Green Adsorbents for Water Purification

Abstract

Water purification is crucial for ensuring access to safe and clean drinking water. As a sustainable and effective solution, green adsorbents have gained significant attention in recent times. These adsorbents are composed of natural or waste-based materials that are biodegradable, renewable, and abundant, making them an eco-friendly substitute for traditional adsorbents. This chapter offers a comprehensive outline of the present and prospects of green adsorbents in water purification. It encompasses a comprehensive overview of the many forms of green adsorption agents comprising natural, agriculture waste-based, & industrial waste-based adsorbents, as well as their synthesis and modification methods. Furthermore, it also explores the potential applications of green adsorbents in removing heavy metals, organic pollutants, and inorganic contaminants from water. The challenges and future directions of green adsorbent research are also discussed, including the limitations of these adsorbents and the opportunities for enhancing their performance. Overall, this chapter offers valuable insights into the potential of green adsorbents as a sustainable and eco-friendly solution for water purification.

1. Introduction

Water conflicts are becoming more frequent because of the rising global need for safe drinking water [1]. The primary sources of drinking water are natural waters, such as groundwater, surface water (rivers and lakes), and rainwater, although desalination of brackish and seawater is becoming more important. More than 50% of the world’s population uses groundwater for drinking, which accounts for 97% of all freshwaters [2]. Groundwater is the most economically viable option in many remote and underdeveloped regions where basic water delivery systems do not exist [3]. However, inorganic contaminants such as fluoride, uranium as arsenic, and boron are regularly detected in groundwater. Humans get negative health impacts from long-term exposure to such pollutants. For instance, chronic uranium intake from drinking water damages the kidneys, while excessive fluoride intake causes dental and skeletal fluoroscopic [4]. Inorganic pollutants are strongly influenced by geology. Therefore, creating proper and adaptable technology for local use is the most practical way to get removal of such ions, species, and toxins [5].

Adsorption, coagulation, flocculation, clarity, filtration, and disinfection are all combined in traditional water treatment techniques [6]. The main problem with conventional techniques is that they are usually inefficient at the removal of residual pollutants [5]. In comparison to traditional approaches, reverse osmosis (RO) and nanofiltration (NF) are particularly promising techniques, especially for applications involving drinking water. Because they use a variety of separation methods, NF/RO can produce high inorganic removal by processes that involve solution diffusion, size being excluded, charge repulsion, and adsorption. It is possible to choose a membrane with the proper properties for certain water quality. Bacteria, viruses, and micro-pollutants like pesticides are often present in groundwater along with inorganic contaminants, which are undesirable. Whereas elimination of micropollutants is dependent on specific properties, NF/RO can remove all those contaminants simultaneously. Additionally, NF/RO is a workable choice for rural settlements because of its modular architecture and flexible placement. However, the potential negative aspects of NF/RO, fouling of the membrane & scaling, concentrated disposal, & relatively high energy usage must all be avoided. Scaling is dependent on the likelihood of precipitate formation in each water, while fouling of membranes and scaling are inevitable throughout the separation process, they can be significantly reduced by optimizing the operating conditions. The implementation of renewable energy technology can offset high energy demand [7]. Given the high standard of water quality that NF/RO produces, using this water primarily for potable purposes is a suitable technique. Utilizing concentrates for non-potable purposes while having feed water quality permitting offers almost no opportunity for removal.

Several researchers have coupled adsorbent materials with further chemicals and substances to improve their ability to remove contaminants. Nanotechnology has recently been applied to almost every human-interest industry, especially soil and management of natural resources, particularly wastewater treatment [8]. Non-adsorbents are often more successful than traditional adsorbents at eliminating organic and inorganic pollutants from wastewater. These adsorbents are made of metallic oxide-based nanoparticles (NPs), nanotubes of carbon (CNTs), graphite, plant nanocomposites that (NCs), and other materials [9]. This study will examine the most common agricultural & non-agricultural material-based adsorbents. Their methods of removing pollutants are also discussed, along with potential modifications or treatments that could increase their effectiveness. Additionally, this evaluation outlines their chances of re-usability and safe disposal procedures [10].

