Introduction

Nanomaterials possess unique optoelectronic, physiochemical, and biological properties compared with their macroscopic counterparts (Menon et al. 2017). Therefore, they have been widely applied in many applications including eliminating pollutants from water and wastewater (Krishnaraj et al. 2014; Kumar et al. 2014; Suresh et al. 2011). For example, the carbon nanotube obtained 50 times higher pseudo-first-order reaction constant than that of granular activated carbon in the degradation of acetaminophen by activating persulfate (1.5 m−1 g−1 vs 0.03 m−1 g−1) (Pham et al. 2020). Also, in order to constrain the fast recombination of photon-generated carriers, tuning the particle size of TiO2 into the nanometer range can enhance the transfer of interfacial charge carrier effectively (Qu et al. 2013).

However, the nanomaterials are mostly synthesized with physicochemical processes which may involve hazardous and costly chemicals as reducing, capping, or stabilizing agents, and the extreme conditions like high temperature and high pressure (Menon et al. 2017; Murphy et al. 2005; Rajeshkumar & Bharath 2017). Considering the drawbacks of traditional strategies, many researchers made efforts to employ environment-friendly and cost-effective protocols for nanomaterial production.

Biosynthesis is an important alternative protocol. Many microbes like bacteria, fungi, and algae can mediate the reduction and oxidation of metal ions with the formation of biogenic metal nanoparticles (Bio-Me-NPs) (Tani et al. 2004; Veeramani et al. 2013). Microorganisms and their intracellular or extracellular enzymes can replace exogenous reducing and capping or stabilizing agent and play a key role in changing the valency and morphology of Bio-Me-NPs (Andeer et al. 2015; Sarkar et al. 2012; Zhang et al. 2015b). In addition, biosynthesis usually can be conducted under ambient conditions, like room temperature, no pressure control, and neutral pH (Furgal et al. 2015; Gericke & Pinches 2006; Matsushita et al. 2018). More interestingly, Bio-Me-NPs may demonstrate some better capacities owing to their unique characteristics like higher specific surface, broader vacancy range, and more crystal defects than their chemically synthesized peers. For instance, the dechlorination of carbon tetrachloride by biogenic FeS derived from Shewanella putrefaciens CN32 was 5 times faster than abiotic FeS, which was ascribed to the higher reductive content (structural Fe(II) and disulfide) and well dispersity of biogenic FeS (Huo et al. 2016). The specific area of biogenic magnetite was 20-fold higher than that of the synthetic magnetite while the adsorption capacity of heavy metals was consequently 30 ~ 40-fold higher (Iwahori et al. 2014). The superior capacity of Bio-Me-NPs is also investigated in advanced oxidation process (AOPs) as catalysts. With respect to the phenol degradation by persulfate activation, the reaction rate constant (10.5 × 10−2 min−1) with biogenic manganese oxide as catalyst is higher than serval abiotic Mn-based catalysts (0.4 ~ 8.7 × 10−2 min−1) (Liu et al. 2016; Saputra et al. 2013, 2014; Tian et al. 2018; Wang et al. 2015).

Several reviews have been published so far on the related topics. Ten years ago, Hennebel et al. (2009a) reviewed the biosynthesis of biogenic Fe, Mn, Pd, Pt, Au, and Ag NPs and discussed their applications in AOPs. In the review of Gautam et al. (2019), the applications of biogenic particles in the remediation of water pollution are highlighted but less content are spent on the progress of the biosynthesis and the morphology of Bio-Me-NPs. Recently, Sadhasivam et al. (2020) reviewed the synthesis of Bio-Fe-NPs and other metals were not included. They also briefly summarized the general application of Bio-Fe-NPs and the water treatment was not a focus. We are here trying to review the advances on the biosynthesis of metal NPs mediated by microbes including bacteria, fungi, and algae; the morphology and composition of the NPs; and the applications on the elimination of organic and inorganic contaminants, mainly consisting of emerging organic contaminants and heavy metals. Besides, the scale-up applicability of Bio-Me-NPs is also discussed.

Types and morphology

The widely studied Bio-Me-NPs mainly include the metals like Fe, Mn, Pb, Au, and Ag. Their biogenic source, morphology, size, and chemical valence are summarized in Table 1 and are further discussed in below sections.

