**5. Natural adsorbents (e.g., clay, zeolites, biochar, and activated carbon)**

Natural resources are explained in **Figure 2** such as volcanic rocks, soil, plant biomass, industrial and agricultural waste, animal shells, microalgae, and fungal biomass are used to create biosorbents (natural adsorbents). These materials share the ability to physically retain arsenic ions or molecules due to their enormous specific surface area. In comparison to other technologies using synthetic membranes and materials requiring large chemical doses, natural adsorbents have a higher capacity to remove arsenic, are reusable, and are less expensive as well as having less of an impact on the environment. Agricultural wastes have the most functional groups that help keep heavy metals out of all the natural components, including proteins, extractives, hemicellulose, lignin, lipids, starch, and simple sugars [19].

Natural green adsorbents such as citrus peel, peel from bananas, rice husk, bagasse from sugar cane, tea waste, avocado peel, Hami melons peel, dragon fruit peels, as well as other peels have been utilized to recover heavy metal ions & organic dyes from contaminated water. One of the most common applications for fruit peels is water treatment. Using solid waste as green adsorbents, harmful compounds such as heavy metals, dyes, & pesticides can be eliminated from contaminated water.

#### **5.1 Clay**

Clay is a multilayered natural adsorbent that contains the minerals vermiculite, smectites (saponite and montmorillonite), pyrophyllite (talc), mica (illite), kaolinite, serpentine, & sepiolite. Adsorption is caused by the minerals' net-negative charge, which negative charge allows the clay material to absorb positively charged ions. Their high porosity and broad surface area account for the majority of their sorption capacities [19].

#### **5.2 Siliceous**

One of the most popular and fairly cost adsorbents is siliceous material. Alunite, dolomite, perlite, glasses, and silica beads are also present. These minerals were used because of the hydrophilic surface's chemical stability and reaction, which was brought on by the presence of a silanol group. However, because of silica beads' lack of resistance towards the utilization of solutions that are alkaline, application as adsorbent was limited to environments with pH values less than 8 [20].

#### **5.3 Zeolites**

In nature, porous aluminosilicates with a range of cavity designs and common oxygen atoms bind together to produce zeolites, which are aluminosilicate porous materials. Zeolite comes in a wide variety of species [21]. The natural species include clinoptilolite and chabazite. On the other hand, clinoptilolite, a heulandite mineral, is the chemical that is most often studied because of its high selectivity for particular contaminants. A unique property of zeolite is its cage-like structure, which makes it ideal for the removal of trace contaminants like phenols and heavy metal ions [22].

*Green Adsorbents for Water Purification DOI: http://dx.doi.org/10.5772/intechopen.112652*

**Figure 3.** *Agricultural waste-based adsorbents.*