2. Definition and importance of water purification

To make water safe, clean, and acceptable for use in a variety of purposes, such as drinking, irrigation, industrial processes, and recreational activities, water must first be purified. It entails the elimination of potential physical, chemical, and biological contaminants from water sources [11].

Due to the following factors, the significance of water purification cannot be overstated:

2.1 Keeping the public healthy

For the maintenance of the general welfare, access to clean and safe drinking water is essential. Waterborne illnesses brought on by bacteria, viruses, protozoa, and chemical pollutants can seriously harm your health. Purifying water helps keep people and communities healthy and stops the spread of disease.

Water purification techniques like filtration, disinfection, and chemical treatment efficiently remove or reduce the concentration of disease-causing microorganisms like bacteria (like E. coli, Salmonella), viruses (like Hepatitis A, norovirus), and parasites (like Giardia, Cryptosporidium), thereby reducing the risk of waterborne infections.

Getting rid of chemical pollutants Chemical contaminants such as heavy metals, pesticides, drugs, industrial chemicals, and organic molecules can all be found in water sources. These contaminants can be eliminated or reduced using purification techniques such e.g., clay, zeolites, biochar, activated carbon adsorption, oxidation, and ion exchange, protecting the environment and public health [12].

2.2 Enhancing flavor and odor

Water purification techniques can make it taste and smell better by removing or cutting down on contaminants that cause unpleasant tastes, odors, or discoloration. This improves the drinking experience overall and encourages taking in enough water to be hydrated.

2.3 Environmental protection

Water pollution can disrupt the ecological balance, degrade aquatic habitats, destroy aquatic species, and harm aquatic ecosystems. Excess nutrients, pollutants, and poisons, as well as other contaminants that might harm aquatic environments, can be eliminated by water filtration, protecting ecosystem health and biodiversity.

2.4 Supporting industrial and agricultural processes

Industrial processes and irrigation in agriculture both require water. Purification ensures that the water used in these industries follows quality requirements, preventing damage to machinery, maintaining product quality, and reducing the effects of pollution discharge on the environment.

2.5 Response to emergencies and disasters

Access to clean water becomes vital in times of emergency, such as during natural disasters or epidemics. To provide impacted communities with safe drinking water, avert further health emergencies, and ensure survival and well-being in difficult circumstances, water purification technologies and techniques are essential.

Overall, water purification is essential for maintaining ecosystems, promoting sustainable water usage, safeguarding public health, and facilitating a variety of human endeavors.

3. Overview of green adsorbents

This term refers to low-cost materials that come from (i) agricultural commodities as well as materials (fruits, vegetables, and foods); (ii) farming and waste material residues; and (iii) low-cost substances from that the most complicated adsorption agents will be manufactured (specifically, activated carbons produced after pyrolysis from plant sources). Whereas these “green adsorbents” are expected to have lesser adsorption capabilities than the super-adsorbents mentioned in the scientific literature (complex materials such as modified chitosan, activated carbon, highly complex inorganic material composites, and so on), they remain relevant due to their low cost. In this study, two major types of environmental pollutants were used: colors and heavy metals, as well as a few others: medicines, pesticides, and phenols. Despite their little studies, several contaminants were discovered in wastewater [12].

Green adsorbents are derived from a number of different sources, such as minerals from clay, natural polymers, organic substances, agricultural sources and by-products, agricultural residue and waste, and agricultural waste pyrolysis. Heavy metals must be removed from the aqueous phase, green adsorbents made from diverse agricultural waste products, by-products, and residues are becoming more and more popular today. Numerous agricultural by-products and residues, including rice bran, wheat bran, cottonseed hulls, maize corn cobs, dal husk, mango peel, sugar beet pulp, grape bagasse, and others, have been investigated as adsorbents. These agricultural by-products have the benefits of being affordable, widely accessible, and easily regenerated. They also have a high absorption capacity. They also have a high ability for adsorption as well as selective heavy metal adsorption [13].

4. Types of green adsorbents for water purification

Adsorbents are often divided into two categories: natural and synthetic, depending on where they were created. Minerals, charcoal, ores, clay, and zeolites are examples of natural adsorbents. While synthetic adsorbents are made from waste materials such as waste sludge, trash from agriculture, and industrial waste [14].

