Keywords

12.1 Introduction

Nowadays, the worldwide attention is toward the threat of heavy metal pollution. It mainly enters the environment from natural sources like volcanic activity, weathering, and deterioration of mineral, rock, etc. as well as anthropique (industrial operation) (Gupta et al. 2001). Mechanization enhanced the living situation; nevertheless, it too influenced the flora and fauna because of discharging large quantity of adulterants into it. Ecologists are bothered with heavy metal availability because their effects are highly toxic, carcinogenic, or mutagenic. Bioaccumulation and consequent biomagnifications of nature at different levels of food chain, making them unavoidable even at very low concentration.

Several pandemics are observed in past history like Minamata because of the presence of methyl mercury poising and contamination of Cadmium found in Injects River in Japan which cause “itai-itai” disease. These are common examples of contamination of heavy metals in aquatic habitat. Heavy metal toxicity realized by it. As claimed by the physiological viewpoint, metals are divided into three main categories: nontoxic and essential metals (Ca and Mg), toxic metals (Hg and Cd), and essential but harmful metals above threshold limit. These essential but harmful type of metals such as cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), nickel (Ni), selenium (Se), zinc (Zn), etc. are essential for typical metabolic functions in life forms. All these elements have their tolerance limit for human consumption. Low levels of these metals can cause deficiency issues and sometimes death in utmost case. Also exceeding its critical level can be detrimental to the organism (Zouboulis et al. 2004; Gupta et al. 2001). Health problems, such as vomiting, skin irritations, stomach cramps, nausea, and anemia, can be caused by the presence of more zinc. In animal metabolism, copper (Cu) played an essential role but brought some significant toxicological concerns, such as cramps, vomiting, convulsions, or even death by immoderate intake of copper (Paulino et al. 2006). Severe problems related to the lung and kidney apart from gastrointestinal distress, pulmonary fibrosis, and skin dermatitis can be caused by exceeding the optimal range of nickel (Borba et al. 2006). Nickel is also known as human carcinogen. Mercury can damage the central nervous system and cause impairment of functions of the lung and kidney, cardiac arrest, and dyspnea because mercury is a neurotoxin. Some researchers proved that cadmium poses severe threat to human health because it is a carcinogen. Kidney dysfunction led by continual exposure of cadmium and extreme range of disclosure will result to death. Lead can cause damage in the central nervous system; it can also harm the human body like the reproductive system, brain functions, kidney, basic cellular processes, and liver (Naseem and Tahir 2001). Cr metal exists in freshwater habitat, where it is found mainly in two forms: chromium (III) and chromium (VI). As a whole, chromium (III) is less poisonous than chromium (VI). Cr (VI) causes severe problem such as expanding the food chain and causing serious health issues varying from lung malignancy to simple skin irritation (Fu and Wang 2011).

12.2 Heavy Metals

The term heavy metal indicates the metal or metalloid has relative density varying from 3.5 to 7 g/cm3. Generally, these elements are considered toxic around lower concentrations. These metals are also required by living organisms in order to maintain their basic metabolic activities. Some of the metals include cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), nickel (Ni), selenium (Se), zinc (Zn), etc. (Fu and Wang 2011). Rapid industrialization has improved one’s standard of living; along with this, direct or indirect discharge of these heavy metals into the atmosphere has also expanded enormously, especially in the developing countries. Every essential element follows a varying optimal range of secure and sufficient human intake concentration. Decrease in this concentration is reported to cause deficiency symptoms and, in exorbitant condition, death. Immoderate level of required elements present in trace quantity can be deleterious to the organism.

12.2.1 Copper (cu)

Copper is an extensively used metal and generally used in electroplating and electrical industries. It plays an important role in biological reaction in the life of human beings (Volesky & Holan 1995). Copper, being the basic element of many enzymes, plays an important role for the process of transferring electrons. Carrier proteins like plastocyanin, azurin, and stellacyanin consist of copper as the core metal (Bashir et al. 2019). Extreme assimilation of copper causes symptoms like vomiting, diarrhea, nausea, and abnormalities of the brain and/or may even lead to death (Bashir et al. 2019). Researches indicate that 30 gm of CuSO4 is potentially lethal in the human liver and can result in Wilson’s disease leading to liver cirrhosis and ultimate demise of the being.