Table 1 The main Bio-Me-NPs reported in literatures
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Bio-Fe-NPs

Microbes play an important role in the geochemical cycle of iron. With its wide valences, Fe could be an electron acceptor or donor, depending on the oxidation environment, e.g., anoxic or oxic conditions. As a result, Fe can exist with the valency of 0, II, III, and IV in Bio-Fe-NPs (Huo et al. 2016; Tuo et al. 2015; Watts et al. 2015). The size of Bio-Fe-NPs may range from few nanometers to dozens of nanometers. With the aid of transmission electron microscope (TEM), scanning electron microscope (SEM), and atomic force microscopy (AFM), researchers identified various shapes of Bio-Fe-NPs, such as hollow tubular, spherical, and irregular forms (Bai et al. 2016; Jung et al. 2008; Pi et al. 2017). As shown in Fig. S1, hollow tubular Bio-Fe-NPs derived from Leptothrix sp. are coating on the sheath of a filamentous bacterium to support its premise (Hargreaves & Alharthi 2016; Rentz et al. 2009). Another research found that the biogenic Mackinawite synthesized by S. putrefaciens CN32 aggregated to form rosette-like particles (Veeramani et al. 2013). A spherical morphology of Bio-Fe-NPs was also reported in previous studies as shown in Fig. S1c (Huo et al. 2016; Cutting et al. 2010).

The crystalline nature of biogenic Bio-Fe-NPs is usually characterized by an X-ray diffraction (XRD) analysis. Many studies found an amorphous or poorly crystalline structure of Bio-Fe-NPs, which usually shows weak diffraction peaks in XRD profiles (Bai et al. 2016; Huo et al. 2016; Pi et al. 2017). When comparing the XRD patterns, researchers noticed that the diffraction peaks of biogenic Fe (oxyhydr)oxides were quite broader than those of synthetic two-line ferrihydrite, indicating that the former may possess a more disordered structure (Whitaker et al. 2018). However, the crystalline of Bio-Fe-NPs can be highly influenced by many factors in biosynthesis including but not limited to the initial rate of Fe reduction, electron carriers, and excreted substances (Cutting et al. 2009). The appearance of lattice fringes in the TEM images has proved that the poorly ordered schwertmannite was transformed into a highly crystalline biogenic magnetite when incubated with G. sulfurreducens (Cutting et al. 2010). Further study suggested that the reduction of ferrihydrite was proceeded in 3 ~ 7-day incubation and stayed in a stable state after 7 days and finally formed a mixed-valent magnetite crystal (FeIIFeIII2O4) which contained low bioavailable Fe(III) for Fe(III)-reducing bacteria (Iwahori et al. 2014). In addition, with the assistance of surface-bound components, the non-magnetic akaganeite was transformed into metallic face-centered cubic magnetic precipitate by Shewanella oneidensis MR-1 under anaerobic conditions (Tuo et al. 2015).

Bio-Mn-NPs

The oxidation of Mn in the natural environment is mainly attributed to the microbes since the rate of microbial oxidation of Mn(II) to Mn(III/IV) is several orders of magnitude (up to 105 times) higher than that of abiotic oxidation (Furgal et al. 2015; Tanaka et al. 2010). Bio-Mn-NPs acquire non-uniform crystallines in different cases. Amorphous structured or poorly crystalline Bio-Mn-NPs (represented by Fig. S2) exist prevalently as mentioned in previous studies (Tanaka et al. 2010; Tian et al. 2018; Wang et al. 2017; Zhou et al. 2018). As indicated by the XRD patterns, plenty of Bio-Mn-NPs were of poor crystallinity and analogously to natural minerals including birnessite (σ-MnO2), manganite (γ-MnOOH), todorokite, and vernadite (Kim et al. 2003; Wang et al. 2009; Xie et al. 2018; Zheng et al. 2016). For instance, the final products in biogenic Mn(II) transformation processes were reported as a mixture of rhodochrosite (MnCO3), sidorenkite [Na3Mn(PO4)(CO3)], and amorphous manganese oxides (Zhou et al. 2016). The well crystal structured Bio-Mn-NP has been also reported. Bio-Mn-NPs produced with Pseudomonas sp. QJX-1 were comprised of discrete layered σ-MnO2 or birnessite with adjacent layers randomly stacked (Bai et al. 2016). Another research found that Bio-Mn-NPs with diffraction peaks at 36.4°, 38.2°, 43.2°, 53.2°, 26.5°, 33.4°, and 38.8° 2θ were ascribed to birnessite and γ-MnOOH (Wang et al. 2009). Actually, the highly crystalline biogenic NPs was reduced at lower rate than amorphous, disordered one since poor crystallinity characterized by greater surface area and structural defects was beneficial to electrons transfer (Wright et al. 2016).

The Mn valence of Bio-Mn-NPs can fluctuate from + II to + IV, largely depending on the reduction capacity of biogenic source and the species of substrate. The main valence state of Bio-Mn-NPs with P. putida MnB-1 was found to be + IV, as demonstrated by XPS analysis (Zhou et al. 2018). Mixed Mn valences such as co-existence of + II and IV or + II, III, and IV, were also frequently detected in previous studies (Du et al. 2020; Nelson et al. 2002; Tian et al. 2018; Wang et al. 2009).

