Heavy metal pollutants pose a serious threat to ecosystems and human health because of their strong toxicity, difficult degradation and significant bioconcentration effects, which can be amplified step by step through the food chain ( Boyd, 2010 ; Timothy & Williams, 2019 ). Industrial activities such as electroplating, tanning and other production processes continuously discharge heavy metal-containing wastewater, and monitoring data show that about 82% of China’s surface water bodies are polluted by heavy metals to varying degrees, with mining, metallurgy and other areas of water pollution being particularly prominent ( Zhang & Wang, 2020 ). Typical pollutants include Pb, As, Cd and other metal elements, and their environmental trends are characterized by multi-media migration ( Chen, Zhang, Zhang, Zhu, & Zhuo, 2022 ). For heavy metal wastewater treatment, the traditional physicochemical technologies (including electrochemical deposition, membrane separation, chemical precipitation, etc.) can achieve a certain degree of treatment effect, but they generally face technical bottlenecks such as high operational energy consumption, significant risk of secondary pollution, and difficulties in selective recycling ( Briffa, Sinagra, & Blundell, 2020 ).

Microbial adsorption, that is, through the chemical composition or structural properties of the microorganisms themselves or products adsorption to remove heavy metals in the medium, and then through the solid-liquid phase separation, thus reducing the metal ion concentration in water ( Goyal, Jain, & Banerjee, 2003 ; Tsuruta, 2004 ). Microbial adsorption has low cost, good effect, short time, reusable, strong specificity and low risk of secondary pollution, so it is feasible and economical. At present, there are many reports on microbial adsorption of heavy metals in water at home and abroad, and this technology is superior in cost, operation, operation and management, and has good development value and application prospects ( Klein & Ziehr, 1990 ).

Microbial adsorption realizes heavy metal immobilization through mechanisms such as binding of functional groups on the cell wall surface, complexation of extracellular polymers and transmembrane transport, and is characterized by high adsorption capacity, fast reaction kinetics and selective adsorption ( Yin, Wang, Lv, & Chen, 2019 ). Studies have shown that bacteria (e.g., Bacillus subtilis), fungi (e.g., Aspergillus niger), and algae (e.g., Chlorella vulgaris) exhibit different adsorption characteristics for heavy metals such as Pb²⁺, Cr⁶⁺, etc., and their efficacies are regulated by multiple factors such as metabolic state of the species, solution pH, temperature, and coexisting ions ( Lin, Zhou, Li, & Dong, 2023 ; Yin et al., 2015 ).

However, microbial adsorption still has certain limitations in practical application, such as susceptible to the influence of the external environment, and the microorganisms are not easy to be separated and recovered after the adsorption of heavy metals, etc. Therefore, this paper summarizes the efficiency and mechanism of heavy metal adsorption by different microorganisms (bacteria, fungi and algae, etc.), the influencing factors of adsorption, describes the limiting factors affecting the application of microorganisms, as well as the techniques and methods to improve the adsorption efficiency in practical application, providing theoretical support and technical reference for improving the efficiency of microorganisms’ adsorption of heavy metal ions for their removal and their practical application.

Ruchhoft (1949) successfully removed the radioactive element plutonium from water by using activated sludge, and because microorganisms multiply to form “colloidal matrix materials with a large surface area capable of absorbing the radioactive element”, it was assumed that the microorganisms had the ability to adsorb Pu, and the concept of microbial adsorption was then proposed. The concept of microbial adsorption was then proposed. Since then, scholars at home and abroad have begun the adsorption of different heavy metals by different microorganisms. Strandberg et al. (1981) found that Saccharomyces cerevisiae and Pseudomonas aeruginosa could adsorb uranium (U⁶⁺). The study of heavy metal adsorption by microorganisms developed rapidly in the late 1990s, with the development of biosorbent materials and the understanding of the biosorption mechanism. Gloab et al. (1991) found the adsorption of Pb²⁺ by Streptomyces; Volesky and Prasetyo (1994) found that Cd²⁺ could be adsorbed by seaweed; and Volesky and May-Phillips (1995) found that the adsorption of Cd²⁺ by Brewer’s bacillus could be adsorbed by Saccharomyces cerevisiae. In 2008, Teclu et al. (2008) found that sulfate-reducing bacteria adsorbed both As³⁺ and As⁵⁺ in water. Microbial adsorption for heavy metal removal has become an emerging technology that is environmentally friendly and does not introduce secondary pollution, and many microorganisms have been found that can adsorb heavy metals, mainly consisting of bacteria, fungi, algae and actinomycetes ( Table 1 ).

