1 Introduction

Life depends on water, and there is a finite supply of freshwater on earth [1, 2]. According to the World Health Organization [3], about 50% of the world’s population is anticipated to reside in water-harassed regions by 2025. Water is necessary for human consumption, but freshwater resources are scarce. Water is necessary for agriculture, industry, and human consumption. It is obvious that industrial and agricultural activities can contaminate water [4, 5]. According to the literature, a variety of contaminants, such as heavy metals (HMs) [6, 7], dyes [8,9,10], pesticides [11], pharmaceuticals [12,13,14,15], personal care products [16], hormones [1], viruses [1], radioactive elements [17], and phenol-derived chemicals [1], can be found in industrial wastewaters, groundwater, and surface water [1]. It is well established in the literature that the presence of these compounds in waters and wastewaters poses a threat to the ecosystem and the general welfare [18,19,20,21,22,23]. In reality, it is our responsibility as scientists and researchers working in the field of environmental science to create the tools, materials, techniques, and technologies needed to manage, purify, and reuse water.

Globally, the viability of water resources and the safety of the environment are now seriously threatened by surface water pollution [24, 25]. Surface water, however, could not be remedied using conventional concentrated treatment methods, such as conventional coagulation and sedimentation for the polluted surface water, due to its abundance and widespread distribution. (e.g., adsorption, extraction, ion exchange, and membrane separation). Additionally, the total nitrogen and total phosphorus concentrations in contaminated surface waters are typically lower than those in raw wastewater, i.e., 10 mg/L and 1.0 mg/L, respectively. As a result, it may not be practical or cost-effective to use the same treatment methods and machinery used to treat domestic or commercial wastewater [26]. In order to stop the degradation of surface water quality (such as eutrophication) and maintain a healthy aquatic ecosystem, it is essential to develop novel remediation technologies.

Nowadays, water contamination is a worldwide problem that affects the majority of nations. Monitoring water quality is necessary so that officials are made aware of water pollution and can take swift action [27,28,29]. Thus, a method for monitoring water quality is thought to be the best way to offer early evaluations of contaminants in water. More research is required to determine the advantages and disadvantages of the different traditional and contemporary approaches to water quality monitoring and remediation techniques. The approaches include cyber-physical systems, virtual sensing, optical techniques, artificial intelligence, the Internet of Things (IoT), micro- and nanorobots and other innovative smart systems [28,29,30,31,32,33,34,35,36,37]. Thus, a method for monitoring water quality is thought to be the best way to offer early evaluations of contaminants in water [27, 38]. Government, non-public sectors, and society all need to treat water pollution as a significant problem. A reliable system for continuously monitoring water quality in real-time is necessary to help officials decide on the best course of action and to produce useful output data. Thus, there is a need to look into earlier water quality monitoring techniques, contrast traditional and contemporary techniques, and examine various techniques from different nations.

The cyber-physical system is a device that seamlessly integrates physical elements into a computational algorithm. The future of embedded computers is cyber-physical system. Contrary to embedded systems, a full-fledged cyber-physical system is typically designed as a network of cooperating components with physical input and output rather than as stand-alone devices. In the end, cyber-physical system provides more advantages because it manages the intricacy of data points produced by numerous sensor nodes, also referred to as sensor arrays, by using a user-friendly decision support system such as fuzzy logic [39]. Lee [40] asserts that Wiener, who created the technology for aiming and launching anti-aircraft guns during World War II, was the one who created the cyber-physical system. IoT, Industrial 4.0, the Industrial Internet, and machine-to-machine communications are just a few of the modern technologies that make up cyber-physical system. Cyber-physical system can be used for a variety of purposes, including healthcare applications where the system gives patients real-time access to medical experts and services [41]. To create effective working and living conditions, cyber-physical system can also be used in big commercial and residential structures [41]. The four components of cyber-physical system for water sustainability—sensing and instrumentation, communications and networking, computing, and control—have been addressed by Wang [42] along with the opportunities and difficulties they present. Imen and Chang [43] conducted additional research on the cyber-physical system for water sustainability and developed a five-level architecture for the cyber-physical system to manage drinking water infrastructure in a smart and sustainable manner. These levels include smart connection level, data-to-information connection level, cyber level, cognition level, and configuration level. A water quality monitoring device using CPS, which included sensing and computing tools for computational modelling, was developed by Bhardwaj et al. [39]. However, a systematic review by Zainurin et al. [27], describes in detail the improvements in water quality monitoring based on different sensing techniques.

