Uptake of aqueous heavy metal ions (HMIs) by various biomasses and non-biological materials: a mini review of adsorption capacities, mechanisms and processes

ABSTRACT Numerous research papers on biosorption have been published in the past four decades. This paper reviews and compares heavy metal uptake capacities of various biological and non-biological materials. Adsorption mechanisms and processes of heavy metal ions (HMIs) onto biomasses are summarised and discussed, respectively. In general, all types of adsorbents exhibit certain uptake capacities for HMIs, but the capacity values for different types of biomasses and non-biological materials vary significantly. For HMIs, the reported values for bacterial biomasses typically range from 0.06 mmol·g−1 to 2.84 mmol·g−1; for fungi and yeasts, 0.03 mmol·g−1 to 2.44 mmol·g−1; for fresh water algae, 0.02 mmol·g−1 to 3.15 mmol·g−1; for marine algae, 0.23 mmol·g−1 to 3.77 mmol·g−1; for other biological materials/derivatives, 0.01 mmol·g−1 to 1.78 mmol·g−1 and for non-biological materials, 0.003 mmol·g−1 to 2.40 mmol·g−1. Thereinto, a few macroalgal species exhibit much higher adsorption capacities for HMIs relative to other types of adsorbents. Statistical analysis of heavy metal uptake capacities of various biological and non-biological materials indicates that marine algae are most suitable for the development of industrial biosorbents for the remediation of diluted HMIs-bearing effluents. Biosorption mechanisms of HMIs by biomasses include physical adsorption, ion exchange, electrostatic interaction, surface complexation and inorganic microprecipitation. the physicochemical properties of biosorbents and HMIs as well as external adsorption conditions significantly influence the adsorption process of HMIs onto biomasses. It could be concluded from numerous previous studies that pseudo-first-order and pseudo-second-order models are the most commonly utilised to characterise adsorption process. Up to date biosorption of HMIs remains largely in the laboratory stage. Combining microscopic mechanisms with macroscopic models may be one of the future research directions for removal of HMIs by biomasses.


Introduction
With the fast development of modern industries [1], effluents containing heavy metal ions (HMIs) are increasingly discharged into the water bodies.This trend has resulted in serious environmental pollution [2].Release of HMIs into the environment without a prior satisfactory treatment is a public concern all over the world due to significant toxicity of HMIs towards human beings and organisms in the aquatic ecosystems [3].Unlike organic contaminants which are susceptible to biological degradation [4,5], HMIs are not converted to harmless end products by bioreduction [6].Recent attentions on the impact of low concentrations of HMIs on public health [7] have encouraged a major research effort to develop effective means to remove and/or recover HMIs from diluted waste streams [8].Traditional techniques for treatment of HMIs bearing industrial effluents generally include chemical precipitation [9], evaporation [10], oxidation-reduction [11], electrochemical methods [12], ion exchange [13], membrane technique [14], coagulation-flocculation [15], flotation [16] and adsorption [17].Nevertheless, most of these conventional physico-chemical methods and/or techniques suffer from diverse drawbacks.The main outstanding issues are summarised as expensive (e.g.ion exchange, electrochemical treatment, membrane technique), not environmentally-friendly (e.g. chemical precipitation, oxidation-reduction, evaporation) and ineffective for large amounts of wastewaters with 10-100 g/m 3 of metal ion concentration (e.g. chemical precipitation, filtration and electrochemical treatment) [18].Biosorption has emerged as a promising alternative technology in the last four decades, in which biomasses are utilised to uptake or accumulate heavy metals from aqueous solutions.The most prominent features of this technology are low operation cost while high efficiency for treatment of high volume and diluted waste streams [19,20].Other advantages may involve easy availability, convenient operating conditions, cost-effective regeneration and no secondary contamination for dispose of exhausted biosorbents.Both living and non-living biomasses have been examined for adsorption purposes [21,22].However, increasing consensus indicates that non-living biomasses excel the living ones in simplicity and ease of operation [23,24].Until now, the commercial biosorption systems utilise only non-living biological systems.An important consideration for practical utilisation of biomasses for uptake of HMIs is the metal amounts removed by biosorbents.Although other factors such as adsorption kinetics and biomass stability play important roles in the selection of biomasses, the uptake capacity of HMIs provides a practical means to compare advantages and/or disadvantages of different adsorbents.There are a large number of biosorption studies reported in recent years, but a comprehensive comparison of the biosorption capacities of varied biomass types has not been fully investigated.Such a comparison may give general insights into the adsorption potential for HMIs by different groups of biomasses including bacteria [25], algae [26,27], fungi [28], yeast [29] and plant derivatives [30].The obtained information can contribute to rational selection of a particular biomass group for development of biosorption systems.Due to a tremendous number of studies employing various experimental conditions, an exhaustive investigation of uptake capacities of all the documented sorbents is burdensome and time consuming.Nevertheless, an attempt has been made here to compare the uptake capacities of HMIs by some representative non-living biomasses that were examined under similar experimental conditions.Based on 171 representative references selected out of more than 1000 papers since 1970s', this mini review is devoted to making a statistical evaluation of uptake capacity of HMIs by different kinds of biomasses and outlining the most effective class.Living biomasses have been excluded from this review because the metal uptake by these systems as well as the constrained experimental conditions are considerably different from those of non-living adsorbents.Adsorption mechanisms and processes of heavy metal ions (HMIs) onto biomasses are summarised and discussed, respectively.