Adsorption of heavy metals by microorganisms is a complex process. The different affinities of the same species of microorganisms for different metal ions and the different tolerances of different species of microorganisms for the same metal ions lead to the diversity of the mechanism and efficiency of microbial adsorption of metals. According to the differences in the distribution of adsorbed ions in microbial cells, they are categorized into three types: intracellular adsorption, extracellular adsorption and surface adsorption ( Figure 1 ) ( Pavel, Martina, & Tomas, 2011 ). Surface adsorption is found in active and inactive microorganisms, while extracellular and intracellular adsorption is mainly found in active microorganisms. In an adsorption system, one or more mechanisms may exist simultaneously. Extracellular adsorption refers to the removal of heavy metal ions by adsorption, precipitation or complexation using extracellular polymeric substances (EPS) secreted by microorganisms, such as glycoproteins, lipopolysaccharides, polysaccharides, and soluble amino acids ( Carpio, Machado-Santelli, Sakata, Ferreira Filho, & Rodrigues, 2014 ). Surface adsorption refers to the process of adsorption of metal ions on the cell surface, especially on the cell wall components (proteins, polysaccharides and lipids, etc.) when chemical functional groups (carboxyl, hydroxyl, phosphoryl, amide, sulfate, amino, and sulfhydryl groups, etc.) interact with the metal ions. Due to the existence of conformational, spatial and other barriers, functional groups do not guarantee the successful adsorption of all heavy metals.

At present, the main mechanisms of surface adsorption of metal ions include ion exchange, surface complexation, physical adsorption (van der Waals force, electrostatic effect), redox or inorganic microprecipitation. Ion exchange refers to the existence of microorganisms on the surface of hydroxyl, carboxyl and amino groups that can be combined with heavy metal ions, and the exchange process of heavy metal ions and functional groups is the ion exchange process. Surface complexation means that heavy metal ions can form complexes with the lone pair of electrons provided by nitrogen, sulfur and oxygen in the functional groups of microbial cell wall components. Electrostatic adsorption is due to the presence of van der Waals forces between the negative charges on the outer surface of microbial cells and the positively charged heavy metal ions. Redox means that some microorganisms with redox ability can change the valence state of heavy metal ions into low toxicity substances by changing the valence state of heavy metal ions. Inorganic microprecipitation refers to the process of forming inorganic precipitates of heavy metal ions on or inside the cell walls of microorganisms through chemical and physical effects. In addition, the differences in the composition of the cell walls of different microorganisms lead to differences in their adsorption mechanisms. For example, Lei et al. (2014) suggested that the ion exchange of chemical groups in the cell wall is the main mechanism for the adsorption and removal of heavy metals (Cu²⁺, Pb²⁺, etc.) from water by Bacillus sp. Intracellular adsorption means that when the concentration of extracellular metal ions is higher than that of intracellular ions, the metal ions can pass through the cell wall and the cell membrane to enter the intracellular through free diffusion. After entering the cell, microorganisms distribute the metal ions to metabolically inactive regions (e.g., vesicles) through compartmentalization, or bind the metal ions to heat-stable proteins to transform them into less active and less toxic forms.

The adsorption of heavy metals by microorganisms is related to the nature of the microorganisms themselves and the nature of the heavy metal species. Different species of microorganisms have different cell structures, and their adsorption mechanisms and adsorption capacities for the same heavy metals are different. For example, in the adsorption of Pb²⁺, the adsorption rate of Bacillus licheniformis was as high as 96.5% by chemical adsorption; when As was adsorbed by Aspergillus niger, its adsorption and removal of As⁵⁺ could reach more than 90%, and its adsorption and removal of As³⁺ could reach 75%, and the adsorption mechanism was oxidization and co-precipitation; in the adsorption of Cd²⁺, the adsorption rate of S. aeruginosa was as high as 99%, and its adsorption sites were mainly carbonyl, carboxyl and carboxylic groups. In the adsorption of Cd²⁺, the adsorption rate of Saccharomyces cerevisiae was as high as 99%, and the adsorption sites were mainly carbonyl, carboxyl and amino groups; the adsorption rate of Penicillium microbialis was as high as 87% when adsorbing Cr⁶⁺, and the main sites were amino, carboxyl, carbonyl and hydroxyl groups; the highest adsorption rate of Bacillus thuringiensis adsorption of Cu²⁺ could be up to 91.8%, and the adsorption rate was mainly on the surface of the cells; the adsorption rate of Aspergillus tamaricus for the adsorption of Zn²⁺ could be up to 99.6%, and the adsorption rate of amines, alcohols and conjugated C=C groups formed covalent bonds with the adsorbent and the matrix layer of the crosslinking network was the main reason for the removal of Zn²⁺ ( Abas, Ismail, Kamal, & Izhar, 2013 ; Goyal et al., 2003 ).

After the adsorption of heavy metals by microorganisms, it is necessary to desorb the adsorbed heavy metals to avoid secondary pollution and at the same time to realize the recovery of precious metals. There are many methods for desorption, and washing with chemical reagents is one of the common methods. Commonly used desorbing agents are hydrochloric acid, sulfuric acid, nitric acid, acetic acid, EDTA, thiourea and carbonate and so on. The desorption efficiency of different desorbing agents for different heavy metals varies, for example, the desorption rate of nitric acid on Pb²⁺ is higher than that of Na₂EDTA on Pb²⁺, while the desorption rate of Na₂EDTA on Ni²⁺ is only 21.1%, and the desorption rate of sodium nitrate on Hg²⁺ is only 3.65%; hydrochloric acid on Mn²⁺ is 92%, while oxalic acid is 92% on Cu²⁺. The desorption rate of hydrochloric acid for Mn²⁺ was 92%, and that of oxalic acid for Cu²⁺ was 42.2%. Therefore, it is necessary to choose desorbing agents with fast desorption speed and high efficiency according to different adsorption systems in the process of practical application.