As reported by Wang et al. Wang et al. [44], physical, chemical, and ecological techniques have all been studied in recent years to remove contaminants from surface waterways (see Table 1 as adapted from Wang et al. [44],). Surface water pollution can be briefly reduced through physical methods such as dredging sediment, mechanical algal removal, aeration, and water diversion, but these effects are transient [45, 46]. The redox potential and pH of surface water must be changed by chemical agents and adsorbents in order for suspended particles and organic matter to be adsorbed and precipitated [47]. Water pollutants will be separated and recovered by agents and pollutants, or they will be changed into harmless compounds. Although the chemical technique works quickly, it requires the addition of numerous expensive chemical agents that are also likely to result in secondary pollution. (e.g., chemical sludge). Additionally, the sewage treatment plants must treat the produced chemical sludge, which adds a significant quantity of additional work and complicates the operation of sewage treatment plants [44].

Table 1 Key methods for remediating surface water
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Ecological remediation is a recent innovative in-situ remediation technique that uses microbes and plants to jointly remove pollutants from the atmosphere [48,49,50,51]. Utilizing the metabolic processes of plants and microbes to ingest, assemble, or degrade environmental pollutants is the primary method of in-situ ecological remediation. When compared to other remediation methods, in-situ ecological remediation has a number of benefits, including cheap costs, fewer negative environmental effects, and no secondary pollution production [44]. For the bioremediation of contaminated surface water, numerous in-situ remediation processes, including ecological floating bed techniques and constructed wetlands, have been devised and have shown satisfactory results [48, 50].

An innovative technique for water remediation known as ecological floating beds is based on the conventional constructed wetland, which is characterized by the predominance of terrestrial or aquatic plant growth on the surface of a body of water [44]. Plants absorb pollutants from the water during their development phase and offer attachment sites for microorganisms to grow through their developed plant roots as an essential part of the ecological floating bed [44]. Using water spinach and sticky rice, Sun et al. [52] examined the viability of ecological floating-bed systems for remediation and discovered that the rates of total nitrogen elimination were 92.3% and 81.2%, respectively. In the meantime, adding the right carrier to the ecological floating bed can encourage plant development and enhance their capacity to withstand contamination stress. Green zeolite was found to be the finest substrate for Acorus calamus L. to uptake metals, and removal efficiencies of Cr and Cd were up to 95.24% and 91.8%, respectively, in research using the plant Acorus calamus L. in the ecological floating bed [53].

As reported by Wang et al. [44], the integrated remediation in rural river network area (RRNA) project, sponsored by the China’s Ministry of Science and Technology, has been started to achieve the integrated remediation of surface/groundwater and soil in the rural river network area (see Fig. 1 as adapted from Wang et al. [44]). Three Chinese institutions (Tongji University, Central South University, and Donghua University) are partners in the 38-months (that is from 2019 to 2022) integrated remediation in RRNA study. Accordingly, the integrated remediation in RRNA will concentrate on developing key technologies for surface water, underground water, and soil environmental remediation in rural river network areas; implementing the integration of remediation technologies for surface/groundwater and soil in rural river network areas; and applying interdisciplinary and methodological knowledge to clarify the transportation and transformation of pollutants in water and soil during surface/groundwater interaction [44].

Fig. 1
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System for integrated remediation in RRNA for contamination surface water/groundwater and soil

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As recently reported by Urso et al. [38], micro- and nanorobots are designed to expedite and enhance the water purification process (remediation). The adsorption of heavy metals and soluble organic contaminants at the solid–liquid interface is improved by the mutual interaction between their active motion and material characteristics [38]. However, details on smart micro- and nanorobots for water remediation (purification) are contained a recent study by Urso et al. [38].

Water treatment uses a variety of technologies (see Fig. 2), each with benefits and disadvantages [1]. However, the emphasis in this present review work is on adsorption. A method of contaminant removal known as “adsorption or biosorption” involves the passive binding of contaminants such as HMs and/or dye ions into non-living biomass (bio-sorbents). The method is similar to bioaccumulation, where pollutants are actively metabolized by living things [54]. It involves the removal of contaminants ions after their binding to both living and non-living biomass. The rate of biosorption is affected by a number of variables, including the size and type of the bio-sorbent, ionic strength, biomass dose, temperature, initial pH, and solvent concentration [55]. A fluid (in this instance, water) and a solid phase are involved in the unit operation of adsorption/bio-sorption (the adsorbent). One or more dissolved contaminants are found in the fluid phase (the adsorbate). Water is purified as the dissolved pollutants are moved from the liquid phase to the adsorbent surface [56,57,58].