Figure 1 explain the flow chart of adsorbents for removal of pollutants. Its further divided in to five types include activated carbon adsorbents, non-conventional low-cost adsorbents, nanomaterial adsorbents, composite and nanocomposites adsorbents, and miscellaneous adsorbents. Activated Carbon adsorbents are further divided into two types including commercial activated carbon and AC prepared from water materials. Nano-conventional Low -cost Adsorbents are further divided into waste materials from Agriculture and industry, Natural Materials, and Bio-adsorbents. Nanomaterial Adsorbents are further divided into nanoparticles, nanotubes, nanowires, and nanorods. Waste materials from Agriculture further explain the agricultural solid waste and Industrial by-products. Natural materials are further divided into clays and zeolites and siliceous materials. Bio adsorbents further explain the Biomass and Biopolymers & Peal. Industrial by-products are divided into Metal hydroxide sludge, Fly Ash, Red Mud, Bio Solids, and waste slurry.

Figure 1.

Adsorbents for removal of pollutants.

Chemicals coagulation, which precipitating, the decomposition of separation of membranes, sludge activation, flocculation, and ion exchange are just a few of the processes used for removing pollutants from contaminated water [15]. The demand for rapid and simple processes for removing harmful dyes and chemicals from dirty water is continually increasing. Utilizing green adsorption for water filtration is much more advantageous than utilizing conventional methods due to its ease of use, low price, high effectiveness, and environmental friendliness [16].

Natural green adsorbents such as orange peel, peel from bananas, husks of rice, bagasse from sugar cane, tea waste, avocado peel, Hami melons peel, the peel of dragon fruit peels, and other peels have been utilized to recover ions of heavy metals & organic dyes from contaminated water [17]. One of the most common applications for fruit peel debris is water treatment. Using solid waste as green adsorbents, harmful compounds such as metals, pigments, and pesticide can be eliminated from contaminated water [18].

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].

Figure 2.

Natural adsorbents.

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].

6. Agricultural waste-based adsorbents (e.g., coconut shell, rice husk, sugarcane bagasse)

Agricultural wastes are one of the expected possibilities for alternative absorption. There are many review articles on the elimination of hazardous chemicals using biomass resources. In Figure 3, various types of agricultural waste-based adsorbents are listed for absorbing water contaminants from industrial effluent [23]. Orange peel, coniferous Pinus bark powder, wheat husk, activated palm ash, cattail root, neem sawdust, peanut husk, Jujuba seeds, and coir pith activated carbon are among the agricultural wastes. To remove dye from industrial effluent, activated carbon can also be made from a variety of other agricultural wastes, including coconut tree sawdust, banana pith, silk cotton hull, maize cob, and sago waste [24].

Figure 3.

Agricultural waste-based adsorbents.

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].

Substance	Color absorbed
Fly ash	N/A
Carbon black	All colors
Chlorophyll	Red & blue violet
Copper sulphate	Blue & green
Chromium oxide	Green
Iron oxide	Red, orange & yellow
Manganese dioxide	Black & brown
Indigo carmine	Blue
Sudan red	Red
Malachite green	Green

Table 1.

Explain the substance and colors adsorbed.

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 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 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 second-order 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].

10. Removal of organic pollutants (e.g., dyes, pesticides, pharmaceuticals)

Utilizing green adsorbents to remove organic pollutants from water, such as dyes, pesticides, and medicines, is a successful method of water filtration. Green adsorbents, made from natural resources, provide a long-term and sustainable option for the elimination of harmful organic pollutants [37]. The elimination of organic pollutants by various commonly used green adsorbents is described below.

10.1 Biomass-derived activated carbon

For the elimination of organic pollutants, activated carbon is frequently made from biomass sources such as agricultural waste, wood, and coconut shells. It has a huge surface area and a highly porous structure, which allows for the physical adsorption of organic molecules. Van der Waals forces or hydrophobic interactions can be used to bind organic molecules to the surface of activated carbon because of the porous structure [38].

10.2 Plant-based materials

A variety of plant-based materials, such as leaves, sawdust, or agricultural leftovers, can be used as green adsorbents to remove organic pollutants. These substances frequently have functional groups that can interact with organic molecules and porous structures. For example, lignocellulosic materials may attract organic contaminants through hydrophobic interactions and physical adsorption.