Uptake of HMIs by biological and non-biological materials
In comparison with many non-biological adsorbents, the advantages of biological adsorbents are summarised as: (i) Rich sources and low preparation cost.(ii) Large specific surface area, abundant pore structure and surface oxygen-containing functional groups.(iii) Good biocompatibility and adsorption selectivity.(iv) High adsorption efficiency and simple operation.(v) Renewable and environmentally-friendly properties.Hundreds of different biomasses with potential HMIs uptake capacities have been investigated and their performance for removal of heavy metals are documented in numerous scientific papers.They are classified here as bacterial biomasses, fungal/ yeast biomasses, algal biomasses (including micro-algae and macro-algae) and other biomass types such as animal and plant shell, plant residues and biopolymers [31].Many factors including types and dosage of biomasses, types and initial concentration of HMIs, contact time, pH, solution temperature and co-existing metal ions significantly influence the uptake capacity of biomaterials for metal ions [32].Undoubtedly types of biosorbents are the most important among the factors mentioned above, determining the nature of HMIs uptake capacity of biomasses [33].An effort is made in this present review to compare HMIs uptake capacities of various biomasses as well as non-biological materials, outlining the most effective biomasses for the potential industrial application.

Heavy metal uptake by bacterial biomasses
Bacterial biomasses have been used in many biosorption systems for the removal and/or recovery of heavy metals.Bacterial biomasses are generally produced as waste byproducts of industrial operations or can be specifically propagated in large scale.The ability of bacterial biomasses in binding HMIs has been known for a long time.Table S1 gives a comparison of HMIs uptake capacities of various bacterial biomasses reported previously.It can be found that the uptake capacities of bacteria for HMIs range between 0.06 mmol•g −1 to 2.84 mmol•g −1 .

Heavy metal uptake by fungal/yeast biomasses
Fungi, including yeasts, has received increased attentions particularly because fungal biomasses arise as a by-product from several industrial fermentations (e.g.Penicillium, and Aspergillus sp.).Utilisation of fungal biomasses for development of biosorbents can bring additional revenues to industries that are currently dumping the fungal/yeast biomasses.Furthermore, yeasts and fungi can also be propagated using unsophisticated fermentation techniques and inexpensive growth media.The HMIs uptake capacities of non-living biomasses of fungi/yeasts are given in Table S2.It can be seen from Table S2 that the related metal uptake varies from 0.03 mmol•g −1 to 2.44 mmol•g −1 .Notably, a group of wood-rotting macro-fungi shows relatively higher HMIs uptake capacities relative to micro-fungi species.Furthermore, macro-fungal biomasses without considerable modification can be conveniently applied in packed-bed operation due to their relatively stable structural strength.There seems to be a potential for further investigating these macro-fungal biomasses.

Heavy metal uptake by microalgal biomasses
Although thousands of microalgal species are identified by now, only a small part of them have been examined for their biosorption potential for HMIs.Microalgae can accumulate and/or adsorb HMIs on the surface of algae cells in a short period of time.This favours practical adsorption process.Removal of HMIs by microalgae demonstrates a certain selectivity, which can be used to treat wastewaters containing multiple heavy metals.The HMIs uptake capacities of non-living microalgal biomasses ranges from 0.02 mmol•g −1 to 3.15 mmol•g −1 (Table S3).Of all these species, Chlorella sp. has been studied extensively.A commercial biosorption system has been developed based on this species by immobilising the micro-algae onto silica gels.

