Phytosorbents in Wastewater Treatment Technologies: Review

Abstract

Turning to green technologies in wastewater treatment is a well-known global trend. The use of natural sorbents of plant origin or phytosorbents in order to purify water from various types of pollutants is becoming more and more popular. This solves several important problems at once: the use of harmless natural materials, reducing the cost of processing, and waste disposal. Moreover, there is a global increase in waste in the agricultural, food, woodworking, and other industries. This review presents data on the modern use of natural materials, mainly vegetable waste, as sorbents in wastewater treatment technologies. Natural materials remove ion metals, dyes, crude oil and petroleum products, and other organic and non-organic contaminants. The techniques of obtaining phytosorbents from plant raw materials are considered. The methods for activation and modification of the various phytosorbents, which provide greater sorption efficiency, are presented. The adsorption mechanisms for various water contaminants are examined, and model descriptions are shown. It has been revealed that the effectiveness of sorption interaction mainly depends on the presence of functional groups. Studies over the past twenty years have shown good prospects for the use of such materials and technologies in practice.

1. Introduction

The fact that more than 80% of all wastewater produced in the world is not treated before being discharged into water bodies is a cause for alarm and urgent action. The state of the world’s oceans and sources of fresh water has a significant impact on human health and quality of life [1]. A great variety of effective technologies used in wastewater treatment (mechanical, biochemical, chemical, and physico-chemical methods) give results, but are usually accompanied by high resource costs and the generation of secondary contaminants. Along with this, waste management is now one of the global challenges for humanity: only 5–10% of the total amount of extracted natural resources and minerals are used [2]. Every country should prioritize nature-based technologies in wastewater treatment in order to protect the environment and maintain economic well-being. Green technologies make it possible not only to reduce the negative impact on nature, but also to save resources. This is achieved through adding environmental practices, careful control of pollutants, recycling, and reuse of production waste [3,4,5,6].

One of the areas studied all over the world as an effective method of wastewater treatment from molecular and ionic contaminants is sorption. Currently, adsorption wastewater treatment is undergoing a new industrial revolution. The technology of production, activation, and modification of sorbents is becoming more complex and reaching a new quality level. The adsorption process is widely used in removing heavy metals, dyes, turbidity, and organic compounds from wastewater. According to a number of authors [7,8,9,10,11,12], the optimization of sorption parameters and the prediction of the sorption performance of natural materials should be carried out by advanced modeling methods, including the use of modern Internet technologies.

Activated natural coal is still the most effective and most widely used sorbent in water treatment. At the same time, reserves of this mineral are completely absent in many regions, and in others the reserves are not unlimited. It should be noted that coal mining involves a number of engineering, environmental, and economic problems [13]. Therefore, many scientists are searching for a worthy alternative to activated carbon; they seek new effective adsorbents of various origins to remove pollutants present in wastewater. In addition to high absorption properties, natural sorbents should be non-toxic, inexpensive, and regenerative. In addition, raw materials should be available, and spent sorbents should be easily recovered [14]. These requirements force researchers to turn to other natural materials (including those of plant origin) that can be effective sorbents for wastewater treatment or excellent and cheap raw materials for the production of such sorbents [15].

According to the classification in [16], plant-based adsorbents (biosorbents or phytosorbents) are classified as organic natural sorbents. These can be hay, grain straw and husk, sawdust and bark, various types of soil (for example, peat), seagrass, leaves, fruit residues (seed, peels, shells), and others. Most of these raw materials are agricultural or food waste products that need to be disposed of or reused. They are an organic part of existing ecosystems and are most consistent with environmental requirements. The use of these materials in the production of adsorbents for water purification solves several simultaneous problems: reducing the volume of natural coal extraction, the use of “green” technologies in wastewater treatment, and the beneficial use of waste.

This review presents systematized data based on publications related to the production, activation, and modification of natural sorbents of plant origin (or phytosorbents—PSs) in wastewater treatment (WWT) technologies. These data describe purification of water from the contaminants characteristic of industrial wastewater: metal ions, dyes, crude oil and petroleum products, pharmaceuticals, and others. The research in this area is very important for the protection of water resources from the discharge of untreated or poorly treated wastewater. The assumed information will greatly facilitate future scientific work. The review covers only peat and terrestrial vegetation, but it does not cover algae and seaweed.

Table 1. Key research directions on the use of PSs in WWT technologies.

Section	Direction of PS Research	Ref.
3.1	Kind of raw materials for the phytosorbents	[14,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56]
3.2	Methods of obtaining phytosorbents	[33,52,53,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85]
3.3	Mechanism of sorption processes	[14,23,28,41,49,52,57,58,86,87,88,89,90,91,92,93,94,95,96,97,98,99]
3.4	Efficacy of wastewater treatment with phytosorbents
3.4.1	Extraction of metal ions	[21,22,24,34,38,40,46,47,50,53,54,58,64,68,87,96,97,98,100]
3.4.2	Removing dyes	[20,43,49,51,52,66,82,83,91,99,101,102,103]
3.4.3	Removing crude oil and petroleum products	[14,15,20,28,42,67,104]
3.4.4	Removing others harmful pollutants	[20,48,72,75,105,106,107,108]
3.5	Regeneration and disposal of phytosorbents	[14,15,16,17,36,61,109]

shows the main directions of research on this topic in modern science.

2. Materials and Methods

Searching for materials: The keywords “phytosorbents”, “sorption/adsorption”, “natural sorbents”, “sorbents modification”, “sorption intensification”, and “wastewater treatment” were used to search for thematic papers in Web of Science, Scopus, as well as Google Scholar, ELIBRARY.RU, and other outlets, with publication dates over the last twenty years.

There are 109 publications that cover scientific research on phytosorbents, their production, activation, modification, and use in wastewater treatment. Basically, the results of laboratory tests are shown. Most of the considered articles were written in English (86) and some in Russian (23). The studies were carried out by scientists from different countries: China, Saudi Arabia, Pakistan, Korea, Nepal, Iran, Romania, Colombia, Algeria, France, Canada, Serbia, Palestine, UK, Bangladesh, Italy, Ethiopia, Hungary, Iraq, Peru, Russia, and others. More than 57% of all these articles were published during the last 5 years. The information provided was gathered between May 2022 and August 2024.

3. Results

3.1. Kind of Raw Materials for the Phytosorbents

Phytosorbents, or PSs, are products obtained from plant-based raw materials after simple or complex processing. These can be a certain type of soil (that is, peat, which is the product of plant decomposition under certain conditions) or plant parts (leaves, bark, fruits, seeds, etc.). They may also be the vegetable waste of some industries, which is generated in a large amount and requires disposal.

Table 2. Kind and origin of raw materials for the phytosorbents.

Kind of Raw Materials for the Phytosorbents	Production Waste	Origin
Peat	No	Peat is a sedimentary loose rock that is an ancient accumulation of dead remains of marsh plants
Leaves and stems (fern, moss, lichen, reindeer moss, tea, bamboo, swamp plants, sugarcane bagasse)	No/Yes	These are plants growing in natural clusters/waste from tea production, sugar, or fodder preparations for animals
Sawdust and bark (pine, poplar, ash, palm)	Yes	Sawdust and bark are waste products that are generated in woodworking enterprises
Branches (maple, birch)	No/Yes	Dry tree branches that can be specially pruned/waste from municipal services of a city which is generated during the pruning of trees
Husk (rice, sunflower, coffee)	Yes	The husks of some plants are an agro-waste product
Straw (rice, wheat, corn)	Yes	Straw is a waste product of crop production at harvest time
Peel (citrus fruits, bananas, cassava, potato)	Yes	The peel is the outer protective layer of a fruit or vegetable that can be removed; this is the waste of the processing agricultural industry, canneries, food industry
Fibrous husk (coconut)	Yes	Fibrous husk is a protective layer of a coconut that can be removed; this is the waste of the processing agricultural industry
Pulp (coffee berries)	Yes	Waste from coffee processing plants
Seed, stones (citrus fruits, grape, dates, rapeseed, pomegranate, olive, ziziphus jujuba)	Yes	This is the waste of the processing agricultural industry, canneries, food industry

presents the main plant feedstocks for sorbent production, most of which are waste from various industries: wood processing plants, agriculture, food enterprises, and others.