12.2.2 Chromium (Cr)

Depending upon the contrasting toxicity, mobility, and availability of chromium, chromium exhibits two consecutive oxidation states present in nature, namely, Cr (III) and Cr (VI) (Volesky & Holan 1995). Occurring in trace amounts in the biochemical system, chromium is suggested to have been involved as a factor of glucose tolerance in the human body for the action of insulin (Bashir et al. 2019). This chromium is more noxious when its valency is six and moderately noxious when its valency is three, because its oxidation property is strong (Rowbotham et al. 2000).

12.2.3 Lead (Pb)

Studies confirm that on subjection to even small amount of lead may affect several crucial organ parts like nervous system, hematopoietic organs, renal, reproductive, and cardiovascular systems (Bashir et al. 2019). It forms complexes with oxo-groups in enzymes, which affect the procedure in the method of hemoglobin synthesis and porphyrin metabolism. Lead can also induce and imbalance the ratio of pro-oxidant and antioxidant which leads to oxidation of protein, peroxidation of lipid, and nucleic acid thereby, causing the cell liable to apoptosis (Flora et al. 2007). Some of the toxicity symptoms are associated with lead such as encephalopathy, seizures, and mental retardation.

12.2.4 Cadmium (Cd)

As per the US Environmental Protection Agency, cadmium has been regarded as a probable human carcinogen (Fu and Wang 2011). A well-known example of toxicity of cadmium is “Itai-Itai”; it is a disease which took place in Japan (Horiguchi et al. 1996); it was characterized by excruciating pain in bones (Volesky & Holan 1995 ). Its exposure leads to severe risks such as renal dysfunction. Increased levels of exposures may even cause death. Studies show that when excessive amount of Cd is assimilated, it exchanges zinc on some key sites of the enzyme leading to renal damage, anemia, bone marrow problems, and destructive to water biome (Bashir et al. 2019). Various guidelines and protocols have been implemented in order to initialize the endurable 29 restrictions for the parts and level of heavy metal that might be available in the effluent wastewater. For the heavy metal, USEPA (Alhendawi 2013) established the Maximum Contaminant Level (MCL) standards, which are enlisted in (Table 12.1).

Table 12.1 Maximum contaminant level (MCL) of hazardous heavy metals
Full size table

12.3 Biosorption

Biosorption is an anabolically static process accountable for the selective isolation of heavy metal by dead/inactive biomaterials (Hansda et al. 2016). It is a physicochemical process, which involves mechanisms, surface complexation, ion exchange, and precipitation. The “bio” prefix indicates toward the collaboration of a biotic entity, viz., life forms or its components or products carried out from it, as used in the spell like biotechnology and bioprocessing. On integrating the term “bio” to a physicochemical expression like “sorption”, it also indicates toward the participation of a process belonging to the abiotic system (Gadd and Griffiths 1977).

The only difference between biosorption and conventional absorption is the quality of absorbent though in the case of biosorption, it is a matter of biotic origin (Hansda et al. 2016). These materials are special biological matters. Biosorption is the easy and economical operation process for heavy metal removal. Studies indicate toward the modification of the biosorbents in order to enhance their biosorption properties (Wang & Chen 2009). These materials can be physically modified with the help of heat treatments, which may result in better biosorption accommodation of the biomass due to the removing surface impurities and generating more active site via denaturing all walls of protein (Cabuk et al. 2005). Chemical and genetic modifications are also seen. Origin, availability, and cost-effectiveness of biomass affect the selection process of an appropriate biosorbent. It is the most crucial step (Hansda et al. 2016).