Fig. 2
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Treatment methods or technologies for contaminants in water and land

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The active binding sites employed, the protein structure, and the functional groups of biosorbents are the variables that affect the solubility of contaminants in solutions. Techniques like surface complexation, chelation, ion exchange, and biologic adsorption are used in the biosorption process to remove trace metals (or HMs) and dyes from contaminated surroundings. Additionally, biosorption is influenced by the environment in which pollutants are present and the particular microbial cell metabolism process in use [2]. Due to the high efficiency of the technique, the capacity to reuse biosorbents, the environmentally friendly nature of the technique, and the capacity to recover the pollutants with little secondary waste, the biosorption of HMs and dyes, especially from polluted water, is widely acknowledged to be effective [55]. Microbial cells, either dead or alive, like those of fungi, algae, and bacteria, are examples of prevalent biosorbents. The cell walls of the microbial biosorbents contain functional groups like carbonyl, hydroxyl, carboxyl, and amino moieties that interact with the pollutants' ions to remove them from contaminated solutions [59]. Toxic trace metals (or HMs) and dyes can be removed by extracellular accumulation and precipitation or by precipitation on the cell membrane, depending on where the pollutant is located. Figure 3 summarizes the biosorption mechanisms based on the configuration of sorbed pollutants and the presence or lack of metabolic activity.

Fig. 3
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Mechanisms of adsorption

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Adsorption is presently used to treat water because it has a number of benefits, including low cost, high efficiency, simplicity in use, the ability to use a variety of solids as adsorbent materials, and the ability to recover both the adsorbent and the adsorbate [1, 21, 60].

The competitive and effective nature of adsorption as a polishing process must be emphasized when contaminants are present in water at concentrations between ng L−1 and mg L−1 [1, 21]. There are many inexpensive materials that can be used as adsorbents to remove pollutants from contaminated land and water. They can be categorized as follows:

  • Agricultural waste-based biosorbents

  • Industrial waste-based biosorbents

  • Natural/organic biosorbents

  • Microbial-based biomass biosorbents

According to the literature in the last few years, the selection, development, and characterization of the adsorbent material; the development and optimization of the adsorption mode; the mathematical modelling; the choice and development of the regeneration process; and the application in actual samples are the crucial aspects of being evaluated to apply adsorption for water treatment. Also, the cost analysis (evaluation) is evidently crucial. Cost, however, is a factor in each of the aforementioned aspects and must be taken into account individually. To give you an idea, the costs for most technologies to treat water vary from 10 to 450 US dollars per m3 of treated water, while those for adsorption are 5.0–200 US dollars per m3 [1, 21]. The filter is responsible for about 70% of these expenses [61]. Hence, the short review will attempt to briefly discuss all of the aforementioned aspects of adsorption for water treatment. Also, some the major challenges as well as the prospects with adsorption for treating water are highlighted. The implication and rationale for consideration of this present review work is that it will serve as a quick guide to the literature, which will encourage new perspectives and original research in the application of adsorption techniques for treating water vis-à-vis innovative techniques for water monitoring and remediation.

2 Adsorbent/Bio-sorbent Materials

Key considerations in the characterization of an adsorption method for the treatment of water include the selection, development, and characterization of the adsorbent material sources [1, 21]. The following features are necessary for an appropriate absorbent for water treatment sources [1]:

2.1 Cost Effectiveness and Availability

Since the adsorbent accounts for 70% of the operating expenses, a significant quantity of the adsorbent material must be quickly produced or bought and transported to the treatment facilities.

2.2 Stability of the Chemical

This is necessary because various water matrices have different chemical properties (conductivity, pH, strength of the ions, etc.), which can affect the adsorbent.

2.3 Mechanical Stability

Since constant water treatment is carried out in columns, the adsorbent also needs to be stable mechanically to prevent high-pressure drops and preferred pathways.

2.4 Decent Textural and Physicochemical Features

A good adsorbent should have a high surface area, a large pore volume, and functional groups on the surface that can interact with pollutants. It should also have good textural and physicochemical properties. The following three points are impacted by these important traits.

2.5 High Adsorption Capacity

This is required because there must be a high pollutant absorption rate per gram of adsorbent. This reduces the amount of adsorbent needed for the therapy, facilitates phase separation following adsorption, and reduces the amount of physical space needed for the procedure.

2.6 High Efficiency

In accordance with the specific rules, a high percentage of contaminants that are removed from the water and transferred to the adsorbent should be achieved.

2.7 Quick Kinetics

The properties of the adsorbent should enable rapid attainment of high adsorption capacity and effectiveness. As a result, the total treatment time is brief, which has an impact on the treatment plant's size and capital expense.