Peat, the reserves of which are huge in countries such as Finland, USA, Canada, Sweden, Russia, and others, is of particular interest. Peatlands have been formed from the accumulation of partially decayed vegetation in waterlogged environments over centuries to millennia. According to researchers, peat is a sedimentary loose rock that is formed in the process of the natural death and incomplete decay of bog plants in conditions of excessive moisture and low access to air. Peatland ecosystems cover only around 3% of the land surface but are important storages of the world’s soil carbon peat [17]. There are a lot of studies on the issue of wastewater purification with peat. Peat is quite often used to treat surface wastewater from oil depots, gas stations, airports, and highways. Peat can also serve as a good raw material for new combined sorbents [14,18,19,20].

Many plants are potential sorbents for water purification. The most suitable for treatment are individual above-ground parts of plants or the plant mass from several components. The leaves and stems of the plants can be natural materials obtained directly from the flora of natural clusters or they can be waste products from agricultural production. The following plants are used to obtain sorbents: fern, moss, lichen, reindeer moss, tea, bamboo, swamp plants, and others. In some cases, the residues from animal feed production can be appropriate [21,22,23,24]. Usually, sawdust and bark are waste products that are generated in woodworking enterprises. Some papers have been published on the study of sorbents obtained from the processing of trees such as pine, poplar, ash, aspen, larch, ailanthus, and palm [25,26,27,28,29,30,31,32,33,34,35]. Phytosorbents can be obtained by cutting and grinding the branches of some trees and shrubs. Branches can be municipal waste, which is generated while pruning trees [15,36].

Agricultural waste, also known as agricultural by-products or agro-waste, refers to the residues generated from various agricultural operations and processes. Crop residues are the leftover plant materials after harvesting, such as straw, stalks, leaves, and husks [37]. Husks are the hard protective shells of grains. They are a waste product after processing some crops, such as rice, sunflower, and others [38,39,40]. A special type of husk is obtained in the process of coffee processing [41]. Likewise, the straw of rice, wheat, and corn is a residual by-product of crop production at harvest time. The approximate ratio of pure crop to straw is 1:1 [42,43,44,45].

The peel is the outer protective removable layer of fruit or vegetables such as citrus fruits, bananas, cassavas, potatoes, and others. This is the waste of the processing agricultural industry or canneries [46,47,48,49,50]. Coconut fibrous husk is a protective layer that can be removed and used for other needs. This is the waste of the processing agricultural industry; it is formed in huge quantities [51]. Coffee berry pulp, as a rule, is a processing waste of coffee plants. The pulp of coffee cherries accounts for about 80% of the weight of the fruit; therefore, after obtaining coffee beans, a large amount of waste remains. In some countries, it is used to produce fertilizers or cheap drinks. This waste is also suitable for sorbent production [41]. In any case, all these materials may well be source materials for obtaining new sorbents.

Seeds of citrus fruits, grape, dates, rapeseed, pomegranate, and other plants are waste products from many industries associated with agriculture and the food industry, in particular, the production of juices [52,53,54]. For example, citrus plants, due to their wide distribution and use, are the main processed horticultural crops around the world. Citrus waste accounts for approximately 40–50% of the total weight of the fruit (including peel, pulp, and seeds), which is a potential source of secondary raw materials. Worldwide, citrus processing plants generate more than 60 million tons of waste [55,56]. Obtaining cheap biochar from seeds, which can completely replace activated charcoal in WWT processes, is one of the urgent tasks of waste disposal and reducing the cost of water treatment technologies. There are also data on the production of sorbents with powder made from citrus seeds.

3.2. Methods of Obtaining Phytosorbents

Different ways to obtain sorbents from plant materials are discussed below.

(1) Natural use. In this case, the natural material is only subjected to repeated washing with ordinary water, drying, and grinding. No chemical reagents are used in these sorbent preparation technologies [57,58]. So, it is possible to make sorbent from peat, sawdust, shredded branches, bark from trees, and the leaves of some plants. The simplicity of sorbent production, non-regeneration, and reuse, all have a positive effect on its low cost. The main disadvantage of natural materials as sorbents is their weak sorption capacity. This is due to high hydrophilicity and insufficient porosity, caused by the presence of secondary natural pollution in pores. Normally, the total sorption capacity of pre-washed and dried natural materials does not exceed 100 mg/g of water-dissolved substances [58,59,60], excluding crude oil absorption, for which it may be higher [14,20,61]. For comparison—activated coal can adsorb up to 1 g/g contamination and more [28,62]. Here, the adsorption capacity of activated coal really depends on the kind of coal, the water contaminant, environmental conditions, and other factors. Or, for example, some synthetic sorbents can adsorb up to 20–30 g/g of petroleum products [63].

Also, in this case there is always a risk of release into the water from PS substances that are secondary pollutants. For example, peat releases humic substances into the treated water, and tree sawdust releases phenols, polysaccharides, and low-molecular-weight carbohydrates to water. Therefore, the use of natural plant materials should be justified and harmless. There are examples of using peat without any special treatment as a filter loading for surface water treatment (rain and tallow), e.g., in road surfaces [47,48,59,60].

(2) Activation and modification. In order to increase the sorption capacity of phytosorbents, special varieties of plant raw material treatment are used: activation and modification. Activation is a way to significantly improve the sorption properties of natural materials by leaching from the sorbent pore of polluting substances without changing or destroying the structure of the PS. The activation method involves a hot water wash (T = 50–70 °C), water steam (T = 100–150 °C), weak reagent solutions (15–20% sulfuric acid, 10–15% hydrochloric acid, alkali metal hydroxides, and others), and short-term physical or physico-chemical exposure (ultrasound, microwave, infrared, and ultraviolet effects). Modification of sorbents is a way to improve sorption properties, usually accompanied by some structural changes in the source material or adsorbent surface. Modification can be achieved by washing in highly concentrated solutions of acids (sulfur, salt, nitrogen, vinegar, and others) or alkalis (usually potassium hydroxide or sodium hydroxide), sometimes in combination with high temperature (T = 70–200 °C); by adding various catalysts (iron salts, H₂O₂, TiO₂, and others); by long-term physical exposure, ultrasound, or microwave; and by physical and chemical effects, such as microwaves in combination with reagents or catalysts [64,65]. In this case, the sorption efficiency can be significantly increased (usually by 1.5–4 times), but also the cost of production of the sorbents can be raised. As a rule, several main criteria are taken into account, such as efficiency, economy, safety, and technology [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,51,66,67]. In general, the high hydrophilicity of natural materials prevents their widespread usage as sorbents. According to a known experience, microwave irradiation effectively dehumidifies, accelerates chemical reactions, and promotes rapid bulk heating of samples. Important aspects in microwave processing are the irradiation power, contact time, and heating temperature, which have a direct influence on the result. If the raw material is overheated, a negative effect can be registered. In this case, contaminants in the pores of the sorbent sinter and reduce the pore size. The sorption surface area of the material will be significantly reduced. Therefore, in each specific case, the sorbent properties determine the selection of treatment parameters [14,68,69,70].

(3) The basis for new sorbents. In some cases, natural plant materials are used as a basis for more complex sorbents. There are ways to treat plant matter more efficiently in order to increase the efficiency of pollutant adsorption. For example, obtain a heat-inactivated hybrid biosorbent [71], bamboo composite sorbent [72], and others. In this case, the cost of producing such adsorbents is significantly higher, but also the efficiency is greater. Usually, new sorbents can be regenerated (recovering of the sorption properties after the water purification cycle) and used repeatedly.

