**7. Industrial waste-based adsorbents (e.g., fly ash, sludge, and sawdust)**

### **7.1 Fly ash**

A form of industrial waste called fly ash can be used to absorb colors shown in **Table 1**. Fly ash, which can contain some dangerous substances like heavy metals, is produced in vast amounts during combustion processes [25]. The sugar industry produces bagasse fly ash, which is free of dangerous metals and is frequently used for color adsorption. Its characteristics vary greatly and are influenced by its source. Adsorption tests on Congo red and MB textile dye solutions revealed exothermic monolayer formation and adsorption processes. Fly ash from combustion plants can effectively filter out colors from dyeing industry effluents [26]. Fly ash was used to remove methylene blue from a solution with an initial dye concentration of 65 mg/l at pH 6.75 and 900 mg/l absorbent. The removal rate ranged from 95 to 99%, with a Langmuir constant of 1.91 mg g−1, Ka value of 48.94 L mg−1, and a linear regression coefficient of 0.999 [27].

**Table 1** contains information on several compounds and the colors they may absorb. Here's a quick rundown: Fly ash: This material absorbs no distinct color. It has nothing to do with color absorption. Carbon Black: Carbon Black can absorb all colors of light. It is thought to be a particularly effective black pigment. Chlorophyll absorbs


#### **Table 1.**

*Explain the substance and colors adsorbed.*

predominantly red and blue-violet light and is found in plants. These wavelengths are required for photosynthesis. Copper Sulphate absorbs light in the blue and green wavelengths. This chemical is frequently utilized in a variety of applications, including fungicides and electrolysis tests. Green light is absorbed by chromium oxide. It's a common pigment in paintings and ceramics. Iron Oxide is a light absorber that absorb red, orange, and yellow light. It is commonly used as an ingredient in a variety of applications, including paints, coatings, & pigments. Manganese Dioxide absorbs the black and brown colors of light. It's used in a variety of sectors, including the manufacture of batteries & ceramics. Indigo Carmine: It absorbs blue light specifically. It is commonly used as a coloring agent in food, textiles, & medicine. Sudan Red is a color that absorbs red light. It is an artificial color that is often used to color plastics, waxes, & oils. Malachite Green: This color absorbs green light. It is commonly used in textile manufacturing as a dye and as a fungicide in fish farming. The table gives a summary of these chemicals and the colors they absorb, helpful for learning their real-world applications and light absorption properties.

#### **7.2 Metal hydroxide sludge**

Metal hydroxide sludge, having maximal adsorption capacities of 45.87 and 61.73 mg/g at 30°C and pH 8–9, is used to remove azo colors from surfaces. pH controls the development of dye-metal complexes and adsorption. Metal hydroxide sludge exhibited a maximum adsorption capacity of 270.8 mg/g at 30°C and an initial pH of 10.4. The elimination of Remazol Brilliant Blue reactive dye from a solution with metal hydroxide has a monolayer adsorption capacity of 91.0 mg/g at 25°C and is inexpensive [28].

#### **7.3 Red mud**

Another industrial byproduct and waste material from the bauxite industry that is used to manufacture alumina is red mud. When the ability of discarded red mud to absorb dye from its solution was investigated, it was discovered to be successful. It was discovered that the Freundlich isotherm followed pH 2 as the pH where the most dye could be removed by adsorption [29]. Red mud was used as an adsorbent

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

to obtain methylene blue, a basic dye, from water solutions. Its adsorption capacity was calculated at 7.8106 mol/g. Wasted red mud was used to separate Congo red from aqueous solutions and acid activated red mud was investigated for Congo red adsorption from sewage [30]. The experimental data closely matched the Langmuir isotherm, and red mud effectively removed Fast Green, Methylene Blue, and Rhodamine B from wastewater. The adsorption procedure followed the Langmuir and Freundlich isotherms and was exothermic [31, 32].