12.3.1 Biosorption Mechanism

Biosorption is the concept of removing the certain ions or other biomolecule from liquid solution by using certain biomolecule or biomass (Bashir et al. 2019). This can be exhibited through several mechanisms, i.e., complexion, chemisorptions, adsorption chelation, ion exchange, and physisorption (Sud et al. 2008). The extent of affinity of a biosorbent for diffused categories explains its dissemination among the two phases (Kaur et al. 2012). The untreated and chemically treated biosorbents consist of various functional groups like acetamide, phenolic, carbonyl, alcohols, amino, esters, and thiols which seem to operate simultaneously for the removal of metal ions (Bashir et al. 2019).

Recent studies report that the basic principle for adsorption of metal ions is surface binding; it takes place because of the biomass mechanism of ion exchange (Matheickal et al. 1997). It has been reported that the ruling mechanism for the biosorption of Cu (II) is ion exchange, by the help of Ecklonia radiata, which involves the swapping of calcium ion (Ca2+) and magnesium ion (Mg2+) present in their cell wall. Heavy metal ions’ removal is result of interaction between the metal cations and active groups present on the cell surface and it causes to the development of a complex chelation group on the surface of the cell (Hansda et al. 2016). Certain researchers reported that uptake of Copper (II) by Zooglearamigera and Chlorella vulgar is which takes place by both adsorption and formation of coordinate bonds between metals and amino (− RNH2) and carboxyl (-RCOOH) groups of the cell wall polysaccharides reported by Aksu et al. (1992).

12.3.2 Factors Affecting Biosorption

Several factors can affect biosorption. The physical and enzymatic treatments like boiling, autoclaving, drying, and mechanical disturbance affect the binding properties of the biosorbent. The chemical treatments like alkali treatment often improves the biosorptive capacity, especially seen in some fungi due to the acetylation of chitin to form chitosan-glucan complexes which according to studies have shown higher metal affinities (Gadd and Griffiths 1977). The investigational frameworks such as pH of the solution, initial adsorbate concentration, biosorbent dose, contact time, agitation speed, temperature and pressure of competing ions, and the nature of the biosorbent affect the process of biosorption (Bashir et al. 2019).

12.3.2.1 Effect of pH

pH plays a vital role in the biosorption from aqueous medium of heavy metal ions (Gueu et al. 2007). The pH of the solution can affect the charge of the surface of the biosorbent and the degree of ionization of the functional group associated with it (Dissanayake et al. 2016). Researches state that the competition between cations and protons for binding site occurs which means the biosorption of metals like Cu, Cd, Ni, Co, and Zn is often reduced at low pH value. Metal ion binding can be effected by the absorbent state of charge. Point of Zero Charge (PZC) of absorbent matter provides details about the charge state of the matter (Ou et al. 2015). This PZC is the pH where surface of absorbent is all over neutral. Surface is neutrally charged or negatively charged and it depends on PZC Value (Below the value- neutral and beyond the value- negative) (Bashir et al. 2019).

12.3.2.2 Effect of Temperature

Over usual ranges, temperature marks a tiny effect on biosorption. High temperature like 50 °C, may increase the rate of biosorption in some cases (Tsezos 1999). However, low temperature will affect the living cell systems and any auxiliary metabolism-dependent process that supports biosorption (Gadd and Griffiths 1977).

12.3.2.3 Characteristics of Biomass

The key factor for the biosorption process is to determine the nature of the biomass. It is evident in some fungal system that chemical treatments like alkali treatment often improves biosorption capacity whereas, physical treatments like autoclaving and drying disrupt its binding properties (Volesky & Holan 1995 ).

12.3.2.4 Biomass Concentration

Concentration of the biomass is considered as one of the most important factors. It is observed that at provided concentration of equilibrium, biomass absorbs more metal ions at lower cell densities. Restriction to metal ion-binding site is seen when there is a higher concentration of biomass.

12.4 Biosorbents

Heavy metals biosorption of aqueous solution is a relatively new operation affirmed an auspicious measure in the expulsion of heavy metal pollutants. The considerable benefits are their high impact in decreasing the heavy metal particles/ions and utilizing modest/inexpensive biosorbents. Biosorption measures especially appropriate to treat adulterated heavy metal sewerage. There are basically three sources of achieving normal biosorption (Apiratikul & Pavasant 2008): (a) from nonliving biomass such as bark, lignin, shrimp, krill, squid, crab shell, and so forth; (b) from algal biomass; and (c) from microbial biomass, for example bacteria, fungi, and yeast. By some researcher, algal biomass is also considered as microbial biomass.