(4) Production of biochar (BC), activated carbon (AC), and hydrocoal (HTC). As a rule, chemical, biological, and thermochemical processes are used for the conversion of natural materials into sorbents. However, the thermochemical approach is often preferred because of accelerated processing, a better product, usage of all raw materials, and energy saving. Hence, charcoal, biochar, and activated carbon are the pyrogenic carbonaceous materials often derived from a separate type of waste or a mixture of several wastes (biomass). In addition, sorbents can be prepared by using a catalyst or template [27]. Biochar (BC) is usually obtained by pyrolysis of biomass (crushed plant raw materials or a mixture of several components)—heated in an oxygen-free environment at temperatures of 400–1000 °C [73,74]. According to reference data, biochar is the lightweight black residue, consisting of carbon and ashes, remaining after the pyrolysis of biomass, and is a form of charcoal [75]. The resulting BC can be used not only in filter plants, but also combined with other wastewater treatment methods such as co-precipitation or the hydrothermal treatment process [76,77]. Vegetable raw materials are the best basis for biochar, as their composition includes various organic components such as proteins, lignocellulose, lipids, etc., as well as inorganic substances like mineral composition, moisture, and volatile fraction. Sawdust, bark and crushed tree branches, fruit seeds, citrus peels, and other agricultural waste can be used as raw materials for biochar production. BC has a number of advantages over conventional fossil charcoal, such as high availability, low cost of renewable raw materials (350–1200 USD/t), high surface density of polar functional groups, and a condensed structure [39,78]. In addition, the performance of biocoal in water treatment technologies is comparable to that of industrial coal [76,77,78,79,80,81].

If you use chemical or physico-chemical activation of raw materials, you can obtain activated carbon (AC) based on plant materials [82]. Another method to obtain a highly efficient coal sorbent, hydrocoal (HTC), is the hydrothermal carbonization of a biomass and water mixture. The mixture of solid and liquid components is reprocessed at a temperature of 100–300 °C and a pressure of 5–70 bar in a special reactor for two hours or more (sometimes up to 72 h). Hydrothermal coking is based on the boiling of wet raw materials. It can be used for industrial or agricultural waste, including non-traditional and renewable biomass sources, which are generated continuously and in large quantities [83,84]. According to published data, the efficiency of BC, AC, and HTC is comparable to the effect of industrial coal, but these data are mostly obtained in laboratory conditions [76,77,78,79,80,81,82,83].

It is known that activated carbon based on pine biomass waste are highly effective in the processes of removing various toxic organic substances (phenol, chloroform, pyridine, etc.) and metals from aqueous solutions. One of the methods for obtaining AC with a developed porous structure based on plant biomass is thermochemical activation by alkali metal hydroxides [62].

Sorption-active materials based on carbon-containing man-made wastes can be an alternative to widely used synthetic sorbents and activated natural coals, obtained mainly from underground minerals, birch wood, and other raw materials [2]. However, the highest quality activated carbon today is activated carbon derived from coconut shells, which are essentially waste. Other such high-density raw materials include the pits of apricot, peach, and other fruits, which, when subjected to carbonation, heat treatment at a temperature of 780–850 °C for 15–24 h, supply of coal without cooling for activation carried out at 920–1050 °C with water vapor, at its consumption equal 8.5–12.0 kg/kg of activated carbon. The resulting AC is characterized by increased indicators of adsorption capacity during the purification of aqueous media from ammonia: 16–17.8 mg/g [85].

In the process of studying activated carbon AUS400 obtained as a result of carbonization of crushed pine bark in the presence of potassium hydroxide at a temperature of 400 °C, it was determined that its maximum sorption capacity for iodine and methylene blue reached 1570 and 697.1 mg/g, respectively [62]. At the same time, the sorption capacity of activated carbon made from coal dust of the well-known Russian industrial grade AG-3 was 650–670 mg/g and 100 mg/g, respectively [85]. In the work, the author indicates that the sorption capacity of activated carbon AG-3 for motor oil (1.0 g/g) is at the level of active charcoal of the BAU-MF brand (1.2 g/g), but is almost 2 times lower than that of industrial charcoal of the OU-A brand (2.3 g/g) [61].

In general, new sorbents based on plant raw materials have good prospects for growth in their production. Considering the fact that most PSs are created from waste, their environmental and economic benefits are obvious. Figure 1 shows the general idea of phytosorbent production.

3.3. Mechanism of the Sorption Process

Despite great interest in phytosorbents, studies of sorption mechanisms lack experimental details. Most PS studies do not have data on the distribution of pores by size, volume, functional groups, and other parameters. There is also a lack of specially developed plant-based complex sorbents for the removal of specific pollutants. You actually have to identify the contaminant first, and then, choose a suitable sorbent, perform some pre-modeling, then optimize, estimate, and design the industrial-scale WWT system [58].

Metal ions extraction. The combined use of batch equilibration adsorption and X-ray absorption spectroscopy (XAS) provides an idea of the mechanism of metal sorption on phytosorbents. For example, an XAS spectroscopic study offered direct evidence for a similar binding environment of Pb and Cu on peat, mainly through carboxyl groups, without excluding hydroxylic groups. Different metals have different competitive effects on the adsorption of other metals. This study provides an insight into the mechanisms of the competitive adsorption of Pb, Cu, and Cd on peat: the competitive ability of metals follows the order Pb > Cu > Cd [86].

Timber includes cellulose, hemicellulose, and lignin, which are active ion-exchange compounds. Scanning electron microscopy (SEM) and infrared spectrophotometry (X-ray) were used on sawdust. Using SEM, the surface structure of maple sawdust was analyzed before and after adsorption of Cu(II) ions. Microphotos taken before the adsorption of metal ions clearly revealed the presence of asymmetrical pores and an open porous structure in maple sawdust, which may be the reason for the high adsorption of copper ions due to the provision of a large internal surface area. X-ray studies show that maple sawdust contains several functional groups that are capable of binding to metal ions, in particular Cu(II) ions [87].

In a study of the sorption of cadmium and copper ions by crushed rosehip and milk thistle seeds by atomic absorption spectrophotometry, an attempt was made to describe the mechanism of the process. The high sorption capacity of plant sorbents is explained by the fact that they contain dietary fibers, including polysaccharides. With a highly developed surface, polysaccharides are able to adsorb a significant amount of metal ions. Sorption processes occurring under static conditions, regardless of the nature of the interaction between the sorbent and the sorbent substance, are determined by diffusion, which causes the penetration of the sorbent substance deep into the sorbent structure. Therefore, one of the important factors influencing the sorption process is the duration of phase contact [88].

Thus, the adsorption of metal ions from aqueous deposits by phytosorbents is influenced by the following most important factors: the availability of adsorbent centers for metal ions, which depends mainly on the porosity of the sorbent; the area of the adsorbent surface; the presence in the composition of the sorbent of natural substances capable of exchanging ions in an aqueous solution or forming stable chemical bonds; the presence of functional groups (hydroxyl, carboxyl, and carbonyl) capable of holding/binding metal ions; the duration of contact between the sorbent and sorbate [89].

Removing dyes. Synthetic dyes are chemical compounds used to give color to various materials, obtained by chemical synthesis. Dyes are used everywhere, and are very often present in wastewater processing at many industrial enterprises [90]. As studies have shown, the mechanism of removing dyes from water by phytosorbents is based on adsorption (due to intermolecular attraction) and chemisorption (due to the formation of chemically stable bonds). For example, when studying the adsorption mechanism of the dye Safranin O (SO) on biochar obtained from rice husks, four options can be considered: (1) Porous diffusion (sufficiently large organic molecules of the dye penetrate into the porous structure of the sorbent and are retained there); (2) H-bonding of dye molecules on functional carboxyl and hydroxyl groups of biochar; (3) mutual connection of aromatic rings of the sorbent and dye by π-π interaction; (4) bond between dye’s N+ cations and sorbent aromatic rings by π+-π interaction. Chemisorption occurs through interaction between the negatively charged surface of sorbents (i.e., carboxyl (COOH) and hydroxyl (OH) groups) and positively charged ions of dye (i.e., N+) [43].