2.8 Regeneration and Reuse Potential

The adsorbent should, if at all possible, be easily regenerated and used more than once to cut down on operational expenses.

Naturally, it is challenging to create an adsorbent with all of the aforementioned qualities. In this regard, various studies have been conducted to create a number of adsorbents for the treatment of water. For the adsorption-based method of treating water, carbon-derived materials such as chars, biochar, activated carbons, coals, and nanomaterials (NMs) have been created [62,63,64]. Chitin and chitosan-derived materials are a different class of adsorbents for the treatment of water [1, 65, 66]. Additionally, biosorbents and agro-industrial pollutants are receiving more attention [67, 68]. The inorganic-based materials are a different family that includes zeolites, layered double hydroxides, and geopolymers [69, 70]. Metal–organic frameworks were created with the intention of removing impurities from water [71].

Additionally, silica-based substances are used as adsorbents for the purification of water [1]. The literature is extensive in terms of preparation methodologies and sophisticated characterization techniques for the bulk of these adsorbent materials. In general, we can state that there are a number of suitable, well-developed, and characterized materials for adsorbents for water treatment. There is no question, however, that in order to achieve all of the aforementioned qualities and create more effective adsorption processes for the treatment of water, it is essential to continuously develop novel adsorbent materials sources [1, 21].

3 The Mode of Operation of Adsorption

The mode of operation of adsorption is crucial because it directly impacts the costs of water treatment, the amount of treated water produced, the amount of space needed for the equipment at the treatment plant, and the length of time needed to decontaminate the water sources [1, 21, 72]. Without a doubt, the primary method for treating water described in the publication (batch adsorbers) is discontinuous batch adsorption [1, 21, 73]. In this method of operation, the adsorbent is inserted into a tank filled with contaminated water. The mixture is stirred until it reaches equilibrium, or a capacity that is very near equilibrium. After that, decantation, filtering, or centrifugation are used to separate the solid from the liquid. This is a liquid phase (purified water) and a solid phase (the adsorbent filled with the contaminant) at the conclusion of the process [1, 21]. This method of operation is helpful for optimizing adsorption parameters like adsorbent dosage, concentration of the contaminant, time of contact, time of equilibrium, pH, and others at the laboratory scale [1, 2, 21]. Large volumes of water, on the other hand, necessitate more physical space and extra unit operations for solid–liquid separation, so it is ineffective for the treatment of large volumes of water sources [1]. The primary large-scale treatment application, fixed-bed operation mode, is only ostensibly recorded in the literature [1, 2, 21]. In this instance, the contaminated water is poured into a column that contains the adsorbent. The process continues until the column achieves saturation, where the concentrations of contaminants at the inlet and outlet are equal. Throughout the trial, the contaminant concentration at the column outlet is tracked. With the help of these statistics, the breakthrough curve can be built, allowing the adsorption process to be scaled up [1, 2, 21, 74]. Variables like bed height and flow rate can be assessed through this method of operation [1, 75]. Fixed-bed operation eliminates the need for extra separation processes and enables the treatment of large water volumes in constrained physical spaces. However, fluid-particle interaction does not always work well. Hydro-dynamic restrictions, such as the development of preferential pathways and high-pressure drops, can affect this working mode [1].

In the background of mode of operation of adsorption, we can state that batch adsorbers should be used on a laboratory scale to optimize the experimental adsorption conditions because they are well documented in the literature. In contrast to batch adsorption, operations in a fixed bed are not extensively studied at the laboratory scale, though their design and efficiency are. Evidently, the commercial scale-up of water treatment procedures can be accomplished through fixed-bed operations. The primary issue at hand is the creation and design of alternative operating modes that enable easier phase separation and more effective fluid-particle contact. As a result, we can benefit from factors such as a smaller physical area needed for the treatment, quicker processing periods, the ability to treat larger volumes of water, and lower costs. Studies involving fluidized beds, spouted beds, simulated moving beds, continuous stirred tank reactors, and multi-batch adsorption reactors are encouraged as possible alternatives sources [1, 21].

4 Modelling of the Adsorption Process

The literature has extensively examined the modelling of the adsorption process for the treatment of water. The study of isotherms, kinetics and thermodynamics is frequently used to clarify a defined adsorption process [76,77,78]. Additionally, mathematical tools such artificial neural networks (ANN), GAMS, fuzzy and neuro-fuzzy networks, and response surface methodology (RSM) can be used to improve the operational parameters [77, 79]. The isotherm curves depict a relationship between the amount of contaminant adsorbed in the adsorbent and the amount of contaminant remaining in the water under thermodynamic equilibrium circumstances [1]. All adsorption systems depend on these curves. Freundlich, Henry, Langmuir, Redlich-Peterson, Sips, Tóth, and other isotherm models are normally used to describe isothermal curves [80].