Bio-Pd-NPs

With electron donors supplied, the bioreduction of Pd(II) can proceed through a metabolic activity. Pd may precipitate on cell surface, debris, periplasmic, cytoplasm, or independently from biomass (Hennebel et al. 2011). Baharak et al. isolated several marine strains with the ability to generate Bio-Pd-NPs, including both Gram-negative and Gram-positive strains (Hosseinkhani et al. 2014). They also found that Bio-Pd-NPs were located in the cytoplasm and periplasmic space of the Gram-negative strains while in the periplasmic space and on the cell wall of Gram-positive strains (shown in Fig. S3). The thinner peptidoglycan layer of Gram-negative bacteria can be favorable for transferring Pd(II) to the cytoplasm, consequently leading to an intracellular precipitation. The precipitation profile was found to be determined by metabolic activities of cells and the electron donor for bioreduction. For instance, smaller and more Pd(0) particles were distributed on the cell surface and inside the periplasm when comparing H2 with formate as electron donors (Windt et al. 2005).

XRD patterns and TEM analysis indicated that Bio-Pd-NP is metallic crystalline in nano-sizes. Bio-Pd-NP production with an indigenous marine bacterial community was manifested as the conversion from Pd(II) ions to metallic Pd(0) and PdO crystals (Hosseinkhani et al. 2014). A cubic crystal metallic Bio-Pd-NP was generated by microbial granules using bio-H2 as electron donor (Suja et al. 2014). Researchers also found that Bio-Pd-NPs produced by both C. pasteurianum BC1 and Citrobacter braakii were in metallic and crystalline phase after a 2-week successive cultivation (Chidambaram et al. 2010; Hennebel et al. 2011).

Bio-Au and Ag-NPs

Bio-Au and Ag-NPs were found to be associated with the cells or precipitated intracellularly which corresponds to the distribution of other Bio-Me-NPs aforementioned (Binoj and Pradeep 2002; Du et al. 2015). Bioreduction of Au and Ag is prone to form well crystalline NPs. Bio-Au and Ag-NPs derived from the Actinomycete Gordonia amicalis HS-11 were depicted with a face-centered cubic (fcc) crystalline (Sowani et al. 2016). Another study also found in the TEM images of Ag nanocrystallites two areas consisting of silver NPs and micro crystalline insoluble Ag salts, respectively (Shahverdi et al. 2007). Biomolecules can largely influence the morphology of Bio-Me-NPs especially in the control of shapes. Zhang et al. (2016) reported that Bio-Au-NPs synthesized by the strain LH-F1 exhibited various sizes and shapes including triangle, hexagon, pentagon, and irregular shapes (Fig. S4), whereas Bio-Au-NPs originated from Escherichia coli K12 were strictly controlled in the circular shape of approximately 50 nm with the aid of stabilizing agents (Srivastava et al. 2013).

Bio-bimetal-NPs

Dosing two kinds of metals in the substrate solution may form the biogenic bimetal NPs, which can enhance the activity of NPs due to the change in geometric and physicochemical properties (De Corte et al. 2011a; Nutt et al. 2005). Fe and Mn elements commonly co-exist in terrestrial and aquatic environments (Huang et al. 2012). Bai et al. (2017) obtained Bio-Fe/Mn-NPs with a strong adsorption capacity of As ions, and the size of Bio-Fe/Mn-NPs was between Bio-Mn-NPs and Bio-Fe-NPs generated under similar conditions. Another study found with XRD and EDX analysis that Bio-Fe/Mn-NPs were not homogenous and showed a poor crystalline (Bai et al. 2016). De Corte et al. also found that Bio-Pd/Au-NPs (seen in Fig. S5) produced by means of decorating Au particles on Bio-Pd-NPs can improve the removal of halogenated organics (De Corte et al. 2011a, 2012).

Composite Bio-Me-NPs

In order to enhance the stability, to avoid the agglomeration, to protect from poisoning, and to reduce metal ions leaching into environment as well, one can immobilize or encapsulate Bio-Me-NPs as composite materials (Parshetti & Doong 2008). Hennebel et al. (2010) immobilized Bio-Pd-NPs on polysulfone membranes in a fed batch configuration and obtained 77% removal efficiency for 20 ppm of diatrizoate within 48 h. Another work of Hennebel et al. (2009c) eliminated 98% trichloroethylene in 22 h by polyurethane cubes empowered with bio-Pd-NPs. Moreover, biogenic jarosite composited with TiO2 via Fe–O–Ti bonds possessed stronger charge transfer capacity in photocatalytic degradation of methyl orange, and the electrical energy consumption derived from artificial light was 4 times lower than that of pristine P25 (Chowdhury et al. 2017). The photoelectrochemical property of Bio-Au-NPs decorated on polymeric graphitic carbon nitride (g-C3N4) as shown in Fig. S6 was strengthened as a result of the synergistic effects between the conduction band of g-C3N4 and the plasmonic band of AuNPs (Khan et al. 2018). In general, the composite strategy compensates for the shortcomings of Bio-Me-NPs and provides more potentials for its further application.

