in the microbial oxidation ecosystem (Bai et al. 2016). The BMO in the BMFO was mainly responsible for Sb(III) oxidation and FeOOH mainly adsorbed Sb(V) (Bai et al. 2017).
In systems with coexisting As(III) and Sb(III), in-situ BMFO preferentially oxidized Sb(III) rather than As(III), but the adsorption efficiency of As was higher (Kashiwabara et al. 2008).
The interaction between Fe and Mn/manganese minerals also formed two-dimensional Fe-Mn layered double hydroxide (Fe-Mn-LDH) (Hou et al. 2019a, b). Fe-Mn-LDH had a large internal surface area and abundant surface hydroxyls. The adsorption capacity of As(III) and As(V) was 1.5 and 0.7 mmol·g −1 , respectively, which was larger than that of other oxides (Otgonjargal et al. 2012). Fe-Mn-LDH has also been shown to be conducive to the adsorption of Sb(V) (Cao et al. 2017). FTIR and XPS results showed that Sb(V) was removed mainly through adsorption on the surface of Fe-Mn-LDH. It has been found that Fe-MnMg-LDH showed good performance for Pb(II) and Cd(II) removal (Cyprian and Abasi 2019). When a variety of heavy metals exist at the same time, Fe-MnMg-LDH adsorption capacity for heavy metals decreased in the order Cd(II) > Cu(II) > Pb(II). Heavy metals were removed from aqueous solution through surface adsorption, surface precipitation and ion exchange. The process also changed the morphology and electronegativity of LDH Zhou et al. 2018). M-Mn-LDHs (M = Mg 2+ , Zn 2+ , Cu 2+ , Ni 2+ , Co 2+ , Fe 3+ and Cr 3+ ) have shown a high catalytic activity for alcohol oxidation . The oxidation ability of the catalysts was related to the manganese valence and lattice oxygen in the LDH. In summary, the interaction between manganese mineral and Fe shows outstanding environmental purification by immobilizing and oxidizing pollutants.

Interaction between manganese minerals and other metals
Co is mainly present in the birnessite structure in the form of Co(III)OOH, replacement of Mn 4+ by Co 3+ not only increased the negative charge of the film and the hydroxyl content, but also decreased the Mn average oxidation state (Mn-AOS) of birnessite (Yin et al. 2011a). The adsorption capacity of Pb 2+ and the oxidation of As(III) was improved by the synergistic effect of Co and Mn (Yin et al. 2011b). Cryptomelane can rapidly oxidize Cr 3+ to HCrO 4 − and remove 45%-66% of total Cr, its adsorption capacity for Pb 2+ was also improved due to the increase of the specific surface area and Mn-AOS of cryptomelane after adsorption Cr 3+ (Li et al. 2015a). The adsorption of Cd(II) on manganese minerals influence its migration in water. Mn oxide shows good affinity for Cd(II) with maximum adsorption capacity of 104.17 mg/g at pH 6.0 and 298 K ). MnO 2 Nanosheet exhibits the maximum adsorption capacity of 348 mg/g, which is ascribed to efficient ion exchange (Liang et al. 2015). Cd(II) was inclined to being fixed on vacant sites than edge sites of birnessite. Biogenic manganese oxides formed in 1 mM Mn(II) efficiently adsorbed Cd(II) at pH 7.0 without obvious release of Mn(II) (Chang et al. 2014). The maximum adsorption capacity of MnO 2 formed in situ was 176 mg/g for Cd(II), which might be attributed to inner-sphere complex formation (Qin et al. 2011). Compared to the single system, the adsorption capacity of birnessite for Cd(II) and As(V) increased in the binary system containing Cd(II) cations and As(V) anions . Nickel is rich in manganese minerals, the incorporation of Ni into manganese mineral also affects the environmental behavior of manganese minerals (Javier and Yusta 2019). The adsorption capacity of Ni-rich birnessite for Pb(II) and Zn(II) decreased, mainly due to the vacancies and edge sites becoming occupied by Ni(II) (Yin et al. 2012). However, the Ni-rich birnessite has a high oxidation capacity and can completely oxidize As(III) in solution at a fast reaction rate.
