**3. Biosorption and its classification**

It is a process that utilizes the biological material or components as adsorbents for the eradication of toxic metal and non-metal ions, and small particulates from the wastewater, which come from numerous industries such as ore refining, metallurgic, fertilizer, paper, batteries, etc. The biological materials that are used as adsorbents in the process of biosorption are called as biosorbents. They can be classified as natural, biological, and waste oriented as shown in **Figure 1**. Some of the natural adsorbents that are having the good properties of an adsorbent are clay, zeolite, and siliceous material and they are easily available in the ecosystem [28]. Most of the biological adsorbents emanate from microorganisms which include bacteria, fungi, algae, and yeast [20]. Other than these, some of the biological adsorbents obtained from biological sources such as chitin and chitosan, peat, and

*Emerging Contaminants*

As the production increased, it also increased the percentage of wastes generated from them [5, 6]. This readily available, cheap, and large amount of waste could be utilized for the removal of heavy metals from the effluents as bioadsorbents either in their natural form or in modified form. So the idea of utilization of agro/food wastes for the removal of heavy metals can suppress both the aforementioned problems to a great extent. Some of the industries such as paper, electroplating, metallurgy, textiles, batteries, metal plating, dyes, pesticides, and fertilizers, etc. are the major contributors of heavy metals into the wastewater [7]. These industries directly or indirectly discharge the heavy metals into the environment, especially in developing countries. The discharge of heavy metals like mercury (Hg), iron (Fe), chromium (Cr), lead (Pb), nickel (Ni), cadmium (Cd), cobalt (Co), zinc (Zn), copper (Cu), arsenic (As) etc. from industries is hazardous to humans [8]. These heavy and toxic metals are a serious concern and are threats to environment due to their toxicity, persistence in nature, and bio-accumulation tendency [9]. Several reputed and standard organizations have set the limits for the release of toxic metals into the water streams. But the addition of heavy metals into the aquatic stream at a higher concentration is increasing day by day by the various industrial activities, thus it increases the human health hazards and environmental pollution. Two tragedies namely, Minamata and Itai-Itai (Jintsu River) occurred in Japan because of the contamination of methyl-mercury and cadmium in aquatic streams [3, 10].

In the past several years various conventional techniques have been used for the eradication of toxic metal ions such as ultrafiltration, reverse osmosis, oxidation, ion exchange, chemical precipitation, reduction, and electrodialysis [11]. But these conventional methods are mainly associated with certain limitations, such as they produce large amount of sludge, are less efficient, are sensitive in operating conditions and are of costly disposal [12]. Thus, the use of agro/food wastes material and by-products of fruits and vegetables such peel, pomace and seeds as a bioadsorbents for the eradication of toxic metals is an emerging and potentially alternative method and in the recent years this method has gained much attention. This technique has several advantages over the conventional method such as low cost, high efficiency, produces less sludge, and regenerates biosorbents [13]. Various types of agro/food waste materials such as sugarcane bagasse, wheat bran, rice bran, rice husk, orange peels, coffee beans, hazelnut shells, groundnut shells, wheat husk, waste tea leaves, maize corn cob, apple peels, banana peels, coconut shells, sugar beet pulp, soybean hulls, cotton stalks etc. have been tried by several researchers for the eradication of heavy metals [14–16]. The agro/food waste materials, especially those containing cellulose, show a high biosorption capacity. Their components such as lignin, hemicellulose, starch, lipids, proteins, hydrocarbons and other functional groups expedite metal complexation that helps in the removal of heavy metals [17]. Because of the several advantages that are associated with the agricultural waste as mentioned above it seems to be a viable option for the removal of heavy metals. These readily available, low cost, promising agro/food waste materials can be effectively utilized for the eradication of various toxic metal ions either in their natural form or after

**2. Conventional methods for the eradication of toxic metal ions**

Several conventional methods are readily available for the removal of heavy metal ions present in effluents that come from different sources. These conventional methods can be classified as physical, chemical, and biological [19]. In the past several years, various industries were extensively using physical and chemical methods, but nowadays, they do not prefer these traditional methods due to

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some modifications [18].

**Figure 1.** *Classification of bioadsorbents and their examples.*

biomass. Waste oriented adsorbents include, bagasse, nutshells, sawdust, sugar beet pulp, bamboo and cassava peel, rice husk, orange and banana peels, egg shell, corn stack, etc. to name a few that come from solid waste, agricultural waste, waste from fruits and vegetables peels, and industrial waste [28]. Some of the examples of natural, biological and waste oriented bioadsorbents are indicated in **Figure 1**.