Biosynthesis of metal NPs

The participation of toxic and costly substances in conventional chemical synthesis of NPs may pose potential threats to human and environmental health. Biosynthesis of NPs with microbes such as bacteria, fungi, and algae through bioaccumulation, precipitation, biomineralization, and biosorption is becoming an attractive alternative (Srivastava et al. 2013). When an applicable concentration of metal ions exists, certain amount of them will be transferred into cells through the endocytosis, the carrier-mediated transport, and the permeation process (Issazade et.al, 2013). Microbes may excrete some enzymes which can mediate the intracellular and extracellular transformation of metals ((Parandhaman et al. 2019). Herein, the processes of biosynthesis with different microbes are discussed below.

Bacteria

Bacterial transformation of metals plays a pivotal role in geochemical cycles of many elements. Many strains of bacteria can mediate the metal transformation and obtain energy for its metabolism process. The organics can serve as reductive and stabilizing agents during the synthesis of Bio-Me-NPs and promote the interaction between NPs and pollutants (Zhou et al. 2016). The transformation of metals by bacteria is also regarded as a detoxification mechanism of metals, reactive species, and other substances that are toxic to cells (Tebo et al. 2005; Kim et al. 2003).

The process of Au NPs synthesized by Shewanella oneidensis was described as a rapid biosorption firstly and a slow reduction process in the second stage (De Corte et al. 2011b). For the generation of biogenic Fe NPs, a conspicuous color distinction with different Fe source was detected (Furgal et al. 2015; Wright et al. 2016). Bio-Fe-NPs derived by S. putrefaciens CN32 with hydrous ferric oxide as precursor resulted in a black precipitate, whereas a grayish white precipitate was displayed with Fe(III)-citrate as precursor, which were further verified as magnetite and vivianite, respectively (Veeramani et al. 2011).

The transformation of Mn(II) can be realized by many manganese-oxidizing bacteria (MnOB) like Pseudomonas putida MnB-1, which can utilize Mn(II) to consume superoxides for self-protection (Zhou et al. 2018). The formation of Bio-Mn-NPs mainly include two steps: (1) Mn(II) is firstly oxidized to an intermediate of Mn(III), which is confirmed by studies with sodium pyrophosphate as a trapping reagent; (2) the formed Mn(III) intermediate is further oxidized to MnOx (Spiro et al. 2010; Tran et al. 2018). The organic pollutants in the aquatic environment can be adsorbed and degraded by the biogenic MnOx (Bio-MnOx), concomitantly releasing oxidative products and Mn(II) ions that can be re-oxidized by Pseudomonas putida MnB-1 (Forrez et al. 2009). In this process, enzymes like multicopper oxidases and peroxidases were requisite actuating force for metal oxidation (Andeer et al. 2015; Zhang et al. 2015b). With the assistance of Mn-oxidizing bacteria, Bio-MnOx has the potential to be regenerated consecutively, which is of great significance in the practical application in water remediation (Furgal et al. 2015).

Fungi

Fungi are well known as “the nanofactory” for its high tolerance to many metals in Bio-Me-NP production (Balaji et al. 2009; Shahverdi et al. 2007). In the biosynthesis of Bio-MnOx produced by fungus strain KR21-2, the fungal hyphae was found to be the main region for Bio-MnOx formation and the optimal concentration of Mn(II) as precursor was up to 1.2 mM (Tani et al. 2004). Instead of exogenous chemicals, biomolecules are deemed to be the prerequisite for the formation and stabilization of NPs. Du et al. stated that fungus Penicillium oxalicum was able to reduce Ag ions to highly crystalline Ag NPs with the characteristics of high stability and great dispersion (Du et al. 2015, 2011). With the assistance of capping agents secreted by fungi, Bio-Au-NPs produced by Magnusiomyces ingens LH-F1 showed a high dispersity (Das et al. 2010; Gangula et al. 2011; Zhang et al. 2016). In the case of Fe NPs generated by Fusarium oxysporum, the biomolecules worked as stabilizing agents to control the shape and size of the formed crystalline magnetite (Bharde et al. 2006).