Algae, a viable natural biomass multiply universally furthermore, bounteously in the intertidal zones of world have pulled in consideration of numerous examiners as organism to be tried and utilized as new adsorbents. A few assets of choosing algae as biosorbent incorporate the extensive accessibility, minimal expense, more metal sorption limit, and quality standard (Apiratikul & Pavasant 2008). An enormous number of analysis deals with the biosorption utilizing algal biomass. Instances of ongoing reports incorporate that utilizing dried marine green microalgae Chaetomorpha linum for biosorption of copper ion (Cu2+) and zinc ion (Zn2+) (Ajjabi & Chouba 2009), utilizing green algae Ulvalactuca for the biosorption of chromium from wastewater (El-Sikaily et al. 2007). Ajjabi and Chouba (2009) examined the biosorption of copper ion (Cu2+) and zinc ion (Zn2+) by dried marine green macroalgae (C. linum). At the ideal, molecule size should be 100–315 mm, biosorbent measurements/dosages (20 g/L), and introductory solution pH 5, the dried algae delivered greatest copper ion (Cu2+) and zinc ion (Zn2+).

12.4.1 Surface Modification and Development of Bioabsorbents

Treating the sewerage sullied with heavy metals in a powerful manner, it is needed to utilize adsorbents having high limit of metal adsorption. In majority of the cases, the successfulness of adsorbents is not just about as high as needed because of their low to medium adsorption limits. Lately, accentuation is provided based on adsorbent surface modification by different procedures to increment adsorption limit. From few decades, many surface modification procedures that have been broadly utilized by investigators include chemical and physical methods. Physical and chemical modifications include acid/base treatment, heat treatment, microwave, and so on (Srivastava et al. 2015).

12.5 Heavy Metal Adsorption Using Microbial Biomass

Adsorption is widely used technique for the removal of heavy metals. Microbes have high efficiency of removing heavy metal and huge surface area-volume ratio. A large number of microbes have been tested for the removal of heavy metal. A large variety of biological materials exploited for their metal desorption capacity.

Absorbents are characterized in following categories-

  1. (a)

    SEM (Scanning electron microscope)

  2. (b)

    TEM (Transmission electron microscope)

  3. (c)

    ESR (Electron spin resonance spectroscopy)

  4. (d)

    NMR (Nuclear magnetic resonance)

  5. (e)

    FTIS (Fourier-transformed infrared spectroscopy)

  6. (f)

    EDS (Energy dispersive x-ray spectroscopy)

  7. (g)

    X-ray diffraction (XRD) analysis

  8. (h)

    X-ray photo-electron spectroscopy (XPS)

  9. (i)

    X-ray adsorption spectroscopy (XAS)

  10. (j)

    Thermogravimetric analysis (TGA)

  11. (k)

    Differential scanning calorimetric (DSC)

All above absorbents have their specific outcomes based on their characteristics.

Microbial biomass binds heavy metal either by active process or by passive process or by the combination of both.

The active process known as bioaccumulation “Bioaccumulation is accumulation of substances such as pesticides, or other chemicals in an organism. It occurs when the rate of absorbance of substance is faster than the substance lost by catabolism and excretion”. Passive process is known as biosorption. “Biosorption is a physiochemical phenomenon that naturally occurs in several biomasses that allows concentrating passively and binding the contaminant to its cellular structure”.

Another major advantage of bioaccumulation is that the recovery of accumulated metals accomplishes by simple physical method without any damage to the biosorbent’s structural integrity. While, bioaccumulation recovered the accumulated metals by destructive means and damaged the structural integrity (Kuyuca & Volesky). Biosorption is easy and cost-effective achievement of a large-scale fermentation process or bulk gain from natural water bodies (Volesky B). Being a surface phenomenon, most of biosorption generally completed within few minutes of contact with biomass.