Additionally, other theories and statistical physics-based models are being created in this regard [81,82,83]. Although the literature contains all of this information, most of it only pertains to particular systems. The thermodynamic equilibrium constant is typically estimated from the isotherm parameters obtained at various temperatures, which is then used to compute the Gibbs free energy change [84]. The Van't Hoff plot is then utilized to determine the enthalpy and entropy changes [84]. Despite being widely used in the literature, this method's validity was questioned by some writers because of its shaky thermodynamic foundation [1, 85]. Specifically, the difficulties here pertain to the development of isotherm models for multicomponent systems and methods for precisely estimating the thermodynamic parameters.

Also, kinetic studies are essential for adsorption in water purification. Plots of adsorption capacity versus time or contaminant content versus time can be used to depict kinetic curves in batch systems. The representation in the case of a fixed-bed can be carried out using either dimensionless contaminant concentration at the column output versus time or dimensionless contaminant concentration at the column outlet versus bed volumes [86]. The adsorption rates are effectively modelled by a number of empirical and semiempirical models [6]. For batch adsorption, it can be referenced in pseudo-first order, pseudo-second order, general order, Avrami and Elovich [1], Bangham [6], and for fixed-bed systems (Adams-Bohart, Clark, Thomas, and Yoon-Nelson) [87]. These models are straightforward mathematically, but they are only useful for a limited variety of experiments [6].

The majority of the models mentioned above are unable to estimate basic mass transport parameters, which hinders scale-up. Diffusional mass transfer models, such as the external mass transfer model, uniform surface diffusion model, surface diffusion model, pore volume diffusion model, and pore volume and surface diffusion model, are also helpful in this regard and well-documented in the literature [88,89,90]. These models are more difficult mathematically, but they can estimate the mass transport properties by taking into consideration the mass transport steps that take place during the adsorption process. Scale-up is therefore feasible [91]. The estimation of mass transfer factors affects the resolution of diffusional models (effective pore diffusion coefficient, external mass transfer coefficient, and surface diffusion coefficient) [1].

There haven't been any fundamentally based correlations to determine the surface diffusion coefficient up until now in the literature. For single systems, diffusional and empirical models are both well established, but there is not enough data to model multicomponent systems. The formation and use of diffusional models for multicomponent systems and the formation of correlations to estimate the surface diffusion coefficient present intriguing challenges in adsorption kinetics for water treatment. The adsorption behaviour for water treatment is also studied using mathematical methods, such as ANN, RSM, fuzzy, and neuro-fuzzy [77, 79]. RSM is used to plan and optimize experiments with the goal of locating an area of interest or the best operating conditions [1]. ANNs are layered algorithms that draw their inspiration from the way the human brain functions. These mathematical tools, which are dependable and strong, can relate non-linearity between input and output variables from a series of trials [1]. Although fuzzy and neuro-fuzzy neural network models are also based on neural networks, they combine the learning and reasoning effects of ANNs to produce a more accurate outcome [1]. As a result of adjusting a number of input variables, such as contaminant concentration, contact time, temperature, stirring rate, adsorbent dosage, pH, and adsorbent characteristics (surface area, type, point of zero charge), it is possible to optimize a determined response variable (for instance, increasing the adsorption capacity or reducing the contaminant concentration in the liquid phase) (binary component, single component, molecular volume or molecular size). It is believed that these procedures are appropriate for representing multi-component systems where it is challenging to anticipate the interactions as well as variables that are not included in diffusional models (such as pH and specific properties of adsorbate and adsorbent). The difficulties here pertain to any advances in mathematics that can speed up calculations, cut down on the number of experiments needed, and shorten computation time. Studies on mixed models that couple isothermal and kinetic models with RSM, ANN, fuzzy, and neuro-fuzzy neural networks are also encouraged. Consecutive cycles should keep the adsorbent's potential, efficiency, and characteristics, and if at all possible, the adsorbate should be recovered. The renewal of adsorbent materials can be accomplished in a number of ways, including thermal, chemical, and microbiological ones [1].