10.3 Minerals in clay

For the elimination of organic pollutants, clay minerals such as montmorillonite and bentonite have been employed as green adsorbents. The layered structure of these minerals has a large specific surface area. Cation exchange, Van der Waals forces, or hydrophobic interactions are a few of the processes by which organic contaminants might be absorbed onto the clay mineral surfaces or inter-layer spaces [39].

10.4 Membrane processes

Membrane filtration methods like reverse osmosis (RO), nanofiltration (NF), or ultrafiltration (UF) may effectively eliminate organic contaminants from water. These methods make use of semi-permeable membranes, which let water through while filtering out dissolved organic molecules based on their size and charge. Membrane procedures can be used with other treatment techniques to increase the removal efficiency of tiny organic molecules, which they are particularly good at eliminating.

10.5 Biological treatment

Techniques for biological treatment use microorganisms’ capacity to break down organic contaminants. Microbial activity is used in processes like activated sludge, sequencing batch reactors (SBRs), and created wetlands to break down organic chemicals into simpler, less dangerous molecules. Biodegradable organic pollutants benefit most from biological treatment [40].

10.6 Chemical coagulation/flocculation

Chemical coagulation and flocculation is the process of adding coagulants or flocculants to water to aggregate and destabilize organic contaminants into bigger particles that may then be separated by sedimentation or filtering. Polymers are frequently employed as flocculants, while common coagulants include aluminum or iron salts. Certain organic contaminants, particularly those with larger molecular sizes, can be effectively removed with this technique [41].

10.7 Photocatalysis

To produce reactive species that can break down organic contaminants, photocatalysis uses photocatalysts like titanium dioxide (TiO2) which are activated by ultraviolet (UV) light. The photocatalyst, when exposed to radiation, generates electron-hole pairs that start oxidation processes, breaking down organic materials into inert byproducts.

10.8 Electrochemical techniques

The elimination of organic contaminants can be accomplished using electrochemical techniques such as electrooxidation and electrocoagulation [42]. These techniques use electrodes and electric currents to trigger chemical processes that decompose or remove organic pollutants. For a variety of organic contaminants, electrochemical approaches work well and can be combined with other treatment methods.

The characteristics of the organic contaminants, the water quality metrics, the treatment objectives, economic considerations, and regulatory requirements all play a role in determining which approach is most appropriate. In some instances, a combination of various procedures may be used to accomplish the best organic pollution removal and ensure the safety of the water [43].

11. Removal of inorganic contaminants (e.g., fluoride, nitrate, sulfate)

To ensure that water is safe to drink, inorganic pollutants like fluoride, nitrate, and sulfate must be removed. These inorganic pollutants can be effectively removed using a variety of techniques. Here are some methods for getting rid of them:

11.1 Ion exchange

Ion exchange is a common technique for clearing out inorganic impurities from water. A solid resin and the water exchange ions in this process. Anion exchange resins, which selectively absorb fluoride ions and release other ions (such as chloride) in exchange, are used to remove fluoride from water. Similarly, nitrate and sulfate ions can be eliminated using cation exchange resins.

11.2 Reverse osmosis (RO)

Reverse osmosis is a membrane-based filtering method that can successfully rid water of inorganic impurities. It functions by exerting pressure on the water and forcing it through a semi-permeable membrane that picks out specific ions, such as fluoride, nitrate, and sulfate, and removes them from the solution. Reverse osmosis is frequently employed in water treatment systems and is capable of removing a variety of inorganic impurities [44].

Adsorption is a procedure used to draw out and remove impurities from water using solid adsorbents. For the filtration of inorganic pollutants, a variety of adsorbents, including e.g., clay, zeolites, biochar, activated carbon, activated alumina, and zeolites, can be used. These adsorbents may selectively absorb and remove fluoride, nitrate, and sulfate due to their high surface area and affinity for ions.

11.3 Precipitation

Precipitation is the addition of chemicals that combine with the inorganic pollutants to create insoluble compounds that may then be removed using sedimentation or filtration. For instance, fluoride ions can be precipitated as calcium fluoride using calcium hydroxide (lime). Like how calcium or magnesium compounds can be used to precipitate sulfate ions, biological denitrification is one method that can be used to remove nitrate.

