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Barley straw Copper 80% (Pehlivan et al., 2012) Coconut shell fibers Chromium 80% (Mohan et al., 2006)

Animal waste products are dumped as solid waste in the environment without being processed or composted or simply washed into water canals, posing a health risk to humans and other living species. The chance to use animal waste for a useful purpose is lost if they are discarded without preparation. Some research has looked into using these animal waste products as a viable adsorbent to adsorb heavy metals from effluent because they are inexpensive and conveniently accessible. Heavy metal adsorption has been researched using animal manure as an adsorbent. Animal bones, pretreated fish bones, crab shells, pretreated arca shells, pretreated crab and arca shells, eggshells, Muscadomestica, and so on are a few other examples (Srivastava et al., 2016). Table 2.6 summarizes the results of the adsorption analysis performed on animal byproducts.

Table 2.6 Biosorption of heavy metals by different animal wastes.

Animal waste	Metal	Adsorption capacity	Reference
Pretreated fish bones	Copper	150.7 mg/g.	(Kizilkaya et al., 2010)
Dried animal bones	Zinc	0.1764 mmol/g.	(Banat et al., 2002)
Crab shell	Cobalt	322.6 mg/g	(Vijayaraghavan et al., 2006)
Pretreated arca shell biomass	Lead	18.33 mg/g	(Dahiya et al., 2008)
Animal bone	Nickel	7.22 mg/g	(Al‐Asheh et al., 1999)

These adsorbents could offer significant advantages compared to currently available economically priced activated carbons and help with waste reduction. Furthermore, research is required to adapt the simulation methodology to larger manufacturing facilities rather than small experimental applications.

Biocomposites

      Biocomposites consist of composite materials up of multiple ingredients that are mixed to make a new product that outperforms the individual constituent materials. They constitute biomass‐based products that are biodegradable, high‐performing, and environmentally friendly and can be utilized for wastewater treatment. Biopolymers such as cellulose, chitosan, starch, chitin, alginate, and others continue to be the most important part of biocomposites. Biopolymers’ advantages include their non‐toxicity, availability, economics, and environmentally friendly nature (Zhang et al., 2013).

Alteration of Biosorbents

      The amount and accessibility of binding sites on the surface of an adsorbent determine the biosorption technique. Usage of biosorbents in their natural state has shown a number of drawbacks due to their poor biosorption potential and unpredictable physical stability. Modifying the surface features of biosorbents can have a huge impact on the biosorbents’ ability to remove metal particles (Gupta et al., 2002). Several researchers concentrated on altering the biomass chemically such that structural stability and effective heavy metal ion biosorption capability can be achieved.

Hydrophobicity, water absorbency, thermal resistance, cation/anion exchange capacity, and ability to resist microbial attack have been improved significantly by modifying the surface of biomass. Physical pretreatment methods such as heating, chilling, drying, lyophilization, and autoclaving are used to alter the cell surface. Several researchers have reported using chemical pretreatments for surface alteration, including washing with mineral and organic acid solutions, detergents, alkaline solutions, and organic compounds (Vieira and Volesky, 2000). These pretreatments assisted in modifying surface binding sites by eliminating or hiding the groups or exposing further sites to bind metals. Biosorptive performance has been boosted by carboxylation, phosphorylation, and amination of amine, carboxyl, and hydroxyl groups, saponification of ester groups, sulfonation, halogenation, oxidation, and so on (Wan Ngah and Hanafiah, 2008). Various biomasses, irrespective of their source, including pine bark, rose petals, spirogyra, walnut shell, rubber leaves, and sawdust, showed promising biosorption capabilities after alkaline pretreatment (Argun et al., 2009).

      Desorption and Regeneration

      The ability of biosorbents to recover after use is one of their most important achievements. Adsorbate is cleared off the biosorbent surface after use, and the biosorbent reverts to its original structure and efficiency (Adewuyi, 2020).Biosorbents’ economic value and sustainability are primarily determined by the number of cycles they can be reused. It is essential to develop an effective desorption method. High performance is not enough for a biosorbent; it also needs to be reusable. As a result, when choosing biosorbents, desorption and regeneration are the important processes to consider. Still, some of them are difficult to regenerate, making their long‐term usage doubtful since they would need to be discarded after a few cycles. Disposing of such materials can result in contamination of the environment. The separation of spent biosorbents and the regeneration and recycling of the medium after the sorption process are very significant.

      Eluent utilization is now the most frequently used desorption mechanism. Choosing the right eluent is essential and depends on the form of biosorbent, adsorbent, and biosorption mechanism. A proper eluent should not harm or modify the biosorbent structure, it should be environmentally friendly and affordable, it should have a high level of adsorbing ability, it should not alter adsorbing or biosorbing substances, and it must be easily disconnected. To dislodge metal ions while concurrently replenishing the filled biosorbent, chemicals like hydrochloric acid and sodium hydroxide and chelators like EDTA have been utilized (Ahemad and Kibret, 2013).

Cost Evaluation

      Approximating the cost of biosorption and biosorbents is a challenging process that is rarely published. Cost assessment depends on many factors that make generalization difficult. Variables such as process handling, storage, energy use, repair, optimization of process, rejuvenation, discharge, and desorption are considered when evaluating cost. The type of water or effluent to be treated, as well as the amount, will affect the cost. Capital costs and operating costs, on the other hand, are determined by the type and size of the treatment plant.

      It is preferred to employ wastes as a substrate for processing biosorbents in order to lower process expenses. Agricultural waste, home trash, and industrial waste, such as bacterial drainage from fermentation, fungal waste from food manufacturing, and waste from other industrial processes, can all be used to make biosorbents. Waste management is a significant environmental issue, and using these wastes addresses the concern while also lowering the cost of manufacturing biosorbents for treating water.

Pretreatment of feedstock is one aspect that increases production costs. Certain products need to be pretreated before manufacturing, and additional pretreatment increases the cost of production. It is important to note that feedstock treatment should be efficient but also inexpensive and accessible. It is critical that

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