Heavy metal uptake by macroalgal (seaweed) biomasses
In recent years, the biosorption potential of marine macroalgae has been paid everincreasing attentions.Development of macro-algae based biosorbents is primarily driven by practical considerations for the field application of biomasses.In comparison with other types of biomass (i.e.bacteria, fungi, yeasts), marine algae is an abundant biomass source for HMIs removal.Macroalgal (seaweed) biomasses has proven to be highly effective as well as reliable and predictable in the removal of HMIs from aqueous solutions.The HMIs uptake values of various non-living macroalgal species in Table S4 range from 0.23 mmol•g −1 to 3.77 mmol•g −1 .It is found that the maximum uptake values for macro-algae are much higher than those of other biomasses.This may be closely associated with the ion-exchange properties of their cell wall matrix polysaccharides and extracellular polymers.

Heavy metal uptake by other types of biomass and non-biological materials
In addition to bacteria, fungi and algae, plenty of other biological materials/derivatives also have exhibited uptake potential for HMIs.In this regard, some inexpensive materials such as straw and bran of wheat [34], plant leaves [35], crustacean shells [36], aerobic granules [37] and Streptomyces rimosus biomass [38] are tentatively employed as biosorbents to treat HMIs-bearing wastewaters.A comparison of non-living biological materials/derivatives in terms of their sorption capacities for HMIs is demonstrated in Table S5.Although these biological materials exhibit certain uptake capacities for HMIs (0.01 mmol•g −1 -1.78 mmol•g −1 , Table S5), practical applications have been seldom reported [39].Notably, non-biological adsorbents derived from synthetic or natural materials are increasingly applied to treat HMIs-bearing effluents in the past two decades.For example, activated carbon (AC) is utilised widely for elimination of organic and inorganic contaminants from industrial effluents.Similarly, water-absorbent resins have been examined as possible adsorbents for HMIs removal due to their low toxicity, minor irritation and few side effects.On the other hand, high costs of AC and other synthetic materials promote some researchers to develop relatively cheap adsorbents based on natural minerals and/or sedimentary rocks (e.g.bentonite, haematite, magnetite, clays, laterite, peat and coal).Advantages of natural materials involve being cheap, abundant and environmentally-friendly as compared with the synthetic adsorbents.It is necessary to compare HMIs uptake properties of various non-biological materials with the abovementioned biomasses.Heavy metal uptake capacities of some non-biological materials in Table S6 varies from 0.003 mmol•g −1 to 2.40 mmol•g −1 .

Comparison of HMIs uptake by various biomasses and non-biological materials
It can be concluded from Tables S1-S6 that HMIs uptake capacities of diverse types of adsorbents vary significantly.For HMIs, the reported adsorption capacity values for bacterial biomasses typically range from 0.06 mmol•g −1 to 2.84 mmol•g −1 ; for fungi and yeasts, 0.03 mmol•g −1 to 2.44 mmol•g −1 ; for fresh water algae, 0.02 mmol•g −1 to 3.15 mmol•g −1 ; for marine algae, 0.23 mmol•g −1 to 3.77 mmol•g −1 ; for other biological materials/derivatives, 0.01 mmol•g −1 to 1.78 mmol•g −1 and for non-biological materials, 0.003 mmol•g −1 to 2.40 mmol•g −1 .Based on Tables S1-S6, statistical analysis of HMIs uptake capacities of various biomasses and non-biological materials are given in Table S7.Table S7 highlights the fact that non-living algae biomasses outperform other biological materials in terms of heavy metal sorption capacity (the average and median values).Table S4 and Table S6 also demonstrate the encouraging potential of macroalgae when compared to traditional inorganic and synthetic adsorbents (Table S7).Activated carbon (AC) and cation-exchange resin (CER) have been routinely used in practical recovery of metal ions.However, the average uptake capacities of macroalgae for HMIs are much higher than those of AC and/or CER (Table S7).
Although fresh water algae and a few bacterial species exhibit similar HMIs uptake values relative to marine macro-algae, limitations in their large-scale propagation and the related operation cost make them less attractive relative to other biosorbents.Kuyucak [40] in his study on cost-comparison of various biosorbents indicated that fresh water algae could cost 9 to 15 times the cost of marine algae.By contrast, large scale cultivation and harvest of macroalgae is a well-established industry in many parts of the world.Moreover, the availability and cost of macroalgae will not be limiting factors in the development of efficient biosorbents.Our previous studies have identified two common Australian macroalgae with high HMIs uptake potential, Durvillaea potatorum and Ecklonia radiata.The batch equilibrium experiments showed that the maximum adsorption capacities of Durvillaea potatorum based biosorbents (DP95Ca) for lead and copper were 1.6 mmol/g and 1.3 mmol/g, respectively.The corresponding values for Ecklonia radiata based biosorbents (ER95Ca) were 1.3 mmol/g and 1.1 mmol/g [41].These HMIs uptake values are comparable to those of commercial ion exchange resins and are much higher than those of natural zeolites and powdered activated carbon.Moreover, Durvillaea potatorum based biosorbents (DPFA5) can be used in multiple-cycle operations without losing much of its uptake capacity [42].Based on previous studies [26,41,42] and the present review, macroalgae biomasses exhibit outstanding heavy metal uptake capacities and should be promising sources for the development of efficient industrial biosorbents.