Algae

Another clean and sustainable microbial route for the biosynthesis of metal NPs is algae. It has been found that algae can transfer metals such as Ag, Mn, Cu, and Au into Bio-Me-NPs (Abboud et al. 2013; Kathiraven et al. 2015; Prasad et al. 2012; Venkatesan et al. 2014). In the photosynthesis process, algae Desmodesmus sp.WR1. can increase the level of pH and dissolved oxygen (DO) by consuming CO2 and consequently expedite the oxidation of Mn2+ to form Bio-Mn-NPs (Wang et al. 2017). Shankar et al. (2016) explained the three stages in biosynthesis of Ag and Au as follows: (1) Ag or Au ions were reduced and formed nucleus; (2) then, small particles aggregated into larger one and the molecular stability was established to a certain extent; (3) the formation of NPs in different shapes took place in the final stage. Similar to the microbes mentioned above, extracellular and intracellular enzymatic reductions contribute largely in this process (Barwal et al. 2011; Jena et al. 2014). The enzymes can also significantly influence the morphology of generated Me-NPs (Aziz et al. 2015). For example, Ag+ reduction mediated by ATP synthase, superoxide dismutase, carbonic anhydrase, ferredoxin-NADP + reductase, etc. formed rounded or rectangular Ag NP intracellular (Barwal et al. 2011). As documented by Xie et al. (2007), Au was reduced by enzymes derived from algae and the single-crystalline NP was in the form of triangular and hexagonal shapes. Likewise, the high consistency of Bio-Ag-NPs produced by chlorella pyrenoidosa was attributed to the morphology control effects of a large amount of enzymes and proteins (Aziz et al. 2015). Additionally, hydroxyl, carboxyl, and amino functional groups on the surface of biomolecules can promote the adsorption of metal ions by electrostatic interaction and further serve as reducing and capping agents (Mahdavi et al. 2013).

As shown above, researchers have demonstrated the capacity of bacteria, fungi, and algae to generate Me-NPs. However, studies have not been conducted to evaluate the applicability of the microbes for a large-scale synthesis of Bio-Me-NPs. Bacteria might be a potential candidate for the scale-up due to their wide variety of strains and high yield efficiency. Algae may find a unique position when the CO2 immobilization is considered. The application of fungi is faced with the challenge of long-term stability due to the problems like microbial contaminations, strict nutrient conditions, and reproduction.

Applications of biogenic metal NPs in water treatment

The migration of heavy metals, textile dyes, pesticides, pharmaceuticals, and personal care products in environment causes severe pollution, which may bring potential threats to the human health once entering the food chain (Lingamdinne et al. 2017; Martins et al. 2017; Mertens et al. 2007; Muthukumar et al. 2017; Qu et al. 2017). Although being detected in low concentrations in water body, these compounds may impose detrimental effects on digestive, immune, and reproductive systems of human (Roberts et al. 2016; Zhang et al. 2003). Due to some special characteristics such as large specific surface areas, weak crystalline, and complex valence, Bio-Me-NPs have been studied to eliminate various pollutants via the mechanisms of adsorption, oxidation, reduction, and catalysis, which is outlined in Table 2.

Table 2 The application of biogenic nanoparticles in wastewater treatment
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Adsorption

The large specific area, wide valent distribution, and unique structure of Bio-Me-NPs attract a lot of research efforts in adsorption (Fig. 1). A wide range of metal ions including Cu(II), Fe(III), Pb(II), Zn(II), and U(VI) can be effectively adsorbed by Bio-Mn-NPs owing to its large surface area with negative charges (Spiro et al. 2010; Zhou et al. 2016). Wang et al. (2009) reported the superior adsorption of heavy metals by Bio-MnOx. With the dosage of Mn oxides as 0.1 mM and the concentration of heavy metals as 30 μM, the adsorption of Zn(II) and Ni(II) of Bio-MnOx is 2 ~ 3 times higher than that of commercial and abotic MnO2. The XRD patterns revealed that the biogenic one contained crystalline birnessite with a high specific area and a cation-exchange capacity, which possibly accounted for its extraordinary adsorption of metals (Wang et al. 2009). It was also found that the Pb binding ability of Bio-MnOx was much higher than that of abotic Mn-bearing materials, suggesting a stronger binding energy per unit surface area of the former (Nelson et al. 2002).

Fig. 1
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Bio-Me-NPs mediated adsorption process

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Notably, a recent study showed that the surface charge of adsorbates may strongly impact the adsorption performance (Bai et al. 2017). In the study, Sb(III) was effectively adsorbed but the adsorption abated after Sb(III) was oxidized into Sb(V). Though the negatively charged Bio-MnOx rejected both Sb(III) and Sb(V) (existing as negatively charged groups), the binding force between Sb(III) and MnOx was higher than the charge repulsion and finally the stable ligand between Sb(OH)3 and Bio-MnOx was formed through the inner-sphere electron transfer.