Chemical reagents, electrical currents, microorganisms or physical waves can all act as regeneration agents. The renewal process can be triggered by leaching or extraction, pH changes, thermal desorption, reaction, or degradation [1]. Although most of these techniques are ineffective for recovering adsorbate, they were effective for regenerating adsorbent. Each of these regeneration techniques, of course, has benefits and disadvantages, and how they are used will rely on the particular adsorption system in question. It is crucial to clarify that regeneration is not always a possibility. For instance, if regeneration produces more waste than the actual adsorption process, if regeneration necessitates more operational effort than adsorption, and if the creation of the adsorbent is simpler than regeneration, therefore, it is preferable to use the refuse treatment techniques of incineration and landfill disposal in these circumstances. The primary issue in this regeneration field is the creation of new, environmentally friendly regeneration methods that reduce waste production, reduce costs, and increase the number of cycles that can be applied to the adsorbent while enabling the recovery of the adsorbate. This is undoubtedly a significant weak spot in the literature when it comes to the use of adsorption to treat actual water samples. The bulk of aqueous phase adsorption studies are concerned with treating synthetic solutions that only contain one adsorbate and water [92,93,94,95,96]. Two or three adsorbates may occasionally be used [97, 98]. In other situations, effluents with multiple pollutants are used [99, 100]. The remedies, however, are always artificial. The removal of pharmaceuticals and other contaminants of growing concern from complex treated wastewaters has been the subject of several adsorption studies, either alone or in combination with other processes using real or pilot set-ups [1, 101]. These pieces, however, make up a very tiny portion of the literature. The aforementioned factors, including adsorbent substance, operation mode, modelling, and regeneration, are of course crucial in actual water and wastewater treatment systems. Additionally, it is crucial to increase the selectivity of the adsorbents for particular contaminants [1, 21]. In a complicated mixture, this enables the adsorbent to only absorb the desired contaminant. Phase separation following adsorption is a crucial step in effective wastewater remediation.

Depending on the operation mode, decantation, filtration, or centrifugation are necessary for solid–liquid separation. In order to avoid these additional unit operations, there is a progress in the development of magnetic adsorbents, which can be easily separated from the liquid by a simple application of a magnetic field [102,103,104,105,106,107]. Concerning the application of adsorption to treat real water and wastewater samples, we agree that, initially, studies should be performed in synthetic solutions because the physicochemical and fundamental aspects of adsorption only can be elucidated in controlled conditions. On the other hand, an adsorbent can be excellent in laboratory studies but often inefficient in real cases. Centrifugation, decantation, or filtration may be required for solid–liquid separation, depending on the method of operation. The development of magnetic adsorbents, which can be easily separated from the liquid by a simple application of a magnetic field, has advanced in order to avoid these extra unit operations [102,103,104,105,106,107]. It is agreed that studies should initially be conducted in synthetic solutions before applying adsorption to actual water and wastewater samples because the physicochemical and basic aspects of adsorption can only be clarified under controlled circumstances. An absorbent, on the other hand, may perform admirably in lab tests but frequently poorly in practical situations. Evidently, adsorption research in laboratory settings is important, and it has been thoroughly recorded and consolidated [1]. However, there aren't many studies that have applications in the actual world. To move forward in this situation, we must first research and perfect adsorption under controlled circumstances before applying it to real-world situations.

5 Regeneration of the Adsorption Process

Adsorption in water purification also relies heavily on the regeneration of the adsorbent and its use with actual water samples [1, 21]. The process's operational expenses and environmental friendliness are directly impacted by the adsorbent regeneration and its repeated use [1, 21]. The characteristics, potential, and efficacy of the adsorbent should be preserved throughout subsequent cycles of the regeneration process, and if at all feasible, the adsorbate must be recovered. The renewal of adsorbent materials can be accomplished in a number of ways, including thermal, chemical, and microbiological ones [1, 21]. Physical impulses, electrical currents, chemical reagents, or microorganisms can all act as regeneration agents. The renewal process may be triggered by leaching or extraction, pH changes, thermal desorption, reaction, or degradation [1, 21].

Although most of these techniques are ineffective for recovering adsorbate, they are effective for regenerating adsorbent. Each of these regeneration techniques has, of course, its benefits and disadvantages, and how they are used will depend on the particular adsorption system in question. It is crucial to clarify that regeneration is not always a possibility. For instance, it is preferable to use the waste treatment techniques of incineration and landfill disposal in situations where regeneration produces more waste than the adsorption process itself, requires more operational work than adsorption, and is simpler to create an adsorbent than regeneration [1, 21].