Biosorption mechanism
Over the past 40 years, by use of various physical and chemical methods as well as advanced instruments researchers have conducted a large number of studies on the adsorption mechanism of HMIs by biomasses and achieved some significant findings [23,43].It is generally believed that biosorption of HMIs is dominated by physical and/or chemical mechanisms, mainly involving physical adsorption, ion exchange, electrostatic interaction, surface complexation and inorganic microprecipitation [44][45][46][47].For a physical adsorption process, the interaction between biomasses and HMIs occurs through van der Waals forces.Large surface area and abundant pore structures facilitate the contact of HMIs with the adsorption sites on the surface of biomasses.Chemical mechanisms generally include ion exchange, electrostatic interaction, surface complexation and inorganic microprecipitation.The basic modes for chemisorption of HMIs by biomasses are summarised as followed.(i) Ion exchange: Ion exchange is a process in which some cations bound on the biomass surface are replaced by HMIs.(ii) Electrostatic interaction: The functional groups with negative charges on the surface of biomasses interact with the positive-charged HMIs via electrostatic attractions.(iii) Surface complexation: The biomass surface contains various functional groups such as hydroxyl, carboxyl, amino, sulphhydryl, phosphoryl and sulphate ester groups [26].The N, O, P, S atoms in the above groups provide lone pair electrons which can bind with HMIs to form complexes or chelates on the biomass surface.(iv) Inorganic microprecipitation: HMIs precipitate on the biomass surface as phosphates, sulphates, carbonates or hydroxides.Although the diversity of biomasses and their complex structures result in various removal mechanisms for HMIs, a complete and detailed theoretical system has not yet formed.Increasing experimental results indicate that in a typical adsorption process the mechanisms mentioned above can act alone or together at the same time [48][49][50][51], depending on the constrained adsorption conditions, on-site environment factors as well as biomass types.

Factors affecting biosorption of HMIs
The adsorption process of HMIs onto biomasses is closely related to the physicochemical properties of biosorbents and HMIs as well as external adsorption conditions.The intrinsic properties of biomasses mainly refer to their chemical composition, structure, porosity, specific surface area, types and quantities of functional groups on their surface.The nature of HMIs involves ion types and their occurrence in aqueous solutions.The external adsorption conditions or parameters include types and initial concentration of HMIs, types and dosage of biomasses, contact time, pH, solution temperature and co-existing metal ions [23,52,53].

Types and initial concentration of heavy metal ions
The removal ratio of HMIs by biomasses varies significantly with different initial concentrations of HMIs.The fixed biomass dosage determines the definite amounts of the adsorbed HMIs.If the initial concentration of HMIs is lower than the optimal metal ion concentration for the constant dosage of biomasses, high removal ratio of HMIs by biomasses will be obtained; otherwise, a low removal ratio of HMIs would occur.
Therefore, optimisation of biomass dosage for a specific initial concentration of HMIs is conducive to achieve preferable adsorption effects at a relatively low cost.Adsorption performance of biomasses is also influenced by different types of HMIs with the same valence state.HMIs characterised by long effective ionic radius, short hydrated ion radius, high electronegativity and low hydration energy are susceptible to biosorption.