12.5.1 Adsorption by Bacterial Biomass

The most abundant and adaptive microorganisms are bacteria. In the living world, bacteria play a crucial role. Approximately 108 g of whole biomass of living world constitute of bacteria. Bacteria are unicellular, are capable of growing under wide range of environment, due to their small size, and are in controlled condition as they are widely used as biosorbent for wastewater treatment. Mostly, all industries belong to food processing and fermentation process releases their by-product in form of bacterial biomass. The bacterial by-products produced by them are employed in bioremediation process and few bacterial species such as Bacillus cherichia, Micrococcus, Pseudomonas, and Streptomyces mainly used for heavy metal remediation.

Bacteria that produce enzyme urease have been biomineralized and characterized by Li et al. 2012. This enzyme is produced by hydrolyzing the bacterial strains, urea resulting in the increased pH of the soil, and production of carbonate, leading to mineralization of heavy metal ion and ultimately conversion to carbonate. After incubating the bacterial strain for 48 hours, the removal efficiency ranged from 88 to 99%. The biosorption ability varied with metals and depends on pH and metal concentration. Biosorption of each metal was rapid and could benefit for treatment of contaminated sites in large scale (Table 12.2).

Table 12.2 Bacterial strains used for removal of heavy metals
Full size table

12.5.2 Adsorption with the Help of Algal Biomass

Algae are found abundantly in freshwater bodies such as seas and oceans. It is large and diverse group of eukaryotic organisms that range from unicellular genera like chlorella to other multicellular genera. Among all types of algae, brown algae gain more attention because it has better sorption capacity and grow up to 50 m in length. Algae have excellent photosynthetic efficiency, fast reproduction cycles, and limited requirement of nutrition. The rate at which the algal cells use up the specified nutrition depends on difference between the concentration inside and outside the cell and the diffusion rate through the cell wall.

Gupta and Rastogi (2008), utilized spirogyra sp. for biosorption of Pb2+. It was observed that Pb2+ was maximum absorbed 140 mg/g at pH 5.0 with the contact time of 100 minute and with 200 mg/l of metal ion concentration, which is initial. Biosorption is endothermic in nature as temperature increased from 20 °C to 40 °C.

12.5.3 Adsorption by Fungal Biomass

The discharged wastewater from chemical industries may contain heavy metal ions that have a toxic effect influence in the living world. Disposing them to the environment is hazardous to both human and ecosystem. New technologies are necessary to being developed to treat wastewater as an alternative to physiochemical process.

Fermentation industries are used to produce metabolites like steroids, antibiotics, enzymes, and chemicals. Fungi have been recognized and trusted as cheap adsorbents for wastewater for removal of heavy metal ion. Fungi’s cell walls are made up of chitin and chitosan, in various reactor systems for heavy metal ion elimination and describe the biosorption process of fungus. To investigate the biosorption mechanism, in order to understand the biosorption process, it is vital to identify the functional groups involved. Heavy metal may be absorbed from aqueous solutions by common filamentous fungus; the heavy metal sorption, Copper, Zinc, Cadmium, Lead, Iron, Nickel, Silver, Thorium, Radium, and Uranium, by fungal biomass. It has been observed to varying extent. The concentration of biomass and metal ions, the presence of ligands in a solution, temperature, and pH are all important factors in fungal biosorption.