When removing masterbatch dye (MB) with a sorbent based on walnut shells (supercritical carbon dioxide), the presence of functional groups such as –OH, –NH, and N–O, which can promote the chemical binding of positively charged dye molecules to the surface of the sorbent, is of great importance. In particular, the sorption of dye molecules with the help of a phytosorbent is affected by the preliminary chemical treatment of the sorbent, which creates a high specific surface area and high-density microporous structure [91].

Removing crude oil and petroleum products. The efficiency of crude oil and petroleum product (CO and PP) recovery depends on the chemical affinity of the sorbent and sorbate material, as well as on the structure of the material [92,93,94,95]. To increase the efficiency of sorption of CO and PPs, it is necessary to select a sorbent that is polar, but at the same time contains functional groups and components close to the composition of oil [96]. In this case, sorbents act on the principle of adsorption—concentration of the adsorbed substance at the interface of phases—and absorption—dissolution of the sorbed substance in the sorbent, and the sorbent usually swells in this case. During sorption in porous phytosorbents, these processes are practically impossible to distinguish, and the process of capillary condensation is also possible in the pores of PSs. Additionally, CO and PP adsorption depends on the form which it takes in water (undissolved/dissolved) [13,28].

As noted above, the sorption ability of wood raw materials (sawdust, bark, branches) is characterized by high porosity and the presence of the high-molecular-weight components cellulose, hemicellulose, and lignin in the structure. At the beginning of the process, a rapid stage of physical adsorption is observed: the power centers of the sorbent’s surface particles intensively interact with the particles of CO and PPs due to the presence of mutual attraction, with the formation of intermolecular bonds (including hydrogen) at the interface of the contacting phases. Physical adsorption is due to the forces (orientation, induction, and dispersion forces) of the molecules’ attraction, which occurs at a very high speed even at low temperatures, so it does not require activation. On the contrary, an increase in the temperature provokes a decrease in sorption, since there is an increase in the kinetic energy of the molecules, which reduces the likelihood of their bonding with the surface of the sorbent. After the saturation of the polar functional groups on the surface of the sorbent, the adsorption of the sorbate slows down, but then, due to the presence of capillary condensation in porous sorbents, the sorption intensity increases again. Particles of CO and PP penetrate into the porous structure of the material and fill all existing voids—the absorption process. Absorption of the petroleum products in the wood sorbents occurs in two directions: vertical and horizontal. Due to the free space inside macromolecules in the structure of cellulose fibers, limited by micelles or polymer chain nodes, CO and PPs are held more firmly in comparison with other fibrous materials [94,95,96].

Thus, the sorption of CO and PPs from water by phytosorbents occurs in three stages: diffusion of oil molecules to the surface of the sorbent, penetration of oil products into the structure of the sorbent under the influence of capillary forces, accumulation of oil products in the porous and rough structure of the sorbent. The sorption of CO and PPs is associated with the properties of the functional groups of the plant sorbent, among which the hydroxyl group plays the main role. Among adsorbents, cellulose occupies a special place, as it is the main component of most plant materials.

Model description. To describe the sorption isotherms of phytosorbents with polluting substances, the classical models of authors such as Langmuir and Freundlich, are most common; less common is the model of Dubinin–Radushkevich, Temkin, Redlich–Peterson, Henry, the BET-model, and others [14,23,41,43,57,58,83,87,97,98,99]. The classic model equations and types of sorption isotherms can be found in the Supplementary Materials (Tables S1 and S2). Examples of sorption isotherms of metals, dyes, and petroleum products for PSs are also given in the Supplementary Materials (Figures S1–S6). Sorption isotherms for PSs can also be described by the various authors’ equations, including polynomial functions [49,52,91].

3.4. Efficacy of Wastewater Treatment with Phytosorbents

3.4.1. Extraction of Metal Ions

Pollutants from various industries (mining, electroplating, mechanical engineering, production of computer and household appliances, production of electrical boards and parts, factories of medical equipment, and many others) enter together with wastewater into the environment in significant quantities. Heavy metals such as copper, chromium, zinc, lead, nickel, cadmium, and other ions are particularly dangerous pollutants in that wastewater. They do not biodegrade, but are accumulated in the ecosystem and living organisms. Therefore, the concentrations of heavy metals in the environment (water and soil) are rather high. They cause the death of living organisms. Drinking water sources containing such toxins are harmful for human health. Adsorption is quite a successful, universal, efficient, and economical method, among others, for removing metal ions from WW [40,57]. Quite a few authors offer results of their research, which can be the basis for WWT.

The article [100] presents a list of reported agro-waste that can be applied as biosorbents to remove heavy metals from wastewater. Their maximum adsorption capacity (MAC) for the metals removed was in the range of 0.0655–1.637 mmol/g. This list includes olive waste (Cd), orange peel and orange bark (Cd), rice hull (Ni, Cd, Cu, Zn, Pb), yellow passion fruit shell (Cr), sugar beet pulp (Cu, Zn), carrot residues (Cu), grapefruit peel (Cd), lemon peel (Cd, Mn, Pb), and lemon resin (Mn, Pb).

The research results of the sorption of various metals from water by phytosorbents are presented in

Table 3. Data from studies of metal ion extraction from aqueous solutions using PSs.