Types and dose of biomasses
Different types of biomasses have exhibited certain adsorption capacities for HMIs as shown in Tables S1-S5.The influence of biomass types on HMIs removal is closely associated with the intrinsic properties of biomasses.As a critical factor for the adsorption process, the dosage of biomasses significantly affects the adsorption balance between the adsorbent and adsorbate in the studied HMIs system.The number of adsorption sites and the adsorbent surface area increase with the increasing amounts of biomasses, resulting in a rise in the quantity of HMIs adsorbed within a certain range.For a given concentration of HMIs, the excessive increase of biomasses augments the number of adsorption sites for HMIs but actually reduces the quantity of HMIs adsorbed per unit mass of biomasses.The reason is that the quantities of heavy metals per unit volume in the constrained aqueous solution remains unchanged before adsorption.When the dosage of adsorbents exceeds a certain level, excessive biomasses will overlap or aggregate with each other, causing poor utilisation of biomasses.Generally, optimisation of the biomass dosage is indispensable for cost reduction of biosorption in the remediation of heavy metal pollution effluents.

Contact time
Relationship between heavy metal adsorption capacity of biomasses and contact time is generally divided into three stages.The first stage is fast adsorption stage, characterised by the largest concentration difference between the bulk and the biomass surface as well as the strongest mass transfer driving force.In this stage, the rapid diffusion of HMIs to the adsorption sites results in swift growth of the adsorption capacity of biomasses for HMIs.The second stage is slow adsorption stage, in which the mass transfer driving force between the bulk solution and the biomass surface gradually decreases as the adsorption proceeds.The corresponding adsorption rate gradually decreases.The third stage is the dynamic equilibrium stage of adsorption [54].For effective removal of HMIs, biosorption generally requires 2 to 4 hours or more time to reach the adsorption equilibrium [53].

Temperature
When internal diffusion and/or external diffusion is the rate controlling step of the whole biosorption process, increase of temperature within a certain range reduces the mass transfer resistance in the transport of metal ions to the biomass surface.In this case, the adsorption efficiency of HMIs onto biomasses gradually increases with the increasing temperature until the optimal operating temperature is reached.When the system temperature exceeds the optimum point, further increase of temperature will lead to a decrease in HMIs adsorption efficiency.This is because the desorption rate of HMIs from the biomass surface exceeds the corresponding adsorption rate at temperatures above the optimum.When the interaction between HMIs and the adsorption sites on the biomass surface is the rate controlling step of the whole adsorption process, effects of solution temperature depend mainly on whether adsorption of HMIs by biomasses is an exothermic or endothermic process.According to Le Chatelier's Principle, augmentation of temperature favours the endothermic process over the exothermic one.For an adsorption experiment, effects of temperature on biosorption of HMIs are generally inferior to other factors.Nevertheless, deviation from the optimal temperature will weaken the saturation adsorption capacity of biomasses.Especially, elevation of temperature leads to considerable increase of the operation costs.Considering operation conditions and the cost of in-depth treatment of effluents, high-temperature operation is not economically viable for removal of HMIs by biomasses.

pH
For most biosorption processes, pH is an important factor affecting heavy metal removal.The pH of the aqueous solution affects the adsorption performance of biomasses mainly by altering the surface charge distribution of adsorbents as well as the chemical forms of HMIs in solutions.It is found that adsorption of HMIs by biomasses is not effective in low pH range.Under acidic conditions, hydrated hydrogen ions occupy some of the adsorption sites on the biomass surface, preventing HMIs from approaching the biomass surface through electrostatic repulsion.This undoubtedly weakens electrostatic interaction and surface complexation between biomasses and HMIs.With the increase of pH in aqueous solution, deprotonation of functional groups on the biomass surface is enhanced accordingly.In this case the biomass surface is negatively charged after deprotonation, effectively improving chemical adsorption of HMIs by biomasses.However, when pH exceeds the upper limit for microprecipitation of HMIs in solutions, the targeted HMIs will occur in the form of insoluble metal oxide/hydroxide particles which weaken the above biosorption.Most related studies have substantiated that the adsorption capacity of biomasses for HMIs increases with the increasing pH within a certain range.Nevertheless, there is not a simple linear relationship between heavy metal adsorption and pH values.An optimal pH range for removal of HMIs by biomasses can be determined by orthogonal experiments and on-site testing [54].

Coexisting metal ions
Multiple metal ions routinely co-exist in the industrial wastewaters, but most of previous studies are focused on biosorption of single heavy metal ion.Simultaneous adsorption of multiple metal ions is not profoundly investigated.Compared with biosorption of single heavy metal ion, mechanisms for removal of multi-component metal ions by biomasses is predictably more complex.When other metal ions are present in aqueous solution, in most cases they compete with the targeted heavy metal ion for adsorption sites and consequently inhibit the uptake of targeted metal ions by biomasses [55].