The valence state of V in birnessite was + 5 in the form of oxygen anion (V 6 O 16 2− , VO 4 3− ), which had tetrahedral symmetry and formed a monodentate-corning shared complex (Yin et al. 2015). The adsorption capacity of V-rich cryptomelane for Pb(II) was significantly improved: At high V contents (V/Mn > 0.07), Pb(II) combined with the V-oxygen anion and formed Pb 5 (VO 4 ) 3 Cl precipitate; With the increase of V content, the binding affinity of V-rich birnessite to Pb(II) significantly increased (Chen et al. 2018a, b). These findings indicate that V-rich cryptomelane plays an important role in the in-situ remediation of environments pollution.
In summary, the interaction between manganese minerals and metals might exhibit a synergistic or inhibitory effect on the migration and transformation of pollutants (Fig. 4). The synergy of manganese minerals and metals results from coordination of adsorption and oxidation, convenient electron transfer, abundant oxygen vacancies and fast migration of lattice oxygen. These findings suggest that in the heavy metal-organic compound pollution system, the interface interaction processes of heavy metals and manganese minerals can change their catalytic properties, thus regulating the migration and transformation of toxic organic pollutants.

Environmental functional materials based on synergy of Mn and metals
Based on the understanding of the synergistic effect among heavy metals, the synthesis of bimetallic composites has been recognized as an effective technology to improve the reactivity of materials for environmental remediation applications (Du et al. 2020;Feng et al. 2016;Li et al. 2020;Lin et al. 2018;Zhao et al. 2018). In turn, the development and use of bimetallic materials for environmental remediation gives a good insight into the synergistic effects derived from metal-mineral interactions Chen et al. 2020a;Tang et al. 2014). Recently, massive Mn-based composites have been designed and applied, the number of articles increased year by year from 2010 to 2019, although there was a slight decline in 2020 and 2021, the number of published articles remained at more than 3500 (Fig. 5).

Removal of heavy metals from water
Based on the synergistic effect of Mn and metals, various Mn-based composites have been designed for heavy metal removal (Xu et al. 2016;Li et al. 2017a;McCann et al. 2018;Wu et al. 2022;Yang et al. 2021;Shao et al. 2022;Chen et al. 2022;Pala et al. 2022). The synergistic effect of oxidation-adsorption between manganese dioxide and other metal oxides have shown good application prospects for As(III) removal ( Fig. 6; Zhang et al. 2019;Shan and Tong 2013;Du et al. 2022;Imran et al. 2021). The manganese dioxide in the Fe-Mn binary composite oxidized As(III) to As(V), and the resultant As(V) was adsorbed by iron oxide (Zhang et al. 2007(Zhang et al. , 2012Liu et al. 2022b). The oxidation of the manganese oxide layer in zero-valent manganese and the adsorption of the iron oxide layer in zero-valent iron showed an outstanding synergistic effect for As(III) oxidation-removal (Amulya et al. 2020). CeMn@9CNTs has good performance for removing As(III) by pre-oxidation-adsorption (Liang et al. 2022b). Ce-Mn binary oxide effectively oxidized As(III) to As(V) (Chen et al. 2018b). The synergistic effect between Mn and Ce played important role in As(III) oxidation. The resultant As(V) was adsorbed by cerium oxide. The synergistic effect of oxidation-adsorption on composite toward As(iii)  Fe-Mn bimetal oxide (FMBO) is a promising adsorbent for Sb removal in drinking water and wastewater (Liu et al. 2015a, b;Li et al. 2022a;Xu et al. 2022b). The manganese oxide in FMBO dominated the oxidation process from Sb(III) to Sb(V), whereas iron oxide was responsible for the adsorption of Sb(III) and Sb(V) (Xu et al. 2011). These studies revealed the mechanism of oxidative-adsorption for Sb(III) by FMBO. Moreover, the addition of Mn(II) greatly improves the performance of FMBO for Sb(V) removal, because Mn(II) promotes the formation of amorphous iron oxide in FMBO, which play a crucial part in the immobilization process of Sb(V) (Yang et al. 2018b). Due to the mesoporous structure together and synergistic effect of FeMn binary component, magnetic mesoporous FeMn binary oxides (MMFMs) showed high performance to removal Sb(III) and Sb(V) ). As Fig. 7 illustrated, high valence Mn(IV) toke a major role in oxidizing Sb(III) to less toxic Sb(V), while the FeOx was responsible to the adsorption of Sb(V) .