The primary issue in this regeneration field is the creation of new, environmentally friendly regeneration methods that reduce waste production, reduce costs, and increase the number of cycles that can be applied to the adsorbent while enabling the recovery of the adsorbate. This is undoubtedly a significant weak spot in the literature when it comes to the use of adsorption to treat actual water samples. The bulk of aqueous phase adsorption studies are concerned with treating synthetic solutions that only contain one adsorbate and water [92,93,94,95]. Two or three adsorbates may occasionally be used [97, 98]. Other times, effluents with a variety of pollutants are applied [99, 100].

The remedies, however, are always artificial. The removal of pharmaceuticals and other contaminants of growing concern from complex treated wastewaters has been the subject of several adsorption studies, either alone or in combination with other processes using real or pilot set-ups [1, 21, 101]. These pieces, however, make up a very tiny portion of the literature. The aforementioned factors, including adsorbent substance, operation mode, modelling, and regeneration, are of course crucial in actual water and wastewater treatment systems. Additionally, it is crucial to increase the selectivity of the adsorbents for particular contaminants [1, 21].

In a complicated mixture, this enables the adsorbent to only absorb the desired contaminant. Phase separation following adsorption is another crucial component of effective wastewater remediation. Centrifugation, decantation, or filtration may be required for solid–liquid separation, depending on the method of operation. The development of magnetic adsorbents, which can be easily separated from the liquid by a simple application of a magnetic field, has advanced in order to avoid these extra unit operations [102,103,104,105,106,107]. We concur that studies should initially be conducted in synthetic solutions before applying adsorption to treat actual water and wastewater samples, owing to the fact that the physicochemical and basic aspects of adsorption can only be clarified under controlled circumstances. An absorbent, on the other hand, may perform admirably in laboratory tests but frequently poorly in practical situations. Undoubtedly, laboratory adsorption studies are important, and they have been thoroughly documented and consolidated. Table 2 contains analysis of the maximum sorption capacity (qm) of some studies on biosorbents for some selected HMs and dyes in water. However, there aren't many research studies that have practical applications. Here, we must move forward; that is, we must research and improve adsorption under controlled circumstances before using it in actual situations sources [1, 21].

Table 2 analysis of the qm of some biosorbents/NMs for some selected HMs and dyes in water
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6 Conclusion

The generation and treatment of effluents are an increasing concern today due to increased urbanization and industrialization. To address the problem of rising environmental risks, a variety of effluent treatment techniques, including physical, chemical, and biological (primary to secondary treatment) methods, are used. The potential for producing secondary pollution is increased by the use of various cleaning techniques. The use of different materials as adsorbents is the most efficient way to treat effluents with the least amount of secondary pollution generation. Adsorption is a vital process that is both effective and affordable. This technique primarily employs the adsorption procedure and mechanism to remove toxicants from effluents. Along with any possible techniques of adsorption for water treatment, some of the major challenges as well as the prospects for adsorption for treating water are highlighted. This also makes it clearer how these sorbents can be modified or given special treatment to increase their efficacy.

The significant study gaps and potential directions for future research on adsorption vis-à-vis innovative techniques for water monitoring and remediation are also highlighted in the next section. However, the main goal of this present review study was to explore the issues and potential benefits of adsorption applications for treating water vis-à-vis environmental safety. This study specifically addressed the present situation and the main challenges with the adsorption applications for environmental safety in relation to water purification. As a result, this review serves as a convenient reference for fresh ideas and original studies in the field. Finally, the review suggests what ought to be the subsequent actions required to advance this aspect.

7 Future Insights on Adsorption Vis-à-Vis Innovative Techniques for Water Monitoring and Remediation

Nevertheless, contemporary adsorption technologies are urgently needed to ensure high-quality water, reduce chemical and biological contaminants, and enhance agricultural and industrial production processes for environmental safety [1, 120,121,122]. Up to this point, the majority of research findings have involved modest laboratory tests. The lack of knowledge about pilot-scale systems is the main weakness of substituting cost-effective adsorbents for activated carbon and other costly treatment technologies [21]. Before promoting the widespread use of unconventional adsorbents, more study is required. Nanotechnology, which includes the adsorption of contaminants using NMs, is one of the best techniques for contemporary contamination treatment processes [68, 123, 124].

It has been effective to conduct research on the treatment of contaminants, and numerous different NMs have been developed. These include a few that are noteworthy; photocatalysts, electrocatalysts, nanofilms, and nano-adsorbents based on the metals [1, 21, 120, 125]. These NMs can also be introduced to biological processes to improve the effluent treatment procedure's efficiency (like microbial fuel cells, algae membranes, and anaerobic fermentation). Each technology varies in how well it removes contaminants and has advantages of its own [1, 21, 120, 125, 126]. Nano-adsorbents can be used to filter out potentially hazardous substances from wastewater [123, 127]. With the help of NM photocatalysts, harmful contaminants can now be treated without the use of expensive synthetic UV radiation, thanks to the modification of the catalyst substance.