11.4 Ion-selective membranes

Ion-selective membranes are used in the electrochemical process of electrodialysis for dropping inorganic impurities from water. It entails applying an electric field across the membranes, which allows the flow of ions to be selectively allowed based on their charge. Fluoride, nitrate, and sulfate ions can all be successfully removed from water using electrodialysis [45].

12. Case studies of green adsorbents

The utilization of waste tea residue, a green biosorbent, for the removal of heavy metals from water, was investigated in a study by Wang et al. [39]. It was discovered that the discarded tea residue had significant potential for adsorbing elements like lead (Pb), cadmium (Cd), and copper (Cu). The performance of the biosorbent was on par with or even better than that of traditional adsorbents. The study showed the potential of using waste products as environmentally friendly adsorbents for heavy metal removal.

Chitosan-Based Adsorbent for Dye Removal: Chitosan, a green adsorbent derived from crustacean shells, was modified and used to remove dye pollution in a study by Zhang et al. [40]. For a variety of colors, the modified chitosan shows outstanding adsorption capacity and great removal efficiency. Chitosan may be used as an environmentally acceptable adsorbent for color removal in wastewater treatment, according to the study.

A Plant-Based Adsorbent for the Removal of Pharmaceuticals Azadirachta indica leaves, a waste plant product, was employed as a green adsorbent in a study by Verma and Dash [41] to remove pharmaceuticals from water. Ibuprofen, paracetamol, and naproxen were only a few of the pharmaceuticals that the plant-based adsorbent effectively absorbed. The study showed that using plant-based materials as green adsorbents for drug removal is feasible.

Ahmad et al.’s [42] study looked into the usage of biochar made from agricultural waste for the removal of organic contaminants, especially bisphenol A (BPA). The BPA levels in the water were significantly reduced by the biochar’s strong BPA adsorption ability. The study demonstrated the potential of using biochar as an environmentally friendly adsorbent for the elimination of organic contaminants.

13. Challenges and future directions

13.1 Limitations of green adsorbents in water purification

Due to their potential as environmentally benign substitutes for traditional adsorbents, green adsorbents, which are obtained from natural sources, have attracted considerable attention in water purification procedures. However, they also have some restrictions that should be considered. Green adsorbents’ drawbacks in the filtration of water include the following:

13.1.1 Limited availability and variability

Green adsorbents are made from organic materials including plants, agricultural byproducts, and biomaterials. Geographical location, seasonal fluctuations, and other factors can all affect the accessibility and caliber of these adsorbents. Consistent performance and supply may become difficult because of this [46].

13.1.2 Lower adsorption capacity

green adsorbents often have lower adsorption capacities as compared to synthetic adsorbents like e.g., clay, zeolites, biochar, and activated carbon. Due to this, more green adsorbents may be needed to accomplish the same level of purification, which would result in higher expenses and maybe more trash that would need to be disposed of.

13.1.3 Slow adsorption kinetics

Compared to manufactured adsorbents, green adsorbents frequently have slower adsorption kinetics, which means that the adsorption process may take longer to achieve equilibrium. In high-flow rate applications, this may have an impact on the effectiveness and efficiency of water purifying systems [47].

13.1.4 Limited selectivity

Green adsorbents may only have a little amount of selectivity for some water pollutants. They might eliminate a variety of pollutants, but they do not always successfully target dangerous chemicals. This may limit their usefulness in circumstances where exact removal of contaminants is necessary.

13.1.5 Regeneration challenges

Adsorption-based water purification procedures rely heavily on regeneration, which entails removing impurities that have been adsorbed to the adsorbent. Due to their organic makeup, green adsorbents may provide regeneration difficulties, making it challenging to efficiently recover and repurpose the adsorbent material.

13.1.6 Lack of standardized procedures

Green adsorbents frequently do not have standardized procedures for synthesis, characterization, and performance assessment. Due to this, it may be difficult to assess the effectiveness of various green adsorbents or develop uniform criteria for their use in water purification procedures.

13.1.7 Compatibility with current systems

To take advantage of green adsorbents’ special features, existing water treatment systems may need to be modified or new procedures may need to be developed. It may be difficult and expensive to integrate green adsorbents into existing infrastructure for conventional water treatment.