The distinct structural property works potently as well. The intrinsic structural defects in Bio-MnOx can provide vacancies for metals and thus can promote the combination of metal cation ions on Bio-MnOx (Cao et al. 2015). Previous studies mentioned that Pb–Mn complexes in triple-corner-sharing inner-sphere surface and double-edge-sharing Pb(II) inner sphere surface were relevant to O ions with unsaturated bonds (Takahashi et al. 2007; Villalobos et al. 2005). Consistent results revealed that octahedral FeO6 composition in the biogenic particles could lead to the strong adsorption of As(III and V) (Bai et al. 2016). Pi et. al. (2017) elaborated that the removal of As by Bio-Fe NPs was due to the adsorption on the surface at first and then co-precipitation as a stable As–Fe complex (Pi et al. 2017). Aside from metal ions, Bio-Me-NPs can also adsorb phosphorus and many other organic contaminants which are often followed with oxidation or reduction as discussed below (Rentz et al. 2009; Zhou et al. 2016).

Oxidation

Some metals in Bio-Me-NPs hold a high valence and thus could exhibit a certain level of oxidation potential. Bio-MnOx, as a widely studied example, is able to oxidize many recalcitrant pollutants like benzophenone-4, estrone, and 17-α-ethinylestradiol (EE2) due to the existence of highly valency Mn (close to IV) (Chang et al. 2018; Furgal et al. 2015). Despite the extensive researches of chemically synthetic Mn(IV)-containing materials, Bio-Mn-NPs show several superior properties.

Large specific area facilitates many interactions. For example, in the work of Forrez et al. (2010), diclofenac was chosen as a model contaminant at initial concentration of 4 mg/L and the dosage of chemical MnO2 and Bio-MnOx is 4.6 vs. 3.5 mg Mn4+ L−1, respectively. The enhanced diclofenac oxidation by Bio-Mn-NPs was attributed to the relatively larger specific surface area than the chemical synthesized MnO2 (98–224 m2 g−1 vs 58–121 m2 g−1) (Forrez et al. 2010). Low crystallinity with more structural defects may be another enhancement factor. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicated that Bio-Mn-NPs held massive Mn(IV) vacancy sites which can provide binding sites for exogenous ions (Spiro et al. 2010). It has been reported that amorphous structure of Bio-Mn-NPs was conducive to surface electron transfer for the oxidation of α-ethinylestradiol (Tran et al. 2018). In Mn2+-enriched cultures, amorphous Bio-Mn-NPs was formed and increased the removal efficiency of bisphenol A by up to 51% (Wang et al. 2017). With the existence of microbes, Bio-Mn-NPs further exerts its superiority. A recent study systematically depicted the ciprofloxacin (CIP) degradation in a Mn redox cycling system (Zhou et al. 2018). As portrayed in Fig. 2, Pseudomonas putida MnB-1 generated Bio-MnOx in two steps through Mn(III) intermediates (Spiro et al. 2010). Both Bio-MnOx and Mn(III) intermediates contributed to the oxidation of CIP; however, Mn(III) intermediates survive only in a short time (Tran et al. 2018). The resulting Mn2+ was detrimental to the oxidation of CIP by blocking the active sites on the surface of Bio-MnOx (Watanabe et al. 2013). The re-oxidation of Mn2+ by bioactivities can eliminate this negative effect, which is also crucial to the microbial redox cycle of Mn (Bai et al. 2016; Forrez et al. 2010). Re-oxidation of Mn2+ decreases the Mn(II) binding on the surface of Bio-MnOx and the process is influenced by DO content, pH, and oxidoreductases (Wang et al. 2017). Similar results were also obtained with the Fe–Mn oxides which can also release synthetic Fe and Mn ions which hinder the further activity (Bai et al. 2017, 2016).

Fig. 2
figure 2

Microbial Mn redox cycle. P stands for pollutants; T for transformation products; M for Bio-MnOx; E for enzymes; MnOB for manganese oxidizing bacteria

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Reduction

Bio-Fe-NPs own an appreciable reductive ability as well. A previous study reported that after being activated by Shewanella alga BrY, the inert ferric oxide was converted into microbial-produced Fe(II) that was capable of reducing 66% of Cr(VI) within 5 min (Vázquez-Morillas et al. 2006). The direct and indirect reduction processes conducted with Bio-Me-NPs are illustrated in Fig. 3. Compared with reactions conducted in pasteurized solution, the reduction of U(VI) by biogenic magnetite and vivianite was suggested as abiotic processes (Veeramani et al. 2011). Lu et al. studied the removal of nitrite through the microbial mediated iron redox cycling and found that both biotic and abiotic reduction occurred concurrently in the removal of nitrite (Lu et al. 2017). Coker et al. (2014) designed a hybrid system by associating hollow-fiber membrane with biogenic Pd-magnetite and achieved considerable removal of azo dye and Cr(VI). Besides the reduction capacity of Fe(II) in magnetite, Pd was able to scavenge electrons and thus to reduce the contaminant or to regenerate Fe(II). Moreover, it is worth mentioning that microbes can act as electron carriers and fix the charged reactants on an appropriate position to promote the reduction corporately (Kondo et al. 2015; Vázquez-Morillas et al. 2006). Some Bio-Fe-NPs may contain S2−, like biogenic minerals such as mackinawite, pyrite, and marcasite. Veeramani et al. (2013) found that the sulfide in biogenic Mackinawite could initiate the reduction of U(VI), whereas Fe(II) in the NPs worked inertly in the process. The contribution of both Fe(II) and disulfide of biogenic FeS in dechloriantion process was also confirmed in another research (Huo et al. 2016). The promoted dechlorination by adsorbed Fe(II) was found previously, whereas a negative effect existed when Fe(II) was excessive (Huo et al. 2016). Other ions such as Ca, Si, and S may impair the reduction process by clogging the combination access (Roberts et al. 2004; Watts et al. 2015). The DO level in water can inhibit the reduction of Cr(VI) by competing the active site and finally oxidizing Fe(II) and Fe(0) (Watts et al. 2015). On the contrary, sulfate and carbonate can promote the reaction by precipitating as green rust that is conductive to electron transfer (Vázquez-Morillas et al. 2006).