The biosorption of metal ions is caused by the development of complexes and ionic interactions between heavy metal ions and the presence of functional groups on the surface of fungal cells (Kapoor & Viraraghavan 1997). In fungal biosorption, the main functional groups involve amine, amide, carboxyl, and phosphate (Akthar et al., 1996). By using Phanerochaete chrysosporium (white rot fungus) from wastewater, Singh et al. (2006) researched on Copper ion (Cu2+), Cadmium ion (Cd2+), and Lead ion (Pb2+). Biosorption equilibrium and biosorption capacity were achieved that are 45.25, 13.24, and 10.72 mg/g for Lead ion (Pb2+), Cadmium ion (Cd2+), and Copper ion (Cu2+), respectively with the contact time of 6 hours at pH 6. Loofah sponge immobilized the P. Chrysosporium, and was utilized for the removal of Copper ion (Cu2+), Zinc ion (Zn2+), and Lead ion (Pb2+) from wastewater (Iqbal & Edyvean 2004). Maximal biosorption of Lead ion (Pb2+), Copper ion (Cu2+), and Zinc ion (Zn2+) was found to be 135.3 mg/g, 102.8 mg/g, and 50.9 mg/g, respectively. For maximum biosorption, optimal pH was 6 and contact time of 60 minutes, respectively. Langmuir isotherm was best fitted isotherm for all the three metal ions (Sing & Yu 1998).

12.5.4 Adsorption by Endophytes

The endophytes are those microorganisms, which live in the living plant tissue. It acts as a biocontrol agent by protecting plant from herbivore (consumption of plant by animals as herbivores adapted to eat plants) by producing compounds which prevent animals from overgrazing on the same plant. Due to peculiar growing environment of endophytes, it experiences high concentration of thrash metals and might have a distinct type of cell wall, which contains special functional group and is required to be an optimistic biosorbents (Xiao et al. 2010; Guo et al. 2010).

El-Gendy (2008) carried out the investigation using ten endophytic fungi isolated from plants grown in industrial regions and these exhibited the potential in treatment of heavy metal from aqueous solution. Those endophytic fungi belong to the eight different genera. Degradation and accumulation of heavy metal were performed by these fungal strains. Among all endophytic fungi (P. declauxi, V.fungicola, A. luchuensis, A. tubingensis, C. lunata, P. lilacinum, D. hawaiiensis, M. elegans, R. oryzae, and P. clavispora), the highest removal of Cu2+ (85.4%) was shown by P. lilacinum with smart removal for Cd2+ (31.43%) (Table 12.3).

Table 12.3 Heavy metals are removed with the help of endophytes
Full size table

12.6 Biosorption Selection Biosorbents

The most effective method to choose the biosorbent, which is appropriate among a huge amount of biomaterial, tried. The choice of an appropriate sorbent for guaranteed partition/portion is an intricate issue. The dominating logical/scientific premise for sorbent choice is the equilibrium isotherm. Dispersion rate is optional in significance. From the perspective of useful application, accessibility and cost-effective are main considerations to be taken into represent choosing the biomaterial for removal purposes (Vieira & Volesky 2000). Right evaluation of the metal-binding limit of a few kinds of biomass is likewise vital. Step by step instructions to assess the sorption execution for a certain biosorbent? Systematically instructions to assess the experimental outcomes announced/researchers reported it from various foundations/background? Volesky and his partners/colleague discuss the connected/related queries (Kratochvil & Volesky 1998; Volesky & Holan 1995).

For solid-liquid sorption system, two kinds of examinations could help to examine: (a) dynamic persistent stream sorption contemplates and (b) equilibrium batch sorption tests (Chen & Wang 2008).

For single solute system, two most acknowledged equilibrium adsorption isotherm models are Langmuir model and the Freundlich model. Qmax, Ce, and b are respectively the most extreme sorption limits concerning complete monolayer coverage (in mmol g − 1), the equilibrium solute concentration (in mmol L−1), and the constant identified with the energy of sorption (or “proclivity”) (in L mmol−1), the Freundlich constants identified with the adsorption limit and intensity of the biosorbent are KF and n, respectively. Biosorbents’ performances might be compared, by utilizing these boundaries from the models (Chen & Wang 2008).