Microbial adsorption is a new technology arising from the intersection of disciplines, which has good application prospects due to its low cost and technical difficulty and environmental friendliness ( Maqsood, Hussain, Mumtaz, Bilal, & Iqbal, 2022 ). However, due to the limited applicability of laboratory studies, few reports have evaluated its feasibility on an industrial scale. The cost of transferring microbial adsorption from the laboratory to the field is high, and its practical application is still limited. Because heavy metals are very toxic to microorganisms in general, the main problem is the selection of microbial strains. At the same time, due to different environmental conditions (such as temperature, pH, other substances and components in water) and other factors will have an impact on the growth and reproduction of microorganisms, biological activity, and so on, which in turn affects the adsorption effect, once the environmental conditions are changed, the process of microbial adsorption is very easy to be limited. Therefore, screening and finding microorganisms that are resistant to heavy metals and have a high capacity for heavy metal enrichment is a top priority for the application of microbial adsorption to industrial water treatment. In recent years, some scholars have adopted genetic engineering means to change the genetic characteristics of microorganisms, and expressed complex proteins (peptides) with strong adsorption of certain heavy metals on the surface of microbial cells or in vivo, so as to improve the adsorption effect of microorganisms on heavy metal ions ( Diep, Mahadevan, & Yakunin, 2018 ; Ranjbar & Malcata, 2022 ).

Zhao et al. (2005) successfully propagated an As-resistant metallurgical engineering bacterium by inserting an As-resistant gene fragment into a Thiobacillus caldus bacterium by in vitro recombination in DNA technology, and this bacterium was able to treat high As wastewater. In addition, nanobiosorbents can be formed by chemically combining biosorbents with nanoparticles ( Boubakri et al., 2017 ). The combination of the two parent materials (nanoparticles and biosorbent material) results in a novel material with multiple enhanced properties. Currently, this technology has been used more for other materials and less for microorganisms. For example, Arshadi et al. (2017) reported the preparation of zero-valent iron nanoparticles modified with aquatic plant Azolla filiculoides for the removal of Pb²⁺ and Hg²⁺ from water, and Thanh et al. (2011) reported the improved performance of pearlite nanocomposites compared with iron and manganese nanomaterials for the removal of As(V). The application of nano-microbial composites is a new and effective way to remove heavy metals from water. In order to ensure the safety of microbial adsorbents, it is necessary to analyze and test the biosafety of adsorbents, which generally includes acute toxicity test, reproductive toxicity test, and teratogenicity-sensitive toxicity test.

In the analysis of the biosafety of microbial adsorbents, because microbial adsorbents are often used to treat polluted water bodies and have obvious impacts on aquatic ecosystems, the acute toxicity of fish is commonly used for evaluation ( Vasylyuk, Shved, Hubrii, Vichko, & Shved, 2022 ). At the same time, fish are more sensitive to changes in the water environment, when there is a certain concentration of exogenous substances in the water body, it can cause a series of physiological and biochemical reactions, including behavioral abnormalities, physiological dysfunction, tissue cell lesions and even death. Therefore, under certain conditions, the safety of the test organisms can be assessed by the short-term exposure effect by exposing the fish to specific types and concentrations of microbial adsorbents, and observing the reaction phenomenon and mortality of the experimental fish within a certain period. In addition, microorganisms have small specific surface area and their surface active sites are often covered by organic matter ( Mohapatra et al., 2019 ); planktonic microorganisms usually have small particles and poor mechanical strength, and are prone to loss of adsorbent during operation, which may cause problems such as swelling of the bacterial body during solid-liquid separation ( Yu, Tong, Sun, & Li, 2007 ); strain selection and breeding are time-consuming; and microorganisms are not easy to be separated and recovered from the aqueous solution.

The specific selection of microbial adsorbents for heavy metals is also a problem in microbial adsorption, and it is difficult to find a microorganism that is tolerant to a variety of heavy metal ions. It is difficult to find a microorganism that is tolerant to a variety of heavy metal ions, and the adsorption capacity and selectivity are not high enough to be applied in practical water treatment projects. In addition, due to the diversity of functional groups and functional differences of microbial adsorbents, the degree of stabilization and unpredictability of adsorption effect is also a factor limiting its application. Therefore, the development and popularization of microbial adsorption were hindered.

In the follow-up research, we can focus on the following aspects: 1) using molecular biology, genetic engineering and other screening of efficient heavy metal-resistant microorganisms and the construction of genetically engineered bacteria; 2) the development of a new type of high-efficiency immobilized bioreactor to reduce the interference and influence of the environment on the microorganisms; 3) designing microbial adsorbents with high surface areas, such as microbial composites; 4) selective and efficient recovery of precious metals after adsorption.