Sorbent; Country	Metal Ions Removed from Water	Sorbent Preparation Process	Sorption Conditions	Description of Results	Ref.
Buxus sempervirens tree leaves (BSTLs); Iran	Copper, Cu (II); Zinc, Zn (II); Nickel, Ni (II)	BSTLs were washed with distilled water and dried in shadow; then, they were ground well and sieved from 0.297 to 1.0 mm; 20 g of leaf powder was soaked in 250 mL NaOH solution for 4 h; it was filtered and washed with double-distilled water to remove excess NaOH, then dried at room temperature and stored in a plastic bag.	The 0.1 g BSTL mixture was loaded, along with 10 mL of metal ion (Cu(II), Ni(II), and Zn(II)) solutions of 100–450 mg/L, into 250 mL Erlenmeyer flasks, and these were shaken at a rate of 300 rpm for 2–90 min at room temperature; pH = 2–5.	The MAC for Cu(II), Zn(II), and Ni(II) on BSTLs in a single system were found to be 19.7, 22.1, and 23.4 mg/g, respectively; pH = 5.	[21]
Aminated tea leaves (ATLs); Nepal	Zinc, Zn(II); Lead, Pb(II)	Tea leaves were washed with boiling water and dried in an oven; powdered and sieved through a 212 µ sieve; charred with concentrated H2SO4; washed thoroughly with distilled water, and then, dried in an oven; and chemically modified by using hydrazine monohydrate.	20 mL of metal solution with different concentrations (25–800 mg/L) was mixed with 25 mg of ATLs and shaken for 24 hrs at 150 rpm at 250 °C in a mechanical shaker; pH = 1–6.	The MAC of the adsorbent was found to be 120.8 mg/g for Pb(II) and 79.76 mg/g for Zn(II).	[22]
Chemically modified Pteris vittata leaves (CMPVL); Pakistan	Zinc, Zn(II)	The leaves were washed, dried, and heated for a while in an oven set to 50 °C; then, 100 g of ground leaf was immersed in a 0.1 M HNO3 solution for 24 h; for the neutralization process, 0.1 M NaOH was used; then, leaves were baked in an oven at 100 °C; for activation of biosorbent, the whole mass was added to a 0.1 M solution of calcium chloride and desiccated in an oven.	Zinc sulphate heptahydrate (ZnSO4·7H2O) was used to prepare the initial solutions; for the batch experiment, 0.1 g of CMPVL was stirred with Zn(II) solution (50 mL) for 2 h; the effect of parameters was investigated: T = 20–50 °C, IC = 20–300 mg/L, and SD = 0.01–0.12 g, pH = 2–8, as well as CT = 10–140 min.	The MAC of Zn(II) was 84.74 mg/g; the optimal conditions were IC = 100 mg/L, CT = 2 h, pH = 6, SD = 0.1 g, and T = 30 °C.	[24]
Date palm waste bark (DPW); Saudi Arabia	Chromium, Cr(VI)	DPW was washed with hot water, followed by rinsing with acetone, sun-dried for a period of 4–5 days, and ground (0.25 to 0.5 mm). Further, it was mercerized with 40 wt.% KOH for a duration of 2 h; then, rinsed with distilled water and dried (24 h) at 60 °C in an oven.	A stock solution of chromium was prepared by dissolving 2.8 g of K2Cr2O7 in one liter of distilled water; pH was changed from 2 to 9 by the addition of 0.1 n solution alkali; the ICs were 25, 50, 75, 100, 125, and 150 mg/L.	Optimal chromium Cr(VI) uptake was achieved at a solution pH of 6.5 over two hours, with removal efficiency of 88% and MAC = 22.26 mg/g.	[34]
Rice husk; Pakistan	Chromium Cr(VI)	The rice husk was collected from a rice mill; it was sun-dried and oven-dried (65 °C) for 2 and 3 days, respectively; then, it was ground thoroughly after drying and sieved (size < 250 µm).	To conduct the isotherm study, IC was varied from 10 to 250 mg/L with CT = 2 h and SD = 0.6 g/L.	Equilibrium time was achieved in 2 h and maximum Cr(VI) adsorption was 78.6% at pH 5.2 and IC = 120 mg/L; MAC= 379.63 mg/g.	[38]
Sunflower husks; Russia	Chromium, Cr(VI)	Sunflower husks were treated with a 2.5 M solution of sulfuric acid at a temperature of 60 °C for 30 min, washed with distilled water, and dried at a temperature of 105 °C to a constant weight.	IC of model solution Cr(VI) was 10 mg/L; the flasks were shaken for 2 h on an ES 20/60 orbital shaker–incubator at room temperature; then, the chromium concentration in the filtered sample was measured.	MAC = 0.44–0.85 mg/g.	[40]
Sugarcane bagasse (SCB) and orange peels (OPs); South Africa	Lead, Pb(II)	The SCB and OPs were washed, air-dried in a clean room for 7–10 days, ground using an electric grinder, and sieved using a sieve with a pore size of 5 µm to obtain particles of uniform size.	The IC of Pb(II) was changed from 10 to 50 mg/L; SD = 0.01–0.35 g/L; pH = 2–12; CT = 60–120 min.	Optimum experimental conditions could achieve up to 100% removal efficiencies for 10–20 mg/L of Pb(II).	[46]
Orange peels; Algeria, Italy, France	Chromium, Cr(VI)	Orange peels were washed, dried in the sun, and crushed; before use, they were dried at T = 105 °C to a constant weight.	IC of Cr(VI) was changed from 10 to 80 mg/L; SD = 10 g/L.	The highest removal efficiency was 99.2%, and it was achieved in an acidic solution (pH = 2) after 5–15 min.	[47]
Pomelo peel; China	Lead, zinc, chromium, copper	Co-pyrolysis of Fenton sludge (FS) and pomelo peel (PP) at different temperatures (300–600 °C) was performed (the mass ratio of 1:1).	Modified PP was used to recover metals from sewage sludge after oxidation in co-pyrolysis.	The recovery rates of Cu, Zn, Cr, and Pb were 71.77–86.85%, 84.0–95.8%, 89.78–104.25%, and 87.24–101.90% respectively.	[50]
Rapeseed waste biomass (RWB); Romania	Lead, Pb(II); mercury, Hg(II)	Rapeseeds were washed, dried for 6 h at 105 °C, ground, and sieved; the obtained rapeseed biomass was then subjected to an oil extraction process, using n-hexane as solvent, to obtain rapeseed waste biomass (RWB); then RWB was air-dried for 3 days at room temperature.	To obtain a stock solution of metal ions (10−2 mol/L), lead nitrate and mercuric nitrate were used; pH = 1.0–6.5; the concentration of metal ions was kept constant 40 mg/L; SD = 4.0–20.0 g/L; CT = 3 h; T = 10–55 °C.	The MAC was higher in the case of Pb(II) (61.97 mg/g) than in the case of Hg(II) (51.32 mg/g); optimal pH = 6.5 for Pb(II) and pH = 4.0 for Hg(II); SD = 4.0 g/L; CT = 3 h; T = 25 °C.	[53]
Pomegranate seed waste; Iran	Chromium, Cr(VI)	The pomegranate seed powder was separated and rinsed with deionized water; after boiling in water for 2 h, the seeds were dried at a temperature between 100 and 105 °C in an air oven for 24 h, then milled; standard sieves with 40–100 mesh sizes were used to sieve the adsorbent.	The adsorption studies were conducted at room temperature (25 ± 2 °C); CT = 15–180 min; pH = 2–6 and IC = 10 mg/L; SD = 0.2–0.6 g/100 mL.	MAC = 3.313–1.6 mg/g.	[54]
Linden and poplar sawdust; Serbia	Iron, Fe(II); manganese, Mn(II); zinc, Zn(II); copper, Cu(II)	The sawdust was sieved through a set of laboratory sieves with a sieve fraction < 0.4 mm.	Sorbent samples (1 g) and 50 mL of metal ion solution were placed in beakers equipped with magnetic stirrers; the contact time was ensured by the stirring.	MAC is achieved at 3.5 < pH < 5. It was found that poplar and linden sawdust have almost equal MACs against copper ions. Removal efficiency of Cu was about 80%; Fe, above 10%; Zn and Mn were between these two.	[58]
Treated olive stone (TOS); Algeria, France	Cadmium, Cd	TOS material was prepared by treatment of olive stones (after washing, drying, and grinding) with concentrated sulfuric acid at room temperature followed by a subsequent neutralization with 0.1 M NaOH aqueous solution.	An aqueous solution of safranine was used. The sorption process occurred in less than 15 min of contact time.	MAC of cadmium was 128.2 mg/g.	[64]
Heat-inactivated hybrid biosorbent from date seed waste (HI HB); Pakistan	Lead, Pb(II)	The pre-washed, oven-dried (60 °C) date seed waste was autoclaved for 15 min at 121 °C; then, 5.0 ± 0.025 mL of Ganoderma lucidum mycelium suspension was added and agitated at 100 rpm, for 7 days at 30 °C (HB); the HB was autoclaved at 121 °C for 15 min, and then, oven dried for 72 h at 70 °C to obtain heat-inactivated hybrid biosorbent (HI HB).	The effects of pH = 2–4.5, SD = 0.05–0.3 g/L, IC = 25–400 ppm, and T = 30–70 °C were checked by varying one parameter while keeping the other parameters constant. The effects of the presence of metal ions (Mg, Al, Cu, and Zn) on the MAC of the immobilized HI HB in a binary system were also studied.	MAC of immobilized HI HB was 365.9 mg/g at IC = 100 mg/L, with the Langmuir isotherm model presenting the best fit.	[68]
Maple wood sawdust; Canada	Copper, Cu(II)	The maple wood sawdust samples were sieved through 20–50 mesh and was used directly for adsorption experiments without any physical or chemical treatments.	The required concentrations of Cu(II) were obtained from the 1000 mg/L reference solution by diluting with distilled deionized water to concentrations of 5.0–100 mg/L.	The isotherms studies revealed that an MAC of 9.51 mg/g for maple wood sawdust was obtained at pH = 6.0. The experimental MAC value was only 6.1 mg/g.	[87]
Spent coffee grounds (SCGs); Korea	Cadmium, Cd(II)	The collected SCGs were air-dried for two weeks, and then, passed through a 0.5 mm sieve; the sieved SCGs were stored in polyethylene bottles until used and were not subjected to any physical or chemical pretreatment prior to use.	The SCGs (1 g) were reacted with 40 mL of Cd(II) solution (IC = 0.1–120 mM) prepared in a 2 mM Ca(NO3)2 solution, using a shaker for 2 h. The amount of ions in the 0.45 µm filtrate was determined using ICP-OES after acidifying with 2% HNO3.	The rate of Cd(II) removal remained constant, at 71.19%; pH = 4–8. MAC =19.32 mg/g.	[96]
Cassava; China	Chromium, Cr(VI)	Activated carbon (ACDCS) based on dewatered cassava sludge (DCS) and ZnCl2 was obtained. The activated DCS was pyrolyzed and carbonized at T = 673–973 K for a time of 30–120 min. After cooling, the obtained samples were fully pickled with 1.0 mol/L HCl solution, and washed.	K2Cr2O7 (2.829 g) was dissolved in 1 L of ultrapure water to prepare a Cr(VI) stock solution (1000 mg/L). The IC of Cr (VI) was 1–100 mg/L. The effects of SD = 0.2–2.5 g, pH = 2–13, CT = 0–180 min, and T = 283–323 K on Cr(VI) removal were investigated.	MAC= 8.01 mg/g; CT = 3 h; SD = 1 g/L; pH = 2.	[97]
Peat; Russia	Copper, Cu(II)	Used: (1) natural peat (NP); (2) mechanically activated peat in a planetary mill (MAP); (3) modified peat with mechanochemical activation by dry sodium percarbonate (MCAP).	The adsorption of copper ions by NP, MAP, and MCAP was studied for IC = 10–150 mg/L with a CT of 0.25–12 h.	MAC = 24.1 mg/g (for NP); MAC = 42.1 mg/g (for MAP); and MAC = 16.0 mg/g (for MCAP).	[98]