A complex formed by the interaction of MnO 2 and polyhydroxy iron (FeOOH) exhibited a stronger affinity for Tl(I) than pure MnO 2 and FeOOH . Because the oxidation of MnO 2 played an important role for Tl(I) removal, and the adsorption of FeOOH loaded on MnO 2 enhanced Tl(I) purification at the same time. Fe-Mn binary oxides showed high efficiency removal capacity for Tl(I) removal, which was ascribed to the strong oxidative power of manganese dioxide and the high adsorption capacity of iron oxides, Tl(I) was removed by the combined surface complexation, oxidation, precipitation and adsorption ).
The Mn-based Layered double hydroxides (LDH) and its derivatives showed an excellent ability for heavy metal removal. The MgMn-LDH prepared by the homogeneous co-precipitation method had an excellent adsorption capacity for Cu(II), and the maximum immobilization capacity for Cu(II) reached 668 mg·g −1 (Chen et al. 2020a). MgMn-LDH and its calcination product (MgMn-LDO) also showed good performance for Cd(II) immobilization (Chen et al. 2019b). The maximum adsorption capacity of a magnetic Mn-based composite (0.3 Ma-MgMn-LDO-a) for Cd(II) was 3.76 mmol·g −1 (422.62 mg·g −1 ) (Chen et al. 2020b). The Cd(II) adsorption equilibrium was reached within 5 min. The Cd(II) removal activity of 0.3 Ma-MgMnLDO-a was significantly higher than Fe 3 O 4 -a and MgMnLDO-a. Fe-Mn binary oxide shows strong adsorb ability for Cd(II) immobilization with the maximum adsorption capacity of 74.76 mg g −1 at pH 6.0, Cd(II) was sequestered by forming outer-sphere complexes with hydroxyl group on the adsorbent surface (Zhong et al. 2016;Wang et al. 2016). The primary adsorption mechanism of biochar modified by iron-manganese oxide for Cd(II) was the precipitation between Cd and carbonate, complexation with the loaded Fe-O and Mn-O groups, and acid oxygen-containing functional groups . The positively-charged Cd(II) enhanced the removal of negative Sb(V) by Fe-Mn binary Oxide (Liu et al. 2015a, b). These findings indicate that the synergistic effect of Mn/manganese minerals and Fe is essential for heavy metal removal. Fig. 7 The mechanism for antimony removal by MMFMs

Elimination of organic pollutants in water
Because of the synergy between metals, Mn-based bimetallic oxides exhibit superior performance for degradation of organic pollutants (Li et al. 2015a, b;Afzal et al. 2017;Hou et al. 2019b;Chen et al. 2019b;Zhang and Wu 2013;Pan et al. 2021;Peng et al. 2021). Hydroxyapatite@Mn-Fe composite was able to eliminate color, turbidity, COD, and BOD 5 from the textile wastewater with removal efficiencies of 93.06, 81.61, 76.05, and 71.88%, respectively (Alali et al. 2022). The catalytic performance of Mn 1.8 Fe 1.2 O 4 nanospheres was obviously higher than that of Mn or Fe single-metal oxide . Mn was inferred to be the primary active site, with Fe(III) acting as the main adsorption site for BPA, The synergy between Fe and Mn plays a key role in activating peroxymonosulfate (PMS) for Bisphenol A degradation. Compared with Fe 3 O 4 -a and MgMnLDO-a, 0.3 Ma-MgMnLDO-a exhibited higher paracetamol removal with a high first-order rate constant of 0.116 min −1 and TOC removal (67.7%), which was ascribed to the synergy of Fe and Mn for the activation of PMS (Chen et al. 2020b).