Remarks about biosorption process
Notably, the constrained experimental conditions for biosorption of HMIs varied widely in previous investigations.Even for the same type of biomasses and HMIs, the adsorption process can be profoundly altered by the changed adsorption parameters.Therefore, up to date it is infeasible to systematically distinguish different adsorption processes based on various types of biomasses.Generally, the whole adsorption process of HMIs on biomasses is divided into three steps [44,56], namely, (i) External diffusion: HMIs are transported from the bulk to the external surface of biomass particles via the convective diffusion.(ii) Internal diffusion: HMIs enter the micro-pores of the biomass particles from the external surface of adsorbents and subsequently diffuse to their inner surface.(iii) Surface adsorption process: HMIs are adsorbed on the inner surface of biomass particles via physical and/or chemical mechanisms [57,58].Detailed adsorption process can be inferred from adsorption kinetic models [59].Various kinetic models including pseudo-first-order, pseudo-second-order, Elovich, intraparticle diffusion and two-constant models have been employed to test the results of adsorption experiments [23,54,59].It could be concluded from numerous previous studies that pseudo-first-order and pseudo-second-order models are the most commonly utilised to characterise adsorption process [26,44,51,54,60].The pseudo-first-order model is the earliest developed equation that explains reversible equilibrium between adsorbates and adsorbents.This kinetic model assumes that the adsorption process is controlled by diffusion mechanisms.By contrast, the pseudo-second-order kinetic model is based on the assumption that the rate-determining step is a chemisorption involving valence forces through sharing or exchange of electrons between adsorbents and adsorbates.Implications of other kinetic models are available in Wu et al. [59].
It should be noted that although biomasses, especially marine algae, exhibit excellent performance on the removal of HMIs in aqueous solutions [45], most of previous studies are restricted to examining or improving the adsorption capacity of biomasses for single heavy metal ion.Investigation of competitive adsorption among multiple mental ions is still in an immature stage [61].Two-dimensional and/or three-dimensional graphs are available in some literature to illustrate the effects of one or two coexisting metal ions on the adsorption of targeted metal ions by biomasses [44,62], however, there are no appropriate mathematical models to simulate or predict the regular pattern for biosorption of multiple HMIs.Application of response surface methodology and artificial neural network methods in modelling and optimisation of biosorption process has been increasingly reported in recent years [63].Development of multiparametric kinetic models for characterisation of the complex biosorption is still rather challenging in the current stage.Moreover, up to date biosorption of HMIs remains largely in the laboratory stage.Real-life application of biomasses for removal of HMIs from the industrial effluents has seldom been reported.Combining microscopic mechanisms with macroscopic models may be one of the future research directions for removal of HMIs by biomasses.

Conclusions
Biosorption is an effective technology for treatment of diluted HMIs-bearing industrial wastewaters.This review compares HMIs uptake capacities of various biological and nonbiological materials documented in literature, including bacterial biomasses, fungi/yeasts, fresh water algae, marine algae, other biological materials/derivatives and non-biological materials.The comparison result indicates that HMIs uptake capacities of different species or materials vary significantly.In terms of HMIs uptake capacities, macro-algae top the list with outstanding values ranging from 0.23 mmol•g −1 to 3.77 mmol•g −1 .The corresponding average and median values are 0.88 mmol•g −1 and 0.76 mmol•g −1 , respectively.These values substantially demonstrate that marine algae have relatively higher heavy metal uptake capacities than other types of biomasses, synthetic adsorbents or inorganic materials.The present study suggests that macro-algae are a promising source for the development of industrial biosorbents for the remediation of diluted HMIs-bearing effluents.Biosorption of HMIs is dominated by physical and chemical mechanisms, mainly involving physical adsorption, ion exchange, electrostatic interaction, surface complexation and inorganic microprecipitation.In addition to physicochemical properties of biosorbents and HMIs, the adsorption process of HMIs onto biomasses is closely related to external adsorption conditions, such as types and initial concentration of HMIs, types and dosage of biomasses, contact time, pH, solution temperature and co-existing metal ions.Generally, the whole adsorption process of HMIs on biomasses is characterised by external diffusion, internal diffusion and surface adsorption.Pseudo-first-order and pseudo-second -order models are the most commonly utilised to characterise the whole biosorption process.Successful industrial applications of biosorption undoubtedly requires more theoretical and technological innovations and significant financial support.