Traditional sorption isotherm found from equilibrium batch contact tests under similar ecological conditions (for example temperature, ionic strength, pH) helps in assessment of sorption system. Identical equilibrium (last, lingering/remaining) concentration must be made at a quantifiable comparison of two unique sorption system. Comparisons at high Cf (e.g., 200 mg L− 1) and low Cf (e.g., 10 mg L−1) are made in some biosorption screens during this way, as an example, appeared in Kratochvil and Volesky (1998). Equilibrium (last) metal concentration something similar (chose: for instance, 10 or 200 mg L−1) decided biosorption performance as far as metal take-up limit. Correlation of qmax is likewise helpful. qmax and b are two effectively or easily interpretable constants of Langmuir isotherm model. Small values of b are reflected within the lofty starting slant/sloop of a sorption isotherm, indicating a beautiful high fondness. In order to find out the efficient adsorbent for adsorption of heavy metals ions it is required to find out high qmax and a steep beginning sorption isotherm slope (for example, low b) (Kratochvil & Volesky 1998).

12.7 Biosorption Models: Kinetics and Isotherms

12.7.1 Biosorption Isotherms

The isotherms of biosorption explain the relation between the concentration of biosorbent and the quantity of it absorbed by the unit mass of the biosorbent by taking constant temperature at equilibrium. This is essential for designing the biosorption systems. The isotherm models give data and facts about the removal capacity of biosorbents. These isotherm models are essential for determining the biosorption parameters and to compare the different biosorbents using different operating conditions.

12.7.1.1 Single Component Isotherm Models

These models give short expression and monocomponent adsorption of ionic metals. These are simple mathematical relationships used to describe different experimental behavior over a large number of experiments under various circumstances. The models are used for substantiating the outcomes that are able to predict metal binding at low as well as high concentrations (Rangabhashiyam et al. 2014). Some important models are as follows:

12.7.1.1.1 Langmuir Model

In 1918, Irving Langmuir gave the Langmuir model. It assumes that under isothermal condition, an adsorbate shows the property of ideal gas. This model is valid for single layer adsorption. The basis of this model is a continuous monolayer of adsorbate molecules occupying a solid homogeneous surface (Langmuir 1918). In this model, adsorption energy is constant.

The Langmuir isotherm model works on certain assumptions.

These are:

  1. 1.

    The surface of adsorbent is uniform, i.e., uniformity of the surface.

  2. 2.

    No interactions between the adsorbed molecules.

  3. 3.

    Molecules get adsorbed at defined sorption sites.

The Langmuir isotherm equation is given as:

$$ {q}_e=\frac{Q_mb{C}_e}{1+b{C}_e} $$
(12.1)

This equation is expressed in linear form as:

$$ \frac{C_e}{q_e}=\frac{1}{bQ_m}+\frac{C_e}{Q_m} $$
(12.2)

Here, qe is the sorbate amount that adsorbs the sorbent per unit mass at equilibrium, Qm is the maximum consumption of sorbent per unit mass of sorbent, Ce is sorbate concentration in solution, and b is Langmuir constant.

12.7.1.1.2 Freundlich Model

Freundlich gave this model and this model describes the process of adsorption by the following equation (Freundlich 1906; Freundlich & Heller 1939).

$$ {q}_e=K{C}_e^{\frac{1}{n}} $$
(12.3)

Here, K and n are Freundlich constants, K is related to capacity of adsorption.

Equation of linear form is given as:

$$ \ln {q}_e=\ln K+\frac{1}{n}\ln {C}_e $$
(12.4)
12.7.1.1.3 Temkin Model

The adsorption process is examined by Said et al. (2018).

It is given by the equation:

$$ {q}_e=\frac{RT}{b}\ln \left({K}_T{C}_e\right) $$
(12.5)

Here, qe is the amount of sorbate adsorbed per unit mass of sorbent at equilibrium, b is Temkin constant, KT is Temkin constant related to binding, and Ce is the concentration of sorbate in solution.

The linear form of equation is given as:

$$ {q}_e={B}_1\mathit{\ln}{K}_t+{B}_1\mathit{\ln}{C}_e $$
(12.6)

Here, \( {B}_1=\frac{RT}{b} \)

R is the universal gas constant (8.314 KJ/mol.K), T is the absolute temperature in (K).