CuO-MgMn-LDO-300 showed good catalytic performance; almost 97% of sulfamethoxazole was removed within 30 min (Chen et al. 2020a). This was attributed to the oxygen vacancies and synergy between Cu and Mn that favored electron transfer (Fig. 8, Chen et al. 2020a). FMO-73 could effectively degrade TC through the transfer of electrons not only from Mn(II) to Mn(III) or from Mn(III) to Mn(IV), but also from Fe(II) to the lattice of Mn(III) or Mn(IV) (Yang et al. 2018c).
The synergistic effect of Fe(III)/Mn(IV) promoted the adsorption process and reactive oxygen species that generation for the complete degradation of organic pollutants. At pH 4.0, the maximum adsorption capacity of p-ASA on the Fe-Mn framework and cubic Fe(OH) 3 was 1.3 and 0.72 mmol·g −1 , respectively (Joshi et al. 2017). In the FMBO and H 2 O 2 system, the synergistic effect of Fe and Mn mediated the generation of the ·OH radical for sulfamethoxazole degradation ). Fe-Mn diatomite had a high catalytic oxidation for phenol and its main intermediates (catechol and hydroquinone), and the degradation efficiency was as high as 100% within 50 min (Son et al. 2017).

Purification of atmosphere pollution
The synergistic effect in Mn-Co composites greatly contributed to its low-temperature reducibility and fast conversion rate for volatile organic carbon oxidation (Li et al. 2021). Compared with pure MnO x and Co 3 O 4 , the oxidation activity of Mn-Co composite for ethyl acetate and n-hexane was highest (Todorova et al. 2011). Because of the synergy of metals, the catalytic performance of Mn-Co composite with a varied mole ratio was much higher than that of the single MnOx and Co 3 O 4 (Tang et al. 2014). The Mn 5 Co 5 composite showed the best activity with T90% for benzene conversions into CO 2 were low to 237 °C with a high space velocity of 120,000 mL·g −1 ·h −1 .
The synergistic effect of Mn and Ce led to low temperature reducibility, high capture capability, fast electron transport and abundant oxygen vacancy Hyok Ri et al. 2021). The doping of Ce obviously improved the performance of birnessite for methanal removal at low temperatures . Ce-MnO 2 with a Ce:Mn mole ratio of 1:10 showed the best activity and achieved complete conversion of methanal at 100 °C. Because of the synergistic effect of MnOx and CeO 2 , the performance of CeO 2 -MnO x composite in catalytic oxidation of benzene was higher than that of pure CeO 2 or MnO x Cuo et al. 2018). Ce-Mn binary oxides supported on fly ash showed higher ratio of Mn 3+ /Mn 4+ and Ce 3+ /Ce 4+ , more lattice defects and surface oxygen vacancies, and greater lattice oxygen mobility, which increased VOC removal from flue gas (Liu et al. 2022a). The excellent catalytic performance of MnO 2 @NiO nanocomposites for oxidation of benzene was also related to the synergistic effect between Mn and Ni . The conversion temperature of 100% benzene was 320 °C under 1000 ppm benzene with a space velocity of 120 L·g −1 ·h −1 , whereas 380 °C was needed for pure MnO 2 . The doping of Ni significantly improved the catalytic performance of α-MnO 2 : Ni-MnO 2 catalysts not only showed a low-temperature performance (T90 = 199 °C), but also showed high catalytic stability and strong water resistance in the toluene oxidation (Dong et al. 2020). Moreover, loading Ni 2+ in the octahedral MnO 6 framework can not only improve redox performance, lattice oxygen mobility and formation of active lattice oxygen, but also promote toluene adsorption.