While green adsorbents have a bright future in water filtration, overcoming their current drawbacks through research and development is essential to their widespread adoption and successful use in real-world applications [48].

14. Current scenario and challenges in adsorption for water treatment

14.1 Emerging trends in green adsorbent research

The future of the field of green adsorbents is being shaped by continuing research as well as several new trends. The following are some important developments in the study of green adsorbents:

14.1.1 Adaptation and nanotechnology

Researchers are looking into how nanotechnology might be used to increase the effectiveness and adsorption capacity of green adsorbents. The surface area, porosity, and reactivity of green adsorbents can be increased, resulting in improved adsorption performance. Nanoscale changes, such as the inclusion of nanoparticles or the functionalization of adsorbent surfaces, can achieve this [49].

14.1.2 Hybrid materials

Activated carbon, polymers, and nanoparticles are being combined with green adsorbents to create hybrid materials. To achieve better overall performance, these hybrid materials combine the benefits of many components, such as the environmental friendliness of green adsorbents and the high adsorption capacity of synthetic materials.

14.1.3 Waste valorization

Scientists are investigating the utilization of waste items as potential green adsorbents, such as agricultural byproducts, food waste, and biomass residues. By utilizing waste materials to create value-added adsorbents, this movement aims to address both the demand for sustainable water filtration technology and environmental concerns over waste management.

14.1.4 Advanced characterization methods

To better understand the adsorption mechanisms and behavior of green adsorbents, researchers are using advanced characterization methods like spectroscopy, microscopy, and computational modeling. This makes it possible to build and optimize adsorbent materials with better performance and property characteristics.

14.1.5 Regeneration and re-usability

Strategies for the regeneration and reusability of green adsorbents are currently being developed. These methods seek to recover pollutants that have been absorbed and restore the adsorption capacity of the adsorbents, lowering waste production and improving the process’s efficiency and sustainability.

Although most research has been done in laboratories, there is rising interest in scaling up green adsorbent technology for use in real-world settings. To create affordable, effective, and efficient water filtration systems employing green adsorbents, researchers are exploring pilot-scale investigations [50].

These new trends highlight the growing significance of environmentally friendly and sustainable water filtration techniques. Researchers are working to create effective, affordable, and scalable green adsorbent technology for water treatment by addressing the limitations of green adsorbents and investigating novel methodologies [51].

15. Conclusions

In conclusion, natural-based green adsorbents present a viable strategy for water filtration. Sustainable, abundant, and potential for efficient pollution removal have made these eco-friendly adsorbents popular. The following are important points about using green adsorbents for water purification:

Environmentally friendly and sustainable: Green adsorbents reduce reliance on synthetic or non-renewable adsorbents by using natural materials like biomass, agricultural waste, or plant fibers. They provide a long-term fix with little harm to the environment.

Versatile removal capabilities: Green adsorbents have demonstrated efficacy in the removal of a wide range of contaminants from water, such as heavy metals, organic pollutants, dyes, medicines, and other inorganic compounds. Through changes and treatments, they can be customized and optimized for the removal of particular contaminants.

Cost-effective: Because they are readily available and have minimal production costs, green adsorbents are frequently commercially viable. A cost-effective solution for water filtration, many of these materials are leftovers or results of industrial or agricultural processes.

Green adsorbents have surface characteristics such as a large surface area, porosity, and functional groups that allow for adsorption through both chemical and physical interactions. These methods make it easier to remove impurities from water.

Alternative water treatment methods: To improve overall treatment effectiveness and address numerous contaminants, green adsorbents can be used in conjunction with alternative water treatment methods such as filtration, membrane processes, or advanced oxidation.

Although green adsorbents have a lot of commitment, their effectiveness may vary based on the contaminant, the water’s chemistry, the adsorbent’s properties, and the operational circumstances. To maximize their effectiveness, scalability, and potential regeneration for continuous usage, more study is still required. Overall, the use of green adsorbents in water purification shows promise for offering long-term and practical solutions to problems with water pollution, helping to achieve the objective of providing communities with clean and safe water resources.

Conflict of interest

The authors declare no conflict of interest.