Note: MAC—maximum adsorption capacity; IC—initial concentration; CT—contact time; T—temperature; SD—sorbent dosage.

.

3.4.2. Removing Dyes

Synthetic dyes are part of the wastewater of the textile, leather, food, chemical, pharmaceutical, and other industries. Various dyes can also be used as indicators of laboratory tests of sorbent materials. Data on the sorption of water-dissolved dyes by biochars, hydrocarbons, or plant-based activated carbons are often published. The raw materials for such sorbents are rice husks and straw, nut shells, fruit seeds, potato, and fruit peels. There is information about the sorption of dyes by other sorbents that are not exposed to high temperatures before the formation of the carbon.

Table 4. Data from studies of dye removal from aqueous solutions using PSs.

Raw Materials; Country	Dye Type	Sorbent Preparation Process	Sorption Conditions	Description of Results	Ref.
Peat; Russia	Methylene blue (MB)	Washing, drying (T = 20 °C), and microwave treatment of the peat samples.	With microwave power from 60 to 600 W for 60 min.	With increasing power, the adsorption of MB decreased by 2 times: from 55 to 28 mg/g.	[20]
Banana peels; Ethiopia	Reactive blue 19	The banana peels were washed, dried, and crushed. Further, 20 g of the sample was subjected to 350 °C at a rate of 10 °C/min for 3 h in a furnace under an N2 environment; then, samples were treated by sulfuric acid solution at T = 50–90 °C.	CT = 20–140 min, pH = 1.0–7.0, SD = 1–4 g/L, and IC = 20–80 mg/L.	The removal efficiency of reactive blue 19 achieved was 70–94%; CT = 60 min, pH = 3, SD = 2 g/L, and IC = 40 mg/L.	[49]
Coconut shells (CS), coconut cellulose (CC), and treated coconut cellulose (MCC); Colombia	Anionic dye— Congo red (CR)	Coconut shells were rinsed, dried (60 °C), and ground to a size of 0.8–0.35 mm (CS); CS submerged in 4% NaOH solution, then mixed (80 °C) for 2 h (CC); CC contacted with 10% NaClO2 solution and liquid CH3COOH added and mixed for 24 h (MCC).	5 mL of the solution was set in contact with the adsorbent at 250 rpm at room temperature for 24 h; the final RC concentration was measured by spectrophotometer; the ICs were 25, 50, 75, 100, 125, and 150 mg/L.	MCC achieved a removal efficiency for CR of 99.9%. CS showed slow kinetics in the initial stages, whereas CC and MCC achieved 78% and 99.98% removal at CT = 120 min, respectively; an equilibrium was reached at 480 and 20 min, respectively. MCC, CC, and CS achieved MACs of 256.12 mg/g, 121.62 mg/g, and 17.76 mg/g, respectively.	[51]
Orange seed (OS) powder; Peru	Dyes from real textile waste- water (TW)	Seeds were washed with distilled water, dried (at 60 °C), and ground using a hand mill. The obtained powder was submitted to fat extraction for 6 h in a Soxhlet using n-hexane.	The ranges were pH = 2–6, SD = 0.5–2.5 g/L, stirring speed = 80–160 rpm, T = 25–35 °C, and CT = 60–120 min. All of the experiments were carried out using 0.05 L of TW.	An adsorption process using fat-free orange seed powder successfully removed (92%) the dyes from TW; optimal CT = 30 min.	[52]
Treated olive stone (TOS); Algeria, France	Safranine	TOS material was prepared by treatment of olive stones with concentrated sulfuric acid at room temperature followed by a subsequent neutralization with 0.1 M NaOH aqueous solution.	An aqueous solution of safranine was used. The sorption process occurred in less than 15 min of CT.	MAC = 526.3 mg/g.	[66]
Aspidosperma polyneuron sawdust; Colombia	Methylene blue (MB)	The sawdust was washed, dried in the sun for 6 h, and then, in an oven at 70 °C for 8 h. The dry sawdust was introduced into a muffle with a heating rate of 5 °C/min up to 250 °C. Activation by sonication with H3PO4 and functionalization with urea (6 M).	To investigate the effect at IC = 60 ppm; pH = 7.	The obtained MAC was 12.4 mg/g due to its favorable physico-chemical properties derived from sonication, activation with phosphoric acid, and functionalization.	[82]
Coconut shell (CS-HC) and oak wood hydrochars (oak-HC); UK	Methylene blue (MB)	About 192 g of the biomass was mixed with 798 mL of distilled water in a 2-L batch autoclave; HC was conducted by heating the biomass–water mixture in the sealed autoclave at three different temperatures of 200, 220, and 250 °C for a residence time of 2 h.	The IC of MB was 50–300 mg/L; CT = 0–240 min; and pH = 2–12.	Efficiency of extracting MB from water on hydrochars: for CS-HC it was 80.22–95.01%, for oak-HC it was 90.18–93.42%.	[83]
Rice straw biochar (RSB) and magnetic rice straw biochar (MRSB); Vietnam	Organic dye— Safranin O (SO)	RSB was made by pyrolysis in a furnace at 500 °C, using a heating rate of 10 °C/min for 2 h in an oxygen-limited environment; the MRSB was produced via the chemical precipitation of Fe2+ and Fe3+ (pH = 10).	To determine the optimal conditions, a series of preliminary tests were performed with various solutions: pH = 2–10, SD = 1–5 g/L, IC = 10–200 mg/L, and CT = 1–720 min.	The MAC of MRSB was found to be 41.59 mg/g; for RSB the MAC was 31.06 mg/g.	[43]
Walnut shell (WS); China	Masterbatch dye (MB)	Supercritical carbon dioxide (SC-CO2) pretreatment technology was developed to prepare porous biochar from WS (200–400 °C); next, the biochar was activated with KOH solution, followed by heat treatment (700 °C) and washing with hydrochloric acid.	IC = 0.1–5 mg/L; activated and non-activated sorbents were tested.	The MAC was from 129.5 to 540 mg/g depending on sample preparation.	[91]
Mandarin peel; Korea	Methyl orange (MO) and fast green (FG)	The mandarin peel biochar (M-biochar) and chemical activated biochars by NH4Cl (MN-biochar) and ZnCl2 (MZ-biochar) were used. The peels were washed, dried, crushed, and heated to T = 700 °C using N2 gas.	The SD was 0.1–3 g/L; pH = 7; 25 mL of solution (MO or FG) was added; the concentration of each dye was 10 mg/L.	The best results were shown by the MZ-biochar, which extracted 93–99% of the MO and 87–99% of the FG. Efficiency of M-biochar was only 1–8%, and of MN-biochar 7–24%.	[99]
Coffee grains, almond shells, pistachio shells, date pits, jute sticks, sunflower shells, peanut shells, and grapevine sticks; Palestine	Methylene blue (MB)	8 samples of biochar (BC) and activated carbon (AC) were prepared from the biomass of plant raw materials: grinding, drying, and washing of raw materials; activation of raw materials with a solution of ZnCl2 during heat treatment at T = 380 °C (in case of AC); production of BC and AC by pyrolysis with T = 700 °C for 1 h.	To investigate the effect, IC = 25–300 mg/L was tested. In these experiments, the adsorption tests were conducted with SD of 0.5% w/v, at a pH of 7.0 for an experimental time of 24 h.	The efficiency of MB removal with biochars was 77.2–99.94%, with the lowest results for almond shells (89%) and date pits (77.2%), for other sorbents it was 90% and higher. The efficiency of dye removal with activated carbon turned out to be excellent, up to 100%, while it was lowest for coffee beans (80%).	[101]
Burmese grape seed (BGS); Bangladesh, Italy	Methylene blue (MB)	The collected BGSs were chopped, air-dried for 2 days, mixed with a solution of H3PO4 (40%) in a weight ratio of 1:2, followed by drying in an air oven at 80 °C for 3 h; then, mixed with 50% (w/v) KOH solution. Then, this was dried at 100 °C for 2 h. Finally, the particles were carbonized at T = 500 °C for 3 h to be converted into biochar.	The SD for each experiment was 5 mg. The influence of pH on adsorption was investigated for pH between 3.0 and 9.0. Kinetic tests were performed using an MB dye with IC = 60 mg/L at T = 27 °C and examined at a time ranging from 5 to 90 min.	The maximum removal percentage was ~85%, and MAC = 166.30 mg/g.	[102]
Ziziphus jujuba stones (ZJS); Algeria	Methylene blue (MB)	The stones were crushed and sieved to obtain a powder, which was washed, and dried at 50 °C for 24 h before undergoing treatment; 1 g of the ZJS powder was mixed with 1 g of the solution of H3PO4 (1 M) and stirred with a magnetic stirrer for 24 h at room temperature.	IC = 50–500 mg/L; 50 mg ZJS mixed with 50 mL solution; CT = 24 h, and pH = 2–12.	The H3PO4 treatment significantly and positively enhanced adsorption performance, with the MAC increasing from 62.25 mg/g for untreated ZJS to 160.85 mg/g for H3PO4-treated ZJS. A 100% efficiency for MB removal was achieved at pH = 10.	[103]