The close contact of Cu with Mn and synergistic effect of Cu and Mn in Cu-Mn composites played a key role in promoting the catalytic activity (Luo et al. 2019). Because Cu doping significantly improved the lattice oxygen activity of ramsdellite MnO 2 , Cu-doped ramsdellite MnO 2 had higher catalytic activity than pure ramsdellite MnO 2 for CO oxidation under full solar spectral radiation (Yang et al. 2018d). The presence of Cu in composite improved the reducibility and the adsorption of methanal, which accelerated the oxidation of methanol . The Cu/Mn composite showed a synergistic effect in terms of activity and selectivity to CO 2 , the water resistance of the composite also increased with the increase of Cu content (Ibrahim et al. 2019). The combination of Cu(II) and Mn(III) in CuMncomposites favored the interface effect, which enhanced benzene catalytic oxidation .
The major synergy between Mn and Fe is important for improving the catalytic oxidation of Mn-Fe composites. Compared with pure metal oxides, p-Mn/Fe 3 O 4 (Mn-Fe composites) exhibits high catalytic performance for benzene oxidation (Du et al. 2019). Fe doping in Mn 3-x Fex4 spinel increased lattice defects, oxygen vacancy concentration, specific surface area and mesoporous rate, these features endow Mn 3-x Fe 4 spinel with high reactivity for benzene oxidation ).

The feasibility of environment pollution based on Mn-based composites
At present, several environmental remediation methods have been developed, such as membrane separation , electrochemical (Gong et al. 2022), biological treatments , etc. However, these methods are selective, expensive and may need special raw materials or devices. Among the current methods, composite based remediation takes advantages of simple operation, and convenient management. The effectiveness and prospect of this method is increasingly prominent and has cost advantages over other methods.
The preparation method of composites determines the cost, performance and operability. The synthesize methods and conditions of some important composite are listed in Table 2. The synthesize methods of Mn-based composites mainly include homogeneous coprecipitation, chemical etching route, hydrothermal, homogeneous chelating sol-gel, and ethylene glycol reduction strategy. Among them, coprecipitation and hydrothermal are frequently-used methods.
The desorption of metals from the composites was usually achieved by using eluent to elute loaded metals in the composites. As Table 3 listed, the common eluents include solutions of HNO 3 , HCl, NaOH, NaCl, NaCl-NaOH, NaOH-NaClO and KMnO 4 . For example, NaOH was applied to desorb Tl(I) and Tl(III) that adsorbed on FeOOH-loaded MnO 2 , while NaClO was used to oxidize Mn(II) to Mn(IV), and the removal rate still reaches 99.7% after 5 cycles use . Conclusions can be drawn that FeOOH-loaded MnO 2 is an excellent recyclable adsorbent for Tl(I) removal.
The recovery rate of metals on composites reflects its reusability and stability. The higher of the reuse rate, the better of the stability of the composite. The GO-MnFe 2 O 4 showed little loss (6.0%) in the removal efficiency after 6 cycles by using HCl solution as eluent, which indicated the high stability of the GO-MnFe 2 O 4 nanocomposite as an economical material for the remediation of Pb 2+ pollution (Verma et al. 2020). The suitable eluent and the optimization conditions are the key factors to realize the reusability and keep stability of composites. Moreover, the metal dissolving solution in the process of pollution treatment also displays the stability of the composites. The leaching of Fe and Mn from nFMBO was less than 2.0% (Lu et al. 2019). Also, only a minor amount of Fe and Mn was leached from FMBO 3 (Yang et al. 2018b). For FeOOH-loaded MnO 2 , the concentrations of released Mn are under drinking water standard when pH was above 5 and the release of iron was under the detection limit .