As previous stated, there is a need to look into earlier water quality monitoring techniques, contrast traditional and contemporary techniques, and examine various techniques from different nations. As reported by Zainurin et al. [27], there is a thorough overview of various approaches for monitoring water quality, including the cyber-physical system approach, electronic sensing methods, virtual sensing systems, IoT approach, and optical techniques. According to the Zainurin et al. [27], cyber-physical systems are appropriate and suitable for use in water quality monitoring systems. In addition, a cyber-physical system connects the physical and digital realms by using software and data to communicate with sensors, environments, and people in the physical world. Figure 4 shows an outline of virtual sensing development stages as adapted from Zainurin et al. [27]; Paepae et al. [128].

Fig. 4
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An outline of virtual sensing development stages

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The potential of early warning in the water quality management system is made possible by the indirect real-time monitoring of water quality [25]. As a result, it is possible to identify water pollution and evaluate the water's quality before it is suitable for consumption. Future monitoring methods will be able to combine advanced optical techniques with cyber-physical system technology to produce systems with high reliability and sensitivity because current monitoring techniques have trouble getting accurate measurements of water quality parameters in real-time and are not cost-effective with continuous data collection [44]. There have been some tool limitations in the past that have necessitated improvements to the current water quality evaluations.

Also, as previously mentioned, micro- and nanorobots are designed to expedite and enhance the water purification process (remediation). Self-propulsion has added an engineering component to micro- and nanomaterials, enabling the creation of groups of intelligent, small-scale machines that move in reaction to exterior cues and collaborate to carry out specific tasks. Micro- and nanorobots have proven successful in water remediation applications, where the effectiveness and speed of the purification process are important. This is because of the interaction between active motion and programmed contaminant removal and degradation processes achieved through material design. Although the effectiveness of micro- and nanorobots against contaminants varying in type and size (from mm- to atomic-scale) is wide, there are still a number of obstacles to overcome for practical uses. For uses involving water remediation, self-propelled micro- and nanorobots are more effective than static materials. For all the contaminants examined, the elimination and degradation efficiencies are also insufficient. As an illustration, it is especially difficult to completely decompose plastic debris because polymers contain UV stabilizers to increase their stability. Combining various degradation processes in a single robot or programming it to specifically target the most enduring contaminant are two feasible solutions. These characteristics increase complexity and manufacturing costs because they call for the integration of numerous components. Thus, in order to satisfy the demands of mass production, the design of micro- and nanorobots must be kept as basic as feasible, for example, by utilizing high-throughput fabrication techniques (for example, 3D bioprinting) [38]. The increase in small-scale machines in the environment should be considered a danger. Utilizing readily available, safe, and impermanent natural materials like microalgae is a desirable choice in this situation. The widespread use of toxic chemicals in agriculture to increase agricultural output puts soil and plants at high risk of contamination. Micro- and nanorobots can be used to clean up these areas. Small-scale machines that are biocompatible and environmentally benign could break down poisons into harmless substances on the spot. The robots require a liquid medium, just like in water remediation, to allow their active movement as well as the removal and degradation of pollutants. Under sunny conditions, the pesticides could be broken down at the water-soil and water-plant interfaces by suspending the robots in water sprinkled on the soil or plant surfaces. They could also be made to specifically target bugs, taking the place of harmful poisons [38]. The enormous promise of using micro- and nanorobots for water remediation has been demonstrated in numerous studies. To satisfy market demands, however, the development of this technology into practical uses requires the combined efforts of scientists from various disciplines.

Since NMs are still more expensive than conventional materials, future studies should concentrate on effective processes that only require small concentrations of NMs (such as activated carbon) [123, 126]. Furthermore, more work is needed to develop low-cost methods of NM synthesis and to conduct large-scale efficiency assessments for practical field uses. The following research voids must be filled in the future:

  • Understanding how adsorbents function in wastewater filtration requires research at the cellular and molecular levels.

  • More work needs to be done on pilot-scale and field-scale experiments in addition to laboratory testing.

  • Cost-effect benefits need to be estimated and computed prior to performing pilot-scale experiments.

  • It is crucial to look into how well the adsorbents mentioned here work in conjunction with other treatment modalities, especially carbon-based adsorbents.

  • Investigating microbial interactions in aquatic environments is crucial.

  • It is necessary to conduct more research on how geo-environmental factors affect sorption processes.