Note: MAC—maximum adsorption capacity; IC—initial concentration; CT—contact time; T—temperature; SD—sorbent dosage.

presents the results of studies on the sorption of some dyes from water using PSs. These are mainly laboratory studies of synthetic dye solutions. However, there is evidence of an experiment with real wastewater from alpaca wool processing (Peru), where a sorbent based on orange seed powder was used [52]. In all the cited works, the influence of the method of obtaining the sorbent, the time of contact with the solution, and the pH on the sorption activity are noted.

3.4.3. Removing Crude Oil and Petroleum Products

Crude oil and petroleum products are one of the main types of pollution and are present in almost all types of wastewater. First of all, this is typical for oil production and oil refining enterprises (pollution concentrations are from 400 to 15,000 mg/L). In addition, in the rain and melt waters of a modern city, petroleum products from vehicle exhaust are definitely present (pollution concentrations are 20–1000 mg/L) [104]. The main industrial sorbents for the collection of CO and PPs have different bases—peat, wood pyrolysis product, polyurethane foam, polypropylene, and others. The modern market of sorbents for the collection of CO and PPs is very diverse, but the oil industry employs only those that are easy to use, have high efficiency, and are low cost. Phytosorbents actively claim this role.

Table 5. Data from studies of crude oil and petroleum product removal from aqueous solutions using PSs.

Raw Materials; Country	Pollutant	Sorbent Preparation Process	Sorption Conditions	Description of Results	Ref.
Moss, reindeer moss	Crude oil, oil product	Washing, drying (T = 20 °C), and microwave treatment of the moss and reindeer moss samples.	With microwave power of 600 W for 1 min for initial concentrations of oil products dissolved in water of 250 mg/L.	Sorption capacity of oil products increased by 10–15%; the MAC was reached at 326–338 mg/g.	[14]
Peat	Crude oil, oil product	Washing, drying (T = 20 °C), and microwave treatment of the peat samples.	With microwave power of 600 W for 1 min for initial concentrations of oil products dissolved in water of 250 mg/L.	Peat sorption capacity of oil products increased by 7.5%; the MAC was reached at 408.1 mg/g.	[14]
			With microwave power from 60 to 600 W for 60 min.	MAC = 2.5–2.73 g/g.	[20]
Maple and birch branches	Petroleum product	Grinding, washing, drying (T = 105 °C), and microwave treatment of the samples.	With microwave power of 600 W for 1 min.	Increase in the MAC for petroleum products by 7.2–30%.	[15]
Pine sawdust	Petroleum product	Grinding, washing, drying (T = 105 °C), and microwave heating of pine sawdust.	With microwave power of 600 W for 1–2 min; initial concentration of model solutions was from 5 to 35 mg/L.	Increase in the MAC for petroleum products by 3.7–4 times for IC of less than 5 mg/L and by 1.2–1.3 times for IC = 16–35 mg/L.	[15,28]
Rice husk	Petroleum product	Combustion in a microwave furnace.	Treatment at T = 500–800 °C for 288–384 h.	The efficiency of petroleum product removal from water was 78–98%.	[67]
Rice straw (RS); Malaysia	Diesel	The RS was cut to 4–5 cm in length from the base throughout the length of the stalk; it was washed twice thoroughly with tap water to remove debris and sun-dried for 7 d (5 h per day) until it reached a constant mass; heat pretreatment (90–140 °C, 10–70 min), and diesel concentration 5–30%.	12 g of RS sample was placed in the holder, which was then placed in a plastic bottle (T = 25 °C); the mixture of diesel (40 mL) and seawater (400 mL) was then poured into the bottle; the mass of each RS sample was measured after 10 min contact time.	The pretreated (at T = 120 °C) RS samples displayed the highest level of MAC (2.3 g/g) and also the most efficient oil absorption (51.67%).	[42]

Note: MAC—maximum adsorption capacity; T—temperature.

presents the results of sorption studies of crude oil and petroleum products by PSs.

3.4.4. Removing Other Harmful Pollutants

Wastewater is a multi-component complex system, it contains not only common pollutants (oil products, metals, various combinations of acids and salts, etc.), but also pharmaceuticals, antibiotics, nutrients, etc. These results can be fully applied in WWT. The main results of the research are shown in

Table 6. Data from studies of the extraction of other harmful pollutants from aqueous solutions using PSs.