Fig. 3
figure 3

Reduction process with different components in Bio-Me-NPs

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Catalysis

The facile and benign route of Bio-Me-NPs synthesis makes it a promising method to generate catalysts with microbes and without involving toxic solvents and complex operations (Fig. 4). Some special characteristics also enable Bio-Me-NPs a higher catalytic performance than the chemically synthesized peers (Jung et al. 2008).

Fig. 4
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Catalysis with Bio-Me-NPs

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The most extensively studied issue is the catalytic reduction ability, in which exogenous reductants like NaBH4, H2, and formate and formic acid are often added to serve as electron donors (Hennebel et al. 2009b; Srinath & Ravishankar Rai 2015; Tuo et al. 2015). With Bio-Au-NPs as a catalyst, MB can be reduced to methylene blue by NaBH4 (Srinath & Ravishankar Rai 2015; Suvith & Philip 2014). Hennebel et al. (2009b) found in the catalytic TCE dechlorination with Bio-Pd-NPs that a rapid process involving radicals was initiated when formic acid was applied as a hydrogen donor, whereas a slow hydride formation process was dominated instead when formate was used as a hydrogen donor. H2 produced by microbes under fermentative conditions can be an excellent alternative to exogenous reducing agents owing to its effective and economical production route (Chidambaram et al. 2010; De Gusseme et al. 2011; Hennebel et al. 2011; Hosseinkhani et al. 2014; Suja et al. 2014). Biocatalysts can be charged by in situ biogenic H2 derived from the oxidation of glucose, which in turn facilitates the production of H2 (Hosseinkhani et al. 2014). Under this circumstance, more than 90% removal of iomeprol, iopromide, and diatrizoate at environmentally relevant concentrations was obtained (Forrez et al. 2011). Except for H2, Bio-Pd-NPs can enhance the removal of Cr(VI) and azo dye by scavenging electrons for generating reductive Fe(II) species or reacting with pollutants directly (Coker et al. 2014). The attachment of noble metal Au on Pd onto biocatalysts can promote the catalytic activity due to the geometric effect that the lattice is constricted to form an optimal intermolecular distance and also due to the electronic effect that Au withdraws the electron density from Pd (De Corte et al. 2011a; Tuo et al. 2015). The catalytic ability of bimetallic bio-Pd/Au has been proved to work in the treatment of hospital wastewater containing diclofenac at neutral and alkaline pH (De Corte et al. 2012).

Recently, Bio-Me-NPs applied in AOPs gain growing interest of research. For instance, the bio-FeMnOx derived from Pseudomonas sp. exhibited a twofold higher catalytic activity in the photo-Fenton degradation of ofloxacin than that of chemically synthesized Fe–Mn oxides (Du et al. 2020). Sunlight-driven photocatalytic decolonization of dyes including congo red, malachite green, direct blue-1, and reactive black-5 was successfully carried out with Bio-Cu-NPs produced by Escherichia sp. SINT7 (Noman et al. 2020). It has been demonstrated that the surface functional groups of the biogenic magnetite enabled its feasible catalytic activity in ozonation as well (Jung et al. 2008). Moreover, the application of Bio-Me-NPs in the activation of peroxymonosulfate (PMS) was also reported, for the degradation of 30 mg/L phenol or 10 mg/L tetracycline with the dosage of the catalyst and PMS as 0.4 and 1 g/L, respectively. The catalytic degradation ability of Bio-MnOx was three times higher in a wide range of pH (3.0 ~ 9.0) than the chemically prepared 3D α-Mn2O3 (Tian et al. 2018). Further study ascribed the enhanced performance to the unique crystal structure of Bio-MnOx, whose contents of Mn(III and IV) increased the generation of SO−4· and OH· radicals (Xie et al. 2018). The catalytic activity of Bio-MnOx could also be regenerated by heat treatment at 400 °C for 2 h, which eliminated the inhibition of substances on the surface (Xie et al. 2018).

