Biosorption and bioaccumulation are environmentally friendly solutions with benefits compared to traditional methods. Owing to their metal‐binding functional groups, abundant natural materials have been considered as viable biosorbents for eliminating heavy metals: for example, microbial biomass, agro waste, and industrial byproducts. Parameters such as pH, temperature, metal ion concentration, biosorbent dose, and agitation rate influence biosorption. After the removal of heavy metals, biosorbents can be regenerated and reused, thus making the process economical (Kanamarlapudi et al., 2018). This chapter provides a summary of some low‐cost biosorbents discussed in recent publications.
Biosorption and Its Mechanism
Biosorption is a simple physical and passive mechanism involving the attachment of biosorbates (metal ions) to the biosorbent surface (of biological origin) (Mrvčić et al., 2012). Simple operation, no demand for supplementary nutrients, low sludge production, lower operational cost, good performance, biosorbent rejuvenation, and no increase in chemical oxygen demand (COD) in water are all advantages of this technology that are major drawbacks of older technologies. Pollutants with parts per billion (ppb) toxicity can be eliminated by biosorption even at diluted concentrations. This process is especially important for removing heavy metals.
The first step in biosorption is to suspend the biosorbent in the biosorbate‐containing solution (metal ions). Equilibrium is achieved after incubation for a given period. The metal‐enriched biomass is separated at this point (Chojnacka, 2010). Biosorption is a dynamic mechanism in which sorbate binds to a biosorbent. Physical attachment of metal ions (Van der Waals interaction or electrostatic forces) or chemical binding (replacement of ions), chelation, precipitation, reduction, and complexation are all possible with a wide range of natural materials as biosorbents. The significant factors that control the biosorption process are (Park et al., 2010) as follows:
Nature of the biosorbent
Category of the biological ligand
Optimum parameters of sorbate and sorbent (temperature, pH, concentration)
Accessibility of binding sites
Biosorbents
Biosorbents are biological elements such as bacteria, agricultural wastes, and municipal byproducts employed in the passive removal of toxins from a system. They have been encouraged since they are made from renewable resources, they are cheap and biologically degradable, and they do not produce toxic compounds after being used. Since they are biological material, they include several functional groups that act as the driving force for hydrophobic interactions, which are mostly pH‐dependent during the sorption process. Biosorbent contains a range of functional sites, including carboxyl, imidazole, sulfuryl, amino, phosphate, sulfate, thioether, phenol, carbonyl, amide, and hydroxylate, in contrast with mono‐functional ion‐exchange resins. A large number of natural materials have been suggested as promising biological agents for removing pollutants from water. The sources can be inactive or dead microbial biomass, as well as living microorganisms. It is well understood how some biosorbents function, whereas others need more research. The toxicity of some of these biomaterials also requires in‐depth analysis (Adewuyi, 2020).
Biosorbents, unlike mono‐functional ion‐exchange resins, possess functional groups that can bind and sequester metal ions. The functional groups may be amide, carbonyl, imidazole, sulfhydryl, thioether, amine, sulfonate, carboxyl, imine, and phosphodiester. The ability of a biosorbent to bind metal ions is characterized as its biosorption capability and defined as the number of metal ions biosorbed per unit weight of the biosorbent. This can be stated by using the mass balance equation as shown here:
(2.1)
The metal's biosorption effectiveness (R %) is calculated using the following equation
(2.2)
where qe = total metal ions adsorbed by the adsorbent (mg/g); Ci = original metal ion concentration in the solution (mg/L); Ce = metal ion concentration in the solution (mg/L); V = medium volume (L); and m = quantity of biomass used during the process of adsorption (g) (Kanamarlapudi et al., 2018).
Types of Biosorbents
A wide range of materials available in nature can be used to remove metals from contaminated water resources as biosorbents. Biosorbents include microbial biomass (live and dead), plant and animal‐derived materials, industrial and agriculture byproducts, biopolymers, and so on. Biosorbents are a less expensive and more efficient way to remove metallic elements from aqueous solutions, especially heavy metals. Choosing the most effective biosorbent type from a wide range of promising and cheap biomaterials is a significant challenge. Biosorbents with the ability to bind metal ions with larger affinities should be developed. The ideal characteristics of biosorbent are (Macek and Mackova, 2011):
Biosorption ability
Economical
Ease of availability
Effortless removal of adsorbed metal ions and promising reuse of the biosorbents.
In the case of biosorbents, proteins, several polyphenols (e.g. chitosan and pectic acid), and polyphenols (e.g. tannins, catechins) are chemical ingredients that bind metal ions efficiently. By applying some easy chemical treatments to biosorbents with poor adsorption properties, the biosorbents can be transformed into adsorbents with outstanding adsorption qualities (Inoue et al., 2017).
Microbial Biomass as Biosorbents
Microorganisms that can tolerate harsh conditions have been used as biosorbents to remove metal ions from wastewater. Most of the microbial groups consist of functional groups that exhibit their ability as biosorbents. A significant number of materials with microbial originshave been studied extensively as biosorbents to remove metal ions. Based on their good performance, low cost, and large available quantities, microorganisms (bacteria, algae, yeasts, and fungi) are receiving increased attention for heavy metal retrieval. Experiments concentrating on the usage of dead and living microorganisms have been carried out to provide alternativetypes of remediation (Hlihor et al., 2014).
When the biosorption process first began to use microorganisms, researchers revealed that inactive/dead microbial biomass could passively bind metal ions. Dead biomass offers various advantages over live microbial biomass: cost‐effectiveness, toxicity limitation, ease of regeneration, displaying an exchange of ions, and a wide range of pH and temperatures (Adewuyi, 2020). Employing dead microbial biomass for metal ion binding is preferred over using alive biomass due to the lack of nutrient requirements and the management of biochemical oxygen demand (BOD) and COD in effluents. Therefore, the use of dead biomass is cost‐effective (Rezaei, 2016).
It became obvious that microbial biomass biosorption is based not only on the microbial biomass's chemical composition but also on external physicochemical factors and the chemical matrix. These biosorbents are capable of effectively retrieving metal ions from solution and efficiently decreasing parts per million (ppm) to ppb levels. As a result, they are thought to be good candidates for treating high‐volume, low‐metal‐ion‐concentration complicated wastewaters (Chen and Wang, 2006).
Bacterial Biomass
Of the microorganisms, bacteria are the world's most abundant, versatile, and diverse creatures. Bacteria can be classified as gram‐negative or gram‐positive based on the cell wall and gram staining. In biosorption, the cell surface structure plays