The remediation method based on composites takes advantages of simple operation and convenient management. Its technical economy depends on the stability and the preparation cost of composites. The improvement of the composites stability can not only reduce the cost and but also decrease secondary pollution. Low-cost composites based remediation can be used as a replacement for current costly methods, but its technical feasibility in engineering application needs further verification.

Research perspectives
There are several different forms of interaction between manganese minerals and metals. The synergy or antagonism derived from the interaction shows important environmental significance. However, few studies have focused on the relationship between the forms of interaction and synergy/ antagonism effect. The synergistic or antagonism processes of manganese minerals and metals also need to be further clarified, as they will be of considerable importance for understanding the environmental effect and resource utilization of manganese minerals.  NaCl -NaOH binary solution meet the standard set by the WHO (0.01 mg·L −1 ) when the initial As(III) concentration of less than 5 mg·L −1 Shi et al. 2022 In some situations, the interaction of manganese minerals with metals has an inhibitory effect on the reactivity of the manganese minerals because the active sites on the manganese minerals are masked by the metals. In order to achieve in-situ self-purification of polluted environment, measures need to be taken to weaken these inhibitory effects. It is important to remediate heavy metal-organic polluted environment by in situ utilization of manganese minerals.
The synergistic effect of manganese minerals and metals is conducive to in-situ utilization of manganese minerals. Based on the synergistic effect, a large number of Mn-based composites have been synthesized for environmental remediation, and it has been verified that the synergy plays an important role in pollutant removal. More effective technologies are needed to further strengthen the synergistic effect and expand the utilization of metal-rich manganese minerals. There are many technologies that could be considered, including external energy reinforcement (ultraviolet light, microwave, PMS, H 2 O 2 ), acid or alkali activation, and high temperature calcination.
Metals that have a synergistic effect with manganese minerals, such as Co, Ni, Cu, and Ce, might also cause environmental pollution. Therefore, monitoring and preventive measures are needed in the in-situ utilization of manganese minerals. The risk of metal dissolution should first be evaluated when Mn-based composites is used in practical application. The practice application of environmentally friendly Mn-based composites is encouraged.
The actual environmental factors have an important impact on the interaction of metals. The function of the synergistic effect might be affected by environmental factors as using Mn-based composites for environmental remediation. To our knowledge, there are few studies have focused on this. Dissolved organic matter, as active components in the environment, might affect the features of manganese minerals, metals and pollutants. Therefore, revealing the effect of dissolved organic matter on the synergistic effect of manganese minerals and metals should be consider in in-situ environment remediation.

Conclusion
Manganese minerals are active in chemical cycling within the environment and have an important role in migration and transformation of metals. The interaction with metals affects the microstructure and properties of manganese minerals, which further regulates the environmental geochemical processes of toxic pollutants. The following conclusions can be drawn from this review: (1) manganese minerals show strong affinity to metals. The interaction mechanism between manganese minerals and metals includes surface adsorption, lattice replacement and formation of associated minerals. (2) The interaction of manganese minerals and metals might exhibit synergistic or inhibitory effects on the migration and transformation of pollutants in the environment. (3) Based on the synergistic effect between Mn and metals, a large number of Mn-based composites have been designed for environmental remediation. (4) The development and use of Mn-based composites for environmental remediation gives a good insight into the synergistic effect derived from metalmineral interactions. (5) The synergistic effect between manganese and metals results from the coordination of adsorption and oxidation, convenient electron transfer, abundant oxygen vacancies and fast migration of lattice oxygen. In the further research, it should pay more attention to the role of manganese minerals in regulating the geochemical fate of combined pollutants in environment. The synergistic effect between manganese and metals should also be fully utilized in expanding the resource utilization of manganese minerals and Mn-based composites for environmental remediation.