12.7.1.1.4 Toth Model

This model is used in a heterogeneous system. It is given by the equation (Said et al. 2018)

$$ {q}_e=\frac{Q_mb{C}_e}{\Big(1+{{\left(b{C}_e\right)}^{t\Big)}}^{\frac{1}{t}}} $$
(12.7)
12.7.1.1.5 Redlich –Peterson Model

Heterogeneous systems are well described by Toth model. This model proves to be effective when pollutants are present at high concentration (Benzaoui et al. 2018).

This model is given by the equation:

$$ {q}_e=\frac{A_R{C}_e}{1+{B}_R{C}_e^{m_R}} $$
(12.8)

Here, AR, BR,and mR are the model parameters, qe is the sorbate amount adsorbed per unit mass of sorbent at equilibrium, Qm is the maximum sorbate uptake per unit mass of sorbent, and Ceis the sorbate concentration in solution.

12.7.2 Kinetic Models

Adsorption kinetics in wastewater treatment is of great significance. It helps us to know about the reaction pathways and mechanisms of adsorption. The kinetic models describe the uptake of solute which in turn controls the time of residence adsorbate at the interface. The residence time is the total time a particular amount of material spends in the reservoir. Information about the kinetics of metal uptake helps us provide the best-suited condition for batch metal removal process (Rangabhashiyam et al. 2014). Some of the models given to outline the adsorption kinetic process are as follows:

12.7.2.1 Pseudo First-Order Kinetic Model

The equation of Lagergren rate is one of the most popular rate sorption equations used to describe the process of adsorption (Lagergren 1898; Langmuir 1918). It is expressed as

$$ \frac{d{q}_t}{dt}={K}_1\left({q}_e-{q}_t\right) $$
(12.9)

Its linear form is expressed as

$$ \ln \left({q}_e-{q}_t\right)=\mathit{\ln}{q}_e-{K}_1t $$
(12.10)

12.7.2.2 Pseudo Second-Order Kinetic Model

It is based on certain assumptions (Ho & McKay 1999):

  1. 1.

    Adsorption monolayer is considered.

  2. 2.

    The energy of adsorption is same for each adsorbent.

  3. 3.

    Adsorption occurs on defined sites.

  4. 4.

    There is no interaction between adsorbed pollutants.

This model is expressed as:

$$ \frac{d{q}_t}{dt}={K}_s\left({q}_{eq}-{q}_t\right) $$
(12.11)

Here, Ks is the adsorption rate constant.

qeq is the pollutant amount adsorbed on the surface of adsorbent.

The equation of linear form is given as

$$ \frac{t}{q_t}=\frac{1}{K_s{q}_{eq}^2}+\frac{t}{q_{eq}} $$
(12.12)

12.7.2.3 Weber and Morris Intra Particle Diffusion Model

It is given by the equation (Weber & Morris 1963):

$$ {q}_t={k}_{id}{t}^{\frac{1}{2}}+C $$
(12.13)

Here, qt is the adsorbed amount at time t, kid is the rate constant, C is the intercept value. This brings idea of the thickness of boundary layer, i.e., the greater the boundary, the larger the effect.

12.8 Conclusion and Future Perspective

Biosorption is facing a major challenge; the failure advertisement is mainly because of the nonspecific peril; it involves the technical alterations, proposed by several investigators. For innovation, solid capitalization is required. The application of biosorbent has not been exploited well at industrial scale; this constitutes the weaknesses that must be faced by biosorbent. All the studies about biosorption until now are enough to provide a base that allows it to be extended. However, this is not widely used process in industry. It is difficult to determine the fact, because at present, few studies are there in which biosorbent is compared with commercial sorbent under similar condition. The use of culturing microbes is a beneficial alternative for removal of metal contaminant as pure biosorptive from commercial effluent. Bacterial, fungal, and algal strains are main types of microorganism that is able to remove organic matter from wastewater. The survey data reveal the biosorption investigations that are quite limited, with only a few types of bacterial, fungal, and algal biomass. Several aspects that affect the biosorption capacity of wastewater are pH, temperature, biomass concentration, ionic strength biological waste, and more components like metal ions, etc. Other parameters that have yet to be examined, like stirring rate and particle size, will require more research.