Raw Materials; Country	Pollutant	Sorbent Preparation Process	Sorption Conditions	Description of Results	Ref.
Peat	Iodine	Washing, drying (T = 20 °C), and microwave treatment of the peat samples.	With microwave power from 60 to 600 W for 60 min.	With increasing power, the MAC of iodine increased by 1.2–1.4 times (from 115 to 150 mg/g).	[20]
			With microwave power of 900 W for 12 min until 450 °C.	Iodine adsorption activity increased from 11.4% to 19.1%.	[75]
Banana peels; Malaysia, Korea, Saudi Arabia	Turbidity	Modified banana peel powder was prepared using a green approach, consisting of microwave treatment at a power of 800 W for 0.5 min.	For all of the experiments, synthetic turbid water with kaolin clay was used. NaCl was also added to the water for intensification of coagulation.	The optimum sorbent dose was found to be 0.4 g/L for modified banana peel, with turbidity removal of up to 90%.	[48]
Composite based on bamboo side shoots; China	Arsenic (As)	Porous α-Fe2O3/Fe3O4/C composite with the bamboo bio-template (PC-Fe/C-B compo-site). Wastewater was filtered through the fixed-bed column.	Influent flow was 5.136 mL/min, pH = 3, As(V) concentration was 20 mg/L, adsorbent particle size < 0.149 mm, adsorption temperature was 35 °C, PC-Fe/C-B dose was 0.5 g, and breakthrough time was 50 min.	MAC = 21.0 mg/g.	[72]
Modified cellulose from corn stalks; Colombia	Nitrate and phosphate	Cellulose was obtained from the CS, and dried for 3 h at 60 °C; it was mixed with 100 mM CTAC; it was rinsed and dried. The biochar was prepared by impregnating the biomass for 24 h with H2SO4 diluted at 50% v/v; the carbonization was performed at 520 °C for 30 min.	Equilibrium experiments were conducted using five different concentrations of the anions from 20 to 100 mg/L for 24 h, using a volume of 100 mL and 2 g/L of adsorbent (T = 25–45 °C). In the study of adsorption kinetics, the contact time changed within the limits of 5–1440 min.	The best MACs obtained for nitrate and phosphate were 15.8 and 23.2 mg/g, respectively.	[105]
Rice and corn husk; Colombia	Nitrate and phosphate	Rice husk biochar with/without activation used; corn straw biochar chemically modified with chemical activation.	No data.	It was found that 95–99% of nitrogen and phosphorus can be removed with biosorbents made from rice husks and corn residues.	[106]
Cassava peels; Uganda	Pharmaceuticals: carbamazepine (CBZ), clarithromycin (CLN), and trimethoprim (TRM)	Dry peels of the cassava were pulverized and soaked in 150 mL of 4.0% w/v NaOH; it was placed in a platinum crucible and heated to 400–900 °C for 20 to 180 min; thus, an activated carbon CPAC was obtained.	Initial concentration of 20 mg/L for all pharmaceuticals and a CPAC dosage of 2.0 g/L were used.	The MACs were 25.907, 84.034, and 1.487 mg/g for CBZ, TRM, and CLN, respectively.	[107]
Orange peels; Hungary, Iraq, and others	Pharmaceuticals: carbamazepine (CAR) and diclofenac (DIC)	The orange peels were washed, dried, and crushed. Then, at a temperature of 550 °C, pyrolysis was performed. Next, activated carbon (MOP) was obtained using 5 m solutions of sulfuric and nitric acid. Then, it was washed to pH = 7 and dried. Also, LMOP was obtained by immobilization of laccase.	Initial concentration of 25 mg/L for two pharmaceuticals and sorbent dosages of 50 mg mixed with 20 mL aqueous solution, were used.	MOPs revealed removal efficiencies of 73.34% and 82.51% for CAR and DIC, respectively. The LMOPs had 81.98% and 90.53% removal percentages for CAR and DIC, respectively.	[108]

Note: MAC—maximum adsorption capacity.

.

3.5. Regeneration and Disposal of Phytosorbents

Many natural sorbents are not regenerated after sorption of contaminants, since traditional regenerators (acute steam, solutions of acids, salts, and alkalis) destroy the natural structure of sorbents. For example, the destruction of peat’s structure by acid solutions leads to a decrease in porosity and filtration capacity, and this makes the filter medium unsuitable for further use [61]. In this case, it is much easier to use used peat briquettes as fuel. This technology is especially suitable after removal of oil and petroleum products: spent loads of the sorption filters are dried, briquetted, and sent for reuse in the boiler houses. In this case, oil and petroleum products increase the calorific value of the fuel material. This technology is suitable for other woodworking waste, which is usually burned in kilns [14].

More resistant materials, such as sawdust, shredded branches, tree bark, and the like, can be regenerated with an acid solution without much destruction of the structure. This type of regeneration is suitable for removing metal ions from used PSs. But the integration of such a process in the life cycle of a PS requires a careful selection of the type and dose of reagents in each case, and also significantly increases the water treatment cost [15,16,17]. In addition, there is a problem with the disposal of spent regenerative solutions saturated with contaminants.

The most suitable for chemical regeneration are biocoal, activated carbon, and hydrocoal, which can withstand several regeneration cycles (usually 1–3 cycles), with a decrease in sorption capacity of no more than 50%. In this case, a thorough economic justification and control of the loading conditions of the filtration facilities are required. Spent BC, AC, and HC can also be disposed of as fuel for WWTP needs.

Most often, spent sorbents of all types are incinerated, which leads to secondary environmental pollution: solid and gaseous combustion products are formed. Disposal of the wastes also has a negative impact on the environment: large areas are allocated for disposal; the release of toxic substances into the atmosphere, as well as seepage into the soil and groundwater, is possible. Such disposal is unsafe and economically unprofitable [36,109]. The recycling of used sorbents on a plant basis may be more environmentally friendly due to their biosphere compatibility, but this problem remains largely unsolved.

Table 7. Advantages and problem areas of PS use.

Advantages

Problem Areas

- Compliance with environmental requirements - Availability in a natural cluster - Can be production waste - Availability of the necessary sorption qualities - Minimal pre-preparation - Low release of secondary contaminants into the water - Often the absence of a regeneration stage - Low production and regeneration costs - Removal of new types of pollutants (e.g., pharmaceuticals)

- Low sorption capacity for natural sorbents - Complex chemical modification is required in order to improve the sorption properties and structure strength - Selection methods of activation, modification, and regeneration individual for each case - Complex activation/modification leads to an increase in the cost - Transition of the pollutants from water into the sorbent (this causes new waste generation, but new waste needs to be disposed) - There is not enough data on the practical application at the WWTP and the further disposal of used PSs

presents the main benefits and trouble spots of the phytosorbents that were noted in the studies referred to in this review.

4. Conclusions

It is generally acknowledged that sorption treatment is effective for all types and concentrations of impurities in water, while there is practically no secondary pollution. However, in order to reduce the cost of purchasing, activating, modifying, and regenerating sorbents, the search for economical, effective, and environmentally friendly sorption materials is currently being actively conducted. Of particular interest are plant-based sorbents (phytosorbents; PSs), which have a number of advantages compared to synthetic sorbents: accessibility, biosphere compatibility, low cost, and others. Along with this, there are problems with the use of the phytosorbents, which must be solved in the future: sorption capacity increase, reuse, and disposal of spent sorbents.

There is a lot of information on the removal of metal ions and dyes from wastewater with the help of PSs (about 80% of all studied sources). There are far fewer studies on the extraction of crude oil, petroleum products, nitrogen, phosphorus, and other compounds (less than 20% of publications). Only recently has there been information on the removal of complex types of pollutants using PSs, such as drugs and toxins. Unfortunately, phytosorbents are rarely used in practice on an industrial scale (except for some types of biochar), because they need even more testing and experiments. There is no doubt that phytosorbents have a great potential for future research as a cheap raw material and the basis of sorbents used in wastewater treatment technologies.