Although Bio-Me-NPs exhibit appreciative properties as a catalyst, the deficiency of aggregation and the leaching of heavy metal ions also need much attention (Jung et al. 2008; Srivastava et al. 2013). Immobilization with appropriate matrix could be a good solution by providing a sturdy support (Srivastava et al. 2013). The strategy of encapsulating the catalysts with polymeric membranes like polyvinylidene fluoride and polysulfone membranes can prevent from poisoning and agglomeration (Hennebel et al. 2010; Parshetti & Doong 2008). De Gusseme et al. (2011) doped Bio-Pd-NPs on the cathode of a microbial electrolysis cell for the dehalogenation of diatrizoate and found that Pd nanoparticles can decrease the cathode overpotential and catalyze the dehalogenation with H2 as an electron donor.

Scaling-up feasibility

Some research efforts have been made on the feasibility of applying Bio-Me-NPs in real applications. Conventionally, Bio-MnOx has been widely involved in the removal of metals from water. The popular reactor for the purpose is biofilter (Nishi et al. 2020; Nitzsche et al. 2015). Recently, researchers tend to apply Bio-Me-NPs to eliminate recalcitrant organics. Forrez et al. (2011) developed an MBR system by incorporating Bio-MnOx in the membrane modules and obtained an effective removal of many emerging contaminates from secondary effluents, such as naproxen (> 95%), morphine (60%), N-acetylsulfamethoxazole (92%), clarithromycin (60%), and diuron (90%). A similar MBR was also applied with Bio-Pd-NPs (Hennebel et al. 2009b, 2011). However, the MBR should overcome the limitation of incorporating large amount of Bio-Me-NPs and the possible membrane fouling issues coming from the direct contact of Bio-Me-NPs, which is still less concerned so far. Zhang et al. (2015a) operated a lab-scale aerated biofilter packed with natural MnOx and two strains of MnOB (Leptothrix sp. and Pseudomonas sp.) and obtained an effective removal of diclofenac and sulfamethoxazole in 1.5 years. Furthermore, they scaled up the biofilter to a pilot scale (250 L) which was installed at a local sewage treatment plants to treat the real secondary effluent and 70 ~ 98% of 15 emerging organic contaminants was eliminated in 500 days (Zhang et al. 2017). To our best knowledge, this reactor is still the largest one for the removal of recalcitrant organics with MnOB and the photo can be found in Fig. 5. This simple reactor worked effectively and continuously owing to the oxidation capacity of natural MnOx and the re-oxidation of Mn2+ by MnOB. However, the population analysis was not conducted so that the contribution of MnOB on the performance of biofilter is not clear. In addition, heterotrophic MnOB was incubated in a downflow hanging sponge reactor to produce Bio-MnOx, and Co and Ni ions in the effluent at an HRT in the range of 1.5–12 h were successfully removed after Bio-Mn-NPs formed (Cao et al. 2015). This reactor was able to remove Mn(II), Ni(II), and Co(II) simultaneously from wastewater in long-term continuous operation. In situ remediation is another promising strategy for groundwater treatment. Pi et al. (2017) injected FeSO4 regularly into an As-contaminated aquifer at the central part of Datong Basin, China, for 25 days to motivate biogenic reduction and As was decreased by 73%, which was caused by surface-adsorption on and co-precipitation with formed biogenic Fe(II) sulfides.

Fig. 5
figure 5

The process scheme (a) and a photo (b) of the pilot biofilter. Reproduced from Zhang et al. (2017) with permission

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Conclusions and outlook

The present work reviews the recent progress in the typical Bio-Me-NPs (Me = Fe, Mn, Pd, Au, or Ag) and their applications in water treatment. In general, the biosynthesis of Bio-Me-NPs with microbes like bacteria, fungi, and algae is environmentally friendly via avoiding toxic chemicals and extreme conditions. Bio-Me-NPs also possess some special characteristics, such as higher specific area, looser crystalline, more defects, and broader valence of metals, which could enable them a better performance in adsorption, oxidation/reduction, and catalysis than their chemically synthesized peers. The incorporation of multi-metals can further enhance the capacity of Bio-Me-NPs. Some efforts have also been made on the possible scale-up applications, such as with the reactor of MBR and biofilter.

However, there are still some issues waiting to be further clarified. For example, the performance of Bio-Me-NPs is strongly connected to their morphology whose regulation methods should be systematically studied. So far, most studies were conducted with a bench scale glassware in a batch mode and further efforts should be given on the continuous and long-term production of Bio-Me-NPs. Moreover, much work is needed to further explore the applicability of Bio-Me-NP in the water treatment at large scale.