### **8. Removal of Cd (II) ions from water samples**

The emission of effluent containing heavy metals, including Cd (II) ions, is extremely dangerous to both the environment and human health. Through a multistep process, a 2-aminopyridine group (2Ap) was chemically added to graphene oxide (GrO). The Gr2Ap adsorbent showed a high ability to adsorb Cd (II) at pH = 6, according to research on the effects of adsorbent quantity, pH, temperature, and equilibrium time on sorption. Additionally, empirical isotherm data were fitted to the Freundlich and Langmuir model to learn more about the adsorption isotherms of metal ions. Additionally, Roginsky-Zeldovich types and pseudo-first- and secondorder kinetics were discovered to be comparable with adsorption kinetic data when the fundamental steps of the metal sorption mechanism were examined. A remarkable potential for eliminating the heavy metal ions from aqueous samples was shown by the Gr2Ap. The method is also effective, simple, affordable, and appropriate for determining the presence of Cd (II) ions in various water and wastewater samples, according to the most recent research [33].

### **9. Applied techniques of green adsorbents in water purification**

#### **9.1 Removal of heavy metals (e.g., Pb, Cd, As, Hg)**

The use of green adsorbents to remove heavy metals from water is a crucial area of water purification research and development. Natural substances or related compounds that can remove heavy metal ions from water are referred to as "green adsorbents." The removal of heavy metals by some common green adsorbents is described below.

#### *9.1.1 Biochar*

Biochar is a substance rich in carbon that is created through the pyrolysis of biomass, such as wood or agricultural waste. Its very porous design provides a substantial surface area for adsorption. Through physical adsorption, in which metal ions are drawn to the surface of the biochar by electrostatic interactions or Van der Waals forces, biochar can absorb heavy metals [34]. Additionally, it can perform chemical reactions like ion exchange or complexation, in which metal ions connect to functional groups on the surface of the biochar.

#### *9.1.2 Chitosan*

Chitosan is a biopolymer made up of glucosamine units that are obtained from the shells of crustaceans. It possesses amino groups that can complex with or chelate with heavy metal ions. To increase its adsorption capability and selectivity for particular heavy metals, chitosan can be chemically altered. It is efficient at removing heavy metals from water, including lead (Pb), cadmium (Cd), arsenic (As), and mercury (Hg) [14].

#### *9.1.3 Chelation*

Chelating agents are organic substances that create stable complexes with heavy metal ions to speed up the elimination of those ions from water. Heavy metals can be removed by natural chelators including organic acids, amino acids, and plant extracts as green adsorbents. These chelating substances generate soluble or insoluble complexes with the metal ions they bind to through coordination bonds, which can then be removed from water by filtration or sedimentation [11].

#### *9.1.4 Phytoremediation*

Phytoremediation is a method for removing heavy metals from contaminated water and soil by using plants. Hyperaccumulation is a technique that some plant species can use to store heavy metals in their tissues. These plants can be grown in artificial wetlands or other treatment facilities, where they accumulate heavy metals in their leaves or stems after absorbing them through their roots. The heavy metals can then be efficiently removed from the environment by harvesting and properly discarding the plants [35].

### *9.1.5 Modified green adsorbents*

Green adsorbents that have been changed can be used to increase their adsorption capacity and selectivity for particular heavy metals. Physical modifications like grinding, sieving, and palletization as well as chemical modifications like acid/base treatment, oxidation, and functionalization with certain groups or polymers are examples of modification procedures. These alterations may increase surface area, the addition of new functional groups, or an improvement in the adsorbent's ability to bind to the desired heavy metals.

### *9.1.6 Biosorption*

Biological materials, such as plant biomass or microbial cells, are utilized as adsorbents in the biosorption process to remove heavy metals from water. Functional groups like carboxyl, amino, and hydroxyl groups can attach to heavy metal ions by a variety of mechanisms, including ion exchange, complexation, and electrostatic interactions, in the cell walls or biomass of these materials. Algae, bacteria, fungi, and agricultural waste including rice husks, coconut shells, and sawdust are a few examples of biosorbents [36].
